Patent Publication Number: US-2013240308-A1

Title: Adjustment assembly installation into automatic slack adjuster housing

Description:
TECHNICAL FIELD 
     The subject invention relates to an automatic slack adjuster that includes an adjustment assembly that is installed from one direction into a blind-bore formed within a slack adjuster housing. 
     BACKGROUND OF THE INVENTION 
     An automatic slack adjuster is used to adjust a brake assembly as brake linings wear. The automatic slack adjuster adjusts the amount of slack, or “free play”, in the brake to ensure that an associated air brake chamber can produce enough actuation force. With too little slack, the brake may drag and overheat, and if there is too much slack, the brake may not be capable of generating enough braking effort to stop the vehicle. 
     Packaging automatic slack adjusters on axles and suspensions can be difficult in many situations due to lack of available space. In one known configuration, the slack adjuster includes a worm gear that is coupled to a camshaft. A worm shaft is in driving engagement with the worm gear. An adjustment assembly couples the worm shaft to a push rod of a brake chamber. The adjustment assembly is configured to automatically adjust slack as brake linings wear by rotating the worm shaft, which in turn rotates the worm gear and camshaft. The slack adjuster is also configured with a manual adjuster that allows for selective manual adjustment of the slack, which is useful when new brake linings are installed for example. 
     The worm shaft, worm gear, and adjustment assembly are mounted within a housing. The housing has a lever coupled to the push rod of the brake chamber. The worm shaft in this configuration is positioned to extend in a vertical direction relative to the worm gear, i.e. generally parallel to the lever. A thru-bore having two open ends is formed in the housing to receive the worm shaft and adjustment assembly. The thru-bore is machined from two different directions and adjustment components are inserted through both open ends for installation purposes. Having two open ends provides flexibility for installation of manual adjuster components. 
     One disadvantage with this known configuration is that the vertical orientation of the worm shaft increases the overall width of the slack adjuster. This makes it difficult to package the slack adjuster in all possible packaging locations. Another disadvantage is that the two open ends of the thru-bore provide potential leak paths for contaminants that may be sprayed onto the slack adjuster by rotating tires. 
     Other known configurations use a worm gear that is orientated to extend in a horizontal direction relative to the worm gear, i.e. the worm shaft extends transversely relative to the lever. These configurations also include a thru-bore with two open ends. The thru-bore is machined from two different directions and adjustment components are inserted through both open ends for installation purposes. While the horizontal orientation of the worm shaft improves packaging, there is still potential for significant leakage of contaminants into the thru-bore, which can result in premature wear and failure of adjustment components. 
     SUMMARY OF THE INVENTION 
     A slack adjuster provides automatic slack adjustment during normal braking operations by adjusting a camshaft via an adjustment assembly. In one example, the slack adjuster comprises a worm gear configured to be coupled to the camshaft, a worm shaft in engagement with the worm gear, and a housing having a lever configured for connection to a brake chamber output rod. The housing includes an opening that receives the worm gear and a blind bore that receives the worm shaft. An adjustment assembly is installed within the blind bore and is coupled to the worm shaft. The adjustment assembly is configured to automatically rotate the worm shaft when brake slack exceeds a predetermined limit. 
     In one example, the blind bore extends in a horizontal direction that is transverse to the lever. 
     In one example, the blind bore is machined from a single direction. The worm shaft and adjustment assembly are then inserted through the only open end of the blind bore. An end cover and seal cooperate to provide a sealed interface once the worm shaft and adjustment assembly are installed. 
     In one example, the adjustment assembly comprises a control disc that is fixed for rotation with the worm shaft and an actuator disc that is coupled to a push rod. The push rod is configured to be coupled to the lever via a coupling member. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a cam brake assembly. 
         FIG. 2  is an exploded view of a cam brake assembly such as that depicted in  FIG. 1 . 
         FIG. 3  is a side view of a slack adjuster in an assembled condition. 
         FIG. 4  is an exploded view of the slack adjuster of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the slack adjuster of  FIG. 3 . 
