(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. See more 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 * r2. 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. See more 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

制动系统

众所周知，踩下制动踏板可以使汽车减速至停止。但这是如何产生的呢？汽车是如何将力从你的腿传递到车轮的呢？汽车是如何将力放大到足够大以致可以将像汽车一样大的东西制动的呢？

制动系统组件

当你踩下制动踏板的时候，汽车通过液体把力从脚传递到制动器。因为制动器需要的真正力量比你的腿能提供的要大的多，所以汽车必须放大脚产生的力有两种方式：

机械杠杆作用

液力放大

制动器通过摩擦把力传递给轮胎，并且轮胎也是通过摩擦把力传递给路面的。在我们讨论制动系统的组成之前，先来介绍以下三条原则：

杠杆

液力

摩擦力

杠杆和液力

在下面的图中，一个力F加在杠杆的左端。左端的杠杆长度（2X）是右端（X）的两倍。因此杠杆右端可施加的力为2F ，但是右端移动的距离(Y)是左端距离(2Y)的一半。改变杠杆的左端和右端的长度可以改变放大系数。

任何液压系统背后的基本原理都是非常简单的：作用在某一点力通过通常是油一类的不可压缩的液体传递到另一点。大多数的制动系统也在这个过程中放大力。下面的是最简单的液压系统：

简单液压系统

在上图中，两个活塞放在两个充满油的玻璃液压缸中并且由充满油的管道相连。如果在一个活塞上施加一个向下的力，那么力将通过管道中的油传递到第二个活塞。因为油液是不可压缩的，所以传递效率很好，大部分的作用力都传递到了另一个活塞。

液压系统的好处连接两液压缸的管道可以是任何长度和形状，这样就可以使管道弯曲的通过两活塞之间的各种部件。管道也可以是分叉的，如果有需要的话，这样一个主缸可以驱动数个副缸。如下图所示：

带有两个副缸的主缸

液压系统的另一个好处是产生放大（或者缩小）力相当地容易。如果你一读过滑车设备工作原理或者齿轮齿数比原理，那么你就会知道在机械系统中把力转化为距离处理是很常见的。在液压系统中，我们所要做的就是相对地改变一组活塞和液压缸的尺寸。如下图所示：

液压增力原理

为了确定上图中的放大因子，先由观察活塞的尺寸开始。假设左边活塞的直径为2英尺（5.08cm而右边的直径为6英尺（15.24cm）。两个活塞的面积是Pi * r2 。因此左面活塞的面积是3.14，而右面的面积是28.26。右面活塞的面积是左边的九倍大。这就意味着无论在左面的活塞上施加多大的力，在右面的活塞上就会输出九倍于左面的力。所以，如果在左边活塞上施加100磅向下的力，那么在右面活塞上将产生900磅向上的力。唯一的补偿是左面的活塞要移动9英尺（22.86cm）来使右面提升1英尺（2.54cm）

一个简单的制动系统

在我们深入了解一个真实的制动系统的各部分之前，让我们先来看一个简化的系统：

我们可以看到踏板到枢轴的距离是液压缸到枢轴距离的4倍，所以施加在踏板上的力在传递到液压缸之前将被增加4倍。我们还可以看到制动缸的直径是踏板缸直径的3倍。这就将力进一步放大了九倍。最终这个系统将腿上的力增加了36倍。所以，如果在踏板上施加10磅的力，将在挤压制动带的轮上产生369磅（162kg）的力。

下面是这种简单系统所存在的问题。要是系统有泄漏该怎么办呢？如果是轻微泄漏，最终将会没有足够的油使制动缸充满，并且制动器将停止工作。如果是严重泄漏，那么在你制动的第一时间，所有的油液将从泄露处喷射而出，并且制动系统将彻底地不起作用。

