Abstract:
An axle driving apparatus for a vehicle includes an axle, a vertical pump shaft, a hydraulic pump having a movable swash plate, a pair of trunnion shafts provided coaxially at respective opposite sides of the movable swash plate, a hydraulic motor having a motor shaft drivingly connected to the axle, and a housing containing the hydraulic pump, the hydraulic motor, and the axle. The housing includes a first housing member and a second housing member separably jointed along a vertical plane perpendicular to a rotary axis of the axle. A rotary axis of the hydraulic pump is substantially parallel to the vertical plane. The hydraulic pump is entirely disposed in the first housing member. A rotary axis of the trunnion shafts is substantially perpendicular to the vertical plane. One of the trunnion shafts is rotatably supported by the first housing member and the other of the trunnion shafts is rotatably supported by the second housing member, whereby the movable swash plate is slantingly rotatably supported in the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/254,746, filed Oct. 21, 2005, which is a continuation of U.S. application Ser. No. 10/847,372, filed May 18, 2004, now U.S. Pat. No. 6,983,815, which is a continuation of U.S. application Ser. No. 10/187,848, filed Jul. 3, 2002, now U.S. Pat. No. 6,752,236, which is a continuation of U.S. application Ser. No. 10/101,071, filed Mar. 20, 2002, now U.S. Pat. No. 6,568,498, which is a continuation of U.S. application Ser. No. 08/875,724, filed Aug. 4, 1997, now U.S. Pat. No. 6,390,227, which is a National Stage of PCT International No. PCT/US95/04097, filed Mar. 30, 1995, the entire disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an axle driving unit suitable for use with a rear-discharge lawn tractor having a mower located beneath the body of the tractor between the front and rear wheels. 
   2. Related Art 
   U.S. Pat. No. 3,969,876 discloses a conventional lawn tractor having a rear discharge system, which has a mower driven by a prime mover mounted on it, and a leaf blower loaded on the rear portion of the tractor. The rear discharge system disposes a chute, for discharging lawn grass cut by the mower, longitudinally between the left and right rear wheels. The rear discharge system has no projection to the outside of the body of the tractor. The cut lawn grass is discharged directly rearwardly of the tractor. This has the advantage of eliminating equipment, such as an auxiliary suction fan. 
   However, because the space between the left and right rear wheels is very narrow, it is very difficult to laterally juxtapose therebetween the cut grass chute and an axle driving apparatus. As disclosed in U.S. Pat. No. 3,969,876, a hydraulic stepless speed change transmission and a differential gear constituting the axle driving unit are separated and are housed in separate housings. The differential gear is disposed in the narrow space between the wheels, and the transmission is disposed at another position on the body of the tractor. The output shaft of the hydraulic stepless speed change transmission and differential gear are connected by a chain belt mechanism. Accordingly, the axle driving unit has the disadvantages of a high manufacturing cost and a long assembly time. Also, since one axle, through which the cut grass chute passes, is much longer than the other axle, this design has the further disadvantage that the one longer axle is easy to deflect. Accordingly, the lifespan of a bearing for the one longer axle provided at the differential gear is reduced. 
   An axle driving unit which houses in a common housing a hydraulic stepless speed change transmission and a differential gear for differentially connecting a pair of axles and integrates them is well-known, as disclosed in, for example, U.S. Pat. No. 4,914,907 and No. 4,932,209. In these patents, the transmission comprises a combination of a variable displacement type hydraulic pump and a fixed displacement hydraulic motor. The hydraulic pump and motor are mounted side by side and longitudinally of the axle with respect to an L-like-shaped center section, whereby the entire axle driving unit is larger in width longitudinally of the axle. Hence, the axle driving unit of this design interferes with the chute of a lawn tractor which has a rear discharge system. As a result, the chute cannot be disposed between the left and right rear wheels. 
   SUMMARY OF THE INVENTION 
   The axle driving unit of the present invention is constructed so that a first shorter axle that mounts a first driving wheel, and a second longer axle that mounts a second driving wheel are supported by a housing provided on a body frame. The housing is attached to the tractor body frame so that it is eccentric or offset to be in proximity to the first driving wheel. The housing is provided with an enlarged region extending forward at approximately a right angle with respect to the axles. A hydraulic stepless speed change transmission is provided in the enlarged region. As a result, the axle driving unit of the present invention can be easily disposed laterally of (or to the side of) the chute of a lawn tractor of the rear discharge type. 
   In order to improve the operating efficiency of the hydraulic stepless speed change transmission, it is desirable to construct the transmission so that it is a hydraulic stepless system fluidly connecting the hydraulic pump and hydraulic motor to each other. In this case, the hydraulic pump can be disposed in the enlarged region, smaller in width and spaced apart from the axle, and the hydraulic motor can be disposed in proximity to the axle. 
   In the enlarged region are provided a pump mounting surface and a motor mounting surface disposed substantially perpendicular or rectangular with respect to each other for mounting the hydraulic pump and hydraulic motor. It is preferable that the hydraulic pump is mounted onto the pump mounting surface so that an input shaft connected to the hydraulic pump is oriented approximately vertically with respect to the axles, and the hydraulic motor is mounted onto the motor mounting surface so that an output shaft connected to the hydraulic motor is oriented approximately horizontally with respect to the axles. In a lawn tractor that includes a prime mover having a vertical crankshaft, the input shaft of the transmission can be connected therewith by use of a simple belt transmitting mechanism. The output shaft of the transmission can be connected with the axle by use of an inexpensive spur gear having a low manufacturing cost. 
   Bearing means are provided in the housing for supporting the first and second axles. A pair of bearing holding portions for supporting distal portions of the first and second axles define the width of the housing to be smaller than the length of the housing including the enlarged region. As a result, the enlarged region is elongated so that the chute can have as large a cross-sectional area as possible. 