         FIG. 6A  is a magnified view of  FIG. 5  showing a manual adjuster in a non-adjusting position. 
         FIG. 6B  is a magnified view of  FIG. 5  showing the manual adjuster in an adjusting position. 
         FIG. 7  is a figure similar to  FIG. 3  but shows a cross-section at a connection between an actuator disc and push rod. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     As shown in  FIGS. 1 and 2 , a brake actuation system  10  is used to actuate a vehicle wheel brake  12  in response to a brake request. In one example, the wheel brake  12  comprises a cam brake with a camshaft  14  that is configured to press a pair of brake shoes  16  into engagement with a brake drum  18  configured to rotate with a vehicle wheel. The camshaft  14  includes an S-shaped cam  20  at one end that, upon rotation, is configured to pivot the brake shoes  16  outwardly into engagement with an inner braking surface  22  of the drum  18 . 
     The brake shoes  16  are coupled to a brake spider  24  via anchor pins  26 . The brake spider  24  is configured to be mounted to an axle housing or non-rotating wheel end component. A return spring  28   a  facilitates returning the brake shoes  16  to the non-applied position upon completion of a braking request. Shoe retaining springs  28   b  couple the brake shoes together adjacent the anchor pins  26 . 
     As shown in  FIG. 3 , the actuation system  10  includes a pressure input member, such as a brake air chamber  30  for example, which has an actuating rod  32  that is coupled to a brake actuation lever  34 . Pressure operating on a diaphragm within the brake chamber  30  in response to a brake request, such as that initiated by depression of a brake foot pedal, forces the actuating rod  32  outwardly, which in turn rotates the actuation lever  34 . The lever  34  extends in a generally vertical direction and is coupled to the camshaft  14 , which rotates the cam  20  to press the brake shoes  16  outwardly into engagement with the brake drum  18  to stop the vehicle. 
     When the brake pedal is released, air pressure is exhausted from the brake chamber  30  and a spring or other return mechanism is configured to return the actuating rod  32  and lever  34  to a retracted, brake-off position. This also causes the brake shoes  16  to return to a non-applied position. In this non-applied position, there is a clearance space S, i.e. slack, between brake friction linings  36  on the brake shoes  16  and the engagement surface  22  on the brake drum  18 . Over time, the thickness of the brake friction linings  36  decreases or wears, and this in turn increases the slack. If slack becomes excessive, it may not be possible to provide a sufficient braking force to the drum  18 , i.e. the actuating rod  32  may not be able to rotate the lever  34  far enough to exert a sufficient braking force against the drum  18 . 
     To ensure that the actuation lever  34  is rotated to produce a desired amount of brake force, the lever  34  is coupled to a coupling member  38 , which can be adjusted throughout the life of the brake shoes to ensure that the cam  20  is properly positioned in relation to the brake shoes to provide the desired braking force. An automatic slack adjuster mechanism  40  is used to automatically readjust the coupling member as the brake linings wear. 
     When an automatic slack adjuster mechanism  40  is installed, a brake chamber stroke measurement is set, which corresponds to the desired or correct shoe-to-drum clearance. As the brake linings  36  wear, this clearance, i.e. slack, increases and the brake chamber actuating rod  32  must travel farther to apply the brakes. Once this clearance or “slack” is increased beyond a predetermined limit, the slack adjuster is configured to automatically adjust during the return stroke to maintain the correct shoe-to-drum clearance. If the air brake chamber push rod stroke is within acceptable limits during operation, no adjustment occurs. 
     In the example shown in  FIGS. 4-6B , the coupling member  38  comprises a worm gear  42  positioned within a housing  44 . The worm gear  42  includes a splined inner opening  46  that is attached to a splined end  48  of the camshaft  14  ( FIG. 2 ) that is opposite of the cam  20 . A first set of worm teeth  50  are formed about an outer circumference of the worm gear  42 . 
