Abstract:
A hydraulic drive device including a hydraulic pump and hydraulic motor disposed in a casing and driving an axle having an interior end disposed inside the casing and abutting against an interior surface of the casing to prevent an inward axial movement of the axle shaft. The device further includes an expansion chamber formed in the casing by a plate having a generally flat portion extending generally perpendicular to the plate to form a thumb stop and an indentation sized to accept a human finger to assist during installation of the plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation of U.S. patent application Ser. No. 12/614,079 filed on Nov. 6, 2009, now U.S. Pat. No. 8,028,520; which is a continuation of U.S. patent application Ser. No. 12/125,495 filed on May 22, 2008; which is a continuation of U.S. patent application Ser. No. 11/548,621 filed on Oct. 11, 2006, now U.S. Pat. No. 7,383,683; which is a divisional of U.S. patent application Ser. No. 11/204,653 filed on Aug. 16, 2005, now U.S. Pat. No. 7,121,092; which is a divisional application of U.S. patent application Ser. No. 10/902,619 filed on Jul. 29, 2004, now U.S. Pat. No. 6,971,234; which is a continuation of U.S. patent application Ser. No. 10/209,703, filed Jul. 31, 2002, now U.S. Pat. No. 6,775,976. These applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to hydrostatic transaxles. 
     Hydrostatic transaxles (“HSTs”), including integrated hydrostatic transaxles (“IHTs”), are known in the art and are more fully described in, among others, U.S. Pat. No. 5,314,387, which is incorporated herein by reference in its entirety. Generally, an HST includes a center section or the like on which is mounted a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons that are in fluid communication through porting formed in the center section. As the hydraulic pump rotates, the pump pistons move axially as they bear against an adjustable swash plate where the degree of axial movement depends upon the angular orientation of the swash plate. Axial movement of the pump pistons forces a hydraulic fluid through the porting, which forces the motor pistons against a thrust bearing to thereby rotate the hydraulic motor. As the hydraulic motor rotates, hydraulic fluid is returned to the hydraulic pump through the porting. In this manner, the rotation of the hydraulic pump is translated to the hydraulic motor and the rotation of the hydraulic motor may be used to drive one or more axles of a riding lawn mower, small tractor, or the like. 
     Zero-turn, hydrostatic transaxles (HZTs) are also known in the art. Generally, an HZT is utilized in connection with a vehicle to provide for the independent control of each of the drive wheels of the vehicle. By way of example, HZTs are described in U.S. Pat. Nos. 5,078,222 and 6,283,235 which are incorporated herein by reference in their entirety. Additionally, Eaton has developed and marketed HZTs as their models 771 and 781. The Eaton model 771 is an assembly with one pump and one motor where two Eaton model 771 assemblies, a right and a left, are required for zero turn drive. The Eaton model 781 consists of two units similar to the Eaton model 771 but joined together to make one assembly. 
     SUMMARY OF THE INVENTION 
     A pair of zero-turn, hydrostatic transaxles (HZTs) that may be joined to form an integrated, zero-turn, hydrostatic transaxle. Each of the hydrostatic transaxles are further comprised of a casing carrying a hydraulic pump driven by an input shaft, a hydraulic motor driven by a the hydraulic pump through porting contained within a center section, and an axle shaft driven by the hydraulic motor. The hydrostatic transaxles are joined along an interface extending generally perpendicularly to the axle shafts. 
