Abstract:
In a hydrostatic device using an axial piston pump, a yoke is mounted so that it contacts the movable swash plate of the hydrostatic transmission. The yoke is biased by a spring-type mechanism to force the swash plate to return to neutral, and the set position of the yoke plate may be externally adjusted. A bias or load arm rotatably fixed to a housing at one end and connected to a spring at the other end is engaged to the yoke to provide the return force to the yoke plate. The yoke plate may have two legs to provide a return to neutral force to the swash plate in either direction, or one leg to provide the return to neutral force in only a single direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/056,439 filed on Mar. 27, 2008 which claims priority to U.S. Provisional Patent Application Ser. No. 60/909,625 filed on Apr. 2, 2007. The prior applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an improved design of a return to neutral mechanism for use in a variable displacement hydraulic unit such as a pump, a hydrostatic transmission (“HST”) or an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles, and such devices can be used in a wide variety of uses, including vehicles and industrial applications. The operation of a hydrostatic application such as a pump, HST or IHT is generally known in the art and will not be described in detail herein. For example, the arrangement of an IHT and the operation of the components thereof are generally described in U.S. Pat. Nos. 5,314,387 and 6,122,996, the terms of which are incorporated herein by reference. 
     In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself. 
     In a typical arrangement, the pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The axial pistons of the pump engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor to drive the motor cylinder block and the motor output shaft. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an IHT, or external thereto. The swash plate is generally controlled by a control arm which is connected via linkage to either a hand control or foot pedal mechanism to control direction and speed. 
     The pump system is fully reversible in a standard HST. As the pump swash plate is moved, the rotational direction of the motor can be changed. The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. 
     A hydraulic pump will also have a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump cylinder block does not create any movement of the hydraulic fluid. The swash plate is in neutral when it is generally perpendicular with respect to the pump pistons. 
     For safety reasons, and for the convenience of the user, it is preferred to have a return to neutral, or zero displacement, feature which forces the swash plate to its neutral position when no force is being applied to the control arm. Such a feature eliminates unintended movement of the vehicle, and returns the unit to neutral in the event of an accident where the vehicle operator is unable to physically disengage the transmission. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved return design for a swash plate used with a variable displacement hydraulic pump, and in particular a simplified internal return to neutral design that uses fewer parts and is easier to install than known designs. This return to neutral design may either be bi-directional, returning the unit to neutral when stroked in either the forward or reverse direction, or uni-directional, providing a return force in only one direction and not the other. The invention is described herein in connection with a hydrostatic transaxle but it could be used in a device having only a pump without the separate hydraulic motor, or with the motor in a separate housing. 
     A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the various ways in which the principles of the invention may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the return to neutral feature of one embodiment of the present invention and an exemplary swash plate, showing the relationship of the two when the swash plate is in a stroked position. 
         FIG. 2  is an end view of the return to neutral feature and swash plate as shown in  FIG. 1 , in conjunction with an exemplary center section, with the swash plate in the stroked position. 
         FIG. 3  is a view similar to that of  FIG. 2 , with the swash plate in the neutral position. 
         FIG. 4  is a perspective view of an exemplary hydrostatic transmission encompassing a second embodiment of this invention. 
         FIG. 5  is a side elevational view of a swash plate and return to neutral mechanism in accordance with the second embodiment of this invention, with the swash plate in the neutral position. 
         FIG. 6  is a view similar to  FIG. 5 , with the swash plate in a first stroked position. 
         FIG. 7  is a view similar to  FIGS. 5 and 6 , with the swash plate in a second stroked position. 
         FIG. 8  is an opposite side elevational view of the swash plate and return to neutral mechanism as shown in  FIG. 5 , and depicting a portion of the housing. 
         FIG. 9  is a partially cross-sectional side view of a portion of the housing and the return to neutral mechanism along the lines  9 - 9  as shown in  FIG. 8 , where the adjustment mechanism is not cross-sectioned for clarity. 
