DRUM BRAKE: FUNCTION, COMPONENTS, WORKING PRINCIPLE AND TYPES

A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

The term drum brake usually means a braking system in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or “band” wrapping around the outside of a drum.

Drum brakes are primarily used at the rear axle of small and compact class vehicles.

FUNCTION

Drum brakes have been around for almost as long as the automobile itself and are still fitted today in a modified and more sophisticated form in modern cars. The term drum brake describes the design principle: namely, an enclosed cylindrical structure.

DRUM BRAKE COMPONENTS

A drum brake comprises the following components:

1. Backing plate: 
Provides a solid base for other components in the drum brake attached to the axle sleeve.

2. Brake drum: 
Bolted to the wheel hub and spins with the wheel. Often made of cast iron, and is resistant to heat and wear. This is what you see when you look at an assembled drum brake and is the component upon which braking force is applied to slow or stop the car.

3. Wheel cylinder: 
Contains two pistons, one at each end of the cylinder, to operate the brake shoes. The cylinder applies pressure to the pistons, which pushes the brake shoes towards the drum, slowing or stopping the car. One cylinder is needed per wheel.

4. Brake shoe: 
Pushes into the drum to create the friction necessary to slow or stop the car. Secured to the backing, but able to slide when pressure from the wheel cylinder is applied. It has a lining attached to it, made up of organic or metallic compounds. The lining is what actually comes in contact with the drum and wears away with use. Each brake contains two shoes. The primary shoe is closer to the front of the vehicle, while the secondary shoe is closer to the rear. Depending on the type and brand, the brake shoes may be interchangeable.

5. Automatic adjuster: 
Keeps the brake shoes at a consistent distance away from the drum, even as the lining wears away.

6. Return springs: 
Pulls the brake shoes back away from the drum when the driver lets off the brake pedal.

The brake drum is fixed to the wheel and turns with it. On braking, the wheel cylinder forces the fixed brake shoes apart and presses them against the brake drum, thus slowing it down. When the brake is released, the return springs move the brake shoes back to their original position.

WORKING PRINCIPLE

When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.

TYPES OF DRUM BRAKES

There are mainly three types – mechanical, hydraulic & pneumatic assisted Drum Brakes.

1. Mechanical:

In the mechanical Drum brake system such as in two-wheeler & auto rickshaw, the brake shoes are actuated by a cam, which is attached to the brake linkage & pedal. When you press the brake pedal, the cam turns. Thus, it causes the brake shoes to expand outwards and rub against the drum. 
The friction between the brake linings and the drum causes the drum to stop rotating, thereby the wheel to stop. When you release the brake pedal, the retracting springs bring the brake shoes back to their original position. This results in a gap between them and the drum and to again spin it freely.

2. Hydraulic:

The hydraulic Drum brake system such as in cars is a bit superior to a mechanical one. In this design, the hydraulic wheel cylinder replaces the cam. In the hydraulic system, instead of a cam, the wheel cylinder’s pistons push the brake shoes outwards. The brake shoes fit on the anchor plate or braking plate. It holds the brakes system parts together and on to the car’s axle. When you press the brake pedal, the oil in the brake master cylinder multiplies the hydraulic force sent to the wheel cylinders. Thus, it causes its pistons to push outwards. The pistons, in turn, cause the brake shoes to expand and rub against the drum. The friction between the brake linings and the drum causes the drum to stop rotating, thereby the wheel to stop.

3. Pneumatic assisted:

The third type – pneumatic assisted Drum-brake system; actuated by air-pressure, which works on the same principle of that of the mechanical Drum brake system. It is also operated by a bigger size cam or the ‘S’ shaped cam and is popularly known as the “S-Cam” brake system. However, high-pressure compressed air actuates a pneumatic piston which turns the cam. Mostly the medium to heavy commercial vehicles use this type of drum brake system.

BASED ON PRINCIPLE

1. Leading/trailing shoe type drum brake

“Leading (or primary) shoe” is a term referring to the shoe that moves in the direction of rotation when it is being pressed against the drum. The other shoe is called the “trailing (secondary) shoe.” The leading shoe is pressed in the same direction as the rotation of the drums, and this rotation helps to press the shoes against the drum with greater pressure for stronger braking force. This is called the servo effect (self-boosting effect) which realizes the powerful braking forces of drum brakes.

Structurally, it has a wheel cylinder housing a piston with which hydraulic pressure is generated to push the two shoes against the drum’s inner surface.

The two shoe function in a way they both become either the trailing shoe or leading shoe depending on whether the vehicle is traveling forward or backward. Drum brakes generate consistent braking force whether the vehicle travels forward or backward. This is because drum brakes generate the same braking force in either direction. Generally, this type is used for the rear brakes of passenger cars.

2. Twin leading shoe type drum brake

This type of drum brake has two-wheel cylinders and two leading shoes. Each wheel cylinder presses on one shoe so that both shoes act as leading ones when the vehicle moves forward, providing superior braking force.
Each of the pistons housed in the wheel cylinders displaces in one direction, so when the vehicle is in reverse both shoes act as trailing ones. This type is used mainly for the front brakes of small-to-mid-sized trucks.
The dual twin leading shoe type has pistons that displace in both directions, making it possible for both shoes to act as leading ones, regardless of the direction of travel. This type is mainly used for the rear brakes of small-to-mid-sized trucks.

3. Duo-servo type drum brake

The duo-servo type features a structure where two brake shoes, called the primary shoe and secondary shoe, are linked via an adjuster. Strong pressure from the servo effect (self-boosting effect) of the primary shoe is transmitted to the linked secondary shoe, thus generated a very large braking force.
This type is mainly used for parking brakes on passenger cars, the center brakes on trucks, and brakes on forklifts.

ADVANTAGES AND DISADVANTAGES

Advantages of Drum brake system:

1. Simple design and parts
2. Easy & cheaper to manufacture
3. Low maintenance cost
4. Comparatively longer life

Disadvantages of Drum Brake system:

1. Low braking force compared to Discs
2. Brakes ‘fade’ when applied for a prolonged time
3. The brake shoe lining made of asbestos is harmful to humans
4. When wet, the braking grip reduces considerably
5. Non-asbestos linings catch moisture, causing Drum brakes to grab suddenly

SAFETY

The brake drum is one of the most important vehicle systems when it comes to safety. It is relatively low-wear and has a long service life. A specialist workshop should be consulted immediately if the deterioration in the braking action of a drum brake becomes noticeable. Drum brakes are only to be replaced by qualified personnel. The manufacturer’s installation instructions must be observed when doing so.

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