鼓式制动器的工作原理和盘式制动器是一样的：制动面接触一个磨砂的表面。在这个系统中，那个表面称作制动鼓

图1.制动鼓的位置

许多汽车的后轮安装鼓式制动器，而盘式制动器安装在前面。鼓式制动器比盘式制动器有更多的零件并且更难检修。但是制造成本相对便宜，还有鼓式制动器容易组装一个紧急使用的制动装置。

在本版本的How StuffWorks中，我们将详尽了解鼓式制动系统是如何工作的。考察紧急制动系统的组成，并且找到鼓式制动器需要何种检修工作。

图2. 有鼓的鼓式制动器

图3.未安装鼓的鼓式制动器

让我们基础开始：

鼓式制动器

鼓式制动器可能看起来比较复杂，它可以是很复杂的，当你打开一个的时候。让我们拆开它，并解释每一块的作用。

图4. 鼓式制动器的组成

如盘式制动器，鼓式制动器有两个制动蹄和一个活塞。但是鼓式制动器也有一个调节机制，紧急刹车机制和大量的弹簧。

首先，基础知识：图5显示只有部分提供的制动力。

图5.工作状态下的鼓式制动器

当你踩下刹车踏板时，活塞推动紧靠着鼓的制动蹄。这是很简单的，但为什么我们需要所有这些弹簧呢？

这使它变的有点复杂许多鼓式制动器是自增力式的。图5表明，当制动蹄与鼓相接触的时候，两者间有一个楔入运动，这起到了产生更多的力量将制动蹄向鼓挤压。

由楔入运动提供的额外制动力使得鼓式制动器可以使用比盘式制动器更小的活塞。

但是由于这种楔入运动，在制动释放的时候制动蹄必须从鼓拉离开。这是使用其中部分弹簧的原因。其它弹簧的作用是将制动蹄固定并且驱动调节臂返回。

制动调节器

为了使鼓式制动器正确的工作，制动蹄必须紧贴着鼓但是不碰到它。如果离鼓太远的话，活塞将需要更多的油液以通过那段距离，并且当你制动时，制动踏板将下行而离地板更近。这就是为什么大多数的鼓式制动器有一个自动调节装置的原因。

图6.调节机构

现在让我们在把调节机构也加进来，这个调节器使用的是上面讨论过的自增力原理。

图7.工作状态下的鼓式制动调节器

在图7中，我们可以看到由于摩擦片的磨损，这使得制动蹄和鼓之间形成更大的空间。每次车停下的时候，相反的是制动蹄被拉的和鼓更紧。当间隙变的足够大时，调节杠杆足够摆动推进调节齿轮先前转动一个齿。调节装置有一个行程，就像一个螺栓，以便当它转动时旋开一点点，延长以填补间隙。当制动蹄进一步磨损，调节器又可以再向前。所以它总是保持制动蹄紧靠着鼓。

有些汽车紧急刹车时有一个被驱动的调节器。如果紧急制动很长一段时间没有使用，这种类型的调节器可以产生调节作用。所以如果你有这种类型的调节器，你应该每周至少使用一次紧急制动装置。

检修

鼓式制动器最常见的检修是更换制动蹄。一些鼓式制动器在背面设置了一个检查孔，通过这个孔，你可以看到制动蹄上还剩余多少摩擦材料。当摩擦材料

磨损到铆钉内1/32英寸（0.8mm）时，必须更换制动蹄。如果摩擦材料和垫板直接连接（无铆钉），那么当摩擦材料只剩下1/16英寸（1.6mm）时，就该换制动蹄了。

图9.制动蹄

正如在盘式制动器中，深的刻痕可能会磨穿到制动鼓。如果一个磨损的制动蹄使用过长的时间，把摩擦片固定到垫板上铆钉可以将制动鼓摸出一条凹槽。一个严重磨损的制动鼓有时可以被修补修复。盘式制动器有最小允许厚度，鼓式制动器有一个最大允许直径。因为接触表面是鼓的内侧。当你将材料从制动器中取出时，制动鼓的直径变大了。

图10.制动鼓