Disc Brakes

INTRODUCTION HISTORY OF DISK BRAKE Ever since the invention of the wheel, if there has been "go" there has been a need for "whoa." As the level of technology of human transportation has increased, the mechanical devices used to slow down and stop vehicles has also become more complex. In this report I will discuss the history of vehicular braking technology and possible future developments. Before Before there there was a "horse"horse-les lesss carria carriage, ge,"" wagons, wagons, and other other animal animal drawn drawn vehicles relied on the animal’s power to both accelerate and decelerate the vehicle. Eventually there was the development of supplemental braking systems consisting of a hand lever to push a wooden friction pad directly against the metal tread of the wheels. In wet conditions these crude brakes would lose any effectiveness. The early early years years of automo automotiv tivee develop developmen mentt were were an intere interesti sting ng time time for the designing engineers, "a period of innovation when there was no established practice and virtually all ideas were new ones and worth trying. Quite rapidly, however, the design of  many components stabilized in concept and so it was with brakes; the majority of  vehicle vehicless soon soon adopted adopted drum brakes brakes,, each each consis consistin ting g of two shoes which could could be expanded inside a drum."

In this chaotic era is the first record of the disk brake. Dr. F.W. Lanchester    patented a design for a disk brake in 1902 in England. It was incorporated into the Lanchester car produced between 1906 through 1914. These early disk brakes were not as effect effective ive at stoppi stopping ng as the contempo contemporar rary y drum drum brakes brakes of that that time time and were were soon soon forgotten. Another important development occurred in the 1920’s when drum brakes were used at all four wheels instead of a single brake to halt only the back axle and wheels such as on the Ford model T. The disk brake was again utilized during World War  II in the landing gear of aircraft. The aircraft disk brake system was adapted for use in automotive applications, first in racing in 1952, then in production automobiles in 1956. United States auto manufacturers did not start to incorporate disk brakes in lower priced non-high-performance cars until the late 1960’s.

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HOW BRAKES WORK  We all know that pushing down on the brake pedal slows a car to a stop. But how does this happen? How does your car transmit the force from your leg to its wheels? How does it multiply the force so that it is enough to stop something as big as a car?

BRAKE BASICS When you depress your brake pedal, your car transmits the force from your foot to its brakes through a fluid. Since the actual brakes require a much greater force than you could apply with your leg, your car must also multiply the force of your foot. It does this in two ways: • •

Mechanical advantage (leverage) Hydraulic force multiplication

The brakes transmit the force to the tires using friction , and the tires transmit that force to the road using friction also. Before we begin our discussion on the components of the brake system, let's cover these three principles:   

Leverage Hydraulics Friction

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LEVERAGE The pedal is designed in such a way that it can multiply the force from your leg several times before any force is even transmitted to the brake fluid.

In the figure above, a force F is being applied to the left end of the lever. The left end of the lever is twice as long (2X) as the right end (X). Therefore, on the right end of  the lever a force of 2F is available, but it acts through half of the distance (Y) that the left end moves (2Y). Changing the relative lengths of the left and right ends of the lever  changes the multipliers.

HYDRAULIC SYSTEMS The basic idea behind any hydraulic system is very simple: Force applied at one  point is transmitted to another point using an incompressible fluid , almost always an oil of some sort. Most brake systems also multiply the force in the process

FRICTION Friction is a measure of how hard it is to slide one object over another. Take a look at the figure below. Both of the blocks are made from the same material, but one is heavier. I think we all know which one will be harder for the bulldozer to push.

