Abstract:
A hydrostatic transmission including a fluid motor and a variable displacement fluid pump in fluid communication with the fluid motor and having a first and a second, much greater, fluid displacement rate. The pump is mounted on a block having a cylindrical surface, a passage through which fluid flows from the pump to the motor, and a fluid bleed hole extending from the passage to the cylindrical surface. The passage and a sump external to the pump are in fluid communication through the bleed hole and the void of an annular element disposed about and in sliding contact with the cylindrical surface when the pump is operating at its first rate and the annular element is in a first position. The bleed hole and the sump are substantially out of fluid communication when the pump is operating at its second rate and the annular element is in a second position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to hydrostatic transmissions intended primarily for use in the lawn and garden industry on tractors, riding lawnmowers, lawn and garden implements and the like. 
     2. Description of the Related Art 
     Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically via positive displacement pumps and motors using oil, and then back to rotary mechanical motion to rotate a drive axle in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of one piston design, and the motor of another. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring of a radial piston pump or the swash plate angle of an axial piston pump. 
     Hydrostatic transmissions have an inherent problem of not achieving, when placed in neutral, a condition in which the pump displacement is completely eliminated. Although the operator may shift the implement into neutral, thereby causing the hydrostatic transmission to be placed in neutral, there may still be some motion, or “creep”, of the implement. During forward or reverse operation of the hydrostatic transmission, this fluid is constantly moving through the system. In neutral, ideally, the displacement of the rotating pump is zero, and no fluid flows to the motor therefrom. Thus, no motion, however slight, is imparted to the axle. Should the rotating pump still have some slight displacement in neutral, fluid in one side of the hydrostatic system will become or remain slightly pressurized and cause the motor to slowly rotate, thereby creating forward or reverse motion of the wheels. What would be desirable is a hydrostatic transmission which allows any fluid displaced by the pump to be vented out of the hydrostatic system when the hydrostatic transmission is placed in the neutral position, thereby eliminating creep. 
     Yet another problem associated with the use of hydrostatic transmission is the “jerking” effect created when the swash plate is moved from neutral to forward or reverse and vice versa. Dampening of the engagement or disengagement of the hydrostatic transmission would eliminate the jerking or at least “soften” the transition to and from neutral. What would be desirable is a hydrostatic transmission which includes a mechanism for dampening the response of the motor to changes in pump displacement rates as the pump approaches and leaves neutral so that such jerking would be eliminated. 
     SUMMARY OF THE INVENTION 
     An advantage provided by the present invention is that any fluid displaced by the pump in neutral is vented out of the hydrostatic system, thereby preventing the occurrence of creep in the forward or reverse direction. 
     An additional advantage provided by the present invention is that it dampens the effect of changes in pump displacement to and from zero by allowing a portion of the hydrostatic fluid to bleed or be vented out of the hydrostatic system as the transmission is shifted from neutral to an operative condition in forward or reverse, and vice versa. 
     The present invention provides a hydrostatic transmission including a fluid motor, a variable displacement fluid pump in fluid communication with the fluid motor, the pump having first fluid displacement rate and a second fluid displacement rate, the second fluid displacement rate being much greater than the first displacement rate, a block on which the pump is mounted and having a cylindrical surface, the block provided with at least one fluid passage, fluid which flows from the pump to the motor being flowed through the passage, the block provided with at least one fluid bleed hole extending from the fluid passage to the cylindrical surface of the block, a fluid sump external to the block, and an annular element disposed about and in sliding contact with the cylindrical block surface, the annular element provided with at least one void and having a first position in which the void is in fluid communication with the fluid passage through the fluid bleed hole, and a second position in which the void is substantially out of fluid communication with the fluid passage. The fluid passage and the sump are in fluid communication through the bleed hole and the void when the pump is operating at its first displacement rate and the annular element is in its first position, and the fluid bleed hole and the sump are substantially out of fluid communication when the pump is operating at its second displacement rate and the annular element is in its second position. 
