History
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, 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 For this reason drum brakes have been superseded in most modern automobiles and light trucks with at least front wheel (often now four wheel) disc brakes.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.
Operation
Drum brakes were once common on all wheels of light vehicles, but on modern designs they are less commonly used, and then usually only found on just the rear wheels, in disc-drum combinations.
The drum brake has 2 brake shoes with a friction material called a lining attached. These shoes expand against the inside surface of a brake drum, and slow the wheel down.
The harder the linings are forced against the brake drum, the higher the braking force that is applied.
They can be expanded mechanically, or hydraulically.
The main advantage claimed for drum brakes is that the shoe mountings can be designed to assist their own operation. This is called self-energizing.
Less hydraulic pressure is then needed to stop the vehicle, which is why many older drum-braked vehicles didn’t use a brake booster.
The main disadvantage of drum brakes is that the friction area is almost entirely covered by lining, so most heat must be conducted through the drum to reach the outside air to cool. With hard use, this can cause overheating, and eventually Brake Dissolve.
Brake Dissolve is the gradual loss of brake stopping power during prolonged or strenuous use. Very high temperatures occur at the brake drum, and that causes deterioration in the frictional value of the lining or pad material. It’s common in drum brakes.
Another problem with drum brakes is that it is difficult to get water out of the drum. If a vehicle is driven through water, it takes longer to get the brakes working effectively.
Three brake designs are in general use:
- single leading shoe,
- twin leading shoe; and
- duo-servo.
Each one uses the wedging or self-energizing action of the brake shoe, to assist the lining to grip the rotating drum when the brakes are applied. The twin-leading shoe has an actuator for each brake shoe. The actuator can be mechanical, however a hydraulic actuator is popular on light vehicles. The hydraulic actuator is called the wheel cylinder.
This arrangement has 2 wheel cylinders, with 1 piston in each cylinder. When the brakes are applied, hydraulic pressure forces each piston to move outwards, pushing on one end of the brake shoe. The direction of rotation of the drum produces a wedging action on both brake shoes, so they are both called leading shoes.
This system was once popular on front wheels because it is very efficient in the forward direction. This is due to the self-energizing or self-wedging action of the shoes as the drum rotates.
Its main disadvantage is that it is only about 30% as efficient in reverse, so it is usually combined with a single leading shoe arrangement on the rear to provide a balanced system.
The single leading shoe system uses a single wheel cylinder with 2 pistons.
When the brakes are applied, both shoes press against the brake drum.
One shoe is called leading shoe, the other is called trailing. The leading shoe tends to be self-energised, while the trailing shoe tends to be forced off the drum.
This arrangement is common on rear wheels as they work equally well in forward and reverse, so it makes an effective handbrake. They can also have a self-adjusting mechanism.
The duo-servo design also uses 1 wheel cylinder with 2 pistons. It is a high energy brake, that is, it exerts large self-energising forces.
The lower ends of the shoes are linked but aren’t firmly anchored to the backing plate. This lets the complete shoe assembly float, within limits.
When the brakes are applied, both shoes are carried around by the drum, until the secondary shoe contacts the anchor pin. The self-energizing force of the primary shoe and its wheel cylinder application force, is now transferred to the secondary shoe through the lower linkage. Force is then being applied to the secondary shoe from both ends - the wheel cylinder at the top, and the linkage from the primary shoe at the bottom.
The primary shoe has the shorter lining and is always fitted ahead of the wheel cylinder in terms of drum rotation. It’s most important that the shoes are fitted correctly, since it’s the secondary shoe that does most of the work.
The linings may also have different frictional values. The colors of the retraction springs indicate different spring strengths. This design is common on rear wheels and it works well in both directions.
Drum brake systems need to be adjusted to allow for wear of the lining. If they are not adjusted, pedal travel will be too long to be safe.
Overview
The drum brake uses brake shoes that have friction material called linings attached to them.
This friction material was once made of asbestos but concerns about health problems associated with asbestos have led to increasing use of non-asbestos alternatives.
