Abstract:
An articulated vehicle with a working device has a first frame having a prime mover mounted thereon and supporting a first transaxle apparatus. The first transaxle apparatus includes an input shalt receiving power from the prime mover, a pair of first axles, and a hydrostatic transmission. The hydrostatic transmission comprises a variable hydraulic pump, a first hydraulic motor fluidly connected to the hydraulic pump via a fluid passage, and a housing with a port fluidly connected to the fluid passage. The second transaxle apparatus includes a pair of second axles having different lengths and a second hydraulic motor. The second hydraulic motor is fluidly connected to the port. Proximal ends of the first and second frames with respect to the vehicle are coupled to each other so that the first anid second flames are rotatable around a vertical axis relative to each other when steered.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a transaxle apparatus having a housing which incorporates a hydrostatic transmission (HST) and a hydraulic actuator arranged outside the housing which can be supplied with hydraulic fluid from the HST. Furthermore, the invention relates to a four-wheel-drive vehicle having first axles, which axles are supported by the housing driven by the HST, and disposed at one of front and rear positions of the vehicle, and second axles, which axles are driven by a hydraulic motor serving as the hydraulic actuator and disposed outside of the housing and the other of front and rear portions of the vehicle. More particularly, the invention relates to a four-wheel-drive articulated working vehicle.  
           [0003]    2. Related Art  
           [0004]    A well-known articulated riding lawn mower has first and second frames which are mutually pivotally coupled at proximal ends thereof so as to turn relatively to each other around a vertically axial pivot steering operation (i.e., manipulation of a steering wheel). The first frame is equipped with a prime mover and a transaxle apparatus which supports first axles driven by power from the prime mover. The second frame is equipped with a working device such as a mower device, an operating section, and an axle casing that supports second axles freely rotatably.  
           [0005]    In the Japanese Patent Laid Open Gazette 2000-270651, for example, is disclosed an articulated four-wheeled lawn mower, which includes as the first frame a rear frame and as the second frame a front frame. On the rear frame, a hydrostatic transmission (hereinafter, “HST”) is disposed, which transfers engine power to rear wheels supported by the rear frame. Moreover, in the rear frame is disposed a power take-off shaft, which receives power from a pump shaft of a hydraulic pump of the HST. The pump shaft revolves synchronously to the engine power output revolution. The revolution of the pump shaft is transferred to the mower device supported by the front frame.  
           [0006]    Generally, as to each of vehicles having the above structure, while the first axles supported by the transaxle apparatus of the first frame (usually serving as a rear frame) is driven by the prime mover, the second axles supported by the axle casing of the second frame (usually serving as a front frame) revolve freely and not in driving association with the power for driving the axles of the first frame. Thus, the vehicle is a so-called two-wheel drive vehicle.  
           [0007]    However, while the two-wheel-drive vehicle which drives only rear wheels exhibits superior steering performance, it lacks stability when working on a slope and roadability when running on a bad road. Further, if the vehicle is an articulated vehicle, the steering performance must be improved because the vehicle is bent at the coupling part of the frames. Moreover, the vehicle is difficult to bail out if it becomes stuck, such as in mud, etc.  
           [0008]    For solving these problems, a four-wheel-drive design, which drives both front and rear wheels, is desirable for the articulated vehicle. The rear frame of the vehicle disclosed in the above document is provided with an HST and a power take-off shaft for transferring power to the working device.  
           [0009]    However, as mentioned above, since the power take-off shaft revolves synchronously with the revolution of the pump shaft, the rotary speed of the pump shaft is fixed as long as the engine speed is fixed. On the other hand, the rotary speed of the rear wheels, which are driven by the power output of the hydraulic motor, is changed variably by a running speed changing operation which adjusts the angle of a movable swash plate of the hydraulic pump. Therefore, the power take-off shaft for driving the working device cannot be used as a front wheel drive shaft. Even if another power take-off shaft for front-wheel-drive, whose rotation is synchronized with the power output of the HST for rear wheel drive, can be connected to the transaxle apparatus mounted in the rear frame, severe limitations exist for such an arrangement to infix additional mechanical transmission system between front and rear transaxle apparatuses, because the turning of front and rear frames must be permitted, as well as in fixing the transmission system for the working device drive therebetween.  
         SUMMARY OF THE INVENTION  
         [0010]    A first object of the present invention is to provide on an articulated vehicle a transaxle apparatus for making the articulated vehicle a four-wheel-drive articulated working vehicle. The transaxle apparatus includes a housing containing an HST and is enabled to supply hydraulic fluid from the HST to a hydraulic actuator arranged outside of the housing.  
           [0011]    To achieve the first object, according to the transaxle apparatus of the present invention, a housing, containing an HST is provided. The HST comprises a hydraulic pump receiving power from the prime mover, a hydraulic motor driven in response to fluid from the hydraulic pump to drive first axles, and a center section. In the center section are provided fluid passages, which are disposed in the housing so as to bring the hydraulic pump and the hydraulic motor into mutual fluidal connection. Also disposed in the center section are ports, which are located on an outer surface of the housing and fluidly connected with the fluid passages so as to introduce fluid flowing in the fluid passages into a hydraulic actuator disposed outside the housing. An axle driven by the hydraulic motor is disposed in the housing.  
           [0012]    The hydraulic actuator may comprise a hydraulic motor for driving a second axle disposed outside the housing so as to constitute a foul-wheel-drive vehicle.  
           [0013]    The center section is detachably attached to the housing, thereby advantageously facilitating its manufacture and preventing fluid from leaking from the fluid passages to the outside of the housing.  
           [0014]    The ports are equipped with tubular elements for supplying pressurized fluid (hydraulic fluid) to the hydraulic actuator (the hydraulic motor) arranged outside the housing. The housing is equipped with openings for exposing the utmost ends of the tubular elements outside the housing. Furthermore, the tubular elements are detachably attached to the center section.  
           [0015]    Accordingly, flexibility of the arrangement of the elements for supplying pressurized hydraulic fluid from the center section in the housing to the outside of the housing in relation to other components (for example, means for transmitting power from the prime mover to a working device) arranged between the first and second frames can be enhanced. Moreover, inexpensive parts such as a fluid hose can be used for the tubular elements. Since the tubular elements are easily detached, they facilitate maintenance.  
           [0016]    Furthermore, removal of the tubular elements can change the vehicle into two-wheel-drive vehicle.  
           [0017]    Moreover, the above-mentioned ports of the transaxle apparatus fluidly connect in parallel the hydraulic motor in the housing and the hydraulic motor, outside of the housing to the hydraulic pump in the housing. This structure is suitable for a vehicle which is designed so that when the vehicle turns, distances from a turning, center of the vehicle to the front and rear axles, namely, to the first axle in the housing and the second axle out of the housing, are different from each other so as to cause a rotary speed difference between the front and rear axles. In this structure, pressurized hydraulic fluid discharged from the hydraulic pump is distributed to both of the hydraulic motors, inside and outside of the housing, in correspondence to the rotary speed difference between the axles.  
           [0018]    Alternatively, the ports of the transaxle apparatus may fluidly connect in series the hydraulic motor in the housing and the hydraulic motor outside of the housing to the hydraulic pump in the housing. This structure is suitable for a vehicle designed so that, when the vehicle turns, distances from the turning center of the vehicle to the front and rear axles, namely, to the first axle in the housing and the second axle outside of the housing, are substantially equal to each other so as not to cause a rotary speed difference between the front and rear axles. According to the series connection structure compared with the above-mentioned parallel connection structure, the entire amount of fluid discharged from the hydraulic pump is supplied to the hydraulic motor in the housing and the hydraulic motor outside of the housing as long as the hydraulic pump is revolving. Thus, even if either of the front or rear wheels gets stuck, as in mud, etc., and the front or rear axle driven by one of the hydraulic motors idles, the other hydraulic motor drives the other axle using all of the fluid discharged by the hydraulic pump, and the vehicle can be freed.  
