The difference is in the technique, but the end result is the same. Automatic slack adjusters — clearance sensing or stroke sensing — ensure maximum braking power is available when needed.
Bendix Spicer manufactures both types of automatic adjusters for s-cam brakes, says John Hawker, technical service engineer.
The difference in automatic slack adjusters lies in how they work, not what they do, he says.
Clearance-sensing adjusters set the amount of space between the friction material and the drum. "Refer to them as pressure-differential slack adjusters," he says. "In other words, when you apply the brakes, as you step on the pedal, air starts up the chamber. The chamber pushes the slack adjuster, and it swings through an arc. The pressure it feels when it starts to swing through the arc is basically the combination of the return spring and the shoe. It senses a controlled, increasing rate of force. Once you get to the point where you start to get heavier resistance (which means the friction is now hitting the drum), it senses the pressure differential. The pressure goes up dramatically. "When that happens it now has an arc, or a pre-distance that is built into the slack. That's clearance. Some clearance adjusts on the down stroke, some on the up stroke."
The advantage of adjusting on the applied stroke is that the drum is cool, says Charlie Fritts, project manager, automatic slack adjusters for Gunite. "[If] you put adjustment on the return stroke, then the heated drum can give you variations in adjustment," he says. Gunite makes a clearance-sensing that adjusts on the applied stroke.
Stroke-sensing automatic slack adjusters "measure the difference in the actuator, the air chamber, that's putting the power into the brake," says Joe Kay, engineering manager, foundation brakes for ArvinMeritor. "The clearance-sensing type is measured between the cam shaft rotation on an s-cam brake and something grounded, such as the axle or the cam bracket.
"The main job of the slack adjuster is to maintain the braking system so that everything is in the correct amount of available stroke from the air chamber, proper running clearance and everything," he says. "The design difference comes down to where the adjustment is actually measured from. There is only an available amount of stroke that the air chamber puts out. The stroke-sensing slack adjuster will try to maintain the proper distance to maximize the air chamber output."
Kay says the strong point of the stroke-sensing design is that the adjuster tries to maintain stroke no matter what "conditions are thrown at it."
"As your brake heats up and the drum expands, the stroke-sensing slack adjuster will try to make sure the operator always has brakes," Kay says.
Craig Frohock, director, North America Brake Systems for ArvinMeritor, suggests there are really two benefits in the stroke-sensing design. "One is from an in-service requirement when roadside inspections are done. What CVSA [Commercial Vehicle Safety Alliance] is checking for is the amount of stroke in the air chamber. Stroke-sensing slack adjusters are designed to function such that it keeps the stroke in predetermined range to meet CVSA criteria."
The second benefit relates to the operating range of strokes in air chambers. "The stroke-sensing slack is designed to keep the brake functioning at the optimum range so you get maximum output of the chamber," Frohock says. "In our opinion, that's really the key benefit the stroke sensing slack is designed for."
Clearance-sensing slack adjusters are much more precise because of their engineering, says Hawker. Stroke-sensing slacks, he says, will "swing out to a certain point and ratchet. When it comes back, it's going to be set to its new point, so it's not a finite adjustment. The clearance sensing slack, on the other hand, has adjustments that are more minute, more finite. "If you want a slack adjuster that inches up by a thousandth of an inch, clearance-sensing does that."
Although some brake manufacturers say four of the top five OEMs in the United States use clearance-sensing technology, Bendix Spicer, which manufactures both types, says stroke-sensing adjusters are predominant in the marketplace because they are less expensive, easier to maintain, easier to manufacture, and more available. "Clearance-sensing gives longer brake life and better performance," says Hawker, "but they have to be maintained — that is, lubricated — properly. And not everybody stocks them because of price."
Historically, maintenance of automatic slack adjusters has been overlooked by end-users, according to manufacturers. "There will always be a learning process," Fritts says. "I think the bigger fleets that have more standardized training, more consistent and up-to-date training, won't have problems with maintaining automatic slack adjusters. Smaller fleets that don't have as much consistent training will."
Gunite's website includes parts catalogs and videos on how to properly install slacks. "All the technicians have to do is go to the site and download what they need," says Tom Parsons, director of product marketing.
One of the most prominent issues today is helping people to realize that automatic slack adjusters should be inspected, not adjusted, as in the past, Parsons says. Mechanics try to adjust automatic slacks, he says, because they don't know any better. "It's also history," he says. "They learned from manual slack adjusters when they were in the market. Automatic slacks haven't been around all that long. Mechanics are just doing what they were taught to do."
The "install 'em and forget 'em" attitude just won't work, says Frohock. "As with every mechanical component, slack adjusters require a certain amount of inspection and maintenance. We've found that regular visual inspections are the easiest way to keep slack adjusters and brake systems in functioning order on a daily basis. That should be done every time the truck is put in service. Ninety percent of the battle is won by visual inspection."
Hawker agrees that misunderstandings about automatic slack adjusters linger in the market. "The biggest issue in the market today is lack of product knowledge and application knowledge," he says. If the slack is installed correctly, he says, "the slack adjuster will take care of itself. Just don't touch it with a wrench. Just grease it regularly and it will do a good job."
Hawker says he keeps telling customers that slacks are not automatic; they still require maintenance. "They are wear-compensating slacks," he says. "What they do is compensate for wear. The biggest problem we have is the mechanic doesn't know how to set up a brake [according to industry standards]."
Hawker says chemicals used on roadways cause problems with adjusters. "It's a magnesium chloride blend that is so corrosive you have to lubricate more often and purge the chemical out of your slack adjusters," he says. "Keep them dry, keep them clean inside."
Hawker sees a change in the market. "Clearance-sensing slacks are becoming more and more prevalent," he says. "As the price of the brake product goes up, many fleet managers believe it becomes more cost-effective to spend a little more money for a slack adjuster that will give you a little more life."
Says Hawker, "it's all economics."