Patent Publication Number: US-8973722-B2

Title: Automatic slack adjuster with clutch release cam

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 12/273,274, filed Nov. 18, 2008, the disclosure of which is expressly incorporated by reference herein. 
    
    
     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&#39;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. 
     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 the Adjust on Release type, when the brake actuator pushrod is retracted, the worm shaft of the worm gear set rotates about is longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. In the Adjust on Apply type, when the brake force is applied, the worm shaft of the worm gear set rotates about is longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. Because the rotation of the slack adjuster relative to the brake camshaft results in reduction of brake lining clearance in the new rest position, the automatic slack adjuster compensates for brake lining and drum wear. 
     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 the Adjust on Release type, when the brake actuator pushrod is retracted, the worm shaft of the worm gear set rotates about is longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. at which the brake linings are maintained closer to the brake drum friction surface. In the Adjust on Apply type, when the brake force is applied, the worm shaft of the worm gear set rotates about its longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. Because the rotation of the slack adjuster relative to the brake camshaft results in reduction of brake lining clearance in the new rest position, the automatic slack adjuster compensates for brake lining and drum wear. 
     In one type of one-way clutch arrangement, the one-way clutch is coupled to the worm shaft through a toothed clutch, which may conically shaped. A heavy coil spring or disc-spring pack is positioned at the opposite end of the worm shaft to keep the conical clutch engaged and to provide adequate torque to 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 art  FIGS. 1-3 , corresponding respectively to FIG. 4 of U.S. Pat. No. 4,380,276 (toothed clutch teeth 63), FIG. 3 of U.S. Pat. No. 5,327,999 (toothed clutch 8), and FIG. 1 of U.S. Pat. No. 5,664,647 (toothed clutch 14). 
     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 (in  FIG. 1 , extension  57 ; in  FIG. 2 , extension  4 ′; in  FIG. 3 , extension  15 ). 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. 
     The slipping of the toothed clutch in response to the application of a large torque to the external extension results in undesired blunting of the teeth in the clutch. As the 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 art  FIGS. 3A-3B , 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. 
     The slipping of the toothed clutch in response to the application of a large torque to the external extension results in undesired blunting of the teeth in the clutch. As the 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 art  FIGS. 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 an automatic disengagement of the one-way clutch teeth and free release of the clutch to permit smooth withdrawal of brake shoes as an external adaptor part is manually operated. 
     In one embodiment of the present invention, the adaptor part and an adjacent end of the automatic slack adjuster&#39;s worm shaft are provided with corresponding axially-oriented lugs which permit the adaptor part to drive rotation of the worm shaft in the clockwise and counter-clockwise directions. Between the lugs, ramps are provided which, when the lugs are displaced circumferentially relative to one another, cause the adaptor part to push the worm shaft to move axially away from the adapter part. This axial displacement of the worm shaft axially lifts the output part of the one-way clutch out of engagement with the input part, disengaging the one-way clutch&#39;s one-way teeth and thereby permitting the worm shaft to smoothly rotate within the automatic slack adjuster housing without damaging the engagement teeth within the one-way clutch. 
     In an alternative embodiment of the present invention, a similar camming action may be obtained by the interaction of camming surfaces of the adaptor part and a rod which extends through a bore in the worm shaft and is fixed to the output part of the one-way clutch. In this embodiment, when the adaptor part is operated in the brake shoe retraction direction, the axial displacement of the cam rod pushes the one-way clutch output part out of engagement with the input part, freeing the worm shaft to be smoothly turned. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 ,  2  and  3 A- 3 C are views of prior art slack adjuster worm gear and one-way adjustment clutch arrangements. 
         FIG. 4A  is an oblique partial cut-away view of an automatic slack adjuster in accordance with an embodiment of the present invention,  FIG. 4B  is an elevation partial cut-away view of the  FIG. 4A  automatic slack adjuster, and  FIG. 4C  is an enlarged view of a detail of the one-way clutch of  FIGS. 4A and 4B . 
         FIG. 5A  is a enlarged view of the components shown in section A of the partial cross-section in  FIG. 4A , and  FIG. 5B  is a transverse cut-away view of the  FIG. 5A  embodiment viewed from the one-way clutch end of the automatic slack adjuster. 
