For effective management movement of any mechanical means - regulating the speed on one or another section of the path, slowing it down when performing maneuvers, and finally, to stop at in the right place– and including emergency ones – all trucks and cars must be equipped with a brake system corresponding to the class of the vehicle. To keep the machine in place during long periods of parking, especially on a slope, a parking brake is provided.
For safe operation of the vehicle, this system must be reliable like no other. It is no coincidence that the list of faults for which the use of a vehicle is prohibited (appendix to the Rules) traffic RF), malfunctions of brake systems are placed in first place.
Classification of car brake systems
Modern cars are equipped with three or four types of brake systems:
- working;
- parking;
- auxiliary;
- spare.
The main and most effective braking system of a car is a working one. It is used throughout the movement to regulate speed and come to a complete stop. Its device is quite simple. It is activated by pressing the brake pedal with the driver's right foot. This procedure ensures simultaneous reduction of engine speed, by removing the foot from the accelerator pedal, and braking. 
Parking brake system, as the name suggests, is designed to keep the vehicle stationary during long periods of parking. In practice, experienced drivers leave the car in first or reverse gear. However, on large slopes this may not be enough.
The hand parking brake is also used when starting off on uneven sections of the road, when right leg should be on the gas pedal, and the left one squeezes the clutch. By smoothly releasing the brake lever with your hand, simultaneously engaging the clutch and adding gas, you can prevent the car from arbitrarily rolling downhill.
The spare brake system is designed to duplicate the main working one in case of its failure. This could be completely standalone device, or represent a part, one of the circuits of the brake drive. Alternatively, a parking system can perform the functions of a spare one.
The auxiliary braking system is installed on heavy-duty vehicles, for example, on domestic KamAZ, MAZ, KrAZ vehicles. It is designed to reduce the load on the main working system during prolonged braking - when driving in the mountains or on hilly terrain.
System design and principle of operation
The main thing in the braking system of any car is the brake mechanisms and their drives. The hydraulic brake drive used on passenger cars consists of: 
- pedals in the cabin;
- working brake cylinders of front and rear wheels;
- pipeline (brake pipes);
- master brake cylinder with reservoir.
The principle of operation is this: the driver presses the brake pedal, driving the piston of the brake master cylinder. The piston squeezes fluid into the pipelines to the brake mechanisms, which in one way or another create resistance to the rotation of the wheels, and thus braking occurs.
When the brake pedal is released, the piston returns via a return spring, and the fluid flows back into the master cylinder - the wheels are released.
On domestic rear-wheel drive cars, the brake system design provides for a separate supply of fluid from the master cylinder to the front and rear wheels.
On foreign cars and front-wheel drive VAZs, the pipeline circuit diagram “left front – right rear” and “right front – left rear” is used.
Types of brake mechanisms used in cars
The vast majority of cars are equipped with friction-type brake mechanisms that operate on the principle of friction forces. They are installed directly in the wheel and are structurally divided into: 
- drums;
- disk.
There was a tradition of installing drum mechanisms on the rear wheels and disc mechanisms on the front. Today, depending on the model, the same types can be installed on all four wheels - either drums or discs.
Design and operation of the drum brake mechanism
System design drum type(drum mechanism) consists of two shoes, a brake cylinder and a tension spring, located on a shield inside the brake drum. Friction linings are riveted or glued onto the pads.
The brake pads with their lower ends are hinged on the supports, and with their upper ends - under the influence of a tension spring - they rest against the pistons of the wheel cylinder. In the unbraked position, there is a gap between the shoes and the drum, allowing the wheel to rotate freely. 
When fluid enters the cylinder through the brake pipe, the pistons diverge and push the pads apart. They come into close contact with the brake drum rotating on the hub, and the friction force causes the wheel to brake.
It should be noted that in the above design, the wear of the front and rear pads occurs unevenly. The fact is that the friction linings of the front pads in the direction of movement at the moment of braking when moving forward are always pressed against the drum with greater force than the rear ones. As a solution, it is recommended to change the pads after a certain period of time.
Disc type brake mechanism
The disc brake device consists of:
- a caliper mounted on a suspension, the body of which houses the outer and inner brake cylinders (there may be one) and two brake pads;
- disk, which is attached to the wheel hub.
