Drum Brake Action; RIGHT side rolling FORWARD shown
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During rolling braking, the brake linings don't merely push up against the drum and hold as they do when the vehicle is standing still. Friction between the moving drum and the brake linings drag the brakes shoes in the direction of the rotation as shown. As a result, several important things happen.
1. When the brakes are applied rolling forward, the drag forces cause the primary shoe (with LESS friction material) to rotate away from the anchor pin.
2. The primary shoe pushes the autoadjuster against the secondary shoe.
3. The secondary shoe comes to a stop against the anchor pin, and neither shoe can rotate farther, even though the drag forces are still in effect.
4. These drag forces create a force pivoting the brake shoe outward, creating increased pressure against the drum, called "wedging" or "self-energizing" action. As the shoes are forced outward, the linings wedge themselves tighter to the drum than would occur if the components weren't free to move.
5. In this process, the secondary shoe has more force applied to it than the primary shoe (which is why the primary needs less friction material). Both shoes experience force from the wheel cylinder pistons and both experience drag forces due to the rotation of the drum. But the secondary shoe also gets additional force from the movement of the primary shoe which is transmitted through the self-adjusting link. For this reason, the secondary shoe does more of the braking work than the primary shoe. To take advantage of this, the secondary lining has a larger surface area. It is also thicker because more wear will occur. This process is called "self-energizing" because it causes the brake shoes to apply more braking force than the wheel cylinder alone could apply. When brakes are designed, this effect is planned for. This is why all the brake components are free to move. Without that freedom, the rotating and wedging action wouldn't occur.
See also:
![brake-drum-action.jpg Drum Brake Action; RIGHT side rolling FORWARD shown
IF THE IMAGE IS TOO SMALL, click it.
During rolling braking, the brake linings don't merely push up against the drum and hold as they do when the vehicle is standing still. Friction between the moving drum and the brake linings drag the brakes shoes in the direction of the rotation as shown. As a result, several important things happen.
1. When the brakes are applied rolling forward, the drag forces cause the primary shoe (with LESS friction material) to rotate away from the anchor pin.
2. The primary shoe pushes the autoadjuster against the secondary shoe.
3. The secondary shoe comes to a stop against the anchor pin, and neither shoe can rotate farther, even though the drag forces are still in effect.
4. These drag forces create a force pivoting the brake shoe outward, creating increased pressure against the drum, called "wedging" or "self-energizing" action. As the shoes are forced outward, the linings wedge themselves tighter to the drum than would occur if the components weren't free to move.
5. In this process, the secondary shoe has more force applied to it than the primary shoe (which is why the primary needs less friction material). Both shoes experience force from the wheel cylinder pistons and both experience drag forces due to the rotation of the drum. But the secondary shoe also gets additional force from the movement of the primary shoe which is transmitted through the self-adjusting link. For this reason, the secondary shoe does more of the braking work than the primary shoe. To take advantage of this, the secondary lining has a larger surface area. It is also thicker because more wear will occur. This process is called "self-energizing" because it causes the brake shoes to apply more braking force than the wheel cylinder alone could apply. When brakes are designed, this effect is planned for. This is why all the brake components are free to move. Without that freedom, the rotating and wedging action wouldn't occur.
See also:
[url=http://www.supermotors.net/registry/media/743832][img]http://www.supermotors.net/getfile/743832/thumbnail/tsb930406brakesgrab.jpg[/img][/url]](https://www.supermotors.net/getfile/280806/fullsize/brake-drum-action.jpg)