Friction force versus weight

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4 To understand why this is, let's take a close look at one of the blocks and the table:

Even though the blocks look smooth to the naked eye, they are actually quite rough at the microscopic level. When you set the block down on the table, the little peaks and valleys get squished together, and some of them may actually weld together. The weight of the heavier block causes it to squish together more, so it is e ven harder to slide. Different materials have different microscopic structures; for instance, it is harder  to slide rubber against rubber than it is to slide steel against steel. The type of material determines the coefficient of friction , the ratio of the force required to slide the block to the block's weight. If the coefficient were 1.0 in our  example, then it would take 100 pounds of force to slide the 100-pound (45 kg) block, or  400 pounds (180 kg) of force to slide the 400-pound block. If the coefficient were 0.1, then it would take 10 pounds of force to slide to the 100-pound block or 40 pounds of  force to slide the 400-pound block. So the amount of force it takes to move a given block is proportional to that   block's weight. The more weight, the more force required. This concept applies for  devices like brakes and clutches, where a pad is pressed against a spinning disc. The more force that presses on the pad, the greater the stopping force.

A SIMPLE BRAKE SYSTEM The distance from the pedal to the pivot is four times the distance from the cylinder to the  pivot, so the force at the pedal will be increased by a factor of four before it is transmitted to the cylinder. The diameter of the brake cylinder is three times the diameter of the pedal cylinder. This further multiplies the force by nine. All together, this system increases the force of your foot by a factor of 36. If you put 10 pounds of force on the pedal, 360  pounds (162 kg) will be generated at the wheel squeezing the brake pads. There are a couple of problems with this simple system. What if we have a leak ? If it is a slow leak, eventually there will not be enough fluid left to fill the brake cylinder, and the brakes will not function. If it is a major leak, then the first time you apply the  brakes all of the fluid will squirt out the leak and you will have complete brake failure. 4

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TYPES OF BRAKES 1. DRUM BRAKES 2. DISC BRAKES (CALLIPER BRAKES)

DRUM BRAKES The drum brake has two brake shoes and a piston. When you hit the brake pedal, the   piston pushes the brake shoes against the drum This is where it gets a little more complicated. as the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force. The extra  braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other  springs help hold the brake shoes in place and return the adjuster arm after it actuates.

DISK BRAKE BASICS:The disk brake has a metal disk instead of a drum. It has a flat shoe, or pad, located on each side of the disk. To slow or stop the car, these two flat shoes are forced tightly against the rotating disk, or rotor. Fluid pressure from the master cylinder forces the pistons to move in. This action pushes the friction pads of the shoes tightly against the disk. The friction between the shoes and the disk slows and stops the disk.

ADVANTAGES OF DISC BRAKES OVER DRUM BRAKES As with almost any artifact of technology, drum brakes and disk brakes both have advantages and disadvantages. Drum brakes still have the edge in cheaper cost and lower  complexity. This is why most cars built today use disk brakes in front but drum brakes in the back wheels, four wheel disks being an extra cost option or shouted as a high  performance feature. Since the weight shift of a decelerating car puts most of the load on the front wheels, the usage of disk brakes on only the front wheels is accepted manufacturing practice. Drum brakes had another advantage compared to early disk brake systems. The geometry of the brake shoes inside the drums can be designed for a mechanical self boosting action. The rotation of the brake drum will push a leading shoe brake pad into   pressing harder against the drum. Early disk brake systems required an outside mechanical brake booster such as a vacuum assist or hydraulic pump to generate the  pressure for primitive friction materials to apply the necessary braking force.

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6 All friction braking technology uses the process of converting the kinetic energy of a vehicle’s forward motion into thermal energy: heat. The enemy of all braking systems is excessive heat. Drums are inferior to disks in dissipating excessive heat: "The common automotive drum brake consists essentially of two shoes which may be expanded against the inner cylindrical surface of a drum. The greater part of heat generated when a brake is applied has to pass through the drum to its outer surface in order to be dissipated to atmosphere, and at the same time (the drum is) subject to quite severe stresses due to the distortion induced by the opposed shoes acting inside the open ended drum.

