Abstract:
An axle driving apparatus consisting of a housing for compactly housing inner portions of axles, a hydrostatic transmission, and a transmitting mechanism for transmitting power to the axles from an output shaft of the hydrostatic transmission. The housing is partitioned by an internal wall into a chamber containing the hydrostatic transmission and a chamber containing the other transmitting mechanisms. The housing is filled with oil in common with both chambers. A trunnion for changing the output rotation of the hydrostatic transmission is disposed in parallel to the axles. A shock absorber is provided to prevent abrupt speed change. A differential locking device is attached to the differential gear to thereby improve the straightforward running capacity of the vehicle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an axle driving apparatus for improving the straightforward running capacity of a vehicle on a muddy road or the like, and more particularly to an axle driving apparatus which is integrally provided with a hydrostatic transmission (hereinafter referred to as the HST); axles; a power transmitting mechanism, which can easily change the speed of the HST; an oil reservoir, which can absorb an increase in the volume of oil due to an increase in the temperature of the HST; and a differential locking device, all of which are provided in a single housing. 
     BACKGROUND OF THE INVENTION 
     Conventionally, an axle driving apparatus consists of a housing for an HST, axles and a power transmitting device for interconnecting the HST and axles. On the center section of the HST is disposed a hydraulic pump, provided with a vertical input shaft, and a hydraulic motor, provided with a horizontal output shaft. A plurality of pistons are disposed in the hydraulic pump cylinder block. The heads of the pistons abut against a movable swash plate. Changing the angle of the movable swash plate changes the pump capacity so as to increase or decrease the number of rotations of the hydraulic motor. The movable swash plate is slanted, thereby enabling the speed of the HST to be changed by rotatably operating trunnions supported in the housing. Each trunnion is disposed on a longitudinally slanted axis of the swash plate, as disclosed in U.S. Pat. No. 5,456,068, for example. 
     A speed change controller, such as a pedal or a lever, which is provided on the vehicle can be operated normally longitudinally thereof so that its motion can be transmitted to a control arm of the axle driving apparatus through a link mechanism, such as a rod, disposed longitudinally of the vehicle. Hence, it is preferable that the control arm swing longitudinally around the lateral axis. One conventional construction is provided with a vertical operating shaft, independent of the trunnions, where both trunnions and the vertical operating shaft interlock with each other. The control arm is provided at one end of the operating shaft so that the control arm swings longitudinally around the vertical axis, and the other end is constructed so that the trunnion projects at the axial end thereof from the front wall of the housing. A control arm is provided at the axial end so that the control arm swings laterally around the longitudinal axis. A complex linkage mechanism, with respect to the vertical operating shaft and trunnions, is required in the first construction described above, thereby increasing the number of parts and assembly time, making the axle driving apparatus too expensive to produce. The second construction described above requires a separate link mechanism for converting the longitudinal motion into a lateral motion, thereby requiring space to provide two link mechanisms in the vehicle, making it difficult to apply the apparatus to a vehicle of small size and increasing the number of parts required. 
     U.S. Pat. Nos. 5,440,951 and 5,515,747 disclose that when the HST and the mechanism for transmitting power to the axles from the HST are housed in the same housing, the housing can be filled with oil to be used as both operating oil for the HST and lubricating oil for the transmitting mechanism. In this case, a foreign object, such as iron powder, created by the rubbing of the transmitting mechanism may flow toward the HST. The iron powder or other foreign object is removed by an oil filter so as not to enter into the HST closed fluid circuit. However, the iron powder or the like may encroach on the piston and swash plate and thereby adversely affect them. The housing is integrated in part with the oil reservoir so as to enable the oil volume in the housing to be adjusted when expanded due to a rise in temperature. However, the greater the quantity of oil, the larger the increase in volume. Thus, the housing must be made larger and the reservoir therefore becomes larger so that the housing itself has to be large in size. 
     U.S. Pat. No. 5,094,077 discloses that in order to prevent the speed change controller equipped on the vehicle from being hastily operated by an operator, a shock absorber is provided on the control arm. The shock absorber should be disposed above the upper wall of the housing because the control arm is configured to vertically and longitudinally swing around the axis on the upper wall of the housing. Therefore, space for disposing the shock absorber without interference with an input pulley or an enlarged portion of the upper wall of the housing is required. 
     Further, where a differential gear is provided between the left and right axles, when one axle is idling, a driving force cannot be transmitted to the other axle. Hence, it is desired to provide a differential locking device on the axle driving apparatus for integrating the differential locking device with the HST and the axles. 
