Abstract:
A vehicle brake actuator including an improved piston assembly having a piston plate including an upstanding annular rim portion and a piston rod including a shank portion having a generally cylindrical head portion and an annular rim portion on a free end of the piston rod head portion which is deformed radially into a cylindrical internal surface of the piston plate upstanding rim portion, permanently attaching the piston plate and rod. The method includes piercing and extruding the upstanding rim portion of the piston plate and forming radially projecting teeth on an external surface of the head portion of the piston rod, driving the head portion of the piston rod into the upstanding rim portion of the piston plate, preventing relative rotation of the piston rod and head. The improved piston assembly of this invention eliminates welding, permitting finishing of the piston rod and plate prior to assembly and reduces cost.

Description:
FIELD OF THE INVENTION 
     This invention relates to vehicle brake actuators, including spring brake actuators, having an improved piston assembly which eliminates welding, reduces cost and results in an improved piston. 
     BACKGROUND OF THE INVENTION 
     Brake actuators are conventionally used on heavy vehicles having pneumatic braking systems, including trucks, buses and tractor trailers. Conventional brake actuators include a cup-shaped flexible diaphragm, which is supported in the housing, and a piston having a piston plate which is spring biased against the diaphragm. Upon actuation of the pneumatic braking system, air pressure drives the flexible diaphragm and the piston rod or push rod of the piston to actuate the braking system of the vehicle. Conventional brake actuators generally also include an emergency chamber, which may be mounted on the service chamber, having a power spring, which actuates the piston of the service chamber when the pneumatic pressure of the vehicle braking system fails or when the vehicle is turned off, providing an emergency braking system for the vehicle and a parking brake. 
     The piston of a brake actuator includes a piston rod or push rod, which is generally welded to the piston plate. The welding of the piston rod to the piston plate, however, creates several problems. First, the heat of welding removes any protective coating which may be applied to the parts, requiring a rust protecting paint to the weld area. Rust may still occur, particularly at the weld area, reducing the strength and durability of the assembly. Second, the butt weld of the piston rod to the piston plate may fail, particularly under the bending load and the extreme conditions encountered with brake actuators for heavy vehicles of the type which utilize brake actors. As will be understood by those skilled in this art, brake actuators are mounted under the carriage of the vehicle or tractor adjacent the axles, wherein the brake actuator is subject to extreme temperature variations and road debris including water, salt and ice. 
     A primary object of the brake actuator piston assembly of this invention is to improve the integrity of the joint between the piston plate and piston rod. A further object is to reduce cost by eliminating the welding of the piston rod to the piston plate, the subsequent coating or painting of the welded joint and permitting the use of a protective finish, such as zinc dichromate finishes. Another object would be to reduce the weight of the piston assembly which requires a piston plate having sufficient thickness to prevent burn-through when the piston rod is butt welded to the piston plate. 
     SUMMARY OF THE INVENTION 
     The improved piston of the type used in vehicle brake actuators of this invention is best described by the method of making the piston assembly. A piston of the type used in vehicle brake actuators includes a generally flat piston plate and a piston rod or push rod which extends generally perpendicular to the piston plate from a mid-portion of the piston plate. In the method of making a piston of this invention, a cylindrical opening is formed in a mid-portion of the piston plate. In the most preferred embodiment of the method of this invention, the piston plate is pierced and extruded, forming an annular upstanding rim portion having a flat free end and a generally cylindrical internal surface defining the opening through the piston plate. 
     The method of this invention further includes forming a piston rod having a shank portion and a generally cylindrical head portion preferably having a diameter slightly greater than the opening through the piston plate. In the most preferred embodiment and method of this invention, the external surface of the cylindrical piston rod head portion includes a plurality of radially extending teeth having a circumferential crest diameter greater than the internal diameter of the piston plate opening. The piston rod head portion further includes a longitudinally projecting annular rim portion on a free end of the head portion, opposite the shank portion. In the most preferred embodiment, the rim portion on the free end of the piston rod is spaced from the radially extending teeth and the internal surface of the rim portion is cylindrical, defining a cylindrical opening or cavity in the free end of the piston rod. 
     The method of this invention then includes driving the free end of the piston rod head portion into the opening in the piston plate, preferably forming an interference fit between the piston rod head portion and the piston plate. In the preferred embodiment, where an upstanding rib or rim portion is formed in the piston plate, the head portion of the piston rod is driven into the opening in the piston plate from the free end of the upstanding rim or rib on the piston plate and the radial teeth bite into the generally cylindrical internal surface of the opening through the plate providing an interference fit. 
