Abstract:
A fluid passage member includes a pump mounting surface onto which a hydraulic pump having a rotary axis is mounted; first and second kidney ports opened at the pump mounting surface; a first fluid extraction port outwardly opened and connected to the first kidney port; a motor mounting surface onto which a hydraulic motor having a rotary axis is mounted so that the rotary axis of the hydraulic motor is parallel to the rotary axis of the hydraulic pump; third and fourth kidney ports opened at the motor mounting surface; and a second fluid extraction port outwardly opened and connected to the third kidney port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/617,278, filed Dec. 28, 2006, which is a divisional of U.S. application Ser. No. 10/948,739, filed Sep. 24, 2004, now U.S. Pat. No. 7,204,779, issued Apr. 17, 2007, the entire disclosures of which are incorporated herein by reference thereto. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a steering transaxle having a hydraulic motor for driving steerable wheels. The invention also relates to a hydraulic driving vehicle, especially a four-wheel driving vehicle, e.g., an agricultural tractor, a riding mower and a construction machine, comprising the steering transaxle. 
         [0004]    2. Related Art 
         [0005]    As disclosed in the Japanese Patent No. 2594951 (document &#39;951) and the Japanese Laid Open Gazettes Nos. 2002-172946 (document &#39;946) and 2000-1127 (document &#39;127), there are conventional well-known four-wheel driving vehicles, each of which has a front steering transaxle and a rear unsteering transaxle. The front steering transaxle supports steerable front wheels and incorporating a front differential gear unit mutually differentially connecting the front wheels, and the rear unsteering transaxle supports unsteerable rear wheels and incorporating a rear differential gear unit mutually differentially connecting the rear wheels, wherein the speed ratio between the steerable wheels and the unsteerable wheels is changed according to the steered angle of the steerable wheels. 
         [0006]    The vehicle disclosed in the document &#39;946 is provided with a large and complicated mechanical (planetary gearing) center differential gear unit interposed between the front steering transaxle and the rear unsteering transaxle. A large middle space of the vehicle is necessary for arranging the center differential gear unit, and two front and rear propeller shafts for transmitting power of the center differential gear unit to the respective front and rear transaxles, thereby causing designing limitation. 
         [0007]    With respect to the conventional hydraulic driving vehicles disclosed in the documents &#39;951 and &#39;127, instead of the center differential gear unit with the front and rear propeller shafts, each of the vehicles is provided with a variable displacement hydraulic motor for driving the front differential gear unit, a fixed displacement hydraulic motor for driving the rear differential gear unit, and a common hydraulic pump for supplying both the hydraulic motors with hydraulic oil one after another, thereby increasing a free space for arranging various parts. 
         [0008]    With respect to the disclosed conventional hydraulic driving vehicles, it is desired for smoothly driving the steerable front wheels that the displacement of the front hydraulic motor is reduced as far as possible so as to correspond to the reduced hydraulic oil after passing the rear hydraulic motor. However, such displacement of the front hydraulic motor causes increase of the rotary speed of the front hydraulic motor, thereby requiring increase of the deceleration ratio between the front hydraulic motor and the steerable front wheels so as to be balanced with the rotary speed of the unsteerable rear wheels. Upsizing of a deceleration gear train between the front hydraulic motor and the front differential gear unit for ensuring the increased deceleration ratio is not desired because it causes the front steering transaxle incorporating the deceleration gear train to expand the vehicle vertically and lengthwise so as to reduce ground clearance below. 
         [0009]    Further, the rotary axis of the front hydraulic motor (its motor shaft) is disposed perpendicularly to that of the axles of the front wheels. If the front hydraulic motor and the front differential gear unit are disposed in a transaxle casing, the transaxle casing must be large lengthwise of the vehicle so as to ensure the length of the motor shaft. 
         [0010]    Further, if a steering transaxle incorporating a variable displacement hydraulic motor, a deceleration gear train and a differential gear unit is to be constructed, it requires many parts and units to be assembled and properly located. Easy assemblage of the steering transaxle such as to overcome the difficulty of assembling and locating many parts and units is desired for reducing labor and time. 
         [0011]    Further, the vehicle disclosed in the document &#39;127 is provided with brakes in the rear transaxle so as to individually brake the respective unsteerable rear wheels. Since the vehicle has the variable displacement hydraulic motor for driving the steerable front wheels, and the fixed displacement hydraulic motor for driving the unsteerable rear wheels, the steerable front wheels are accelerated and the unsteerable rear wheels are not reduced during turning of the vehicle. In brief, the high speed of the unsteerable rear wheels that occurs during straight traveling of the vehicle is kept even during turning of the vehicle, thereby increasing a centrifugal force applied onto the turning vehicle and making the vehicle unstable. Further, in this condition during turning of the vehicle, if the vehicle brake-turns, i.e., if one of the brakes is actuated for braking, a braking shock may occur. 
       SUMMARY OF THE INVENTION 
       [0012]    A first object of the invention is to provide a hydraulic steering transaxle which is compact and can be easily assembled. 
         [0013]    To achieve the first object, according to a first aspect of the invention, a steering transaxle comprises: left and right drive shafts drivingly connected to respective steerable wheels; a differential gear unit differentially connecting the drive shafts to each other; a transaxle casing having an opening, the transaxle casing incorporating the differential gear unit and the drive shafts; a cover for covering the opening of the transaxle casing; a variable hydraulic motor having a movable swash plate, the hydraulic motor being provided on the inside of the cover so as to drive the differential gear unit; a motor control mechanism for controlling the movable swash plate, the motor control mechanism being provided on the outside of the cover; and a hydraulic oil port for oil supply and delivery of the hydraulic motor, the hydraulic oil port being provided on the outside of the cover. Due to the external arrangement of the motor control mechanism and the hydraulic oil port, the transaxle casing can be slimmed. 
         [0014]    In the first aspect, preferably, according to a second aspect of the invention, the steering transaxle further comprises: left and right axle support units steerably provided on left and right ends of the transaxle casing; left and right axles supported by the respective axle support units, the steerable wheels being provided on the respective axles; and a pair of deceleration gear trains each of which is interposed between each of the drive shafts and each of the axles, each of the deceleration gear trains being disposed in each of the axle support units. Due to the arrangement of the deceleration gear trains in the left and right axle support units, the transaxle casing can be further slimed. 
         [0015]    In the first aspect, preferably, according to a third aspect of the invention, the cover, the hydraulic motor with the movable swash plate, the motor control mechanism and the hydraulic oil port constitute an assembly unit, which is detachably attached to the transaxle casing so that, by covering the opening with the cover, the hydraulic motor is disposed in the transaxle casing so as to be drivingly connected to the differential gear unit, thereby facilitating assembly and disassembly of the steering transaxle. 
         [0016]    In the third aspect, preferably, according to a fourth aspect of the invention, the movable swash plate includes a pair of trunnion shafts, one of which is supported by the cover in the assembly unit. When the assembly unit is attached to the transaxle casing, the other trunnion shaft comes to be supported by the transaxle casing. Therefore, of the two trunnion shafts, only the other trunnion shaft is required to be located and assembled to the transaxle casing, thereby further facilitating assembly and disassembly of the steering transaxle. 
         [0017]    In the fourth aspect, preferably, according to a fifth aspect of the invention, the hydraulic motor includes a motor shaft disposed in parallel to the drive shafts, so that a gear provided on the motor shaft meshes with an input gear of the differential gear unit so as to drivingly connect the hydraulic motor to the differential gear unit. The meshing gears on the motor shaft and the differential gear unit can be economic spur gears. 
         [0018]    In the third aspect, preferably, according to a sixth aspect of the invention, the assembly unit includes a center section of the hydraulic motor fixed to the cover, so that, when the assembly unit is attached to the transaxle casing, the center section is sandwiched between the cover and the transaxle casing. 
         [0019]    In the sixth aspect, preferably, according to a seventh aspect of the invention, the movable swash plate includes a pair of trunnion shafts both of which are supported by the cover in the assembly unit. Therefore, no trunnion shaft needs to be located and assembled to the transaxle casing when the assembly unit is attached to the transaxle casing, thereby further facilitating assembly and disassembly of the steering transaxle. 
         [0020]    In the seventh aspect, preferably, according to an eighth aspect of the invention, the hydraulic motor includes a motor shaft disposed perpendicular to the drive shafts, and a bevel gear provided on the motor shaft meshes with a bevel input gear of the differential gear unit so as to drivingly connect the hydraulic motor to the differential gear unit. 
         [0021]    In the first aspect, preferably, according to a ninth aspect of the invention, the hydraulic motor includes a cylinder block whose rotary center axis is disposed coaxially to the drive shafts. The cylinder block is directly connected to an input portion of the differential gear unit. Therefore, the hydraulic motor, the differentially gear unit and the drive shafts are disposed coaxially so as to slim the steering transaxle. 
