While automatic brake adjusters don’t need a lot of attention, they are far from “set it and forget it.” Like their manual counterparts, periodic inspection and lubrication are critical to optimal performance.
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.
“Dimensions and the geometry have to be maintained throughout the life of the pad in order to keep the performance and efficiency of the brake up,” said Joe Kay, director of brake engineering for Meritor. “That’s where the automatic slack adjuster comes into play.”
Inadequate stroke can have negative implications on braking power, making the ASA a vital component.
Keith McComsey, director of marketing and customer solutions for wheel ends at Bendix Spicer Foundation Brake, said ASAs often are not greased regularly during scheduled maintenance and inspections.
“Slack adjusters that don’t perform properly through lack of lubrication can have a direct impact on both safety and regulatory compliance,” he said.
Properly lubricating the ASA purges old grease, water and contaminants from the adjuster and protects its internal gear sets, clutches and other components from wear.
McComsey said lubrication with an NGLI grade-2 lubricant at least every 30,000 miles should take less than a minute per adjuster. “You can also make things less complicated by greasing the ASAs every time you do a preventive maintenance inspection,”he said.
Mitch Forbes, senior technical service representative for Haldex, recommends at least an annual lubrication interval with a standard chassis grease applied by hand. Routine visual inspections and operational checks, including brackets and control arms, should be performed at each preventive maintenance service interval. Adjusters or anchor brackets that show damage or fail operational checks should be replaced immediately, Forbes said.
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, which Kay said can accelerate brake stroke issues.
“You want to make sure the clevis pins are inspected, because the more vibration they get, the more wear they get,” he said. “Pins wearing inside the holes can add extra clearances, and those extra clearances can add up to extra stroke when applying the brake.”
McComsey also recommends checking the brake shoe return springs to ensure a proper return to the zero-stroke position.
“If the stroke is too long, the brake chamber would essentially go to its maximum length, and the force of the brake drops,” Kay said. “[Stroke] is one of the first things that would indicate that something’s not right with this particular brake.”
Duty cycle also can affect a slack adjuster’s life, but Kay said that in typical line-haul applications, it’s common for ASAs to last for the life of the truck with basic maintenance.
However, segments that feature more starting and stopping would need a maintenance cycle that includes ASA replacement. “Every component on the brake is going through a fatigue cycle,” he said.
No manual adjustments
Proper maintenance of automatic slack adjusters also means no manual adjustments. “The only times that an automatic brake adjuster needs to be adjusted is during a foundation brake repair or initial setup,” Forbes said. “Manually adjusting a brake adjuster causes premature wear on internal components and leads to the adjuster becoming inoperable.”
McComsey said there are many reasons an ASA may fall out of adjustment, but it is unlikely that a manual adjustment will correct the problem.
“Manual adjustment may seem to temporarily solve the out-of-adjustment issue, but there may be other factors at play that must be addressed,” he said. “Otherwise, the condition will reoccur.”