Abstract:
A slack adjuster for a vehicle brake includes a slack adjuster body having a cylindrical inner surface at least partially defining a chamber in the body. A worm gear is received in the chamber in the body. At least one low friction ring is interposed between the worm gear and the body and supports the worm gear for rotation in the chamber in the body. A method of remanufacturing a slack adjuster includes the steps of removing a worm gear and at least one low friction ring from a body of the slack adjuster; setting aside the removed low friction ring; and putting the worm gear and at least one new low friction ring into the body.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a slack adjuster. A slack adjuster is an integral part of actuating linkage for a vehicle air brake system on heavy duty vehicles, such as buses and trucks. The slack adjuster transmits braking force to a brake shaft, which applies the braking force to a cam and thereby to the brake shoes and drum associated with a road wheel of the vehicle.  
         [0002]     One common type of slack adjuster includes a worm and a worm gear that are in meshing engagement within a housing or body. The worm gear is a metal gear that rotates within a cylindrical opening in a metal body. Annular shoulders on the worm gear engage the cylindrical surface of the body, to support the worm gear for rotation within the body. The brake shaft extends through the adjuster and is fixed for rotation with the worm gear by a splined connection. The worm and the worm gear are relatively rotatable to effect adjustment of the linkage to accommodate clearance that develops in the system with extended usage of the brake.  
         [0003]     In this type of slack adjuster, the braking force is applied to the body of the slack adjuster through an actuator. The body transmits the braking force, through the worm and the worm gear, to the splined connection with the brake shaft. The brake shaft is thus rotated to actuate the brake.  
         [0004]     The worm gear is subjected to the entire braking force passing through the brake adjuster. The resulting force presses the worm gear against the body which causes a substantial amount of friction. This friction in turn causes galling of the body material and worm gear as they interface. When the damage from galling becomes severe the performance of the slack adjuster can be greatly reduced.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention relates to a slack adjuster for a vehicle brake. The adjuster includes a slack adjuster body having a cylindrical inner surface at least partially defining a chamber in the body. A worm gear is received in the chamber in the body. At least one low friction ring is interposed between the worm gear and the body and supports the worm gear for rotation in the chamber in the body.  
         [0006]     The present invention also relates to a method of remanufacturing a slack adjuster including the steps of removing q worm gear and at least one low friction ring from a body of the slack adjuster; setting aside the removed low friction ring; and putting the worm gear and at least one new low friction ring into the body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, in which:  
         [0008]      FIG. 1  is a perspective view of a slack adjuster in accordance with a first embodiment of the invention;  
         [0009]      FIG. 2  is a front elevational view of the slack adjuster of  FIG. 1 ;  
         [0010]      FIG. 3  is a side elevational view of the slack adjuster of  FIG. 1 ;  
         [0011]      FIG. 4  is a radial sectional view of the slack adjuster of  FIG. 1 ;  
         [0012]      FIG. 5  is an exploded sectional view of the slack adjuster of  FIG. 1 ;  
         [0013]      FIG. 6  is an axial sectional view of the slack adjuster of  FIG. 1 ;  
         [0014]      FIG. 7  is an axial sectional view of a slack adjuster in accordance with a second embodiment of the invention;  
         [0015]      FIG. 8  is an exploded sectional view of the slack adjuster of  FIG. 7 ; and  
         [0016]      FIG. 9  illustrates graphically a method of remanufacturing a slack adjuster in accordance with the invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0017]     The present invention relates to a slack adjuster and is applicable to various slack adjuster constructions. As representative of the present invention,  FIGS. 1-6  illustrate a slack adjuster  10  in accordance with a first embodiment of the invention  
         [0018]     The slack adjuster  10  ( FIGS. 1-3 ) is connected between brake linkage  12  that forms a part of an actuator, and a brake shaft  14 . The application of an actuating (braking) force to the linkage  12  will operate through the adjuster  10  to rotate the brake shaft  14  and apply the brakes by engaging brake shoes with the brake drum  16 . The slack adjuster  10  is adjustable, in a known manner, to take up the slack in the brake system which occurs after prolonged use.  
