The brakes function by absorbing in friction the energy possessed by the moving car. In so doing they convert the energy into heat. There are two types of brakes, the drum brake, and the disc brake. Either or both types may be fitted. But where both types are used it is usual for the disc brakes to be fitted to the front wheels. In both drum and disc brakes, a hydraulic system applies the brakes. The hydraulic system connects the brake pedal to the brake parts at each wheel.
1 、 Drum Brakes
The drum brake consists of a pair of semicircular brake shoes mounted on a fixed backplate and situated inside a drum. This drum is fixed to the road wheel and rotates with it. One end of each shoe is on a pivot and the spring holds the other end in contact with the piston of a hydraulic cylinder. (In front brakes it is usual to use two hydraulic cylinders in order to equalize the pressures exerted by the shoes, as shown in Fig.7.1)Each shoe is faced with a material, known as brake lining, which produces high frictional resistance.
The hydraulic system comprises a master cylinder and slave cylinders which are the cylinders on the road wheels. The slave cylinders are connected to the master cylinder by tubing and the whole system is filled with hydraulic fluid. A piston in the master cylinder is connected to the brake pedal so that when the driver depresses the pedal the fluid is forced out to each slave cylinder and operates their pistons. The fluid pushes the pistons out of their cylinders. They ，in turn, push against the inner ends of the brake shoes and force them against the drums in each wheel. We say that the brakes are on. This friction of the shoes against the drums, which are fixed to the road wheels, slows down or stops the car.
As the brake pedal is allowed to come up, the hydraulic fluid returns to its original position, the pistons retract, and a spring attached to each brake shoe returns it also to its original position.
Free of the brake drum. Now we say that the brakes are off.(Fig.7.2)
The brakes may also be operated by mechanical linkages from the foot pedal and hand brake lever. The common practice is to operate both front and rear brakes hydraulically with a secondary mechanical system operating the rear brakes only from the hand lever. One of the great advantages of hydraulic operation is that the system is self-balancing, which means that the same pressure is automatically produced at all four brakes, whereas mechanical linkages have to be very carefully adjusted for balance. Of course, if more pressure is put on one of the brakes than on the others there is a danger that the car will skid.
The mechanical linkage on the rear brakes is a system of rods or cables connecting the handbrake lever to the brake-shoe mechanism, which work entirely independently of the hydraulic system. Drum brakes are prone to a reduction in the braking effort, known as “fade”, caused by the overheating of the brake linings and the drum. Fade can affect all or only some of the brakes at a time, but it is not permanent, and full efficiency returns as soon as the brakes have cooled down. However, fading is unlikely to occur except after the brakes have been used repeatedly in slowing the car from a high speed or after braking continuously down a steep hill. Descending such a hill, it would have been preferable to use engine braking by changing down into a lower gear. Drum brakes can be made less prone to fade by improving the cooling arrangements, by arranging for more air to be deflected over them, for example.
2 Disc Brakes
The disc brake consists of a steel disc with friction pads operated by slave hydraulic cylinders. The steel disc is attached to the road wheel and rotates with it. Part of this steel disc is enclosed in a caliper. This caliper contains two friction pads，one on each side of the disc, and two hydraulic cylinders, one outside each pad. The pads are normally held apart by a spring, but when the driver depresses the brake pedal, pistons from the hydraulic cylinders force the pads against the sides of the disc. Because the disc is not enclosed all the way around, the heat generated when the brakes are applied is dissipated very much more quickly than it is from brake shoes which are entirely enclosed inside a drum. This means that disc brakes are less prone to fade than drum brakes. (Fig.7.3)
3 、Anti-lock Brake System (ABS)
The function of an anti-lock, or anti-skid, the braking system is to prevent the wheels from locking under hard braking. Maximum braking force is obtained just before the wheels lock and skid. Such an anti-skid system ate useful on slippery surfaces, such as ice and snow, where the wheels may lock easily. Locked wheels are dangerous because the car needs a much longer distance to stop. Locked wheels also can cause the driver to lose control.
The system uses a sensor that knows when one wheel (or a pair of wheel) is skidding. (Fig.7.4) The sensor sends a signal to a computer, which signals a modulator valve. The modulator connects to the hydraulic system and can momentarily release the brake pressure and prevent the wheels from locking. (The pressure release is so fast that a driver is seldom aware of it.) the pressure is then reapplied until the sensor again senses that the wheel is about to lock up. Thus, this system keeps the wheels as close to lock up as possible, without actually allowing the wheels to lock up and skid. This is called an incipient lock up. Maximum braking occurs at that point. If any part of the system should fail to work, the system goes into a “fail-safe” mode. The brakes operate normally, as they would on a car that is not equipped with ABS. Today, ABS is an optional or standard feature that typically is found on expensive luxury cars and sports cars. In the future, ABS may be available for all cars.