         [0022]    In the first aspect, preferably, according to a tenth aspect of the invention, the motor control mechanism is operatively connected to an active portion of a linkage between a steering operation device and each of the steerable wheels so as to control the slant angle of the movable swash plate according to the steered angle of the steerable wheels. 
         [0023]    In the first aspect, preferably, according to an eleventh aspect of the invention, the motor control mechanism includes: a rotary shaft supported by the cover to interlock with the active portion; a cam provided on the rotary shaft; and a control lever supported by the cover to interlock with the movable swash plate. The rotary shaft is rotated by movement of the active portion so that the cam acts to move the control lever with the movable swash plate. Therefore, the motor control mechanism becomes simple and economic. 
         [0024]    In the eleventh aspect, preferably, according to a twelfth aspect of the invention, the motor control mechanism further includes: a torque spring, one end of the torque spring engaging with the control lever; and a retaining member for retaining the other end of the torque spring. When the control lever is rotated by the action of the cam, the one end of the torque spring engaging with the control lever is moved away from the other end of the torque spring retained by the retaining member so as to cause a biasing force of the torque spring for returning the control lever with the movable swash plate. 
         [0025]    In the twelfth aspect, preferably, according to a thirteenth aspect of the invention, the position of the retaining member retaining the other end of the torque spring can be adjusted. Therefore, the position of the movable swash plate, i.e., the displacement of the hydraulic motor during straight traveling of a vehicle can be adjusted. 
         [0026]    In the first aspect, preferably, according to a fourteenth aspect of the invention, the cover is disposed on a proximal side of the transaxle casing lengthwise of a vehicle having the steering transaxle. Alternatively, according to a fifteenth aspect of the invention, the cover is disposed on a distal side of the transaxle casing lengthwise of a vehicle having the steering transaxle. Any of the proximal and distal sides of the transaxle casing can be optionally selected for arranging the cover with the exposed motor control mechanism and hydraulic oil port in consideration of positions of other members, like a tie rod or a power steering actuator, adjacent to the transaxle casing, thereby ensuring a good layout of the steering transaxle. 
         [0027]    A second object of the invention is to provide a hydraulic driving vehicle having front and rear hydraulic motors for steerable wheels and unsteerable wheels, which can turn or brake-turn at a moderate speed while regulating the speed ratio between the steerable wheels and the unsteerable wheels. 
         [0028]    To achieve the second object, according to a sixteenth aspect of the invention, a vehicle comprises: left and right steerable wheels; a fixed displacement hydraulic motor for driving the steerable wheels; left and right unsteerable wheels; a variable displacement hydraulic motor for driving the unsteerable wheels; and a hydraulic pump for supplying oil to the fixed displacement hydraulic motor and the variable displacement hydraulic motor. The displacement of the variable displacement hydraulic motor is changed according to the steered angle of the steerable wheels. Therefore, to regulate the speed ratio between the steerable wheels and the unsteerable wheels during turning of the vehicle so that the vehicle can turn at a moderate speed, the unsteerable wheels are decelerated instead of the steerable wheels being accelerated. 
         [0029]    In the sixteenth aspect of the invention, preferably, according to a seventeenth aspect of the invention, the vehicle further comprises a pair of brakes for braking the respective unsteerable wheels. The brakes can be actuated for braking individually. When one of the brakes is actuated for braking, the displacement of the variable displacement hydraulic motor is changed in addition to the change thereof according to the steered angle of the steerable wheels. Therefore, the speed of the vehicle in brake-turn can be further moderated. 
         [0030]    Alternatively, to achieve the second object, according to an eighteenth aspect of the invention, a vehicle comprises: left and right steerable wheels; a variable displacement hydraulic motor for driving the steerable wheels; left and right unsteerable wheels; a variable displacement hydraulic motor for driving the unsteerable wheels; and a hydraulic pump for supplying oil to the hydraulic motors. The displacement of at least one of the hydraulic motors is changed according to the steered angle of the steerable wheels. Any of the hydraulic motors may be selectively changed in displacement during turning of the vehicle correspondingly to various conditions. 
         [0031]    In the eighteenth aspect, preferably, according to a nineteenth aspect of the invention, the vehicle further comprises a pair of brakes for braking the respective unsteerable wheels. The brakes can be actuated for braking individually. When one of the brakes is actuated for braking, the displacement of at least one of the hydraulic motors is changed in addition to the change of displacement according to the steered angle of the steerable wheels. Any of the hydraulic motors may be selectively changed in displacement during turning and brake-turning of the vehicle correspondingly to various conditions. Further, it is possible to reduce the degree of displacement change of each of the hydraulic motors. 
         [0032]    In the nineteenth aspect of the invention, preferably, according to a twentieth aspect of the invention, the displacement of one of the hydraulic motors is changed according to the steered angle of the steerable wheels, and the displacement of the other hydraulic motor is changed when one of the brakes is actuated for braking. Therefore, the degree of displacement change of each of the hydraulic motors corresponding to turning and brake-turning of the vehicle is reduced. 
         [0033]    These, further and other objects, features and advantages will appear more fully from the following detailed description with reference to accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0034]      FIG. 1  is a diagram of a four-wheel driving vehicle  100  equipped with an unsteering transaxle  10  and a steering transaxle  1 , showing its mechanical and hydraulic structure, wherein a variable displacement hydraulic motor  30  is disposed in steering transaxle  1 , and a fixed displacement hydraulic motor  82  in unsteering transaxle  10 . 
           [0035]      FIG. 2  is a fragmental sectional plan view of steering transaxle  1 , showing that variable displacement hydraulic motor  30  supported by a motor support cover  31  is drivingly connected to a differential gear unit  25  in a transaxle casing  21 . 
           [0036]      FIG. 3  is a cross sectional view taken along III-III line of  FIG. 2 . 
           [0037]      FIG. 4  is a cross sectional view taken along IV-IV line of  FIG. 2  when motor support cover  31  is removed from transaxle casing  21 . 
           [0038]      FIG. 5  is the same view as  FIG. 4  when transaxle casing  21  is covered with motor support cover  31 . 
           [0039]      FIG. 6  is a sectional rear view of a right axle support unit  23 R attached to a right end of a right casing part  21 R of transaxle casing  21 . 
           [0040]      FIG. 7  is a sectional rear view of an alternative right axle support unit  23 R′ attached to a right end of an alternative right casing part  21 R′ of transaxle casing  21 . 
           [0041]      FIG. 8  is a fragmental sectional plan view of steering transaxle  1 , showing that variable displacement hydraulic motor  30  supported by an alternative motor support cover  71  is drivingly connected to differential gear unit  25  in transaxle casing  21 . 
           [0042]      FIG. 9  is a fragmental sectional plan view of steering transaxle  1 , showing that an alternative variable displacement hydraulic motor  130  supported by an alternative motor support cover  131  is drivingly connected to an alternative differential gear unit  125  in an alternative transaxle casing  121 . 
           [0043]      FIG. 10  is a cross sectional view taken along IX-IX line of  FIG. 9 . 
           [0044]      FIG. 11  is a sectional view of unsteering transaxle  10  with an HST  80 . 
           [0045]      FIG. 12  is a sectional view of a center section  87  for HST  80 . 
           [0046]      FIG. 13  is a diagram of an alternative four-wheel driving vehicle  100 A equipped with an alternative unsteering transaxle  10 A and an alternative steering transaxle  1 A, showing its mechanical and hydraulic structure, wherein mechanical auxiliary transmissions  19  and  95  are disposed in respective transaxles  1 A and  10 A. 
           [0047]      FIG. 14  is a diagram of an alternative four-wheel driving vehicle  100 B equipped with an alternative unsteering transaxle  10 B and an alternative steering transaxle  1 B, showing its mechanical and hydraulic structure, wherein a fixed displacement hydraulic motor  230  is disposed in steering transaxle  1 B, and a variable displacement hydraulic motor  282  in unsteering transaxle  10 B. 
           [0048]      FIG. 15  is a fragmental sectional plan view of steering transaxle  1 B, showing that fixed displacement hydraulic motor  230  supported by an alternative motor support cover  231  is drivingly connected to differential gear unit  25  in transaxle casing  21 . 
           [0049]      FIG. 16  is a sectional view of unsteering transaxle  10 B with an HST  280 . 
           [0050]      FIG. 17  is a diagram of an alternative four-wheel driving vehicle  100 C equipped with unsteering transaxle  10 B and steering transaxle  1 B, showing its mechanical and hydraulic structure, wherein variable displacement hydraulic motor  282  is servo-controlled. 
