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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
制動系統(tǒng)
眾所周知，踩下制動踏板可以使汽車減速至停止。但這是如何產(chǎn)生的呢？汽車是如何將力從你的腿傳遞到車輪的呢？汽車是如何將力放大到足夠大以致可以將像汽車一樣大的東西制動的呢？
制動系統(tǒng)組件
當(dāng)你踩下制動踏板的時候，汽車通過液體把力從腳傳遞到制動器。因為制動器需要的真正力量比你的腿能提供的要大的多，所以汽車必須放大腳產(chǎn)生的力
有兩種方式：
機械杠桿作用
液力放大
制動器通過摩擦把力傳遞給輪胎，并且輪胎也是通過摩擦把力傳遞給路面的。 在我們討論制動系統(tǒng)的組成之前，先來介紹以下三條原則：
杠桿
液力
摩擦力
杠桿和液力
在下面的圖中，一個力F加在杠桿的左端。左端的杠桿長度（2X）是右端（X）的兩倍。因此杠桿右端可施加的力為2F ，但是右端移動的距離(Y)是左端距離(2Y)的一半。改變杠桿的左端和右端的長度可以改變放大系數(shù)。
任何液壓系統(tǒng)背后的基本原理都是非常簡單的：作用在某一點力通過通常是油一類的不可壓縮的液體傳遞到另一點。大多數(shù)的制動系統(tǒng)也在這個過程中放大力。下面的是最簡單的液壓系統(tǒng)：
簡單液壓系統(tǒng)
在上圖中，兩個活塞放在兩個充滿油的玻璃液壓缸中并且由充滿油的管道相連。如果在一個活塞上施加一個向下的力，那么力將通過管道中的油傳遞到第二個活塞。因為油液是不可壓縮的，所以傳遞效率很好，大部分的作用力都傳遞到了另一個活塞。
液壓系統(tǒng)的好處連接兩液壓缸的管道可以是任何長度和形狀，這樣就可以使管道彎曲的通過兩活塞之間的各種部件。管道也可以是分叉的，如果有需要的話，這樣一個主缸可以驅(qū)動數(shù)個副缸。如下圖所示：
帶有兩個副缸的主缸
液壓系統(tǒng)的另一個好處是產(chǎn)生放大（或者縮小） 力相當(dāng)?shù)厝菀?。如果你一讀過滑車設(shè)備工作原理或者齒輪齒數(shù)比原理，那么你就會知道在機械系統(tǒng)中把力轉(zhuǎn)化為距離處理是很常見的。在液壓系統(tǒng)中，我們所要做的就是相對地改變一組活塞和液壓缸的尺寸。如下圖所示：
液壓增力原理
為了確定上圖中的放大因子，先由觀察活塞的尺寸開始。假設(shè)左邊活塞的直徑為2英尺（5.08cm而右邊的直徑為6英尺（15.24cm）。兩個活塞的面積是Pi * r2 。因此左面活塞的面積是3.14，而右面的面積是28.26。右面活塞的面積是左邊的九倍大。這就意味著無論在左面的活塞上施加多大的力，在右面的活塞上就會輸出九倍于左面的力。所以，如果在左邊活塞上施加100磅向下的力，那么在右面活塞上將產(chǎn)生900磅向上的力。唯一的補償是左面的活塞要移動9英尺（22.86cm）來使右面提升1英尺（2.54cm）
一個簡單的制動系統(tǒng)
在我們深入了解一個真實的制動系統(tǒng)的各部分之前，讓我們先來看一個簡化的系統(tǒng)：
我們可以看到踏板到樞軸的距離是液壓缸到樞軸距離的4倍，所以施加在踏板上的力在傳遞到液壓缸之前將被增加4倍。我們還可以看到制動缸的直徑是踏板缸直徑的3倍。這就將力進一步放大了九倍。最終這個系統(tǒng)將腿上的力增加了36倍。所以，如果在踏板上施加10磅的力，將在擠壓制動帶的輪上產(chǎn)生369磅（162kg）的力。
下面是這種簡單系統(tǒng)所存在的問題。要是系統(tǒng)有泄漏該怎么辦呢？如果是輕微泄漏，最終將會沒有足夠的油使制動缸充滿，并且制動器將停止工作。如果是嚴(yán)重泄漏，那么在你制動的第一時間，所有的油液將從泄露處噴射而出，并且制動系統(tǒng)將徹底地不起作用。
鼓式制動器的工作原理和盤式制動器是一樣的：制動面接觸一個磨砂的表面。在這個系統(tǒng)中，那個表面稱作制動鼓
圖1.制動鼓的位置
許多汽車的后輪安裝鼓式制動器，而盤式制動器安裝在前面。鼓式制動器比盤式制動器有更多的零件并且更難檢修。 但是制造成本相對便宜，還有鼓式制動器容易組裝一個緊急使用的制動裝置。
