Drum brakes– such a device is familiar to many motorists. This type of braking system is becoming a thing of the past, giving way to more technologically advanced and efficient ones.
Photo: Drum brake mechanism
Terminology
Drum brakes are a system of mechanisms aimed at reducing speed or completely stopping a vehicle. In addition, this complex protects the car from spontaneous movement.
History of origin and development
First mechanisms
Despite the fact that disc brakes were invented even earlier, it was drum brakes that began to be equipped on new cars. After all, they turned out to be much easier to produce, which is important, since the industry was not so developed as to produce complex mechanisms.
The first drum brakes consisted of a drum rigidly fixed to the hub, around which a strong and flexible tape was wound. During braking, it stretched onto the surface of the drum and stopped the car.
But this design turned out to be unsuccessful, since the tape wore out very quickly, and the dirt and small debris that accumulated under it damaged the drum itself.
Louis Renault
The honor of invention in 1902 belongs to drum brakes, where the pads were located inside the drum. This significantly increased braking efficiency, as well as reliability, because such a design excluded the possibility of dust and other contaminants getting inside. Renault's system was based on the use of cables and levers.
Photo: Repair kit for drum brake VW Golf (1997)
30s
The evolution of drum brakes during these years led to the appearance of compact brake cylinders, of which two were sometimes installed per mechanism. However, a significant part of automakers did not switch to the new design, using the cable type in the future.
50s
This period was marked by the launch of drum brakes with a self-adjusting function. This greatly simplified the situation, since previously, due to rapid wear, the pads had to be frequently tightened due to a decrease in braking efficiency.
60-70s
At this time, the power of cars was growing, as was their weight, which led to the need to install disc brakes, since the friction properties of the drum system became insufficient. However, despite the transition of some auto companies to disc brakes on both axles, most continued to install drum brakes on the rear axle.
Nowadays
Today, the drum design is universally inferior to the disk one, but in some budget models drum mechanisms continue to be preserved.
Design
Over time, new design solutions appeared and were used various materials, however, the brake layout drum type was preserved. It consists of a number of elements.
Photo: Drum brake device
- Brake drum– it is made of cast iron with high strength values, and its internal surface is carefully ground. The drum is installed on the support shaft or wheel hub, and the bearing is pressed inside.
- Brake cylinder (hydraulic)- This is a cast iron body with pistons integrated inside, equipped with rubber cuffs that prevent brake fluid from leaking out. It also installs a bleed valve designed to bleed air from the system.
- Brake pads– elements made in the shape of a crescent, with friction linings. They press against the drum and stop vehicle. Friction linings are made with the addition of rubber (synthetic), modifiers, resins, ceramics and fibers (mineral and organic).
- Protective disk– it is mounted on the rear beam or hub, and the brake pads complete with the cylinder are movably fixed to it.
- Springs (coupling)– secured to the pads from below and above. Their task is to work on compression and prevent the pads from diverging during movement.
- Spacer (block)– it is not used in all brake systems, but only in those where there is only 1 brake cylinder. It is a metal plate with special cutouts, which is necessary for the operation of the second block when tensioning the parking brake handle, as well as for installing the self-supply.
- Retainer- a metal rod with a set of blocks, springs and plates installed on it, created in exactly this sequence. IN in this case While pressing the pad against the brake disc, it will remain possible to move it vertically.
- Supply of pads– a pair of eccentrics placed in the body of the protective disk. During rotation, eccentrics promote closer contact between the shoe and the drum. Previously this system was widely used, but is now almost never used.
- Self-feeding mechanism– it is necessary to level out the degree of wear of the pads and their supply to the drum. Typically used simple system from the Volkswagen company, which is a wedge that falls inward and spreads the pads apart. Ford has developed more complex design With metal plate and chopped teeth. But it is less reliable.
Advantages of the drum design
Photo: Brake drum Renault Logan
Despite the fact that disk mechanisms are better, drum mechanisms also have a number of strengths:
- Greater resource - it is achievable due to the protection of the pads hidden in the drum, in contrast to the external ones on the discs;
- Possibility of expansion - by increasing the dimensions (width and height) of the drum, it is easy to achieve high efficiency, whereas the size of the disc is limited by the rim;
- Simplicity - despite the fact that this design is more complex than a disk design, it is easier to integrate it with a parking brake;
- Heat generation – it is much lower in drum structures, which allows the use of cheaper brake fluids;
Due to these advantages, drum brakes are still used on some car models.