           [0019]    A second object of the present invention is to provide a four-wheel-drive articulated working vehicle with the above-mentioned transaxle apparatus, including first and second frames, each of which has opposite proximal and distal ends with respect to the vehicle. The first and second frames are coupled mutually at the proximal ends thereof so as to be rotated in relation to each other around a vertically axial pivot in the coupling part therebetween by a steering operation. A prime mover is mounted on the first frame, and a working device is attached to the distal end of the second frame.  
           [0020]    To achieve the second object, according to tile vehicle of the present invention, the transaxle apparatus including the HST for supporting and driving a pair of first axles serves as a first transaxle apparatus supported by the first frame on which the prime mover is mounted. The hydraulic motor disposed in the housing of the first transaxle apparatus serves as a first hydraulic motor. A second transaxle apparatus with a second hydraulic motor, which supports and drives a pair of second axles, is supported by the second frame provided on the distal end thereof with the working device. The second hydraulic motor is fluidly connected to the above-mentioned ports of the center section of the HST disposed in the first transaxle apparatus. As means for receiving power from the prime mover a rotor is disposed at the junction between the first and second frames so as to locate a rotation axis of the rotor on the vertical axial pivot. The lengths of the pair of second axles are different from each other, and a transmission element for drivingly connecting the prime mover to the working device crosses the longer the pair of second axles.  
           [0021]    Due to the above structures, fluid connection of the HST of the first transaxle apparatus to the second hydraulic motor can be ensured without interfering with the transmission system from the prime mover to the working device, thereby realizing a four-wheel-drive articulated working vehicle.  
           [0022]    Moreover, the four-wheel-drive articulated working vehicle is designed so that distances from the vertically axial pivot in the coupling part to an axis of the first axles and to an axis of the second axles are substantially equal to each other. The vehicle can be simplified by applying the series fluid connection as the fluid connection of the first and second hydraulic motors through the ports to the hydraulic pump. Accordingly the entire amount of fluid discharged from the hydraulic pump is supplied to each of the first and second hydraulic motors as long as the hydraulic pump is revolving. Thus, even if one of the drive wheels gets stuck, as in mud, etc., and either the first or second axles driven by one of the hydraulic motors idles, the other hydraulic motor drives the other axles using the entire amount of fluid discharged from the hydraulic pump so that the vehicle can be freed.  
           [0023]    These and other objects, features, and advantages of the invention will become more apparent upon a reading of the following detailed description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0024]    [0024]FIG. 1 is a side view of a riding lawn mover as an embodiment of a four-wheel-drive articulated working vehicle according to the present invention.  
         [0025]    [0025]FIG. 2 is a plan view partly in section of the vehicle of FIG. 1.  
         [0026]    [0026]FIG. 3 is a rear view partly in section of a front transaxle apparatus provided in the vehicle of FIG. 1.  
         [0027]    [0027]FIG. 4 is a plan view of the front transaxle apparatus of the present invention from which an upper housing half is removed.  
         [0028]    [0028]FIG. 5 is a fragmentary rear view partly in section of the front transaxle apparatus of the present inventions showing a hydraulic motor disposed therein.  
         [0029]    [0029]FIG. 6 is a sectional left side view of the front transaxle apparatus of the present invention.  
         [0030]    [0030]FIG. 7 is a right side view of a rear transaxle apparatus.  
         [0031]    [0031]FIG. 8 is a plan view partly in section of the rear transaxle apparatus according to the first embodiment of the present invention from which an upper housing half is removed, showing that a center section having ports for series connection is disposed therein.  
         [0032]    [0032]FIG. 9 is a rear view partly in section of the real transaxle apparatus according to the first embodiment.  
         [0033]    [0033]FIG. 10 is a fragmentary sectional plan view of the rear transaxle apparatus according to the first embodiment, showing the fluid passage structure formed in the center section disposed therein.  
         [0034]    [0034]FIG. 11 is a fragmentary sectional side view of the rear transaxle apparatus according to the first embodiment of the present invention.  
         [0035]    [0035]FIG. 12 is a hydraulic circuit diagram showing the hydraulic motor of the rear transaxle apparatus according to the first embodiment of the present invention and the hydraulic motor of the front transaxle apparatus are fluidly connected to the hydraulic pump of the rear transaxle apparatus in series.  
         [0036]    [0036]FIG. 13 is a hydraulic circuit diagram of the motor of FIG. 12, showing a case where the hydraulic motor of the front transaxle apparatus is exchanged for a variable displacement type.  
         [0037]    [0037]FIG. 14 is a plan view partly in section of a rear transaxle apparatus according to a second embodiment of the present invention from which an upper housing half is removed, showing that a center section having ports for parallel connection is disposed therein.  
         [0038]    [0038]FIG. 15 is a rear view partly in section of a portion of the rear transaxle apparatus according to the second embodiment where a third passage is passed.  
         [0039]    [0039]FIG. 16 is a rear view partly in section of another portion of the transaxle of FIG. 15 where a fourth passage is passed.  
         [0040]    [0040]FIG. 17 is a fragmentary sectional plan view of the rear transaxle apparatus according to the second embodiment, showing fluid passage structure formed in the center section.  
         [0041]    [0041]FIG. 18 is a fragmentary sectional side view of the rear transaxle apparatus according to the second embodiment.  
         [0042]    [0042]FIG. 19 is a hydraulic circuit diagram showing the hydraulic motor of the rear transaxle apparatus according to the second embodiment and the hydraulic motor of the front transaxle apparatus are fluidly connected in parallel to the hydraulic pump of the rear transaxle apparatus.  
         [0043]    [0043]FIG. 20 is a plan view partly in section of a four-wheel-drive articulated working vehicle in which front transaxle apparatuses having respective hydraulic motors are provided to right and left front wheels, respectively.  
         [0044]    [0044]FIG. 21 is a rear view partly in section of the right and left front transaxle apparatuses provided to the working vehicle.  
         [0045]    [0045]FIG. 22 is a plan view partly in section of the front transaxle apparatuses  400 R ( 400 L) provided to the working vehicle.  
         [0046]    [0046]FIG. 23 is a hydraulic circuit diagram showing that a hydraulic motor the rear transaxle apparatus according to the first embodiment of the present invention and a circuit which fluidly connects hydraulic motors of both the front transaxle apparatuses to each other in parallel are fluidly connected in series to the hydraulic pump to the rear transaxle apparatus.  
         [0047]    [0047]FIG. 24 is a hydraulic circuit diagram of the present invention in a case that variable displacement hydraulic motors serve as both the hydraulic motors.  
         [0048]    [0048]FIG. 25 is a hydraulic circuit diagram showing the hydraulic motor of the rear transaxle apparatus according to the first embodiment and the hydraulic motors of both the front transaxle apparatuses are fluidly connected in series to the hydraulic pump of the rear transaxle apparatus.  
         [0049]    [0049]FIG. 26 is a hydraulic circuit diagram showing the hydraulic motor of the rear transaxle apparatus according to the second embodiment of the present invention and the hydraulic motors of both the front transaxle apparatuses are fluidly connected in parallel to the hydraulic pump of the rear transaxle apparatus.  
         [0050]    [0050]FIG. 27 is a hydraulic circuit diagram showing the hydraulic motor of the rear transaxle apparatus according to the second embodiment of the present invention and a circuit, which fluidly connects in series the hydraulic motors of both the front transaxle apparatuses to each other, are fluidly connected in parallel to the hydraulic pump of the rear transaxle apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    Description will be given of a four-wheel-drive articulated working vehicle according to the present invention.  