The conventional disk brake, on the other hand, consists essentially of a flat disk  on either side of which are friction pads; equal and opposite forces may be applied to these pads to press their working surfaces into contact with the braking path of the disks. The heat produced by the conversion of energy is dissipated directly from the surfaces at which it is generated and the deflection of the braking path of the disk is very small so that the stressing of the material is not so severe as with the drum."

The result of overheated brakes is brake fade...the same amount of force at the  pedal no longer provides the same amount of stopping power. The high heat decreases the relative coefficient of friction between the friction material and the drum or disk. Drum brakes also suffer another setback when overheating: The inside radii of the drum expands, the brake shoe outside radii no longer matches, and the actual contact surface is decreased. Another advantage of disk brakes over drum brakes is that of weight. There are two different areas where minimizing weight is important. The first is unsprung weight. This is the total amount of weight of all the moving components of a car between the road and the suspension mounting points on the car’s frame.

Auto designs have gone to such lengths to reduce unsprung weight that some, such as the E-type Jaguar, moved the rear brakes inboard, next to the differential, connected to the drive shafts instead of on the rear wheel hubs. The second "weighty" factor is more of an issue on motorcycles: gyroscopic weight. The heavier the wheel unit, the more gyroscopic resistance to changing direction. Thus the bike’s steering would be higher effort with heavier drum brakes than with lighter disks. Modern race car disk   brakes have hollow internal vents, cross drilling and other weight saving and cooling features. Most early brake drums and disks were made out of cast iron. Current OEM motorcycle disk brakes are usually stainless steel for corrosion resistance, but aftermarket racing component brake disks are still made from cast iron for the improved friction qualities. Other exotic materials have been used in racing applications. Carbon fiber composite disks gripped by carbon fiber pads were common in formula one motorcycles and cars in the early 1990’s, but were outlawed by the respective racing 6

7 sanctioning organizations due to sometimes spectacular failure. The carbon/carbon  brakes also only worked properly at the very high temperatures of racing conditions and would not get hot enough to work in street applications. A recent Ducati concept show bike uses brake disks of silesium, developed by the Russian aerospace industry(3), which claim to have the friction coefficient of cast iron with the light weight of carbon fiber. Another area of development of the disk brake is the architecture of the brake caliper. Early designs had a rigidly mounted caliper gripping with opposed hydraulic  pistons pushing the brake pads against a disk mounted securely to the wheel hub. Later  developments included a single piston caliper floating on slider pins. This system had improved, more even pad wear. Most modern automobiles and my 1982 Kawasaki motorcycle uses this type caliper. Current design paradigm for motorcycle brakes h ave up to six pistons, opposed to grip both sides of a thin, large radius disk that is "floating" on  pins to provide a small amount of lateral movement; two disks per front wheel. Improvements in control have been made available with the application of AntiLock Brake technology. Wheel sensors convey rotation speed of each wheel to a computer that senses when any of them are locked up or in a skid, and modulates individual wheel brake hydraulic pressure to avoid wheel skidding and loss of vehicular  control. The use of exotic materials for additional weight savings would be likely for the future of motor vehicle braking. Disks mounted to the wheel’s rim gripped by an internally located caliper is not necessarily a new design (Porsche, 1963) (4) but could be a futuristic looking option for motorcycle wheels. Electric vehicles of the future will likely utilize regenerative braking, the electric motors become generators to convert kinetic energy back to electricity to recharge the batteries. As production vehicles  become increasingly quicker, the need for "whoa" will always accompany the "go."