2108 ViewsThe drum brake mechanism has been known to engineers and car owners for quite a long time. Generally speaking, drum brakes appeared much earlier than disc brakes, and they can be found on cars of the past much more often than now. Today we will talk about the design, structure and principle of operation of drums, as well as what is good and bad about them.
Let's take it apart
Rear drum brakes can be found on most cars. domestic production. Along with the disc brakes that are installed at the front, they function successfully on the car and do not fail, no matter how much the car is driven.
No matter how much they say that the brake drum is much worse than the disc one, this design, it seems, is still long years will not outlive itself and will great solution to reduce the cost of the finished car that comes off the assembly line.
To better understand how a drum brake system functions, it is worth understanding what its operating principle is and what type of device it has. The main working element that is included in drum brakes is the shoe itself. When you press the pedal, the shoe creates friction that acts on the inner diameter of the drum.
It is difficult to say how much this pressure is, but it can be established with certainty that the temperature of mechanical elements sometimes reaches hundreds of degrees. However, no matter how high the temperature, the drum must be equipped with a ventilation system. Such a system does not allow the temperature to go beyond the required limits, and therefore the drum runs up to hundreds of thousands of kilometers.
When the block rubs against the inner diameter of the drum device, it cannot be said that this diameter is subject to gradual wear. This is due to constant friction and increased temperature, which ultimately leads to depletion of the walls and a significant change in their size. For this purpose, it is important to constantly monitor that the diameter of the brake drum is always within the normal range. It is not worth reminding about the negative effect that untimely maintenance entails: a critical change in the size of the drum will lead to the failure of the system.
No matter how hard you have to press the pedal, the shoe is pressed with great force against the drum along its entire diameter using special springs that are attached to the caliper. This creates uniform friction and the pad is pressed tightly enough. By adjusting the pressure on the pedal, you can easily control the force with which the shoe is pressed against the drum. Thus, the brake drum is subjected to a strong impact, due to which thermal energy, and the car gradually reduces speed and stops.
Benefits and Features
No matter how often the topic of comparing drum brakes with disc brakes is discussed, the discussion always remains open. Perhaps this is due to the following: no matter how many disadvantages such a braking system has, it provides exactly the same number of advantages to the motorist.
Let's start with the positives. The brake drums are made of fairly high-quality iron, and the size of the walls always remains quite noticeable. This makes it possible to achieve a fairly high resource of the entire system and a long service life. Indeed, if we compare the service life of drum and disc brake systems, it turns out that the drum lasts much longer. How much is average difference in their resources? If you believe the experience of motorists themselves, then it is about ten to fifteen thousand kilometers.
The second advantage is the low cost of spare parts and components. Indeed, any of the functional elements here are much cheaper, and the range of components themselves always remains wide. Speaking about the cost of spare parts, one cannot fail to mention the fact that servicing drum brakes is easier and more convenient for an inexperienced technician.
This fact suggests that drum systems are simpler and less expensive to maintain. This is what prompted manufacturers of inexpensive cars to equip their creations with such systems.
Let's move on to the cons. The main disadvantage of the drum brake system is the low efficiency of braking itself. Indeed, such a minus is very seriously felt when braking from high speeds, especially when a disc system is installed on the front axle.
It is difficult to imagine a car without a brake system. Not so long ago, drum brakes were considered the most used. Let's look at the structure of the mechanism, the principle of operation, as well as the most common malfunctions. Let's study some operating tips on how to properly monitor and prevent wear and tear of the part.
A copper spray has been applied to the drum brake of the Volkswagen Polo Sedan to eliminate squeaking.
Design and principle of operation of drum brakes
In the modern automotive industry, “drums” are no longer as popular as they were twenty years ago; they are being replaced by more modern and reliable disk ones. However, among the budget class of cars, such a braking system device as “drums” is still found. As a rule, they are installed at the rear, and disk ones at the front. Such systems gained popularity among manufacturers for their low cost of production, as well as the ability to easily integrate a handbrake.
Disc brake on the left, drum brake on the right
However, when it comes to maintenance, drum systems are quite complex, because there is much more details and components than those of the same disk ones, although the operating principle of both is identical. The drum brake mechanism consists of a rotating part (the drum itself), as well as a stationary mechanism, such as brake pads and a shield. So, detailed device mechanism, what it consists of:
Directly the drum itself, installed on the wheel hub.
Brake pads on which friction linings are additionally installed.
Brake cylinder with fittings, cuffs, pistons.
Special tension springs (for pads).
