Slack adjuster: manual vs automatic checks

A slack adjuster is the lever that links the brake chamber pushrod to the brake camshaft on an air-braked trailer or truck. When the chamber pushes, the adjuster turns the camshaft and the S-cam spreads the shoes against the drum. As the lining and drum wear, it takes up the extra gap so the brake still applies within a safe stroke. This guide explains the difference between a manual and an automatic slack adjuster, what to check at each service, and what to send us when you need a replacement.

Manual or automatic slack adjuster

There are two kinds, and they are serviced in different ways.

A manual slack adjuster holds a fixed setting. A mechanic turns the adjusting nut by hand to set the shoe clearance, and must do it again as the lining wears. The York 730101/3 we supply is this type: a manual adjuster with a SAE 10 spline, in 5, 6 or 7 inch arm lengths, listed for all York brake sizes. Details are on the York 730101/3 slack adjuster page.

An automatic slack adjuster resets itself during braking. The UK government's automatic slack adjuster maintenance guide describes two designs:

• Clearance sensing adjusters keep a set gap between the lining and the drum. Haldex and Bendix make this type.
• Stroke sensing adjusters keep the chamber pushrod stroke at a set length. BPW and Meritor make this type.

The BPW 05.174.65.13.0 is an automatic adjuster from the BPW Transpec range. It is a multi-hole version with lever lengths of 120, 135, 150, 165 and 180 mm. See the BPW 05.174.65.13.0 slack adjuster page.

Several countries require automatic adjusters on new heavy vehicles. In the United States, 49 CFR 393.53 covers air-braked commercial vehicles built on or after 20 October 1994. In the UK, the guide above applies the rule to trailers over 3.5 tonnes built from 1 April 1995. Check the rules that apply to your own fleet and permits.

Why you should not keep adjusting an automatic one

This is the most common mistake in the workshop. If the stroke on an automatic adjuster is too long, turning the adjusting nut brings it back into range for a while, but it does not fix the cause.

The Bendix technical tip on automatic slack adjusters says a properly installed, working and greased automatic adjuster should not need manual adjustment after setup or a reline. Manual adjustment usually hides the real problem, so the brake goes out of adjustment again. The UK guide adds that repeated manual adjustment wears the adjuster's internal parts and leads to early failure.

The US inspection standard, Appendix A to Part 396, takes the same view. A brake found out of adjustment must be checked for the reason first. It lists examples such as broken welds, loose mounting hardware, cracked drums and worn bushings.

Manual adjustment of an automatic unit is normal only when you fit it, repair the brakes or reline the shoes.

What to check at each service

The UK guide lists five checks for every maintenance inspection. They take a few minutes per wheel end:

1. Measure the pushrod stroke with the brake fully applied and compare it with the maker's limit. As a rough guide, the stroke should not be more than two thirds of the chamber's maximum stroke.
2. Look at the control arm, bracket and yoke for cracks, bending and broken bolts. These can mean the adjuster was fitted wrongly or the camshaft bearing is worn.
3. Check that the adjuster moves freely and returns fully. If it does not return, it cannot adjust.
4. Check the camshaft for end float and wear. The guide gives 0.5 mm as the most end float you should see.
5. Grease the adjuster until clean grease comes out, unless it is a sealed type, and grease the clevis pin.

The stroke should be the same on both sides of an axle. The UK guide says to replace an adjuster that needs manual adjustment in service or one that over-strokes.

Our article on trailer brake lining wear covers the lining and drum checks that go with these.

When to replace the adjuster and the camshaft parts

Replace the adjuster when it fails the checks above, when the stroke keeps growing after a correct setup, or when the body or arm is cracked. Look at the camshaft at the same time. A worn camshaft bush lets the cam move and the new adjuster will not hold its setting.

For BPW axles we supply the BPW 09.801.06.09.2 brake camshaft kit with iron bush, and the BPW 09.801.06.09.1 camshaft bearing repair kit. Other BPW and York brake parts are listed on the brakes parts page, and the BPW axle and suspension page and York brake assembly page show what we supply for those axles.

When you fit a new automatic adjuster, follow the maker's setup steps, because they differ by brand. Match the lever length and spline to the old unit.

What to send us for a quotation

The adjuster must match the camshaft spline, the lever length and the axle. Send as many of these as you have:

• The part number cast or stamped on the old adjuster.
• Whether it is manual or automatic, and the brand.
• The axle brand and type, plus a photo of the axle nameplate.
• The lever length from the camshaft centre to the clevis pin hole, and the spline count.
• The quantity you need.

If the number has worn off, our guide on how to find the part number on an old trailer part shows where else to look. You can also search the number on the spare parts search. For the wider range of trailer parts, see the trailer and truck spare parts guide. When you are ready, request a quotation and we reply by email with availability and lead time.