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Introduction to Drum Brakes

Drum brakes use the brake transmission mechanism to make the brake shoes to press the brake friction pads on the inside of the brake drum, thus generating braking force to slow down the wheels or stop them within the shortest distance as needed to ensure driving safety and to protect the car from automatically sliding when parked reliably.

1.Concept Introduction

Drum brakes, also known as block brakes, rely on the brake blocks to be compressed on the brake wheels to achieve braking.

Drum brakes were an early design of braking system, and their drum design was used on horse-drawn carriages in 1902, and did not begin to be widely used in the automotive industry until about 1920.

The mainstay of drum brakes is the internally tensioned type, which has a brake block (brake shoe) located on the inside of the brake wheel, and when the brake is applied, the brake block opens outward and rubs the inside of the brake wheel to achieve the purpose of braking.

In the past three decades, drum brakes have gradually given way to disc brakes in the car segment.

However, due to the relatively low cost, they are still used in some economic cars, mainly for braking the rear wheels and parking brakes with a relatively small load.

 

2. Classification and composition of drum brakes

According to the direction of brake shoe movement
Drum brakes use the brake shoes to squeeze the brake drum to obtain braking force, and can be divided into two types: internally tensioned and externally applied.

Internal tension drum brakes use the inner cylindrical surface of the brake drum as the working surface, and are widely used in modern automobiles; external beam drum brakes use the outer cylindrical surface of the brake drum as the working surface, and are currently used only as parking brakes on a very small number of cars.
By actuating device
Figure 1 by actuator
Figure 1 by actuator
Drum brakes can be divided into wheel cylinder brakes and cam brakes according to the form of brake shoe opening device (also called actuating device), as shown in Figure 1. The wheel cylinder brake uses a hydraulic brake wheel cylinder as the brake shoe actuator, which is mostly used in hydraulic braking systems; the cam brake uses a cam as the actuator, which is mostly used in pneumatic braking systems.

By brake shoe force
Fig. 2 Force by brake shoe
Figure 2: Brake shoe force
Wheel cylinder brakes are divided into different types of brake shoes, such as leading trailing shoe, double leading shoe (one-way action, two-way action), double trailing shoe, and self-increasing force (one-way action, two-way action), as shown in Figure 2.


(1) Leading shoe brake (leading trailing shoe brake)

Figure 3 Leading trailing shoe type
Figure 3 Leading shoe brake
The structure of the leading trailing shoe brake is shown in Figure 3. The brake base plate 5 is fixed to the rear axle housing or the front axle steering knuckle flange, and the lower part of the brake base plate is equipped with two eccentric adjusting screws 1, and the two brake shoes 11 and 12 have holes at the lower end, which are set on the eccentric adjusting screws and locked with lock nuts 3.
The middle of the brake base plate is equipped with two brake shoe brackets 4 to limit the axial position of the brake shoe.

The upper end of the brake shoe is pulled against the top block of the brake wheel cylinder 9 with a return spring 10.

The brake wheel cylinder, which is the brake shoe actuator, is also screwed to the brake base plate. The brake drum is mounted on the flange of the wheel hub and rotates with the wheel.


The leading shoe brake has a stable braking performance, simple and reliable structure, easy to install, and is widely used as the front and rear wheel brakes of trucks and rear wheel brakes of cars.

(2) Two leading shoe brake

Figure 4 One-way double leading shoe brake
Figure 4 One-way two leading shoe brake
When the brake drum is rotating in the forward direction, the two leading shoe brakes with both brake shoes as leading shoes are called double leading shoe brakes. This is shown in Figure 4.

The two brake shoes are each actuated by a single piston brake wheel cylinder 2, and the two sets of brake shoes, brake wheel cylinders, supporting pins and adjusting cams are arranged centrally symmetrically on the brake base plate in lieu of the axisymmetrical arrangement in a collar-shoe brake. The two brake wheel cylinders of equal diameter can be connected by oil pipes, so that the oil pressure is equal. In this way, when the car is moving forward, both brake shoes are collar shoes; but in reverse, both brake shoes become slave shoes. It can be seen that this double-collar shoe brake has a one-way effect, good braking efficiency in forward, braking efficiency in reverse is greatly reduced, and inconvenient to install the parking brake, so it is generally not used as a rear wheel brake; however, the two brake shoes are subjected to the same force, uniform wear, and the brake shoes act on the brake drum force is balanced, that is, the one-way double-collar shoe brake is a balanced brake.
Figure 5 Two-way double-collar shoe brake
Figure 5 Two-way double-collar shoe brake

If the two brake shoes of the one-way double-collar shoe brake can be made to interchange the positions of the supporting pins and the actuation points, then the same braking effect can be obtained when braking in reverse as when braking in advance.

The design of the dual two leadingshoe brake is based on this idea, and the brake shoes of this type of brake are both leadingshoe when the brake drum rotates in forward and reverse, as shown in Figure 5.
If the left and right side wheel brakes of a car equipped with a double lead shoe brake are installed in pairs, it becomes a double trailing shoe brake in which both brake shoes are from the brake drum when it is rotating in the forward direction.

