What is Drum Brake?
A 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.
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.
Brake shoes fitted with brake linings (friction material) that press against the drums from the inside to generate braking force (decelerate and stop) are set inside of the drums.
With this system, friction is generated by pressing the brake linings against the inside surfaces of the drums. This friction converts kinetic energy into thermal energy. Drum rotation helps to press the shoes and the lining against the drum with more force, offering superior braking force in comparison with disc brakes.
On the other hand, it is very important to design the components so that the heat from the thermal energy is dissipated efficiently into the atmosphere.
There are three types of drum brakes depending on how the brake shoes are pressed onto the drums; leading/trailing shoe type, twin leading shoe type, and duo-servo type.
Structure of drum brakes
Drum brakes are a brake system with brake drums (rotor) that rotate with the wheels. Inside each drum are brake shoes fitted with brake linings (friction material). Pistons (pressure mechanism) press against the drums from the inside to generate braking force, thus making it possible to decelerate and stop the vehicle.
Diagram of Drum Brakes
Drum Brakes Parts
Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.
1. Backing plate
The backing plate provides a base for the other components. The backplate also increases the rigidity of the 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 the backplate 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 adjusters were also added in recent years.
2. 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.
3. 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 secondary shoe. The trailing shoe is known as the primary 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.
4. 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.
How Do Drum Brakes Work?
Drum brakes slow down the car by pressing a brake shoe against the inside of the drum brake, the rotating component that spins with the wheels. The drum is made from an iron alloy that is specially formulated to be extremely long-lasting. The brake shoes consist of a friction lining bonded to a metal backing plate.
The drum brake may look complicated, and it can be pretty intimidating when you open one up. Let’s break it down and explain what each piece does.
When you hit the brake pedal, the piston pushes the brake shoes against the drum. That’s pretty straightforward, but why do we need all of those springs?
This is where it gets a little more complicated. Many drum brakes are self-actuating. As the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force.
The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster’s arm after it actuates.
How to tell if your brake shoes need changing?
Inconsistent brake pedal feel. If the rear brakes are drum brakes, the driver may feel vibrations under braking. The initial bite when the brakes are still cold may be lacking. This is because the brake shoes are so worn that they cannot firmly press against the drum.
The hand brake feels loose. If the hand brake requires a hard yank to keep the car from rolling off, chances are the brake shoes need replacing. On a steep incline, if the car moves an inch or so before coming to a rest, the drum or brake shoes would need to be inspected. There is a possibility it could be a loose hand brake cable, which is a much easier fix.
Scraping noise while braking. When brake shoes are worn away so much that only the metal shoe platform remains, under braking, you would hear a metallic scraping noise. Once it is this severe, you may have to machine the drum or replace it together.
Why drum brakes?
All cars sold in the United States use disc brakes for the front wheels because about 70-80% of the vehicle’s stopping power is done by the front wheels.
Even so, the venerable drum brake is still used by some of today’s vehicles, but that is getting less and less every year. One of the reasons for doing this is because the parking brake can be located inside the rear drum brakes whereas, with a rear disc brake, it is more expensive to include a parking brake.
The manufacturer can thus offer plain-Jane vehicles by offering drum brakes with a parking brake built-in at the rear wheels.
Drum brake advantages
Brake shoes today are still being used. Here are some advantages that drum brakes have over disc brakes:
- Drum brakes can provide more braking force than an equal diameter disc brake.
- Drum brakes last longer because drum brakes have increased friction contact area than a disc.
- Drum brakes are cheaper to manufacture than disc brakes.
- Rear drum brakes generate lower heat.
- Drum brakes have a built-in self-energizing effect that requires less input force (such as hydraulic pressure
- Wheel cylinders are simpler to recondition than with disc brake calipers.
- Brake shoes can be remanufactured for future use.
- Drums have slightly lower frequency of maintenance due to better corrosion resistance.
Drum brake disadvantages
- Excessive heating can happen due to heavy braking, which then can cause the drum to distort, and thus cause vibration under braking.
- Under hard braking, the diameter of the drum increases slightly due to thermal expansion, the driver must press the brake pedal farther.
- Brake shoes can overheat to the point where they become glazed.
- Excessive brake drum heating can cause the brake fluid to vaporize.
- Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction is enough, the brake will stay engaged due to self-application, even when the external application force is released.
- Another disadvantage of drum brakes is their relative complexity.
- Maintenance of drum brakes is more time-consuming, compared to disc brakes.