           [0051]      FIG. 18  is a fragmental sectional plan view of steering transaxle  1 B, showing that fixed displacement hydraulic motor  230 A supported by an alternative motor support cover  271  is drivingly connected to differential gear unit  25  in transaxle casing  21 . 
           [0052]      FIG. 19  is a diagram of an alternative four-wheel driving vehicle  100 D equipped with an alternative unsteering transaxle  10 C and steering transaxle  1 , showing its mechanical and hydraulic structure, wherein servo-controlled variable displacement hydraulic motor  30  is disposed in steering transaxle  1 , and servo-controlled variable displacement hydraulic motor  282  and a mechanical auxiliary transmission  95   a  are disposed in unsteering transaxle  10 C. 
           [0053]      FIG. 20  is a table representing a control pattern of movable swash plates  35  and  282   b  of hydraulic motors  30  and  282  in vehicle  100 D. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0054]    As shown in  FIG. 1 , a four-wheel driving vehicle  100  is equipped with a steering transaxle  1  and an unsteering transaxle  10 . In the following description, for convenience, steering transaxle  1  is referred to as a front transaxle, and unsteering transaxle  10  as a rear transaxle, however, they may be exchanged in the fore-and-aft direction of vehicle  100 . The same is said about later-discussed alternative vehicles. 
         [0055]    Front steering transaxle  1  incorporates a variable displacement hydraulic motor  30  for driving front steerable wheels  22 L and  22 R, and rear unsteering transaxle  10  is provided with a fixed displacement hydraulic motor  82  for driving rear unsteerable wheels  92 L and  92 R. Hydraulic motors  30  and  82  are fluidly connected in tandem to a variable displacement hydraulic pump  81  so as to constitute a hydrostatic transmission (HST) circuit for driving four wheels  22 L,  22 R,  92 L and  92 R. Hydraulic pump  81  has a movable swash plate  81   b  operatively connected to a speed control lever  15  provided on vehicle  100 . 
         [0056]    An output shaft of an engine  4  is drivingly connected to an input shaft (a pump shaft)  81   a  of hydraulic pump  81 . Pump shaft  81   a  also serves as a driving shaft of a charge pump  16 . Hydraulic pump  81  and motor  82  constitute an HST  80  for driving rear unsteerable wheels  92 L and  92 R of rear unsteering transaxle  10 . HST  80  includes outwardly open ports  87   f  and  87   g  fluidly connected to a drive mode switching valve  89  which is shiftable between a two-wheel drive position and a four-wheel drive position. Pipes  88   a  and  88   b  are extended from valve  89  to external ports  37   c  and  37   d  of front steering transaxle  1  for hydraulic oil supply and delivery to and from hydraulic motor  30 . 
         [0057]    When valve  89  is set at the two-wheel drive position, ports  87   f  and  87   g  are mutually connected so as to make HST  80  into a closed circuit isolated from hydraulic motor  30 , thereby transmitting the power of engine  4  to only rear unsteerable wheels  92 L and  92 R. When valve  89  is set at the four-wheel drive position, ports  87   f  and  87   g  are connected to respective pipes  88   a  and  88   b  so as to realize the four-wheel driving HST circuit including hydraulic pump  81  and motors  82  and  30 , thereby transmitting the power of engine  4  to all wheels  22 L,  22 R and  92 L ad  92 R. 
         [0058]    HST  80  for driving unsteerable wheels  92 L and  92 R integrally provided in unsteering transaxle  10  may be alternatively separated from unsteering transaxle  10 , or only one of hydraulic pump  81  and motor  82  may be provided in unsteering transaxle  10 . 
         [0059]    As shown in  FIGS. 1 and 2 , steering transaxle  1  includes a transaxle casing  21  and left and right axle support units  23 L and  23 R steerably mounted on respective left and right ends of transaxle casing  21 . A steering wheel  14  is interlockingly connected to axle support units  23 L and  23 R so as to steer them. Axle support units  23 L and  23 R incorporate respective deceleration gear trains (see  FIG. 6 ) and support respective front wheels  22 L and  22 R drivingly connected to the deceleration gear trains at the outsides thereof so that front wheels  22 L and  22 R serve as steerable wheels. 
         [0060]    As shown in  FIG. 5 , transaxle casing  21  consists of a left casing part  21 L and a right casing part  21 R fastened to each other by bolts. Right casing part  21 R is provided at the top portion thereof with a penetrating hole  21   u  through which a center pin  6  is passed so as to suspend transaxle casing  21  from a chassis of vehicle  100 . 
         [0061]    As shown in  FIGS. 1 and 2 , transaxle casing  21  incorporates a differential gear unit  25  and left and right drive shafts  24 L and  24 R mutually differentially coupled by differential gear unit  25 . Transaxle casing  21  has an opening  21   d  between left and right casing parts  21 L and  21 R, which is covered with a motor support cover  31 . Variable displacement hydraulic motor  30  is fixed onto an inside surface of motor support cover  31  so as to be disposed in transaxle casing  21 . A motor control mechanism  40  for controlling the displacement of hydraulic motor  30  (the slant angle of a movable swash plate  35  of hydraulic motor  30 ), and hydraulic oil ports  37   c  and  37   d  for supply and delivery of oil to and from hydraulic motor  30  are provided on an outside surface of motor support cover  31  so as to be exposed to the outside of transaxle casing  21 . Hydraulic oil ports  37   c  and  37   d  serve as pipe connectors connected to hydraulic oil pipes  88   a  and  88   b.    
         [0062]    Hydraulic motor  30 , motor control mechanism  40  and hydraulic oil ports  37   c  and  37   d  are integrated with motor support cover  31  so as to constitute an assembly unit, which is removed from transaxle casing  21  by removing motor support cover  31  from transaxle casing  21 , thereby facilitating their easy maintenance or adjustment. Further, by removing hydraulic motor  30  together with motor support cover  31  from transaxle casing  21 , parts including differential gear unit  25  disposed in transaxle casing  21  are exposed so as to facilitate their easy maintenance. 
         [0063]    Since the deceleration gear train in each of axle support units  23 L and  23 R has a large deceleration ratio, hydraulic motor  30  is allowed to have a small displacement. As a result, hydraulic motor  30  can be downsized and lightened, and its motor shaft  33  can be rotated at high speed and low torque by the hydraulic oil reduced in pressure by driving hydraulic motor  82  preceding hydraulic motor  30 . 
         [0064]    Differential gear unit  25  will be described. As shown in  FIG. 2 , differential gear unit  25  differentially coupling drive shafts  24 L and  24 R to each other is disposed in right casing part  21 R. Differential gear unit  25  comprises a differential casing  25   a , a bull gear  25   b , a pinion shaft  25   c , bevel pinions  25   d  and bevel differential side gears  25   e . Differential casing  25   a  is journalled at left and right ends thereof by transaxle casing  21  (right casing part  21 R) through respective bearings  21   b  and relatively rotatably penetrated by drive shafts  24 L and  24 R at the respective left and right ends thereof. Bull gear  25   b  is fixed on the outer periphery of differential casing  25   a  so as to serve as an input gear of differential gear unit  25 . Pinion shaft  25   c  is supported in differential casing  25   a  perpendicularly to drive shafts  24 L and  24 R. Pinions  25   d  are oppositely pivoted on pinion shaft  25   c . Each of differential side gears  25   e  is fixed on each of drive shafts  24 L and  24 R and disposed in differential casing  25   a  so as to mesh with both pinions  25   d.    
         [0065]    Bull gear  25   b  meshes with a motor gear  33   c  fixed on motor shaft  33  of hydraulic motor  30  so as to constitute a deceleration gear train between hydraulic motor  30  and differential gear unit  25 . Since motor shaft  33  and drive shafts  24 L and  24 R are parallel, mutually meshing bull gear  25   b  and motor gear  33   c  may be inexpensive spur gears or helical gears, and the width of steering transaxle  1  lengthwise of vehicle  100  can be shortened regardless of the length of motor shaft  33 . 
         [0066]    Hydraulic motor  30  will be described. As shown in  FIGS. 2 and 4 , right casing part  21 R has a portion  21   e  projecting rearward behind differential gear unit  25  such as to form a motor chamber  21   c  therein. Opening  21   d  is provided at the rear end of portion  21   e  so as to open motor chamber  21   c.    
         [0067]    As shown in  FIGS. 2 and 3 , a center section  32  is integrally formed of motor support cover  31 . Hydraulic motor  30  is mounted on the inside surface of center section  32 , and hydraulic oil ports  37   c  and  37   d  on the outside surface of center section  32 . 
