Automatic slack adjuster with ball detent clutch

An improved self-adjusting automatic slack adjuster for reducing slack in the brake of a vehicle is provided, in which an easily accessible external operating feature actuates a ball detent clutch to permit the automatic slack adjuster's adjustment mechanism to be readily disengaged, so as to allow smooth release and retraction of the brake linings of a vehicle brake without damage to the adjustment mechanism's components. When the torque applied to the external feature exceeds a predetermined torque, the balls of the ball detent clutch move out of their retaining detents, thereby disengaging the slack adjuster's worm shaft from the adjustment mechanism and permitting brake shoe retraction without resistance from the adjustment mechanism.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to automatic slack adjusters which eliminate excess motion in a brake actuator mechanism used to apply the brake.

Over the life of the brake linings of a brake, such as a pneumatic drum brake used on commercial vehicle axles, as the brake's friction linings wear the clearance between the brake linings and their respective friction surfaces (for example, the inner surface of a brake drum) increases. This increasing clearance requires an ever-increasing range of motion from the brake actuator mechanism to move the brake linings from their rest position to the point at which the linings contact the friction surface.

It has become commonplace to include an automatic slack adjuster in the mechanical path between the brake actuator and the brake linings so as to eliminate excess lining travel slack as the brake linings wear. Such adjusters typically are: (i) located on a portion of a brake camshaft which is outside of the brake (typically splined to the camshaft); and (ii) coupled to a pushrod of a brake actuator such that when the brake actuator push rod is extended or retracted, the slack adjuster rotates about the longitudinal axis of the brake camshaft. An example of such a brake and slack adjuster arrangement is shown in FIG. 1 of U.S. Pat. No. 4,380,276. Thus, by extending or retracting the brake actuator pushrod, the slack adjuster causes the brake camshaft to rotate about its longitudinal axis, which in turn rotates a brake actuation cam affixed to the end of the brake camshaft located within the drum brake. The rotation of the cam either presses the brake linings into engagement with the brake drum inner friction surface or allows the brake linings to withdraw radially inward, away from the friction surface. Because the brake camshaft is used to rotate the cam which presses the brake linings radially outward, the brake camshaft is also known as the brake cam.

Automatic slack adjusters can be designed to transmit brake actuator force to the brake camshaft in the brake application direction with no relative motion between the adjuster and the brake camshaft. When the brake actuation force is withdrawn, if there is greater than desired distance between the brake linings and the brake drum friction surface, the slack adjuster is permitted to rotate relative to the brake camshaft an angular distance sufficient to remove some or all of this undesired slack, i.e., limiting the distance the brake linings withdraw from the brake drum friction surface so that the lining-drum clearance is maintained at a desired minimum.

Automatic slack adjusters as described above, where the slack adjuster rotates relative to the brake camshaft when the brake actuation force is withdrawn, are said to adjust on release. There is also the other category of automatic slack adjusters which rotate relative to the brake camshaft during the phase when the actuation force is applied, with no relative rotation when the actuation force is withdrawn, this category being said to Adjust on Apply.

In many automatic slack adjusters, a one-way clutch is used to accomplish the rotary adjusting movement, with a worm shaft located in the adjuster turning a worm gear (also known as a worm wheel) coupled to the brake camshaft. In one type of one-way clutch arrangement, the one-way clutch is coupled to the worm shaft through a toothed clutch or a friction clutch located coaxially with the worm shaft. A heavy coil spring or disc-spring pack biases the one-way clutch to keep it engaged so that a torque applied through the one-way clutch can turn the worm shaft. The worm shaft turns the worm wheel, which is coupled to brake camshaft, in order to decrease the brake lining clearance and thus compensate for lining wear. Examples of such arrangements are shown in prior artFIGS. 1-3, corresponding respectively to FIG. 4 of U.S. Pat. No. 4,380,276 (toothed clutch teeth63), FIG. 3 of U.S. Pat. No. 5,327,999 (toothed clutch8), and FIG. 1 of U.S. Pat. No. 5,664,647 (toothed clutch14).

A further type of one-way clutch is a ratchet and pawl arrangement, in which a pawl has to be manually retracted to retract the brake linings. This design has the problem that if the operator does not remember to retract the pawl when manually servicing the brake, an attempt to retract the brake linings can result in damage to the one-way clutch.

Regardless of the type of automatic slack adjuster, typically an external extension of the worm shaft projects outside the automatic slack adjuster housing to permit manual brake lining clearance adjustment during the installation of the slack adjuster or of new brake linings (inFIG. 1, extension57; inFIG. 2, extension4′; inFIG. 3, extension15). The extension usually is shaped as a square or hexagon to facilitate gripping and turning with a wrench or other tool. In order to advance the brake lining, the worm shaft must be rotated in a first direction (designated the clockwise direction for the purpose of this description). In order to retract the brake lining, the worm shaft must be rotated in the opposite, or counter-clockwise, direction.

