Abstract:
A brake shoe for a drum brake is provided that improves the direction of the brake actuating forces to reduce stress, improve efficiency and permit thicker brake linings. The brake shoe has a web having first and second ends with the first end configured for pivotally coupling to an associated brake spider. A brake table is supported on the web. The second end of the web is configured to engage an associated cam follower that causes the brake shoe to move between positions of engagement and disengagement with an associated braking surface at either of first and second radially offset positions on the second end of the web such that the cam followers can engage two brake shoes in the brake at offset positions and improve the direction of the force vectors.

Description:
BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     This invention relates to vehicle brakes. In particular, the invention relates to a drum brake in which the brake actuating means engages the brake shoes at offset positions to improve the direction of the brake actuating forces. 
     b. Background Art 
     Referring to  FIG. 1 , a conventional prior art drum brake  10  is illustrated. In a conventional drum brake, a drum  12  rotates with a wheel or wheels proximate to one end of an axle. The drum  12  defines a radially inner braking surface  14 . A brake spider  16  is disposed about the axle and a pair of brake shoes  18 ,  20  are pivotally mounted at one end to the brake spider  16 . The opposite end of each brake shoe  18 ,  20  is engaged by an actuating member such as a cam  22  to move the brake shoes  18 ,  20  between positions of engagement and disengagement with the braking surface of the brake drum. 
     In a conventional S-cam drum brake as shown in  FIG. 1 , rollers  24 ,  26 , or cam followers, are disposed between the brake shoes  18 ,  20  and the cam  22  to transfer actuating forces from the cam  22  to the brake shoes  18 ,  20 . Force is applied by the cam  22  through the rollers  24 ,  26  along the direction indicated by arrows  28 ,  30 , respectively, which extend from a “power circle” of the cam  22  defined by a radius from the rotational axis of the generally involute cam  22 . The force vector represented by arrows  28 ,  30  may be resolved into two components—a component represented by arrows  32 ,  34  comprising the effective brake actuation force tangent to the pivot arc of the corresponding brake shoe  18 ,  20  and a component represented by arrows  36 ,  38  comprising the divergence between the direction of the force exerted by the cam  22  on rollers  24 ,  26  and the effective brake actuation force. 
     The divergence between the actuating forces applied by the cam  22  and represented by arrows  28 ,  30  and the effective brake actuating force represented by arrows  32 ,  34  has several drawbacks. First, the force component represented by arrows  36 ,  38  creates mechanical stress in the webs of the brakes shoes  18 ,  20  and is particularly acute when the force component  36 ,  38  is directed outward in trailing brake shoes. The increased stress can lead to cracks in the brake shoe webs and costly downtime and repairs. Second, the actuation efficiency of the brake  10  is less than optimal because the effective brake actuation force is less than the force exerted by the cam  22 . As a result, more fluid pressure is required to actuate the brake leading to relatively large air chambers in brake actuators and/or requiring brake linings with relatively high friction coefficients. Third, the amount of rotation of cam  22  and, therefore, the displacement of brakes shoes  18 ,  20 , is relatively limited thereby limiting the potential thickness of the brake linings and requiring more frequent maintenance and/or repair. 
     The inventor herein has recognized a need for a brake that will reduce one or more of the above-identified deficiencies and/or provide improved performance. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention relates to vehicle brakes. In particular, the invention relates to a drum brake in which the brake actuating means engages the brake shoes at offset positions to improve the direction of the brake actuating forces. 
     A brake shoe for use in a drum brake in accordance with one embodiment of the invention includes a first web having first and second ends. The first end is configured for pivotally coupling to an associated brake spider of the drum brake. The brake shoe further includes a brake table supported on the first web. The second end of the first web is configured to engage an associated cam follower that causes the brake shoe to move between positions of engagement and disengagement with an associated braking surface at either of first and second radially offset positions on the second end of the first web. 
     A brake shoe for use in a drum brake in accordance with another embodiment of the invention includes a first web having first and second ends. The first end is configured for pivotally coupling to an associated brake spider of the drum brake. The brake shoe further includes a brake table supported on the first web. The second end of the first web defines first and second radially offset portions. Each of the first and second portions is configured to engage an associated cam follower that causes the brake shoe to move between positions of engagement and disengagement with an associated braking surface and to locate the can follower at corresponding first and second radially offset positions. 