An anti-lock braking system is using the body of a rubber balloon while hitting the brakes, which will give brake oil pressure, feeding through to the ABS body, using the air in the middle of the air layer to return the pressure, make wheels evade the locked points. When the wheel will arrive next locked point, brake oil pressure makes the balloon repeat function, so can function eight to thirty times in one-second equivalent constantly brake, relax, namely, similar to the “mechanical braking “. Therefore, the ABS anti-lock braking system can avoid the orientation losing control, the wheel’s lateral sliding coming up and the wheels rubbing on one point with the ground without being locked in the emergency brake，so it can make the brake friction efficiency achieves ninety percent. It also can reduce the braking consumption and prolong the brake wheel drum, disc and tire twice with the service life of the vehicle in the ABS. On the dry tarmac road or on snowy or rainy days, the slippery performance reached 80% – 90%, 30% -10%, 15% – 20%. The ordinary braking system on a wet road surface brake, or in the emergency brake, it’s easy for wheels to be locked owing to the braking force exceeds the friction force of the tires and ground. In recent years, the consumers of the vehicle emphasis on safety, so most of the cars have ABS listed as standard. Without ABS, an emergency brake usually causes a tire locked, then, the rolling friction becomes sliding friction, as a result, the braking force dramatically decreased. And if the front wheels are locked first, the vehicle will lose the steering ability; if the rear tires are first locked, the vehicle is easy to slide laterally, so the direction becomes impossible to control. Through electronic or mechanical control, the ABS system controls the braking fluid pressure at a fast speed to avoid the wheels locked. Ensure the tire have the biggest braking force and the turning ability when braking, and make the vehicle have the ability to evade the obstacles in emergency braking. With the rapid development of the automotive industry, safety increasingly becomes the important basis for people to choose and buy cars. At present the widespread adoption of holding brake system (ABS) that people can fully meet the safety of requirements. The automobile brake prevents embrace system, referred to as the ABS, is to improve the car an important device passive safety. Someone said the brake prevent embrace system is auto safety measures relay belts after another major progress. The automobile braking system is the bus passengers safety is the most important relationship to one of the second systems. With the rapid development of the automotive industry, automobile safety for people appeals more and more attention. Automobile brake prevents embrace system, is another major progress to improve safety.
ABS braking system is controlled by an automobile microcomputer, when braking, it can keep the wheel rotating to help the driver control vehicle parking safely. The anti-locked braking system detects wheel speed by a speed sensor, and then send the wheel speed signals to the microcomputer. The microcomputer controls wheel slipping rate by increasing or decreasing the brake pressure repeatedly according to the input wheel speed to keep the wheel rotate. In the braking process, keeping the wheel rotate not only ensures the ability to control driving direction but also provides higher brake force than the locked wheel in most circumstances.
3.2、The working principle of ABS
It includes control devices and ABS warning lights, in different ABS system, the structure of brake pressure adjusting device and working principle of electronic control devices are often different, the internal structure and control logic of ABS system usually includes the wheel speed sensors, brake pressure adjusting device, electronic identical and so on. In a common ABS system, each wheel is installed a rotational speed sensor on the wheel speed, input the signal to the electronic control device. The electronic control unit states monitoring and determination according to each wheel speed sensors’ signal about each wheel movement and has formed the corresponding control instruction. The Brake pressure adjusting device is mainly composed of the pressure regulating solenoid valves, electric pump composition, and liquid container components compose an independent whole, braking main cylinder and the cylinder brake wheel connected by the brake pipe. The Brake pressure adjusting device is controlled by the electronic unit to control all brake wheel cylinders’ brake pressure. The working process of ABS can be divided into general braking, brake pressure kept brake pressure decrease and brake pressure increase stage. In the general braking phase, ABS doesn’t intervene brake pressure control, pressure regulating electromagnetic valve assembly in various into liquid solenoid valves are no electricity and is open, each produced liquid are no electricity and the electromagnetic valve is in the closed position, the electric pump also operates without electricity, and brake the main cylinder to each brake wheel cylinder brake lines are in communication condition, and the brake wheel cylinder to liquid brake lines are in close condition, the brake wheel cylinder brake pressure change with the output pressure brake master cylinder, the braking process at this time is completely the same with conventional braking system braking process.
