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
History
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 1950s introduction of self adjusting drum brakes. In the 1960s and 1970s brake drums on the front wheels of cars were gradually replaced with disc brakes and now almost all cars use disc brakes on all wheels. However, drum brakes are still often used for handbrakes as it has proved very difficult to design a disc brake suitable for holding a car when it is not in use. Moreover, it is very easy to fit a drum handbrake inside a disc brake so that one unit serves for both footbrake and handbrake.
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. The Federal government began to regulate asbestos production, and there was a period of time where owners complained of poor braking with the non-asbestos linings. Eventually technology advanced to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings.
Servo design
Drum brakes, depending on the way the shoes are hinged, can have a "self-servo" characteristic. This increases stopping power without any additional effort by the driver because the rotation of the drum drags the shoes around with it, increasing the force holding them together. In rear brakes (as illustrated above) only one shoe will have this characteristic. Front drum brakes may use two actuating cylinders which allow both shoes to utilize the servo characteristic and which also increase the front axle braking force, required to compensate for forward weight shift and also to avoid premature rear-wheel locking. Servo action can be used to make a very powerful brake (as on the rear axles of large commercial vehicles), but it does reduce the ability of the driver to modulate the brakes sensitively. (The disc brake has no self-servo effect because the pads act perpendicularly to the rotating disc.)
Advantages
Drum brakes are still used in modern cars. There can be engineering and cost advantages. Drum brakes allow simple incorporation of a parking brake. They are often applied to the rear wheels since most of the stopping happens in the front of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels used disk brakes as the main brakes. In this situation, a small drum is usually fitted within or as part of the brake disk.
An advanced technology hybrid car using drum rear brakes is the Toyota Prius. (4-wheel discs are used in certain markets - Hybrid vehicles greatly reduce everyday wear on braking systems owing to their energy recovery motor-generators, see regenerative braking).
Disadvantages
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 is known as brake fade and can lead to driver panic and brake failure in extreme circumstances. Under normal driving conditions it is seldom noticed, especially when drums of appropriate size are fitted. The Pontiac GTO is one vehicle often cited as having undersized drums.
Before 1984, it was common to re-arc brake shoes to match the arc within brake drums; the machinery used has been phased out. This practice, however, was controversial as it removed friction material from the brakes and caused a reduction in the life of the shoes as well as creating hazardous asbestos dust. It is much better to use shoes for the proper diameter drum, and if the procedure was needed, the drums were so worn that they should have been replaced, as the thickness of the drum contributes to the strength and the heat absorption and dissipation ability of the drum.
Adjustment
Early drum brakes (before about 1955) required periodic adjustment to compensate for drum and shoe wear. If not done sufficiently often the symptom would be long brake pedal travel ("low pedal"), which could also be caused by low hydraulic fluid level. Low pedal can be a severe hazard when combined with brake fade as the brakes can become ineffective when the pedal bottoms out.
Self adjusting brakes may use a mechanism that engages only when the vehicle is being stopped from reverse motion. This is a traditional method suitable for use where all wheels use drum brakes (most vehicles now use disc brakes on the front wheels). By operating only in reverse it is less likely that the brakes will be adjusted while hot (when the drums are expanded), which could cause dragging brakes that would accelerate wear and reduce mileage.
Self adjusting brakes may also operate by a ratchet mechanism engaged as the hand brakes is applied, a means suitable for use where only rear drum brakes are used. If the travel of the parking brake actuator lever exceeds a certain amount, the ratchet turns an adjuster screw that moves the brake shoes toward the drum.
text taken from http://en.wikipedia.org/wiki/Drum_brake
picture taken from http://www.howstuffworks.com/
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