Abstract:
This invention relates to an improved structure for a piston assembly adapted for use in a wheel cylinder of a drum brake assembly, the drum brake assembly having a pair of brake shoes which are adapted to be urged into frictional engagement with a brake drum. The piston assembly includes a piston body formed from a first material, and a piston cover formed from a second material and integrally molded in situ with the piston body. The piston cover includes an end wall defining an abutment surface which is adapted to engage an upper end of the brake shoe during brake actuation. The piston cover further includes a plurality of circumferentially spaced projections extending from the end wall of the piston cover into the piston body and integrally molded and embedded within the piston body. Each of the projections of the piston cover defines first and second side edges which are in contact with the piston body to thereby assist in securing the piston cover to the piston body.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to vehicle drum brake assemblies and in particular to an improved structure for a piston assembly adapted for use in a wheel cylinder of such a vehicle drum brake assembly. 
     Most vehicles are equipped with a brake system for slowing or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies can be actuated by hydraulic, pneumatic, or mechanical pressure generated by an operator of the vehicle depressing a foot pedal, pulling a hand lever, and the like. The structure and operation of both drum brake assemblies and disc brake assemblies, as well as the actuators therefor, are well known in the art. 
     A typical drum brake assembly includes a backing plate which is secured to a fixed, non-rotatable component of the vehicle, such as the vehicle axle housing. A pair of opposed arcuate brake shoes are supported on the backing plate for selective movement relative thereto. Each of the brake shoes has a friction pad secured thereto. The drum brake assembly further includes a cylindrical brake drum which is secured to the vehicle wheel for rotation therewith. The interior of the brake drum is hollow, defining an inner cylindrical braking surface. The brake drum is disposed adjacent to the backing plate such that the brake shoes extend within the inner cylindrical braking surface. To effect braking action, the brake shoes are moved outwardly apart from one another such that the friction pads frictionally engage the inner cylindrical braking surface of the brake drum. Such frictional engagement causes slowing or stopping of the rotational movement of the brake drum and, therefore, the wheel of the vehicle in a controlled manner. 
     One or more actuating mechanisms are provided in the drum brake assembly for selectively moving the brake shoes outwardly apart from one another into frictional engagement with the cylindrical braking surface of the brake drum. Usually, a hydraulically or pneumatically actuated service brake mechanism is provided for selectively actuating the drum brake assembly under normal operating conditions. Such a service brake mechanism can include a hydraulically actuated wheel cylinder having a pair of opposed pistons which abut and move the brake shoes apart from one another into frictional engagement with the cylindrical braking surface of the brake drum. 
     A mechanically actuated parking and emergency brake mechanism is also usually provided for selectively actuating the drum brake assembly. The parking and service brake mechanism can include an actuating lever pivotally supported on one of the brake shoes. The actuating lever is connected to a cable which, when pulled, moves the brake shoes apart from one another into frictional engagement with the cylindrical braking surface of the brake drum. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved structure for a piston assembly adapted for use in a wheel cylinder of a drum brake assembly, the drum brake assembly having a pair of brake shoes which are adapted to be urged into frictional engagement with a brake drum. The piston assembly includes a piston body formed from a first material, and a piston cover formed from a second material and integrally molded in situ with the piston body. The piston cover includes an end wall defining an abutment surface which is adapted to engage an upper end of the brake shoe during brake actuation. The piston cover further includes a plurality of circumferentially spaced projections extending from the end wall of the piston cover into the piston body and integrally molded and embedded within the piston body. Each of the projections of the piston cover defines first and second side edges which are in contact with the piston body to thereby assist in securing the piston cover to the piston body. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a portion of a prior art drum brake assembly. 
     FIG. 2 is a sectional view of a first embodiment of a piston assembly constructed in accordance with this invention and adapted for use in the prior art drum brake assembly illustrated in FIG.  1 . 
     FIG. 3 is a plan view of the end of the cover provided on the piston assembly illustrated in FIG.  2 . 
     FIG. 4 is a sectional view of a second embodiment of a piston assembly constructed in accordance with this invention. 
