Patent Publication Number: US-6338293-B1

Title: Reduced oil volume piston assembly for a hydrostatic unit

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of hydrostatic units such as transmissions, pumps and motors. More particularly, this invention relates to means for reducing the oil volume of pistons slidably mounted in the cylinder block bores of hydrostatic units. 
     The oil volume in each piston bore is compressed to the operating pressure of the hydrostatic unit at some time during each rotation of the cylinder block. The fact that oil is compressible and takes energy to compress results in energy losses for units that do not have solid or sealed pistons. It is known in the hydrostatic art to utilize solid pistons to improve efficiency. However, solid pistons are relatively heavy, which reduces the maximum speed at which they can operate due to higher block tipping forces and higher centrifugal forces that cause piston burn as the pistons reciprocate longitudinally in the piston bores. 
     It is also known in the hydrostatic art that the oil-containing volume of a piston can be reduced by forming the piston with a hollow or cavity therein, then sealing the cavity so oil cannot enter. This is conventionally done by welding a cap member on the hollow piston body. Unfortunately, such welded pistons are generally costly to produce. Direct displacement (non-servo) units typically do not utilize pistons with conventionally reduced oil-containing volume because of the higher cost. Another problem with hollow welded pistons lies in the variation in control moments that occurs with changes in the rotational speed of the cylinder block. Since direct displacement units do not have a servo to control the swashplate, the operator feels the control moments to a greater degree and therefore experiences greater operator fatigue. 
     Therefore, a primary objective of the present invention is the provision of a reduced oil volume piston and cylinder block assembly that improves the efficiency of a hydrostatic unit without unduly increasing its cost. 
     Another objective of the present invention is the provision of a piston and cylinder block assembly wherein the filler material for the piston is retained in the cylinder block, rather than in the piston, so that the filler material has no impact on the centrifugal forces on the piston and causes no additional block tipping forces. 
     Another objective of the present invention is the provision of a reduced volume piston and cylinder block assembly that is economical to produce, as well as reliable and durable in use. 
     These and other objectives will be apparent to one skilled in the art from the drawings, as well as from the following description and claims. 
     SUMMARY OF THE INVENTION 
     The present invention relates to piston and cylinder block assemblies for hydrostatic units. These assemblies reduce the oil volume of the pistons. The reduced oil volume piston and cylinder block assembly includes a cylinder block with a central bore and a plurality of cylindrical piston bores radially spaced from the central bore, a plurality of pistons having one hollow end slidably mounted in the piston bores, and a plurality of stems correspondingly disposed in the piston bores and extending into the cavity at the hollow end of the pistons so as to displace or reduce the oil-containing capacity or volume of the piston. 
     The stem of this invention is formed separately from the piston body, as described below in three different embodiments. In the first embodiment, the stem is a separate component that is inserted in each piston bore. In the second embodiment, the stems are cast as an integral part of the cylinder block. In the third embodiment, the stems extend into the pistons because the stems are attached to a ring that is fixed to the bottom of the cylinder block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a first embodiment of the present invention. 
     FIG. 1A is partial sectional view taken of the area  1 A— 1 A in FIG.  1  and shows how fluid can flow around the bottom of the stem of this invention. 
     FIG. 2 is an exploded assembly view of the stem and piston return spring shown in FIG.  1 . 
     FIG. 3 is a cross sectional view of a second embodiment of this invention in which the stem is integrally cast into the cylinder block. 
     FIG. 3A is a partial perspective view of the stem area of the cylinder block of FIG.  3 . 
     FIG. 4 is a cross sectional view of a third embodiment of this invention in which a separate stem ring assembly is fixed to the bottom of the cylinder block. 
     FIG. 5 is an exploded assembly view of the stem ring assembly and cylinder block of the embodiment shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the figures and the description that follows, like reference numerals are used to refer to like parts and features. The first embodiment of the invention is shown in FIGS. 1 and 2. Referring to FIG. 1, a piston and cylinder block assembly  10  includes a central bore  14  extending therethrough and a plurality of cylindrical piston bores  16  radially spaced from the central bore  14  and equally spaced angularly around the central bore  14 . Each of the piston bores  16  has a central longitudinal axis  18 . 
     The piston and cylinder block assembly  10  further includes a plurality of piston assemblies  20  that include a piston  22  with a slipper  24  pivotally attached by a ball and socket connection. While the drawings show the socket incorporated in the upper end of the piston  22 , one skilled in the art will appreciate that a ball end can be provided on the piston and the socket incorporated into the slipper without detracting from the present invention. As is conventional, a slipper retainer guide  26  tiltingly supports the slippers  24  on a spherical central hub surface  28  of the cylinder block  12 . 
     The body of the piston  22  is elongated and cylindrical. The end of the piston opposite that which is connected to the slipper  24  has a cavity  30  formed therein. Preferably, the cavity  30  is defined by cylindrical bore that extends into the body of the piston  22 . 
     The portion of the piston bore  16  that slidably receives the piston  22 , does not extend all the way through the cylinder block  12 . Instead, a reduced diameter bore  32  and a bottom wall  34  are present below the main piston bore  16 . As is conventional, arcuate ports  36  are provided on a bottom “running” surface  38  on the cylinder block  12 . As best understood in view of F in FIGS. 1A,  3 A, and  5 , the ports or fluid passages  36  are in fluid communication with the piston bores  16 . 