Linings can be riveted, or more often, bonded to the brake shoes.
The composition of the friction material affects brake operation. Linings which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased.
Linings which maintain a stable friction co-efficient over a wide temperature range, generally require higher pedal pressures to provide efficient braking. This may necessitate the use of a booster.
Background
Drum brake lining normally come under one of three categories based on the composition of materials used in their manufacture. In addition, it is also depends on the curing method undertaken.
The majority of brake linings sold today will have manufacturer's codes stenciled on the edge of the lining. This will identify its co-effiecient of friction as outlined in the manufacturer's specification. To ensure the maximum life and performance from brake linings, it is recommended that repairers only use good quality brake lining from reputable suppliers.
The brake lining surface, as well as being smooth to ensure maximum surface contact for better braking, it also conducts heat away to the cooling air thus help maintaining it retardation/stopping ability. By allowing the linings to radiate heat away it helps prevent brake fade by the linings becoming overheated and loosing it frictional qualities.
WARNING
Asbestos in brake components
Beware many older brake pads and linings were constructed with asbestos in the material. Asbestos fibers are a known health risk that can lead to lung diseases after many years of exposure. As the linings or pads worn down asbestos fibers were present and could easily be inhaled. Reputable manufacturers have ceased using asbestos in their linings. Most governments around the world have now legislated against the inclusion of asestos in new brake linings. It is virtually impossible to know if asbestos has been used. In this case, all brake lining servicing should be carried out as though there is asbestos in the material.
Cleaning methods can vary from using special vacuum cleaners to a washing method of remove the brake dust. As a matter of importances,
NEVER USE AN AIR HOSE to blow dry dust off any brake component.
All of the brake unit components, except the brake drum, are mounted on a backing plate bolted to the vehicle axle housing or suspension.
The backing plate is usually pressed from heavy gauge steel. It has a raised outer edge that fits into a groove or recess in the brake drum and helps keep out any dust or dirt.
Some vehicles have manual brake adjusters so openings are usually provided to allow for adjustments without having to remove the wheel and brake drum.
The wheel cylinder is located inside the brake drum, and bolted to the backing plate. It converts hydraulic pressure from the master cylinder into mechanical force that pushes the brake linings against the brake drum.
Wheel cylinders are either:
- Single pistion/single action; and
- Dual action/double cylinder with a piston at each end.
They are usually made of cast iron or aluminium alloy. Some are sleeved with stainless steel to be longer-wearing and more resistant to corrosion. Cast iron and aluminium cylinders are suspectible to pitting and potential fluid loss if the brake fluid is contaminated, particularly with water vapour. This also lowers the boiling point of the brake fluid.
The wheel cylinder cups seal the cylinder against fluid loss. They operate under difficult conditions of extreme pressures and temperatures.
They may be fitted with a spreader and light expansion spring to keep the lips in contact with the cylinder during retraction and while at rest. This helps keep air out of the system.
Most wheel cylinders are fitted with bleed nipples to allow air to be bled from the system after assembly.
A flexible cover, or boot, allows for piston movement, and also keeps out dust and moisture.
Majority of manufacturers that utilised drum brakes used a larger diameter wheel cylinder on the front braking stations while using a smaller diameter wheel cylinder on the rear. This allows for the braking proportion for the vehicle, normally 60/40; 60% on front brakes and 40% on the rear brakes.
Leaking wheel cylinders are the cause of many brake problems such as unreliable stopping, brake shoe damage and even partial brake system failure.
Whenever you open a hydraulic brake system and introduce air into the lines, master cylinder or wheel cylinders you will need to bleed the air out of the system.
Depending on the manufacturer, rear wheel cylinders can be piped independantly of each other or in series. Independently piped systems are the most common. These have a bleeder screw on each cylinder to allow the expulsion of any trapped air bubbles in the system. Air bubbles trapped in the wheel cylinders will affect the functionality of the brake stations.
