Chapter 8 Drum Brakes

Arcing

The process of grinding or forming drum brake linings to conform to the drum diameter and provide clearance where needed.

Backing plate

The mounting surface for all other parts of a drum brake assembly except the drum

Brake shoes

The curved metal parts of a drum brake assembly that carry the friction material lining

Cam-ground lining

A brake shoe lining that has been arced or formed so that it is thinner at the ends than at the center, and the lining surface is not a portion of a circle with a constant radius

Composite

A drum made of different materials, usually cast iron and steel or aluminum, to reduce weight. The friction surfaces and the hubs are cast iron, but supporting parts are made of lighter metal.

Cup expander

A metal disc that bears against the inner sides of wheel cylinder seals to hold the seal lips against the cylinder bore when the brakes are released. This keeps air from entering the cylinder past the retracting pistons and seals

Drum web

The closed side of a brake drum.

Duo-servo brake

A drum brake that develops self-energizing action on the primary shoe, which in turn applies servo action to the secondary shoe to increase its application force. Brake application force is interrelated for the primary and the secondary shoes. Also called a dual-servo or a full-servo brake

Hold-down springs

Small springs that hold drum brake shoes in position against the backing plate while providing flexibility for shoe application and release.

Leading shoe

The first shoe in the direction of drum rotation in a leading-trailing brake. When the vehicle is going forward, the forward shoe is the leading shoe, but the leading shoe can be the front or the rear shoe depending on whether the drum is rotating forward or in reverse and whether the wheel cylinder is at the top or the bottom of the backing plate. The leading shoe is self-energizing

Leading-trailing brake

A drum brake that develops self-energizing action only on the leading shoe. Brake application force is separate for the leading and the trailing shoes. Also called a partial-servo or a nonservo brake

Overload spring

Spring in the end of the cable in cable-operated adjusters that lets the cable move without breaking if the pawl or star wheel is jammed

Pawl

A hinged or pivoted component that engages a toothed wheel or rod to provide rotation or movement in one direction while preventing it in the opposite direction.

Piston stop 

A metal part on a brake backing plate that keeps the wheel cylinder pistons from moving completely out of the cylinder bore

Primary shoe

The leading shoe in a duo-servo brake. The primary shoe is self-energizing and applies servo action to the secondary shoe to increase its application force. Primary shoes have shorter linings than secondary shoes.

Return spring

A strong spring that retracts a drum brake shoe when hydraulic pressure is released

Secondary shoe

The trailing shoe in a duo-servo brake. The secondary shoe receives servo action from the primary shoe to increase its application force. Secondary shoes provide the greater braking force in a duo-servo brake and have longer linings than primary shoes

self-adjusters 

A cable, lever, screw, strut, or other linkage part that provides automatic shoe adjustment and proper lining-to- drum clearance as a drum brake lining wears.

self-energizing operation

The action of a drum brake shoe when drum rotation increases the application force of the shoe by wedging it tightly against the drum.

Shoe anchor

The large pin, or post, or block against which a drum brake shoe pivots or develops leverage.

Star-wheel

A small wheel that is part of a drum brake adjusting link. Turning the star wheel lengthens or shortens the adjuster link to position the shoes for proper lining-to-drum clearance.

Table

The outer surface of a brake shoe to which the lining is attached.

Trailing shoe

The second shoe in the direction of drum rotation in a leading-trailing brake. When the vehicle is going forward, the rear shoe is the trailing shoe, but the trailing shoe can be the front or the rear shoe depending on whether the drum is rotating forward or in reverse and whether the wheel cylinder is at the top or the bottom of the backing plate. The trailing shoe is non-self-energizing, and drum rotation works against shoe application.

Web

The inner part of a brake shoe that is perpendicular to the table and to which all of the springs and other linkage parts attach.