     SUMMARY OF THE INVENTION 
     The axle driving apparatus of the present invention is partitioned by an internal wall provided within the housing, into a first chamber for housing therein the HST and a second chamber for housing therein axles and a transmitting mechanism which transmits power from an output shaft of the HST to the axles. Both chambers are filled with common oil. An oil filter is disposed therebetween to allow the chambers to communicate with each other. One chamber communicates with an oil reservoir. Trunnions for the swash plate to change the output rotation of the HST are supported between the internal wall and a side plate fixed to the housing. The trunnions are disposed laterally of and in parallel to the axles. One of the trunnions projects outwardly from the housing so as to fix an arm. The shock absorber is connected thereto, thereby preventing hasty speed change. A differential locking device is attached to a differential gear differentially connecting the left and right axles. During the normal running of the vehicle, the differential rotation can be performed. When one axle is idling, both axles are adapted to be directly connected to each other. 
     These and other objects of the invention will become more apparent from the detailed description and examples which follow. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a plan view of an axle driving apparatus; 
     FIG. 2 is a partially sectional plan view of the same in which an upper half housing thereof is removed; 
     FIG. 3 is a sectional view looking in the direction of arrows  3 — 3  in FIG. 2; 
     FIG. 4 is a sectional view looking in the direction of arrows  4 — 4  in FIG. 2; 
     FIG. 5 is a sectional view looking in the direction of arrows  5 — 5  in FIG. 2; 
     FIG. 6 is a sectional view looking in the direction of arrows  6 — 6  in FIG. 2; 
     FIG. 7 is a sectional view looking in the direction of arrows  7 — 7  in FIG. 2; 
     FIG. 8 is an enlarged sectional plan view of a principal portion of the mechanism of a braking device; 
     FIG. 9 is an enlarged sectional side view of a principal portion of the same; 
     FIG. 10 is a enlarged sectional view of only a part of a principal portion of the same; 
     FIG. 11 is a left side view of a center section of the present invention; 
     FIG. 12 is a plan view of the same; 
     FIG. 13 is a sectional view looking in the direction of arrows  13 — 13  in FIG. 11; 
     FIG. 14 a sectional view looking in the direction of arrows  14 — 14  in FIG. 11; 
     FIG. 15 is a sectional view looking in the direction of arrows  15 — 15  in FIG. 11; 
     FIG. 16 is a sectional view looking in the direction of arrows  16 — 16  in FIG. 12; 
     FIG. 17 is a sectional view looking in the direction of the arrows  17 — 17  in FIG. 12; 
     FIG. 18 is a sectional view looking in the direction of the arrows  18 — 18  in FIG. 12; 
     FIG. 19 is a sectional view looking in the direction of the arrows  19 — 19  in FIG. 12; 
     FIG. 20 is a sectional view looking in the direction of the arrows  20 — 20  in FIG. 12; 
     FIG. 21 is a bottom plan view of the center section from which the charge pump has been removed; 
     FIG. 22 is sectional view of a differential gear and a differential locking device; 
     FIG. 23 is a side view of a slider of the differential locking device; 
     FIG. 24 is a side view of a ring gear of the same; and 
     FIG. 25 is a perspective exploded view of the differential gear of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-7 show the construction of an axle driving apparatus. The housing of the axle driving apparatus comprises an upper half housing  1  and a lower half housing  2  joined to each other along a horizontal, flat joint surface along the periphery of the upper and lower half housings  1 , 2 . A bearing for a motor shaft  4  is provided on the joint surfaces of both upper half housing  1  and lower half housing  2 . Bearings for axles  7  are shifted upwardly from the joint surface of both upper half housing  1  and lower half housing  2  and are disposed in upper half housing  1  to rotatably support axles  7 . Axles  7  are differentially connected by a differential gear unit  23  and project laterally outwardly of the housing. 
     The interior of the housing is partitioned by an internal wall  8  into a first chamber R 1  for housing therein an HST and a second chamber R 2  for housing therein a gear-type drive train which transmits power to differential gear unit  23  from motor shaft  4  to axles  7 . First chamber R 1  and second chamber R 2  are filled with common oil which forms an oil sump. As shown in FIG. 7, an oil feed lid  6  is provided on an upper wall of upper half housing  1  above differential gear unit  23 . The housing can be filled with operating oil through lid  6 . As shown in FIG. 6, an oil flow port  75  is provided in the upper portion of upper half housing  1 . Upper half housing  1  communicates through a piping  9 , of rubber hose or the like, with the interior of an oil reservoir  10  mounted at a predetermined position on the vehicle, thereby enabling the volume of operating oil in oil reservoir  10  to be adjusted. 