         [0052]    [0052]FIGS. 1 and 2 show a working vehicle equipped at a front portion thereof with a mower device  3  serving as a working device. A front frame  11  is provided with a front transaxle apparatus from which front wheel axles  12 L and  12 R are extended in a transverse direction and fixed to respective front wheels  13 . A rear frame  21  is provided with a rear transaxle apparatus from which rear wheel axles  22 L and  22 R are extended in a transverse direction and fixed to respective rear wheels  23 .  
         [0053]    A rear end portion of the front frame  11  is horizontally rotatably coupled to a front end portion of the rear flame  21  through a coupling part  50 . Coupling part  50  constitutes a pivot point of rotation of both the frames. Thus, the working vehicle including the horizontally turnable front and rear frames  11  and  21  is bendable at the intermediate portion thereof, thereby being a so-called articulated vehicle.  
         [0054]    A steering column  14 , a steering wheel  4 , and a pedal  15  are arranged in a front portion of front frame  11 , and a seat  9  is disposed behind steering column  14 , thereby constituting an operation part  16  on front frame  11 . Mower device  3  is vertically movably provided at a distal end of front frame  11 , that is, at a downwardly forward position from operation part  16 . Mower device  3  is driven by an engine  5 .  
         [0055]    As shown in FIGS. 1 and 2, on rear frame  21  is disposed engine  5  covered with a bonnet  8 . A rear transaxle apparatus is arranged under engine  5 .  
         [0056]    On the rear frame  21  end, a first engine output pulley  94  is fixed to an output shaft  93  of engine  5 , an HST input pulley  292  is fixed to a pump shaft  231  of a hydraulic pump incorporated in the rear transaxle apparatus, and a second engine output pulley  96  (shown in FIG. 1) is fixed to output shaft  93  under first engine output pulley  94 .  
         [0057]    On the front frame  11  end, a working device drive power input pulley  111  is fixed to a power input shaft  112  of mower device  3  as a working device, and an idle pulley  98  is rotatably supported through a hearing (not shown) on a support shaft  97  suspended from front frame  11 .  
         [0058]    Moreover, as shown in FIGS. 1 and 2, regarding coupling part  50 , a cylindrical pivotal connector  28  is disposed on the laterally middle front end of rear frame  21  and not-relatively rotatably supports a joint shaft  55  in the vertical direction. A platy pivotal connector  18 , which is U-shaped in side view, is pivotally coupled to joint shaft  55 . Thus, rear frame  21  and front frame  11  are pivotally coupled so as to be horizontally turnable. In this way, pivotal connectors  18  and  28  are provided at the respective proximal ends of frames  11  and  21 , each of which faces to the proximal side of the vehicle, and are pivotally connected to each other through joint shaft  55  so as to constitute coupling part  50 . Thus, both frames  11  and  21  are arranged in tandem and coupled so as to be turnable around joint shaft  55 , thereby enabling the vehicle to be steered.  
         [0059]    A lower end of joint shaft  55  is extended below pivotal connector  18  so as to support a power output pulley  57  and a power input pulley  56  rotatably thereon through bearings (not shown).  
         [0060]    As shown in FIGS. 1 and 2, on rear frame  21  end, a rear drive transmission belt  92  is wound around the first engine output pulley  94  and HST input pulley  292  and a first working-device drive transmission belt  58  is wound around the second engine output pulley  96  and power input pulley  56 .  
         [0061]    On front frame  11  end, second working-device drive transmission belt  59  is wound around an idle pulley  98  (FIG. 2), a working-device drive power input pulley  111 , and power output pulley  57 .  
         [0062]    Due to this construction, engine output is transmitted to HST input pulley  292  through rear drive transmission belt  92  from first engine output pulley  94  so as to rotate pump shaft  231 . The engine output is also transmitted to working-device drive power input pulley  111  through second engine output pulley  96 , first working-device drive transmission belt  58 , power input pulley  56 , power output pulley  57  integrally rotating power input pulley  56 , and second working-device drive transmission belt  59 , so as to rotate a power input axis  112 , thereby rotating mowing blades  17 .  
         [0063]    As shown in FIG. 2, at a position shifted leftward from lateral middle of front frame  11  is disposed front transaxle apparatus, which supports left and right front wheel axles  12 R and  12 L so as to extend right front wheel axle  12 R longer than left wheel axle l  21 .  
         [0064]    As shown in FIGS. 2 and 3, a pair of left and right collars  99   a  and  99   b  are freely rotatably on right front wheel axle  12 R at a substantially laterally middle position of front frame  11 . The lower surfaces of second working-device drive transmission belt  59  comes into contact with the respective upper surfaces of collars  99   a  and  99   b.    
         [0065]    Hence, front transaxle apparatus supports the pair of axles whose lengths are different from each other and second working-device drive transmission belt  59 , serving as a transmission element for drivingly connecting engine  5  and mower device  3  to each other, crosses the longer axle of the pair; in other words, second working-device drive transmission belt  59  changes direction by contacting collars  99   a  and  99   b  on the longer axle.  
         [0066]    In this way, second working-device drive transmission belt  59  passes above front wheel axle  12 R so as not reduce the road clearance. Moreover, since collars  99   a  and  99   b  are idled, second working-device drive transmission belt  59  is not damaged by friction.  
         [0067]    Next, description will be given of the front transaxle apparatus. As shown in FIG. 6, an upper housing half  46  and a lower housing half  47  are vertically joined to each other so as to form one housing, which provides the external appearance of the front transaxle apparatus  10  and contains within its interior a fluid sump and for incorporating the hydraulic motor, etc.  
         [0068]    As shown in FIG. 4, a counter shaft  139 , on which a reduction-gear train  135  is freely provided divides the hollow interior of the housing into a first chamber  10   a,  which incorporates a differential gear unit  120 , and a second chamber  10   b,  which incorporates a hydraulic motor  40 . Driving force of hydraulic motor is transmitted to differential gear unit  120  through reduction-gear train  135 .  
         [0069]    As shown in FIG. 5, hydraulic motor is integrally disposed within front transaxle apparatus. On a vertical portion of center section  62  is formed a motor mounting surface  63   m  (shown in FIG. 16) on which a cylinder block  43  is rotatably and slidably supported. A plurality of pistons  42  are reciprocally movably fitted through respective biasing springs into a plurality of cylinder bores in cylinder block  43 . A thrust bearing  44   a  of a fixed swash plate  44  abuts against the heads of pistons  42 . An opening  44   b  is provided in the center of fixed swash plate  44  so as to allow motor shaft  41  to pass therethrough. Fixed swash plate  44  is fixedly sandwiched between upper housing half  46  and lower housing half  47 .  
         [0070]    Motor shaft  41  is rotatably supported by a sealed bearing  45  held on the joint surface between upper housing half  46  and lower housing half  47 . Motor shaft  41  is not-relatively rotatably engaged with cylinder block  43  so as to be disposed horizontally on the rotary axis of cylinder block  43  and serve as an output shaft.  
         [0071]    In this way, front transaxle apparatus  10  contains an axial piston type hydraulic motor  40 .  
         [0072]    Moreover, as shown in FIG. 6, a pair of first and second kidney-ports  62   a  and  62   b  are formed in motor mounting surface  63   m  formed on the vertical portion of center section  62 . First and second kidney-ports  62   a  and  62   b  are connected to, respectively, horizontal first and second fluid passages  53   a  and  53   b  bored within center section  62 . As shown in FIG. 4, first fluid passage  53   a  and second fluid passage  53   b  are connected to respective caps  54   a  and  54   b  to which hydraulic hoses are connected. Thus, hydraulic motor  40  is fluidly connected to hydraulic pump  30  through hydraulic hoses (not shown).  
