CAPS approved, this popular series has been written by a team of Mechanical Technology specialists who are familiar with...
Charles Goodwin André Lategan Daniel Meyer
Mechanical Technology Grade 12 Learner’s Book
© Future Managers 2013 All rights reserved. No part of this book may be reproduced in any form, electronic, mechanical, photocopying, or otherwise, without prior permission of the copyright owner. ISBN 978-1-77581-000-1 To copy any part of this publication, you may contact DALRO for information and copyright clearance. Any unauthorised copying could lead to civil liability and/or criminal sanctions.
Telephone: 086 12 DALRO (from within South Africa); +27 (0)11 712-8000 Telefax: +27 (0)11 403-9094 Postal Address: P O Box 31627, Braamfontein, 2017, South Africa www.dalro.co.za First published 2007 Second edition 2012 Revised 2013
FutureManagers Published by Future Managers (Pty) Ltd PO Box 13194, Mowbray, 7705 Tel (021) 462 3572 Fax (021) 462 3681 E-mail: [email protected]
Website: www.futuremanagers.net ii
Please note that this copy is for sample purposes only and will still undergo final editing. Contact Future Managers for more details on when final copies can be ordered or ‘like’ our Facebook page to be kept up to date.
Contents How to use this book............................................................................................................................................................................................................ iv Chapter 1 – Safety.............................................................................................................................................................1 Chapter 2 – Tools............................................................................................................................................................27 Chapter 3 – Materials.....................................................................................................................................................51 Chapter 4 – Terminology..............................................................................................................................................63 Chapter 5 – Joining methods........................................................................................................................................79 Chapter 6 – Forces....................................................................................................................................................... 105 Chapter 7 – Maintenance........................................................................................................................................... 123 Chapter 8 – Systems and control............................................................................................................................... 139 Chapter 9 – Turbines................................................................................................................................................... 167
How to use this book Outline of the curriculum Mechanical Technology Grade 12 Learner’s Book has been based on the new FET Curiculum for Mechanical Technology which entails 9 topics.
Spider diagrams Each chapter is introduced by a spider diagram which is a diagrammatical summary of the content covered in a particular chapter. The following spider diagram is an example from Chapter 6: Calculation of forces in engineering components
Calculating moments in engineering components
Advanced tests on various mechanical principles
Calculation of stress, strain and modulus of elasticity
Concepts of stress, strain and modulus of elasticity
Explanation of icons The following icons are used throughout the book to help you to recognise important concepts or activities.
Did you know?
Pause for thought
Besides the various icons, explanatory notes, Pause for thought and Did you know? boxes have been placed in the margin to give further insights.
The importance of Mechanical Technology in the world South Africa and many other countries in the world currently suffer from huge skills’ shortages and desperately need skilled engineers, technologists, technicians and artisans. An introduction to Mechanical Technology aims to produce learners who have been exposed to skills, knowledge, attitudes and values (SKAVs) which will equip them for further study in Mechanical Engineering and related sectors. The subject Mechanical Technology focuses on technological processes from conceptual design through to the process of practical problem solving to produce or improve on products which can enhance our quality of life.
Explanation of the key words You may encounter many unfamiliar words in this course. For this reason, key words have been included in the margins to explain the meanings of words that appear in bold print in the text. The key words also cover acronyms (words made up of the first letters of the name of something) and abbreviations that are used in the book.
Assessment activities The assessment activities comprise individual, pair and group tasks. Some are pen-and-paper activities and some are practical tasks. The solutions to some tasks can be found in the text but others will require you to do further research. It is very important that you read the instructions carefully before attempting any of the tasks.
Message from the authors You have the good fortune to be one of the first learners to choose Mechanical Technology as one of your FET subjects. It will definitely stand you in good stead for your future studies. To help you succeed in this subject, it is essential to apply the following principles: • Go through your notes and make sure that you understand the work. • Learn the important concepts and definitions. • Solve as many problems as you can. • You will find that regular revision will help you to understand and remember the work better. Do not hesitate to refer to other relevant reading material to broaden your understanding of the subject. Above all, think and work hard. We wish you well for your studies this year. THE AUTHORS
Safety Topic 1
Lathes Milling machines
OHS Act Grinding machines
Cutting machines Safety
Press machines Joining
Introduction Safety in the workplace was introduced in Grades 10 and 11. In Grade 12, learners must be fully aware of all the safety precautions to be taken during performance-based activities in order to avoid injuries or incidents. Learners must demonstrate an understanding of the Occupational Health and Safety (OHS) Act where applicable.
Occupational Health and Safety Act 1993 (OHSA) The aim of the Occupational Health and Safety Act, Act 85 of 1993 (OHSA) is to provide for the health and safety of employees at work. The following groups are excluded from the Act: • Parties covered by the Merchant Shipping Act. • People employed in mines, mining areas or any works defined in the Mine Health and Safety Act 29 of 1996. The provisions of the Occupational Health and Safety Act of 1993 (OHSA) are as follows: • The employer must provide and maintain a safe working environment. • Suppliers/manufacturers must ensure that items used in the workplace do not pose a safety or health risk. • Employers must inform the workforce of hazards in the workplace. Employees must: • Take reasonable care to ensure their own and others’ health and safety. • Carry out any lawful order and obey the health and safety rules and procedures laid down by the employer. • Report unsafe/unhealthy situations. • Report incidents that may affect health or which may cause injury. The Act also refers to the following: • The appointment of health and safety representatives. • The establishment of health and safety committees. • Reporting of incidents/diseases and inspections.
Extracts from the Occupational Health and Safety Act of 1993 (OHSA) concerning General Health and Safety Regulation Personal protective equipment and facilities 1. Subject to the provisions of section 8(2)(d), every employer, self-employed person or user, as the case may be, shall make an evaluation of the risk attached to any condition or situation which may arise from the activities of such employer, self-employed person or user, as the case may be, and to which persons at a workplace or in the course of their employment or in connection with the use of machinery are exposed, and he shall take such steps or precautionary measures as may be necessary to render the condition or situation safe and without risk to the health of persons. 2. Taking into account the nature of the hazard that is to be countered, and without derogating from the general duties imposed on employers and users by sub-regulations (1) and (2), the personal protective equipment and facilities contemplated in sub-regulation (2) shall include, as may be necessary:
• suitable goggles, spectacles, face shields, welding shields, visors, hard hats, protective helmets, caps, gloves, gauntlets, aprons, jackets, capes, sleeves, leggings, spats, gaiters, protective footwear, protective overalls, or any similar personal protective equipment or facility of a type that will effectively prevent bodily injury; • waterproof clothing, high visibility clothing, chemical resistant clothing, low temperature clothing, chain-mail garments, waders, fire-retardant or flameproof clothing, ice jackets, or any similar personal protective equipment of a type that will effectively protect the wearer thereof against harm; • harnesses, nets, fall arresters, life lines, safety hooks or any similar equipment of a type that will effectively protect persons against falls; • mats, barriers, locking-out devices, safety signs or any similar facility that will effectively prevent slipping, unsafe entry or unsafe conditions; • protective ointments, ear muffs, ear plugs, respirators, breathing-apparatus masks, air lines, hoods, helmets or any similar personal protective equipment or facility of a type that will effectively protect against harm: provided that hearing protective equipment shall be of a type that conforms to SANS 1451; Parts I and II: provided further that respiratory protective equipment shall be of a type that conforms to SANS 033-99; • suitable insulating material underfoot where persons work on a floor made of metal, stone or concrete or other similar material; and such personal protective equipment or facilities as may be necessary to render the persons concerned safe. It is important to protect yourself by wearing the appropriate protective clothing. To begin with, a correctly fitting, fire-retardant overall (one- or two-piece) is the first line of defence, as it protects your body and clothing from workshop dirt and sparks. It also prevents your clothing from becoming entangled in machine parts. Steel-capped safety shoes or boots are also important as they protect your feet from falling objects. Depending on the activities you will be conducting in the workshop, the following items of personal protective wear are also necessary: • chrome leather gloves and aprons – essential when welding or working with heated material • protective eyewear – important when conducting any work which can be hazardous to your eyes, for example, shaded lenses should be worn when welding to protect your eyes from intense heat and light, and clear protective lenses should be worn when doing grinding work or machining operations • ear plugs or ear muffs to protect your ears – especially if you are working in a noisy environment. It is important to get into the habit of wearing the necessary protective equipment. Neglecting to wear the appropriate equipment even once may result in permanent injuries or worse. Figure 1.1 illustrates some basic, personal protective safety wear. Overall
Safety shoes Ear plugs
Figure 1.1: Basic personal protective wear
Grinding machines Angle grinders
Handle Disc Safety guard
Figure 1.2: Angle grinder
When working with angle grinders, remember the following safety precautions: • The same safety precautions applicable to other types of grinders are applicable to angle grinders. • The safety guard must be in place before you start the grinding process. • Protective shields must be placed around the grinding object to protect people passing by. • Use the right blade for the grinding job. • Do not force the grinding stone on the object. • Make certain that there are no cracks in the stone before you start the job. • Protective clothes and eye protection are essential when working with an angle grinder. • Wear ear plugs or muffs. • Wear safety boots with steel toe caps. • Wear overalls or other close-fitting clothing. • Wear gloves.
Bench grinders Perspex shield Tool rest On/off switch
Head Wheel guard Maximum gap 3 mm Grinding wheel Stand
Figure 1.3: Bench grinder
Safety When you use a bench grinder, remember the following: • Use a machine only when the guards have been correctly fitted. • Make sure that there is no oil or grease on the floor around the machine which could cause you to slip. • Check that the tool rest is not more than 3 mm from the surface of the grinding wheel. Gaps exceeding 3 mm increase the risk of material being drawn in between the tool rest and grinding wheel. • When starting the machine, do not stand in front of the wheel. Before you start grinding, let the machine idle for a few seconds. The risk of the grinding wheel rupturing is higher at start-up than when it is running at its operating speed. • If the wheel is running unevenly, dress it with an emery-wheel dresser. • Grind only on the face of a straight grinding wheel and never on the side of the wheel. • Use wheels for their intended purpose only. Certain types of grinding wheels should only be used for their corresponding materials. Most grinding wheels are only suitable for grinding ferrous metals. The appropriate degree of coarseness should also be selected for the finish required of the material. • Approach the wheel carefully and gradually and do not ‘jab’ materials onto it. Jabbing puts uneven pressure on the wheel surface, causing uneven wear or structural damage to the wheel. • Never ‘force grind’ so that you cause the motor to slow or stop. • Adjust the tool rest only when the wheel is stationary. • Clamp workpieces and holding devices safely and firmly. • Never allow the wheel to stand in cutting fluid as this may cause the wheel to run ‘off balance’ when you switch the machine on again.
Idle Run without any load
Exceed Be more than
Stationary At rest and not rotating
Mounting of grinding wheels The following steps are advised: • Select the correct type of wheel for the job. • Inspect the wheel for cracks and tap it to apply the ‘ringing test’. Never use a grinding wheel which is damaged or not properly dressed. • Make sure that the wheel’s speed does not exceed the manufacturer’s recommendation. • Never force the wheel onto the spindle. • Use only one smooth paper spacer on each side of the wheel. • Use true and correctly recessed flanges of the same size and at least one-third the diameter of the wheel. • Gently tighten the grinding wheel with a spanner, only enough to hold it firmly. • Replace the guards correctly. • Stand aside and set the machine in motion. Let the machine idle before you dress the wheel, using an emery-wheel dresser. • Finally, stop the machine and reset the tool rest to within 2 mm of the wheel surface. • Ensure that the tool rest is parallel to the wheel surface.
Grinding wheels All power-operated grinding machines should be clearly marked to indicate the recommended speed (in revolutions per minute) of the spindle. This speed should not allow the peripheral speed of the wheel to exceed the manufacturer’s recommendation.
Spindle The spindle of the bench grinder is the rotating shaft onto which the grinding wheel is attached
Peripheral speed The peripheral speed of the grinding wheel is the speed along the circumference of the grinding wheel
Rupturing Breaking or bursting suddenly
Other safety measures are: • Every grinding wheel should have a guard which can withstand the force of a rupturing wheel. • Bench grinders must have a transparent shield to protect the operator’s eyes. • Each machine must carry a notice prohibiting persons from performing, inspecting or observing grinding work without wearing suitable protection for the eyes.
Surface grinders Prohibiting Not allowing
Horizontal table Horizontal adjuster On/off switch
Figure 1.4: A surface grinder
Remember the following safety precautions when using surface grinders: • The safety precautions applicable to other types of grinders are applicable when using a surface grinder. • Protective clothes and eye protection are essential when working with a surface grinder. • Before operating the surface grinder, be sure you have been taught how to control it and are aware of the potential dangers associated with it. • Do not operate the surface grinder unless all guards and safety devices are in place and working correctly. • Make sure that you understand the operating instructions applicable to your machine. • Never clean or adjust the machine while it is in motion. • Report any dangerous aspect of the machine immediately and stop using it until it has been repaired by a qualified person. • You may have to stop your machine in an emergency. Learn how to do this quickly and automatically.
Assessment 1 1. 2. 3. 4.
When working with an angle grinder, you must follow safety rules. Name six. Name five safety precautions to observe when working with a grinding wheel. Name five steps to follow when installing a grinding wheel. Name five safety precautions to remember when working with a surface grinder.
Cutting machines Drill presses Motor and gearbox
Depth gauge Feed lever
Figure 1.5: A drill press
Observe the following safety precautions when using a drill press: • Choose a drill bit correctly sharpened for the type of work you need to do and the material of the workpiece. • Do not leave the key in the chuck when you are not at the machine. • Never leave the machine running if it is unattended. • Clamp the workpiece securely to the table and do not hold it by hand. • Never try to stop the workpiece by hand if it slips from the clamp. • A drill should run at the correct speed for the job. • Do not force a drill bit into the workpiece – this may cause broken or splintered drill bits and possible injuries. • Use a brush or wooden rod to remove chips from the drill. Do not use your fingers, waste material or rags. • When reaching around a revolving drill, be careful that your clothes do not get caught in the drill or drill chuck. • Do not use a drilling machine with a faulty switch. • Do not wear loose clothing or jewellery when drilling.
Did you know? A drill press is also called a drilling machine.
Portable electric drilling machine
Figure 1.6: Portable drilling machine
Mechanical Technology Observe the following safety precautions when using a portable drilling machine: • Wear safety goggles. • Choose the correct size and type of drill bit. A metal bit is used to drill into iron and a masonry bit to drill into a brick wall. • The bit must be placed in the chuck of the drill and fixed in place by using the chuck key. Make sure that the bit is centred. • Place the key in the key holder provided at the bottom of the drill’s handle. • If you are drilling into metal, mark the position with a centre punch. • Stand firmly with your legs slightly apart and one leg more forward than the other. • Hold the drill firmly and squeeze the trigger. • If you are drilling through a metal plate you should decrease the pushing pressure when you come close to drilling through the last piece of the metal plate.
Power saw switch
Figure 1.7: A power saw
Observe the following precautions when using a power saw: • Ensure that all guards are in place. • Make sure that there is no oil, grease or obstacles around the machine. • Select the right blade for the material to be cut. • When changing blades, ensure that the machine is switched off at the main switch. • Remove or replace the blade gently. Quick movements, such as pulling off the blade, may result in a badly cut hand. • Do not adjust guides while the machine is running. • All material must be clamped properly before cutting is started. • Long pieces of material must be supported at the end. • Always stop the machine when you leave it unattended.
Centre lathes and milling machines Toolpost Top slide or compound slide Cross slide Cross-feed handle Saddle or carriage Thread dial Power-feed control Leadscrew engager
Feed rod Carriage handwheel
Figure 1.8: A centre lathe Adjustable overarm Arbor support
Machine table Knee and saddle Handwheel
Figure 1.9: A milling machine
Observe the following safety precautions when working with a centre lathe or milling machine: • Make sure that all guards are in place. • Do not use a machine or come close to its moving parts while wearing loose clothing. • Keep any cleaning material such as waste and rags away from rotating parts. • Check that there is no oil or grease on the floor around the machine. • Do not leave spanners or keys on rotary parts. Always disconnect, remove or stand clear of handwheels, levers or chuck keys before setting your machine or feeds in motion. • Never apply a spanner to revolving work. • Always clamp workpieces and holding devices safely and firmly. A loose fit, especially of spanners and keys, may cause slipping and result in injury. • Do not use your hands to remove cuttings while a machine is in motion. Use a wire hook or a brush once the machine has stopped. • Never adjust the cutting tool while a machine is running.
Mechanical Technology • Resist the habit of leaning on machinery. This dangerous, ‘automatic’ practice often results in serious injury. • Do not attempt to stop a machine by placing your hand on the chuck while the machine is slowing down. • Pay attention to cutting-fluid control before switching on a machine.
Assessment 2 1. When working with a portable drilling machine, you must adhere to safety rules. Name six. 2. Name five safety precautions to follow when working with a drill press. 3. Power saws are dangerous power tools. Name five safety precautions that must be observed when working with them. 4. Give four safety precautions to follow when working with a lathe or a milling machine.
Blade safety mechanism
Figure 1.10: An electrical guillotine Safety mechanism Cutting blade
Figure 1.11: A manual guillotine
Manual and electrical shearing machines Where the opening at the point of operation of a pair of shears or a guillotine is greater than 10 mm, the machine should be fitted with one of the following: • A fixed guard to prevent hands or fingers from reaching through, over, under or around the guard. • A self-adjusting guard which automatically adjusts to the thickness of the material being worked. • Some machines have manual or automatic moving guards which completely enclose the point of operation so that the working stroke cannot be opened unless the ram or blade is stationary. • Another safety device is the automatic sweep-away or push-away that pushes any part of the machine operator’s body out of the danger zone when the working stroke starts. Nowadays, there is even an electronic presence-sensing device which stops the working stroke if the device senses a foreign object in the danger zone.
Hydraulic press Pressure meter
Return springs Plunger Platform
Hydraulic press cylinder
Figure 1.12: A hydraulic press
Observe the following safety precautions when using a hydraulic press: • The predetermined pressure must never be exceeded. This operating pressure is always less than the maximum safe pressure and is shown by a pressure gauge on the apparatus. • Pressure gauges must be tested regularly and adjusted or replaced if any malfunction occurs. • The platform on which the workpiece rests must be rigid and square with the cylinder of the press. • The platform must rest on the supports provided and should not be supported by the cable by which it is raised or lowered. • Objects to be pressed must be placed in suitable jigs. Ensure that the direction of pressure is always at 90° to the platform. • To prevent damage to soft material, the prescribed equipment must be applied. • Relieve the cylinder of all pressure after use by opening the valve.
Mechanical Technology Also remember: • The level of the hydraulic fluid in the reservoir should be checked regularly. If fluid has to be added frequently, it is an indication that there may be an internal leak. • Regularly inspect the apparatus for rigidity and tighten all nuts and bolts. • Pins and/or other equipment that keep the platform at a desired height on the frame must be inspected regularly for damage. • When the apparatus is equipped with cables to alter the working height of the platform, the cable and pulleys must be inspected for damage and lubricated with grease.
Assessment 3 1. A hydraulic press is an important tool in the workshop. Name five precautions to observe when working with this equipment. 2. Which safety devices are used in conjunction with guillotines?
Joining (arc, spot, gas) Arc welding Safety rules to observe when working with arc welders • Always wear personal protective equipment. • Make sure the area where you are going to work is clear of obstructions. • Use as small a rod as possible when tackling the job. This will ensure a much better and neater weld upon completion of the job. • Only weld in well-ventilated areas. • Seek medical attention if someone suffers a burn. • Do not weld near flammable materials or liquids. • Do not weld on petrol tanks or any containers that contain flammable liquids or gas. • Radiation from the arc is dangerous to the eyes. Always wear a welding mask. • Avoid striking an arc when other people are close to you. • An electric shock is always a possibility.
Figure 1.13: Spot welding machine
Safety rules to observe when working with a spot welder • Wear protective clothes. • Wear goggles. • Wear gloves.
• The area must be cordoned off when the machine is being used. • Make certain that the time and current settings are appropriate to the type and thickness of the material being welded. • Make certain that the alloy tips are kept in good condition and are not damaged during operation. • Make certain that the copper tips are constantly cooled (through the circular liquid cooling system) to prevent overheating.
Gas welding Oxygen regulater Oxygen flashback arrestor
Acetylene regulater Acetylene flashback arrestor
Cylinders secured in a cylinder trolley
Acetylene hose Oxygen hose Acetylene cylinder
Parallel hose clips Nozzle
Torch mounted flashback arresters
Universal cutting torch
Figure 1.14: Oxy-acetylene welding plant
The following are some safety precautions that should be followed when using oxy-acetylene apparatus: • Welding or flame-cutting operations may not be undertaken unless: – An operator has been instructed on how to use the oxy-acetylene welding plant safely. – The workplace is effectively partitioned off. – An operator uses protective equipment. – Effective ventilation is provided and maintained. – Masks or hoods maintaining a supply of safe air for breathing are provided and used by the persons performing such operations. • The following precautions must also be considered: – Any vessel that contains a substance which, when subjected to heat, may ignite or explode (or react to form dangerous or poisonous substances) must not be welded or heated until it has been properly cleaned.
Mechanical Technology – Where hot work involving welding, cutting, brazing or soldering operations is carried out at places other than workplaces, steps must be taken to ensure proper and adequate fire precautions. When in doubt, the manufacturer’s instructions are always the final authority on safety precautions and procedures. African Oxygen (Afrox) supplies safety booklets on all aspects of welding safety, free of charge, at their outlets and depots. – Never use damaged equipment. – Never use oil or grease on or near oxygen equipment. – Never use oxygen or fuel gas to blow dirt or dust off clothing or equipment. – Never light a torch with matches or a lighter. Always use a striker. – Always crack cylinders before assembling the regulators to remove any dust. – Always make sure regulators have their adjusting screws released by turning them anticlockwise untill free before opening the cylinder valves. Stand to the side of a regulator and not in front of it when opening cylinder valves. – Always wear proper welding goggles, gloves and clothing when operating oxy-acetylene equipment. – Always have a fire extinguisher handy when operating oxy-acetylene equipment. – Always use the proper regulator for the gas in the cylinder. – Always use cylinders in the upright position only. – Always keep the valve wrench on the acetylene cylinder valve when in use. – Open the cylinder valve a maximum of 1½ turns. – Do not carry lighters, matches or other flammable objects in your pockets when welding or cutting. – Always be aware of others around you when using a welding torch. – Be careful not to let welding hoses come into contact with the torch flame or the sparks from cutting.
Assessment 4 1. Name four safety precautions to observe when working with an electrical welder. 2. Name four safety rules that should be considered when working with a spot welder. 3. Name six safety precautions to be observed when welding with a gas welding apparatus.
Handling gas cylinders
Figure 1.15: Oxy-acetylene cylinders
Every gas cylinder should have a label which contains the following particulars: • name of the manufacturer • country of origin • year of manufacture • manufacturer’s serial number • name, number and date of the standard of design • design gauge pressure in pascals • maximum permissible operating pressure in pascals • operating temperature • mark of an approved inspection authority. No portable gas container should be used, filled, handled, modified, repaired or inspected in any way other than in compliance with standards set by the South African Bureau of Standards (SABS). The following safety precautions must be observed when handling gas cylinders: • Store full cylinders and empty ones separately. • Keep cylinders in a cool place and protect them from sunlight and other sources of heat. • Always store and use acetylene cylinders in an upright position. • Store oxygen cylinders and acetylene cylinders separately. • Never stack cylinders on top of one another. • Do not bang or work on cylinders. • Never allow cylinders to fall. • Do not allow oil or grease to come into contact with oxygen fittings as this forms a flammable mixture. • Keep the caps on the cylinders for protection. • The thread on an oxygen cylinder is a right-hand thread. • The thread on an acetylene cylinder is a left-hand thread.
Cylinder testing register The following are the requirements for cylinder testing: Pressure vessels must be tested every 4 years, inspected every 2 years, tested before commissioning, tested after each major repair and tested after being out of use for longer than 2 years. Location: Record of inspections and tests Manufacturer’s particulars: Manufacturer:
Country of origin:
Year of manufacture:
Hydraulic test pressure:
Name and number of code of manufacture: Maximum permissible working pressure: Inspection/test date:
Mechanical Technology The following form is used by inspection authorities when testing cylinders. Items
Clean and free of carbonised oil and other matter
Chemically reactive matters Internal surfaces and all seams External and internal plates: cleaned All plates hammer tested All welds checked Corrosion, internal and external Internal pitting Oil deposits Red mark on pressure gauge (P.W.P.) Safety valve set, locked or sealed Liquid level indicator Reducing valve Leaks detected Other defects detected Remarks I certify that this pressure vessel has been inspected and/or tested in accordance with legal requirements and that the employer/manager has been informed of all weaknesses and defects.
Vessel May be used May not be used
Hydraulic pressure test
Signature of competent person
Major repair/out of use
Assessment 5 1. Which nine particulars must be visible on a gas cylinder? 2. Name five precautions that should be considered when handling gas cylinders.
Specific safety measures when dealing with the following machines and equipment Metal-arc gas shield welders Gas hose Flow meter
Continuous wire reel Wire-feed unit Power cable
Shielding gas cylinder
Figure 1.16: Metal-arc gas shield welders
The following safety measures must be adhered to: • An operator should be instructed to use a machine safely. • A workplace is effectively partitioned off. • An operator uses protective equipment. • Effective ventilation is provided and maintained. • Masks or hoods maintaining a supply of safe air for breathing are provided and used by the people performing such operations. • The insulation of electrical leads is satisfactory. • The holder which contains the wire is completely insulated to prevent accidental contact with current-carrying parts. • The operator is completely insulated by means of boots, gloves and rubber mats. • The argon gas cylinder is fixed in an upright position. • When welding operations are carried out at places other than workplaces, steps must be taken to ensure proper and adequate fire precautions. When in doubt, the manufacturer’s instructions are always the final authority on safety precautions and procedures.
Hardness tester Hardness indicator meter
Platform height adjuster Activating knob
Figure 1.17 (a): Rockwell hardness tester
Platform adjuster Activating panel
Figure 1.17 (b): Brinell tester
Mechanical Technology The following safety measures must be adhered to: • The area around the tester must be cordoned off. • Only one person should be in the confined space. • The person should wear protective clothing (an apron). • The tester should wear safety goggles. • The tester should wear gloves. • The hardness tester should be mounted on a rigid spot on a worktable. • A cover should be placed around the area where the metal ball will be launched onto the material to be tested.
Tensile tester Handwheel
Figure 1.18: Mini tensile tester
The following safety measures must be adhered to: • The area around the tester must be cordoned off. • Only one person should be in the confined space. • The person should wear protective clothing (an apron). • The tester should wear safety goggles. • Make certain that the dial indicator is mounted properly. • Ensure that the front section of the indicator touches the bottom section of the tester. • Move the protective cover over the specimen that must be tested. • For steel and duralumin, use half-turn increments of the handwheel moving through the range. • For aluminium, one-fifth turn increments are acceptable. • For plastics, one-fifth turn increments for the first three turns are acceptable. Thereafter, use increments of two turns until the material is crushed.
Torsion tester Twisting rod Twisitng indicator
Figure 1.19: Torsion tester
The following safety measures must be adhered to: • Fasten the tester to a workbench. • Determine the strength of the bolts keeping the framework together. • Get the specification (torsion) of the different materials and of the rod size you would like to test. • When you add pieces of different mass, you should attach them very gently otherwise you could get a skew reading of the torsion on the rod.
Moments and forces testers Frame
Beam Mass pieces
Moment of a force
Figure 1.20: Moments and forces testers
The following safety measures must be adhered to: • Determine the strength of the bolts keeping the framework together. • Get the specification. • When you add different mass pieces, you should attach them very gently otherwise you could get a skew reading on the tester.
Mechanical Technology Beam-bending tester
Frame Dial indicator Bending bar
Figure 1.21: Beam-bending tester
The following safety measures must be adhered to: • Ensure the beam is clamped parallel to the backboard. • Do not leave plastic beams loaded for any length of time. (They tend to creep if left loaded.) • Gently drop the weights onto the hanger. This helps to ‘bounce’ the beam and reduces inaccuracies due to friction. • To get true deflection, take 10 mm off the dial gauge reading at each step.
Assessment 6 1. Name three safety precautions to observe when working with a metal-arc gas shield welder. 2. Name three safety precautions that must be considered when the following testing equipment is used: • Brinell tester • Tensile tester • Torsion tester • Beam-bending tester
Cylinder leakage tester Pressure indicator
Air hose coupler
Pressure adjuster Plug adaptor
Figure 1.22: Cylinder leakage tester
The following safety measures must be adhered to: • Get the specifications of the engine to be tested. • The cylinder to be tested should be on Top Dead Centre. • The crankshaft should be locked so that it cannot turn during the testing procedure. • Predetermined compressed air should be used or else the machine will be damaged. • Do not play with compressed air. • Make certain that the dipstick is in its hole. • Make certain that the radiator cap is screwed onto the radiator. • Make certain that the oil filler cap is screwed on. • The adaptor which screws into the plug hole must be tightened, otherwise all the compressed air will escape.
Pressure testers Pressure meter indicator Spark plug adaptor
Pressure relief valve
Figure 1.23: Compression tester
The following safety measures must be adhered to: • Make certain that the connections (pipes) are not broken. • Screw the adaptor into the plug hole without stripping the hole. • Do not over-tighten the adaptor. • Do not store the tester when the meter still contains pressure. • Always release the pressure before it is stored. • Do not drop the meter. You will damage it and impede its accuracy.
Connector to the radiator
Figure 1.24: Radiator tester
Mechanical Technology Safety measures when working with the instrument • Make certain that all adaptors have no cracks. • Make sure that the pipes are leak-free and that the connections are tightened properly. • Determine the specifications for the objects you want to test first. • Never pump the radiator to a point that exceeds the predetermined pressure of the radiator or the cap.
Assessment 7 1. Name three safety precautions to consider for each of the following testers: • Cylinder leakage tester • Compression tester • Radiator tester
Spring compressors and testers (valve and coil) Lever
Valve cotter clamp
Figure 1.25: Valve lifter
The following safety measures must be adhered to: • Make certain that the front part is adjusted to fit tightly over the valve retainer. • Tighten the adjuster behind the valve lifter. • Determine the specifications of the cylinder head valve spring. • Do not stretch or compress the spring further/more than indicated in the specification. You could damage the spring. Load cell
Figure 1.26: Valve spring compressor
Figure 1.27: Coil spring compressor
The following safety measures must be adhered to: • Make certain that the diameter of the compressor bolts can take the pressure of the coil spring. • If the pressure exceeds the strength of the coil spring, the puller will snap and damage the car and/or people may be hurt.
Assessment 8 1. Name two safety precautions to observe when using the following testers: • Valve lifter • Coil spring compressor
Gas analysers Pipe to the exhaust system Filter
Gas analyser meter
Figure 1.28: Gas analyser
The following safety measures must be adhered to: • The inlet hose should not be stepped on or restricted in any way. • The hose connections must be airtight and the valve on the condenser should be in the horizontal position (closed). • The vehicle being tested should have no leaks in the exhaust, manifolds or vacuum systems. (This will result in the analyser giving lean readings or no readings at all. If you are unsure, test the analyser on another ‘good’ vehicle.) • From time to time, condensate should be blown out of the hoses and pickup probe with compressed air. • The hoses should be disconnected from the analyser or else the pump will be damaged. • The condenser should be drained after each test, using the valve. • When the paper filter becomes light grey, it should be changed. (Take care when inserting filter paper into the housing. Make sure the window is properly located before tightening the large nut.) • The fuel filter on the condenser stand should be changed regularly. • On a 12-volt analyser, the battery clamps should be clean.