Brake shield (depending on the modification, can be installed either on the hub or directly on the beam).
Various supports (with adjuster) and stands for pads.
Parking brake systems (cable, lever).
In some models, two working cylinders are used for reliability during operation.
In essence, there is no global difference between brake systems in principle; however, there are some subtleties, given the presence in the drum additional details. We have already found out that the main parts are pads, as well as cylinders, where there is one and where there are two, that’s not the point.
The principle of operation is as follows. When you press the brake, the fluid in the cylinders is compressed and the piston “forces” the pads to press against the drum. And what happens is that the pads, pressing against each other, seem to jam. But, taking this into account, the pads need to move back somehow, which is why they began to use return springs.
The use of regulators is due to the fact that it is necessary to constantly maintain optimal distance to the drum from the pads. For example, if the pads are worn out, the piston will require more fluid to travel the distance, causing the pedal to go deeper (to the floor). Therefore, even when the pads wear out, the regulator does not allow them to “move away” too far.
Nowadays, automatic regulators have begun to be used almost everywhere. Whenever the machine stops, the pads, as required, are pressed as much as possible against the drum. In reverse action, when there is no pressure on the pedal, the adjuster moves one “tooth” to increase the clearance. In principle, the regulator is somewhat similar to a simple threaded bolt. Although there are even simpler designs, in the form of a simple spring or bracket connected to return springs.
As for the operation of the handbrake, there is nothing complicated here either.
The handbrake lever, which is connected to the shoes by a tie bar, is actuated by a tensioned cable. That is, the “handbrake” itself rises, the cable is pulled, which is pulled by the lever, the latter, in turn, acts on the spacer bar, which pushes the pads apart and moves them in the opposite direction.
What cars have drum brakes?
The presented brake systems are almost universally used in class A, since the weight of the cars is small, and therefore over efficient systems braking is unnecessary here. Drums are also used in most budget class B models - these are KIA RIO 4, Hyundai Solaris in mid-spec versions, domestic Lada Granta, Kalina, Priora, Largus, VAZ 2107-15 family, Vesta, Xray, Renault Kaptur, Duster, Clio, Logan, Sandero, Nissan Almera, Skoda Fabia, Volkswagen Polo Sedan, Chevrolet Aveo, Lacceti, Cobalt, Geely MK, Opel Corsa, Daewoo Nexia, Lanos.
Among the A segment are Daewoo Matiz, Smart, Citroen C1, Lifan Smile, Chevrolet Spark, Peugeot 107, KIA Picanto.
Among the SUVs are UAZ Patriot, Lada Niva, Nissan Terrano, Navara, Mitsubishi L200, Volkswagen Amarok, Great Wall Wingle.
Brake drum Volkswagen Amarok
Pros, cons, and differences of drum brakes
Poor contact. Even if two pistons are used, the pads have a huge contact area, and they are simply not able to hold the pads evenly, resulting in unstable contact.
Loads. No matter how stupid it may sound now, strong pressure in the cylinders can “break” the drum. The fact is that the pads work as if outwards, that is, it is quite likely that with a lot of force the drum can “break”.
Poor grip. Considering that the drum body is closed, which means wear products from the friction linings remain inside. Getting on the surfaces of rubbing parts, it significantly impairs adhesion.
Overheat. Let us remember that the drum is closed and, accordingly, there is no airflow. During emergency braking, the temperature reaches 650 degrees. Due to this, the drum expands, and the brake has to be pressed “to the floor”.
The pads get stuck and freeze. It is not uncommon that after a long cocked handbrake or aggressive use of the brakes before stopping (the friction linings get very hot), the pads can stick. They stick, as is already clear, to the part of the drum that they rub against. A similar problem occurs in winter, when the handbrake freezes. Driving your wheels through a puddle or snow will cause moisture to get on the pads. And if you tighten the handbrake, the pads will simply freeze, given the sub-zero temperature.
In this case, it is difficult to remove a jammed wheel; you will have to jack it up, remove the wheel and use a screwdriver or pry bar to move the pads. In some cases, it is enough to water the drum warm water(suitable in winter). You can also try to “rock” the car back and forth, the main thing is not to overdo it, so as not to “burn” the clutch.
By the way, disc brakes are free from this problem.
Even despite such obvious disadvantages, drums still have certain advantages:
Great braking force, of course, this point looks somewhat contradictory, taking into account statements about weak contact, but there are still certain advantages. For example, if you increase not only the diameter of the drum, but also its width, you will be able to significantly increase common plane contact with the pads.