         FIG. 6  is cross-section view of an automatic slack adjuster in accordance with an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIGS. 4A-4C  are partial cut-away views of a clearance-sensing automatic slack adjuster of the self-setting type  100 , and  FIGS. 5A-5B  provide enlarged views of the cut-away areas of  FIGS. 4A-4C . The automatic slack adjuster  100  has a main gear set comprising a worm shaft  110  meshing with a worm gear  120 , a brake actuator pushrod receiving hole  130  for coupling the automatic slack adjuster  100  to a brake actuator pushrod (not illustrated), and a splined coupling  140  either integrally formed with or otherwise coupled to the worm gear  120  to receive an end of a brake camshaft (not illustrated). The worm shaft  110  is provided with a heavy coil spring or a disc spring pack  150  preloaded to bias the worm shaft load face  155  away from its corresponding housing mating face  156  by a designed gap. 
     Concentrically located adjacent to worm gear  120  is a reference wheel  125 , which is non-rotationally fixed to stationary reference arm  137 , the reference arm being secured to a fixed point, such as the axle of the vehicle. The reference wheel  125  meshes with adjustment wheel  135 , which through a worm shaft extension  136  drives the control worm wheel  180 , coupled via one-way clutch  170  to the worm shaft  110 . The control worm wheel  180  is formed with saw-tooth teeth  185  on an end face. The control worm wheel teeth  185 , which function as the input part to the one-way clutch  170 , cooperates with corresponding saw-tooth teeth  186  on the output side of the one-way clutch. The output part of the one-way clutch is a ratchet wheel  190 , keyed to the worm shaft  110  by a polygon section  115  (or another alternate feature, such as a roll pin or other component sufficient to preclude rotation of the output side of the one-way clutch relative to the worm shaft  110 ). The output part of the one-way clutch (ratchet wheel  190 ) is held axially against a step  195  on the worm shaft  110  by a light spring  200 . 
     In the preferred embodiment, which is of the “Adjust on Apply” type, prior to application of a braking force, as shown in  FIG. 5B  the wheel of adjuster screw  135  rests in the slack adjuster housing a predetermined distance a from the outer cover  138 . This distance corresponds to the desired brake lining clearance. As the brake actuation pushrod rotates the automatic slack adjuster, the adjuster screw  135  begins to climb along the circumference of fixed reference wheel  125 , driven by the engagement of its worm threads  136  with the control worm wheel  180 . At the same time, the adjuster screw  135  begins to move axially outward toward the outer cover  138 . An alternate variant to the preferred embodiment is of the “Adjust on Release type” in which prior to application of a braking force, as shown in  FIG. 5B  the wheel of adjuster screw  135  rests against the outer cover, a predetermined distance a from the housing. This distance corresponds to the desired brake lining clearance. As the brake actuation pushrod rotates the automatic slack adjuster, the adjuster screw  135  begins to climb along the circumference of fixed reference wheel  125 , driven by the engagement of its worm threads  136  with the control worm wheel  180 . At the same time, the adjuster screw  135  begins to move axially inward toward the housing. 
     Prior to application of a braking force, as shown in  FIG. 5B  the adjuster wheel  135  rests in the slack adjuster housing a predetermined distance a from the outer cover  138 . This distance corresponds to the desired brake lining clearance. As the brake actuation pushrod rotates the automatic slack adjuster, the adjuster screw  135  begins to climb along the circumference of fixed reference wheel  125 , driven by the engagement of its worm threads  136  with the control worm wheel  180 . At the same time, the adjuster screw  135  begins to move axially outward toward the outer cover  138 . If there is no excessive wear in the brake, the linings will touch the brake drum when the adjuster screw  135  closes the predetermined distance a, and thus the adjuster wheel  135  will not rotate control worm wheel  180 . 
     Prior to application of a braking force, as shown in  FIG. 5B  the adjuster wheel  135  rests in the slack adjuster housing a predetermined distance a from the outer cover  137 . This distance corresponds to the desired brake lining clearance. As the brake actuation pushrod rotates the automatic slack adjuster, the adjuster screw  135  begins to climb along the circumference of fixed reference wheel  125 , driven by the engagement of its worm threads  136  with the control worm wheel  180 . At the same time, the adjuster screw  135  begins to move axially outward toward the outer cover  137 . If there is no excessive wear in the brake, the linings will touch the brake drum when the adjuster screw  135  closes the predetermined distance a, and thus the adjuster wheel  135  will not rotate control worm wheel  180 . 
