Hydrolic Breaking System

Content 1.Introduction 2.Break drum 3.Wheel 4.Master cylinder piston 5.Wheel cylinder piston 6.Wheel cylinder 7.Master cylinder 8.Hydraulic break 9.General operation 10 . Component specification 11 . Special consideration 12 . Functioning 13 . Manufacturing 14 . Advantages 15 . Disadvantages 16 . Brake fluid 17. Principles of hydrolics 18 . Theory of hydrolics

HYDROLIC BREAK  The hydraulic brake is an arrangement of  braking  braking mechanism which uses brake uses brake fluid, fluid, typically containing ethylene glycol, glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle. The most common arrangement of hydraulic brakes for   passenger vehicles consists of a brake a brake pedal, pedal, a vacuum assist module, a master cylinder , hydraulic lines, and a  brake rotor and/or  rotor and/or  brake  brake drum. drum. At one time, passenger vehicles commonly employed disc  brakes on the front wheels and drum brakes on the rear  wheels. However, four wheel disc brakes have becoming more popular, replacing drums on all but the most basic vehicles.

HYDROLIC BREAK  The hydraulic brake is an arrangement of  braking  braking mechanism which uses brake uses brake fluid, fluid, typically containing ethylene glycol, glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle. The most common arrangement of hydraulic brakes for   passenger vehicles consists of a brake a brake pedal, pedal, a vacuum assist module, a master cylinder , hydraulic lines, and a  brake rotor and/or  rotor and/or  brake  brake drum. drum. At one time, passenger vehicles commonly employed disc  brakes on the front wheels and drum brakes on the rear  wheels. However, four wheel disc brakes have becoming more popular, replacing drums on all but the most basic vehicles.

PARTS OF HYDROLIC BREAKING SYSTEM 1. BREAK DRUM 2. WHEEL 3. MASTER CYLINDER PISTON 4. WHEEL CYLINDER PISTON 5. WHEEL CYLINDER 6. MASTER CYLINDER

BREAK DRUM

A drum brake is a brake in which the friction is caused by a set of shoes of shoes or pads that press against the inner surface of a rotating drum.  The drum is connected to a rotating wheel. wheel.  The modern automobile drum brake was invented in 1902 by Louis Renault, Renault, though a less-sophisticated drum brake had been used by Maybach a year earlier. In the first drum brakes, the shoes were mechanically operated with levers and rods or cables. From the mid1930s the shoes were operated with oil pressure in a small wheel cylinder and pistons (as in the picture), though some vehicles continued with purely-mechanical systems for decades. Some designs have two wheel cylinders. cylinders.

WHEEL A wheel is a circular device that is capable of  rotating on its axis, facilitating movement or transportation whilst supporting a load (mass ( mass), ), or performing labor in machines. Common examples are found in transport applications. A wheel, together with an axle overcomes friction by facilitating motion by rolling. rolling. In order for wheels to rotate, a moment needs to be applied to the wheel about its axis, either by way of gravity, or by application of another external force. More generally the term is also used for other circular objects that rotate or turn, such as a ship's wheel, wheel, steering wheel and flywheel. flywheel.

WHEEL AND MASTER CYLINDER PISTON A piston is a component of reciprocating engines, pumps and gas compressors. It is located in a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of  compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

Wheel cylinder It is the important part of the hydraulic breaking system. The construction of wheel cylinder is very simple as shown in fig. it consist of cylinder body, piston, rubber caps.  The cylinder body contains two holes which provide connection for the pipe line and bleeder valve.  The rubber caps avoid the leakage of the fluid out of the wheel cylinder. The piston transmits fluid pressure to the brake shoes for the application of breaks. Hence the brake fluid pressure forces the piston a part for applying the brakes.

Master cylinder  The diameter and length of the master cylinder has a significant effect on the performance of  the brake system. A larger diameter master cylinder delivers more hydraulic fluid to the slave cylinders, yet requires more brake pedal force and less brake pedal stroke to achieve a given deceleration. A smaller diameter master cylinder has the opposite effect.A master cylinder may also use differing diameters between the two sections to allow for increased

fluid volume to one set of slave cylinders or the other.

