Introduction drum brake:
Drum brakes primarily consist of a brake drum and backing plate, on which the brake shoes and wheel cylinders are mounted. By pressing on the brake pedal, the pistons of the wheel cylinders are pushed outward. These pistons push the brake shoes against the brake drum. Once the brake pedal is released, return springs ensure that the brake shoes return to their resting position.
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Different designs of drum brakes:
Simplex
In the Simplex design, both brake shoes are pushed outward by one wheel cylinder with two pistons. There is a pivot point between the lower parts of the brake shoes.
Duplex
The duplex system uses two wheel cylinders, each with one brake piston. Each brake shoe is actuated by its own brake cylinder. When driving forward, both brake shoes have a self-energizing effect, making the braking force greater than with simplex brakes. When driving in reverse, there is no self-energizing effect on either brake shoe. Therefore, there is a significant difference in braking force between forward and reverse driving.
Duoduplex
The duoduplex system also uses two brake cylinders, but they are double-acting like simplex brakes. As a result, the same braking force is achieved both when braking while moving forward and in reverse.
Servo
Servo brakes feature a single dual wheel cylinder (at the top) with an adjustment mechanism at the bottom. With the servo system, particularly strong braking can be achieved. The primary shoe presses on the secondary shoe through the self-energizing effect and via the adjuster pin, which shifts to the right. This gives it a self-energizing effect as well. When braking in reverse, only the right brake shoe is self-energizing.
Duoservo
The duoservo brake is equipped with a single dual wheel cylinder. The difference with the servo brake is that the adjuster pin can now move both to the right and to the left. This means the two brake shoes are self-energizing both when braking while moving forward and in reverse.
Adjusting the brake shoes:
The brake shoes should be as close as possible to the brake drum. If this is not the case, the brake pedal will have to be pressed deeper before the shoes make contact with the brake drum.
- Manual adjustment:
To compensate for wear, it is necessary to adjust the brake shoes according to the prescribed maintenance schedule. Where and how this needs to be done depends on the brake design; for example, with gears or a pin with a conical end. Instead of gears, eccentrics may also be used to adjust the brake shoes. The adjustment of the brake shoes is usually done at the back of the brake backing plate. - Automatic adjustment:
There are also drum brakes with an automatic adjustment mechanism. A specially toothed rod is clamped by a spring-loaded lifter. The head of the rod is anchored on the brake backing plate. When the shoes move outward during braking, the lifter pulls the rod along.
Measuring drum brake wear:
The internal diameter of the brake drum increases as it wears. In addition to the linings of the brake shoes, the drum’s diameter should also be checked for wear. The manufacturer’s specifications for the vehicle provide the values determined by the manufacturer. If the measured size is smaller, equal to, or larger than the manufacturer’s specification, the brake drums should be replaced.
The image below shows a special caliper for brake drums. Distance pins can be seen at the outer sides of the measuring jaws. These distance pins should be placed at the deepest point of the brake drum. Move the caliper back and forth a few times to find the deepest point. The largest distance displayed by the caliper is decisive.
The next two images are an enlargement of the caliper shown above. On the caliper, two scales can be seen. The top scale (Innen / Inside) indicates the internal diameter. We read from this because we are measuring the internal diameter of the brake drum. The bottom scale is for the external diameter (Aussen / Outside). This is also indicated in the accompanying drawing.
The top scale starts at 4 cm (40 mm) because the measuring jaws, including the distance pins, are a total of 4 cm thick. Since the internal diameter of the brake drum is being measured, we read from the top scale.
The measurement value is displayed in the image.
First, check where the 0 of the vernier scale is closest. It is slightly past 20.1 cm (201 mm), which means there is a digit after the decimal point. The digit depends on which line of the vernier scale coincides with the line on the main scale. In this case, the 2 of the vernier scale coincides. This means the digit after the decimal point is a 2, so the measured value is 20.12 cm (therefore 201.2 mm).
For the vehicle associated with this measurement, the manufacturer’s data states a minimum size of 201 mm. The measured value of the brake drum exceeds the wear limit, so the brake drum must be replaced.
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