     The free end of the longitudinal annular rim on the piston rod is then deformed radially outwardly, preferably by swaging, forming a flush mounting of the piston rod to the piston plate which receives the diaphragm in the brake actuator. In the most preferred embodiment, the head portion of the piston plate includes a radial rib adjacent the teeth, opposite the rim portion, which is driven against the flat free end of the radial rim of the piston plate, forming a very secure and accurate assembly. As will be understood by those skilled in this art, the overall length of the piston must be accurately controlled and the radial flange on the piston rod head portion assures that the overall length of the piston is constant for each assembly. 
     The brake actuator of this invention includes a housing defining a service chamber, a flexible diaphragm is supported within the housing chamber and a piston including a piston rod and a generally flat piston plate is biased against the flexible diaphragm, generally by a return spring. In the most preferred embodiment, the piston plate includes an upstanding annular rim portion, preferably having a generally flat free end and a generally cylindrical internal surface defining an opening through the piston plate. A piston rod including a shank portion and a generally cylindrical head portion is disposed within the piston plate rim portion opening from the free end, forming an interference fit, and an annular rim portion on the free end of the piston rod, opposite the shank portion, is deformed radially outwardly into the internal surface of the piston plate, permanently attaching the piston plate to the piston rod and forming a flush assembly. 
     The method of making a piston of the type used in a vehicle brake actuator of this invention thus eliminates the requirement for welding the piston rod to the piston plate, permitting the use of pretreated components, such as a zinc dichromate coating on the piston plate and piston rod. The piston rod is preferably cold headed from steel equivalent to a Grade 2 cold headed bolt. The push rod head portion is firmly secured in the extruded longitudinal rim portion of the piston plate, reducing bending of the push rod as the push rod reciprocates in an arcuate motion to actuate the braking system of the vehicle. Further, the thickness of the piston plate may be reduced without sacrificing the integrity of the joint, thereby reducing the overall weight of the piston. 
     The improved piston and method of forming a piston assembly for a brake actuator of this invention thus significantly improves the integrity of the joint between the piston rod and the piston plate while reducing cost and weight. Other advantages and meritorious features of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims, and the drawings, a brief description of which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a conventional dual diaphragm spring brake actuator having the improved piston assembly of this invention; 
     FIGS. 2A and 2B are cross-sectional views of the piston plate illustrating the preferred method of forming the piston plate; 
     FIG. 3 is a side perspective view of the improved piston plate and piston rod of this invention prior to assembly; 
     FIG. 4 is a side partially cross-sectioned view of the piston plate during assembly; and 
     FIG. 5 is a side partially cross-section view of the improved piston following assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a dual diaphragm spring brake actuator  20  of the type disclosed in U.S. Pat. No. 4,960,036, assigned to the assignee of the present application, having the improved piston assembly of this invention. The illustrated dual diaphragm spring brake actuator includes a service chamber  22 , which normally actuates the vehicle braking system as described below, and an emergency chamber  24  which actuates the vehicle braking system when the vehicle pneumatic pressure drops below a predetermined pressure in an emergency situation or as a parking brake when the vehicle is turned off. As will be understood, however, the improved piston assembly of this invention may be utilized with any type of brake actuator, such as a separate service chamber, a piston-type brake actuator, and the like. 
     The dual diaphragm spring brake actuator  20  illustrated in FIG. 1 includes a service chamber  22  and an emergency chamber or spring chamber  24 . The disclosed embodiment of the brake actuator  20  includes a flange case  26  which may, for example, be formed of cast aluminum, having a central web portion  27  which separates the service chamber  22  from the emergency chamber  24 . The flange case includes radial flanges  28  and  30  for attachment of the cover  32  or head of the emergency chamber and the service chamber housing  34 . The spring chamber  24  includes a cup-shaped flexible diaphragm  36  and a power spring  38  which actuates the brake of the braking system when the pneumatic pressure falls below a predetermined pressure as described below. The power spring  38  reacts against the head  32  and spring piston  40 . The emergency chamber further includes a piston  42  having a piston rod  44  and a piston head  46 . In the disclosed embodiment, the peripheral edge  48  of the cup-shaped flexible diaphragm  36  is received on the flange  28  of the flange case  26  and secured in place by the skirt portion  50  of the head  32  by crimping or the like as disclosed in the above-referenced U.S. patent. The flange case  26  includes a pneumatic port  52  connected to the pneumatic braking system of the vehicle (not shown). During normal operation of the brake actuator, the emergency chamber  24  is pressurized through port  52 , biasing the diaphragm  36  upwardly as shown in FIG. 1, compressing the power spring  38 . When the pressure in the emergency chamber  24  falls below a predetermined pressure or when the vehicle is turned off, the power spring  38  expands against the spring piston  40 , inverting the diaphragm  36  and driving the piston rod  44  through opening  54  in the flange case and driving the piston assembly  58  to actuate the brakes as described below. 