     As shown in FIG. 6, an oil bore  8   a  is open at a predetermined position in internal wall  8  which partitions first chamber R 1  from second chamber R 2 . An oil filter  18  covers oil bore  8   a . In this embodiment, as shown in FIGS. 2 and 6, oil bore  8   a  and oil filter  18  are disposed on internal wall  8  between the portion containing the HST and the portion containing the right side axle  7 , thereby enabling oil to flow between first chamber R 1  and second chamber R 2  through oil filter  18 . Accordingly, oil filling the housing can be used as both operating oil for the HST and lubricating oil for the gears and bearings. When the oil enters into first chamber R 1  from second chamber R 2 , foreign objects such as iron powder which are harmful to the HST, are filtered by oil filter  18 . 
     Internal wall  8  is provided within the housing so that first chamber R 1  is disposed in front of axles  7  and to the side of the drive train for transmitting power from motor shaft  4  to differential gear unit  23 . Internal wall  8 , as shown in FIG. 4, comprises (1) an internal wall portion  1   a  erected integrally with the upper inner surface of upper half housing  1  and is positioned at the end surface on the same plane as the joint surface of the housing parts  1 ,  2  and (2) an internal wall portion  2   a  erected integrally with the inner bottom surface of lower half housing  2  and positioned at the end surface on the same plane as the joint surface of the housing. When both upper half housing  1  and lower half housing  2  are joined together, the end surfaces of both internal wall portion  1   a  and internal wall portion  2   a  join each other to form internal wall  8 , thereby partitioning the interior of the housing into first chamber R 1  and second chamber R 2 . 
     The HST is housed in first chamber R 1 . The HST comprises a hydraulic pump P, a hydraulic motor M and a center section  5 . Center section  5  is elongated and is longitudinally disposed in first chamber R 1 . A vertical surface  91  is formed at the front of center section  5  on which hydraulic motor M is disposed. A horizontal surface  90  is formed along the top of center section  5  on which hydraulic pump P is disposed. A pump shaft  3  is substantially vertically disposed on center portion  5  and is positioned between motor shaft  4  and axles  7  which extend substantially horizontally and in parallel to each other. A pump mounting surface  40  is formed on horizontal surface  90  of center section  5  for hydraulic pump P. A cylinder block  16  is rotatably and slidably disposed on pump mounting surface  40 . Pistons  12  are fitted into a plurality of cylinder bores in cylinder block  16  and are reciprocally movable by biasing springs. The heads of pistons  12  abut against a thrust bearing  11   a  held to the movable swash plate  11 . At the center of movable swash plate  11 , an opening  11   b  is provided through which pump shaft  3  perforates. Pump shaft  3 , used also as an input shaft, is disposed on the rotary axis of cylinder block  16  and is fixed thereto as that pump shaft  3  and cylinder block  16  rotate together. Pump shaft  3  projects at the upper axial end thereof outwardly from the upper wall of upper half housing  1 . An input pulley  43  with a cooling fan  44  is fixed to pump shaft  3 . Input pulley  43  is given power from a prime mover (not shown) through a belt transmitting mechanism (not shown). 
     As seen in FIG. 6, the piston abutting surface of movable swash plate  11  is disposed perpendicular to the rotary axis of cylinder block  16 . Movable swash plate  11  is shown in the neutral position. Movable swash plate  11  can be tilted from side to side so as to enable the discharge amount and discharge direction of oil from hydraulic pump P to be changed. As seen in FIG. 4, for example, movable swash plate  11  is integrally provided with trunnions  35 L and  35 R, which project laterally from both sides of swash plate  11  and are disposed in parallel to axles  7 . Movable swash plate  11 , as shown in FIGS. 2 and 4, is slantingly rotatably supported between the two parallel walls of internal wall portion  1   a  in upper half housing  1  and the side wall of the upper half housing  1 . A recess  1   b  is bored in the side surface of internal wall portion  1   a . Recess  16  has an inner diameter about equal to the outer diameter of a bearing bush fitted on trunnion  35 L. As best seen in FIG. 4, trunnion  35 L is rotatably supported in recess  1   b . In order to bore recess  1   b  in internal wall portion  1   a , an opening  1   c  is formed in the side wall of upper half housing  1 . A machining tool for boring recess  1   b  is inserted into upper half housing  1  through opening  1   c.  A side plate  15  for closing opening  1   c  is detachably fixed onto the outer surface of the side wall of upper half housing  1  through sealing members (not shown). Trunnion  35 R extends into a hollow cylindrical portion integrally formed in side plate  15  so as to be rotatably supported therein. Movable swash plate  11  is longitudinally tilted around trunnions  35 L and  35 R within first chamber R 1 , enabling the output of hydraulic pump P to be changed. 
