Abstract:
In a hydrostatic device using an axial piston pump, a return plate is mounted so that it contacts the movable swash plate of the hydrostatic transmission. The plate is biased by a spring-type mechanism to force the swash plate to return to neutral, and the set position of the plate may be externally adjusted to compensate for irregularities. A second plate may be used where necessary based on the arrangement of the return plate with respect to the pump cylinder block.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/290,838 filed on May 14, 2001, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This invention relates to an improved design of a variable displacement hydraulic unit such as a pump or hydrostatic transmission (“HST”), and in particular to an improved return to neutral feature. Hydrostatic transmissions and other hydraulic units using an axial piston design are well known in the art. While this invention will be generally described in connection with an HST, it is understood that this invention could be applied to a variety of hydrostatic units, such as stand-alone pumps using external hoses. The invention described herein can also be adapted for use in an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles, and a wide variety of uses, including vehicles and industrial applications. 
     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. 
     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, causing rotation thereof. The axial pistons of the motor are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. 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 which the vehicle operator uses to control direction and speed. 
     The pump system is fully reversible in a standard HST. As the 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. 
     The hydrostatic pump described herein has a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid. Where the pump pistons move vertically, the swash plate is in neutral when it is generally horizontal with respect to the pump pistons. The swash plate need not be horizontal in the neutral position, depending on the orientation of the pump, but it will be generally perpendicular to the pump pistons in the neutral position. 
     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 devices are important for vehicle safety, to eliminate unintended movement of the vehicle, and to return the unit to neutral in the event of an accident where the vehicle operator is unable to physically disengage the transmission. Such return to neutral devices generally involve a spring mechanism engaged to the control arm to force the control arm to a neutral position, which then returns the swash plate to a neutral position. These may be located external to the housing or internally. 
     One example of a device used to maintain a hydrostatic unit in the zero displacement mode is shown in U.S. Pat. No. 5,207,144. While that design incorporates a spring mechanism to force a return to neutral, the reciprocal follower used to contact the swash plate does not separately pivot itself, leading to binding problems. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved return design for a swash plate used with a variable displacement hydraulic pump, and this invention could be adapted for use with any swash plate or equivalent structure in any hydrostatic application. The swash plate has a neutral position wherein the thrust bearing engaging the pump pistons is generally perpendicular to the pistons. This invention uses a separate member such as a plate which directly engages the swash plate. This separate member, or return plate, rotates about an axis with movement of the swash plate; it is also engaged to a preload spring mechanism which acts to force the return plate to a set position that in turn forces the swash plate to a conforming position, which is preferably but not necessarily the neutral position. The preload spring keeps the return plate biased against the housing sockets and the swash plate. The separate return plate can be mounted in a variety of places with respect to the swash plate or can be of different sizes and the location of its axis of rotation simply needs to be altered to reflect such changes. 
     The present invention not only returns the unit to a set position, but also helps to maintain the unit in this position. Specifically, a stroking force applied to the swash plate through a control arm or similar mechanism causes rotation of the swash plate and the swash plate, in turn, presses on one side of the return plate. The return plate then transmits a restoring force from the spring mechanism to the swash plate, through one contact point. When the stroking force is removed and the swash plate is rotated back to the set position, both contact points are engaged against the swash plate. The force balance between the two contact points keeps the swash plate at the desired set position. The force balance eliminates the dead band found in other return to neutral devices. An optional adjustment feature can be incorporated at the return plate hinge or the swash plate contact points, and can be accessed from outside the housing by means of an external screw. This adjustability eliminates many of the problems heretofore seen with other designs, as the present unit may be adjusted to compensate for design tolerances, wear or contamination, any one of which may otherwise make the actual set position differ from the desired set position. 
     A second embodiment has the return plate being fitted around the pump cylinder block to provide a more compact design. With such an arrangement, however, the cylinder block prevents mounting the preload spring along the required line of action relative to the return plate. In this embodiment, a second plate, referred to as a preload plate, is used to transmit force from an offset mounted spring to the return plate through two contact points. The correct spring force line of action on the return plate is obtained by the geometry of the preload plate contact points and the spherical pivot of the preload plate. This embodiment enables the use of a more compact design where such may be appropriate. 
     Further objects and benefits of the invention will be apparent to one skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a hydraulic pump using a return plate in accordance with the present invention. 
     FIG. 2 is a perspective view of a swash plate and single return plate in accordance with this invention, and mounted in a transmission housing, with the unit in a stroked position. 
     FIG. 3 is perspective view of the swash plate and return plate of FIG. 2, where the hydrostatic unit is in the neutral position. 
     FIG. 4 is a perspective view of a center section, pump and swash plate incorporating this invention, with the unit in the neutral position. 
     FIG. 5 is a perspective view of the center section, pump and swash plate of the present invention, with the unit in a stroked position. 
