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 humbler pieces of braking technology, and certainly much older than disc brakes, have almost completely disappeared from everyday life in the 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 is the Achilles heel of the “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 brake wheel housing outward, causing the brake pads to diverge, adjacent to the inner lining of the 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. Because the contact patch extends around the entire circumference of the drum, the braking force transmitted to drum brakes is greater than that of a brake rotor of the 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 their simplicity of design and cheaper production, drum brakes cannot compete with disc brakes in maintenance. 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.
- 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 drums is relevant, 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.
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 domestic cars. 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 a brake drum is much worse than a disc one, this design, it seems, will not become obsolete for many years and will be an excellent solution for reducing the cost of a 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 is released, 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. What is the 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.
Readers know that currently there are two types of brakes most widely used in the automotive industry - disc and drum. If everything is clear with disc brakes, the design, operating principle and operating efficiency of drum brakes still remains a mystery to many. In today's article we will talk about the main components of drum brakes, describe the algorithm of their operation, and also find out the main advantages and disadvantages of their use.
Drum brakes
What are drum brakes made of?
The design of drum brake mechanisms is noticeably more complex than the design of their disc “brothers”. The main internal parts of such brakes are:
- Brake drum. An element made of high-strength cast iron alloys. It is installed on the hub or support shaft and serves not only as the main contact part that interacts directly with the pads, but also as a housing in which all other parts are mounted. The inside of the brake drum is ground to ensure maximum braking performance.
- Pads. Unlike the brake pads of disc brakes, the pads used in drum mechanisms have a semicircular shape. Their outer part has a special asbestos coating. If brake pads are installed on a pair of rear wheels, then one of them is also connected to the parking brake lever.
- Compression springs. These elements are attached to the upper and lower parts of the pads, preventing them from moving in different directions at idle.
- Brake cylinders. This is a special body made of cast iron, on both sides of which working pistons are mounted. They are activated by hydraulic pressure generated after the driver presses the brake pedal. Additional parts of the pistons are rubber seals and a valve to remove air trapped in the circuit.
- Protective disk. The part is a hub-mounted element to which the brake cylinders and pads are attached. They are secured by using special clamps.
- Self-feeding mechanism. The basis of the mechanism is a special wedge that deepens as the brake pads are ground down. Its purpose is to ensure constant pressure of the pads to the surface of the drum, regardless of the wear of their working surfaces.
Drum brake device
The components we have listed are generally accepted. Most of the largest manufacturers use them. There are a number of parts that are installed privately by some companies. These, for example, are the pad supply mechanism, all kinds of spacers, etc. It makes no sense to dwell on them in detail.
Operating principle of drum brakes
The basic sequence of operation of drum mechanisms is approximately as follows. The driver, if necessary, presses the pedal, creating increased pressure in the brake circuit. Hydraulics press on the master cylinder pistons, which engage the brake pads. They “diverge” to the sides, stretching the tension springs, and reach points of interaction with the working surface of the drum. Due to the friction that arises in this case, the speed of rotation of the wheels decreases and the car slows down. The general operating algorithm for drum brakes looks exactly like this. There are no significant differences between systems with one piston and two.
Advantages and disadvantages of drum brakes
Despite the seemingly general obsolescence of the design, many automakers still use drum brakes on their models. The point is that there are many advantages that have a beneficial effect on the use of a car.
- Firstly, Drum brakes last 2-3 times longer than disc brakes. This applies not only to the pads, but also to the brake discs themselves, which wear out no less.
- Secondly, Drum mechanisms are not afraid of water ingress, while the highly heated surfaces of disc brakes, when suddenly cooled with water, can become covered with microcracks, which leads to their rapid failure.
- Third, It is much easier to install a parking brake into a drum brake system than to integrate it into a disc system. Of course, simplicity significantly reduces the costs associated with the production of the overall structure.
The main disadvantage of drum-type brakes is their lower efficiency compared to disc mechanisms. It is unsafe to use them on cars with powerful, high-speed engines under the hood, as well as on models with high weight.
Conclusion
To summarize, let’s say that in the near future, drum brakes, of course, will “give way” to more advanced disc systems. Already, many manufacturers install drum brakes exclusively on budget models, combining the vast majority of their new products with various variations of disc systems.
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 of completely 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 working surface area of the 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
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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. Looking for simpler and more efficient options to solve the problem of a shoe that won't wedge, they came to the conclusion that it was possible to put 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
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One of the main advantages of drum mechanisms is that they are closed from the environment - neither dirt nor dust gets inside. It’s hard to disagree with this, but with a caveat – if we are talking about 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. The upper return spring was then removed.
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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 products get so packed on the ring that its magnetism begins to be lacking, and the ABS system throws up the “I don’t see the sensor” error. 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.