     FIG. 5 is a plan view of the end of the cover provided on the piston assembly illustrated in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in FIG. 2 a first embodiment of a piston assembly, indicated generally at  60 , constructed in accordance with the present invention. The piston assembly  60  is adapted to be used in a wheel cylinder of a drum brake assembly, such as that shown in prior art FIG.  1 . The general structure and operation of the prior art drum brake assembly, indicated generally at  10  in FIG. 1, is conventional in the art. Thus, only those portions of the prior art drum brake assembly  10  which are necessary for a full understanding of this invention will be explained and illustrated in detail. Although this invention will be described and illustrated in connection with the particular vehicle prior art drum brake assembly disclosed in FIG. 1, it will be appreciated that this invention may be used in connection with other drum brake assemblies. 
     The illustrated prior art drum brake assembly  10  includes a rigid backing plate  11  which is generally flat and circular in shape. The backing plate  11  is adapted to be secured to a fixed, non-rotatable component of the vehicle, such as an outer end of a housing (not shown) for enclosing a rotatable axle. To accomplish this, a relatively large opening  12  is formed through a central portion of the backing plate  11 . The central opening  12  is provided to permit the outer end of the rotatable axle to extend therethrough to the driven vehicle wheel (not shown). A plurality of relatively small holes  13  are also formed through the backing plate  11 , located about the central opening  12 . The small holes  13  are provided to permit threaded bolts (not shown) to extend therethrough to secure the backing plate  11  to the outer end of the axle housing. 
     An abutment block assembly, indicated generally at  15 , is provided on an outer surface of the backing plate  11 . The abutment block assembly  15  includes an inner spacer plate  16 , which is disposed adjacent to the outer surface of the backing plate  11 , and an outer retainer plate  17 , which is disposed adjacent to the spacer plate  16 . The spacer plate  16  and the retainer plate  17  are secured to the backing plate  11  by a pair of rivets  18  or similar fasteners which extend therethrough to the inner surface of the backing plate  11 . 
     The prior art drum brake assembly  10  further includes first and second brake shoes, indicated generally at  20  and  20 ′. Structurally, the brake shoes  20  and  20 ′ are essentially mirror images of one another, and like reference numbers are used to indicate similar parts. The brake shoes  20  and  20 ′ include respective web portions  21  and  21 ′ which are generally flat and crescent-shaped. Arcuate table portions  22  and  22 ′ are secured to the opposed outer curved surfaces of the web portions  21  and  21 ′, such as by welding. A friction pad  23  is secured to the outer arcuate surface of the table portion  22  of the brake pad  20 , while a friction pad  23 ′ is secured to the outer arcuate surface of the table portion  22 ′ of the brake pad  20 ′. A circular aperture  24  is formed through the upper end of the web portion  21  of the brake shoe  20  and through the upper end of the web portion  21 ′ of the brake shoe  20 ′. 
     The first and second brake shoes  20  and  20 ′ are supported on the backing plate  11  by respective pivot pin and spring-clip assemblies, indicated generally at  25  and  25 ′, which are conventional in the art. As shown by the dotted lines in FIG. 1, the lower ends of the web portions  21  and  21 ′ of the brake shoes  20  and  20 ′, respectively, are slightly curved. The curved lower ends of the web portions  21  and  21 ′ are received between the backing plate  11  and the retainer plate  17  and extend into abutment with the opposed side surfaces of the spacer plate  16 . A first coiled spring  26  has hooked ends which extend through respective openings  27  formed through the lower ends of the web portions  21  and  21 ′ so as to urge such lower ends thereof into abutment with the opposed side surfaces of the spacer plate  16 . 
     The prior art drum brake assembly  10  includes a service brake mechanism for actuating the drum brake assembly  10  under normal operating conditions. The service brake mechanism includes a hydraulically actuated wheel cylinder  30  which is disposed between the upper ends of the web portions  21  and  21 ′ of the brake shoes  20  and  20 ′, respectively. The wheel cylinder  30  includes a mounting plate  31  having a pair of threaded openings  31 A formed therein. A pair of bolts (not shown) extend through a pair of openings (not shown) provided in the backing plate  11  and threaded ends thereof are received in the threaded openings of the mounting plate  31  to secure the wheel cylinder  30  to the backing plate  11 . 