         [0073]    As shown in FIG. 5, a bypass operation lever  65  for opening first fluid passage  53   a  and second fluid passage  53   b  to the fluid sump is disposed above upper housing half  46  in order to enable the axles ( 12 L and  12 R) to idle when the vehicle is towed. Bypass operation lever  65  is fixed at a basal portion thereof to an upper end of a vertical bypass lever shaft  66  rotatably supported by an upper wall of upper housing half  46 . Bypass lever shaft  66  extends at a lower end thereof to the interior of center section  62  so as to be horizontally slidably in center section  62 . A flat surface  66   a  is formed in a lower end side of bypass lever shaft  66  so as to contact an end face of a push pin  67  which is allowed to contact the rotationally sliding surface of cylinder block  43 .  
         [0074]    As shown in FIG. 6, a feeding-and-discarding port  46   a  is formed in the upper portion of upper housing half  46  so as to enable hydraulic fluid to be fed or discharged from and to a reservoir tank (not shown).  
         [0075]    As shown in FIGS. 4 and 5, a drive output gear  131  is fitted with spline onto an end of motor axis  41  opposite to center section  62  so as to be rotated integrally with motor shaft  41 . On the side of drive output gear  131  facing section  62  is integrally formed a brake rotor  133  whose diameter is larger than that of drive output gear  131 . Brake rotor  133  is sandwiched between brake pads  134   a  and  134   b  (FIG. 4) so as to brake rotating motor shaft  41 .  
         [0076]    As shown in FIG. 4, a counter shaft  139  is arranged parallel to motor shaft  41 , a wide, small diameter gear  137  fits loosely on counter shaft  139 , and a large diameter gear  136  is engaged with a toothed side portion of small diameter gear  137 , thereby forming reduction-gear train  135 .  
         [0077]    Regarding reduction-gear train  135 , while large diameter gear  136  engages with drive output gear  131 , small diameter gear  137  engages with a ring gear  121  of differential gear unit  120 , thereby transmitting driving force of motor shaft  41  to differential gear unit  120  through reduction-gear train  135 .  
         [0078]    Moreover, differential gear unit  120  comprises ring gear  121 , which engages with small diameter gear  137  of reduction-gear train  135 , pinions  123 , which are rotatably supported by respective pinion shafts  122  which project inward from an inner periphery of ring gear  121 , and side gears  124  fixed to respective front wheel axles  12 L and  12 R and laterally engaged with each of pinions  123 . Due to this construction, the driving force from motor shaft  41  is transmitted to front wheel axles  12 L and  12 R through reduction-gear train  135 , ring gear  121 , pinions  123 , and side gears  124 .  
         [0079]    As shown in FIGS. 4 and 5, an end of motor axis  41 , which is opposite to cylinder block  43 , is extended outside of the housing so as to be fixedly provided thereon with a cooling fan  191  for cooling fluid collected in the front transaxle apparatus.  
         [0080]    Description will now be given of the rear transaxle apparatus. As shown in FIG. 2, hydraulic motor  40  incorporated in front transaxle apparatus  10 , which drives front wheel axles  12 L and  12 R, is fluidly connected through hydraulic hoses  81   a  and  81   b  to a hydraulic motor incorporated in the rear transaxle apparatus  20 , which drives rear wheel axles  22 L and  22 R.  
         [0081]    As shown in FIGS. 8 and 9, real transaxle apparatus comprises a housing which is formed by an upper housing half and a lower housing half  247  vertically separably joined to each other so as to form a hollow interior into which the hydraulic motor, etc., is incorporated.  
         [0082]    The housing forms a bearing portion for a later-discussed motor shaft  241  on the joint surface thereof between housing halves  246  and  247 , and forms a bearing portion for journaling rear wheel, axles  22 L and  22 R in the upper housing half above the joint surface. Rear wheel axles  22 L and  22 R are differentially connected at inner ends thereof to each other through a differential gear unit  220 , and extended outward from respective left and right outside walls of the housing.  
         [0083]    As shown in FIG. 8, rear transaxle  20  apparatus is integrally formed therein with an internal wall  248  which divides the inner space of rear transaxle  20  apparatus into first and second chambers  20   a  and  20   b.  In first chamber  20   a  is disposed an HST  290 , and in second chamber  20   b  are disposed a drive train  249  comprising a gear train which transmits power to differential gear unit  220  from motor shalt  241 , differential gear unit  220 , and inner side ends of rear wheel axles  22 L and  22 R.  
         [0084]    Internal wall  248  comprises a longitudinal portion parallel to rear wheel axles  22 L and  22 R, and a perpendicular portion extended perpendicularly to the longitudinal portion. These two portions are provided continuously so as to arrange first chamber  20   a  adjacent to second chamber  20   b.  An upper wall portion of internal wall  248  extends downward from an inner upper wall surface of upper half housing  246 , and a lower portion of internal wall  248  rises from the inner bottom surface of lower half housing  247  through the joint surface. By joining upper and lower housings  246  and  247 , end faces of both the upper and lower wall portions are also joined to each other so as to form internal wall  248 , thereby dividing the inner space into first and second chambers  20   a  and  20   b  which are independent of each other.  
         [0085]    In the housing, first chamber  20   a  is disposed in front of rear wheel axle  22 R and on a lateral side of drive train  249  which transmits power to differential gear unit  220  from motor shaft  241 .  
         [0086]    In first chamber  20   a  is detachably settled a center section  260  of the HST. A longitudinal portion of center section  260  is extended rectangularly to rear wheel axles  22 L and  22 R, and a vertical surface is formed on a front portion of the longitudinal portion so as to serve as a motor mounting surface  260   m,  onto which the hydraulic motor is mounted. A horizontal surface is formed on the rear portion of center section  260  so as to serve as a pump mounting surface  260   p,  onto which the hydraulic pump is mounted. In the center of pump mounting surface  260   p  is vertically supported a pump shaft  231 .  
         [0087]    Description will now be given of the hydraulic pump arranged on center section  260 .  
         [0088]    As shown in FIG. 9, a cylinder block  233  is rotatably and slidably disposed on pump mounting surface  260   p  which is formed at the horizontal portion of center section  260 .  
         [0089]    Pistons  232  are reciprocally movably fitted through respective biasing springs into a plurality of cylinder bores in cylinder block  233 . A thrust bearing  234   a  of a movable swash plate  234  abuts against the heads of pistons  232 . An opening  234   b  is provided at the center of movable swash plate  234  so as to allow a pump shaft  231  to pass therethrough. A control arm  238  engages with a side of movable swash plate  234  so that a tilt angle of movable swash plate  234  is adjusted by rotating a control shaft  237  serving as a rotary shaft of control arm  238 .  
         [0090]    In order that pump shaft  231  may function as an input shaft, pump shaft  231  is rotatably supported by a bearing  235  engaged in an opening  236  formed above first chamber  20   a  in upper hall housing  246  and is not-relatively rotatably engaged with cylinder block  233 , thereby being arranged vertically on the rotary axis of cylinder block  233 .  
         [0091]    In this way, an axial piston type variable displacement hydraulic pump is constructed in rear transaxle apparatus.  
         [0092]    As shown in FIG. 9, the upper end of pump shaft  231  projects outwardly from the rear transaxle apparatus. An HST input pulley  292  and a cooling fan  291  are fixed onto the upper end of pump shaft  231 . Thus, while cooling the hydraulic fluid accumulated in rear transaxle apparatus  20  by cooling fan  291 , driving force of the engine is inputted into HST input pulley  292  through a transmission clement so as to rotate pump shaft  231 .  