     A stem  40  is formed separately from the block  12  and the pistons  22 . The stem  40  is adapted to be inserted into the piston bore  16  prior to or in conjunction with the insertion of the piston assembly  20  into the cylinder block  12 . Referring to FIG. 2, the stem  40  is a rigid and solid elongated member that includes a neck  42  and a head  44 . The head  44  is enlarged with respect to the diameter of the neck  42 . In the embodiment of FIGS. 1 and 2, the neck  42  of the stem  40  is cylindrical. However, other shapes will suffice for the invention, realizing that the oil volume displaced will differ as a result of their shape. The cylindrical shaped neck  42  is preferred because of the high volume of oil that displaces from the hollow piston  22  and the ease with which it can be manufactured. 
     The head  44  is shaped so as to partially cover the opening of the cavity  30  without completely covering it when the stem  40  is inserted into the cavity. The head  44  has a plurality of spaced flange members  46 ,  48  thereon protruding radially outward and downward from the neck  42 . Each of the flange members  46 ,  48  has an L-shaped longitudinal cross section and an arcuate transverse cross section. Preferably, the head has a maximum dimension across the flanges  46 ,  48  (i.e.—in a direction transverse to the neck  42  and thereby to the piston  22 ) that is less than the outer diameter of the piston  22  and is adapted to be received in the reduced diameter bore  32 . The maximum dimension across the flanges  46 ,  48  is also larger than the diameter of the cavity  30 , as shown in FIG. 1, so that the head  44  cannot enter the cavity  30  of the piston  22 . 
     The neck  42  has a bottom end  43 . The flange members  46 ,  48  attach to the bottom end  43  of the neck  42  and protrude outward and downward therefrom. Each of the flange members  46 ,  48  has an L-shaped longitudinal cross section and an arcuate transverse cross section. Thus, a gap is provided under and around the bottom end  43  of the stem  42  so that fluid can pass by the stem  40 . As best seen in FIG. 2, the flanges  46 ,  48  also are angularly spaced. Notches or flats  47  are formed in the outer perimeter of the head  44  between the flanges  46 ,  48 , thereby leaving gaps that allow fluid to pass in and out of the piston bores  16  as the pistons  22  reciprocate. See FIG.  1 A. 
     The piston assembly  20  further includes a spring  50 . The spring  50  is preferably a coiled compression spring, which has an inner diameter sufficient to pass over the neck  43  of the stem  40  and an outer diameter adapted to be loosely received in the cavity  30  of the piston  22 . Thus, the spring  50  can be coiled around the neck of the stem  42  and positioned between the end wall  52  of the cavity  30  and the head  44  of the stem  40 . In the hydrostatic art such a spring is generally referred to as a piston return spring, however, in this invention the spring  50  performs another important function. The force of the spring  50  holds the stem  40  in place in the cylinder block  12  as the piston  22  reciprocates. The spring force effectively locks the stem  40  to the block  12  so that it does not move relative to the block. 
     There are two possible methods for assembling the piston and cylinder block assembly  10  of FIGS. 1 and 2. One way is to place the spring  50  over the stem  40  and insert these two items into the cavity  30  of the piston  22 . Then, the block  12  can be placed over the above-mentioned components. This method requires a special fixture to align all of the piston assemblies  20  with their respective piston bores  16 . A second and more preferred method of assembling the components is to drop stems  40  into the reduced diameter bores  32  through the piston bores  16 . Then, the springs  50  are installed over the neck  42  of the stem  40 . The upper surface of the head  44  serves as seat for the spring  50 . Then, the remainder of the piston assembly  20  is guided over the springs  50  in the piston bores  16 . Preferably, the piston assemblies  20  are inserted in the slipper retainer guide  26 , which assists in simultaneously aligning the piston assemblies  20  with their respective piston bores  16 . 
     A second embodiment of the invention  10 A is shown in FIG. 3 and 3A. The cylinder block  12 A of this embodiment is formed in a conventional “lost foam” casting process such that the stem  40 A is integrally formed with the cylinder block  12 A as a single piece casting. The cast cylinder block is then conventionally machined so as to finish the piston bore and the other features, while leaving the central post of neck  42 A of the stem  40 A. Thus, in this embodiment, the filler material that displaces or reduces the oil volume in the piston cavity  30  is integrally attached to the block  12 A. The cost of this embodiment is low and the reliability is high. Attaching the filler material to the cylinder block  12 A has no impact on the centrifugal forces on the piston  22  and does not add to the block tipping forces because it does not extend out of the cylinder block  12 A with the pistons  22 . 
     FIGS. 4 and 5 illustrate a third embodiment of this invention. The piston and cylinder block assembly  10 B of this embodiment includes a main block  13 , a central opening  14  and a plurality of piston bores  16 . A stem ring assembly  54  includes an annular base plate  56  having a plurality of arcuate ports  36 B extending therethrough. The base plate  56  has a lower or “running” surface  58  and an upper surface  60 . The stems  40  are rigidly attached to the upper surface  60  of the base plate  56 , preferably extending at right angles thereto. The flanges  46 ,  48  bridge the arcuate ports  36 B and a gap exists between the bottom end  43  of the neck  42  and the upper surface  60 , as best seen in FIG.  5 . 
     An annular groove  62  is formed in the bottom surface of the main block  13 . The groove  62  has a width that is the same as the reduced diameter bore  32  in the main block  13 . The groove  62  is concentric with the central bore  14  and registers with each of the piston bores  16 . 
     To make the cylinder block  12 B of this embodiment, the stem ring assembly is fabricated first as a separate component. The stems  40  on the stem ring assembly  54  are aligned with the piston bores  16  and the base plate  56  is inserted into the groove  62 . Then, the base plate  56  is braised or otherwise rigidly attached or affixed to the bottom of the main block  13 . The lower surface  58  of the base plate  56  acts as the running surface for the cylinder block  12 B. 
     Thus, it can be seen that the present invention at least achieves its stated objectives.