Abstract:
An extended male slipper servo pad provides an improved connection between the swashplate and a positioning mechanism in a variable displacement hydraulic unit. The swashplate assembly includes a swashplate having a socket formed therein and a male slipper servo pad pivotally attached to the swashplate at the socket. The male slipper servo pad has a ball end secured in the socket and a pad end having a substantially flat planar surface thereon directed away from the ball end.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of variable displacement hydraulic units, such as hydrostatic pumps and motors. More particularly, this invention relates to an extended male slipper servo pad pivotally mounted to the swashplate of such units so as to provide sliding surface area contact with the positioning mechanism. The invention results in a unique swashplate assembly that has few parts and is economical to produce. 
     Various arrangements are known for connecting the swashplate of a variable displacement hydraulic unit, such as a pump or motor, to a positioning means or mechanism such as a servo piston or a bias piston. In one such arrangement a cammed button is press fitted into the swashplate. This provides a sliding line contact on the servo piston or bias piston. A second arrangement involves a domed servo piston or bias piston running against the swashplate. This provides a sliding point contact. Pin and link connections have also been tried. Another known arrangement involves attaching a female slipper to a male piston in a crimping or swedging operation. The male piston end of this piston-slipper assembly is then pressed into a cylindrical hole in the swashplate. With this arrangement, multiple operations are required to provide a swashplate assembly that is ready for connection with the positioning mechanism. Therefore, there is a need for an improved connection of the swashplate to the positioning mechanism in a variable displacement hydraulic unit. 
     A primary objective of the present invention is the provision of an improved connection between the swashplate and swashplate positioning mechanism of a variable displacement hydraulic unit. 
     Another objective of the present invention is the provision of an extended male slipper having a ball end pivotally attached to the swashplate and a pad end adapted to provide surface area contact with the positioning mechanism. 
     A further objective of the present invention is the provision of a connection between the swashplate and the swashplate positioning mechanism that is economical to produce and reliable in use. 
     These and other objectives will be apparent from the drawings, as well as from the description and claims that follow. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to an extended male slipper servo pad pivotally mounted to the swashplate of variable displacement hydraulic units so as to provide sliding surface area contact with the swashplate positioning mechanism. The extended male slipper servo pad is pivotally secured in a socket formed in the swashplate. The slipper servo pad has a substantially spherical ball end with a major diameter disposed in the socket, an elongated neck portion, and a pad end having a substantially flat planar surface thereon directed away from the ball end. The substantially flat planar surface of the pad end provides surface area contact with a mating planar surface on the swashplate positioning means, which can include a servo piston and/or a biased piston. 
     In the first embodiment of the invention, the swashplate socket has a reduced diameter portion adjacent the entrance of the socket and an enlarged diameter portion adjacent to the reduced diameter portion so as to form a shoulder therebetween for retaining the ball end of the slipper servo pad, which can be press fitted into the socket. In another embodiment, a sleeve or bushing having a malleable ramped skirt portion is interposed between the ball end of the male slipper servo pad and the socket during installation. The ramped skirt portion, which has an outer diameter slightly greater than the diameter of the socket, bends or deforms inwardly to automatically crimp the sleeve on the ball end of the slipper servo pad and retain the same in the socket. Both embodiments provide quick and easy ways to connect the swashplate with a piston member of a positioning mechanism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of portions of a hydraulic unit equipped with the present invention in a zero displacement position. 
     FIG. 2 is a cross-sectional view similar to FIG. 1, but shows the swashplate pivoted to its maximum displacement or full stroke position. 
     FIG. 3 is an enlarged cross-sectional view that shows in greater detail the extended male slipper servo pad arrangement of this invention for positioning the swashplate. The slipper servo pad provides surface area contact with the servo piston. 
     FIG. 4 is an enlarged cross-sectional view of the area  4 — 4  in FIG.  3  and shows in even greater detail the means and method for pivotally attaching the male slipper servo pad to the swashplate. 
     FIG. 5 is a cross-sectional view similar to FIG. 1 but shows another embodiment of this invention. 
     FIG. 6 is an enlarged cross-sectional view illustrating how the self-crimping bushing receives the male slipper servo pad and is automatically crimped thereonto as the bushing is driven into the swashplate socket by the slipper servo pad. 
     FIG. 7 is a cross-sectional view that shows the male slipper servo pad pivotally attached to the swashplate by the self-crimping bushing. The slipper servo pad provides substantial surface area contact with the servo piston. 