         [0093]    Description will now be given of the hydraulic motor  240  arranged on center section  260 .  
         [0094]    As shown in FIG. 8, a cylinder block  243  is rotatably and slidably disposed on motor mounting surface  260   m  which is formed at the vertical portion of center section  260 .  
         [0095]    A plurality of pistons  242  are reciprocally movably fitted into a plurality of cylinder bores in cylinder block  243  through respective biasing springs. The heads of pistons  242  abut against a thrust bearing  244   a  of a fixed swash plate  244  which is fixedly sandwiched between upper housing half  246  and lower housing half  247 . An opening  244   b  is provided in the center of fixed swash plate  244  so as to allow motor shaft  241  to pass therethrough.  
         [0096]    In order that motor shaft  241  may function as an output shaft, motor shaft  241  is rotatably supported by a sealed bearing  245  sandwiched between upper housing half  246  and lower housing half  247 , and is not-relatively rotatably engaged with cylinder block  243 , thereby being arranged horizontally on the rotary axis of cylinder block  243 .  
         [0097]    In this way, an axial piston type fixed displacement hydraulic motor is constructed in rear transaxle apparatus  20 .  
         [0098]    Moreover, as shown in FIG. 8, the end portion of motor shaft  241  opposite to center section  260  is fitted with a drive output gear  212  in spline fitting such that drive output gear  212  rotates with motor shalt  241 . The portion of motor shaft  241  outward from drive output gear  212  is fitted with a brake rotor  213  in spline fitting. By pressing brake rotor  213  between brake pads  214   a  and  214   b,  rotating motor shaft  241  is braked. In this embodiment, as mentioned above, brake devices including brake rotor  213  are provided in respective transaxle apparatuses  10  and  20 , although it may be considered that at least one of transaxle apparatuses  10  and  20  is provided therein with the brake device. These two brake devices can be used effectively, namely, one brake device is for braking during running of the vehicle, and the other for a brake at the time of parking. With this structure, a mechanical link interlocked with a running brake pedal and a mechanical link interlocked with a parking brake lever are distributed so as to be simplified. Moreover, the braking effect may be enhanced if both the front and rear brake devices are connected to the running brake pedal so as to be actuated for braking simultaneously.  
         [0099]    As shown in FIG. 8, a counter shaft  239  is arranged parallel to motor shaft  241 , a wide, small diameter gear  217  fits loosely on counter shaft  239 , and a large diameter gear  216  is engaged on a toothed side of small diameter gear  217 , thereby constituting a reduction-gear train  215 .  
         [0100]    Regarding reduction-gear train  215 , large diameter gear  216  engages with drive output gear  212 , small diameter gear  217  engages with a ring gear  221  of a differential gear unit  220 , thereby transmitting the driving force from motor shaft  241  to differential gear unit  220  through reduction-gear train  215 .  
         [0101]    Moreover, differential gear unit  220  comprises ring gear  221 , which engages with small diameter gear  217 , pinions  223  rotatably supported by respective pinion shafts  922  which project inward from an inner periphery of ring gear  221 , and left and right side gears  224  fixed to respective rear wheel axles  22 L and  22 R and engaged with each of pinions  223 . Due to this construction, the driving force of motor shaft  241  is transmitted to rear wheel axles  22 L and  22 R through reduction-gear train  215 , ring gear  221 , pinions  223 , and side gears  224 .  
         [0102]    Description will now be given of a hydraulic circuit structure inside of center section  260  and a manifold block  268 , which is attached to the undersurface of center section  260 .  
         [0103]    First, a first embodiment of a hydraulic circuit structure is described. According to the first embodiment, hydraulic motor  40  in front transaxle apparatus  10  and hydraulic motor  240  in rear transaxle apparatus  20  are fluidly connected in series to hydraulic pump in  230 .  
         [0104]    As shown in FIG. 8, into pump mounting surface  260   p  in the horizontal portion of center section  260  are bored a first kidney-port  261   a  and a second kidney-port  261   b  opposite to each other. These kidney-ports  261   a  and  261   b  are open at a position above which openings of the cylinder bores of cylinder block  233  pass.  
         [0105]    As shown in FIG. 10, into motor mounting surface  260   m  in the vertical portion of center section  260  are bored a first kidney-port  262   a  and a second kidney-port  262   b  opposite to each other. These kidney-ports  262   a  and  262   b  are open at a position where openings of the cylinder bores of cylinder block  243  pass leftward.  
         [0106]    As shown in FIGS.  9  to  11 , in center section  260  are bored an upper first fluid passage  271  and a lower second fluid passage  272  parallel to each other in the longitudinal direction of center section  260 . First fluid passage  271  connects first kidney-port  261   a  at pump mounting surface  260   p  to first kidney-port  262   a  at motor mounting surface  260   m.  Second fluid passage  272  is connected at the front end thereof to second kidney-port  262   b  at motor mounting surface  260   m.    
         [0107]    Moreover, as shown in FIGS. 9 and 10, manifold block  268  is attached to the undersurface of center section  260 . In manifold block  268  from a side surface thereof are bored a third fluid passage  273  and a fourth fluid passage  274  parallel to each other and perpendicular to first and second fluid passages  271  and  272 . Into openings of third and fourth fluid passages on the left side surface of manifold block  268  are fitted respective caps  283  and  284  so as to constitute connection ports  273   a  and  274   a.  As shown in FIG. 9, ends of caps  283  and  284  project outward from lower housing half  247  so as to be connected to hydraulic hoses (not shown) outside of lower housing half  247 . The axes of connection ports  273   a  and  274   a  are disposed in a substantially horizontal plane, namely, they are not slant upward or downward, thereby facilitating the connection work of piping comparatively. That is, the arrangement of connection ports  273   a  and  274   a  in the horizontal plane solves the problems of the reduction of the ground clearance in the case of piping with downward ports and interference of piping with a transmission belt or a frame in the case of piping with upward ports.  
         [0108]    Moreover, as shown in FIG. 9, between center section  260  and manifold block  268  are bored a vertical fifth fluid passage  275  which connects second fluid passage  272  to third fluid passage  273  and a vertical sixth fluid passage  276 , which connects second kidney port  262   b  in pump mounting surface  260   p  to fourth fluid passage  274 .  
         [0109]    Incidentally, a bypass operation lever (not shown) for opening first fluid passage  271  and second fluid passage  272  to the fluid sump is disposed at rear transaxle apparatus  20  in order to enable axles  22 L and  22 R to idle when the vehicle is towed.  
         [0110]    Due to the above-mentioned fluid passages the hydraulic motor in the front transaxle apparatus  10  and the hydraulic motor  240  in the rear transaxle apparatus  20  are fluidly connected in series to the hydraulic pump  230  in the rear transaxle apparatus  20 .  
         [0111]    That is, as shown in FIG. 2, hydraulic hose  81   a  connects cap  54   a  on the front transaxle apparatus  10  to cap  283  on rear transaxle apparatus  20 , and hydraulic hose  81   b  connects cap  54   b  on front transaxle apparatus  10  to cap  284  on rear transaxle apparatus  20 , thereby forming a hydraulic circuit shown in FIG. 12. The kind of fluid communication means between the front and rear transaxle apparatuses  10  and  20  is not limited. However, like hoses  81  a and  81   b  according to this embodiment, the means is preferably flexible and resistant to considerably high pressure so as not interfere with the bending of the vehicle body.  