         FIG. 10  is a plan view of the adjustment mechanism shown in  FIG. 9 . 
         FIG. 11  is a perspective view of the second embodiment of this invention, depicting the return to neutral structure and exemplary swash plate, showing the relationship of the two when the swash plate is in a neutral position. 
         FIG. 12  is a perspective view similar to  FIG. 11 , showing the swash plate in a stroked position. 
         FIG. 13  is a side elevational view of a third embodiment of this invention, depicting the swash plate in the neutral position. 
         FIG. 14  is a view similar to  FIG. 13 , with the swash plate in a first stroked position. 
         FIG. 15  is a view similar to  FIGS. 13 and 14 , with the swash plate in a second stroked position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of this invention and in particular return to neutral mechanism  10  is disclosed in  FIGS. 1-3 . A second embodiment of the invention and in particular return to neutral mechanism  110  is disclosed in  FIGS. 4-12 . A third embodiment of the invention and in particular return to neutral mechanism  210  is disclosed in  FIGS. 13-15 . 
     The general arrangement of the hydrostatic transmission used with these embodiments will be discussed with regard to hydrostatic transmission  120  shown in  FIG. 4 . Pump cylinder block  12  is rotatably mounted on center section  114 , which includes internal hydraulic porting (not shown) to transfer hydraulic fluid between pump cylinder block  12  and motor cylinder block  15 . A plurality of pump pistons (not shown) are mounted in cylinder block  12 . Center section  114  and the other components could take on a variety of other shapes and arrangements. By way of example only, the pump and motor cylinder blocks need not be at right angles to one another but could also be in a parallel or back-to-back arrangement, and center section  114  could be formed in the shape of a plate or other structure, or could be formed as part of the housing. 
     Center section  14  is depicted in  FIGS. 2 and 3  in connection with the first embodiment of this invention and while it is a different shape than center section  114 , both can operate in essentially the same manner. In both cases, a motor running surface  18  is provided with a pair of kidney ports  19  to connect motor cylinder block  15  to the internal hydraulic porting (not shown). It will also be understood that the various gears and other components that would be used in connection with this invention if used with a transaxle are not depicted herein. 
     As shown in, e.g.,  FIG. 4 , swash plate  122  is used to control the output of hydraulic pump cylinder block  12 ; a swash plate bearing (not shown) located inside swash plate  122  engages with pump pistons (not shown). In neutral, swash plate  122  is generally perpendicular to the rotational axis of pump cylinder block  12 . 
     Trunnion  124  extends from one side of swash plate  122  and includes a portion  129  engaged to a control mechanism (not shown) located external to housing  150  for causing rotation of swash plate  122 . Trunnion  124  extends from the side of swash plate  122  opposite to the side where the return to neutral mechanism  110  is located. A second support trunnion  126  can be used to support swash plate  122  within housing  150  and would be located on the same side of swash plate  122  as return to neutral mechanism  110 . 
     A similar arrangement can be used with the other embodiments depicted herein; for example, swash plate  22  in  FIGS. 1-3  can use two support trunnions  24 ,  26  that are similar in function to that previously described. It will be understood that return to neutral mechanism  10  or  110  could be on the same side as the trunnion  24  or  124 , depending on factors such as housing size and the like. It will also be understood that other methods of supporting a swash plate, such as a cradle bearing, are known and are interchangeable with the use of a pair of opposing trunnions. 
     In a first embodiment, return to neutral mechanism  10  comprises yoke  32  engaged to load arm  31  and swash plate  22 . At one end of load arm  31 , spring  30  is secured to spring attachment opening  36  and to a fixed point in the housing (not depicted in this embodiment), thus providing the return force to load arm  31  and yoke  32 . In operation load arm  31  pivots about adjustment mechanism  60 , and in particular about the axis of protrusion  49 , described in more detail below. Yoke  32  comprises a pair of arms  33   a  and  33   b  joined by a curved surface, culminating in a preferably curved end  35   a  and  35   b , respectively. Yoke  32  is secured to a side of load arm  31  in a manner to permit its rotation with respect thereto. 