     FIG. 8 is a cross-sectional view of the self-crimping bushing of the embodiment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings and the description that follows, similar components are designated with similar reference numerals. Portions of a variable displacement axial piston unit,  10  constructed according to the present invention are shown in FIG.  1 . Although the invention is shown and described as being applied to a variable displacement open circuit pump, one skilled in the art will appreciate that the invention is applicable to variable displacement hydraulic motors. The invention is also applicable to closed circuit pumps or motors. 
     The hydraulic unit  10  has a housing  12  and an end cap  14  detachably mounted thereto by conventional fasteners (not shown). The major rotating components of the hydraulic unit  10  are conventional and are not particularly relevant to the invention. Thus, the following conventional components have been omitted from the drawings to simplify them: a shaft, a cylinder block assembly including a cylinder block housing a plurality of axially reciprocating pistons, and a valve plate for controlling the flow of the working fluid. The fluid displacement or consumption of the hydraulic unit  10  is determined or controlled by a swashplate  16  that movably mounts in the housing  12  so as to pivot along a tilt axis  18  in a well known conventional manner. Positioning means forcibly position or pivot the swashplate  16  about the tilt axis  18 . Generally, the positioning means includes one or more hydraulically operated servo pistons  20 . In the examples shown in the drawings and described below, the positioning means includes a servo piston  20  and a spring-loaded bias piston  22 . The bias piston  22  urges the swashplate  16  to pivot to its maximum angle and the servo piston  20  located on the opposite side of the tilt axis  18  destrokes the open circuit pump to modulate its displacement. 
     The swashplate  16  has a bottom surface  24  that is generally directed toward the bottom of the housing  12  and a substantially planar top surface  26  that is generally directed toward the end cap  14 . A substantially cylindrical socket  28 A extends into the swashplate  16 , preferably perpendicularly from its top surface  26 . The socket  28 A registers with the servo piston  20 . A second socket  28 B registers with the bias piston  22 . Since sockets  28 A and  28 B are preferably identical, only socket  28 A is described in detail below. 
     As best seen in FIGS. 3 and 4, the socket  28 A has a reduced diameter portion  30  adjacent its entrance. An enlarged diameter portion  32  resides inwardly adjacent the reduced diameter portion  30 , so that a shoulder  34  resides therebetween. The bottom wall  36  of the socket  28  preferably is a frustoconical surface having an included angle of approximately 60 degrees. This is approximately the same angle as the point on a standard drill bit. A fluid passageway  38  extends into the socket  28 A from the bottom surface  24  of the swashplate  16 . The entrance of the socket  28 A at the top surface  26  of the swashplate  16  preferably has a lead-in chamfer  40  formed thereon. The chamfer  40  preferably forms an angle of approximately 30 to 60 degrees, and more preferably approximately 45 degrees, with respect to a central longitudinal axis  42  of the socket  28 . Preferably the socket  28 A is perpendicular to the top surface  26  of the swashplate  16 . The socket  28 A is offset from the tilt axis  18  of the swashplate  16 . 
     The socket  28 A or  28 B constitutes one element of the unique means and methods for connecting the positioning means to the swashplate  16  in this invention. The other element is a male slipper servo pad  44  (hereinafter “slipper”). The slipper  44  has a pad end  46  and a generally spherical ball end  48  connected by an intermediate elongated neck portion  50 . The slipper  44  has a central longitudinal axis  52 . The ball end  48  of the slipper  44  has a major diameter D 1  in a plane perpendicular to the central longitudinal axis  52 . The ball end  48  of the slipper  44  has an undercut radius at its trailing end, which blends into the intermediate neck portion  50 . The pad end  46  is preferably a circular or annular disk that has an outside diameter larger than the diameter D 1  of the ball end  48 . The pad end  46  has a substantially planar surface  54  thereon that engages the substantially planar forward surface  56  of the servo piston  20 . Thus, the positioning force transmitted by the servo piston  20  on the swashplate  16  is advantageously distributed over a substantial surface area of contact. 
     The enlarged diameter portion  32  of the socket  28 A has a diameter D 2  that is greater than the major diameter D 1  of the ball end  48  of the male slipper  44 . On the other hand, the reduced diameter portion  30  of the socket  28 A has a diameter D 3  that is slightly smaller than the major diameter D 1  of the ball end  48 . 