When braking, the pistons of the working cylinders hydraulically press the brake pads against the rotating disc, stopping the latter.
Comparative characteristics
Drum brakes are simpler and cheaper to manufacture. They have a property called the mechanical self-reinforcement effect. That is, with prolonged pressure on the pedal with your foot, the braking effect increases many times over. This occurs due to the fact that the lower parts of the pads are connected to each other, and the friction of the front pad on the drum increases the pressure of the rear pad on it.
However, the disc brake mechanism is smaller and lighter. Temperature resistance is higher, they cool faster and better due to the provided window openings. And replacing worn disc pads is much easier than replacing drum pads, which is important if you carry out the repairs yourself.
Working principle of the parking brake
He is purely mechanical device. It is activated by raising the handbrake lever to a vertical position until the latch clicks. In this case, tension occurs on two metal cables running under the bottom of the car, which tightly press the brake pads of the rear wheels to the drums.
To release the car from the parking brake, press the locking button with your finger and lower the lever down to its original position.
Don't forget to check the position of the handbrake before you start driving! Driving with the handbrake not released will quickly damage the brake pads.
Car brake system care
As one of the most important nodes, the car's braking system requires constant attention and care. Here, literally any malfunction can lead to unpredictable consequences on the road.
Some diagnoses can be made based on the behavior of the brake pedal. Thus, an increased stroke or a “soft” pedal most likely indicates that air has entered the hydraulic drive system as a result of a brake fluid leak. Therefore, it is necessary to periodically monitor the liquid level in the tank.
Its increased consumption may be a consequence of damage to hydraulic hoses and tubes, as well as ordinary evaporation over time. This causes air to enter the system and cause brake failure.
Parts that have become unusable must be replaced, and the system will have to be pumped by bleeding air from each working cylinder on the wheels and adding fluid. The process is long and tedious.
When the car pulls to the side when braking, it indicates possible exit failure of one of the working cylinders or excessive wear of the linings on a particular wheel. If the brake mechanisms are dirty, a characteristic noise may occur when you press the pedal.
All these malfunctions can be easily eliminated independently or by contacting service center. And to minimize the troubles described above, take care of your brakes and use engine braking more often, especially on steep and long descents. Prolonged activation of the main working system leads to overheating of parts and causes various breakdowns.
TO category:
Car brake control
Drum brake mechanisms and their elements
The drum brake mechanism has symmetrical shoes (usually two), bearing friction brake linings on the outer cylindrical surfaces, which, under the action of the drive device, are pressed against the inner cylindrical surface of the drum. Diagrams of the most common drum brake mechanisms are shown in Fig. 34. They are classified by the type and number of drive devices, as well as by the number of degrees of freedom of the pads. The block has one degree of freedom if it rotates around a fixed geometric axis. This is achieved either by pivoting the pad with an axis fixed in the caliper, or by placing the radius end of the pad in the corresponding cylindrical socket of the caliper.
Rice. 34. Drum brake diagrams s
For pads with two degrees of freedom, the geometric axis of their rotation can move, which allows the pad to self-align, and therefore ensures a better fit to the drum and more uniform wear of the lining. Pads with two degrees of freedom either rest with their rounded end on the beveled plane of the caliper and slide along it, or are connected to the latter using an intermediate link, which, in turn, has a fixed geometric axis of rotation relative to the caliper. Sometimes this link is the second brake pad.
The effectiveness of different drum brake mechanisms with the same size and equal drive forces varies greatly. The most effective is the brake mechanism, which has one clamping and second servo block with sliding supports and one drive device in the form of a double-sided wheel cylinder. With a braking mechanism of this type, the servo effect reaches the greatest value. However, the higher the efficiency of the braking mechanism, the more sensitive it is to changes in the friction coefficient of the friction pair. Since the friction coefficient is a variable value and depends on many factors (speed and temperature in the friction zone, the magnitude of the driving force, the rigidity of the brake parts, etc.). The most effective brake mechanisms are usually the most unstable. During their operation, vibrations, squeaks, etc. occur more often. In this regard, the area of use of such brake mechanisms is gradually narrowing.