     The wheel cylinder  30  includes a pair of opposed piston assemblies  32 A and  32 B which respectively abut the upper ends of the web portions  21  and  21 ′ of the brake shoes  20  and  20 ′. A second coiled spring  33  has hooked ends which extend through respective openings  28  formed through the upper ends of the web portions  21  and  21 ′ of the brake shoes  20  and  20 ′, respectively, so as to urge such upper ends thereof into abutment with the piston assemblies  32 A and  32 B. The wheel cylinder  30  is connected to a conventional source of pressurized hydraulic or pneumatic fluid (not shown) for operating the prior art drum brake assembly under normal operating conditions. 
     The prior art drum brake assembly  10  further includes a hollow cylindrical brake drum  40  which is secured to a wheel (not shown) of the vehicle for rotation therewith. The interior of the brake drum  40  defines a cylindrical braking surface  41 . When installed, the brake drum  40  is disposed adjacent to the backing plate  11  such that the brake shoes  20  and  20 ′ extend within the cylindrical braking surface  41 . To effect braking action, the brake shoes  20  and  20 ′ are moved outwardly apart from one another so as to frictionally engage the cylindrical braking surface  41  of the brake drum  40 . Such frictional engagement causes slowing or stopping of the rotational movement of the brake drum  40  and, therefore, the wheel of the vehicle in a controlled manner. 
     The hydraulically actuated wheel cylinder  30  is used to operate the drum brake assembly  10  under normal service conditions. When it is desired to actuate the drum brake assembly  10 , pressurized hydraulic or pneumatic fluid is supplied to the wheel cylinder  30 . Typically, this is accomplished by the operator of the vehicle depressing the brake pedal in the driver compartment of the vehicle. When such pressurized hydraulic or pneumatic fluid is supplied to the wheel cylinder  30 , the piston assemblies  32 A and  32 B are moved apart from one another. As a result, the upper ends of the brake shoes  20  and  20 ′ are also moved apart from one another, essentially pivoting about the abutment block assembly  15 . This movement causes the friction pads  23  and  23 ′ of the brake shoes  20  and  20 ′, respectively, to move into frictional engagement with the cylindrical braking surface  41  of the brake drum  40 . The abutment block assembly  15  functions to transfer the braking torque from the leading brake shoe (i.e., the left brake shoe  20  when the brake drum  40  is rotating in a counter-clockwise direction) to the frame of the vehicle. When the brake shoes  20  and  20 ′ are moved apart from one another, the second coiled spring  33  is expanded. Thus, when the pressurized hydraulic or pneumatic fluid to the hydraulic actuator  30  is subsequently released, the second coiled spring  33  retracts the brake shoes  20  and  20 ′ inwardly toward one another and out of frictional engagement with the cylindrical braking surface  41  of the brake drum  40 . 
     The prior art drum brake assembly  10  also includes an automatic adjusting mechanism to compensate for thinning of the friction pads  23  and  23 ′ of the brake shoes  20  and  20 ′, respectively, resulting from wear caused by repeated use. The illustrated automatic adjusting mechanism is conventional in the art and includes a pair of struts  42  and  43  having opposed slotted ends. The upper ends of the web portions  21  and  21  ′ of the brake shoes  20  and  20 ′, respectively, are received within the slotted ends of the struts  42  and  43 . A star wheel portion  42 A is threaded onto the strut  42 . An adjuster lever  44  is pivotally mounted on the web portion  21  of the brake shoe  20 . The adjuster lever  44  has a protruding arm portion  44   a  which extends into cooperation with the star wheel portion  42   a  of the strut  42 . A third coiled spring  45  has hooked ends which extend through respective openings formed through the adjuster lever  44  and the lower end of the web portion  21  so as to urge the protruding arm portion  44   a  of the adjuster lever  44  into engagement with the star wheel portion  42   a  of the strut  42 . 
     As is known, when the brake shoes  20  and  20 ′ are moved outwardly apart from one another such that the friction pads  23  and  23 ′, respectively, frictionally engage the cylindrical braking surface  41  of the brake drum  40 , the adjusting lever  44  is pivoted. When a sufficient amount of wear has occurred on the friction pads  23  and  23 ′, the adjusting lever  44  will be pivoted a sufficient amount so as to rotate the star wheel portion  42 A relative to the struts  42  and  43 . Such relative rotation causes the opposed ends of the struts  42  and  43  to be moved slightly farther apart from one another, together with the associated upper ends of the brake shoes  20  and  20 ′. Thus, the automatic adjusting mechanism functions to maintain a predetermined clearance between the friction pads  23  and  23 ′ of the brake shoes  20  and  20 ′, respectively, and the cylindrical braking surface  41  of the brake drum  40  as wear occurs during operation of the prior art drum brake assembly  10 . 
