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Picking up the slack

GUELPH, Ont. - Every single year, CVSA inspectors write more tickets for "brakes out of adjustment" than for any other defect. Automatic slack adjusters are now state-of-the-industry, but the word "au...


Actuator RodClevisHousingBoot & StrapActuator PistonWorm GearPull PawlAdjusting PawlGrease FittingWormAdjusting Nut (or Hex)(Illustration by ArvinMeritor)Worm Seal
Actuator Rod

Clevis

Housing

Boot & Strap

Actuator Piston

Worm Gear

Pull Pawl

Adjusting Pawl

Grease Fitting

Worm

Adjusting Nut (or Hex)

(Illustration by ArvinMeritor)

Worm Seal

GUELPH, Ont. – Every single year, CVSA inspectors write more tickets for “brakes out of adjustment” than for any other defect. Automatic slack adjusters are now state-of-the-industry, but the word “automatic” is a bit of a misnomer.

There are still some keys to properly selecting, installing and maintaining your slacks that can keep you from handing over your razor-thin profits to North America’s various jurisdictions and enforcement agencies.

The first, and most important, issue stressed by all manufacturers, is that no automatic slack adjuster can compensate for problems and deficiencies in the foundation braking system. So make sure the braking system is in a good state of repair when new slack adjusters are installed.

When selecting new auto slacks, you can really go with any manufacturer. However, if you exercise a little brand loyalty, the replacement process may get a little easier. Each OEM generally has a different clevis design, so going with new adjusters that are compatible (some manufacturers do build units that can accommodate other clevis designs) will simplify things.

Most clevis’ on automatic brake slack adjusters are double action units. The clevis is the key to engaging the push rod when the brakes are applied and they also generally make the constant adjustments that are intended to keep auto slacks in line. One of the only exceptions to this would be Haldex, which has a different actuating mechanism. When the component is installed, the technician must rotate a control arm into position and then all of the adjustments are made internally.

Most manufacturers also have a similar adjusting hex that essentially work the same way. After tightening the lining down until it just contacts the drum, use the adjusting hex to back off approximately one half of a turn. The technician should hear a loud ratcheting sound during this final step. There is one notable exception to this procedure: the ArvinMeritor system is unique in that on the side of the adjuster there is an assembly called a “pawl”. It is an internal locking mechanism for the auto slack. To adjust it properly, simply pry out on the pawl while backing off the adjuster, otherwise you’ll strip the internal gears.

In either case this will generally put the brakes close to where they need to be and then the adjuster will tighten them down in the course of operation.

All automatic slack adjusters feature a grease fitting and their own unique lubrication requirements. However, one key with all manufacturers is to avoid any grease high in molybdenum. While quite popular in Canada – they are simply too slippery for auto slacks. n


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