     To pivotally attach the slipper  44  to the swashplate  16 , the assembler positions the slipper  44  with its ball end  48  at the entrance of the socket  28 A. The chamfer  40  provides guidance into the socket  28 A. Then an axial force is applied to the pad end  46  of the slipper  44  to push the ball end  48  through the reduced diameter portion  30  of the socket  28 A. Once the major diameter D 1  is forward of the shoulder  34  and disposed in the enlarged diameter portion  32  of the socket  28 A, the shoulder  34  retains the ball end  48  of the slipper  44  in the socket  28 A and the pad end  46  is free to pivot about the central longitudinal axis  52 . The sizes of the diameters D 2  and D 3  can be adjusted relative to the diameter D 1  of the ball end  48  so as to arrive at a reasonable press-in force and a desired pull-off strength for the joint. For example, the following dimensions have been found to work well in a 100 cc per revolution open circuit pump: 
     D 1 =12.137 mm; 
     D 2 =12.23 mm; and 
     D 3 =12.1 mm. 
     A second socket  28 B and slipper  44  are provided on the opposite side of the tilt axis  18  adjacent the piston member  58  of the bias piston  22 . A passageway  38 B intersects the socket  28 B. The surface  54  on the slipper  44  engages the substantially planar surface  60  on the bias piston  22 , as best seen in FIG.  2 . Again, surface area contact is provided between the piston  22  and the slipper pad end  46 . 
     FIGS. 5-8 illustrate another embodiment of this invention. In this embodiment, the swashplate  16  has one or more sockets  28 C,  28 D formed therein. Fluid passageways  38 C,  38 D extend from the bottom surface  24  of the swashplate  16 A so as to be in fluid communication with the sockets  28 C,  28 D respectively. Since the sockets  28 C and  28 D are identical except for their location on the swashplate  16 A, only the first socket  28 C will be described in detail below. As best seen in FIGS. 6 and 7, the socket  28 C has a substantially cylindrical shape. A main diameter portion  62  extends inwardly from the top face  26  of the swashplate  16 A. The main diameter portion  62  has a diameter D 8 . The entrance of the socket  28 C has a lead-in chamfer  64  thereon. The chamfer  64  has an included angle of approximately 60 degrees to 120 degrees, more preferably approximately 60 degrees to 90 degrees. The main diameter portion  62  terminates in a bottom wall  66 . 
     Referring to FIG. 8, this embodiment includes a bushing or sleeve  70  formed of a suitably malleable material, including but not limited to brass. The bushing  70  has a first end  72  and a second end  74 . The bushing  70  includes a main diameter portion  76  generally adjacent the first end  72  and a ramped skirt portion  78  generally adjacent the second end  74 . The bushing  70  has a central longitudinal axis  80  and a fluid passageway  82  that extends through the bushing  70  along its central longitudinal axis  80 . A cavity  83  for receiving the ball end  48  of the male slipper  44  extends into the second end  74  of the bushing  70 . The cavity  83  includes a semi-spherical concave hollow  84  and a counterbore  86 . The semi-spherical hollow has a diameter D 4 , while the counterbore  86  has a diameter D 5 . The main diameter portion  76  of the bushing  70  is designated by reference numeral D 6 . The ramped skirt  78  has an outer diameter designated by the reference numeral D 7 . 
     The use of the bushing  70  to pivotally attach the male slipper  44  to the swashplate  16 A can best be understood in view of FIGS. 6-8. The ball end  48  of the slipper  44  is loosely inserted into the cavity  83  of the bushing  70 . This loose subassembly is then positioned at the entrance of the socket  28 C. An axial force F is applied to the pad end  46  of the slipper  44  to press the subassembly into the socket  28 C. The lead-in chamfer  64  assists in guiding the bushing  70  into the main diameter portion  62  of the socket  28 C. The diameter D 8  of the main diameter portion  62  is large enough to slidably receive the diameter D 6  of the bushing  70 . However, once the major diameter D 1  of the ball end of the slipper  44  passes the lead-in chamfer  64 , the main diameter D 8  engages the ramped skirt portion  78  of the malleable bushing  70 . Thus, the malleable ramped skirt portion  78  is automatically crimped, deformed, or bent inwardly around the back of the ball end  48  of the slipper  44  as the subassembly is pressed into the socket  28 C. The ramped skirt portion  78  also provides a light press fit between the subassembly and the socket  28 C. 
     The ramped skirt portion  78  has a substantially frustoconical leading edge  79 . The ramped skirt portion  78  extends outwardly at an angle of approximately 15 to 45 degrees, more preferably approximately 20 to 30 degrees, and most preferably approximately 25 degrees, with respect to the main diameter portion  76 . Although the entire bushing  70  is malleable in the preferred embodiment described, one skilled in the art would appreciate that only the skirt portion  78  needs to be malleable. 
     Thus, it can be seen that the present invention at least achieves its stated objectives. 
     In the drawings and specifications, there has been set forth a preferred embodiment invention, and although specific terms are employed, these are used in a generic and descriptive sense only and not for purposes of limitation. Changes in the form and proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the scope of the invention as defined in the following claims.