     FIG. 6 is a perspective view of a swash plate and a portion of the return to neutral feature of the present invention, where the swash plate is in a stroked position. 
     FIG. 7 is a partial cross-sectional view of a second embodiment of this invention, with certain elements removed for clarity. 
     FIG. 8 shows a cross-sectional view of a second embodiment of this invention. 
     FIG. 9 shows a perspective view of a swash plate and return mechanism of a second embodiment of the invention, where the swash plate is in the neutral position. 
     FIG. 10 is a side view of certain components of the second embodiment of this invention, with the swash plate in the neutral position. 
     FIG. 11 is a side view of the components shown in FIG. 10 with the swash plate in a stroked position. 
     FIG. 12 is an exploded perspective view of the components of the second embodiment of this invention. 
     FIG. 13 is a plan view of the return plate of the second embodiment of this invention. 
     FIG. 14 is a plan view of the preload plate of the second embodiment of this invention. 
     FIG. 15 is a side view of a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cross-sectional view of a standard hydraulic pump as may be used in a hydrostatic application. FIGS. 4 and 5 show certain components of a typical hydrostatic application incorporating the present invention, namely a hydrostatic pump rotatably mounted on a center section. The operation of a hydrostatic application such as a pump, HST or IHT are generally known in the art and will not be described in detail herein. For example, the arrangement of pump  12 , center section  14  and the hydrostatic motor are generally described in U.S. Pat. No. 5,314,387, the terms of which are incorporated herein by reference. As noted, this invention could be used in a device having only a pump  12  without the separate hydraulic motor, or with the motor in a separate housing. 
     Pump cylinder block  12  is rotatably mounted on center section  14 , which includes a plurality of hydraulic porting  20  to transfer hydraulic fluid to another component, such as external hoses (not shown) or a hydraulic motor (not shown). A plurality of pump pistons  16  are mounted in cylinder block  12 , which is driven by input shaft  26 . The motor (not shown) would be mounted on motor running surface  33  of center section  14 . The above elements are generally mounted internal to housing  18 . Center section  14  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  14  could be formed in the shape of a plate or other structure, or could be formed as part of housing  18 . Similarly, for convenience only the upper portion of housing  18  is shown in these figures; the embodiment shown is of a horizontal split line, where upper housing  18  and a corresponding lower housing (not shown) are joined at a split line perpendicular to pump input shaft  26 . It will be understood that other housing arrangements and designs could be substituted for this housing shown within the scope of this invention. 
     Pump pistons  16  are engaged and rotate against swash plate bearing  28 . When the unit is in neutral, swash plate bearing  28  is generally perpendicular to input shaft  26 . Trunnion arm  24 , which may extend out of housing  18 , is used to control the direction of swash plate  22 , which can rotate about an axis parallel to the plane of the page, as shown in FIG. 1. A slider block  60  may be provided on the side of swash plate  22  and connected to trunnion arm  24  or the like to rotate swash plate  22 . Swash plate  22  is mounted on and moves against cradle bearings  27  which engage housing  18 . 
     Return plate  19  is mounted inside housing  18  in contact with swash plate  22 . Spring  23  forces return plate  19  against swash plate  22  and pivot housings  32 . Return plate  19  includes a pair of projections  25  and a pair of pivot pins  30 . The position shown in FIG. 3, where both pins  30  contact pivot housings  32  and projections  25  engage swash plate  22  due to the force of spring  23  on return plate  19 , may be referred to as the set position, which is most likely the neutral position. In certain applications the set position may not be set at neutral but could rather be set at a stroked position, depending on the design requirements. 
     When the unit is stroked in one direction, as can be seen most clearly in FIGS. 2 and 6, swash plate  22  will press against one of the projections  25 , causing return plate  19  to pivot along the axis perpendicular to the page, as shown in FIG. 1, and thus causing compression of spring  23 . The return force of spring  23  acts to counter the rotation of swash plate  22 , biasing return plate  19  to the set position, which in turn forces swash plate  22  to the set position. 
     Pins  30  may be formed as an integral part of return plate  19 , or secured to return plate  19  in some other manner. Pins  30  are mounted in pivot housings  32  which may be formed as part of transmission housing  18 , or as a separate bracket  34  attached to housing  18  through screws  35 , and act as a hinge to allow a separate pivoting of return plate  19 . The pivot axis of return plate  19  is different from the pivot axis of swash plate  22 , and in the embodiment shown they are perpendicular. The ability of return plate  19  to pivot about such a separate pivot axis (as opposed to, e.g., sliding) reduces the risk of binding of return plate  19  as spring  23  is compressed, as shown in FIG.  6 . Other hinge mechanisms could also be used to create the pivot. It is important to note that return plate  19  is not constrained by shaft  21 ; rather, it is located by pins  30 , thus providing the pivoting action for return plate  19 . Spring  23  and shaft  21  need not extend through return plate  19 ; they can be so extended for ease of manufacturing and assembly. 
