Any brake system consists of a drive and a brake mechanism.
The drive can be mechanical, hydraulic, pneumatic, electric and combined. The electric drive is currently not used in its pure form.
Braking mechanisms, in turn, can be drum, disc and tape. The latter, by the way, can be seen only on caterpillar vehicles.
Medium and heavy duty trucks use a pneumatic drive (for the sake of the length of all drives). But we will focus on the hydraulic brake drive - the most popular drive for passenger cars.
Note
A drive is a set of mechanisms and parts designed to transmit force, thrust or movement.
This system is based on the non-compressible property of fluids.
The hydraulic brake drive consists of the following elements (which are shown in Figure 7.1):
Note
Figure 7.1 shows an example of brake control. In addition to the hydraulic drive, an anti-lock braking system and a parking brake system are presented, the design and description of which will be given below.
Figure 7.1 Example of a hydraulic brake control of a passenger car.
Principle of operation of this system is as follows: the brake pedal through the rod is connected to the piston, which moves inside the master brake cylinder (shown in Figure 7.2), filled with brake fluid, as, indeed, all pipes and hoses of the brake system. When you press the brake pedal, the piston moves inside the cylinder, presses on the fluid, which transmits the force to the wheel brake cylinders of the brake mechanisms. Everything is simple. If it is not clear, you should not immediately be embarrassed, because the operation of each element of the brake system will be described later in the chapter.
Figure 7.2 Example of master brake cylinder with expansion tank.
In addition to the fact that with the help of the main brake cylinder, the force is transmitted from the pedal to the brakes, it also ensures the separation of the circuits (see below).
An expansion tank, which is necessary to compensate for the expansion of the brake fluid when it heats up and to prevent air from entering the hydraulic brake system, is installed above the master brake cylinder (in order to achieve this, you shall always monitor the level of the brake fluid in the reservoir and do not allow it to fall below the "MIN" mark).
The expansion tank is divided into two tanks (or has one tank, but with a dividing wall), that communicate with the master brake cylinder of the system through two holes. The pistons have O-rings that are pressed by springs.
Note
In addition to the groove, the pistons have annular cavities and flat angular grooves that connect to the tank at any position of the pistons. This prevents air from entering inside the hydraulic line.
Note
One of the most dangerous, from the point of view of hit of air in the master brake cylinder, is the moment of the mode of deceleration, which is often carried out sharply by a pedal throw. In this case, the fluid due to its certain viscosity returns to the master cylinder relatively slowly, and the pistons under the action of springs tend to break away from the fluid. This results in a discharge in the line. At the same time, it is almost impossible to exclude air hit in the highway only by consolidations therefore on the back party of pistons or in them cavities filled with liquid have.
Technologies are evolving and, in order not to burden the driver with constant checks of the brake fluid level, a float is installed in the expansion tank and a brake fluid level sensor is connected to it. As soon as this very level falls below the set, the corresponding alarm will light up on the dashboard in the car passenger compartment, and maybe even the sound alarm will be actuated.
Note
It is almost impossible to exclude air entering the hydraulic drive of the brake system. Thus, during the replacement of any elements of brake system it is necessary to drain a part of brake liquid. And it is impossible to fill the system with fresh brake fluid without air ingress. Also, when the brakes overheat, the fluid may start boiling, forming air bubbles.
There so-called breathers in order to remove air from the hydraulic brake are installed in the uppermost points of each element of the brake system. The breather is a hollow bolt acting as a valve, but with a "manual drive". While unscrewing the breather, the “valve” opens, and while screwing it, the valve closes.
Note
Information on how to perform operations to remove air ("pumping", simply put) from the hydraulic brake system, can be found in any book on car repair and maintenance.
Working cylinder is the final element of the hydraulic drive. In case if the brakes are drum, then the working cylinder is a separate part. In case if the brakes are disc, then it is integrated into the caliper of the brakes (see Figures 7.3 and 7.4).
The brake fluid pressure regulator is installed in the brake hydraulic drive system. What is it intended for? The rear wheels shall start braking a moment before the front wheels. This is done in order to prevent the car from skidding if its wheels hit a slippery surface. But the process of car movement is quite complicated, and the car can be loaded with luggage or passengers. This will increase the load on the rear axle. In case if the rear axle becomes heavier, then it will demand more effort to stop it. That's just so that the driver is not burdened with thoughts about the distribution of braking forces between the front and rear axles, in the hydraulic brakes "hit" the regulator.
The pressure regulator adjusts pressure of brake liquid in system of rear brake mechanisms depending on change of loading on back wheels.