     The present invention further provides a hydrostatic transmission including a fluid motor, a variable displacement fluid pump in fluid communication with the fluid motor, the pump having first fluid displacement rate and a second fluid displacement rate, the second fluid displacement rate being much greater than the first displacement rate, a block on which the pump is mounted, the block having a flat surface against which the pump is slidably engaged when the pump is operating at its first and second fluid displacement rates, the block provided at least one fluid passage which opens to the flat block surface, fluid which flows from the pump to the motor being flowed through the passage, a fluid sump external to the block, and means for placing the passage and the sump in fluid communication when the pump is operating at its first fluid displacement rate and providing a gradual motor response to changes between the pump first and second fluid displacement rates. 
     The present invention also provides a method for dampening the response of a fluid motor to changes in a fluid pump between neutral and drive positions in a hydrostatic transmission, and ensuring that no fluid is pumped by the pump to the motor in the pump neutral position, including: rotating the pump while maintaining its sliding engagement against a block having a passage therethrough; operating the rotating pump at a first displacement rate in its neutral position, in which the passage and a sump are in fluid communication, whereby fluid displaced by the pump in its neutral position is directed to the sump; gradually decreasing the fluid communication between the passage and the sump while changing from the pump neutral position to the pump drive position; operating the rotating pump at a second displacement rate greater than the first displacement rate in its drive position, in which the passage and the sump are substantially out of fluid communication, whereby fluid displaced by the pump in its drive position is directed to the motor through the passage for driving the motor; and gradually increasing the fluid communication between the passage and the sump while changing from the pump drive position to the pump neutral position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional top view of one embodiment of a reversible hydrostatic transmission module according to the present invention; 
     FIG. 2 is a sectional side view of the hydrostatic transmission module of FIG. 1 along line  2 — 2  thereof; 
     FIG. 3 is a sectional side view of the hydrostatic transmission module of FIG. 1 along line  3 — 3  thereof; 
     FIG. 4 is a side view of the hydrostatic transmission module of FIG. 1 along line  4 — 4  thereof; 
     FIG. 5 is a sectional top view of the hydrostatic transmission module of FIG. 1 attached to one embodiment of a differential axle unit, the assembly forming one embodiment of hydrostatic transaxle; 
     FIG. 6A is a top view of the center section or block for the hydrostatic transmission module of FIG. 1, showing a first embodiment of the inventive hydrostatic dampening and neutral bleed mechanism; 
     FIG. 6B is an enlarged, fragmentary view of the center section or block of FIG. 6A, showing in section the inventive mechanism in a fully neutral position; 
     FIG. 6C is an enlarged, fragmentary view of the center section or block of FIG. 6A, showing in section the inventive mechanism in a fully engaged, forward position; 
     FIG. 6D is an enlarged, fragmentary view of the center section or block of FIG. 6A, showing in section the inventive mechanism in a dampened, reverse position; 
     FIG. 7A is an upper perspective view of the center section or block, and the inventive mechanism of FIG. 6A, also showing the control device for the mechanism; 
     FIG. 7B is an upper perspective view of a hydrostatic transmission center section or block and a second embodiment of the dampening and neutral bleed mechanism, also showing the control device for the mechanism; 
     FIG. 8A is another upper perspective view of the center section or block, mechanism and control device of FIG. 7A; 
     FIG. 8B is another upper perspective view of the center section or block, mechanism and control device of FIG. 7B; 
     FIG. 9A is a side view of the center section or block, mechanism and control device of FIG. 7A; and 
     FIG. 9B is a side view of the center section or block, mechanism and control device of FIG.  7 B. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplifications set out herein illustrate particular embodiments of the invention such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. The invention includes any alterations and further modifications in the illustrated devices and described methods and further applications of the principles of the invention which would normally occur to one skilled in the art to which the invention relates. 
     Referring first to FIG. 5, transaxle  160  comprises hydrostatic transmission  20  and axle mechanism  180 . Axle mechanism  180  includes casing  166  having upper and lower halves, split along a horizontal plane coincident with the axes of axles  162  and  164 . Disposed within casing  166  are reduction gear train  188  and differential mechanism  172 . Axles  162  and  164  extend outwardly from differential mechanism  172  through a pair of openings in either end of casing  166  at which point axles  162  and  164  are sealed by seals  168  and supported by bearings  170 . 
