Abstract:
A hydrostatic transmission may be placed in a free-wheeling state by rotating a bypass actuator that lifts the motor from a motor running surface whereby hydraulic fluid flows into a transmission cavity. A bypass arm connected to the bypass actuator and to a bypass rod may be used to engage and disengage the bypass actuator via operation of the rod. Further, a bypass latch may be used to lock the bypass arm in an engaged position whereby the bypass actuator is engaged and the transmission is disengaged. The bypass latch arm is preferably linked to the brake pedal whereby actuation of the brake pedal disengages the bypass actuator through operation of the latching arm. The system may be designed to provide dynamic braking through transmission resistivity prior to application of a brake force to the axles or wheels by unlatching the bypass arm prior to actuating the brake arm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/082,750, filed Feb. 25, 2002 now U.S. Pat. No. 6,651,427, which is a division of U.S. patent application Ser. No. 09/637,304, filed Aug. 11, 2000, now U.S. Pat. No. 6,374,604 and, therefore, claims the earlier filing date under 35 U.S.C. 120. Both of these applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to hydrostatic transmissions (“HST”) and transaxles including hydrostatic transmissions, and in particular to a bypass system for such transmissions. Specific exemplary embodiments discussed herein relate to riding lawn mowers and similar small tractors. 
     BACKGROUND OF THE INVENTION 
     The description of art in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art”with respect to this invention, unless specifically designated as such. 
     Riding lawn mowers and small tractors  10  of the type discussed herein and depicted in FIGS. 1A and 1B generally use an engine having a vertical output shaft  12  that is connected to the transaxle  14  via a conventional belt and pulley system  16 . A standard HST for such a transaxle includes a hydraulic pump that is driven by the engine output shaft  12 , and a hydraulic motor, both of which are usually mounted on a center section. Rotation of the pump by an input shaft creates an axial motion for the pump pistons. The oil pressure created by this axial motion is channeled via porting to the hydraulic motor, where it is received by the motor pistons. The axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft that drives the vehicle axles through differential gearing. For additional background, the reader is referred to U.S. Pat. No. 5,201,692, (“the &#39;692 patent”) issued to Johnson et al., and incorporated herein by reference in its entirety. 
     A problem with the typical HST arrangement is that a “neutral gear” does not exist, as it is merely a point where the hydraulic pressure in the pump goes to zero. 
     However, at this point the oil remains in the transmission, preventing the vehicle from being freely rolled. The present invention is an improvement over prior art methods used to place a transaxle into neutral gear and enable movement of the tractor without the motor running or, more generally, without the transmission being engaged. 
     Prior art has solved the problem by diverting the oil through a hydraulic valve from the pressure side to the vacuum side of the HST center section. The problem with such a design is that the hydraulic valve allows for the movement of only a limited amount of oil due to inherent design limitations, such as the diameter of the hydraulic valve through which the oil is diverted. Machining such a valve also requires precise tolerances, thus increasing the manufacturing costs of the unit. 
     The &#39;692 patent solves this neutral gear problem by providing a mechanism whereby the motor block is mechanically lifted from its running surface. This mechanism allows the oil to bypass the vacuum-pressure (hydraulic) circuit and to exit the case completely. This mechanism operates to enable the vehicle to free-wheel more easily than is possible with prior art hydraulic valve methods. 
     FIG. 1C (which is similar to FIG. 2 of the &#39;692 patent) is a section view through the transaxle  14 . To activate the bypass feature disclosed in the &#39;692 patent, a bypass arm  18  is manipulated by the user to rotate a bypass actuator  20 . The bypass actuator  20  includes a rod  22  which is shaped at its base  24  so that rotation of the rod  22  forces a bypass plate  26  to press against the base of the motor  29 , thereby breaking its seal to the motor running surface (See FIGS. 2 and 5 of the &#39;692 patent). It will be apparent to those of skill in the art that a pin or multiple pins, or other mechanical means may be used to lift the motor block. These mechanical lifting mechanisms allow the oil to flow between the motor and the transmission cavity. 
