Aluminum brake drums are constructed similar to the centrifuse drums. They consist of an aluminum casting with a cast-iron liner for a braking surface. This design allows heat to be transferred to the surrounding atmosphere more readily and also reduces weight.

Cooling fins or ribs are added to most brake drums. The fins or ribs increase the surface area of the outside portion of the brake drum, allowing the heat to be transferred to the atmosphere more readily, which keeps the drum cooler and helps minimize brake fade.

For good braking action, the brake drum should be perfectly round and have a uniform surface. Brake drums become out-of-round from pressure exerted by brake shoes and from heat developed by application of the brakes. The brake drum surface becomes scored when it is worn by braking action. When the braking surface is scored or the brake drum is out-of-round, it may be necessary to machine the brake drum until it is smooth and true again. Care must be taken not to exceed the maximum allowable diameter according to the manufacturer's specification. Each drum is stamped with the maximum diameter information and. if exceeded, it should be discarded and replaced with a new one.

Figure 7-12. - Brake drum construction.

BRAKE SHOE ADJUSTERS. - Brake shoe adjusters maintain correct drum-to-lining clearance, as the brake linings wear. Automatic brake shoe adjusters normally function when the brakes are applied with the vehicle moving in reverse. If there is too much lining clearance, the brake shoes move outward and rotate with the drum enough to operate the adjusting lever. This lengthens the adjusting mechanism, and the linings are moved closer to the brake drum, thereby maintaining the correct lining-to-drum clearance.

Many vehicles use a star wheel (screw) type brake shoe adjusting mechanism. This type consists of a star wheel (adjusting screw assembly), adjuster lever, adjuster spring, and an adjusting mechanism. The adjustment system may grouped as follows (fig. 7-13):

Cable type - The cable type self-adjusting system (fig. 7-13) uses a braided steel cable and the expanding action of both brake shoes to accomplish the self-adjusting action in forward and reverse directions. A one-piece cable is attached to the adjusting lever and passes through a cable guide on the primary shoe. The