Multimeters LCD display screen
Range selector switch
10A DC terminal socket
VΩmA terminal socket Common terminal socket
Figure 1.29: Multimeter
The following safety precautions should be taken: • Keep the meter dry. • Keep the meter away from dust and dirt. • Use and store the meter in environments where the temperature is normal. • Do not drop the meter as it could be damaged and this will affect its operation. • Use only charged cells of the correct size. (Always remove old batteries as they can leak and corrode the wiring.)
Assessment 9 1. Name three safety precautions to observe when using the following special tools: • Gas analyser • Multimeter
Bearing and gear puller Bearing pullers are used to remove and replace bearings and bushes.
Figure 1.30: Bearing and gear pullers
Figure 1.31: Bearing and gear pullers
The following safety measures must be adhered to: • Make certain that the puller is the right one for the job. • Make certain that the puller is strong enough to remove the bearing or gear. • You must tighten the clamps or fingers around the object, otherwise it could slip and damage the objects or hurt you in the process. • Do not use a hammer on the puller. • Use the right spanner to tighten the clamps and to pull off the object. • Make certain that the puller is at a 90º angle to the horizontal before you start to pull.
Assessment 10 1. Name four safety precautions to consider when working with a bearing and gear puller.
Tools Topic 2
Spring tester Compression tester
Introduction Tools are very important to complete different tasks in the workplace. In Grade 10, hand tools was explained. In Grade 11, power tools and machines were explained. In this chapter in Grade 12, we will explain advanced engineering equipment in the workplace.
Gas analyser Function To analyse the gas emitted by the exhaust pipe of a motor car and determine the amount of CO (carbon monoxide) being processed by the engine. The reading will determine whether the petrol mixture of the engine is adjusted according to the manufacturer’s specifications. Pipe to the exhaust system Filter
Gas analyser meter
Figure 2.1: A CO gas analyser
The test procedure • Connect the analyser to the 12-volt battery terminals of the vehicle. Polarity does not have to be observed. • The LCD will display ‘000’ during the 30-second, preheat period and will go through the auto-zero self-test programme for 35 seconds, until 0.00 is displayed. • Do not connect the armoured hose of the condenser pickup to the rear of the machine until 0.00 is displayed. • Insert the silicone hose probe and clamp it onto the exhaust tailpipe with the stand to the right or left of the exhaust fumes to prevent the heat from affecting the stand. • Unroll the armoured hose and press it onto the brass inlet at the back of the analyser. • Observe the readings on the display and make adjustments according to the vehicle manufacturer’s specifications. • It is important to use the analyser with the condenser stand otherwise you will damage the analyser. • If the analyser is accidentally switched off at any time, the pickup hose must be removed from the back of the analyser. This allows the analyser, when switched on again, to auto-zero without containing exhaust gas in the pump chambers and hose. If the pickup hose is not removed, the auto-zero will start above 0 and give inaccurate readings. Never connect and disconnect clamps without removing the pipe at the back of the analyser. In addition, attention should be paid to the following: • The inlet hose should not be stepped on or restricted in any way.
• The hose connections must be airtight and the valve on the condenser must be in the horizontal position (closed). • From time to time, condensate must be blown out of the hoses and pickup probe with compressed air. • The hoses should be disconnected from the analyser to avoid damage to the pump. • The condenser should be drained after each test, using the valve. • When the paper filter becomes light grey, it should be changed. (Take care when inserting filter paper into the housing. Make sure the window is properly located before hand-tightening the large nut.) • The fuel filter on the condenser stand should be changed regularly. • On a 12-volt analyser, the battery clamps should be clean.
Assessment 1 1. Explain the function of a gas analyser. 2. Explain how you would connect the gas analyser to a motor car’s engine to test the CO of the exhaust gas.
Brinell hardness tester Function • To test how hard different types of materials are. • Hardness refers to a material’s ability to resist plastic deformation, usually by penetration. The term ‘hardness’ may also refer to a material’s resistance to bending, scratching, abrasion or cutting. • The Brinell Hardness Test involves indenting the test material with a 10 mm piece of hardened steel or a carbide ball, by subjecting it to a load of 3 000 kg. (For softer materials, the load can be reduced to 1 500 kg or 500 kg to avoid excessive indentation.) The full load is normally applied for 10 to 15 seconds, in the case of iron and steel, and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a lowpowered microscope. Rockwell Hardness Test The Rockwell Hardness Test method involves indenting the test material with a diamond cone or hardened steel-ball indenter. The indenter is forced into the test material under a preliminary minor load (F0), usually 10 kgf. Kgf is a method of expressing load in terms of mass. The permanent increase in depth of penetration, resulting from the application and removal of the additional major load, is used to calculate the Rockwell Hardness Number. Hardness indicator meter
Platform height adjuster Activating knob
Figure 2.2 (a): Rockwell hardness tester
Platform adjuster Activating panel
Figure 2.2 (b): Brinell tester
Mechanical Technology Procedure to use the Brinell hardness tester • The hardness of a material cannot be measured in standard units like the those used to measure mass, length or time. We can only define hardness. • For many years, the hardness of a material has been assessed by its resistance to scratching or cutting, for example, if material B scratches material C but not material A. The relative hardness of minerals can be assessed by referring to the Mohs’ scale, which ranks the ability of materials to resist scratching by other materials. • Similar methods to assess hardness are still used today. An example is the file test. In this test, a file, tempered to a desired hardness, is rubbed on the test material’s surface. If the file slides without biting or marking the surface, the test material is considered harder than the file. • Hardness tests like these are limited and do not provide accurate numeric data or scales, particularly for modern metals and materials. The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape when a specific force is applied for a specific time. Care • Oil the platform adjuster to avoid rust. • When the tester is used, it should be mounted properly to ensure the correct reading. • Do not exceed the pressure that is prescribed for testing certain materials. The machine could be damaged. • The vice in which the sample material is clamped should be stable to ensure a correct reading.
Assessment 2 1. Explain the principles of the Brinell hardness tester.
Multimeter Function • The meter can be used to test different electrical components and concepts. • The switch on the meter selects the function and desired ranges and also turns the meter on and off. To extend the life of the battery, the switch should be in the ‘OFF’ position when the meter is not in use. • To test current, voltage, resistance, continuity, transistors and diodes.
Panel description LCD display screen
Range selector switch
10A DC terminal socket VΩmA terminal socket Common terminal socket
Figure 2.3: Multimeter
Use of a multimeter How to test current flow DC current measurement • Connect the black test lead to the COM jack and the red test lead to the VΩmA jack. (If the current is equal to or greater than 200 mA, use the 10A jack instead.) • Set the range switch to the desired current range. • Open the circuit in which the current is to be measured and connect the test leads in series with the circuit. • Read the current value on the LCD display along with the polarity of the red test lead.
How to test voltage DC voltage measurement • Connect the red test lead to the VΩmA jack and the black test lead to the COM jack. • Set the range switch to the desired range. If the voltage is not known beforehand, set the range switch at the highest range position and then reduce the range in increments until the resolution is satisfactory. • Connect the test leads across the device or circuit to be measured. • Read the voltage value on the LCD display along with the polarity of the red test lead.
How to test resistance Resistance measurement • Connect the red test lead to the VΩmA jack and the black test lead to the COM jack. • Set the range switch to the desired Ω range. • If the resistor to be measured is connected to a circuit, disconnect the circuit’s power and discharge all capacitors before measuring the resistance. • Connect the test leads across the resistor to be measured and read the resistance value on the LCD display.
How to test transistors Transistor test • Set the range switch to the hFE range. • Determine whether the transistor to be tested is a PNP or NPN type and locate the emitter, base and collector leads. Insert the leads into the proper holes of the hFE socket on the front panel. The meter will display the approximate hFE value.
How to test continuity Diode and continuity measurement • Connect the red test lead to the VΩmA jack and the black test lead to the COM jack. • Set the range switch to the range. • Connect the red test lead to the anode of the diode to be tested and the black test lead to the cathode of the diode.
Mechanical Technology • The approximate forward voltage drop of the diode will be displayed in mV. If the connection is reversed, only the figure ‘1’ will be shown on the LCD. • Connect the test leads to the two terminals of the circuit to be tested. • If the resistance is less than about 50 Ω, the buzzer will sound. Care of the multimeter The following precautions should be taken: • Keep the meter dry. • Keep the meter away from dust and dirt. • Use and store the meter in environments where the temperature is normal. • Do not drop the meter as it could be damaged and its operation could be affected. • Use only charged cells of the correct size. (Always remove old batteries as they can leak and corrode the wiring.)
Assessment 3 1. Explain how to use a multimeter to test for DC current. 2. Explain how you would measure DC voltage using a multimeter. 3. Explain, step-by-step, how you would test resistance using a multimeter. 4. Describe, step-by-step, how you would use a multimeter to test diodes and continuity in a wire. 5. Name four points that have to be considered in the care of a multimeter.
Pressure testers Cooling tester Function • A pump is used on a cooling system to test the system for leaks. • To pump compressed air into the cooling system of a motor car to determine whether there are any water leaks in the system. • To test if the pressure cap on the cooling system operates according to the prescribed pressure of the system.
Connector to the radiator
Figure 2.4: Cooling pressure tester
Use of the cooling pressure tester • Unscrew the radiator cap. Determine the type of adaptor to be used for testing the radiator. • Fix the adaptor to the tester and screw it onto the radiator. • Pump the tester to the predetermined pressure of the radiator cap. • Let the tester stand for a while. • Note the reading on the meter. If the reading drops, it indicates that the cooling system has a leak. • You can use the cooling tester to determine if a cylinder-head gasket is leaking. • Screw the tester into the cooling system. • Pump until the predetermined pressure is reached. • Start the car. • Rev the engine and note the meter reading on the tester. • If the meter reading increases, it indicates that the cylinder-head gasket is leaking. Care • The meter should always be stored in its container to protect it from damage. • Check the connecting pipes regularly to determine if there are any leaks or if the pipes are damaged. • Determine the pressure of the cooling system before pumping air into the system. You could damage the pressure meter gauge if you exceed the predetermined pressure.
Oil tester Function • To use an indicator to determine the operating oil pressure in an engine. • To test the operating pressure of the oil in the lubricating system of an engine by means of an oil pressure meter. Oil pressure meter
Oil pressure meter pipe
Figure 2.5: Oil pressure gauge and oil pressure tester
Procedure to use an oil pressure meter An oil pressure gauge can provide an excellent indication of the health of various systems in an engine. The key is to establish baseline readings when the engine is healthy and then to be aware of any changes perceived over time.
Mechanical Technology How to test the oil pressure of an engine • Remove the oil sender unit. • Connect the oil pressure gauge to the hole of the sender unit. • Start the engine. • Activate the pump. • Check the reading. • Check the reading and compare it to the vehicle’s specifications. • If the reading is incorrect, replace the oil pump. Care • The meter should always be stored in its container to protect it from damage. • Check the connecting pipes regularly to determine whether there are any leaks or whether the pipes are damaged. • Make certain that the connector hole, where the oil sender unit is situated, has been screwed in correctly and is not crooked. • Make certain that the relief valve is deactivated before the meter is stored.
Assessment 4 1. Explain how you would test the cooling system of an engine. 2. Explain how you would test the oil pressure of an engine.
Fuel pressure tester/meter Function • To use an indicator to determine the fuel operating pressure in the system. • To test the pressure of the fuel in the fuel line that runs to the direct injection system. Pressure meter
Pressure relief valve
Return pipe to tank
Figure 2.6: Fuel pressure tester
Procedure to use the fuel pressure tester • Disconnect the pipe running from the tank to the fuel pump. • Connect the fuel pump pressure meter to the inlet. • Activate the pump. Note the reading. • Keep the pump at the predetermined stroke and check whether the reading decreases.
• • • • • •
If it does, the inlet valve in the pump is leaking. Remove the pressure gauge. Remove the pipe from the fuel pump at the delivery side of the pump. Connect the fuel pressure gauge on the delivery side. Activate the pump. Keep the pump at the predetermined stroke and check whether the reading decreases. • If it does, the delivery valve in the pump is leaking. • Check the reading and compare it with the vehicle’s specifications. • If the reading is incorrect, replace the pump. Care • Store the pressure meter in its container. • Make certain that the pipes are in good condition. • Check the clamps around the pipes to ensure that they are tight. • The fuel pump sucks the fuel from the petrol tank and pumps it to the carburettor under pressure. • If the fuel pressure in the petrol pump is too high, the pipes could burst. • If the pressure is too low, the vehicle will not move due to fuel starvation.
Assessment 5 1. Explain how you would test the pressure in the fuel system.
Cylinder leakage tester Function • Use a meter with compressed air to determine if any compressed air escapes the engine. • The function of the cylinder leakage tester is to check whether gases leak from the cylinder in the engine during the compression stroke.
Air hose coupler
Pressure adjuster Plug adaptor
Figure 2.7: Cylinder leakage tester
Procedure to use the cylinder leakage tester • Turn the engine until both valves on cylinder 1 are closed. (Piston 1 is on the power stroke.) • Unscrew the spark-plug. • Screw the spark-plug adaptor into the spark-plug hole. • Use a spanner to lock the crankshaft pulley so that it cannot turn. • Close the relief valve on the tester. • Connect the compressed air pipe to the tester and to the adaptor in the sparkplug hole.
Mechanical Technology • Connect the compressed air hose to the main supply. • Open the relief valve on the tester slowly. • Note the reading on the tester. The reading will determine the amount of gas leakage occurring in the engine. Determining the leakage • Listen to the carburettor for a hissing noise. (Inlet valve is leaking.) • Listen to the exhaust pipe for a hissing noise. (Exhaust valve is leaking.) • Listen for a hissing noise in the dipstick hole. (Piston ring is worn.) • Remove the filler cap on the tappet cover and listen for a hissing noise. (Rings are worn.) • If you see bubbles in the radiator water, the cylinder-head gasket is blown or the cylinder block is cracked. Care • Store meter in a dry place. • Check the thread on the adaptors regularly. • Check pipes for damage. • Check that the clamps on the pipes are tight. • Always relieve the pressure on the tester after performing a test on a cylinder.
Assessment 6 1. What is the function of a cylinder leakage tester? 2. Explain, step-by-step, how to connect a cylinder leakage tester. 3. How would you determine where the leakage is when using a cylinder leakage tester?
Torsion tester Function A torsion tester allows you to investigate the relationship between the momentum or torque applied to material and the influence of the material or member length on torsional deflection. To test how torque and the material type affect the torsional deflection.
Figure 2.8: A torsion bar used in a car’s suspension
Twisting rod Twisitng indicator
Figure 2.9: Torsion tester
How to use a torsion tester • Measure the diameter of the material using a vernier caliper or a micrometer. • Place the material between the two drill chucks, allowing a length of 500 mm between centres. • Zero the protractor using the pointer arm. • Add masses in 50 g increments to the load arm, noting the angle of twist (to the nearest 0,25°) on the protractor until either the load arm hits the end stop or a maximum load of 500 g has been used. • Plot a graph depicting the angle of twist versus load. • Repeat the above and use different materials. • Compare the results. Care • All nuts and bolts on the tester should be secured properly. • Do not over-tighten the chuck where the sample material is clamped. • Store the equipment in a dry place to prevent rust. • When adding different mass pieces, work very carefully or you could obtain an incorrect reading of the torsion of the specimen. • Before you store the tester, remove all mass pieces.
Moments and forces tester Principles In order to understand moments and forces testers, it is necessary to know what the terms ‘force’ and ‘moment’ mean. There are two types of forces acting on a body: • Load – Loads are caused by acceleration due to gravity. (The weight of a body is termed a load.) • Shear – Shear stress refers to a state where the stress is parallel or tangential to the face of the material as opposed to normal stress. Function To determine the reactions on either side of a simply loaded beam. To illustrate the concept of the triangle of forces.
Mechanical Technology Procedure to use testing equipment By using this equipment, you can test the reaction on either side of a simply loaded beam. Load cells
Frame Supported beam
Reaction of a supported beam
Figure 2.10: A supported-beam kit
• Assemble the equipment as indicated in Figure 2.10. • Zero the load cells. • Apply loads to the beam at any position via the plastic hangers. • Record the load cell reading. • Repeat for other loads and load positions. Use the calculation methods you learnt in Grade 11 to confirm your findings.
Triangle of forces Load cells
Mass pieces on a string
Pulleys Equilibrium of three forces
Figure 2.11: Equilibrium of three forces
• Assemble the equipment as indicated in Figure 2.11. • Zero the load cell and compression cell. • Put the weight hanger in any position along the string and add a known mass (300 g). • Take the reading on the load cell. • Obtain the geometry of the system of forces by measurement and then by calculation. Ensure that the pulley is free-running. Try other geometries by rearranging the pulleys and load cells (as long as the pulleys allow the string to be attached to the load cell vertically). Care • All nuts and bolts on the tester should be secured properly. • Store the equipment in a dry place to prevent any rust. • When adding different mass pieces, do so very gently or you could obtain an incorrect reading on the load cells. • Before you store the tester, remove all mass pieces. • Do not bump the load cells as this may cause damage.
Beam bending Function To use a tester to determine the deflection of various given pieces of material. To investigate the deflection of beams. In the simplest case, the equipment can be used as an illustration of Young’s modulus for a material. For example, an aluminium beam will deflect roughly three times more than a steel beam of the same section, under the same load conditions, since the modulus for aluminium is a third of that of steel.
Frame Dial indicator Bending bar
Figure 2.12: A beam-bending test kit
Procedure to use the beam-bending test kit • Assemble the equipment as directed in the kit, selecting an appropriate beam. • Using a ruler and a dry-wipe marker, draw a line across the beam 200 mm from the root. • Add a 10 g weight hanger to the dial indicator and slide the dial gauge down onto the beam until it reads 10 mm. Remove the weight hanger and zero the outer scale using the bezel. With plastic beams, this may take several attempts. • Add a 100 g mass to the dial indicator and record the dial indicator reading.
Mechanical Technology • Repeat the previous step until you reach 500 g, increasing the mass in 100 g increments. • Plot a graph of deflection (x-axis) versus load (y-axis) to prove the relevant theories. Care • All nuts and bolts on the tester should be secured properly. • Store the equipment in a dry place to prevent any rust. • When adding the different mass pieces, work very gently on or you could obtain an incorrect reading on the dial indicator. • Before you store the tester, remove all mass pieces. • Do not keep the specimen in a bent position after the testing has been completed.
Tensile tester Function To demonstrate the fundamentals of the tensile test of different materials. The tensile tester is a destructive tester which subjects a piece of material to an increasing axial load while measuring the corresponding elongation of the material. The test is designed to give the yield stress, ultimate tensile stress and elongation percentage of a piece of material.
Figure 2.13: A tensile tester
Procedure to use the tensile tester • Place the piece of material between the two anchor points. • Tighten the screws. • Turn the handwheel until the dial indicator starts to move. • Zero the dial indicator. • Note the measurement on the ruler near the top part of the anchor point. • Turn the handwheel one full rotation. (The reading on the ruler should be 1 mm.) • The top of the sample material will now be stretched by 1 mm. • The bottom of the specimen is connected to large springs, the deflection of which is measured on the dial indicator.
• The specimen’s elongation is calculated by subtracting the dial indicator reading from the number of turns. For example, if the handwheel has been turned three times and the dial indicator reading is 2,83 mm, the elongation is 3,00 – 2.83 = 0,17 mm. • The dial indicator also provides an indication of the force applied to the specimen. Since the springs have a combined rate of 100 N/mm, each dial indicator division is equal to one newton. In other words, if the dial indicator reads 2,83 mm, the force is 283 N. • Towards the end of the test, the material will yield rapidly and an accurate dial indicator reading may not be easily read. If the reading does not stabilise after 20–30 seconds, take the specimen to fracture point by turning the handwheel until the specimen snaps. • Wind the handwheel back until the ends of the snapped specimen touch and read the length on the scale at the back. Note • For steel and duralumin, use half-turn increments of the handwheel through the range. • For aluminium, one-fifth-turn increments are acceptable. • For plastics, one-fifth-turn increments for the first three turns are acceptable. Thereafter, use increments of two turns until the material snaps. Care • Store the equipment in a dry place to prevent any rust. • Do not keep the specimen in a stretched position when you store it. • Oil the adjusting wheel and thread. • Check the dial indicator and clean the shaft.
Assessment 7 1.
Explain the use of the following testers: • Torsion tester • Beam-bending tester • Mini tensile tester.
Compression testers Function By using a pressure tester, you can determine the condition of a motor engine. To test the compression of an engine when the piston moves from TDC to BDC or from BDC to TDC during the power stroke. Pressure meter indicator Spark-plug adaptor
Pressure relief valve
Figure 2.14: Compression tester
Mechanical Technology Procedure to use a compressor tester • Run the engine until it reaches normal operating temperature. When a test is done on a cold engine, it usually shows lower readings. • Remove the high tension (HT) leads and take out all the spark-plugs. • Mark each spark-plug so that you know from which cylinder it was taken. • The ignition system must then be disabled. If this is not done, it will continue to generate high tension voltages into the HT leads which will have nowhere to go with the plugs removed. • These high voltages will find another route to earth and can damage the ignition system or even the car’s ECU (engine control unit). • Unplug the low tension connections to the coil or to the distributor. • It is also good practice to unplug the fuel injectors or disable the fuel pump, especially on cars fitted with a catalytic converter. This prevents unburned fuel from getting into the exhaust system during the test. • Screw the gauge into cylinder 1 and rest it where you can see the dial while you crank the engine.
Figure 2.15: Using a compression tester
• Open the throttle fully, either by pressing the accelerator or wedging open the linkage under the bonnet. If the throttle is not open, air cannot get into the cylinder and the readings will be far too low. • Crank the engine until the gauge stops rising and count the revolutions while you do so. • It should normally take no more than 10 engine revolutions (5 compression cycles) to obtain a full reading. • You can count the cycles by watching the gauge too – each jump of the needle is one compression stroke. • Write down the final reading and also make a mental note of how quickly the gauge rose on the first few cycles. • Make use of the release valve and release the pressure to zero. • Repeat the procedure for the other three cylinders. • Make sure that each cylinder reaches its highest reading after the same number of engine revolutions. If all readings are good, you can finish the test there. • If any cylinders are low, you can do a ‘wet’ test. This involves squirting a few drops of oil into the cylinder and repeating the test. • The oil will help seal leaky rings and increase the reading but it will not solve the problem, which lies in the valves or head gasket.
Care • Store the meter in a dry place. • Check the thread on the adaptors regularly. • Check pipes for damage. • Ensure that the clamps on the pipes are tight. • Always relieve the pressure on the tester after you have performed a test on a cylinder.
Assessment 8 1. Explain, step-by-step, the use of a compression tester on a motor vehicle.
Spring tester Function The tester is used to test the properties of a compression spring. By using Hooke’s law, you can determine the following: Strain (the amount by which a body is deformed) is directly proportional to the stress its deformation causes, provided the limit of the proportionality is not exceeded.
Upper platform Lower platform
Figure 2.16: A spring tester
Procedure to use a spring tester • Place the spring between the two jaws. • Take up the slack of the spring. • The spring is compressed by the top jaw. • Set the indicator to zero. • Turn the lever on the machine so that it compresses the spring 2 mm (0,002 m). (The scale is on the front panel of the machine.) Take a reading from the indicator. • Repeat in 2 mm (0,002 m) steps until you reach the end of the travel of the spring. Care • All moving parts should be cleaned properly before the machine is stored. • Lubricate the moving parts. • Secure all bolts before you use the tester.
Assessment 9 1. Explain Hooke’s law. 2. What is the function of a spring tester? 3. Explain, step-by-step, how you would use a spring tester to test the tension of spring when you compress it.
Mig/Mag welders Function Mig welding is an automatic or semi-automatic process in which a wire connected to a source of direct current acts as an electrode to join two pieces of metal as they are continuously passed through a welding gun. A flow of an inert gas, originally argon or CO2 (or a mixture of argon and CO2), is injected at the same time as the wire electrode. The wire is continuously fed through a gun to the weld pool by a wire feeder. Either solid wire (GMAW) or cored wire (FCAW-GS – flux-cored arc welding, gas shielding) can be used. This inert gas acts as a shield, keeping airborne contaminants away from the weld zone. Mig/Mag welders use the heat produced as electricity jumps across the gap from one conductor to another. As the electricity passes through this gas, intense and concentrated heat (3 600 to 4 000 °C) is produced.
Figure 2.17: Mig/Mag welder
Procedure to use the Mig/Mag welder The advantage of MIG welding is that it allows metal to be welded much more quickly than traditional ‘stick welding’ techniques. This makes it ideal for welding softer metals such as aluminium. When this method was first developed, the cost of the inert gas made the process too expensive for welding steel. Over the years, the process has evolved, however, and semi-inert gases such as carbon dioxide can now be used to provide the shielding function which now makes MIG welding costeffective for welding steel.
Gun Trigger Nozzle
Contact tip Gas shield
Work Weld puddle
Figure 2.18: Mig/Mag welding process
Care • Close the bottles after use. • Remove the pressure on the gas cylinder after use. • Work in a clean, dry place. • Ensure that there is no water on the floors. • Shields should surround the welding process to protect you from the rays. • See to loose connections on the machine. Check for plugs that are broken. • Always store the gas cylinder in an upright position. • Always disconnect the welder from the electricity supply when it is not in operation.
Assessment 10 1. Name six points that you should observe when caring for a Mig/Mag welding machine.
Depth micrometer Function A depth micrometer is used to measure the depth of a workpiece accurately. It works on the principle that you put a measuring tool in an open container, pipe or cylinder to measure the depth of the objects. How to measure Take care not to lift the micrometer off the surface of the work to be measured. Another error which could occur is in the reading of a depth micrometer, as its scale reads in the opposite direction compared to other micrometers. A depth micrometer is equipped with interchangeable rods of pre-set lengths for measuring extra depth. With each change, the micrometer must be checked for accuracy.
Figure 2.19: A depth micrometer
Use of the depth micrometer When using the 0–25 mm rod, the micrometer is laid on a flat surface and a zero reading is taken. For larger rods, gauge blocks are usually set up. Reading the depth micrometer • Take care when reading a depth micrometer as it differs from the outside micrometer. The actual reading is hidden by the thimble. • The scale on the barrel reads in the opposite direction to the outside micrometer and the scale on the thimble is also opposite in direction.
Example of the reading on the micrometer Simple calculations Depth micrometer: read what you do not see full mm
1 full turn = 0,5 mm 2 full turns = 1 mm If the 0,5 mm between two full mm cannot be seen, then add 0,5 mm to the reading. If it can be seen, do not add.
0,5 mm can be seen, so do not add
Figure 2.20: A depth micrometer reading
Reading = 6,47 mm
Reading: 13 + 0,5 + 0,15 = 13,65
Reading : 25 + 16 + 0 + 0,24 = 41,24
Figure 2.21: Examples of depth micrometer readings
Care of a depth micrometer • Store in a clean, dry place. • Do not let the micrometer drop. You will get an incorrect reading. • Clean with a rag before use. • Make certain that the extension rods are secured properly. • Zero the micrometer after you have fitted a new extension rod.
Assessment 11 1. State the function of a depth micrometer. 2. Use the following information and draw the micrometer scales: 6,46 mm, 63,12 mm.
Screw-thread micrometer Function The screw-thread micrometer is specifically designed to measure the pitch diameter of a screw thread. Adjustable anvil
Thimble Thimble lock nut
Figure 2.22 Screw-thread micrometer
Use Figure 2.22 illustrates the screw-thread micrometer which is basically similar to an ordinary outside micrometer. The difference is that the anvils of the screw-thread micrometer are adapted to the form of the screw thread to be measured. The spindle of the micrometer is pointed so that it fits into the angle of the screw thread and is free to rotate, while the anvil is V-shaped and also fits into the screw-thread angle. The shape of the spindle and anvil is adapted so that when measurements are taken the point of the spindle as well as the root and crest of the V-anvil do not rest on the root or crest of the screw thread, but the flanks of the spindle and V-anvil make contact with the flanks of the screw thread. When the micrometer is set at zero,
Mechanical Technology the pitch lines of the spindle and V-anvil coincide as shown in Figure 2.22. When micrometer readings are taken, they indicate the pitch diameter of the screw thread. The same readings that you will get from an ordinary outside micrometer can be used on the thread micrometer. Checking the outside diameter (crest diameter) An ordinary outside micrometer may be used to check the outside or crest diameter of the screw thread. However, care must be taken that the anvils of the micrometer have sufficient diameters for at least two screw threads, to ensure that, during measurement, the micrometer is at right angles to the centre line of the screw thread. See Figure 2.23.
Figure 2.23: Checking the crest diameter
Checking the root diameter The root diameter of the screw thread may be tested using a micrometer, the anvils of which are modified to measure the root diameter. See Figure 2.24:
Figure 2.24: Micrometer with modified anvils for measuring the root diameter of a screw thread
Checking the effective diameter (pitch diameter) • The effective diameter, also called the pitch diameter, is one of the most important elements in the correct fitting of screw threads. If the pitch of the screw thread cut on the bolt differs slightly from the thread cut into the nut into which the bolt should fit, there will not be a good fit between the two parts. • To ensure an acceptable fitting, the outside diameter, as well as the root diameter, may be left unchanged while only the effective diameter is adjusted. Thus the relationship between the pitch of the thread and the effective diameter is of particular importance in fitting screw threads.
Examples of thread micrometer readings
Reading = 10 mm + 0,5 mm + 0,14 mm = 10,64 mm
Reading = 5 mm + 0,5 mm = 5,5 mm Figure 2.25: Thread micrometer readings
Care • Store in a clean, dry place. • Do not let the micrometer drop. You will get an incorrect reading. • Clean with a rag before use. • Clean the thread of the micrometer regularly. • Zero the micrometer before you use it.
Assessment 12 1. State the function of a screw-thread micrometer. 2. Determine the following readings of a thread micrometer:
Materials Topic 3
Engineering materials in environmental context
Introduction In Grade 10 and Grade 11, materials, heat treatment, heat-treatment processes and properties of engineering materials were discussed in detail. In Grade 12, we will look at what really happens to the material during the heat-treatment process.
Engineering material in the environmental context Did you know? The first blast furnaces did not have charging bells (open top) to prevent environmental damage: soot and dust from the smelting process escaped and caused respiratory and health problems for the inhabitants of the surrounding areas.
Mining is not an environmentally-friendly industry. There are clear negative impacts on the environment caused by mining, e.g. the Big Hole of Kimberley, the unrestored, unrehabilitated mine dumps of Gauteng or the planned exploration for earth (shale) gas in the Karoo. Shale gas is extracted by combining two established technologies – hydraulic fracturing and horizontal drilling. Mining causes severe scarring of sensitive environments, e.g. coal mining, to supply South Africa’s power stations, as well as air and noise pollution, dust, and contaminated soil and water, especially groundwater. Once groundwater is contaminated, it is almost impossible to clean (great cause of concern for the farmers of the Karoo and other parts of South Africa in their attempts to secure South Africa’s food security. South Africa recently became a food importer for the first time in its history). On the other hand, mining for iron ore, semi-precious and precious metals provides work for an estimated 600 000 South Africans and people from neighbouring states. Because of the nature of their work and living conditions, HIV/Aids and lung diseases are the most important illnesses affecting mineworkers. Accidents in foundries, factories, mines and workplaces are a great concern for trade unions and the Department of Labour.
Did you know? Hematite is the most important iron ore in South Africa and is largely an iron oxide.
The Department of Environmental Affairs endeavours to control air and water pollution, global warming and the release of greenhouse gases such as CFCs, carbon dioxide, water vapour and nitrous oxides and methane. Many mining and industrial companies have become socially aware of the need to rehabilitate land such as mine dumps to minimise dust. They have planted trees and greened areas to decrease carbon dioxide levels in the air. The production of smokeless coke also helps. We can play a part by participating in Arbour Day, by planting trees and by remembering the three Rs (Reduce, Recycle and Re-use).