     A bypass rod  28  is connected to the bypass arm  18  to facilitate manipulation of the bypass arm  18 . FIG. 1D shows the bypass rod  28  in an unlocked position. The prior art means for activating the bypass mechanism includes pulling the bypass rod  28  through the tractor hitch plate  30  and securing it in place with a weldment (or cross pin)  32 . To secure the bypass rod  28  in the hitch plate  30 , the hitch plate  30  must include a relatively complex stamp out, e.g., a keyhole  34 . See FIG. 1F which shows the bypass rod  28  extending through the keyhole  34  of the hitch plate  30 . Perhaps more troublesome is the requirement to have tight tolerances for setting the cross pin  32  relative to the hitch plate  30  and, the keyhole  34 . This tolerance requirement is especially frustrating because manufacturing tolerances involving the hitch plate  30  and the vehicle frame are much looser. 
     While the present invention relates to hydrostatic transaxles and transmissions generally, it will be better understood within the discussion of exemplary embodiments directed toward riding lawnmowers and similar small tractors. 
     SUMMARY OF THE INVENTION 
     A primary object of the present invention is directed toward a bypass system, and in particular, a bypass latch, for a hydrostatic transaxle or transmission. A hydrostatic transaxle according to one embodiment of the present invention comprises a casing including a hydrostatic transmission located in the casing. The transmission comprises a motor on a motor running surface and a pump connected to the motor via a hydraulic circuit. A bypass actuator, in a preferred embodiment, is rotatably positioned to lift the motor from the motor running surface when the actuator is rotated such that hydraulic fluid flows out of the motor (a part of the hydraulic circuit). Alternatively, other components can be shifted to allow fluid to flow out of the hydraulic circuit. A bypass arm is preferably positioned external to the casing (though need not be) and affixed to the bypass actuator&#39; and rotatable therewith. A latch arm that is rotatable about an axis parallel to an axis about which the bypass actuator rotates is adapted to releasably engage the bypass arm. The latch arm is coupled with a brake rod such that operation of the brake rod disengages the latch arm from the bypass arm, whereby the bypass actuator is disengaged and the transmission is thereby engaged. 
     Accordingly, an embodiment of the invention is directed toward a bypass  15  assembly comprising a latch arm having a latching end for releasably engaging the bypass arm and a brake end coupled with a brake rod. A returning spring, preferably an extension spring, is connected to the latch arm to apply a returning force to bias the latch arm toward an at-rest position. In a preferred embodiment the at-rest position is the position where the brake end of the latch arm abuts, or is stopped against, a brake arm. A second spring, preferably a compression spring, cooperating with the brake rod is used to apply a braking force to the brake arm when the brake rod is actuated. In a preferred embodiment, the compression spring transmits a force to the braking end of the latch arm prior to transmitting a braking force to the wheels or axles of the tractor. A slidable member adapted to control the timing of the force may be positioned on the brake rod. 
     One advantage of the design is that it is integral with the transmission. One method of integrating a bypass latch with a hydrostatic transmission according to the present invention comprises placing the hydrostatic transmission in a casing and connecting a bypass actuator to the transmission. This allows the transmission to roll more freely when the bypass actuator is engaged than when the bypass actuator is disengaged. A bypass arm positioned external to the casing is connected to operate the bypass actuator. A latch arm adapted to releasably engage the bypass arm is positioned external to the casing as well. The method of integrating further includes linking the latching arm to a brake rod such that operation of the brake rod causes the latching arm to release the bypass arm, whereby the bypass actuator is disengaged. 
     By integrating the design, the manufacturer could provide a means to actuate the bypass that is less expensive than the prior art. The current means of activation involves a bent wire form with a weldment (or cross pin). A straight wire form, rod, or stamping could be used with the proposed invention thereby reducing tolerance requirements and costs. 
     Another advantage of the invention is that it can be deactivated by the pressing of the brake pedal. Many vehicles require the operator to press the brake pedal prior to starting the tractor. The present invention, which deactivates the bypass actuator when the brake pedal is applied, insures that the transmission is ready for operation. This is particular useful when the user has perhaps forgotten that the bypass has been left “on,” i.e. the transmission is disengaged. 
     Other objects and advantages in accordance with the present invention will be apparent to those of skill in the art from the teachings disclosed herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the interest of enabling one of skill in the art to practice the invention, exemplary embodiments are shown and described. For clarity, details apparent to those of skill in the art without undue experimentation are generally omitted from the drawings and description. 
     FIG. 1A depicts a plan view of a typical lawn tractor. 
     FIG. 1B shows a lawn tractor similar to that of FIG. 1A with a cutout to show a transaxle connected to an engine output via a belt system. 