Wear resistance. Yes, given the smaller clutch, the result is less wear. That is why the pads on the drums often last at least 70,000 km. Somewhere even more, up to 150,000 km, of course it all depends on operating conditions.
Protection from dirt. Dust, moisture, and dirt from the outside simply do not penetrate here (the only exception is for those systems where “ventilation fins” are made).
Based on the above, we can say that the differences are only in the design (is there airflow), the size and shape of the pads, as well as, in principle, differences in configurations and fastening methods. Otherwise, their main task is identical.
Malfunctions
There are about seven main problems that every car owner has to face sooner or later. So:
1. Wear of pads and drum. The situation is especially dangerous when wear occurs simultaneously; often in such cases, the wheel is simply blocked. By the way, if the wear of the drum walls is not great, it is enough to grind off the protruding sides and adjust the shoe tension system.
Worn drum pads Volkswagen Passat 1996
As for the pads, they should be changed in the following cases:
- if the clutch is applied with glue, then the permissible wear is 1.6 mm;
- if the clutch is held on by rivets, the permissible wear is 0.8 mm.
2. Distortions of the pads, by the way, often cause rapid wear of the inner walls of the drum, uneven abrasion, which is why you have to purchase a new part.
3. Breakage of racks, springs, spacer bars.
Scheme
4. Broken cable or broken handbrake lever.
Drum spring. Photo — drive2.ru
5. Disconnecting friction linings.
6. Damage to the cylinder, cuff, pipeline. As a result, depressurization, leakage of brake fluid.
In case of partial depressurization, it is possible that the system will become “aired” and its performance will deteriorate. If the fluid leaks completely, the brakes will fail.
7. Corrosion of springs is dangerous, as they can “hang” and not work as they should.
There are no operating rules for drum brakes as such. But, it is important to periodically inspect this unit for integrity and presence of damage and wear. So:
Check the condition of the pads at least every 20,000 km.
Likewise, it is worth checking the condition of springs, struts, struts, and levers.
Don't forget to monitor the amount of brake fluid.
Also pay attention to the presence of smudges around the cylinders; the cuffs or pipeline may be torn.
I would like to give some tips on “rolling in” only the installed pads. So:
Choose an area where you can safely accelerate and brake sharply without danger to other drivers.
Do ten cycles: acceleration to 60-70 km/h, sharp braking to 10 km/h. The main thing is to do this without stopping, slow down the speed to 10, immediately dial up to 60-70.
After this, give the brakes a rest and drive for 5 km. in quiet mode, without the need to press the brake.
Remember, under no circumstances stop after 10 cycles. Otherwise, particles of the heated clutch will remain on the walls of the drum. This will disrupt the contact area and adhesion.
Conclusion
In conclusion, I would like to emphasize how important it is to do routine or even periodic “inspections” of this unit, which, without exaggeration, affects driving safety. Worn parts can lead to serious consequences and costly repairs.
Have you ever wondered what actually happens inside a drum brake when it operates and why disc brakes are generally considered superior to their older design counterpart? Let us explain.
Most people probably know how disc brakes work. Let us briefly recall the system operation algorithm: After pressing the brake pedal, the master brake cylinder, through the brake fluid in the hydraulic lines, begins to increase the pressure in the calipers, where one or more pistons, using the pressure applied to them, begin to press one or two pads against the disc (brake disc).
With the help of frictional forces, the car begins to slow down, ensuring that you don't end up crashing into the rear bumper of the car in front or into a wall/post/tree. Simple and effective. Read more on the topic:
But what about brake drums? These more modest elements of braking technology, and certainly much older than disc brakes, have almost completely disappeared from use. Everyday life automotive community. Even trucks and buses resort to the services of these “servants” less and less. Now such brake schemes can only be found on very inexpensive cars or specific equipment. Why did it happen? What lies behind Achilles' heel"drums"?
How do drum brakes work?
The working process begins in exactly the same way as on disc mechanisms - with fluid transmitting pressure from the master cylinder to the brake actuator. From this moment on, all the main differences appear.
Instead of a brake cylinder, like a disc brake, in drum brakes the fluid flows into what is called a wheel cylinder, which is mounted inside a cast iron brake drum.
The fluid pushes two pistons from the body of the working brake cylinder outward, causing the brake pads to diverge, adjacent to interior lining brake drum. Since the drum is attached to the hub, the friction caused begins to slow down the rotation of the wheel.
