Expansible chamber devices – Collapsible chamber wall portion – Wall portion formed of flexible material
Reexamination Certificate
2000-03-22
2001-09-18
Look, Edward K. (Department: 3745)
Expansible chamber devices
Collapsible chamber wall portion
Wall portion formed of flexible material
C029S888051
Reexamination Certificate
active
06289786
ABSTRACT:
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.
REFERENCES:
patent: 4960036 (1990-10-01), Gummer et al.
patent: 5218763 (1993-06-01), Marker et al.
Greenly Gregory A.
Plantan Ronald S.
Howard & Howard
Indian Head Industries Inc.
Leslie Michael
Look Edward K.
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