     FIG. 1C is a section view through a prior art transaxle showing a bypass actuator. 
     FIG. 1D is an enlarged view of FIG. 1B showing a prior art design for manipulating and locking a bypass arm. 
     FIG. 1E shows the bypass arm of FIG. 1D secured in an engaged position via a cross pin on the bypass rod. 
     FIG. 1F shows the bypass rod of FIG. 1E extending through a keyhole stamp out in the tractor hitch plate. 
     FIG. 2 shows a bypass latch according to one embodiment of the present invention. 
     FIG. 3 depicts an enlarged view the transaxle of FIG. 2, showing the bypass arm disengaged i.e., transmission engaged. 
     FIG. 4 is similar to FIG. 3 but with the bypass arm in the “on” or latched position. 
     FIG. 5 shows a rear view of a vehicle employing the present invention such that only a circular stamp out is needed in the hitch plate to allow the bypass rod to pass through. 
     FIG. 6 shows a cutout view of FIG. 5 to more clearly show the transaxle. 
     FIG. 7 shows a top view of a transmission similar to that show in FIG. 2, but with the bypass rod extending in the opposite direction. 
     FIG. 8 is a side view of the transmission shown in FIG. 7 showing the bypass in the “off” position. 
     FIG. 9 is a top view of the transmission shown in FIG. 7 with the bypass rod is being pushed to activate the bypass actuator and the bypass arm is starting to engage the latch arm. 
     FIG. 10 shows a top view of the transmission shown in FIG. 9 with the bypass arm latched in the “on” position, thereby disengaging the transmission. 
     FIG. 11 shows a top view of the transmission of FIG. 7 with the bypass arm being unlatched. 
     FIG. 12 is a side view of the transmission shown in FIG. 11 but with the bypass fully disengaged and the brake arm activated. 
     FIG. 13 is a perspective view of the bypass latch system according to the present invention, showing the bypass in the on position. 
     FIG. 14 is an isometric view of an alternative bypass latch using a different latch arm with a latching end having a tab generally perpendicular to the axis of the arm. 
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention is discussed in relation to lawn tractors, however, other uses will be apparent from the teachings disclosed herein. The present invention will be better understood from the following detailed description of exemplary embodiments with reference to the attached drawings, wherein like reference numerals and characters refer to like parts, and by reference to the following claims. 
     FIG. 2 depicts a transaxle  14  with bypass latch  36  shown installed in the tractor  10 . The bypass arm is shown in the “off” position or the disengaged position. Some vehicles may have the transaxle  14  (or more generally the transmission) rotated 180 degrees from what is shown in FIG.  2 . If the transaxle is rotated, then the bypass rod  28  may extend forward, rather than rearward as shown. The bypass rod  28  may also be rotated relative to the transaxle  14  to provide access to the bypass actuator from another direction without changing the orientation of the transaxle  14 . Thus, a design wherein the bypass rod  28  extends out the back of vehicle  10  is only one of many options for providing customer access. For example, in an embodiment wherein the bypass rod  28  extends to one of the sides, the attachment point of the bypass arm  18  need only be rotated 90 degrees to provide the desired coupling between the bypass actuator  20  and the bypass rod  28 . Other mechanisms to attain the desired rotary motion will be apparent to those of skill in the art. 
     FIG. 3 depicts an enlarged view of FIG. 2, showing a hydrostatic transaxle according to one embodiment of the present invention. The transaxle  14  comprises a casing  38  including a hydrostatic transmission located in the casing (the transmission is not shown in FIG.  3 ). Reference should be made to the &#39;692 patent for additional detail regarding the general background of the invention. The transmission comprises a motor on a motor running surface and a pump connected to the motor via a hydraulic circuit. A bypass actuator  20  is rotatably positioned to lift a transmission component when the actuator  20  is rotated such that hydraulic fluid flows out of the hydraulic circuit. In a preferred embodiment the motor  29  is lifted from the motor running surface when the bypass actuator  20  is rotated. A bypass arm  18  is preferably positioned external to the casing  38  and affixed to the bypass actuator  20  and rotatable therewith. A latch arm  40 , that is rotatable about an axis  42  (See FIG. 13) which is parallel to an axis  44  about which the bypass actuator  20  rotates, is adapted to releasably engage the bypass arm  18 . The latch arm  40  is coupled with a brake rod  46  such that operation of the brake rod  46  disengages the latch arm  40  from the bypass arm  18 , whereby the bypass actuator  20  is disengaged and the transmission is thereby engaged as the motor (or other transmission component) reseats itself. 
