The automatic slack adjuster (ASA) is an important part of keeping truck brakes operating at peak performance and up to regulatory standards and like other parts, inspection and lubrication are critical for optimal performance.
How does the slack adjuster work?
When the brake is applied, a rod pushes out on the slack adjuster, which then turns the S-cam to force the brake shoes apart to apply the friction necessary to slow the truck. The automatic slack adjuster is designed to compensate for brake lining and drum wear to maintain a constant actuator stroke.
Automatic slack adjusters often are not greased regularly during scheduled maintenance and inspections.
All too often, we see automatic slack adjusters that are not being greased regularly during scheduled maintenance and inspections,” said Frank Gilboy, product manager, automatic slack adjusters at Bendix Spicer Foundation Brake LLC, a joint venture between Bendix Commercial Vehicle Systems LLC and Dana Commercial Vehicle Products, LLC. “And slack adjusters that don’t perform properly through lack of lubrication can have a direct impact on both safety and regulatory compliance.”
Greasing the ASA serves two vital purposes: Forcing the new lubricant in purges the old grease from the adjuster, along with any water or contaminants that have found their way in; and it protects the adjuster’s internal gear sets, clutches, and other components from wear. Both are necessary to keep the ASA working properly to maintain correct brake stroke and provide optimal stopping performance.
Lubrication is a simple process, regardless of the adjuster’s manufacturer. Just locate the ASA’s grease fitting, attach the hose from a grease gun, and inject new grease until old grease is forced through the release opening. The release location varies depending on the maker – for the Bendix® Versajust® LS™, it’s on the boot; for others, it’s a hole in the casting – and manufacturer instructions will offer advice on how much grease to purge from the adjuster, and what lubricant to use.
It should easily take less than a minute per adjuster,” Gilboy said. “Bendix recommends greasing with an NGLI grade-2 lubricant every 30,000 miles, but you can also make things less complicated by greasing the ASAs every time you do a preventive maintenance inspection.”
Pre & post trip inspections
As part of pre- and post-trip inspections, drivers should check the slack adjuster for correct operation, lose or missing hardware, a missing cotter pin, bent or damaged components such as the control arm or attaching anchor brackets, and worn clevises, clevis pins and clevis pin bushings.
Fleets that run lightly loaded are more susceptible to vibration.
Once the ASA itself has been inspected, use the following checklist to examine other wheel-end components and fix any situations that can lead to overstroke. Where component replacement is necessary, Bendix stresses the use of OEM-equivalent equipment to ensure correct operation and best performance.
- Check for proper lubrication of the brake camshaft and cam tube.
- Inspect the cam head and rollers for abnormal wear.
- Evaluate the brake’s friction blocks for wear level and cracks, replacing the friction if necessary.
- Inspect the camshaft bushings for wear by checking radial and axial endplay, following the brake manufacturer’s guidelines. If necessary, replace the bushings.
- Check the brake drums for maximum diameter and surface finish, and replace if necessary.
- Check the brake shoe return springs to ensure a proper return to the “zero stroke” position.
- Inspect the return spring on the air chamber or on the service side of double diaphragm spring brakes.
- Inspect the clevis pins connecting the chamber to the automatic slack adjuster to ensure they turn freely and are coated with an anti-seize lubricant.
No manual adjustments
Proper maintenance of automatic slack adjusters also means no manual adjustments.
Manual adjustment may seem to temporarily solve the out-of-adjustment issue, but there may be other factors at play that must be addressed. As always, it’s critical to maintain, adjust, or replace slack adjusters.