     At the outer surface of side plate  15 , a plurality of fins  15   a  (see FIG. 3) for receiving cooling wind from cooling fan  44  are disposed in the direction of the flow of the cooling wind. Wind blowing across fins  15   a  lowers the temperature of oil stored in first chamber R 1 . 
     The axial end of trunnion  35 R projects outwardly from side plate  15 . A control arm  38  (discussed below) is fixed onto the axial end and is connected through a link or wire (not shown), to a speed change lever mounted at the driver&#39;s seat of the vehicle, so as to rotate around the lateral axis of the vehicle body. This simplifies the transmitting mechanism for slantwise control of movable swash plate  11 . A neutral return coiled spring  31  is fitted onto trunnion  35 R in first chamber R 1 . Both ends of neutral return coiled spring  31  project forwardly between an engaging pin  39  and around an eccentric shaft  33  mounted onto the inner surface of side plate  15  (see FIG.  2 ). Engaging pin  39  projects from an arm  11   d  which projects forwardly from movable swash plate  11 . 
     Accordingly, when control arm  38  is rotated in order to change the speed of the vehicle, arm  11   d  rotates together therewith and one end of neutral return coiled spring  31  moves away from the other end toward engaging pin  39 . The other end of neutral return coiled spring  31  is retained by eccentric shaft  33  so as to apply a biasing force to control arm  38  which tends to return to the neutral position. When the operating force to the speed change lever is released, the restoring force created at one end of neutral return coiled spring  31  returns engaging pin  39  toward eccentric shaft  33  so as to be held in a neutral position. A portion of eccentric shaft  33  extending outwardly from side plate  15  is fixed thereto through an adjusting nut  33   a , which can be released to properly rotatably shift eccentric shaft  33 , thereby shifting arm  11   d  around trunnion  35 R through neutral return coiled spring  31 . This enables movable swash plate  11  to be adjusted to the accurate neutral position. 
     Control arm  38  is fixed to the end of trunnion  35 R which extends outside of the housing, as shown in FIG.  3 . Control arm  38  is substantially V-shaped, with a first retaining portion  38   a  and a second retaining portion  38   b . First retaining portion  38   a  projects upwardly to connect with a speed changing member such as a lever or pedal (not shown), and with trunnion  35 R when the speed change force is applied. Second retaining portion  38   b  projects slantwise rearwardly of the vehicle to connect with one end of a movable portion  73   a  of a shock absorber  73 . Shock absorber  73  and control arm  38  are formed to straddle right axle  7 . The base of a fixed portion  73   b  of shock absorber  73  is pivotally supported to a mounting pin  74   b . Mounting Pin  74   b  is mounted to the rear end of a support plate  74  fixed through mounting bolts  74   a  to the lower surface of a sleeve for right axle  7 . Thus, shock absorber  73  connects with control arm  38  so as to prevent a rapid speed change operation. Further, the operating force of the speed changing member, when released, does not rapidly return swash plate  11  to its neutral position, due to the spring force of neutral return coiled spring  31 . This prevents an abrupt stop of the vehicle caused by the braking action of the HST. 
     Because shock absorber  73  is disposed longitudinally along one side of the housing, it is not necessary to consider the height of input pulley  43  or an enlarged portion of the housing. A reasonable connection and arrangement is provided allowing control arm  38  to be swung along a lateral axis of the apparatus. 
