Abstract:
An axle driving system which houses in a housing thereof a hydrostatic transmission, axles, and a driving gear train for connecting output means of the hydraulic transmission and axles, so as to transmit power from a driving source to the hydrostatic transmission and to change the speed, thereby driving the axles. A first chamber therein contains the hydrostatic transmission and a second chamber therein contains the driving gear train. Both the first and second chambers are independent of each other so as to prevent a foreign object, such as iron powder produced in the driving gear train, from entering the hydrostatic transmission. The system includes an L-like-shaped center section on which the hydrostatic transmission is offset such that an imaginary plane which includes a motor mounting surface passes in proximity to the axis of a pump shaft. The pump shaft is disposed perpendicular to the axles. The motor shaft is disposed in parallel thereto. A hydraulic pump is positioned between the hydraulic motor and the axles, so that the housing for the hydrostatic transmission, axles and driving gear train, is smaller in width to thereby make the system more compact.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an axle driving system in which a hydrostatic transmission (hereinafter referred to as an “HST”), axles and a power transmitting mechanism are integrally provided in a housing, and more particularly to an axle driving system in which the width of the portion of the housing which houses the HST and power transmitting mechanism is smaller than in conventional systems. 
     2. Related Art 
     A conventional axle driving system houses the HST, axles and a driving gear train for interlocking the HST with the axles in a common housing. The HST is constructed so that a hydraulic pump is disposed on a horizontal portion of a center section which is L-like-shaped and a hydraulic motor is disposed on the vertical portion of the same. The hydraulic motor is positioned to one side of the axle. The hydraulic pump and hydraulic motor are fluidly connected to each other by a closed fluid circuit formed in the center section. The hydraulic pump is driven by a prime mover provided on the vehicle so as to drive the hydraulic motor and then the axles through a driving gear train. Such a construction is disclosed, for example, in U.S. Pat. Nos. 5,163,293 and 5,335,496. 
     The hydraulic pump and hydraulic motor in the conventional technique, are disposed side-by-side and to one side of the axles. As such, the width of the HST is larger which results in the lateral width of the common housing for both the pump and motor also being larger. Furthermore, an output shaft of the hydraulic motor extends to one side of the vehicle to transmit power therefrom to a differential gear unit through gears of a driving gear train, so as to drive the axles. An unused space is formed at a side of the gear train and between the HST pump and the axles. 
     Further, when the HST and the driving gear train for driving the axles by the output shaft of the HST are housed in a common housing, a foreign object, such as iron powder produced by the driving gear train, may enter into the HST. This can adversely affect operation of the HST or various parts thereof. 
     SUMMARY OF THE INVENTION 
     The axle driving system of the present invention is constructed so that the HST center section is formed in such a manner that the extended phantom plane of the motor mounting surface of the center section passes in the vicinity of the axis of the pump shaft of the hydraulic pump. The pump shaft extends substantially perpendicular to the axles. The motor shaft of the hydraulic motor extends substantially in parallel thereto. The hydraulic pump is disposed between the hydraulic motor and the axles. Hence, the width of the housing is made smaller so as to be compact in size. The axle driving system, which is smaller in lateral width, is provided with a wide swinging space for the running wheels of the vehicle and is extremely effective for a vehicle having freely steerable wheels mounted thereon. 
     Further, the present invention divides the housing into two separate chambers for housing the HST and for housing a driving gear train and axles. A partition for dividing the two chambers is provided with an oil filter so that both chambers can be filled with common oil. This improves the durability of the HST and reduces the manufacturing cost. 
    
    
     The above and other related objects and features of the invention will be apparent from a reading of the following description of the preferred embodiments including the accompanying drawings and the appended claims. 