         [0019]     The slack adjuster  10  ( FIGS. 4 and 5 ) includes a body  20  having a cylindrical inner surface  22 . The surface  22  defines a circular central opening or chamber  24  centered on an axis  26 .  
         [0020]     A worm gear  30  is received in the chamber  24  and is rotatable in the chamber about the axis  26 . The worm gear  30  has a generally cylindrical configuration including an intermediate gear tooth portion  32  and first and second end portions  34  and  36 . The intermediate portion  32  of the worm gear  30  includes a set of spiral gear teeth  42 . The worm gear  30  has a central opening  38  that receives the brake shaft  14  in a splined connection  40 .  
         [0021]     The intermediate portion  32  of the worm gear  30  has an annular, radially extending surface  46  that forms an edge of the gear tooth portion of the worm gear. The surface  46  faces axially outward of the worm gear  40 . The first end portion  34  of the worm gear  30  has a cylindrical surface  48  that extends axially outward from the radially extending surface  46 . The two surfaces  46  and  48  form a first shoulder  50 , on the worm gear  30 , facing away from the central gear tooth portion  32  of the worm gear.  
         [0022]     The central portion  22  of the worm gear  30  has an annular, radially extending surface  52  that forms an edge of the gear tooth portion of the worm gear. The surface  52  faces axially outward of the worm gear  30 . The second end portion  36  of the worm gear  30  has a cylindrical surface  54  that extends axially outward from the radially extending surface  52 . The two surfaces  52  and  54  form a second shoulder  56 , on the worm gear  30 , facing away from the central gear tooth portion  32  of the worm gear.  
         [0023]     The slack adjuster  10  includes one or more bearings or low friction rings for reducing friction between the worm gear  30  and the body  20 . In the embodiment illustrated in  FIGS. 1-6 , the slack adjuster  10  includes two low friction rings  60  and  62 . In the embodiment illustrated in  FIGS. 7-8  and described below, the slack adjuster  10   a  includes one low friction ring  80 . In other embodiments of a slack adjuster in accordance with the invention, more than two low friction rings may be included, and the low friction rings may be of a different type than as illustrated herein.  
         [0024]     The two low friction rings  60  and  62  shown in  FIGS. 5 and 6  are identical to each other. In other embodiments they may be different, for example, of differing axial widths to fit a particular slack adjuster. Each low friction ring  60  or  62  is an annulus or ring having a cylindrical inner side surface  64 , a cylindrical outer side surface  66 , and parallel, radially extending inner and outer edge surfaces  68  and  69 . A low friction ring of the present invention may be split as illustrated at  71  ( FIG. 4 ), or may be solid (unbroken around its circumference).  
         [0025]     The low friction rings of the present invention are made from a material having a lower coefficient of friction than the worm gear itself on metal. One suitable material is nylon. Another suitable material is bronze. Other materials may be used.  
         [0026]     Each low friction ring  60  or  62  is located in one of the shoulders  50  or  56  of the worm gear  30 . The inner side surface  64  of the first low friction ring  60  engages the cylindrical surface  48  of the first shoulder  50  of the worm gear  30 . The inner edge surface  68  of the ring  60  engages the annular side surface  46  of the first shoulder  50 .  
         [0027]     The inner side surface  64  of the second low friction ring  62  engages the cylindrical surface  54  of the second shoulder  56  of the worm gear  30 . The inner edge surface  68  of the ring  62  engages the annular side surface  52  of the second shoulder. The outer edge surfaces  69  of the rings  60  and  62  engage the inner wall of the body  20  and the cover  74 . The rings  60  and  62  thus provide a low-friction fit between the worm gear  30  and the body  20  in an axial direction as well.  
         [0028]     The outer diameter of the low friction rings  60  and  62  is greater than the outer diameter of any portion of the worm gear  30  including the first and second end portions  34  and  36  and the central gear tooth portion  32 . As a result, the gear teeth  42  on the worm gear  30  have a smaller diameter than the low friction rings  60  and  62 .  
         [0029]     The slack adjuster  10  also includes a worm  70  located in the body  20  and in meshing engagement with the worm gear  30 . The worm  70  projects through a cutout  72  in the inner surface  22  of the body  20  to enable the worm to mesh with the worm gear  30 . The slack adjuster  10  also includes a cover  74  that closes the open end of the chamber  24  and helps to retain the worm gear  30  and the low friction rings  60  and  62 .  
