1、Automotive Brake System analysisAbstract:As traffic tools of the modernization and absolute number increase sharply, traffic accidents are also increasing. Car accident has become severe global social issues. Undoubtedly, advanced auto safety facilities are the driving safety indispensable safeguard
2、. So, we should start, from technology to research and development of high performance, high safety car, also want to strengthen the regular inspection in cars, so timely maintenance investigation, make cars often in good technical status, so as to improve the safety performance of the car. The brak
3、ing system is the most important system in cars.If the brakes fail, the result can be disastrous. Here we introduced automobile brake system Key words: automobile, brake system, ABSThe brake systemThe braking system is the most important system in cars. If the brakes fail, the result can be disastro
4、us. Brakes are actually energy conversion devices, which convert the kinetic energy (momentum) of the vehicle into thermal energy (heat).When stepping on the brakes, the driver commands a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert thou
5、sands of pounds of pressure on each of the four brakes. Two complete independent braking systems are used on the car. They are the service brake and the parking brake.The service brake acts to slow, stop, or hold the vehicle during normal driving. They are foot-operated by the driver depressing and
6、releasing the brake pedal. The primary purpose of the brake is to hold the vehicle stationary while it is unattended. The parking brake is mechanically operated by when a separate parking brake foot pedal or hand lever is set. The brake system is composed of the following basic components: the “mast
7、er cylinder” which is located under the hood, and is directly connected to the brake pedal, converts driver foots mechanical pressure into hydraulic pressure. Steel “brake lines” and flexible “brake hoses” connect the master cylinder to the “slave cylinders” located at each wheel. Brake fluid, speci
8、ally designed to work in extreme conditions, fills the system. “Shoes” and “pads” are pushed by the slave cylinders to contact the “drums” and “rotors” thus causing drag, which (hopefully) slows the car.Disk brake The typical brake system consists of disk brakes in front and either disk or drum brak
9、es in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder .Basically, all car brakes are friction brakes. When the driver applies the brake, the control device forces brake shoes, or pads, against the rotating brake drum or disks at wheel. Frict
10、ion between the shoes or pads and the drums or disks then slows or stops the wheel so that the car is braked. In most modern brake systems , there is a fluid-filled cylinder, called master cylinder, which contains two separate sections, there is a piston in each section and both pistons are connecte
11、d to a brake pedal in the drivers compartment. When the brake is pushed down, brake fluid is sent from the master cylinder to the wheels. At the wheels, the fluid pushes shoes, or pads, against revolving drums or disks. The friction between the stationary shoes, or pads, and the revolving drums or d
12、isks slows and stops them. This slows or stops the revolving wheels, which, in turn, slow or stop the car.The brake fluidThe brake fluid reservoir is on top of the master cylinder. Most cars today have a transparent r reservoir so that you can see the level without opening the cover. The brake fluid
13、 level will drop slightly as the brake pads wear. This is a normal condition and no cause for concern. If the level drops noticeably over a short period of time or goes down to about two thirds full, have your brakes checked as soon as possible. Keep the reservoir covered except for the amount of ti
14、me you need to fill it and never leave a cam of brake fluid uncovered. Brake fluid must maintain a very high boiling point. Exposure to air will cause the fluid to absorb moisture which will lower that boiling point. The brake fluid travels from the master cylinder to the wheels through a series of
15、steel tubes and reinforced rubber hoses. Rubber hoses are only used in places that require flexibility, such as at the front wheels, which move up and down as well as steer. The rest of the system uses non-corrosive seamless steel tubing with special fittings at all attachment points. If a steel lin
16、e requires a repair, the best procedure is to replace the compete line. If this is not practical, a line can be repaired using special splice fittings that are made for brake system repair. You must never use copper tubing to repair a brake system. They are dangerous and illegal.Drum brakeDrum brake
17、s, it consists of the brake drum, an expand, pull back springs, a stationary back plate, two shoes with friction linings, and anchor pins. The stationary back plate is secured to the flange of the axle housing or to the steering knuckle. The brake drum is mounted on the wheel hub. There is a clearan
18、ce between the inner surface of the drum and the shoe lining. To apply brakes, the driver pushes pedal, the expand expands the shoes and presses them to the drum. Friction between the brake drum and the friction linings brakes the wheels and the vehicle stops. To release brakes, the driver release t
19、he pedal, the pull back spring retracts the shoes thus permitting free rotation of the wheels. The brake system assemblies are actuated by mechanical, hydraulic or pneumatic devices. The mechanical leverage is used in the parking brakes fitted in all automobile. When the brake pedal is depressed, th
20、e rod pushes the piston of brake master cylinder which presses the fluid. The fluid flows through the pipelines to the power brake unit and then to the wheel cylinder. The fluid pressure expands the cylinder pistons thus pressing the shoes to the drum or disk. If the pedal is released, the piston re
21、turns to the initial position, the pull back springs retract the shoes, the fluid is forced back to the master cylinder and braking ceases. The primary purpose of the parking brake is to hold the vehicle stationary while it is unattended. The parking brake is mechanically operated by the driver when
22、 a separate parking braking hand lever is set. The hand brake is normally used when the car has already stopped. A lever is pulled and the rear brakes are approached and locked in the “on” position. The car may now be left without fear of its rolling away. When the driver wants to move the car again
23、, he must press a button before the lever can be released. The hand brake must also be able to stop the car in the event of the foot brake failing. For this reason, it is separate from the foot brake uses cable or rods instead of the hydraulic system.Caliper There are two main types of caliper:float