     The slack adjuster mechanism  40  includes a worm shaft  52 , which has a second set of worm teeth  54  that are directly engaged with the first set of worm teeth  50 . The worm shaft  52  is positioned within the housing  44  such that the worm shaft  52  is extending in a generally horizontal direction relative to the lever  34 . In other words, the worm shaft  52  extends transversely relative to the lever  34 , as opposed to extending generally parallel to the lever  34 . 
     The slack adjuster mechanism  40  further includes an actuator disc  60 , a control disc  62 , a bushing  64 , and an end cover  66 . The housing  44  includes a blind-bore or blind-hole  70 , i.e. a hole having only one open end, which receives these components, as well as the worm shaft  52 , as a sub-assembly. The blind-hole  70  is solely formed by a machining operation from one side of the housing  44  such that the hole  70  does not extend entirely through the housing  44  as shown in  FIG. 5 . This provides a fully enclosed end within the housing itself, and eliminates any cap, plug, or cover that is typically required to seal off an open end to protect the internal components from contaminants. 
     The worm shaft  52  includes a gear portion  72  that includes the second set of worm teeth  54 , a reduced neck portion  74  that supports the bushing  64 , and a splined shaft portion  76  that is coupled to the control disc  62  via a mating splined bore  78  formed within the control disc  62 . As shown in  FIG. 6A , one end face of the bushing  64  engages an end face on the worm shaft between the neck portion  74  and gear portion  72 . An opposite end face of the bushing  64  engages a shoulder  80  formed within the housing  44 . The bushing  64  includes a bore  82  that rotatably supports the worm shaft  52  at the neck portion  74 . 
     The control disc  62  includes a first end face  84  that faces the bushing  64  and a second end face  86  that faces the end cover  66 . The control disc  62  includes a plurality of control teeth  88  on the second end face  86 . A resilient member  90 , such as a coil spring for example, has one spring end in engagement with the first end face  84  and a second spring end in engagement with the bushing  64 . The resilient member  90  loads the control teeth  88  into engagement with actuator teeth  92  on the actuator disc  60 . As such, the actuator  60  and control  62  discs are always in contact with each other. In one rotational direction, i.e. a brake apply direction, the actuator teeth  92  skip past the control teeth  88 . In an opposite rotational direction, i.e. a brake return direction, the actuator teeth  92  drive against the control teeth  88 , causing the control disc  62  to rotate, and hence the worm shaft  52  to rotate. 
     The actuator teeth  92  are formed on a first end  94  of the actuator disc  60 , which faces the control disc  62 . A second end  96  of the actuator disc  60  faces the end cover  66 . The actuator disc  60  has an outer circumference that is close to the diameter of the blind-hole  70  such that the actuator disc  60  sits within the blind-hole in a light press-fit or slightly loose fit. A snap ring  98  is installed within the blind-hole to hold the actuator disc  60  at a desired axial location. 
     The end cover  66  includes a hub portion  100  at one end and a knob portion  102  that extends outwardly of the housing  44  at an opposite end. The hub portion  100  includes an outer surface  104  that has a slightly smaller diameter than an inner diameter of a center bore  106  of the actuator disc  60 . This allows the hub portion  100  to be inserted into the center bore  106  without contacting the actuator disc  60 . The hub portion  100  includes a blind-bore  108  that has a splined inner circumferential surface  110 . An end face  112  of the hub portion  100  abuts against the control disc  62 . 
     A seal  114  holds the end cover  66  within the bore  70 . An optional snap ring  116  ( FIG. 4 ) could also be used to provide additional securement for the end cover  66 . The seal  114  abuts against a flange portion  118  of the end cover  66 . The flange portion  118  is generally centrally located on the end cover and is defined by a diameter that is larger than the hub  100  and knob  102  portions. An outer surface  120  of the seal  114  sealing engages a surface  122  that defines the bore  70 . 
     The knob portion  102  extends outwardly of the housing  44 . In one example, the knob portion  102  is configured to provide a grasping surface and/or a tool engagement surface  124  to provide a manual adjustment feature. This will be discussed in greater detail below. 