     Pressure oil from hydraulic pump P is sent to hydraulic motor M through an oil passage in center section  5 . Hydraulic motor M, as shown in FIG. 5, is constructed so that a motor mounting surface  41  is formed along vertical surface  91  of center section  5 . A cylinder block  17  is rotatably slidably mounted to motor mounting surface  41 . A plurality of pistons  13  are reciprocally movably inserted into a plurality of cylinder bores in cylinder block  17  through biasing springs. A thrust bearing, held to a fixed swash plate  37 , abuts against the heads of pistons  13 . Fixed swash plate  37  is fixedly positioned between upper half housing  1  and lower half housing  2 . Motor shaft  4  is disposed on the rotary axis of cylinder block  17  and is fixed thereto so that motor shaft  4  and cylinder block  17  move together. One end of motor shaft  4  is supported in a shaft bore provided at the center of motor mounting surface  41  of center section  5 . The other end of motor shaft  4  perforates through internal wall  8 , formed at the joint surface of upper half housing  1  and lower half housing  2 , so as to enter into second chamber  2 . Motor shaft  4  is rotatably supported by a bearing  76  fitted into internal wall  8 . Bearing  76  includes an oil-tight seal in order to partition first chamber R 1  and second chamber R 2 . In particular, an O-ring  77  is provided on the outer periphery of bearing  76 . 
     A brake disc  19  is fixed to one axial end of motor shaft  4  positioned in second chamber R 2 . As shown in FIG. 9 a brake pad  98  is fitted into the inner surface of upper half housing  1  positioned at one side of the upper portion of brake disc  19 . At the opposite side of brake disc  19 , a brake operating shaft  97  is supported which perforates the wall of upper half housing  1  from the outside to the inside thereof through a support plate  92 . Brake pad  98  and the end surface of brake operating shaft  97  are opposite to each other. Brake disc  19  is sandwiched therebetween. Brake operating shaft  97  is supported in parallel to motor shaft  4 . A brake arm  93  is fixed to the end of brake operating shaft  97  outside of the housing. A spring  94  is fitted onto brake operating shaft  97  between brake arm  93  and support plate  92 , so as to bias the end surface of brake operating shaft  97  away from brake disc  19 . 
     A flange  97   a  is formed within the housing at one end of brake operating shaft  97 . A plurality (four in this embodiment) of groves  97   b  are provided at the surface of flange  97   a  facing the inner surface of the housing. Cam grooves  92   a , each V-shaped in cross-section and arcuate when viewed in plan are provided at the end surface of support plate  92 , opposite to grooves  97   b . As shown in FIG. 10, balls  95  are interposed between cam grooves  92   a  and grooves  97   b . In such construction, when brake arm  93  is rotated, brake operating shaft  97  rotates along its longitudinal axis. Balls  95 , held by recesses  97   b , slowly ride onto the shallowest portions of cam groove  92   a  from the deepest portions thereof. This causes brake operating shaft  97  to slidably move, due to the thrust generated thereon by balls  95 , toward brake disc  19  thereby sandwiching brake disc  19  between brake pad  98  and the end surface of brake operating shaft  97  so as to brake motor shaft  4 . Flanges  92   b , which extend radially and are V-shaped, are integrally provided at the outer end of support plate  92  (see FIG.  8 ). Elongate slots  92   c , which are oval-arcuate shaped are open in flanges  92   b  around brake operating shaft  97 . Bolts  96  are inserted into elongate slots  92   c , thereby fixing support plate  92  onto the outer surface of the side wall of upper half housing  1 . Bolts  96  may be unscrewed to properly rotate flanges  92   b  around brake operating shaft  97 , thereby enabling balls  95  to adjust the length of time during which balls  95  ride on cam groove  97   b.    
     Next, explanation will be given on the construction of center section  5  for loading thereon hydraulic pump P and hydraulic motor M in accordance with FIGS. 11 through 21. Center section  5  is longitudinally elongated and is provided at one side thereof with a bolt bore  5   h  and at another side thereof with two bolt bores  5   h . Three mounting bolts are inserted into bolt bores  5   h  and are used to fix center section  5  to the inner wall of upper half housing  1  in first chamber R 1 . At the center of pump mounting surface  40  and at the rear and upper surface of upper half housing  1  is formed a bearing bore for rotatably supporting the lower end of pump shaft  3 . A pair of arcuate ports  40   a  and  40   b  are open longitudinally through center section  5  around a bearing bore. Feed or discharge oil is communicated with cylinder block  16  through parts  40   a  and  40   b . At the center of motor mounting surface  41 , vertically disposed in front of pump mounting surface  40 , is bored a bearing bore for rotatably supporting one end of motor shaft  4 . A pair of arcuate ports  41   a  and  41   b  are open vertically and around the bearing bore, thereby communicating feed or discharge oil with cylinder block  17 . 