Assessment 1 The purpose of the crossword puzzle below is to help you to reflect on Grade 10 and Grade 11 knowledge which you should understand in this new section of work. There are questions that you will not be able to answer but this has been done on purpose to encourage you to read ahead (pre-read) this chapter.
Crossword ACROSS 1 Aluminium is produced from an ore called ................... 2. Which element determines the properties of steel? 3. When you cut steel from colour-coded stock, you should always cut the steel from the ............. end. 4. The graphite in grey cast iron is in the form of ......................
5. The weight of an object changes if gravity changes. Its mass will ................... 6. Carbon in the form .................. makes white cast iron very hard. 7. The most difficult steels to cut or form are the ........ 8. Weld shrinkage distortion can often be corrected by ................................ 9. A copper alloy is easier to machine if it contains a little ........................... 10. Alloys of copper and zinc make up the class of metals known as ............ 11. Stainless steels are defined by the presence of .................... 12. The ability of a metal to be stretched into a thin wire is called .......... DOWN 1. What element added to aluminium produces an alloy having high load capacity and good fatigue strength? 2. Steel contains at least 98% of ........ 3. When the film of corrosion on copper turns a pale blue-green colour, it is called ............... 4. Steels containing one or more metals in addition to iron are called ........... steels. 5. The ability of metal to resist deformation is called .............. 6. Which is not a property of high-alloy cast iron? 7. The ability of a metal to resist penetration is called ................... 8. The two major classifications of malleable cast iron are ferrite and ............. cast iron. 1
7 8 8 9 10
Terminology in relation to the iron-cementite equilibrium diagram Carbon content As we learned in Grade 10 and Grade 11, steels are divided into three classes according to the amount of carbon they contain. The classes are known as lowcarbon steel, medium-carbon steel and high-carbon steel. In figure 3.1 below, some of their uses are listed As the carbon content increases, the mechanical strength, hardness and hardening properties of steel improve, while properties like elasticity, ductility, forging, welding and cutting ability are adversely influenced.
Did you know? To convert degrees Celsius (°C) to degrees Fahrenheit (°F): °F = 9/5 × °C + 32. To convert degrees Fahrenheit (°F) to degrees Celsius: °C = 5/9 (°F − 32). To convert from Kelvin (K) to degrees Celsius (°C): °C = K – 273. To convert degrees Celsius (°C) to Kelvin (K): K =°C + 273.
Low 0,10 – 0,25%
Bolts, screws, rivets
Annealing, hardening, tempering
Medium 0,25 – 0,55%
Crankshafts, tie rods, pliers, open-ended spanners,screwdrivers
Surface hardening, (case hardened) tempering
High 0,55 – 1,00%
Cutting tools, springs, shafts, hammers, axes
Brittle, poor weldability
Figure 3.1: Classes of steel
Temperature Temperature is the level of heat energy in a material as measured by a thermometer or thermostat and recorded on any of several temperature scales, e.g. Celsius, Fahrenheit or Kelvin. The freezing temperature of water on the Celsius scale is 0 °C and the boiling temperature is 100 °C. The freezing temperature of pure/ clean water on the Kelvin scale is 273 K and boiling temperature is 373 K at normal atmospheric pressure.
Austenite This is a solid solution of iron and carbon or iron carbide. The crystal formation is created when the carbon dissolves in the steel crystals to form smaller crystals between the higher and lower critical points. It is a high-temperature form of steel called face-centred cubic (FCC) structured iron. Grain boundary
Figure 3.2: Microscopic view of austenite (greatly enlarged)
Ferrite Ferrite is the microstructure of iron or steel which is mostly pure iron or steel and appears light grey or white when etched and viewed under a microscope.
Did you know? A microstructure is a structure that is only visible at high magnification under a microscope.
Figure 3.3: Ferrite crystal of carbon steel
Cementite Cementite also known as iron carbide, a compound of iron and carbon (Fe3C) found in steel and cast iron. When the carbon content rises above 0,83%, the carbon combines with the pearlite crystals to form a very hard combination of cementite crystals.
Figure 3.4: Pearlite and cementite crystals
Pearlite Pearlite is a combination of ferrite and cementite, occurring in alternating layers in the microstructure. It is the type of crystal formed before hardening when the steel contains 0,83% carbon.
Figure 3.5: Microscopic view of pearlite (greatly enlarged)
Martensite This is the structure obtained when austenite is quenched suddenly. The needle-like structure below shows a pattern of fine untempered martensite.
Figure 3.6: Microscopic view of martensite (greatly enlarged) Structure
Soft, grain structure coarse
Good strength, not as hard as martensite, ductile and tough
Intensely hard and brittle
Soft and ductile
Extremely hard, strong and brittle
Good ductility, very hard but not as hard as bainite, fairly strong and tough, resistant to deformation
Figure 3.7: Table of the characteristics for the various types of steel structures
Critical temperature This is the temperature where a phase change, structural change or a crystalline structure change takes place. It greatly depends on the carbon or alloy content. Critical temperatures are very important when heat treating a material.
Materials Cooling Temperature in °C
Did you know? The points where one phase changes to another are called critical points by heat treaters and transformation points by metallurgists.
Figure 3.8: Critical temperature diagram of 0,83% carbon steel
The figure above shows the grain structure in heating and cooling cycles. The centre section shows quenching from different temperatures and the resultant grain structure.
Lower critical point (AC1) This is the lowest temperature to which steel must be heated to be hardened. The lower critical point is always about 721 °C in equilibrium or slow cooling but the higher critical point changes as the carbon content changes.
Second arrest point (AC2) If heating continues uniformly, a second arresting or critical point (AC2) occurs, although the effect at AC2 is not as prominent as at AC1. When the temperature rises slowly above 700 °C (AC1) until it reaches 800 °C, it now displays a bright red colour. The changes at this point are entirely dependent on the carbon content. At this stage, a partially annealing temperature is reached in most steel types.
Did you know? AC1, AC2 and AC3 are abbreviations for a French term ‘arrêt de chauffage’ which means ‘heating ends’.
Higher critical temperature (AC3) This is the highest temperature to which steel can be heated to obtain maximum hardness. Percentage carbon
Critical temperature for hardening and annealing in °C
915 – 980
885 – 925
850 – 900
815 – 870
790 – 850
780 – 825
760 – 810
745 – 790
0,90 or higher
730 – 780
Figure 3.9: Critical temperatures in steels
Mechanical Technology Decalescence point • The temperature where carbon steel changes from pearlite to austenite when heated (700 °C for 0,83% carbon steel). Recalescence point • The temperature where carbon steel changes back from austenite to pearlite when the steel is cooled (700 °C for 0,83% carbon steel). Critical temperature of ordinary carbon steel • When a steel or iron sample is heat treated, it undergoes structural changes. At this stage, the nature of the structural changes is not as important as the temperature at which they take place. When an artefact consisting of ordinary medium-carbon steel is heat treated, internal structural changes take place at 700 °C and it takes up an entirely new form at a temperature of about 800 °C. For this particular sample, 700 °C is regarded as the lower critical temperature and 800 °C as the higher critical temperature. The critical temperature of any type of sample which is subjected to heat treatment must be known for the process to be performed successfully. The following points should be considered: • The lower critical temperature for all ordinary carbon steels is 700 °C. • The higher critical temperature fluctuates in accordance with the carbon content of the steel. • Steel or iron with a carbon content of 0,87% has only one critical temperature, i.e. 700 °C.
Did you know? Cementite is the silvery speckle in white cast iron after it is fractured (and is intensely hard). A mixture of a certain proportion of these two elements is called pearlite because under the microscope it frequently has the appearance of ‘mother of pearl’, hence the name.
Changes during the hardening of carbon tool steel Carbon steel which has been fully annealed consists mainly of two parts: One of the elements is iron or ferrite (derived from the word ferrous, meaning containing or resembling iron) and the other is a carbide of iron known as cementite. When carbon steel in the fully annealed state is heated, usually to a temperature between 680 °C and 720 °C (depending on the carbon content), the alternate bands or layers of ferrite and cementite form many alternating layers side-by-side, like layers of bread and meat in an endless sandwich. The layers in the steel are so thin that they are only visible under a microscope. They look like alternating sheets of white and black paper (viewed from the edge). The pearlite layers begins to merge into each other. The temperature at which this occurs is known as the lower critical point (AC1). The merging process continues until the pearlite is thoroughly ‘dissolved’, forming what is known as austenite. If the temperature of the steel continues to rise, the pearlite and any excess ferrite or cementite will also begin to dissolve into austenite until finally only austenite will be present. The merging process continues until the pearlite is completely dissolved to form austenite. If the temperature of the steel continues to rise and there is (apart from the pearlite) any remaining ferrite or cementite present, it will also ‘dissolve’ until eventually only austenite is present. The temperature at which the excess ferrite or cementite is completely dissolved into austenite is called the upper critical point (AC3 ). If the steel is now suddenly cooled by plunging it into a bath of cold water or oil, a new structure is formed when the austenite is transformed into martensite and this provides the steel with the property of hardness.
Figure 3.10: The iron-carbon equilibrium diagram
Steel characteristic changes at critical temperatures (AC1, AC2, AC3) The change that takes place when heating steel is of great importance when explaining the reasons for the effect of the different heat-treatment processes on the metal. This aspect often causes considerable confusion. If a steel bar containing, say, 0,3% carbon is gradually heated in a furnace and the time limit of the heating process is observed, it will be noted that the temperature rises uniformly at first. When, however, the temperature reaches 700 °C (a dull, red heat) it will remain stationary for a while and will then rise at a slower rate until reaching 800 °C (a bright red colour). Hereafter, the temperature will continue to rise constantly if the heating can be maintained, as was initially the case. This first arrest point (AC1) is called a critical point or point of decalescence.
Mechanical Technology Now observe a piece of steel heated to 900 °C (a bright reddish-yellow colour) and then allowed to cool in dim light so that the colour can be observed effectively. On cooling, the steel will lose its brilliance first. The cooling will continue normally until the temperature is reached (AR1) which more or less coincides with the temperature where the steel experienced the arrest point during heating (AC1). At this point, it will appear as if the steel has stopped cooling and, by taking careful note, the steel will appear to have an extra glow as if it was heated. The rate of cooling will continue normally after this point; so noting that time is important during the cooling process. Figure 3.11 shows a graph representing the steps for the duration of heating and cooling of steel against time taken. We must remember that the point at which this arrest in temperature drop takes place is known as the recalescence point (AR1) and indicates that the change in the internal structure of the steel has taken place. In all heat treatment, time and temperature are both important in producing the desired change in the steel. Uniform rise in temperature
Halt in temp rise (point of decalescence AC1)
Halt in temperature drop (point of recalescence AR1) AC3 AC1
Temperature in °C
Uniform drop in temperature
Uniform drop in temperature Uniform rise in temperature
Figure 3.11: The temperature – time graph
Effect of heating and cooling on the structure of steel When a sample of steel or iron containing a small percentage of carbon is heated and its temperature rise is measured, it can be seen that, after a certain time, although we continue to soak the sample in the heat, the temperature stops rising for a short period of time and then starts to rise again at a uniform rate.
Evidently, at this arrest or critical point (AC1) the heat was absorbed without causing a rise in temperature but has caused a change in the internal structure of the sample (steel or iron). If heating is continued, a second arrest or critical point (AC2) occurs but the effect is not as noticeable as at the AC1 point. At higher temperature still, a third critical point (AC3) occurs which is comparable to the first (AC1). The AC3 (third critical) point is the most important, as the sample’s grain structure is at its smallest and is called austenite.
Identification of material uses with enhanced properties in practical applications Crankshafts Crankshafts can be forged from a steel bar generally through roll forging or casting in ductile steel. With forged crankshafts, vanadium alloyed steels are mostly used because of their air cooled properties, especially after reaching high strengths without additional heat treatment, excluding the case hardening of the bearing surfaces. The low alloy content makes the material cheaper than high alloy steels. Carbon steels are also used but require extra heat treatment to achieve the sought-after properties. Case-hardening (carburising, nitriding and cyaniding) is a surfacehardening process. The objective is to produce a hard case over a tough core. Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and, at the same time, must be tough enough internally at the core to withstand the applied loads, such as gears, cams, cylinder sleeves, etc. Almost all production crankshafts use induction hardened bearing surfaces, since that technique provides good results with minimum costs. It also permits the crankshaft to be reground without re-hardening. Then again, high performance crankshafts, (billet crankshafts, a term to describe crankshafts that are made from casting a special grade of iron alloy, in specific, tend to use nitriding instead. Nitriding is slower and thereby more costly and in addition it puts certain demands on the alloying metals in the steel to be able to create stable nitrides. The advantage of nitriding is that it can be done at low temperatures, it produces a very hard surface and the process leaves some compressive residual stress in the surface which is good for fatigue properties. The low temperature during treatment is advantageous in that it doesn’t have any negative effects on the steel. With crankshafts that operate on roller bearings, the use of carburisation tends to be favoured due to the high contact stresses in such an application. Like nitriding, carburisation also leaves some compressive residual stresses in the surface. Gas carburising is a heat-treatment process for steel camshaft billets (see above for the explanation of this term). In this procedure, the camshaft is placed in a furnace with a carbon-gas atmosphere and heated to a specific temperature. After the camshaft surface ‘skin’ has absorbed a desired amount of additional carbon, it is removed from the furnace and quenched to attain the proper temper. Tempering is needed to minimise retained austenite and increase camshaft performance. The material for the casting is a special grade of iron alloy, which is used primarily for non-roller camshafts because of its excellent anti-wear properties. Certain camshafts are nitride; in this case, the hardened ‘skin’ depth is shallower. These camshafts will be used for less critical applications. Carburising inclines to be the more costly process of the two. This process is critical to safeguard a high quality completed camshaft as well as delivered durability to high performance camshafts. Camshaft The camshaft is driven by a chain, belt or set of gears from the adjacent crankshaft, at half engine speed. This shaft is made of either forged steel or cast iron, machined and hardened to give maximum resistance to the cam lobes against wear. The cams are spaced at intervals to match the firing order. As the camshaft rotates, each cam in turn lifts a tappet and push-rod causing the corresponding rocket to pivot and push the valve down. The valve is closed by a spring when further rotation of the cam allows the tappet to descend. Some engines have two springs on each valve.
Mechanical Technology For efficient operation, the valves must return to their seats. To ensure this, a gap known as tappet clearance is allowed between the closed valve and its rocker. This allows the valve gear to expand when it is hot. Piston rings Grey cast iron and steel piston rings are manufactured in different processes. To overcome the problem of low toughness and brittleness in cast piston rings, the rings must be heat treated. Heat treatment and proper piston ring fit ensure long life and less wear. Heat treatment provides maximum hardness which extends life. Rings fit precisely into piston ring grooves, reducing ring band wear. The ‘correct’ heat treatment temperature is 480 – 520 °C, with slow heating, the temperature being held for 1 hour per 25,4 mm of thickness but at least 10 minutes for very thin rings. The outer surface of the piston ring is selectively and superficially heat treated in one of a variety of ways to form an austenitic metal layer on the surface of the ring. The ring is then rapidly cooled in an appropriate environment, resulting in the transformation of the austenitic compound into a martensitic compound adjacent to the base material at the point of heat treatment.
1. What does the hardness of steel depend upon? 2. What structure in steel determines the hardness of steel? 3. What structure in steel determines the ductility of steel? 4. How are the following formed in steel? 4.1 Martensite 4.2 Pearlite 5. When plain carbon steel is heated at a uniform rate, its temperature rises evenly to 700 °C. The temperature then remains constant for a while. This point is called the 5.1 Recalescence point 5.2 Cooling point 5.3 Lower critical point 5.4 Decalescence point 5.5 Higher critical point 6. Explain the following terms which are used in connection with heat treating: 6.1 Austenite 6.2 Recalescence point 6.3 Decalescence point 7. Describe the term ‘heat treatment’ as you understand it. 8. Explain briefly the reasons for performing the following heat-treatment processes on carbon steel: 8.1 Hardening 8.2 Case hardening 8.3 Annealing 8.4 Tempering 8.5 Normalising 9. Draw a neat, simple iron-carbon diagram which will indicate the following: 9.1 The AC1 and AC3 lines 9.2 The different steel groups which are represented in the diagram in their respective sequences. 9.3 Explain what is meant by the first and third arrest points of carbon steel. 10. Pearlite is a combination of __________ and _________. 11. The carbon content of ferrite is _________. 12. The carbon content of cementite is _____________. 13. The carbon content of pearlite is _____________.
Terminology Topic 4
Cutting procedures for lathes Apply cutting methods to make an artifact
Milling machine calculations
Terminology and procedures for using lathes and milling machines Screw-thread micrometer The screw-thread micrometer below is specifically designed to measure the pitch diameter of a screw thread. The anvil and spindle tips are shaped to match the included angle (form) of the screw thread to be measured. The screw-thread micrometer has a V-shaped anvil which fits over a thread form and a cone-shaped spindle that fits into the opposite thread groove. Spindle Screw
Screw thread Anvil
Figure 4.1: Screw-thread micrometer
Basic screw-thread terminology Helix angle
Minor or root dia
Major or crest dia
Single depth Thickness of thread
Screw Thread angle
Figure 4.2: Screw-thread terminology
Figure 4.2 illustrates the parts of a screw thread. Below are the definitions of the parts.
Major, crest or basic diameter is the larger or outside diameter of the screw thread. It is also known as the full diameter of the screw thread. It is the nominal size by which it is recognised e.g. M 20.
Minor, root or core diameter is the smaller diameter of a screw thread, measured at its root or bottom. It is equal to the major diameter minus twice the depth of the screw thread, measured at right angles to the axis. Pitch, mean or effective diameter is the diameter of an imaginary cylinder. The surface of this cylinder would pass through the threads at such points as to make equal the width of the threads and the width of the spaces between the threads. The mean diameter is the outside diameter minus the depth of the screw thread. Pitch (P) The pitch (P) of a screw thread is the distance from any given point on the screw thread to a corresponding point on an adjacent thread, measured parallel to the axis of the screw thread. A screw thread pitch gauge is used to measure the pitch of a screw thread. Lead (L) The lead of a screw thread is the distance that the nut on a screw thread will move (advance) along the screw thread axis when turned through one complete revolution (turn). The lead and the pitch of a single start screw thread are equal. The lead is calculated by multiplying the number of starts of the screw thread by the pitch. Lead = number of starts × pitch. Crest The crest of the screw thread is the top (outside) surface where the two sides (flanks) of a screw thread join. Root The root of the screw thread is the bottom surface where the sides (flanks) of adjacent threads join. Axis The axis of the screw thread is the centre line through the screw thread lengthwise. Depth of screw thread The depth of the screw thread is the distance between the crest and the root of the screw thread, measured perpendicular to the axis. Screw-thread angle The screw-thread angle of the screw thread is the angle included between the sides (flanks) of the screw thread measured in a plane through the axis. Form of screw thread The form of the screw thread is the cross-section of thread cut by a plane containing the axis. Series of screw thread The series of the screw thread is the standard number of threads per 25 mm for various diameters. A single thread is composed of one ridge. The lead is equal to the pitch. Multiple threads are composed of two or more ridges running side-by-side. Multiple screw threads are used where quick motion, but not great strength, is important. A right-hand screw thread requires a bolt or nut to be turned clockwise or to the right to tighten it. A left-hand screw thread requires a bolt or nut to be turned anticlockwise or to the left to tighten it.
Mechanical Technology Screw-thread fit Screw thread fit describes how tightly a bolt and nut fit together. There are four general screw thread fit classes: • Class 1 fit is recommended only for screw thread work in which shake or play is not objectionable. This fit is used in parts that are essential for rapid assembly. • Class 2 is for threaded parts that can be put together with the fingers (hand tight). There may be a little looseness between the parts. A Class 2 fit is recommended for most interchangeable screw thread work. • Class 3 is for a higher grade of threaded parts, requiring greater accuracy. It is recommended only in cases where the high cost of precision tools and continual checking are warranted. • Class 4 is for the finest threaded work. A screwdriver or wrench may be necessary to assemble the parts. These screw thread fits are not adaptable to quantity production. Calculating the cutting depth It stands to reason that, in engineering practice, the screw-thread shape be different from the standard shape or form. This adaptation is made to prevent sharp corners at the crest or at the root of the screw thread. The crest and the root of the screw thread are rounded to smooth movement. This is also done to avoid the mating threaded parts jamming or getting stuck. Pitch line
Axis of thread
Depth = 0,86603 × pitch
Figure 4.3: Calculating screw-thread depth
Cutting procedures Screw cutting on a centre lathe Screw-thread cutting on a centre lathe is executed by turning the workpiece at the same time as giving a uniform longitudinal movement to the saddle of the lathe. To cut dissimilar pitches on the lathe, trains of gears are provided. The principle of screw cutting involves gearing the headstock so that, by turning the spindle once, the leadscrew will turn sufficiently to carry the saddle forward a distance equal to the pitch of the thread to be cut. The modern centre lathe is fitted with a quick-change gearbox to speed up the changing of gears for thread cutting.
Cutting an outside metric V-screw thread using the cross-slide method
Set up the workpiece in the centre lathe and turn the part to be threaded to the major (nominal) diameter of the thread. Set the compound slide to 30° (equal to half the screw-thread angle) to the right and set the cutting tool up accurately in the tool post (centre and square to the workpiece height).
3. Consult the index plate of the quick-change gear box and shift the levers accordingly for the necessary pitch of the screw thread. 4. Start the centre lathe and set the cutting tool at touching point on the workpiece. Set the graduated (micrometric) dials of the cross feed and compound slide to zero. 5. Move the cutting tool a short distance off, to clear the end of the workpiece and feed the compound slide 0,05 mm inwards. 6. With the centre lathe revolving (turning), engage the half nuts at the correct line on the threading (chasing) dial, putting the first cut of the screw thread in progress. The cutting tool will now scrape (scratch) the workpiece. 7. At the end of the cut, withdraw the cutting tool quickly and disengage the half nut lever. Return the carriage to the starting point of the screw thread. 8. Stop the centre lathe and check the screw-thread pitch with a screw thread pitch gauge. 9. Repeat the process of cutting with successive cuts until the required depth is reached and the screw thread is completed. (HINT: Bring the cross-feed collar back to zero for each cut). 10. Each consecutive cut is set by means of the compound slide. 11. A very light finishing cut can be made by adjusting the cross-slide feed screw. 12. On completion, check the finished screw thread with a ring gauge for the correct fit. Chuck Workpiece
Cross slide ___ = 30˚ 2
Figure 4.4: Cutting tool set at right angle to the workpiece
The left-hand V-screw thread The left-hand screw thread turns counterclockwise (anticlockwise) when advancing. The centre lathe is set-up exactly as for cutting a right-hand screw thread, except that the tool is fed from left to right (from the headstock to the tail-stock) instead of from right to left. Left-hand threads are used for the cross-feed screw of centre lathes and screw threads in oxy-acetylene welding equipment.
Turning exercise Turn the following artefact from mild steel on a centre lathe. The exercise includes step turning, groove cutting, centre drilling, chamfer and dual-centre work.
Figure 4.5: Turning test sample
Milling machine calculations Gears play an extremely important part in mechanical devices of all kinds. To meet a variety of gearing requirements, many types of gears have been developed. Although most gear cutting is done on specialised machine tools, a spur gear may be cut on the milling machine by using straightforward machining technology. Gears and gear cutting in general involve a number of terms, numerous dimensions and calculations. To cut gears, you must be familiar with gear terminology and the calculations involved in gear cutting.
Indexing on the dividing head Simple indexing When indexing is required, the number of turns of the crank needed must be calculated to move the workpiece the required distance to cut the number of teeth or grooves on the circumference of the workpiece. Because the dividing head has a 40 to 1 ratio, the indexing movement calculation is the crank T = 40 N Number of turns = where N = number of divisions (e.g. number of teeth or grooves) Example Calculate the indexing for the following: (i) 10 teeth (ii) 8 teeth (iii) 2 grooves To calculate the number of turns (i)
40 Number of turns = N 40 Number of turns = 10 Number of turns = 4 full turns
40 (ii) Number of turns = N 40 Number of turns = 8 Number of turns = 5 full turns 40 (iii) Number of turns = N 40 Number of turns = 2 Number of turns = 20 full turns
Cutting a gear To cut a spur gear, the following method can be used: • Mount a gear blank on a mandrel between centres of the diving head and tailstock on the machine table as close to the column as possible. • Fit the correct involute cutter on the arbor. • Calculate and attach the proper index plate to the dividing head and set the dividing head for the correct number of teeth. • Centre the cutter over the blank gear so that the cutter line of the gear teeth is radialled to the gear axis and lock the table in position. • Raise the machine table and bring the gear blank in position under the cutter, start the milling machine and lift the table until the cutter touches the gear blank. (Test with tissue paper placed between the cutter and the gear blank, or chalk on the workpiece). • Set knee graduated sleeve (vertical feed dial) to zero and move the table back to the starting position. • Raise the table to an amount equal to the required tooth depth. (Raise the table a lesser amount if two or more cuts are to be made) and lock in position. • Move the table so that the cutter is close to the gear blank and engage the automatic longitudinal feed. • Make the cut across the gear blank. • At the end of the cut, stop the machine, disengage the feed and reverse the table to the starting position. • Index the gear blank for the next groove (tooth) and repeat until all teeth have been cut. • When all the teeth have been formed, the machine can be set for a finishing cut if required.
Did you know? A mandrel is a work holding device.
Square Machine table
Figure 4. 6: Centering an involute cutter on a gear blank
Mechanical Technology Milling a keyway or slot
Did you know? A staggeredtooth cutter should be used to mill deep slots and grooves. It lessens vibration and results in better workmanship.
Milling a slot includes many operations, from cutting slots in the heads of small screws to milling narrow and wide slots in the workpiece. A keyway is a slot milled on the centre and parallel to the axis of a shaft for the purpose of receiving a key. Procedure: 1. Place a staggered-tooth side milling cutter on the arbor. 2. Fasten the workpiece in the vice. 3. If the workpiece is rectangular, move the table to locate the work in the proper position. Hold the end of the steel ruler against the side of the cutter. Move the saddle to set the cutter the correct distance from the side of the work. 4. Use a piece of tissue paper or mark the contact point with chalk as a feeler between the workpiece and the cutter. Raise the knee of the milling machine until the revolving cutter tears the tissue paper away or scrapes the chalk away. Caution: be sure that the paper is long enough so that your fingers will clear the revolving cutter. 5. Set the graduated dial on zero to its index line. 6. Move the workpiece back from the cutter. Raise the knee of the milling machine to take the correct depth of cut. 7. Start the milling machine. Engage the table feed. 8. To cut a keyway in a round shaft or bar, mount the staggered-tooth, side milling cutter on the arbor. 9. Centralise the cutter (as discussed in Grade 11). 10. Raise the table to take the proper depth of cut; this depth can be found in the machinist handbook.
Figure 4.7: Using a steel rule to measure the location of a slot or keyway on a rectangular workpiece
Side and face cutter Arbor
Figure 4.8: Centralising a cutter to cut a keyway on a shaft
Example A 15 mm-wide keyway must be cut on a shaft 60 mm in diameter. Sketch and describe how a 15 mm-wide side and face cutter can be centred on the shaft to cut the keyway. Width of the key (W) W = D 4 W = 60 4 W = 15 mm
Mechanical Technology Solution:
Side and face cutter
Arbor with spacers/collars
Ruler Keyway Workpiece Square Milling machine table
Figure 4.9: Centring a milling cutter on a workpiece
• Measure, from the side of the square to the side of the cutter, a distance equal to half the shaft diameter minus (–) half the width of the cutter: = 30 – 7,5 mm = 22,5 mm • When the distance of 22,5 mm is measured between the square and the cutter, the centre of the cutter will coincide with the centre of the shaft.
Milling processes Methods of milling In peripheral milling, the workpiece can be fed either with or against the direction of the cutter rotation. In face milling, however, the characteristics of the two methods are usually combined. The feeding motion is normally partly with and partly against the direction of the cutter rotation. The two methods are known as up-cut milling and down-cut milling.
Up-cut milling (conventional milling) In up-cut milling, the cutter turns against the direction of feed as the workpiece moves toward it from the side where the teeth are moving upward. The separating forces produced between the cutter and workpiece oppose the motion of work. In up-cut milling, the cutter teeth come up from the bottom of the cut. Thus the chip is very thin at the beginning where the tooth contacts the workpiece. The chip increases steadily in thickness. The chip reaches its maximum thickness where the tooth leaves the workpiece.
Figure 4.10 : Up-cut milling
Down-cut milling (climb milling) If down-cut milling is used, all looseness in the table-feed screw must be eliminated. The motion of the cutter tends to pull the workpiece into the cutter. In down-cut milling, the maximum chip thickness is obtained close to the point where the tooth contacts the workpiece. No build-up pressure is developed in down-cut milling. Therefore no heavy burrs form on the surface of the metal.
Figure 4.11: Down-cut milling
Assessment 1 1. Draw neat sketches to demonstrate the difference between up-cut milling and down-cut milling.
Plain or slab milling One of the simplest operations that can be performed on the horizontal milling machine is plain or slab milling. This operation consists of machining a plain, flat, horizontal surface with cylindrical milling cutters. These cutters have a length that is usually greater than the diameter. Helical cutters generally produce a much better surface than cutters with straight teeth.
Mechanical Technology Set-up 1. Clean the surface of the machine table. 2. Setup the vice in the centre of the machine table. 3. Bolt the vice down on the machine table. 4. Check the dial-test indicator to confirm whether the vice is square on the machine table. 5. Check if the workpiece fits in the vice. (If the workpiece is too small, use parallel bars to lift it.) 6. Tighten the vice and tap the workpiece with a soft-faced hammer to seat it properly on the parallel bars. Push the parallel bars to ascertain if the work piece is seated properly. 7. Move the saddle as close as possible to the column. 8. Choose a plain milling cutter of the smallest diameter to cover the workpiece. The diameter of the cutter must be big enough to permit the arbor to clear the work piece. A small cutter needs less time to make a cut than a large cutter. 9. Clean the tapered hole in the spindle nose and the arbor’s taper shank before mounting the arbor. 10. In removing the arbor nut, the arbor support should be clamped in position to prevent the arbor from springing out. 11. Mount the cutter on the arbor as close as possible to the column. Ensure that the end thrust caused by the helix angle of the cutter teeth will be against the spindle bearings. 12. Insert the key into the keyway of the arbor and cutter. Serious damage can be done to the arbor if the cutter slips. Never rely on friction between cutter and the arbor. 13. Clean the spacing collars and slide them onto the arbor. The bearing collars should be as close as possible to the cutter. 14. Tighten the nut on the arbor with your fingers. 15. Move the overarm into position, slide the arbor support onto the overarm and lock them in position. 16. Lock the spindle and tighten the arbor nut with a spanner. 17. Adjust the machine table to the proper height for the cut desired. Adjust the trip dogs for the length of cut to be made. 18. Calculate the cutting speed for the type of cutter and material.