Also in the functional part of the brake mechanism, so-called tension springs play an important role. Two springs are installed at either end of the two pads. As the name suggests, these springs return the brake pads to their original position after the brake pedal is released.
As the pads wear out, a special supply system will select the excess distance between the drum and the pads, which will allow the efficiency and speed of the brake system not to decrease over time and natural wear of the components. However, experts say this - the front pads in drum brakes are pressed against the surface with greater force, which increases their wear.
Are there any advantages to a drum mechanism over a disk mechanism?
It would seem that this is simply impossible. How can an archaic system be better than a more modern one? But there are several undeniable advantages of drum brakes that cannot be taken away from them:
1. Since the contact patch extends around the entire circumference of the drum, the braking force transmitted to drum brakes is greater than that of brake disc same size.
2. Don’t take it as a joke, but we read on specialized websites that using drum brakes saves weight, money for the production of an element for an auto company, and ultimately money in the wallets of car owners.
If we knew about the last two points for a long time - indeed, it is difficult to find a simpler and cheaper design, then we didn’t even know about the weight. Somehow the cast iron bass drum did not inspire too much confidence in this. However, if you consider that in addition to the brake hydraulics, the disc brake also has a huge one (also cast iron), then that’s what it looks like. With the same weight, the drum brake will be more powerful due to the larger contact patch of the pads in it. But with the same power, it will be lighter than its modern counterpart.
3. Finally, another undeniable advantage is that brake pads, as a rule, do not wear out much longer than on conventional disc brakes.
Disadvantages of drum brakes
1. Despite the simplicity of the design and more cheap production, in service, drum brakes cannot compete with disc brakes. They require very complicated setup. Fiddling with drums was like art in some ways. Only a master could adjust the worn-out brakes perfectly. This setup also took a fair amount of time.
Shoe drum brakes:
A- mechanism with one-sided supports;
b- with spaced supports;
V- self-reinforcing mechanism;
G- mechanism with expanding fist
Shoe drum brake mechanisms, despite their external similarity, differ significantly from each other in design and properties. The figure shows the basic diagrams of drum shoe brakes. They mainly differ in the location of the pad supports and the nature of the driving forces that push the pads apart and press them against the drum from the inside. The difference in design also predetermines the difference in properties.
Drum mechanism with equal drive forces and one-sided arrangement of shoe supports:
1 - brake drum;
2 - friction lining;
3 - block;
4 - brake shield;
5 - brake cylinder;
6 - return (tension) springs;
7 - brake adjustment eccentric
The illustration shows a drum brake with equal drive forces and single-sided shoe support.
The support disk is fixed to the bridge beam. At the bottom of the support disk there are two fingers on which eccentric washers are attached. The position of the fingers is fixed with nuts. The lower ends of the pads are placed on eccentric washers. The adjusting eccentrics are secured to the support disk with bolts that are kept from arbitrary rotation by pre-compressed springs. The tension spring presses each shoe against its adjusting eccentric. The spring fixes the adjusting eccentric in any position when turning it by the head of the bolts. Thus, each shoe is centered relative to the brake drum by adjusting eccentrics and eccentric pin washers. The upper ends of the pads are in contact with the pistons of the working cylinder. The pads are held against lateral movement by guide brackets with leaf springs.
The length of the friction linings attached to the front and rear pads is not the same. The front pad is longer than the rear pad. This was done to ensure uniform wear of the linings, since the front pad works longer as a primary pad and creates more braking torque than the rear one. The brake drum is attached to the wheel hub. For easy access to the pads, the drum is removable.
When braking, the fluid pressure in the wheel cylinder pushes the pistons in the opposite direction; they act on the upper ends of the pads, which overcome the spring force and are pressed against the drum. When releasing the brakes, the pressure in the cylinder decreases due to return spring, the pads are returned to their original position.
The mechanism has a special drive lever connected at the upper end to one brake pad, and through a bar to the other. The parking cable is connected to the lower end of the lever. When the cable is pulled, the lever turns and presses first one block to the drum, and then another through the bar.
Brake car with spaced supports made according to the diagram (see Fig. b). It has two identical brake pads, each mounted on a corresponding support pin. The pads are tightened by springs. The ends of the pads are in contact with the pistons of the wheel cylinders. The working cylinders are connected to the main brake cylinder and to each other by a pipeline. The mechanism has automatic device gap regulation.
