The drum-type brake mechanism is functionally designed to change the speed limit of the vehicle. In addition, a drum brake mounted on the rear wheelset provides the parking brake function.
The main structural element of this type of brake mechanism, which actually gives it its name, is a drum, or metal bowl, mounted on the wheel hub.
The drum-type brake mechanism (Fig. 1) consists of the following main parts:
Brake drum, the material for the manufacture of which is high-strength cast iron. The inner surface of the drum, which is in direct contact with the remaining elements of the mechanism, is thoroughly polished. It is mounted on the support shaft (in this case, a bearing is pressed into the drum) or the wheel hub.
Brake pads (item 4). They are made of metal and have a crescent shape. The working surface of the brake pad is equipped with a friction lining (asbestos based).
Brake hydraulic cylinder (item 2). This is a hollow cast-iron cylinder with two working pistons, filled with working (brake) fluid. The cylinder is equipped with a bleed valve to remove air from the brake system. To prevent brake fluid from leaking, sealing collars are used.
Upper (item 1) and lower (item 5) tension springs that work in “compression”. Their main operating function is to prevent the brake pads from moving apart in the “rest” mode.
A protective disc mounted directly on the hub (rear beam).
Spacer bar (item 3), which is a metal plate of a specific configuration (having special cutouts). The functional purpose of this element is to install a “self-feeding” mechanism. In addition, when installing a brake device on the rear wheel pair, the spacer bar operates the second brake shoe, while ensuring the functioning of the parking brake. It is used in drum-type brake mechanisms with one brake cylinder.
A “self-feeding” mechanism (in the form of two eccentrics located in the body of the protective disk), which ensures the separation of brake pads with worn friction linings.
Drum brakes - operating principle
The operating principle of the drum brake mechanism is as follows:
When the driver presses the brake pedal, pressure builds up in the brake system circuit.
Under the influence of brake fluid pressure, the pistons of the brake cylinders, overcoming the resistance of the tension springs, initiate the divergence of the brake pads.
The brake pads, diverging and tightly fitting the friction linings to the working surfaces of the brake drums, reduce their rotation speed, thereby slowing down the rotation of the vehicle wheels.
The braking efficiency of drum-type brakes is slightly lower than that of disc brakes. Thus, the difference in braking distance can differ significantly (up to 20%). And there are several, quite objective reasons for this:
Instructions for repair and adjustment of rear drum brakes on the R-11 (sedan, hatchback)
In the book “Repair and Operation Manual for Nissan-Primera” 1990-1999. release" in paragraph 4 on page 190 literally the following is written:
“The rear drum brakes have a leading and driven shoe, which are actuated by the brake wheel cylinders in a pair of piston. In addition, they are equipped with a self-adjusting slack adjuster. As the brake pad linings wear, the brake pedal automatically activates an adjuster mechanism that extends the pad spreader bar, thereby compensating for changes in the gap between the lining and the drum.
The mechanical pull of the handbrake operates these same pads through a lever attached to the driven pad.”
1 - bleeding valve, 5 - 8 N m
2 - brake pipeline with union nut, 15 - 18 N m
3 - bolt, 6 - 10 N m
4 - anchor pin of the pads
5 - plug
6 - wheel brake cylinder
7 - bolt, 40 - 55 N m
8 - upper return spring
9 - locking latch
10 - pad lever
11 - brake pads
12 - lower return spring
13 - anchor spring pads
14 - spring cup
15 - rear fixing spring of the regulator
16 - pad adjuster and next to it - front securing spring
In words - “no problem”, but in practice everything is exactly the opposite, especially if the drum and pads have worked thoroughly and are worn out.
In order to remove the brake drum, first of all, lower the handbrake to the “zero” position. If in the “zero” position a springy tension of the cable is felt on the handle, then under the handbrake handle (behind the rubber curtain) slightly unscrew (with a deep socket 10) the cable tension adjusting nut. We unscrew this adjusting nut just enough so that the cable tension on the handbrake handle in the “zero” position is no longer felt.
Only one tool will help here - our dear Soviet hammer. But you need to use it wisely with the placement... We do not hit the edge of the drum with a hammer very hard, in the direction of pressing the working brake cylinder and a little “towards ourselves” (like: with a hammer blow we want to press the working brake cylinder and at the same time pull off the edge of the brake drum). Then we rotate the brake drum until we hear the rustling of the brake pad. If the drum rustles (or the drum jams), hit it again with the EXACT same hammer and turn the drum again.
