Drum brakes. What are they and what is the principle of operation

Brakes are critical to the safety of any vehicle. And of course, for every motorist, knowledge about the design and various aspects of the functioning of the braking system will not be superfluous. Although we have already addressed this topic more than once, for example, we will return to it again. This time we will take a closer look at the operation of the drum-type brake system and, in particular, we will pay attention to the brake drum itself.

Briefly about the story

The history of drum brakes in their modern form goes back over a hundred years. Their creator is the Frenchman Louis Renault.

Initially, they worked solely due to mechanics. But in the twenties of the last century, the invention of the English engineer Malcolm Lowhead came to the rescue - a hydraulic drive.

Then a vacuum booster appeared, and a cylinder with pistons was added to the design of the drum brake. Since then, drum-type brakes have continued to improve, but the basic principles of their operation have been preserved to this day.

Soon after the Second World War, disc brakes came to the fore, which have several advantages - they are lighter and more efficient cooling, they are less dependent on temperature, they are easier to maintain.

However, drum brakes are not a thing of the past. Due to the ability to achieve very significant braking forces, they are still successfully used in trucks and buses. In addition, they are much more convenient for organizing a parking brake.

Therefore, drum-type brakes are placed on the rear wheels of most passenger cars. They are also relatively inexpensive, have a fairly simple device, and the closed design provides protection from dirt and water.

Of course, there are also disadvantages - the drum actuator works more slowly than the disk one, it is not sufficiently ventilated, and overheating can lead to deformation of the drum.

Design features of drum brakes

A wheel (working) cylinder, a brake regulator and brake shoes are placed on a fixed support shield, between which the upper and lower return springs are stretched. In addition, there is a parking brake lever. Typically, the parking brake is actuated by a metal cable connected to the lower end of the lever. The hydraulic drive to turn on the handbrake is rarely used.

When the brake pedal is depressed, pressure builds up in the hydraulics of the brake system. Brake fluid fills the cavity in the central part of the cylinder and pushes the pistons out of it from opposite ends.

Steel piston pushers put pressure on the pads, pressing them against the inner surface of the rotating drum. As a result of friction, the rotation of the wheel slows down. When the brake pedal is released, the return springs move the shoes away from the drum.

When the handbrake is applied, the cable pulls and turns the lever. He pushes the pads, which with their friction linings are pressed against the drum, blocking the wheels. Between the brake shoes there is a special expansion bar, which is used as an automatic parking brake adjuster.

Vehicles with disc brakes on the rear wheels are additionally equipped with a separate drum-type parking brake. To avoid sticking or freezing of the pads to the drum, do not leave the car for a long time with the handbrake engaged.

More about drums

The drum is the rotating part of the brake mechanism. It is mounted either on the rear axle or on the wheel hub. The wheel itself is attached to the drum, which thus rotates with it.

The brake drum is a cast hollow cylinder with a flange, made, as a rule, from cast iron, less often from an alloy based on aluminum. For greater reliability, the product may have stiffening ribs on the outside. There are also compound drums, in which the cylinder is cast iron, and the flange is made of steel. They have increased strength compared to cast ones, but their use is limited due to their higher cost.

In the vast majority of cases, the working surface is the inner surface of the cylinder. The exception is the parking brake drums of heavy trucks. They are placed on the cardan shaft, and the pads are outside. In an emergency, they can serve as a backup braking system.

In order for the friction pads of the pads to fit as tightly as possible and provide effective braking, the working surface of the cylinder is carefully processed.

To eliminate beats during rotation, the product is balanced. For this purpose, grooves are made in certain places or weights are attached. The flange may be a solid disk or have a hole in the center for the wheel hub.

In addition, to fix the drum and wheel on the hub, the flange has mounting holes for bolts and studs. Drums of the usual type are mounted on the hub.

However, occasionally there are designs in which the hub is an integral part. In this case, the part is mounted on an axle. On the front axle of cars, drum-type actuators have not been used for a long time, but they are still installed on the rear wheels, structurally combining them with a parking brake. But on massive vehicles, drum brakes still dominate.

This is explained simply - by increasing the diameter and width of the cylinder, and consequently, the area of ​​the friction surfaces of the pads and the drum, you can significantly increase the power of the brakes.

It is clear that in the case of a heavy truck or a passenger bus, the task of effective braking is a priority, and all other nuances of the braking system are secondary. Therefore, brake drums for trucks often have a diameter of more than half a meter, and weigh 30-50 kg or even more.

Possible problems, selection and replacement of drums

1. Braking has become less effective, the braking distance has increased.

2. The vehicle vibrates heavily during braking.

3. Beating is felt on the steering wheel and brake pedal.

4. Loud creaking or grinding noise when braking.

If you experience these symptoms, have your rear brakes checked immediately and in particular the condition of the drums.

Cracks

Cast iron, from which drums are most often made, is very hard, but at the same time quite brittle metal. Careless driving, especially on bad roads, contributes to the appearance of cracks in it.

There is another reason for their occurrence. The frequent intermittent loads and sudden temperature changes that are characteristic of drum brakes cause a phenomenon called material fatigue over time.

In this case, microcracks may appear inside the metal, which after a while sharply increase in size. If the drum is cracked, it must be replaced. No options.

Deformation

Another reason to replace the drum is a violation of geometry. If an aluminum alloy product is warped due to overheating or a strong impact, you can still try to straighten it. But with a cast-iron part, there is no choice - only a replacement.

Worn working surface

Any drum is subject to gradual natural wear. With uniform wear, the inner diameter increases, the pads are pressed against the working surface worse, which means that braking efficiency decreases.

In other cases, the working surface wears unevenly, it may take the form of an oval, scratches, grooves, chips and other defects may appear. This happens due to insufficiently tight fit of the pads, the ingress of foreign solid objects into the brake mechanism, for example, pebbles, and for other reasons.

If the depth of grooves or scratches is 2 mm or more, then the drum will have to be replaced with a new one. Less deep defects can be tried to be eliminated with the help of a groove.

About groove

To carry out the groove, you will need a lathe and a fairly serious experience working on it. Therefore, for such work, it is better to find a professional turner. First, approximately 0,5 mm of the working surface is removed.

After that, a thorough inspection and assessment of the feasibility of further turning is carried out. In some cases, it may turn out that there is no point in continuing.

If the degree of wear is not too great, then approximately 0,2 ... 0,3 mm is removed to smooth out the existing defects. The work is completed by polishing using a special grinding paste.

Choice for replacement

If the drum needs to be replaced, choose according to your car model. It's best to check the catalog number. Parts have different sizes, differ in the presence, number and location of mounting holes.

Even minor differences from the original may cause the brakes to work incorrectly or not work at all after installing the drum.

Avoid purchasing products from unknown manufacturers from dubious sellers so that you don’t end up having to pay twice. High-quality ones can be bought in the Chinese online store.

On passenger cars, both drums on the rear axle should be changed at once. And do not forget to make the necessary adjustments after installation.