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"

With the recent accident on the N12 still fresh in our minds, the following is an article which relates to poor trailer maintenance and what you can do to rectify it.

It is a well-known fact that the average age of a trailer in this country is anything between 12 and 15 years. Anybody who has been to a transport workshop on a Saturday will always see a lot of activity servicing the prime mover – the truck. But where are the trailers? There is hardly ever any space for them in the workshop, because most of the time they get loaded for the next trip.

There is no doubt in my mind that it is an accepted part of the transport culture that proper trailer maintenance is of no importance, which is quite ironic as it carries the core of your business - your stock.

Rusty and dented drop side doors, missing mud flaps, scratched and dented chevron boards, and damaged reflectors are all signs that the trailer gets the raw end of the deal.

Imagine this: A new powerful 400-horsepower truck from Europe – fitted with the latest disc brake design – standing in the yard, coupled to a trailer with standard 16 ½ x 7 drum brakes. The trailer might have loose mounting brackets for the brake valves and for the spring brakes (boosters), the load sensing valve has lost the linkage arm, permanently running fully open or at half pressure. The slack-adjusters settings are most probably wrong and a garden hose is used in the place of high pressure piping. One axle has automatic slack adjusters fitted, while the other axle runs with manual slack-adjusters. This is a common sight.

Let’s face it, a trailer is not forever, but by following a few simple trailer maintenance rules will pay off at the end:

• Do not compromise on safety critical items like your brake drums, shoes and linings;
• Do not mix brake linings on the same axle;
• Remove worn out brake linings and get them relined;
• Replace worn cam shafts bushes, damaged rollers and return springs;
• When replacing the bearings, replace the cup and cone together;
• Don’t use pirate, reconditioned or below standard trailer valves;
• Use automatic slack-adjusters;
• Convert the old braking system to EPS and ABS;
• Make sure your load sensing valve is working;
• Do use the same type of brake boosters with the same lever length.

JMR operates certified trailer workshops in Alrode, Vryheid and Empangeni. Click here for a complete list of our services.

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" ?