BRAKE LINES AND HOSES The arteries of the brake system are the steel lines and flexible rubber hoses that route hydraulic pressure to each brake when the driver steps on the brake pedal. The lines and hoses must withstand pressures that can range from a few hundred pounds per square inch up to almost 2000 psi! If a line or  hose can't take the pressure and blows, all braking ability in the affected brake circuit will be lost. A slow leak in a brake line or hose is almost as bad as a sudden failure because over time, enough fluid may be lost to allow air to enter the hydraulic system. Air in the fluid is bad because air is compressible. This increases the amount of pedal travel that's necessary to apply the brakes, and may increase it to the point where the pedal hits the floor before the brakes apply. The first indication of a leak in a brake line or hose may be a low fluid level in the master cylinder  reservoir. Other clues may include wet spots on the driveway, dampness on the back of a drum brake, or  a brake warning light that comes on. If a leak is suspected, the entire brake system should be inspected for a leak. The most likely leak points are the brake calipers, wheel cylinders and rubber brake hoses, though steel lines can also rust through and leak. Fluid can sometimes be pulled into the engine through a leak in the power brake vacuum booster. If 

there's any fluid inside brake booster vacuum hose, the brake booster needs to be replaced. Rubber brake hoses also need to be inspected for  age cracks, bulges, swelling or other damage that would indicate a need for replacement. Rubber hoses have an expansion resistant inner lining that should not give under pressure. If the inner liner leaks, fluid will force its way under the outer liner causing a bubble or blister to appear when the brakes are applied.

Although it doesn't happen very often, sometimes, internal damage or deterioration in a rubber hose allows a small flap of material to lift up and plug the line. This prevents brake pressure from reaching the wheel causing a brake pull when the brakes are applied. The same thing can also happen to steel brake lines. Debris in the brake fluid or a crushed or kinked line can block the passage of hydraulic pressure to the brakes. In some cases, pressure will get through but when the brakes are released, the blockage prevents pressure from releasing back to the master cylinder, causing the brake to drag.

hydraulic brake The hydraulic brake is an arrangement of  braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator  of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle. The most common arrangement of hydraulic brakes (for passenger vehicles) consists of a brake pedal, a vacuum assist module, a master cylinder , hydraulic lines, a "slave cylinder " , and a brake rotor and/or  brake drum. Typical passenger vehicles employ disc brakes on the front wheels and drum brakes on the rear wheels. However, four wheel disc brakes are becoming more popular.

Why do you use brake oil in hydraulic brakes can you use water in the place of brake oil to transmit the pressure?

  No. Water under pressure doesnt have the mass for the stress placed on brake fluid or hydrolic fluid. Water is  best used for cooling. It will still cool at 700 degrees F, at this point it is considered a plasma. Hydolic fluids such as   brake fluid, transmission fluid and hydrolic fluids or oils will exert the same pressure as what is   placed upon it. It is one of the major differences   between oil and water. Also, water under pressure freezes, oil does not.

General operation When the brake pedal is pressed, leverage multiplies the force applied from the pedal to a vacuum booster. The booster multiplies the force again and acts upon a piston in the master cylinder. As force is applied to this piston, pressure in the hydraulic system increases, forcing fluid through the lines to the slave cylinders. The two most common arrangements of slave cylinders are a pair of opposed pistons which are forced apart by the fluid pressure (drum brake), and a single piston which is forced out of its housing (disc brake). The slave cylinder pistons then apply force to the brake linings (generally referred to as shoes for drum brakes and pads for disc brakes). The force applied to the linings cause them to be pushed against the drums and rotors. The friction between the linings and drum/rotor causes a braking torque to be generated, slowing the vehicle.

Component specifics (For typical light duty automotive braking systems)

The brake pedal is a simple lever. One end is attached to the framework of the vehicle, a rod extends from a point along its length, and the foot pad is at the other end of the lever. The rod either extends either to the master cylinder (manual brakes) or to the vacuum booster (power brakes). The master cylinder  is divided internally into two sections, each of which pressurizes a separate hydraulic circuit. Each section supplies pressure to one circuit. Passenger vehicles typically have either a front/rear split brake system or a diagonal split brake system.

