Drum Brake Axle

Drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum. Drum brake axle uses the brake system by this traditional brake way . History[edit] Several schemes of operation of the drum brake, in black...

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Drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

Drum brake axle uses the brake system by this traditional brake way .


History

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Several schemes of operation of the drum brake, in black is highlighted  the distribution force during the braking phase.

The modern automobile drum brake was first used in a car made by Maybach in 1900, although the principle was only later patented in 1902 by Louis Renault. He used woven asbestos lining for the drum brake lining, as no alternative dissipated heat like the asbestos lining, though Maybach had used a less sophisticated drum brake. In the first drum brakes, levers and rods or cables operated the shoes mechanically. From the mid-1930s, oil pressure  in a small wheel cylinder and pistons (as in the picture) operated the brakes, though some vehicles continued with purely  mechanical systems for decades. Some designs have two wheel cylinders.

As the shoes in drum brakes wear, brakes required regular manual adjustment until the introduction of self-adjusting drum brakes in the 1950s.  Drums are prone to brake fading with repeated use.[1][2]

In 1953, Jaguar fielded three cars equipped with disc brakes at Le Mans, where they won, in large part due to their superior braking over drum-equipped  rivals.[3] This spelled the beginning of the crossover of drum brakes to disc brakes in passenger cars. From the 1960s to the 1980s, disc brakes gradually replaced drum  brakes on the front wheels of cars. Now practically all cars use disc brakes on the front wheels, and many use disc brakes on all four wheels.

In the United States, the Jeep CJ-5 (manufactured by AM General) was the final automobile (produced for the United States Postal Service) to use front drum brakes when it was phased out in 1984. However, drum brakes are  still often used for handbrakes, as it has proven very difficult to design a disc brake suitable for holding a parked car.[citation needed] Moreover, it is very easy to fit a drum handbrake inside a disc brake so that one unit serves as both service brake and handbrake.

Early 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 United States Federal Government  began to regulate asbestos production, and brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements; however, technology eventually advanced to compensate.  A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries also limit the use of asbestos in brakes.


Components

Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.


Backing plate

The backing plate provides a base for the other components. The back plate also increases the rigidity of whole set-up, supports the housing,  and protects it from foreign materials like dust and other road debris. It absorbs the torque from the braking action, and that is why back plate is also called the "Torque Plate". Since all braking operations exert  pressure on the backing plate, it must be strong and wear-resistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also added in recent years.

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Back plate made in the pressing shop.


Brake drum

The brake drum is generally made of a special type of cast iron that is heat-conductive and wear-resistant. It rotates with the wheel and axle. When a driver applies the brakes, the lining pushes radially against the inner  surface of the drum, and the ensuing friction slows or stops rotation of the wheel and axle, and thus the vehicle. This friction generates substantial heat.


Wheel cylinder

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Cut-away section of a wheel cylinder.

One wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. The leading shoe  (closest to the front of the vehicle) is known as the primary shoe. The trailing shoe is known as the secondary shoe. Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes,  forcing them against the drum. When the driver releases the brakes, the brake shoe springs restore the shoes to their original (disengaged) position. The parts of the wheel cylinder are shown to the right.

Brake shoe

Main article: Brake shoe

Brake shoes are typically made of two pieces of steel welded together. The friction material is either riveted to the lining table or attached  with adhesive. 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 shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle and has the lining positioned differently  from the secondary shoe. Quite often, the two shoes are interchangeable, so close inspection for any variation is important.

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Brake shoe assembly

Linings must be resistant to heat and wear and have a high friction coefficient unaffected by fluctuations in temperature and humidity. Materials that make up the brake shoe include, friction modifiers (which can include graphite and cashew nut shells), powdered metal such as lead, zinc, brass, aluminium and other metals that resist heat fade,  binders, curing agents and fillers such as rubber chips to reduce brake noise.

In the UK two common grades of brake shoe material used to be available. DON 202 was a high friction material that did not require a brake  power servo. The disadvantage was that the lining was prone to fading on steep hills (calculate the kilowatts dissipated by a one-ton car descending a 15% hill at a constant 60 mph) A harder lining, the famous VG95 was produced  but this required a brake servo. The other snag was that the parking brake would often fail the annual MOT test unless the high friction linings were installed just for the test.


Drum brake designs

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Rendering of a drum brake

Drum brakes are typically described as either leading/trailing or twin leading.

Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or primary/secondary (for duo servo systems) the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point. In this design, one of the brake shoes always experiences the self-applying effect,  irrespective of whether the vehicle is moving forwards or backwards.This is particularly useful on the rear brakes, where the parking brake (handbrake or  footbrake) must exert enough force to stop the vehicle from traveling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't always the case, the self-applying effect can  securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite  pivot may be made in the form of a double-lobed cam that is rotated by the action of the parking brake system.

Front drum brakes may be of either design in practice, but the twin leading design is more effective.This design uses two actuating cylinders arranged so that both shoes use the self-applying  characteristic when the vehicle is moving forwards. The brake shoes pivot at opposite points to each other. This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.

The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more braking force at the front of the vehicle when it is moving forwards, with less at the rear. This helps prevent the rear wheels  from locking up, but still provides adequate braking at the rear.

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Shimano Nexus front hub with roller brake


The brake drum itself is frequently made of cast iron, though some vehicles have used aluminum drums, particularly for front-wheel applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces fade. Aluminum  drums are also lighter than iron drums, which reduces unsprung weight. Because aluminum wears more easily than iron, aluminum drums frequently  have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.

Roller brakes are specially designed drum brakes for bicycles, mounted to the side of the wheel's hub.


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