     Washer  29  is an optional safety feature in that it acts as a supplemental means for maintaining return plate  19  in the proper position, e.g., during assembly or if the unit receives an external force. Washer  29  may be secured by a screw  31  or similar device. Other methods of maintaining return plate  19  in place could also be used, such as housing projections or a bracket, as shown in FIGS. 9 and 12. 
     Spring  23  is shown in this embodiment as being mounted around rod  21 , which is supported by housing  18  and center section  14 . It is understood that other support mechanisms for spring  23 , or even other arrangements of the spring could be used in accordance with this invention. Any device to provide a spring return force to return plate  19  could be used in place of coil spring  23  shown. 
     The adjustability of the internal return to neutral feature of the present invention is shown in FIGS. 4 and 5. Specifically, adjustment screw  39  extends through hole  36  in bracket  34  to contact pin  30 . Rotation of screw  39  in either direction will move return plate  19 , allowing the set position of return plate  19  to adjusted as needed. Adjustment screw  39  extends outside of the transmission housing  18 , through an opening that should be sealed in some manner to prevent oil leakage. Different adjustment mechanisms could also be used within the spirit of this invention. For example, if pin  30  was a different shape, a wedge device could be inserted between it and the bracket, and movement of the wedge in or out would provide the adjustment. 
     A second embodiment of this invention is shown in FIGS. 7-14, where identical element numbers denote common elements. This embodiment allows for a different arrangement of elements to accommodate smaller housing designs or the use of additional equipment which may require certain space within the housing. 
     In this embodiment, return plate  40  is shaped to fit around cylinder block  12  with pivot pins  44  and projections  45  on opposite sides of cylinder block  12 . However, this arrangement precludes the desired location of the return spring element between pivot pins  44  and projections  45 . Thus, the second embodiment uses a preload plate  42  which is directly engaged to the spring  23  and which engages return plate  40  at projections  37 . As swash plate  22  is moved out of the set position, it will exert a force upon one or the other of the projections  45 , causing a rotation of return plate  40  about its pivot point, which in this embodiment is about an axis between pins  44 . In this embodiment, pins  44  are formed as a part of return plate  40  and are mounted in pivot housings  43 . Pivot housings  43  are shown as being formed separately from main housing  18 , although they could also be formed integrally therewith. The optional safety function similar to that served by washer  19  of the first embodiment is served by projections  46  which are shown as being formed as integral to support bracket  50 . Bracket  50  is shown as a separate element secured within housing  18 ; it may also be formed integrally as a portion of the housing or center section  14 . Preload plate  42  has a spherical or multi-axis pivot  53  that mates with slot  51  formed on bracket  50 . Pivot  53  allows preload plate  42  to contact return plate  40  at projections  37  with generally equal forces as return plate  40  is moved by swash plate  22  and by changes to adjustment screw  52 . The function of pivot  53  may also be accomplished by other support arrangements that would enable the motions of pivot  53  as disclosed. Slot  51  allows pivot  53  and thus preload plate  42  to move generally perpendicular to pin  21  to prevent binding of preload plate  42 . Slot  51  could be replaced by a socket in bracket  50  to receive pivot  53  and a longer slot in preload plate  42  to provide for clearance for pin  21 . 
     As shown most clearly in FIGS. 7 and 9, adjustment screw  52  extends through pivot housing  43  and can extend out of the transmission housing  18  to permit adjustment. It may be sealed through an o-ring at the head thereof or some other known method. 
     Preload plate  42  is engaged to spring  23 , which could be any type of spring return mechanism. Preload plate  42  also includes a series of projections  37  to engage return plate  40  and bias it to the set position, which would force swash plate  22  to the set position. The location of projections  37  on preload plate  42  closer to pins  44  than to spring  23  acts to prevent pins  44  from lifting out of pivot housings  43  when the unit is in stroke. One could modify the radius of projections  37  or use a series of projections  37  on preload plate  42  in conjunction with modifying the location of the pivot point of preload plate  42  with respect to the pivot point of return plate  40  to change the return force as the unit moves away from the set position. As an example, a reduced return force in stroke could make it easier for an operator to maintain the unit in stroke compared to a similar unit without such a modification, while achieving the appropriate amount of return force as the unit nears the set position. 
     The various embodiments shown in FIGS. 1-14 depict a cradle-mounted swash bearing, but other designs could be used. For example, FIG. 15 shows a trunnion mounted swash plate  22 ′ having a first trunnion  61  which would extend out of the device housing to be attached to a control device or the like (not shown) and a second trunnion  62  which would be rotatably mounted in a opening in the housing or some similar structure. Return plate  40 ′ would be shaped to accommodate the shape of swash plate  22 ′. Other elements could be substantially identical to the embodiment shown in FIGS. 7-14. It will be understood by one of skill in the art that trunnion mounted swash plate  22 ′ could also be used with the embodiment shown in FIGS. 1-6. 
     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.