Automobile hydrolic brake-system

 Automobile hydraulic-brake system. 1) Brake pedal; 2) piston; 3) master cylinder; 4) hydraulic line; 5) brake cylinder; 6) brake  piston; 7) brake band; 8) wheel; 9) return spring.

A front/rear split system uses one master cylinder  section to pressurize the front slave cylinders, and the other section to pressurize the rear slave cylinders. A split circuit braking system is now required by law in most countries for safety reasons; if one circuit fails, the other circuit can stop the vehicle. The diameter and length of the master cylinder has a significant effect on the performance of the brake system. A larger diameter master cylinder delivers more hydraulic fluid to the slave cylinders, yet requires more brake pedal force and less brake pedal stroke to achieve a given deceleration. A smaller  diameter master cylinder has the opposite effect. A master cylinder may also use differing diameters between the two sections to allow for increased fluid volume to one set of slave cylinders or the other. The vacuum booster  or  vacuum servo is used in most modern hydraulic brake systems. The vacuum booster is attached between the master cylinder and the brake pedal and multiplies the braking force applied by the driver. These units consist of a hollow housing with a moveable rubber  diaphragm across the center, creating two chambers. When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered. The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed. A return spring keeps the diaphragm in the starting

position until the brake pedal is applied. When the brake pedal is applied, the movement opens an air  valve which lets in atmospheric pressure air to one chamber of the booster. Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure. This force, in addition to the driver's foot force, pushes on the master cylinder piston. A relatively small diameter booster unit is required; for a very conservative 50% manifold vacuum, an assisting force of about 1500 N (150 kgf) is produced by a 20cm diaphragm with an area of 0.03 square metres.

The diaphragm will stop moving when the forces on both sides of the chamber reach equilibrium. This can be caused by either the air valve closing (due to the pedal apply stopping) or if "runout" is reached. Runout occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated by the now stagnant differential pressure. After the runout point is reached, only the driver's foot force can be used to further apply the master cylinder  piston. The fluid pressure from the master cylinder travels through a pair of steel brake tubes to a compensator, which performs two functions: It equalizes pressure between the two systems, and it provides a warning if  one system loses pressure. The compensator has two

chambers (to which the hydraulic lines attach) with a piston between them. When the pressure in either line is balanced, the piston does not move. If the pressure on one side is lost, the pressure from the other side moves the piston. When the piston makes contact with a simple electrical probe in the center of the unit, a circuit is completed, and the operator is warned of a failure in the brake system. brake tubing carries the pressure to the brake units at the wheels. Since the wheels do not maintain a fixed relation to the automobile, it is necessary to use hydraulic brake hose from the end of the steel line at the vehicle frame to the caliper at the wheel. Allowing steel brake tubing to flex invites metal fatigue and, ultimately, brake failure. A common upgrade is to replace the standard rubber hoses with a set which are externally reinforced with braided stainless-steel wires; these have negligible expansion under  pressure and can give a firmer feel to the brake pedal with less pedal travel for a given braking effort. Steel lines are preferred for most of the system for  their rigidity. Any pressure induced distortion in the lines results in less useful volume and pressure of  fluid reaching the slave cylinders, with reduced braking effectiveness. Finally, the fluid pressure enters the Slave Cylinders and use one or more pistons to apply force to the braking unit.

Special considerations Air brake systems are bulky, and require air  compressors and reservoir tanks. Hydraulic systems are smaller and less expensive. Hydraulic fluid must be non-compressible. Unlike air  brakes, where a valve is opened and air flows into the lines and brake chambers until the pressure rises sufficiently, hydraulic systems rely on a single stroke of a piston to force fluid through the system. If any vapor is introduced into the system it will compress, and the pressure may not rise sufficiently to actuate the brakes. Hydraulic braking systems are sometimes subjected to high temperatures during operation, such as when descending steep grades. For this reason, hydraulic or  brake fluid must resist vaporization at high temperatures. Water vaporizes easily with heat and can corrode the metal parts of the system. If it gets into the brake lines, it can degrade brake performance dramatically. This is why light oils are used as hydraulic fluids. Oil displaces water and protects metal parts against corrosion, and can tolerate much higher temperatures before vaporizing.