     The service chamber also includes a cup-shaped flexible diaphragm  60  having a peripheral edge  62  which is received on the flange  64  of the service chamber housing and the clamp  68  secures the flange  64  of the service chamber housing  34  to the flange  30  of the flange case  26  in sealed relation with the peripheral edge  62  of the diaphragm  60  located therebetween. The flange case  26  includes a service chamber port  70  which receives air pressure from the pneumatic braking system upon actuation of the vehicle brakes. The pneumatic pressure inverts the cup-shaped flexible diaphragm  60 , driving the piston plate  72  downwardly in FIG. 1, which drives the piston rod  58  through opening  76  in the service chamber housing  34 , actuating the braking system of the vehicle (not shown). The free end of the piston rod is connected to a clevis  78  and the clevis  78  is connected to the braking system of the vehicle by clevis pin  80 . The service chamber housing  34  is mounted on a bracket (not shown) under the vehicle carriage by mounting bolts  82 , lock nuts  84  and washers  86 . Upon release of the brake by the vehicle operator, the pneumatic pressure from the vehicle through port  70  returns to zero and the return spring  88  returns the piston  58  and the diaphragm  60  to the position shown in FIG.  1 . 
     Thus, the operation of the brake actuator  20  illustrated in FIG. 1 may be briefly described as follows. During normal operation of the vehicle, when the brake is actuated in the vehicle by the operator, air pressure is delivered to port  70  in the flange case  26 , inverting the cup-shaped flexible diaphragm  60  which drives the piston rod or push rod  58  through opening  76  in the service chamber housing  34 , actuating the brake. Upon release of the brake pedal, the pressure through port  70  returns to zero and the return spring  88  returns the piston  58  to the position shown in FIG.  1 . The air pressure through line  52  retains the pressure in emergency chamber  24 , maintaining the compression of power spring  38  as shown in FIG.  1 . However, when the pneumatic pressure in emergency chamber  24  falls below a predetermined pressure when the vehicle is turned off or when the pneumatic braking system fails, the power spring  38  expands, driving the piston rod  44  through opening  54  in the web  27  of the flange case  26 , driving the piston rod  58  of the emergency chamber through opening  76  in the service chamber housing  34 , actuating the brake. The dual diaphragm spring brake actuator  20  thus operates during normal braking of the vehicle and during emergency situations to stop a runaway truck. 
     The clevis  78  may be connected directly to the vehicle brake or more commonly to a slack adjuster, such as an automatic slack adjuster commonly used in vehicles of the type having brake actuators. Thus, the clevis  78  must follow the free end of the slack adjuster (not shown), which is an arcuate motion, subjecting the piston assembly  58  to substantial bending forces. That is, the piston rod  74  will rock in an arcuate motion during braking of the vehicle, which results in a bending force between the piston rod  74  and the piston plate  72 . Further, as will be understood by those skilled in the art, the brake actuator  20  will be subject to extreme conditions. As described above, brake actuators are normally mounted beneath the vehicle chassis where the brake actuator is subject to extreme vibrational loads, temperature variations and road debris, including water, salt, ice and dirt. Although most manufacturers of brake actuators include a stone shield which partially seals the opening  76  in the service chamber housing  34 , moisture will still enter the service chamber  22  requiring protection of the components. In a conventional brake actuator of this type, the piston rod  74  is welded to the piston plate  72  and later coated with a protective coating, such as a protective paint. Nevertheless, failures still occur in the weld between the piston rod  58  and the piston plate  72 . 