24、ing calipers and fixed calipers.calipers must be replace if they show signs of leaking brake fluid.Single piston flouting calipers are the most popular and also least costly to manufacture and service .a floating caliper“floats” or moves in a track in its support so that it can center itself over th
25、e rotor.as you apply brake pressure,the hydraulic fluid pushes in two directions,it forces the piston against the inner pad which in turn pushes against the rotor.it also pushes the caliper in the opposite direction against the other side of the rotor.floating calipers are also available on same veh
26、icles with two pistons mounted on the same side.two-piston-floating calipers are found on more expensive cars and can provide an improved braking “feel”.Four-piston-fixed calipers are mounted rigidly to the support and not allowed to move.instead,there are two pistons on each side that press the pad
27、s against the rotor.four-piston caliper have a better feel and are more efficient,but are more expensive to produce and cost more to service.this type of caliper is usually found on more expensive luxury and high performance cars.Disk rotorThe disk rotor is made of iron with highly machined surface
28、where the brake pads contact it.just as the brake pads wear out over time,the rotor also undergoes some wear,usually in the form of ridges and groves where the brake pad rubs against it.this wear pattern exactly matches the wear pattern of the pads as they seat themselves to the rotor.when the pads
29、are replaced,the rotor must be machined smooth to allow the new pads to have an even contact surface to work with.Brake pad There are two brakes pads on each caliper. They are constructed of a metal “shoe” with the ;lining riveted or bonded to it.the pads are mounted in the caliper ,one on each side
30、 of the rotor.brake linings used to be made primarily of asbestos because of its heat absorbing properties and quiet operation.Anti-lock Brake System Anti-lock brake systems make braking safer and more convenient, Anti-lock brake systems modulate brake system hydraulic pressure to prevent the brakes
31、 from locking and the tires from skidding on slippery pavement or during a panic stop. Anti-lock brake systems have been used on aircraft for years, and some domestic car were offered with an early form of anti-lock braking in late 1990s. Recently, several automakers have introduced more sophisticat
32、ed anti-lock system. Investigations in Europe, where anti-lock braking systems have been available for a decade, have led one manufacture to state that the number of traffic accidents could be reduced by seven and a half percent if all cars had anti-lock brakes. So some sources predict that all cars
33、 will offer anti-lock brakes to improve the safety of the car. Anti-lock systems modulate brake application force several times per second to hold the tires at a controlled amount of slip; all systems accomplish this in basically the same way. One or more speed sensors generate alternating current s
34、ignal whose frequency increases with the wheel rotational speed. An electronic control unit continuously monitors these signals and if the frequency of a signal drops too rapidly indicating that a wheel is about to lock, the control unit instructs a modulating device to reduce hydraulic pressure to
35、the brake at the affected wheel. When sensor signals indicate the wheel is again rotating normally, the control unit allows increased hydraulic pressure to the brake. This release-apply cycle occurs several time per second to “pump” the brakes like a driver might but at a much faster rate. In additi
36、on to their basic operation, anti-lock systems have two other things in common. First, they do not operate until the brakes are applied with enough force to lock or nearly lock a wheel. At all other times, the system stands ready to function but does not interfere with normal braking. Second, if the
37、 anti-lock system fail in any way, the brakes continue to operate without anti-lock capability. A warning light on the instrument panel alerts the driver when a problem exists in the anti-lock system. The current Bosch component Anti-lock Braking System (ABS), is a second generation design wildly us
38、ed by European automakers such as BWM, Mercedes-Benz and Porsche. ABS system consists of : four wheel speed sensor, electronic control unit and modulator assembly. A speed sensor is fitted at each wheel sends signals about wheel rotation to control unit. Each speed sensor consists of a sensor unit a
39、nd a gear wheel. The front sensor mounts to the steering knuckle and its gear wheel is pressed onto the stub axle that rotates with the wheel. The rear sensor mounts the rear suspension member and its gear wheel is pressed onto the axle. The sensor itself is a winding with a magnetic core. The core
40、creates a magnetic field around the winding, and as the teeth of the gear wheel move through this field, an alternating current is induced in the winding. The control unit monitors the rate o change in this frequency to determine impending brake lockup. The control units function can be divided into
41、 three parts: signal processing, logic and safety circuitry. The signal processing section is the converter that receives the alternating current signals form the speed sensors and converts them into digital form for the logic section. The logic section then analyzes the digitized signals to calcula
42、te any brake pressure changes needed. If impending lockup is sensed, the logic section sends commands to the modulator assembly. Modulator assembly The hydraulic modulator assembly regulates pressure to the wheel brakes when it receives commands from the control unit. The modulator assembly can main
43、tain or reduce pressure over the level it receives from the master cylinder, it also can never apply the brakes by itself. The modulator assembly consists of three high-speed electric solenoid valves, two fluid reservoirs and a turn delivery pump equipped with inlet and outlet check valves. The modu
44、lator electrical connector and controlling relays are concealed under a plastic cover of the assembly. Each front wheel is served by electric solenoid valve modulated independently by the control unit. The rear brakes are served by a single solenoid valve and modulated together using the select-low
45、principle. During anti-braking system operation, the control unit cycles the solenoid valves to either hold or release pressure the brake lines. When pressure is released from the brake lines during anti-braking operation, it is routed to a fluid reservoir. There is one reservoir for the front brake circuit. The reservoirs are low-pressure accumulators that store fluid under slight spring pressure until the return delivery pump can return the fluid through the brake lines to the master cylinder. .此处忽略！
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