"Brake fade" is a condition caused by overheating in which braking effectiveness reduces, and may be lost. It may occur for many reasons. The pads which engage the rotating part may become overheated and "glaze over", becoming so smooth and hard that they cannot grip sufficiently to slow the vehicle, vaporization of the hydraulic fluid under temperature extremes, and thermal distortion may cause the linings to change their shape and engage less surface area of the rotating part. Thermal distortion may also cause permanent changes in the shape of the metal components, resulting in a reduction in braking capability that requires replacement of the affected parts.

Functioning Diagram of the Hydractive system, showing centre spheres and stiffness valves At the heart of the system, acting as pressure sink as well as suspension elements, are the so called 'spheres', five or six in all; one per wheel and one main accumulator as well as a dedicated brake accumulator on some models. On later cars fitted with antisink or Activa suspension, there may be as many as nine spheres. They consist of a hollow metal ball, open to the bottom, with a flexible desmopan rubber membrane, fixed at the 'equator' inside separating top and bottom. The top is filled with nitrogen at high pressure, up to 75 bar, the bottom connects to the car's LHM fluid circuit. (See hydraulic accumulator). The high pressure pump powered by the engine pressurizes the circuit and an accumulator sphere. This part of the circuit is between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type, can power steering, clutch, gearchange etc.

Pressure goes from this circuit to the wheel spheres, pressurizing the bottom part of the spheres and rods connected to the wheel suspension. Suspension works by the rod pushing LHM into the sphere, compacting the nitrogen in the upper part of the sphere, the damping is provided by a two-way 'leaf valve' in the opening of the sphere. LHM has to squeeze back and forth through this valve which causes resistance and controls the suspension movements, it is the simplest damper and one of the most efficient. Car height correcting works by height correctors connected to the anti-roll bar, front and rear.  These height correctors allow for more fluid to travel under pressure to the rod/sphere system when detecting that the suspension is lower than its expected ride height (e.g. the car is loaded). When the car is too high (e.g. after unloading) fluid is returned to the system reservoir via low-pressure return lines. Height correctors act with some delay in order not to correct regular suspension movements. Rear brakes are powered from the rear suspension spheres. Because the pressure there is proportional to the load, so is the braking power.

Figure 3-46.—Hand brake valve. Figure 3-48.Air-over-hydraulic power cylinder.

 The air-over-hydraulic brake system has an trailer brakes without applying the truck or tractor air-over-hydraulic power cylinder (fig. 3-48) that brakes. The hand brake valve or

independent trailer contains an air cylinder and a hydraulic cylinder in control valve, as shown in figures 3-44 and 3-46, tandem. Each cylinder is fitted with a piston and a provides the operator control of the trailing load at all common rod.  The air piston is of greater diameter than times. the hydraulic piston. This difference in the two pistons Figure 3-47.-Typical airover-hydraulic brake system. 3-26 This system combines the use of compressed air and hydraulic pressure for brake operation.

Manufacturing

 The whole high pressure part of the system is manufactured from steel tubing of small diameter, connected to valve control units by Lockheed type pipe unions with special seals made from desmopan rubber, a type of rubber compatible with the LHM fluid. The moving parts of the system e.g. suspension strut or steering ram are sealed by extremely small tolerances between the cylinder and piston for tightness under pressure. The other plastic/rubber parts are return tubes from valves such as the brake control or height corrector valves, also catching seeping fluid around the suspension push-rods. The metal and alloy parts of the system rarely fail even after excessively high mileages but the rubber components (especially those exposed to the

air) can harden and leak, typical failure points for the system.

Spheres are subject to no mechanical wear but suffer pressure loss, mostly from nitrogen naturally diffusing through the membrane. They typically keep between 60,000 and 100,000 km. Spheres originally used to have a

valve on top and be rechargeable. Newer spheres do not have this valve anymore, but it can be retrofitted. Though a rechargeable sphere has a longer lifespan, the membranes will eventually wear out though this can take over 20 years. A ruptured membrane means suspension loss at the attached wheel, however ride height is unaffected. Or in the case of the accumulator sphere, reliance on the high pressure pump as the only source of  braking pressure to the front wheels.

