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Automatic Slack Adjuster

 

Start Position....       Pre-Set Running Clearance   Forward Stroke
 

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. 

how it works 11b asa   3.39 crop
     
Excess Clearance Phase    Forward Stroke
how it works 10b asa 

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 throughout 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.

3.42 crop  3.49crop 
.     
Elasticity Phase    Forward Stroke
how it works 9.30b asa 

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.

  3.58cropb
.     
Elasticity Phase   Return Stroke
 how it works 10b asa

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.

  4.09 crop 
.     
Pre-Set Running Clearance Phase    Return Stroke
how it works 11b asa

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.

 
  4.18crop
.     
Adjustment Phase   End Position Return Stroke
how it works 12 asa

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. 

how it works 12 asa end

 

4.29 crop

 

Self-Setting Automatic Slack Adjuster

 

Start Position. Pre-Set Running Clearance   Forward Stroke
  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.    3.31sasa
.
   
Excess Clearance Phase .    Forward Stroke
  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.    
.

   
Elasticity Phase     Forward Stroke
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.   
 ..      
Elasticity Phase     Return Stroke
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.    
 .      
Pre-Set Running Clearance Phase    Return Stroke
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    
 .      
Adjustment Phase   End Position Return Stroke
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.

 

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.

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