The brake fluid is subject to very strict requirements, because it is operated in very aggressive conditions: under the influence of high and low temperatures. Therefore, the main indicator, which characterizes the brake fluid is its boiling point: the higher it is, the better. In general, boiling for brake fluid is a way to reduce the braking efficiency to almost zero: once the fluid boils, air bubbles appear, the pedal fail, and braking doesn’t start. Also do not forget that low temperatures can cause the brake fluid to freeze, which will also result in a loss of braking efficiency.
Note
The brake fluid is very hygroscopic. This means that it has the ability to absorb moisture, which is contained in the air. The ingress of moisture into the brake fluid will lead to a decrease in its boiling point and the appearance of ice crystals during freezing, which will instantly reduce the efficiency of the brake system as a whole.
Operating manual is supplied along with the car. In case if it is not inside the car, this manual can be purchased separately. This manual will specify the type of brake fluid, which is classified by the US Department of Transportation - DOT. Currently, the most common brake fluids are DOT3, DOT4, DOT4 + and DOT5.1. Moreover, top-class fluid may be added to the master brake cylinder expansion tank (for example, DOT4 may be added to the tank with DOT3 fluid), of course, provided that both fluids are produced by the same manufacturer. Conversely, do not add DOT3 liquid to the DOT4 tank.
At one time it was a very common type of brakes. Its arrangement is very simple (Figure 7.3): brake flap, which is non-rotating and rigidly fixed on the steering knuckle* (in case if the front wheels are steerable) or on an axle journal (in case if it is the rear axle), the brake flap has a wheel brake cylinder and brake pads that rest on one end and rest on the other in pistons of the wheel brake cylinder. Friction pads are glued or riveted on the brake pads. All these parts are covered with a brake drum, which rotates along with the wheel.
In case if it is suddenly necessary to reduce speed or to stop the car, the driver, pressing a brake pedal, through the hydraulic actuator acts on pistons of the wheel brake cylinder which, moving, spread brake pads, pressing them to a surface of a brake drum.
There are several layouts of brake pads.
* If anyone has forgotten, you can see what the steering knuckle looks like in Chapter 6, Figure 6.4.
Figure 7.3 Example of drum braking mechanism.
Interesting
In the drum brake mechanism, which is shown in Figure 7.3 (one of the most common layout of pads), the two pads are installed in series. One brake pad is front, the other is rear (in the course of movement). Both these brake pads are installed on axes from below. And from above these pads rest against pistons of the working cylinder. When braking, the forces will act so that the front brake pad will be wedged, and the rear pad will try to move the forces from the drum. This can cause uneven wear of the friction pads. Also, this effect leads to the fact that the working surfaces of the friction pads are not fully used.
Pull-back springs are installed in order for the brake pads to return to their original position after braking.
The clearance between the pads and the drum is often adjusted automatically. This is implemented simply: the pistons of the wheel cylinders, while moving outwards under the action of fluid pressure, will choose the axial gap between them and the elastic rings, and then pull the rings behind them. The movement of the pistons will continue until the pads rest against the drum. When the pedal is released, the pull-back springs will only be able to move the pistons back at a distance, which corresponds to the axial clearance between the piston and the ring, as they are unable to move the ring. The size of the gap, as it was mentioned above, corresponds to the required gap between the pad and the drum. Thus, as the pads wear, the ring will move along the cylinder, maintaining a constant amount of clearance in the mechanism.
Drum braking mechanisms have a number of advantages over disc brakes, but there are many disadvantages.
Advantages:
Disadvantages:
Disc brakes are the only promising alternative to drum mechanisms for passenger cars. In such mechanisms, the brake disc is attached to the wheel hub and rotates along with it. In order to stop the car, it is necessary to bring under pressure brake liquid to the piston in a brake support. The piston, while acting on the brake pads, will transmit to them the force of the brake fluid, press against the disc and start slowing down the wheel. The principle of operation of the disc brake is somewhat similar to a bicycle brake, where the surface of the wheel rim acts as a disc, and the stop is made by pressing the brake pads towards the rim. The arrangement of the disc braking mechanism can be seen in Figure 7.4.
Figure 7.4 Example of disk braking mechanism.
Note
There are two types of brake calipers: fixed and floating. In the first case, the brake caliper has two pistons that are located on either side of the brake disc. The caliper itself is rigidly attached to the steering knuckle. The pistons act on the inner and outer brake pads. Brake fluid is supplied to each of the pistons. In the second case, the brake caliper has a piston or pistons on one side only. It has the ability to move along the axis of rotation of the disk. Thus, while braking, the piston moves and presses on the inner pad. After that the pad rests on the disk, and the pressure in the hydraulic drive continues increasing. The caliper will begin to move and press the outer pad to the disk. The first option is stronger, but more expensive. The second option is cheaper, but not as reliable.