Advantages

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  The suspension is self-leveling and rideheight is adjustable; this provides aerodynamic benefits because of the stable ride-height and extra clearance over rough terrain.   The ride comfort is excellent (the ride is described as floating above the road surface) but the suspension never 'wallows', giving precise handling and roadholding (like a sports car)

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Large loads do not seriously affect the dynamics of the suspension system and handling is not affected thereby. Compact suspension design.

Maintenance for relatively easy.

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Inexpensive in mass production; for vehicles that would otherwise have a conventional power steering pump, hydropneumatic suspension adds no new equipment and in many cases results in a lower unsprung mass.

Upon body roll, the pressure exerted between the tyres of the same axle is not subject to the same differential as on some other cars; the pressure in one suspension strut equals the pressure in the other through Pascal's law, potentially giving the 'light' tyre more footprint pressure.

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Can be conveniently interconnected in the roll plane to improve roll stiffness and thus roll stability limit, especially for heavy vehicles. Can be connected in the pitch plane to improve braking dive and traction squat. If they are interconnected in the threedimensional full car model, the interconnected hydro-pneumatic suspension could realize enhanced roll and pitch control during excitations arising from steering, braking/traction, road input and crosswind, as with the Hydractive arrangement Flexibility in the suspension strut design in the interconnected suspension system to realize desirable vertical, roll and pitch properties for different types of vehicles.

Horizontal orientation of the rear suspension cylinders below the level of the boot floor means that the full width of the boot is available for loads.

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Mechanical steel spring suspension systems that try to replicate some of the inherent advantages of hydropneumatic suspension (multilink, adjustable shock absorbers) end up more complex to build and maintain than the straightforward hydropneumatic layout. People who are prepared to carry out simple maintenance can acquire a luxury car for a fraction of the cost as hydropneumatic suspension scares potential buyers and dealers.

Disadvantages

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Hydropneumatic suspension systems are expensive to repair or replace, if poorly maintained. Older designs of hydropneumatic suspension systems can lead to significant body roll. Failure of the hydraulic system will cause a drop in ride height and, possibly, the failure of suspension completely. However, an acute failure will not lead to acute brake failure as the accumulator sphere holds enough pressure to ensure safe braking far beyond the braking needed to bring a vehicle with a failed system to a standstill. The novelty (and unfamiliarity) of the hydropneumatic suspension system leads to people being wary of using, purchasing or trusting it.

Hydractive

Hydractive Suspension is a new automotive technology introduced by the French manufacturer Citroën in 1990. It describes a development of the 1955 Hydropneumatic suspension design using additional electronic sensors and driver control of suspension performance. The driver can make the vehicle stiffen (sport mode) or ride in outstanding comfort (soft mode). Sensors in the steering, brakes, suspension, throttle pedal and gearbox feed information on the car's speed, acceleration, and road conditions to on-board computers. Where appropriate - and within milliseconds - these computers switched an extra pair of suspension spheres in or out of  circuit, to allow the car a smooth supple ride in normal circumstances, or greater roll resistance for better handling in corners. This development keeps Citroën in the forefront of  suspension design, given the widespread goal in the auto industry of an Active Suspension system. All auto suspension is a compromise between comfort and handling. Auto manufacturers try to balance these aims and

locate new technologies that offer more of  both. Hydractive 1 and Hydractive 2

Citroën hydractive (Hydractive 1 and Hydractive 2) suspension was available on several models, including the XM and Xantia, which had a more advanced sub-model known as the Activa.

Brake fluid Brake fluid is a type of hydraulic fluid used in brake applications in motorcycles, automobiles, light trucks and some advanced bicycles. It is used to transfer force under pressure from where it is created through hydraulic lines to the braking mechanism near the wheels. It works because liquids are not appreciably compressible. Braking applications produce a lot of  heat so brake fluid must have a high boiling point to remain effective and must also not freeze under normal temperatures. These requirements eliminate most water-based solutions. In the USA brake fluid comes in a number of  forms, standardized under by the United States Department of Transportation (DOT). DOT 2 is essentially castor oil; DOT 3, DOT 4, and DOT 5.1 are composed of various mineral oils, glycol esters and ethers; some are synthetic oil based, and DOT 5 is silicone-based. As of 2006, most cars produced in the U.S. use DOT 3.

