By John Smith
Mark Kromer admits that manufacturers had to scramble to meet the trucking industry’s earliest requirements for automatic slack adjusters (ASAs). Quite frankly, some of the first designs didn’t last as long as they should, recalls the engineering manager for the Specialty Products Group at Bendix Commercial Vehicle Systems.
But ASAs certainly evolved during the years that followed.
“We do a much better job of controlling clearances,” notes Ron Plantan, principal engineer for wheel ends at Bendix Spicer Foundation Brake. Related seals have improved dramatically, retaining the all-important lubricants and offering a barrier to the outside world.
Everything from ball joints to linkages are now protected from the elements.
Improvements like this have become more important than ever, particularly when the equipment travels through areas where highly corrosive anti-icing compounds, such as calcium chloride, are used, which can leach into the smallest crevices.
Granted, the development of a product is always a delicate balance between engineering and price. Suppliers and those who sell ASAs say that they are under constant pressure to find ways to reduce the related costs. Some representatives candidly admit that truck and trailer manufacturers see ASAs as a commodity.
“All of our competition, as well as us, we get the year-over-year requests for three percent [cost cuts],” Kromer observes.
It is a matter of finding the best solution that money can buy – and selecting an ASA that will help to combat any headaches.
Shops need to consider some significant differences between the individual designs before making a selection.
Stroke-sensing ASAs maintain the complete stroke of the brake, from the free stroke that exists before the shoes contact the drum, to the stroke associated with the stretching of the various components during an application. In contrast, a clearance-sensing unit monitors the clearance between the shoe and the drum, and the ASA’s clutch decouples once it identifies higher levels of torque at the camshaft. If the stroke at this point is longer than a pre-set amount, the unit makes an adjustment.
And while most models maintain their required tension with teeth and springs, Bendix leverages two shafts that are wrapped in a spring.
Technicians need to pay attention to the differences. With a stroke-sensing ASA, the choice of piston will be dictated by a combination of chamber size, lining, drum width and camshaft length. But a clearance-sensing ASA will maintain the same clearance whether the installation involves a Type 24 or Type 30 brake chamber, Kromer says.
Then again, the choice of replacement components also can be somewhat dictated by the equipment that is already in place.
“ArvinMeritor might have a different stroke on the air chamber than a Haldex or Gunite,” explains Joe Kay, ArvinMeritor Commercial Vehicle System’s engineering manager – foundation brakes, North America, referring to the need to use the same style and type of ASA on opposing ends of an axle. A decision to put a different design on a single wheel end could lead to a potential imbalance in the brakes, which in turn could affect everything from the life of the friction material to the stopping distance of the vehicle.
The components also can be designed to meet the needs of specific applications.
Off-road vehicles need to be equipped with ASAs that offer a stroke value that is more stable to account for the higher weights and heavier stops, Kromer explains. An adjuster designed for over-the-road equipment may compensate for expanding brake drums a little more quickly, but that could lead to dragging brakes in a heavier application.
The ASAs also come in different lengths, Kay adds. A standard linehaul application may have a 5.5-inch lever, but a heavier application could require 6.5- or 7-inch designs, such as those used on a transit bus.
“If you don’t get the right body configuration, there may be interference somewhere on the suspension or the axle. You may not even see it until the brake is actuated,” he says.
“We generally recommend that you maintain like for like,” Plantan agrees, “but you can swap out different styles.” When making the alterations, it is simply a matter of ensuring there is enough clearance when the brake is at full stroke, and that the body of the slack adjuster is not coming into contact with suspension components such as differential lock housings. Pushrods will also need to be properly cut to ensure the proper geometry.
There are other important steps to protect the all-important geometry. Shims will ensure that the slack adjuster can be centered properly on the clevis, and any remaining washers still should be used to ensure the component cannot move beyond acceptable levels of end play, and prevent any of the damaging side loads that would otherwise occur.
Then it’s a matter of maintaining the equipment once it is installed. They may be “automatic,” but ASAs are not “maintenance-free.”
Once the related springs or teeth wear out, customers will begin to report units that are “seized” and technicians will note how an ASA can’t adjust or maintain stroke. But the comments mean the same thing. Depending on the designs, the internal teeth simply may have worn down to the point that they slip past each other, or the wrapped spring may be unable to maintain the required amount of friction.
Regular grease applications are recommended to keep the wearable components lasting as long as possible. An NLGI 2 grease often is recommended to ensure a balance between the need to retain the lubricant and ensuring that it will flow freely. Escaping lubricants, after all, will create a cavity that can be filled with moisture or contaminants.
Some common shop procedures can even create maintenance challenges of their own. For example, shop personnel often will tighten the adjustment instead of actually checking the stroke. That may be understandable. A single technician can adjust every ASA on a vehicle in about 15 minutes. In contrast, a decision to measure the stroke at every wheel end will require two people and about 25 minutes of time.
Just be aware that the practice comes at a cost. The higher loads introduced by the adjustments can reduce the life of the component, Plantan says.
There is also a reason that maintenance teams should abandon the traditional practice of manual de-adjustments, where the slack adjuster is de-adjusted a few times to ensure that the ASA is functioning. The practice can override what is typically a one-way clutch or a mechanism that is always supposed to be engaged.
Shops even need to be careful not to automatically blame the slack adjuster when dealing with a report of excessive stroke.
“If you have friction material that swells and grows when it gets hot, the perceived symptom can be the slack adjuster,” Kay says. The hidden causes of excessive stroke can include everything from an air chamber’s springs to worn bushings in the camshaft. “You’ve got lining material and shoes and S-cams and the drums themselves,” he says. “They all have a function in how the slack adjusts.”