Rice. 36. Static characteristics of brake mechanisms
In recent years, with the spread of automated brake drives, which allow increasing driving force, brake mechanisms with low servo action are increasingly used. It should be noted that pads with two degrees of freedom have greater servo action than those with one. However, such pads, especially those with a sliding support, are very prone to vibration and squeaking. In addition, the angle of the pad support must be such that the pad returns to its original position after braking.
One of the simplest is a drum brake mechanism with articulated pad supports and a cam drive device. Its design is shown in Fig. 37. The pads of this type of brake have equal movements determined by the shape of the expansion cam (mechanisms of this type are sometimes called equal movement brakes). As a result, the braking moments created by both pads are equal, and the driving force acting on the release pad is significantly greater than that acting on the pressing pad. The total braking torque of this brake when rotating the brake drum in both directions is almost the same; The wear on both pads is almost identical. The advantages of such a braking mechanism include its high stability, as well as the fact that the forces applied to the brake drum from the shoes are practically balanced and do not create additional load on the wheel bearings. The disadvantage of a brake with equal movements is the need for significant driving force and a relatively low coefficient useful action cam drive device. According to domestic researchers, the efficiency of a cam drive device ranges from 0.60 to 0.80. To reduce friction between the fist and the block, a roller is installed, and sliding bearings are used in the fist supports, which increases the efficiency of the drive device to 0.75-0.90. In practice, due to dirt getting into the cam supports and the axles on which the rollers rotate, the efficiency of the cam drive device is at the lower limit. It should also be noted that the increased labor intensity Maintenance such a brake mechanism due to the need to periodically lubricate the knuckle supports.
Rice. 37. Brake mechanism of the ZIL-130 car:
1 - brake power supply slave; 2 - friction lining; 3 - rivet; 4 - brake block; 5 - expansion fist; 6 - adjustment lever; 7 - worm cash; 8 - worm; 9 - pad release spring; 10 - caliper; 11 - block axis
Rice. 38. Brake mechanism of the GAZ-21 car:
1 - brake pad; 2- rivet; 3 - friction lining; 4 - eccentric adjusting washer; 5 - wheel cylinder; b - tension spring; 7 - block retainer; 8 - block axis; 9 - caliper
The brake mechanism, which is shown in diagram II of Fig., has become widespread. 34. It has articulated pad supports and a drive device in the form of a double-sided wheel brake cylinder (Fig. 38). Here, equal driving forces are applied to the pads, but the braking torque created by the pressing pad is greater than the pushing torque. Accordingly, there is more wear on the pressure pad lining. This braking mechanism is equally effective when the drum rotates in both directions. With equal drive force, it provides greater braking torque than the cam brake mechanism described above, due to greater servo action and higher (up to 0.95-0.98) efficiency of the drive device.
The disadvantage of this braking mechanism is the presence external force, loading the wheel bearings, as well as the unequal durability of the friction linings.
To eliminate these shortcomings, stepped wheel cylinders are used, which create different drive forces. Sometimes the pad on the pressing pad is made smaller or thinner than on the pressing pad.
The design of the third fairly common brake mechanism is shown in Fig. 39. This is a brake mechanism with sliding pad supports and two drive devices in the form of one-sided wheel cylinders. Both pads are pressed when the brake drum rotates forward and pressed when it rotates backward, as a result of which the effectiveness of the braking mechanism when the car is moving in reverse is much less.
Rice. 39. Brake mechanism of the Moskvich-408 car:
1 - brake pad; 2 - friction lining; 3 - pressure spring; 4 - tension spring; 5 - wheel cylinder; 6 - caliper
Rice. 40. Wedge drive device of the drum brake mechanism:
1 - body; 2 - roller return spring; 3 - plunger; 4 - plunger head; 5 - pin; 6 - dust cover; 7 - dog; 8- pawl spring; 9 - clamp; 10 - roller; 11 - roller holder; 12 - rod; 13 - seal; 14 - rod return spring; 15 - brake chamber housing
This is a significant drawback of such a brake. In addition, the use of two spaced apart drive devices makes it difficult to drive the parking brake system. However, the equality of the pad moments, uniform wear and large servo action make it possible to successfully use a mechanism of this type on the front wheels passenger cars.