     A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth illustrative embodiments that are indicative of the various ways in which the principles of the invention may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, reference may be had to preferred embodiments shown in the following drawings in which: 
         FIG. 1  illustrates a perspective view of an exemplary, integrated, zero-turn, hydrostatic transaxle constructed in accordance with the principles of the subject invention further illustrating an exemplary, outboard, disk brake mechanism and various casing attachment mechanisms; 
         FIG. 2  illustrates a perspective view of the integrated, zero-turn hydrostatic transaxle of  FIG. 1  with an exemplary bracket attachment mechanism; 
         FIG. 3  illustrates a perspective view of the integrated, zero-turn hydrostatic transaxle of  FIG. 1  with an exemplary, inboard, disk brake mechanism; 
         FIG. 4  illustrates an exploded view of exemplary casing members and center sections of the integrated, zero-turn hydrostatic transaxle of  FIG. 1 ; 
         FIG. 5  illustrates an exploded view of the integrated, zero-turn hydrostatic transaxle of  FIG. 3  particularly illustrating the exemplary, inboard, disk brake mechanism and attachment hardware; 
         FIG. 6  illustrates a perspective view of a further exemplary embodiment of the integrated, zero-turn hydrostatic transaxle of  FIG. 1  wherein a single plate replaces the cap members of the casings; 
         FIG. 7  illustrates a perspective view of yet another exemplary embodiment of the integrated, zero-turn hydrostatic transaxle of  FIG. 1  wherein a single internal plate replaces the cap members of the casings; 
         FIG. 8  illustrates an exploded view of the integrated, zero-turn hydrostatic transaxle of  FIG. 6 ; 
         FIG. 9  illustrates an exploded view of the integrated, zero-turn hydrostatic transaxle of  FIG. 7 ; 
         FIG. 10  illustrates a perspective view of an exemplary, zero-turn, hydrostatic transaxle used to form the integrated zero-turn, hydrostatic transaxle of  FIG. 1  further illustrating an exemplary, inboard, disk brake mechanism and outboard control arm mechanism; 
         FIG. 11  illustrates a perspective view of the exemplary zero-turn, hydrostatic transaxle of  FIG. 10  further illustrating an exemplary, inboard, cog brake mechanism and outboard control arm mechanism; 
         FIG. 12  illustrates a perspective view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 10  further illustrating an exemplary, inboard, disk brake mechanism and inboard control arm mechanism; 
         FIG. 13  illustrates a top view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 12 ; 
         FIG. 14  illustrates a perspective view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 10  further illustrating an exemplary, outboard, disk brake mechanism and outboard control arm mechanism; 
         FIG. 15  illustrates a top view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 14 ; 
         FIG. 16  illustrates a side view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 12  with the cap member removed; 
         FIG. 17  illustrates an exploded view of the exemplary, zero-turn, hydrostatic transaxle of  FIG. 12  particularly illustrating an exemplary center section, filter mechanism, and attachment hardware; 
         FIG. 18  illustrates a cross-sectional view of the exemplary, zero-turn, hydrostatic transaxle along line A-A of  FIG. 15  with an exemplary, outboard control arm mechanism and outboard brake mechanism; 
         FIG. 19  illustrates a cross-sectional view of the exemplary, zero-turn, hydrostatic transaxle along line A-A of  FIG. 15  with an exemplary, inboard control arm mechanism and inboard brake mechanism; 
         FIG. 20  illustrates a cross-sectional view of the exemplary, zero-turn, hydrostatic transaxle along line B-B of  FIG. 15 ; 
         FIG. 21  illustrates a cross-sectional view of the exemplary, zero-turn, hydrostatic transaxle along line C-C of  FIG. 13 ; 
         FIG. 22  illustrates a cross-sectional view of the exemplary, zero-turn, hydrostatic transaxle along line D-D of  FIG. 13 ; 
         FIG. 23  illustrates an exploded view of an exemplary bypass mechanism and internal expansion tank cover for use in connection with the integrated, zero-turn, hydrostatic transaxle of  FIG. 1 ; 
         FIG. 24  illustrates a pump end view of exemplary center sections for use in connection with the integrated, zero-turn, hydrostatic transaxle of  FIG. 1 ; 
         FIG. 25  illustrates a motor end view of the exemplary center sections of  FIG. 24 ; 
         FIG. 26  illustrates a top view of the exemplary center sections of  FIG. 24 ; 
         FIG. 27  illustrates a cross-sectional view of the exemplary center sections along lines E-E of  FIG. 26 ; 
         FIG. 28  illustrates an exploded view of an exemplary filter assembly for use in connection with the integrated, zero-turn hydrostatic transaxle of  FIG. 1 ; 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the figures, wherein like reference numerals refer to like elements, there is illustrated a zero-turn, hydrostatic transaxle generally used to drive a vehicle, such as a walk behind mover, snow thrower, riding mower, tractor, or other vehicle desiring a zero turn radius. As particularly illustrated in  FIGS. 1-9 , the zero-turn, hydrostatic transaxle is comprised of a pair of generally mirror image HZTs  10 L and  10 R that are each used to independently drive a single axle shaft  24 . While the HZTs  10 L and  10 R can be used independently, the HZTs  10 L and  10 R may be adapted to be attached to one another in a manner described hereinafter to form an integrated, zero-turn, hydrostatic transaxle. 
