HYDRAULIC BRAKES Page 1

BRAKEA brake is a mechanical device which inhibits motion. Most commonly brakes use friction to convert kinetic energy into heat, though other methods of energy conversion may be employed. For example regenerative braking converts much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into different forms, for example by transferring the energy to a rotating flywheel.Brakes are generally applied to rotating axles or wheels, but may also take other forms such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing.Since kinetic energy increases quadratically with velocity (), an object moving at 10m/s has 100 times as much energy as one of the same mass moving at 1m/s, and consequently the theoretical braking distance, when braking at the traction limit, is 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.Almost all wheeled vehicles have a brake of some sort. Even baggage carts and shopping carts may have them for use on a moving ramp. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to reduce their speed in flight. Notable examples include gliders and some World War II-era aircraft, primarily some fighter aircraft and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. The Saab B 17 dive bomber used the deployed undercarriage as an air brake.Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking then conduct it to the air gradually. When travelling downhill some vehicles can use their engines to brake. When the brake pedal of a modern vehicle with hydraulic brakes is pushed, ultimately a piston pushes the brake pad against the brake disc which slow the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the wheel down. CHARACTERSTICS Brakes are often describe according to several characteristics include: Peak force The peak force is the maximum decelerating effect that can be obtained. The peak force is often greater than the traction limit of the tires, in which case the brake can cause a wheel skid. Continuous power dissipation Brakes typically get hot in use, and fail when the temperature gets too high. The greatest amount of power (energy per unit time) that can be dissipated through the brake without failure is the continuous power dissipation. Continuous power dissipation often depends on e.g., the temperature and speed of ambient cooling air. Fade As a brake heats, it may become less effective, called brake fade. Some designs are inherently prone to fade, while other designs are relatively immune. Further, use considerations, such as cooling, often have a big effect on fade. Smoothness A brake that is grabby, pulses, has chatter, or otherwise exerts varying brake force may lead to skids. For example, railroad wheels have little traction, and friction brakes without an anti-skid mechanism often lead to skids, which increases maintenance costs and leads to a "thump thump" feeling for riders inside. Power Brakes are often described as "powerful" when a small human application force leads to a braking force that is higher than typical for other brakes in the same class. This notion of "powerful" does not relate to continuous power dissipation, and may be confusing in that a brake may be "powerful" and brake strongly with a gentle brake application, yet have lower (worse) peak force than a less "powerful" brake. Pedal feel Brake pedal feel encompasses subjective perception of brake power output as a function of pedal travel. Pedal travel is influenced by the fluid displacement of the brake and other factors. Drag Brakes have varied amount of drag in the off-brake condition depending on design of the system to accommodate total system compliance and deformation that exists under braking with ability to retract friction material from the rubbing surface in the off-brake condition. Durability Friction brakes have wear surfaces that must be renewed periodically. Wear surfaces include the brake shoes or pads, and also the brake disc or drum. There may be tradeoffs, for example a wear surface that generates high peak force may also wear quickly. Weight Brakes are often "added weight" in that they serve no other function. Further, brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in some circumstances. "Weight" may mean the brake itself, or may include additional support structure. Noise Brakes usually create some minor noise when applied, but often create squeal or grinding noises that are quite loud.

FUNCTIONS OF VEHICLE BRAKING

There are two main functions of brakes :

(a) To slow down or stop the vehicle in the shortest possible time at the time of need.

(b) To control the speed of vehicle at turns and also at the time of driving down on a hill slope.

PRINCIPLE OF VEHICLE BRAKING

Braking of a vehicle depends upon the static function that acts between tyres and road surface. Brakes work on the following principle to stop the vehicle :

The kinetic energy due to motion of the vehicle is dissipated in the form of heat energy due to friction between moving parts (wheel or wheel drum) and stationary parts of vehicle (brake shoes).

The heat energy so generate4d due to application of brakes is dissipated into air. Brakes operate most effectively when they are applied in a manner so that wheels do not lock completely but continue to roll without slipping.

CLASSIFICATION OF BRAKES

On the Basis of Method of Actuation

(a) Foot brake (also called service brake) operated by foot pedal. (b) Hand brake it is also called parking brake operated by hand.

On the Basis of Mode of Operation (a) Mechanical brakes (b) Hydraulic brakes (c) Air brakes (d) Vacuum brakes (e) Electric brakes.

On the Basis of Action on Front or Rear Wheels

(a) Front-wheel brakes (b) Rear-wheel brakes.

On the Basis of Method of Application of Braking Contact

(a) Internally expanding brakes (b) Externally contracting brakes.

SHORT NOTES ON MISCELLANEOUS BRAKING SYSTEMS

AIR BRAKES

Air brakes are applied by the pressure of compressed air. Air pressure applies force on brakes shoes through suitable linkages to operate brakes. An air compressor is used to compress air. This compressor is run by engine power.VACUUM BRAKES Vacuum brakes are a piston or a diaphragm operating in a cylinder. For application of brakes one side of piston is subjected to atmospheric pressure while the other is applied vacuum by exhausting air from this side. A force acts on the piston due to difference of presure.ELECTRIC BRAKES

In electrical brakes an electromagnet is used to actuate a cam to expand the brake shoes. The electromagnet is energized by the current flowing from the battery. When flow of current is stopped the cam and brake shoes return to their original position and brakes are disengaged. Electric brakes are not used in automobiles as service brakes sure. This force is used to operate brake through suitable linkages. FRICTION BRAKESA friction brake is a type of automotive brake that slows or stops a vehicle by converting kinetic energy into heat energy, via friction. The heat energy is then dissipated into the atmosphere. In most systems, the brake acts on the vehicle's roadwheel hubs, but some vehicles use brakes which act on the axles or transmission. Friction brakes may be of either drum or disc type.DRUM BRAKEA drum brake is a vehicle brake in which the friction is caused by a set of brake shoes that press against the inner surface of a rotating drum. The drum is connected to the rotating roadwheel hub.

DISC BRAKEThe disc brake is a device for slowing or stopping the rotation of a road wheel. A brake disc (or rotor in U.S. English), usually made of cast iron or ceramic, is connected to the wheel or the axle. To stop the wheel, friction material in the form of brake pads (mounted in a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop.ELECTROMAGNETIC BRAKESElectromagnetic brakes slow an object through electromagnetic induction, which creates resistance and in turn either heat or electricity. Friction brakes apply pressure on two separate objects to slow the vehicle in a controlled manner.PARKING BRAKES OR EMERGENCY BRAKESParking brakes or emergency brakes are essentially mechanical brakes operated by hand. These are used to prevent the motion of vehicle when parked at a place or when parked on slopes. In cars, these brakes are generally attached to rear wby pushing a button (provided on lever) and pressing the lever down. heels. In this type, a cable connects the hand lever to the brake. Brakes are applied by pulling the lever and released.

DRUM BRAKEA drum brake is a brake that uses friction caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum.The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or "band" wrapping around the outside of a drum.HISTORYThe modern automobile drum brake was invented in 1902 by Louis Renault. He used woven asbestos lining for the drum brakes lining, as no alternative dissipated heat like the asbestos lining, though Maybach has used a less sophisticated drum brake. In the first drum brakes, levers and rods or cables operated the shoes mechanically. From the mid-1930s, oil pressure in a small wheel cylinder and pistons (as in the picture) operated the brakes, though some vehicles continued with purely mechanical systems for decades. Some designs have two wheel cylinders.The shoes in drum brakes wear thinner, and brakes required regular adjustment until the introduction of self-adjusting drum brakes in the 1950s. In the 1960s and 1970s, disc brakes gradually replaced drum brakes on the front wheels of cars. Now practically all cars use disc brakes on the front wheels, and many use disc brakes on all wheels. However, drum brakes are still often used for handbrakes, as it has proven very difficult to design a disc brake suitable for holding a parked car. Moreover, it is very easy to fit a drum handbrake inside a disc brake so that one unit serves as both service brake and handbrake.Early brake shoes contained asbestos. When working on brake systems of older cars, care must be taken not to inhale any dust present in the brake assembly. The United States Federal Government began to regulate asbestos production, and brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements; however, technology eventually advanced to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries also limit the use of asbestos in brakes.COMPONENTSDrum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.BACKING PLATEThe backing plate provides a base for the other components. It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes, and assorted hardware. Since all braking operations exert pressure on the backing plate, it must be strong and wear-resistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also added in recent years.