     The housing  44  ( FIGS. 3-4 ) includes a clevis attachment  130  that is coupled to the push rod  32  of the air chamber  30  in a known manner. A push rod  132  ( FIG. 4 ) has a first end  134  coupled to the clevis attachment  130  in a known manner and a second end  136  that is in engagement with the actuator disc  60  as shown in  FIG. 7 . In the example shown, the actuator disc  60  includes a pair of grooves  128  ( FIG. 4 ) formed in an outer surface on one side of the actuator disc  60 . A mating portion of the push rod  132 , adjacent the second end  136 , cooperates with these grooves  128  such that the push rod  132  and actuator disc  60  are always in contact. The housing  44  includes an opening on an upper surface that extends into the blind bore to allow the push rod  132  to engage the actuator disc  60 . 
     The slack adjuster mechanism  40  operates in the following manner. A brake request is initiated, for example, by pressing a brake pedal. In the brake apply direction, if there is significant lining wear, i.e. lining wear has exceed a predetermined limit, the push rod  132  is lifted and rotates the actuator disc  60  such that the actuator teeth  92  skip past the control teeth  88  at least one increment, i.e. at least one tooth increment. The adjustment is made on the brake return stroke as the actuator teeth  92  and control teeth  88  are in driving engagement upon opposite rotation, i.e. the actuator  60  and control  62  discs are locked together on the return stroke. This causes the worm shaft  52  to provide incremental adjustment by rotating the worm gear  42 , which in turn rotates the camshaft  14  to adjust the position of the cam  20 . 
     The slack adjustment mechanism  40  can also be manually adjusted as needed. In a normal, non-adjustment position ( FIG. 6A ), the seal  114  holds the end cover  66  in place so that the end cover cannot fall out of the hole  70 . The splined inner circumferential surface  110  of the bore  108  is axially spaced from the splined shaft portion  76  of the worm shaft  52 . In this position the end cover  66  can rotate freely within the hole  70 . 
     To manually adjust the worm gear, the end cover  66  is pressed axially inward in a direction toward the worm shaft  52 . This movement results in two simultaneous actions. First, the control disc  62  is disengaged from the actuator disc  60 . Second, the splined inner circumferential surface  110  of the end cover  66  is slid into engagement with the splined shaft portion  76  of the worm shaft  52 , as shown in  FIG. 6B . Rotation of the end cover  66  in this position results in rotation of the worm shaft  52  to adjust slack by rotating the worm gear  42  and camshaft  14 . To rotate the end cover  66 , the knob portion  102  is rotated by using a tool, for example. 
     This manual adjustment configuration is advantageous for several reasons. The end cover  66  serves to seal the open end of the blind-hole  70  to prevent contaminants from adversely affecting the internal components. Also, a single tool can be used to provide the adjustment. Finally, the end cover does not interface with other components during non-manual adjustment operations. As such, the torque required to adjust the slack is low. Also, there is no wear or damage to adjuster components during manual adjustment because the end cover has disengaged the automatic adjustment system, i.e. the control disc  62  is disengaged from the actuator disc  60 . Application Ser. No. ______, directed to manual adjustment, is filed on an even date herewith and is assigned to the assignee of the present application. 
     Another advantage with the subject slack adjuster mechanism  40  is provided by the blind-hole  70 . By assembling the internal slack adjuster components as a sub-assembly that is inserted into the blind-hole  70 , overall packaging size is reduced. The compact sub-assembly of internal components, i.e. actuator disc  60 , control disc  62 , spring  90 , bushing  64 , and end cover  66 , allow for a very short worm shaft  52  and a narrower housing  44 . Further, by using a blind-hole as opposed to having a hole open at both ends, a potential leak path is eliminated. This is critical due to the location of the slack adjuster in relation to the vehicle tires which spray water and road debris onto the slack adjuster. Finally, the blind-hole configuration offers a design where all machining can be accomplished from one direction. All the internal components are loaded in the same direction, which simplifies assembly and shortens the time for manufacturing purposes. 
     The cam operated brake is shown merely as an example, it should be understood that other brake configurations could also be used with the subject slack adjuster. Further, although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.