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a partial cross-sectional plan view of a first embodiment of an axle driving system of the present invention, from which an upper half housing is removed; 
     FIG. 2 is a cross-sectional view looking in the direction of the arrows  2 — 2  in FIG. 1; 
     FIG. 3 is a cross-sectional view looking in the direction of the arrows  3 — 3  in FIG. 1; 
     FIG. 4 is a cross-sectional view looking in the direction of the arrows  4 — 4  in FIG. 1; 
     FIG. 5 is a cross-sectional view looking in the direction of the arrows  5 — 5  in FIG. 1; 
     FIG. 6 is a cross-sectional view looking in the direction of the arrows  6 — 6  in FIG. 1; 
     FIG. 7 is a top plan view of a center section of the present invention; 
     FIG. 8 is a side elevational view of the same; 
     FIG. 9 is a bottom plan view of the same; 
     FIG. 10 is a cross-sectional view looking in the direction of the arrows  10 — 10  in FIG. 7; 
     FIG. 11 is a cross-sectional view looking in the direction of the arrows  11 — 11  in FIG. 8; 
     FIG. 12 is a cross-sectional view looking in the direction of the arrows  12 — 12  in FIG. 8; 
     FIG. 13 is a cross-sectional view looking in the direction of the arrows  13 — 13  in FIG. 7; 
     FIG. 14 is a cross-sectional view looking in the direction of the arrows  14 — 14  in FIG. 7; 
     FIG. 15 is a cross-sectional rear view of a portion of the present invention surrounding a brake operating shaft; 
     FIG. 16 is a cross-sectional view looking in the direction of the arrows  16 — 16  in FIG. 15; 
     FIG. 17 is a cross-sectional view looking in the direction of the arrows  17 — 17  in FIG. 15; 
     FIG. 18 is a perspective view of the brake operating shaft and a biasing member of the present invention; 
     FIG. 19 is a plan view of a second embodiment of the axle driving system of the present invention from which an upper half housing is removed; 
     FIG. 20 is a cross-sectional view looking in the direction of the arrows  20 — 20  in FIG. 19; 
     FIG. 21 is a sectional view looking in the direction of the arrows  21 — 21  in FIG. 19; 
     FIG. 22 is a side view of an alternative embodiment of the center section of the present invention; 
     FIG. 23 is cross-sectional view looking in the direction of the arrows  23 — 23  in FIG. 22; 
     FIG. 24 is a cross-sectional view looking in the direction of the arrows  24 — 24  in FIG. 22; and 
     FIG. 25 is a cross-sectional view looking in the direction of the arrows  25 — 25  in FIG.  22 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Explanation will now be given on the entire construction of an axle driving system according to the present invention in which the housing thereof comprises an upper half housing  1  and a lower half housing  2  which are joined together along a horizontal, flat peripheral joint surface of each half housing. Along the joint surface of the upper and lower half housings is provided bearings for a motor shaft  4  and a counter shaft  26 . Axles  7  are disposed in parallel to the joint surface of the housing. The bearings for axles  7  are shifted upwardly from the joint surface and are disposed in upper half housing  1  so as to rotatably support axles  7 . Axles  7  are differentially coupled with a differential gear unit  23 . Each axle  7  projects outwardly from one end of left and right side walls of the housing, respectively. 
     The interior of the housing is divided by an inner wall  8  into a first chamber R 1  for housing therein an HST and a second chamber R 2  for housing therein (1) a driving gear train comprising a plurality of gears for transmitting power from motor shaft  4  to differential gear unit  23 , (2) differential gear unit  23 , and (3) axles  7 . Inner wall  8  comprises a longitudinal portion which is in parallel to axles  7  and a perpendicular portion which extends at a right angle to the longitudinal portion of inner wall  8 . Both portions of inner wall  8  are continuously provided so that first chamber R 1  is disposed adjacent to second chamber R 2 . Inner wall  8  also comprises a vertical wall portion which extends downwardly from the interior of upper half housing  1  toward the joint surface of the housing and rising from the interior of second half housing  2  toward the same. The end surfaces of both the vertical wall portions of inner wall  8  abut against each other when both upper and lower half housings  1  and  2  are joined, thereby forming two divided, independent chambers within the housing. 