Back plate made in the pressing shop. BRAKE DRUMThe brake drum is generally made of a special type of cast iron that is heat-conductive and wear-resistant. It is positioned close to the brake shoe without actually touching it, and rotates with the wheel and axle. When a driver applies the brakes, the lining pushes against the inner surface of the drum, generating friction heat that can reach as high as 600 F (316C). WHEEL CYLINDEROne wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes, forcing them against the drum. When the driver releases the brakes, the force of the brake shoe return springs returns the piston to its original position. The parts of the wheel cylinder are as follows:

Cut-away section of a wheel cylinder. BRAKE SHOEBrake shoes are typicaslly made of two pieces of sheet steel welded together. The friction material is either riveted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three V"-shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle and has the lining positioned differently than the secondary shoe. Quite often the two shoes are interchangeable, so close inspection for any variation is important.Linings must be resistant against heat and wear and have a high friction coefficient unaffected by fluctuations in temperature and humidity. Materials that make up the brake shoe include, friction modifiers (which can include graphite and cashew nut shells), powdered metal such as lead, zinc, brass, aluminium and other metals that resist heat fade, binders, curing agents and fillers such as rubber chips to reduce brake noise. AUTOMATIC SELF-ADJUSTERThe self-adjuster is used to adjust the distance between the brake shoe and the drum automatically as brake shoes wear.

Sectional layout showing the push rods, nut adjuster and lever pawl. IN OPERATION NORMAL BRAKINGWhen the brakes are applied, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released, return springs pull the shoes back to their rest position. AUTOMATIC SELF-ADJUSTMENTAs the brake linings wear, the shoes must travel a greater distance to reach the drum. When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum. Here, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.EMERGENCY BRAKEThe parking brake (emergency brake) system controls the brakes through a series of steel cables that are connected to either a hand lever or a foot pedal. The idea is that the system is fully mechanical and completely by passes the hydraulic system so that the vehicle can be brought to a stop even if there is a total brake failure. Here the cable pulls on a lever mounted in the brake and is directly connected to the brake shoes. This has the effect of by passing the wheel cylinder and controlling the brakes directly. SELF-APPLYING CHARACTERISTICDrum brakes have a natural "self-applying" characteristic, better known as "self-energizing." The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc. Disc brake systems usually have servo assistance ("Brake Booster") to lessen the driver's pedal effort, but some disc braked cars (notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos.Note: In most designs, the "self-applying" effect only occurs on one shoe. While this shoe is further forced into the drum surface by a moment due to friction, the opposite effect is happening on the other shoe. The friction force is trying to rotate it away from the drum. The forces are different on each brake shoe resulting in one shoe wearing faster. It is possible to design a two-shoe drum brake where both shoes are self-applying (having separate actuators and pivoted at opposite ends), but these are very uncommon in practice.

DRUM BRAKE DESIGNS

Rendering of a drum brakeDrum brakes are typically described as either leading/trailing or twin leading.Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or primary/secondary (for duo servo systems) the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point. In this design, one of the brake shoes always experiences the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards. This is particularly useful on the rear brakes, where the parking brake (handbrake or footbrake) must exert enough force to stop the vehicle from travelling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't always the case, the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double-lobed cam that is rotated by the action of the parking brake system.Front drum brakes may be of either design in practice, but the twin leading design is more effective. This design uses two actuating cylinders arranged so that both shoes use the self-applying characteristic when the vehicle is moving forwards. The brake shoes pivot at opposite points to each other.This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more braking force at the front of the vehicle when it is moving forwards, with less at the rear. This helps prevent the rear wheels from locking up, but still provides adequate braking at the rear.

Shimano Nexus front hub with roller brakeThe brake drum itself is frequently made of cast iron, though some vehicles have used aluminum drums, particularly for front-wheel applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces fade. Aluminum drums are also lighter than iron drums, which reduces unsprung weight. Because aluminum wears more easily than iron, aluminum drums frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.

ADVANTAGESDrum brakes are used in most heavy duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes allow simple incorporation of a parking brake.Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disc brakes as the main brakes. The vast majority of rear disc braking systems use a parking brake in which the piston in the caliper is actuated by a cam or screw. This compresses the pads against the rotor. However, this type of system becomes much more complicated when the rear disc brakes use fixed, multi-piston calipers. In this situation, a small drum is usually fitted within or as part of the brake disc. This type of brake is also known as a banksia brake.In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering motor-generators (see regenerative braking), so some hybrid vehicles such as the GMC Yukon Hybrid and Toyota Prius (except the third generation) use drum brakes.Disc brakes rely on pliability of caliper seals and slight runout to release pads, leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and, adjusted correctly, often have less drag when released.Certain heavier duty drum brake systems compensate for load when determining wheel cylinder pressure; a feature rare when discs are employed (Hydropneumatic suspension systems as employed on Citron vehicles adjust brake pressure depending on load regardless of if drum or discs are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load.Due to the fact that a drum brakes friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake self-applying nature, large friction surface contact area, and long life wear characteristics (%life used/kW of braking power). As a driveshaft parking/emergency brakeDrum brakes have also been built onto the transmission's driveshaft as parking brakes (e.g., Chryslers through 1956). This provides the an advantage that it is completely independent of the service brakesbut suffers a severe disadvantage in that, when used with a bumper jack (common in that era) on the rear, and without proper wheel blocks, the differential's action can allow the vehicle to roll off the jack.

DISADVANTAGESDrum brakes, like most other brakes, convert kinetic energy into heat by friction. This heat should dissipate into the surrounding air, but can just as easily transfer to other braking system components. Brake drums must be large to cope with the massive forces involved, and must be able to absorb and dissipate a lot of heat. Heat transfer to air can be aided by incorporating cooling fins onto the drum. However, excessive heating can occur due to heavy or repeated braking, which can cause the drum to distort, leading to vibration under braking.The other consequence of overheating is brake fade. This is due to one of several processes or more usually an accumulation of all of them.1. When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, so the shoes must move farther and the driver must press the brake pedal farther.2. The properties of the friction material can change if heated, resulting in less friction. This can be a much larger problem with drum brakes than disc brakes, since the shoes are inside the drum and not exposed to cooling ambient air. The loss of friction is usually only temporary and the material regains its efficiency when cooled, but if the surface overheats to the point where it becomes glazed the reduction in braking efficiency is more permanent. Surface glazing can be worn away with further use of the brakes, but that takes time.3. Excessive brake drum heating can cause the brake fluid to vaporize, which reduces the hydraulic pressure applied to the brake shoes. Therefore, the brakes provide less deceleration for a given amount of pressure on the pedal. The effect is worsened by poor maintenance. Brake fluid that is old and has absorbed moisture has a lower boiling point, so brake fade occurs sooner.Brake fade is not always due to overheating. Water between the friction surfaces and the drum can act as a lubricant and reduce braking efficiency. The water tends to stay until heated sufficiently to vaporize, at which point braking efficiency returns. All friction braking systems have a maximum theoretical rate of energy conversion. Once that rate is reached, applying greater pedal pressure doesn't change it in fact, the effects mentioned can substantially reduce it. Ultimately, this is what brake fade is, regardless of the mechanisms of its causes. Disc brakes are not immune to any of these processes, but they deal with heat and water more effectively than drums.Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and self-amplification are high enough, the brake stay engaged due to self-application, even when the external application force is released.While disc brake rotors can be machined to clean the friction surface (i.e., 'turning'), the same generally cannot be done with brake drums. Machining the friction surface of a brake drum increases the diameter, which would require oversized shoes to maintain proper contact with the drum. However, since oversized shoes are generally unavailable for most applications, worn or damaged drums generally must be replaced.Another disadvantage of drum brakes is their relative complexity. A person must have a general understanding of how drum brakes work and take simple steps to ensure the brakes are reassembled correctly when doing work on drum brakes. And, as a result of this increased complexity (compared to disc brakes), maintenance of drum brakes is generally more time-consuming. Also, the greater number of parts results in a greater number of failure modes compared to disc brakes. Springs can break from fatigue if not replaced along with worn brake shoes. And the drum and shoes can become damaged from scoring if various components (such as broken springs or self-adjusters) break and become loose inside the drum.Also, drum brakes do not apply immediately when the wheel cylinders are pressurized, because the force of the return springs must be overcome before the shoes start to move towards the drum. This means that the very common hybrid disc/drum systems only brake with the discs on light pedal pressure unless extra hardware is added. In practice, a metering valve prevents hydraulic pressure from reaching the front calipers until pressure rises enough to overcome the return springs in the drum brakes. If the metering valve were left out, the vehicle would stop only with the front discs until return spring pressure on the rear shoes were overcome.

