According to the National Transportation Safety Board (NTSB), a recent runaway truck accident in Pennsylvania, which killed two people and injured others, was likely the result of improper brake maintenance, including manually adjusting automatic slack adjusters (ASAs), a practice that should be avoided.
“We’ve seen cases where ASAs have been adjusted and backed off 1/4 turn repeatedly as routine maintenance,” says Dick Radlinski of Link-Radlinski, based in East Liberty, Ohio. “That’s not a good practice, and can ultimately damage some ASAs by wearing out their clutch mechanisms.” Radlinski should know – he’s the former chief of the Vehicle Stability Control Division of the National Highway Traffic Safety Administration.
While ASAs often are blamed for out-of-adjustment conditions, chances are better that there are other problems in the brake system. Improper installation, inadequate system maintenance and poor visual inspections can cause problems related to adjuster function. In addition, other problems – such as wear at quick-connect collars, clevis pin bushings, and camshaft bushings and splines – can lead to excessive brake stroke. In short, no slack adjuster can compensate for maintenance neglect.
Fleet executives also should be advised that, just because ASAs are automatic, they shouldn’t be ignored. A procedure for checking their operation can be found in Recommended Practice (RP) 609B, Manual and Automatic Slack Adjuster Removal, Installation and Maintenance, from the Technology & Maintenance Council (800-ATA-LINE).
The RP warns that ASAs should not have to be manually adjusted, except for initial installation and at brake reline. Further, ASAs generally aren’t field-serviceable, and in any case, the practice is not recommended. If the entire brake system is shipshape, and an ASA isn’t doing its job, it’s time for ASA replacement.
Here’s how to check: If the power stroke exceeds the maximum allowable, you’ll need to determine whether the problem is in the slack itself, or in another foundation brake component. A good way to find out to first measure free stroke: Free stroke is determined by using a lever to pull on the slack arm until the brake shoes contact the drum. If free stroke is over 5/8 inch, suspect a malfunctioning slack adjuster.
Then apply, or have someone apply, the brakes several times, and observe whether the splined adjusting ring is rotating in the direction needed to remove brake slack. If there’s no adjustment going on, the slack’s bad.
If free stroke is between 3/8 and 5/8 inch, the out-of-spec power stroke is being caused by a foundation brake problem. The problem, in many cases, is slop that can’t be felt with a lever, but which shows up under the pressure of an actual brake application.
An easy way to keep on top of the performance of your ASAs, according to the North American Brake Safety Conference, is to use visual stroke indicators, such as Brake Sentry (mechanical) or e-stroke by MGM (electronic), or equivalent.
And in no case should ASAs be adjusted just for the heck of it. “The warnings in existing materials available to owners, drivers, mechanics and inspectors of air-braked vehicles equipped with ASAs have not been successful in communicating the inherent dangers of manually adjusting ASAs to correct out-of-adjustment brakes,” warns NTSB.
Remember, ASAs actually can amplify weaknesses in brake maintenance procedures. If ASAs are to be expected to do their job, brakes need to be maintained to a high standard.
