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Machine Design – I (MU)

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Machine Design & Design Considerations

1

Machine Design &Design Considerations • TOPICS • 1.1

Introduction to Machine Design

1.2

Classification of Machine Design

1.3

Design Process Design Considerations Aesthetic Considerations in Design Ergonomic Considerations in Design Communication Between Man (User) and Machine Working Environment Design for Manufacture (DFM)

1.10

Design Considerations for Casting

1.11

Design Considerations for Forging

1.12

Design Considerations for Machining

1.13

Design for Assembly (DFA)

1.14

Requisites of Design Engineer

1.15

Standards and Codes in Design

1.16

Preferred Series

1.17

Sources of Design Data

1.18

Creativity in Design

1.19

Role of Tolerances and Fits in Design, Manufacturing and Assembly

1.20

Tolerances

1.21

Fits

1.22

List of formulae Exercise

Machine Design – I (MU)

1.1 • • •

• • • •

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Machine Design & Design Considerations

INTRODUCTION TO MACHINE DESIGN The term design can be defined as the formulation of a plan for the satisfaction of human need. Design means to create something new or arrange existing things in a new order to satisfy a recognized need of society. Mechanical engineering design essentially means the design of the parts, products and systems of mechanical nature. It deals with all the disciplines of mechanical engineering, such as, machine design, thermal engineering, fluid power engineering, refrigeration and air conditioning, etc. Machine design deals with the design of machines, mechanisms and their elements. The design of machines or mechanisms ultimately comes to the design of their individual elements. Machine Design is the process of selection of the materials, shapes, sizes and arrangements of mechanical elements so that the resultant machine will perform the prescribed task. Design of machine element can be defined as the selection of material and the values for independent geometrical parameters so that the element satisfies its functional requirements and undesirable effects and kept within the permissible limits. The concept of machine design is illustrated in Fig. 1.1.

Fig. 1.1 : Concept of Machine Design

Machine Design – I (MU)

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Machine Design & Design Considerations

For example, the process of design of a belt drive consists of : 1.

Selection of arrangement of mechanical elements like : pulleys, belt, shafts, keys, bearings, etc;

•

2.

Selection of shapes of these mechanical elements;

3.

Selection of materials for these mechanical elements; and

4.

Selection of sizes of these mechanical elements.

Most of the problems in mechanical engineering design or specifically in machine design, do not have a unique right answer. There are nearly endless number of workable designs, none of which could be called an `incorrect' answer. But of the `correct' answers, some are obviously better than others.

1.2

CLASSIFICATION OF MACHINE DESIGN

On the basis of methods used and objectives, the machine design can be classified broadly as follows [Fig. 1.2] :

Fig. 1.2 : Classification of Machine Design 1.

System Design :

•

• 2.

System design is the design of any complex mechanical system. Each mechanical system consists of number of sub-systems and each sub-system consists of number of mechanical elements. Examples of the system design are : design of car, design of EOT crane, design of conveyer, etc.

Product Design :

• •

Product design is the design of a product which is a sub-system of any mechanical system. Examples of the product design are : design of gearbox, design of brake, design of clutch, etc.

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Machine Design & Design Considerations

Element Design :

•

Element design is the design of any mechanical element which is a part of mechanical sub-system or product.

• 4.

Examples of element design are : design of gear, design of shaft, design of key, etc.

Empirical Design :

•

Empirical design is the design using empirical formulae and relations. These empirical formulae are developed based on the past experience and practice.

•

Empirical design is preferred where design equations are not available or are too complex.

•

Empirical design does not involve too many calculations and is normally too much on the safer side.

•

Examples of empirical design are : design of gear box casing and design of machine tool body.

5.

Optimum Design :

•

For any design problem, a large number of design solutions are available which fulfill the requirements.

•

An optimum design is the best design solution from the possible design solutions.

•

An optimum design minimizes the undesirable effects like : cost, weight, size, etc or maximizes the useful parameters like : load carrying capacity, power transmitting capacity, etc.

6.

Computer Aided Design :

In computer aided design, computer system is used to assist in the creation, modification, analysis, and optimization of a design.

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Machine Design & Design Considerations

DESIGN PROCESS

Fig. 1.3 : Design Process

The general procedure that is followed in machine design is illustrated in Fig. 1.3. It consists of following steps : Step 1 : Definition of Problem : Define the design problem giving all input parameters, output parameters, and constraints. Step 2 : Synthesis : Once the problem is defined, the next step is synthesis. Synthesis is the process of selecting or creating the mechanism for the machine and the shapes of the mechanical

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Machine Design & Design Considerations

elements so as to get the desired output with given input. Step 3 : Analysis of Forces : Draw the force body diagram of each element of the machine. Find out the forces (including moments and torque) acting on each element by force analysis. Step 4 : Selection of Material : Select the suitable material for each element. Four basic factors that are to be considered while selecting the material are : availability, cost, mechanical properties, and manufacturing considerations. Step 5 : Determination of Mode of Failure : Before finding out the dimensions of the element, it is necessary to know the type of failure by which the element will fail when put into the use. Step 6 : Selection of Factor of Safety : Based on the application, select the factor of safety. Knowing factor of safety and material strength, determine the permissible or design stresses. Step 7 : Determination of Dimensions : Find the dimensions of each element of the machine by considering the forces acting on the element and the permissible stresses. Step 8 : Modification of Dimensions : Modify the dimensions of the elements on the higher side, if required, based on the following considerations : (i) Selection of standard parts available in the market; (ii) Convenience of assembly; and (iii) Convenience of manufacturing. Step 9 : Preparation of Drawings : •

Prepare working drawing of each element or component with minimum two views showing following details : (i) Dimensions; (ii) Dimensional tolerances; (iii) Surface finish; (iv) Geometrical tolerances; and (v) Special production requirements like heat treatment.

•

Prepare assembly drawing giving part numbers, overall dimensions, and part list.

•

The component drawing is supplied to the shop floor for manufacturing purpose, while assembly drawing is supplied to the assembly shop.

Step 10 : Preparation of Design Report : •

Prepare design report containing details about step 1 to step 8.

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Machine Design & Design Considerations

DESIGN CONSIDERATIONS Design considerations are the characteristics which influence the design of the element or, perhaps, the entire system. Normally, a number of such characteristics have to be considered in any design problem. In a given design problem, the design engineer should identify the various design considerations and incorporate them in the design process in their order of importance. For example, in the design of a spring, two most significant design considerations are : strength and stiffness. Some of the important design considerations are as follows : 1. Strength 7. Ergonomics 2. Rigidity 8. Aesthetics 3. Reliability 9. Manufacturing 4. Safety 10. Conformance to Standards 5. Cost 11. Assembly 6. Weight 12. Friction and Wear 13. Life 18. Flexibility 14. Vibrations 19. Size and Shape 15. Thermal Considerations 20. Stiffness 16. Lubrication 21. Corrosion 17. Maintenance 22. Noise The various design considerations, listed above, are discussed as follows :

1.

Strength :

• • 2.

The machine elements are subjected to any one or combination of loads like : forces, bending moments, and torque. A machine element should have sufficient strength to avoid failure either due to yielding or due to fracture, under the loads.

Rigidity :

A machine element should have sufficient rigidity so that its linear as well as angular deflections, under the loading, are within the permissible limits. 3.

Reliability :

• • 4.

The reliability is defined as the probability that a component, system, or device will perform without failure for a specified period of time under the specified operating conditions. A machine element should have reasonably good reliability so that it can perform its function satisfactorily over its life span.

Safety :

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Machine Design & Design Considerations

A machine element should be designed such that it ensures safety of the users and machine. 5.

Cost :

• • 6.

The life cycle cost of the machine element consists of : production cost, operating cost, maintenance cost, and disposal cost. A machine element should have a minimum possible life cycle cost.

Weight :

A machine element should have a minimum possible weight. 7.

Ergonomics :

• •

8.

Ergonomics is defined as the scientific study of the man-machine-working environment relationship and the application of anatomical, physiological, and psychological principles to solve the problems arising from this relationship. The objective of ergonomics is to make the machine fit for user rather than to make the user adopt himself or herself to the machine. If the user in likely to communicate directly with the machine element, it should be designed with an ergonomic considerations.

Aesthetics :

Aesthetics deals with the appearance of the product. In a present days of buyer's market, with a number of products available in the market are having most of the parameters identical, the appearance of the product is often a major factor in attracting the customer. This is particularly true for consumer durables like : automobiles, domestic, refrigerators, television sets, music systems, etc. 9.