HYDRAULIC BRAKES The brakes which are actuated by the hydraulic pressure (pressure of a fluid) are called hydraulic brakes. Hydraulic brakes are commonly used in the automobiles.

PRINCIPLE Hydraulic brakes work on the principle of Pascals law which states that pressure at a point in a fluid is equal in all directions in space. According to this law when pressure is applied on a fluid it travels equally in all directions so that uniform braking action is applied on all wheels.

CONSTRUCTION AND WORKING OF HYDRAULIC BRAKES When brake pedal in pressed, the force is transmitted to the brake shoes through a liquid (link). The pedal force is multiplied and transmitted to all brake shoes by a force transmission system. Figure 6.1 shows the system of hydraulic brake of a four wheeler automobile. It consists of a master cylinder, four wheel cylinders and pipes carrying a brake fluid from master cylinder to wheel cylinder. applied on all four wheels.

Hydraulic Brake

The master cylinder is connected to all the four-wheel cylinders by tubing or piping. All cylinders and tubes are fitted with a fluid which acts as a link to transmit pedal force from master cylinder to wheel cylinders.

BRAKE FLUID

The fluid filled in the hydraulic brake system is known as brake fluid. It is a mixture of glycerine and alcohol or caster oil and some additives . Master cylinder consists of a piston which is connected to peal through connecting rod. The wheel cylinder consists of two pistons between which fluid is filled. Each wheel brake consists of a cylinder brake drum. This drum is mounted on the inner side of wheel. The drum revolves with the wheel. Two brake shoes whichare mounted inside the drum remain stationary. Heat and wear resistant brake linings are fitted on the surface of the brake shoes.

MASTER CYLINDER

A master cylinder from a Geo StormIn automotive engineering, the master cylinder is a control device that converts non-hydraulic pressure (commonly from a driver's foot) into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic system.As piston move along the bore of the master cylinder, this movement is transferred through the hydraulic fluid, to result in a movement of the slave cylinder. The hydraulic pressure created by moving a piston (inside the bore of the master cylinder) toward the slave cylinder compresses the fluid evenly, but by varying the comparative surface-area of the master cylinder and/or each slave cylinder, one can vary the amount of force and displacement applied to each slave cylinder, relative to the amount of force and displacement applied to the master cylinderVEHICLE APPLICATIONSThe most common vehicle uses of master cylinders are in brake and clutch systems. In brake systems, the operated devices are cylinders inside of brake calipers and/or drum brakes; these cylinders may be called wheel cylinders or slave cylinders, and they push the brake pads towards a surface that rotates with the wheel (this surface is typically either a drum, or a disc, a.k.a. a rotor) until the stationary brake pad(s) create friction against that rotating surface (typically the rotating surface is metal or ceramic/carbon, for their ability to withstand heat and friction without wearing-down rapidly). In the clutch system, the device which the master cylinder operates is called the slave cylinder; it moves the throw out bearing until the high-friction material on the transmission's clutch disengages from the engine's metal (or ceramic/carbon) flywheel. For hydraulic brakes or clutches alike, flexible high-pressure hoses or inflexible hard-walled metal tubing may be used; but the flexible variety of tubing is needed for at least a short length adjacent to each wheel, whenever the wheel can move relative to the car's chassis (this is the case on any car with steering and other suspension movements; some drag racers and go-karts have no rear suspension, as the rear axle is welded to the chassis, and some antique cars also have no rear suspension movement).A reservoir above each master cylinder supplies the master cylinder with enough brake fluid to avoid air from entering the master cylinder (even the typical clutch uses brake fluid, but it may also be referred to as "clutch fluid" in a clutch application). Most modern light trucks and passenger cars have one master cylinder for the brakes which contains two pistons; but many racing vehicles, as well as some classic and antique cars, have two separate master cylinders, each with only one piston (much like hydraulic clutches typically have only 1 piston per master cylinder). Each piston in a master cylinder operates a brake circuit, and for modern light trucks and passenger cars, usually a brake circuit leads to a brake caliper or shoe on only two of the vehicle's wheels, and the other brake circuit provides brake-pressure to power the other two brakes. For safety, this is done so that usually only two wheels lose their braking ability at the same time; it results in longer stopping distances and should be fixed immediately, but at least gives some braking ability, which is preferable to having no braking ability.

WHEEL CYLINDERA wheel cylinder is a component in a drum brake system. It is located in each wheel and is usually at the top, above the shoes. Its responsibility is to exert force onto the shoes so they can contact the drum and stop the vehicle with friction. What connects these wheel cylinders to the shoes are usually small rods shaped like a birds beak. It is very similar to a master cylinder and functions in pretty much the same way, consisting of just a simple little plunger on the inside. On older vehicles these will begin to leak and hinder the performance of the brakes, but are normally inexpensive and easy to replace.The wheel cylinder consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out pushing the shoes into contact with the drum. Wheel cylinders must be rebuilt or replaced if they show signs of leaking.HYDRAULIC FLUIDSHydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids include excavators and backhoes, hydraulic brakes, power steering systems, transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low compressibility.

FUNCTIONS AND PROPERTIES

The primary function of a hydraulic fluid is to convey power. In use, however, there are other important functions of hydraulic fluid such as protection of the hydraulic machine components. The table below lists the major functions of a hydraulic fluid and the properties of a fluid that affect its ability to perform that function.FunctionProperty

Medium for power transfer and control Low compressibility (high bulk modulus) Fast air release Low foaming tendency Low volatility

Medium for heat transfer Good thermal capacity and conductivity

Sealing Medium Adequate viscosity and viscosity index Shear stability

Lubricant Viscosity for film maintenance Low temperature fluidity Thermal and oxidative stability Hydrolytic stability / water tolerance Cleanliness and filterability Demulsibility Antiwear characteristics Corrosion control

Pump efficiency Proper viscosity to minimize internal leakage High viscosity index

Special function Fire resistance Friction modifications Radiation resistance

COMPOSITION BASE STOCKThe original hydraulic fluid, dating back to the time of ancient Egypt, was water. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its inherent lubrication properties and ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks.Natural oils such as rapeseed (also called canola oil) are used as base stocks for fluids where biodegradability and renewable sources are considered important.Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils. OTHER COMPONENTSHydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc. BIODEGRADABLE HYDRAULIC FLUIDSEnvironmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon grapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant. BRAKE FLUIDBrake fluid is a subtype of hydraulic fluid with high boiling point, both when new (specified by the equilibrium boiling point) and after absorption of water vapor (specified by wet boiling point). Under the heat of braking, both free water and water vapor in a braking system can boil into a compressible vapor, resulting in brake failure. Glycol-ether based fluids are hygroscopic, and absorbed moisture will greatly reduce the boiling point over time. Silicone based fluids are not hygroscopic, but their inferior lubrication is not suitable for all braking systems. SAFETYBecause industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees Celsius, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.Fire resistance is a property available with specialized fluids. TRADE NAMESSome of the trade names for hydraulic fluids include Arnica, Tellus, Durad, Fyrquel, Houghto-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube,Valvoline Ultramax and Skydrol.

