Abstract:
A speed adjusting mechanism that includes a return to neutral feature for returning a control member of a pump to neutral after the control member has been moved to a forward or reverse position. The speed adjusting mechanism is characterized by the use of a torsion spring that enables the mechanism to be composed of just a few parts.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/590,390 filed Jul. 21, 2004, which application is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention herein described relates generally to hydraulic pumps and more particularly to a return to neutral mechanism used therewith. The invention has particular applicability to piston pumps used in hydrostatic transmissions employed in axle driving apparatus such as zero-turn transaxles.  
       BACKGROUND  
       [0003]     Transaxle and hydrostatic transmission assemblies are known in the art. Generally, a hydrostatic transmission assembly includes a rotating hydraulic pump and a rotating hydraulic motor. The hydraulic pump usually is a piston-type pump including a plurality of reciprocating pistons which are in fluid communication through hydraulic porting with the hydraulic motor. Rotation of the hydraulic pump against a moveable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the hydraulic motor to drive the motor. The rotation of the hydraulic motor may be used to drive a wheel axle of a riding lawn mower, small tractor and the like.  
         [0004]     The position of the swash plate with respect to the hydraulic pump pistons can be changed to vary the speed and change the direction of rotation of the hydraulic motor and thus the speed and direction of rotation of the vehicle axle. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction. Additionally, the angle at which the swash plate addresses the hydraulic pump pistons can be changed to increase or decrease the amount of hydraulic fluid that is forced from the hydraulic pump to the hydraulic motor to change the speed at which the hydraulic motor rotates.  
         [0005]     The position of the moveable swash plate typically is varied by rotating a trunnion arm that is coupled to the swash plate. The trunnion arm projects from the pump housing for coupling to a speed change lever or a speed change pedal of the vehicle through a cable, lever or other linkage. In this manner, movement of the speed change lever/pedal results in rotational movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle.  
         [0006]     Return to neutral mechanisms heretofore have been used to cause the swash plate of the pump to assume a neutral position even after having been shifted to a forward and/or reverse position. The return to neutral mechanism is normally implemented as an integral part of the vehicle linkage.  
         [0007]     In U.S. Pat. No. 6,782,797 there is disclosed a return to neutral mechanism that is adapted to be mounted to the casing of a hydraulic pump or hydrostatic transaxle. The mechanism is integrated into a speed adjusting mechanism that includes a return arm adapted to be mounted to the casing in a fixed position indicative of a neutral position of the trunnion arm coupled to the swash plate of the pump. Additionally, the speed adjusting mechanism includes a control arm that is mounted to and moves the trunnion arm. A pair of scissor return arms are provided that are adapted to move the control arm in cooperation with the return arm for the purpose of moving the trunnion arm to the neutral position. To provide mounting flexibility, the control arm is adapted to be mounted to the trunnion arm in any one of a plurality of different positions and the return arm is capable of being mounted to the casing in a corresponding position such that the return arm can be aligned with the control arm to establish the neutral position.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a hydraulic drive apparatus including a speed adjusting mechanism that includes a return to neutral feature for returning a control member of a pump to neutral after the control member has been moved to a forward or reverse position. The speed adjusting mechanism is characterized by the use of a torsion spring that enables the mechanism to be composed of just a few parts.  
         [0009]     In accordance with the invention, a speed adjusting mechanism is provided for a hydraulic drive apparatus including a casing having a hydraulic pump mounted therein and a trunnion arm extending outside the casing and being rotatable for controlling the output of the pump. The speed adjusting mechanism comprises a return member mountable to the casing, a control arm connectable to the trunnion arm and movable between forward and reverse positions to move the trunnion arm between forward and reverse positions, respectively, and a torsion spring operatively coupled to the return member and the control arm for biasing and returning the control arm to a neutral position after the control arm has been moved to at least one of the forward or reverse positions.  
         [0010]     More particularly, the return member includes at least one spring stop for preventing outward movement of at least one leg of the torsion spring when the control arm is rotated in a first direction while permitting inward movement of the at least one leg of the torsion spring when the control arm is rotated in a second direction opposite the first direction. The spring stop can be a slot in the return member through which the at least one leg of the torsion spring extends. The control arm includes at least one abutment for engaging and urging at least one spring leg of the torsion spring to cause it to move inwardly against the biasing force of the torsion spring when the control arm is rotated in a first direction.  
