Slack adjusters are the link between a brake chamber (booster) or actuator, and the S-Cam brake camshaft. Its arm is fastened to the push rod with a yoke (clevis) and its spline is mounted on the foundation brake cam shaft.
The slack adjuster transforms and multiplies the force developed by the brake booster into torque which applies the brake via the brake cam shaft. The length of the lever arm of the slack adjuster is always perpendicular between the centre line of the brake cam shaft opening and the centre line of the clevis pin opening in the arm. They provide an operating lever between the brake booster and the brake actuating cam.
Clevis pin holes in the slack adjuster arm are measured in inches (except for the BPW slack adjusters where the increments are in steps of 15 mm). The bottom hole is normally five-inch (127 mm) from the centreline of the cam shaft while the next hole up is 6” (152 mm) from the centreline of the cam shaft. Only these two holes are required in South Africa on a foundation brake.
Both slack adjusters have an adjusting mechanism, providing a means of adjusting for brake lining wear. In this article we will focus on the manual slack adjusters and will be followed by a separate article dedicated to automatic slack adjusters.
To compensate for the gradual wear on the brake shoe lining, manual slack adjusters are equipped with an adjusting mechanism, which provides a means of adjusting for brake lining wear. Slack adjuster models are available in various arm configuration, lengths and spline types. Manual slack adjusters require manual adjustment to compensate for brake lining wear and the associated drum to lining clearance.
Adjustment is usually performed by rotating the hexagon nut on the slack adjuster body to set a lining to drum clearance of about 0.50 mm – 0.76 mm.
The checking procedure involves:
- Park the vehicle (truck/trailer) on level ground;
- Turn off the parking brake so can move the slack adjusters;
- Pull hard on each slack adjuster that you can reach;
- If a slack adjuster moves more than 25mm where the push-rod is attached, it probablyrequires adjustment.
- Out-of-adjustment brakes are the most common problem found in roadside inspection.
- Be safe and check the slack adjuster movement.
Should you require any additional information, contact your nearest JMR branch.
Click here to read the article "The facts about Automatic Slack Adjusters"