In recent years, a new design of drum brake mechanisms has been created for brake systems with pneumatic drive. In it, the pads are unclenched not with a traditional fist, but with a wedge drive device (Fig. 40). Since the wedge rod is made floating, such a brake mechanism has more high efficiency than the cam driven brake mechanism described above. The support of the pads is either sliding or hinged. A very promising design is a brake mechanism with two wedge drive devices, one of which has a conventional brake chamber, and the other a chamber with a spring energy accumulator. The advantages of a brake mechanism with a wedge drive device are more uniform and less wear on the parts of the rubbing pair, more high efficiency, smaller size of the brake chambers, resulting in significantly less consumption compressed air. However, the wedge drive device also has disadvantages: increased manufacturing costs and the need for good dirt protection.
The most important elements of the brake mechanism are the parts that make up its friction pair - the brake drum and friction linings. Brake performance and maintenance different conditions almost entirely depend on the quality of these parts.
The specificity of the operation of the brake drum is that, due to the extremely low thermal conductivity of the friction lining material, over 95% of the heat released during braking is absorbed by the drum. Tests have shown that the temperature of the brake drums of heavy vehicles on long descents can reach 250 - 360 °C. The thermal stresses in the drum arising from such temperatures are aggravated by the action of cyclic loads from the pads. Note also that for safety reasons, the strength of the brake drum must be guaranteed. Brake drums trucks and buses are usually made of cast iron and often have ribs on the outer surface to increase strength, rigidity and heat dissipation. On passenger cars, to reduce weight, a combined drum is used - a stamped steel or aluminum cast disk cast into a cast-iron rim.
The use of cast iron for the manufacture of brake drums is due to the fact that this material, paired with modern friction linings, provides a high coefficient of friction, works well in compression, and has sufficient thermal conductivity. Less critical transmission brake drums are sometimes made of stamped steel.
The friction lining is made of a complex asbestos composition, which consists of a filler - asbestos fibers and a binder - synthetic resins or their mixtures with various organic substances. Sometimes zinc or brass particles are added to the composition, which increase mechanical strength linings improve its thermal conductivity, but they intensify drum wear.
At present, asbestos-friction brake linings are mainly manufactured by combustion molding. In recent years, experiments have been conducted on the use of metal-ceramic and metal-resin (semi-metallic) linings. However, such linings are so far used only in special brake mechanisms. Vehicle. Possessing high heat resistance, they have insufficient efficiency in a cold state, cause increased wear on the drum, and create vibrations and squeaking brakes.
Friction linings of automobile brake mechanisms must have the following properties:
– high friction coefficient, stable when sliding speed, specific pressure and temperature change over the entire range of real operating conditions;
– high wear resistance; low moisture and oil absorption, the ability to quickly restore efficiency after getting wet;
– strength and reliability, the ability to operate without cracks, tears, and application of drum material to the surface of the lining, without scuffing and excessive wear of the drum material;
– lack of tendency to vibrations and “squeaking”. Great importance has a method of attaching friction linings to the pads. Highly rigid truck linings are usually riveted or screwed. This method of fastening is convenient for repairs, but reduces the working area of the lining and its durability, since the working thickness is reduced. Thinner and therefore elastic linings of passenger cars are often glued. The glued pad works almost until it is completely worn out, but its removal and replacement is very labor-intensive.
During operation, the friction linings and the drum wear out, which entails an increase in the gap between them in the released state. An increased gap leads to a delay in the brake response, an increase in the strokes of the actuating elements of the drive, and, consequently, to an overconsumption of the working fluid in it. Hydrostatic brake actuators may fail for this reason.
To avoid such phenomena, modern brake mechanisms are equipped with devices for manual or automatic regulation the size of the gap in the friction pair. The principle of operation of these devices is to periodically change the position of the released block. There are two types of adjustments: factory, which is made after assembling a new brake or after replacing its parts, and operational, which eliminates the effects of wear. For operational adjustments of brake mechanisms with hydraulic cylinders, washers with a spiral or eccentric profile installed on the brake caliper are used. The rotation of such a washer 4 (Fig. 38) causes a corresponding angular movement of the block resting on it. For brake mechanisms with a cam drive device, a worm pair in the adjusting lever is used for this purpose (Fig. 37). Rotation of the worm shaft brings the lever, and therefore the expanding fist 5, to a new angular position, and the shoes move closer to the drum. In a wedge brake mechanism, this is achieved by increasing the length of the plunger by rotating the plunger head (Fig. 40).