Manufacturing :

In a design of machine element, the selection of manufacturing processes must be given a due importance. The manufacturing processes should be selected such that the machine element can be produced with minimum manufacturing cost and, as far as possible, with existing manufacturing facilities. 10.

Conformance to Standards :

A design of machine element should conform to the national and / or international standards and codes. 11.

Assembly :

A machine element or a product should be designed such that it facilitates to minimize the assembly cost and time. 12.

Friction and Wear :

Friction and wear are major contributing factors for reducing the life of machine elements and increasing the power loss. The friction can be reduced by improving the surface finish, adequately lubricating the surfaces, and replacing the sliding motion by rolling motion. The wear can be reduced by increasing the surface hardness. 13.

Life :

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A machine element should be designed for an adequate life. 14.

Vibrations :

A machine element should be designed so as to keep the vibrations at minimum level. 15.

Thermal Considerations :

A machine element should be able to withstand the temperature to which it may be subjected. In addition, it should dissipate the heat generated, if any. 16.

Lubrication :

In a design of machine elements, due consideration must be given for the lubrication of the elements, if there is relative sliding or rolling motion between the elements. 17.

Maintenance :

A machine element should be such that it can be easily repaired or serviced. 18.

Flexibility :

A machine element should be flexible so that the modifications can be carried out with minimum efforts. 19.

Size and Shape :

As far as possible, standard sizes and shaped should be adopted for machine element. 20.

Stiffness :

Whenever a stiffness is a functional requirement like in springs, a machine element should be designed with a precise value of required stiffness. 21.

Corrosion :

A machine element should be a corrosion resistance. This can be achieved by a proper selection of material and adapting the surface coating. 22.

Noise :

A machine element should be designed such that the noise during operation is at minimum possible level. 1.5

AESTHETIC CONSIDERATIONS IN DESIGN

•

Each product is to be designed to perform a specific function or a set of functions to the satisfaction of customers.

•

The parameters that are normally considered by the customer while selecting the product are : Functional Performance Durability Initial and Running Costs Ability to Withstand Adverse Conditions Service Support Available

1. 2. 3. 4. 5.

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Machine Design & Design Considerations

6. Comfort to User 7. Appearance •

In a present days of buyer's market, with a number of products available in the market are having most of the parameters identical, the appearance of product is often a major factor in attracting the customer.

•

This is particularly true for consumer durables like : automobiles, domestic refrigerators, television sets, music systems, etc.

•

Aesthetics is defined as a set of principles of appreciation of beauty. It deals with the appearance of the product. Appearance is an outward expression of quality of the product and is the first communication of the product with the user. At any stage in the product life, the aesthetic quality cannot be separated from the product quality. The growing importance of the aesthetic considerations in product design has given rise to a separate discipline, known as ‘industrial design’. The job of an industrial designer is to create new shapes and forms for the product which are aesthetically appealing.

• • •

1.5.1 Guidelines in Aesthetic Design : •

For any product, there exists a relationship between the functional requirement and the appearance of a product.

•

The aesthetic quality contributes to the performance of the product, though the extent of contribution varies from the product to product.

•

For example, the chromium plating of the automobile components improves the corrosion resistance along with the appearance. Similarly, the aerodynamic shape of the car improves the performance as well as gives the pleasing appearance.

•

The following guidelines may be used in aesthetic design (design for appearance) : 1.

The appearance should contribute to the performance of the product. For example, the aerodynamic shape of the car will have a lesser air resistance, resulting in the a lesser fuel consumption.

2.

The appearance should reflect the function of the product. For example, the

3. 4. 5.

aerodynamic shape of the car indicates the speed. The appearance should reflect the quality of the product. For example, the robust and heavy appearance of the hydraulic press reflects its strength and rigidity. The appearance should not be at too much of extra cost unless it is a prime requirement. The appearance should be achieved by the effective and economical use of materials.

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Machine Design & Design Considerations

The appearance should be suitable to the environment in which the product is used.

1.5.2 Aspects of Aesthetic Design : The various aspects of the aesthetic design, which are discussed below, are also related to : functional requirements, ergonomic considerations, manufacturing considerations, assembly considerations and cost, in addition to the aesthetic considerations. These aspects are not very rigid. 1. 2. 3. 4. 5. 6.

Form (Shape) Symmetry and Balance Colour Continuity Variety Proportion

7. 8. 9. 10. 11. 12.

Contrast Impression and Purpose Style Material and Surface Finish Tolerance Noise

1.

Form (Shape) : There are five basic forms of the products, namely, step, taper, shear, streamline and sculpture, as shown in Fig. 1.4. The external shape of any product is based on one or combination of these basic forms.

Fig. 1.4 : Basic Types of Product Forms

(i)

Step form :

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The step form is a stepped structure having vertical accent. It is similar to the shape of a multistorey building. (ii)

Taper form : The taper form consists of tapered blocks or tapered cylinders.

(iii) Shear form : The shear form has a square outlook. (iv) Streamline form : The streamline form has a streamlined shape having a smooth flow as seen in automobile and aeroplane structures. (v)

Sculpture form : The sculpture form consists of ellipsoids, paraboloids and hyperboloids.

2.

Symmetry and Balance : • • • •

Most of the life forms in the nature are approximately symmetrical about at least one axis. The human eye is thus conditioned to see the things in symmetrical form and tends to reject asymmetrical shapes as ugly. Hence in many products, symmetry about at least one axis improves the aesthetic appeal of the product. However, wherever functional requirement demands asymmetry, balance in the product improves the aesthetic feeling.

Fig. 1.5 : Arrangements of Control Panel

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Fig. 1.5 shows three arrangements of a control panel : (i)

Arrangement (a) : It is symmetrical but is ergonomically poor, as control knobs are placed on either side of the panel.

(ii)

Arrangement (b) : It is ergonomically good but looks unbalanced because bulk of the display ‘mass’ is towards the right of the panel, and hence aesthetically poor.

(iii)

Arrangement (c) : It is ergonomically good as well as aesthetically pleasing because of the sense of balance of mass about the central axis.

Colour : • •

Colour is one of the major contributors to the aesthetic appeal of the product. Many colours are linked with different moods and conditions. The selection of the colour should be compatible with the conventions. Morgan has suggested the colour code given in Table 1.1. Table 1.1 : Morgan Colour Code

Colour

4.

•

Red

Meaning Danger, Hot

•

Orange

Possible’ Danger

•

Yellow

Caution

•

Green

Safe

•

Blue

Cold

•

Grey

Dull

Continuity : • •

A product which has good continuity of elements is aesthetically appealing. For example, a fillet radius at the change of cross section adds the continuity to the product, and hence improves the appearance, as shown in Fig. 1.6.

(a) Poor Appearance

Fig. 1.6

(b) Better Appearance

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Variety : •

Variety is particularly useful while marketing the range of products. The variety releives the user of the boredom.

•

For example, in a consumer appliances, the functionally identical products are manufactured in a number of varieties by a single manufacturer.

Proportion : •

Proportion is concerned with the relationship, in size, between connected items or elements of items.

(a) Poor Appearance

(b) Better Appearance Fig. 1.7 : Spanner

•

The product which is out of proportion, is not aesthetically pleasing.

•

Normally, the proportions of the product are developed from the sound functional requirements, but can sometimes override the functional aspect.

•

The spanner, shown in Fig. 1.7(a), satisfies the functional requirement and is also easy to manufacture. But it is out of proportion, and hence poor in appearance.

• 7.

8.

The spanner shown in Fig. 1.7(b) is in proportion and aesthetically pleasing.

Contrast : •

Contrast is a distinction between the adjacent elements of the product which have clearly different characteristics and functions.

•

The contrast improves the aesthetic appeal of the product.

Impression and Purpose : •

The product not only should look nice but also should look as if it will work.

•

The product should give the impression of the satisfactory performance or purpose.

•

The taper shape gives the impression of strength and stability as shown in Figs.1.8 and 1.9 respectively.

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(a) Impression of Weakness

Machine Design & Design Considerations

(b) Impression of Strength Fig. 1.8

(a) Impression of Unstability

(b) Impression of Stability Fig. 1.9

• 9.

Style : • •

10.

Similarly, the streamline shape gives the impression of speed. Style is a visual quality of the product which sets it apart from the rest of the functionally identical products. Good style will skillfully reflect a current public mood, which may be influenced by the technological developments, or by a prevailing social or environmental climate.