     The first and second chambers R 1  and R 2  are filled with lubricating oil which is used in common therewith to form an oil sump. As shown in FIG. 6, an oiling lid  6  is provided on an upper wall of upper half housing  1  above differential gear unit  23  so as to enable operating oil to be supplied through lid  6 . As shown in FIG. 5, an oil flow-through port  75  is mounted on a wall surface of upper half housing  1  constituting first chamber R 1 , so that first chamber R 1  and an external reservoir tank  10  fluidly communicate with each other through a piping  9  made of a rubber hose or the like so as to enable operating oil in the oil sump to be maintained at a predetermined amount. The amount can be adjusted by flowing an incremental volume of oil into reservoir tank  10  when the temperature of the oil rises when the HST is driven. 
     An oil filter  18  is disposed on inner wall  8  which partitions first chamber R 1  from second chamber R 2 . In a first embodiment, as shown in FIGS. 1 and 5, oil filter  18  is disposed at the joint surfaces of the vertical portions of inner wall  8  to house therein the HST and right side axle  7 , thereby enabling oil to flow through oil filter  18  between first chamber R 1  and second chamber R 2 . Accordingly, oil provided in the housing can be used in common as operating oil for the HST and as lubricating oil for the gears and bearings. Also, when oil flows from second chamber R 2  into first chamber R 1 , harmful foreign objects such as iron powder, flowing into the HST is filtered by oil filter  18 . 
     First chamber R 1  is disposed in front of axles  7  and to the side of the geared transmission for transmitting power from motor shaft  4  to differential gear unit  23 , provided in the housing. A center section  5  of the HST is mounted in first chamber R 1  and is separate therefrom. Center section  5  is disposed in a manner such that its longitudinal direction is substantially perpendicular to axles  7 . The front portion forms a vertical surface  91  on which a motor mounting surface  41  is formed on which a hydraulic motor is disposed. The rear portion forms a horizontal surface  90  on which a pump mounting surface  40  is formed on which a hydraulic pump is disposed. Accordingly, the hydraulic pump is disposed between the hydraulic motor and axles  7 . A pump shaft  3  is supported vertically in the center of pump mounting surface  40  and is positioned between the hydraulic motor and axles  7 . 
     The axial piston type hydraulic pump of the present invention includes a cylinder block  16  which is rotatably, slidably disposed on pump mounting surface  40  of center section  5 . Pistons  12  are fitted into a plurality of cylinder bores and move in reciprocation through biasing springs. A movable swash plate  11  having a thrust bearing  11   a  abuts against the heads of pistons  12 . At to the center of movable swash plate  11  is formed an opening  11   b  through which pump shaft  3  perforates. Pump shaft  3  also serves as an input shaft and is disposed along the rotational axis of cylinder block  16  and is not relatively rotatably retained thereto. The upper end of pump shaft  3  projects outwardly from the upper wall of upper half housing  1  and fixedly supports an input pulley  43  having a cooling fan  44 . Input pulley  43  is given power from a prime mover (not shown) of the vehicle to which the axle driving system is mounted through a belt transmission mechanism (also not shown). 
     The piston abutting surface of movable swash plate  11  is desirably slantingly movable from a horizontal state with respect to the rotational axis of cylinder block  16 , thereby enabling the amount and direction of discharged oil from the hydraulic pump to be changed. The rear surface of movable swash plate  11  is convex and the inner surface of a lid member  15  fixed to upper half housing  1 , which closes an opening in the upper wall, is made concave to match with the convex rear surface of movable swash plate  11 . Movable swash plate  11  is constructed to be of a cradle type which, when slantingly moved, slides while coming into close contact with the concave surface of upper half housing  1 . 
     In order to slantingly operate movable swash plate  11 , as shown in FIGS. 1 and 3, a control shaft  35  extending in parallel to axles  7  is rotatably supported on the right side wall of upper half housing  1  opposite to the driving gear train for transmitting power to differential gear unit  23 . A control arm  38  is mounted onto one end of control shaft  35  outwardly extending from the housing. A swinging arm  39  is mounted to the other end of the same, inside the housing. The swinging arm  39  comprises a first arm  39   a  and a second arm  39   b  which extend radially from control shaft  35 . A projection  39   c  is provided at the utmost end of second arm  39   b , as shown in FIG.  2 . Since control shaft  35  coincides at the axis thereof with the axis of slanting motion of movable swash plate  11 , it is possible to directly engage projection  39   c  with a groove  11   d  formed on a side surface of movable swash plate  11 . In such a construction, when control arm  38  is rotated longitudinally of the vehicle body, swinging arm  39  rotates longitudinally around control shaft  35  so as to enable movable swash plate  11  to be slantingly moved to thereby change the output of the hydraulic pump. 