AIRCRAFT HYDRAULIC SYSTEMSAs aircraft performance increased in mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort. The hydraulic actuators are controlled by valves; these in turn are operated directly by input from the aircrew (hydro-mechanical) or by computers obeying control laws (fly by wire). Hydraulic power is used for other purposes. It can be stored in accumulators to start an auxiliary power unit (APU) for self-starting the aircraft's main engines. Many aircraft equipped with the M61 family of cannon use hydraulic power to drive the gun system, permitting reliable high rates of fire.The hydraulic power itself comes from pumps driven by the engines directly, or by electrically driven pumps. In modern commercial aircraft these are electrically driven pumps; should all the engines fail in flight the pilot will deploy a propeller-driven electric generator called a Ram Air Turbine (RAT) which is concealed under the fuselage.This provides electrical power for the hydraulic pumps and control systems as power is no longer available from the engines. In that system and others electric pumps can provide both redundancy and the means of operating hydraulic systems without the engines operating, which can be very useful during maintenance/. SPECIFICATION Aircraft hydraulic fluids fall under various specifications: Common petroleum-based: Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from 65 F (54C) to 275 F (135C), red color Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to 40 F (40C) degrees. Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity. Phosphate-ester based: Skydrol hydraulic fluid meets Military Specifications . The list below contains most of the existing industry specifications and approvals: S.A.E.-Ac974 S.A.E. - AS1241 Boeing Seattle - BMS3-11 Boeing Long Beach - DMS2014 Boeing Long Island- CDS5478 Lockheed - LAC C-34-1224British Aerospace - BAC M.333.B Bombardier - BAMS 564-003Below are some of the more common aircraft Phosphate-ester based hydraulic fluids. Skydrol 500B-4 (Type IV class 2): The Skydrol 500 series of fluids has the longest service history among phosphate ester products. The first version, Skydrol 500, was introduced in 1952. Steady improvements to the formulation led in 1978 to the current version, Skydrol 500B-4 which contains the same breakthrough anti erosion additive and acid scavenger found in Skydrol LD-4 . Skydrol 500B-4 is the most worker friendly of the aviation phosphate esters; it is least irritating to skin and less prone to form mists which can be irritating to the respiratory tract . This has given the product enormous popularity for use in work shops and indoor test stands. Skydrol LD-4 (Type IV class 1): Was also introduced in 1978, and is today the largest selling aviation phosphate ester fluid in the world. At the time of its introduction it was a breakthrough product, solving problems of valve erosion and thermal stability common in earlier fluids. Its excellent thermal stability under real world conditions has given it a reputation as the gold standard among Type IV fluids. In recent years it has received an additional qualification of 5000 psi approval under Boeing BMS 3-11 (Type V, Grade B and Grade C). Skydrol LD-4 features low density, excellent thermal stability, valve erosion prevention, and deposit control. Skydrol 5 (Type V): Introduced in 1996, Skydrol 5 was the first Type V fluid qualified under the Boeing BMS 3-11 specification. Skydrol 5 offers higher temperature capability than Type IV fluids, the lowest density, and better paint compatibility. Skydrol 5 does not have universal air frame manufacturer approval. Skydrol PE-5 (Type V): Skydrol PE-5, introduced in 2010, has full approval from Airbus and Boeing for use in all of their aircraft models. Skydrol PE-5 was developed to meet and exceed the more demanding Type V fluid requirements. It features the longest fluid life of any commercially available fluid, low density and low viscosity at low temperature; an unbeatable combination of the best features for optimum fluid performance. Exxon HyJet IV-A plus (Type IV): Exxon HyJet IV-A plus is a fire-resistant phosphate ester hydraulic fluid designed for use in commercial aircraft. It is the best-performing Type IV fluid and approaches to a great extent many of the performance capabilities of Type V fluids, including high temperature stability, fluid life, low density, and rust protection. It is superior to all other Type IV fluids in these respects. Exxon HyJet IV-A plus meets the specifications of all major aircraft manufacturers and SAE AS1241. Exxon Hyjet V (Type V): Exxon HyJet V is a Type V fire-resistant phosphate ester hydraulic fluid, which is superior in thermal and hydrolytic stability to commercially available Type IV hydraulic fluids. Better stability means the extent of fluid degradation in aircraft systems will be less than Type IV fluids, in-service fluid life will be longer, and aircraft operator maintenance costs will be lower. HyJet V provides excellent high and low temperature flow properties (kinematic viscosities) and rust protection. HyJet V has also demonstrated an improvement over the erosion protection performance afforded by Type IV fluids.CONTAMINATIONSpecial, stringent care is required when handling aircraft hydraulic fluid as it is critical to flight safety that it stay free from contamination. It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy aircraft maintenance checks to check contamination. APPLICATION OF BRAKEWhen brake pedal is pressed to apply the brakes, the piston in the master cylinder forces the brake fluid. This increases the pressure of fluid. This pressure is transmitted in all the pipes and upto all wheel cylinders according to Pascals law. This increased pressure forces out the two pistons in the wheel cylinders. These pistons are connected to brake shoes. So, the brake shoes expand out against brake drums. Due to friction between brake linings and drum, wheels slow down and brakes are applied. Two pipes carrying braked fluid are connected to front wheel cylinders which may be same as rear wheel cylinders. The front wheels may also have same type of brakes (drum brakes) as shown in the rear wheels. But, in modern cars, there are disc brakes in the front wheels and drum brakes in the rear wheel.RELEASE OF BRAKESWhen pedal is released, the piston of master cylinder returns to its original position due to retractor spring provided in master cylinder. Thus, fluid pressure drops to original value. The retractor spring provided in the wheel cylinders pulls the brake shoes and contact between drum and brake linings is broken. Therefore, brakes are released.ADVANTAGES AND DISADVANTAGES OF HYDRAULIC BRAKES

Advantages

(a) Equal braking action on all wheels. (b ) Increased braking force. (c) Simple in construction. (d) Low wear rate of brake linings. (e) Flexibility of brake linings. (f) Increased mechanical advantage

Disadvantages(a) Whole braking system fails due to leakage of fluid from brake linings.

(b) Presence of air inside the tubings ruins the whole system.

ADJUSTMENT OF BRAKES

When pedal is pressed to apply brake, there should be atleast 1/2 inch free pedal movement before breaking action starts. This may vary from company to company.The brakes are adjusted as per the above mentioned recommendation before they are ready to use. This is done by following a definite procedure.

(a)List the wheels by screw jack.

(b) Loosen the lock nut for the forward brake shoe and keep it in this position.

(c) Turn the eccentric with other wrench towards the front of automobile till the brake shoe touches the drum.

(d) Release the eccentric while turning the wheel with one hand, till wheel turns freely.

(e) Hold the eccentric in this position and tighter the lock nut.

(f) Repeat the same operation to adjust other shoe, but turn the eccentric in the backward direction of the vehicle.

(g) Above procedure is repeated for all the four wheels.

MECHANICAL DRUM BRAKES BRAKE SHOE REPLACEMENT

Take off the wheel, remove the grease cap and split pin, undo the spindle nut then take off the drum.

Remove the worn brake shoes. DO NOT discard the springs.

Before replacing the brake shoes, check that there is grease on the backing plate under the lever assembly (at the bottom), use molybdenum grease as required.

Assemble the new shoes to the backing plate with the springs between the shoes as shown on the drawing below. The longer spring must be assembled in the lower position, the shorter spring in the top position. Note that left hand side brakes are a mirror image to right hand side brakes. Fit the drum over the brake shoe and reassemble.