在本版本的How StuffWorks中，我們將詳盡了解鼓式制動系統(tǒng)是如何工作的?？疾炀o急制動系統(tǒng)的組成，并且找到鼓式制動器需要何種檢修工作。
圖2. 有鼓的鼓式制動器
圖3.未安裝鼓的鼓式制動器
讓我們基礎(chǔ)開始：
鼓式制動器
鼓式制動器可能看起來比較復(fù)雜，它可以是很復(fù)雜的，當(dāng)你打開一個的時候。讓我們拆開它，并解釋每一塊的作用。
圖4. 鼓式制動器的組成
如盤式制動器，鼓式制動器有兩個制動蹄和一個活塞。 But the drum brake also has an adjuster mechanism, an emergency brake mechanism and lots of springs .但是鼓式制動器也有一個調(diào)節(jié)機制，緊急剎車機制和大量的彈簧 。
首先，基礎(chǔ)知識： 圖5顯示只有部分提供的制動力。
圖5.工作狀態(tài)下的鼓式制動器
當(dāng)你踩下剎車踏板時，活塞推動緊靠著鼓的制動蹄。 That's pretty straightforward, but why do we need all of those springs?這是很簡單的，但為什么我們需要所有這些彈簧呢？
這使它變的有點復(fù)雜許多鼓式制動器是自增力式的。圖5表明，當(dāng)制動蹄與鼓相接觸的時候，兩者間有一個楔入運動，這起到了產(chǎn)生更多的力量將制動蹄向鼓擠壓。
由楔入運動提供的額外制動力使得鼓式制動器可以使用比盤式制動器更小的活塞。
但是由于這種楔入運動，在制動釋放的時候制動蹄必須從鼓拉離開。這是使用其中部分彈簧的原因。其它彈簧的作用是將制動蹄固定并且驅(qū)動調(diào)節(jié)臂返回。
制動調(diào)節(jié)器
為了使鼓式制動器正確的工作，制動蹄必須緊貼著鼓但是不碰到它。如果離鼓太遠的話，活塞將需要更多的油液以通過那段距離，并且當(dāng)你制動時，制動踏板將下行而離地板更近。這就是為什么大多數(shù)的鼓式制動器有一個自動調(diào)節(jié)裝置的原因。
圖6.調(diào)節(jié)機構(gòu)
現(xiàn)在讓我們在把調(diào)節(jié)機構(gòu)也加進來，這個調(diào)節(jié)器使用的是上面討論過的自增力原理。
圖7.工作狀態(tài)下的鼓式制動調(diào)節(jié)器
在圖7中，我們可以看到由于摩擦片的磨損，這使得制動蹄和鼓之間形成更大的空間。每次車停下的時候，相反的是制動蹄被拉的和鼓更緊。當(dāng)間隙變的足夠大時，調(diào)節(jié)杠桿足夠擺動推進調(diào)節(jié)齒輪先前轉(zhuǎn)動一個齒。調(diào)節(jié)裝置有一個行程，就像一個螺栓，以便當(dāng)它轉(zhuǎn)動時旋開一點點，延長以填補間隙。當(dāng)制動蹄進一步磨損，調(diào)節(jié)器又可以再向前。所以它總是保持制動蹄緊靠著鼓。
有些汽車緊急剎車時有一個被驅(qū)動的調(diào)節(jié)器。如果緊急制動很長一段時間沒有使用，這種類型的調(diào)節(jié)器可以產(chǎn)生調(diào)節(jié)作用。所以如果你有這種類型的調(diào)節(jié)器，你應(yīng)該每周至少使用一次緊急制動裝置。
檢修
鼓式制動器最常見的檢修是更換制動蹄。一些鼓式制動器在背面設(shè)置了一個檢查孔，通過這個孔，你可以看到制動蹄上還剩余多少摩擦材料。當(dāng)摩擦材料
磨損到鉚釘內(nèi)1/32英寸（0.8mm）時，必須更換制動蹄。如果摩擦材料和墊板直接連接（無鉚釘），那么當(dāng)摩擦材料只剩下1/16英寸（1.6mm）時，就該換制動蹄了。
圖9.制動蹄
正如在盤式制動器中，深的刻痕可能會磨穿到制動鼓。如果一個磨損的制動蹄使用過長的時間，把摩擦片固定到墊板上鉚釘可以將制動鼓摸出一條凹槽。一個嚴(yán)重磨損的制動鼓有時可以被修補修復(fù)。盤式制動器有最小允許厚度，鼓式制動器有一個最大允許直徑。因為接觸表面是鼓的內(nèi)側(cè)。當(dāng)你將材料從制動器中取出時，制動鼓的直徑變大了。
圖10.制動鼓
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