     A brake shoe kit adapted for use in a drum brake in accordance with one embodiment of the invention includes a first brake shoe. The first brake shoe includes a web having first and second ends. The first end is configured for pivotally coupling to an associated brake spider of the drum brake and the second end is configured for engagement with an associated first cam follower through which the first brake shoe is moved between positions of engagement and disengagement with an associated braking surface. The second end includes a radially extending edge having radially inner and outer ends. The first brake shoe further includes a brake table supported on the web. The brake shoe kit further includes a second brake shoe. The second brake shoe includes a web having first and second ends. The first end of the web of the second brake shoe is configured for pivotally coupling to the brake spider of the drum brake and the second end of the web of the second brake shoe is configured for engagement with an associated second cam follower through which the second brake shoe is moved between positions of engagement and disengagement with the braking surface. The second end of the web of the second brake shoe includes a radially extending edge having radially inner and outer ends. The second brake shoe further includes a brake table supported on the web of the second brake shoe. The webs of the first and second brake shoes are configured to engage corresponding ones of the first and second cam followers so that a distance between the radially outer end of the radially extending edge of the web of the first brake shoe and a radially center point of engagement between the web of the first brake shoe and the first cam follower is different than a distance between the radially outer end of the radially extending edge of the web of the second brake shoe and a radially center point of engagement between the web of the second brake shoe and the second cam follower. 
     A brake in accordance with the invention represents an improvement as compared to conventional brakes. In particular, by adjusting the position of the cam followers and offsetting the cam followers, the force applied by the cam to the cam followers and brake shoes may be substantially tangent to the pivot arc of the brake shoes and reduces any divergence between the effective brake actuation force and the force applied by the cam. As a result, mechanical stress in the brake shoes webs is reduced as well as downtime and repair costs resulting from web cracking. The actuation efficiency of the brake is also increased enabling a reduction in the air chamber size for the brake actuator and/or the use of brake linings with lower coefficients of friction. The inventive brake also allows increased travel of the cam followers and allows the brake shoes to retract further thereby permitting the use of thicker brake linings and improving the life of the brake shoes. 
     The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a prior art brake. 
         FIG. 2  is a plan view of a brake in accordance with one embodiment of the present teachings. 
         FIG. 3  is a plan view of a brake in accordance with another embodiment of the present teachings. 
         FIG. 4  is an enlarged view of one end of a web for a brake shoe in accordance with the present teachings. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 2  illustrates a brake  40  in accordance with one embodiment of the present invention. Brake  40  is provided to slow rotation of one or more vehicle wheels. Brake  40  is particularly adapted for use in heavy vehicles. It should be understood, however, that brake  40  may be used on a wide variety of vehicles and in non-vehicular applications. Brake  40  is configured to act against an annular brake drum  42  that rotates with the vehicle wheel or wheels at one end of an axle (not shown). Brake  40  may include a brake spider  44 , one or more anchor pins  46 , brake shoes  48 ,  50 , return and retaining springs  52 ,  54 , and means, such as cam  56  and rollers or cam followers  58 ,  60 , for moving brake shoes  48 ,  50  between positions of engagement and disengagement with a braking surface. 
     Spider  44  is provided to mount the various components of brake  40 . Spider  44  defines a central aperture  62  having a center axis  64  which may be coincident with the rotational axis of the vehicle wheel. The aperture  62  is configured to receive a vehicle axle extending therethrough and along axis  64 . Spider  44  may further define bores (not shown) on either side of aperture  62  configured to receive anchor pin  46  and a camshaft (not shown) supporting cam  56 . 
     Anchor pin  46  is provided to pivotally mount brake shoes  48 ,  50  to brake spider  44 . Anchor pin  46  may comprise a round pin and may be mounted on and extend from brake spider  44 . Referring to  FIG. 3 , in an alternative embodiment, a brake  40 ′ may include multiple anchor pins  46  with each of brake shoes  48 ,  50  pivotally coupled to a separate anchor pin  46 . 
     Brake shoes  48 ,  50  are provided for selective engagement with a braking surface  66  of drum  42  in order to apply a braking torque to the drum and one or more vehicle wheels. Brake shoes  48 ,  50  may together comprise a brake shoe kit adapted for use in brake  40  or  40 ′. Brake shoes  48 ,  50  are supported on anchor pin(s)  46  and thereby pivotally coupled to spider  44  at one end. Each brake shoe  48 ,  50  may include one or more webs  68 , a brake table  70 , and one or more brake linings  72 . 