Support disk servo brakes(see Fig. c) mounted on the gearbox; It has two pads, an expansion mechanism and an adjustment mechanism. The upper ends of the pads are pressed by tension springs to the pushers of the expansion mechanism, and the lower ends to the supports of the adjusting mechanism. The force of the tension springs of the left block is less than the force of the springs of the right block. The adjusting mechanism can move together with the pad supports by 3 mm relative to the screw. In the released position, the block is pressed against the body by strong springs and the specified gap is set on the side of the left block. When the brake lever moves, the force from it is transmitted through the rod to the double-arm lever. The position of the brake lever in the braked state is fixed by a latch on the gear sector. The short arm of the double-armed lever presses on the expansion rod, which, moving into the body, spreads the pushers of both pads with balls. The left shoe, which has weaker tension springs, is pressed against the drum first. If braking occurs when the car moves forward, then this block is captured by the drum and its lower end moves the right block until it comes into contact with the drum (the movement of the block, which does not exceed 3 mm, occurs counterclockwise). Both pads act as primary pads, with the driving force for the right pad being the friction force transmitted from the left pad. Since the braking torque of the transmission parking brake is increased by the main gear, its dimensions are smaller than those of wheel brakes or brakes installed after the cross-axle differential.
Brake with equal pad movements(see Fig. d). The pads rest on axles with eccentric journals. The axles are installed and secured with nuts in brackets riveted to the support disk. When installing the brake, the axis rotates and thereby moves the end of the shoe relative to the drum. The tension spring presses the pads against the expansion fist. Two friction linings are riveted to the pads. The brake drum is cast iron and attached to the wheel hub with studs. The expanding fist is manufactured as one piece with the shaft and installed in a bracket. A lever is attached to the splined end of the shaft. The lever contains a worm gear, which serves to regulate the gap in the brake mechanism.
When the brakes are released, there is a gap between the pads and the drum. When braking, the air pressure is perceived by the brake chamber membrane mounted on the bracket, and its rod turns the shaft with the expansion cam behind the lever. The pads are pressed against the drum, causing the wheel to brake. The expansion knuckle profile is designed to ensure movement of the ends of the pads at equal distances. This ensures a balanced brake mechanism, equal braking torques and pad wear.
Brake mechanism with wedge release device and automatic clearance adjustment:
1 - block;
2 - expansion wedge;
3 - brake valve;
4 - brake chamber;
5 - spring
A number of cars use brake mechanisms with a wedge release device and automatic clearance adjustment. A caliper is mounted on the support disk, into the cylindrical holes of which two pushers are inserted. Adjusting bushings are located inside each pusher. On the outer surface of each adjusting sleeve there is a spiral thread with a triangular tooth profile, and on inner surface a thread is cut into which the adjusting screw is screwed. When initially adjusting the brake mechanisms, turn the adjusting screws to set the gap between the brake drum and the shoes, the value of which is then maintained automatically. Ratchets are pressed to the adjusting bushings, which have teeth that engage with the outer teeth of the adjusting bushings.
The spreading device consists of a wedge, two rollers (the axes of which are located in the separator), a thrust washer and a dirt cap. When braking, the force from the brake chamber rod is transferred to the wedge, as a result of which it moves in the axial direction and pushes the pushers apart using rollers. The adjusting bushings and screws moving at the same time press the pads against the drum, and the ratchet pawl jumps over the teeth of the adjusting bushings. When the brakes are released and the pushrods and their associated parts move in the opposite direction, the adjusting bushings rotate under the force generated in the engagement between the ratchet pawls and the bushings, causing the screws to come out. The necessary gaps are established between the pads and the drum. As the gap between the shoes and the drum increases, the ratchet pawls engage with another pair of teeth on the adjusting sleeve, which automatically restores the gap in the brake mechanism.
Brake drums for wheel and transmission brakes are usually cast from gray cast iron. Some brakes have a drum disc stamped from sheet steel and connected to a cast iron drum by casting into a one-piece structure. Brake drums passenger cars made from aluminum alloy with a cast iron ring poured inside. Drums are sometimes equipped with ribs that increase the rigidity of the structure and improve heat dissipation. Drum brake pads have a T-shaped cross-section for rigidity. Sometimes the block rests loosely with its lower end on the platform and is not fixed. Such a block self-aligns relative to the drum when braking.
Friction linings are made of materials with a high coefficient of friction (up to 0.4), high heat resistance and good wear resistance. Previously, hot linings were mainly molded from fibrous asbestos mixed with organic binders (resins, rubber, oils). Currently, the use of asbestos in brake linings is prohibited by law, since asbestos is recognized as a carcinogenic material.