     As will be understood by those of skill in the art, and as particularly illustrated in  FIGS. 16-22 , each HZT  10  generally operates on the principle of an input shaft  12  rotatably driving a hydraulic pump  14  which, through the action of its pump pistons  16 , pushes hydraulic fluid to a hydraulic motor  18  through porting formed in a center section  20  to cause the rotation of the hydraulic motor  18 . The rotation of the hydraulic motor  18  causes the rotation of a motor shaft  22  which rotation is eventually transferred through a gearing system or the like to drive the axle shaft  24 . A motive force from, for example, an engine may be supplied directly to the input shaft  12  or indirectly by means of a pulley  26 . For a more detailed description of the principles of operation of such a hydrostatic transaxle, the reader is referred to U.S. Pat. Nos. 5,201,692 and 6,122,996 which are incorporated herein by reference in their entirety. 
     To house these components, each HZT  10  is provided with a casing wherein the casings of each HZT  10 L and  10 R are generally mirror images of one another. In one embodiment, the casing is comprised of first casing members  28 L and  28 R and second casing members  30 L and  30 R (in the form of end caps) that are joined along a substantially vertical junction surface  32 , as is illustrated in  FIGS. 1-4 . In this embodiment, for accepting fasteners  52 , each of the HZTs  10  can be provided with a plurality of bosses  54  (illustrated as three by way of example only) having fastener accepting openings. The fasteners  52  are passed through the fastener accepting openings of adjacent bosses  54  (which may be formed in both the first and second casing sections or one of the casing sections alone) to mate the HZTs  10 L and  10 R to form the integrated unit. The casing of each HZT  10 L and  10 R can also be provided with a flat surface  56  that engages the flat surface  56  of the opposite HZT  10  to provide an additional point of contact between the HZTs  10 . Thus, the individual HZTs  10 L and  10 R also may be joined along a substantially vertical junction surface to thereby form the integrated, zero-turn, hydrostatic transaxle assembly. 
     To maintain the attachment between the HZTs  10 L and  10 R, a bracket  58  may be fastened between each of the HZT casings as illustrated in  FIGS. 1-3 . For this same purpose and by way of further example, a rod  59  having opposing threads that are adapted to engage correspondingly threaded apertures formed in the casings of the HZTs  10  may be utilized. Still further, a threaded rod may pass through un-threaded openings in the casings and nuts may be threaded to the rod to maintain the attachment between the HZTs  10 . In yet another configuration, one or more bosses on the front portions of the casings of the HZTs  10  may be fastened to a vehicle frame to resist torque induced by movement of the axle shafts  24  and maintain the orientation of the HZTs  10 L and  10 R with respect to one another. This fastening technique may be used alone or in conjunction with other fastening techniques such as the aforementioned bracket  58  or threaded rod  59 . 
     As illustrated in  FIGS. 6 and 8 , the casing may alternatively be arranged such that the second casing sections  30  are replaced by a single, unitary casing section  31  to which each of the first casing sections  28  are attached. In this case, the casing section  31  generally comprises a plate having openings for accepting the fasteners and the junction or sealing surfaces  32  between the casing section  31  and the first casing sections  28  lie in parallel, vertical planes. In this embodiment, there is minimal fluid transfer between the two units because of the high tolerances involved in the fit of various shafts into the bores. It will be appreciated that the illustrated bores need not be through holes but could be partially bored to accept the shafts of each unit while leaving an intermediate sealing surface. Bearings may be inserted into the bores, but these may or may not be necessary depending upon anticipated loads. The casing section  31  (as well as the plate member  33  described below) may be fabricated from bar stock, be die cast, or the like. 
     Still further, as illustrated in  FIGS. 7 and 9 , the casing may comprise a plate member  33  adapted to be attached over the interface of one or both of the first casing sections  28  at a vertical junction surface. In this embodiment, the first casing sections  28  of both HZTs  10  would be attached directly to one another at a single sealing surface using fasteners that pass through the openings in adjacent bosses. As a result of the joining of the first casing sections  28 , the plate member(s)  33  would be located internally with respect to the attached casing sections  28 . The plate member(s)  33  could be used to prevent movement of fluid from one HZT  10  to the other HZT  10  or allow for minimal leakage across bearings, cross holes, portings, and/or the like to allow for a single fluid fill. In the embodiment particularly illustrated in  FIG. 8 , cross holes are provided to accept the various shafts of the HZT  10 . 