TYPES OF DISK BRAKES

The three types of disk brakes are:-

1. FLOATING CALIPER DISK BRAKES 2. FIXED CALIPER DISK BRAKES 3. SLIDING CALIPER DISK CALIPER 

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8 MAIN PARTS:

The main components of a disc brake are: • • •

The brake pads The caliper , which contains a piston The rotor, which is mounted to the hub

BRAKE PAD

CALIPER AND ROTOR 

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WORKING OF DISC BRAKES FLOATING-CALIPER DISK BRAKES

The caliper is the part that holds the break shoes on each side of the disk. In the floating-caliper brake, two steel guide pins are threaded into the steeringknuckle adapter. The caliper floats on four rubber bushings which fit on the inner and outer ends of the two guide pins. The bushings allow the caliper to swing in or out slightly when the brakes are applied When the brakes are applied, the brake fluid flows to the cylinder in the caliper  and pushes the piston out. The piston then forces the shoe against the disk. At the same time, the pressure in the cylinder causes the caliper to pivot inward. This movement  brings the other shoe into tight contact with the disk. As a result, the two shoes “pinch” the disk tightly to produce the braking action

STAGES OF WORKING

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. FIXED-CALIPER DISK BRAKE

This brake usually has four pistons, two on each side of the disk. The reason for the name  fixed-caliper  is that the caliper is bolted solidly to the steering knuckle. When the brakes are applied, the caliper cannot move. The four pistons are forced out of their caliper bores to push the inner and outer brake shoes in against the disk. Some brakes of this type have used only two pistons, one on each side of the disk 

SLIDING-CALIPER DISK BRAKE

The sliding-caliper disk brake is similar to the floating-caliper disk brake. The difference is that sliding-caliper is suspended from rubber bushings on bolts. This  permits the caliper to slide on the bolts when the brakes are applied.

Proper function of the brake depends on (1) the rotor must be straight and smooth, (2) the caliper mechanism must be properly aligned with the rotor, (3) the pads must be  positioned correctly, (4) there must be enough "pad" left, and (5) the lever mechanism must push the pads tightly against the rotor, with "lever" to spare. Most modern cars have disc brakes on the front wheels, and some have disc brakes on all four wheels. This is the part of the brake system that does the actual work of stopping the car  The most common type of disc brake on modern cars is the single-piston floating caliper. In this article, we will learn all about this type of disc brake design

SELF ADJUSTMENT OF DISK BRAKES: Disk brakes are self adjusting. Each piston has a seal on it to prevent fluid leakage. When the brakes are applied, the piston moves toward the disk. This distorts the   piston seal. When the brakes are released, the seal relaxes and returns to its original  position. This pulls the piston away from the disk. As the brakes linings wear, the piston over travels and takes a new position in relation to the seal. This action provides self  adjustment of disk brakes.

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EMERGENCY BRAKES In cars with disc brakes on all four wheels, an emergency brake has to be actuated  by a separate mechanism than the primary brakes in case of a total primary brake failure. Most cars use a cable to actuate the emergency brake.

Some cars with four-wheel disc brakes have a separate drum brake integrated into the hub of the rear wheels. This drum brake is only for the emergency brake system, and it is actuated only by the cable; it has no hydraulics.

DISK BRAKE VENTS A moving car has a certain amount of kinetic energy, and the brakes have to remove this energy from the car in order to stop it. How do the brakes do this? Each time you stop your car, your brakes convert the kinetic energy to heat generated by the friction between the pads and the disc. Most car disc brakes are vented.

Vented disc brakes have a set of vanes, between the two sides of the disc, that  pumps air through the disc to provide cooling . 11

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WHY DISK BRAKES? 

Why disk brakes in a truck or bus that travels in excess of 65 mph? Improved road handling, higher engine ratings and torque, reduced drag and rolling resistance resulting in faster acceleration and higher average speeds Higher vehicle speeds with full loads Higher traffic density, greater chances of emergency braking Extremely high kinetic energy needed to brake on wet roads, high front axle loads effecting vehicle directional stability The power and behavior of drum brakes cannot be improved Disk brakes provide optimum braking while retaining directional stability