   It is desirable that the second longer axle be supported at an intermediate portion thereof by a bearing device provided on the tractor body frame. As a result, the second longer axle is stably supported. It is also desirable that the second axle be composed of at least two axle parts separably connected with each other through a coupling. Hence, the axle driving unit becomes superior in assembly efficiency and transformation efficiency. The axle to which the other (first) driving wheel is mounted is stably supported by a bearing device provided on the tractor body frame. 
   The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  a side view of a lawn tractor of the present invention. 
       FIG. 2  is a cross sectional view of the lawn tractor of  FIG. 1  taken along line  2 - 2 . 
       FIG. 3  is a cross sectional view taken along line  3 - 3  in  FIG. 2 . 
       FIG. 4  is a cross sectional view taken along line  4 - 4  in  FIG. 3  showing a first embodiment of the axle driving unit, from which an upper half housing is removed. 
       FIG. 5  is a sectional view taken on the line  5 - 5  in  FIG. 4 . 
       FIG. 6  is a sectional view taken on the line  6 - 6  in  FIG. 4 . 
       FIG. 7  is a sectional view taken on the line  7 - 7  in  FIG. 4 . 
       FIG. 8  is a sectional view taken on the line  8 - 8  in  FIG. 4 . 
       FIG. 9  is a sectional view taken on the line  9 - 9  in  FIG. 8 . 
       FIG. 10  is a sectional view taken on the line  10 - 10  in  FIG. 4 . 
       FIG. 11  is a perspective view of a center section of the present invention. 
       FIG. 12  is a partially sectional plan view of a second embodiment of the axle driving unit, from which an upper half housing is removed. 
       FIG. 13  is a sectional view taken on the line  13 - 13  in  FIG. 12 . 
       FIG. 14  is a sectional view taken on the line  14 - 14  in  FIG. 12 . 
       FIG. 15  is a sectional view taken on the line  15 - 15  in  FIG. 12 . 
       FIG. 16  is a perspective view of a center section of the second embodiment. 
       FIG. 17  is a partially sectional plan view of a third embodiment of the axle driving unit, from which an upper half housing is removed. 
       FIG. 18  is a sectional view taken on the line  18 - 18  in  FIG. 17 . 
       FIG. 19  is a perspective view of a center section of the third embodiment. 
       FIG. 20  is a sectional side view of a fourth embodiment of the axle driving unit. 
       FIG. 21  is a sectional plan view taken on the line  21 - 21  in  FIG. 20 . 
       FIG. 22  is a sectional front view taken on the line  22 - 22  in  FIG. 21 . 
       FIG. 23  is a sectional side view of a fifth embodiment of the axle driving unit. 
       FIG. 24  is a sectional plan view taken on the line  24 - 24  in  FIG. 23 . 
       FIG. 25  is a sectional front view taken on the line  25 - 25  in  FIG. 23 . 
       FIG. 26  is a sectional side view of a sixth embodiment of the axle driving unit. 
       FIG. 27  is a sectional plan view taken on the line  27 - 27  in  FIG. 26 . 
       FIG. 28  is a sectional front view taken on the line  28 - 28  in  FIG. 26 . 
       FIG. 29  is a sectional side view of a seventh embodiment of the axle driving unit. 
       FIG. 30  is a sectional plan view taken on the line  30 - 30  in  FIG. 29 . 
       FIG. 31  is a sectional front view taken on the line  31 - 31  in  FIG. 29 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description of the various embodiments, description of parts designated with the same reference numerals will not be repeated, unless specifically noted otherwise. In  FIGS. 1 through 3 , a lawn tractor of the present invention is so constructed that an engine E is loaded on the front of a body frame  1 , and left and right freely steerable driven (or front) wheels  2  are suspended under the front of the body frame. An axle driving unit provided with left and right driving (or rear) wheels  3 L and  3 R is suspended at the rear of body frame  1 . A grass catcher  4  is mounted to the rear end of body frame  1 , and a mower  5  is attached beneath the body of the tractor between the front and rear wheels through an elevation device (not shown). Mower  5  is connected at a rear discharge port  5   a  thereof with an inlet port of catcher  4  through a chute  6 . Chute  6  extends slantwise upwardly from the rear discharge port  5   a  of the mower  5 , passes laterally or to the side of the axle driving unit and between the left and right rear driving wheels  3 L and  3 R, and is connected to the inlet of catcher  4 . Accordingly, lawn grass cut by mower  5  is blown rearwardly to pass between left and right rear driving wheels  3 L and  3 R through chute  6 , and is then stored in catcher  4 . The lawn tractor preferably includes a height-adjustable seat that can be raised and lowered. 
   As shown in  FIG. 1 , two pulleys  8  and  9  are fixed onto an output shaft  7  of engine E. Pulley  8  transmits a driving force through a belt  10  to a third pulley  12  fixed onto an input shaft  11  of mower  5 , thereby rotating cutter blades  5   b  of mower  5  (see  FIG. 2 ). The other pulley  9  is adapted to transmit a driving force through a belt  13  to a fourth pulley  15  fixed onto a pump shaft or input shaft  29 , projecting upwardly from a housing for the axle driving unit. Reference numeral  16  designates a cooling fan fixed to pulley  15  in order to cool the axle driving unit. 
   As shown in  FIG. 3 , the axle driving unit is suspended from a first mounting member  1   a  and a second mounting member  1   b , both parts of body frame  1 . The axle driving unit is eccentrically disposed toward one of the two sides of body frame  1 .  FIG. 3  shows the axle driving unit disposed in a position displaced toward driving wheel  3 L relative to the center of the space between left and right driving wheels  3 L and  3 R. The left side of the housing of the axle driving unit is fixed to first mounting member  1   a , and the right side to second mounting member  1   b  that downwardly extends from a laterally intermediate portion of first mounting member  1   a . As shown in  FIG. 3 , body frame  1  of the tractor includes first mounting member  1   a  provided longitudinally of body frame  1  and at a portion adjacent to the inside of driving wheel  3 L. The second mounting member  1   b  is suspended from about the center of the space between driving wheels  3 L and  3 R. A bearing  20  is provided at a portion of body frame  1  adjacent to the inside of driving wheel  3 R. 