     In each of the illustrated embodiments, vertically extending from the top of the first casing member  28  is the input shaft  12  and horizontally extending from and supported by the first casing member  28  is the axle shaft  24 . Thus, the axis of the axle shaft  24  is generally perpendicular to the substantially vertical junction surfaces of the casing. Similarly, the plane of the pump running surface  34  of the center section  20  is generally perpendicular to the substantially vertical junction surfaces while the plane of the motor running surface  36  of the center section  20  is generally parallel to the substantially vertical junction surfaces. The axis of the motor shaft  22  is also seen to be generally parallel to the axis of the axle shaft  24 . It is to be understood, however, that this arrangement of components is merely illustrative and that the components can be otherwise arranged without departing from the scope of this invention. 
     For placing the hydraulic pump  14  in fluid communication with the hydraulic motor  18 , the center section  20  includes hydraulic porting P, as is illustrated in  FIGS. 25-28 . As will be further seen in these figures as well as  FIG. 24 , the center sections  20 L and  20 R of each of the HZTs  10 L and  10 R, respectively, are generally mirror images of one another. However, since the input shafts  24  are rotated in the same direction when the vehicle is driven in the forward or reverse direction, the intersection of the kidneys, formed on the running surface  34 , and the cross passages of the porting P are symmetrical as seen in  FIG. 26 . It will be appreciated, however, that the center sections  20 L and  20 R can be full mirror images of one another in the case where the angular rotation of the swash plates of each HZT are made non-symmetrical, i.e., the angle of rotation of the swash plates are reversed with respect to one another. 
     The hydraulic porting P is in further fluid communication with a source of makeup fluid, such as a fluid sump or a charge gallery, for example, by means of check plugs  60 . Generally, the hydraulic porting P comprises a high pressure side through which fluid moves from the hydraulic pump  14  to the hydraulic motor  18  and a low pressure side through which fluid returns from the hydraulic motor  18  to the hydraulic pump  14 . Since the center sections  20 L and  20 R are generally mirror images of one another, it will be appreciated that similar hydraulic porting P will be utilized when both the HZTs  10 L and I 0 R are placed in the forward or reverse direction. This arrangement of the center section porting P provides each of the HZTs  10 L and  10 R with nearly identical hydraulic efficiencies. 
     To minimize the introduction of impurities, such as metal shavings, into the hydraulic circuit when makeup fluid is drawn into the hydraulic circuit, an upward facing filter assembly  62 , illustrated in  FIG. 28 , may be positioned adjacent to the center section  20  through which fluid may pass from the sump to the hydraulic porting P. The upward facing filter assembly  62  reduces the potential that air is ingested into the hydraulic porting P as it provides an upward facing exit path for the air. This is especially the case when the filter assembly  62  is positioned in a generally non-turbulent area of operation within the HZT  10 . 
     By way of example, the filter assembly  62  may be comprised of an upper filter member  64  that carries the filtering mesh. The upper filter member  64  is positioned adjacent to the center section  20 . Attached to the upper filter member  64 , for example by being snap-fit thereto, is a lower filter member  66  that forms a seal with the upper filter member  64  such that make-up enter the interior formed by the joined upper and lower filter members  64  and  66  substantially via the filtering mesh. The attached upper filter member  64  and lower filter member  66  may be maintained in position relative to the center section  20  by means of the check plugs  60  the ends of which extend into the interior formed by the joined upper and lower filter member  64  and  66 . Carried by the lower filter member  66  may be a magnet  68  and a deflector shield  70  for protecting the lower filter member  66  from fluid expelled via the check plugs  60 . The magnet  68  is preferably molded into the lower filter member  66  although it may be attached to the lower filter member  66  using an adhesive, for example, as shown in FIG. 1 of U.S. Pat. No. 5,613,409 which is incorporated herein by reference in its entirety or by snap-fit engagement, a staking process, or the like. The deflector shield  70  is attached to the lower filter member  66  by tabs  69  that are formed during the molding process. The deflector shield  70  may also be retained by heat staking to plastic posts, fasteners, or the like. 