     In both the above embodiments, If there is no excessive wear in the brake, the linings will touch the brake drum when the adjuster screw  135  closes the predetermined distance a, and thus the adjuster wheel  135  will not rotate control worm wheel  180 . 
     On the other hand, if there is excessive slack present due to brake lining wear, the brake actuator pushrod will cause the automatic slack adjuster to continue to rotate in brake application direction  160 , and adjuster wheel  135  will continue to climb reference wheel  125  until the brake linings come into full contact with the brake drum. As noted above, when full contact of the brake linings with the brake drum is reached, the worm shaft  110  is displaced axially until its load face  155  rests again the mating face  156  of the housing. This same axial motion also axially separates the toothed faces of the one-way clutch  170 , disengaging the control worm wheel  180 ) from the worm shaft  110 . Accordingly, any further motion of the adjuster wheel  135  about reference wheel  125  will cause the adjuster wheel worm shaft threads  136  to rotate control wheel  180  about the worm shaft  110 , but will not result in adjustment of the worm shaft  110 . 
     On the other hand, if there is excessive slack present due to brake lining wear, the brake actuator pushrod will cause the automatic slack adjuster to continue to rotate in brake application direction  160 , and adjuster wheel  135  will continue to climb reference wheel  125  until the brake linings come into full contact with the brake drum. As noted above, when full contact of the brake linings with the brake drum is reached, the worm shaft  110  is displaced axially until its load face  155  rests again the mating face  156  of the housing. This same axial motion also axially separates the toothed faces of the one-way clutch  170 , disengaging the control worm wheel  180 ) from the worm shaft  110 . Accordingly, any further motion of the adjuster wheel  135  about reference wheel  2  will cause the adjuster wheel worm shaft threads  136  to rotate control wheel  180  about the worm shaft  110 , but will not result in adjustment of the worm shaft  110 . 
     During brake application, the rotational input to the one-way clutch  170  is in the free-wheel direction, and thus as the input part teeth  185  tend to slide lout of engagement with the teeth  186  of the ratchet wheel  190 , the output part effectively pushes the ratchet wheel  190  against the light spring  200 . If the force pushing the ratchet wheel is sufficiently high, the one-way clutch skips forward by a tooth. Then, during the brake return stroke, as the input part rotates in the brake release direction, the teeth  185  re-engage teeth  186 . The rotation of the output part  190  thus also rotates the input part  180  in the brake release direction. Because the output part  190  is held in a rotationally-fixed relationship to the worm shaft  110  by polygon-shaped region  115 , the worm shaft  110  is rotationally displaced relative to the worm wheel  120 , such that the worm wheel  120  does not return to its original rest position, but instead comes to rest in a position corresponding to the brake cam shaft and the brake shoe being closer to the brake drum, i.e., in a position which reduces excess clearance. 
     During brake application, the rotational input to the one-way clutch  170  is in the free-wheel direction, and thus as the input part teeth  185  tend to slide lout of engagement with the teeth  186  of the ratchet wheel  190 , the output part effectively pushes the ratchet wheel  190  against the light spring  200 . If the force pushing the ratchet wheel is sufficiently high, the one-way clutch skips forward by a tooth. Then, during the brake return stroke, as the input part rotates in the brake release direction, the teeth  185  re-engage teeth  186 . The rotation of the output part  190  thus also rotates the input part  190  in the brake release direction. Because the output part  190  is held in a rotationally-fixed relationship to the worm shaft  110  by polygon-shaped region  115 , the worm shaft  110  is rotationally displaced relative to the worm wheel  120 , such that the worm wheel  120  does not return to its original rest position, but instead comes to rest in a position corresponding to the brake cam shaft and the brake shoe being closer to the brake drum, i.e., in a position which reduces excess clearance. 
     As described above, when the brake application force reaches a predetermined value, worm shaft  110  initially moves backward until it reaches housing mating face  156 . In doing so, the worm shaft pushes the output part  190  of the one-way clutch out of engagement with the teeth  185  of the input part, with no further adjustment. In this way a clearance sensing function is realized, by disengagement of the one-way clutch. 