When the external extension is rotated in the clockwise (advance) direction, the toothed clutch remains engaged, and the worm shaft rotates with little resistance from the one-way clutch permitting the worm shaft to rotate with little resistance. When the external extension is rotated in the counter-clockwise (retracting) direction, the one-way clutch is rotated in its “lock-up” direction, and therefore the toothed clutch coupling strongly resists rotation of the worm shaft. The strong resistance requires application of high torque loads to the external extension, up to the point at which the toothed clutch begins to slip, disconnecting the one-way clutch from the worm shaft.

The slipping of the toothed clutch in response to the application of a large torque to the external extension often results in damage to the one-way slack adjuster, for example, in the case of toothed adjusters mounted on the worm shaft, the undesired blunting of the teeth in the clutch. As these clutch teeth wear, the torque capacity of the automatic slack adjuster decreases, progressively reducing the useful service life of the automatic slack adjuster. Attempts have been made to reduce this undesired deterioration of the clutch teeth, for example, by altering the angle of the clutch teeth or rounding the tips of the teeth as shown in prior artFIGS. 4a-4b, corresponding to FIGS. 3-4 of U.S. Pat. No. 5,664,647. However, these slight teeth geometry changes have not been fully successful in addressing the wear concerns.

In view of the foregoing, it is an objective of the present invention to provide an improved automatic slack adjuster with superior manual adjustment provisions. In addressing these and other objectives, the present invention provides a solution to the problems of the prior art by providing for controlled disengagement of the one-way clutch teeth to permit withdrawal of brake shoes as an external adaptor part is manually operated without incurring damage to the one-way clutch components.

In one embodiment of the present invention, the one-way clutch includes a clutch wheel concentrically arranged about an end of the worm shaft, a hex wheel concentrically arranged on the worm shaft adjacent to the clutch wheel and engaging the worm shaft in a non-rotating manner, and a power spring arranged to press the clutch wheel into non-rotating contact with the hex wheel with sufficient force that during a brake application event, the clutch wheel and the hex wheel do not rotate relative to one another. The one-way clutch is also provided with a mechanism which facilitates retraction of the brake linings without damage to the one-way clutch during manual operation of the external extension. In this embodiment, the clutch wheel and the hex wheel are provided with a ball detent clutch arrangement between their respective contact faces, arranged such that when a predetermined torque applied to the external extension is exceeded, the ball detent clutch elements overcome the pressure applied by the power spring, pushing the clutch wheel and the hex wheel axially apart and permitting the hex wheel to rotate the brake lining-retraction direction on the balls of the ball detent clutch, independent from the non-rotating clutch wheel. Preferably, the predetermined torque which must be exceeded by the manual actuation of the external extension is a torque which is higher that that normally observed between the clutch wheel and the hex wheel during brake application operations. This ensures that the ball detent clutch does not permit the clutch wheel and the hex wheel to move relative to one another during normal brake application events.

A ball detent clutch includes a series of balls arrayed in a pattern such as a circle, located between two parallel members. When at rest, the balls reside in detents on at least one of the parallel members. A normal force (provided, for example, by a spring) is applied to bias the parallel members toward one another. If a torque is applied to one of the parallel members to rotate the members, the torque is transferred through the balls to the opposing the parallel member. The strength of the spring biasing the parallel surfaces toward one another governs how much torque may be applied before, at a predetermined torque, the tangential force on the balls cause the balls to push apart the parallel members and allow the parallel members to begin to rotate relative to one another. In one type of ball detent clutch, the balls reside in recesses or holes in one of the parallel members, with a portion of each ball extending above the surface of the face of the member so that they can engage corresponding detents in the opposite parallel member, and when the predetermined torque is exceeded, the balls rise out of the detents on the surface of the opposite member and permit the parallel members to move relative to one another while the balls rotate in place within their respective recesses or holes. Once the predetermined torque is exceeded, torque is no longer transferred from one parallel member to the opposing member, i.e., the clutch is released.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 5is partial cut-away view of a clearance-sensing automatic slack adjuster100. The automatic slack adjuster100has a housing105containing a main gear set comprising a worm shaft110meshing with a worm gear120, a brake actuator pushrod receiving hole130for coupling the automatic slack adjuster100to a brake actuator pushrod101(schematically illustrated with brake actuator102), and a splined coupling140either integrally formed with or otherwise coupled to the worm gear120to receive an end of a brake camshaft103(splined end of the brake cam illustrated inside splined coupling140).

