while the other rod connects the bell crank and the adjusting lever. In this configuration. the self-adjuster works only in reverse direction. As the vehicle is backing up and the brakes are applied, the adjusting process is as follows:

1. The secondary, shoe moves away from the anchor because of the self-energizing action.

2. The pivot point of the bell crank is moved in the direction of rotation.

3. The lever moves up on the star wheel through the connection of the linkage. If enough clearance is available between the brake shoes and the drum, the lever will engage another tooth on the star wheel. As the brakes are released. the shoes retract and the return spring helps force the adjusting lever down, rotating the star wheel and expanding the adjusting screw to remove excess shoe-to-drum clearance.

Lever type - The lever type self-adjusting system (fig. 7-13) is similar to the link type, in that it operates in reverse direction only. While the link type system uses linkage rods to perform the adjusting process, the lever type uses a stamped metal lever to engage the star wheel and actuating link to connect to the anchor pin. The adjusting process is the same as the link type system.

Brake Shoe Energization

The primary function of the brake drum assembly is to force the brake shoes against the rotating drum to provide the braking action. When the brake shoes are forced against the rotating drum. they are pulled away from their pivot point by friction. This movement, called self-energizing action (fig. 7-14), draws the shoes tighter against the drum.

As the brake actuating mechanism forces the brake shoes outward, the top of the brake shoe tends to stick or wedge to the rotating brake drum and rotates with it. This effect on brake shoes greatly reduces the amount of effort required to stop or slow down the vehicle.

With most drum brake designs, shoe energization is supplemented by servo action. When two brake shoes are linked together, as shown in figure 7-14, the application of the brakes will produce a self-energizing effect and also a servo effect. Servo action is a result of the primary (front) shoe attempting to rotate with the brake drum. Because of the fact that both shoes are linked together, the rotating force of the primary shoe applies the secondary, (rear) shoe.

In the forward position, the anchor point for both brake shoes is at the heel of the secondary, shoe. As the vehicle changes direction from forward to reverse, the toe of the primary shoe becomes the anchor point, and the direction of self-energization and servo action changes (fig. 7-14). The most popular brake drum configurations (fig. 7-15) are as follows:

Single anchor, self-energizing servo action (fig. 7-15) - In this configuration both brake shoes are self-energizing in both forward and reverse directions. The brake shoes are self-centering and provide servo action during brake application. This system is provided with one anchor pin. which is rigidly mounted to the backing plate and is nonadjustable. Both forward and reverse torque is transmitted to the backing plate through the anchor pin. One wheel cylinder with dual pistons is used in this system.

Single anchor, self-centering (fig. 7-15) - In this configuration only the primary brake shoe is self-energizing in the forward direction; therefore. it provides the majority of the brake force. This system is self-centering in that the lower brake shoe anchor does not fix the position of the brake shoes in relation to the drum. The shoes are allowed to move up and down as needed. Some systems provide eccentric cams for front and rear brake shoe adjustments. One wheel cylinder is provided in this system.

Double anchor, single cylinder (fig. 7-15) - In this arrangement, each brake shoe is anchored at the bottom by rotating eccentric-shaped anchor pins. Only the primary shoe is self-energizing. and the system does not develop servo action. Spring clips are used in the middle of the shoe to hold the shoes against the backing plate. Brake shoes are adjusted manually by rotating the anchor pins. One wheel cylinder is provided in this arrangement.

Double anchor. double cylinder (fig. 7-15) - In this system the brake shoes are provided with an anchor at each heel. The anchors are eccentric-shaped to allow for adjustment and centering. Each shoe has a single piston wheel cylinder mounted at the toe of the brake shoe. Which allows both shoes to be self-energizing in the forward direction only. Eccentrics mounted in the middle of the shoe also allow for brake adjustment.

Disadvantages of Drum Brakes

The drum brake assembly, although well suited for wheeled vehicles, has some disadvantages. One 7-12