     As shown most clearly in  FIG. 2 , swash plate  22  includes an end portion  25 , which may be integrally formed therewith, and having a generally curved shape culminating in two stops  27 , which are connected by curved interface  23 . Interface  23  preferably has a radius complementary to that of curved surface  38  on yoke  32 . It will be understood that these two surfaces will not actually contact one another when the unit is in neutral, as shown in  FIG. 3 , but that there would be a small gap between them, and the contact between yoke  32  and swash plate  22  will be through arms  33   a ,  33   b  contacting the two stops  27  when the unit is in neutral. The geometry of these components, such as yoke  32 , load arm  31 , location of spring attachment opening  36 , and the like can be modified to change the restoring moment of yoke  32  as a function of the swash angle, depending on the specific application requirements. 
     The location of the neutral position for swash plate  22  may be adjusted by the externally accessible adjustment mechanism  60 , which is similar in operation to the adjustment mechanism  160  discussed in detail below in connection with the second embodiment of this invention. In general, adjustment mechanism  60  extends through the housing (not shown in this embodiment) so that shoulder  41  engages an internal surface of the housing and threaded portion  43  and adjustment hex  34  are located outside the housing. An off-center protrusion  49  is located on the internal end of adjustment mechanism  60  and is mounted in opening  39  formed in one end of load arm  31 . Since protrusion  49  is off-center with respect to the axis of rotation of adjustment  60 , the position of load arm  31  changes as adjustment mechanism  60  is rotated. 
     The return to neutral mechanism  10  is bidirectional. One of the arms of yoke  32  can be easily shortened so that only one of the stops  27  is contacted by yoke  32 , in the event one wishes to provide for a unidirectional return to neutral; i.e., providing a return force only when the swash plate is stroked in one direction but not the other. Such a feature is described below in connection with further embodiments. 
     A second embodiment of this invention showing a bidirectional return to neutral mechanism  110  is depicted in  FIGS. 4-12 . The relationship of the return to neutral mechanism  110  and housing  150  can best be understood in connection with the second embodiment of the invention as depicted in, e.g.,  FIGS. 8 and 9 . This same connection to the housing could be used in connection with the first embodiment of return to neutral mechanism  10 , but the housing is not depicted in  FIGS. 1-3  for clarity. It will be understood that many of the same components as described above may be used and similar reference numerals are used for components that may be identical to those previously discussed. For example, the shape of center section  114  is not critical to this invention and different center sections could be used or, as noted above, the invention could be used in a design that does not use a center section. 
     In this second embodiment, return to neutral mechanism  110  comprises load arm  131 , which is sandwiched between housing  150  and center section  114 . Load arm  131  may also be retained in place by other methods, such as a retaining ring on adjustment mechanism  160 . At one end of load arm  131 , spring  130  is secured to spring attachment hole  136  and to a fixed point, which may be a fastener  151  attached to housing  150 , as shown in  FIG. 8 , thus providing the return force to load arm  131  and yoke  132 . The other end of load arm is supported in housing  150  by adjustment mechanism  160 , described below. In operation load arm  131  pivots about adjustment mechanism  160 , and in particular about the axis of protrusion  149 . 