     For attaching the center section  20  to the first casing member  28 , fasteners  40  (e.g., bolts) may be passed through openings  42  formed in the center section  20  to mate with attachment points  44  (e.g., threaded holes) formed in the first casing member  28 . In an embodiment illustrated in  FIGS. 4 ,  16 ,  17  and  24 - 28 , the center section  20  is formed with three extensions  46  each having an opening  42 . A first one of the extensions  46   a  extends from a side of the center section  20  proximate to the motor running surface  36 , a second one of the extensions  46   b  extends from a side of the center section  20  proximate to the pump running surface  34 , and a third one of the extensions  46   c  extends from the bottom of the center section  20 . The axis of the openings  42  are parallel to the axis of the opening  72  through which the motor shaft  22  passes. 
     For use in orienting the center section  20  within the first housing section  28 , a side of the center section  20  may be provided with a protuberance  48 , e.g., a machined diameter, that extends from the center section  20  proximate to the pump running surface  34 . The protuberance  48  is adapted to mate with a center section locator  50  formed in the first casing member  28  and to thereby establish an arbitrary X-Y orientation of the central axis of the protuberance  48  and one locating point of the center section  20 . The axis of the protuberance  48  is also parallel to the axis of the openings  42  and to the axis of the opening  72  through which the motor shaft  22  passes. Meanwhile, on extension  46   a  are a pair of flats  47 , located on the top and bottom of extension  46   a  as illustrated in  FIG. 28 , that are adapted to mate with features  49  formed in the first casing member  28  to locate the center section  20  rotationally, as illustrated in  FIG. 16 . The mating of the fasteners  40  to the first casing member  28  then provides a Z-axis locator for the center section  20  as illustrated in  FIGS. 18 and 19 . 
     For adjusting the amount of oil that is pushed from the hydraulic pump  14  to the hydraulic motor  18  via the high pressure side of the hydraulic porting P, each HZT  10  includes a moveable swash plate  74  against which the pump pistons  16  travel. The direction of rotation of the hydraulic pump  14  is fixed by the rotation of the input shaft  12 . The hydraulic pump  14  is nearly always rotated in one direction. As will be understood by those of ordinary skill in the art, the swash plate  74  may be moved to a variety of positions to vary the stroke of the pump pistons  16  and the direction of rotation of the hydraulic motor  18 . Generally, as the swash plate  74  angle is varied in one direction from the neutral position the stroke of the pump pistons  16  is varied, which then drives the hydraulic motor  18  in a direction determined by the hydraulic porting at a speed determined by the volume of the fluid displaced by the pump pistons  16  and the torque delivered by the input shaft  12 . As will be appreciated, rotation of the hydraulic motor  18  results from the motor pistons  19  moving against a thrust bearing  76  under the influence of the hydraulic fluid. As the angle of the swash plate  74  is decreased to pass through the neutral position, the direction of rotation of the hydraulic motor  18  is reversed and the speed of the hydraulic motor  18  is again determined by the volume of fluid displaced by the pump pistons  16  and the torque delivered by the input shaft  12 . 
     Since the speed of rotation of the hydraulic motor  18  is dependent upon the amount of hydraulic fluid pumped thereinto by the hydraulic pump  16  and the direction of rotation of the hydraulic motor  18  is dependent upon the direction of angular rotation of the swash plate  74 , the positioning of the swash plate  74  is seen to control the speed and direction of rotation of the hydraulic motor  18  and, as will be apparent, the speed and direction of rotation of the axle shaft  24 . While it is true that the direction of rotation of the hydraulic motor  18  will be affected by the rotation of the hydraulic pump  16 , the variation of rotation from one direction to another is accomplished completely by the swash plate  74 . 
     For moving the swash plate  74 , the swash plate  74  is supported by a pair of trunnion arms  78  that are rotatably supported in the casing of the HZT  10  as illustrated in  FIGS. 18 and 19 . As will be appreciated, rotation of the trunnion arms  78  changes the angular orientation of the swash plate  74  with respect to the pump pistons  16 . To rotate the trunnion arms  78  and, accordingly, move the swash plate  74 , a speed adjusting mechanism is coupled to one of the trunnion arms  78 . A control arm  80  of the speed adjusting mechanism may be connected, via a driving link, to a lever or a pedal provided on a vehicle whereby movement of the lever or pedal is translated to the control arm  80  to cause the rotation of the trunnion arms  78  and movement of the swash plate assembly. A further, exemplary speed adjusting mechanism with a return to neutral mechanism  41  is illustrated in FIG. 8 of U.S. patent application Ser. No. 09/789,419 and which is incorporated herein by reference in its entirety. 