     At the utmost end of first arm  39   a , opposite to projection  39   c , is disposed an engaging pin  39   d . A bush  51  is fitted onto control shaft  35  within the housing. A neutral position return spring  31  of the torsion coil type is fitted onto bush  51 . Both ends of neutral position return spring  31  cross and extend in the direction of first arm  39   a  so as to put between both ends an eccentric shaft  33  mounted onto an inside wall of upper half housing  1  and engaging pin  39   a . Accordingly, when control arm  38  and swinging arm  39  rotate to change the speed of the vehicle, one end of neutral position return spring  31  is moved to widen a gap between both ends, but the other end of spring  31  is retained by the eccentric shaft  33 , so that control lever  38  is given a biasing force to return to a neutral position. When the operating force on control arm  38  is released, a restoring force generated at one end of neutral position return spring  31  holds engaging pin  39   d  by eccentric shaft  33  in the specified neutral position. A portion of eccentric shaft  33  extending outwardly of the housing is formed into an adjusting screw and eccentric shaft  33  is preferably rotatably shifted therethrough, so that swinging arm  39  shifts around control shaft  35 , thereby enabling movable swash plate  11  to be adjusted to put it into an accurate neutral position. 
     Control arm  38 , as shown in FIG. 2, is provided with an arm  38   b  for connecting a shock absorber  73 . A vertical arm  38   a  connects to a speed changing member (not shown), such as a lever or a pedal provided on the vehicle, through a link mechanism (not shown) on the vehicle. Arm  38   b  is pivotally supported by a movable member of shock absorber  73 . A casing thereof is pivotally mounted onto a support plate  74  fixed to a lower surface of an axle housing portion of lower half housing  2 . Shock absorber  73  prevents control arm  38  from abruptly changing speed and also prevents the speed changing member (not shown) from abruptly returning to the neutral position when operating force is released so as to exert a sudden braking action onto the HST. Also, shock absorber  73  is positioned somewhat forwardly slanted and extends along the right side wall of upper half housing  1  straddling axles  7 , thereby effectively utilizing an otherwise unused or dead space surrounding axles  7 . 
     Pressurized oil discharged from the hydraulic pump is sent to the hydraulic motor through an oil passage in center section  5 . The hydraulic motor is constructed as shown in FIG.  4 . In detail, a cylinder block  17  is rotatably, slidably mounted on motor mounting surface  41  formed on vertical surface  91  of center section  5 . A plurality of pistons  13  are movably mounted in reciprocation in a plurality of cylinder bores in cylinder block  17 , through biasing springs. The heads of pistons  13  abut against a fixed swash plate  37  which is fixedly disposed between upper half housing  1  and lower half housing  2 . Motor shaft  4  is not relatively rotatably retained on the rotational axis of cylinder block  17  and extends substantially horizontally. One end of motor shaft  4  is supported in a bearing bore in motor mounting surface  41  of center section  5 . The other end is supported by a bearing  76  on inner wall  8  formed along the joint surfaces of upper half housing  1  and lower half housing  2 . The utmost end of motor shaft  4  enters into second chamber R 2 . Bearing  76  is a sealing bearing for partitioning first chamber R 1  from second chamber R 2 . An O-ring  77  is disposed between the outer periphery of an outer ring and inner wall  8 . 