         [0068]    Movable swash plate  35  has front and rear trunnion shaft shafts  35   a  and  35   b . Trunnion shaft  35   a  is rotatably supported by transaxle casing  21  (right casing part  21 R), and trunnion shaft  35   b  by motor support cover  31 . 
         [0069]    The interior space of transaxle casing  21  is filled with oil so as to be tightly fluidly isolated from the outside of transaxle casing  21 , including motor chamber  21  c covered with motor support cover  31 , but excluding hydraulic oil ports  37   c  and  37   d.    
         [0070]    Left and right bearing support walls  31   a  and  31   b  are integrally formed of motor support cover  31  so as to project in parallel with center section  32  therebetween. Bearings  33   a  and  33   b  are fitted in respective bearing support walls  31   a  ad  31   b  so as to journal left and right ends of motor shaft  33 . Center section  32  has a central axial penetrating hole  32   a  through which motor shaft  33  is rotatably passed at its intermediate portion. 
         [0071]    As shown in  FIGS. 2 and 3 , center section  32  is formed therein with upper and lower oil passages  32   c  and  32   d . Ports  37   c  and  37   d  are fitted onto respective outer ends of oil passages  32   c  and  32   d . Center section  32  is also formed therein with kidney ports  38   c  and  38   d  open at a right surface thereof in communication with respective oil passages  32   c  and  32   d.    
         [0072]    As shown in  FIG. 2 , a cylinder block  34  is disposed between center section  32  and bearing support wall  31   a , and slidably rotatably fitted to center section  32  through a valve plate  36  so as to be fluidly connected to kidney ports  38   c  and  38   d . Cylinder block  34  is not-relatively rotatably fitted on motor shaft  33 , which serves as the rotary center axis of cylinder block  34 . 
         [0073]    Pistons  39  are reciprocally fitted in cylinder block  34  around motor shaft  33 , and a spring  34   b  is wound around motor shaft  33  in cylinder block  34  so as to bias cylinder block  34  toward center section  32 . 
         [0074]    Movable swash plate  35  freely penetrated by motor shaft  33  is disposed between cylinder block  34  and bearing support wall  31   a , and pivotally supported on cylinder block  34 . Movable swash plate  35  has a thrust bearing  35   c  fitting heads of pistons  39 . By rotating movable swash plate  35  around trunnion shafts  35   a  and  35   b , strokes of pistons  39  are changed so as to change the displacement of hydraulic motor  30 . 
         [0075]    As shown in  FIG. 2 , motor gear  33   c  meshing with bull gear  25   b  of differential gear unit  25  is disposed between center section  32  and bearing support wall  31   b.    
         [0076]    Transaxle casing  21  (right casing part  21 R) is formed with a boss  21   a  between motor chamber  21   c  and the chamber incorporating drive shaft  24 R in front thereof. When motor support cover  31  is fitted onto transaxle casing  21  (right casing part  21 R), trunnion shaft  35   a  is rotatably fitted into boss  21   a , thereby easily locating hydraulic motor  30  in transaxle casing  21  (casing part  21 R). 
         [0077]    Motor control mechanism  40  for controlling the displacement of hydraulic motor  30  will be described. As shown in  FIGS. 2 ,  4  and  5 , motor control mechanism  40  provided on motor support cover  31  is disposed on the proximal (rear) side of transaxle casing  21  in the longitudinal direction of vehicle  100  so as to be prevented from colliding with obstacles during forward traveling of vehicle  100 . 
         [0078]    To ensure a good balance or compactness, a tie rod and a linkage from steering wheel  14  (such as a power steering cylinder) are appreciated to be disposed opposite to motor control mechanism  40  with respect to transaxle casing  21 . Therefore, the tie rod and the linkage from steering wheel  14  come into a space in front of transaxle casing  21 , i.e., on the distal side of transaxle casing  21  in the longitudinal direction of vehicle  100 . Alternatively, if protection of the tie rod and the like is preferred to that of motor control mechanism  40 , they may be disposed behind transaxle casing  21  (on the proximal side of transaxle casing  21  in the longitudinal direction of vehicle  100 ), and motor control mechanism  40  with motor support cover  31  in front of transaxle casing  21  (on the distal side of transaxle casing  21  in the longitudinal direction of vehicle  100 ). Motor support cover  31  may be also disposed at the front surface of transaxle casing  21  if an engine oil pan or a linkage for suspending a mid-mount working machine, e.g., a mower, must be disposed behind transaxle casing  21 . The same is said about later-discussed various alternative steering transaxles. 
         [0079]    As shown in  FIGS. 2 and 5 , trunnion shaft  35   b  supported by motor support cover  31  projects at its rear end rearward from motor support cover  31 . A control lever  42  has a boss portion  42   a  fixed on the rear end of trunnion shaft  35   b . Doglegged control lever  42  includes a first arm  42   b  extended rightward from boss portion  42   a , and a second arm  42   c  extended downwardly leftward from boss portion  42   a.    
         [0080]    On the right side of trunnion shaft  35   b , motor support cover  31  is bored by a penetrating hole  31   g  through which a camshaft  41  is rotatably passed. A rear end of camshaft  41  projects rearward from motor support cover  31  and its upper portion is cut away so as to form its lower portion into a semicircular cam  41  a having a flat surface and left and right angled ends of the flat surface. The angled ends serve as cam profiles of cam  41   a.    
         [0081]    An arm  43  is fixed at its bottom end to camshaft  41  in front of cam  41   a . A link rod  44  is pivotally connected at one end thereof to a top end of arm  43 . Link rod  44  is connected at the other end thereof to an active portion of a linkage between steering wheel  14  and steerable wheels  22 L and  22 R. As shown in  FIG. 1 , for example, an arm  14   a  rotated according to the rotation of steering wheel  14  may be operatively connected link rod  44 . Alternatively, link rod  44  may be connected to a steerable portion (such as a later-discussed steered casing  23   b ) of one of axle support units  23 L and  23 R. 
         [0082]    Referring to  FIG. 5 , when vehicle  100  travels straight, the flat surface of semicircular cam  41   a  is disposed horizontally and first arm  42   b  of control lever  42  is put on the flat surface of cam  41   a . When vehicle  100  turns either left or right so as to move link rod  44  rightward (as expressed by an arrow R in  FIG. 5 ), arm  43  rotates clockwise so as to raise the left cam profile of cam  41   a , thereby pushing up first arm  42   b . Therefore, control lever  42  is rotated counterclockwise so as to rotate movable swash plate  35  in the direction for reducing its slant angle, i.e., for reducing the displacement of hydraulic motor  30  (for increasing the rotary speed of hydraulic motor  30 ). 
         [0083]    If the turning direction of vehicle  100  is the other of left and right such as to move link rod  24  leftward (as expressed by an arrow L in  FIG. 5 ), arm  43  rotates counterclockwise so as to raise the right cam profile of cam  41   a , thereby pushing up first arm  42   b  and rotating control lever  42  counterclockwise. Consequently, whether vehicle  100  turns left or right, swash plate  35  rotates in the direction for reducing its slant angle, i.e., for reducing the displacement of hydraulic motor  30  (for increasing the rotary speed of hydraulic motor  30 ). 
         [0084]    As shown in  FIGS. 2 and 5 , a torque spring  45  is wound around boss portion  42   a  of control lever  42  so as to bias control lever  42  toward its initial position defining the maximum slant angle of movable swash plate  35 , i.e., the maximum displacement of hydraulic motor  30 . Both end portions  45   a  and  45   b  of spring  45  are twisted to cross each other and extended so as to pinch pins  42   d  and  46 . Pin  42   d  projects from second arm  42   c  of control lever  42 . If control lever  42  is rotated for reducing the slant angle of swash plate  35 , i.e., reducing the displacement of hydraulic motor  30 , pin  42   d  pushes one of end portions  45   a  and  45   b  of spring  45  away from the other retained by pin  46 , thereby causing the biasing force of spring  45  for returning control lever  42  to the initial position. 
         [0085]    Pin  46  has a pivot portion  46   a  planted in motor support cover  31  and an eccentric portion  46   b . Correctly, the portion of pin  46  pinched by end portions  45   a  and  45   b  of spring  45  is eccentric portion  46   b . Pivot portion  46   a  is screwed into motor support cover  31  and fastened to motor support cover  31  by nuts  47 . By loosening nuts  47 , pivot portion  46   a  can be rotated relative to motor support cover  31  so as to revolve eccentric portion  46   b  around pivot portion  46   a , thereby adjusting the initial position of control lever  42 . 