Adjust the wheel bearings by tightening the spindle nut until firm while turning the hub slowly to seat the bearings. Loosen the spindle nut and then re-tighten by hand (not with a wrench) to a finger-tight condition, then align the first notch with the hole in the shaft and insert the split pin. Put the wheel back on the hub and tighten the wheel nuts in a cross star pattern as shown below.

BACK SIDE PLATE

Check the override/park brake cable is adjusted correctly. The brakes are now ready to use. Note that the brakes may require several stops to bed in the new shoes before they are fully effective.

MECHANICAL BRAKE ADJUSTEMENTS AND SYSTEM SETTINGS

With overrun braking systems the adjustment sequence is always:

Drum >> Compensator >> Brake Rod >>Coupling

N.B. in the case of over centre handbrakes it is ESSENTIAL that the lever is tied down and the locking pins or stop pegs are fitted as per the manufacturer's instructions BEFORE any adjustment is carried out. Brake Link Diagram (enlarge)

A. Before commencing adjustment - ensure that the handbrake is fully off and that the coupling drawtube is fully extended.

B. Linkages - slacken all nuts.

C. Drum - adjust the linings (AL-KO through the back plate, Knott/Lockheed by the hexagonal nut on outside of back plate), so that resistance can be felt when rotating the drum forwards. Slacken adjuster until drum turns freely in the forward direction.

N.B. ALWAYS rotate the drum in the FORWARD direction, NEVER in REVERSE, otherwise the auto reverse mechanism will be activated and correct adjustment will be impossible.

D. Repeat - on other brake units on the trailer.

E. If an over centre handbrake is fitted - remove any locking pins or stop pegs, untie the lever. Apply the hand brake three or four times to ensure that brake shoes are centralised. Refit pins or pegs and retie lever down after last application and before proceeding to (F) below.

F. Anchor plates - check that the nuts securing outer bowden cable are tight.

G. Compensator - clean off any old grease and dirt. Adjust any slack out of the inner cable, but do NOT put under tension. Check that assembly runs parallel to the axle tube and that nuts and lock nuts are tight. Smear with clean grease.

H. Brake rod - should pass through the centre of the anchor plate by at least 50mm in order to provide correct support. If rod length is excessive, additional support at the front of the trailer will be required. .Adjust so that the overrun lever/brake link just butts up against the rear end of the draw tube shaft. Tighten all lock nuts.

N.B. this applies to all over centre and gas strut handbrakes and some AL-KO spring cylinder models. Other earlier models from Knott and Bradley, particularly those operating with rods instead of cables, may require some clearance between the draw tube and brake lever - consult the manufacturer's instructions.

I. Spring cylinder - if fitted with a ratchet handbrake adjust the locknuts to give the required clearance.

J. If fitted with an over centre handbrake - remove any locking pins or stop pegs and untie the lever.

K. Testing - fully apply the handbrake several times to test its operation. With a ratchet handbrake , the adjustment is correct if slight and equal braking resistance can be felt at each hub when turning the wheels in a forward direction with the handbrake on the second or third tooth. The same applies to gas strut models when the handbrake lever is held manually in position equivalent to first or second tooth .For over centre models apply the handbrake and turn each wheel in the reverse direction until it locks. All brakes should lock firmly. If not, their adjustment is not tight enough and they should be readjusted as per (C) above.

L. Remove the axle stands and jack - Check torque settings on wheel bolts.See "Wheel Bolt Tightening" for exact figures and tightening sequence.

M. Road test - if practicable - ensuring that braking is smooth and progressive at various road speeds. (N.B. always carry out brake testing taking due account of other road users. (This is assuming that the tests cannot be carried out on private roads.)

N.B. It is important that reference is also made to the brake and hub manufacturers' maintenance instructions.

CONSTRUCTION AND WORKING OF MECHANICAL BRAKESInternal expanding shoe brakes are most commonly used in automobiles. In an automobile, the wheel is fitted on a wheel drum. The brake shoes come in contact with inner surface of this drum to apply brakes.The construction of internal expanding mechanical brake is shown in Figure 6.2. The whole assembly consists of a pair of brake shoes along with brake linings, a retractor spring two anchor pins a cam and a brake drum. Brake linings are fitted on outer surface of each brake shoe. The brake shoes are hinged at one end by anchor pins. Other end of brake shoe is operated by a cam to expand it out against brake drum. A retracting springbrings back shoes in their original position when brakes are not applied. The brake drum closes inside it the whole mechanism to protect it from dust and first. A plate holds whole assembly and fits to car axle. It acts as a base to fasten the brake shoes and other operating mechanism.HOW BRAKE ARE APPLIED AND RELEASEDWhen brake pedal is pressed, the cam turns through brake linkages. Brake shoes expand towards brake drum due to turning of cam. The brake linings, rub against brake drum and therefore motion of wheels is stopped. The pedal force is transmitted to the brake shoes through a mechanical linage. This mechanism also removed, the retractor spring brings back shoes in original position and brakes are released multiplies the force to apply the brakes effectively.

DISC BRAKEA disc brake is a wheel brake which slows rotation of the wheel by the friction caused by pushing brake pads against a brake disc with a set of calipers. The brake disc (or rotor in American English) is usually made of cast iron, but may in some cases be made of composites such as reinforced carboncarbon or ceramic matrix composites. This is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads, mounted on a device called a brake caliper, is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes convert motion to heat, and if the brakes get too hot, they become less effective, a phenomenon known as brake fade. Modern motor cars are fitted with disc brakes instead of conventional drum type brakes. In Santro car and Maruti-800, front wheels are provided with disc brakes whereas rear wheel are provided with drum brakes. A disc brake consists of a rotating disc and two friction pads which are actuated by hydraulic braking system as described earlier. The friction pads remain free on each side of disc when brakes are no applied. They rub against disc when brakes are applied to stop the vehicle. These brakes are applied in the same manner as that of hydraulic brakes. But mechanism of stopping vehicle is different than that of drum brakes.