Glycol based fluids are two times less compressible than silicone type fluids, even

when heated. Less compressibility of brake fluid will increase pedal feel (firmness), but in either case this effect is minimal. The U.S. Army has used silicone brake fluid exclusively since 1982 successfully. Glycols are hygroscopic and will absorb water from the atmosphere, reducing the boiling point of the fluid and degrading hydraulic efficiency. Changing fluid on a regular basis will greatly increase the performance of the brake system, but this is often not a concern in passenger cars. On the other hand, changing fluid at least every several years will preserve the life of  brake system components (by removing accumulated water and other contaminants) and increase the overall reliability of the brake system. Polyethylene glycol and other brake fluid ingredients may be corrosive to paint and finished surfaces such as chrome and thus care should be taken when working with the fluid. Additionally, polyethylene glycol, in the concentrations found in DOT brake fluids, reacts violently, producing a large fireball, with some household chemicals, notably pool care products.

Hotwheelscollectors.com cites that hobby modellers use brake fluid as a safe (if  somewhat slow) paint stripper. It is less likely to harm skin and will not harm plastics.

Components Mineral-based •

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Alkyl ester Aliphatic amine Diethylene glycol Diethylene glycol monobutyl ether Diethylene glycol monoethyl ether Diethylene glycol monomethyl ether Dimethyl dipropylene glycol Polyethylene glycol monobutyl ether Polyethylene glycol monoethyl ether Polyethylene glycol monomethyl ether Polyethylene oxide  Triethylene glycol monobutyl ether  Triethylene glycol monoethyl ether  Triethylene glycol monomethyl ether

Silicone-based •

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Di-2-ethylhexyl sebacate Dimethyl polysiloxane  Tributyl phosphate

PRINCIPLES OF HYDRAULICS

1A1. Increasing use of hydraulic power in modern submarines. In the development of  the submarine from pre-war classes, many changes and improvements have occurred. One of the outstanding differences is the large variety of submarine devices which are now operated by hydraulic power. In early classes, there was no hydraulic system, and power requirements were met by means of air or electricity. Along with constantly improving submarine design has gone a constant extension and diversification of the use of  hydraulic power. 1A2. Other sources of power available on submarines. Why this noticeable trend toward hydraulics? Obviously

hydraulic actuation is not the only means of transmitting  power throughout the submarine, and the tasks now being done by the hydraulic system were originally performed by hand, electricity, or compressed air. a. Hand power . Some equipment on a submarine is still operated exclusively by hand, but this practice is rapidly disappearing. This is because the power requirements exceed that which manual effort can provide over long  periods of time, and because power operation is faster and can be remotely controlled, thus greatly reducing the communication necessary between crew members.  b. Electric power . Since the electrical plant occupies such a  prominent place in the submarine power system and must  be used for propulsion in any event, it would be reasonable to expect that electricity would also be used to operate all of the auxiliary equipment as well. Electricity is ideally adapted for submarine equipment that has few or no moving parts, such as lamps, radios, cooking facilities, and similar devices. But electricity is not so ideal when it is necessary to move heavy apparatus such as rudders, and bow and stern planes, because heavy, bulky electrical units are required. Also it is not ideal when instantaneous stopping of a driving mechanism is demanded, since electric motors have a tendency to "overtravel c. Pneumatic power . Since compressed air must also be used aboard a submarine for certain functions, this system, which consists of the compressors, high and low  pressure air bottles and air lines, provides another source of  auxiliary power. However, pneumatic or compressed-air 

 power also has definite shortcomings. Pressure drop caused  by leakage, and the mere fact that air is a compressible substance, may result in "sponginess" or lag in operation. The high pressure necessary for compressed-air storage increases the hazard from ruptured lines, with consequent danger to personnel and equipment. Another disadvantage of air systems is that the air compressors require greater  maintenance and are relatively inefficient.

d. Comparative advantages of hydraulic power . Hydraulic systems possess numerous advantages over other systems of power operation. They are light in weight; they are simple and extremely reliable, requiring a minimum of attention and maintenance. Hydraulic controls are sensitive, and afford precise controllability. Because of the low inertia of moving parts, they start and stop in complete obedience to the desires of the operator, and their operation is positive. Hydraulic systems are self-lubricated; consequently there is little wear or corrosion.  Their operation is not apt to be interrupted by salt spray or water. Finally, hydraulic units are relatively quiet in operation, an important

consideration when detection by the enemy must be prevented.

e. Comparative summary . If we draw up a table of the characteristics of the three power systems, a comparison will reveal the superiority of hydraulics for the operation of  auxiliary mechanisms

Pascal's law In the physical sciences, Pascal's law or Pascal's principle states that "a change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container."