   The housing for the axle driving unit comprises an upper half housing  21  and a lower half housing  22  joined to each other through a peripheral joint or junction surface. When the housing is mounted in an operating position on body frame  1  of the tractor, the joint surface is substantially horizontally disposed. 
   Shorter first axle  17  projects from the left side of the housing for the axle driving unit, and longer second axle  18  projects from the right side of the housing. Driving wheel  3 L mounts at one axial end of first axle  17 , and driving wheel  3 R mounts at one axial end of second axle  18 . 
   A first mounting boss  210   a  is formed at a portion of the axle driving unit housing positioned near the distal portion (portion closest to the axle driving unit) of first axle  17 , and a second mounting boss  210   b  is formed at the portion of the housing positioned near the distal portion of second axle  18 . 
   First mounting member  1   a  is connected to first mounting boss  210   a , and second mounting member  1   b  is connected to second mounting boss  210   b . Thus, the housing is eccentrically mounted in the working or operating position, offset to one side toward driving wheel  3 L. Such a layout ensures a sufficiently wide space between the axle driving unit housing and driving wheel  3 R so that chute  6  can be offset from the center of the tractor body. As a result, the axle driving unit and chute  6  can be arranged laterally, side by side of each other within body frame  1 . This enables chute  6  to longitudinally extend within body frame  1 . 
   Second axle  18  is supported at the distal portion thereof ( 18   a ) by the housing, and at an intermediate portion of a longer portion ( 18   b ) projecting outwardly from the housing by a bearing  20 . Hence, the distance between the bearing for supporting the distal portion of first axle  17  and that for supporting the distal portion of second axle  18  can be greatly reduced, thereby reducing the width W of the axle driving unit housing. Consequently, chute  6  can be designed with a relatively increased cross-sectional area. 
   Alternatively, second axle  18  may be formed as one long axle. However, it is favorable for assembly of the axle driving unit and facilitation of transportation to divide axle  18  into part  18   a  supported at the housing, and part  18   b  supported by bearing  20 , as shown in  FIG. 3 . Parts  18   a  and  18   b  are connected by a coupling  19 . 
     FIGS. 4-11  show the construction of an axle driving unit for speed-change-driving rear driving wheels  3 L and  3 R. The housing of the axle driving unit is formed by connecting upper half housing  21  and lower half housing  22  with each other. Lubricating oil is charged into the housing; and is used as operating oil for a hydraulic stepless speed change transmission T to be discussed below. First axle  17  and part  18   a  of the second axle are rotatably supported in upper half housing  21  positioned above the joint surface. As shown in  FIG. 10 , cylindrical bores  211  are formed at left and right side walls of upper half housing  21  to form first bearing supports, respectively. Rolling bearings  100  fitted into cylindrical bores  211  support the distal portion of first axle  17  and part  18   a  of second axle  18 , cylindrical bores  211  being closed at their outer ends with oil seals. 
   As best seen in  FIG. 10 , semicircular concave circular-arc surfaces  212  are formed on the inner surface of an upper wall of upper half housing  21 . On the inner surface of lower half housing  22  are integrally formed projections  220  that project beyond the joint surface. Semicircular concave circular-arc surfaces  221  are formed at the end surfaces of projections  220  that are positioned opposite to concave surfaces  212 , respectively, to form a pair of second bearing holders. Bushings  101  are inserted into the second bearing holders, so that bushings  101  support the base ends of first axle  17  and part  18   a  of second axle  18 , respectively. 
   As shown in  FIG. 10 , concave circular-arc surfaces  213  larger in radius than surfaces  212  are formed in upper half housing  21  from concave circular-arc surfaces  212 , and concave circular-arc surfaces  222  larger in radius than surfaces  221  are formed in lower half housing  22  from concave circular-arc surfaces  221 . Concave circular-arc surfaces  213  and  222  are combined to form annular cavities. Rolling bearings (not shown) may be built-in to the cavities, instead of the aforesaid bushings  101 , to support the base ends of first and second axles  17  and  18 . The choice of bushings or bearings depends upon the size of the load applied to axles  17  and  18 . When the load is small, bushings  101 , as shown in the embodiment of  FIG. 10 , are used to reduce manufacturing cost. When the load is large, the rolling bearings (not shown) are used to increase the support strength for the axles. 
   As shown in  FIG. 10 , first axle  17  and part  18   a  of second axle  18  abut concentrically against each other in the housing, differentially connected to each other by a differential gear  23  contained in the housing. Axle  17  and part  18   a  of axle  18  project laterally outwardly from the housing. 
   As best seen in  FIGS. 8 and 10 , an input gear  48  larger in width than differential gear  23  has at the center a through-bore  480 . The base ends of first axle  17  and part  18   a  of second axle  18  are inserted into through-bore  480  to be supported by input gear  48 . Bevel gears  49 , spline-engaged with first axle  17  and part  18   a  of second axle  18 , and input gear  48  prevent axial movement of the axles. Pinions  50  engageable with bevel gears  49 , and through-bores  481  for containing therein pivotal pins  51  for pinions  50 , are provided at both sides of input gear  48 . A flat portion formed at the circumferential surface of the end of each pivotal pin  51  abuts against a flat portion provided at each through-bore  481 , thereby restraining each pivotal pin  51  from rotating. Accordingly, differential gear  23  is formed from a smaller number of parts. 