     It is to be further appreciated that the control arm  80  may be located on either the outboard or inboard side of the casing of HZT  10 , as illustrated in  FIGS. 18 and 19 , respectively. To this end, the first casing member  28  may be provided with a pair of opposed bearing seats  82  in which the trunnion arms  78  are carried. The casing may then have openings adjacent to both of the bearing seats  82 , illustrated in  FIG. 19 , by which the control arm  80  can be attached to one of the trunnion arms  78 . Thus, depending upon the desired location for the control arm  80 , the control arm  80  would be mated to one of the trunnion arms  78  by way of one of the openings and the opposite opening would be closed with a seal  84 . Alternatively, the casing can have an opening adjacent to just one of the bearing seats  82 , as illustrated in  FIG. 18 . In this case, it will be appreciated that the location of the single opening will dictate whether the control arm  80  is mounted on the inboard side or the outboard side of the casing of the HZT  10 . It will be further appreciated that when it is desired to have an inboard control arm  80  on an integrated, zero-turn, hydrostatic transaxle assembly, sufficient spacing is to be provided between the joined casings of the HZTs  10 L and  10 R, similar to but larger than the spacing illustrated in  FIGS. 1 and 2 . The spacing is used to accommodate the control arms  80  (as well as any inboard braking mechanisms that are described hereinafter). 
     For limiting the range of motion of the control arm  80 , the control arm  80  may be provided with a slot  86  that cooperates with a stop  88 , such as a bolt or the like, attached to the casing as illustrated in  FIG. 14 . It will also be appreciated that the control arm  80  may be locked into the neutral position, for example during shipment of the HZT  10  and/or during assembly into a vehicle. To this end, as illustrated in  FIG. 1 , a nut  90  may be attached to the stop  88  to frictionally engage the control arm mechanism and thereby prevent its movement. The slot  86  of the control arm  80  may be asymmetrical to thereby allow a greater speed to be imparted to the axle  24  in the forward direction as compared to the reverse direction. 
     To provide a space for hydraulic fluid to expand into during operation of the HZT  10 , each HZT  10  may include an internally located expansion tank  92  as illustrated in  FIGS. 16 and 17 . In the illustrated embodiment, the expansion tank  92  is positioned within the HZT casing adjacent to a bull gear  94  that is used to drive the axle shaft  24 . Venting of the expansion tank  92  to atmosphere is accomplished via a breather tube  96  that extends from a top of the casing of the HZT  10 . Such an expansion tank may be seen in U.S. patent application Ser. No. 10/062,734, that is incorporated herein by reference in its entirety. Fluid may be added to the HZT  10  by means of an oil fill port  98  that is also formed on the top of the casing of the HZT  10 . Further, the expansion tank cover  91  may be provided with an indentation  93  and a thumb stop  95  (that extends below the sealing surface) by which the expansion tank cover  91  may be grasped for insertion into the first casing section  28 . The indentation  93  is particularly sized to accept a finger of the installer. In this manner, the expansion tank cover  91  may be installed while allowing the user to avoid contacting sealant carried on the sealing surface of the cover  91 . 
     To enable the vehicle on which the HZTs  10  are mounted to roll or “freewheel” without resistance from the hydraulic fluid, each HZT  10  may include a hydraulic bypass. Generally, when an HZT  10  does not have a motive force being applied to it, the hydraulic pump  14  and the hydraulic motor  18  are not being rotated. Therefore, any attempt to roll the vehicle would transmit rotational energy through axle shaft  24  to the motor shaft  22 , via any internal gearing, thereby causing the hydraulic motor  18  to rotate. The rotation of the hydraulic motor  18 , and the action of motor pistons  19  against motor thrust bearing  76 , causes fluid to flow through the hydraulic porting P of the center section  20  to the hydraulic pump  14 . However, with the hydraulic pump  14  being in neutral, the resultant pressure causes resistance to motion of the motor shaft  22  and the axle shaft  24  and prevents the user from easily pushing the vehicle. 
