Brake drums may be made of pressed steel, cast iron, a combination of the two metals, or aluminum. Cast-iron drums dissipate the heat generated by friction faster than steel drums and have a higher coefficient of friction with any particular brake lining. However, cast-iron drums of sufficient strength are heavier than steel drums. To provide lightweight and sufficient strength, use CENTRIFUSE brake drums (fig. 7-12). These drums are made of steel with a cast-iron liner for the braking surface. A solid cast-iron drum of the same total thickness as the centrifuse drum would be too weak, while one of sufficient strength would be too heavy for the average vehicle.

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.