Obviously, the double trailing shoe brake forward braking efficiency is lower than the collar from the shoe brake and double collar shoe brake, but its braking efficiency is less sensitive to changes in the friction factor, that is, good stability of braking efficiency, only in a few to ensure the reliability of the brake on the senior car used.

If the two brake shoes of the one-way double-collar shoe brake can be made to interchange the positions of the supporting pins and the actuation points, then the same braking effect can be obtained when braking in reverse as when braking in advance.

The design of the dual two leadingshoe brake is based on this idea, and the brake shoes of this type of brake are both leadingshoe when the brake drum rotates in forward and reverse, as shown in Figure 5.
If the left and right side wheel brakes of a car equipped with a double lead shoe brake are installed in pairs, it becomes a double trailing shoe brake in which both brake shoes are from the brake drum when it is rotating in the forward direction.

Obviously, the double trailing shoe brake forward braking efficiency is lower than the collar from the shoe brake and double collar shoe brake, but its braking efficiency is less sensitive to changes in the friction factor, that is, good stability of braking efficiency, only in a few to ensure the reliability of the brake on the senior car used.

(3) Self-energizing brake (servo brake)

Self-energizing brakes can be divided into one-way self-energizing brakes (uni-servobrake) and two-way self-energizing brakes (doo-servo brake), which differ in structure only in the number of pistons in the brake wheel cylinders.
The uni-servo brake is only self-reinforcing when the car is moving forward, using a single piston brake wheel cylinder; the bi-directional self-reinforcing brake can be self-reinforcing when the car is moving forward or braking in reverse, using a double piston brake wheel cylinder.

The force-boosting principle of the self-boosting brake is to use the floating articulated brake shoes of the adjustable top rod body to replace the fixed eccentric pin brake shoes, and to use the front shoes to push the rear shoes, so that the total friction torque can be increased to play the role of automatic force-boosting.

As shown in Figure 6 is a one-way self-increasing force brake. The upper end of the first brake shoe 1 and the second brake shoe 6 is pulled together by the respective brake shoe return spring 2, and riveted to the upper end of the web on both sides of the concave curved surface of the cleat 3 supported by the support pin 4.

The lower end of the two brake shoes to the concave plane were floating support in the adjustable top bar body ends of the straight groove on the bottom surface, and tensioned by the tensioning spring 8.

Figure 7 Two-way self-increasing force
Figure 7 Bi-directional self-increasing force

Figure 7 shows a bi-directional self-energizing brake.

The upper end of the brake shoe is riveted with cleats 4 on both sides, and the cleats are pulled against the support pins by the front and rear shoe return springs 6 and 3, and the lower ends of the two brake shoes are pulled against the bottom surface of the straight slots at both ends of the adjustable top bar body 8 by the tensioning spring 9.

The adjustable top bar body is floating.

The brake wheel cylinder is in a position slightly below the support pin.

Under the same conditions of basic structural parameters and brake wheel cylinder operating pressure, the self-increasing brake has the best braking efficiency due to the use of frictional force, but its braking efficiency is the most dependent on the friction factor, and therefore its stability is the worst; in addition, the growth of the braking torque of the self-increasing brake in the braking process appears to be too rapid in some cases.

Therefore, one-way self-reinforcing brakes are only used on the front wheels of medium and light vehicles, while two-way self-reinforcing brakes are widely used on the rear wheels of cars because they can also be used as parking brakes.

3.Working Principle

In the car brake drum, generally only one wheel cylinder, in the brake wheel cylinder from the total pump fluid force, the wheel cylinder piston at both ends will simultaneously top to the left and right brake shoes hoof end, the force is equal. But because the wheel is rotating, the brake drum acts on the brake shoe pressure is not symmetrical, resulting in self-increasing force or self-reducing force.

Therefore, the industry will increase the force of the side of the brake shoe called the collar shoe, the side of the brake shoe called the self-reducing force from the shoe, the friction torque of the collar shoe is 2 to 2.5 times from the shoe, the two brake shoe friction lining wear degree is not the same.
In order to maintain good braking efficiency, the brake shoes and the brake drum to have an optimal clearance value.

As the friction lining wears, the gap between the brake shoes and the brake drum increases, and a mechanism for adjusting the gap is required.

In the past, the clearance of drum brakes needed to be adjusted manually, using a stopper to adjust the clearance.

After the improvement of the car drum brakes are automatically adjusted, the friction lining will automatically adjust the gap with the brake drum after wear.

When the clearance increases and the brake shoe is pushed out beyond a certain range, the gap adjustment mechanism will pull the adjusting rod (pawl) to the


position where the next tooth is engaged with the adjusting tooth, thus increasing the length of the connecting rod, displacing the brake shoe position and restoring the normal clearance.
Drum brakes for cars are generally used on the rear wheels (disc brakes for the front wheels).

Drum brakes in addition to the relatively low cost, there is a benefit, is to facilitate the combination with the parking (parking) brake together, where the rear wheel for the drum brake car, the parking brake is also combined in the rear wheel brake.

This is a mechanical system, it is completely separate from the car brake hydraulic system: the use of hand lever or parking pedal (American cars) to tighten the steel cable, the drum brake bar to extend the brake shoes, to play the role of parking brake, so that the car will not skid; release the steel cable, the return spring to restore the brake shoes to their original position, the braking force disappeared.