     The driving gear train for transmitting power from motor shaft  4  to differential gear unit  23 , as shown in FIGS. 1 and 6, comprises a gear  25  fixed onto motor shaft  4  where it enters into second chamber R 2 , a larger diameter gear  24  supported onto a counter shaft  26  and permanently engageable with gear  25 , a smaller diameter gear  21  supported on counter shaft  26  and integrally rotatable with larger diameter gear  24 , and ring gear  22  of differential gear unit  23  which is permanently engageable with smaller diameter gear  21 . Counter shaft  26  is disposed in second chamber R 2  adjacent to pump shaft  3  and perpendicular thereto. One end of counter shaft  26  is supported by a side wall of the housing at the joint surface of upper half housing  1  and lower half housing  2 . The other end is supported by inner wall  8  at the joint surface thereof. The rotational output speed of motor shaft  4  is reduced by larger diameter gear  24 , smaller diameter gear  21  and ring gear  22  so as to drive axles  7  through differential gear unit  23 . Larger diameter gear  24  on counter shaft  26  is disposed as close as possible to the outside surface of ring gear  22  and is overlapped axially therewith, thereby reducing the longitudinal length of the housing. In this embodiment, the HST is disposed to one side of the driving gear train at the right side thereof. At a further right side thereof is disposed a speed changing mechanism for the HST. The hydraulic pump thereof is positioned substantially in the lateral and longitudinal center of the housing. Differential gear unit  23  is disposed in an enlarged portion of the housing. 
     A brake disc  19  is fixed on the utmost end of motor shaft  4  in second chamber R 2 . As shown in FIGS. 1,  15 ,  16  and  17 , a brake pad  29  and a wedge shaped member  70  are interposed between the upper portion of the front surface of brake disc  19  and the inner surface of upper half housing  1  and are supported thereto, movable only in the direction of the rotational axis of motor shaft  4 . In a space surrounded by inner wall  8  and the surface of brake disc  19  opposite to brake pad  29  (at the left side of brake disc  19  in FIG.  15 ), a biasing member  72  and a brake operating shaft  14  are disposed. Brake operating shaft  14  is vertically disposed and is rotatably supported by upper half housing  1  and lower half housing  2 . The upper end of brake operating shaft  14  projects upwardly from the housing and has a brake arm  27  fixed thereto. On an outside surface of an intermediate portion of brake operating shaft  14  in the housing is formed a flat cutout  14   a  which is D-like-shaped when viewed in cross-section. Arch-like biasing member  72  is fitted into cutout  14   a  and is restricted from axial movement by cutout  14   a  and is guided at both sides by the inner surface of upper half housing  1  so as to be slidable only axially of motor shaft  4 . Accordingly, when brake arm  27  is rotated to the left or to the right, brake operating shaft  14  is rotated. One longitudinal end of cutout  14   a  pushes the rear surface of biasing member  72  and brake disc  19  is interposed between brake pad  29  and biasing member  72  to exert a braking action on motor shaft  4 . Wedge member  70  abuts at the lower surface thereof against the upper end of an adjusting bolt  71 . Adjusting bolt  71  screws into lower half housing  2  and projects outwardly from lower half housing  2 , thereby screwably tightening a lock nut at the intermediate portion of bolt  71  for locking wedge member  70 . Wedge member  70  is raised or lowered in the housing as adjusting bolt  71  is rotated so as to advance or retract in the direction of the rotational axis of motor shaft  4 . As brake pad  29  is worn, the interval between brake pad  29  and brake disc  19  can be properly maintained by adjusting bolt  71  which is vertically disposed in lower half housing  2 . 
     Next, explanation will be given on the construction of center section  5  in accordance with FIGS. 7 through 14. Center section  5  is larger longitudinally than conventional center sections. Center section  5  has three bolt bores  5   h  which are open vertically between a front portion of center section  5  and a rear portion thereof. Center section  5  is fixed to upper half housing  1  through bolts. At the center of pump mounting surface  40  formed on horizontal surface  90  on an upper surface of a rear portion of center section  5  is formed a bearing portion so as to enable the lower portion of vertical pump shaft  3  to be rotatably supported therewith. Pump shaft  3  is perpendicularly disposed with respect to axles  7 . A pair of arcuate ports  40   a  and  40   b  are open at both sides of the bearing for suppling and for discharging oil from cylinder block  16 . 
