Drum Brake Components | Advantages and disadvantages of drum brake

June 27, 2024 Roberto Magalhães

Drum brake components Advantages and disadvantages of drum brake

Drum brake introduction:

A drum brake is a brake in which friction is caused by a set of shoes or pads that press on a rotating drum-shaped part called a brake drum.

The term “drum brake” generally means a brake in which the shoes press against the inner surface of the drum. When the shoes press against the outside of the drum, it is often called a clamp brake. Where the drum is compressed between two shoes, similar to a conventional disc brake, it is sometimes called a “caliper drum brake”, although such brakes are relatively rare. A related type of brake uses a flexible belt or “band” wrapping around the outside of a drum, called a band brake.

Main parts of drum brake:

Some of the main components of the drum brake assembly are the backing plate, brake drum and shoe, wheel cylinder, and various springs and pins.

Back plate

The backplate serves as the base on which all components are mounted. It attaches to the axle and forms a solid surface for the wheel cylinder, brake shoes and assorted hardware. Since all braking operations put pressure on the backing plate, it needs to be very strong and wear-resistant. Levers for emergency or parking brakes and automatic brake shoe adjuster have also been added in recent years.

Brake drum

The brake drum is usually made from a special type of cast iron that is heat conductive and wear resistant. It is positioned very close to the brake shoe, without actually touching it, and rotates with the wheel and axle. As the liner is pushed against the inner surface of the drum, frictional heat can reach up to 316°C (600°F).

Wheel Cylinder

A wheel cylinder is used for each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. When hydraulic pressure from the master cylinder acts on the piston cup, the pistons are pushed toward the shoes, forcing them against the drum. When the brakes are not being applied, the piston is returned to its original position by the force of the brake shoe return springs.

Brake shoe

Brake shoes are typically made from two pieces of sheet steel welded together. The friction material is riveted to the facing table or fixed with adhesive. The crescent-shaped part is called the Web and contains differently shaped holes and slots for return springs, mounting hardware, parking brake linkage, and self-adjusting components. All application force from the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table usually has three “V” shaped notches or tabs on each side, called spikes. The ends rest on the support pads of the backing plate on which the shoes are installed. Each brake assembly has two shoes, a primary and a secondary. The primary shoe is located at the front of the vehicle and has the lining positioned differently than the secondary shoe. Often the two shoes are interchangeable, so careful inspection for any variation is important.

Advantages of drum brake:

Drum brakes are used on most heavy trucks, some medium and light trucks, and some cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels, as most of the stopping force is generated by the vehicle's front brakes and therefore the heat generated at the rear is significantly less. Drum brakes allow for the simple incorporation of a parking brake.

Disadvantages of drum brake:

When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, which means the brake shoes need to move more and the brake pedal needs to be depressed more. The properties of the friction material may change if heated, resulting in less friction. This can be a much bigger problem with drum brakes than disc brakes, as the shoes are inside the drum and are not exposed to cooling ambient air. The loss of friction is usually only temporary and the material regains its efficiency when cooled, but if the surface overheats to the point that it becomes glassy, the reduction in braking efficiency is more permanent. The vitrified surface can be worn away with subsequent use of the brakes, but this takes time.