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Why are disk brakes more efficient? Flat brake disk (axial brake) under high pressure versus round brake drum (radial  brake) during braking Full friction surface of the brake pad on the plane brake disk   No loss of brake power due to overheating or partial contact from brake drum  parts expansion Disk brakes can withstand higher loads and its efficiency is maintained considerably longer even under the highest stresses Higher residual brake force after repeating braking Brake disks can withstand extremely high temperatures Full contact of brake pads achieve maximum effect  No vitrification of brake pads. Dangerous fading o r slipping is almost completely eliminated

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Why do disk brakes have a better braking behavior? Driver friendly braking behavior. Sensitive braking in all situations and better  Sensitive brake application and better brake feeling Uniform braking from small fluctuations in brake forces Retardation values retained even under heavy stresses Minimal "pulling to one side" due to uneven brake forces Disk brake axial arrangement permits a simple and compact design Linear characteristics lead to an even progression of brake force Basic design principle makes for higher efficiency Low hysteresis is particularly suitable to ABS control cycles

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Why are disk brakes more economical? Clear economic benefits due to long service life and reduced maintenance downtime Long service life of disks and pads versus drum brakes Shorter service downtime due to quick pad changes Good access for visual brake components checks Maintenance free brake components Optimized installation space in the wheel rim resulting in the largest possible  brake disks and pads Optimized cooling resulting from open sliding caliper design with internally ventilated brake discs Even and safe brake pad wear resulting from simple pad guide with level braking faces

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Why do disk brakes have higher safety reserves? Minimal braking effect from high temperatures and extreme driving requirements Minimal heat fading  No brake disk distortion from extreme heat due to internal ventilation with directional stability and large power reserve under high stress The decisive safety aspects of the disk brake design are shorter braking distances High power and safety reserves for emergencies Constant braking power under high stresses

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14 Shortened braking distance under emergency braking with considerably improved directional stability

LIMITATIONS 

BRAKING SYSTEMS FAILS IF THERE IS LEAKAGE IN THE BRAKE LINES

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THE BRAKE SHOES ARE LIABLE TO GET RUINED IF THE BRAKE FLUID LEAKES OUT

TESTING OF DISK BRAKES The individual components are subjected to extensive test on the test bed. The optimum arrangement of components on the axle beam, operational reliability and convincing performance are requirements that must be met prior series production.

Today, all MAN city, inter-city buses and coaches utilize the MAN disk brake system on all wheels with ABS. The disk brake system is used with and without retarders Brake performance is tested on the test track and in racing to ensure their practice. Only after these extensive tests can the disk brake be cleared for production . The brake disks are subjected to the highest stresses from contact pressure. The broad   brake disks with radial cavities made of heat resistant special gray cast iron, are still operational in temperatures in excess of 1380 degrees F

DISK BRAKES IN PRACTICE... All friction brakes (drum and disk) have wear parts and are unsuitable for continuous  braking, i.e. on long down hill runs Disk brakes are no substitute for an engine brake or retarder. Economic driving with sensible use of the engine brake or retarder optimizes the service life of pads and disks that are equal to those in comparable drum brakes Economic use of technology means driving and braking with anticipation and economic thinking. Disk brakes offers safety in any situation

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CONCLUSION Many trucks and buses are equipped with air actuated sliding caliper disk brakes The high contact forces are transmitted mechanically via needle mounted actuating device Depending on size the actuating pressure is transmitted evenly to the brake pads via one or two plungers The easy action, fully sealed guides between the axially moving sliding caliper and fixed brake anchor plate are maintenance free. Integrated automatic adjustment with wear display. There are no brake shafts, external levers or cylinder brackets, as the brake cylinders are directly attached. The high efficiency of 95% is achieved by only a few moving parts and low friction  bearings Asbestos free brake pads 19 to 23 mm thick, depending on version extremely heat resistant brake disks (34 to 45 mm) made of special gray cast iron with internal ventilation The brake disks are 330 to 432 mm in diameter and permissible wear of 6 to 10 mm allowed; depending on version .The service and parking brakes use the same actuating unit and differ only in the shape of the brake cylinder.

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