HISTORYDisc-style brakes development and use began in England in the 1890s. The first caliper-type automobile disc brake was patented by Frederick William Lanchester in his Birmingham, UK factory in 1902 and used successfully on Lanchester cars. Compared to drum brakes, disc brakes offer better stopping performance, because the disc is more readily cooled. As a consequence discs are less prone to the "brake fade"; and disc brakes recover more quickly from immersion (wet brakes are less effective). Most drum brake designs have at least one leading shoe, which gives a servo-effect. By contrast, a disc brake has no self-servo effect and its braking force is always proportional to the pressure placed on the brake pad by the braking system via any brake servo, braking pedal or lever, this tends to give the driver better "feel" to avoid impending lockup. Drums are also prone to "bell mouthing", and trap worn lining material within the assembly, both causes of various braking problemsDisc-style brakes development and use began in England in the 1890s. The first caliper-type automobile disc brake was patented by Frederick William Lanchester in his Birmingham factory in 1902 and used successfully on Lanchester cars. However, the limited choice of metals in this period meant that he had to use copper as the braking medium acting on the disc. The poor state of the roads at this time, no more than dusty, rough tracks, meant that the copper wore quickly, making the disc brake system non-viable (as recorded in The Lanchester Legacy). It took another half century for his innovation to be widely adopted.The 1950 Crosley Hot Shot is often given credit for the first U.S. production disc brakes but the Chrysler Crown Imperial actually had them first as standard equipment at the beginning of the 1949 model year. The Crosley disc was a Goodyear development, a caliper type with ventilated rotor, originally designed for aircraft applications. Only the Hot Shot featured it.Lack of sufficient research caused enormous reliability problems, especially in regions requiring the use of salt on winter roads, such as sticking and corrosion. Drum brake conversions for Hot Shots were quite popular.The Chrysler four-wheel disc brake system was more complex and expensive than Crosley's, but far more efficient and reliable. It was built by Auto Specialties Manufacturing Company (Ausco) of St. Joseph, Michigan, under patents of inventor H.L. Lambert, and was first tested on a 1939 Plymouth. Unlike the caliper disc, the Ausco-Lambert used twin expanding discs that rubbed against the inner surface of a cast-iron brake drum, which doubled as the brake housing.The discs spread apart to create friction against the inner drum surface through the action of standard wheel cylinders.Chrysler discs were "self energizing," in that some of the braking energy itself contributed to the braking effort. This was accomplished by small balls set into oval holes leading to the brake surface. When the disc made initial contact with the friction surface, the balls would be forced up the holes forcing the discs further apart and augmenting the braking energy. This made for lighter braking pressure than with calipers, avoided brake fade, promoted cooler running, and provided one-third more friction surface than standard Chrysler twelve-inch drums. But because of the expense, the brakes were only standard on the Chrysler Crown Imperial through 1954 and the Town and Country Newport in 1950. They were optional, however, on other Chryslers, priced around $400, at a time when an entire Crosley Hot Shot retailed for $935. Today's owners consider the Ausco-Lambert very reliable and powerful, but admit its grabbiness and sensitivity.Reliable caliper-type disc brakes were developed in the UK by Dunlop and first appeared in 1953 on the Jaguar C-Type racing car. The 1955 Citron DS featuring powered inboard front disc brakes was the first French application of this technology, while the 1956 Triumph TR3 was the first English production car to feature modern disc brakes The first production car to have disc brakes at all 4 wheels was the Austin-Healey 100S in 1954. The first British company to market a production saloon (US: sedan) fitted with disc brakes to all four wheels was Jensen Motors with the introduction of a Deluxe version of the Jensen 541 with Dunlop disc brakes. The first German production car with disc brakes was the 1961 Mercedes-Benz 220SE coupe featuring British-built Girling units on the front. The next American production automobile equipped with caliper-type disc brakes was the 1963 model year Studebaker Avanti (the Bendix system optional on some of the other Studebaker models). Front disc brakes became standard equipment in 1965 on the Rambler Marlin (the Bendix units were optional on all American Motors "senior" platform models), the Ford Thunderbird,and the Lincoln Continental A four-wheel disc brake system was also introduced in 1965 on the Chevrolet Corvette StingrayCompared to drum brakes, disc brakes offer better stopping performance, because the disc is more readily cooled. As a consequence discs are less prone to the "brake fade" caused when brake components overheat; and disc brakes recover more quickly from immersion (wet brakes are less effective). Most drum brake designs have at least one leading shoe, which gives a servo-effect; see leading/trailing drum brake. By contrast, a disc brake has no self-servo effect and its braking force is always proportional to to the pressure placed on the brake pad by the braking system via any brake servo, braking pedal or lever; this tends to give the driver better "feel" to avoid impending lockup. Drums are also prone to "bell mouthing", and trap worn lining material within the assembly, both causes of various braking problems.Many early implementations for automobiles located the brakes on the inboard side of the driveshaft, near the differential, but most brakes today are located inside the road wheels. (An inboard location reduces the unsprung weight and eliminates a source of heat transfer to the tires.)Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Discs have now become the more common form in most passenger vehicles, although many (particularly light weight vehicles) use drum brakes on the rear wheels to keep costs and weight down as well as to simplify the provisions for a parking brake. As the front brakes perform most of the braking effort, this can be a reasonable compromise. The first motorcycles to use disc brakes were racing vehicles. The first mass-produced road-going motorcycle to sport a disc-brake was the 1969 Honda CB750. Disc brakes are now common on motorcycles, mopeds and even mountain bikes Historically, brake discs were manufactured throughout the world with a strong concentration in Europe and America.

BRAKE DISCThe brake disc is the disc component of a disc brake against which the brake pads are applied. The design of the disc varies somewhat. Some are simply solid cast iron, but others are hollowed out with fins or vanes joining together the disc's two contact surfaces (usually included as part of a casting process). The weight and power of the vehicle determines the need for ventilated discs. The "ventilated" disc design helps to dissipate the generated heat and is commonly used on the more-heavily-loaded front discs.Many higher-performance brakes have holes drilled through them. This is known as cross-drilling and was originally done in the 1960s on racing cars. For heat dissipation purposes, cross drilling is still used on some braking components, but is not favored for racing or other hard use as the holes are a source of stress cracks under severe conditions.Discs may also be slotted, where shallow channels are machined into the disc to aid in removing dust and gas. Slotting is the preferred method in most racing environments to remove gas and water and to deglaze brake pads. Some discs are both drilled and slotted. Slotted discs are generally not used on standard vehicles because they quickly wear down brake pads; however, this removal of material is beneficial to race vehicles since it keeps the pads soft and avoids vitrification of their surfaces.As a way of avoiding thermal stress, cracking and warping, the disc is sometimes mounted in a half loose way to the hub with coarse splines. This allows the disc to expand in a controlled symmetrical way and with less unwanted heat transfer to the hub.On the road, drilled or slotted discs still have a positive effect in wet conditions because the holes or slots prevent a film of water building up between the disc and the pads. Cross-drilled discs may eventually crack at the holes due to metal fatigue. Cross-drilled brakes that are manufactured poorly or subjected to high stresses will crack much sooner .

MOTORCYCLES AND SCOOTERSThis section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.

A drilled motorcycle brake discMotorcycle and scooters disc brakes have become increasingly sophisticated since their introduction in 1962 on the Lambretta TV175 Series 3. Motorcycle discs are usually stainless steel, drilled and occasionally slotted, to help remove rain water. Many motorcycle discs are of a floating design where the disc rides on small dowels and is allowed to slightly move laterally. This allows for better disc centering when used with a fixed caliper. It can also prevent heat transfer to the wheel hub under hard braking. This allows the disc to expand while heating up without increasing tension in such a way that the disc would become warped. Calipers have evolved from simple "single-piston" units to two-, four- and even six-piston items. Since (compared to cars) motorcycles have a higher centre of gravity:wheelbase ratio, they experience more weight transference when braking. The front brake(s) provide most of the required deceleration, while the rear brake serves mainly to "balance" the motorcycle during braking. A modern sports bike will typically have twin front discs of large diameter, but only a very much smaller single rear disc. This is because the rear wheel can only transfer a fraction of the stopping power due to the weight transfer to the front that occurs when braking. The same effect lets the front wheel transfer a lot more stopping power before locking up.

BICYCLEBICYCLE BRAKE RAKE DISC BRAKES

Mountain bike front disc brake

Rear disc brake caliper and rotor on a mountain bikeMountain bike disc brakes range from simple, mechanical (cable) systems, to expensive and powerful, six-piston hydraulic disc systems, commonly used on downhill racing bikes. Improved technology has seen the creation of the first vented discs for use on mountain bikes, similar to those on cars, introduced to help avoid heat fade on fast alpine descents. Although less common, discs are also used on road bicycles for all-weather cycling with predictable braking, although drums are sometimes preferred as harder to damage in crowded parking, where discs are sometimes bent. Most bicycle brake discs are made of steel. Stainless steel is preferred due to its anti-rust properties. Some lightweight discs are made of titanium or aluminium. Discs are thin, often about 2mm. Some use a two-piece floating disc style, others use a floating caliper, others use pads that float in the caliper, and some use one moving pad that makes the caliper slide on its mounts, pulling the other pad into contact with the disc. Because the "motor" is small, an uncommon feature of bicycle brakes is that the pads retract to eliminate residual drag when the brake is released. In contrast, most other brakes drag the pads lightly when released so as to minimise initial operational travel.

OTHER VEHICLES SLIPPERY RAIL DISC BRAKESDisc brakes are increasingly used on very large and heavy road vehicles, where previously large drum brakes were nearly universal. One reason is that the disc's lack of self-assist makes brake force much more predictable, so peak brake force can be raised without more risk of braking-induced steering or jackknife on articulated vehicles. Another is disc brakes fade less when hot, and in a heavy vehicle air and rolling drag and engine braking are small parts of total braking force, so brakes are used harder than on lighter vehicles, and drum brake fade can occur in a single stop. For these reasons, a heavy truck with disc brakes can stop in about 120% the distance of a passenger car, but with drums stopping takes about 150% the distance.In Europe, stopping distance regulations essentially require disc brakes for heavy vehicles. In the U.S., drums are allowed and are typically preferred for their lower purchase price, despite higher total lifetime cost and more frequent service intervals.