     To solve this problem, a bypass mechanism  100  may be associated with the hydraulic circuit to allow fluid to flow between the high pressure side and the low pressure side of the center section  20  porting. The bypass mechanism  100 , illustrated in  FIG. 23 , may be activated via rotation of a bypass arm  102  that is located proximate to the top of the casing of the HZT  10 . The bypass arm  102  is linked to a bypass actuator  104  that, in turn, interfaces with the center section  20  at its distal end. The degree of movement of the bypass arm  102  may be controlled by providing the control arm  102  with a notch  103  the shoulders of which are adapted to engage a stop  105  formed on the casing to limit how far the bypass arm  102  may be rotated. 
     In order to locate the relatively featureless bypass actuator  104  within the casing, a retaining ring  110  is attached to a groove in the bypass actuator  104 . Once the bypass actuator  104  and retaining ring  110  are installed, a second retaining ring  106  is installed to keep retaining ring  110  in place. A seal  112  may also be placed adjacent to the retaining ring  110 . 
     The bypass arm  102  interfaces with bypass actuator  104  by means of a tapered flat surface that prevents relative rotation between the bypass actuator  104  and the bypass arm  102 . Push nut  108  aids in maintaining engagement between the bypass arm  102  and the bypass actuator  104 . In this manner, rotation of the bypass actuator  104 , via the bypass arm  102 , can be used to move a puck, pin, or the like to lift the hydraulic motor  18  off of the motor running surface of the center section  20  to break the hydraulic circuit and thereby allow for freewheeling as described in U.S. Pat. Nos. 5,201,692, 5,423,182, and 5,497,623 which are incorporated herein by reference in their entirety. 
     To drive the axle shaft  24 , gearing may be provided that functions to drivingly couple the axle shaft  24  to the motor shaft  22 . By way of example, with reference to  FIGS. 16 and 17 , the motor shaft  22  may include a drive gear  114  that drivingly engages one or more reduction gears  116  that drive the bull gear  94  which, in turn, drivingly engages the axle shaft  24 . In the illustrative embodiment, two reduction gears  116   a  and  116   b  are provided wherein the first reduction gear  116   a  engages the drive gear  114  and drives the second reduction gear  116   b  that is set within the inside diameter of the first reduction gear  116   a . The second reduction gear  116   b  drives the bull gear  94 . 
     As further illustrated in  FIG. 22 , a proximal end of the axle shaft  24  is carried by an inboard bushing  118  positioned within the first casing section  28  adjacent to the bull gear  94 . Axial movement of the axle shaft  24  in an inward direction towards the bull gear  94  is prevented since the proximal end of the axle shaft  24  is restrained by contacting an interior wall of the first casing section  28 . Axial movement of the axle shaft  24  in an outward direction may be prevented through the use of a retaining ring positioned adjacent to the inward side of the bull gear  94 . The first casing section  28  also includes an axle horn in which is carried an outboard bushing  120  that provides additional support for the axle shaft  24 . A seal and retaining ring pack  122  is positioned in the axle horn on the outboard side of the bushing  120 . It is to be understood that the distal end of the axle shaft  24  is adapted to have a vehicle wheel mounted thereto. 
     For allowing a brake mechanism  123  to be mounted to either the inboard or outboard side of the casing of the HZT  10 , the motor shaft  22  can extend from the inboard side or the outboard side of the first casing section  28  as seen in  FIGS. 20 and 21 . It will be appreciated that the brake mechanism  123  may be a disc brake mechanism, as illustrated in  FIG. 10 , a cogged parking brake as illustrated in  FIG. 11 , or the like. As further illustrated in  FIGS. 20 and 21 , the motor shaft  22  may be provided with a configuration that depends upon whether the brake mechanism  123  is to be mounted on the inboard or outboard side of the casing. In this regard, three motor/brake shaft options are available. First, the motor/brake shaft could extend simultaneously from both the inboard and outboard side of the casing of the HZT  10  (not shown). Second, as illustrated in  FIG. 21 , the second casing section  30  can have an opening to accommodate the motor shaft  22  for inboard mounting thereof and the motor/brake shaft would not extend through the first casing section  28 . Third, as illustrated in  FIG. 20 , the second casing section  30  can be used to cover and support one end of the motor/brake shaft while the opposite end of the motor/brake shaft extends from the first casing section  28  to the outboard side of the HZT  10 . It will be appreciated that the first option increases the flexibility of the HZT  10  while the second and third options provide for a lower cost motor/brake shaft while eliminating the need for extra machining and seals. 