     Webs  68  support brake table  70 . Webs  68  may also provide a connection point for return spring  52  and retaining spring  54 . Webs  68  may be made from metals and metal alloys such as steel. Webs  68  are arcuate in shape and extend between opposite ends of brake shoes  48 ,  50 . It should be understood that the number of webs  68  in each brake  48 ,  50  may vary and each brake shoe  48 ,  50  may therefore include a plurality of webs  68  that extend generally parallel to one another. Webs  68  may be secured to brake table  70  using welds or other conventional fastening means. Each web  68  may have one end  74  that defines a semicircular recess  76  configured to receive a corresponding anchor pin  46 . In accordance with one aspect of the present teachings, the opposite end  78  of each web  68  may be configured to engage rollers  58 ,  60  at either of first and second radially offset positions. Referring to  FIG. 4 , end  78  of web  68  may be divided into a plurality of radially offset portions such as portions  80 ,  82 . Each portion  80 ,  82  may be configured to engage a roller  58 ,  60  and thereby locate rollers  58 ,  60  at corresponding radially offset positions. For example, web  68  may define one more semicircular recesses  84 ,  86  configured to receive rollers  58 ,  60  with each portion  80 ,  82  including a corresponding recess  84 ,  86 . End  78  of web  68  may further define a radially extending edge  88  having radially inner and outer ends  90 ,  92 . Recesses  84 ,  86  may be formed along edge  88  and may be disposed between ends  90 ,  92  of edge  88 . Recesses  84 ,  86  each define radially innermost and radially outermost points of engagement  94 ,  96  and  98 ,  100 , respectively, for rollers  58 ,  60  and a radially center point of engagement  102 ,  104 , respectively, approximately midway between the corresponding radially innermost and outermost points of engagement  94 ,  96  and  98 ,  100 . Referring to  FIGS. 2-3 , rollers  58 ,  60  may be positioned such that roller  58  is received within recess  84  of web  68  of brake shoe  48  while roller  60  is received within recess  86  of web  68  of brake shoe  50 . As a result, and with reference to  FIG. 4 , the distance in brake shoe  48  between the radially inner end  90  of edge  88  and the radially center point of engagement  102  of web  68  of brake shoe  48  and roller  58  is different than the distance in brake shoe  50  between the radially inner end  90  of edge  88  and the radially center point of engagement  104  of web  68  of brake shoe  50  and roller  60 . Similarly, the distance in brake shoe  48  between the radially outer end  92  of edge  88  and the radially center point of engagement  102  of web  68  of brake shoe  48  and roller  58  is different than the distance in brake shoe  50  between the radially outer end  92  of edge  88  and the radially center point of engagement  104  of web  68  of brake shoe  50  and roller  60 . 
     In the embodiments illustrated in  FIGS. 2 and 3 , the webs  68  of each brake shoe  48 ,  50  are identical in construction such that the webs  68  and brake shoes  48 ,  50  are interchangeable within brake  40  or  40 ′. In particular, each web  68  contains recesses  84 ,  86  with recess  84  disposed radially outward of recess  86 . In an alternative embodiment, however, the web(s)  68  of brakes shoes  48 ,  50 —and particularly the shape of ends  78  of brake webs  68 —may differ. For example, the web(s)  68  of brake shoe  48  may include only one of recesses  84 ,  86 —such as radially outer recess  84 —while the web(s)  68  of brake shoe  50  include the other of recesses  84 ,  86 —such as radially inner recess  86 . 
     Brake table  70  is provided to support brake linings  72 . Table  70  is supported on webs  68  and may be arcuate in shape. Table  70  may be made from conventional metals and metal alloys including steel. 
     Brake linings  72  are provided for frictional engagement with braking surface  66  of drum  42 . Linings  72  may be made from conventional friction materials. Brake linings  72  are disposed on brake table  70  and may be secured to brake table  70  using a plurality of rivets or other conventional fasteners. 
     Return spring  52  is provided to bias brake shoes  48 ,  50  to a position of disengagement from the braking surface  66  of drum  42 . Retainer springs  54  are provided to retain brake shoes  48 ,  50 —and particularly webs  68 —on anchor pin(s)  46 . Springs  52 ,  54  are conventional in the art. The ends of spring  52  may engage pins (not shown) extending from webs  68  of brakes shoes  48 ,  50  while the ends of springs  54  extend through corresponding apertures in webs  68  of brake shoes  48 ,  50 . 