         [0011]     In a preferred embodiment, a speed adjusting mechanism is provided with unidirectional return to neutral functionality wherein the torsion spring biases and returns the control arm to neutral after the control arm is rotated in a first direction, and wherein the control arm can rotate independent of the torsion spring when rotated in a second direction opposite the first direction.  
         [0012]     In another preferred embodiment, the return member includes first and second spring stops. The first spring stop is configured to prevent outward movement of a first leg of the torsion spring and the second spring slot is configured to permit inward movement of a second leg of the torsion spring when the control arm is rotated in a first direction. Conversely, when the control arm is rotated in a second direction opposite the first direction, the first spring stop is configured to permit inward movement of the first leg of the torsion spring and the second stop is configured to prevent outward movement of the second leg of the torsion spring.  
         [0013]     The control arm includes laterally spaced apart first and second of abutments, referred to as ears, for engaging and urging the first and second legs of the torsion spring when the control member is rotated. The first abutment is configured to engage and urge the first leg of the torsion spring to cause it to move inwardly against the biasing force of the torsion spring when the control arm is rotated in a first direction. The second abutment configured to engage and urge the second leg of the torsion spring to cause it to move inwardly against the biasing force of the torsion spring when the control arm is rotated in a second direction opposite the first direction.  
         [0014]     The foregoing and other features of the invention are hereinafter described in detail in conjunction with the accompanying drawings which set forth an exemplary embodiment illustrating one of the various ways in which the principles of the invention may be employed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of an exemplary hydraulic drive apparatus with which can be used a speed control mechanism corresponding to the present invention.  
         [0016]      FIG. 2  is a cross-sectional view of the hydraulic drive apparatus of  FIG. 1 .  
         [0017]      FIG. 3  is a cross-sectional view of the speed control mechanism.  
         [0018]      FIG. 4  is a perspective view of the speed control mechanism of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings in detail and initially to  FIGS. 1 and 2 , an exemplary hydraulic drive apparatus is indicated generally by reference numeral  10 . The hydraulic drive apparatus  10  is an integrated hydrostatic transmission (herein referred to as IHT) that comprises a housing  14  containing a reversible pump  18  fluidly connected to a motor  22  in a closed loop hydraulic circuit. An input shaft  26  ( FIG. 2 ) for driving the pump  18  extends vertically from a top  30  of the housing  14  and includes a fan  34  and pulley  38 . The pulley  38  can be connected by a belt to a prime mover, such as an internal combustion engine (not shown) of a vehicle, for turning the input shaft  26 . An output shaft  42  driven by the motor  22  extends horizontally from a side of the housing  14 . As will be appreciated, the pump  18  supplies pressurized hydraulic fluid to an inlet of the motor  22  for driving the output shaft  42 .  
         [0020]     IHT  10  can be mounted to a frame  43  of a vehicle, generally indicated by reference number  44 , using bolt holes  45  through which bolts can extend to secure the IHT  10  to the vehicle frame  43 . A wheel  46  can be mounted to an outer axle portion of the output shaft  42  of the IHT  10  for supporting the vehicle  44  for movement over the ground. In the illustrated embodiment, the output shaft  42  has a tapered portion provided with a key  54  that engages in a key slot in the hub of a wheel  46  so that the wheel  46  will rotate with the output shaft  42 . The wheel  46  can be secured in place by means of a nut  58  which is threaded onto the end of the axle output shaft  42 .  
         [0021]     The housing  14  of the IHT  10  includes an upper housing part  62  and a lower housing part  66  secured together with bolts  70 , or by other suitable means. A sealing gasket  72  is typically provided at the interface of the upper housing portion  62  and the lower housing portion  66 . The upper housing portion  62  and the lower housing portion  66  together form a generally circular opening into which the motor  22  is received. The opening is closed by a mounting plate  74  that carries the motor  22 . The mounting plate  74  is secured to the upper housing portion  62  and the lower housing portion  66  by bolts  78 , or by other suitable means. The mounting plate  74  may be provided with a bypass valve  76  for creating a short fluid circuit in the motor  22  to allow free rotation of the output shaft  42  when desired.  
         [0022]     The upper housing portion  62 , the lower housing portion  66 , and the mounting plate  74  together form a sealed interior space of the housing  14  that contains the motor  18  and pump  22 . The interior of the housing  14  serves as a shared sump for the motor  18  and the pump  22 , and further may function as a reservoir.  