   The front portions (portions toward the front or forward end of the lawn tractor) of upper and lower half housings  21  and  22  are enlarged in a direction perpendicular to the longitudinal axes of the axles. The housing of the axle driving unit is therefore longer (length L 1 ) than it is wide (width W; see  FIG. 4 ). A center section  25  for a hydraulic stepless speed change transmission T is mounted in the enlarged region. As shown in  FIG. 11 , center section  25  is a single and elongated piece having an upper surface  250  and a side surface  251  which are adjacent and perpendicular to each other. 
   A pump mounting surface  40  is formed at the front portion (toward the front or forward end of the lawn tractor) of upper surface  250  for mounting thereon a hydraulic pump. At the rear portion of side surface  251  a motor mounting surface  41  is formed for mounting a hydraulic motor. As shown in  FIG. 8 , pump mounting surface  40  and motor mounting surface  41  are partially overlapped with each other by a longitudinal length OL. The center of motor mounting surface  41  extends in parallel to pump mounting surface  40  and is offset downwardly therefrom by a height H 1 . 
   As shown in  FIG. 11 , housing mounting faces  42  are formed on upper surface  250  of center section  25 , approximately level with mounting surface  40 . Therefore, housing mounting faces  42  can be ground when pump mounting surface  40  is ground, so that the processing time for the pump mounting surface can be reduced. Bolt insertion bores are provided at housing mounting faces  42  and center section  25  is fixed to the inner wall of the enlarged region of upper half housing  21  through connecting bolts inserted into the bores. Pump mounting surface  40  of center section  25  extends horizontally with respect to the axles, and is disposed spaced apart from the axles. Motor mounting surface  41  extends vertically with respect to the axles, and is disposed in proximity to the axles. 
   Alternatively, pump mounting surface  40  and motor mounting surface  41  may be provided integral to the inner wall by increasing the thickness of the inner wall in the enlarged region of lower half housing  22 . However, as shown in this embodiment, it is preferable to use center section  25  separate from the housing to facilitate processing of the housing, and to prevent oil from leaking out of the housing. 
   As shown in  FIG. 5 , a valve plate  102  is mounted onto pump mounting surface  40 . A cylinder block  36 , constituting the hydraulic pump, is rotatably disposed on valve plate  102 . Pistons  36   a  are fitted into a plurality of cylinder bores of cylinder block  36 , for reciprocating movement through biasing springs. Pump shaft  29  engages with a spline bore provided on the rotary axis of cylinder block  36 . Pump shaft  29  is rotatably supported by upper half housing  21  and pump mounting surface  40 . Pulley  15  is fixed to a projection of pump shaft  29  projecting outwardly from upper half housing  21 . The heads of pistons  36   a  abut against a thrust bearing  28   a  of a movable swash plate  28 , thereby forming an axial piston type variable displacement hydraulic pump. Alternatively, the hydraulic pump may be of a radial piston type or a gear type. 
   Pump shaft  29  is inserted into a longitudinally extending through-bore formed at the axial center of movable swash plate  28 . A convex circular-arc surface is formed at the rear of movable swash plate  28 , and slidably contacts with a concave circular-arc surface formed at the inner surface of the upper wall of upper half housing  21 . As a result, movable swash plate  28  is movable in a longitudinal slantwise direction along the concave circular-arc surface. As movable swash plate  28  moves with respect to the rotary axis of cylinder block  36  along the contact surface, the amount and flow direction of oil discharged from the hydraulic pump changes. As shown in  FIG. 5 , a control shaft  35  for movably operating movable swash plate  28  extends horizontally and is rotatably supported by a lid  38  that closes a side opening of upper half housing  21 . At an outer end of control shaft  35  is fixed a control lever  35   a  that is connected in association with a speed change operating tool (not shown). At an inner end of control shaft  35  is fixed a base of a swinging arm  35   b . A ball  37 , fixed to the utmost end of swinging arm  35   b , engages through a joint block with an engaging groove  28   b  of movable swash plate  28 , control shaft  35  being rotated for movement of movable swash plate  28 . 
   Movable swash plate  28  shown in this embodiment is of a cradle type that moves slantwise along the concave circular-arc surface of the inner surface of the upper wall of upper half housing  21 . To enable a trunnion-type swash plate to be mounted instead of cradle-type swash plate  28 , a bearing bore is positioned on the same axis as control shaft  35  at a portion of the inner wall of upper half housing  21 . Where the trunnion-type movable swash plate is used, each trunnion shaft is supported by the bearing bore and lid  38 . The cradle-type movable swash plate is advantageous in that it is inexpensive to produce, while the trunnion-type is advantageous in that it requires a decreased operating force. Swash plates of both types are easily exchangeable. 
   Referring to  FIG. 6 , a valve plate  103  is mounted onto motor mounting surface  41  formed on side surface  251  of center section  25 . A cylinder block  44  of the hydraulic motor is rotatably disposed on plate  103 . A plurality of pistons  44   a  are fitted for reciprocating movement into a plurality of cylinder bores of cylinder block  44 . 
   The heads of pistons  44   a  abut against a thrust bearing  45   a  at a fixed swash plate  45 , fixed between upper half housing  21  and lower half housing  22 . A motor shaft  24  engages with a spline bore provided on the rotary axis of cylinder block  44  to form an axial-piston type fixed displacement hydraulic motor. Alternatively, the hydraulic motor may be of a radial piston type or a gear type. 
   The rotation axis of cylinder block  44  is positioned in the same plane as the joint surface of the upper and lower half housings. One end of motor shaft  24  is supported by motor mounting surface  41 , the other end being supported by a bearing sandwiched between upper half housing  21  and lower half housing  22 . 
   When the hydraulic pump and the hydraulic motor are disposed on a center section having the configuration described above, pump shaft  29  and motor shaft  24  are perpendicular to each other. Pump shaft  29  is offset from motor shaft  24  by a length L 2  in the direction apart from the axles (see  FIG. 8 ). 
