Drum Brakes

Drum Brake Operation

•  The most important feature contributing to the effectiveness of the braking force supplied by the drum brake is the brake shoe pressure or force directed against the drum.

•  With the vehicle moving in either the forward or reverse direction with the brakes on, the applied force of the brake shoe pressing against the brake drum increasingly multiplies  itself.

–  This is because the brake’s anchor pin acts as a brake shoe stop and prohibits the brake shoe from its tendency to follow the movement of the rotating drum.

–  The  result is a wedging action between the brake shoe and brake drum.

–  The  wedging action combined with the applied brake force creates a self-multiplied brake force.

Drum Brake Components

Backing plate:

– The backing plate provides a foundation for the brake shoes and associated hardware.

Master cylinder:

• The  master cylinder converts mechanical pressure on the brake pedal to hydraulic pressure on the wheel cylinders. 

Wheel cylinders:

–  Wheel cylinders convert hydraulic pressure from the master cylinder into a mechanical force at the brakes.

Brake shoes and linings:

–  Each drum in the drum braking system contains a set of shoes.

–  Each shoe consists of a rim welded to the web to provide a stable surface for the lining.

Primary shoe:

–   The primary shoe (or leading shoe) is the one that is toward the front of the vehicle.

–  The friction between the primary shoe and the brake drum forces the primary shoe to shift slightly in the direction the drum is turning.

Secondary shoe:

–  The shifting of the primary shoe forces it against the bottom of the secondary shoe, which causes the secondary shoe to contact the drum.

–  The secondary shoe (or trailing shoe) is the one that is toward the rear of the vehicle.

Mechanical Components:

–  Springs are normally installed on the anchor.

–  While shoe brake springs look the same, they are not interchangeable.

–  Sometimes to help distinguish between them, they are color coded.

– Shoe hold-downs:

–  To unlock or lock the straight pin hold downs, depress the locking cup and coil spring, and rotate the pin or lock 90 degrees.

Drums:

–  brake-001.html">Brake drums are made of heavy cast iron with a machined surface inside against which the linings on the brake shoes generate friction when the brakes are applied.

Drum Brake Designs:

•  There are two drum brake designs:

–  Duo-servo (or self-energizing).

–  In duo-servo drum brakes the self-energizing force is transferred from one shoe to the other with the wheel rotation in either direction.

–  Non-servo (or leading-trailing).

–  The basic difference between this type and the duo-servo brake is that both brake shoes are held against a fixed anchor at the bottom by a retaining spring. Non-servo brakes have no servo action. The non-servo drum brake is often used on smaller cars. 

Automatic adjusted brakes:

–  Adjusters, whether cable, crank, or lever, are installed on one shoe and operated whenever the shoe moves away from its anchor.

Drum Brake Inspection

Drum removal:

–  If the brake shoes have expanded too tightly against the drum or have cut it the friction surface of the drum, the drum might be to tight for removal.

–  In such a case, the shoes must be adjusted inward before the drum is removed.

Drum Parking Brakes

Integral parking brakes:

– Integral parking brakes are for vehicles with rear wheel drum brakes.

Types of parking brake systems:

–  Hand operated

–  Foot operated

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