         [0086]    Each of symmetric axle support units  23 L and  23 R will be described with reference to representing right axle support unit  23 R shown in  FIG. 6 . Each of axle support units  23 L and  23 R includes a kingpin casing  23   a , a steered casing  23   b  and an axle casing  23   c . Kingpin casing  23   a  is fixed to an outer end of each of transaxle casings  21 L and  21 R and bent downward. Steered casing  23   b  is relatively rotatably disposed around the downwardly extending portion of kingpin casing  23   a . Upper and lower bearings  51   a  and  51   b  are interposed between kingpin casing  23   a  and steered casing  23   b  therearound. Steered casing  23   b  has a vertically cut open surface to which a bowl-like axle casing  23   c  is jointed. 
         [0087]    In the bent portion of kingpin casing  23   a , a bevel gear  52   a  is formed on the outer end of each of drive shafts  24 L and  24 R. A transmission shaft  52  is disposed on the central axis of the downwardly extending portion of kingpin casing  23   a . A bevel gear  52   b  fixed on the top end of transmission shaft  52  is rotatably supported by kingpin casing  23   a  through a bearing  51   c , and meshes with bevel gear  52   a . Transmission shaft  53  projects downward from the bottom end of the downwardly extending portion of kingpin casing  23   a  so as to be fixedly provided on its bottom end with a bevel gear  52   c  and rotatably supported on a bottom portion of steered casing  23   b  through a bearing  51   d.    
         [0088]    Bevel gear  52   c  meshes with a large bevel final gear  52   d  fixed on an axle  53 . Mutually joined steered casing  23   b  and axle casing  23   c  enclose final gear  52   d . Final gear  52   d  is rotatably supported through a bearing  51   c  by steered casing  23   b , and through a bearing  51   f  by axle casing  23   c . An outer portion of axle  54  projecting outward from axle casing  23   c  is formed into a flange  55  to be fixed to each of wheels  22 L and  22 R. 
         [0089]    In this way, gears  52   a ,  52   b ,  52   c  and  52   d  in each of axle support units  23 L and  23 R decelerate the output rotation of hydraulic motor  30  transmitted to axle  54  of each of steerable wheels  22 L and  22 R through each of drive shafts  24 L and  24 R and transmission shaft  53 . Such an arrangement of deceleration gears in axle support units  23 L and  23 R allow reduction of the displacement of hydraulic motor  30 , which enables high-speed and low-torque rotation of hydraulic motor  30  for saving load on hydraulic pump  81 , and facilitates compactness of transaxle casing  21  disposed just below the vehicle chassis. 
         [0090]    At least one of steered casings  23   b  (or axle casings  23   c ) is operatively connected to steering wheel  14 . Steering casings  23   b  of both axle support units  23 L and  23 R are mutually connected by a tie rod. Steered casing  23   b  of either axle support unit  23 L or  23 R may serve as the above-mentioned active portion to be operatively connected to arm  43  through link rod  44  so as to change the displacement of hydraulic motor  30  in association with the operation of steering wheel  14 . 
         [0091]      FIG. 7  illustrates an alternative right casing part  21 R′ of transaxle casing  21  and an alternative right axle support units  23 R′. They represent respective alternative left ones provided in the left portion of transaxle casing  21 , which are laterally symmetric with the illustrated right ones. A steered casing  61  is laterally rotatably fitted onto an outer end portion of right casing part  21 R′ of transaxle casing  21  through upper and lower coaxial kingpins  62   a  and  62   b . A bearing casing  68  and steered casing  61  with a ring gear  66   e  therebetween are fastened together by bolts so as to enclose a space  69  for incorporating a planetary gear mechanism  64  including ring gear  66   e.    
         [0092]    Planetary gear mechanism  64  comprises a sun gear  66   a , a planetary gear (or planetary gears)  66   d  and a carrier  66   b  in addition to ring gear  66   e . Sun gear  66   a  is fixed on a distal end of sun gear shaft  63   a  rotatably supported by steered casing  61  through a bearing  67   a , and by carrier  66   b  through a bearing  67   b . Between steered casing  61  and casing part  21 R′, a universal joint  63  couples sun gear shaft  63   a  to drive shaft  24 R. The bending pivot point between sun gear shaft  63   a  and drive shaft  24 R is disposed between upper and lower kingpins  62   a  and  62   b.    
         [0093]    Carrier  66   b  surrounding sun gear  66   a  and sun gear shaft  63   a  is rotatably supported by steered casing  61  through a bearing  67   c , and by bearing casing  68  through a bearing  67   d . Planetary gear (gears)  66   d  is (are) pivoted by carrier  66   b  through a pivot pin (respective pivot pins)  66   c . Planetary gear (gears)  66   d  meshes (mesh) with sun gear  66   a  and an internal gear formed on the inner periphery of ring gear  66   e . A flange  65  to be fixed to each of steerable wheels  22 L and  22 R is rotatably fitted on an outer end of steered casing  68 . Flange  65  has an axle portion  65   a  axially fitted into carrier  66   b  and fastened to carrier  66   b  by a bolt (or bolts). 
         [0094]    In this way, carrier  66   d  fixed to flange  65  and each of wheels  22 L and  22 R is rotated by revolution of planetary gear (gears)  66   d  around sun gear  66   a  that is being rotated by each of drive shafts  24 L and  24 R. 
         [0095]    At least one of steered casings  61  is operatively connected to steering wheel  14 . Steering casings  61  are mutually connected by a tie rod. One of steered casings  61  may serve as the above-mentioned active portion to be operatively connected to arm  43  through link rod  44  so as to change the displacement of hydraulic motor  30  in association with the operation of steering wheel  14 . 
         [0096]    When the axle supporting structure shown in  FIG. 6  is employed, axles (central axes) of steerable wheels  22 L and  22 R are disposed lower than drive shafts  24 L and  24 R. When the axle supporting structure shown in  FIG. 7  is employed, axles (central axes) of steerable wheels  22 L and  22 R are disposed as high as drive shafts  24 L and  24 R. Any of the two structures may be selected at need. 
         [0097]      FIG. 8  illustrates variable displacement hydraulic motor  30  in an alternative motor support cover  71  attached to transaxle casing  21 . Unless being specified, parts and members designated by the same reference numerals as those of  FIG. 2  are identical in structure or function to those of  FIG. 2 . 
         [0098]    Motor support cover  71  supporting hydraulic motor  30  is fastened to transaxle casing  21  through a center section  72  of hydraulic motor  30 . A movable swash plate  75  of hydraulic motor  30  has trunnion shafts  75   a  and  75   b  rotatably supported by motor support cover  71 . Trunnion shafts  75   a  and  75   b  are disposed in parallel to drive shafts  24 L and  24 R. 
         [0099]    Motor support cover  71  includes a casing part  71   a  and a casing cover part  71   b . Casing part  71   a  is penetrated by a hole  71   f  into which trunnion shaft  75   b  is rotatably fitted. Casing cover part  71   b  having a recess  71   g , into which trunnion shaft  75   a  is rotatably fitted, is fixedly fitted onto an (left) open side surface of casing part  71   a  so that casing part  71   a  and casing cover part  71   b  encloses a motor chamber  71   c  for incorporating hydraulic motor  30 . By removing casing cover part  71   b  from casing part  71   a , the open side surface of casing part  71   a  is exposed so as to facilitate disassembly of hydraulic motor  30 . 
         [0100]    Hydraulic motor  30  has a motor shaft  73  perpendicular to drive shafts  24 L and  24 R. Motor shaft  73  is rotatably passed through a penetrating hole  72   a  of center section  72 . Motor shaft  73  is journalled at its rear end by cover part  71   a  through a bearing  73   a . A front end of motor shaft  73  is rotatably fitted into a boss portion  21   h  formed in transaxle casing  21  (right casing part  21 R) through a bearing collar  73   b . A motor gear  73   c  is fixed on motor shaft  73  between bearing collar  73   b  and center section  72 . Differential gear unit  25  has an alternative bevel bull gear  25   g  fixed on differential casing  25   a  meshing with motor gear  73   c.    
         [0101]    Center section  72  is a member separated from motor support cover  71  and transaxle casing  21  (right casing part  21 R) and fixedly sandwiched between motor support cover  71  and casing part  21 R. Hydraulic oil passages  78   c  and  78   d  are bored in center section  72 , and hydraulic oil ports  78   a  serving as pipe connectors are externally fitted on center section  72  so as to communicate with respective passages  78   c  and  78   d.    
         [0102]    A cylinder block  74  is not-relatively rotatably fitted on motor shaft  73  serving as the rotary center axis of cylinder block  74 , and slidably rotatably fitted to the rear end of center section  72  through a valve plate  76 . Pistons  79  are reciprocally fitted into respective cylinder holes  74   a  formed in cylinder block  74 . Hydraulic oil passages  78   c  and  78   d  fluidly communicate with cylinder holes  74   a  through valve plate  76 . Heads of pistons  79  project rearward from cylinder block  74  and abut against movable swash plate  75  therebehind. Motor shaft  73  is freely passed through movable swash plate  75 . 