Material and Surface Finish : • • • • •

The material and surface finish of the product contribute significantly to the appearance. The material like, stainless steel gives better appearance than the cast irons, plain carbon steels or low alloy steels. The brass or bronze give richness to the appearance of the product. The products with better surface finish are always aesthetically pleasing. The surface coating processes like : spray painting, anodizing, electroplating, etc. greatly enhances the aesthetic appeal of the product.

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Machine Design & Design Considerations

Tolerance : • •

12.

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Proper tolerancing of the mating parts improve the aesthetic appeal of the product. Unwanted clearance or interference hampers the aesthetic appeal.

Noise :

Unwanted noise is disturbing and is suggestive of some malfunction within the product, and hence it greatly reduces the aesthetic appeal. 1.6

ERGONOMIC CONSIDERATIONS IN DESIGN

•

In a machine design, the machine is considered as an entity in itself. However, in reality, the man (operator), machine and working environment form the system and this system needs to be considered as a single unit.

•

Ergonomics is defined as the scientific study of the man-machine-working environment relationship and the application of anatomical, physiological and psychological principles to solve the problems arising from this relationship.

•

The word ‘ergonomics’ is formed from two Greek words : ‘ergon’ (work) and ‘nomos’ (natural laws).

•

The final objective of the ergonomics is to make the machine fit for user rather than to make the user adapt himself or herself to the machine. It aims at decreasing the physical and mental stresses to the user.

1.6.1 Areas Covered Under Ergonomics : The different area covered under the ergonomics are : 1. 2. 3. 4.

Communication Between Man (User) and Machine; Working Environment; Human Anatomy and Posture While Using the Machine; and Energy Expenditure in Hand and Foot Operations.

1.7

COMMUNICATION BETWEEN MAN (USER) AND MACHINE

•

Fig. 1.10 shows the man-machine closed loop system. The machine has a display unit and a control unit.

•

A man (user) receives the information from the machine display through the sense organs.

•

He (or she) then takes the corrective action on the machine controls using the hands or feet.

•

This man-machine closed loop system in influenced by the working environmental

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Machine Design & Design Considerations

factors such as : lighting, noise, temperature, humidity, air circulation, etc.

Fig. 1.10 : Man-Machine Closed Loop System

•

The communication system between the man (user) and the machine consists of the displays and the controls.

•

The man-machine system has two important units : 1.Displays 2.Controls.

•

The considerations in the design of the displays and the controls are discussed below :

1.7.1 Design of Displays : Displays are the devices through which the man (user) receives the information from the machine. A good display device is one which allows the proper combination of speed, accuracy and sensitivity of display. The display devices can be broadly classified into two categories : 1. 2. 1.

Qualitative Displays Quantitative Displays. Qualitative Displays : • • •

The displays which indicate only the condition or state without giving the values are known as qualitative displays. The examples of the qualitative displays are : traffic signals and on-off indicators. The qualitative displays are of following types :

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(i) Circular dial [Fig. 1.11(a)] ; (ii) Straight legend [Fig. 1.11(b)] ; or (iii) Coloured lights [Fig. 1.12].

(a) Circular Dial (b) Straight Legend Fig. 1.11 : Qualitative Display by Pointer

Fig. 1.12 : Qualitative Display by Coloured Lights

• 2.

Quantitative Displays : • • •

(i)

The qualitative display by a light can be made more effective by the use of flashing light, sometimes accompanied by the auditory warning. The displays which give the quantitative measurements or numerical information are known as quantitative displays. The examples of the quantitative displays are : voltmeters, ammeters, speedometers, energy meters, watches, etc. The quantitative displays are of the following types : (i) Moving pointer - fixed scale type displays [Fig. 1.13]; (ii) Fixed pointer - moving scale type displays [Fig. 1.14]; and (iii) Digital displays [Fig. 1.15].

Moving pointer - fixed scale type displays :

The moving pointer - fixed scale type displays [Fig. 1.13] are easy to read than the fixed pointer - moving scale type displays [Fig. 1.14], and hence they are more common in use.

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(a) Circular Scale

(ii)

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Machine Design & Design Considerations

(b) Semi-Circular Scale (c) Horizontal Scale Fig. 1.13 : Moving Pointer-Fixed Scale Type Displays

(d) Vertical Scale

Fixed pointer - moving scale type displays :

Whenever the values are to be read over the wide range, the fixed pointer-open window type displays [Fig. 1.14(b) and (c)] are more efficient than the moving pointer-fixed scale type displays.

(a) Circular Scale

(b) Open Window with (c) Open Window with Horizontal Scale Vertical Scale Fig. 1.14 : Fixed Pointer-Moving Scale Type Displays

(iii) Digital displays : The digital display [Fig. 1.15] is most accurate of all the displays.

Fig. 1.15 : Digital Display

1.7.2 Ergonomic Considerations in Design of Displays : The basic objective in the design of the displays is to minimize the fatigue to the user. The ergonomic considerations in the design of the displays are as follows : 1.

The scale should be clear and legible.

2.

The size of the numbers or letters on the scale should be taken such that, Height of the number or letter ≥

Machine Design – I (MU)

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6. 7. 8. 9.

10.

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The scale should be divided in a linear progression such as 0 – 10 – 20 – 30… and not as 0 – 5 – 25 – 45….. The number of subdivisions between the numbered divisions should be as minimum as possible. The vertical numbers should be used for the moving pointer type displays with circular scales as shown in Figs. 1.13(a) and 1.13(b), while the radially oriented numbers should be used for the fixed pointer type displays with circular scales as shown in Fig. 1.14(a). The vertical numbers should be used for the vertical and horizontal scales as, shown in Figs. 1.13(c), 1.13(d), 1.14(b) and 1.14(c). The numbering should be in clockwise direction on a circular scale, from left to right on a horizontal scale and from bottom to top on a vertical scale. The pointer should have a knife-edge with a mirror in a dial to minimize the parallax error while taking the readings. When a display and its associated control are to be placed near each other, the control device should be placed either below or to the right of the display, as shown in Fig. 1.16, so that the user's hand, operating the control is less likely to interfere while reading the display. Whenever straight scales are to be used, the horizontal scales are preferred over the vertical scales because the vertical scales are more prone to the reading errors.

(a) Control Device Below Display (b) Control Device to the Right of Display Fig. 1.16 : Arrangements for Easy Reading of Display

1.7.3 Design of Controls : • •

Controls are the devices through which the man (user) conveys his instructions to the machine.

Selection of control devices : The type and size of the control device selected for a given application depends upon the following factors :

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Machine Design & Design Considerations

The required speed of operation; The required accuracy of the control; The required operating force; The required range of the control; The required direction of the control; and The convenience of the user.

•

Types of controls : The various types of controls used in machines are : Crank, hand-wheel, star-wheel hand-lever, foot pedal, knob, push-button, toggle switch, joystick, etc. (Refer Fig. 1.17).

Fig. 1.17 : Types of controls

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1.7.4 Ergonomic Considerations in Design of Controls : The ergonomic considerations in the design of the controls are as follows : The control devices should be logically positioned and easily accessible. The control operation should involve minimum and smooth moments. The control operation should consume minimum energy. The portion of the control device which comes in contact with user's hand should be in conformity with the anatomy of human hands. The proper colours should be used for control devices and backgrounds so as to give the required psychological effect. The shape and size of the control device should be such that the user is encouraged to handle it in such a way as to exert the required force, but not excessive force, damaging the control or the machine.

1. 2. 3. 4. 5. 6.

1.8

WORKING ENVIRONMENT

The working environment affect significantly the man-machine relationship. It affects the efficiency and possibly the health of the operator. The major working environmental factors are : 1. 2. 3. 4.

Lighting, Noise, Temperature, Humidity and Air Circulation.

1.

Lighting : • • • • • • •

The amount of light that is required to enable a task to be performed effectively depends upon the nature of the task, the cycle time, the reflective characteristics of the equipment involved and the vision of the operator. Codes of practice are available that recommend the amount of light necessary for a certain task. The intensity of light in the surrounding area should be less than that at the task area. This makes the task area the focus of attention. Operators will become less tired if the lighting and colour schemes are arranged so that there is a gradual change in brightness and colour from the task area to the surroundings. The task area should be located such that the operator can occasionally relax by looking away from the task area towards a distinct object or surface. The distinct object or surface should not be so bright that the operator's eyes take time to adjust to the change when he or she again looks at the task. Glare often causes discomfort and also reduces visibility, and hence it should be minimised or if possible eliminated by careful design of the lighting sources and their positions.

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2.