         [0112]    According to the hydraulic circuit shown in  12 , in center section  260  arranged in rear transaxle apparatus  20 , first kidney-port  261   a  of pump-mounting-surface  260   p  is connected through first fluid passage  271  to first kidney-port  262   a  of motor mounting surface  260   m.  Also, second kidney-port  262   b  in center section  260  of motor mounting surface  260   m  is connected to first kidney-port  62   a  in center section  62  of front transaxle apparatus  10  to motor mounting surface  63   m  through a string of fluid passages  299   a  which consists of second fluid passage  272 , fifth fluid passage  275 , third fluid passage  273 , hydraulic hose  81   a,  and first fluid passage  53   a  provided in center section  62  of front transaxle apparatus  10 .  
         [0113]    Second kidney-port  62   b  formed in center section  62  of front transaxle apparatus  10  to is connected to second kidney-port  261   b  formed in pump-mounting-surface  260   p  in center section  260  through second fluid passage  53   b  provided in center section  62 , hydraulic hose  81   b,  and a string of fluid passages  299   b  which consists of fourth fluid passage  274  and sixth fluid passage  276  in the rear transaxle apparatus  20 .  
         [0114]    As mentioned above in the hydraulic circuit structure according to the first embodiment, hydraulic motors  40  and  240  which are arranged in front and rear transaxle apparatuses  10  and  20 , respectively, are fluidly connected in series to hydraulic pump  230 . This in series connection form is suitable for an articulated vehicle in which coupling part  50  serves as a turning center of the vehicle and is arranged at an equidistant position from both the front and rear axles of the vehicle.  
         [0115]    In this way, in front transaxle apparatus  10  and rear transaxle apparatus  20  are driven front wheel axles  12 L and  12 R and rear wheel axles  22 L and  22 R, respectively, thereby realizing a four-wheel-drive vehicle which is excellent in both steering performance and running performance over bad ground conditions.  
         [0116]    Especially, a four-wheel-drive working vehicle provided with the in series hydraulic connection has the capability of freeing its running wheels from mud. For example, even if the vehicle travels in a swamp and a front wheel is stuck in mud, hydraulic fluid discharged from hydraulic pump  30  bypasses hydraulic motor  40  in front transaxle apparatus  10  so as to idle the unloaded front wheels, and then flows into hydraulic motor  240  in rear transaxle apparatus  20  so as to drive the loaded rear wheels, whereby the vehicle can escape from the mud smoothly.  
         [0117]    Alternatively, caps  283  and  284  may be connected mutually through a hydraulic hose bypassing hydraulic motor  40  so as to make a rear-wheel-drive vehicle which drives with only the driving force of hydraulic motor  240  in rear transaxle apparatus.  
         [0118]    When the rotary speed (peripheral speed) of front wheel axles  12 L and  12 R is substantially identical to that of rear wheel axles  22 L and  22 R, hydraulic motors  20  and  240  in respective front and rear transaxle apparatuses  10  and  20  preferably have the same displacement (amount of discharge). With this composition the same reduction gears may be applicable to both front and rear transaxle apparatuses  10  and  20 . Of course, hydraulic motors of different volume can also be applied in this case, however, the mechanical deceleration ratio of front transaxle apparatus must be different from that of rear transaxle apparatus so as to substantially equalize the rotary speed (peripheral speed) of front wheel axles  12 L and  12 R with that of rear wheels axles  22 L and  22 R.  
         [0119]    In addition, as shown in FIG. 13, front transaxle apparatus for driving the front wheels may be modified so that the tilt angle of swash plate  44   c  of hydraulic motor is adjustable and swash plate  44   c  is interlockingly connected to steering wheel  4  through a wire, a link, or similar structure so as to correlate the tilt angle of swash plate  44   c  and the turning angle of steering wheel  4 , thereby increasing the rotary speed of the front wheel axles.  
         [0120]    This structure is particularly effective for improving steering performance of a vehicle having an Ackerman type steering device or a chassis layout wherein a difference of rotary speed is generated between the front wheels and the rear wheels at the time of left or right turning, namely, coupling part  50  is not located equidistant from the front and rear axles of the vehicle.  
         [0121]    Thus, regarding vehicles having the front and rear transaxle apparatuses with a layout wherein a difference of rotary speed is generated between the front wheels and rear wheels at the time of turnings and fluidly connecting in series the hydraulic motors in both the transaxle apparatuses, steering performance can be improved by making the hydraulic motor which actuates steer able wheels (the front wheels) variable in displacement, and increasing the rotary speed of this hydraulic motor in correspondence to the angle of the steering wheel.  
         [0122]    Moreover, in hydraulic circuit shown in FIGS. 12 and 13, bypass valves  40   v  and  240   v  are provided to front and rear hydraulic motors  40  and  240 , respectively, so that the fluid passages are opened to the fluid sump by the above-mentioned bypass operation lever, thereby enabling towage of the vehicle. Towing the vehicle can be achieved if at least one of front and rear transaxle apparatuses  10  and  20  is provided with either bypass valve  40   v  or  240   v,  respectively. However, according to this embodiment, both front and rear transaxle apparatuses  10  and  20  are provided with respective bypass valves  40   v  and  240   v.  Therefore, at the time of assembling, extraction of air can be done from each transaxle apparatus and  20  comparatively easily. Moreover, the vehicle can be towed even in low-temperatures and with high consistency of hydraulic fluid, because hydraulic fluid discharged from each of the idling hydraulic motors  40  and  240  is bypassed near motor  40  or  240  so as not to be considerably resistant to towage of the vehicle.  
         [0123]    Description will now be given of a hydraulic circuit structure according to a second embodiment, wherein hydraulic motor  40  in front transaxle apparatus  10  and hydraulic motor  240  in rear transaxle apparatus  20  are fluidly connected in parallel to hydraulic pump  230 .  
         [0124]    As shown in FIG. 14, in a horizontal portion of a center section  360  are bored a first kidney-port  361   a  and a second kidney-port  361   b  opposite to each other. These kidney-ports  361   a  and  361   b  are open at a position where openings of the cylinder bores of cylinder block  233  pass.  
         [0125]    On the other hand, as shown in FIG. 17, in the vertical portion of the center section  360  are bored a first kidney-port  362   a  and a second kidney-port  362   b  opposite to each other. These kidney-ports  362   a  and  362   b  are open at a position where openings of the cylinder bores of cylinder block  243  pass.  
         [0126]    As shown in FIGS. 15, 16, and  18 , in the center section are bored an upper first fluid passage  371  and a lower second fluid passage  372  parallel to each other in the longitudinal direction of center section  360 .  
         [0127]    As shown in FIG. 15, in center section  360  is bored a third fluid passage  373  perpendicular to first fluid passage  371  so as to be connected to first fluid passage  371 . An opening of third fluid passage  373  on a side surface of the center section  360  is closed by a plug  373   a.    
         [0128]    As shown in FIG. 16, in center section  360  are bored a slant fourth fluid passage  374 , which connects second kidney-port  361   b  to second fluid passage  372 . An opening of fourth fluid passage  374  on the side face of center section  360  is closed by a plug  374   a.    
         [0129]    Moreover, as shown in FIGS.  15  to  17 , a manifold block  368  is attached to the undersurface of center section  360 . From a side surface of manifold block  368  are bored a fifth fluid passage  375  and a sixth fluid passage  376  forward and backward parallel to each other and perpendicular to first and second fluid passages  371  and  372 . Caps  385  and  386  are fitted into respective openings of fifth and sixth fluid passages  375  and  376  so as to form respective connection ports  375   a  and  376   a.  As shown in FIGS. 15 and 16, ends of caps  385  and  386  opposite to manifold block  368  project outward from a lower housing half  347  so as to be connected to hydraulic hoses (not shown) outside lower housing half  347 . Axes of connection ports  375   a  and  376   a  are disposed in a substantially horizontal plane (i.e., a plane which is oriented neither upward nor downward) so as to facilitate piping thereto.  