         [0103]    Motor control mechanism  40  including camshaft  41  and control lever  42 , similar to that of  FIG. 2 , is provided on casing part  71   a  of motor support cover  71 . However, since trunnion shaft  75   b  serving as the rotary center shaft of control lever  42  is disposed in parallel to drive shafts  24 L and  24 R, camshaft  41  also becomes parallel to drive shafts  24 L and  24 R. Therefore, arm  43  fixed on camshaft  41  is rotatable perpendicularly to drive shafts  24 L and  24 R (lengthwise of vehicle  100 ). Arm  43  may be interlockingly connected to any active portion of the linkage between steering wheel  14  and each of steerable wheels  22 L and  22 R. The only important function for arm  43  is to transmit the degree of left and right turning of steerable wheels  22 L and  22 R. 
         [0104]    Hydraulic motor  30  including center section  72 , and motor control mechanism  40  are integrated with motor support cover  71  so as to be made into an assembly unit, thereby facilitating their easy maintenance or adjustment. When this assembly unit is assembled with transaxle casing  21 , center section  72  is fixedly fitted onto the rear end of right casing part  21 R so as to cover an opening  21   d ′ formed in right casing part  21 R. The front end of motor shaft  73  is inserted into boss portion  21   h  of casing part  21 R through bearing collar  73   b , thereby easily drivingly connecting hydraulic motor  30  to differential gear unit  25 . By separating center section  72  from casing part  21 R, hydraulic motor  30  together with motor support cover  71  can be easily removed from transaxle casing  21 . 
         [0105]    To ensure a good balance or compactness, the tie rod and the linkage from steering wheel  14  are preferably disposed opposite to motor support cover  71  with respect to transaxle casing  21 , i.e., in front of transaxle casing  21 . Alternatively, if the tie rod, an engine oil pan, a linkage for supporting a mid-mount working machine or so on must be disposed behind transaxle casing  21 , motor support cover  71  with motor control mechanism  40  may be disposed in front of transaxle casing  21 . 
         [0106]      FIGS. 9 and 10  illustrate steering transaxle  1  having an alternative transaxle casing  121  incorporating an alternative hydraulic motor  130  and an alternative differential gear unit  125 . Transaxle casing  121  has a rear opening  121   d  covered with a motor support cover  131 . As shown in  FIG. 9 , transaxle casing  121  has a front wall  121   f  toward which a center section  132  integrally formed of motor support cover  131  is extended. In transaxle casing  121 , hydraulic motor  130  and differential gear unit  125  are distributed right and left, opposite to each other, with respect to center section  132 . 
         [0107]    Center section  132  has a penetrating hole  132   c  through which drive shaft  24 R is freely passed. Further, drive shaft  24 R is disposed on the center axis of cylinder block  134  of hydraulic motor  130  and freely penetrates cylinder block  134 , a movable swash plate  135 , and a guide block  136  supporting movable swash plate  135 . 
         [0108]    Differential gear unit  125  includes a differential casing  125   a . A boss portion  125   z  is integrally formed on the right end of differential casing  125   a , and extended into penetrating hole  132   c  around drive shaft  24 R so as to rotatably fit center section  132  through a bearing  121   b . Differential casing  125   a  is journalled at the left end thereof by a bearing  121   a  which is fitted in a support block  131   a  fastened to motor support cover  131  by a bolt. Boss portion  125   z  surrounding drive shaft  24 R projects rightward from penetrating hole  132   c . Boss portion  125   z  is spline-fitted into a center axial hole  134   c  of cylinder block  134  slidably rotatably fitted onto the right surface of center section  132 , thereby serving as an axial motor shaft of hydraulic motor  130 . In this way, differential casing  125   a  is rotatable integrally with cylinder block  134 . Further, differential casing  125   a  and hydraulic motor  130  are disposed coaxially around drive shaft  24 R so as to facilitate compactness of steering transaxle  1 . 
         [0109]    Upper and lower hydraulic oil passages  132   a  and  132   b  are bored in center section  132 , and outwardly open at a rear surface of motor support cover  131 . Hydraulic oil ports  133   a  and  133   b  serving as pipe connectors are externally fitted on the rear surface of motor support cover  131  so as to communicate with respective passages  132   a  and  132   b.    
         [0110]    Parallel cylinder holes  134   a  are bored in cylinder block  134  around center axial hole  134   c . Pistons  139  are reciprocally fitted into respective cylinder holes  134   a  and abut at head thereof against a thrust bearing  135   a  of movable swash plate  135 . Guide block  136  fixed to motor support cover  131  has an arcuate guide surface  136   a  to which movable swash plate  135  is slidably fitted. In this way, movable swash plate  135  is made as a cradle type. 
         [0111]    Hydraulic motor  130  is provided with a motor control mechanism  140  assembled with motor support cover  131 . Referring to motor control mechanism  140 , upper and lower projections  135   b  are formed on the rear end of movable swash plate  135 , as shown in  FIG. 10 . An arm  137   a  is clamped at one end thereof between projections  135   b . Arm  137   a  is disposed perpendicular to drive axles  24 L and  24 R and rotatably supported by motor support cover  131 . The other end of arm  137   a  is formed into a boss fixed on an inner front end portion of a control shaft  137 . By rotating control shaft  137 , arm  137   a  is rotated together with control shaft  137  so as to change the slant angle of movable swash plate  135 , thereby changing the displacement of hydraulic motor  130 . 
         [0112]    A torque spring  145  is wound around the boss of arm  137   a  on control shaft  137 . A second arm  137   b  is branched from arm  137   a , and a pin  142   d  projects from second arm  137   b . Both end portions  145   a  and  145   b  are twisted to cross each other and extended to clamp pin  142   d  and an eccentric portion  146   b  of a retaining pin  146  when movable swash plate  135  is disposed at its initial position for defining the maximum displacement of hydraulic motor  130 . If control shaft  137  is rotated for reducing the displacement of hydraulic motor  130 , pin  142   d  of arm  137   b  rotating together with control shaft  137  pushes one of end portions  145   a  and  145   b  away from the other end portion  145   b  or  145   a  retained by eccentric portion  146   a  of retaining pin  146  so as to generate the biasing force of spring  145  for returning movable swash plate  135  to its initial position for ensuring the maximum displacement of hydraulic motor  130 . 
         [0113]    Retaining pin  146  has a pivot portion pivotally planted in motor support cover  131 . Eccentric portion  146   a  is eccentrically extended from the pivot portion. The pivot portion of retaining pin  146  is normally fastened to motor support cover  131  by a nut, for example. By loosening the nut and letting the pivot portion rotate relative to motor support cover  131 , eccentric portion  146   a  revolves around the pivot portion so as to adjust the initial position of arm  137   a  with movable swash plate  135 . 
         [0114]    A boss portion  142   a  of a control arm  142  is fixed on the rear end of control shaft  137  behind motor support cover  131 . A camshaft  141  is disposed in parallel to control shaft  137  and rotatably supported by motor support cover  131 . The rear end of camshaft  141  projecting rearward from motor support cover  131  is formed into a semicircular cam  141  a whose flat surface confronts an upper edge of control arm  142 . An arm  143  is fixed on camshaft  141 . Arm  143  may be interlockingly connected to any active portion of the linkage between steering wheel  14  and each of wheels  22 L and  22 R if arm  143  is rotated accordingly to the degree of left and right turning of wheels  22 L and  22 R. 
         [0115]    In this way, motor control mechanism  140 , including arms  137   a  and  137   b , control shaft  137 , control arm  142 , camshaft  141  and arm  143 , is assembled together with motor support cover  131  and hydraulic motor  130 . Such a resultant assembly unit can be easily attached or removed to and from transaxle casing  121 . 
         [0116]    Cylinder block  134  having center axial hole  134   c  for passing boss portion  125   z  of differential casing  125   a  and drive shaft  24 R becomes diametrically large so as to be reduced in rotary speed, thereby saving the size or parts count of a deceleration gear for ensuring a deceleration ratio between hydraulic motor  130  and axles of steerable wheels  22 L and  22 R. 
         [0117]    Incidentally, as shown in  FIG. 10 , transaxle casing  121  has hole  21   u  through which center pin  6  is passed for hanging down steering transaxle  1  from a chassis of vehicle  100 . 
         [0118]    Unsteering transaxle (main transaxle)  10  integrally provided with HST  80  including mutually fluidly connected hydraulic pump  81  and motor  82  will now be described with reference to  FIGS. 1 ,  11  and  12 . A casing of unsteering transaxle  10  is divided into an HST casing  80   a  and a differential gear casing  90 R by a plate-like center section  87 . 