         [0030]     When the low friction rings  60  and  62  and the worm gear  30  are in the body  20 , the low friction rings support the worm gear for rotation relative to the body, about the axis  26 . The outer side surfaces  66  of the low friction rings  60  and  62  engage the cylindrical inner surface  22  of the body  20 . No portion of the worm gear including the first and second end portions  34  and  36  and the central gear tooth portion  32  engages the inner surface  22  of the body  20 . As a result, the worm gear  30  can rotate within the body  20  without engaging the body. When a load is applied to the worm gear  30  that tends to urge the worm gear in a direction radially toward the body  20 , the low friction rings  60  and  62  transmit that load to the body without engagement of the worm gear with the body.  
         [0031]     The low friction rings  60  and  62  thus reduce wear on the worm gear  30  and on the body  20 . Specifically, slack adjusters  10  including the low friction rings  60  and  62 , in repeated testing, show 5 times as much useful life as those without the low friction rings. This 400% increase in useful life is attained with minimal increase in cost or manufacturing complexity compared to the overall cost of the slack adjuster. Also, a less forceful actuator can be used, and still maintain proper slack adjuster operation.  
         [0032]     In addition, the amount of wear experienced by the worm gear  30  and the body  20  is so low that a slack adjuster  10  including the low friction rings  60  and  62  is remanufacturable. The slack adjuster can be remanufactured (rebuilt) in the following manner, as illustrated graphically in  FIG. 9 .  
         [0033]     The cover  74  on the end of the body  20  is removed. The worm gear  30  and the low friction rings  60  and  62  are removed. The used low friction rings  60  and  62  are set aside. New low friction rings  60  and  62  are placed on the same worm gear  30  or in the same body  20 . The worm gear and the new low friction rings are placed again in the chamber in the body. The removed cover  74  is replaced. At this point, a fully functional slack adjuster  10  is available for reuse or sale, using the previous body and worm gear.  
         [0034]      FIGS. 7 and 8  illustrate a slack adjuster  10   a  constructed in accordance with a second embodiment of the invention. The slack adjuster  10   a  is generally similar in construction to the slack adjuster  10  ( FIGS. 1-6 ), and parts that are the same or similar are given the same reference numeral with the suffix “a” added.  
         [0035]     In the slack adjuster  10   a , a single low friction ring in the form of a wear sleeve  80  is supported on the body  20   a , in place of the two low friction rings  50  and  62  that are on the worm gear. The sleeve  80  is preferably made from nylon, but may alternatively be made from another low friction material, such as oil-impregnated bronze.  
         [0036]     The sleeve  80  has a cylindrical configuration including parallel inner and outer side surfaces  82  and  84 . The sleeve  80  is fitted in the chamber  24   a  in the body  20   a . The chamber  24   a  may be made larger in diameter that the chamber  24 , to accommodate the sleeve  80 . The sleeve  80  is fixed in the body  20   a  in a manner not shown. The sleeve  80  has a cutout  86  that aligns with the cutout in the body  20   a  to enable the worm to mesh with the worm gear.  
         [0037]     The end portions of the worm gear  30  are formed as lands  88  that are larger in diameter than the central gear tooth portion  32   a . The lands  88  have cylindrical outer side surfaces that engage the cylindrical inner surface  22   a  of the sleeve  80 .  
         [0038]     When the worm gear  30   a  rotates in the body  20   a , the metal of the lands  88  of the worm gear rides against the low friction material of the sleeve  80 . The friction between these parts is substantially less than the friction between a metal worm gear and a metal body. Test results show a reduction in wear and an increase in product life similar to that experienced with the first embodiment ( FIGS. 1-6 ). In addition, the adjuster  10   a  is remanufacturable in the manner shown in  FIG. 9  and described above with reference to the adjuster  10 .  
         [0039]     From the above description of the invention, those skilled in the art will perceive improvements, changes, and modifications in the invention. Such improvements, changes, and modifications within the skill of the art are intended to be included within the scope of the appended claims.