A railroad bogie and disc brakesStill-larger discs are used for railroads and some airplanes. Passenger rail cars and light rail often use disc brakes outboard of the wheels, which helps ensure a free flow of cooling air. In contrast, some airplanes have the brake mounted with very little cooling and the brake gets quite hot in a stop, but this is acceptable as there is then time for cooling, and where the maximum braking energy is very predictable.For automotive use, disc brake discs are commonly manufactured out of a material called grey iron. The SAE maintains a specification for the manufacture of grey iron for various applications. For normal car and light-truck applications, SAE specification J431 G3000 (superseded to G10) dictates the correct range of hardness, chemical composition, tensile strength, and other properties necessary for the intended use. Some racing cars and airplanes use brakes with carbon fiber discs and carbon fiber pads to reduce weight. Wear rates tend to be high, and braking may be poor or grabby until the brake is hot.

RACING

Reinforced carbon brake disc on a Ferrari F430 Challenge race car.In racing and very-high-performance road cars, other disc materials have been employed. Reinforced carbon discs and pads inspired by aircraft braking systems such as those used on Concorde were introduced in Formula One by Brabham in conjunction with Dunlop in 1976. Carboncarbon braking is now used in most top-level motorsport worldwide, reducing unsprung weight, giving better frictional performance and improved structural properties at high temperatures, compared to cast iron. Carbon brakes have occasionally been applied to road cars, by the French Venturi sports car manufacturer in the mid 1990s for example, but need to reach a very high operating temperature before becoming truly effective and so are not well suited to road use. The extreme heat generated in these systems is easily visible during night racing, especially at shorter tracks. It is not uncommon to be able to look at the cars, either live in person or on television and see the brake discs glowing red during application ceramic composites.

Mercedes Benz AMG carbon ceramic brake

Porsche Carrera composite ceramic brakeCeramic discs are used in some high-performance cars and heavy vehicles.The first development of the modern ceramic brake was made by British engineers working in the railway industry for TGV applications in 1988. The objective was to reduce weight, the number of brakes per axle, as well as provide stable friction from very high speeds and all temperatures. The result was a carbon-fibre-reinforced ceramic process which is now used in various forms for automotive, railway, and aircraft brake applications.Due to the high heat tolerance and mechanical strength of ceramic composite discs, they are often used on exotic vehicles where the cost is not prohibitive to the application. They are also found in industrial applications where the ceramic disc's light weight and low-maintenance properties justify the cost relative to alternatives. Composite brakes can withstand temperatures that would make steel discs bendable.Porsche's Composite Ceramic Brakes (PCCB) are siliconized carbon fiber, with very high temperature capability, a 50% weight reduction over iron discs (therefore reducing the unsprung weight of the vehicle), a significant reduction in dust generation, substantially increased maintenance intervals, and enhanced durability in corrosive environments over conventional iron discs. Found on some of their more expensive models, it is also an optional brake for all street Porsches at added expense. It is generally recognized by the bright yellow paintwork on the aluminum six-piston calipers that are matched with the discs. The discs are internally vented much like cast-iron ones, and cross-drilled.DISC DAMAGE MODESDiscs are usually damaged in one of four ways: scarring, cracking, warping or excessive rusting. Service shops will sometimes respond to any disc problem by changing out the discs entirely, This is done mainly where the cost of a new disc may actually be lower than the cost of labour to resurface the original disc. Mechanically this is unnecessary unless the discs have reached manufacturer's minimum recommended thickness, which would make it unsafe to use them, or vane rusting is severe (ventilated discs only). Most leading vehicle manufacturers recommend brake disc skimming (US: turning) as a solution for lateral run-out, vibration issues and brake noises. The machining process is performed in a brake lathe, which removes a very thin layer off the disc surface to clean off minor damage and restore uniform thickness. Machining the disc as necessary will maximise the mileage out of the current discs o Excessive lateral run-out.Measuring this is accomplished using a dial indicator on a fixed rigid base, with the tip perpendicular to the brake disc's face. It is typically measured about 12in (12.7mm) from the outside diameter of the disc. The disc is spun. The difference between minimum and maximum value on the dial is called lateral runout. Typical hub/disc assembly runout specifications for passenger vehicles are around 0.0020in (50.8m). Runout can be caused either by deformation of the disc itself or by runout in the underlying wheel hub face or by contamination between the disc surface and the underlying hub mounting surface. Determining the root cause of the indicator displacement (lateral runout) requires disassembly of the disc from the hub. Disc face runout due to hub face runout or contamination will typically have a period of 1 minimum and 1 maximum per revolution of the brake disc.Discs can be machined to eliminate thickness variation and lateral runout. Machining can be done in situ (on-car) or off-car (bench lathe). Both methods will eliminate thickness variation. Machining on-car with proper equipment can also eliminate lateral runout due to hub-face non-perpendicularity.Incorrect fitting can distort (warp) discs; the disc's retaining bolts (or the wheel/lug nuts, if the disc is simply sandwiched in place by the wheel, as on many cars) must be tightened progressively and evenly. The use of air tools to fasten lug nuts is extremely bad practice, unless a torque tube is also used. The vehicle manual will indicate the proper pattern for tightening as well as a torque rating for the bolts. Lug nuts should never be tightened in a circle. Some vehicles are sensitive to the force the bolts apply and tightening should be done with a torque wrench.Often uneven pad transfer is confused for disc warping. In reality, the majority of brake discs which are diagnosed as "warped" are actually simply the product of uneven transfer of pad material. Uneven pad transfer will often lead to a thickness variation of the disc. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly; this is pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.17mm (0.0067in) or greater (on automobile discs).This type of thickness variation has many causes, but there are three primary mechanisms which contribute the most to the propagation of disc thickness variations connected to uneven pad transfer. The first is improper selection of brake pads for a given application. Pads which are effective at low temperatures, such as when braking for the first time in cold weather, often are made of materials which decompose unevenly at higher temperatures. This uneven decomposition results in uneven deposition of material onto the brake disc. Another cause of uneven material transfer is improper break in of a pad/disc combination. For proper break in, the disc surface should be refreshed (either by machining the contact surface or by replacing the disc as a whole) every time the pads are changed on a vehicle. Once this is done, the brakes are heavily applied multiple times in succession. This creates a smooth, even interface between the pad and the disc. When this is not done properly the brake pads will see an uneven distribution of stress and heat, resulting in an uneven, seemingly random, deposition of pad material. The third primary mechanism of uneven pad material transfer is known as "pad imprinting." This occurs when the brake pads are heated to the point that the material begins to break-down and transfer to the disc. In a properly broken in brake system (with properly selected pads), this transfer is natural and actually is a major contributor to the braking force generated by the brake pads. However, if the vehicle comes to a stop and the driver continues to apply the brakes, the pads will deposit a layer of material in the shape of the brake pad. This small thickness variation can begin the cycle of uneven pad transfer.Once the disc has some level of variation in thickness, uneven pad deposition can accelerate, sometimes resulting in changes to the crystal structure of the metal that composes the disc in extreme situations. As the brakes are applied, the pads slide over the varying disc surface. As the pads pass by the thicker section of the disc, they are forced outwards. The foot of the driver applied to the brake pedal naturally resists this change, and thus more force is applied to the pads. The result is that the thicker sections see higher levels of stress. This causes an uneven heating of the surface of the disc, which causes two major issues. As the brake disc heats unevenly it also expands unevenly. The thicker sections of the disc expand more than the thinner sections due to seeing more heat, and thus the difference in thickness is magnified. Also, the uneven distribution of heat results in further uneven transfer of pad material. The result is that the thicker-hotter sections receive even more pad material than the thinner-cooler sections, contributing to a further increase in the variation in the disc's thickness. In extreme situations, this uneven heating can actually cause the crystal structure of the disc material to change. When the hotter sections of the discs reach extremely high temperatures (12001300F or 649704C ), the carbon within the cast iron of the disc will react with the iron molecules to form a carbide known as cementite. This iron carbide is very different from the cast iron the rest of the disc is composed of. It is extremely hard, verybrittle, and does not absorb heat well. After cementite is formed, the integrity of the disc is compromised. Even if the disc surface is machined, the cementite within the disc will not wear or absorb heat at the same rate as the cast iron surrounding it, causing the uneven thickness and uneven heating characteristics of the disc to return.

