Drum brake

thumb|upright=1.35|Drum brake (upper right) with the drum removed (lower left, inside facing up), on the front of a [[Ford Falcon Sprint]]

thumb|A rear drum brake on a [[Kawasaki W800 motorcycle]]

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

The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or "band" wrapping around the outside of a drum. Drum brakes are manufactured in small automobiles or cheap vehicles, as they are less expensive to make than a traditional disc brake.

History

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

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 material dissipated heat more effectively, 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. Drum brakes are also prone to brake fade with repeated use.

Jaguar Cars fielded three cars equipped with disc brakes at Le Mans in 1953, where they won, in large part due to their superior braking over drum-equipped rivals. This spelled the beginning of the end for drum brakes in passenger cars. From the 1960s to the 1980s, disc brakes gradually replaced drum brakes on the front wheels of cars (which receive the majority of braking force). 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 1986. However, drum brakes are still often used on the rear wheels, and for parking brakes. Some vehicles utilize a "drum-in-hat" parking brake, where the brake shoes are arranged inside the center portion (hat) of a disc brake rotor, which acts as the drum.

Early brake shoes contained asbestos. After the United States Federal Government began to regulate asbestos production, brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements, but brake technology eventually advanced to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries have also prohibited 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.

thumb|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

thumb|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

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.

thumb|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 lining 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. 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.

In operation

Normal braking

When the brakes are applied, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released, return springs pull the shoes back to their rest position.

Automatic self-adjustment

As the brake linings wear, the shoes must travel a greater distance to reach the drum. In systems fitted with automatic adjusters, when the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum. Here, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum. Typically the adjusters only operate when the vehicle is going in reverse and the brakes are engaged.

On vehicles without automatic adjusters, it is required to periodically manually adjust the brakes to take up any excess gap between the shoes and the drum.

Parking/emergency brake

The parking (or emergency) brake system controls the brakes through a series of steel cables that are connected to either a hand lever or a foot pedal. The idea is that the system is fully mechanical and completely bypasses the hydraulic system so that the vehicle can be brought to a stop even if there is a total brake failure. Here the cable pulls on a lever mounted in the brake and is directly connected to the brake shoes. This has the effect of bypassing the wheel cylinder and controlling the brakes directly.

Self-applying characteristic

Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing." The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.

Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc.

Certain heavier-duty drum brake systems compensate for load when determining wheel cylinder pressure; a feature which is rare when discs are employed (hydropneumatic suspension systems as employed on Citroën vehicles adjust brake pressure depending on load regardless of if drum or discs are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load. Most other brands have used load sensing valves in the hydraulics to the rear axle for decades.

Due to the fact that a drum brake's friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes, but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake's self-applying nature, larger friction surface contact area, and long life wear characteristics (% life used / kW of braking power).

To list advantages of drum brakes:

less expensive to produce.
slightly lower frequency of maintenance due to better corrosion resistance compared to disks.
built-in self energizing effect requires less input force (such as hydraulic pressure).
wheel cylinders are somewhat simpler to recondition compared to calipers.
minor weight savings, primarily from much smaller and lighter hydraulic cylinders vs. calipers.

As a driveshaft parking/emergency brake

Drum brakes have also been built onto the transmission's driveshaft as parking brakes (e.g., Chryslers through 1962). This provides the advantage that it is completely independent of the service brakes—but suffers a severe disadvantage in that, when used with a bumper jack (common in that era) on the rear, and without proper wheel blocks, the differential's action can allow the vehicle to roll off the jack.

Land Rover have used a drum brake on the gearbox output shaft for over fifty years. The advantage is that all four wheels can be braked with the parking brake.

Disadvantages

Drum brakes, like most other brakes, convert kinetic energy into heat by friction. Asbestos fibers would break off or become separated over time and with the high temperatures induced by braking. Wet brushes and aerosol sprays were commonly used to reduce dust. Safety regulators sometimes recommended using vacuum hoses to suck away the dust, or enclosures with interior lighting and space to use tools inside them, but these were rare and cumbersome. Distinctive shoes designed to protect against asbestos were also recommended. There is evidence that auto mechanics had disproportionate levels of mesothelioma.