Figure 4.12: Plain or slab milling
Face milling Face milling can be executed with a wide range of face-milling cutters. Face milling is the production of a surface that is parallel to the face of the cutter and at right angles to the cutter. This type of milling operation can be performed on both the vertical as well as the horizontal milling machine. Set-up 1. The face cutter should be approximately 25,4 mm bigger in diameter than the width of the face to be milled. This enables the whole surface to be milled at once. 2. Mount the workpiece in the vice, on the mandrel, angle plate or fixture. Locate the workpiece in the centre of the machine table. Ensure that the holding device is correctly aligned. 3. Set the machine for the correct speed and feed. Carbide tips
Figure 4.13: Face milling
Straddle milling The operation of milling the opposite sides of a workpiece at the same time is called straddle milling. This operation is used to mill square heads and hexagonal heads on bolts. It is done mainly with two side and face cutters, spaced at the correct distance apart on the milling-machine arbor. Side and face-milling cutters
Vice movable jaw
Shoulder width determined by collars or spacers
Vice fixed jaw
Figure 4.14: Straddle milling
Gang milling Gang milling consists of machining surfaces of manufactured workpieces by means of two or more milling cutters mounted on an arbor. Sometimes a combination of milling cutters may be used for plain milling and side milling simultaneously. When helical cutters are used to produce a flat surface, these cutters should be equal in diameter. Cutters of uneven diameters will produce a rib on the workpiece. Overarm Arbor support Plain milling cutter Arbor
Side and face cutter Clamped workpiece
Figure 4.15: Gang milling
Assessment 2 1. Calculate the indexing for the following: (i) 25 divisions (ii) 17 divisions (iii) 28 divisions (iv) 39 divisions (v) 98 divisions 2. Name four main parts of a universal dividing head. 3. Name two advantages or two disadvantages of down-cut milling. 4. Choose the correct answer. A. We fit braces to a horizontal milling machine to (i) Protect the operator (ii) Guard the cutter (iii) Prevent the table from being raised (iv) Improve rigidity of the overarm during heavy cuts B. We usually mount the cutter of a horizontal milling machine on the machine’s (i) Arbor (ii) Knee (iii) Spindle (iv) Overarm 5. What is the end milling cutter used for? 6. Explain what you understand by the gear ratio of a dividing head being 40:1. 7. True or False: The Brown and Sharpe index plate has holes on both sides. 8. What is the purpose of the dividing head on a milling machine? 9. Give two important features of a universal milling machine compared with a plain milling machine. 10. How many cutters are usually found in a set of involute gear-milling cutters?
Using advanced instructions to make an artefact
Milling exercise Mill shoulders in the mild steel block below with the following dimensions: length 70 mm, width 60 mm and height 38 mm. The tolerances are 0,02 mm and 0,03 mm.
Figure 4.16: Milling test sample
Joining methods Topic 5
Applications Defects Visual inspection
Joining methods Manufacture joints using MIG welding
Apply advanced permanent joining applications
Introduction In Grades 10 and 11, we learned about permanent and semi-permanent joining methods by means of fasteners and basic welding/soldering techniques. In Grade 12, we will focus on advanced joining methods as well as inspection and testing thereof.
Analysis of the possible defects of welding joints by visual inspection It is necessary to inspect and test welds to assess the quality, strength and properties of the joints. A visual inspection, both during welding and afterwards, will give you a good idea of the probable strength of the weld. Visual inspection is carried out by the welder only after some welding experience has been obtained.
Defects Imperfections; things that are wrong or not perfect
The following are the visual requirements of acceptable welds: • Shape of profile: The profile of a butt weld must be slightly convex and must have a reinforcement that is within the limits specifications. The profile must merge smoothly into the adjacent surface of the metal. In the case of fillet welds, the leg length and the throat thickness (the distance between the root and weld face of the weld) should not be less than those laid down in the code of practice, which is a document published by the South African Bureau of Standards. This code of practice specifies the requirements for welds and provides standardised welding conventions for the welding/engineering industry. The profile must merge smoothly into the adjacent surface of the parent plate. • Uniformity of surface: The weld face must be uniform in appearance throughout its length. At points where the welding has been interrupted and restarted, the weld must be smooth and must show no pronounced humps or craters. • Overlap: There must be no overlap at the toes of the weld. • Undercutting: Grades A and B welds must be completely free from undercutting. In grade C welds, slight intermittent undercutting may be present, provided that it does not produce a notch effect. • Penetration bead: In butt welds made from one side only and without the use of a backing bar, a slight penetration bead should be present but the absence of a penetration bead in isolated places may be disregarded, provided that there is full penetration. • Root groove: In butt welds made from one side only and without the use of a backing bar, a root groove may, at the discretion of the welder, be present, provided that it has a rounded outline and does not penetrate below the level of the adjacent surface of the parent plate. • Freedom from cracks: The weld metal, the heat-affected zone and the surrounding parent metal must be free from cracks that arise from uncontrolled expansion and contraction. • Freedom from surface defects: The weld face must be free from porosity, craters or cavities and slag inclusion. More detail on these defects is provided later.
Applications MIG/MAG welding
Inert Unreactive element found in Group 8 of the Periodic Table of Elements
MIG stands for metal inert gas. MIG welding is also known as metal active gas (MAG) welding. MIG welding machines are direct current (DC) welding machines. Instead of using flux-coated electrodes like metal-arc welding machines, MIG machines use a continuous wire electrode feed. The wire electrode is shielded by an inert gas. The inert gas takes the place of the flux coating in metal-arc welding electrodes. Inert gases are used to shield the molten pool because they do not react with the weld metal and they shield the molten pool from Figure 5.1: A MIG welding machine atmospheric gases.
Most MIG welding machines consist of four basic components: • a shielding-gas cylinder and gas flow meter/regulator • a power source (transformer/DC rectifier) • a wire-feed controller and feed wire • a gun and accessories. Many MIG welders combine the transformer and wire-feed unit in the same housing but some are supplied as independent units, as shown in Figure 5.2. Gas hose
Continuous wire reel
Power cable Gun conduit
Figure 5.2: Components of a MIG welding machine
Gas cylinder and regulator/flow meter The cylinder normally contains 17 kg of inert shielding gas. The inert gas that is usually used is a mixture of argon and carbon dioxide (CO2). UHP (ultra high purity) argon is too expensive to be used on its own commercially, so it is mixed with CO2. The cylinder must be fitted with a regulator to reduce the cylinder pressure as well as a flow meter to control the flow rate of shielding gas (in litres/ minute).
Power source The power source of most MIG welding machines is an alternating-current (AC) welding transformer which has a bridge rectifier attached to it in order to convert the AC to DC. Adjusting knobs adjust the power source voltage.
Figure 5.3: The power source of a MIG welding machine
Mechanical Technology Wire-feed controller and feed wire The wire-feed controller feeds the consumable electrode wire to the welding gun at a constant predetermined speed. The speed can be adjusted to suit the welding conditions. Normally, the higher the voltage, the faster the wire speed needs to be. Higher voltages produce more heat, which in turn melts the consumable wire at a faster rate and faster feed speeds are necessary. Wire reels usually contain 15 kg of wire and are available in a large range of thicknesses and alloy types for welding different materials. Motor Tensioner roller
Wire liner Wire-feed roller Consumable wire reel
Figure 5.4: The wire-feed controller of a MIG welding machine
Simultaneous Occurring at the same time
The welding gun is attached to the power source and wire feeder by means of a flexible conduit. This pipe contains a sheath for the feed wire, a shielding-gas hose and electrical connections to the power source. The welding gun has a trigger which remotely controls the simultaneous supply of gas, power and wire feed. The welding gun contains two main consumable parts, namely the gas shroud and nozzle. Regular spraying with anti-spatter spray and brushing clean with a wire brush will prolong the lifespan of these parts.
Figure 5.5: Anti-spatter spray Nozzle
Figure 5.6: A MIG welding gun
The process of MIG welding As the welding gun trigger is pressed, the power, wire feed and gas flow are engaged simultaneously. The intense heat of the arc melts the wire and the parent metal in a molten pool. As the MIG wire is consumed, more is fed into the molten pool and in so doing a weld bead is deposited. Figures 5.7 and 5.8 show the difference between metal-arc welding and MIG welding.
Did you know? Consumable components are components which are discarded (thrown away) after they have become worn out.
Shielding gas Weld pool
Figure 5.7: Metal-arc welding
Mechanical Technology Gas shroud
Continuous feed electrode wire
Did you know? TIG welding stands for tungsten inert gas welding. TIG welding is similar to MIG welding but does not include the continuous wire feed. The electrode is made from tungsten (an element with a very high melting point) which is used to establish and maintain a welding arc. The filler rod is fed into the molten pool manually. The process resembles gas welding but has an electrical arc as its source of heat.
Molten weld pool
Inert shielding gas
Figure 5.8: MIG welding
Without shielding gas, it is difficult to control the welding arc. Atmospheric gases also react with the weld, causing a lot of spatter and a poor quality weld.
Figure 5.9: MIG welding without shielding gas
Assessment 1 1. What type of shielding gas is used when MIG welding? Why is shielding gas used? 2. Sketch the MIG welding set-up and label the four important components. 3. State four important safety precautions to be taken when MIG welding.
Defects The following section deals with defects which can occur during the welding process. Commercially, defects are categorised according to severity. Welding runs are either accepted or rejected, depending on the criteria set by engineers and designers. There is a specific career in weld testing which we will look at later in the chapter. Numerous errors can occur during welding. We will only look at six main defects: 1. Porosity 2. Slag inclusion 3. Weld craters/faulty restart 4. Incomplete penetration 5. Lack of fusion 6. Undercutting
Porosity Porosity refers to gas pores (tiny bubbles) found in the solidified weld bead. As you can see in Figure 5.10, these pores may vary in size and are randomly distributed. Pores can occur either under or on the weld surface (the latter being called surface porosity). The most common causes of porosity are: • atmospheric contamination • surface contamination • dirty or wet electrodes when arc welding • rusted MIG wire. Atmospheric contamination Atmospheric contamination during MIG welding can be caused by: • inadequate shielding-gas flow • excessive shielding-gas flow (this can cause aspiration of air into the gas stream) • a severely clogged gas nozzle or a damaged gas supply system (leaking hoses, fittings, etc.) • excessive wind in the welding area (this can blow away the gas shield: see Figure 5.9). The atmospheric gases primarily responsible for porosity in steel are nitrogen and excessive oxygen. Inspect the gas supply regularly to ensure there are no leaks, thus ensuring continuity of the shielding gas and minimising atmospheric gases from coming into contact with the weld. Surface contamination Surface contamination can be caused by dirty, oxidised (rusted), oily, wet or painted surfaces. In all these cases, the gases formed by the melted surface impurities become entrapped in the weld surface, usually resulting in a brittle weld.
Brittle Hard but easily broken
Mechanical Technology Dirty or wet electrodes when arc welding Even if the welding electrodes have not come into contact with water, they are hygroscopic, which means that they tend to absorb moisture from the atmosphere if not stored correctly. Rusted MIG wire MIG wire which is not often used may start rusting around the outer surface of the reel. When the wire has rusted, welding should not be carried out as the rust can damage the wire liner of the feed mechanism (see Figure 5.4). Porosity
Figure 5.10: Porosity in a fillet weld
Slag inclusion Slag The layer on top a weld, resulting from the melted flux
Slag inclusions are non-metallic solids entrapped in the weld metal or between the weld metal and the base metal. Slag inclusions are solid regions within the weld cross-section or at the weld surface where the once-molten flux used to protect the molten metal is mechanically trapped within the solidified metal. This solidified slag represents a portion of the weld’s cross-section where the metal is not fused to itself. This can result in a weakened condition which reduces the serviceability of the component. Inclusions may also appear at the weld surface. Like incomplete fusion, slag inclusions can occur between the weld and base metal or between individual weld passes. In fact, slag inclusions are often associated with incomplete fusion. Slag inclusions can be avoided by thoroughly chipping off the slag from previous weld runs and brushing the weld bead with a wire brush before doing any further welding. Slag inclusion can also result from incorrect current settings. To remove slag inclusion, grind out the offending part of the weld and re-weld the section.
Figure 5.11: Slag inclusion in a butt weld
Weld craters/faulty restarts Weld craters form where welding is resumed at the bottom of the previous weld instead of at the top. When this happens, not enough weld filler material is added to the beginning of the new weld run and therefore a depression or crater is formed between the two weld runs. Always restart a weld run at the top of the previous weld run.
Figure 5.12: A weld crater in a fillet weld
Incomplete penetration This type of defect happens when: 1. The weld bead does not penetrate the entire thickness of the base plate. 2. Two opposing weld beads do not inter-penetrate. 3. The weld bead does not penetrate the toe of a fillet weld but only bridges across it. Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the welding current being too low and can be eliminated by simply increasing the amperage. Other causes include travel speed which is too slow and an incorrect torch angle. Both will cause the molten weld metal to roll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edge of the molten pool. Incomplete penetration can also be the result of poor edge preparation or an insufficient root gap when setting up a weld joint. Both conditions make it difficult for the electrode to approach the weld root and therefore may result in inadequate fusion and root penetration.
Root gap The gap between two welded plates
Insufficient penetration to the weld root
Figure 5.13: Incomplete penetration in welded joints
Mechanical Technology Lack of fusion Figure 5.14 shows a lack of fusion between the weld metal and the surfaces of the base plate. The most common cause of lack of fusion is poor welding technique. Either the molten pool is too large (travel speed is too slow) or the weld metal has been permitted to roll in front of the arc. Again, the arc must be kept on the leading edge of the molten pool. When this is done, the molten pool will not get too large and cannot cushion the arc.
Multi-pass welding When several welding runs are used to fill a welding joint
Another cause is using a very wide weld joint. If the arc is directed down to the centre of the joint, the molten weld metal will only flow and cast against the side walls of the base plate without melting them. The heat of the arc must be used to melt the base plate. This can be done by making the joint narrower or by directing the arc towards the side wall of the base plate. When multi-pass welding thick material, a split bead technique should be used whenever possible after the root passes. Large weld beads bridging the entire gap must be avoided. Lack of fusion can also occur in the form of a rolled over bead crown. Again, this is generally caused by a very low travel speed and attempting to make too large a weld in a single pass. However, it can also be caused by the welding voltage being too low. Excessive mill scale (iron oxide) can also hamper fusion even though light mill scale can be welded over in mild steel.
Lack of fusion
Figure 5.14: Lack of fusion in a butt weld
Undercutting As shown in Figure 5.15, undercutting is a defect which appears as a groove in the parent metal, directly along the edges of the weld. It is most common in lap fillet welds but can also be found in fillet and butt joints. This type of defect is most commonly caused by improper welding parameters, particularly the travel speed and arc voltage. When the travel speed is too high, the weld bead will be very peaked because of its extremely fast solidification. The forces of surface tension draw the molten metal along the edges of the weld bead and pile it up along the centre. Melted portions of the base plate are affected in the same way. The undercut groove occurs where melted base material has been drawn into the weld and not allowed to wet back properly because of rapid solidification. Decreasing the arc travel speed will gradually reduce the size of the undercut and eventually eliminate it. Undercutting can also be avoided by raising the arc voltage or using a leading torch angle (an angle close to 90°). In both cases, the weld bead will become flatter and wetting will improve.
However, if the arc voltage is raised to excessive levels, undercutting may again appear. When the arc becomes excessively long, it also becomes too wide. This results in an increased amount of base material being melted. However, the heat transfer of a long arc is relatively poor, so actually the arc supplies no more total heat to the weld zone. The outermost areas are cooled very quickly and again proper wetting is prevented. The arc length should be kept short, not only to avoid undercutting but to increase penetration and weld soundness. Excessive welding currents can also cause undercutting.
Figure 5.15: Undercutting
Assessment 2 Sketch six different types of welding defect and state one cause for each type.
Testing welds Although visual inspection is important, it has shortcomings. To check the quality of welded joints more thoroughly, it may be necessary to conduct certain tests. These tests are divided into two broad categories, namely destructive and nondestructive testing, such as the free-bend test. Weld testing is a specialised career path and provides well-paid employment opportunities.
Destructive testing Destructive testing, as the name implies, requires that a test piece is destroyed in the testing process. This method can provide useful information about the quality of a welding sample but is not suitable for large scale testing, for example, on a welding installation. Tests are usually carried out in laboratories by metallurgists but basic tests can be conducted in your workshop. There are numerous destructive tests which can be performed on welded joints, for example, the free-bend test, the guided-bend test, the nick-break test and the impact and tensile tests. We will first look at the free-bend and nick-break test.
Metallurgists People who study the properties and production of metals
Nick-break test The nick-break test determines the internal quality of the weld metal and can reveal internal defects (if present) such as slag inclusions, porosity, lack of fusion and oxidised or burnt metal. To perform the nick-break test on a butt weld, make a hacksaw cut at both edges, through the centre of the weld (see Figure 5.16). The cuts should be about 6,5 mm deep. Next, place the saw-nicked specimen on two steel supports. Use a sledgehammer to break the specimen by striking it in the zone where you made the saw cuts. The weld metal exposed in the break should be completely fused, free from slag inclusions and contain no gas pockets greater than 1,6 mm. There should not be more than one pore or gas pocket per square centimetre (of exposed broken surface of the weld). Saw cuts
Figure 5.16: Nick-break testing
Caution: Destructive tests may involve large stresses being placed on test pieces and should only be performed under the supervision of your teacher.
The nick-bend test measures the ductility of the weld deposit and the heat-affected area adjacent to the weld. It is also used to determine the percentage of elongation of the weld metal. You may recall (from Chapter 3 in the Grade 11 Learner’s Book) that ductility is the property of a metal that allows it to be drawn out into a thin wire. To prepare a welded specimen for the free-bend test, you must machine the welded face flush with the surface of the test plate. When the weld area of a test plate is machined, perform the machining operation in the opposite direction that the weld was deposited. The next step in the test is to mark two lines on the face of the filler deposit. Locate these lines 1,6 mm from each edge of the weld metal, as shown in Figure 5.17. Measure the distance between the lines to the nearest 0,001 mm and let the resulting measurement equal x. Then bend the ends of the test piece until each leg forms an angle of 30° to the original centre line. With the marked lines on the outside, bend the test piece with a hydraulic press.
Joining methods Weld face machined flat
1,6mm Marked lines Testing
Figure 5.17: Nick-bend testing
When the proper precautions have been taken, a hydraulic press or hammer can be used to complete the bending operation. If a crack more than 1,6 mm develops during the test, stop the bending because the weld has failed; otherwise, bend the specimen flat. After completing the test, measure the distance between the marked lines and call that measurement y. You can then calculate the percentage of elongation using the formula: y − x × 100 = % elongation A satisfactory weld will have a maximum elongation of 15% and no cracks greater than 1,6 mm on the face of the weld. Elongation refers to the degree of strain that the test piece has undergone. We look at the concept of strain in Chapter 6.
Machinability test Machinability testing can be applied to welds in various contexts but all are used to determine the weld’s hardness and thus its strength. The simplest method for determining comparative hardness is the file test. It is performed by running a file under manual pressure over the piece being tested. Information may be obtained as to whether the metal tested is harder or softer than the file or other materials that have been given the same treatment. The machinability test makes use of hardness testing machines such as the Rockwell, Brinell or Vickers hardness testing machines to evaluate the strength of the weld. Of particular interest is the hardness around the heat-affected zone (HAZ). The hardness in and around the HAZ can assist you in evaluating the brittleness of the weld and, thus, whether the weld has the desired strength. Often, a series of measurements in a given pattern is made at a given distance from the sample edge or from top of the weld. The progression of the hardness values can then be plotted in a graph.
Mechanical Technology Tensile testing is also used to measure the strength of a welded joint. A test piece of a welded plate is located midway between the jaws of the tensile testing machine (Fig. 5.18). The width and thickness of the test specimen are measured before testing and the cross-sectional area is then calculated. The tensile test specimen is then stretched until it breaks. As the specimen is tested in this machine, the load in newtons is registered on a gauge and the load at the point of breaking is recorded. The tensile stress in the test specimen is calculated by dividing the tensile load by the cross-sectional area of the test specimen. The higher the breaking stress (measured in pascals), the stronger the welded joint. Chapter 6 goes into greater detail in calculating stress and strain. Tensile load = L
Tension test specimen
Area = A = W × T Tensile strength = Tensile load = L Area A
Figure 5.18: Tensile testing
Non-destructive testing In contrast to destructive testing, non-destructive testing does not involve the destruction of a test piece. Non-destructive testing is carried out on site or in the workshop. The three non-destructive tests we will look at are X-ray testing, liquid dye penetrant testing and ultrasonic testing.
Visual inspection during welding The following items should be observed during metal-arc welding and oxy-acetylene welding.
Inspection during arc welding The following should be observed during arc welding: • rate of rod burning and the progress of the weld • amount of penetration and fusion • the way the weld metal is flowing (no slag inclusions) • the sound of the arc, indicating correct current and voltage for the particular weld.
Inspection during oxy-acetylene welding The following should be observed during oxy-acetylene welding: • correct flame for the work on hand • correct angle of the blowpipe and rod, depending on the method being used • depth of fusion and amount of penetration • the rate of progress along the joint. Visual inspection after welding will be dealt with at the end of the chapter (after the practical assessments).
X-ray testing X-rays are produced in the following way: a glass tube with two arms and a central bulb is evacuated (the air is removed). In one arm, there is a filament which can be heated to white heat (this will be referred to as the cathode). In the other arm, there is a thick copper stem ending in a target made of platinum and inclined at an angle of 45° to the axis of the tube (this will be referred to as the anode). A high voltage of between 60 000 and 180 000 volts is placed across the ends of the tube.
Platimum target Cathode rays
High voltage Electrons
Heated filament (cathode)
Figure 5.19: An X-ray tube
The filament (cathode) is white hot and emits negatively charged particles. This enables the high voltage to send a current through the tube. The current causes a stream of negatively charged particles, called cathode rays, to flow from the filament to the positively charged target (anode) (see Figure 5.19). On hitting the target, the rays are converted by a complex process of physics to X-rays and subsequently reflected, as shown in Figure 5.19.
Did you know? X-rays are electromagnetic radiations of short wavelengths. They occur just above the ultraviolet spectrum, approximately between 0,01 – 10 nanometers. X-rays can penetrate solid substances.
Did you know? The silver molecule in silver halides is precipitated when exposed to light (photons) and hence forms a dark area on the negative of photographic film.
Figure 5.20: A chest X-ray – X-rays penetrate less dense material more easily and hence show the skeletal bones and not soft tissue
When testing welds, a certain proportion of the rays are absorbed, depending on the thickness of the substance and on its density. The denser and thicker the substance, the less the proportion of rays which get through. In the same way that visible light exposes a sheet of photographic paper (films covered with silver halides), X-rays also expose photographic film. When X-ray testing is done, the photographic film is sealed in an envelope (so that light cannot expose it) and placed behind the object being tested. The X-ray (or gamma ray) source is placed in front of the object being tested as shown in Figure 5.21. Radioactive source
Pause for thought The use of gamma ray sources is overtaking the use of X-ray sources because they do not require a power source to operate them. Gamma ray sources use a radioactive plutonium sample to produce gamma rays (high energy photons) to expose photographic film. This makes them more portable than X-ray testers.
Figure 5.21: X-ray or gamma ray testing
When the tester is standing behind lead shields and as far as possible away from harmful exposure, the source is activated for a brief moment and the X- rays or gamma rays penetrate the test piece. As they pass through areas of lower density (air pockets, cracks or inclusions), the rays expose the film as lighter on the negative, indicating a welding defect. Photographic films are useful because they provide a permanent record of the shadow which can be carefully studied. Just as in photography, an incorrectly exposed film will contain no detail, so the X-ray or gamma ray photograph will show no detail of defects in the object unless the correct exposure is given. This is entirely a matter of practice. In order to make sure that a correctly exposed negative is achieved (so that even the smallest defects will be shown), a penetrometer is placed on the upper surface next to the source. A penetrometer is a small wire of the same material as the object (steel, for example)
and about 1 to 2 of its thickness. If it appears as a shadow on the 100 100 negative, you can be sure that any defects or holes of the size 1 to 2 100 100 of the thickness of the object will be indicated. In the same way, small lead letters are arranged on the outside of the envelope (stuck together with masking tape) to mark important details of the test, such as date, location and so on. Radiation does not pass through lead easily and therefore the markings are permanently exposed on the photographic film for reference. Great practice is necessary to interpret the X-ray films of welds correctly and to distinguish between various defects shown up as shadows. Gas holes causing porosity are usually regular in shape while any included slag is usually very irregular. In this way, you can determine whether there has been penetration to the full depth and correct fusion between parent metal and weld metal or between layer and layer in a multi-layer weld. The X-rays will also tell you whether there are regions of entrapped slag, blowholes or other porous defects. In addition, defects such as contraction cracks will be clearly shown.
Liquid dye penetrant testing The liquid dye penetrant testing method uses coloured liquid dyes and fluorescent liquid penetrant to check for surface flaws. This system can be used to detect surface flaws only in metals, plastics, ceramics and glass. This method will not detect sub-surface flaws. The liquid dye penetrant is sprayed onto the clean surface to be inspected. After allowing a short time for the liquid to penetrate, the excess amount of dye is removed with a cleaner (solvent), the surface is washed with water and allowed to dry. When the surface is dry, a developer is sprayed on the surface to bring out the colour in the dye penetrant which has penetrated any cracks or pin holes. Fluorescent liquids are also used. A fluorescent liquid is applied to the surface to be inspected. After a short time, the excess fluorescent liquid is removed with a cleaner and the surface is washed and dried. A black-light source (ultraviolet light) is then brought up to the surface. Areas where the fluorescent liquid has penetrated will show up clearly under the ultraviolet light. For convenience, the dye, the cleaner and the developer are available in aerosol spray cans. Some solvents used in the cleaners and developers contain high percentages of chlorine, a known health hazard, to make the liquids non-flammable. Solvents and developers containing chlorine should be used with great care. Because the penetration ability of the dye varies according to the materials being tested and because it is affected by the ambient temperature, it is important to allow sufficient time (from three to 60 minutes) to enable accurate inspection. At room temperatures, the recommended time is between three and ten minutes.
Figure 5.22: Liquid dye penetrant testing Figure 5.23: Liquid dye penetrant aerosols
Ultrasonic testing Ultrasonic testing uses high frequency sound waves to penetrate test pieces. This testing technique can detect internal flaws as well as surface flaws. The principle involved is the same as the echo-ranging principle used by dolphins and bats to navigate, namely sonar. Ultrasonic testing is commonly used to find flaws in materials and to measure the thickness of objects. A high frequency sound wave (ultrasonic wave) is sent into the metal for very short periods (l to 3 microseconds). Did you know? A special ultrasonic gel is placed on the surface of the test piece to ensure that there is no air gap between the transponder and metal surface. This is because sound travels more slowly through air than through metal. An air gap would cause false readings as the ultrasonic waves would take longer to return to the transponder
When the wave is stopped, the unit which was used to send the sound wave then acts as a receiver to listen to the ultrasonic wave as it is reflected through the metal. This is called a transceiver (transmitter-receiver). Frequencies of 2 to 10 MHz are common but for special purposes other frequencies can also be used. Inspection may be manual or automated and is an essential part of modern manufacturing processes. Most metals can be inspected as well as plastics and aerospace composites. Lower frequency ultrasound (50 kHz to 500 kHz) can also be used to inspect less dense materials such as wood, concrete and cement. Each wave is visually represented on an oscilloscope. The oscilloscope is calibrated to pick up only flaws of a size which would be considered harmful. The oscilloscope wave pattern is also calibrated to show the distance between the searching unit and any flaw found, as shown in Figure 5.24. The transponder is moved in a zig-zag motion alongside the weld being tested to broaden its detection range.
Welding defect wave
Figure 5.24: Ultrasonic testing
This system is fast and results are determined at the time of testing. The operator must have some training to ensure consistent interpretation of results. Another advantage of this system is that no additional materials are needed and the test piece is not damaged.
Assessment 3 1. What is the difference between destructive and non-destructive testing? 2. Use illustrations to describe the nick-break test. 3. Use illustrations to describe the X-ray test.
Applying advanced permanent joining applications Learning to MIG weld Safety precautions You should observe the following safety precautions before starting any MIG welding. If in doubt, refer to the welding machine manufacturer’s safety instructions. • Always wear the correct personal protective safety wear. This includes a fireretardant (cotton) overall, chrome leather gloves, apron and spats. Safety boots and an appropriately shaded arc welding helmet are also essential to ensure personal safety. • Make sure that the welding area is adequately ventilated. • Make sure that there are no fire hazards in the workshop and that adequate fire protection is in place. • Ensure that your equipment is absolutely safe. Do not use welding equipment without the direct supervision of your teacher. Getting started Begin by ensuring that the gas shroud and nozzle of the MIG gun are clean (use a wire brush) and spray them with anti-spatter spray. Cut off the excess wire that protrudes through the gas shroud, as shown in Figure 5.25.
Mechanical Technology Use side cutters to cut excess wire off at this point
Caution: Always wear a correctly shaded welding helmet when MIG welding. The light generated by MIG welding is extremely bright. Looking directly at this light for even a split second can burn the corneas of your eyes, causing ‘arc eyes’.
Figure 5.25: Cutting off excess wire
Fire-retardant Preventing the outbreak of fire
Figure 5.26: A self-darkening (photoelectric) welding helmet
Set the voltage and wire-feed speed according to the thicknesses of the wire and the metal being welded. The following table is a guide for feed settings. The table is based on the types of MIG machine you are likely to encounter at your school. Experience will teach you to adjust the voltage setting to suit the wire speed and vice versa. As a general rule, thin, tall weld beads indicate that your power setting is too low while broad, flat weld beads show that your power setting is too high. A grey block in the table indicates that the wire is not suitable for the steel thickness. Common wire thickness is 0,6 mm – 1,2 mm for light engineering work.
Steel thickness (mm)
Did you know? Having ‘arc eyes’ is very painful. It feels as though someone is sticking pins into your eyes. Before you start welding, warn people in the area and use a welding screen to protect passers-by.
Wire speed (metres per minute) 0,6 mm wire
0,8 mm wire
Once you have set up, hold the MIG gun at an angle of approximately 20° to the vertical, 10 mm above the sheet of scrap material on which you will be welding. The wire should touch the weld metal.
Figure 5.27: The correct set-up of the MIG gun
Draw a straight line with boilermaker’s chalk on the material you will be welding. This will help you to weld in a straight line. Once you have depressed the trigger, push the gun forward along the chalk line, in a weaving motion as indicated in Figure 5.28.
Figure 5.28: Weaving motion
During MIG welding, pull the gun towards yourself or push it away from your body (see Figure 5.29). The latter method is recommended as it improves the coverage of the shielding gas.
Figure 5.29: Results of pushing or pulling the MIG gun
Mechanical Technology One of the main advantages of MIG welding over metal-arc welding is that the weld bead does not leave a slag layer which should be cleaned off after welding. MIG welding is widely used commercially for this reason. MIG welding is also more suitable than metal-arc welding because thinner material can be more easily welded without the formation of defects.
Manufacturing various joints using a variety of joining techniques which include gas metal-arc welding
Practical assessment 1 Conduct the following procedure in the presence of your teacher. Tack and braze together two pieces of off-cut square tubing in a fillet joint as shown in Figure 5.30.
Figure 5.30: A fillet joint
The following score sheet can be used for self-assessment, peer assessment and assessment by your teacher.
Start run: /5
End run: /5
Your name: Peer 1: Peer 2: Teacher: /100
Practical assessment 2 Conduct the following procedure in the presence of your teacher. Cut two pieces of off-cut 1,6 mm plate (50 × 100 mm) and tack together to form a square bevel joint as shown in Figure 5.31. Then MIG weld and finish according to the welding symbol below. Practise a few times before you attempt the assessment.
Figure 5.31: A square bevel joint
End run: /3
Start run: /3
Interpretation of tail info /1
Interpretation of welding symbol /1
Arrow side orientation/1
Interpretation of weld symbol /1
Interpretation of finish symbol /1
The following score sheet can be used for self-assessment, peer assessment and assessment by your teacher.
