Return to neutral device for a hydraulic apparatus

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. A bias arm comprising a generally U-shaped member having spring mounted on either leg thereof may be engaged to the return plate.

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.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a cross-sectional view of a standard hydraulic pump as may be used in a hydrostatic application.FIGS. 4 and 5show 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 pump12, center section14and 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 pump12without the separate hydraulic motor, or with the motor in a separate housing.

Pump cylinder block12is rotatably mounted on center section14, which includes a plurality of hydraulic porting20to transfer hydraulic fluid to another component, such as external hoses (not shown) or a hydraulic motor (not shown). A plurality of pump pistons16are mounted in cylinder block12, which is driven by input shaft26. The motor (not shown) would be mounted on motor running surface33of center section14. The above elements are generally mounted internal to housing18. Center section14and 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 section14could be formed in the shape of a plate or other structure, or could be formed as part of housing18. Similarly, for convenience only the upper portion of housing18is shown in these figures; the embodiment shown is of a horizontal split line, where upper housing18and a corresponding lower housing (not shown) are joined at a split line perpendicular to pump input shaft26. It will be understood that other housing arrangements and designs could be substituted for this housing shown within the scope of this invention.

Pump pistons16are engaged and rotate against swash plate bearing28. When the unit is in neutral, swash plate bearing28is generally perpendicular to input shaft26. Trunnion arm24, which may extend out of housing18, is used to control the direction of swash plate22, which can rotate about an axis parallel to the plane of the page, as shown inFIG. 1. A slider block60may be provided on the side of swash plate22and connected to trunnion arm24or the like to rotate swash plate22. Swash plate22is mounted on and moves against cradle bearings27which engage housing18.

Return plate19is mounted inside housing18in contact with swash plate22. Spring23forces return plate19against swash plate22and pivot housings32. Return plate19includes a pair of projections25and a pair of pivot pins30. The position shown inFIG. 3, where both pins30contact pivot housings32and projections25engage swash plate22due to the force of spring23on return plate19, 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 inFIGS. 2 and 6, swash plate22will press against one of the projections25, causing return plate19to pivot along the axis perpendicular to the page, as shown inFIG. 1, and thus causing compression of spring23. The return force of spring23acts to counter the rotation of swash plate22, biasing return plate19to the set position, which in turn forces swash plate22to the set position.

Pins30may be formed as an integral part of return plate19, or secured to return plate19in some other manner. Pins30are mounted in pivot housings32which may be formed as part of transmission housing18, or as a separate bracket34attached to housing18through screws35, and act as a hinge to allow a separate pivoting of return plate19. The pivot axis of return plate19is different from the pivot axis of swash plate22, and in the embodiment shown they are perpendicular. The ability of return plate19to pivot about such a separate pivot axis (as opposed to, e.g., sliding) reduces the risk of binding return plate19as spring23is compressed, as shown inFIG. 6. Other hinge mechanisms could also be used to create the pivot. It is important to note that return plate19is not constrained by shaft21; rather, it is located by pins30, thus providing the pivoting action for return plate19. Spring23and shaft21need not extend through return plate19; they can be so extended for ease of manufacturing and assembly.

Washer29is an optional safety feature in that it acts as a supplemental means for maintaining return plate19in the proper position, e.g., during assembly or if the unit receives an external force. Washer29may be secured by a screw31or similar device. Other methods of maintaining return plate19in place could also be used, such as housing projections or a bracket, as shown inFIGS. 9 and 12.

Spring23is shown in this embodiment as being mounted around rod21, which is supported by housing18and center section14. It is understood that other support mechanisms for spring23, 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 plate19could be used in place of coil spring23shown.

The adjustability of the internal return to neutral feature of the present invention is shown inFIGS. 4 and 5. Specifically, adjustment screw39extends through hole36in bracket34to contact pin30. Rotation of screw39in either direction will move return plate19, allowing the set position of return plate19to adjusted as needed. Adjustment screw39extends outside of the transmission housing18, 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 pin30was 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 inFIGS. 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 plate40is shaped to fit around cylinder block12with pivot pins44and projections45on opposite sides of cylinder block12. However, this arrangement precludes the desired location of the return spring element between pivot pins44and projections45. Thus, the second embodiment uses a preload plate42which is directly engaged to the spring23and which engages return plate40at projections37. As swash plate22is moved out of the set position, it will exert a force upon one or the other of the projections45, causing a rotation of return plate40about its pivot point, which in this embodiment is about an axis between pins44. In this embodiment, pins44are formed as a part of return plate40and are mounted in pivot housings43. Pivot housings43are shown as being formed separately from main housing18, although they could also be formed integrally therewith. The optional safety function similar to that served by washer19of the first embodiment is served by projections46which are shown as being formed as integral to support bracket50. Bracket50is shown as a separate element secured within housing18; it may also be formed integrally as a portion of the housing or center section14. Preload plate42has a spherical or multi-axis pivot53that mates with slot51formed on bracket50. Pivot53allows preload plate42to contact return plate40at projections37with generally equal forces as return plate40is moved by swash plate22and by changes to adjustment screw52. The function of pivot53may also be accomplished by other support arrangements that would enable the motions of pivot53as disclosed. Slot51allows pivot53and thus preload plate42to move generally perpendicular to pin21to prevent binding of preload plate42. Slot51could be replaced by a socket in bracket50to receive pivot53and a longer slot in preload plate42to provide for clearance for pin21.

