This invention relates to an automatic slack adjuster of the type used for commercial vehicle braking systems.
Commercial vehicle braking systems typically incorporate a slack adjuster in each brake assembly to adjust the clearance between the brake linings and the rotating brake element, such as a brake drum or rotor. The brake assembly includes an actuator, such as an air chamber, that urges the brake linings into engagement with the brake element. As the linings wear, the clearance between the brake linings and brake element increases requiring the air chamber push rod to move a greater distance to apply the brakes. It is desirable to maintain a relatively constant clearance throughout the life of the brake linings to provide consistent braking performance. Slack adjusters are employed to keep the distance that the air chamber push rod must move within a specified range as the linings wear to maintain a consistent clearance.
Automatic slack adjusters have been developed to adjust for the clearance between the brake linings and brake element during normal vehicle operation. One automatic slack adjuster available from the Assignee of the present invention incorporates a worm gear in engagement with a gear. The gear is connected to a camshaft that moves the brake lining into engagement with the brake element upon actuation of the air chamber. When excess clearance occurs, the worm gear is rotated by an adjustment assembly connected between the worm gear and the push rod. Rotating the worm gear adjusts the rotational position of the slack adjuster relative to the camshaft, which adjusts the push rod travel.
The worm gear is disposed within a bore in the housing. An end portion of the worm gear has longitudinally extending teeth. A cylinder-shaped actuator is disposed within the bore and includes an inner diameter having longitudinally extending teeth engaging the teeth of the worm. An actuator piston is disposed within an internal actuator cavity and is retained therein by a piston retaining ring. An end of the actuator rod is pinned to the actuator piston. A spring loaded pawl assembly is supported by the slack adjuster housing and includes an end having teeth that engage the outer diameter of the actuator. The outer diameter of the actuator has helical teeth that cooperate with the teeth on the pawl assembly.
The actuator rod moves the piston along a length defined by the actuator cavity in response to the brake being applied and released. The actuator fits loosely within the bore to permit lateral movement of the actuator within the bore. When excess clearance has developed, the actuator rod will pull the actuator in a direction away from the worm gear with the actuator piston. As a result, the actuator will “jump” a tooth relative to the pawl assembly teeth such that on the brake release the worm will rotate the gear relative to the slack adjuster for the next brake apply thereby taking up the clearance.
As can be appreciated for the above description of prior art slack adjusters, the slack adjuster utilizes numerous components adding cost and complexity to the assembly of the slack adjuster. For example, the actuator requires machining on both the inner and outer diameters, and the spring loaded pawl requires numerous parts. Therefore, what is needed is a simplified slack adjuster that reduces the cost of the assembly.