• • • • •

•

• •

•

• •

For an operator to perform the task efficiently, he should neither feel hot nor cold. When the heavy work is done, the temperature should be relatively lower and when the light work is done, the temperature should be relatively higher. The optimum required temperature is decided by the nature of the work. The deviation of the temperature from the optimum required reduces the efficiency of the operator.

Humidity and air circulation : •

1.9

The noise at the work place cause annoyance, damage to hearing and reduction of work efficiency. The high pitched noises are more annoying than the low pitched noises. Noise caused by equipment that a person is using is less annoying than that caused by the equipment, being used by another person, because the person has the option of stopping the noise caused by his own equipment, at least intermittently. The industrial safety rules specify the acceptable noise levels for different work places. If the noise level is too high, it should be reduced at the source by maintenance, by the use of silencers and by placing vibrating equipment on isolating mounts. Further protection can be obtained by placing the sound-insulating walls around the equipment. If required, ear plugs should be provided to the operators to reduce the effect of noise.

Temperature : • •

4.

Machine Design & Design Considerations

Noise : •

3.

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Humidity has little effect on the efficiency of the operator at ordinary temperatures. However, at high temperatures, it affects significantly the efficiency of the operator. At high temperatures, the low humidity may cause discomfort due to drying of throat and nose and high humidity may cause discomfort due to sensation of stuffiness and over sweating in a ill-ventilated or crowded room. The proper air circulation is necessary to minimize the effect of high temperature and humidity. DESIGN FOR MANUFACTURE (DFM)

One of the aspects of the concurrent engineering is integrating the design and manufacturing in the product design stage. This is called Design For Manufacture (DFM). The major objective of DFM is to ensure that the product and the manufacturing processes are designed together. This optimizes the manufacturing phase of the product life cycle, which results in improving product quality as well as reducing the product cost.

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1.9.1 Guidelines To Be Followed In Design For Manufacture (DFM) : The general guidelines to be followed in design for manufacture are discussed below : 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1.

Minimize total number of parts in a product :

•

A product cost is related to the number of parts in a product. Reducing the number of parts in a product normally reduces the cost of the product.

•

In addition, it also increases the reliability of the product.

•

A part can be eliminated if, there is no need for relative motion between parts, no need for adjustment between the parts, and no need for materials to be different.

•

The number of parts can be reduced by :

•

2.

Minimize total number of parts in a product, Minimize variety of parts, Use standard parts, Use modular design, Design parts to be multifunctional, Design parts for multiple use, Select least costly material, Design parts for ease of manufacture, Shape the parts for minimizing the operations, Design for general purpose tooling.

(i)

Combining two or more parts into an integral design;

(ii)

Use of snap fits to replace fasteners;

(iii)

Use of press fits to reduce the number of fasteners; and

(iv)

Including labels in the mold and/or combining information from the labels into one label.

It is important to note that, sometimes the reduction of too many parts may increase the cost of the product because the remaining parts may become too heavy or complex. Sometimes, it may make the disassembly also difficult.

Minimize variety of parts :

Minimizing the variety of parts reduces the manufacturing cost, improves the quality of the parts and minimizes the inventory requirement. 3.

Use standard parts :

The standard (off the shelf) parts are always less expensive than the custom-made parts. Therefore, as far as possible, standard parts should be used in a product.

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Machine Design & Design Considerations

Use modular design :

•

A module is a self-contained component with a standard interface with other components in the product. • Product consisting of 4 to 8 modules with 4 to 10 parts per module are preferred for automatic assembly. • Advantages of modular design : The advantages of the modular design are as follows : (i) It customizes the product by using different combinations of standard modules. (ii) It is relatively resistant to obsolescence, since a new generation product can utilize most of the old modules. (iii) It results in easier service and repair because the defective module can be replaced by a new one. (iv) It simplifies final assembly because there are fewer parts to assemble. • Disadvantage of modular design : The major disadvantage of the modular design may be cost, because extra fittings are required. 5.

Design parts to be multifunctional :

• • 6.

Design parts for multiple use :

• • 7.

The parts should be designed such that they can be used in more than one product. For example, the same shaft and gear can be used in different products. The multiple use parts ultimately reduce the product cost.

Select least costly material :

• • 8.

In order to minimize the number of parts, the parts should be designed to fulfill more than one function. For example, a part can be designed to serve as a structural member as well as a spring.

In many products, 50 to 60 percent of the total product cost is attributed to the materials. The least costly material which satisfies the functional requirements should be selected.

Design parts for ease of manufacture :

• •

The manufacturing process should be selected such that the minimum number of economical operations are required to give the part a final shape. Finishing operations such as grinding, lapping, honing, etc. should be avoided wherever possible.

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9.

10.

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Machine Design & Design Considerations

Shape the parts for minimizing the operations :

•

The parts should be shaped such that, they can produced with minimum number of operations.

•

For example, holes should be spaced in the parts such that they can be made in one operation.

Design for general purpose tooling :

•

Whenever possible, parts should be designed to use general purpose tooling rather than special purpose tooling.

•

An exception to this is a high volume production, where special purpose tooling may be cost effective.

1.10 DESIGN CONSIDERATIONS FOR CASTING •

One of the shortest routes from raw material to finished part is casting.

•

In casting, a molten metal is poured into a mould which approaches the shape of the part. Heat is extracted through the mould and the molten metal solidifies into the shape.

•

The poor shape of the casting can adversely affect its strength more than the composition of the material.

1.10.1 Guidelines to be Followed in Design of Castings : The general guidelines to be followed in the design of the castings are discussed below :

1.

1.

Design parts to be in compression than in tension,

2.

Strengthen parts under tension by use of external devices,

3.

Shape the casting for orderly solidification,

4.

Avoid abrupt change in cross-section,

5.

Provide more thickness at the boss,

6.

Round off the corners,

7.

Avoid concentration of metal at junctions,

8.

Avoid thin sections,

9.

Make provision for easy removal of pattern from the mould.

Design parts to be in compression than in tension :

•

The cast iron is much stronger in compression than in tension. Hence, design the parts such that stressed areas of the parts are under compression, as shown in Fig. 1.18(a), rather than under tension, as shown in Fig. 1.18(b).

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(a) Good (Part Under Compression) Fig. 1.18 2.

Machine Design & Design Considerations

(b) Poor (Part Under Tension)

Strengthen parts under tension by use of external devices :

When the parts are to be subjected to the tensile stress, these are strengthened by the use of external devices like tie rods, as shown in Fig. 1.19.

Fig. 1.19 3.

4.

Shape the casting for orderly solidification :

•

The main consideration in the design of the castings is that, the shape of the castings should allow for orderly solidification.

•

The solidification should progress from the remotest area towards the area where molten metal is fed in.

Avoid abrupt change in cross-section :

•

Wherever possible, the section thickness should be uniform throughout.

•

If the thickness is to be different at two sections, the change should be gradual as shown in Fig. 1.20(b) and not abrupt, as shown in Fig. 1.20(a).

•

The abrupt change in cross-section results in heavy stress concentration.

•

The ratio of the thickness of adjoining sections should not exceed 2.

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(a) Poor

Machine Design & Design Considerations

(b) Good Fig. 1.20

5.

Provide more thickness at the boss :

The thickness of the boss should be more than the thickness of the pad and the transition should be gradual as illustrated in Fig. 1.21

(a) Poor

(b) Good Fig. 1.21

6.

Round off the corners :

• •

All the corners should be rounded as illustrated in Figs. 1.22 and 1.23. It improves the endurance strength of the part and reduces the formation of brittle chilled edges.

(a) Poor

(b) Good

(a) Poor

Fig. 1.22 7.

(b) Good Fig. 1.23

Avoid concentration of metal at junctions :

•

Avoid the concentration of metal at the junctions. Whenever there is concentration of metal, the metal on the surface solidifies first whereas the central portion solidifies much later.

•

This produces shrinkage cavity in the central portion which reduces the strength of the part.

•

This can be avoided by two ways, as illustrated in Fig. 1.24 : (i)

by providing core hole in the centre [Fig. 1.24(b)], or

(ii)

by offsetting the ribs [Fig. 1.24(c)].

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(a) Poor 8.

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Machine Design & Design Considerations

(b) Good (Core Hole) Fig. 1.24

(c) Good (Offset Ribs)

Avoid thin sections :

Avoid very thin sections. The minimum permissible thickness of the castings depends upon the casting process. 9.