     FIGS. 2-5 illustrate an improved piston assembly  58  and a method of making the piston assembly. In the preferred method of making the piston assembly of this invention, an opening  90  is pierced in the piston plate as shown in FIG. 2A and the area around the opening is then extruded as shown in FIG.  2 B. The extrusion step forms an upstanding annular rib or rim portion  92 , preferably having a generally cylindrical internal surface  94  and a generally flat free end  96 . The piston rod  74  in the preferred embodiment of the piston assembly and method of this invention includes an enlarged generally cylindrical head portion  98  having relatively sharp radially projecting teeth  100 , a radial flange  102  adjacent the shank portion  97  and a longitudinal annular rim  104  surrounding a cylindrical cavity or recess  106  as best shown in FIG.  3 . In the preferred embodiment, the diameter of the head portion  98  is selected to form an interference fit with the internal surface  94  of the piston plate  72 . In the most preferred embodiment, the interference fit is provided by the radially projecting teeth  100 , wherein the circumference defined by the crest diameter is greater than the internal diameter of the generally cylindrical opening  94  in the piston plate  72 . In a typical application, the crest diameter of the teeth  100  will be approximately 0.03 inches greater than the internal diameter of the generally cylindrical internal surface  94  of the piston plate. However, the diameter of the annular longitudinal rim  104  may be equal to or slightly less than the diameter of the internal cylindrical surface  94 . 
     After forming the piston plate  72  and the piston rod  74  as shown in FIGS. 2 and 3, the generally cylindrical head portion  98  is driven into the opening in the piston plate from the upstanding rim  92 , as shown in FIGS. 3 and 4. Because of the interference fit between the head  98  of the piston rod  74  and the cylindrical surface  94  of the piston plate, the teeth  100  bite into the cylindrical surface  94  as shown in FIG.  4 . As the head  98  is driven into the opening in the piston plate  72 , the radial flange  102  on the piston head engages the relatively flat free end  96  of the upstanding rim  92 , accurately locating the piston rod in the piston plate and very accurately controlling the overall length of the piston assembly  58 . As best shown in FIG. 4, the end of the longitudinal lip  104  of the piston rod head extends slightly above the adjacent surface  73  of the piston plate  72 . The final step in the assembly of the piston rod to the piston plate is to radially deform the longitudinal rim  104  as shown in FIG. 5 as by swaging. This forms a very secure assembly as discussed more fully hereinbelow. 
     The piston rod and piston plate assembly  58  shown in FIG. 5 has several important advantages over the prior art methods of attaching the piston rod to the piston plate by welding as described above. Where the piston rod is welded to the piston plate, the welding operation will remove any finish applied to the components. The piston rod  74  of this invention may be formed by conventional cold heading techniques and may include a protective finish, such as zinc dichromate. Similarly, the piston plate  72  may include a protective finish, such as zinc dichromate, which will protect the components from corrosion due to salt and moisture. The piston assembly  74  of this invention is also able to withstand bending loads, particularly where the piston plate  72  includes an upstanding annular rim portion  92 . Further, as discussed above, the overall length of the piston assembly  58  may be very accurately controlled. As will be understood by those skilled in this art, the overall length of the piston must be accurately controlled because the piston actuates the brakes. The shank portion  97  of the piston rod may include indicia, such as grooves  108  and  110 , which indicate an overstroke condition as is presently conventional in this art. Further, the end of the shank portion may be threaded as shown at  112  for attachment of the piston to the yoke  108  shown in FIG.  1 . As will be understood, the yoke  78  includes an internally threaded bore (not shown) and the yoke is fixed to the threaded portion  112  by nut  114 . The piston assembly  58  is also less expensive than a welded assembly because of the elimination of the welding step. Thus, the piston assembly  58  and method of forming the piston has several significant advantages over the prior art. 
     As will be understood by those skilled in this art, various modifications may be made to the brake actuator and method of forming a piston of this invention within the purview of the appended claims. For example, as described above, the improved piston assembly  58  may be used with any brake actuator, particularly in service chambers of the type described herein. As will be understood, the piston  42  in the emergency chamber  24  is not subject to the bending loads described above. Therefore, the piston head  46  may be conventionally secured to the piston rod  44  with a screw (not shown). However, the piston assembly  42  may also be replaced by the improved piston assembly of this invention. Further, the configuration of the piston plate  72  and the shank portion  97  may be modified as required by the particular application. Other protective finishes other than zinc dichromate may also be used and various steels may be used to form the piston plate and piston rod. Finally, as described above, the thickness of the piston plate may be reduced because the welding step has been eliminated, thereby further reducing the weight of the piston assembly. Having described the preferred embodiments of the invention, the invention is now claimed as follows.