Your name: Peer 1: Peer 2: Teacher: /80 Convert to: /50
Practical assessment 3 Conduct the following procedure in the presence of your teacher. Tack weld together two pieces of 50 × 6 mm flat bar according to the welding symbol as shown in Figure 5.32. Practise a few times before you attempt the assessment. MIG weld
Figure 5.32: A V-bevel butt weld
End run: /3
Start run: /3
Interpretation of tail info /1
Interpretation of dimension (60) /1
Interpretation of dimension (20) /1
Interpretation of dimension (3) /1
Arrow side orientation/1
Interpretation of weld symbol /1
Interpretation of dimension (6) /1
The following score sheet can be used for self-assessment, peer assessment and assessment by your teacher.
Your name: Peer 1: Peer 2: Teacher: /80 Convert to: /50
Visual inspection of welded joints You can use the following checklist to inspect the completed welds. Checklist
Is there fusion between the weld metal and the parent metal? Is there an indentation, denoting undercutting along the line where the weld joins the parent metal (line of fusion)? Has penetration been obtained right through the joint, indicated by the weld metal appearing through the bottom of the V or U on a single V- or U-joint? Has the joint been built up on its upper side (reinforced), or has the weld a concave side on its face, denoting lack of metal and thus weakness? Is the metal full of pinholes and burnt, indicating an incorrect flame? In arc welding, has spatter occurred, indicating too high current, too high arc voltage or too long arc length? Are the dimensions of the weld correct and have they been tested by gauges such as the gauge shown in Figure 5.33?
Figure 5.33: Using a gauge to check the leg length of a fillet weld
Assessment 4 1. Which aspects should be inspected during arc welding and oxy-acetylene welding? 2. Name seven visual requirements of a good weld.
Forces Topic 6
Calculation of forces in engineering components
Calculating moments in engineering components
Advanced tests on various mechanical principles
Calculation of stress, strain and modulus of elasticity
Concepts of stress, strain and modulus of elasticity
Basic calculations of forces found in engineering components System of forces In Grade 11, you learned how to calculate components (such as resultant and equilibrant) in a triangle of forces. In Grade 12, we will use exactly the same principles to calculate resultant and equilibrant forces in a system of forces with no more than four forces.
Calculating the resultant of a system of forces To calculate the resultant of a system of forces, the forces in the system must first be resolved into their individual horizontal and vertical components. However, remember to use the correct sign to indicate a + or – position on the Cartesian plain. Basic trigonometry can be used to find the respective X and Y components as follows: sin = opposite (X or Y depending on orientation) hypotenuse (force) cos = adjacent (X or Y depending on orientation) hypotenuse (force) The arithmetic sum of the X and Y components is then used to find the magnitude of the resultant using Pythagoras, i.e. R2 = X2 + Y2. The direction of the resultant force is calculated by using the formula: tan = opposite (sum of X or Y components depending on orientation) adjacent (sum of X or Y components depending on orientation)
Example 1 Calculate the magnitude and direction of the resultant force (R) in the system of forces below.
Y = 60 sin 0°
X = 60 cos 0°
Y = 120 sin 45°
X = 120 cos 45°
Y = 80 sin 90°
X = 80 cos 90°
Y = 40 sin 60°
X = 40 cos 60°
Arithmetic sum of components: Y = 29,7 N X = –44,9 N Use Pythagoras to calculate R as follows: R2 = X2 + Y2 R2 = 44,92 + Y2 R2 = 2 016 + 882 R2 = 2 898 R = 53,8 N The resultant, therefore, can be represented as a single force representing the entire system as follows:
Calculate the direction of the resultant as follows: tanθ = opp (X) adj (Y) tanθ = 44,9 29,7 tanθ = 1,51 θ = 56,5° The resultant, therefore, has a magnitude of 53,8 N in the direction of 56,5° west of north or 303,5°
Mechanical Technology Calculating the equilibrant of a system of forces The equilibrant has the same magnitude as the resultant and acts in the same line of action but in the opposite direction. The equilibrant in the example above therefore has a magnitude of 30,1 N in the direction of 350,6° – 180° = 170,6°
Calculate the resultant and equilibrant of the following systems for forces: a.
Calculating moments found in engineering components: uniformly distributed loads ( UDLs) A UDL is a load which is spread over a certain distance; a brick resting on a beam, for instance, is a good example of a UDL because the mass is spread over its entire length, not only at a certain point, as is the case with point loads. Until now, we have only looked at beams with point loads. The reality, however, is that very few beams have only point loads. It is, therefore, important to know how to calculate reactions and bending moments of UDLs. The first step in solving any problem relating to UDLs is to convert the UDL into a point load. This is done by multiplying the force of the load per metre by the length across which the force acts. UDLs are indicated by a broken arrow and placed on top of a beam. Figure 6.1 demonstrates how the conversion should be done. 2N
Figure 6.1: Representing UDLs 6N 2N
Figure 6.2: Converting UDL to point load
In Figure 6.2, an imaginary point load is created in the centre of the UDL. The load of the UDL is 3 N/m acting across 2 m; therefore, the equivalent point load is 6 N. The imaginary point load is always represented by a dotted line. Label the drawing indicating that the new imaginary point load is in the centre of the UDL.
Take note If there is already another point load in the centre of the UDL, indicate the new imaginary point load above the existing point load.
Mechanical Technology Example 2 A beam is subjected to two point loads and one UDL and is supported at each end by RL and RR. 1N
Figure 6.3: Example 2
Calculate: 1. The magnitudes of reactions RL and RR. 2. The bending moments at points A, B and C. Solution: Start by converting the UDL into a point load as follows: 6N 1N
Figure 6.4: Converting 3 N/m UDL to 6 N point load
Calculating reactions: Take moments about reaction left (RL) RR × 8 m = (1 N × 2 m) + (6 N × 3 m) + (3 N × 6 m) RR = (2 + 18 + 18) 8 RR = 4,75 N Take moments about reaction right (RR) RL × 8 m = (3 N × 2 m) + (6 N × 5 m) + (1 N × 6 m) RL = (6 + 30 + 6) 8 RL = 5,25 N
Test: The sum of up forces must equal the sum of down forces. RL + RR = down forces 5,25 N + 4,75 N = 1 N + 6 N + 3 N 10 N = 10 N 2. Calculating bending moments: Bm A: (5,25 × 2) – (1 × 0) Bm B: (5,25 × 4) – (1 × 2) – (6 × 1) Bm C: (5,25 × 6) – (1 × 4) – (6 × 3) – (3 × 0)
= 10,5 Nm = 13 Nm = 9,5 Nm
Assessment 2 A beam is subjected to two point loads and one UDL and is supported at each end by RL and RR. 2N
Figure 6.5: A beam with 2 point loads and one UDL
Calculate: 1. The magnitudes of RL and RR. 2. The bending moments at points A, B and C.
Concepts of stress, strain and modulus of elasticity Before we can begin to discuss the concepts of stress and strain, it is important to know some basic definitions. You might not understand them initially but they will become clearer as we proceed. Memorising the basic definitions will help you to understand the work and will assist you when you attempt the calculations later in the chapter. • Load: is an external force acting upon matter. • Stress: may be defined as the internal resistance in a body to an external force or load. It is directly proportional to the applied load and inversely proportional to the cross-sectional area of the body. • Tensile stress: is an internal force present in material when an external tensile load is applied. • Compressive stress: is an internal force present in a material resisting an external load.
Take note It is interesting that the stress in example 2 is lower than in example 1, even though the load is 2 kN bigger. What is the reason for this?
• Shearing stress: an internal force in material which resists a shearing load or force between two shearing planes. • Mass: is an indication of the reluctance of a body to move; it indicates the quantity of matter which that body contains. • Original length: is the exact length of an object before an external load is applied. • Change in length: is the length by which an object is shortened or lengthened when an external load is applied. • Strain: is the ratio between the change in length and the original length and is expressed as a constant. • Hooke’s law: strain is directly proportional to the stress it causes, provided the limit of proportionality is not exceeded. • Young’s modulus of elasticity: theoretically, this may be defined as the ratio between the stress and the strain in a metal, providing that the limit of elasticity is not exceeded; it is indicated by the letter E.
Calculating stress and strain (Hooke’s law) Hooke’s law defines the relationship between stress and strain as follows: Strain is directly proportional to the stress it causes, provided the limit of proportionality is not exceeded.
Calculating stress As you learned in Grade 10, stress is calculated by dividing the load (N) by the cross-sectional area over which it acts (m2). Stress is measured in pascals (Pa). Example 3 Calculate the stress in a Ø 20 mm round bar if it is subjected to a load of 35 kN. Step 1: Write down the formula. Stress = Load Cross-sectional area You will not need to manipulate the formula as stress is already the subject of the formula. Step 2: Write down all the information you are given about the problem. Stress = ? Load = 35 kN Cross-sectional area = π × 202 4 Step 3: Convert all the variables to their correct units. Stress = ? Load = 35 × 103 N (Correct units are newtons.)
Forces Cross-sectional area = π202 4 × 106
(area converted to m2)
Using a diameter of 20 mm gives an answer in mm2. To convert to the correct units, the diameter must be divided by 106 because there are 106 square millimetres in one m2 or 106 mm2 in 1 m2. Step 4: Substitute the variables in the formula and solve the equation. Stress = Load Cross-sectional area Stress = 35 × 103 π202 4 × 106
Stress = 35 × 103 × 4 × 106 π202
Stress = 111,41 × 106 Pa Step 5: Convert the solution to the appropriate units according to engineering notation. Stress = 111,41 M Pa
Assessment 3 1. Calculate the compressive stress in a Ø 15 mm round bar if it is subjected to a compressive load of 30 kN. 2. Calculate the load which causes a tensile stress of 50 MPa in a round bar with a diameter of 10 mm.
Calculating strain Strain is the ratio between the change in length and the original length and is expressed as a constant. Strain has no units; it is merely a ratio of how much an engineering component has elongated or compressed under a certain load. This is important to know as it gives you information about how metals react to the loads placed on them. The value of strain is higher in soft, malleable materials than in harder and tougher materials when subjected to the same loads because they deform more easily. Strain on its own, however, has little value without the knowledge of how much stress is present in the material. In the next section, we will look at how stress and strain are proportional to each other according to Young’s modulus of elasticity.
Mechanical Technology Strain is calculated by dividing the change in length (∆l) by the original length(l) of an object which has undergone deformation according to the following formula:
Take note Strain has no units because the mm units cancel each other out in the equation.
Strain = change in length (∆l) Original length (l) Strain is a constant and has no units. It is important when calculating strain however, to make sure that both x and l are converted to the same units. For example, if x is measured in mm then l must also be measured in mm. The solution to a strain problem may be expressed as a decimal fraction or as an exponent. Example 4 Calculate the strain in a 0,5 m long cable which elongated by 0,1 mm under a certain load. As with stress calculations, a few simple steps should be followed. Step 1: Write down the formula. Strain = Change in length (∆l) Original length (l) Step 2: Write down what you know. Strain = ? (∆l) = 0,1 mm l = 0,5 m Step 3: Convert (∆l) and l into the same units. Strain = ? ∆l = 0,1 mm l = 500 mm (There are 1 000 mm in 1 m, so 0,5 m × 1 000 = 500 mm) Step 4: Substitute the values and solve the equation. Strain = Change in length (∆l) Original length (l) Strain = 0,1 mm 500 mm Strain = 0,0002 or 2 × 10-4
Assessment 4 1. Calculate the strain in a steel rod which has elongated by 0,5 mm under a tensile load if its original length was 2 m. 2. A brass bush is pressed into a machine part. The compressive force has shortened the bush by 0,01 mm. Calculate the strain in the bush if its original length was 50 mm.
Calculating compressive/tensile stress Compressive stress occurs over the full length of a bar which is subjected to a compressive (pushing) force. Figure 6.6 shows the stress at section xx in a bar which has been subjected to a compressive load.
Figure 6.6: Compressive stress in a bar
Tensile stress occurs over the full length of a bar which is subjected to a tensile (pulling) force. Figure 6.7 shows the stress at section xx in a bar which has been subjected to a tensile load.
Figure 6.7: Tensile stress in a bar
Both compressive and tensile stresses are calculated using exactly the same method as in Assessment 3. The nature of the stress (compressive or tensile) has no influence on the magnitude of the stress, only on the direction.
Calculating Young’s modulus of elasticity Young’s modulus of elasticity is theoretically defined as the ratio between the stress and the strain in a metal, providing that the limit of elasticity is not exceeded. It is indicated by the letter E. Basically, what this means is stress in metal is proportional to strain up to a certain point (the limit of elasticity). After this point, the elongation in the metal starts to increase rapidly. This is because there is an internal shearing process happening, known as slip. The metal molecules move past one another, resulting in elongation of the metal. This results in permanent deformation called plastic deformation.
Mechanical Technology Young’s modulus is calculated by dividing stress by strain for any given component. The units are also measured in Pa, as with stress. The method of calculation is, as shown earlier, (the same) as for stress and strain, with the final substitution of their values into the following formula. Young’s modulus (E) = Stress Strain
Figure 6.8: A tensile tester
Example 5 Calculating Young’s modulus A Ø 30 mm round bar lengthens by 0,2 mm in a tensile test under a load of 35 kN. Calculate Young’s modulus for the bar if its original length was 60 mm. Step 1: Write down the formula. Stress = Load Cross-sectional area Step 2: Write down all the information you are given about the problem. Stress = ? Load = 35 kN Cross-sectional area = π302 4 Step 3: Convert all the variables to their correct units. Stress = ? Load = 35 × 103 N Cross-sectional area = π302 4 × 106
Step 4: Substitute the variables into the formula and solve the equation. Stress = Load Cross-sectional area Stress =
35 × 103 π302 4 × 106
Stress = 35 × 103 × 4 ×106 π302 Stress = 140 × 109 π × 900 Stress = 49 514 871,18 Pa Step 5: Convert the solution to the appropriate units in accordance with engineering notation. Stress = 49,51 MPa Calculate strain: Step 1: Write down the formula. Strain = Change in length(∆l) Original length (l) Step 2: Write down what you know. Strain = ? x = 0,2 mm l = 60 mm Step 3: Substitute the values and solve the equation. Strain = Change in length (∆l) Original length (l) Strain = 0,2 mm 60 mm Strain = 0,0033 Calculate Young’s modulus: Step 1: Write down the formula for Young’s modulus. Young’s modulus (E) = Stress Strain
Mechanical Technology Step 2: Substitute the values for stress and strain and solve. E = Stress Strain E = 49,51 × 106 0,0033 E = 14 854 461 355,24 Pa E = 14,85 × 109 E = 14,85 GPa
Assessment 5 1. A Ø 32 mm round bar lengthens by 0,5 mm in a tensile test under a load of 100 kN. Calculate Young’s modulus for the bar if its original length was 120 mm. 2. A Ø 10 mm steel cable stretches by 0,6 mm under a load of 56 kN. Calculate Young’s modulus for the cable if its original length was 20 m.
Calculating change in length (∆l) Change in length is simply calculated by manipulating the strain calculation formula as in the following example: Example 6 A steel stud is pressed into a machine part by a compressive force resulting in strain of 0,0045. Calculate by how much it was shortened if its original length was 50 mm. Step 1: Write down the formula. Strain = Change in length (∆l) Original length (l) Step 2: Write down what you know. Strain = 0,0045 ∆l = ? L = 50 mm Step 3: Substitute the values and solve the equation. 0,0045 = Change in length (∆l) 50 Change in length (∆l) = 0,0045 × 50 mm (∆l) = 0,23 mm
The following diagram is a graph of stress (y-axis) against strain (x-axis), known as the stress/strain diagram.
KEY A Limit of proportionality Strain
B Elastic limit C Yield point D Maximum stress E Break stress Stress
Figure 6.9: The stress/strain diagram
Performing advanced tests on various mechanical principles Testing bending moments The beam tester is used to determine the deflection (bent shape) of various given pieces of material at any given bending moment. In the simplest case, the equipment can be used as an illustration of Young’s modulus of elasticity for a material. For example, an aluminium beam will deflect roughly three times more than a steel beam of the same section, under the same load conditions, since the modulus for aluminium is a third of that of steel.
Figure 6.10: A beam tester
• Assemble the equipment as directed in the kit, selecting an appropriate beam. • Using a ruler and a dry-wipe marker, draw a line across the beam 200 mm from the root. • Add a 10 g weight hanger onto the dial indicator and slide the dial gauge down onto the beam until it reads 10 mm. Remove the weight hanger and zero the outer scale using the bezel. With plastic beams this may take several attempts.
Mechanical Technology • Add a 100 g mass to the dial indicator and record the dial indicator reading. • Repeat the previous step until you reach 500 g, increasing the mass in 100 g increments. • Plot a graph of deflection (x-axis) versus load (y-axis) to prove the relevant theories.
Testing shear forces Shear force testing is used to establish the strength of a material or bonding material which is subjected to shear forces (sliding forces). A test device, as shown in the image, is commonly used to determine the shear strength. Two skids are pressed together. The top tool provides the applied force on the test medium that is sufficient to shear the specimen. The bottom tool holds the specimen and is mounted on a movable skid to ensure that only shear forces affect the specimen. The test rig measures the forces which caused the specimen to shear as well as the shear stresses during testing and the ultimate (or breaking stress) when the specimen fails.
Figure 6.11: A shear tester
Testing stress/strain and elasticity Knowing Young’s modulus of elasticity for any given component is very important because it indicates whether the material can safely bear up under the applied load. Values for Young’s modulus of elasticity for various metals have been determined experimentally by using tensile testers which can measure both stress and strain in standard samples. Comparing the calculated values of a particular component to these experimental values will give a good indication of the strength of the particular component.
Figure 6.12: A tensile tester
Charting values for stress vs. strain for most engineering materials should yield a very similar graph to the stress/strain diagram in Figure 6.9.
Maintenance Topic 7
The effect of lack of maintenance on operating systems Replacement of belt and chain drives and clutches
Identification of the most suitable preventative maintenance in operating systems
Belt-, chaindrives and clutches
Grading of oil according to viscosity (SAE Standards)
Introduction In Grades 10 and 11, we learned about the basic concepts of friction, wear and lubrication. We also looked at the basic principles of maintenance. In Grade 12, we will explore properties of lubricants and learn about the maintenance of drives and clutches.
The effect of lack of maintenance on operating systems The primary effect of lack of maintenance on operating systems is ultimately the failure of particular parts or the system as a whole. As you learned in Grade 10, maintenance may be: • preventive • predictive • reliability-centred. Failure to conduct routine maintenance (regardless of which maintenance methodology is used) can have very serious or even catastrophic consequences. The most important effects are: • risk of injury or death (e.g. failed brakes) • financial loss due to damage suffered as a result of part failure • loss of valuable production time.
Identification of the most suitable preventive maintenance in operating systems Preventive maintenance is defined as: • Care and servicing by personnel for the purpose of maintaining equipment and facilities in a satisfactory operating condition by providing for systematic inspection, detection and correction of emerging failures, either before they occur or before they develop into major defects. • Maintenance, including tests, measurements, adjustments and parts replacement, performed specifically to prevent faults from occurring. Preventive maintenance can be described as maintenance of equipment or systems before a fault occurs. It can be divided into two subgroups: planned/scheduled maintenance and condition-based maintenance. The main difference between the subgroups is the determination of maintenance time, or the determination of the moment when maintenance should be performed. While preventive maintenance is generally considered to be worthwhile, there are risks involved, such as equipment failure or human error, when preventive maintenance is performed, just as in any maintenance operation. Preventive maintenance, such as scheduled overhaul or scheduled replacement, provides two of the three proactive failure management policies available to the maintenance engineer. Common methods of determining what preventive (or other) failure management policies should be applied are: OEM (Original Equipment Manufacturers) recommendations, requirements of codes and legislation within a jurisdiction, what an ‘expert’ thinks ought to be done or the maintenance which has already been done to similar equipment and, most importantly, measured values and performance indications.
In simple terms: Preventive maintenance is conducted to keep equipment working and/or extend the life of the equipment.
Corrective maintenance, sometimes called ‘repair’, is conducted to get equipment working again. The primary goal of maintenance is to avoid or mitigate the consequences of failure of equipment. This may be done by preventing the failure before it actually occurs, which planned maintenance and condition-based maintenance help to achieve. This is designed to preserve and restore equipment reliability by replacing worn components before they actually fail. Preventive maintenance activities include partial or complete overhauls at specified periods, oil changes, lubrication, etc. In addition, workers can record equipment deterioration so that they know when to replace or repair worn parts before these parts cause system failure. The ideal preventive maintenance programme would prevent all equipment failure before it occurs.
Did you know? Automotive gears are gears found in manual transmissions and industrial gears are gears found in the gearboxes of industrial machinery.
Assessment 1 1. Define preventive maintenance and state its primary goal. 2. Give an example of preventive maintenance.
Properties of lubricants Viscosity The viscosity of a liquid can be thought of as its ‘thickness’ or a measure of its resistance to flow. The viscosity should be high enough to maintain a lubricating film but low enough for the oil to flow around the engine parts under all conditions.
Did you know? SAE is the abbreviation for the Society of Automotive Engineers.
Viscosity index The viscosity index is a measure of how much the oil’s viscosity changes as the temperature changes. The viscosity of an oil should be adequate to maintain a hydrodynamic lubricating film under all working conditions. Industrial gear lubricants are usually considered for their intended applications in terms of their viscosities at 40 °C (rather than 100 °C), as they generally operate at lower temperatures than automotive gear lubricants. As automotive gear lubricants are more likely to be exposed to outside temperatures than industrial gear lubricants, low temperature viscosity is important in determining the particular SAE ‘W’ viscosity grade of a lubricant. Viscosity can also influence gear noise and the ease of changing gears. The oil should be adhesive enough to cling to the gear teeth and resist removal by transfer, wiping or centrifugal force. The oil film should also be resistant to rupture under heavy loads. The viscosity index is a measure of how much the oil’s viscosity changes as the temperature changes. A higher viscosity index indicates that the viscosity changes less with temperature than a lower viscosity index. The viscosity index (VI) is a petroleum industry term. It is a lubricating oil quality indicator, an arbitrary measure for the change of kinematic viscosity with temperature. The viscosity of liquids decreases as temperature increases. The viscosity of a lubricant is closely related to its ability to reduce friction. Generally, you want the thinnest liquid/oil which still forces the two moving surfaces apart. If the lubricant is too thick, it will require a lot of energy to move the surfaces (such as with honey). If, however, the lubricant is too thin, the surfaces will rub together and friction will increase.
Kinematics A branch of mechanics which describes the motion of objects without the consideration of the masses or forces that bring about the motion
Mechanical Technology The VI indicates how a lubricant’s viscosity changes with variations in temperature. Automotive lubricants must reduce friction between engine components when the engine is started from cold as well as when it is running. The best oils (with the highest VI) do not vary much in viscosity over such a temperature range and, therefore, will perform well throughout. The VI scale was set up by the SAE. The temperatures chosen arbitrarily for reference are 40 °C and 100 °C. The original scale stretched only between VI = 0 (worst oil) and VI = 100 (best oil) but better oils have recently been produced, leading to VIs which are greater than 100. VI-improvement additives and higher quality base oils are widely used nowadays. The VIs of synthetic fluids range from 80 to more than 400. An oil’s VI should be high enough to keep the viscosity within permitted limits at any temperature. The VI is also an indicator of the quality of the base oils used in a gear-oil formulation. For multigrade, automotive gear oils such as the SAE 75W90, it is important to use a highly shear-stable, VI-improvement additive in the formulation to ensure that oil viscosity is maintained under conditions of very high shear, particularly as required by Japanese transmission manufacturers.
Pour point Pour point is the lowest temperature at which a liquid remains ‘pourable’ (meaning it still behaves like a fluid). The pour point is, therefore, an index of the lowest temperature of its utility for a given application. If the pour point of the oil is higher than the ambient temperature in which the gears are operating, then they will run the risk of ‘dry’ operation during the time immediately after start-up. The gears will cut channels in the lubricant (called channelling) and run virtually unlubricated until frictional heat has caused the oil to flow more readily. Obviously, severe damage can be done to the gear teeth during this period.
Flash-point Flash-point is an important property of motor oil and is defined as the lowest temperature at which the oil gives off vapours which can ignite. It is dangerous for the oil in a motor engine to ignite and burn, so a high flash-point is desirable. At a petroleum refinery, fractional distillation separates a motor-oil fraction from other crude-oil fractions, removing the more volatile components and, therefore, increasing the oil’s flash-point (reducing its tendency to burn).
Assessment 2 1. What is the difference between viscosity and the viscosity index? 2. Define the pour point of a lubricant. 3. Why is it important for engine oil to have a high flash-point?
Grading of oil according to viscosity (SAE standards) Transmission oil There two main types of transmissions in motor vehicles, namely manual and automatic transmission. Each uses different types of oils for various reasons. To understand transmission oils better, you need to know more about transmissions or gearboxes.
Manual transmission oils The first application in which these oils are used is in manual transmissions (often called gearboxes). A gearbox is an arrangement of gears, bearings and shafts in an enclosed housing which allows the driver to adjust the speed and torque of the vehicle.
Torque A measure of the turning effort of the gears which is necessary for climbing hills or starting from rest
Bigger gears rotate more slowly but carry greater turning effort. By manually controlling the various combinations of larger and smaller gears with the gear lever, a driver can progressively shift from low gear (low speed, high torque) to top gear (high speed, low torque).
Selector for gear changes
Splined input shaft
Figure 7.1: A manual transmission
The types of gears usually found in manual transmission are spur or helical, which means that the lubrication conditions are not too severe. Some manual transmissions are fitted with yellow metal bushings which are vulnerable to corrosive attack by the sulphur components of high EP gear-oil formulations.
EP gear-oil Extreme pressure gear oil
Automotive gear-lubricant classifications Automotive gear-oil classifications provide a convenient way of comparing the performance of various gear oils. The American Petroleum Institute (API) is regarded as the world’s authority on oil classification.
Mechanical Technology The API uses the following designations to identify the load-carrying capabilities of automotive manual transmission and axle lubricants: • Firstly, there is API GL-6, which is not included here as it is obsolete (no longer used) and not referred to anymore. • API GL-5 covers high EP lubricants, particularly for hypoid gears in axles operating under high speed, high load conditions. • API GL-4 covers milder EP lubricants for spiral bevel (and some hypoid) gears in axles operating under moderate speeds and loads. These oils may be used in selected manual transmission and trans-axle applications. • API GL-3 covers service for manual transmissions and spiral bevel axles operating under mild to moderate speeds and loads. • API GL-2 covers service for automotive-type worm-gear axles. • API GL-1 covers service for manual transmissions operating under such mild conditions that straight mineral oil will suffice. • API MT-1 is the only current member of a new classification series specifically for manual transmissions, hence the letters ‘MT’. This classification designates lubricants intended for non-synchronised manual transmissions used in buses and heavy-duty trucks. API MT-1 provides protection against thermal degradation, component wear and oil-seal deterioration which is not provided by lubricants meeting only the requirements of API GL-4 or GL-5. The API GL specifications are described in the product guides of all oil manufacturers. The following table is a generalisation of the viscosity properties which are required in gearbox oil at a range of different temperatures (outside temperatures). However, the manufacturer’s specifications should always be observed when topping up or replacing gearbox oil. Gearbox oil SAE viscosity
Temperature -29 °C
Figure 7.2: Viscosity of gearbox oil for different temperature ranges
Automotive synchromesh transmissions Synchromesh transmissions are found in most modern manual gearboxes and are used to ease the gear-changing process. Once a vehicle is in motion, it is obvious that for one gear to mesh with another quietly and without damage, both gears must be rotating at nearly the same speed when they come together. Synchromesh transmissions use the friction between the mating conical surfaces of the collar and the free-running gear wheel to slow down or speed up the wheel. The cones on the gear wheel and collar, which are made of yellow-metal alloy or sintered-steel alloys, are the most critical elements of the synchroniser design, from a lubrication
perspective. The high temperatures generated by the sliding contact can cause glazing of the surfaces with high EP additive formulations, resulting in difficulty with gear changes. For this reason, mild EP products are recommended for synchromesh transmissions.
Automatic transmission oils ATFs (automatic transmission fluids) are probably the most complex and sophisticated of all lubricating oils. Typically, they can contain over fifteen additives. These additives must be delicately balanced in a carefully chosen base oil. They must be capable of functioning in a wide variety of transmissions and vehicle combinations, as well as being compatible with the range of materials present in the transmission, including aluminium, steel, copper, bronze, brass, tin, silver, plastics, paper and rubber. ATFs must also be multifunctional and be able to perform functions such as: • transmitting power – in the torque converter • acting as hydraulic fluid – transmitting energy (hydrostatic) in order to move various components such as the servo cylinder (which can then lock or unlock various gears into place) • acting as a heat-transfer medium – transfer heat from within the transmission to the outside and so assist in cooling it down – otherwise temperatures in some areas would quickly reach 600 °C+ • acting as a lubricant – for gears and bearings. Torque converter Planetary gears Clutch assemblies
Figure 7.3: A cutaway model of an automatic transmission and torque converter
The viscosity of the oil needs to be sufficiently high over a wide range of temperatures in order to provide the desired hydrodynamic film. Yet it should not be high enough to interfere with the flow of oil, cause viscous losses and overheating or affect shift times. It should also be able to flow readily at low temperatures in order to reach components quickly at start-up, allow easy cold starts and avoid oil pump cavitation.
Cavitation The general term used to describe the behaviour of spherical voids in fluid
Mechanical Technology Engine oil Engine oils can be classified according to the type of engine in which they will be used. There are several different types, such as four-stroke petrol engines, two-stroke petrol engines and CI or diesel engines. Each type of engine has very specific requirements for its lubricating oil. In this chapter, we will look only at the lubrication of the four-stroke petrol engine. Passenger car motor oil (PCMO) is probably the best known type of oil, yet PCMOs are little understood by their everyday users. A PCMO must perform many functions, most of them aligned with the basic fundamentals of lubrication theory which you learnt about in Grade 10. A PCMO should do the following: • Separate moving components by forming a film of lubricant between them. This reduces friction and wear. • Clean deposits and sludges from engine surfaces and prevent the formation of varnish or lacquering on the piston. • Suspend contaminants such as soot, dirt, worn metal particles and acidic byproducts of combustion until the oil is drained and these contaminants can be removed with the used engine oil. In the case of suspending contaminants, it holds them as microscopic particles, preventing them from blocking filters. • Minimise wear in areas of the engine where loads are high or speeds are low, where the hydrodynamic lubrication film is broken and boundary lubrication, or some metal-to-metal contact, can occur. • Resist oxidation, that is temperature-induced thickening. An engine oil is exposed to extreme temperatures. Bulk crankcase temperature of an engine oil will be somewhere around 100 °C. The temperature of an engine oil flowing through the main and big-end bearings will be around 150 °C and the oil which forms the lubricating layer separating the cylinder and the piston rings will experience temperatures around 300 °C or higher before being scraped back down into the crankcase. • Cool parts of the engine, especially bearings and the underside of the piston. It is therefore exposed to additional thermal stresses which place further demands on its ability to resist oxidation. • Flow at low temperatures. In other words, it must circulate immediately on startup to form a lubricating film throughout the engine, whatever the temperature. • Thin out as little as possible with increasing temperature so that it forms a lubricating film under extreme temperatures. The engine oil’s viscosity determines how thick the oil film separating engine components will be. There are a number of different viscosity conditions which oil will experience within an engine. The SAE has drawn up a standard classifying the viscosity grades for engine oils. The SAE J300 specification establishes the viscosities which an oil should have under four different temperature conditions if it is to be described as meeting the SAE viscosity grades. There are two ranges of viscosity grades laid down by the SAE J300. The first is the ‘W’ grades, widely known as ‘winter grades’. This common description came about because, in areas like the central US, a single-viscosity grade oil would previously have been used in the engine during winter and changed to a summer grade during the hotter months. The W grades are the SAE 0W, 5W, l0W, 15W 20W and 25W grades.