   As shown in  FIGS. 4 ,  9 , and  11 , a pair of kidney-shaped ports  40   a  and  40   b  is open on pump mounting surface  40  of center section  25  to take in or discharge oil in cylinder block  36 . A pair of kidney-shaped ports  41   a  and  41   b  is also open on motor mounting surface  41  to take in or discharge oil in cylinder block  44 . Within center section  25  are provided an L-like shaped oil passage  25   a  and a straight oil passage  25   b  for connecting kidney-shaped ports  40   a  and  41   a , and  40   b  and  41   b  with each other, respectively, to circulate the operating oil between the hydraulic pump and hydraulic motor, thereby making a closed circuit. 
   The hydraulic pump and hydraulic motor are fluidly connected with each other through the above-mentioned closed circuit, and the combination of these members forms a hydraulic stepless speed change transmission. Capacity of the hydraulic pump is changed by rotatably operating control lever  35   a , thereby enabling the hydraulic motor to obtain stepless output rotation. 
   Operating oil supply means is provided for replenishing oil that has leaked out from center section  25 . The supply means may be the hydraulic pump itself, or a charge pump  31 . 
   Charge pump  31 , as shown in  FIGS. 5 and 7 , is a trochoid pump which is contained in a charge pump casing  30  attached to a charge pump mounting surface  43  formed on the lower surface of center section  25 . A wave washer  34  is interposed between a stepped portion at the outer periphery of charge pump casing  30 , and a bottom surface of a lid  33  that closes an opening  223  in lower half housing  22 . Wave washer  34  biases charge pump  31  so that charge pump  31  is in contact with charge pump mounting surface  43 . The lower end of pump shaft  29  passes through center section  25  and projects from charge pump mounting surface  43 , and a pin is mounted on shaft  29 . An engaging bore is open at the center of an internal gear of charge pump  31 . The lower end of pump shaft  29  is inserted into the engaging bore, and pump shaft  29  engages with the internal gear. Charge pump  31  is fixed to the lower end of pump shaft  29  and is driven by pump shaft  29 . 
   Charge pump casing  30  is disposed in an oil sump formed by the housing. A suction port  30   a  of charge pump  31  is open at the lower surface of charge pump casing  30 . Suction port  30   a  connects with the oil sump in the housing through a groove  33   a  formed by partially cutting out a wave washer mounting portion at lid  33 . An annular oil filter  32  is fixed between charge pump mounting surface  43  and the bottom surface of lid  33 . Since oil filter  32  surrounds charge pump  31  and charge pump casing  30 , existing oil is cleaned and then taken in by charge pump  31  through groove  33   a  and suction port  30   a . When oil filter  32  is maintained and inspected, lid  33  is removed from lower half housing  22 , and oil filter  32  is removed from the housing through an opening  223  of lower half housing  22 , oil filter  32  being smaller in outline than opening  223 . 
   The pressure oil discharged from charge pump  31  is directly guided into a supply oil passage  25   c  open in charge pump mounting surface  43 . If the introduced pressure exceeds the pressure corresponding to a biasing force of the wave washer biasing means, the pressure causes charge pump casing  30  to move away from or detach from charge pump mounting surface  43  against the biasing force of wave washer  34 . This creates a gap between charge pump  31  and charge pump mounting surface  43 . Pressure oil is then released in part from the gap into the oil sump to adjust its pressure below the biasing force of wave washer  34 . Charge pump  31  is detached from charge pump mounting surface  43  to adjust the discharge pressure below the biasing force while maintaining fluid communication between the discharge port and the oil supply port. 
   As shown in  FIG. 8 , due to the form of center section  25 , pump mounting surface  40  is positioned in the second plane P 2  in upper half housing  21 , in parallel to and spaced apart by a height H 1  from the first plane P 1  coincident with the joint surface of the housing. Hence, a wide space is formed between charge pump mounting surface  43  positioned opposite to pump mounting surface  40 , and the inner surface of the bottom of lower half housing  22 . As a result, charge pump  31  and oil filter  32  can be contained in this space with ample room. Height H 2  from the axis of the axles to the bottom of lower half housing  22  is reduced to ensure sufficient ground clearance. 
   As shown in  FIG. 9 , check valves  150  are disposed at the open ends of oil passages  25   a  and  25   b  in the closed circuit of center section  25 . Check valves  150  mutually communicate at the inlet ports through one transverse passage  25   d . Transverse oil passage  25   d  communicates at the intermediate portion with a supply oil passage  25   c  open at charge pump mounting surface  43  of center section  25 . Oil introduced from the discharge port of charge pump  31  to supply oil passage  25   c  reaches the inlet side of each check valve  150 . The oil pressure pushes out check valves  150 , positioned at the low pressure side of oil passages  25   a  and  25   b , so that oil is supplied from the outlet side into the closed circuit. 
   Check valves  150  are slidably provided with push pins  151  that project outward from center section  25 . An axial end of each push pin  151  comes in contact with a single connecting plate  52  in the housing, and a release rod  53  is fixed at the center of connecting plate  52 . Release rod  53  projects outwardly from upper half housing  21  at one end. Connecting plate  52  is outwardly biased by a spring  54  interposed between plate  52  and center section  25 . Release rod  53  is manually pushed in, so that push pins  151  simultaneously push out check valves  150  so that the inlet port is in fluid communication with the outlet port. Hence, oil passages  25   a  and  25   b  communicate with each other through transverse oil passage  25   d , thereby enabling the hydraulic motor to idle. 
   As shown in  FIGS. 4 and 8 , motor shaft  24  is disposed in parallel to axles  17  and  18 . A counter shaft  26  is provided between the axles and motor shaft  24 , and extends in parallel to the axles and motor shaft  24 . A gear  240  is provided on motor shaft  24  and engages with a larger diameter gear  46  fixed onto counter shaft  26 . A smaller diameter gear  47  on counter shaft  26  engages with a ring gear  48  of differential gear  23 . Thus, a driving force output from motor shaft  24  is transmitted to axles  17  and  18  through a gear system speed reduction transmission and differential gear  23 . 