Those. hit “in the same place” with a hammer, turning the drum until the edge of the brake pad appears from under the drum. When the edge of the block appears, we proceed to circular tapping of the drum in the direction “slightly towards ourselves” and with hammer blows we finally pull the drum off the blocks.
Next, take any file and completely cut off the annular protrusion-skirt along the entire circumference of the brake drum, which interferes with the removal and installation of the brake drum. (This circular protrusion-skirt, harmful in all respects, was formed as a result of the brake pads rubbing the groove-groove on the working surface of the brake drum.)
After the harmful ring-skirt is removed, you can check how significant the working gap is between the brake pads and the drum. If the drum, put on the pads, “dangles freely,” then this is no good - the gap must be reduced to such a minimum value that the drum “barely” fits on the brake pads.
And here we are faced with the second serious problem (or rather an English design mistake), which is that the so-called “self-adjusting” clearance adjuster can increase the release of the pads only if they are opened and compressed (near the working brake cylinder) with an amplitude 1.5-2.0 mm. But a brake drum placed on the pads will not allow them to move apart by more than 1 mm. Those. in other words: at 1.5 - 2 mm of pad wear, the bar itself will automatically move apart by only 0.2 mm (!!!). Here you have the drums “loosely dangling” on the pads, because of which when braking “the rear does not hold up” and the car “pecks on the nose” even when the brake pedal is lightly pressed.
On dry road surfaces, such delayed rear brakes are not very critical, because Powerful front disc brakes stop the car quite well on dry asphalt. But on slippery roads, weak rear brakes reduce the braking effect by almost half. It’s one thing when only 2 front wheels “cling” to a slippery road surface (because the rear wheels are weakly braked by pads), and quite another when all 4 wheel tires try to cling to the road.
In general, the “self-adjusting” slack adjuster for “worn” pads and “worn” drum must be moved apart manually. At the same time, you should not try to solve this problem by twisting the adjusting wheel on the regulator with your fingers (and even more so with pliers!!!). The compression force of the garter springs is so great that the only way to turn the wheel is to open and close the brake pads (near the brake wheel cylinder) with 2 long, strong screwdrivers, while watching the adjustment wheel automatically turn. Each release and compression of the brake pads rotates the wheel by 1 tooth.
If the decompression and compression of the pads does not entail turning the wheel, then you need to completely remove the brake pads and thoroughly wash (with gasoline or something else) the entire structure of the “self-adjusting” slack adjuster from grease and dirt. Moreover, you need to pay attention to the fact that you should not lubricate the clearance adjuster, because dust from the brake pads will stick to the lubricant and the lubricant will turn into dirt, preventing the adjustment wheel from turning.
For a better understanding of the “self-extension” mechanism of the gap adjuster, several photos are shown below. (Although they are blurry, because I took the photo with a mobile phone, they will still help you understand the tricks of this “magic wand”...)
Photo 1.
Rear left drum brake assembly. The “self-adjusting” slack adjuster and its front retaining spring are clearly visible
Photo 2.
A “self-adjusting” slack adjuster, washed from dirt, attached by springs to the back of the brake pads of the right rear wheel. The front retaining spring is engaged on the back side of the pads temporarily - only for the purpose of ensuring the clarity of the photo. Its correct position is as in photo 1.
Photo 3.
Disassembled slack adjuster. In the middle lies the “highlight” of its automation: a corner converter for the direction of movement. It is bent at an angle of not 90 degrees, but 100. When you press the adjusting wheel (brake pads) on it, the second part of the angle presses the pusher plate.
Photo 4.
Shown is the pusher plate and the small pressure ratchet removed from it.
Photo 5.
The working position of the “highlight” of the automation is shown - the corner-converter of the direction of movement
Photo 6.
The position of the parts when the adjuster is compressed on both sides by the brake pads: the converter angle pressed the pusher plate, and the pressure ratchet caught on the lower tooth of the adjusting wheel.
Photo 7.
The position of the parts when the adjuster is stretched on both sides by the brake pads: the pusher plate pressed on the converter corner, which was freed from compression by the pads, and the pressure ratchet rotated upward the tooth of the adjusting wheel engaged from below.
When the principle of automatic extension (extension) of the regulator inherent in the mechanism is understood, it becomes clear how it can be controlled. We unclench and compress the pads 4-5 times (spread the pad adjuster by 4-5 teeth), put the brake drum on the pads, pull the handbrake several times (in the cabin) to center the position of the pads and remove the drum. If the working gap needs to be further reduced, we repeat this simple procedure again.