     In order to connect arcuate ports  40   a  and  40   b  at pump mounting surface  40  with arcuate ports  41   a  and  41   b  at motor mounting surface  41 , a first linear oil passage  5   a  and a second linear oil passage  5   b  are bored in a thick portion of center section  5 , in parallel to each other. As shown in FIG. 12, the center of pump mounting surface  40  is positioned along an imaginary vertical plane (line  16 — 16 ) disposed along motor mounting surface  41 . Half of cylinder block  16  mounted on pump mounting surface  40  (as shown in FIG. 2) overlaps, when viewed from above, with half of cylinder block  17  disposed on motor mounting surface  41 . This arrangement permits the HST and first chamber R 1  which contains the HST to be smaller in lateral width. A third linear oil passage  5   c  communicates horizontally and perpendicularly with an intermediate portion of second oil passage  5   b . Arcuate port  40   a  at pump mounting surface  40 , as shown in FIG. 18, is shallow and directly communicates with first oil passage  5   a.  Arcuate port  40   b  is deeper to communicate with third oil passage  5   c . Arcuate port  41   a  at motor mounting surface  41  is deeper at the upper portion thereof to communicate with first oil passage  5   a  and shallow at the lower portion thereof, as shown in FIGS. 16 and 17. Arcuate port  41   b  communicates, at the lower portion thereof, with second oil passage  5   b . Thus, first oil passage  5   a  communicates with arcuate port  40   a  and with arcuate port  41   a , while second oil passage  5   b  and third oil passage  5   c  communicate with arcuate port  40   b  and with arcuate port  41   b , so as to form a closed fluid circuit in center section  5 . 
     With reference to FIG. 17, check valves  54  and  55  are disposed at the open ends of first oil passage  5   a  and second oil passage  5   b  respectively. Both first oil passage  5   a  and second oil passage  5   b  are closed by plug members  64   a  in which check valves  54  and  55  are disposed, respectively. The open end of third oil passage  5   c  is closed by a plug member  64   b . Check valves  54  and  55  communicate at the inlet sides thereof with oil passage  5   d  through oil bores  54   b  and  55   b  provided at casings  54   a  and  55   a . The open end of oil passage  5   d  is positioned in a recess  5   g  formed at the lower surface of center section  5 . At the lower surface of center section  5 , opposite to pump mounting surface  40 , a charge pump casing  46  is mounted through a plurality of mounting bolts  69 . A trochoid-type charge pump  45  is housed (see FIG. 4) in a recess formed at a center of the upper surface of charge pump casing  46 . Trochoid-type charge pump  45  is provided with internal teeth and external teeth. The lower end of pump shaft  3  projects downwardly from center section  5  and engages with the external teeth so as to drive charge pump  45 . Charge pump  45 , however, may be of an external gear type or other known type. 
     As seen in FIGS. 18 and 19, charge pump  45  has a discharge port  45   a  and an intake port  45   b . Intake port  45   b  communicates with an opening  46   b  (FIG. 17) into which the open end of a cylindrical oil filter  56  is inserted (see FIGS.  5  and  6 ). Oil filter  56  is disposed under hydraulic motor M in first chamber R 1 . Oil filter  56  is insertable into charge pump casing  46  which is in the housing from the exterior thereof through an insertion bore open at the front wall of lower half housing  2 . Oil filter  56  is interposed between charge pump casing  46  and a plug member  48  which closes the insertion bore at the front wall of lower half housing  2 . Plug member  48  can be removed to facilitate maintenance and inspection of oil filter  56 . A pair of oil joints  49  and  50  project from the a side surface of charge pump casing  46  (FIG.  13 ). The ends of joints  49  and  50 , as shown in FIG. 3, are exposed at a lower portion of the outside surface of lower half housing  2 . Oil joints  49  and  50  function as an oil pressure source for hydraulic actuators equipped outside of the vehicle. 
     Oil joint  50  is formed to serve as an oil takeout port and communicates with discharge port  45   a  of charge pump  45  through an oil passage  46   a  as shown in FIG. 13. A first relief valve  57 , for setting the oil pressure of discharge port  45   a , is housed in charge pump casing  46  and is connected to an oil passage  46   c  which is branched from oil passage  46   a . Relief oil discharged from first relief valve  57  flows into recess  5   g  at the lower surface of center section  5  through oil passage  46   c . Oil joint  49  is formed to be an oil return port and to communicate with recess  59  of center section  5  through oil passages  46   d  and  46   e . A second relief valve  58  for setting the oil pressure in recess  5   g  to be supplied to the closed circuit of the HST is housed in charge pump casing  46  and connects with recess  5   g  through an oil passage  46   f . Relief oil discharged from second relief valve  58  is discharged outwardly from charge pump casing  46  through an oil passage  46   g.    