SAE J300 Viscosity Grades for Engine Oils set two low-temperature viscosity conditions. These must be met by any oil carrying one of the W viscosity grades: • Low Temperature Cranking Viscosity, which ensures that the oil will allow the engine to be cranked at the minimum speed needed to start it. • Low Temperature Pumping Viscosity, which ensures that the oil will be able to be pumped (or flow) to all sections of the engine at the low temperatures sometimes found at start-up. The second range of SAE J300 viscosity grades is the high temperature grades, also sometimes called, somewhat confusingly, the ‘Non-W’ grades. These are the SAE 20, 30, 40, 50 and 60 grades. SAE J300 Viscosity Grades for Engine Oils set two high-temperature viscosity conditions for the high-temperature viscosity grades: 1. a viscosity at 100 °C 2. a viscosity at 150 °C under high rates of shear as experienced in an engine bearing. The following table is a generalisation of the viscosity properties which are required in engine oil at a range of different temperatures (outside temperature). However, the manufacturer’s specifications should always be observed when topping up or replacing gearbox oil. Engine oil SAE viscosity
Temperature -29 °C
Figure 7.4: Viscosity of engine oil for different temperature ranges
Differential oils Power from the transmission is transferred along a shaft (called the drive shaft) to the rear axle (in rear-wheel-drive vehicles). In order to transmit rotational motion to the rear wheels, the drive needs to make a 90° change in direction. This is achieved by means of a series of bevel gears called a differential. The differential allows for a change of direction in the transmitted power and also enables the driving wheels to turn at different speeds when cornering. (Remember that the outside wheel has further to roll than the inside wheel.) Speed reduction depends on the numbers of teeth used in the crown wheel and pinion. In passenger cars and most trucks, the differential is the final stage of gear reduction.
Figure 7.5: A cutaway view of a differential
Spiral bevel gears (with curved teeth) are used in differentials. When the axis of the pinion is set below the axis of the crown wheel, giving a lower propeller shaft, this is known as a hypoid differential. Hypoid gears are not only difficult to lubricate because their motion is more one of sliding than rolling and the sliding motion tends to rub away the lubricant, but the service required of a differential is typically severe, for example, high speeds, stop/starts and shock loads due to potholes. Therefore, gear lubricants containing sulphur-phosphorus EP additives are required to provide the load-carrying capacity necessary to lubricate hypoid differentials.
Axis of pinion set below the centre of the crown wheel
Figure 7.6: A hypoid gear
Differential oils are usually classified using the rating GL1 – GL6. However, as with gearbox oil, the manufacturer’s specifications should always be observed when topping up or replacing the differential oil.
Cutting fluid Cutting fluid is the fluid used in conjunction with machining processes such as lathe work or milling. It is usually a compound of water-soluble oil and water. A wide range of cutting fluid products is available but most have a very low viscosity (below SAE 20). The fluid has a milky white appearance and is applied directly to the workpiece by means of a movable spout. The cutting fluid is recycled as a pump circulates it from the machine’s splash tray and sump back to the spout.
Figure 7.7: Applying cutting fluid to a vertical milling cutter
Advantages of cutting fluid The use of cutting fluids has many advantages: • The workpiece and cutting tool are kept cool. • The life of the cutting tool is prolonged. • A better finish is imparted to the workpiece. • Cuttings are washed away, keeping the cutting tool free of debris. • The machine is protected because the cutting process is eased. • The machine operator is protected from very fine metal chips and dust. • Productivity is increased because the cutting process is faster. • The soluble oil prevents corrosion.
The application of cutting fluid Cutting fluids should be applied to the cutting tool in order for them to reach all the areas that needing cooling and lubricating. Care should be taken not to cause the fluid to splash. This can occur, for example, if the stream of cutting fluid runs onto the chuck of a centre lathe. Splashes of soluble oil on the floor of the workshop are slippery and should be cleaned up immediately. Often sawdust is used to absorb spilt cutting fluid. Industrial degreasers can also be used when cleaning spills.
Maintenance of cutting fluid The following guidelines should always be followed for safe and effective work with cutting fluid: • Avoid contamination of the cutting fluid by draining and regularly replacing it. • Always clean the machine’s splash tray of metal cuttings after use. • Regularly wipe cutting-fluid splashes off machine parts (only when the machine is stationary). • Ensure that the sump is topped up from time to time and check that there is sufficient flow of cutting fluid to the cutting tool.
Grease When selecting grease for various components (such as wheel bearings), one must use the grease products which meet the requirements of the original equipment manufacturer (OEM). OEM specifications for grease normally include viscosity at operating or ambient temperature, additive requirements and base oil type. Viscosity is, however, one of the most important aspects to consider; equally important is the velocity of the bearing or component involved. There are very sophisticated calculations, tables and graphs to determine the correct viscosity for use at a range of operating temperatures. Other factors to be considered in the selection of the most appropriate grease are operating temperature, special environmental considerations for different environmental conditions (e.g. salt water in boat trailer wheel hubs), etc. However, all greases have a very high viscosity compared to motor oil.
Assessment 3 1. Classify the viscosity of each of the following types of lubricants and state why you think they have that viscosity. a. Transmission oil b. Engine oil c. Differential oil d. Cutting fluid e. Grease
Belt and chain drives and clutches Belts, chains and clutches are all components which transfer rotary motion. The friction experienced by these components causes wear and tear. Because of this, the components need to be adjusted or replaced periodically.
Maintenance of belt drives Belts tend to stretch with prolonged use. They will need to be tightened periodically and checked for correct alignment.
Figure 7.8: Incorrectly aligned pulleys
Incorrectly aligned pulleys will drastically shorten the life of belts because of the extra wear that is placed on them. When machines are serviced, it is important to check the pulleys for alignment and adjust them to achieve the correct tension. When belts become too worn, they should be replaced.
Replacement of belt drives Once belts have worn out, they must be removed and replaced with new belts. It is extremely important to ensure that the machine is switched off and locked out. Locking out means that the machine’s start switch cannot be activated because it has been physically locked. It is therefore impossible to start the machine without the knowledge of the servicing technician. This should be done to prevent accidents. Once the machine and work area have been secured, the tension on the belt must be released. This may involve loosening an adjusting screw or releasing the belt tensioner. The belt can then be removed and replaced with a new belt of the correct type and length. The belt can then be re-tensioned and aligned to ensure maximum service life.
Maintenance of chain drives Chain drives are much stronger than belt drives and have a much longer service life. Like gear drives, they also provide positive traction when transferring rotary motion. To maintain optimal service life, a chain drive also needs to be tensioned correctly. This is achieved, in some cases, by using a spring-loaded chain tensioner. The length of the chain can also be adjusted by adding or removing chain links. As with belt drives, the sprockets that the chain runs on must also be correctly aligned. Chains should be cleaned periodically and lubricated with suitable oil. On inspection, it may be necessary to replace some of the links or the entire chain because of wear.
Replacement of chain drives As chains usually bear relatively high loads, they tend to stretch. A stretched chain loses strength, generates friction, vibration, excessive noise and may ultimately break or derail. When a chain needs to be replaced, the following should be observed. It is extremely important to ensure that the machine is switched off and locked out.
Mechanical Technology Locate the link in the chain and remove the spring retainer or cotter pins (depending on what type of chain it is) and then remove the pin link plate. This will break the chain and allow you to remove it. Take note that certain chain systems must have their sprockets in alignment, such as crankshafts and camshafts in engines. Care should be taken in these instances to ensure that the sprockets are aligned when the new chain is installed. Failure to do so could result in very expensive repairs later on. Select the correct length and size of replacement chain and make sure that it is sufficiently lubricated before installion. When the new chain has been run over the sprockets, insert the chain link and tension the chain. Never lubricate a chain manually while it is in motion. Always check the alignment of the chain to make sure that it is running true.
Figure 7.9: A chain drive
Maintenance of clutches Clutches are used to couple and decouple rotating shafts. The basic principle on which clutches operate is that two rotating surfaces are brought together until the friction between them causes them to rotate at the same speed. The friction which occurs on engagement and disengagement eventually wears down the fibre plate between the plates. When this happens, this fibre plate, known as the clutch plate, will need to be replaced. In the case of a motor vehicle’s clutch, the flywheel is one of the plates against which the clutch plate presses. As a result of the friction involved, it too wears down, resulting in grooves. These grooves will need to be removed by a precision machining process known as skimming before a new clutch plate can be fitted. The only maintenance associated with the mechanical clutch actuation is to check the distance between the release bearing and the pressure plate periodically. This clearance is commonly called ‘clutch pedal free-play’ and refers to the distance that the pedal moves before the slack is taken from the linkage and the release bearing begins to disengage the clutch from the flywheel. This distance can be measured by standing a steel ruler on the floorboard and measuring the height of the pedal in the released position. Take the slack from the clutch linkage (depress the pedal until resistance is felt) and re-measure. The difference between the two measurements is the amount of clutch linkage free-play (measured at the pedal). Generally, the clearance should be approximately 20 – 25 mm. This clearance can be maintained by adjustment of the clutch linkage. If not, the clutch and pressure plate should be replaced as a set.
On systems that have hydraulically actuated clutches, the fluid level in the master cylinder reservoir should be checked at least every 6 months or whenever a vehicle service is performed. As with many brake master cylinder reservoirs, you normally check the fluid level through the translucent plastic body of the reservoir. Fill to the line in the reservoir. NOTE: Although the level should drop slowly with clutch wear, the need to add large amounts of fluid constantly points to the probability of a leak. If a leak is suspected, the system should be thoroughly checked to prevent a hydraulic system failure.
Figure 7.10: Measuring clutch pedal free-play
Replacement of clutches When clutches need to be replaced, it is advisable to purchase a clutch kit. A clutch kit contains the clutch plate and other components which may also need to be replaced, such as the pressure plate and release bearing. Different vehicle models have very specific methods of replacing the clutch assembly. The basic process, however, is to remove the bell housing first. The clutch assembly can then be dismantled and replaced with the new components. As stated earlier, it is advisable to skim the flywheel before installing the new clutch assembly. This type of maintenance should also be accompanied by checking and maintaining the hydraulic cylinders or mechanical systems which activate the clutch.
Note Do not attempt to do any clutch maintenance or repair work/replacement unless under the direct instruction and supervision of a competent instructor/ teacher.
Crankshaft Diaphragm spring Flywheel
Pressure plate Flywheel and pressure plate disengaged
Flywheel and pressure plate engaged
Figure 7.11: A mechanical clutch assembly
Assessment 4 1. 2. 3. 4.
Explain how you would conduct maintenance work on a belt drive. Describe the process of replacing a chain drive. What is the correct method of checking the amount of clutch pedal free-play? Why is it important to skim the flywheel when the clutch of a motor vehicle is overhauled?
Systems and control Topic 8
Methods of repair
Systems and control
Electrical/ electronic control
Introduction In Grades 10 and 11, we looked extensively at mechanical, hydraulic, pneumatic and electrical/electronic systems and at control methods and components. We also dealt with the development of systems in use today which are based on the ingenuity of our ancestors (indigenous knowledge). It is important to use your current knowledge to evaluate current systems critically, and components and products with the goal of constant innovation and development of products and systems. Did you know? Mechatronics is the field of study describing systems that integrate mechanical devices (mechanical engineering), electrical and electronic circuits (electrical engineering) and elements of information technology e.g. robotic arms in a motor vehicle (manufacturing plant).
Did you know? Gears can only mesh if they have the same module, as this fixes the size and pitch of the teeth. The gear teeth guarantee that no slip will take place between the two gears (unless the teeth are stripped).
We must remember that a machine which includes systems and control is designed to work and can be any man-made device. All machines are envisioned to save effort (the force that is applied to lift or remove a load), that is, to be more efficient than manual (physical) labour. A machine does not constantly save us energy; however, energy may be used in smaller quantities spread over a longer period.
Mechanical drives Mechanical drives convey one rotary motion to another by means of belt drives, gears, pulleys, rope drives or chain drives. The prime purpose of a mechanical system is to transmit power and motion, and the various ways in which this can be achieved will be investigated. Mechanical drives comprise a vast field of activities. They include power generation, land, marine (sea) and air transport, manufacturing and fabrication plants. Gears Gear drives work on the principle that the turning motion of one gear may be transferred to another gear if the gears are mounted so that they mesh (engage). Gear trains (simple and compound systems) form an indispensable part of many power transmission systems. The most common profile used in gears is the involute tooth profile. The foremost reason for embracing the involute tooth form (profile) is that the teeth are very strong, the velocity (speed) ratio between meshing (mating) gears is constant and the teeth can be accurately machined with modern gearcutting machinery. It is important to know and remember that all gear teeth mesh with a rolling and sliding action which is why good lubrication (as discussed in Chapter 7) is vital.
Assessment 1 1. Investigate how an involute curve is generated. (Hint: Consult your Drawings teacher.) Involute curve
Figure 8.1: Involute curve
Systems and control
If a gear system consists of two gears, it is called a simple or singular gear train. A driver and a driven gear are shown in Figure 8.2. If one gear turns clockwise, the other will automatically turn anticlockwise, as shown below. When gears rotate, the same number of teeth of each gear will pass the point of mesh (D) in the same time. That means that for any period of time: No of teeth on driver × revs of driver = No of teeth on driven × revs of driven Simple calculations
Figure 8.2: Simple gear train
NA × TA = NB × TB NA = rotational frequency (revs) of gear A TA = number of teeth on gear A NB = rotational frequency (revs) of gear B TB = number of teeth on gear B The following formulae are also true for a simple gear train: NA × DA = NB × DB NA = rotational frequency (revs) of gear A DA = diameter of gear A NB = rotational frequency (revs) of gear B DB = diameter of gear B Example A gear with 32 teeth meshes with a gear of 59 teeth. Calculate the speed of the larger gear if the smaller gear rotates at 180 r/min. Solution: NA × TA = NB × TB TA × NA Thus NB = ___ TB NB = 32 59
NA = rotational frequency (revs) of gear A TA = number of teeth on gear A NB = rotational frequency (revs) of gear B TB = number of teeth on gear B
NB = 97,627 rev/min = 1,627 rev/sec
Mechanical Technology If the driven is to turn in the same direction as the driver, then an idler (intermediate) gear is used. If more than two gears are used, the formula can be expanded as follows: NA × TA = NB × TB = Nc × Tc
Figure 8.3: Gear train with idler gear
Example A gear drive consists of 3 gears; gear A with 20 teeth, turns at 100 r/sec and meshes with an idler gear B, with 50 teeth. Gear B meshes with gear C, turns at 25 r/sec. Calculate: The rotational frequency (revs) of gear B. The number of teeth on gear C Solution: Given: NA = 20 teeth TA = 100 rev/sec NB = 50 teeth TB = ? Nc = ? Tc = 25 rev/sec a. The rotational frequency (revs) of gear B NA × TA = NB × TB 100 × 20 = NB × 50 NB = 100 × 20 50 NB = 40 revs/min b. The number of teeth on gear C NB × TB = Nc × Tc 40 × 50 = Nc × 25 Nc = 40 × 50 25 Nc = 80 teeth Compound gears When a gear system consists of more than two gears (more than one pair of gears engaged), where the intermediate shaft has two gears fixed to it, it is called a compound gear train.
Systems and control
If the gear with TA teeth rotates at NA rev/min then, as previously discussed, the gear with which it engages, having TB teeth will rotate at: TA × NA NS = ___ TS
NB = NC TB
Figure 8.4: Compound gear train
As can be seen above, the intermediate gears are mounted on the same shaft and therefore have the same rotational frequency (turning speed), that is NC = NB. The speed (rotational frequency) of TD can be determined as above. TC × NC ND = ___ TD TA But remember, gear B and gear C are on same shaft NC = ND = ___ × NA TD TC TA Therefore: ND = ___ × ___ × NA TD TB
Take a close look at the directions of rotations in Figure 8.4 above. The same reasoning can be followed for gear trains with more than one intermediate shaft. If we consider equation (1), where NA and NC are drivers and NB and ND are driven gears, then we can formulate the following general equation as: Rev of final driven gear
product of number of teeth on drivers product of number of teeth on driven gears
× revolutions of first gear
Mechanical Technology The velocity ratio (VR) of a gear train is the ratio of the speed of the first driver to the last driven gear. For the gear train in Figure 8.4, NA VR = ____ ND TA NA TC × ___ × …………………(1) From (1): ND = ___ TD TB TC TA NA ___ × ___ = ___ TB ND TD = VR ………………………………………(2) In other words, product of teeth on driven gears product of teeth on driving gears
VR = =
speed of first driver speed of final driven gear
Food for thought: Revolutions of driven = number of teeth on driver Revolutions of driver number of teeth of driven Example In a compound gear train, the first driver rotates at 600 rev/min. If the velocity ratio is 15, calculate the rotational frequency of the final driven gear. Solution: VR
N1 = ___ Nfinal …………….. Equation (2)
N1 Nfinal = ___ VR Nfinal = 600 15 Nfinal = 40 rev/min Power transfer We cannot discuss power if we do not understand forces and work done. In Grades 10 and 11, forces were discussed extensively. The purpose of this discussion is to refresh your memory and also to serve as a basis for the following discussion on power transfer.
Systems and control A force is unseen but its effects (output) can be seen. A force may cause a body to move, stop and change shape or direction. The SI unit for force is newton (N). A tensile force causes an elongation of a body, whilst a compression force tends to crush or shorten the body it acts on. The sliding-down motion in a body is called a shearing force. Machine components are constantly subjected to these kinds of forces. When a force moves an object over a distance, we see work done. The SI unit of work (energy), is called the joule (symbol J). The joule is defined as the work done when a force of 1 newton is exerted over a distance of 1 metre in the direction of the force. Hence, if a force (F), in newton, is exerted over a distance (s), in metres, in the direction of the force:
Did you know? That the SI unit commemorates the English physicist James P Joule (18181889), famous for his experiments on the relationship between mechanical and thermal energy.
Work done = Force(F) × Distance(s) = newton (N) × metre (m) = joules (J) = (Nm) Example A force of 10 N moves an object over a distance of 5m. Solution: Work done = Force (F) × Distance(s) = 10 (N) × 5 (m) = 50 Nm = 50 joules (J) In the abovementioned example, we saw work of 50 Nm (50 joules (J)) done. No mention was made of time taken though and it could have taken as long as a week/ month or have been as short as a second for the work to be done. If the work took a long time to do, we say that very little power was exerted. The shorter the time taken to do the work, the more power is required. Power = Work done = Force(F) × Distance(s) Time taken Time taken = joule second = J/s = watt (W) Example: A force of 10 N moves an object over a distance of 5 m in 2 seconds. Solution: Work done = = =
Force(F) × Distance(s) 10 (N) × 5 (m) 50 Nm = 50 joules (J)
work done per second
= = = = =
Work done Time taken in seconds 50 Nm in 2 seconds 25 Nm/sec 25 watt (watt, the unit of power, is equal to 1 joule per second)
Food for thought: Power has a specific duration (rate) as it refers to the amount of work done per second. Power is the rate of doing work and is measured in watt (W). Power = work done per second Force(F) × Distance(s) Power = Time taken in seconds Work done Power = Time taken in seconds Power = 2πNT (watt) Power =
2πNT (If rotational speed be 60 N revolution per minute)
Where: T = torque in Nm = Force × radius N = rotational frequency of body in rev/s
Mechanical Technology Example A motor vehicle hauls a trailer at 75 km/h when exerting a steady pull of 800 N. Calculate: a) the work done in 20 minutes b) the power required Solution: a) the work done in 20 minutes Distance travelled = distance × time Distance travelled = 75 (km/h) × 20/60 (h) (60 minutes in the hour) Distance travelled = 25 km Work done = Force × Distance Work done = 800 (N) × 25 km (25 × 1000 m) = (km) Work done = 20 000 000 J = 20 MJ b) the power required Work done in joule Power = Time taken in seconds 20 000 000 joule Power = 20 minutes × 60 (seconds) Power = 16 667 W = 16, 667 kW
Pulleys Did you know? That a pulley is also called a sheave freely mounted in pulley blocks. A single rope is passed over each pulley in turn. One end is fastened to either the top or bottom pulley block, depending upon the number of pulleys. The effort is applied to the free end of the rope and the load is attached to the lower pulley block.
The pulley block arrangements: Of the machines used for lifting loads, the pulley block (also called block and tackle) is perhaps the most common. The pulley block which interests us consists of a number of pulleys. The modest arrangement of the pulley block system is shown below on the right of Figure 8.5. The single pulley serves only to reverse the direction of the pull. The pulling force on the rope is equal to the load (W) and the distance travelled by the effort to that through which the load moves at the same time. The effort (F) has to be greater than the load (W) to allow for friction on the pulley so that mechanical advantage (= W / F) is less than unity. The only advantage of the single pulley system is that a person, when pulling a rope downwards, is able to make use of his own weight when exerting the pull and thus finds it easier than lifting the load directly. In the two-pulley system, it will be seen that the load (W) is supported by two portions of rope so that the tension in each portion is W/2. The tensions in the rope on either side of the top pulley must be equal (assuming frictionless pulley and spindle).
Fixed block Standing part
Figure 8.5: A pulley block system
Systems and control
Belt drives A belt is used when a shaft has to be driven from a parallel shaft that is too far away for the use of gear wheels. The illustration below shows a belt drive in which A is the driver pulley and B the driven pulley. The transfer motion from pulley A to the belt and again from the belt to pulley B is dependent upon friction at each area of contact between pulley and belt. Slack side A
Tight side Driver pulley
Figure 8.6: A belt drive
If DA and DB = diameters of pulleys A and B respectively and NA and NB = speeds in revolutions/second of pulley A and pulley B respectively, then the linear speed of the rim of pulley A = π DA NA and the linear speed of the rim of pulley B = π DB NB If there is no slipping, the linear speed of the rim of each pulley is the same as the speed of the belt, hence π DA NA = π DB NB speed of driver pulley A speed of driven pulley B
= diameter of driven pulley B diameter of driver pulley A
i.e. the speeds of the pulleys are inversely proportional to their diameters. When the driving torque is applied to the driver pulley A, the side of the belt approaching the pulley tightens and that leaving the pulley slackens. Thus, with the driver pulley rotating clockwise, as in the illustration above, the lower side of the belt has a larger tension than the upper side. If F1 and F2 are the tensions on the tight and slack sides respectively on the belt, effective force due to friction = F1 – F2 and power transmitted = net force (newton) × speed of belt (metre/second) = (F1 – F2) × π DA NA (watt) or (π DB NB). One of the main disadvantages of a belt is its liability to slip. This disadvantage can be avoided by the use of a chain drive (the chain’s links engage with the teeth on the driver and driven wheels) or by adding a belt tensioning device or jockey.
Did you know? Chain drive wheels are called sprockets.
Mechanical Technology Example An electric motor rotates at 1 000 rev/min. It drives a pulley with a diameter of 1 200 mm, mounted on a machine at 200 rev/min. Calculate the diameter of the pulley on the motor. Solution: Peripheral speed of motor shaft = Peripheral speed of machine shaft π DA NA = π DB NB π DA × 1000 = π × 1200 × 200 π × 1200 × 200 DA = π × 1000 DA = 240 mm Example A belt-driven pulley has a diameter of 500 mm and its speed is 300 rev/min. The tension on the tight side of the belt is 1800 N and on the slack side is 400 N. Calculate the power transmitted by the belt. Solution Effective friction force = 1800 – 400 = 1400 N Linear speed of belt = π × 0,5 (m) × 300/60 (rev /sec) = 7,855 m/s Power transmitted = 1400 (N) × 7,85 (m/s) = 10997 W, say, 11KW (rounded off) Alternatively Speed = 300/60 = 5 rev/sec and torque = 1400 (N) × 0,25 (m) = 350 Nm Hence, we have power transmitted = 2πNT = 2π × 5 (rev/s) × 350 (Nm) = 10997 W or 11KW (rounded off)
Figure 8.7: A V-belt
Figure 8.8: A flat belt
Systems and control
Hydraulics As you will recall from Grades 10 and 11, the word hydraulics is derived from the Greek word ‘hudro’ and means water. Hydraulics refers to the transmission and control of forces and movement by means of fluid. Fluid is used to transmit energy. Generally, mineral oil is used although a synthetic fluid, water or oil-water emulsion can be used. Hydrostatics means equilibrium conditions in fluids (still fluid) while hydrodynamics means mechanics of moving fluid (flow theory). Pure hydrostatics is the transfer of force in hydraulics. An example of hydrodynamics is the conversion of flow energy in turbines in hydroelectric power plants. Pressure, one of the most important measurements in hydraulics, is defined as force per area. F = Force in N Pressure = Force (F) Area (A) A = Area in mm2 P = Pressure in pascal (Pa) When a force acts on an enclosed fluid, pressure occurs in the fluid. The pressure is related to the amount of force applied to the surface vertically and the area. Pressure = Force Area The pressure acts equally and simultaneously on all sides. This is valid with the omission of gravitational force. The gravitational force is calculated according to the fluid level. Due to the high pressure at which a hydraulic system operates, this fraction may be neglected. F1
Figure 8.9: Hydraulic force transmission in a hydraulic jack
Because the pressure is distributed equally on all sides, the shape of the tank is not important. In order to operate with pressure created by an external force influence, we use a system as shown in the illustration above. If a force F1 is applied on area A1, a pressure is created: F1 Pressure = ___ A1
Mechanical Technology The pressure affects all parts of the system and therefore also surface A2: F2 = P × A2 Thus F2 F1 ___ = ___ A2 A1 or
F2 ___ = F1
A1 ___ A2
The pressure in such a system always depends on the size of the load and the effective surface area. The pressure will always rise until it can overcome the resistance in opposition to the movement of fluid. If it is possible to achieve the pressure necessary to overcome the load F2 by means of a force F1 and surface area A1, then the load F2 can be raised. The relationship between distances S1 and S2 is opposite to that of the surfaces. The greater the force on the piston, the higher the pressure rises. The pressure rises only until, related to the cylinder area, it is in a position to overcome the load. If the load remains constant, the pressure will not increase further. The pressure acts according to the resistance which is opposed to the flow of the fluid. If the necessary pressure has built up, the load can be moved. The speed at which the load is moved depends on the volume of fluid fed to the cylinder. The volume of liquid displaced by the large piston (ram) is equal to the liquid displaced at the small piston (plunger).
d = diameter a = area f = force h = height v = volume p = pressure
d = diameter a = area f = force h = height v = volume p = pressure
Figure 8.10: A basic close-loop hydraulic system
Volume = Area of large piston (ram) × height of displacement (h) = Area of small piston (plunger) × height of displacement (h) πd2h πD2h Volume = ____ = ____ 4 4
Systems and control
Example In a hydraulic press, a force of 100 N acts on the small piston which moves 90 mm. Calculate the displacement of the large piston if the area of the small piston is 0,2 m2 and that of the large piston 1,8 m2. Find the force exerted by the large piston. Solution (i)
πd2H πD2h _____ = ______ 4 4 6 1,8 × 10 × h = 0,2 × 106 × 90 0,2 × 106 × 90 ___________ h = 1,8 × 106 h = 10 mm
F = A1
F ____ = 100 1,8 0,2
100 × 1,8 0,2
Example Calculate the mass in kg that can be raised by a hydraulic press with a plunger diameter of 30 mm. The force on the plunger is 50 N while the ram diameter is 120 mm. Solution 50 N
= F2 D
50 N = F 1202 302 F
50 × 1202 302
Mechanical Technology This load must be raised against gravity 800 N Mass = 10 m/s Mass = 80 kg
Gravitational force = 9,81 m/s, but for your purposes we use 10 m/s.
What will the pressure be in the fluid of the hydraulic press mentioned in the example above? f Fluid pressure (P1) = a 50 (N) = __________ π × 0,032 (m)
4 = 70 735, 53 Pa = 70,735 kPa
Is the pressure on the ram the same? F A 800 (N) = _______ π × 0,122 4 = 70 735, 53 Pa = 70,735 kPa (Yes, the pressure is the same)
Fluid pressure (P2) =
Did you know? That isothermal simply means ‘at one temperature’. The change in volume is covered by Boyle’s law. If P1 and V1 are the original pressure and volume and P2 and V2 are the new pressure and volume, then the law of expansion or contraction is: P1 V1 = P2 V2 = constant
Boyle’s law, enunciated by Sir Robert Boyle, an Irish scientist, in 1661, states that the volume of a given mass of gas at a constant temperature is inversely proportional to the pressure; thus, if the pressure on a given mass of gas is doubled, the volume is halved. Hence, if V is the volume and P is the pressure, then the gas remains at a constant temperature.
Definition of Boyle’s law The volume of a given mass of gas is inversely proportional to the pressure on it, if the temperature remains constant. The volume of a gas can be changed by altering: • its pressure • its temperature • both its pressure and temperature. If the volume is changed by altering the pressure and keeping the temperature constant, the change is known as isothermal. Example The volume of a gas is 6 m3 at a pressure of 300 kPa. Calculate the volume of the gas if the pressure is increased to 1000 kPa while the temperature remains constant. Solution
P1 V1 = P2 V2 6 × 300 = 1000 × V2 P1 V1 V2 = ______ P2 6 × 300 V2 = 1000 V2 = 1,8 m3
Systems and control
Example A gas whose original pressure and volume were 300 kPa (kN/m2) and 0,14 m3 is expanded until the new volume 0,7 m3 is reached while the temperature remains constant. What will the new pressure be? Solution P1 V1 = P2 V2
P1 V1 OR P2 = _____ V2
300 × 0,14 = P2 × 0, 7 P1 V1 P2 = _____ V2 P2 = 300 × 0,14 0, 7
P2 = 60 kPa (kN/m2)
Assessment 2 1. 2. 3.
A gas occupies a volume of 0,2 m3 at a pressure of 2,9 MPa. Calculate the pressure, if the volume is changed to 0,12 m3 at a constant temperature, and the volume if the pressure is changed to 5,1 MPa at a constant temperature. In an isothermal process, a mass of gas has its volume reduced from 4 100 mm3 to 2 500 mm3. If the initial pressure of the gas is150 KPa (KN/m2), calculate the final pressure. Some gas occupies a volume of 2,0 m3 in a cylinder at a pressure of 300 KPa. A piston sliding in the cylinder compresses the gas isothermally until the volume is 0,67 m3. If the area of the piston is 400 cm3, determine the force on the piston when the gas is compressed. (Hint: We learnt earlier that an isothermal process means at a constant temperature and that Boyle’s law applies, i.e. P1V1 = P2V2.)
Electrical/electronic control Basic operating principles of: Vehicle management systems/ECU The heart of an electronic control system is the electronic control unit (ECU). All electronic systems use different types of sensors to supply the ECU with relevant data. The sensor data is read by the ECU which then compares it with preprogrammed information contained in the memory. A response is calculated and the various actuators connected to the ECU are adjusted as directed. The results are checked and the process is repeated many times over every second. An ECU can be used to control the engine fuel system, ignition system and exhaust emission controls and is collectively called a (vehicle) engine management system. Anti-lock brake systems (ABS) are controlled by an ECU and sometimes automatic transmissions too.
Did you know? The word ABS covers a variety of electronically controlled systems which are aimed at offering ideal braking in demanding situations.
Electronic control can be exercised either by a central electronic unit (ECU), or individual electronic control units can be incorporated as sub-systems to each of the controls, such as steering, brakes, etc., where they can be used to transmit
Mechanical Technology information to, and receive it from, the other sub-systems. The latter generally offers the advantages of compactness and because, provided the central computer can be eliminated, the wiring harness can be made simpler and installation easier. Because electric motors are amenable to electronic control, they are now being considered for use as actuators. However, it might be more practicable to substitute electric motors for engine-driven hydraulic pumps, the former being potentially both lighter and more compact.