Ultimately, on my car, by opening and closing the brake pads with 2 screwdrivers, I adjusted the gaps so that the brake drums became barely possible to put on the brake pads by hand (not with a hammer!).
At the same time, in the “zero” position of the handbrake, the drums on the hub rotated freely, without touching the pads, and after the first click of the handbrake, the efforts to turn the brake drums by hand became approximately the same. Those. At the first click of the handbrake, we check that the set working gaps are equal.
After adjusting the rear brakes in this way, the handling of the car was literally transformed. Instead of sharp braking of the car at the end of the brake pedal stroke with the hood squatting to the ground - uniform braking of the car throughout the entire stroke of the brake pedal, excellent throttle response of the brake pedal, smooth stopping of the car at the end of braking, the hood squatting to the ground not “everywhere and every time”, but only during sudden emergency braking.
Moreover, the car reacts sensitively to pressing the brake pedal not only in dry, but also in wet weather. Water practically does not get onto the working surfaces of the brake drums (unlike the discs), and in the rain they work without a “delayed response effect”.
In general, the fundamental idea of a brake system design, in which there are discs in front and drums in the rear, is quite justified in wet weather, but the complexity of servicing drum brakes gradually leads to the fact that disc brakes are increasingly being installed on all 4 wheels. The logic here is not that “discs hold up better,” but that “rather than driving all the time with only front brakes (because the rear doesn’t hold up anyway), it’s better to put discs in the back.”
So, whoever is “lucky” with the rear drums - adjust them correctly and then you will probably like them.
There are two widely used types of brakes on modern cars - drum and disc. Why isn't there only one of them left on the market? Let's look at the features, advantages and disadvantages of each.
Drum mechanisms
Historically, drum brakes appeared earlier. Moreover, on the first cars, brake mechanisms of this type were installed only on the rear wheels. The operating principle of the drum mechanism has changed slightly since then. Just like a century ago, two crescent-shaped pads are moved apart and pressed by special friction linings against the inner surface of a hollow cylinder called a brake drum.
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But the Renault-Nissan alliance, which is firmly established in Russia, brought . Here, the self-supply of the pads and the constant correct tension of the handbrake cables are carried out by a spacer bar of a rather complex design. Moreover, it proved to be quite reliable even on dirty and salty roads. The alliance uses it on different cars.
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So, having reached the limit of perfection in terms of design, including a modern ABS system, drum brakes are not losing ground, settling on the rear axle of inexpensive cars.
Advantages of drum brakes:
- cheap proven design
- good protection against dirt
- long service life of the pads due to the large working surface area. (The first rear brake pads have already covered 120 thousand km.)
- simple and convenient combination with the parking brake mechanism.
Disadvantages of drum brakes:
- significant dimensions and weight of the mechanism
- tendency to overheat under extremely high load conditions
- relatively unstable friction coefficient of the pads, depending on temperature, humidity, etc. Although availability largely compensates for this shortcoming.
- uneven pad wear - the front one wears out more
- sometimes, if the quality of the friction lining material is poor, they may be attached to the drums.
- If large grains of sand get in, a grinding noise may occur during operation.
Disc brakes
In our country, disc brakes appeared en masse along with Zhiguli cars. Before this, they even smelled like burnt drum brakes after racing. Disc brakes were first installed on the front wheels as they were more heavily loaded with braking forces. At first, disc brakes were structurally more complex than they are now, and each brake pad was pressed to the disc by its own individual cylinder. Over the years, such a complex design remained only on expensive, high-speed or heavy cars. And on mass-produced cars, a lightweight, cheap and simple single-piston design began to be used. Ventilated discs have increasingly begun to be used, which improves the cooling of the parts of the brake mechanism itself and protects the wheel bearings from overheating.
The design couldn't be simpler. The retractable hydraulic cylinder piston presses two flat brake pads against the rotating disc. Wear compensation occurs automatically in a very simple and reliable way. In general, the mechanism does not require any maintenance up to complete. This is followed by a fairly simple replacement procedure and no adjustments before further use. The characteristics of disc brakes, which are so pleasant for both the manufacturer and the consumer, led to the fact that they began to be installed on the rear axle of relatively expensive cars and crossovers. At the same time, we had to solve the issue with the parking brake drive, which was done in two ways. The first is that a small drum brake mechanism is located in the disc hub, and the second is that a lever-screw mechanism presses the main piston to the pads using a cable or electric drive. Advantages of disc brakes:
Disadvantages of disc brakes:
conclusionsOf course, disc brakes are the future. After all, it is difficult to imagine silent and streamlined electric cars rushing along the glassy smooth surface of the highway and at the same time braking with squeaky drum brakes. But for now, on inexpensive cars, drum brakes on the rear axle, and even imported ones, require less care than disc brakes. And possible uneven operation is compensated by ABS. Share in the comments which brake mechanisms you prefer. |
Drum brakes are a type of braking system that consists of a rotating drum.