         [0023]     The housing  14  further includes cooling fins  82  at various locations, generally on the top surface  30  near the fan  34 , for assisting in dissipating heat generated during operation of the pump  18  and motor  22  of the IHT  10 . The housing  14  can further include an externally mounted oil filter  86  for filtering the hydraulic fluid of the IST  10 . The externally mounted oil filter  86  can be easily accessed for replacement when the filter becomes dirty. A breather  90  is provided for allowing air to exit and enter the housing  14  as necessary to compensate for thermal expansion and contraction of the hydraulic fluid. The breather  90  can also function as a fill cap for adding hydraulic fluid to the shared sump. Although not shown, a drain plug can be included at the bottom of the housing  14  for draining the hydraulic fluid so that fresh hydraulic fluid can be added to the housing  14 .  
         [0024]     In the illustrated embodiment, the pump  18  is a variable flow reversible piston pump. The input shaft  26  is supported in the upper housing portion  62  by a bearing  91 , and a seal  92  is provided to seal the shaft  26  to the housing  14  to prevent fluid from escaping from the interior of the housing  14 . A cylinder barrel  93  having a plurality of pistons  94  mounted for reciprocal movement therein is coupled to the input shaft  26  for rotation therewith. The input shaft  26  is supported by a thrust bearing  95  below a swash block  96  (also commonly referred to as a swash plate).  
         [0025]     The swash block  96  is arranged such that the pistons  94  of the cylinder barrel  93  abut its upper surface. As will be appreciated, the swash block  96  can be inclined in first and second directions from a neutral position, or zero-inclination. Inclining the swash block  96  in the first direction (e.g., a forward position) results in the pump  18  pumping fluid in a first direction, while inclining the swash block  96  in the opposite direction (e.g., a reverse position) results in the pump  18  pumping fluid in the opposite direction.  
         [0026]     As seen in  FIG. 2 , the swash block  96  has a cam slot  98  for receiving a cam pin  99  of a trunnion arm  100  shown in  FIG. 3  (trunnion arm  100  not shown in  FIG. 2 ). The trunnion arm  100  includes a trunnion shaft  101  and a linkage  102  provided with the cam pin  99  at an end thereof for connecting the linkage  102  to the cam slot  98  in the swash block  96 . As best seen in  FIG. 1 , the trunnion shaft  101  is rotatably mounted in a wall of the housing  14  by a bushing and protrudes from the housing  14  through a hole H in the lower housing part  66 . It will be appreciated that the inclination of the swash block  96  is controlled by rotation of the trunnion arm  100 . For a further description of the linkage between the trunnion arm  100  and the swash block  96 , reference may be had to U.S. Pat. No. 6,772,591, which is hereby incorporated herein by reference.  
         [0027]     In accordance with the present invention, a speed adjusting mechanism  103  is provided for controlling the swash block  96 . The speed adjusting mechanism  103  also functions as a return to neutral mechanism for biasing and returning the swash block  96  to the neutral position after the swash block  96  has been moved to at least one of the forward or reverse positions. It will be appreciated that, although not shown in  FIGS. 1 and 2 , the speed adjusting mechanism  103  can be mounted to the housing  14  and connected to trunnion shaft  101 , as will now be described.  
         [0028]     As shown in  FIGS. 3 and 4 , the speed adjusting mechanism  103  comprises a return member  104  and a control arm  108 . The return member  104  and control arm  108  are operatively coupled together by a torsion spring  110  having first and second spring legs  111  and  112 . The torsion spring  110  is configured to bias and return the control member  108 , and consequently the swash block  96 , to the neutral position.  
         [0029]     The return member  104  has a generally U-shape portion  113  having a bottom  114  and opposed side walls  115  and  116 . The bottom  116  of the U-shape portion  113  has an opening for passage of a fastener, such as a screw  120 , that is used to secure the return arm  104  to the housing  14 . Preferably, the opening is a slot  122  to provide for angular adjustment of the return member  104  relative to the rotational axis of the trunnion shaft  101  so that the return member  104  can be adjusted to correspond to the neutral position of the trunnion arm  100 . The return member  104  also has a trunnion mount portion  126  that extends laterally from the top of side wall  116  of the U-shape portion  113 . The trunnion mount portion  126  has a hole  130  into which a spindle portion  134  on an axially outer end of the trunnion shaft  101  extends to define a pivot axis for the above noted angular adjustment of the return member  104 . The screw  122  and the spindle portion  134  of the trunnion shaft  101  that extends into the hole  130  of the return member  104  fix the return member  104  against rotational movement relative to the housing  14 .  