Figure 8.11: an ECU for vehicle management
Anti-lock brake systems (ABS) The most efficient braking takes place when the wheels are revolving. Once the brakes lock the wheels and the wheels begin to skid, braking is much less effective. Did you know? That in many countries anti-lock brake systems are known as anti-skid mechanisms.
ABS systems are used on many motor vehicles, commercial vehicles and trailers. Anti-skid systems relieve hydraulic pressure on wheels which are about to skid. This action reduces the braking effort that would have caused a skid. The main purpose of anti-skid braking systems is to provide safer vehicle handling in difficult conditions. When the wheels are skidding, it’s not possible to steer the motor vehicle correctly and a tyre that is still rolling, not sliding, on the surface will provide a better braking performance. ABS does not generally operate under normal braking but comes into play in poor road surface conditions such as water, snow or ice and also during emergency stops. Figure 8.12 shows the anti-skid mechanism on a front wheel and Figure 8.13 indicates the rear wheel mechanism. The working operation for both wheels is identical. On the front wheel, there is a magnetic wheel attached to the brake disc. As the wheel and disc rotate, the magnetic wheel produces an alternating current in the sensor.
Splash shield Brake disc with rotor tone wheel assembly
Figure 8.12: Anti-lock brake system on the front wheel
Systems and control
Sensor Drive ring Brake drum
Figure 8.13: Anti-lock brake system on the rear wheel
Apportioning valves (pressure valves)
Hydraulic power source Electronic controller Sensor Modulator
Figure 8.14: Anti-lock braking system (ABS) on a standard motor vehicle
The sensor is a coil of wire or a winding and the rotor carries a magnetic field through the stator windings. This produces an alternating current (AC) in the stator windings. In the same way, the magnetic wheel produces AC current in the sensor. A similar action takes place in the other wheels. These AC signals from the motor vehicle wheels are fed to the ECU. When brakes are applied, the ECU compares the AC signals from the wheels. The AC frequency increases with speed. As long as the AC frequency from the wheels is about the same, normal braking is indicated. However, if the AC frequency from any wheel shows a rapid decrease in frequency, it means that the wheel is slowing down too fast. The wheel is starting to skid. When the ECU senses this rapid drop of AC frequency, it signals the modulators at the front of the motor vehicle. The hydraulic pressure from the master cylinder to the wheel cylinders or calipers passes through these modulators. When the ECU senses that a wheel is about to skid, it ‘tells’ the modulator for that wheel to ‘ease up’. In other words, the ECU signals the modulator to reduce the hydraulic pressure to the brake for that wheel. When the pressure is reduced, the braking effect on that wheel is reduced, so the skid is prevented.
Traction control Traction control systems prevent the wheels from spinning if the torque transmitted to any wheel rises above that which can be transmitted by the tyre. If one or more wheels spin, the consequent loss of co-efficient of friction between the tyre and the road tends to cause the vehicle to become unstable and go out of control. The sensor for detecting the onset of the wheel spin is usually common to both the ABS and traction control systems but, of course, for the latter function, it sends a signal of impending wheel spin, instead of wheel lock, to the electronic control. On receipt of such a signal, the computer orders application of the relevant brake until the tendency to spin is nullified, and thus maintains the vehicle in a stable condition. Traction control systems are used in road vehicles as a safety feature in premium high performance vehicles, which otherwise need sensitive throttle input to prevent spinning driven wheels when accelerating in wet, icy or snowy conditions. Traction control systems are used in racing cars for performance enhancement, allowing maximum traction under acceleration without wheel spin. They are also used in production motor bikes. In off-road vehicles, traction control is used instead of, or in conjunction with, mechanical limited slip or locking differential. Wheel sensors Control module Modulator unit
Wheel sensor Gear pulser
Figure 8.15: Traction control on a standard motor vehicle
Air bag control Air bags are a passive safety feature which protect the driver and passengers in a motor vehicle collision. ‘Passive’ means that the driver and passengers in the vehicle do not need to activate the air bags or do anything to be protected by them.
Systems and control
This is the difference between air bags and other safety features such as seat belts, which need action like buckling up. (In fact, one of the downfalls of safety belts is that people often don’t buckle up). An air bag is a large fabric bag which fills with air and provides protection for the head and upper body of the driver and passengers of a motor vehicle during a collision. In head-on collisions, drivers and passengers are thrown forward inside the vehicles. When an air bag is activated, it inflates instantly and creates a firm barrier which counters the forward motion of the driver or front-seat passenger. Air bags are designed to prevent the occupants from hitting the windscreen or dashboard of the vehicle, thereby eliminating injuries or reducing their severity. An air bag is also known as a supplementary restraint system (SRS), or a supplementary inflatable restraint (SIR). Air bags are designed to work in conjunction with seat belts. However, an air bag on its own can provide some protection for a motor vehicle occupant who is not wearing a seat belt.
Crash sensor activated
Nitrogen gas Filters
Air bag inflation device
Figure 8.16: Air bag control system for front seat passengers
Central locking In addition to conventional locks, keyless-entry systems and keypads, some motor vehicles today have a number of different ways of unlocking the motor vehicle doors. How do motor vehicles keep track of all those different locking and unlocking systems and exactly what happens when the motor vehicle doors unlock? The mechanism which unlocks your motor vehicle doors is truly quite fascinating. It has to be extremely trustworthy because it is going to lock and unlock your motor vehicle doors tens of thousands of times over the working lifespan of your motor vehicle.
Mechanical Technology Methods of unlocking motor vehicle doors: • The conventional way with a key. • By pressing the unlock button inside the motor vehicle. • By using the combination lock on the outside of the motor vehicle door. • By pulling up the knob on the inside of the motor vehicle door. • With a keyless-entry remote control. • By electronic signal from a control centre. We will learn just what’s inside your door that makes it unlock. Door knob
Keyless-entry remote control
Combination lock Unlock button
Figure 8.17: Methods of unlocking motor vehicle doors
The lock/unlock switch sends power to the actuators which unlock the motor vehicle door. But in more complicated systems that have several ways to lock and unlock the doors, the body controller decides when to do the unlocking. The body controller is a mini computer system in your motor vehicle. It controls a number of the smaller functions which make your motor vehicle people-friendly – for example, it ensures the interior lights stay on until you start the motor vehicle and it beeps at you if you have not fastened your safety belt or it beeps at you if you leave your headlights on or you forget the keys in the ignition. The body controller also monitors all of the possible sources of an ‘unlock’ or ‘lock’ signal. It monitors a door-mounted touchpad and unlocks the doors when the correct code is entered. It monitors a radio frequency and unlocks the doors when it receives the correct digital code from the radio transmitter in your key fob and also monitors the switches inside the car. When it receives a signal from any of these sources, it provides power to the actuator which locks or unlocks the doors. In fact, what is happening inside the motor vehicle door? Inside a motor vehicle door The actuator is positioned below the latch. A rod connects the actuator to the latch and another rod connects the latch to the door knob which sticks up out of the top of the door. When the actuator moves the latch up, it connects the outside door handle to the opening mechanism. When the latch is down, the outside door handle is disconnected from the mechanism so that it cannot be opened. To unlock the door, the body controller supplies power to the door-lock actuator for a timed interval.
Systems and control
Door knob Door knob connecting rod Door handle
Figure 8.18: A motor vehicle door inside panel removed Connecting rod for unlocking door knob
Figure 8.19: Inside of a motor vehicle door showing mechanism
Figure 8.20: A central-locking actuator
The actuator can move the metal hook shown in Figure 8.20 to the left or right. When mounted in the motor vehicle, it is vertical, so the hook can move up or down. It imitates your motions when you pull the inside door knob up or push it down.
Assessment 3 1. The pressure in the hydraulic fluid in a 230 mm diameter cylinder is 0,895 MPa. Calculate in newton the force exerted when the plunger moves outwards. 2. In the figure below, the force applied to the plunger causes a force of 900 N on the ram in a hydraulic press. The ram moves 10 mm upwards. The area of the plunger is 0,2 m2 and the area of the ram is 1,8 m2. Ram moves 10 mm upwards
Plunger moves × mm downwards
Ram 900 N 10 mm
Figure 8.21: A hydraulic system
Calculate: 2.1 The diameter of the ram in mm 2.2 The force applied on the plunger 2.3 The distance the plunger moves downward in mm 3. The volume of a gas is 8 m3 at a pressure of 600 kPa. Calculate the volume of the gas if the temperature is increased to 2000 kPa while the temperature remains constant. 4. A flat-belt drive consists of a 200 mm diameter driving pulley and a 100 mm diameter driven pulley. Determine the driven pulley speed if the driving pulley rotates at 750 rev/min. 5. What is the main function of a mechanical system? 6. Complete the following sentences: 6.1 When pressure is applied to a hydraulic fluid, the liquid will … 6.2. Two spur gears will only mesh when … 6.3. The purpose of intermediate gears is to … 7. Which precision measuring tool can be used to measure the depth of a screw thread? 8. State four methods of unlocking motor vehicle doors. 9. Briefly explain the chief purpose of air bags as found in motor vehicles. 10. What is an ECU and what is its function?
Systems and control
Investigation 1 Investigate and write an essay on how the speed variations are obtained in the motor vehicle gearbox below. Make reference to the components of the motor vehicle gearbox as shown below in Figure 8.19 and use your knowledge on gears, gear ratios and rotational frequencies. Clutch release fork 3rd/4th synchroniser
Ball spigot bearing
5th speed synchroniser
Clutch release bearing
1st/2nd 4 pinion synchroniser Final drive 4th speed differential pinion 2nd speed
Figure 8.22: Sketch of a motor vehicle gear box
Investigation 2 The diagram below depicts a hydraulic brake as in a motor vehicle. Apply all your knowledge and understanding of linkages, pressure, rams and plungers in hydraulics and investigate and explain in your own words how this brake system operates. Brake pedal Master cylinder Tube T
Pipeline To other wheels
Lever system Brake oil
Inner rim of the wheel
Figure 8.23: Hydraulic brake in a motor vehicle
Relevant methods of repairing Integrated electrical/mechanical systems Electronic ignition Electronic ignition defects are limited to component failure. Should a component fail in most systems, the system resets itself to operate in ‘fail-safe’ or ‘limphome’ mode. This allows the driver to continue the journey with limited engine performance. A major disadvantage of any electronic ignition system is the need for dedicated equipment to perform fault diagnosis. In the case of programmed engine management systems, special equipment may be needed to reprogram the settings for the motor vehicle. The ABS warning light ABS systems are equipped with a warning light. This warning light will come on when the system is not operating. When the motor vehicle is first started, the ABS warning light is lit up and the system runs through a self-check procedure. As the motor vehicle drives away, the ABS warning lamp will remain ‘on’ until a speed of approximately 7 km/h is reached. If the ABS system is working correctly, the warning light will remain off until the motor vehicle stops. The system continually monitors itself when the motor vehicle is in motion. Should a fault occur, the warning light will illuminate again. If this happens, the system returns to normal braking operation and the motor vehicle should receive urgent attention to ascertain the cause of the problem.
Fault finding Regular maintenance and servicing backed by a sound knowledge of how your motor vehicle works and a measure of common sense help to ensure a long motor vehicle life. A starting problem is one of the most likely problems that can occur at some point in the life of a motor vehicle. It can be caused by a multitude of malfunctions, from the battery (power supply) to a sensor. Cognisance must be taken that many motor vehicles are of the older type and the newer models are of the electronic type, with onboard computers, etc. A car needs four things to run: • fuel • compression • spark • timing. Engine turns but will not start: • Fuel tank empty. • Fuel-line fault or fuel pump. • Faulty spark-plugs. • Ignition system damp or wiring faulty. • Battery problem. • Air filter clogged. • Poor cylinder compression. • Timing belt broken. • Choke incorrectly adjusted.
Systems and control
Ignition (Spark) It’s always a good idea to change the spark-plug and see if it fixes the fault. Remove spark plug and check for sparking. Check the spark-plug colour; this tells you how the engine is running. If it’s running rich (black sooty deposits = carbon fouled), clean the air filter. If it’s running ‘lean’ check the exhaust and silencer; it could also be a fuel starvation (blocked) fault in the fuel line or carburettor. When choke is on, the mixture will be rich. Fuel Fuel (petrol) ages and old petrol can cause trouble. There are also bad batches from time to time; try to dilute it with fresh fuel (petrol) or drain and replace it. Over a long time, a petrol tank can build up rust, sludge and dirt inside. This normally settles in the bottom of the tank. In case of emergency, check if there is sufficient fuel in the fuel tank. Operate the accelerating pump to see if the fuel reaches the carburettor (if fuel is injected), loosen the supply line at carburettor and turn engine to see if pump operates. If the pump does not supply fuel, check fuel line for loose or blocked fittings and pipes or cracked fuel pump. If fuel does reach the carburettor (is injected), check if the needle valve is not stuck, screen not blocked or that the float level is not wrong. Too much fuel will also cause a motor vehicle not to start. If you have a damaged ECU, or if your coolant temperature sensor (which may tell the computer the coolant temperature is too low and adjust fuel accordingly) is bad, for example, it can cause the injectors to feed too much fuel and flood the engine. There are other factors that can cause this as well. Vacuum leaks can cause too MUCH air. The air/fuel mixture has to be just right for your motor vehicle to run. If a vacuum line is open or broken, it may be hard for your motor vehicle to start. Ignition timing When your motor vehicle is stranded, then you will look for the following faults in connection with the timing: • Remove the distributor cap and swing the engine. • If the rotor does not turn, you know that the following problems may exist: – The distributor shaft is broken. – The cam belt is broken. – The timing chain is broken. • The timing belt can also skip a tooth and the engine will not start either.
Power supply (battery) Batteries are normally reliable for a number of years. However, batteries do sometimes fail to provide sufficient voltage to operate the motor vehicle starting system. Such failure can arise for various reasons. Reason If the lights or any other system have been left switched on and the battery has discharged. In this case, the battery is ‘run down’ and a quick check of switch positions (is the switch, e.g. light switch, still in the on position?) will reveal the most likely cause. A hydrometer test, or a voltage check at the terminals, will confirm that the battery has discharged. The remedy is to recharge the battery, probably by removing it and replacing it temporarily, with a service battery.
Mechanical Technology Reason If the battery is no longer capable of ‘holding’ a charge, or the battery is discharged because the alternator charging rate is too low. It would be necessary to check the battery more thoroughly. A high rate discharge tester which tests the battery’s ability to provide a high current for a short period is very effective but it can only be used if the battery is at least 70% charged. The state of charge of the battery can be determined by using a hydrometer. If the specific gravity (relative density) reading shows that the battery is less than 70% charged, the battery should be recharged at the recommended rate. After charging and leaving time for the battery to settle and the gases to disperse, the high rate discharge test can be applied. Many different forms of high rate discharge testers are available and it is important to read the instructions for use carefully. Alternator test Before starting with the alternator test procedure remember: • Do not run engine with a disconnected alternator lead. Note that for the test that follows the engine is switched off and the ammeter is connected before restarting the engine. • Do not disconnect the alternator while the engine is running. • Always disconnect the alternator and battery when using an electric welder on the motor vehicle because failure to do so may cause damage to the alternator electronics by stray current. • Take care not to reverse the battery connections. With the above in mind, we can do the alternator test. As with any test, a thorough visual check should be made first. In the case of the alternator, this will include: • Check the drive belt for tightness and condition. • Check leads and connections for tightness and condition. • Ensure that the battery is properly charged. • Check all fuses in the circuit.
Hydraulics Fluid It is important always to use only the type of fluid which the motor vehicle manufacturer recommends for use in their motor vehicles. Brake fluid has a boiling temperature of not less than 190° and a freezing temperature not higher than –40°. Throughout this range of temperatures, the viscosity or thickness of the fluid must remain constant. It should not attack the rubber seals or corrode the metal parts. Brake fluid is hygroscopic; this means that it absorbs water from the atmosphere. Water in brake fluid affects its boiling and freezing temperatures which is one of the reasons why brake fluid needs to be changed at recommended intervals. Pressure Oil pressure in motor vehicles without a gauge can be tested by attaching one to the oil system. The tests and readings should be in accordance with the motor vehicle manufacturer’s specifications. Relief valves The purpose of the relief valve is to limit the maximum pressure of the oil supplied by the pump to the system.
Systems and control
As engine speed increases, the oil pump produces higher pressure than is required by the engine lubricating system. The pressure relief valve is therefore fitted in the system to remove the excess pressure and maintain it at a level appropriate for the bearings and seals used. Therefore, the relief valve performs two important functions: • It acts as a pressure regulator. • It acts as a safety device for the lubricating system. The main types of pressure relief valves in use are the ball valve, the plate and the plunger or poppet valve. Each is held in the closed position by a spring. As the oil pressure in the oil gallery rises above the setting of the relief valve, the valve opens against the spring pressure allowing the oil to bypass the system and return back to the sump via the return outlet. The force on the spring determines the oil pressure in the lubrication system.
Piston The piston forms an integral part of the piston pump and operates on the principle that a piston reciprocating in a bore will draw in hydraulic fluid as it is retracted and expel it on the forward stroke. A radial pump has the pistons arranged radially in the cylinder block while in axial units the pistons are parallel to each other and to the axis of the cylinder block. Piston pumps are highly efficient units available in a wide range of capacities from very small to large.
Seals All the joint faces, such as sump to crankcase and the moving surfaces of protruding shafts, must be made oil-tight to prevent oil loss between the shafts and their housings. Where the surface does not move, gaskets are used to ensure a water-, oil- and gas-tight situation, for example, between the cylinder head and the cylinder block seal. Gaskets are usually made from waxed gasket paper, cork, plastic or other materials that are resistant to heat, oil and water. Revolving shafts are sealed by springloaded synthetic rubber lip seals or felt sealing rings, often in combination with oil deflectors.
Pipes and pipe connectors The hydraulic pressure created in the master cylinder is conveyed to the wheel brakes through strong metal pipes, where they can be clipped firmly and securely to the motor vehicle, and through flexible hoses where there is relative movement between parts, e.g. axles and steered wheels and the motor vehicle frame.
Pneumatics Vacuum Vacuum, or ‘the lack of air’, makes possible the manufacture of a wide range of everyday products and processes such as electric light bulbs, electronic tubes, packing of food products, production of medicines, vitamins and the manufacture and movement of paper, to name but a few. With an air pump on vacuum duty, we ‘push’ a volume of air from a closed vessel and deliver it to the atmosphere. As each ‘push’ takes place, an equal volume of air is removed but at a reduced density. It is important to note that the volume at each ‘push’ is identical but the density of the air in that volume progressively decreases.
Mechanical Technology Pressure Pressure and volume are the most important factors in a properly functioning pneumatic system. Pressure is defined as force per unit area. It is measured in pascal and the unit is written as Pa. Air which fills motor vehicles tyres is a gas and obeys the laws of gases (remember Boyle’s law). When you inflate a tyre, you are pressing in more air than the tyre would like to hold. The air inside the tyre resists this constant pressure by pushing outward on the casing of the tyre. This outward push of the air is pressure. Air, of course, like all gases, is highly compressible (contrary to liquids, like hydraulic fluid). Compressing more air into the same space means that you can squeeze more air into a smaller volume, or you can squeeze more air into the same space. You need more force to squeeze extra air into a tyre, as the pressure within the tyre increases. The greater the force exerted, the greater the pressure in the tyre.
Valves Check valve The check valve allows compressed air from the compressor into the air receiver (tank). It is a one-way valve and thus stops air leaking back when the compressor is stopped. Pressure-reducing valve This valve is also known as the safety valve. The main purpose of this valve is to allow compressed air to escape into the atmosphere if the pressure in the air receiver should rise above the allowed safe pressure level. The pressure-reducing valve limits the maximum system pressure and thus prevents system failure. Therefore, it is understandable that it is a protective device. Directional control valve The directional control valve pressurises and exhausts the two cylinder connections interchangeably, to control direction and movement.
Pistons The compressor is used to pressurise air in a pneumatic system. The most commonly used form of compressor is the piston compressor. On the air intake stroke, the descending piston causes air to be sucked into the chamber through the inlet valve. When the piston starts to rise again, the trapped air forces the inlet valve to close and so becomes compressed. When the air pressure starts to rise sufficiently, the outlet valve opens and the trapped air flows into the compressed air system. The cycle repeats itself.
Diaphragms The diaphragm membrane provides for separation of the process fluid and the compressed air power source. To perform adequately, diaphragms should be of sufficient thickness and of appropriate material to prevent degradation or permeation in specific process fluid applications.
Vacuum meters Vacuum gauge (kPa) This is the vacuum reading taken directly from a dial gauge and is usually denoted from zero to 100 kPa. The sea-level reading for a perfect vacuum would be 101,3 kPa.
Turbines Topic 9
Introduction In Grade 10, the basic principles and operation of the different engines were covered. In Grade 11, principles and operations of different pumps were explained. In Grade 12, we will explain different types of turbines, like water, steam, gas and superchargers, their components and the function thereof.
Types of turbines, their components and their fuctions Water turbines Principle of operation Flowing water is directed onto the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts over a distance (force acting over a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine. Water turbines are divided into two groups. reaction turbines and impulse turbines. The precise shape of water turbine blades is a function of the supply pressure of water and the type of impeller selected.
Reaction turbines Generator Stator Rotor
Shaft TURBINE Water flow
Figure 9.1: Reaction turbine with a generator
Reaction turbines are acted on by water which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction) or they must be fully submerged in the water flow. Newton’s third law describes the transfer of energy for reaction turbines. Most water turbines in use are reaction turbines and are used in low (300m/984 ft) head applications.
Power The power available in a stream of water is: P = η • ρ= • g • h • q where: • P = power (J/s or watts) • η = turbine efficiency • ρ = density of water (kg/m³) • g = acceleration of gravity (9,81 m/s²) • h = head (m). For still water, this is the difference in height between the inlet and outlet surfaces. Moving water has an additional component added to account for the kinetic energy of the flow. The total head equals the pressure head plus velocity head. • q = flow rate (m³/s)
Pumped storage Some water turbines are designed for pumped storage hydroelectricity. They can reverse flow and operate as a pump to fill a high reservoir during off-peak electrical hours and then revert to a turbine for power generation during peak electrical demand. This type of turbine is usually a Deriaz or Francis in design. Efficiency Large modern water turbines operate at mechanical efficiencies greater than 90% (not to be confused with thermodynamic efficiency).
Types of water turbines
Figure 9.2: Types of water turbines
Figure 9.2 shows various types of water turbine runners. From left to right: Pelton wheel, two types of Francis turbine and a Kaplan turbine. Reaction turbines: • Francis • Kaplan, propeller, bulb, tube, Straflo • Tyson • Gorlov.
Mechanical Technology Impulse turbines: • waterwheel • Pelton • Turgo • Michell-Banki (also known as the Crossflow or Ossberger turbine) • Jonval turbine • reverse overshot waterwheel • Archimedes’ screw turbine.
Design and application
Figure 9.3: Turbine application chart
Turbine selection is based mostly on the available water head and less so on the available flow rate. In general, impulse turbines are used for high head sites and reaction turbines are used for low head sites. Kaplan turbines with adjustable blade pitch are well adapted to wide ranges of flow or head conditions since their peak efficiency can be achieved over a wide range of flow conditions. Small turbines (mostly under 10 MW) may have horizontal shafts and even fairly large bulb-type turbines up to 100 MW or so may be horizontal. Very large Francis and Kaplan machines usually have vertical shafts because this makes best use of the available head and makes installation of a generator more economical. Pelton wheels may be either vertical or horizontal shaft machines because the size of the machine is so much less than the available head. Some impulse turbines use multiple water jets per runner to increase specific speed and balance shaft thrust. Typical range of heads • • • • • •
Hydraulic wheel turbine Archimedes’ screw turbine Kaplan Francis Pelton Turgo
0,2 < H < 4 (H = head in m) 1 < H < 10 2 < H < 40 10 < H < 350 50 < H < 1300 50 < H < 250
Specific speed The specific speed (ns) of a turbine characterises the turbine’s shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criterion for matching a specific hydro site with the correct turbine type. The specific speed is the speed at which the turbine turns for a particular discharge Q with the unit head, and thereby is able to produce unit power.
Affinity laws Affinity laws allow the output of a turbine to be predicted based on model tests. A miniature replica of a proposed design, about 0,3 m in diameter, can be tested and the laboratory measurements applied to the final application with high confidence. Affinity laws are derived by requiring similitude between the test model and the application. Flow through the turbine is controlled either by a large valve or by wicket gates arranged around the outside of the turbine runner. Differential head and flow can be plotted for a number of different values of gate opening, producing a hill diagram used to show the efficiency of the turbine at varying conditions. Runaway speed The runaway speed of a water turbine is its speed at full flow and with no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.
Figure 9.4: Francis turbine
Figure 9.4 shows a Francis turbine at the end of its life, showing cavitation pitting, fatigue cracking and a catastrophic failure. Earlier repair jobs that used stainless steel weld rods are visible. Turbines are designed to run for decades with very little maintenance of the main elements; overhaul intervals are in the order of several years. Maintenance of the runners and parts exposed to water include removal, inspection and repair of worn parts. Normal wear and tear includes pitting from cavitation, fatigue cracking, and abrasion from suspended solids in the water. Steel elements are repaired by welding, usually with stainless steel rods. Damaged areas are cut or ground out, then welded back up to their original or an improved profile. By the end of their lifetime, old turbine runners may have a significant amount of stainless steel added this way. Elaborate welding procedures may be used to achieve the highest quality repairs. Other elements requiring inspection and repair during overhauls include bearings, packing box and shaft sleeves, servomotors, cooling systems for the bearings and generator coils, seal rings, wicket gate linkage elements and all surfaces.
Mechanical Technology Environmental impact Environmental impacts of reservoirs Water turbines are generally considered a clean power producer, as the turbine causes essentially no change to the water. They use a renewable energy source and are designed to operate for decades. They produce significant amounts of the world’s electrical supply. Historically, there have also been negative consequences, mostly associated with the dams normally required for power production. Dams alter the natural ecology of rivers, potentially killing fish, stopping migrations and disrupting peoples’ livelihoods. For example, American Indian tribes in the Pacific Northwest had livelihoods built around salmon fishing but aggressive dam-building destroyed their way of life. Dams also cause less obvious, but potentially serious, consequences, including increased evaporation of water (especially in arid regions), build-up of silt behind the dam and changes to water temperature and flow patterns. Some people believe that it is possible to construct hydropower systems which divert fish and other organisms away from turbine intakes without significant damage or loss of power; historical performance of diversion structures has been poor. In the United States, it is now illegal to block the migration of fish, for example, the endangered great white sturgeon in North America, so fish ladders must be provided by dam builders. The actual performance of fish ladders is often poor.
Assessment 1 1. 2. 3. 4. 5. 6. 7. 8. 9.
Explain the theory of operation of a waterwheel. Water turbines are divided into two groups. Name them. The shape of water turbine blades is determined by two factors. Name them. Explain the principle of impulse turbines. Explain what is meant by pump storage. Name four types of reaction turbines. Name seven types of impulse turbines. Define affinity laws. Define runaway speed.
Steam turbines Function A steam turbine is a mechanical device which extracts thermal energy from pressurised steam and converts it into useful mechanical work.
Figure 9.5: The rotor of a modern steam turbine used in a power plant
Turbines The steam turbine has completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt) primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to driving an electrical generator – it doesn’t require a linkage mechanism to convert reciprocating motion to rotary motion. The steam turbine is a form of heat engine which obtains improved thermodynamic efficiency by using multiple stages in the expansion of the steam (as opposed to one stage in the Watt engine) which results in greater efficiency.
Thermodynamics A branch of physics that studies the effects of changes in temperature, pressure and volume on physical systems at the macroscopic scale
The first steam engine, the classic Aeolipile made by Heros of Alexandria, was little more than a toy. Another steam turbine was created by Italian Giovanni Branca, in 1629. Anglo-Irishman Charles A. Parsons invented the modern steam turbine in 1884. His first model was connected to a dynamo which generated 7,5 kW of electricity. His patent was licensed and the turbine was scaled up shortly afterwards by an American, George Westinghouse. Many variations of steam turbine have been developed since. The de Laval turbine, invented by Gustaf de Laval, accelerated the steam to full speed before running it against a turbine blade. This was effective because the turbine was simpler, less expensive and did not need to be pressure-proof. It could operate with any steam pressure. It was, however, considerably less efficient. The Parson’s turbine turned out to be relatively easy to scale up. Within Parson’s lifetime, the generating capacity of a unit was scaled up 10 000 times.
Types Steam turbines are made in a variety of sizes, ranging from small 1 hp (0,75 kW) units, which are rarely used as mechanical drives for pumps, compressors and other shaft-driven equipment, to 2 million hp (1,5 million kW) turbines used to generate electricity. There are several classifications for modern steam turbines.
Did you know? Horsepower (or hp) is the common unit of power, or, in other words, the rate at which work is done.
Figure 9.6: A steam turbine rotor
These types include condensing, non-condensing, reheat, extraction and induction turbines.
Condensing turbines Condensing turbines are most commonly found in electrical power plants. These turbines use exhaust steam in a partially condensed state. The steam is usually composed of more than 90% partially condensed steam, at a pressure well below atmospheric pressure and then sent to a condenser.
Non-condensing turbines Non-condensing (or back-pressure) turbines are most widely used for processsteam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. Desalination Removing salt from seawater
Non-condensing turbines are commonly found at refineries, pulp and paper plants and desalination facilities where large amounts of low-pressure process steam are available.
Reheat turbines Reheat turbines are used almost exclusively in electrical power plants. In a reheat turbine, steam flow occurs in a high-pressure section of the turbine and is returned to the boiler where additional heat is added. The steam then goes back into an intermediate-pressure section of the turbine and continues its expansion.
Did you know? Desalination is also called ‘desalting’. It involves removing dissolved salts from seawater and groundwater. Desalination makes otherwise unusable water fit for human consumption, irrigation, industrial applications and various other purposes. Unfortunately, existing desalination technology needs a lot of energy so the process is expensive.
Extracting-type turbines are common in all applications. In an extracting turbine, steam is released at various stages of the turbine and used for industrial processes or sent to boiler-feed water heaters to improve overall cycle efficiency. A valve may control extraction flows.
Induction turbines Induction turbines introduce low-pressure steam at an intermediate stage to produce additional power.
Casing or shaft arrangements These arrangements include single-casing, tandem-compound and cross-compound turbines. Single-casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem-compound turbines are used where two or more casings are directly coupled together to drive a single generator. A crosscompound turbine features two or more shafts which are not in line, driving two or more generators that often operate at different speeds. A cross-compound turbine is typically used for many large applications.
Principle of operation and design Isentropic Isentropic processes occur where the entropy is constant
The ideal steam turbine uses an isentropic process or constant entropy process in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly isentropic, however, as most steam turbines are able to convert only 20% – 90% of the energy available from the steam into useful work.
Turbines The interior of a turbine has several sets of blades (or ‘buckets’ as they are commonly referred to). One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the sizes and configurations of the sets varying to use the expansion of steam efficiently at each stage.
Turbine efficiency Impulse turbine
Moving buckets Rotor
Entropy Central to the second law of thermodynamics; it is a process too complex to explain in the context and scope of this curriculum but is a very interesting study in physics should you wish to persue further research on your own
Stationary Still; not moving Steam pressure
Figure 9.7: A schematic diagram outlining the difference between impulse and reaction turbines
To maximise turbine efficiency, the steam is expanded, so generating work, in a number of stages. These stages are characterised by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of reaction and impulse turbines. Typically, higherpressure sections are the impulse type and lower-pressure stages are the reaction type.
