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How Our Slack Adjusters Work

What an Automatic Slack Adjuster Does

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.

See How Webb Brake Adjusters Work

Start Position

Pre-Set Running Clearance Phase

In the start position the toe of the rack touches the top of the cut-out. On the forward stroke the ASA rotates. The rotation of the ASA means the toe of the rack changes position and now touches the bottom of the cut-out. This is the Pre-set Running Clearance - the rotation of the ASA when no adjustment occurs.

 

Forward Stroke

Excess Clearance Phase

 

If there is Excess Clearance or wear, the stroke will be longer and the ASA will rotate further. The rack will remain touching the bottom of the cut-out through­ out the forward stroke, therefore rotating the pinion in the slip direction of the one-way clutch assembly. As the pinion slips in this direction, the one-way clutch ensures that even though the conical clutch is engaged, the worm shaft does not rotate. The rotation of the pinion in this phase directly corresponds to the adjustment which will be made.

 

Forward Stroke

Elasticity Phase

 

When the brake linings touch the brake drum the ASA continues to rotate due to Elasticity in the brake. The increasing camshaft torque acts through the worm wheel and worm shaft, the coil spring is compressed and the conical clutch disengages. The rack continues to turn the one-way clutch assembly, which now rotates freely being disengaged from the worm shaft.

Forward Stroke

Elasticity Phase

 

On the return stroke the ASA starts to rotate in the opposite direction. The rack is held down, keeping the same position in the cut-out, and rotates the one way clutch assembly, still disengaged from the worm shaft, in the opposite direction. As the brake linings move away from the brake drum the camshaft torque drops, the coil spring de-compresses and the conical clutch re-engages.

Return Stroke

Pre-Set Running Clearance Phase

 

The rack moves with the housing during this phase, therefore the rack does not rotate the pinion. As the ASA rotates through the return stroke, the toe of the rack will move from the bottom to the top of the cut­ out.

Return Stroke

Adjustment Phase

 

The ASA continues to rotate through the return stroke. Held by the cut-out, the rack drives the one-way clutch assembly. The conical clutch is engaged, so the worm shaft rotates and indexes the worm wheel within the housing. This in turn indexes the camshaft to a new position, meaning the Excess Clearance (slack) has been Adjusted and the pre-set running clearance maintained.

End Position

Return Stroke

The ASA must return to its original position for Adjustment to be made. The Adjustment ratio of the ASA mechanism is such that Excess Clearance is not always adjusted for in a single brake application. Over a number of brake applications the correct clearance will be maintained.

Start Position

Start Position

In the start position the cut-out slot in the regulator is open. On the forward stroke the S-ASA rotates. The rotation of the S-ASA closes the cut-out as the regulator pinion rotates but the regulator worm shaft is held by the engaged conical clutch. This is the Pre-Set Running Clearance - the S-ASA rotates but no adjustment occurs.

 

Forward Stroke

Excess Clearance Phase

 

If there is Excess Clearance or wear, the stroke will be longer and the S-ASA will rotate further. With the slot now closed, the one-way clutch in the regulator rotates in the slip direction. As the conical clutch is engaged the regulator worm shaft and clutch worm wheel do not rotate. The rotation of the one-way clutch in the slip direction directly corresponds to the adjustment which will be made.

Forward Stroke

Elasticity Phase

 

When the brake linings touch the brake drum the S-ASA continues to rotate due to Elasticity in the brake. The increasing camshaft torque acts through the main worm wheel and main worm shaft, the coil spring is compressed and the conical clutch disengages. The regulator worm shaft and clutch worm wheel then rotate freely because the one-way clutch is no longer under load and does not slip.

Forward Stroke

Elasticity Phase

 

On the return stroke the S-ASA starts to rotate in the opposite direction. The conical clutch is still disengaged from the worm shaft so the regulator worm shaft and clutch worm wheel rotate freely. As the brake linings move away from the brake drum the camshaft torque drops, the coil spring de-compresses and the conical clutch re-engages.

Return Stroke

Pre-Set Running Clearance Phase

 

The S-ASA continues to rotate. The regulator worm shaft is held by the re-engaged conical clutch but the regulator pinion rotates, opening the slot.f

Return Stroke

Adjustment Phase

 

The S-ASA continues to rotate through the return stroke. The one-way clutch assembly is now rotating in the non-slip direction and drives the regulator worm shaft, rotating the clutch worm wheel. As the conical clutch is engaged the main worm shaft rotates and indexes the main worm wheel within the housing. This in turn indexes the camshaft to a new position, meaning the Excess Clearance (slack) has been Adjusted and the pre-set running clearance maintained.

End Position

Return Stroke

The S-ASA must return to its original position for Adjustment to be made. The Adjustment ratio of the S-ASA mechanism is such that Excess Clearance is not always adjusted for in a single brake application. Over a number of brake applications the correct clearance will be maintained.