The efficiency of disc brakes is much higher than the efficiency of drum brakes. They are easier to maintain and better dissipate the heat, which is released during braking.
Designers decided to increase efficiency of heat removal from the brake mechanism, because overheating of brake mechanisms is considered as one of the most dangerous aspects. The way to solve the problem is simple in terms of ideas and not so simple on practice. Many holes and channels have been made in the disk, through which it is possible to pass air in order to cool the disk from the inside (Figure 7.5). Efficiency was increased many times. But everything has its price. And in this case they had to pay for low resistance to temperature changes in order to receive improved cooling. There is a possibility of appearance of cracks on the disc when, for example, water gets on it during very active braking. The reason is that these holes are stress concentrators. It turns out that the principle "It tears where it is thin" works.
Figure 7.5 Example of a ventilated brake disk.
Grooves can often be seen on the brake discs (Figure 7.6). The reason why the disc is manufactured with such design features lies in the fact that while performing braking a layer of waste material (dust) is created on the friction linings of the brake pads. This layer impairs the braking efficiency. The layer of the worked out dust is cut off by grooves, cleaning thereby a working surface of a frictional pad.
Figure 7.6 Example of a ventilated brake disk with special grooves.
But as time goes on, some cars become more powerful and heavier. They have high dynamic performance and very high power-to-weight ratio*. And the process of stopping all this mass, which is moving at great speed, requires the use of super-efficient braking mechanisms. The main indicator, which influences the efficiency of the brakes, as it was already mentioned above, is the temperature. There are two methods for solving the problem of brake overheating. The first method is to manufacture the brakes with their own cooling system. But this makes their design and subsequent maintenance too complicated. The second method is to install composite brake discs; such as disks of powder-metal type. Such disks are not affected by overheating at all. But the fee for disks of powder-metal type is their fragility and high cost.
Note
* Power-to-weight ratio is a specific indicator, which characterizes how many kilowatts or horsepower are per unit weight of a car (per kilogram or per ton). For example, in case if the car weighs 2 tons and its engine capacity is 300 hp, the power-to-weight ratio of this car is 150 hp/t. But a car weighing 1 ton and engine capacity of 150 hp will have the same power-to-weight ratio, and therefore will not be inferior in dynamic characteristics to a more powerful, but also heavier "rival".
Note
Disc brakes are often equipped with a friction pad wear sensor. In the simplest form, this sensor is a spring-loaded plate, which when the maximum thickness of the friction lining reaches the disk and starts creaking. In a more complicated version, this sensor in the literal sense of the word is installed. The readings of this sensor are displayed in the corresponding place on the instrument panel.
The greater the weight of the car, the more effort was required in order to apply the brake pedal for reducing the speed or stopping the car effectively enough. It would be an unforgivable mistake not to use the physical processes that occur during engine operation No mistakes were made - a vacuum booster was installed. Why is it vacuum? It uses the vacuum, which is created inside the engine intake manifold. The device of such booster is simple (figure 7.7): there is a case, which is divided by a diaphragm into two chambers - vacuum and atmospheric. On the brake pedal rod, inside the booster, there is a follow-up valve*, which opens or closes the access of atmospheric pressure to the atmospheric chamber. In addition, a pull-back spring of the amplifier diaphragm is installed. Master brake cylinder is consistently installed after the booster.
*Follow-up valve is not shown in Figure 7.7 for the ease of perception.
Figure 7.7 Vacuum booster of brakes in assembly with brake pedal and master brake cylinder.
Примечание
Due to various design features of the engines, the vacuum can be supplied not only from the intake manifold, but also from a special vacuum pump. For example, a vacuum pump is used for all diesel engines, because they have a small amount of vacuum in the intake manifold.
How does it work? This is quite simply: in the initial position (when no one is going to brake) the pressure in both chambers is the same and equal to the pressure, which is created inside the intake manifold. As soon as there is a need to brake, you will need to press the brake pedal. The movement of the pedal will be transmitted through the push rod to the follow-up valve. This valve will close the channel, which connects the atmospheric chamber with the vacuum unit. Further movement will connect the atmospheric chamber with the atmosphere. There will be a pressure drop, which will start acting on the diaphragm and move it, overcoming the force of the return spring. And the diaphragm, in turn, will move the piston rod of the master brake cylinder.
Примечание
This design of the vacuum booster provides a significant increase of the force (it can reach five times the magnification) on the piston rod of the master brake cylinder, which is proportional to the force on the brake pedal. In a word: the harder you press the pedal, the stronger and more efficient the vacuum booster will work.
As soon as the driver releases the brake pedal, the bleeder valve will close. The pressure in both chambers of the booster will be equalized, and the diaphragm will return to the initial position under the action of a pull-back spring.