     Cam  56 , together with rollers  58 ,  60 , provides an actuating assembly or means for moving brake shoes  48 ,  50  between positions of engagement with and disengagement from the braking surface  66  of the drum  42 . In the illustrated embodiment, cam  56  comprises a doubled lobed S-cam that engages rollers  58 ,  60 . Cam  56  is connected to one end of a camshaft (not shown) and rotates about a rotational axis  106  responsive to forces imposed by a brake actuator (not shown) on the camshaft. 
     Rollers  58 ,  60  are provided to transfer brake actuation forces from cam  56  to brake shoes  48 ,  50 . Rollers  58 ,  60  are circular in cross-section and are configured to be received within recesses  84 ,  86  of webs  68  formed at end  78  of shoes  48 ,  50 , respectively. Rollers  58 ,  60  engage webs  68  and cam  56  and follow the surface of the cam  56  as it rotates thereby causing shoes  48 ,  50  to pivot about a pivot axis  108  ( FIG. 2 ) or axes  110 ,  112  ( FIG. 3 ) defined at the center of anchor pins  46 . In accordance with the present invention, rollers  58 ,  60  are offset from one another. A plane  114  contains both axis  64  and the rotational axis  106  of cam  56 . Referring to  FIG. 2 , plane  114  may also contain the pivot axis  108  at the center of anchor pin  46 . Alternatively, and with reference to  FIG. 3 , in embodiments where multiple anchor pins  46  are employed the pivot axes  110 ,  112  of the anchor pins  46  may be equidistant from plane  114  on either side of plane  114 . Another plane  116  containing rotational axis  106  of cam  56  extends perpendicular to plane  114 . The center of roller  58  is disposed on one side of plane  116  while the center of roller  60  is disposed on the other side of plane  116 . Further, because rollers  58 ,  60  are offset from one another, a distance d 1  between axis  64  and the center of roller  58  is different than a distance d 2  between axis  64  and the center of roller  60 . In the illustrated embodiment distance d 1  is greater than distance d 2 . Similarly, and with reference to  FIG. 2 , a distance d 3  between the center of anchor pin  46  (i.e. pivot axis  108 ) and the center of roller  58  is different than a distance d 4  between the center of anchor pin  46  and the center of roller  60 . In the illustrated embodiment distance d 3  is greater than distance d 4 . Likewise, and with reference to  FIG. 3 , in a brake in which brake shoes  48 ,  50  are mounted on separate anchor pins  46  centered at points on either side of plane  114  and equidistant from plane  114 , the distance d 5  between the center of the anchor pin  46  supporting brake shoe  48  and the center of roller  58  is also different than the distance d 6  between the center of the anchor pin  46  supporting brake shoe  50  and the center of roller  60 . Although the distances between the anchor pin or pins  46  and the rollers  58 ,  60  differ in the illustrated embodiment, in certain embodiments the distances may be equal despite the offset position of the rollers by, for example, arranging the two anchor pins  46  in  FIG. 3  in a corresponding offset relationship. Although the distances from the anchor pin(s)  46  to rollers  58 ,  60  in  FIGS. 2 and 3  differ resulting in asymmetrical forces within the brake  40  or  40 ′, the impact is substantially less than the asymmetrical forces already present in the brake  40  or  40 ′ due to the typical dynamic self-energizing action of the leading brake shoes. The exact position of the rollers  58 ,  60  may be further optimized to account for friction in the roller journals. 
     A brake  40  or  40 ′ in accordance with the invention represents an improvement as compared to conventional brakes. In particular, by adjusting the position of the rollers  58 ,  60  and offsetting the rollers  58 ,  60 , cam  56  applies actuating forces (represented by arrows  118 ,  120 ) to rollers  58 ,  60  and brake shoes  48 ,  50  in directions perpendicular to plane  114  (and parallel to plane  116 ). These forces are substantially tangent to the pivot arc of the brake shoes  48 ,  50  and therefore reduce or eliminate any divergence between the effective brake actuation force and the force applied by the cam  56 . As a result, mechanical stress in the brake shoes webs  68  is reduced as well as the resulting downtime and repair costs resulting from web cracking. The actuation efficiency of the brake  40  or  40 ′ is also increased enabling a reduction in the air chamber size for the brake actuator and/or the use of brake linings  72  with lower coefficients of friction. The inventive brake  40  or  40 ′ also allows increased travel of the cam followers  58 ,  60  and allows the brake shoes  48 ,  50  to retract further thereby permitting the use of thicker brake linings  72  and improving the life of the brake  40  or  40 ′. 
     While the invention has been shown and described with reference to one or more particular embodiments thereof, it will be understood by those of skill in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.