     At the front portion of horizontal surface  90  is formed a vertical surface  91 , a phantom plane which includes vertical surface  91  crosses near the longitudinal axis of pump shaft  3 . Center section  5  is substantially L-like-shaped when viewed in cross section. As shown in FIG. 8, a pair of arcuate ports  41   a  and  41   b  are also vertically open on motor mounting surface  41  formed on front vertical surface  91 , so that oil is adapted to be supplied to or discharged from cylinder block  16  through ports  41   a  and  41   b . At the center of motor mounting surface  41  is provided a bearing for motor shaft  4  which is disposed in parallel to axles  7 . 
     In order to connect arcuate ports  40   a  and  40   b  on pump mounting surface  40  with arcuate ports  41   a  and  41   b  on motor mounting surface  41 , a first linear oil passage  5   a  and a second oil passage  5   b  are vertically and forwardly bored in a thick portion of center section  5  so as to reduce the lateral length of center section  5 . 
     Motor mounting surface  41  is positioned in front of the substantial center of pump mounting surface  40  so as not to increase the lateral length of the HST when the hydraulic motor is disposed thereon. A third linear oil passage  5   c  crosses and communicates with an intermediate portion of second linear oil passage  5   b . Arcuate port  40   a  on pump mounting surface  40  is, as shown in FIG. 14, made thinner to communicate with first linear oil passage  5   a . Arcuate port  40   b  is made deeper to communicate with third linear oil passage  5   c . Arcuate port  41   a  at the upper portion of motor mounting surface  41  communicates with first linear oil passage  5   a . Arcuate port  41   b  at the lower portion of the same communicates with second linear oil passage  5   b . Second linear oil passage  5   b  communicates with third linear oil passage  5   c , whereby arcuate ports  40   a ,  41   a ,  40   b  and  41   b  communicate to form a closed fluid circuit so as to circulate operating oil between the hydraulic pump and the hydraulic motor. 
     Check valves  54  and  55  are disposed at the open ends of first linear oil passage  5   a  and second linear oil passage  5   b  and are closed with lids  64 , as shown in FIG. 10. A lid  65  closes the open end of third linear oil passage  5   c . When subjected to pressure, lids  64  and  65  abut against projections  2   a  and  2   b  formed on the inner wall of lower half housing  2 . A first communication oil passage  5   d  is vertically bored in center section  5  so as to communicate with inlet ports of check valves  54  and  55 . Oil passage  5   d  communicates with a terminal end of a second communication oil passage  5   g  which is horizontally bored in center section  5 . A fore end of second communication oil passage  5   g  communicates with an inlet port  45   a  into which discharged oil from a charging pump  45  is guided, as shown in FIG. 12. A plug  66 , as shown in FIG. 9, closes the open end of first communication oil passage  5   d.    
     Charge pump  45 , as shown in FIG. 3, comprises a pump casing which has internal teeth for retaining the lower end of pump shaft  3  extending from the horizontal lower surface of center section  5  and external teeth engageable with the internal teeth and which is brought into close contact with the horizontal lower surface of center section  5 . The pump casing is biased upwardly by a spring interposed between the lower surface of the pump casing and the inner bottom surface of lower half housing  2  and serving also as a relief valve for maintaining a specified value of pressure of oil discharged from charge pump  45  and filled in the closed fluid circuit. An annular oil filter  56  is disposed between the inner bottom surface of lower half housing  2  and the horizontal lower surface of center section  5  in a manner of surrounding charge pump  45 , thereby filtering operating oil taken therein. 
     As shown in FIGS. 5,  10  and  13 , in order to fill the closed fluid circuit with operating oil after the axle driving system is assembled, oiling pipes  52  and  53  are disposed on the horizontal lower surface of center section  5 . Oiling pipe  52  communicates with the deep portion of arcuate port  41   a  through an oil passage vertically bored from the horizontal lower surface of center section  5 . Oiling pipe  53  directly communicates with second linear oil passage  5   b . Oiling pipes  52  and  53  are exposed at the lower ends thereof from the lower outer surface of lower half housing  2  and are closed by lids after the closed fluid circuit is filled with operating oil. 