   As shown in  FIG. 4 , a braking friction plate  63  is fixed on motor shaft  24 , an arm  64  is fixed to upper half housing  21 , and a brake actuator  65  is provided at one end of arm  64 . An operating lever (not shown) is rotated to bring brake actuator  65  in press contact with braking friction plate  63 , thereby enabling motor shaft  24  to be braked. 
   The axis of counter shaft  26  is positioned in the first plane P 1 . It is supported at both axial ends by a pair of bearings interposed between upper half housing  21  and lower half housing  22 . 
   As shown in  FIG. 8 , a pocket projects from the bottom of lower half housing  22  to define height H 2 . The lower portion of larger diameter gear  46  on counter shaft  26  is contained within this. pocket. 
   The axes of axles  17  and  18  may be disposed in the first plane P 1 . However, it is preferable to dispose the axes above the first plane P 1  as shown in this embodiment. This is because, even when a large diameter input gear  48  is used in order to enlarge the last gear ratio, the lower portion of input gear  48  will not extend lower than the lower portion of larger diameter gear  46 . Therefore, the pocket for gear  46  need not be further enlarged, thereby enabling height H 2  to be as small as possible so that ground clearance is ensured. 
   As shown in  FIG. 4 , differential gear unit  23  is displaced in the housing toward part  18   a  of second axle  18 . A space is thereby formed at one lateral side of a second axle holder. The larger diameter gear  46  is partly disposed in this space so that the length of the housing does not have to be increased. As shown in  FIG. 8 , a partition  214  for covering an upper portion of input gear  48  is integrally formed in upper half housing  21  and an oil flow-through bore  215  is formed at partition  214 . An opening  216  is formed at a portion of the upper wall of upper half housing  21  positioned above partition  214 , and covered with a cover member  55 . Cover member  55  is provided with a breather  56  and an oil check rod  57  that is also used as an oil charge plug. A predetermined amount of oil is charged into the housing through cover member  55  so that the boundary of oil and air is formed in a space enclosed by partition  214  and cover member  55 . Air mixed in the oil when charged into the housing is collected in an air reservoir through oil flow-through bore  215 . Partition  214  is filled at the lower portion with oil, so that, even when the various gears rotate, the air in the air reservoir is scarcely mixed in the oil. When the axle driving unit is operated for a long time, the oil volume expands. The volume of air in the air reservoir decreases to accommodate the expanded volume of oil. 
   A second embodiment of the axle driving unit is shown in  FIGS. 12-16 . The second embodiment is almost the same in construction as the first embodiment so that the same parts are designated with the same reference numerals and the description is omitted. As such, only three points of different construction will be described. 
   In the case where a lawn tractor includes a wide space underneath the tractor where the mower is disposed, the enlarged region of the housing is extended forwardly as much as possible. The width of the housing is made as small as possible, thereby enabling the grass chute to increase in volume. 
   Therefore, a first difference is to elongate the enlarged region of the housing, and to suitably form center section  25  for such an enlarged region. Pump mounting surface  40  and motor mounting surface  41 , formed on upper surface  250  and side surface  251  of center section  25 , are formed so that motor mounting surface  41  laterally overlaps pump mounting surface  40 . As a result, center section  25  can be contained in the enlarged region having width Wa (see  FIG. 12 ). Cylinder block  36  of the hydraulic pump is disposed further away from axles  17  and  18  than in the first embodiment. Length L 2  from pump shaft  29  to motor shaft  24  is larger in this embodiment (see  FIG. 13 ) than in the first embodiment (see  FIG. 8 ). There is no overlap OL in this embodiment as there was in the first embodiment (see  FIG. 8 ). 
   A third mount boss  210   c  is provided at the utmost end of the enlarged region of the housing. Mount boss  210   c  is connected to a third mounting member (not shown) hanging from body frame  1 . Therefore, even when the entire length L 1  of the housing becomes larger, the axle driving unit remains in a proper operating position. 
   A second difference is with respect to motor shaft  24 . As in the first embodiment, braking friction plate  63  is disposed on one end of motor shaft  24  that extends outwardly from the housing. The difference in this embodiment is that the other end of motor shaft  24  is provided with a spline, and it extends into a through-open bore provided at the center of motor mounting surface  41 . A bushing is interposed between the joint surfaces of the housing to support a rotary shaft  59  (see  FIG. 12 ). The spline end of motor shaft  24  is spline-engaged with one end of rotary shaft  59  so that the driving force of motor shaft  24  is taken out of the housing through rotary shaft  59 . 
   The outer end of rotary shaft  59  is an indented spline. Braking friction plate  63  may be mounted on this end of rotary shaft  59 , or rotary shaft  59  may be used as a power take-out shaft. If such construction is not required, rotary shaft  59  can be removed and the bore formed at the joint surfaces of the housing can be closed by a seal cap. 
   A third difference is found in the movable swash plate of the hydraulic pump. Movable swash plate  60  of a trunnion-type is used in place of that of a cradle-type. One trunnion shaft  60   a  of movable swash plate  60  is supported by a lid  38  mounted to upper half housing  21 , the other trunnion shaft  60   b  being supported by a bearing bore provided at the inner wall of upper half housing  21  (see  FIG. 14 ). Opposite sides of moveable swash plate  60  include step portions  61 , whereby movable swash plate  60  is prevented from moving toward an axial direction along a rotary axis of trunnion shafts  60   a  and  60   b . Trunnion shaft  60   a  projects outwardly from lid  38  and a control lever  35   a  is mounted onto the projection. 
   A third embodiment of the axle driving is shown in  FIGS. 17-19 . The construction of the third embodiment compares to that of the second embodiment as follows. 