         [0130]    Between center section and manifold block  368  are bored a vertical seventh fluid passage  377  (FIG. 15), which connects a junction point between second and fourth fluid passages  372  and  374  to fifth fluid passage  375 , and a vertical eighth fluid passage  378  (FIG. 16), which connects third fluid passage  373  to sixth fluid passage  376 .  
         [0131]    Due to the above mentioned fluid passage structure, hydraulic motor  40  in front transaxle apparatus  10  and hydraulic motor  240  in rear transaxle apparatus  20  are fluidly connected in parallel to the hydraulic pump  230 .  
         [0132]    That is, as shown in FIG. 2, cap  54   a  provided in front transaxle apparatus is connected to cap  385  provided in rear transaxle apparatus  20  through a hydraulic hose  81   a,  and cap  54   b  in front transaxle apparatus  10  to the cap  386  in rear transaxle apparatus  20  through a hydraulic hose  81   b,  thereby forming a hydraulic circuit shown in FIG. 19.  
         [0133]    According to the hydraulic circuit shown in FIG. 19, in center section  361   a  arranged in rear transaxle apparatus  20 , the first kidney-port  361 , formed to pump mounting surface  360   p,  is connected through first fluid passage  371  to first kidney-port  362   a  and to motor mounting surface  360   m.  First kidney-port of  361   a,  formed in center section  361   a  to pump mounting surface  360   p,  is connected to first kidney-port  62   a,  formed to the motor mounting surface  63   m,  through a string of fluid passages  399   a,  which branch from first fluid passage  371  (as shown in FIG. 19) and consist of third fluid passage  373 , sixth fluid passage  376 , hydraulic hose  81   a,  and first fluid passage  53   a  provided in center section  62  of front transaxle apparatus  10 .  
         [0134]    On the other hand, in the center section arranged in the rear transaxle apparatus, the second kidney-port  362   b  formed to the motor mounting surface  360   m  is connected to the second kidney-port  361   b  formed to the pump mounting surface  360   p  through a string, of fluid passage  399   b  which consists of the second fluid passage  372  and fourth fluid passage  374 .  
         [0135]    Moreover, since the fourth fluid passage  374  is connected to the seventh fluid passage  377 , the second kidney-port of  361   b  formed to the pump mounting surface  360   p  is connected to the second kidney-port  62   b  formed to the motor mounting surface  63   m  through a string of the fluid passage  399   c  which consists of the fourth fluid passage  374 , the seventh fluid passage  377 , and the fifth fluid passage  375  (as shown in FIG. 19), hydraulic hose  81   b,  and second fluid passage  53   b  provided in center section  62  of front transaxle apparatus.  
         [0136]    In this way, in the hydraulic circuit structure according to the second embodiment, hydraulic motors  40  and  340  arranged in respective front and rear transaxle apparatuses  10  and  20  are fluidly connected in parallel to hydraulic pump  230 . Particularly, in this parallel connection structure is suitable for a vehicle which turns left and right while generating a difference in rotary speed between the front wheels and the real wheels.  
         [0137]    Due to the above structure, in front transaxle apparatus and rear transaxle apparatus  10  are driven front wheel axles  12 L and  12 R and rear wheel axles  22 L and  22 R, respectively, thereby making a four-wheel-drive vehicle which excels in steering performance and running performance over bad ground conditions.  
         [0138]    Alternatively, although not shown, caps  385  and  386  may be plugged so as to make a rear-wheel-drive vehicle which drives with only the driving force of hydraulic motor  340  of rear transaxle apparatus  20 .  
         [0139]    Moreover, as shown in FIG. 19, the vehicle provided with the in parallel hydraulic connection structure may be modified by providing differential gear units  120  and  220  in front and rear transaxle apparatuses  10  and  20  with respective differential-lock devices  125  and  225  for restricting differential rotation of right and left axles and by providing operation levers for differential-lock devices  125  and  225  on the vehicle, so as to restrict the differential rotation of the axles when any of the running wheels are stuck.  
         [0140]    In the in parallel connection, hydraulic fluid is distributed between the two hydraulic motors  40  and  340 , whereby a larger amount of hydraulic fluid flows to the lighter-loaded of the hydraulic motors  40  and  340 . For this reason, when a right front wheel actuated by hydraulic motor  40  is stuck, for example, the vehicle becomes impossible to free because hydraulic fluid doesn&#39;t flow to hydraulic motor and the rear axles aren&#39;t actuated; by operating differential-lock device  125 , load for driving a left front wheel is applied to hydraulic motor  40  so as to supply a suitable amount of hydraulic fluid to rear hydraulic motor  340  so as to drive the rear wheels, thereby enabling the vehicle to be freed. Incidentally, in the case where differential-lock devices  125  and  225  are provided to respective front and rear transaxle apparatuses  10  and  20 , a common differential-lock pedal may be provided for both the differential-lock devices so as to actuate the devices simultaneously, or two pedals may be separately provided for the respective differential-lock devices.  
         [0141]    Description will be given of a second embodiment of the working vehicle having rear transaxle apparatus  20 .  
         [0142]    As shown in FIG. 20, in the working vehicle according to the second embodiment, a pair of left and right front transaxle apparatuses  400 L and  400 R are provided to front frame  11 . Left and right front transaxle apparatuses  400 L and  400 k include respective front-wheel axles  412 L and  412 R, and are fluidly connected to rear transaxle apparatus  20  through a distribution device  80 , hydraulic hoses, etc.  
         [0143]    As shown in FIG. 21, an upper housing half  446  and a lower housing half  447  are joined to each other so as to form a housing of each of front transaxle apparatuses for incorporating a hydraulic motor. Left and right front transaxle apparatuses  400 L and  400 R share the same structure and are supported on front frame  11  through respective stays  19   a  and  19   b  so as to orient front-wheel axles  412 L and  412 R opposite to each other.  
         [0144]    As shown in FIG. 22, each of the front transaxle apparatuses  400 L and  400 R incorporates a hydraulic motor  440 , which is fluidly connected to hydraulic pump  230  in rear transaxle apparatus  20  (not shown). Rotation of a motor shaft  441  of hydraulic motor  440  is output to the outside of the housing through each of front wheel axles  412 L and  412 R.  
         [0145]    As shown in FIG. 22, into each of front transaxle apparatuses is integrally assembled hydraulic motor  440 , which is so constructed that a cylinder block  443  is rotatably slidably mounted on a motor mounting surface  463   m  formed on a vertical portion of a center section  462 . A plurality of pistons  442  are reciprocally movably fitted into a plurality of cylinder bores in cylinder block  443  through respective biasing springs. The heads of pistons  442  abut against a fixed swash plate  444  which is fixedly sandwiched between upper housing half  446  and lower housing half  447 . An opening  444   b  is provided in the center of fixed swash plate  444  so as to allow motor shaft  441  to pass therethrough.  
         [0146]    So that motor shalt  441  may function as an output shaft, motor shaft  441  is rotatably supported by a sealed bearing  445  which is sandwiched between upper housing half  446  and lower housing half  447 , and is not-relatively rotatably engaged with cylinder block  443  so as to he disposed horizontally on the rotary axis of cylinder block  443 .  
         [0147]    Thus, an axial piston type fixed displacement hydraulic motor is constructed in each of front transaxle apparatuses.  
         [0148]    Moreover, as shown in FIG. 22, a pair of first and second kidney-ports  462   a  and  462   b  are formed in a vertical portion of center section  462  from a motor mounting surfaces  463   m.  A first fluid passage  453   a  and a second fluid passage  453   b  are horizontally formed in center section  462  so as to be fluidly connected to respective kidney-ports  462   a  and  462   b.  First fluid passage  453   a  and second fluid passage  453   b  are connected to respective caps  454   a  and  454   b  to be connected to respective hydraulic hoses. Thus, each of hydraulic motors is fluidly connected to the hydraulic pump  200  in rear transaxle apparatus through the hydraulic hoses (not shown).  