     Differential mechanism  172  is of a type known in the art and includes ring gear  174 , bevel gears  177  and  178 , and pin  176 . Differential  172  is connected to pinion  186  which is splined to countershaft  184 . The opposite end of countershaft  184  is similarly splined to gear  182  which is enmeshed with pinion gear  190  splined to gear train input shaft  194 . 
     Further included in transaxle  160  is space  198  which contains mechanical disconnect mechanism  200  of the type disclosed in U.S. Pat. No. 5,701,738, issued Dec. 30, 1997, and assigned to the assignee of the present application. The disclosure of this patent is expressly incorporated herein by reference. Additionally, transaxle  160  includes brake mechanism  204 . The operation of the brake itself is the subject of U.S. Pat. No. 6,123,182, issued Sep. 26, 2000, and assigned to the assignee of the present application. The disclosure of this patent is expressly incorporated herein by reference. Transaxle  160  is further connected to hydrostatic transmission  20 , as described hereinbelow. 
     Referring now to FIGS. 1 through 4, hydrostatic transmission  20  comprises a separate, self-contained casing  28  having two casing halves  30  and  74  split along horizontal interface  82  which is coplanar with the axis of motor output shaft  26 . Casing halves  30  and  74  are connected together by a plurality of bolts  76  extending through lower casing half  74  and threadedly received in bores provided in upper casing half  30 . Disposed within self-contained casing  28  is hydrostatic pump and motor mechanism  34  comprising center section, or block,  32  having pump mounting surface  128  and motor mounting surface  36  and internal passages  126  and  234  (FIG. 6A) hydraulically connecting each of arcuate slots  236  and  240  (FIG. 6A) in pump face  128  and motor mounting face  36 . Pump and motor mechanism  34  further includes axial piston motor  24  and variable displacement pump  22 . 
     Axial piston motor  24  comprises rotatable cylinder  42  having a plurality of pistons  40  therein sliding against fixed swash plate assembly  54  and thrust bearing  52 . Face  44  of rotatable cylinder  42  interfaces with motor mounting face  36  of center section  32 . Motor output shaft  26  extends through cylinder  42  and is supported by bearings  48  in center section  32 . The axis of output shaft  26  is oriented 90° relative to the axis of pump input shaft  84 , as shown in FIG.  3 . Motor output shaft  26  is also supported by sleeve and bearing assembly  56 , particularly sleeve  58 , press fitted to casing  28  and extending through portion  62  into a recess in axle casing  166 . 
     Connection of transmission  20  with gear train  188  occurs through reduced end  158  of gear train input shaft  194  being received within bore  66  in the end of motor output shaft  26 . A firm connection between shafts  194  and  26  occurs through the compression spring  156  cooperating with mechanical disconnect mechanism  200  (FIG.  5 ). Compression spring  156  is retained on shaft  26  by ring  64 , disposed in groove  154 , and flat washer  152 . Transmission casing  28  is mounted to transaxle casing  166  at two locations  38  and  60  by corresponding overlapping extensions on casings  28  and  166  and bolts (not shown) which are driven from the bottom. 
     With reference to FIGS. 2 and 3, pump  22  is in mechanical communication with pump swash plate assembly  98 , particularly swash plate  90 . Swash plate assembly  98  includes swash plate  90 , bearings  106 , and bearing housing plates  112  and  114  encasing bearings  106 . Swash plate  90  further includes arcuate bearing strips  92  with inner surfaces  94  attached to arcuate swash plate upper surface  88  and outer surface  96  interfacing with upper casing half  30 . Pump swash plate assembly  98  will be tilted through the action of control rod  138  and control arm  142  (FIG. 5) in order to vary the displacement of pump  22 . The operation of transmission  20  is more fully described hereinbelow. 