Rice. 41. Automatic clearance adjuster for GAZ-24:
During factory adjustment, in addition to these devices, pad supports are also used. So, in the brake mechanisms shown in Fig. 37 and 38, the axes of the pads are made in the form of eccentrics and their rotation changes the position of the pads.
In recent years, widespread automatic devices for adjusting the gap in the brake mechanism. Such devices significantly reduce the labor intensity of brake system maintenance and increase traffic safety by constantly maintaining the brake mechanisms in a state of technical readiness.
The principle of operation of automatic regulators is based on limiting the reverse motion of the brake pads when releasing the brakes, if their working stroke, due to the increased gap, turns out to be greater than the prescribed value. Automatic regulators are built into the drive device or installed directly on the block. Examples of their designs are shown in Fig. 41-13.
The piston reverse limiter built into the wheel brake cylinder (Fig. 41) is a split spring ring, loosely placed on the piston neck and inserted into the cylinder with a large interference fit (the force required to move it in the cylinder is 60 kgf). The width of the piston neck is greater than the width of the ring, as a result of which axial movement of the piston relative to the ring is ensured by a given amount (from 1.2 to 2.1 mm). If the gap in the brake is greater than the specified value, then when braking, the piston at the end of its stroke will move the ring to a new position (the pressure force in the drive is sufficient for this). When releasing the brakes, the release spring of the shoes will not be able to overcome the tension of the ring, and the piston together with the block will be installed closer to the drum.
Rice. 42. Automatic vehicle slack adjuster BA3-2103:
1 - brake pad; 2 - yatulka; 3 - friction washer; 4 - spring support cup; 5- spring; 5 - nut; 7 - axis; 8 - brake caliper
Rice. 43. Automatic cam drive adjustment lever
Autonomous block backstop, shown in Fig. 42, consists of friction washers that compress the rib of the brake pad under the action of a powerful spring, as well as a threaded bushing inserted with a large gap into the hole of the rib of the pad and an axle that is welded to the brake caliper. The reverse motion of the block is limited by friction between its edge and the washers.
The structure of the automatic adjustment lever of the cam drive device is shown in Fig. 43. When braking, the body of the adjusting lever turns counterclockwise and the gear rack, resting its tooth against the cutout of the disk connected to the fixed lever, turns the gear and the outer cone half-coupling. In this case, under the influence of force on the brake chamber rod, the disc springs are compressed and the outer cone half-coupling does not touch the inner one, which is integral with the worm. When released, the gear rack is held in a new position, as a result of which the worm, the cone half of which is connected to the outer cone half under the action of springs, rotates through a small angle. The worm wheel meshed with it and placed on the splines of the expansion fist also rotates. Thus, the fist rotates and the gap between the lining and the drum decreases. This process occurs every time you brake. The amount by which the gap is reduced depends on its original value. So, with an initial gap between the lining and the drum of 1.6 mm, after 40 brakings the gap decreases by 1.1 mm, and with an initial gap of 0.5 mm - by only 0.1 mm.
The automatic gap regulator of the wedge drive device works in a similar way, in which, with a large stroke of the plunger, the pawl jumps to the next tooth and, during the reverse stroke, turns the plunger head, as a result of which the pin extends and brings the shoe closer to the drum.
TO Category: - Car brake control
Drums, of course, long ago lost the evolutionary war to discs, but to this day they are quite actively used on inexpensive and lightweight machines. All Ladas, Renault Logan, VW Polo sedan, Skoda Rapid, Daewoo Matiz – the list is quite modern models using these archaic but durable brake mechanisms will be very long. This means it’s useful to know how they work, why they break and how they are repaired. After theoretical preparation, we will go to the repair zone, where we will examine the drums of a rare Chinese sedan Chery Jaggi, better known in Russia under the name QQ.
Drum brake design
Drum brakes have not changed fundamentally since their mass appearance in 1902 thanks to Louis Renault. True, those brakes had a cable drive, and therefore they were exclusively mechanical. Plus they did not have automatic adjustment, so the driver had to regularly check the gap between the pads and the drum. But the fundamental design, I repeat, has changed minimally.
