Drum brake
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A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against the inner surface of a rotating drum. The drum is connected to a rotating wheel.
The modern automobile drum brake was invented in 1902 by Louis Renault, though a less-sophisticated drum brake had been used by Maybach a year earlier. In the first drum brakes, the shoes were mechanically operated with levers and rods or cables. From the mid-1930s the shoes were operated with oil pressure in a small wheel cylinder and pistons (as in the picture), though some vehicles continued with purely-mechanical systems for decades. Some designs have two wheel cylinders.
The shoes in drum brakes are subject to wear and the brakes needed to be adjusted regularly until the 1950's introduction of self adjusting drum brakes. Self adjusting brakes operate by a ratchet mechanism engaged as the hand brakes is applied. If the travel of the handbrake acutator lever exceeds a certain amount, the rachet turns an adjuster screw that moves the brake shoes toward the drum. In the 1960s and 1970s brake drums on the front wheels of cars were gradually replaced with disc brakes and now many cars uses disc brakes on all wheels.
Another type of drum brake is where a friction belt is wrapped around the outside of the drum and tightened. This type predated the modern drum brake, and was later often used for the parking brake on the central drive shaft. This type of band brake is also used in automatic transmissions and aerobic exercise cycling equipment.
Drum brakes with internal shoes have a particular disadvantage; when the drums are heated by hard braking the diameter of the drum increases due to the expansion of the material and the brakes must be further depressed to obtain effective braking action. This increase of pedal motion is known as brake fade and can lead to brake failure in extreme circumstances. For this reason drum brakes have been superseded in most modern automobiles and light trucks with at least front wheel (often now four wheel) disc brakes.
Drum brakes are still used in some modern cars owing to weight and cost advantages. An advanced technology hybrid car using drum rear brakes is the Toyota Prius. (Hybrid vehicles greatly reduce everyday wear on braking systems owing to their energy recovery motor-generators.)
Early type brake shoes contained asbestos. When working on brake systems of older cars, care must be taken not to inhale any dust present in the brake assembly.