     When a brake mechanism is positioned on the inboard side of both the HZTs  10 L and  10 R, an integrated brake unit can be utilized as illustrated in  FIG. 5 . By way of example, the integrated brake unit may comprise a first brake disk  124 L mounted to the motor shaft  22  of HZT  10 L that is cooperable with a second brake disk  124 R mounted to the motor shaft  22  of HZT  10 R. The brake disks  124  may be provided with splines that are adapted to mate with corresponding splines formed on the motor shafts  22 . Furthermore, when the HZTs  10 L and  10 R are mated, the spacing between the motor shafts  22  is not sufficient to allow the brake disks  124  to separate from their engagement with their respective motor shaft  22 . It is contemplated that the spacing between the motor shafts  22  may be such that the brake disks  124  are in slipping engagement with one another when the brake mechanism is not activated. 
     To drive the brake disks  124  into frictional engagement with one another, a brake actuator  126 , which can be a wire form, stamped metal, powdered metal piece, constructed using a cold heading process, etc., may be mounted to one of the HZT casings. Generally, the actuator  126  comprises an arm that is used to rotate the brake actuator  126  and a cam which, when the actuator  126  is rotated, is used to drive the brake disks  124 R and  124 L into frictional engagement. More specifically, the cam of the actuator arm  126  is used to drive a brake puck  128 , via a protecting brake puck plate  130 , into a first one of the brake disks  124  to, in turn, drive the first one of the brake disks  124  into the second one of the disk brakes  124 . A second brake puck  132 , associated with the second one of the disk brakes  124 , is used to prevent movement of the second one of the disk brakes  124  under the influence of the driving first one of the disk brakes  124  to thereby maintain the frictional engagement. It will be appreciated that additional brake disks (not illustrated) may be utilized. It is to be further appreciated that the illustrated brake mechanism can also provide for the use of a brake yoke. 
     For maintaining the positioning of the brake pucks  128  and  132  within the brake mechanism, the casings of the HZTs  10  may include a grooved portion  134  sized and arranged to accept the brake puck. It will be appreciated that the positioning of the corresponding brake disk  124  functions to prevent the brake puck from dislodging from the groove  134  in which it is positioned. A further groove  136  may be provided in the casing of the HZT  10  in which the actuator  126  is positioned. This groove  136  may extend into and add to the grooved portions  134  to thereby allow the cam of the actuator  126  to be positioned behind the brake puck and brake puck plate  130 . It is to be understood that the wire form, brake actuator  126  may be used in other configurations such as with a single or multiple disk brake and a brake yoke in place of a mating housing. 
     For maintaining the brake actuator  126  on the casing of the HZT  10 , a retaining bracket  138  may be provided. The retaining bracket  138  may be attached to the casing by means of the fastener  139  used to mate the first and second casing sections  28  and  30 . A separate fastener  140  adapted to mate with the second casing section  30  may also be utilized for this same purpose. The brake puck plate  130 , the brake puck  128 , and the brake disk  124  also function to keep the actuator  126  retained on the casing of the HZT  10  given the proximity of these components to one another and the mating features formed in the housing and shaft of the actuator  126 . 
     To provide for the easy mounting of the HZT  10  to a vehicle frame, the first casing section  28  of each HZT  10  includes a plurality of fastener accepting openings  142 . As illustrated in  FIGS. 12-15 , a pair of fastener accepting openings  142  can be positioned on opposing sides of the first casing section  28  and a further plurality of fastener accepting openings  142  can be positioned on the axle shaft horn of the first casing section  28 . While illustrated with four fastener accepting openings  142  being formed on the axle shaft horn of the first casing section  28 , it is to be appreciated that this is not intended to be limiting. Rather, any number of fastener accepting openings  142  can be formed and/or utilized in the attachment process. Still further, fastener accepting openings could be formed on a bracket  58  for use in mounting the HZTs  10 L and  10 R to a vehicle frame. 
     For use in cooling the EZTs I 0 L and  10 R, a fan  150  may be mounted to one or both of the input shafts  12  adjacent to the pulley  26  as is illustrated in  FIGS. 1 and 3 . When two fans  150  are utilized, the diameters of the fans  150  need to be such that they do not contact each other while turning. Alternatively, if the fans  150  do have overlapping diameters, the fans  150  need to be vertically spaced to prevent blade contact. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangement disclosed is meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any equivalents thereof.