Derivation Pressure is the result of a force applied over a specific area and that pressure is therefore measured by the formula P =

 F / A or "pressure equals force divided by area". When a force is applied to an incompressible fluid, the area in question is the contact area between any two molecules of  the fluid. That area is the same for any pair of molecules within the fluid. Because an incompressible fluid accepts and applies forces evenly throughout itself, the pressure will be equal at all points within the fluid. The molecules that are in contact with the surface of the container will  push against that surface with the same pressure as between any two molecules anywhere else within the container   because they have the same contact area with the molecules of the container as with each other. If we consider that this container and its fluid contents are subject to gravity as an additional force then we must consider that the difference of  pressure due to a difference in elevation within a fluid column is given by:

where

ΔP is the hydrostatic pressure (given in pascals in the SI system), or the difference in pressure at two points within a fluid column, due to the weight of the fluid; ρ is the fluid density (in kilograms per cubic meter in the SI system);

g is acceleration due to gravity (normally using the sea level acceleration due to Earth's gravity in meters per second squared); Δh is the height of fluid above the point of  measurement, or the difference in elevation between the two points within the fluid column (in meters in SI). The intuitive explanation of this formula is that the change in pressure between two elevations is due to the weight of  the fluid between the elevations.  Note that the variation with height does not depend on any additional pressures. Therefore Pascal's law can be interpreted as saying that any change in pressure applied  at any given point of the fluid is transmitted undiminished  throughout  the fluid.

Applications

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 The underlying principle of the hydraulic press Used for amplifying the force of the driver's foot in the braking system of most cars and trucks.

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Used in artesian wells, water towers, and dams.

Scuba divers must understand this principle. At a depth of 10 meters under water, pressure is twice the atmospheric pressure at sea level, and increases by about 105 kPa for each increase of 10 m depth.

Comparison of Air, Electricity & Hydraulics

FACTOR

AIR

ELECTRICI HYDRAULI  TY CS

Reliability

Poor

Good

Good

Weight

Light

Heavy

Light

Installation Simple

Simple

Simple

Control Valves Mechanism

Switches Valves and solenoids

Maintenanc Constant Difficult, Simple e attention requiring necessar skilled y personnel Vulnerabilit High Good y pressure bottle dangero us; broken lines cause failure and

Safe; broken lines cause failure

danger to personne l and equipme nt Response

Slow for both starting and stopping

Rapid starting, slow stopping

Instant starting and stopping

Controllabil Poor ity

Fair

Good

Quietness of  Operation

Poor

Good

Poor

THEORY OF HYDRAULICS 1B1. Familiarity of hydraulic principles. For many centuries, man has utilized hydraulic principles to satisfy common, everyday needs. Opening a faucet to fill a sink with water a practical application of hydraulics. Water moves through a dam in accordance with wellknown principles of fluid motion. There are hydraulic principles that explain the action of  fluids in motion and others for fluids at rest. We are chiefly concerned, however; with that branch of  hydromechanics which is called simply Hydraulics and is defined in engineering textbooks as the engineering application of fluid mechanics. It includes the study of the  behavior of enclosed liquids under pressure, and the harnessing of the forces existing in fluids to do some  practical task such as steering a submarine or opening the outer door of a torpedo tube.

Examples of hydraulically operated equipment are familiar to all. Barber or dentist chairs are raised and lowered hydraulically; so is an automobile when placed on a hydraulic rack for a grease job. Stepping on the brake pedal in an automobile creates the hydraulic power which

stops the rotation of the four wheels and brings the car to a halt. For an understanding of how a hydraulic system works, we must know the basic principles, or laws, of hydraulics, that is, of confined liquids under pressure. This will be made easier, however, if we first examine the somewhat simpler  laws governing the behavior of liquids when unconfined , that is, in open containers.