We will describe here the most common, classic design of the drum brake mechanism. There is a brake flap that is rigidly attached to the rear axle housing or wheel axle, and it does not rotate. There is also a drum that is attached to the wheel hub and rotates with it and the wheel.
The brake pads are installed on the brake flap. On one side, the pads rest on the axles, on the other, on the pistons of the working brake cylinder (this is clearly visible in the photographs). When the brake pedal is pressed, the brake fluid pushes the pistons in the working cylinder apart, which in turn pushes the brake pads apart. The pads are pressed against the surface of the drum and the car slows down. Friction linings are glued or riveted to the pads. To prevent the pads from falling out, pressure springs are installed.
A nice feature of this design is that one of the pads has the property of wedging (it is called active). To give an example, imagine a car wheel, spin it well and try to insert an object between the wheel and the arch with your hand: on one side the object will be pushed out, and on the other it will be pulled even more into the space between the wheel and the arch, thereby wedging wheel. The same situation applies to the pads.
The second block (passive) is repelled by the drum, and its efficiency is lower than the first - this, on the contrary, is an unpleasant moment. To compensate for the difference, the friction lining of the passive pad is larger in size than the active pad.
The downside of a jammed pad is that the braking force does not increase in proportion to the force on the pedal. Simply put, you press the brake pedal and get a completely different, much greater deceleration than expected. This is not the case with disc brakes.
To ensure that the pads return to their original position after braking, return springs are installed on them. Often, if the rear brake mechanism is drum, then the same pads are activated when the parking brake (“handbrake”) is applied. One of the pads has an additional lever to which a cable is attached, when moving the cable the pads are moved apart.
On modern cars, the drum brake mechanism is self-adjusting. That is, you don’t need to crawl under the car every few thousand kilometers or after repairs, like on a ZIL 130, to measure the gap between the friction linings and the drum.
However, even on modern cars the parking brake still needs to be adjusted. Therefore, the spacer strut, thanks to which the pads are moved apart when the handbrake is tightened, tends to lengthen or shorten due to the rotation of the nut (it is also clearly visible in the photo). Another positive aspect of drum brakes is the area work surface friction linings - in any case, it is larger compared to disc brakes.
But due to the specific operating conditions (see above), the wear of the linings is uneven, which means that the force will also change with wear. In turn, no one bothers to increase the working area of the linings by increasing not only the diameter of the drum, but also its width, and this is an indisputable plus. This is skillfully used by truck designers, for whom it is more important to brake 20 tons within the limits of decency than the delicate connection between the driver’s foot and the acceleration of the vehicle’s deceleration.
Test drives / Singles
Nicknamed “barge”: test drive GAZ-24 Volga
From afar, for a long time... So much has been written about the history of the Volga that I am simply ashamed to start this conversation again. But I’ll start it: they pay me a salary for this, and repetition, as they say, is the mother of something...
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Moreover, even if a car has disc brakes installed all around, then with a high degree of probability the handbrake brake mechanism is implemented using a drum circuit. They simply make a groove in the disc and create their own small drum and place it inside the pad.
A few words about outdated drum brake designs. In search of simpler and effective options execution engineers, in order to solve the problem with a block that does not wedge, came to the conclusion that it is possible to install two working cylinders on two opposite sides of the brake flap (and many other cars with drum brakes front and rear). In this case, both pads became jammed, but only when moving forward.
AZLK designers used drum mechanisms with floating shoes. Floating because they do not rest on an axle, each on its own, but on a hinge connecting both blocks. Therefore, when the pistons move them apart, they are stabilized relative to the drum due to efforts. And the wedging effect of the active block is reduced due to the transfer of force through the hinge to the passive block.
Pros and cons of drums
Articles / History
Brakes a hundred years ago: how drums turned out to be more effective than discs
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One of the main advantages of drum mechanisms is its closeness from environment– neither dirt nor dust gets inside. It’s hard to disagree with this, but with a caveat – if we're talking about about the dirt outside. All the wear products of the pads that appear inside the drum cannot simply “get out” from there. All the beauty of being closed by a drum is visible in the photographs of the experimental subject.