     In addition to the service brake mechanism described above, the prior art drum brake assembly  10  further includes a mechanically actuated parking and emergency brake mechanism. The parking and emergency brake mechanism includes an actuating lever, indicated generally at  50 . The actuating lever  50  is pivotally supported on the web  21 ′ of brake shoe  20 ′ by a pivot pin assembly which includes a pivot pin  51  having an enlarged head and a reduced diameter body. The body of the pivot pin  51  is inserted through respective aligned apertures formed through the actuating lever  50  and the web portion  21 ′ of the brake shoe  20 ′. An E-clip  52  is installed in a groove formed about the end of the body of the pivot pin  51  to retain it in the apertures such that the actuating lever  50  is pivotally supported on the brake shoe  20 ′. 
     The actuating lever  50  further includes an upstanding pin  53 , and a hooked lower end portion  54 . The hooked end portion  54  facilitates the connection of one end of an actuating cable  55  thereto. The actuating cable  55  is conventional in the art and is connected to a hand operated lever (not shown) or similar manually operable parking and emergency brake mechanism for selectively actuating the drum brake assembly  10 . The construction of the prior art drum brake assembly  10  thus far described is conventional in the art. 
     Turning now to FIG. 2, the structure of a first embodiment of a piston assembly, indicated generally at  60 , in accordance with this invention will be discussed. As shown therein, the piston assembly  60  includes a piston body  62  which defines an axis X and is formed from a first material, and a piston cover  64  formed from a second material. Preferably, the piston body  62  is molded from a plastic and more preferably, the piston body  62  is molded from a synthetic resin material, such as a phenolic resin. However, the piston body  62  can be formed from other materials, such as for example, ceramic. Preferably, the piston cover  64  is formed from a metal, such as for example, steel or aluminum, and is integrally molded in situ with the piston body  62 . The cover  64  is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover  64  can be formed from other materials which are different from the material of the piston body  62 , such as for example, aluminum. 
     In the illustrated embodiment, the piston body  62  is provided with a first annular groove  62 A and a second annular groove  62 B. The first annular groove  62 A is adapted to receive an elastomeric boot seal (not shown), and the second annular groove  62 B is adapted to an elastomeric annular fluid seal (not shown). 
     The piston cover  64  includes an end wall  66  which defines an annular abutment surface and which is adapted to engage the associated upper ends of the brake shoes  20  and  20 ′ as discussed above. In the illustrated embodiment, the end wall  66  of the piston cover  64  fully covers an end wall  62 C of the piston body  62 . Alternatively, the end wall  66  of the piston cover  64  can cover less then the entire area of the end wall  62 C of the piston body  62  if desired. However, it is preferred that the end wall  66  of the piston cover  64  cover at least that portion of the end wall  62 C of the piston body  62  which engages the upper ends of the web portions  21  and  21 ′ of the respective brake shoes  20  and  20 ′. 
     The piston cover  64  further includes a plurality of angled “tangs”  68  formed thereon. As used herein, a tang is defined as a projection having a portion which extends from the end wall  66  thereof. Each tang  68  defines a first side edge  68 A and a second side edge  68 B, shown in FIG.  3 . In the illustrated embodiment, four equidistantly spaced angled tangs  68  which extend radially inwardly and axially inwardly are provided on the piston cover  64 . However, the number and/or the spacing of the tangs  68  may be varied as desired. In addition, the configuration of the tangs  68  can be other than illustrated. For example, the tangs  68  could be curved, stepped, or L-shaped. 
     Preferably, the tangs  68  are angled in the range of about 30 degrees to about 60 degrees relative to the axis X, with approximately 45 degrees being the preferred angle. However, the tangs  68  can be angled in the range of 5 to 85 degrees. When the plastic body  62  of the piston assembly  60  is molded about the cover  64 , the tangs  68  are embedded therein as best shown in FIG.  2 . Thus, the tangs  68  assist in mechanically securing the cover  64  to the piston assembly  60  when the piston body  62  thereof is molded thereabout. 
