(完整版)汽车制动系统-英文文献及翻译

Brake systems

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 Image Gallery

Layout of typical brake system.

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brake images

.

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, we'll cover these three principles:

?

Leverage

?

Hydraulics

?

Friction

Leverage and Hydraulics

In the figure below, 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.

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.

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. Here you can see

the simplest possible hydraulic system:

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Simple hydraulic system

In the figure above, two pistons (shown in red) are fit into two glass cylinders filled

with oil (shown in light blue) and connected to one another with an oil-filled pipe. If you

apply a downward force to one piston (the left one, in this drawing), then the force is

transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the

efficiency is very good -- almost all of the applied force appears at the second piston. The

great thing about hydraulic systems is that the pipe connecting the two cylinders can be

any length and shape, allowing it to snake through all sorts of things separating the two

pistons. The pipe can also fork, so that one

master cylinder

can drive more than one slave

cylinder if desired, as shown in here:

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Master cylinder with two slaves

The other neat thing about a hydraulic system is that it makes force multiplication (or

division) fairly easy. If you have read

How a Block and Tackle Works

or

How Gear Ratios

Work

, then you know that trading force for distance is very common in mechanical

systems. In a hydraulic system, all you have to do is change the size of one piston and

cylinder relative to the other, as shown here:

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Hydraulic multiplication

To determine the multiplication factor in the figure above, start by looking at the size

of the pistons. Assume that the piston on the left is 2 inches (5.08 cm) in diameter (1-inch /

2.54 cm radius), while the piston on the right is 6 inches (15.24 cm) in diameter (3-inch /

7.62 cm radius). The area of the two pistons is Pi * r

2

. The area of the left piston is

therefore 3.14, while the area of the piston on the right is 28.26. The piston on the right is

nine times larger than the piston on the left. This means that any force applied to the

left-hand piston will come out nine times greater on the right-hand piston. So, if you apply

a 100-pound downward force to the left piston, a 900-pound upward force will appear on

the right. The only catch is that you will have to depress the left piston 9 inches (22.86 cm)

to raise the right piston 1 inch (2.54 cm).

A Simple Brake System

Before we get into all the parts of an actual car brake system, let's look at a simplified

system:

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A simple brake system

You can see that 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.

You can also see that 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.

Drum brakes work on the same principle as disc brakes:

Shoes press against a

spinning surface

. In this system, that surface is called a drum.

Figure 1. Location of drum brakes.

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drum brake

pictures

.

Many cars have drum brakes on the rear wheels and disc brakes on the front. Drum

brakes have more parts than disc brakes and are harder to service, but they are less

expensive to manufacture, and they easily incorporate an emergency brake mechanism.

In this edition of

HowStuffWorks

, we will learn exactly how a drum brake system

works, examine the emergency brake setup and find out what kind of servicing drum

brakes need.

Figure 2. Drum brake with drum in place

Figure 3. Drum brake without drum in place

Let's start with the basics.

The Drum Brake

The drum brake may look complicated, and it can be pretty intimidating when you

open one up. Let's break it down and explain what each piece does.

Figure 4. Parts of a drum brake

Like the

disc brake

, the drum brake has two brake shoes and a piston. But the drum

brake also has an

adjuster

mechanism, an

emergency brake

mechanism and lots of

springs

.

First, the basics:

Figure 5

shows only the parts that provide stopping power.

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Figure 5. Drum brake in operation

When you hit the brake pedal, the piston pushes the brake shoes against the drum.

That's pretty straightforward, but why do we need all of those springs?

This is where it gets a little more complicated. Many drum brakes are

self-actuating

.

Figure 5 shows that 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.

Brake Adjuster

For the drum brakes to function correctly, the brake shoes must remain close to the

drum without touching it. If they get too far away from the drum (as the shoes wear down,

for instance), the piston will require more fluid to travel that distance, and your brake pedal

will sink closer to the floor when you apply the brakes. This is why most drum brakes have

an

automatic adjuster

.

Figure 6. Adjuster mechanism

Now let's add in the parts of the adjuster mechanism. The adjuster uses the

self-actuation principle we discussed above.

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Figure 7. Drum brake adjuster in operation

In

Figure 7

, you can see that as the pad wears down, more space will form between

the shoe and the drum. Each time the car stops while in reverse, the shoe is pulled tight

against the drum. When the gap gets big enough, 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.

Some cars have an adjuster that is actuated when the emergency brake is applied.

This type of adjuster can come out of adjustment if the emergency brake is not used for

long periods of time. So if you have this type of adjuster, you should apply your

emergency brake at least once a week.

Servicing

The most common service required for drum brakes is

changing the brake shoes

.

Some drum brakes provide an inspection hole on the back side, where you can see how

much material is left on the shoe. Brake shoes should be replaced when the friction

material has worn down to within 1/32 inch (0.8 mm) of the rivets. If the friction material is

bonded to the backing plate (no rivets), then the shoes should be replaced when they

have only 1/16 inch (1.6 mm) of material left.

Photo courtesy of a local

AutoZone

store

Figure 9. Brake shoe

Just as in disc brakes, deep scores sometimes get worn into brake drums. If a

worn-out brake shoe is used for too long, the rivets that hold the friction material to the

backing can wear grooves into the drum. A badly scored drum can sometimes be repaired

by refinishing. Where disc brakes have a minimum allowable thickness, drum brakes have

a

maximum allowable diameter

. Since the contact surface is the inside of the drum, as

you remove material from the drum brake the diameter gets bigger.

Figure 10. Brake drum