     A by-pass operating arm  60 , as shown in FIG. 5, is disposed above upper half housing  1  so as to open first and second linear oil passages  5   a  and  5   b  into the oil sump for idling axles  7  when hauling the vehicle. In detail, as shown in FIGS. 1 and 4, by-pass operating arm  60  is fixed at the base thereof to a by-pass shaft  61  vertically, pivotally supported to an upper wall of upper half housing  1 . Bypass shaft  61  extends at the lower end thereof toward the surface of center section  5  opposite to motor mounting surface  41  and forms a flat surface at the periphery of the lower portion. 
     A through bore  5   f  (see FIG. 8) is open on motor mounting surface  41  of center section  5  and is slightly above the center thereof and between accurate ports  41   a  and  41   b . A push pin  62  is slidably supported by center section  5  in the direction of rotation of the axis of cylinder block  17 . Push pin  62  can at one end abut against a rotatably slidable surface of cylinder block  17  which comes into close contact with motor mounting surface  41 , and abuts at the other end against flat surface  61   a  of by-pass lever shaft  61 . 
     When the vehicle is hauled, an operator operates by-pass operating arm  60  outside of the housing causing by-pass shaft  61  to rotate. Flat surface  61   a  pushes push pin  62  toward cylinder block  17 . Push pin  62  releases cylinder block  17  from motor mounting surface  41 , and the closed fluid circuit communicates with the oil sump in the housing through arcuate ports  41   a  and  41   b , thereby obtaining free rotation of motor shaft  4 . 
     Next, explanation will be given on a second embodiment of the present invention in accordance with FIGS. 19 through 25, in which similar parts have been given the same reference numerals as used in the description of the first embodiment. In the second embodiment, the center section is formed in two pieces rather than in one piece as is center section  5  in the first embodiment. In this embodiment, center section  5 ′ is formed of a first piece  5   ′a  and a second piece  5   ′b  which are coupled together. On horizontal surface  90  of first piece  5   ′a  is formed pump mounting surface  40 . A pair of kidney-shaped ports  40   a  and  40   b  are open on pump mounting surface  40 . On a side surface of a vertical portion  91  of second piece  5   ′b  is formed motor mounting surface  40 , on which a pair of kidney-shaped bores  41   a  and  41   b  are open. Communicating oil passages  100  and  101  are bored in first piece  5   ′a . The terminal ends thereof are open on the side surface. Inside second piece  5   ′b  are bored oil passages  102  and  103  which communicate with the pair of kidney-shaped ports  41   a  and  41   b . The terminal ends of the passages  102  and  103  are open on the side surface. Oil passages  100  and  102 ,  101  and  103  connect with each other through the joint surfaces when horizontal portion  90  is coupled with vertical portion  91 , thereby forming a closed fluid circuit for circulating therein operating oil between the hydraulic pump and hydraulic motor. 
     Center section  5 ′ is not provided with bolt insertion bores  5   h  as shown in the first embodiment, but is sandwiched between upper half housing  1  and lower half housing  2  so as to be restrained from vertical and lateral movements, thereby being fixedly positioned in the housing. 
     The advantages of a two-piece center section  5 ′ include that the manufacturing and processing costs and the number of assembly processes are reduced, which reduces the overall cost of the system. Further, fewer parts are required in that bolts for securing the center section in the housing are not required. 
     When oil leaks caused from the closed fluid circuit in center section  5 ′, oil in first chamber R 1  is taken into the closed fluid circuit through oil filter  56  and check valves (not shown). In this embodiment, control shaft  35  for slantingly rotating swash plate  11  of the hydraulic pump is vertically and rotatably supported by an upper wall of upper half housing  1 . Such construction for engaging control shaft  35  with swash plate  11  is the same as, for example, that described in U.S. Pat. No. 5,495,712 which is incorporated herein by reference thereto in its entirety. 
     As seen from the above description, the axle driving system of the present invention can be applied to drive the axles of a vehicle so as to sufficiently reduce the mounting space thereof. Vehicles on which this axle driving system may be used include agricultural working vehicles such as lawn and garden tractors, and transportation vehicles. 
     Although several embodiments have been shown and described, they are merely exemplary of the invention and are not to be constructed as limiting the scope of the invention which is defined by the appended claims.