Make provision for easy removal of pattern from the mould :

• • 1.10.2

Casting should be designed so that the pattern can be removed from the mould and the casting from the permanent mould without difficulty. A minimum draft or taper of 3° should be provided on the vertical surfaces so that the pattern can be removed from the mould. Factors Affecting Selection of Casting Process :

There are number of casting processes. The selection of the proper casting process depends upon the following factors : (i) Complexity of the shape of part ; (a) external and internal shape, (b) minimum wall thickness, (ii) Required quantity of parts ; (iii)

Cost of the pattern or die ;

(iv)

Required tolerances ;

(v)

Required surface finish ;

(vi)

Strength ;

(vii) Weight ; (viii) Required overall quality. 1.11 DESIGN CONSIDERATIONS FOR FORGING •

Forging is a deformation process in which a solid metal is forced under pressure to undergo extensive plastic deformation into finished or near-to-finished shape.

•

It is normally carried out on a hot workpiece.

•

The forging brings the metallurgical changes in metal. It produces a fibre structure.

•

Forging processes are used for producing parts for high performance applications.

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•

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Machine Design & Design Considerations

Advantages of Forgings : The forging offers the following advantages :

(i)

The fibre lines of the forged parts can be arranged in the required direction. Hence it improves the strength and the toughness in the required direction.

(a) Cast

(b) Machined Fig. 1.25 : Crank Shaft

(c) Forged

Fig. 1.25 shows the crank shafts manufactured by casting, machining and forging. There are no fibre lines in cast parts. In machined parts, the original fibre lines of rolled stock are broken. In forged parts, the fibre lines are arranged in the required direction to withstand the external load. (ii)

The forging reduces the grain size, which results in improving the strength and toughness of the parts.

(iii)

The forging produces the parts without shrinkage cavities, blow holes and machining scratches, which increases the endurance strength of the parts.

(iv)

The forging can produce the parts with thin section and that too without reducing the strength. This results in lightweight construction.

(v)

The forging can produce the parts with close tolerances. This reduces the material removal during the finishing processes.

1.11.1

Guidelines to be Followed in Design of Forgings :

The general guidelines to be followed in the design of the forgings are discussed below : 1. 2. 3. 4. 5. 6. 1.

Keep fibre lines parallel to tensile and compressive forces and perpendicular to shear forces, Avoid deep machining cuts, Keep vertical surfaces of forged parts tapered, Keep the parting line in one plane, Provide adequate fillet and corner radii, Avoid thin sections.

Keep fibre lines parallel to tensile and compressive forces and perpendicular to shear forces :

The forged parts should be designed such that the fibre lines are parallel to tensile and compressive forces and perpendicular to shear forces. This improves the strength and toughness of the parts.

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2.

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Machine Design & Design Considerations

Avoid deep machining cuts :

Whenever possible, the deep machining cuts into forged parts should be avoided. The machining cuts break the fibre lines, making the parts weak. 3.

Keep vertical surfaces of forged parts tapered :

• • 4.

The vertical surfaces of a forged parts must be tapered to permit the removal of forging from the die cavity. The draft angle of 5° to 7° is provided on the external surfaces and 7 ° to 10° is provided on the internal surfaces.

Keep the parting line in one plane :

There are two important terms used in forgings : the parting line (PL) and the forging plane (FP).

Fig. 1.26

•

The parting line (PL) is the plane where two die halves meet and the forging plane (FP) is the plane perpendicular to the die motion [Fig. 1.26].

•

In the design of the forged parts, wherever possible, the parting line should be in one plane, as shown in Fig. 1.27. This minimizes the forging cost.

•

When the parting line is not in one plane, as shown in Fig. 1.28, the unbalanced forging forces tend to displace the two die halves. Such forces can be balanced by providing a counter lock or by forging the parts simultaneously in a mirror-image position.

Fig. 1.27 5.

Fig. 1.28

Provide adequate fillet and corner radii :

•

The forged parts should be provided with adequate fillet and corner radii.

•

The sharp corners on the parts require excessive forging force, and hence also reduce the die life.

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Machine Design & Design Considerations

Avoid thin sections :

•

The forged parts should not be thin. The thin sections require excessive forging force, and hence also reduce the die life. Removal of such parts from the die cavity is also difficult.

•

For steel forgings, the minimum permissible thickness is 3 mm.

1.12 DESIGN CONSIDERATIONS FOR MACHINING •

Machining processes are the most versatile and most common manufacturing processes. Almost all parts are subjected to some kind of machining process.

•

The machining processes are broadly classified into two categories : 1. 2.

1.

Metal-cutting processes Surface-finishing processes

Metal-cutting processes :

The metal-cutting processes are : shaping, milling, turning, boring, drilling, reaming, broaching, slotting, hobbing, etc. 2.

Surface finishing processes :

The surface-finishing processes are : grinding, honing, lapping, buffing, polishing, etc. • The machining cost forms the significant portion of the total cost of the part. The total cost of the part can be reduced by optimizing the machining processes. 1.12.1

Guidelines to be Followed in Design for Machining :

The general guidelines to be followed in designing the parts for machining are discussed below : 1. 2. 3. 4. 5. 6. 7. 8. 9.

Machine only functional surface, Select widest tolerances and roughest surface finish that fulfills functional requirement, Use minimum number of machines, As far as possible design for existing machining facilities, Machining should be completed in minimum machining positions, Part should be rigid, Use holes parallel or perpendicular to the axis of the part, Use standard size tooling, Use ends of blind holes conical,

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Machine Design & Design Considerations

10. Avoid continuation of threading up to the bottom of the hole, 11. Avoid intersection of finished surfaces to form internal corners, •

For Parts With Rotational Symmetry :

12. Use concentric cylindrical surfaces, 13. Avoid internal features in long parts, 14. Avoid parts with very large or very small L/D ratios, 15. For internal corners on part specify the radii equal to the radius of the tool tip, •

For Parts With Non-Rotational Symmetry :

16. Wherever possible, all machined surfaces should be perpendicular or parallel to each other as well as to the base, 17. Avoid cylindrical bores in long parts, 18. Avoid extremely long and thin parts, 19. Wherever possible restrict plain surface machining processes like : slotting, grooving, etc., to one surface of the part. 1.

Machine only functional surface :

In a part, as far as possible, the surface should be machined only when it is needed for functioning of the part. The machining area should be kept as minimum as possible, as illustrated in Fig. 1.29.

(a) Poor

(b) Good Fig. 1.29

2.

Select widest tolerances and roughest surface finish that fulfills functional requirement :

Select the widest tolerances and the roughest surface that will give the acceptable performance for operating surfaces. This will reduce the machining cost. Fig. 1.30 shows the relative increase in cost associated with closer tolerances and better surface finish.

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Machine Design & Design Considerations

Fig. 1.30 : Effect of Tolerances and Surface Finish on Relative Cost Increase 3.

Use minimum number of machines :

The part should be designed such that it can be machined using minimum number of machines. 4.

As far as possible, design for existing machining facilities :

Whenever possible, avoid the machine processes that the company shop is not equipped to carry out. In an era of increasing automation with high capital cost, the product should be designed to fit the existing factory. 5.

Machining should be completed in minimum machining positions :

Whenever possible, the part should be designed such that all the machining can be done in one position. If the position needs to be changed, one of the already machined surface should be used as reference surface. 6.

Part should be rigid :

The part should be designed such that it is sufficiently rigid to withstand the machining forces. 7.

Use holes parallel or perpendicular to the axis of the part :

As far as possible, the auxiliary holes should be parallel or perpendicular to the axis of the part as illustrated in Fig. 1.31.

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(a) Poor

Machine Design & Design Considerations

(b) Good Fig. 1.31

8.

Use standard size tooling :

In a part, as far as possible, the hole dimensions (i.e. diameter and length) should be selected such that they can be machined with standard drills or boring bars. 9.

Use ends of blind holes conical :

Wherever possible, the ends of the blind holes should be conical as illustrated in Fig. 1.32.

(a) Poor

(b) Good Fig. 1.32

10

Avoid continuation of threading up to the bottom of the hole :

In a threaded blind holes, the threads should not continue up to the bottom of the hole [Fig. 1.32]. 11.

Avoid intersection of finished surfaces to from internal corners :

Ensure that the surfaces to be finished are raised and never intersect to form internal corners. [Fig. 1.33].

(a) Poor

(b) Good Fig. 1.33

•

For Parts With Rotational Symmetry :

12

Use concentric cylindrical surfaces :

As far as possible, the cylindrical surfaces should be concentric and plane surfaces should be normal to the axis of the part [Fig. 1.34].

(a) Poor

(b) Good Fig. 1.34

13.

Avoid internal features in long parts :

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Machine Design & Design Considerations

14.

Avoid parts with very large or very small L/D ratios :

15.