SCARRINGScarring (US: Scoring) can occur if brake pads are not changed promptly when they reach the end of their service life and are considered worn out. Once enough of the friction material has worn away, the pad's steel backing plate (for glued pads) or the pad retainer rivets (for riveted pads) will bear directly upon the disc's wear surface, reducing braking power and making scratches on the disc. Generally a moderately scarred / scored disc, which operated satisfactorily with existing brake pads, will be equally usable with new pads. If the scarring is deeper but not excessive, it can be repaired by machining off a layer of the disc's surface. This can only be done a limited number of times as the disc has a minimum rated safe thickness. The minimum thickness value is typically cast into the disc during manufacturing on the hub or the edge of the disc. In Pennsylvania, which has one of the most rigorous auto safety inspection programs in North America, an automotive disc cannot pass safety inspection if any scoring is deeper than .015 inches (0.38mm), and must be replaced if machining will reduce the disc below its minimum safe thickness.To prevent scarring, it is prudent to periodically inspect the brake pads for wear. A tire rotation is a logical time for inspection, since rotation must be performed regularly based on vehicle operation time and all wheels must be removed, allowing ready visual access to the brake pads. Some types of alloy wheels and brake arrangements will provide enough open space to view the pads without removing the wheel. When practical, pads that are near the wear-out point should be replaced immediately, as complete wear out leads to scarring damage and unsafe braking. Many disc brake pads will include some sort of soft steel spring or drag tab as part of the pad assembly, which is designed to start dragging on the disc when the pad is nearly worn out. The result is a moderately loud metallic squealing noise, alerting the vehicle user that service is required, and this will not normally scar the disc if the brakes are serviced promptly. A set of pads can be considered for replacement if the thickness of the pad material is the same or less than the thickness of the backing steel. In Pennsylvania, the standard is 1/32".CRACKINGCracking is limited mostly to drilled discs, which may develop small cracks around edges of holes drilled near the edge of the disc due to the disc's uneven rate of expansion in severe duty environments. Manufacturers that use drilled discs as OEM typically do so for two reasons: appearance, if they determine that the average owner of the vehicle model will prefer the look while not overly stressing the hardware; or as a function of reducing the unsprung weight of the brake assembly, with the engineering assumption that enough brake disc mass remains to absorb racing temperatures and stresses. A brake disc is a heat sink, but the loss of heat sink mass may be balanced by increased surface area to radiate away heat. Small hairline cracks may appear in any cross drilled metal disc as a normal wear mechanism, but in the severe case the disc will fail catastrophically. No repair is possible for the cracks, and if cracking becomes severe, the disc must be replaced. RUSTINGThe discs are commonly made from cast iron and a certain amount of what is known as "surface rust" is normal. The disc contact area for the brake pads will be kept clean by regular use, but a vehicle that is stored for an extended period can develop significant rust in the contact area that may reduce braking power for a time until the rusted layer is worn off again. Over time, vented brake discs may develop severe rust corrosion inside the ventilation slots, compromising the strength of the structure and needing replacement.

CALIPER

Disc brake caliper (twin-piston, floating) removed from brake pad for changing padsThe brake caliper is the assembly which houses the brake pads and pistons. The pistons are usually made of aluminium or chrome-plated steel.Calipers are of two types, floating or fixed. A fixed caliper does not move relative to the disc and is thus less tolerant of disc imperfections. It uses one or more single or pairs of opposing pistons to clamp from each side of the disc, and is more complex and expensi A floating caliper (also called a "sliding caliper") moves with respect to the disc, along a line parallel to the axis of rotation of the disc; a piston on one side of the disc pushes the inner brake pad until it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc. Floating caliper (single piston)expensive than a floating caliper.A floating caliper (also called a "sliding caliper") moves with respect to the disc, along a line parallel to the axis of rotation of the disc; a piston on one side of the disc pushes the inner brake pad until it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc. Floating caliper (single piston).Designs are subject to sticking failure, caused by dirt or corrosion entering at least one mounting mechanism and stopping its normal movement. This can lead to the caliper's pad's rubbing on the disc when the brake is not engaged or engaging it at an angle. Sticking can result from infrequent vehicle use, failure of a seal or rubber protection boot allowing debris entry, dry-out of the grease in the mounting mechanism and subsequent moisture incursion leading to corrosion, or some combination of these factors. Consequences may include reduced fuel efficiency, extreme heating of the disc or excessive wear on the affected pad. A sticking front caliper may also cause steering vibration.PISTONS AND CYLINDERSThe most common caliper design uses a single hydraulically actuated piston within a cylinder, although high performance brakes use as many as twelve. Modern cars use different hydraulic circuits to actuate the brakes on each set of wheels as a safety measure. The hydraulic design also helps multiply braking force. The number of pistons in a caliper is often referred to as the number of 'pots', so if a vehicle has 'six pot' calipers it means that each caliper houses six pistons.Brake failure can result from failure of the piston to retract, which is usually a consequence of not operating the vehicle during prolonged storage outdoors in adverse conditions. On high-mileage vehicles, the piston seals may leak, which must be promptly corrected. The brake disc must have enough surface to perform well, and the coefficient of friction is the most important factor to be considered when designing a brake system.

BRAKE PADSBrake pads are designed for high friction with brake pad material embedded in the disc in the process of bedding while wearing evenly. Friction can be divided into two parts: Adhesive and abrasive.Depending on the properties of the material of both the pad and the disc and the configuration and the usage, pad and disc wear rates will vary considerably. The properties that determine material wear involve trade-offs between performance and longevity.The brake pads must usually be replaced regularly (depending on pad material), and some are equipped with a mechanism that alerts drivers that replacement is needed, such as a thin piece of soft metal that rubs against the disc when the pads are too thin causing the brakes to squeal, a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin, or an electronic sensor.Generally road-going vehicles have two brake pads per caliper, while up to six are installed on each racing caliper, with varying frictional properties in a staggered pattern for optimum performance.

BRAKE SQUEALSometimes a loud noise or high pitched squeal occurs when the brakes are applied. Most brake squeal is produced by vibration (resonance instability) of the brake components, especially the pads and discs (known as force-coupled excitation). This type of squeal should not negatively affect brake stopping performance. Simple techniques like adding chamfers to linings, greasing or gluing the contact between caliper and the pads (finger to backplate, piston to backplate), bonding insulators (damping material) to pad backplate, inclusion of a brake shim between the brake pad and back plate, etc. may help to reduce squeal. Cold weather combined with high early-morning humidity (dew) often worsens brake squeal, although the squeal stops when the lining reaches regular operating temperatures. Dust on the brakes may also cause squeal; there are many commercial brake cleaning products that can be used to remove dust and contaminants. Finally, some lining wear indicators, located either as a semi-metallic layer within the brake pad material or with an external squealer "sensor", are also designed to squeal when the lining is due for replacement. The typical external sensor is fundamentally different because it overall brake squeal can be annoying to the vehicle passengers, passers-by, pedestrians, etc. especially as vehicle designs become quieter. Noise, vibration, and harshness (NVH) are among the most important priorities for today's vehicle manufacturers.Apart from noise generated Brake judderBrake judder is usually perceived by the driver as minor to severe vibrations transferred through the chassis during braking.The judder phenom