Concentrically located adjacent to worm gear120is a reference wheel125used to perform automatic adjustments of brake lining clearance. Reference wheel125is held on an inner circumferential surface by a plurality of one-way toothed pawls (not illustrated) against rotation relative to stationary reference arm137in one direction when the automatic slack adjuster is moved, and is free to move in the other direction as it ratchets over the pawls.

The reference wheel125meshes with adjustment wheel (not illustrated), which through a worm shaft extension136drives a control worm wheel180which is concentrically located on an end of worm shaft110. The control worm wheel180is coupled to an output wheel190, which is keyed to the worm shaft110by a polygon section (those of skill in the art will recognize that alternate features, such as a roll pin or other such component, would be sufficient to preclude rotation of the output wheel190relative to the worm shaft110). The control worm wheel180and the output wheel190biased toward one another by a power spring200. Interposed between the control worm wheel180and the output wheel190are the balls210of a ball detent clutch, where the opposing faces of the control worm wheel180and output wheel190form the races upon which the balls210roll when displaced out of detents240in the opposing faces. An external adaptor part201coupled to the output wheel190is provided for manual rotation of the worm gear, for example when retraction of the brake linings is desired. In this embodiment, the adaptor part201is formed as a circular plate, with an external hex-shaped projection suitable for turning by a wrench.

FIGS. 6-7show a detailed cross-section view of two embodiments of a clutch, including inFIG. 6the top half, above the worm shaft centerline A-A, of a conventional friction cone clutch220, and inFIG. 7the half below the worm shaft centerline A-A, the ball detent clutch230ofFIG. 4. In both embodiments, power spring200biases control worm wheel180(the input part) against output wheel190(the output part) (here, control worm wheel180also has an end portion181which provides the contact surface).

In the upper friction cone embodiment220, there is no means present for releasing the friction cone engagement when a technician applies a brake lining retraction torque to the adaptor part. Thus, in order to overcome the friction force generated by the power spring200to rotate the worm gear in the brake lining release direction, the technician must apply a large torque to the adaptor part (and hence, the output wheel190, resulting in damage to the opposing mating surfaces of the control worm wheel180and the output wheel190. The damage caused by the sliding of these components' friction surfaces across one another deforms the surfaces, thereby reducing the torque capacity of the clutch and hastening the need for repair and/or replacement. Similar damage and loose of capacity occurs with alternative clutches, as well, such as when conical toothed clutches are used, and their teeth are deformed when a technician applies enough torque to overcome the engagement of the opposing clutch teeth.

In contrast, in the lower ball detent clutch embodiment230, damage is avoided by the plurality of balls210(a single ball illustrated in theFIG. 7cross-section view) interposed between the opposing faces of control worm wheel180and output wheel190. In this embodiment, the ball210is located in a recess in control worm wheel end portion181, with enough of the ball210extending beyond the face of end portion181(here, by approximately 1 mm) to engage a detent240in the face of output wheel190. During normal brake application operations, the power spring200biases the two wheels toward one another with sufficient axial force to ensure that, as control worm wheel180is rotated by adjustment wheel135and worm shaft extension136, the output wheel190is driven by the balls210acting transversely on the detents240. The force generated by the power spring200is selected, however, to not be so high as to prevent the ball detent clutch from releasing the output wheel190when manual operation to retract the brake linings is desired.

When a technician applies a torque to the external adaptor part201to withdraw the brake linings, no significant motion occurs until the applied torque exceeds a predetermined value. The predetermined torque value in turn is determined by the predetermined amount of force generated by power spring200. When the applied torque exceeds the predetermined value, the balls210are cammed up out of the detents240, moving axially as the balls210rise out of the detents240. In order to move axially, either the balls move axially deeper into their recesses in end portion180, or the balls' axial motion is transferred to the control worm wheel, overcoming the force of the power spring. When the balls210are out of the detents240, the output wheel190is free to be driven by the external adaptor part201to rotate relative to the control worm wheel180at a considerably lower torque level, and without any opposing face interaction or consequent damage. Accordingly, because the worm shaft110is non-rotatably coupled to the output wheel190, the technician may rotate the external adaptor part201with low resistance to rotate the worm shaft110, thereby retracting the brake linings without damage to the automatic slack adjuster. Thus, the present invention provides an automatic slack adjuster with a greatly enhanced service life and resistance to long-term degradation of the torque capacity of its adjustment mechanism.

In addition to the reduction in internal damage to the automatic slack adjuster provided by the ball detent clutch arrangements of the present invention, the present invention also lowers automatic slack adjuster initial manufacture cost and long-term servicing costs by eliminating the need for expensive specialty lubricants previously required to help minimize friction and thus ensure adjustment.