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.

Manual 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:

1. Park the vehicle (truck/trailer) on level ground;
2. Turn off the parking brake so can move the slack adjusters;
3. Pull hard on each slack adjuster that you can reach;
4. If a slack adjuster moves more than 25mm where the push-rod is attached, it probablyrequires adjustment.
5. Out-of-adjustment brakes are the most common problem found in roadside inspection.
6. 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"

Automatic slack adjusters perform the same function as the manual slack adjuster, except that it automatically adjusts for lining wear. It does not require periodic manual adjustment, however the slack adjuster does provide for manual adjustment if necessary.

Automatic slack adjusters are fast becoming the industry standard because they offer the most reliable and consistent operation. The result is consistent brake adjustment and optimum brake performance, which increases profits through less downtime and lower operating costs. In South Africa trailers above 3500 kg GVM, first registered from 14 February 2004 must comply with SANS 20013. This requires that they have automatic slack adjusters fitted.

There are many different automatic slack adjusters on the market today which JMR supplies. Contact your nearest JMR branch to find out more.

Two types of automatic slack adjusters are available: the stroke adjuster type and the clearance adjuster type.

Stroke Adjuster:                   

Measures the stroke of the pushrod and keeps the stroke constant. It does not take the elasticity of the brake or the driver’s way of braking (hard or soft) into consideration.

Clearance Adjuster:

Measures the clearance between the drum and lining and keeps this constant. The moment the lining comes into contact with the drum the adjusting mechanism disengages and this means that the brake adjusters’ automatic adjustment is not affected by the elasticity of the brakes or the driver’s way of braking. This minimises the risk of insufficient or excessive clearance between the lining and the drum.

Please Note:

Most automatic slack adjusters are clearance sensitive adjusters and not stroke sensitive adjusters. These adjusters will only adjust the brakes if the clearance is bigger than a set limit irrespective of the stroke of the brake booster.

All adjustments on a clearance sensitive adjuster are done on the return stroke and not during the brake application stroke. It is important to know which slack adjuster you have on the cam shaft as each one operates different from one another. Always remember to keep the same brand on the same axle – do not mix.

Automatic slack adjusters should not have to be manually adjusted except when performing maintenance on the brakes and during installation of the slack adjusters.  Manual adjustment of an automatic slack adjuster should only be used to correct the adjustment in an emergency situation as it is likely the brake will soon be back out of adjustment since the procedure usually does not fix the underlying adjustment problem.

Manual adjustment of slack adjusters is dangerous because it gives the operator a false sense of security about the effectiveness of the braking system. Installing an automatic - slack adjuster incorrectly can result in many different problems with the most commonly occurring problem being over-adjustment. If that is the case the adjuster must be treated as a new installation in order to correct the problem.

Facts about automatic slack adjusters:

1. Do not manually adjust slack adjusters for any reason other than initial adjustment after brake overhaul;

2. Never use an impact wrench to adjust or internal slack adjuster damage will occur;

3. Grease the slack adjuster if required;

4. Automatic Slack Adjusters need to be replaced in pairs;

5. When re-lining brakes, remove the slack adjuster from the S-cam and rotate the worm wheel 90 degrees before refitting. This prevents the gear wheel from operating in the same range all the time and reduces wear of the worm shaft;

6. Make sure that the adjustment arm is securely fitted to the bracket.

Doesn’t it make sense to treat your automatic slack adjusters with utmost care?

Did you read the article "Manual Slack adjusters - What you need to know" ?