     As seen in FIG. 17, when charge pump  45  is driven, oil flowing into recess  5   g  through the oil passage  46   c  is adjusted by second relief valve  58 . This causes check valve  54  or  55  to open through oil passage  5   d  at the low pressure side of one of oil passages  5   a ,  5   b  or  5   c , thereby forcibly supplying operating oil into the closed fluid circuit for the HST. 
     When the vehicle is stopped on a sloping surface, and the HST is put in the neutral position without the parking brake exerted, the force causing the driving wheels of the vehicle to roll acts on the closed fluid circuit of the HST to generate pressure so as to cause negative pressure in the closed fluid circuit and possibly causing the vehicle to move. In order to prevent such a phenomenon, a check valve  47  (see FIG. 15) is housed in charge pump casing  46  which can supply operating oil to the closed fluid circuit of the HST even when charge pump  45  is not driven. Check valve  47  communicates at the inlet side thereof with intake port  45   b  through an oil passage  46   h  and at the outlet side with recess  5   g  through an oil passage  46   i . When charge pump  45  is driven to flow operating oil into recess  5   g  though oil passages  46   c  and  46   e , check valve  47  closes between oil passage  46   h  and oil passage  46   i . When charge pump  45  is not driven, causing negative pressure on the low pressure side of the closed circuit, check valve  47  is open to enable oil filtered by filter  56  to be guided from intake port  45   b  and oil passages  46   h  and  46   i  into recess  5   g . Check valve  54  or  55 , at the negative pressure side of the closed fluid circuit, is open through oil passage  5   d , whereby oil is supplied to the closed fluid circuit. Thus, oil can be maintained in the closed fluid circuit at all times. 
     In order to place operating oil into the closed fluid circuit after the axle driving apparatus is assembled, oiling pipes  52  and  53  are disposed at the lower surface of center section  5  as shown in FIGS. 11,  15 ,  17  and  20 . At the lower surface of center section  5 , a fourth vertical passage  5   e  is bored to communicate with the upper deep portion of arcuate port  41   a . A fifth vertical oil passage  5   f  is bored to communicate with second oil passage  5   b . Oiling pipes  52  and  53  are mounted into oil passages  5   e  and  5   f  respectively and are opened at the lower ends thereof outwardly from the bottom wall of lower half housing  2  and closed at the open ends by use of plug members after the closed fluid circuit is filled with operating oil. 
     As shown in FIGS. 2 and 5, a by-pass arm  60  for opening the interior of the closed circuit to the oil sump, in order to enable the axle to be idle during hauling of the vehicle, is disposed in the upper portion of upper half housing  1 . In particular, by-pass arm  60  is fixed at its base onto the upper end of a by-pass shaft  61 , which is vertically, rotatably and pivotally supported to the upper wall of upper half housing  1 . By-pass shaft  61  extends at its lower end into a thick portion of motor mounting portion  41  of center section  5 . A flat surface  61   a  is formed at a part of the outer periphery of the lower end of by-pass shaft  61 . A through-bore  5   i  (see FIG. 11) is open at motor mounting surface  41  of center section  5  slightly above the center thereof and between arcuate port  41   a  and  41   b . A push pin  62  (see FIG. 5) is slidably supported into through-bore  5   i  along the rotary axis of cylinder block  17 . One end surface of push pin  62  can abut against the rotary sliding surface of cylinder block  17  in close contact with the motor mounting surface  41 . The other end surface abuts against flat surface  61   a  of by-pass shaft  61 . 
     Thus, when an operator operates a by-pass operating lever (not shown) equipped on the vehicle when the vehicle is hauled, by-pass shaft  61  is rotated through by-pass arm  60 . Push pin  62  is pushed toward cylinder block  17  by the flat surface of the lower end of by-pass shaft  61 . Push pin  62  moves the cylinder block  17  above motor mounting surface  41 . First oil passage  5   a  and second oil passage  5   b  communicate with the oil sump of the housing through arcuate ports  41   a  and  41   b  respectively, thereby enabling motor shaft  4  to freely rotate. 
