BRAKES

Brakes are devices whose function it is to slow and stop and automobile. They are mandatory for the safe operation of vehicles. When a car is in motion, it has kinetic energy or energy derived from this motion. In order for the car to slow down, this energy must be decreased. This is accomplished by transforming it into another form. In the case of brakes, this form is heat. In short, brakes transform the kinetic energy of the car into heat energy, thus slowing its speed and, if enough energy is transferred, bringing it to a stop.

Brakes have been refined and improved ever since their invention. The increase in traveling speeds as well as the growing weights of cars has made these improvements essential. The faster a car goes and the heavier it is, the harder it is to stop. An effective braking system is needed to accomplish this task. Today's cars often use a combination of disc brakes and drum brakes. Disc brakes are usually located on the front two wheels and drum brakes on the back two wheels. Detroit is searching for even better engineered braking systems that will allow automobiles to decelerate in a shorter distance, while still allowing drivers to maintain control of their car.

THE DEVELOPMENT OF BRAKES

Brakes operate by converting the kinetic energy (motion) of an automobile into heat energy. How effectively this is achieved depends on the type of braking system. There are two main types of brakes that have been used in cars. These are disc brakes and drum brakes.

Disc brakes operate in a similar fashion to that of a bicycle. It involves pushing a block against a spinning wheel. This contact causes friction, which changes kinetic energy into heat energy. Automobiles use two of these blocks, one on each side of the wheel, which helps keep the wheel more stable. When the brake pedal is pushed, the blocks (often called brake shoes) push up against the wheel disc. The actual wheel is attached to this disk and the two spin together. Therefore, when the disc is slowed, the wheel also slows.

Drum brakes have their blocks located in the inside of a drum. Like the disc in disc brakes, the drum in drum brakes is attached to the wheels. When the brake pedal is pressed the curved brake shoes (blocks) are pushed outward so that they make contact with the rotating drum. Just as with disc brakes, this causes friction which turns kinetic energy into heat energy, thus slowing the car.

Not only are their different types of brakes, but there are various systems that operate these brakes. These include mechanical, hydraulic, and power brake systems.

Mechanical brake systems are the most simple of the three. In a mechanical system, when the brake pedal is pressed, it pulls on a line that is connected directly to the brake shoe assembly, causing it to come into contact with either the disc or drum. When the pedal is released, the tension on the line is released, causing the brake shoe to release its contact with the disc or drum. This type of system is the system found on most bicycles.

Hydraulic brake systems involve a master brake cylinder and brake cylinders at each of the wheels. When the brake pedal is pressed, it moves a piston located in the master cylinder. This movement forces brake fluid through tubes to each of the individual wheel cylinders. The pistons in these cylinders are then moved, causing the brake shoes to come into contact with either the disc or drum.

Power brake systems are basically the same as hydraulic brake systems with one key difference. In power braking systems, the movement of the piston in the master cylinder has a little help. During the intake stroke of the engine, a vacuum is created. This vacuum is then used to increase the pressure applied to the piston in the master cylinder.

Another development in brakes came with the invention of the anti-lock brake system. This device helps drivers maintain control when braking in wet or slippery conditions. How it does this is when the brake pedal is pressed, a computer pumps the brake shoes in and out of contact with the disc or drum many times per second. With normal brake systems, when the brakes are applied in slippery conditions, the wheels lock up and the car can easily lose control since it cannot be steered as effectively. With an anti-lock braking system the wheels never lock up so drivers can still steer and therefore better maintain control of their cars.

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Air Brakes

Train air brakes are a combination of mechanical devices operated by compressed air, arranged in a system and controlled manually or pneumatically, by means of which the motion of cars and locomotives is retarded or stopped. The air is supplied by a compressor mounted on the locomotive. It is delivered to the cars through a brake pipe in each locomotive and car, and to the flexible hoses and couplings between them. A series of valves, air reservoirs and pistons transforms changes of pressure in the brake pipe into application or release of pressure by the brake shoes against the wheels

Malfunctions can happen at many different places and from various causes, most of which would be invisible or unrecognizable to a fire agency inspector. On the other hand, the results or symptoms are usually visible and should be recognized by the inspector. He/she should also know what must be done to correct the problem or to isolate the offending car from the train brake system. He/she should never attempt to take such action personally, but should merely satisfy him/herself that company employees have corrected the situation.

The most obvious result of an air brake malfunction is smoke being given off by brake shoes dragging against wheels. Another indicator is an extended brake cylinder piston when those on all other cars are retracted (train brakes released) or a retracted piston when the others are extended (brakes set). On some new cars the brake cylinders are mounted on the wheel trucks and are not readily visible from alongside the car.