     Pump  22  includes pump cylinder  116  rotatably driven by input shaft  84  and having a plurality of cylinders  68  within which are disposed pistons  80 . Pistons  80  are urged against the face of swash plate  90  by springs  110 . Shaft  84  is sealed by seal  86  and is rotatably supported by bearings  78 . Note that pump shaft  84  extends through swash plate assembly  98  and is splined to pump cylinder  116  via splined portion  108  on shaft  84  and splined portion  118  on pump cylinder  116 . Distal end  120  of shaft  84  is supported by bearing  122  in center section  32 . Screws  76  connect center section  32  to upper casing half  30 . Also located on upper casing half  30  is neutral switch  150 . 
     Referring now to FIGS. 3 and 4, shift lever  136  is attached to rotatable control arm  142  by screw  130 , external of casing  166 , received in control rod  138 . Shift lever  136  is returned to neutral by a conventional return-to-neutral spring mechanism  134 , while adjustable plate  132  permits fine adjustment of neutral position. Control arm  142  is attached to control rod  138  and includes first end  143  extending into arm  104  and second end  145  extending in the opposite direction; both ends  143  and  145  are perpendicular to control rod  138 . Second end  145  of control arm  142  swings through an arc about control rod  138  when shift lever  136  is rotated. Pin  144  attaches to second end  145  of control arm  142  and extends into slot  148  disposed on periphery  140  (FIG. 3) of swash plate  90 . Friction roller  146  fits over pin  144  and freely rotates about pin  144  to engage with slot  148  of swash plate  90 . 
     Selectively positioning control arm  142  causes swash plate  90  to tilt, and in turn, pistons  80 , orbiting about input shaft  84 , reciprocate causing hydrostatic fluid in each cylinder  68  to pressurize as respective piston  80  retracts. Swash plate  90  tilts and rotates against a pair of low friction bearings attached to the casing as previously described. 
     With reference to FIGS. 3 and 6A through  6 D, lower surface  124  of center section  32  is provided with a pair of openings  238  to provide makeup oil to pump  22 . In addition, a filter and check valves (not shown) are provided as is customary in the art for controlling the ingress and quality of the make-up oil. Pump input shaft  84  is received within bore  242  and integral bosses  50  of center section  32  accommodate and provide support for mounting screws  76 . Blind drilled passageways  126  and  234  are sealed by plugs  232 . 
     Referring to FIGS. 6A through 6D, surrounding pump mounting surface  128  of center section  32  is annular element  100  having a ring structure. Annular element  100  includes protrusion  244  containing slot  102  for receipt of arm  104 . Arm  104  is allowed limited rotation due to its combination with control arm  142 . Element  100 , in addition to protrusion  244  and slot  102 , further includes a pair of voids  220  extending from inner surface  101  of element  100  to outer surface  103  thereof. Inner surface  101  is in sliding contact with cylindrical outer surface  129  of pump mounting face  128 . Cylindrical surface  129  includes a pair of fluid bleed holes  222  extending from arcuate slots  236  and  240 . As stated previously, arcuate slots  236  and  240  are in fluid communication with a pair of openings in lower surface  124  of center section  32  and internal passages  234  and  126 . Center section  32  also includes bearing cradle  224  having raised shoulder  226  (FIG.  6 A). The structure and operation of bearing cradle  224  is disclosed in U.S. patent application Ser. No. 09/498,692, filed Feb. 7, 2000, the complete disclosure of which is incorporated herein by reference. 
     Arm  104 , which may be an extension of control arm  142 , moves annular element  100  to a position in which voids  220  and fluid bleed holes  222  are radially aligned, thereby allowing the motive fluid to vent from the hydrostatic fluid circuit to the interior of casing  28  when transmission  20  is in neutral. As stated above, control arm  142  has first end  143 , which is the end attached to control rod  138  and which extends to form arm  104 . If annular element  100  were not present, control arm  142  would terminate at first end  143  at the point of connection to control rod  138 , as opposed to extending beyond the connection point to form arm  104 . Arm  104  is operatively connected to annular element  100  at slot  102 . Arm  104  is in fitted engagement with slot  102  such that when arm  104  moves, annular element  100  rotates around cylindrical surface  129  of pump mounting surface  128 . 