As shown most clearly inFIGS. 7 and 9, adjustment screw52extends through pivot housing43and can extend out of the transmission housing18to permit adjustment. It may be sealed through an o-ring at the head thereof or some other known method.

Preload plate42is engaged to spring23, which could be any type of spring return mechanism. Preload plate42also includes a series of projections37to engage return plate40and bias it to the set position, which would force swash plate22to the set position. The location of projections37on preload plate42closer to pins44than to spring23acts to prevent pins44from lifting out of pivot housings43when the unit is in stroke. One could modify the radius of projections37or use a series of projections37on preload plate42in conjunction with modifying the location of the pivot point of preload plate42with respect to the pivot point of return plate40to 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 inFIGS. 1–14depict a cradle-mounted swash bearing, but other designs could be used. For example,FIG. 15shows a trunnion mounted swash plate22′ having a first trunnion61which would extend out of the device housing to be attached to a control device or the like (not shown) and a second trunnion62which would be rotatably mounted in a opening in the housing or some similar structure. Return plate40′ would be shaped to accommodate the shape of swash plate22′. Other elements could be substantially identical to the embodiment shown inFIGS. 7–14. It will be understood by one of skill in the art that trunnion mounted swash plate22′ could also be used with the embodiment shown inFIGS. 1–6.

Another embodiment of a transmission having an internal return to neutral feature is shown inFIGS. 16–23. This embodiment is preferably for use in a zero turn transaxle10such as that depicted inFIG. 17, where a transmission120is secured to a pair of axle housings122, each having an output axle121mounted therein. The transmission120comprises a main housing96having a cover98mounted thereon and secured thereto by means of bolts101; it will be understood that the specific design of the transmission120and axle housings122are not required for the present invention and this invention could be readily used with other transaxle designs. For example, while this embodiment is shown as a dual hydrostatic transmission having a pair of pumps and motors, it could be used with a single pump and motor combination as well.

Transmission120includes a hydrostatic transmission100mounted in housing96and having a pair of hydraulic pump cylinder blocks118mounted on a mounting member or center section116. A plurality of pump pistons124are mounted in each pump cylinder block118. Pump swash plates126are moveable to control the hydraulic output of the cylinder blocks118.

A pair of hydraulic motor blocks206are mounted on opposite ends of center section116. A plurality of motor pistons208are mounted in each motor cylinder block206. Pump input shaft106drives at least one and can drive both of the pump cylinder blocks118. In this embodiment, first input shaft106is connected to and drives second input shaft107through gears160. Hydraulic porting (not shown) is formed in center section116to connect each pump cylinder block118to its respective motor cylinder block206. A motor shaft1110is engaged to and driven by each motor cylinder blocks206, and each motor shaft110extends into the respective axle housing122, where it engages a drive train (not shown) to drive output axle121.

In this embodiment, the return to neutral feature140forces pump swash plates126to the neutral position when the corresponding trunnion arm132is not under stroke. In most cases, this means returning the swash plates126to a generally horizontal position, such as is shown inFIGS. 18 and 20, where there is insufficient axial displacement of the pump pistons124to cause rotation of axle shafts121.

Because this embodiment depicts a dual hydrostatic transmission, it will be understood that there are two identical return to neutral features140depicted herein, and identical numerals are intended to depict identical structure.

A return plate142is mounted in housing96and has a pair of oppositely extending pins143formed therewith. As shown inFIG. 21, plate142can rotate within housing96about an axis extending through the two pins143. At the opposite end of plate142are two projections145extending perpendicularly upward from the top surface thereof to contact surface141of swash plate126.

Plate142also has a mating feature148comprising a generally curved surface having a pair of lips152and153extending downwardly therefrom. Arm144having a generally circular cross-section is mounted in the housing96and comprises a generally U-shaped member having a curved cross-piece150that mates with mating feature148on plate142and is held in place by lips152and153. Springs146are mounted around each end of arm144, and are located at one end in holes149and act against cover98. Thus, when swash plate is moved into either the forward or reverse position by movement of trunnion arm132, springs146will be compressed and will then provide a counteracting spring force in the opposite direction in order to return swash plate126to the horizontal or neutral position.

In order to keep arm144and its associated springs146in position during assembly, a mating feature147is provided in housing96. Feature147provides a location for springs146to be positioned during installation of cover98so that springs146may be more easily located in mating holes149in cover98. Springs146on the other side are preferably maintained in an identical manner.

A neutral adjust means154penetrates housing96to contact one pin143of return plate142, so that the return-to-neutral mechanism140may be adjusted to establish a set point to coincide with a selected position, which would in most cases be the neutral position. Neutral adjust means154preferably has an o-ring156or other means of preventing oil leakage. Neutral adjust means154includes a locking device in the form of a nut158so that once neutral adjust means154is adjusted to an appropriate position, nut158may be tightened onto housing96to prevent further movement of neutral adjust means154that might tend to change the set point of return-to-neutral mechanism140.

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.