In any of the above situations, the offending car should be isolated from the train brake system and its own brakes released until the cause of the malfunction can be determined and corrected. So long as only one car (or a very few in a long train) is isolated, the safety of the train for shortage of braking power is not affected. Trains must have 85% effective brakes and no more than three consecutive cars with inoperative brakes. Isolation is accomplished by closing the cutout cock between the brake pipe and the control valve. Brake release is done by pulling or pushing on the release rod which releases the air in the brake cylinder. This can only be done by a railroad employee, never by a fire agency inspector. The brake shoes may not separate from the wheels until the train starts to move but there will be no pressure on them and the piston will retract.

Retaining valves (retainers) control the exhaust of brake cylinder air. In their normal operating position the air is exhausted directly and quickly when the engineer returns brake pipe pressure to normal. Their purpose is to retain a steady pressure or a controlled slow release depending on which position the handle is set. These valves were originally developed as a safety measure, which would allow the engineer to recharge the air system without losing all braking action on the train. This was quite necessary to avoid runaways by heavy trains on long downgrades. Modern dynamic brakes have largely taken over the function of retainers, which are seldom used in normal operation now.

Retainers are still required to be installed on all railroad rolling stock as a back-up safety system.

Since retainers create prolonged brake shoe pressure on the wheels, they cause overheating and sparking. Agency inspectors should, therefore, report any they observe in other than normal position.

FRICTIONAL BRAKES

Frictional brakes are most common and can be divided broadly into "shoe" or "pad" brakes, using an explicit wear surface, and hydrodynamic brakes, such as parachutes, which use friction in a working fluid and do not explicitly wear. Typically the term "friction brake" is used to mean pad/shoe brakes and excludes hydrodynamic brakes, even though hydrodynamic brakes use friction.

Friction (pad/shoe) brakes are often rotating devices with a stationary pad and a rotating wear surface. Common configurations include shoes that contract to rub on the outside of a rotating drum, such as a band brake; a rotating drum with shoes that expand to rub the inside of a drum, commonly called a "drum brake", although other drum configurations are possible; and pads that pinch a rotating disc, commonly called a "disc brake". Other brake configurations are used, but less often. For example, PCC trolley brakes include a flat shoe which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating drum, and the Ausco Lambert disc brake uses a hollow disc (two parallel discs with a structural bridge) with shoes that sit between the disc surfaces and expand laterally.

PUMPING BRAKES

Pumping brakes are often used where a pump is already part of the machinery. For example, an internal-combustion piston motor can have the fuel supply stopped, and then internal pumping losses of the engine create some braking. Some engines use a valve override called a Jake brake to greatly increase pumping losses. Pumping brakes can dump energy as heat, or can be regenerative brakes that recharge a pressure reservoir called a hydraulic accumulator.

ELECTROMAGNETIC BRAKES

Electromagnetic brakes are likewise often used where an electric motor is already part of the machinery. For example, many hybrid gasoline/electric vehicles use the electric motor as a generator to charge electric batteries and also as a regenerative brake. Some diesel/electric railroad locomotives use the electric motors to generate electricity which is then sent to a resistor bank and dumped as heat. Some vehicles, such as some transit buses, do not already have an electric motor but use a secondary "retarder" brake that is effectively a generator with an internal short-circuit. Related types of such a brake are eddy current brakes, and electro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called “electromagnetic brakes” as well).

DESCRIPTION OF PARTS OF A DRUM BRAKE

Drum: cylindrical part attached to the wheel, against which the brake shoes are pressed to stop the car.

Brake lining: frictional part on the outside edges of the brake shoes.

Return spring: part of the brake mechanism that returns the brake shoes to their initial position.

Piston: cylindrical part that transmits the pressure to and receives pressure from the brake shoes.

Wheel cylinder: type of roller that applies a uniform pressure to the wheel then the brake is activated.

Brake shoe: part on which the brake lining is mounted.

Brake pads: part activated by the piston.

Wheel hub: central part crossed by the axle.

Stud: metal pin.

Disk: round, flat, piece of metal, pressed against the wheel to slow or stop the car.

Brake line: system liquid-transporting tubes.

Splash shield: protector that prevents dirt from fouling the braking system.

Brake linings are probably the most misunderstood part of a brake system.

The output of any brake is directly related to the coefficient of friction (µ) between the lining and the disc or drum.

The challenge is to know what the instantaneous value of µ is during any given stop.

Any design calculations you do, go right out the window if the lining you use does not have the µ value you assumed.

 

The equation for a disc brake                                                          

The best method for determining the actual value of µ for a given lining is from a dynamometer test.

THE STOPPING DISTANCE OF A BRAKE

does not depend on:

•           Type of brakes

•           Size of brakes

The stopping distance of a brake depends on

•           Tire to road friction

•           Vehicle balance

•           Skill of driver

•           System Reaction Time

BRAKE FADE

Brake fade is the loss of performance resulting from the lining friction decreasing as the lining and rotor or drum rises in temperature.