     Accordingly, an embodiment of the invention is directed toward a bypass assembly  48  such as shown in FIG.  13 . The assembly  48  comprises a latch arm  40  rotatable about an axis  42  parallel to an axis  44  about which the bypass arm  18  rotates. The latch arm  40  comprises a latching end  50  for releasably engaging the bypass arm  18  and a brake end  52  coupled with a brake rod  46 . In a preferred embodiment, the brake end  52  is slidably coupled with the brake end  46 , but other means for movably coupling the two will suffice. A return spring  54 , preferably an extension spring, is connected to the latch arm  40  to apply a returning force to bias the latch arm  40  toward an at-rest position (See FIGS.  3  and  7 ). In a preferred embodiment, the at-rest position is the position where the brake end  52  of the latch arm  40  abuts, or is stopped against, a brake arm  56 ; intervening structure, such as spacers is acceptable. The brake end  52  need not directly rest against the brake arm  56 . A compression spring  58  cooperating with the brake rod  46  is used to apply a braking force to the brake arm  56  when the brake rod  46  is actuated. In a preferred embodiment, the compression spring  58  transmits a force to the braking end  52  of the latch arm  40  (via a bushing  60  positioned on the brake rod  46 ) prior to the compression spring  58  transmitting a braking force, via the brake arm  56 , to the wheels  59  or axles of the tractor  10 . 
     More generally, the bushing  60  may be a slidable member adapted to apply the brake force (from the brake rod  46 ) to the latch arm  40  before the brake arm  56  is activated. Preferably the slidable member is positioned on the brake rod  46  between the compression spring  58  and the brake arm  56 . In a preferred embodiment, the bushing  60  and the brake arm  56  are designed such that an end of the bushing will pass through an opening  62  in the brake arm  56 , but an opposing end of the bushing  60  will not pass through the opening  62 . 
     In FIGS. 3 and 7, the latch arm  40  is positioned against the brake arm  56  and the spacer  60  (bushing) as well, i.e. the latch arm  40  is not directly against the brake arm  56 . The bypass rod  28  is positioned rearward in FIG.  3  and forward in FIG.  7 . The vehicle will not roll easily in this condition, where the transmission is engaged, without engine input because of back driving the gears, the pump, the pulleys, etc. To engage the bypass actuator  20 , in the orientation shown in FIG. 3, the user pulls the bypass rod  28 . 
     FIG. 4 shows the same transaxle  14  as FIG. 3 but with the bypass  20  latched in the “on” position. Since the bypass is engaged, the vehicle may be rolled relatively easier even though there is no engine input. FIG. 5 shows a rear view of the vehicle  10  showing the hitch plate  30 . The stamp out through which the bypass rod  28  extends is not visible behind the end of the bypass rod  28 . FIG. 6 is a cutout of FIG. 5 to more clearly show the transaxle  14 . The mower deck  11  is shown roughly aligned with the wheel axles. Note that the preferred stamp out in the present invention is circular. The present invention avoids the need for a keyhole stamp out and for a cross pin on the bypass rod  28  to secure the bypass actuator  20  in an engaged position. 
     FIG. 7 shows a top view of a transmission similar to the transaxle of FIG. 3 (with the axles removed), but with the bypass rod  28  extending in the opposite direction, i.e., it is rotated 180° in a plane normal to the axis  44  of the actuator  20 . Accordingly, the bypass rod  28  would be pushed in toward the case  38  of transmission to activate the bypass actuator  20 . Thus a rider could reach the rod  28  from the tractor seat  64  (See, e.g., FIG.  1 A). While activation of the actuator  20  has been described in relation to pushing and pulling the bypass rod  28  relative to the casing  38 , this is only for convenience. Other methods for manipulating the bypass arm  18  are within the spirit of the invention and will be understood by those of skill in the art. Furthermore, the latching assembly  48  may be readily adapted to operate with other chosen methods of bypass-arm manipulation so as to couple operation of the brake to the latch. The bypass arm  18  is in the disengaged or “off” position and the latch arm  40  is at an at-rest position against the brake arm  56  and directly against the spacer  60 . 