     As shown in FIGS. 2 and 7, the drive train for transmitting power from motor shaft  4  to differential gear  23  is constructed with a gear  25  provided on a portion of motor shaft  4  entering into second chamber R 2 , for engaging with a larger diameter gear  24 , fixed onto a counter shaft  26 . A smaller diameter gear  21  is also fixed onto counter shaft  26  and engages with an input gear  22 . Power from motor shaft  4  is reduced in speed by gears  25 ,  24  and  21  to drive differential gear unit  23  by input gear  22 . Larger diameter gear  24 , on counter shaft  26 , is disposed to the side of input gear  22  and overlaps in part therewith. Counter shaft  26  is rotatably housed in lower half housing  2  and is supported at both axial ends in a recess formed on the side wall of lower half housing  2  and a recess formed on the internal wall  2   a  of lower half housing  2 , as shown in FIG. 2, so as to be rotatably supported when lower half housing  2  is joined with upper half housing  1 . 
     As best seen in FIGS. 2 and 22, the distal ends of axles  7  are rotatably supported by ball bearings in axle housing portions projecting from upper half housing  1 . The proximate end of each axles  7  is sleeved by a bearing bush. One half of each bearing bush is received in a recess in upper half housing  1 . The other half is received by a projection of lower half housing  2  which enters into upper half housing  1 . Axles  7  are rotatably supported to receive power transmitted through differential gear  23 . As shown in FIG. 2, the HST is disposed to the right side of the drive train. A control arm  38  for movable swash plate  11  is disposed to the right side of the HST. Hydraulic pump P is positioned substantially at the lateral and longitudinal center of the apparatus and is disposed so as to avoid the enlarged portion of differential gear  23 . This enables the housing to be compact. 
     Differential gear unit  23  is shown in FIGS. 22 through 25. As seen in FIG. 24, the center of input gear  22  has a shaft bore  22   a  for receiving axles  7  therein. Bores  22   b  for receiving differential pinions  80  and fitting-in bores  22   a  for receiving the differential locking device are disposed at both sides of input gear  22 . Spline-fitted bevel-type output gears  81 L and  81 R are disposed at the proximate end of axles  7 . Spindles  80   a  of the bevel-type differential pinions  80  are retained in bores  22   b  of input gear  22  in which differential pinions  80  are also housed. Differential pinions  80  engage with output gears  81 L and  81 R so as to form differential gear unit  23 . No differential casing is otherwise provided. The differential locking device is provided opposite to the drive train at one side (preferably the right side) of differential gear  23  unit. 
     Between output gear  81 R and the proximate end of right axle  7  is interposed a collar  83  on which a slider  82  is axially slidably fitted. Slider  82  is cup-like shaped to wrap around output gear  81 R. At the outer peripheral side surface of slider  82 , projections  82   a  are integrally provided. Projections  82   a  are permanently engageable with insertion bores  22   c  of input gear  22 . At the inner peripheral side surface of slider  82  are formed a plurality of projections  82   b  which are engageable with a plurality of recesses  81   a  formed in the outer periphery of output gear  81 R. An insertion groove  82   c  is formed on the cylindrical portion of slider  82  opposite to projections  82   a , so as to fit the tip of a fork  84  into groove  82 . The base of fork  84  is slidably fitted onto a shaft  85  which is journalled to both side walls in lower half housing  2 . At the side surface of the base of fork  84  is formed a cam surface  84   a , which abuts against a pin  87  provided on shaft  85  so as to constitute a cam mechanism. An arm  86  is fixed to shaft  85 . Arm  86  projects outwardly from the housing so as to connect with a differential locking pedal (not shown) provided on the vehicle. 
     In such construction, when the operator presses the differential locking pedal, shaft  85  rotates through arm  86 , and pin  87  rotates to push to the right in the drawing of FIG.  22 . As a result, cam surface  84   a  abuts against pin  87  so as to slidably move fork  84 . At the same time, slider  82  slides, while maintaining projections  82   a  in insertion bores  22   c  of ring gear  22 . Projections  82   b  engage with recesses  81   a  of output gear  81 R and input gear  22  is differentially locked and coupled with axles  7 . As a result, axles  7  are uniformly driven when the vehicle runs on any road surface. 
     The axle driving apparatus of the present invention can be used for driving the axles of a vehicle to improve the operability of changing the speed of the vehicle. An example of a moving vehicle which may utilize the above-mentioned axle driving apparatus is a farm or other working vehicle, such as a tractor with a mower attachment, or other vehicle for transportation. 
     While one embodiment of the present invention has been shown and described, the invention should not be limited to the specific construction thereof, and is meant to be merely exemplary.