For internal corners on part, specify the radii equal to the radius of the tool tip :

•

For Parts With Non-Rotational Symmetry :

16.

Wherever possible, all machined surfaces should be perpendicular or parallel to each other as well as to the base :

17.

Avoid cylindrical bores in long parts :

18.

Avoid extremely long and thin parts :

19.

Wherever possible, restrict plain surface machining processes like : slotting, grooving, etc. to one surface of the part :

1.13 DESIGN FOR ASSEMBLY (DFA) •

Another important aspect of the concurrent engineering is integrating the design and assembly in the product design stage. This is called Design For Assembly (DFA). Minimizing the cost of the assembly is one of the main design functions.

1.13.1 Guidelines to be Followed in Design for Assembly : The general guidelines to be followed in design for assembly are discussed below : 1. 2. 3. 4. 5. 6. 7. 1.

Minimize total number of parts in product, Use symmetrical parts in product, Exaggerate asymmetry, if functional requirement demands, Use slotted holes to accommodate variations in parts, Minimize assembly direction, Maximize assembly compliance features in parts, Design the parts for the method of assembly.

Minimize total number of parts in product :

Reducing the number of parts in a product simplifies the assembly. Therefore it reduces the assembly time as well as the cost of the assembly. 2.

Use symmetrical parts in product :

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Machine Design & Design Considerations

(a) Asymmetry Part

(b) Symmetrical Part Fig. 1.35

• • • 3.

Symmetrical parts require less handling and hence also reduce the assembly time, especially in manual assembly. Symmetry is advantageous particularly when the product is to be assembled in the field. Fig. 1.35(a) shows an asymmetrical cover plate with only one correct assembly orientation. The redesigned symmetrical cover plate, shown in Fig. 1.35(b), has four correct assembly orientations.

Exaggerate asymmetry, if functional requirement demands :

• •

Some parts will function only if assembled with a particular orientation. In such cases, it is necessary to design the parts with asymmetry. If the asymmetry is difficult to distinguish as shown in Fig. 1.36(a), the assembler may try to force the parts together with the wrong orientation. Exaggerated asymmetry, as shown in Fig. 1.36(b), ensures the correct orientation for assembly.

(a) Asymmetry

(b) Exaggerated Asymmetry Fig. 1.36

• 4.

Use slotted holes to accommodate variations in parts :

• • 5.

When incorrect assembly would result in a safely hazard, the asymmetry should be exaggered such that incorrect assembly should be virtually impossible. Slotted holes and similar features can be used to accommodate variations in parts. For example, a motor base plate or cover plates may have slotted holes to allow adjustments.

Minimize assembly direction :

• •

All parts should be designed so that they can be assembled from one direction. The need to rotate parts in assembly require extra time and motion, and hence require additional fixtures and transfer stations.

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• • 6.

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Machine Design & Design Considerations

This will increase the assembly time as well as the cost of the assembly. The best way of assembly is to assemble in Z-direction.

Maximise assembly compliance features in parts :

Excessive assembly force may be required if the parts are not perfect. The addition of the compliance features, like tapers, chamfers, radii, etc. to the parts reduce the assembly force and hence simplify the assembly. 7.

Design the parts for the method of assembly :

•

The parts can be assembled into products by one of the following methods : (i)

Manual assembly,

(ii)

Mechanically aided manual assembly,

(iii) Special purpose automatic assembly, (iv) Programmable automatic assembly with robots and parts magazines. •

The method of assembly depends upon the volume of production, the number of parts in product, the variety of parts in product and the likelyhood of design changes. The following guidelines are to be followed in the automatic assembly. (a)

In automatic assembly, small parts are fed and oriented by belts, tracks, rotary disks, reciprocating arms and magnetic devices. Therefore, the parts must have sufficient strength and rigidity to withstand feeding forces. Thin, weak and brittle parts should be avoided.

(b)

Minimize the number of finished surfaces that must be protected from damage due to scratching.

(c)

Use flanges or projections to protect finished surfaces from damage due to scratching.

(d)

If possible, design the largest and most rigid part of the assembly to serve as a base or fixture. This eliminates the need for assembly fixture.

1.14 REQUISITES OF DESIGN ENGINEER A design engineer is expected to possess the following qualities : 1. Sound and in depth knowledge of design principles and methods. 2. Adequate knowledge of other areas such as, strength of materials, theory of machines, mechanics, materials and their heat treatment processes, surface coating processes, etc. 3. Adequate knowledge of manufacturing processes, fits, tolerances, and ability to communicate with shop floor people. 4. Ability to read and prepare drawings.

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Machine Design & Design Considerations

5.

Adequate knowledge of CAD tools like finite element analysis, geometric modeling (2D, 3D), and simulations. 6. Ability to work in a team and lead the team. 7. Ability to effectively communicate and sell the ideas to others. 8. Problem solving ability. 9. Innovativeness and greater creativity. 10. Ability to keep the pace with the rapid technological developments. 11. Ability to understand the problems and requirements of end users. 12. Awareness of environmental problems and related laws. In short, the design engineer should be versatile and should be well conversant with all the phases of product life cycle. 1.15 STANDARDS AND CODES IN DESIGN •

Standard is a set of specifications, defined by a certain body or an organization, to which various characteristics of a component, a system, or a product should conform. The characteristics include : dimensions, shapes, tolerances, surface finish, materials, method of testing, method of use, method of packing and storing, etc.

•

The purpose of standardization is to establish the norms intended to achieve uniformity, specified quality, interchangeability, safety, and to put reasonable limit on the variety.

•

Code is a set of specifications or procedure for the design, analysis, testing, and manufacturing of a component, a system, or a product.

The different standards used in the mechanical engineering applications are : 1.

Standards for sizes and shapes of components like, nuts and bolts, bearings, keys, belts, chains, gears, etc.

2.

Standards for products like electric motors, engines, gear boxes, pressure vessels, etc.

3.

Standards for fits, tolerances, and surface finish of components.

4.

Standards for conventional representation of components on the drawing. Based on the defining body or organization, the standards can be divided into three catagories. 1.

Company Standards :

These standards are defined or set by a company or a group of companies for their use. 2.

National Standards :

These standards are defined or set by a national apex body and are normally followed throughout the country. The examples are standards prepared by : (i) Bureau of Indian Standards (BIS) (ii) American Society of Mechanical Engineers (ASME)

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Machine Design & Design Considerations

(iii) (iv) (v)

American Gear Manufacturers Association (AGMA) American Welding Society (AWS) American National Standards Institute (ANSI).

3.

International Standards :

These standards are defined or set by an international apex body and are normally followed all over the world. The examples are standards prepared by : (i) International Standards Organization (ISO) (ii) International Bureau of Weights and Measures (IBWM). Some of the standards are advisory in nature and are used as guidelines, whereas others, especially those set by the national or international organizations, are obligatory and sometimes enforced by law. Some of the advantages of standardization are as follows : (i)

Interchangeability of the components is possible.

(ii)

It reduces the inventory of components required.

(iii)

It ensures certain minimum specified quality.

(iv)

Easy and quick replacement of the components is possible.

(v)

Sometimes it ensures the safety.

(vi)

It results in overall cost reduction.

1.16 PREFERRED SERIES •

With the acceptance of standardization, there is a need to keep the standard sizes or dimensions of any component or product in discrete steps.

•

The sizes should be spread over the wide range, at the same time these should be spaced properly. For example, if shaft diameters are to be standardized between 10 mm and 25 mm, then sizes should be like : 10 mm, 12.5 mm, 16 mm, 20 mm, 25 mm and not like : 10 mm, 11 mm, 13 mm, 18 mm, 25 mm. This led to the use of geometric series known as series of preferred numbers or preferred series.

•

Preferred series are series of numbers obtained by geometric progression and rounded off. There are five basic series with a step ratios of : , , , , and . The five basic series of preferred numbers (known as preferred series) are designated as : R5, R10, R20, R40, and R80. These series were first introduced by the French engineer Renard.

•

Each series is established by taking the first number one and multiplying it by a step ratio to get the second number. The second number is then multiplied by a step ratio to get the third number. The procedure is continued until the complete series is built up.

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Machine Design & Design Considerations

•

The example of preferred number series are : standard shaft diameters, power rating of coupling, centre distances of standard gear boxes, etc.

