Brake shoes are typically made from two pieces of steel, welded together. The friction material is attached to the lining table either by an adhesive or riveting. The crescent shaped piece is called the web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three “V" shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the brake shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary brake shoe is located toward the front of the vehicle and has the lining positioned differently than the secondary brake shoe. Quite often the two brake shoes are interchangeable, so close inspection for any variation is important.
Linings must be resistant against heat and wear and have a high friction coefficient. This coefficient must be as unaffected as possible by fluctuations in temperature and humidity. Materials which make up the brake shoes include friction modifiers, powdered metal, binders, fillers and curing agents. Friction modifiers such as graphite, alter the friction coefficient. Powdered metals such as lead, zinc, brass, aluminum and other metals increase a material's resistance to heat fade. Binders are the glues that hold the friction material together. Fillers are added to friction material in small quantities to accomplish specific purposes, such as rubber chips to reduce brake noise.
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