     Turning now to FIG. 4, the structure of the second embodiment of a piston assembly  60 ′ in accordance with this invention will be discussed. As shown therein, the piston assembly  60 ′ includes a piston body  62 ′ which defines an axis X′ and is formed from a first material, and a piston cover  64 ′ formed from a second material. Preferably, the piston body  62 ′ is molded from a plastic and more preferably, the piston body  62 ′ is molded from a synthetic resin material, such as a phenolic resin. However, the piston body  62 ′ can be formed from other materials, such as for example, ceramic. Preferably, the piston cover  64 ′ is formed from a metal, such as for example, steel or aluminum, and is integrally molded in situ with the piston body  62 ′. The cover  64 ′ is preferably formed from stainless steel or carbon steel, and may be electroplated with zinc for corrosion protection. However, the cover  64 ′ can be formed from other materials which are different from the material of the piston body  62 ′, such as for example, aluminum. 
     The piston body  62 ′ is provided with a first annular groove  62 A′ and a second annular groove  62 B′. The first annular groove  62 A′ is adapted to receive an elastomeric boot seal (not shown), and the second annular groove  62 B′ is adapted to an elastomeric annular fluid seal (not shown). 
     In the illustrated embodiment, the piston cover  64 ′ includes an end wall  66 ′ having a slot or recess  70 ′ provided therein, which defines an annular abutment surface and which is adapted to receive the associated upper ends of the brake shoes  20  and  20 ′. To accomplish this, the recess  70 ′ defines a width W which is slightly greater than the thickness of the web portions  21  and  21 ′ of the respective brake shoes  20  and  20 ′. The recess  70 ′ extends inwardly from the end wall  66 ′ a predetermined axial distance D. In the illustrated embodiment, the recess  70 ′ extends across the entire face of the end wall  66 ′ of the cover  64 ′. Alternatively, the location, configuration, and/or design of the recess  70 ′ can be other than illustrated if desired. 
     In the illustrated embodiment, the piston cover  64 ′ further includes a plurality of angled tangs  68 ′ formed thereon. Each tang  68 ′ defines a first side edge  68 A′ and a second side edge  68 B′, shown in FIG.  5 . In the illustrated embodiment, four equidistantly spaced angled tangs  68 ′ which extend radially inwardly and axially inwardly are provided on the piston cover  64 ′. However, the number and/or the spacing of the tangs  68 ′ may be varied as desired. In addition, the configuration of the tangs  68 ′ can be other than illustrated. For example, the tangs  68 ′ could be curved, stepped, or L-shaped. 
     Preferably, the tangs  68 ′ are angled in the range of about 30 degrees to about 60 degrees relative to the axis X′, with approximately 45 degrees being the preferred angle. However, the tangs  68 ′ can be angled in the range of 5 to 85 degrees. When the plastic body  62 ′ of the piston assembly  60 ′ is molded about the cover  64 ′, the tangs  68 ′ are embedded therein as best shown in FIG.  2 . Thus, the tangs  68 ′ assist in mechanically securing the cover  64 ′ to the piston assembly  60 ′ when the piston body  62 ′ thereof is molded thereabout. 
     In the illustrated embodiment, the end wall  66 ′ of the piston cover  64 ′ fully covers the end wall  62 C′ of the piston body  62 ′. Alternatively, the end wall  66 ′ of the piston cover  64 ′ can cover less then the entire area of the end wall  62 C′ of the piston body  62 ′ if desired. However, it is preferred that the end wall  66  of the piston cover  64 ′ cover at least that portion of the end wall  62 C′ of the piston body  62 ′ which engages the upper ends of the web portions  21  and  21 ′ of the respective brake shoes  20  and  20 ′. 
     Although this invention has been described and illustrated in connection with a particular prior art drum brake assembly  10  illustrated in FIG. 1, it will be appreciated that this invention may be used in connection with other drum brake assemblies. For example, this invention may be used in a “duo-servo” type of drum brake assembly, or in a “dual mode” drum brake assembly, wherein the service brake is of the leading/trailing type and the parking and emergency brake is of the duo-servo type. Some examples of the above brake assemblies are shown in U.S. Pat. No. 5,070,968 to Evans, and U.S. Pat. No. 5,275,260 to Evans et al., the disclosures of which are incorporated herein. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been described and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.