         [0030]     The control arm  108  is mounted at a central portion thereof to the trunnion shaft  101  and fixed for rotation therewith by a square slot  136  in the control arm  108  that engages a square portion  138  of the trunnion shaft  101 . Although typically the square slot  136  and square portion  138  are of corresponding shape, any other non-circular configuration can be used to prevent relative rotation between the control arm  108  and the trunnion shaft  101 . For example, the control arm  108  and trunnion shaft  101  can be keyed together. A washer  139 , a nut  140 , and a lock nut  141  are provided for securing the control arm  108  to the trunnion shaft  101 . The nut  140  and lock nut  141  are threaded onto a threaded shank portion  142  ( FIG. 1 , threads not shown) of the trunnion shaft  101 . Holes  144  are provided for connection of the control arm  108  to a cable or other linkage that may extend to a speed control lever, pedal, or other suitable control device that can be operated by the vehicle operator.  
         [0031]     The control arm  108  also has laterally spaced apart abutments, hereinafter referred to as ears  146  and  150 , that extend from the central portion of the control arm  108  for engaging respective legs  111  and  112  of the torsion spring  110 , as hereinafter described.  
         [0032]     The torsion spring  110  has a central coil portion  152  that is mounted coaxial with the trunnion shaft  101  between the trunnion mount portion  126  of the return member  104  and the control arm  108 . As shown in  FIGS. 3 and 4 , the central coil portion  152  surrounds the nut  140  and lock nut  141 . The first and second legs  111  and  112  of the torsion spring  110  extend from opposite ends of the central coil portion  152  of the torsion spring  110  and have generally parallel end portions that pass through respective slots  154  and  158  in the sidewall  115  of the U-shape portion  113  of the return member  104 . The outer ends of the slots  154  and  158  serve as positive stops for preventing any movement of respective legs  111  and  112  beyond the slot. However, either leg  111  and  112  od the spring  110  can be moved inwardly toward the other in the opposite direction with the slot guiding movement of the leg of the spring in a plane perpendicular to the axis of the trunnion shaft  101 .  
         [0033]     The legs  111  and  112  of the spring  110  also are engaged by the ears  146  and  150  of the control arm  108 . The ears  146  and  150  are laterally spaced apart such that an inner side of each ear engages an outer side of a respective leg of the spring  110 . Consequently, when the control member  108  is rotated clockwise, ear  146  will engage and urge leg  111  of the torsion spring  110  inwardly against the biasing force, while the other ear  150  is free to move away from leg  112  of the torsion spring  110 . Upon release of the control member  108 , the biasing force of the spring  110  will cause spring leg  111  to urge ear  146  counterclockwise until leg  111  reaches the outer end of the slot  154 . Conversely, when the control arm  108  is rotated counterclockwise, ear  150  will engage and urge leg  112  of the torsion spring  110  inwardly against the biasing force, while the other ear  146  is free to move away from leg  111  of the torsion spring  110 . Upon release of the control member  108 , the biasing force of the spring  110  will cause spring leg  112  to urge ear  150  clockwise until spring leg  112  reaches the outer end of the slot  158 .  
         [0034]     It will now be appreciated that the speed adjusting mechanism  103  as described above generally maintains the pump  18  in its neutral position until the control arm  108  is moved to a forward or reverse position, and returns the pump  18  to neutral after the control arm  108  has been moved to at least one of the forward or reverse positions.  
         [0035]     In some applications, it can be desirable to have the speed adjusting mechanism  103  return the pump  18  to neutral from only one of the operating directions. As an example, in some applications it may be desirable to only return the pump  18  to neutral when it is operated in the reverse direction. To achieve unidirectional return to neutral functionality, one of the ears  146  and  150  can be removed or reduced in length such that it does not engage a respective leg of the torsion spring  110  when the control arm  108  is rotated. As such, the control arm  108  can freely rotate independent of the torsion spring  110  in one direction, while when rotated in the other direction the remaining ear engages the respective leg of the torsion spring and functions to return the pump  18  to neutral as described above. As an alternative to removing or reducing in length an ear of the control arm  108 , one of ears  146  or  150  can include a slot or notch into which a respective spring leg can extend when the control arm  108  is rotated to thereby prevent loading of the torsion spring  110 .  
         [0036]     It will be appreciated that the hydraulic drive apparatus of the present invention can be a hydrostatic transmission as described in detail above, or may be a hydraulic pump, hydraulic pump unit, or any other hydraulic drive apparatus as desired. The  
         [0037]     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.