     The location of the neutral position for swash plate  122  may be adjusted by modifying the set position of load arm  131 ; this is accomplished by means of the externally accessible adjustment mechanism  160 , seen most clearly in  FIGS. 9 and 10 . In  FIG. 9  certain components such as housing  150  are sectioned, but adjustment mechanism  160  is not sectioned merely for clarity. Adjustment mechanism  160  comprises bearing surface  152  extending through an opening  154  in housing  150  so that shoulder  141  engages an internal surface of housing  150 . An off-center protrusion  149  is located on one end of adjustment mechanism  160  internal to housing  150 ; protrusion  149  is mounted in opening  139  formed in one end of load arm  131 . A threaded portion  143  and adjustment hex  134  are located at the opposite end of adjustment mechanism  160  and are located outside the housing so that a user can adjust mechanism  160  externally, and then lock the unit in the selected position by means of locknut  144 . Seal  145  is used to prevent leakage through opening  152 . Since protrusion  149  rotates with shoulder  141  but is located off-center with respect to the axis of rotation of adjustment  160 , it will move the set position of load arm  131  as adjustment mechanism  160  is rotated. While it is generally intended that the adjustment mechanisms  60  and  160  disclosed herein are used to locate neutral, it will be understood that these mechanisms could also be set to be biased to an off-neutral position, so that yoke  32  or  132  would return swash plate  22  or  122  to some preselected, non-neutral position. 
     Yoke  132  comprises a pair of arms  133  joined by a curved surface, each arm culminating in a preferably curved end  135 . In the second embodiment, the shapes of yoke  132  and load arm  131  and the relationship between these elements and with swash plate  122  are slightly different than the first embodiment. Yoke  132  includes two arms  133   a ,  133   b  extending from the main body thereof to engage swash plate  122  and, in particular pockets  128  formed in surfaces  127 . Pockets  128  act as the stops and are shaped to receive curved ends  135   a ,  135   b  of each arm  133   a ,  133   b ; using a curved interaction surface such as pocket  128  as the stop improves the interaction between yoke  132  and swash plate  122 , thereby narrowing the dead band. 
     Housing interface  155  shown in  FIG. 8  may be formed on an internal surface of housing  150  and permits the use of a smaller swash plate than the embodiment shown in, e.g.,  FIGS. 2 and 3 . In the bidirectional embodiment depicted in, e.g.,  FIG. 8 , there will be a clearance between housing interface  155  and curved portion  138  of yoke  132 . Housing interface  155  is not depicted in  FIGS. 5-7  in order to more clearly show the geometry of the other elements. 
     Yoke  132  is secured to load arm  131  by means of a protuberance  146  shaped to engage a pocket  148  on load arm  131 ; this arrangement is generally less expensive to manufacture than the structure shown in the first embodiment and also maintains the forces between yoke  132  and load arm  131  in the same plane. 
     A further embodiment is depicted in  FIGS. 13-15 , which show a uni-directional return to neutral mechanism  210 , which is similar in many ways to mechanism  110  previously discussed. Many of the same components may be used and similar reference numerals are used for components that may be identical to those previously discussed. For example, load arm  131  and its mounting within the housing can be same as previously described. 
     In this embodiment, yoke  232  includes protuberance  246  mounted into pocket  148  on load arm  131 . Yoke  232  includes, however, only one arm  233   a , with the other arm removed. Thus, when swash plate  122  is stroked in the direction shown in  FIG. 15 , a return force is provided by the interaction of arm  233   a  with swash plate  122 , and more particularly with the interaction of curved end  235   a  with pocket or stop  128  formed in swash plate surface  127 . In this unidirectional embodiment as opposed to the prior bidirectional embodiment, curved portion  238  of yoke  232  interacts with housing interface  155  so that there is no clearance between these two elements. 
     When swash plate  122  is stroked in the first direction such as is depicted in  FIG. 14 , there is no contact between swash plate  122  and yoke  232 , so that no return force is provided in this direction. Note that the same swash plate  122  as previously described is used in this embodiment, to minimize the number of components needed for different applications. If desired, one could use a different swash plate having only the one stop  128  needed. Similarly, the same housing as in the prior embodiments could be used. As shown in  FIGS. 13-15 , a housing interface  155  may be used in this unidirectional embodiment 
     It is to be understood that the above description of the invention should not be used to limit the invention, as other embodiments and uses of the various features of this invention will be obvious to one skilled in the art. This invention should be read as limited by the scope of its claims only.