In today's article we will discuss rear drum brakes in more detail, and specifically answer the following questions:
- What are rear drum brakes?
- What is the operating principle of a drum brake system?
- Why do drum brakes squeak?
- What to do if drum brakes squeak?
- Repair kit to repair squeaky drum brakes?
- How to replace drum brakes with disc brakes?
- Repair kit for replacing rear drum brakes with disc brakes.
basic information
Any car, both small and large, must be equipped with a braking system. The main task of the braking system is to change the speed of movement of vehicles, at the command of the car owner or the electronic guidance system. The second purpose of the braking system is to keep the car stationary in relation to the road while it is parked.
Depending on the design of the friction part, a distinction is made between disc and brake drum mechanisms. The braking mechanism has a fixed and rotating part. The role of the moving part of the drum braking structure is played by the stopping drum, and not by the rotating band or braking pad. The moving part of the brake disc design has the form of a disk, and is not rotated by the brake pads. The axles of modern passenger cars usually have a disc braking structure.
The disc braking mechanism consists of the following elements:
- Brake disc;
- Two non-rotating pads that are installed in the middle of the caliper on both sides.
Let's look at the main advantages and disadvantages of drum and disc brakes. So, the advantages of drum brakes:
- Long service life, since dirt and dust do not penetrate into the drum;
- Greater braking effort, which is good for trucks;
- Not an expensive repair kit.
Disadvantages include:
- Slowness;
- Pads sticking.
Advantages of disc brakes:
- Greater braking efficiency;
- Small mass;
- Small sizes;
- Low brake fluid temperature;
- High level of reliability;
- Stability.
The only drawback is poor protection from dirt and dust.
What is the principle of operation of the braking system?
Let's look at the principle of operation of the braking system using the example of a hydraulic operating unit. When the brake is pressed, the load is transferred to the amplifier, which creates additional reinforcement on the main cylinder. The piston of the main brake cylinder collects all the fluid in the car wheel cylinders using pipelines. Moreover, at the same moment, the fluid pressure in the brake drive increases. Thanks to the pistons of the car wheel cylinders, the brake pads move to the discs, or as they are also called drums.
After pressing the brake, the fluid pressure increases, as a result of which the braking mechanisms are activated, causing the rotation of the car wheels to slow down and generate braking force at the point of contact of the car tires with the road surface.
Moreover, the more force is applied to the brake pedal, the more efficiently and quickly the car wheels will stop. The liquid pressure at the moment of stopping can reach from ten to fifteen megapascals.
At the end of braking, the pedal moves to the reverse position under the influence of the return spring. The piston of the main braking cylinder also switches to the reverse position. Parts of the springs are retracted from the drums using blocks. Brake fluid passes into the main braking cylinder from the car wheel cylinders thanks to pipelines. Thus, the pressure of the braking system decreases.
Drum brakes squeak
To eliminate the squeaking of rear drum brakes, you need to purchase a suitable repair kit. So, what repair kit should you buy if your drum brakes squeak?
- Anti-recoil struts;
- Set of socket heads;
- Coarse-grained skin;
- Hammer;
- Balloon wrench;
- Torque wrench;
- Screwdriver;
- Support stands;
- Jack.
So, what should you do if your drum brakes squeak?
- First of all, prepare the repair kit;
- Place the car on a flat surface and remove the parking brake;
- Place wheel chocks under the car wheels;
- Loosen the bolts securing the car wheels;
- Place the car on a jack;
- Remove the wheel;
- Carefully remove the protective cap that covers the wheel bearing. To do this you will need a screwdriver and a hammer.
- Unscrew the nut that secures the drum;
- Remove the drum;
- Using coarse sandpaper, remove the edge created by the functioning of the pads;
- Clean the drum from rust;
- Install everything in reverse order. Use a torque wrench when tightening the bearings.
- Check if the rear brakes are squealing.
Algorithm for replacing rear drum brakes with disc brakes
Before you proceed to replacing the rear drum brakes, you need to purchase a repair kit. So, how do you replace brakes with disc brakes?