   Center section  25  is not connected to upper half housing  21  by bolts, but inserted in part between upper half housing  21  and lower half housing  22 . Therefore, center section  25  is positioned in the enlarged region in a free-standing state. Housing mounting faces  42 ′ project from the left and right side surfaces  251 . The upper surface of center section  25  and the lower surface opposite thereto form housing mounting faces. Since center section  25  is free-standing, bolts are not required so that assembly is simplified and manufacturing cost is lowered. 
   In order for center section  25  to be free-standing, pump shaft  29  and motor shaft  24  are completely supported by the housing. Upper end of pump shaft  29  is supported by a bearing  104  attached to upper half housing  21 . Lower end of pump shaft  29  passes through mounting surface  40  and charge pump mounting surface  43  and is supported by a bearing  105  attached to lower half housing  22  (see  FIG. 18 ). Motor shaft  24  passes through motor mounting surface  41  and the two ends are supported by bearings  106  and  107  inserted between both upper half and lower half housings  21  and  22  (see  FIG. 17 ). 
   In the axle driving unit of an embodiment to be discussed below, as in the second and third embodiments, the enlarged region is made as elongated as possible in order to allow greater volume for the chute of the rear discharge lawn tractor. 
   A fourth embodiment of the axle driving unit will be described in accordance with  FIGS. 20 ,  21  and  22 . A center section  25  of substantially L-like shape in sectional side view is disposed in an elongated enlarged region extending across upper half housing  21  and lower half housing  22 , and is fixed to upper half housing  21 . A pump mounting surface  40  is formed on a substantially horizontal upper surface  250 , and a motor mounting surface  41  on a substantially vertical side surface  251  of center section  25 . Pump mounting surface  40  is positioned apart from axles  17  and  18 , and motor mounting surface  41  is positioned near the axles. Pump shaft  29  extends substantially vertically with respect to axles  17  and  18 , and motor shaft  24  extends substantially horizontally with and perpendicular to axles  17  and  18 . 
   A movable swash plate  60  at the hydraulic pump is of a cradle-type and uses the same operating mechanism as that in the first embodiment so that it is manually controllable along a concave circular-arc surface of an inner wall of upper half housing  21 . 
   An oil filter  32  is interposed between the lower surface of center section  25 , opposite to pump mounting surface  40 , and the inner surface of the bottom wall of lower half housing  22 . Oil in the housing is filtered by oil filter  32  and guided to a supply port (not shown) open at the lower surface of center section  25 . 
   A pair of kidney-shaped ports  40   a  and  40   b  open at pump mounting surface  40 , and a pair of kidney-shaped ports  41   a  and  41   b  open at motor mounting surface  41  are connected with each other through a pair of substantially L-like-shaped oil passages  25   a  and  25   b , respectively. As shown in  FIG. 9 , check valves are disposed at the open ends of oil passages  25   a  and  25   b . In this embodiment, operating oil is supplied by operation of the hydraulic pump itself, but a charge pump as described in the previous embodiments may alternatively be used. 
   The axis of motor shaft  24  is positioned in the same plane as the joint surface of the housing. Motor shaft  24  is rotatably supported by bearings interposed between upper half housing  21  and lower half housing  22 . 
   First axle  17  and part  18   a  of second axle  18  are rotatably supported by lower half housing  22 . Distal portions of first axle  17  and part  18   a  of second axle  18  are supported by bearings  100  held into cylindrical bores of lower half housing  22 . The bases of axles  17  and  18  are supported by bushings  101  disposed in lower half housing  22 . Bushings  101  are fixedly interposed between legs  217 , long enough to extend beyond the joint surface of the housing, and the concave circular-arc surface of lower half housing  22  (see  FIG. 22 ). 
   Differential gear  23  is displaced toward part  18   a  of second axle  18  in the housing. Motor shaft  24  is substantially perpendicular to first axle  17 , and passes above axle  17  as it extends toward the rear of the housing. The utmost end of motor shaft  24  projects from the housing, and a braking friction plate  63  is attached to the utmost end of motor shaft  24 . 
   A counter shaft  26 , extending in parallel to axles  17  and  18 , is disposed at the rear of the housing. The axis of shaft  26  is positioned in the same plane as the joint surface of the housing. Shaft  26  is rotatably supported by bearings sandwiched between upper half housing  21  and lower half housing  22 . 
   Since motor shaft  24  and counter shaft  26  are substantially perpendicular to each other, a smaller diameter bevel gear  240   a  on motor shaft  24  and a larger diameter bevel gear  46   a , engageable with bevel gear  240   a , on counter shaft  26  are used to connect the shafts together in a driving manner. The driving force is transmitted from a smaller diameter gear  47  on counter shaft  26  to axles  17  and  18  through a ring gear  48  on differential gear  23 . 
   A fifth embodiment of the axle driving unit will be described in accordance with  FIGS. 23 ,  24  and  25 . Center section  25 , disposed in an elongated enlarged region of the housing, has a substantially flat body. Center section  25  is attached to a side of upper half housing in a substantially horizontal manner. Pump mounting surface  40  and motor mounting surface  41  are formed on upper surface  250  of center section  25 ; the former is positioned apart from axles  17  and  18  and the latter is positioned near the axles. Pump shaft  29  of the hydraulic pump and motor shaft  24  of the hydraulic motor extend in parallel to each other, and are substantially vertical, being at a right angle to axles  17  and  18 . 
   A pair of kidney-shaped ports  40   a  and  40   b  open at pump mounting surface  40 , and a pair of kidney-shaped ports  41   a  and  41   b  open at motor mounting surface  41  are connected to each other through a pair of straight oil passages  25   a  and  25   b.    
   Movable swash plate  28  of the hydraulic pump is of a cradle-type, and is manually controllable along the concave circular-arc surface of the inner wall of upper half housing  21  by use of an operating mechanism as in the first embodiment. A fixed swash plate  45  of the hydraulic motor is fixedly fitted into a concave formed at the inner wall of upper half housing  21 . 