It is important to know that while sitting in the driver's seat, you shall always check the technical condition of the vacuum booster. How to do it? It can be done easily…
The following procedures must be performed in order to check the operation of the vacuum booster of brake system:
1. Start the engine for 1-2 minutes and then stop it. The first time you press the brake pedal, it is fully depressed. But the next time you press the pedal stroke becomes larger with each press, then the booster works properly. In case if the pedal stroke height remains the same, the booster is not working properly.
Figure 7.8 Illustration to c. 1.
2. With the engine idle, depress the brake pedal several times. Then press the brake pedal and start the engine. In case if the pedal moves down slightly, this is normal operation of the booster. In case if the pedal movement is not changed, then the booster is working good.
Figure 7.9 Illustration to c.2.
3. With the engine running, press the brake pedal and then stop the engine. Hold the pedal pressed for about 30 seconds. In case if the height of the pedal does not change, the booster works fine, if the pedal rises - the booster is defective.
Figure 7.10 Illustration to c. 3.
Perform the three tests described above. In case if at least during one of the three tests vacuum booster does not work properly, check the check valve, vacuum hose, and booster for damage.
The parking brake (in everyday life - "handbrake") is intended for holding the car during parking both on a flat, and on an inclined surface. In passenger cars, the parking brake system can perform a function of emergency brake system.
Parking brake systems differ in the type of drive as well as in the type of braking mechanism.
According to the type of drive parking brake systems can be as follows:
Simple and cheap layout of impact on the brakes of the rear wheels involves the use of cables and a lever (or pedal), which is located inside the passenger compartment. The principle of operation of this system is as follows: while moving the parking brake lever, the drive cables are moved. They act on the brake mechanisms, thus preventing the wheels from turning (the elements of the parking brake system are shown in Figure 7.1). The lever inside the passenger compartment is mounted on the ratchet gear, which allows fixing it in the working position. A signal switch is installed under the lever on the instrument panel in order to inform the driver that the parking brake is currently applied.
Note
From the lever inside the passenger compartment the force is transmitted to the brakes by the means of cables. One, two or three cables are used in the design of the parking brake drive. The most common layout has three cables: one front (central) and two rear. The front cable is connected to the lever in the passenger compartment, the rear cables - with the brakes of the right and left wheels, respectively. An equalizer or balance weight is used for connecting the front cable to the rear and evenly transmitting the force.
As well as any other mechanism, the parking brake needs to be adjusted. For this purpose, adjustment of length of cables by means of the adjusting nut, which is installed often on the equalizer, is provided. Return of the system to the initial position (removal from a brake) is made at transfer of the manual lever to the corresponding position by means of a pull-back spring. The spring can be located on the front cable, the equalizer or directly on the brake mechanism.
The braking mechanisms of the parking system can be of different types. Thus, in case if drum brakes are applied behind, the parking brake system uses brake mechanisms of working brake system. Only in this case blocks are pressed not at the expense of the wheel brake cylinder, but at the expense of influence of a cable of system on the driving lever of pads. In case if the brakes are of disc type, then there are two options: either transfer the force via the drive lever to the brake caliper of the service brake system and thereby fix the wheel, or make the rear brake disc composite (meaning that the brake disc will be made so that its inner surface will be act as a brake drum).
Interesting
It is advisable to always use the parking brake in order to prevent acidization of the cables. Because, in case of an emergency and unforeseen situation on the road, it will not be possible to use the parking brake system as emergency one, with sour drive cables.
Such a parking brake system is often used in sports cars, when the parking brake is applied for a short time and is necessary in order to achieve the desired trajectory of the car while turning.
In fact, the same lever is used in the passenger compartment. The same rear brakes as in the mechanical drive, but instead of cables — hoses and pipes that are filled with brake fluid. One or two master brake cylinders are connected to the lever in the passenger compartment. The principle of operation is the same as in a regular service brake system. It was discussed above.
Figure 7.11 Parking brake with hydraulic drive.
During the age of automation and computerization, the heavy burden of applying the parking brake was shifted to the electromechanical servo.
There is only a key or a button instead of the lever inside the passenger compartment. The cables were replaced by electrical wiring, and the actuators were installed on the elements of the service brake system. With this system, the driver does not need to count the number of ratchet gear clicks while tightening the "handbrake". There is no need to worry about adjusting the parking brake cables - just press a button or pull a key and the electronics will do the job.
In addition to this simplification, a system of assistance when starting from a place under a slope, which uses a parking brake was added. As a result of this the driver got rid of one more headache - car rollback at the beginning of movement.
Figure 7.12 Electro-mechanical drive of parking brake.
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