         [0149]    Although not shown, a bypass operation lever for opening first fluid passage  453   a  and second fluid passage  453   b  to the fluid sump is included with each front transaxle apparatuses so as to idle front wheel axles  412 L and  412 R when the vehicle is towed.  
         [0150]    As shown in FIG. 22, on an end portion of motor shalt  441  opposite to the center section  462  is provided a drive output gear  431  in spline fitting, whereby drive output gear  431  rotates integrally with motor shaft  441 . On a portion of drive output gear  431  toward center section  462  is integrally formed a brake rotor  433  whose diameter is larger than that of drive output gear  431 , so that rotating motor shalt  441  is braked by pressing brake rotor  433  between brake pads  434   a  and  434   b.    
         [0151]    Moreover, as shown in FIG. 22, bearing  439   a  and  439   b  rotatably support front-wheel axle  412 L (or  412 R) in parallel to motor shaft  441 . A deceleration gear  421  is fixed onto front-wheel axle  412 L (or  412 R) and engages with drive output gear  431 . The diameter of deceleration gear  421  is larger than drive output gear  431  so as to reduce the rotary speed of motor shaft  441  greatly so as to enable each of front transaxle apparatuses to incorporate a hydraulic motor having a small capacity.  
         [0152]    Alternatively, although not shown, instead of front-wheel axle  412 L (or  412 R), upper and lower housing halves  446  and  447  may be formed on a side thereof opposite to the center section  462  with an opening on an axial extension of motor shaft  4411  and motor shaft  441  may be extended through the opening to the outside of the housing, so as to be fixed to front wheel  13 . In brief, motor shaft  441  may replace front wheel axle  412 L (or  412 R).  
         [0153]    As shown in FIG. 20, front transaxle apparatuses constructed as described above are fluidly connected to rear transaxle apparatus through distribution device  80 , hydraulic hoses, etc., so as to drive respective front-wheel axles  412 L and  412 R, thereby rotating left and right front wheels  13 .  
         [0154]    There are several types of fluidal connection between front transaxle apparatuses  400 L and  400 R and rear transaxle apparatus  20 . These fluidal connection types will be described as follows.  
         [0155]    According to an embodiment shown in FIG. 23, employing rear transaxle apparatus according to the first embodiment (shown in FIGS.  8  to  11 ), hydraulic motor  240  of rear transaxle apparatus and a circuit, which fluidly connects in parallel hydraulic motors  440  of both front transaxle apparatuses  400 L and  4001  to each other are fluidly connected in series to the hydraulic pump of rear transaxle apparatus.  
         [0156]    Due to this structures front-wheel axles  412 L and  412  of front transaxle apparatuses can be driven differentially.  
         [0157]    According to an embodiment shown in FIG. 24, employing a fluidal connection similar to that of FIG. 23, both hydraulic motors  440  of front transaxle apparatuses  4001  and  400 R are variable displacement hydraulic motors having respective movable swash plates  444   c.  This structure is particularly effective for a vehicle having an Ackerman type steering device or chassis layout wherein a difference in rotary speed is generated between the front wheels and the rear wheels at the time of turning of the vehicle, namely, that coupling part  50  is not located at an equidistant position from both front and rear axles, because a difference of rotary speed can be generated between front and rear wheels by adjusting movable swash plates  444   c  so as to improve steering performance of the vehicle.  
         [0158]    According to an embodiment shown in FIG. 25, employing rear transaxle apparatus according to the first embodiment, hydraulic motor  240  of rear transaxle apparatus  20  and hydraulic motors  440  of both front transaxle apparatuses  400 L and  400 R are all fluidly connected in series to hydraulic pump  230  of transaxle apparatus  20 . Moreover, both hydraulic motors  440  of front transaxle apparatuses  400 L and  400 R are variable displacement hydraulic motors having respective movable swash plates  444   c.    
         [0159]    This structure is particularly effective for a vehicle having an Ackerman type steering device or a chassis layout wherein a difference in rotary speed is generated between the front wheels and the rear wheels at the time of turning of the vehicle, namely, that coupling part  50  is not located at an equidistant position from both front and rear axles, because a difference in rotary speed can be generated between front and rear wheels by adjusting movable swash plates  444   c  so as to improve steering performance of the vehicle.  
         [0160]    According to a hydraulic circuit shown in FIG. 26 employing, rear transaxle apparatus according to the second embodiment (shown in FIGS.  14  to  18 ), hydraulic motor  340  of rear transaxle apparatus  20  and hydraulic motors  440  of both front transaxle apparatuses are all fluidly connected in parallel to hydraulic pump of rear transaxle apparatus.  
         [0161]    Due to this structure, front-wheel axles  412 L and  412  of front transaxle apparatuses can be driven differentially.  
         [0162]    Moreover, the hydraulic circuit in rear transaxle apparatus  20  is fluidly connected to the hydraulic circuit of front transaxle  400 L, and  400 R apparatuses through a control valve  80   a.  If any of front wheels  13  is stuck, control valve  80   a  stops the supply of hydraulic fluid to front transaxle apparatuses  400 L and  400 R, and hydraulic motor  340  rotates rear wheel axles  22 L and  22 R, whereby the vehicle is freed. Furthermore, differential-lock device  225  is provided to restrict the differential rotation of rear wheel axles  22 L and  22 R so as to correspond to the situation where one of rear wheels  23  is stuck.  
         [0163]    According to an embodiment shown in FIG. 27, employing rear transaxle apparatus  20  according to the second embodiment, hydraulic motor  340  of rear transaxle apparatus  20  and a circuit. Which fluidly connects in series hydraulic motors  440  of both front transaxle apparatuses  400 L and  400 R to each other, are fluidly connected in parallel to hydraulic pump of rear transaxle apparatus in parallel. Moreover, both hydraulic motors of front transaxle apparatuses  400 L and  400 R are variable displacement hydraulic motors having respective movable swash plates  444   c.    
         [0164]    This structure is particularly effective for a vehicle having an Ackerman type steering device or a chassis layout wherein a difference in rotary speed is generated between the front wheels and the real wheels at the time of turning of the vehicle, namely, that coupling part  50  is not located at an equidistant position from both front and rear axles, because a difference in rotary speed can be generated between front and rear wheels by adjusting movable swash plates  444   c  so as to improve steering performance of the vehicle.  
         [0165]    Description will now be given of a layout of front transaxle apparatuses.  
         [0166]    As shown in FIG. 21, inner ends of front wheel axles  412 L and  412 R, which are opposite to respective font wheels  13 , are inserted in respective front transaxle apparatuses  400 L and  400 R.  
         [0167]    Front transaxle apparatuses are supported on left and right side portions of front frame  11  through stays  19   a  and  19   b,  respectively, so as to ensure a lateral interval  401 L between both front transaxle apparatuses  400 L and  400 R.  
         [0168]    This interval  401 L is wider than a lateral width  402 L of second working-device drive transmission belt  59  at the position where belt  59  passes front transaxle apparatuses.  
         [0169]    With arranging front transaxle apparatuses  400 L and  400 R as described above, even if a working device such as mower device  3  is raised so as to change the vertical height where second working-device drive transmission belt  59  passes second working-device drive transmission belt  59  interferes with neither front wheel axles  412 L and  412 R nor front transaxle apparatuses. Therefore, the problem of second working-device actuation transmission belt  59  rubbing against front wheel axle  412 L,  412 R, etc., and wearing out is not generated.