     FIG. 8 is a side view of the transmission shown in FIG. 7; the bypass is in the “off” position. The compression spring  58  is not compressed and the brake arm  56  is not engaged. 
     FIG. 9 is a top view of the transmission shown in FIG.  7 . The bypass rod  28  is shown being pushed to activate the bypass actuator  20 . The bypass arm  18  is starting to engage the latch arm  40 . The extension spring  54  is being stretched and exerting a force on the latch arm  40  brake end  52  to bias the latch arm  40  toward the at-rest position. Spring  54  also forces the latching end  50  in the opposite direction as that of the brake end  52  since the latch arm  40  rotates about axis  42  (through bolt  66 ) which is positioned between the brake end  52  and the latching end  50 . 
     FIG. 10 shows the bypass arm  18  latched in the “on” position, thereby disengaging the transmission so the tractor rolls relatively more freely. The extension spring  54  exerts a force to maintain the latching end  50  of the latching arm  40  in engagement with the bypass arm  18 . When the brake rod  46  is being actuated, the force transmitted to the latching arm  40  (via the compression spring  68 ) will overcome the force from the extension spring  54  and rotate the latch arm  40  out of engagement with the bypass arm  18 . The springs in the hydraulic motor act to return the bypass actuator  20 , and the bypass arm  18 , to a disengaged position. 
     FIG. 11 shows the bypass arm  18  being unlatched. The brake rod  46  is activated (via operator foot pedal typically) moving the compression spring  58  in contact with the bushing  60 . As the brake rod  46  and compression spring  58  move further forward, the bushing  60  slides through the brake arm  66  and makes contact with the latch arm  40  at its brake end  52 . As the bushing  60  (or spacer) applies a force to the latch arm  40 , the latch arm  40  starts to disengage from the bypass arm  18 . The brake arm  56  has not yet rotated, i.e., the compression spring  58  has not yet applied a force (or, a sufficient force) to the brake arm  56  when the latch arm  40  disengages the bypass arm  18 . As the compression spring  68  on the brake rod  46  is further compressed (or pulled further forward by the brake rod  46 ), the brake arm  56  begins to be activated. Activating the brake arm  56  applies a braking force to the wheels  59  (or axles depending on the system). Thus, a system that utilizes dynamic braking (via the hydrostatic transmission) prior to conventional wheel/axle braking is achieved. By removing or otherwise adapting the bushing  60  or the latch arm  40  or both, the latch arm  40  can be made to disengage the bypass arm  18  contemporaneously with the onset of the brake arm  56  activation. The latching assembly can also be made to disengage the bypass arm  18  after activation of the brake arm  56  or, as previously discussed, before activation of the brake arm  56 . 
     FIG. 12 is a side view of the transmission shown in FIG. 11 but with the bypass arm (not shown) fully disengaged, the compression spring  58  fully compressed and the brake arm  56  activated. By comparison, FIG. 8 is a similar view prior to the brake arm  56  being activated. 
     FIG. 13 is a view of a bypass latch assembly  48  according to an embodiment of the present invention showing the bypass mechanism  48  in the engaged position, so that rod  22  is rotated to force plate  26  towards the cylinder block of motor  29 , thus lifting motor  29  off the motor running surface  51  of center section  53 . FIG. 14 is an isometric view of a bypass assembly  48  with an alternative bypass latch arm  40 ′ having a different latching end  50 ′ from that shown in FIG.  13 . The latching end  50 ′ has a tab  68  extending generally perpendicular to the axis of the arm  40 ′ (i.e., the tab  68  extends generally parallel to axis  42 ). The tab  68  need not extend perpendicular to the axis of the latch arm  40  but may be at a predetermined angle measured relative to the axis of the latch arm  40  or measured relative to an axis parallel to axis  42 . The tab  68  allows the manufacturer to use looser manufacturing tolerances while achieving the desired latching capability. For example, the bypass arm  18  may be allowed out of plane movement, rather than being rigidly held to a motion in a common plane with the latching arm  40 . Another alternative comprises adapting the latching end of the bypass arm  18  to latchingly engage the latching arm  40  even though both arms move out of plane. 
     While the invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. The scope of the claimed invention is intended to be defined by following claims as they would be understood by one of ordinary skill in the art with appropriate reference to the specification, including the drawings, as warranted.