         [0119]    HST casing  80   a  incorporates HST  80  including hydraulic pump  81  and motor  82  slidably rotatably fitted onto center section  87 . Hydraulic pump  81  has movable swash plate  81   b  operatively connected to speed control lever  15 . Hydraulic pump  81  has axial pump shaft  81   a  freely passing movable swash plate  81   b  and projecting outward from HST casing  80   a  to be drivingly connected to engine  4 . Pump shaft  8   l a is journalled by HST casing  80   a  and center section  87 . Pump shaft  81   a  penetrates center section  87  and extends opposite to HST casing  80   a  and adjacent to differential gear casing  90 R so as to serve as the drive shaft of charge pump  16  disposed opposite to hydraulic pump  81  with respect to center section  87 . Hydraulic motor  82  has axial motor shaft  82   a  disposed in parallel to pump shaft  81   a  and journalled by HST casing  80   a  and center section  87 . 
         [0120]    As shown in  FIG. 12 , center section  87  is formed therein with kidney ports  87   a  and  87   b  fluidly open to hydraulic pump  81 , and with kidney ports  87   d  and  87   e  fluidly open to hydraulic motor  82 . Ports  87   f  and  87   g  are outwardly open in center section  87  so as to be fluidly connected to drive mode switching valve  89 . In center section  87  are bored an oil passage  87   h  interposed between port  87   f  and kidney port  87   a , an oil passage  87   c  interposed between kidney ports  87   b  and  87   d , and an oil passage  87   j  interposed between kidney port  87   e  and port  87   g . In this way, center section  87  is formed therein with a part of the closed circuit of HST  80 . 
         [0121]    In center section  87  is also bored a charge oil passage  87   k  which can communicate with oil passages  87   h  and  87   c . A pair of check valves  99  are interposed between oil passage  87   c  and charge oil passage  87   k , and between oil passage  87   h  and charge oil passage  87   k , respectively. Charge oil passage  87   k  is supplied with oil from charge pump  16 , as shown in  FIG. 1 . Each of check valves  99  is opened to pass oil from charge oil passage  87   k  into corresponding oil passage  87   h  or  87   c  when corresponding oil passage  87   h  or  87   c  is hydraulically depressed. 
         [0122]    HST casing  80   a  also incorporates a PTO shaft  85  disposed in parallel to pump shaft  81   a  and motor shaft  82   a . PTO shaft  85  projects outward from HST casing  80   a  adjacent to center section  87 . In HST casing  80   a , a PTO clutch  85   a  and a PTO brake  85   b  are provided on PTO shaft  85 . PTO clutch  85   a  has a clutch input gear  86   b  relatively rotatably provided on PTO shaft  85 . A gear  86   a  is fixed on pump shaft  81   a  and meshes with gear  86   b  so as to transmit the power of engine  4  to PTO shaft  85  through PTO clutch  85   a.    
         [0123]    Motor shaft  82   a  is extended from center section  87  into differential gear casing  90 R opposite to hydraulic motor  82  so as to be fixedly provided on its end with a bevel motor gear  82   c . Differential gear casing  90 R incorporates a differential gear unit  84  differentially connecting axles  83 L and  83 R to each other. Unsteerable wheels  92 L and  92 R are fixed onto outer ends of respective axles  82 L and  82 R. A deceleration gear train including large and small gears  93   a  and  93   b  is interposed between motor shaft  82   a  and differential gear unit  84 . A deceleration gear shaft  93  is disposed in parallel to axles  83 L and  83 R. Large bevel gear  93   a  is fixed on deceleration gear shaft  93  and meshes with bevel motor gear  82   c . Small bevel gear  93   b  is formed on deceleration gear shaft  93  and meshes with a bull gear  84   a  of differential gear unit  84 . 
         [0124]    Differential gear casing  90 R has a large opening open at one of left and right sides (in this embodiment, the left side) thereof, and casing cover  90 L is attached to differential gear casing  90 R so as to cover the large opening, thereby supporting deceleration gear shaft  93  and differential gear unit  84 . By removing casing cover  90 L from differential gear casing  90 R, the large opening of differential gear casing  90 R is exposed so as to facilitate disassembly of differential gear unit  84  and deceleration gear shaft  93  with the deceleration gears. An axle casing  11 L incorporating axle  83 L is fixed at its proximate end to casing cover  90 L, and an axle casing  11 R incorporating axle  83 R to differential gear casing  90 R. The interior space of the joint portion of left axle casing  11 L and casing cover  90 L serves as a left brake chamber  27 L for incorporating a left brake  5 L for braking left axle  83 L, and the interior space of the joint portion of right axle casing  11 R and differential gear casing  90 R serves as a right brake chamber  27 R for incorporating a right brake  5 R for braking right axle  83 R, so that left and right brakes  5 L and  5 R are disposed symmetrically. 
         [0125]    With respect left brake  5 L in brake chamber  27 L between axle casing  11 L and casing cover  90 L, brake pads  8 L fitted to axle casing  11 L and brake disks  9 L fitted on axle  83 L are alternately aligned in the proximal portion of axle casing  11 L, and a pressure ring  7 L disposed around axle  83 L is fitted to casing cover  90 L so as to confront brake disks  9 L and pads  8 L. A brake shaft  3 L is rotatably supported by axle casing  11 L and casing cover  90 L, and a brake arm  12 L is fixed on the outer end of brake shaft  3 L. Brake shaft  3 L is partly formed into a cam  3   a  confronting a part of pressure ring  7 L. By rotating brake arm  12 L for braking, cam  3   a  acts to move pressure ring  7 L so as to press brake disks  9 L and pads  8 L against one another, thereby braking axle  83 L. Similar to left brake  5 L, right brake SR in brake chamber  27 R between differential gear casing  90 R and axle casing  11 R includes brake pads  8 R, brake disks  9 R, pressure ring  7 R, a brake shaft  3 R with a cam  3   a , and a brake arm  12 R. A parking brake manipulator is operatively connected to both brake arms  12 L and  12 R so as to brake axles  83 L and  83 R simultaneously. 
         [0126]    An alternative four-wheel driving vehicle  100 A shown in  FIG. 13  will be described. The same parts and members as those of  FIG. 1  are designated by the same reference numerals of  FIG. 1 , and so description of them will be omitted. 
         [0127]    Vehicle  100 A is equipped with a steering transaxle  1 A and an unsteering transaxle  10 A distributed before and behind. Steering transaxle  1 A and unsteering transaxle  10 A incorporate respective mechanical (multi-speed stage) auxiliary transmissions  19  and  95 , which are operatively connected to an auxiliary speed control lever  17 . 
         [0128]    In steering transaxle  1 A, an alternative motor supply cover  31 A supports hydraulic motor  30  and mechanical auxiliary transmission  19  driven by hydraulic motor  30  so as to constitute an assembly unit to be detachably attached to transaxle casing  21 . When the assembly unit is attached to transaxle casing  21 , motor supply cover  31 A is disposed to cover the opening (such as opening  21   d  or  21   d ′) of transaxle casing  21  so as to drivingly connect mechanical auxiliary transmission  19  to differential gear unit  25  in transaxle casing  21  through the opening of transaxle casing  21 . In this way, in steering transaxle  1 , mechanical auxiliary transmission  19  is drivingly interposed between hydraulic motor  30  and differential gear unit  25 . 
         [0129]    In unsteering transaxle  10 A, mechanical auxiliary transmission  95  is drivingly interposed between motor shaft  82   a  of hydraulic motor  82  and deceleration gear shaft  93 . Alternatively, it may be drivingly interposed between deceleration gear shaft  93  and differential gear unit  84 . 
         [0130]    An alternative four-wheel driving vehicle  100 B shown in  FIG. 14 ,  15  and  16  will be described.  FIGS. 14 ,  15  and  16  illustrate the same parts and members as those of  FIGS. 1 ,  2  and  11 , which are designated by the same reference numerals of  FIGS. 1 ,  2  and  11 , and so description of the parts and members will be omitted. Further,  FIGS. 3 ,  6  and  12  are applied for description of vehicle  100 B. 
         [0131]    The important distinctive point of vehicle  100 B compared with vehicle  100  is that an alternative steering transaxle  1  incorporates a fixed displacement hydraulic motor  230 , and an HST  280  in an unsteering transaxle  10 B includes a variable displacement hydraulic motor  282  fluidly connected to hydraulic pump  81 . In this regard, hydraulic motor  230  has a fixed swash plate  231   c  and axial motor shaft  33  in parallel to drive shafts  24 L and  24 R as shown in  FIG. 15 , and hydraulic motor  282  has a movable swash plate  282   b  and axial motor shaft  82   a  as shown in  FIG. 16 . 
