Within the air brake systems of trucks and trailers, actuators play a vital, but largely unsung, role. Brake actuators are the devices that convert the compressed air force within a truck or trailer's air reservoir into a mechanical force, which activates the brake. Without the actuator, air brakes couldn't function, says Prakash Jain, ArvinMeritor's technical support manager for braking systems.

“Once a driver puts his foot on the brake pedal, a signal activates air pressure from the truck's air tanks,” he says. “That air moves through the actuator, triggering a relay valve that converts the air pressure into physical braking force. That means the actuator plays a key role within the air brake system.”

Air brake systems are changing, however, and with them air brake actuators. New long-stroke air brakes have necessitated change in current actuator designs, with more changes coming as new air disc brake systems for trucks and trailers enter the North American market.

Duane Stocksdale, product manager for Haldex Brake Systems, says the development of long-stroke air brakes has led to change for actuators. There are different kinds of actuators on the market, one for standard stroke, 2.5-in. brakes and one for the 3-in. long-stroke.

The most common actuator size is the 30/30 double diaphragm spring brake (DDSB), says Stocksdale. This indicates a Type 30 service chamber combined with a Type 30 parking brake, where the service chamber works during normal service braking and the spring brake is used for parking and emergency braking. The numerical values, such as 30/30 or 24/30, refer to the effective area of the diaphragm that converts compressed air into mechanical force, with the stroke dimension varying depending on the size of the chamber.

The term “stroke” relates to the performance of the service chamber, Stocksdale adds. “Stroke” is the generating force through the push rod, which in turn rotates the automatic brake adjuster (“slack adjuster”) that's connected to the camshaft of the brakes. Stroke is key to maintaining optimum lining-to-drum clearance to apply the brakes during a braking, he says.

The designated stroke dimension is the maximum distance the push rod can move while continuing to provide acceptable brake performance, he adds. As brake lining material wears, however, the lining-to-drum clearance increases, which means the push rod has to move a longer distance to rotate the slack adjuster to apply the brakes. Beyond a certain distance, brakes are deemed to be out of adjustment, which means they can't provide the proper amount of braking force.

According to Jain, long-stroke brakes were designed to give truck brakes a greater margin of effectiveness to better combat brake fade.

Generally, air brake systems and actuators are the same for trucks, tractors and trailers. The only difference occurs if the brake is going to perform two functions. For example, actuators on tractor front axles have one valve, since they provide stopping power only. But actuators for trailer rear axles have two valves — one for stopping power and one for parking brake power. Virtually all of these applications use a double-diaphragm actuator design, adds Stocksdale.

The biggest change in actuators over the years has been their durability, says Jain. In the past, actuator diaphragms were replaced every 50,000 miles; now they're part of the “sealed for life” brake service chamber, meaning no servicing is necessary.

Actuators are expected to undergo much more change in the future, and according to Stocksdale will be a key element in the development of electronically controlled braking systems. “Actuators may very well become the platform for maximizing electronic control of pneumatic actuation on the future heavy truck/trailer brake system.”

Jain adds that as air disc brake technology comes to market, more changes will be in store for actuators. While the design will remain the same, size may decrease. Also, actuators that serve a dual role as relays in both stopping and parking brake functions may be required to exert more parking force. But that development, which depends largely on potential shifts in government regulations, could be at least 10 years away.


ArvinMeritor 256

Bendix Comm Vehicle Systems 257

Dana/Commercial Axle 262

Federal Mogul/Abex 260

Haldex Brake Systems 258

Hendrickson Int'l 264

Holland Neway Int'l 261

MGM Brakes 259

Roadranger System 263