If in disc brakes the remains of the friction linings are simply blown out of the mechanism, then in drum brakes almost everything remains in place. And further. Anyone who has driven trucks or ancient cars with “drums” in a circle in their life must remember: if you drive through a deep puddle or ford, then you need to press the brakes several times to dry them out, otherwise they simply won’t happen. There is no such circus with discs.
Drums also overheat easily and, unlike disks, cannot be quickly cooled by incoming air. In this case, it is difficult to warp the drum itself (which cannot be said about discs), but the braking efficiency of hot drums decreases very significantly.
In terms of dynamics, drums are also inferior to discs, since the latter are lighter. Plus, the maximum braking force of drums is greatly limited - excessive pressure on the pads can simply “break” the drum. Discs can be compressed much more strongly.
Example of rear drum brake repair
Everything here is, in general, quite predictable. Drums are usually disassembled for two manipulations: replacing the pads or repairing the jammed mechanism itself.
This time we received a car with a non-working rear right brake and no parking brake. The experienced eye of the foreman found no brake fluid leaks. Therefore, the probability of a stuck brake wheel cylinder has increased to 99%. The decision was made immediately - disassembly and more detailed diagnostics.
Unscrew the nuts and remove the wheel. Fortunately, the drum did not stick and came off quite easily. The owner of the car felt better when he found out that it was too early to change the pads. But then the bad news came. The parking brake strut is soured, therefore, it is impossible to adjust the location of the pads, and this is the reason for the missing handbrake. Further. The pistons in the working cylinder were jammed, which is why the car did not brake. Verdict - replacement of the working cylinder. The owner faced the difficulties courageously and blessed us to start immediately.
Since it is necessary to replace the working cylinder, we clamp the brake hose to prevent all the brake fluid from leaking out of the circuit. Unscrew the connecting nut and disconnect the brake pipe from the working cylinder. Using narrow-nose pliers, remove the lower spring from the brake pads. Then we disconnected the parking brake cable from the brake shoe lever.
Using the same narrow-nose pliers, they pressed, turned and removed the pressure springs of both pads. The springs are fixed on the finger: each has a small support cover with a slot, and the outer end of the finger is flattened. Accordingly, during installation, the spring is compressed, the end of the pin passes through the slot, and in order to fix the spring, it is turned. But that will come later, now it’s dismantling.
After removing the pressure springs, both pads can be removed from the brake flap and the working cylinder. This is what we do by moving them slightly apart to overcome the force of the upper return spring. Then we unscrewed the fastening bolts and removed the working brake cylinder. We removed the spacer from the pads, cleaned it thoroughly and designed it so that the parking brake could be adjusted. Then they removed the top return spring.
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During the process, the grooves on the friction linings drew attention to themselves. Exactly the same were on the working surface of the brake drum, and such wear inevitably reduces braking efficiency. In order not to risk the health and well-being of the car owner, the drums were sent for grinding. It’s too early to change the pads - they will level out.
The photographs clearly show the ring gear of the rear wheel speed sensor. Recently, automakers have often installed a conventional ring with magnetized sectors instead of a ring gear. Everything is fine, but sometimes dirt, dust, and wear debris accumulate so much on the ring that its magnetism begins to be lacking, and ABS system I get the error “I don’t see the sensor”. This can be treated by thoroughly cleaning the ring and resetting the error. But we digress.
We install a spacer stand on the pads - clean, designed and lubricated. We connect the upper return spring to both pads. First of all, we connect the parking brake cable to the lever on the shoe, then we hang the shoes on the brake flap. Install a new brake slave cylinder. We screw in, but do not tighten the bolts of its fastening and do not forget about the bleeder fitting.
Drum brake location
Drum brakes work on the same principle as disc brakes: The brake shoe presses against a rotating surface. Only in this design this surface is called a drum.
In most cars drum brakes are installed on the rear wheels, and disc wheels - on the front. Drum brake design includes larger number parts compared to disk ones, so they are more difficult to maintain. However, they are cheaper to manufacture and easier to integrate with the handbrake.
In this article we will talk about how drum brakes work, how to maintain them, and look at installing a handbrake mechanism.
Let's start with the basics.
Drum brake with drum removed
Drum brake
Drum Brake Components
The drum brake looks like complex design, but everything is much simpler if we look in more detail. We suggest disassembling the brake and seeing how it works.