•

Table 1.2 shows the step ratios for basic series and Table 1.3 shows preferred numbers of each basic series. Table 1.2 : Step Ratios Series

Step Ratio

R5

= 1.58

R10

= 1.26

R20

= 1.12

R40

= 1.06

R80

= 1.03

Table 1.3 : Preferred Numbers of Basic Series

R5 1.00

R10 1.00

R20 1.00 1.12

1.25

1.25 1.40

1.60

1.60

1.60 1.80

2.00

2.00 2.24

2.50

2.50

2.50 2.80

3.15

3.15 3.55

4.00

4.00

4.00

R40 1.00 1.06 1.12 1.18 1.25 1.32 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.12 2.24 2.36 2.50 2.65 2.80 3.00 3.15 3.35 3.55 3.75 4.00 4.25

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R5

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Machine Design & Design Considerations

R10

R20 4.50

5.00

5.00 5.60

6.30

6.30

6.30 7.10

8.00

8.00 9.00

10.00

10.00

10.00

R40 4.50 4.75 5.00 5.30 5.60 6.00 6.30 6.70 7.10 7.50 8.00 8.50 9.00 9.50 10.00

If the product is to be manufactured in the minimum number of sizes, R5 series may be used. If the number of sizes required increases, then accordingly R10, R20, R40, or R80 series may be used. In addition to five basic series, some derived series like R10/3, R20/3 are also used sometimes. • Advantages of preferred series : The advantages of preferred series are as follows : 1. The difference in two successive terms has a fixed percentage. 2. Provides small steps for small quantities and large steps for large quantities. It is in conformation with the mode of variation found in nature. 3.

The product range is covered with minimum number of sizes without restricting the choice of the customers.

1.17 SOURCES OF DESIGN DATA During the process of designing the machine elements, systems, or products, the design engineer needs variety of information such as : • Available materials and their properties; • Design procedures as per various national and international standards and codes; • Standard sizes and shapes of components like screws, bolts, nuts, circlips, etc; • Standard sizes and load ratings of standard components like rolling contact bearings, chains, belts, ropes, etc, • Types of fits and tolerances; • Surface finish; etc. It is really a difficult task for design engineers and design office to get latest information or data required during the design process. The various sources of design data are as follows :

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1. 2. 3. 4. 5. 6.

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Machine Design & Design Considerations

Textbooks and reference books : e.g. J.E. Shigley and C.R. Mischke, “Mechanical Engineering Design”, McGrow-Hill Book Company, 1989. Handbooks : e.g. Dudley D.W., “Handbook of Practical Gear Design”, McGraw-Hill International Book Company, 1984. National and International Standards and Codes : e.g. IS 2825-1969 : Code For Unfired Pressure Vessels. Manufacturer's Catalogue : e.g. SKF catalogue of Ball and Roller Bearings. Charts : e.g. Charts of Theoretical Stress Concentration Factors Kt. Technical Journals : e.g. ASME Journals.

1.18 CREATIVITY IN DESIGN •

Creativity is an ability to synthesize new combinations of ideas and concepts into meaningful and useful forms. Creativity is one of the most important quality which a design engineer should possess. Most creative ideas occur by a slow, deliberate process that can be cultivated and enhanced with study and practice.

•

In a creative process, initially the idea is only imperfectly understood. It is followed by a slow process of clarification and exploration as the entire idea takes shape. The creative process can be viewed as moving from an amorphous idea to a well- structured idea, from the unorganized to the organized, from the implicit to the explicit. In a creative thinking, an individual should fill the mind and imagination with the context of the problem and then relax and think of something else.

•

A creative experience normally occurs when the individual is not expecting it and thinking about something else. It is important to note that every individual is born with an inherent measure of creativity, which can be unfolded by persistence and hardwork. Some of the positive steps one can take to enhance one's creative thinking are as follows : 1. 2. 3.

1.

Develop a creative attitude Unlock imagination Be persistent

4. Develop an open mind 5. Suspend judgement at early stage 6. Set problem boundaries

Develop a creative attitude :

To be creative, it is essential to develop confidence that one can provide a creative solution to a problem. Confidence comes with success, so one can start with small problems and build up self-confidence with small success.

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Machine Design & Design Considerations

Unlock imagination :

One must ask questions “WHY?” and “WHAT IF?” to unlock the imagination and sharpen the observation power. 3.

Be persistent :

Creativity requires hardwork. Many problems will not succumb to the first attack but requires persistence. Edison made the famous comment, "invention is 95 percent perspiration and 5 percent inspiration." 4.

Develop an open mind :

One should be receptive to ideas from any and all sources. Even a simple suggestion has a potential to become a solution of the problem and hence suggestions should be encouraged all the time. 5.

Suspend judgement at early stage :

Creative ideas develop slowly and hence, critical judgment on the ideas should be avoided at an early stage. 6.

Set problem boundaries :

Proper definition of problem and its boundaries enhances creative process. In order to achieve a truely creative solution to a problem, a person should utilize two thinking styles : vertical (or convergent) thinking and lateral (or divergent) thinking. 1.19

ROLE OF TOLERANCES AND FITS IN DESIGN, MANUFACTURING AND ASSEMBLY

•

The tolerances and fits are very significant in the process of design, manufacturing and assembly. The proper selection of tolerances and fits on component reduces the cost of the component and improves its performance.

•

The tolerances and fits are discussed in subsequent sections.

1.20

TOLERANCES

•

In a design process, the number of dimensions are assigned to the component or machine element and it is expected that the manufacturing has to be done as per the specified dimensions.

•

However, it is not possible to manufacture a component to the exact dimensions specified by the design engineer.

•

The dimensional variations occur due to the following reasons : (i)

Human errors in setting and operating the machines;

(ii) Errors in machines and measuring instruments; and (iii) Variations in material properties.

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Machine Design & Design Considerations

•

For example, as per the specifications of the design engineer, the shaft diameter is, say, 30 mm. However, due to above stated reasons, it is not possible to machine the shaft to the diameter of 30 mm and it can be machined only in the range 29.95 mm to 30.05 mm.

•

Tolerance can be defined as the permissible variation in the dimensions of the component. It is the difference between the maximum and minimum size limits of the component.

1.20.1 Terminology used in Relation to Tolerances : •

The terminology used in relation to tolerances, shown in Fig. 1.37, is explained below.

•

Basic size :

•

Deviation :

It is the basic dimension specified by the design engineer. It is the algebraic difference between the size and the corresponding basic size. •

Upper deviation :

It is the algebraic difference between the maximum size limit and the corresponding basic size. It is denoted by ‘ES’ for the hole and ‘es’ for the shaft. •

Lower deviation :

It is the algebraic difference between the minimum size limit and the corresponding basic size. It is denoted by ‘EI’ for the hole and ‘ei’ for the shaft.

Fig. 1.37 : Terminology in Relation to Tolerances •

Fundamental deviation :

It is either upper or lower deviation, depending on which is closer to the basic size.

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1.20.2

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Machine Design & Design Considerations

Types of Tolerances :

There are two types of tolerances : 1. 2. 1.

2.

Unilateral Tolerances Bilateral Tolerances Unilateral Tolerances :

•

In unilateral tolerances, one of the limits of tolerance is zero, while the other value takes care of all permissible variation in basic size.

•

For example,

Bilateral Tolerances :

• • 1.20.3

In bilateral tolerances, the variations are given in both directions from the basic size. For example, B.I.S. System of Designation of Tolerances :

•

According to the Bureau of Indian Standards, tolerance is specified by an alphabet (capital or small), followed by a number. For example, H7 or f6.

•

This designation of tolerance consists of two parts : 1.

Fundamental Deviation

2.

Magnitude of Tolerance

Fig. 1.38 : Designation of Tolerance 1.

Fundamental Deviation :

The fundamental deviation gives the location of tolerance zone with respect to zero line. It is indicated by an alphabet : capital letters (A to Z) for hole and small letters (a to z) for shaft. The different alphabets representing fundamental deviations for holes and shafts, is shown in Fig. 1.39.

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Machine Design & Design Considerations

Fig. 1.39 : Alphabets Representing Fundamental Deviations 2.

