How Does Drum Brake Work?

A mechanic is replacing a drum brake.

The Drum brake was once the standard method for stopping vehicles. With the introduction of the disc brake, its use became secondary to the disc brake. Disc brakes, of course, come with greater braking force. Hence, automobile manufacturers consider it a safer type of brake than the drum brake.

The drum brake, however, did not completely fade from usage. You can still find it in many new cars. Yet, it now plays a secondary billing as an alternate brake. At present, it is mostly found in many economy cars. Moreover, it is usually positioned on the car’s rear wheels. Front-wheel brakes, however, that do the hard works of stopping a vehicle, are generally disc brakes. 

The drum brake makes use of friction as all other types of brakes. It consists of a set of pads or shoes that presses outward the brake drum. The brake drum is a rotating cylinder-shaped component of the drum brake system.

As you step on the brake pedal, the pneumatic brake fluid found in the master cylinder compresses. It is squeezed under pressure through the brake lines. Then, it is pushed into the brake cylinder. This enables the wheel cylinder’s pistons to expand outwardly. Each piston is then driven against any of the two long and curved brake shoes. This forces the brake shoe against the brake drum. 

The brake shoe lining, then, touches the drum’s inner surface. Friction between the inner surface of the drum and the brake shoe lining ensues. Due to this friction, the wheel reduces its motion, in turn, slowing down or stopping the vehicle.  

The brake shoe usually consists of a friction material pad and a metal backing. The friction-material pad makes contact with the drum. The friction between these two parts causes the wheel to slow down and halt.  

Once you remove your foot from the brake pedal, the brake shoe draws back inwardly. This inward motion is caused by the retracting springs. This inward motion also ends the friction. The vehicle, then, will continue to move.  

A self-adjusting screw maintains the minimum gap between the brake shoe and the drum.

Through constant use, the shoe brake’s lining may wear out. This leads to an increase in the gap between the brake shoe and the drum. Mechanics usually adjust the adjuster to maintain the minimum gap.

Drum brake may vary according to how the shoes press onto the drum. If the shoes press on the outside rim of the drum, then it is referred to as “pinch drum brake.” Such brakes, however, are very few and rare. Another related type is the band brake. This type of drum brakes makes use of a flexible band or belt to wrap around the drum. 

Components of Drum Brake

The drum brake system consists of various components. These components need to function in synchrony. The following are the important components of the drum brake. 

1) Brake Drum

The brake drum, as discussed earlier, is made of a unique type of cast iron. This iron is heat-conductive and wear-resistant as well. It is attached to the wheel and axle and rotates with them. When you step on the brakes, the lining radially pushes against the drum’s inner surface. The friction generated stops or slows the wheel and axle’s rotation. This friction, of course, produces a substantial amount of heat.

2) Backing Plate

Without the backing plate, there will be no base for other components. The backing plate, of course, provides that base. The whole set up of the drum brake system also becomes sturdy and rigid because of the backing plate. It supports the housing. It also protects it from debris and foreign materials. Moreover, it absorbs the force (torque) from the braking action. Hence, it is often referred to as “Torque Plate.”

The backing plate needs to be sturdy and wear-resistant. This is because it absorbs a lot of pressure. It also contains the brake-shoe adjuster and levers for emergency use. And these components of the backing plate, of course, are recent additions. 

3) Wheel Cylinder

Each wheel has a wheel cylinder on each brake. Two pistons, however, operate the shoes. Each piston is positioned at each end. The primary shoe is the leading shoe. It is closest to the front of the car. The trailing shoe, on the other hand, is called “secondary shoe.”

The piston cup is acted upon by the hydraulic pressure within the master cylinder. This pressure pushes the pistons against the shoes. In turn, the shoes press toward the drum. Once you release the brake, the shoes spring back to their original position. This happens because of the shoe springs that pull back the brake shoes.     

4) Brake Shoes

We’ve already talked about the brake shoe earlier. Yet, to further describe the brake shoes, they are two pieces of steel that are welded together. There are basically two shoes on each brake assembly. They are the primary and the secondary. The primary, of course, is found near the vehicle’s front. It has linings that are positioned differently from that of the secondary shoe. These two shoes are often interchangeable, likewise.

The shoes have friction material that is riveted to the lining table. This material can also be attached with adhesive. 

The brake shoes also have a crescent-shaped piece referred to as the “Web.” It contains holes and slots. These holes and slots come in various shapes for hold-down hardware and return springs. Parking brakes linkage and self-adjusting components are also slotted into these holes. Moreover, all the application force is received by the web to the lining table and braking lining. 

The lining table’s edge has 3 “V-shaped” notches. On every side. These notches are called “nibs.” These nibs are set against the backing plate’s support pads on which the shoes are installed. 

5) Self-adjuster

The self-adjuster is responsible for maintaining the appropriate gap between the drum and the brake shoe. It is there to ensure that that the gap is there if you don’t press on the brake pedal. The gap lining may increase through wear and tear. Thus, sometimes the gap lining must be adjusted to get the standard minimum gap.

6) Retracting Spring

Once you step on the brake pedal, the brake shoes press against the drum. There should be a mechanism to bring back the shoes to their original positions. This mechanism is called the “return spring” or “retracting spring.” This retracting spring is there for the primary and secondary shoe. 

Replacement and Maintenance for Drum Brake

The brake shoe is the most commonly replaced parts of the drum brake. Some drum brake systems allow you to see the amount of material left on the brake shoes. You can do this through the inspection hole that comes with some drum brakes. 

The brake shoes, of course, bear the brunt of constant use. Thus, the friction material found on the brake shoe usually wears out. If it wears out down to 0.8 mm of the rivets, then you need to replace the brake shoes. If the friction material is attached without rivets, then the wearing out should not go beyond 1.6 mm of material left. 

It is essential to have regular checkups of the brake shoes. If the brake shoes are left for a long time, even if they are already worn out, then they may damage other components. The rivets, for example, that keep the friction material attached to the backing, can cut grooves into the drum. Drums can be repaired, however, by refinishing.

Pros and Cons of Using Drum Brakes

The use of drum brakes comes with its own pros and cons. Some of these pros and cons are the following:

Pros:

Cons:

Why Some Cars Use Drum Brakes Instead of Disc Brakes?

Conclusion

Previous drum brakes made use of asbestos. Thus, in the past, they’d posed serious dangers to maintenance workers who used to replace them. Breathing asbestos fibers can cause mesothelioma. This is a kind of cancer that occurs in the thin layer of tissues that usually cover the internal organs of a person. Many drum brakes manufacturers, however, have already switched to the use of alternative materials. This is due to stricter regulations on the use of asbestos. 

Brake drums, at present, are not that much popular anymore. Some modern cars, however, like the economy cars, still have drum brakes. But these brakes only have secondary braking roles on these vehicles.

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