Like a disc brake, a drum brake has two pads and a piston. But a drum brake also houses a brake adjuster, a handbrake mechanism, and a variety of springs.
When you press the brake pedal, the piston presses the pads against the drum. Everything is simple enough, but what are all these springs for?
In fact, the situation is a little more complicated. Many drum brakes are self-acting. The brake pads make contact with the drum, causing a sort of jamming action to occur, causing the pads to be pressed harder against the drum.
The additional braking force that this wedging provides allows the use of a smaller piston compared to disc brakes. However, due to jamming, the brake pads must be moved away from the drum after braking is completed. Springs are used for this. Other springs hold the pads in position and return the brake adjuster to its place after it is activated.
Brake regulator
Brake adjuster mechanism
For a drum brake to function correctly, the shoes must be close to the drum, but not in contact with it. If they are moved too far (for example, when the pads are worn), the piston will need more fluid to overcome that distance, and the brake pedal will “go to the floor” when pressed. For this reason, most drum brakes use an automatic adjuster.
Let's look at the structure of the regulator mechanism. The regulator is also self-acting.
When the pad wears out, more space is created between it and the drum. Every time the car stops, the pads are pressed against the drum as much as possible. As the gap increases, the adjuster lever moves the gear one tooth. The regulator, like the bolt, has a thread. When turning, it unscrews, reducing the gap. With further wear of the pads, the adjuster is unscrewed further, ensuring that the pads are close to the drum.
In some cars, the regulator is activated when the handbrake is applied. But the adjustment of such a mechanism may go wrong if the handbrake is not used for a long time. If you have such a system, put the car on the handbrake at least once a week.
Hand brake
The handbrake, in addition to the main braking system, can be activated by other means. The drum brake design allows the use of a simple cable drive mechanism.When using the handbrake, the cable pulls the lever, which presses the pads.
Service
Brake shoe
For the most part, drum brake maintenance involves replacing the brake pads. Some drum brakes have a service hole on the side that allows you to determine pad wear. Brake pads need to be replaced when the thickness of the friction material on the rivets is 0.8 mm. If the friction material is applied to the backing plate (without rivets), then the pads must be replaced when the thickness of the friction material is 1.6 mm.
Just like disc brakes, worn pads can leave grooves in the drums. If worn pads are used for a long time, the rivets can damage the drum. Drums with deep grooves can be resharpened. If for disc brakes there is a minimum permissible thickness, then for drum brakes - the maximum permissible diameter. The contact surface in drum brakes is located inside the drum. As material is removed, the diameter increases.
- Overheat. Since the friction surfaces are not blown with air (unlike the design of disc brakes), they are cooled much worse. Here it must be said that the temperature of the drums during emergency braking can reach 500-600 degrees. Under these conditions, the drum expands, the distance to the pads increases and the pedal needs to be pressed harder. They tried to combat overheating of the drums by installing additional fins on the outside - they were blown with air and “drained” some of the heat. However, this design still cannot withstand any competition with disc brakes.
Do they have any advantages?
For all its disadvantages, drum brakes also have undeniable advantages:
- Protection from dirt. The pads here work in a confined space, and dirt from the outside does not penetrate there.
- High braking force. We said above that the efficiency of drum brakes and the maximum pad pressure are lower than that of disc brakes. However, the closed design makes it possible to make the friction area very large by increasing the diameter and width of the drum. Because of this, brake drums have had no alternative for large trucks and buses for a very long time.
- Wear resistance of pads. The worse adhesion of the pads to the drum does its job: the pads wear out more slowly, although the quality of braking suffers from this.
Why are they still being installed?
The first two advantages of drum brakes have long been almost irrelevant. Engineers learned to make discs and pads more resistant to wear, and bulky drums gradually fell out of use among truck and bus manufacturers. European models lost them in the late 90s - early 2000s. However, the Russian “Lawn”, for example, still has drum brakes front and rear, but very soon it will become history.
As for SUVs, for which the dirt resistance of the drums is relevant, then expensive models(Toyota Land Cruiser, Mitsubishi Pajero) lost them back in the 80s, and on cheaper ones, as well as on pickup trucks, drums are still found, but only on the rear axle. Why behind? It's simple: because a lot of dirt flies onto the rear wheels from the front ones.
