汽车制动系统(机械、车辆工程毕业论文英文文献及翻译)

Automobile Brake System

The braking system is the most important system in cars. If the brakes fail, the result

can be disastrous. 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 thousands 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 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

“master

cylinder” which is located under the hood, and is directly connected to the brake pedal,

converts driver foot’s 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, specially 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.

The typical brake system consists of disk brakes in front and either disk or drum

brakes in the rear connected by a system of tubes and hoses that link the brake at each

wheel to the master cylinder (Figure).

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. Friction between the shoes or pads and the drums or disks then slows or stops

the wheel so that the car is braked.

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In most modern brake systems (see Figure 15.1), 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 connected to a brake pedal in the driver’s 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 disks slows

and stops them. This slows or stops the revolving wheels, which, in turn, slow or stop

the car.

The 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 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 ashort 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 time 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

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 line 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 brakes, it consists of the brake drum, an expander, 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 clearance between the inner

surface of the drum and the shoe lining. To apply brakes, the driver pushes pedal, the

expander 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

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brakes, the driver release the pedal, the pull back spring retracts the shoes thus

permitting free rotation of the wheels.

Disk brakes, it has a metal disk instead of a drum. A flat shoe, or disk-brake pad, is

located on each side of the disk. The shoes squeeze the rotatin g disk to stop the car.

Fluid from the master cylinder forces the pistons to move in, toward the disk. This

action pushes the friction pads tightly against the disk. The friction between the shoes

and disk slows and stops it. This provides the braking action. Pistons are made of

either plastic or metal. There are three general types of disk brakes. They are the

floating- caliper type, the fixed-caliper type, and the sliding-caliper type. Floating-

caliper and sliding-caliper disk brakes use a single piston. Fixed-caliper disk brakes

have either two or four pistons.

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, the 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 returns to the

initialposition, 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 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, 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.

Anti-lock Brake System

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Anti-lock brake systems make braking safer and more convenient, Anti-lock brake

systems modulate brake system hydraulic pressure to prevent the brakes 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 1990’s. Recently, several

automakers have introduced more sophisticated anti-lock system. Investigations in

Europe, where anti-lock brakin g 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 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 signal 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 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 a

t a much faster rate.

In addition 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 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

Ⅱ

), is a second

generation design wildly used by European automakers such as BWM, Mercedes-

Benz and Porsche. ABS

Ⅱ

system consists of : four wheel speed sensor, electronic

control unit and modulator assembly.

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