1B2. Liquids in open containers. a. Density  and specific gravity . The first characteristic of  an unconfined liquid which interests us is its density. The density of a fluid is the weight of a unit volume of it . The unit of volume normally used in this text is the cubic foot; the unit of  weight normally used is the pound. The standard of density, to which the atmospheric pressure.

Let us fill a container with a cubic foot of pure water (see Figure 1-1). We weigh

 Figure 1-1. Liquids of different densities. the contents and find it to be 62.4 pounds. This is the density of water . Under the same conditions, a similar  volume of oil, such as is used in a submarine's hydraulic system weighs approximately 50 pounds; therefore its density is less than that of water. Under the same conditions, a cubic foot of mercury weighs 845.9 pounds; its density obviously exceeds that of water.

When we speak of the weight of substance, we actually mean the force, or gravitational pull, exerted on the substance at the earth's

surface. Every material responds to the earth's gravitational attraction. To express the relative density, or specific gravity , of various liquids and solids, gravitational pull upon them is compared to the gravitational pull upon an equal volume of water. Water, therefore, is said to have a specific gravity of 1 and the specific gravity of any other substance is its density relative to that of water. Oil has a specific gravity of (50 x 1)/62.4, or approximately 0.8; that is, its density is 0.8 of that of water. This explains why oil floats on water. Mercury, on the other hand, has a specific gravity of (845.9 x 1)/62.4 or about 13.5; that is, its density is 13.5 times as great as that of water; consequently, it sinks rapidly. These calculations of the weights of water, oil, and mercury were made at zero degrees centigrade (32 degrees Fahrenheit) and at sea level. At other temperatures and altitudes, different results would be obtained. In some engineering calculations, cubic centimeters and grams are used instead of cubic feet and pounds. This does not affect specific gravity, as the relationship between the weight of a unit volume of any other material and of water would be the same no matter what measuring unit were used.  b. Force and pressure . A liquid has no shape of its own. It acquires the shape of its container up to the level to which

it fills the container. However, we know that liquids have weight. This weight exerts a force upon

Figure 1-2. Weight of an isolated column of water.

Increase of force with area. We are now ready to consider a remarkable fact which follows from the principles just discussed, and which is illustrated in a simplified manner in Figure 111. Here a cylinder whose base has an area of  1 square inch is connected to another cylinder whose base has an area of 10 square inches. Again a force of 1 pound is applied to the

piston in the smaller cylinder; and again the pressure exerted is 1 pound per square inch. Now, since this pressure is transmitted equally in all directions throughout the confined liquid, an upward pressure of 1 pound per square inch will be exerted on the piston in the larger cylinder; and since this larger piston has a total area of 10 square inches, the total force exerted on the larger piston is 10 pounds. Actually, what is happening is that an upward force of 1 pound is being exerted against each square inch of bottom surface of the larger piston; and since the area of this surface is 10 square inches, the total force is equal to the downward pressure on the small piston (1 pound per square inch) multiplied by the area of the larger piston (10 square inches); or, 1 (pounds per square inch) X 10 (square inches) = 10 pounds (total force exerted on larger piston). In other words, the ratio between the force applied to the smaller piston and the force applied to the

Figure 1-10. Transmission of equal pressures to equal areas.

larger piston is the same as the ratio between the area of the smaller piston and the area of  the larger piston. Expressed as a proportion, then, we have: Force on larger piston/Force on smaller piston = Area of larger piston/Area of smaller piston This means that the mechanical advantage obtainable by such an arrangement is equal to the ratio between the areas of the two pistons.

Figure 1-11. Equal pressure transmitted to larger area.

It is this principle, discovered by Pascal, which makes possible the tremendous forces attainable in certain hydraulic devices, such as the hydraulic press, and hydraulic hoists.

Figure 1-12. Multiple units from a single source of power .

Now let us once more consider the arrangement shown in Figure 1-10. Since the cylinders (and pistons) are of equal area, pushing the liquid down a distance of 1 inch in