PRATT Industries TRUSS - Self-Adjusting Slack Adjuster; Operation, Troubleshooting, and Checks

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position in these conditions, because the slide bogey may start to slide back under the trailer when you pull
forward picking up on the tractor. By locking the slide bogey in the forward position, you eliminate this from
happening in bad conditions.
Self-Adjusting Slack Adjuster
Operation
Upon brake application the slack adjuster rotates and moves the
shoes into contact with the drum. The clearance notch (A)
corresponds to the normal lining-to-drum clearance. As the brake
application continues the rack (B) moves upwards and rotates the
one-way clutch (C) which slips in this direction. As the brake torque
increases the coil spring (D) load is overcome and the wormshaft is
displaced axially releasing the cone clutch. When the brake begins
its return stroke the coil spring load returns to normal and the cone
clutch is again engaged. The rack is pulled back to its original
position in the notch, and any additional travel brought about by
lining wear causes the rack to turn the locked one-way clutch and
rotates the wormshaft through the locked cone clutch. The worm
shaft then rotates the wormwheel and camshaft, adjusting the
brakes.
Trouble Shooting
TIGHT OR DRAGGING BRAKES
Check Foundation Brake Components For:
• Control arm anchor bracket not positioned properly.
• Brake chamber not fully releasing
- Spring brake not fully releasing.
- Pushrod binding on chamber housing
- Air supply not exhausting completely
• Extreme differences in lining-to-drum clearances between shoes on same wheel.
• Broken shoe return spring.
EXCESSIVE CHAMBER PUSH ROD TRAVEL
Check Foundation Brake Components For:
• Loose, broken or bent control arm anchor bracket.
• Worn camshaft bushings
• Binding camshaft.
• Worn clutch assembly.
Operational Check
Functional operation of the slack adjuster can be performed on vehicle by:
1. Block wheels to prevent vehicle from rolling.
2. Check that the push rod is fully retracted; apply air to release spring brake.
3. Manually de-adjust brakes (turn adjustment hex counterclockwise) to create an excessive clearance
condition. (A ratching sound will occur)
4. Make a full service brake application, on release; allow sufficient time for brake to fully retract. During
the brake release, observe rotation of the adjustment hex (attaching a wrench on the hex will make this