The Automatic Slack Adjuster (ASA) is a vital part of the foundation brake. It acts as a lever, forming the connection between the actuator and the camshaft.  The function of the ASA is to compensate for wear in both brake lining and brake drum while maintaining a constant stroke of the actuator.

When the brake is applied the actuator moves the ASA forward. The worm wheel rotates the camshaft, in turn moving the brake lining toward the brake drum. This is the pre-set running clearance.

The ASA continues to move until the brake lining touches the drum. If lining and/or drum has worn the ASA mechanism engages and accounts for slack or wear in the system. This is the excess clearance for which adjustment is made.

When the brake lining contacts the drum, the reacting force causes the ASA mechanism to dis-engage. This dis-engagement means adjustment is not made for any elasticity within the foundation brake.

When the brake is released the reacting force drops and the ASA mechanism re-engages. If excess clearance (wear) is in the system, the worm wheel in the mechanism will not return to its original position - it will have indexed. This means the camshaft will also have indexed, keeping a constant gap between lining and drum. 

The ASA, however, does return to its original position, ensuring constant stroke is maintained.