Magnitude of Tolerance :

The magnitude of tolerance is designated by a number called grades. There are total 18 grades of tolerances, designated as : IT01, IT0, IT1, IT2, IT3, IT4, IT5, IT6, IT7, IT8, IT9, IT10, IT11, IT12, IT13, IT14, IT15, IT16. The actual magnitude of tolerance depends upon the basic size and IT grade. The magnitude of tolerance increases with IT grade. Therefore, lower the grade, closer is the tolerance. Each IT grade has an equation from which the magnitude of tolerance can be calculated. For example, for IT1, the magnitude of tolerance is equal to (0.8 + 0.02D) microns, where D = basic size in mm. For IT5, the magnitude of tolerance is equal to (0.45 D1/3 + 0.001D) microns. Table 1.4 shows the relative magnitudes of tolerances for tolerance grades between IT5 to IT16. Table 1.4 : Relative Magnitudes of Tolerances

Tolerance Grade Relative Magnitude of Tolerance Tolerance

IT5 IT6 IT7 IT8 IT9 IT10 7i

10i

16i

25i

40i

64i

IT11

IT12 IT13 IT14 IT15

IT16

100 i

160i

1000i

250i

400i

640i

–––––→ Increases

Table 1.5 shows the tolerance grades and the suggested manufacturing methods for producing the corresponding tolerance grades. Table 1.5 : Tolerance Grades and Manufacturing Methods

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Tolerance Grade

•

Machine Design & Design Considerations

Suggested Manufacturing Methods

IT16

Sand Casting

IT 15

Forging, Sand Casting

IT14

Die Casting

IT 13

Drilling, Rough Turning

IT12

Drilling, Rough Turning, Rough Shaping

IT11

Drilling, Rough Turning, Rough Shaping

IT 10

Shaping, Milling, Planning, Slotting.

IT 9

Boring, Reaming and Turning on Automatic Lathes

IT 8

Boring, Reaming and Turning on Centre and Turret Lathes

IT 7

Broaching, High Precision Turning, Surface Grinding

IT 6

Grinding, Honing

IT 5

Lapping, Fine Grinding, Fine Honing

IT 4

Lapping

The tolerance for a hole of 50 mm diameter as the basic size, with the fundamental deviation denoted by an alphabet H and the magnitude of tolerance equivalent to grade 7 is designated as 50H7. Similarly, the tolerance for the shaft of 30 mm diameter as the basic size, with the fundamental deviation denoted by an alphabet g and the magnitude of tolerance equivalent to grade 6 is designated as 30g6. The tables are available in standards and data books, which give tolerances for holes and shafts of different diameters for various fundamental deviations and grades. Readers should refer design data book for tolerances on various hole and shaft sizes.

• •

1.21 •

FITS

When two parts (hole and shaft) are assembled, the type of assembly resulting by virtue of the difference between their sizes before assembly is called a fit.

1. 21.1 Types of Fits : Depending upon the relative sizes of the hole and shaft, the fits are broadly classified into three types : 1. 2. 3.

Clearance Fit Transition Fit Interference Fit

Machine Design – I (MU)

(a) Clearance Fit

1-49

(b) Interference Fit

Machine Design & Design Considerations

(c) Transition Fit

Fig. 1.40 : Types of Fits 1.

Clearance Fit :

• • • 2.

Interference Fit :

• • • 3.

It is the fit, which always provides a positive clearance between the hole and the shaft over the entire range of tolerances. In this case, the tolerance zone of the hole is entirely above that of the shaft, as shown in Fig. 1.40(a). The examples of clearance fit are : sliding contact bearings, pin and bush, piston and cylinder, shaft and pulley, etc. It is the fit, which always provides an interference over the entire range of tolerances. In this case, the tolerance zone of the hole is entirely below that of the shaft, as shown in Fig. 1.40(b). The examples of interference fit are : brass bush in gear, bearing bushes, valve seats, etc.

Transition Fit :

• • •

It is the fit, which may provide either a clearance or an interference, depending upon the actual value of individual tolerances of the mating parts. In this case, the tolerance zones of the hole and the shaft overlap, as shown in Fig. 1.40(c). The examples of transition fit are : spigot and recess of rigid coupling, bimetallic gear blanks, key and keyway, etc.

1.21.2 B.I.S. System of Designation of Fits : According to the Bureau of Indian Standards, fit is specified by the basic size common to two mating parts followed by the symbols for tolerance of each part. For example, 50 H7-g8 or 50 H8/g7 1. 21.3 Fit Systems : There are two basic systems of fit : 1. 2. 1.

Hole Basis System Shaft Basis System Hole Basis System :

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(a) Clearance Fit

• • • 2.

Machine Design & Design Considerations

(b) Transition Fit Fig. 1.41 : Hole Basis System

(c) Interference Fit

In hole basis system, as shown in Fig. 1.41, the size of the hole is the basic size, whose lower deviation is zero. The clearances and interferences are obtained by associating various shaft diameters. The examples of hole basis system are : 50H7-g8, 30H7-k6. The hole basis system is advantageous, because, normally holes are machined by standard drills and reams and their dimensions are fixed.

Shaft Basis System :

(a) Clearance Fit

• • • 1.21.4

(b) Transition Fit Fig. 1.42 : Shaft Basis System

(c) Interference Fit

In shaft basis system, as shown in Fig. 1.42, the size of the shaft is the basic size whose upper deviation is zero. The clearances and interferences are obtained by associating various hole diameters. The examples of shaft basis system are : 50G8-h7, 30K8-h7. The hole basis system is useful, where shafts are made from standard bright bars. Selection Of Fits :

The examples of clearance, transition and interference fits used in various applications are given in Table 1.6 as guidelines. Table 1.6 : Selection of Fits

Type of Fit Clearance Fits

Fits

Applications

H7-d8, H8-d9, H8-d10, Loose running fits used for loose pulleys and H11-d11 plummer-block bearings.

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Machine Design & Design Considerations

H6-e8, H7-e8, H8-e8

Easy running fits used for properly lubricated bearings requiring appreciable clearance. Examples are : high speed, heavy duty bearings.

H6-f6, H7-f7, H8-f8

Normal running fits used for grease or oil lubricated bearings having low temperature rise. Examples are : bearings used in gear boxes and small electric motors.

H6-g5, H7-g6, H8-g7

Close running fits used in precision equipments, pistons, slide valves, etc.

H6-h5, H7-h6, H8-h7, Precision sliding fits used for non-running parts. H11-h11 Transition Fits

Interference Fits

H6-j5, H7-j6, H8-j7

Push fits used for very accurate locations with easy assembly/disassembly. Examples are : spigot and recess of the rigid coupling, composite gear blank, etc.

H6-k5, H7-k6, H8-k7

True transition fits used for keyed shafts, non-running lock pins, etc.

H6-p5, H7-p6, H7-p7

Light press fits with easy assembly/ disassembly used for non-ferrous parts.

H6-r5, J7-r6

Medium drive fits with easy disassembly for ferrous parts.

H6-s5, H7-s6, H8-s7

Heavy drive fits for permanent or semi-permanent assembly of ferrous parts.

H6-t5, H7-t6, H8-t7

Force fits on ferrous parts for permanent assembly.

1.22 LIST OF FORMULAE Preferred Series and Step Ratio

Series

Step Ratio

R5

= 1.58

R10

= 1.26

R20

= 1.12

R40

= 1.06

R80

= 1.03

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●

1.

What is machine design ?

2.

Classify the machine design.

3.

Machine Design & Design Considerations

EXERCISE ●

Explain the various phases involved in the process of design of machine elements.

4.

What is synthesis ?

5.

Explain the various considerations influencing the machine component design.

6.

Explain the importance of aesthetic considerations in design.

7.

What is aesthetics in design.

8.

State the guidelines to be followed in aesthetic design.

9.

Discuss the aesthetic considerations in design with respect to:

(i)

Shape

(ii) Colour (iii) Surface finish (iv) System and balance 10.

Distinguish between 'machine design' and 'ergonomic design'.

11.

Explain the term 'ergonomics'.

12.

Which areas are covered under ergonomics ?

13.

Explain man-machine relationship. How does working environment affects this relationship ?

14.

What are the different types of displays ?

15.

State the ergonomic considerations in the design of displays.

16.

What are the different types of controls ?

17.

State the ergonomic considerations in the design of controls.

18.

What is design for manufacture (DFM) ? Explain the general principles to be followed while designing the parts for manufacture.

19.

Explain the guidelines to be followed in the design of the parts for the following processes :

Machine Design – I (MU)

(i)

Casting

(ii)

Forging

(iii)

Machining

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Machine Design & Design Considerations

20.

What is Design For Assembly (DFA) ? Explain the general principles to be followed while designing the parts for assembly.

21.

What is standardization ? Explain its significance.

22.

What are preferred number series ? State their advantages and applications.

23.

What is creativity in design ?

24.

What is tolerance ? Explain the types of tolerances.

25.

Explain the B.I.S. system of designation of tolerances.

26.

What do you understand by fits ? How fits are classified ?

27.

Explain the B.I.S. system of designation of fits.

28.

How fits are selected for a given application ?



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Note Note

Machine Design & Design Considerations