- First, we prepare the repair kit;
- Raise the back side of the car on a jack;
- Remove the corresponding wheel;
- Release the handbrake;
- Mark the location of the drum and hubs;
- Insert 2 screwdrivers into the holes of the brake flap;
- We clean the brakes using a special fluid;
- Remove the return spring from above in front of the block;
- Remove the spring plate that secures the block. To do this, you need to press the plate and rotate it ninety degrees.
- Remove the block and the adjusting bar;
- Disconnect the handbrake cable from the rear shoe lever;
- Remove the adjusting bar spring;
- Disconnect the return spring;
- Remove the adjusting bar;
- Remove the bracket that secures the pad;
- Remove the adjusting lever;
- Remove the handbrake lever;
- We install new parts, doing everything in reverse order;
- We install the wheel on the vehicle.
Not really
Drum brakes are gradually leaving the arena under the onslaught of disc brakes. Nevertheless, “drums” are still found on the rear wheels of many modern cars. This is explained not only by their low cost, but also by the fact that, in addition to a number of disadvantages, they also have certain advantages. Let's look at the first and second.
What kind of means did the first automakers use to stop cars? Most of them will seem quite bizarre now. For example, on the legendary Ford T model, produced from 1908 to 1927 and selling more than 15 million copies, a transmission brake was used - a special steel band compressed the gearbox shaft. The transmitted torque dropped and the drive wheels slowed down accordingly. However, drum brakes were additionally installed on the rear wheels. It’s easy to guess that they were far from ideal: the brakes were operated by a lever system, quickly wore out and overheated.
The main elements of a drum brake are the support plate (1), the drum itself (2) and the shoes (3). The design of the modern version is shown in more detail in the figure:
The operating principle of a drum brake is simple. When you press the brake pedal, the pressure in the circuit (brake line) increases, the pistons of the working brake cylinder move apart and spread the brake pads to the sides, which then come into contact with the inside of the drum connected to the wheel. Accordingly, the friction force created by the pads slows down not only the drum, but also the wheel. As soon as the brake pedal is released, the return springs will return the pads to their place and nothing will interfere with the rotation of the drum, and therefore the wheel.
A clear example of how a drum brake works:
Drum brake designs may vary. For example, the pad supports can be spaced on different sides and have manual or automatic adjustment of the pad position.
In addition, the design of rear drum brakes is often somewhat more complicated, since it also requires the installation of a hand (parking) brake. Two cables are attached to the handbrake and a special lever mechanism on the blocks, which, when tensioned, move the blocks apart. The latter are pressed against the drum and ensure the immobility of the wheels.
The “handbrake” is a mechanical braking system, outdated in form, but not at all in content. For example, if the “hydraulics” of the brakes suddenly fail, then the only mechanism capable of stopping the car will be the hand brake. Here's how it works (click the red pointer arrow):
Adjusting the position of the pads is necessary to ensure that the brakes maintain their functionality for as long as possible. Although the pads should not be constantly pressed against the drum, they should still be as close to it as possible. However, the friction linings on the pads wear out over time and the distance gradually increases, which leads to a slower response speed, or even to insufficient contact between the pads and the drum.
A manual adjuster uses only an adjustment screw, while the most common version of an automatic adjuster adds an adjusting lever and gear. When the pads, when triggered, begin to diverge to a greater distance than usual, the lever automatically engages the gear and thereby tightens the adjusting screw, which increases the distance between the pads, again bringing them as close as possible to the drum. Automatic adjustment is very convenient because it does not require disassembling the drum brake structure.
A clear example of the operation of an automatic pad position regulator:
A significant disadvantage of drum brakes is their tendency to overheat. This is explained by the design itself, because air access is closed on one side by a support shield, and on the other by a drum. In this regard, windswept disc brakes are much more effective.
So why are the “drums” still in service? Everything is simple - because they are still coping with the task. Nowadays they are mainly used on budget cars and in most cases only on the rear axle, since disc brakes are installed on the front wheels, which play the main role in braking. “Drums”, which are more than a hundred years old, are reliable and, compared to disc brakes, are cheaper both to manufacture and to maintain. Moreover, the main disadvantage, the closed design, can in some situations also be considered an advantage. For example, on a dusty road, disc brakes will very quickly become dirty, while drum brakes, on the contrary, will remain clean and fully functional.
Nevertheless, there is no point in once again abandoning the achievements of the modern automotive industry, because if attention is still paid to disc brakes and their improvement, then the “drums” are essentially marking time. In other words, they are trying to increase the efficiency of the former, and only maintain the efficiency of the latter.