In the braking process, when the electronic control detects that the wheels primarily tend to embrace dies according to the wheel speed sensors of the wheel speed signal input, ABS came into the braking anti-lock process. For example, when the electronic control unit judges the front-right wheel tends to embrace, the electronic control unit will make control scrape the dynamic pressure front-right wheel failure into liquid solenoid valves electricity, make the fluid electromagnetic valve closed, brake main cylinder output brake fluid no longer enter into brake wheel cylinder, right now, right at the end of a fluid electromagnetic valve still energized and closed, the right brake wheel cylinder brake fluid also won’t outflow, the right brake wheel cylinder scraping dynamic pressure stays certain, and other arms tend to be dead wheel with still brake pressure braking main cylinder and the increase of output pressure increases; If the right brake wheel cylinder brake pressure keep certain, the electronic control unit front-right wheel failure still tend to judge lock, the electronic control unit and move out liquid solenoid valves also electrify into a state of opening, the part right brake wheel cylinder brake wave will pass is open from fluid electromagnetic valve flow back to liquid container, make right brake wheel cylinder brake pressure holding diminishes quickly front-right wheel failure will start to eliminate death trend, with the right brake wheel cylinder brake pressure decreases, the front-right wheel under the action of inertia force will speed up gradually; accelerate gradually When the electronic control device determines the lock front-right wheel failure to completely eliminate according to the wheel speed sensors input signal, the electronic control unit makes right into the fluid electromagnetic valve and a liquid solenoid valves are without electricity, make into fluid electromagnetic valve to open, use liquid into closed electromagnetic valve, also make electric pump operation, energized to brake wheel cylinder pump brake fluid, output by brake main cylinder brake fluid electromagnetic valve into the right brake wheel cylinders, make right brake wheel cylinder brake pressure increased rapidly, opening up a front-right wheel failure and slow rotation. ABS control the sliding rate of wheels which tend to be locked by holding the brake force of wheels that tends to be locked repeatedly, adhesion coefficient is at peak within the scope of the sliding rate, until the bus speed reduced to very low or brake main cylinder pressure no longer tend to be locked. Brake pressure to adjust cycle of frequency can reach 3 ~ 20HZ. In the ABS each has on the fluid and the fluid electromagnetic valve corresponding at each brake wheel cylinder, but by the electronic control device to control, therefore, respectively, the brake wheel cylinder brake pressure can be an independent adjustment so that four wheels are not occurred breaking lock phenomenon. Although various ABS structure form and working process are not exactly the same, are based on the brake holding pressure adaptive cycle adjustment of the wheel which tends to be locked, to prevent the controlled wheels holding in death occurred when brake.
3.3、 ABS function
The braking performance is one of the main auto performance, it is also related to the security of driving. Evaluate the braking performance of a car, the basic index is brake acceleration, braking distance, and direction of braking time and braking stability.
Braking stability points to that the vehicle can still be specified in the direction of the auto brake when driving in the direction of the track. If cars proceed high-speed brake (especially when an emergency brake) and make the wheel fully embrace die, it is very dangerous. If the front wheel is locked, it will make cars lose steering ability. If the rear wheel is locked, it will appear to swing tail or switching (running deviation, sideslip) especially in the road is the all wet slippery case, it will cause the traffic safety great harm. Automobile braking force depends on the brake friction, but the braking force can make the car brake slow, and also restricted by the ground adhesion coefficient. When brake produces braking force increases to a certain value, the tires will appear slipping on the ground. Its sliding rate
δ = (Vt – Va) / Vt x 100%
δ: delta – sliding rate;
Vt – theoretical speed of the car;
Va – the actual speed of the car.
According to the experiments confirm, when the wheel sliding rate delta range from 15% to 20%, the ground adhesion coefficient reaches the maximum, therefore, in order to get the best braking effect, we must control the slip rate in 15% ~ 20% range.
ABS function will namely decrease the brake force when the wheels will embrace dies, and when the wheel will not hold died and increase braking force, so repeated action, the braking effect is the best.
3.4、The problems needing attention when ABS is used
(1) After replacing the brake or replace hydraulic brake system components, it should exhaust the air in the brake pipe, lest affect the braking system work normally.
(2)The car equipped with ABS should be replaced every year. Otherwise, brake fluid hygroscopic is very strong, water will not only reduce the boiling point to make it easy to produce corrosion, and still can cause braking performance recession.
(3) To examine ABS braking system should pull power firstly.