     With automatic slack adjusters, manual shoe adjustment, either advancing or back-off, is typically accomplished by manually turning the worm shaft  110  clockwise or counter-clockwise, respectively. 
     In this embodiment of the invention, an adaptor part  210  projects out of the automatic slack adjuster housing and is provided with a hexagonal end for applying a wrench or similar tool. On the inside of the housing, the adaptor part and worm shaft are rotationally coupled by corresponding driving lugs  225 ,  226 . The openings  227  between the lugs are wider than the lugs, so that in each direction of rotation one set of lug side faces are in contact with one another, and there is a gap between the opposite lug side faces. The adaptor part  210  also is provided with a feature which has a slit to hold a leaf spring. The leaf spring is positioned such it exerts a force on the worm shaft lugs  226  to bias the lugs  226  into contact with the corresponding side faces of adaptor part lugs  225  which are engaged when turning in the brake shoe advance direction (in this embodiment, clockwise). It may be noted that the function of the leaf spring may alternatively be accomplished by the use of a torsion spring. 
     In the adaptor part the opening between adjacent lugs is constructed so that when brake shoes are to be manually advanced, one set of drive faces of the adjuster part lugs  225  are in contact with the corresponding faces of the lugs  226  of the worm shaft. In this condition there is no axial contact between the adaptor and worm shaft. 
     When the adaptor part  210  is manually rotated in the brake release direction (in this embodiment, counter-clockwise), the worm shaft lugs  226  encounter a ramp construction  228  provided in the webbing between the adaptor lugs  225 . The ramps  228  are arranged such that as soon as the adaptor part  210  starts to turn counter-clockwise, the ramps contact the end faces of the worm shaft lugs  226 . As the worm shaft lugs  226  move toward contact with the corresponding side faces of the adaptor part lugs  225 , the ramps  228  affect a cam action which pushes the worm shaft  110  axially away from the adaptor part  210 . This axial motion of the worm shaft  110  results in a corresponding axial motion of the one-way clutch output part  190 . Before the worm shaft  110  contacts the mating face  156  in the housing, the clutch output part  150  is pushed by worm shaft step  195  axially a distance sufficient to disengage it from the input part  180 . Thus, by rotating adaptor part  210  in the counter-clockwise direction, the ramps  228  cause the worm shaft to automatically take the teeth  186  of output part  190  out of engagement with the teeth  185  of the input part so that the worm shaft  110  is may turn freely, thereby permitting brake back-off to be accomplished in smooth manner without any forced slippage of clutch teeth. 
     An alternative embodiment of the present invention is illustrated in  FIG. 6 . In this embodiment, an output part disengaging actuator, here in the form of a slender coaxial rod  300  slidingly positioned within a hole running through the length of the worm shaft  100 , is used to axially displace the output part of the one-way clutch to disengage the output part from the input part. The inner end  310  the rod may rest against an actuating surface of the output part  190  of the one-way clutch, or as shown in  FIG. 6 , may be coupled to the output part. As with the  FIG. 4A  embodiment, the output part  190  is biased into engagement with control worm wheel  180  by a light spring  150 . One of ordinary skill will recognize that the output part disengaging actuator need not be acted directly by the adaptor part, or need not act directly on the output part, but may alternatively act indirectly through intermediate elements to transfer motion from the adaptor part to the output part of the one-way clutch. 
     The manual adjustment adaptor part  320  in this embodiment is constructed with a cam surface  330  on inside surface. The cam surface  330  engages with a mating cam  340  on the adaptor part end of the coaxial rod  300 . When the adaptor is turned in the brake back-off direction (in this embodiment, counter-clockwise), the interaction of the cam  340  with cam surface  330  pushes the coaxial rod  300  toward the one-way clutch, moving the output part  190  axially out of engagement with the input part. The adaptor part  320 , which is also arranged to engage and rotate the worm shaft  110 , is thereby free to turn the worm shaft in a smooth manner without any forced slippage of clutch teeth. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. For example, in the  FIG. 5A  embodiment, it does not matter whether the ramp is provided between the adaptor part lugs or between the worm shaft lugs, as long as the required camming action is provided to lift the one-way clutch output part axially out of engagement with the control worm wheel&#39;s one-way teeth. Because such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.