     A second embodiment, shown in FIGS. 7B,  8 B, and  9 B, utilizes a protrusion  244  on annular element  100 ′, as does the first embodiment, but includes gear teeth  248  which are intermeshed with gear teeth  250  on arm  246 . Arm  246 , like arm  104 , is connected to control arm  142 , and may even be an extension thereof, the operation of arm  246  is similar to that of arm  104  with shift lever  136  through control arm  142  moving arm  246  into the neutral, forward, or reverse positions. The difference is that enmeshed gear teeth  248  and  250  provide operative engagement between arm  246  and element  100 ′, versus an end of arm  104  being received in slot  102  of annular element  100 . 
     The operation of hydrostatic pump and motor mechanism  34 , through movement of swash plate  90  to effectuate variable rotational movement of motor cylinder barrel  42 , will now be described with reference to FIGS. 2,  3  and  6 A. Customarily, pump cylinder barrel  116  is driven by a power source through input shaft  84 . Typically, input shaft  84  includes a first end keyed to common hub  252  of pulley  70  and fan  72  with pulley  70  being belt driven by a power source (not shown), thereby providing power to input shaft  84 . The other end of input shaft  84  includes splined portion  108  disposed on the surface of input shaft  84  and engages matching splined portion  118  formed within pump cylinder barrel  116 . Swash plate  90 , selectively controlled by shift lever  136 , which is external to transmission casing  28 , initiates motive fluid displacement within pump cylinder barrel  116  to transfer power from input shaft  84  to drive axles  162 ,  164 . 
     In operation, when shift lever  136  is moved in either direction, control arm  142  moves in an opposite direction, thereby causing swash plate  90  to pivot in a direction corresponding to that of shift lever  136 . As control arm  142  moves, arm  104  is moved in the same direction as shift lever  136 , thereby moving annular element  100  through the operative connection at slot  102 . This motion allows voids  220  to either become radially aligned with fluid bleed holes  222  or to move out of radial alignment, depending upon whether the operator is selecting a neutral position, or a forward or reverse drive position. When shift lever  136  is moved to the neutral position, arm  104  causes annular element  100  to move in such a manner that voids  220  and fluid bleed holes  222  are in complete alignment, thereby allowing any motive fluid being displaced by pump  22  to bleed from center section  32  to the oil sump. When shift lever  136  is moved from neutral to forward, for example, annular element  100  is moved to a position in which voids  220  and fluid bleed holes  222  are not in alignment, thereby preventing motive fluid being displaced by the pump from being vented into the fluid sump, as shown in FIG.  6 C. Similarly, when shift lever  136  is moved from neutral to reverse, voids  220  and holes  222  are not in alignment, as shown in FIG.  6 D. 
     Although the neutral bleed aspect of annular element  100  has been discussed, element  100  and its movement, in addition to holes  222 , provides a mechanism for dampening the change from neutral to one of the forward or reverse positions or vice versa. As element  100  approaches the neutral position, motive fluid begins to bleed from center section  32  to the oil sump as voids  220  and holes  222  approach alignment; however, as element  100  is moved away from the neutral position, motive fluid continues to bleed, albeit at a decreasing rate, as voids  220  and holes  222  move out of alignment. In either scenario, the change from one displacement rate to another occurs gradually as the fluid slowly begins to bleed or slowly stops bleeding. An example of a position where some damping is occurring is shown in FIG. 6D, in which voids  220  are not in complete alignment with holes  222 , but are close enough for motive fluid to bleed, or seep, between surfaces  101  and  129  to voids  220  where it bleeds away. As annular element  100  rotates about cylindrical surface  129  of pump mounting surface  128 , voids  220  and holes  222  move further apart thus slowing and eventually stopping motive fluid from bleeding from center section  32  though holes  222 . This action occurs as annular element  100  is moved in either a forward or a reverse direction (FIG.  6 D). The opposite of the above occurs when element  100  approaches neutral thereby damping the change until neutral is reached and motive fluid is allowed to bleed to prevent motion of the implement. 
     While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.