Patent Publication Number: US-6220144-B1

Title: Extended slipper for hydrostatic pump and motor rotating cylinders

Description:
This is a continuation of copending application Ser. No. 08/582,656 filed on Jan. 4, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     Hydrostatic transmissions have two or more hydrostatic units utilizing rotating cylinder blocks of similar construction that are hydraulically connected. One cylinder block, normally referred to as the pump, is connected to a rotatable input shaft, and the other cylinder block, referred to as a motor, has a power output shaft. These cylinder blocks or groups typically are of the piston/swashplate design or are of a bent-axis design. Each of these designs have certain beneficial design features, but both also have certain structural and functional shortcomings. This invention combines the best of these two designs, and substantially eliminates the shortcomings of each. Further advantages are realized by this invention that are not present in either of the prior designs. 
     Specifically, the ball and socket joint at the piston/slipper interface was characteristic of both bent-axis and swashplate designs. An extended arm that reaches toward and sometimes inside the piston bore was characteristic of only the bent-axis design. A hydrostatic bearing-surface that slides between the slipper and the swashplate was characteristic of the swashplate design. 
     The prior swashplate designs had substantial side-load on the pistons creating greater friction between the pistons and bores, which adversely affected torque efficiency. Only low swashplate angles were available which limited power output. The load carrying point of the shaft (i.e., the “sweet spot”) was located by the length of the cylinder in a position which invited failure at the interface between the cylinder block and the shaft. The greater length of existing blocks required a longer shaft which invited shaft deflection which reduced the life of shaft bearings. 
     Therefore, a principal object of this invention is to provide a rotating cylinder for a swashplate-type hydrostatic transmission that will create a substantial reduction in piston surface pressure between the pistons of the cylinder and the piston bores, and to thereby improve the torque efficiency of the machine being driven by the transmission. 
     A further object of this invention is to provide a rotating cylinder for a swashplate-type hydrostatic transmission wherein higher swashplate angles are achievable to improve power output and overall efficiency of the machine. 
     A still further object of this invention is to provide a rotating cylinder for swashplate-type hydrostatic transmissions which moves the load-carrying point of the shaft closer to the valve plate end of the cylinder block to increase the strength of the interface between the cylinder block and the shaft to prevent failure of the interface. 
     A still further object of this invention is to provide a rotating cylinder block for swashplate-type hydrostatic transmissions which will have a sweet spot closer to the valve plate end of the cylinder block, and which will reduce the overhang length of the piston, which allows for a reduction in the overall length of the machine package. 
     A still further object of this invention is to provide a rotating cylinder for a swashplate-type hydrostatic transmission which will permit a reduction in the length of the shaft which in turn will increase the bearing life for the shaft bearings. 
     These and other objects will be apparent to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     The cylinder block for hydrostatic transmission of this invention has a rotatable piston housing with a plurality of piston bores arranged in a circular pattern in the housing. A piston element is slidably mounted in each bore. Each of the piston elements has an outer end extendible out of an open end of each of the bores. A well or socket having a rounded bottom portion is in the outer end of each of the pistons. A piston slipper element comprising an arcuate base (a ball) is rotatably mounted in the socket. An arm extends outwardly from the ball and has a laterally extending flat planar surface at the other end. The flat planar surface slidably engages a swashplate having a planar control surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view through the cylinder block of this invention. 
     FIG. 2 is a schematic drawing similar to FIG. 1 showing the dimensional relationship between the length of the slipper and the diameter of the piston; 
     FIG. 3 is a drawing similar to that of FIG. 1 showing a modified form of the invention; and 
     FIG. 4 is a drawing of a prior art configuration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, the numeral  10  designates a cylinder block housing having an end cap or center section  12  at one end thereof. The conventional fluid conduits in member  12  have not been shown but are well understood in the art. See U.S. Pat. No. 5,218,886. The other end of member  10  has a center opening  14  with areas  14 A,  14 B and  14 C to receive conventional bearings, seals or the like. A shaft  16  extends inwardly into housing  10  through opening  14 . Shaft  16  has conventional connection  18  to be attached to a source or recipient of rotational power (not shown). Splines  20  on the center portion of shaft  16  serve to power or rotate the cylinder block of this invention as will be discussed hereafter. A bearing socket  22  appears in member  12  and receives the inner end of shaft  16 . Appropriate bearings (not shown) can be utilized in conjunction with the bearing socket  22  as required. A conventional valve plate  24  is placed on the member  12  and has suitable ports therein (not shown) which are well known in the art to service the components in the cylinder block housing  10 . The valve plate  24  has a center opening  26  to accommodate shaft  16 . 
     The numeral  28  designates a cylinder block which has a splined hub  30  in its center portion adapted to slidably receive, for assembly purposes, the splines  20  on shaft  16 . Hub  30  has an outer end with a shoulder  31  of reduced outer diameter (FIG.  1 ). The inter-connection between the splined hub  30  and the splines  20  permit the shaft  16  to rotate the cylinder block  28  as is conventional. 
     Cylinder block  28  has a plurality of cylindrical bores  32  each of which has a piston  34  slidably mounted therein. The length of the bores  32  is greater than the length of pistons  34 . A conduit  35  at the inner end of each of the bores  34  serves to permit hydraulic fluid to be in communication with the bottoms of the bores  32 . The conduits  35  are in communication with appropriate conduits in the member  12  for the supply of hydraulic fluid to the piston bores  32 . Spherical-shaped wells or sockets  36  are formed in the outer end of each of the pistons  34 . 