Impulse turbines An impulse turbine has fixed nozzles that channel the steam flow into high-speed jets. These jets contain significant kinetic energy which the rotor blades convert into shaft rotation as the steam-jet changes direction. A pressure drop occurs only across the stationary blades, with a net increase in steam velocity across the stage.
Reaction turbines In a reaction turbine, the rotor blades are arranged to form convergent nozzles. This type of turbine uses the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. The steam leaves the stator as a jet which fills the entire circumference of the rotor. The steam then changes direction and increases its speed, relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature. This is reflected in the work performed in driving the rotor.
Kinetic Relating to or resulting from motion (movement)
Convergent Coming closer together
Stator The stationary part of the steam turbine encased in its housing
Operation and maintenance
RPM Revolutions (turns) per minute
Impingement Coming into contact with
When warming up a steam turbine for use, the main steam stop-valves (after the boiler) have a bypass line to allow superheated steam to bypass the valve slowly and proceed to heat up the lines in the system along with the steam turbine. A turning gear is also engaged when there is no steam available in the turbine to rotate the turbine slowly to ensure that even-heating takes place. This prevents uneven expansion. After rotating the turbine by the turning gear first, allowing time for the rotor to assume a straight plane (no bowing), the turning gear is disengaged and steam is sent to the turbine, first to the astern blades then to the ahead blades, slowly rotating the turbine at 10 to 15 RPM to warm the turbine. Problems with turbines are now rare and maintenance requirements are relatively low. However, an imbalance of the rotor can lead to vibration which, in extreme cases can lead to a blade freeing itself and punching through the casing. It is essential that turbines be turned with dry steam. Water entering the steam and being blasted onto the blades (moisture carryover) can cause rapid impingement and erosion of the blades. This can lead to imbalance and failure. Water entering the blades is likely to destroy the thrust bearing of the turbine shaft. To prevent this, condensate drains are installed in the steam piping leading to the turbine, along with controls and baffles in the boilers.
Speed regulation It is essential to control a turbine with a governor as turbines need to be run up slowly to prevent damage. Some applications (such as those that generate alternating-current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip which causes the closing of the nozzle valves controlling the flow of steam to the turbine. If those valves fail, the turbine may continue accelerating until it breaks apart. Turbines are expensive to manufacture, requiring precision and special quality materials.
Synchronous Existing or occurring at the same time
Power stations use large steam turbines which drive electric generators to produce most of the world’s electricity. There are two types of power station: a fossil fuel station and a nuclear power station. The turbines used for electric power generation are directly coupled to generators. As the generators have to rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3 000 RPM for 50 Hz systems, and 3 600 RPM for 60 Hz systems. Some large nuclear sets rotate at half those speeds and have a four-pole generator rather than the more common two-pole generator.
Speed reduction Reciprocating Rfers to engines, such as internal combustion engines, that convert the up-and-down (reciprocating) motion of the pistons into rotary motion
Steam turbines are also used in ships because they can be small and lightweight, require little maintenance and have low vibration. In the past, steam turbine locomotives were also tested but with limited success. A steam turbine is efficient only when operating in thousands of RPM, while the application of the power in propulsion applications may only be in hundreds of RPM, requiring expensive and precise reduction gears to be used. Although the purchase cost is high, the fuel and maintenance requirements costs are much lower. Another advantage is the small size of a turbine when compared to a reciprocating engine of equivalent power.
Figure 9.8: The Turbinia – the first steam turbine-powered ship
Assessment 2 1. Name the function of a steam turbine. 2. Which type of engine replaced the reciprocating-piston steam engine? 3. Who invented the reciprocating-piston steam engine? 4. Who invented the first steam engine and when; which country did this person come from? 5. Research the history of the steam engine and then write a 150-word summary of your findings. 6. Name five classifications for modern steam turbines. 7. Explain the operation and design principle of an ideal steam turbine. 8. Explain an impulse turbine. 9. Explain a reaction turbine. 10. Speed regulation is essential to turbines. How is speed controlled? 11. How is most of the world’s electricity produced?
Gas turbines Function A gas turbine, also called a combustion turbine, is a rotary engine which extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine with a combustion chamber in-between. (The term ‘gas turbine’ is sometimes used to refer only to the turbine element.)
Mechanical Technology Spinner Intake turbines
Compressor turbine rotor
Figure 9.9: A jet engine (gas turbine)
Energy is released when air is mixed with fuel and ignited in the combustor. The resulting gases are directed over the turbine’s blades, spinning the turbine and powering the compressor. The gases are finally passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure. Energy is extracted in the form of shaft power, compressed air and thrust in any combination and is used to power aircraft, trains, ships, generators and even tanks.
Theory of operation Gas turbines are described by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure and expansion over the turbine occurs isentropically back to the starting pressure. In practice, friction and turbulence cause: • non-isentropic compression – for a given overall pressure ratio, the compressor delivery temperature is higher than ideal • non-isentropic expansion – although the turbine-temperature drop needed to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work • pressure losses in the air intake, combustor and exhaust – these losses reduce the expansion available to provide useful work.
Figure 9.10: The idealised Brayton cycle
As with all cyclical heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, ceramic or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat which is otherwise wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. Combined heat and power (co-generation) uses waste heat for hot water production. Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines usually have only one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see Figure 9.9), not counting the fuel system. More sophisticated turbines (such as those in jet engines) may have multiple shafts (called ‘spools’), hundreds of turbine blades, movable stator blades and a vast system of complex piping, combustors and heat exchangers. Generally, the smaller the engine, the higher the rotation of the shaft(s) needed to maintain tip speed. Tip speed is the difference between the rotational speed of the tip of a turbine blade and the actual velocity of the steam. Tip speed determines the maximum pressure that can be gained in the turbine, independent of the size of the engine. Jet engines operate around 10 000 RPM and micro-turbines around 100 000 RPM. Thrust bearings and journal bearings are a critical part of the design of a gas turbine. Traditionally, they have been hydrodynamic oil bearings or oil-cooled ball bearings. Recently, foil bearings have become common in micro-turbines and auxiliary power units (APUs).
Hydrodynamic Describes forces acting on or exerted by fluids
Figure 9.11: A gas turbine used for power production
Figure 9.11 shows a GE H-series power-generation gas turbine. This 480-megawatt unit has a rated thermal efficiency of 60% in combined cycle configurations. Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems.
Baseload plants Fossil fuel plants
Simple-cycle gas turbines in the power industry require a smaller capital investment than combined-cycle gas, coal or nuclear plants and can be designed to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for baseload plants. The other main advantage is that they can be turned on and off within minutes, thus supplying power during increased demand. Large simple-cycle gas turbines may produce several hundred RPM. The power turbines in the largest industrial gas turbines operate at 3 000 or 3 600 RPM to match the AC-power-grid frequency and to avoid the need for a reduction gearbox. Such engines require specialised building. They can be efficient up to 60% when waste heat from the gas turbine is recovered by a conventional steam turbine in a combined-cycle configuration. They can also be run in a co-generation configuration where the exhaust is used for space or water heating or to drive an absorption chiller for cooling or refrigeration. Here cogeneration can be over 90% energy-efficient.
Gas turbines for mechanical drive applications Two-shaft gas turbines are often used to drive compression trains, for example, in gas pumping stations or natural gas liquefaction plants. The first shaft bears the compressor and the high-speed turbine (often referred to as ‘gas generator’), while the second shaft bears the low-speed turbine (or ‘power turbine’). This arrangement increases speed and power output flexibility.
Scale jet engines (micro-turbines) Scale jet engines are also known as: • miniature gas turbines • micro-jets.
Figure 9.12: A micro-turbine
Many model engineers enjoy the challenge of re-creating today’s tiny working models. Naturally, the idea of re-creating a powerful engine, such as a jet’s, has fascinated hobbyists since the first full-size engines were powered up by Hans von Ohain and Frank Whittle, back in the 1930s. Re-creating machines on a much smaller scale is not easy. The laws of physics governing the behaviour of machines do not always scale up or down at the same rate as the machine’s size. An automobile engine, for example, will not work if reproduced to the size of a human hand. With this in mind, the pioneer of modern micro-jets, Kurt Schreckling, produced one of the world’s first micro-turbines, the FD3/67. This engine can give out 22 newtons of thrust and can be built by most mechanically minded people with basic engineering tools, such as a metal lathe.
Micro-turbines Micro-turbines are becoming widespread for distributing power and for combined heat and power applications. They range from hand-held units producing less than a kilowatt to commercial-sized systems that produce tens or hundreds of kilowatts. Part of their success is due to advances in electronics which allow unattended operation and interfacing with the commercial power grid. Electronic powerswitching technology eliminates the need for the generator to be synchronised with the power grid. This allows, for example, the generator to be integrated with the turbine shaft and to double as the starter motor.
Eliminates Gets rid of
Micro-turbine systems have many advantages over piston-engine generators, such as higher power density (with respect to weight), extremely low emissions and few moving parts (and sometimes just one moving part). Those micro-turbine systems with foil bearings and air-cooling operate without oil, coolants or other hazardous materials. However, piston-engine generators are quicker to respond to changes in output-power requirements. Piston-engine generators accept most commercial fuels such as natural gas, propane, diesel and paraffin kerosene. They can also use renewable energy when fuelled with biogas from landfills and sewage treatment plants. Micro-turbine designs usually consist of a single-stage radial compressor, a single-stage radial turbine and a recuperator. Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials. Exhaust heat can be used for water heating, drying processes or absorption chillers, which create cold air for air-conditioning from heat energy instead of electric energy. Typical micro-turbine efficiencies are from 25% to 35%. When in a combined heatand-power co-generation system, efficiencies of greater than 80% are commonly achieved.
Auxiliary power units Auxiliary power units (or APUs) are small gas turbines designed for auxiliary power of larger machines, usually aircraft. They are well suited for supplying compressed air for aircraft ventilation, start-up power for larger jet engines and electrical and hydraulic power. The Perrys’ APUs are large electric motors that provide manoeuvring help in close waters or emergency backup if the gas turbines are not working.
Manoeuvring To do with movement
Gas turbines in vehicles
Figure 9.13: Rover JET1
Gas turbines are used on ships, locomotives, helicopters and in tanks. Many experiments have been conducted with gas turbine-powered automobiles. In 1950, designer F. R. Bell and Chief Engineer Maurice Wilks, from British car manufacturers Rover, unveiled the first car powered by a gas turbine engine. The two-seater JET1 had the engine positioned behind the seats, air intake grilles on either side of the car and exhaust outlets on the top of the tail (back). During tests, the car reached top speeds of 140 kph, at a turbine speed of 50 000 RPM. The car ran on petrol, paraffin and diesel oil but fuel-consumption problems prevented it becoming a production car. JET1 is on display at the London Science Museum. Coupé A car with a fixed roof, two doors and a sloping back
Fictional Not based on actual events; made up
Hybrid Something made from a combination of two elements
Naturally aspirated Naturally aspirated engines use a carburettor and not fuel injection
Rover and the British Racing Motors (BRM) Formula One team joined forces to produce a gas turbine-powered coupé. In 1963, they entered it in the 24-hour race at Le Mans (in France). It was driven by Graham Hill and Richie Ginther. It averaged 173 kph and had a top speed of 229 kph. In 1971, Lotus principal, Colin Chapman, introduced the Lotus 56B F1 car, powered by a Pratt & Whitney gas turbine. Colin Chapman had a reputation for building unusual championship-winning cars but had to abandon the project because of problems with turbo lag. The fictional Batmobile is often said to be powered by a gas turbine or a jet engine. American car manufacturer Chrysler developed several prototype gas turbinepowered cars from the 1950s to the early 1980s. Chrysler built 50 Chrysler turbine cars in 1963 and conducted the only consumer trial of gas turbine-powered cars. In 1993, General Motors introduced the first commercial gas turbine-powered hybrid vehicle – as a limited production run of the EV-1. A Williams International 40 kW turbine drove an alternator which powered the battery-electric powertrain. The turbine design included a recuperator. Gas turbines offer a high-powered engine in a small and light package. However, they are not as responsive and efficient as small piston engines over the wide range of RPM and powers needed in vehicle applications. Also, turbines have been more expensive historically to produce than piston engines, though this is partly because piston engines have been mass-produced in huge quantities over a long time while small turbines are rarities. It is also worth noting that the main advantage of jets and turbo-props for aeroplane propulsion – their superior performance at high altitude compared to piston engines, particularly naturally aspirated ones – is irrelevant in automobile applications. Their power-to-weight advantage is far less important. Their use in hybrids reduces the responsiveness problem and the development of the continuously variable transmission may also help solve this problem.
The Marine Turbine Technologies (MTT) Turbine SUPERBIKE appeared in 2000 (hence the designation of Y2K Superbike by MTT) and is the first production motorcycle powered by a jet engine. This engine is a Rolls-Royce Allison model 250 turboshaft engine, producing about 283kW. The Superbike was speed-tested to 365 kph and apparently the test team ran out of road during the test. The Superbike holds the Guinness World Record for the most powerful production motorcycle and most expensive production motorcycle (with a price tag of US$185 000). Gas turbines have been used more successfully in military tanks. In the 1950s, a Conqueror heavy-tank was fitted with a Parsons 650-hp gas turbine. They have been used as auxiliary power units in several other production models. Today, the Soviet/ Russian T-80 and US M1 Abrams tanks use gas-turbine engines. Several locomotive classes have been powered by gas turbines, the most recent being Bombardier’s JetTrain.
Naval use Gas turbines are used in many naval vessels where they are valued for their high power-to-weight ratio and their ships’ resulting acceleration and ability to get underway quickly. The first gas turbine-powered naval vessel was the Royal Navy’s Motor Gun Boat MGB 2009 (formerly MGB 509), converted in 1947. The first large gas turbine-powered ships were the Royal Navy’s Type 81 (Tribal class) frigates, the first of which (HMS Ashanti) was commissioned in 1961.
Frigate A warship, usually lighter than a destroyer
The Swedish Navy produced six Spica-class torpedo-boats between 1966 and 1967, powered by three Bristol Siddeley Proteus 1282s, each delivering 4 300 hp. They were later joined by six upgraded Norrköping-class ships with the same engines. With their rear torpedo tubes replaced by anti-shipping missiles, they served as missile boats until the last was retired in 1986. The first US gas turbine-powered ships were the US Coast Guard’s Hamilton-class High Endurance Cutters. The first of these ships, the USCGC Hamilton, was commissioned in 1967. Since then, they have powered the US Navy’s Perry-class frigates, Spruance-class and Arleigh Burke-class destroyers and Ticonderoga-class guided-missile cruisers. USS Makin Island, a modified Wasp-class amphibious assault ship, is to be the US Navy’s first amphibious craft powered by gas turbines. Three Rolls-Royce gas turbines power the 118 WallyPower, a 118-foot (36 m) superyacht. These engines combine to give a total of 12 356 kW, allowing the boat to maintain speeds of 60 knots or 112 kph. Another example of commercial usage of a gas turbine in a ship is the Stena Discovery, using the GE LM2500, a gas turbine made by General Electric.
Amateur gas turbines A popular hobby is to construct gas turbines from automotive turbochargers. A combustion chamber is fabricated and plumbed between the compressor and turbine. Several small companies manufacture small turbines and parts for the amateur.
Advances in technology Gas turbine technology has steadily advanced and research is actively producing ever smaller gas turbines. Computer design, specifically CFD (computer fluid dynamics software) and finite element analysis (simulation software) along with material advances, has allowed higher compression ratios and temperatures, more efficient combustion, better cooling of engine parts and reduced emissions. The challenge to technology is to get a catalytic combustor running properly in order to achieve minimal carbon monoxide (CO) emissions. Additionally, compliant foil bearings were commercially introduced to gas turbines in the 1990s. They can withstand over 100 000 ‘start/stop’ cycles and eliminate the need for an oil system.
Assessment 3 1. Name the function of a gas turbine. 2. Research the gas turbine and then write half an A4 page on your findings. 3. Explain the theory of how gas turbines operate. 4. Compare micro-turbine systems to piston engines. 5. Write up your research on the use of gas turbines in motor vehicles from 1950. 6. Why are gas turbines used on naval vessels? 7. Gas turbine technology has steadily advanced since its inception and continues to evolve. Do research on smaller gas turbines.
Did you know? The Indianapolis 500, also known as the Indy 500, is an American car race held annually since 1911. The race always takes place at the Indianapolis Motor Speedway in Indianapolis, Indiana, and draws crowds of over 400 000.
Pole position A racing term for the best position at the start of the race
Shards Sharp pieces
Turbochargers History The turbocharger was invented by Swiss engineer Alfred Buchi who had worked on steam turbines. His patent for the internal combustion turbocharger was applied for in 1905. Diesel ships and locomotives with turbochargers began appearing in the 1920s. One of the first applications of a turbocharger to a non-diesel engine was when General Electric engineer Sanford Moss attached a turbo to a V12 Liberty aircraft engine. The engine was tested at Pikes Peak in Colorado at an altitude of 4 267 m above sea level to demonstrate that it could eliminate the power losses usually experienced in internal combustion engines as a result of altitude. Turbochargers were first used in production aircraft engines in the 1930s, prior to World War II. The purpose behind most aircraft-based applications was to increase the altitude at which an aeroplane could fly, by compensating for the lower atmospheric pressure present at high altitude. Aircraft such as the Lockheed P-38 Lightning, Boeing B-17 Flying Fortress and B-29 Superfortress all used exhaustdriven ‘turbo-superchargers’ to increase high-altitude engine power. It is important to note that turbo-supercharged aircraft engines actually used a gear-driven centrifugal type supercharger in series with a turbocharger. Turbo-diesel trucks were produced in Europe and America (notably by Cummins) after 1949. The turbocharger hit the automobile world in 1952 when Fred Agabashian qualified for pole position at the Indianapolis 500 and led for 160 km (100 miles) before tyre shards disabled the blower.
The first production turbocharged car engines came from General Motors. The A-body Oldsmobile Cutlass Jetfire and Chevrolet Corvair Monza Spyder were both fitted with turbochargers in 1962. Both of these engines were abandoned within a few years and GM’s next turbo engine came more than ten years later.
Function A turbocharger is a dynamic compressor in which air or gas is compressed by the mechanical action of impellers (vaned rotors) which are spun using the kinetic movement of air, imparting velocity and pressure to the flowing medium.
Figure 9.14: A turbocharger
The mechanical concept of a turbocharger revolves around three main parts. A turbine is driven by the exhaust gas from a pump, most often an internalcombustion engine, to spin an impeller whose function is to force more air into the pump’s intake or air supply. The third basic part is a centre hub rotating assembly which contains bearings, lubrication, cooling and a shaft that directly connects the turbine and impeller. The shaft, bearings, impeller and turbine can rotate at speeds of hundreds of thousands of RPM. The lubrication system can be either a closed system or be fed from the engine’s oil supply. The lubrication system may double as the cooling system or separate coolant may be pumped through the centre housing from an outside source. Oil lubrication and water cooling using engine oil and engine coolant are commonplace in automotive applications.
Figure 9.15: The position of the turbocharger in a car
Conical Shaped like a cone
The turbine and impeller are each contained within their own folded conical housing on opposite sides of the centre hub-rotating assembly. These housings collect and direct the gas flow. The size and shape can dictate some performance characteristics of the overall turbocharger. The area of the cone to radius from A centre hub is expressed as a ratio (AR, R , or A:R). Often, the same basic turbocharger assembly will be available from the manufacturer with multiple AR choices for the turbine housing and sometimes the compressor cover as well. This allows the designer of the engine system to tailor the compromises between performance, response and efficiency to application or preference. Both housings resemble snail shells and thus turbochargers are sometimes referred to as ‘snails’. Change air cooler
Compressed air flow Turbocharger oil inlet
Exhaust gas outlet
Ambient air inlet Compressor wheel Oil outlet
Figure 9.16: How a turbocharger is plumbed into a car
By spinning at a relatively high speed, the compressor turbine draws in a large volume of air and forces it into the engine. As the turbocharger’s output-flow volume exceeds the engine’s volumetric flow, air pressure in the intake system begins to build. The speed at which the assembly spins is proportional to the pressure of the compressed air and total mass of air flow being moved. Since a turbo will spin faster than is needed, the speed must be controlled and thus it is also the property used to set the desired compression pressure. A wastegate is the most common mechanical control system and is often aided by an electronic boost controller. The installation of a turbocharger is done to improve upon the size-to-output efficiency of an engine by solving one of its cardinal limitations. A naturally aspirated car engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder. Since the number of air and fuel molecules determine the potential energy available to force the piston down on the combustion stroke, and because of the relatively constant pressure of the atmosphere, there is ultimately a limit to the amount of air and consequently fuel, filling the combustion chamber. Turbine section Turbine housing
Turbine exhaust gas outlet
Turbine wheel Turbine exhaust inlet
Compressor ambient air inlet
Compressor air discharge Compressor wheel Compressor section
Figure 9.17: The inside of a turbocharger
This ability to fill the cylinder with air is its volumetric efficiency. Since the turbocharger increases the pressure at the point where air is entering the cylinder and the amount of air brought into the cylinder is largely a function of time and pressure, more air will be drawn in as the pressure increases. The intake pressure, in the absence of the turbocharger determined by the atmosphere, can be controllably increased with the turbocharger. The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as forced induction. Centrifugal superchargers operate in the same fashion as a turbo. However, the energy to spin the compressor is taken from the rotating output energy of the engine’s crankshaft as opposed to exhaust gas. For this reason, turbochargers are ideally more efficient, since their turbines are actually heat engines, converting some of the heat energy from the exhaust gas that would otherwise be wasted, into useful work. Superchargers use output energy to achieve a net gain which is at the expense of some of the engine’s total output.
The basics One of the surest ways to get more power out of an engine is to increase the amount of air and fuel that it can burn. One way to do this is to add cylinders (or make the current cylinders bigger). Sometimes these changes may not be feasible – a turbo can be a simpler, more compact way to add power, especially for an aftermarket accessory.
Fuel efficiency Since a turbocharger increases the horsepower output of an engine, the engine will also produce increased amounts of waste heat. This can be a problem when fitting a turbocharger to a car which was not designed to cope with high-heat loads. This extra waste heat, combined with the lower compression ratio (more specifically, expansion ratio) of turbocharged engines, contributes to slightly lower thermal efficiency. This has a small but direct impact on overall fuel efficiency. Turbochargers allow an engine to burn more fuel and air by packing more into the existing cylinders. The typical boost provided by a turbocharger is 41,3 to 55,2 kPa. Since normal atmospheric pressure is 96,52 kPa at sea level, about 50% more air enters the engine. Therefore, you would expect to get 50% more power. It’s not perfectly efficient, so you might get 30% to 40% improvement instead. One cause of the inefficiency is having to spin the turbine. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that, on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little power.
Reliability As long as the oil supply is clean and the exhaust gas does not become overheated because of lean mixtures or advanced spark timing on a petrol engine, a turbocharger can be very reliable. However, care of the unit is important. Replacing a turbo that lets go and sheds its blades is expensive. The use of synthetic oils is recommended in turbo engines. Synthetic oils are designed and engineered to withstand the relatively harsh conditions under which turbochargers operate.
Lean petrol mixture A relatively low concentration of petrol vapour in air
Advanced spark timing The spark timing occurs before it should
Sever Cut off
After high-speed operation of the engine, it is important to let the engine run at idle speed for one to three minutes before turning it off. For example, Saab, in its owner manuals, recommends a period of just 30 seconds. This lets the turbo-rotating assembly cool from the lower exhaust gas temperatures. Not doing this will also result in the critical oil supply to the turbocharger being severed when the engine stops while the turbine housing and exhaust manifold are still very hot. This may lead to coking (or carbonisation) of the lubricating oil trapped in the unit, when the heat soaks into the bearings, and later, failure of the supply of oil when the engine is next started, causing rapid bearing wear and failure. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure. A turbo timer is a device designed to keep a car engine running for a pre-specified time, to allow it to cool down. Oil coking is completely eliminated by foil bearings because they require no additional lubrication, therefore coking is not possible.
Figure 9.18: A turbine vane
Inertia Newton’s First Law states: “All bodies preserve their state of being at rest or of moving uniformly straight ahead, unless they are compelled to change their state by applied forces. In this context, rotational inertia describes the resistance the turbine’s rotor has to start rotating and accelerate to higher speeds”
A lag is sometimes felt by the driver of a turbocharged vehicle as a delay between pressing the accelerator pedal and feeling the turbo kick in. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly-driven compressor in a positive-displacement supercharger does not experience this problem. (Centrifugal superchargers do not build boost at low RPMs like a positive-displacement supercharger will.) Conversely, at light loads or at low RPM, a turbocharger supplies less boost and the engine is more efficient than a supercharged engine.
Boost Boost refers to the increase in manifold pressure that is generated by the turbocharger in the intake path or specifically intake manifold which exceeds normal atmospheric pressure. This is also the level of boost as shown on a pressure gauge, usually in bar, psi (pounds per square inch) and kPa. Pascals are the standard metric unit of pressure. One kilopascal (kPa) is 1 000 pascals and one pascal is 1 newton per square metre (N/m2). Boost is also referred to as ‘pounds of boost’. This is representative of the extra air pressure that is achieved over that which would be achieved without the forced induction.
Turbines Boost pressure is limited to keeping the entire engine system, including the turbo, inside its design operating range by controlling the wastegate which shunts the exhaust gases away from the exhaust side turbine. In some cars, the maximum boost depends on the fuel’s octane rating and is electronically regulated using a knock sensor.
Did you know? With air being pumped under pressure by the turbocharger into the cylinders and then being further compressed by the piston, there is more danger of knocking. Knocking happens because as the air is compressed its temperature increases. The temperature may increase enough to ignite the fuel before the spark-plug fires. Cars with turbochargers often run on higher-octane fuel to avoid knock. Engine knock occurs when the compressed air/fuel mixture is ignited too soon. This happens in the compression stroke when the piston is too far away from top dead centre. The expansion of combusted gases and increased cylinder pressure, due to the upward movement of the piston, results in a very high compression ratio and hence excess pressure on the engine components which causes a knocking effect. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.
Types of supercharger
Figure 9.19: The Eaton supercharger, a modified Roots supercharger
There are three types of supercharger: Roots, twin screw and centrifugal. The main difference is how they move air to the intake manifold of the engine. Roots and twin-screw superchargers use different types of meshing lobes and a centrifugal supercharger uses an impeller which draws air in. Although all these designs provide a boost, they differ considerably in efficiency. Each type of supercharger is available in different sizes, depending on whether you just want to give your car a boost or compete in a race.
The Roots supercharger The Roots supercharger is the oldest design. Philander and Francis Roots patented the design in 1860 as a machine that would help ventilate mine shafts. In 1900, Gottleib Daimler included a Roots supercharger in a car engine. Fill side Inlet
Outlet Discharge side
Figure 9.20: A Roots supercharger
Figure 9.21: A 1942 Ford pick-up with a Roots supercharger
As the meshing lobes spin, air trapped in the pockets between the lobes is carried between the fill side and the discharge side. Large quantities of air move into the intake manifold and ‘stack up’ to create positive pressure. For this reason, Roots superchargers are really nothing more than air blowers and the term ‘blower’ is often used to describe all superchargers. Roots superchargers are usually large and sit on top of the engine. They are popular in ‘muscle cars’ and hotrods because they stick out of the bonnet of the car. However, they are the least efficient supercharger for two reasons: they add more weight to the vehicle and they move air in discrete bursts instead of in a smooth and continuous flow.
The twin-screw supercharger A twin-screw supercharger operates by pulling air through a pair of meshing lobes that resemble a set of worm gears. Like the Roots supercharger, the air inside a twin-screw supercharger is trapped in pockets created by the rotor lobes. A twinscrew supercharger compresses the air inside the rotor housing. This is because the rotors have a conical taper which means the air pockets decrease in size as air moves from the fill side to the discharge side. As the air pockets shrink, the air is squeezed into a smaller space.
Figure 9.22: A twin-screw supercharger
This makes the twin-screw superchargers more efficient but they cost more because the screw-type rotors require more precision in the manufacturing process.
Centrifugal supercharger Any of these superchargers can be added to a vehicle as an after-market enhancement. Several companies offer kits which come with all of the parts necessary to install a supercharger as a do-it-yourself project. In the world of funny cars and fuel racers, such customisation is an integral part of the sport. Several auto manufacturers also include superchargers in their production models. Centrifugal superchargers are the most efficient and the most common of all forcedinduction systems. They are small, lightweight and are attached to the front of the engine instead of the top. They also make a distinctive whine as the engine revs up – a quality that may turn heads on the street. A centrifugal supercharger powers an impeller, a device similar to a rotor, at very high speeds to draw air into a small compressor housing quickly. An impeller is similar to a rotor. Impeller speeds can reach 50 000 to 60 000 RPM. As the air is drawn in at the hub of the impeller, centrifugal force causes it to radiate outward. The air leaves the impeller at high speed but low pressure. A diffuser – a set of stationary vanes which surround the impeller – converts the high-speed, lowpressure air to low-speed, high-pressure air. Air molecules slow down when they hit the vanes, which reduces the velocity of the airflow and increases pressure.
Did you know? Volkswagen has recently released a ‘Twincharger’ engine in a Golf GT. The Twincharger comes with both a supercharger and a turbocharger. At low RPM, the supercharger blasts air into the cylinders to enhance low-end torque. At high RPM, when exhaust gases have been produced in sufficient quantity, the turbocharger kicks in to increase top-end performance. The GT, which is available only in Europe, reaches 100 kph in 7,9 seconds. It can also reach 219 kph while still delivering 16,64 km per litre.
Figure 9.23: A centrifugal supercharger
Supercharger advantages and disadvantages The biggest advantage of having a supercharger is the increased horsepower. A supercharger attached to an otherwise normal car or truck will make it behave like a vehicle with a larger, more-powerful engine. What if a motorist is trying to decide between a supercharger and a turbocharger? This question is hotly debated by auto engineers and car enthusiasts but, in general, superchargers have some advantages over turbochargers. • Superchargers do not suffer lag – a word used to describe the time between the driver pressing down on the accelerator and the engine’s response. Turbochargers result in lag because it takes a few moments before the exhaust gases reach a velocity which is sufficient to drive the impeller/turbine. Superchargers have no lag time because they are driven directly by the crankshaft. • Certain superchargers are more efficient at lower RPM while others are more efficient at higher RPM. Roots and twin-screw superchargers, for example, provide more power at lower RPM. Centrifugal superchargers which become more efficient as the impeller spins faster, provide more power at higher RPM. • Installing a turbocharger requires extensive modification of the exhaust system but superchargers can be bolted to the top or side of the engine. This makes them cheaper to install and easier to service and maintain. • No special shutdown procedure is required with superchargers. Because they are not lubricated by engine oil, they can be shut down normally. Turbochargers must idle for about 30 seconds or so prior to shutdown so the lubricating oil has a chance to cool down. With that said, a good warm-up is important for superchargers as they work most efficiently at normal operating temperatures. Superchargers are common additions to the internal-combustion engines of aeroplanes. This makes sense when you consider that aeroplanes spend most of their time at high altitudes where significantly less oxygen is available for combustion. With the introduction of superchargers, aeroplanes were able to fly higher without losing engine performance. In general, superchargers offer a few advantages over turbochargers.
Assessment 4 1. Research the history of the turbocharger and write a summary of your findings. 2. Name the function of the turbocharger. 3. Explain the working operation of a turbocharger. 4. What impact does a turbocharger have on fuel efficiency? 5. Explain what is meant by the following: (a) lag (b) boost. 6. Name three types of supercharger. 7. Explain the operation of the following: (a) twin-screw supercharger (b) centrifugal supercharger. 8. Compare the advantages and disadvantages of a supercharger.