   Pump shaft  29  and motor shaft  24  are rotatably supported by bearings longitudinally juxtaposed at upper half housing  21 , and bearings longitudinally juxtaposed at center section  25 . 
   A counter shaft  26  is disposed in the same plane as the joint surface of the housing. A pair of bearings for supporting counter shaft  26  are sandwiched between a pair of legs  252  downwardly projecting from the lower surface opposite to motor mounting surface  41 , and a pair of legs  224  upwardly projecting from the inner surface of the bottom wall of lower half housing  22  (see  FIG. 23 ). With this construction, there is no need for a bearing holding portion for counter shaft  26  to be provided at the housing. Therefore, the width Wa of the enlarged region can be further restricted (see  FIG. 24 ). One end of counter shaft  26  projects outwardly from the housing for attachment of a braking friction plate  63 . 
   A substantially vertical motor shaft  24  passes downwardly through center section  25 . The lower end portion of motor shaft  24  is positioned just above, and at about a right angle to, the axis of the axles (see  FIG. 25 ). A small diameter bevel gear  240   a  is fixed on the lower end of motor shaft  24 , and a larger diameter bevel gear  46   a , engageable with bevel gear  240   a , is fixed on counter shaft  26 , thereby connecting shafts  24  and  26 . 
   A driving force is transmitted from a smaller diameter gear  47  on counter shaft  26  to a ring gear  48  of differential gear  23 , differential gear  23  being displaced toward part  18   a  of axle  18  in the housing. The axes of axles  17  and  18  are positioned in the same plane as the joint surface of the housing. The terminal and bore of first axle  17  are supported only by bearing  100  and bushing  101  interposed between upper half housing  21  and lower half housing  22 . Differential gear  23  is eccentrically disposed to further reduce the axial length of part  18   a  of second axle  18  so that part  18   a  of second axle  18  is supported by only bushing  101  sandwiched between upper half housing  21  and lower half housing  22 . 
   A sixth embodiment of the axle driving unit will be described in accordance with  FIGS. 26 ,  27  and  28 . The construction is basically the same as that of the fifth embodiment so that only the following four points are described which are different from that of the fifth embodiment. 
   A first modified point is the position of braking friction plate  63 . The upper end of substantially vertical motor shaft  24  passes through the upper wall of upper half housing  21 , and braking friction plate  63  is mounted to this upper end. This allows the braking device to be smaller and more compact because motor shaft  24  has a lower transmitting torque than counter shaft  26 . As a result, the axle driving unit becomes smaller in width. Braking friction plate  63  is disposed on the same side as pump shaft  29  that projects from the housing, and is in proximity to cooling fan  16 . This enables friction plate  63  to be effectively cooled by the ventilation from cooling fan  16 . The ventilation from cooling fan  16  also blows away the dust collected on braking friction plate  63 . 
   A second modified point is with respect to center section  25 . One leg  252  downwardly projects from the surface of center section  25  opposite to motor mounting surface  41 . Leg  252  and lower half housing  22  support a bearing for the end of counter shaft  26 , and a bearing at the other end of counter shaft  26  is sandwiched between upper half housing  21  and lower half housing  22 . Leg  252  may be formed separately from center section  25  and fixed below its plane. 
   A third modified point is with respect to differential gear  23 ′. An input gear  48 ′ of differential gear  23 ′ is freely fitted on part  18   a  of second axle  18 , and is disposed in proximity to one side wall of the housing, thereby considerably reducing the axial length of part  18   a  of second axle  18 . A differential case  48   a  is attached to the opposite side wall of the housing. In differential case  48   a , the base ends of first axle  17  and part  18   a  of second axle  18  face each other. Side gears  49  engage with pinions  50  pivoted to the base sides of first axle  17  and part  18   a  of second axle  18 . 
   A fourth modified point is in the layout of first axle  17  and part  18   a  of second axle  18  and the construction of the housing support with respect to the joint surface of the housing. The axes of axles  17  and  18  can be disposed in substantially the middle portion of the height of the housing and still retain the balance of the axle driving unit. The protrusion formed in the bottom portion of lower half housing  22  for input gear  48 ′ can thus be made smaller in volume to ensure sufficient height from the ground. 
   A seventh embodiment of the axle driving unit will be described in accordance with  FIGS. 29 ,  30  and  31 . 
   Center section  25 , as in the fifth and sixth embodiments, is substantially shaped like a flat plate, and mounted to upper half housing  21 . The body of center section  25  is substantially horizontally disposed in lower half housing  22 . Pump mounting surface  40  is formed on the substantially horizontal upper surface of center section  25  spaced apart from axles  17  and  18 . Motor mounting surface  41  is formed on the substantially horizontal lower surface of center section  25  in proximity to axles  17  and  18 . 
   Motor shaft  24  of the hydraulic motor is journalled at its upper end to center section  25 , and at its lower end to lower half housing  22 . If it is difficult to mount the hydraulic motor onto motor mounting surface  41 , the lower end of motor shaft  24  may be journalled to a fixed swash plate  45 , and fixed swash plate  45  may be connected to the lower surface of center section  25 . 
   Motor shaft  24  extends in parallel to pump shaft  29 , and substantially vertically passes through the upper wall of center section  25 . On the upper end of motor shaft  24  is fixed a smaller diameter bevel gear  240   a  engageable with a larger diameter bevel gear  46   a  on counter shaft  26 . 
   In order to support counter shaft  26  in the same plane as the joint surface of the housing, a pair of bearings is provided between a pair of legs  252 ′, upwardly projecting from the surface of the center section opposite to motor mounting surface  41 , and a pair of legs  218  projecting from the inner wall of upper half housing  21 . 
   Although several embodiments have been described, they are merely exemplary of the invention and not to be construed as limiting, the invention being defined solely by the appended claims and their equivalents.