     Slipper elements  38  are comprised of spherical balls  40  which are rotatably inserted in the sockets  36 . Arms  42  extend outwardly from balls  40  and terminate in slide elements  44  which have a flat planar surface  46 . 
     Swashplate  48  is conventionally mounted in housing  10  and has a center opening  50  to receive shaft  16  and to permit the swashplate to be angularly moved, if required, within housing  10  with sufficient clearance so as not to impinge on shaft  16 . Center opening  50  has a peripheral tapered wall  52  which serves to further permit the swashplate to be angularly disposed with respect to shaft  16  without touching shaft  16 . The swashplate  48  is of solid construction and has an inner flat planar surface  54  that engages the flat planar surface  46  on slipper elements  38  as discussed heretofore. 
     It should be noted that the slipper elements  38 , depending upon the angular position of swashplate  48 , are able to extend into the cylinder bores  32  primarily because of the length of the arms  42  which are a part of the slipper elements. This arrangement of structure permits the swashplate  48  to assume a greater angular position with respect to shaft  16  than would otherwise be the case. Further, this arrangement of structure results in a cylinder block  28  of a shorter length than would otherwise be the case. In addition, the sweet spot  56 , which is the point of maximum concentration of lateral or side forces within the device is moved in a direction closer to valve plate  24  within the body of cylinder block  28 . This results in a structurally superior cylinder block as compared to a sweet spot that was further removed from the valve plate  24 . 
     The numeral  58  designates the center axis of the housing  10 , block  28  and shaft  16 . The numeral  60  designates the center axis of the bores  32  and pistons  34 . The line  62  is an imaginary line between the center of balls  40 . The sweet spot  56  is located within block  28  and is at the intersection of line  62  and axis  58 . 
     By more centralizing the side load forces between the shaft  16  and the cylinder block  28  (by moving the sweet spot  56  further into the cylinder block  28  as discussed above) a substantial reduction in the surface pressure on the pistons  34  is achieved. This results in less friction between the pistons and the piston bores  32 . 
     The higher swashplate angles which are achievable by this structure, as discussed above, improves the power output and the overall efficiency of the transmission. Further, the higher swashplate angles give the variable motor a great stroking ratio from maximum stroke to minimum stroke. This advantage is available for both the pumps and the motors. 
     A further advantage of moving the sweet spot  56  further into the body of the cylinder block  28  is that it increases the strength of the interface between the splined hub  30  of the cylinder block and the splines  20  of shaft  16 . This is a very significant improvement of the invention since this interface in prior art devices is notorious for failure. 
     The position of the sweet spot  56  as described above, in combination with the reduction of the overhang length of the piston results in a significant potential for reducing the overall length of the machine package. It has been demonstrated in models of this invention that the overall length of the cylinder block can be reduced by 1.5 inches. By this reduction in length, the length of the shaft  16  is also reduced. This reduces the possibility of shaft deflections which in turn would increase the bearing life of bearings supporting the shaft. 
     FIG. 3 shows a structure very similar to that of FIG. 1, and like reference numerals are shown in each drawing, except that the shoulder  31  on hub  30  of FIG. 1 has been eliminated on hub  30 A in FIG.  3 . This is because the arms  42  on slipper elements  38  are longer in FIG. 3 than in FIG.  1 . This allows the swashplate  48  in FIG. 3 to be displaced at a greater angle with respect to axis  58 , thus moving the sweet spot  56 A in FIG. 3 closer to valve plate  24  than is the sweet spot  56  in FIG.  1 . With the sweet spot located “deeper” into the cylindrical block as shown in FIG. 3, the shoulder  31  can be eliminated. 
     Both the devices of FIGS. 1 and 3 are an improvement over the prior art device of FIG. 4 which shows cylinder block  28 A with shoulder  31 A on hub  30  which is substantially “longer” than the shorter shoulder  31  of FIG.  1 . Shoulder  31  is very susceptible to damage because the sweet spot  56 B of the device of FIG. 4 is substantially beyond the end of hub  30 . Also, the hub  30  of FIG. 3 is easier to fabricate than either of the hubs  30  of FIG. 1 or  4 . The hub  30  and shoulder  31  of FIG. 1 is an improvement over that of FIG. 4 because shoulder  31  of FIG. 1 is shorter, and hence stronger, than the hub  31 A of FIG.  4 . The advantages of the device of FIGS. 1 and 3 over that of FIG. 4 reside in the movement of the sweet spots of FIGS. 1 and 3 in a direction towards valve plate  24 . 
     Further, the device of FIGS. 1 and 3 permit a greater range of inclination for the swashplate with respect to axis  58  which provides for a greater range of speeds to be produced. Heretofore, an inclination of 25° of the swashplate was attainable only through an expensive bent axis arrangement. (See U.S. Pat. No. 1,137,283). The structure of FIG. 1 permits more swashplate inclination than that of FIG. 4, and FIG. 3 permits inclination very close to 25°. 
     It should be noted that the slippers  38 A of FIG. 4 shows balls  40 A integral with pistons  34 . Balls  40 A rotatably meet in sockets  36 A in slide elements  44 A. 
     FIG. 2 shows the preferred dimensional relationship between the diameter of pistons  34  and the effective length of slippers  38  as measured from the geometric center of balls  40  to flat surface  46 . The length L of slipper  38  is greater than the piston diameter D. 
     From the foregoing, it is seen that this invention will achieve at least its stated objectives.