         [0132]    With respect to steering transaxle  1 B having fixed displacement hydraulic motor  230 , a motor support cover  231  supporting hydraulic motor  230  is slimmed in comparison with motor support cover  31  shown in  FIG. 2  because hydraulic motor  230  needs no mechanism for controlling the slant angle of swash plate  231   c . A motor shaft support portion  231   a  formed of motor support cover  231  and a motor shaft support block  231   b  fixed to motor support cover  231  journal respective ends of motor shaft  33  of hydraulic motor  230 . Center section  32  formed of motor support cover  231  between motor shaft support portion  231   a  and motor shaft support block  231   b  passes motor shaft  33  therethrough and fits cylinder block  34  through valve plate  36  so as to constitute hydraulic motor  230 , similar to that of  FIG. 2 . Hydraulic motor  230  is disposed between center section  32  and fixed swash plate  231  c on motor shaft support block  231   b,  and motor gear  33   c  meshing with bull gear  25   b  of differential gear unit  25  is fixed on motor shaft  33  between motor support portion  231   a  and center section  32 . 
         [0133]    As shown in  FIG. 14 , a motor control unit  29  is provided for movable swash plate  282   b  of hydraulic motor  282  for reducing the rotary speed of unsteerable wheels  92 L and  92 R to balance with the rotary speed of steerable wheels  22 L and  22 R during turning of vehicle  100 B. A link rod  44 A is operatively interposed between steering wheel  14  and one of axle supply units  23 L and  23 R so as to steer axle supply units  23 L and  23 R according to rotation of steering wheel  14 . Link rod  44 A is also operatively connected to motor control unit  29  so as to change the slant angle of swash plate  282  according to the steered angle of axle supply units  23 L and  23 R. Instead of link rod  44 A connected to one of axle supply units  23 L and  23 R, motor control unit  29  may be operatively connected to any active portion of the linkage from steering wheel  14  to each of steerable wheels  22 L and  22 R. 
         [0134]    When vehicle  100 B travels straight, movable swash plate  282   b  is disposed at the minimum slant angle for defining the minimum displacement of hydraulic motor  282 . During turning of vehicle  100 B, the slant angle of movable swash plate  282   b  is increased to increase the displacement of hydraulic motor  282 , thereby decelerating unsteerable wheels  92 L and  92 R. 
         [0135]    Referring to  FIG. 17 , in an alternative vehicle  100 C having fixed displacement hydraulic motor  230  and variable displacement hydraulic motor  282 , left and right brakes  5 L and  5 R for braking respective axles  83 L and  83 R are operated individually, thereby enabling brake-turn of vehicle  100 C. In this regard, left and right brake pedals  59 L and  59 R are interlockingly connected to respective brake arms  12 L and  12 R. 
         [0136]    The slant angle of movable swash plate  282   b  of hydraulic motor  282  is controlled by actuation of a servo actuator  18 . Vehicle  100 C is equipped with a controller  19  from which a communication line  45  is extended to servo actuator  18 . The angle of link rod  44 A operatively interposed between steering wheel  14  and one of axle support units  23 L and  23 R is detected by a potentiometer, and its detection signal is inputted to controller  19 . Controller  19  outputs a command signal to servo actuator  18  according the detection signal about the angle of link rod  44 A. Alternatively, any active portion of the linkage from steering wheel  14  to each of steerable wheels  22 L and  22 R may serve as a detection target for controlling servo actuator  18 . 
         [0137]    Brake pedals  59 L and  59 R and an accelerator pedal  58  are provided with respective potentiometers electrically connected to controller  19 . Due to the setting of controller  19  for controlling servo actuator  18 , when steering wheel  14  is fully rotated and neither brake pedal  59 L nor  59 R is depressed, the periphery speed of steerable wheels  22 L and  22 R becomes about 1.5 to 1.7 times as large as that of unsteerable wheels  92 L and  92 R. If steering wheel  14  is fully rotated and one of brake pedals  59 L and  59 R is depressed, the slant angle of swash plate  282   b  is further increased to reduce the rotary speed of unsteerable wheels  92 L and  92 R so that the periphery speed of steerable wheels  22 L and  22 R becomes about twice as large as that of unsteerable wheels  92 L and  92 R. Therefore, reduction of depression of accelerator pedal  58  is unnecessary for brake-turn of vehicle  100 C. The deceleration rate of unsteerable wheels  92 L and  92 R by controlling of servo actuator  18  for brake-turn of vehicle  100 C may be inversely proportional to the traveling speed of vehicle  100 C. Further, vehicle  100 C includes an adjusting dial  17  for setting the actuation speed of servo actuator  18 . For example, adjusting dial  17  may be rotatable among low, medium and high speed positions. 
         [0138]      FIG. 18  illustrates transaxle casing  21  with an alternative motor support cover  271  incorporating an alternative fixed displacement hydraulic motor  230 A, in comparison with  FIG. 8 . Hydraulic motor  230 A has motor shaft  73 , which is perpendicular to drive shafts  24 L and  24 R. An open end of motor support cover  271  toward transaxle casing  21  is fixed to center section  72  so as to form a motor chamber  271   c  enclosed by motor support cover  271  and center section  71 . Hydraulic motor  230 A with a fixed swash plate  271   d  and a retainer  271   b  holding fixed swash plate  271   d  is supported between center section  72  and motor support cover  271  in motor chamber  271   c . Other part and members designated by the same reference numerals of  FIG. 8  are identical with those of  FIG. 8 , and so description of them is omitted. 
         [0139]    In comparison with motor support cover  71  of  FIG. 8 , motor support cover  271  of  FIG. 18  is slimmed because it supports fixed displacement hydraulic motor  230 A and no mechanism for controlling the slant angle of swash plate  271   d.    
         [0140]    An alternative vehicle  100 D shown in  FIG. 19  includes variable displacement hydraulic motor  30  for driving steerable wheels  22 L and  22 R and variable displacement hydraulic motor  282  for driving unsteerable wheels  92 L and  92 R. A servo actuator  18 A for controlling movable swash plate  35  of hydraulic motor  30  is connected to controller  19  through a communication line  45 A, and a servo actuator  10 B for controlling movable swash plate  282   b  of hydraulic motor  282  is connected to controller  19  through a communication line  45 B. 
         [0141]    Controller  19  controls servo actuator  18 A and  18 B based on the detection of the angle of link rod  44 A (or the detection of movement of any active portion of the linkage from steering wheel  14  to each of steerable wheels  22 L and  22 R) so that the periphery speed of steerable wheels  22 L and  22 R is balanced with that of unsteerable wheels  92 L and  92 R during turning of vehicle  100 D. Further, controller  19  controls servo actuator  18 A and  18 B based on the detection of depression of brake pedals  59 L and  59 R as well as the detection of the angle of link rod  44 A so that the periphery speed of steerable wheels  22 L and  22 R is balanced with that of unsteerable wheels  92 L and  92 R during the brake-turn of vehicle  100 D. 
         [0142]    Referring to  FIG. 20 , an example of control pattern of movable swash plates  35  and  282   b  will be described. While vehicle  100 D travels straight, slant angles D 2  of swash plates  35  and  282   b  are kept in constant. As steering wheel  14  is rotated for turning, the slant angle of swash plate  35  is reduced from angle D 2  so as to accelerate steerable wheels  22 L and  22 R while angle D 2  of swash plate  282   b  is kept. When steering wheel  14  is fully rotated without depression of brake pedals  59 L and  59 R, the reduced slant angle of swash plate  35  reaches minimum angle D 1  so that the periphery speed of steerable wheels  22 L and  22 R becomes about 1.5 to 1.7 times as large as that of unsteerable wheels  92 L and  92 R, for example. As one of brake pedals  59 L and  59 R is depressed during the full rotation of steering wheel  14 , the slant angle of swash plate  282   b  is increased from angle D 2  so as to decelerate unsteerable wheels  92 L and  92 R while minimum angle D 1  of swash plate  35  is kept. When the one of brake pedals  59 L and  59 R is fully depressed, the increased slant angle of swash plate  282   b  reaches maximum angle D 3  so that the periphery speed of steerable wheels  22 L and  22 R becomes about twice as large as that of unsteerable wheels  92 L and  92 R, for example. 
         [0143]    In comparison with the control of movable swash plate  35  of hydraulic motor  30  in vehicle  100  as shown in  FIG. 1  and with the control of movable swash plate  282   b  of hydraulic motor  282  in vehicle  100 B or  100 C as shown in  FIG. 14  or  17 , the movement of each of movable swash plates  35  and  282   b  in vehicle  100 D can be reduced because of cooperation of movable swash plates  35  and  282   b.    
         [0144]    Further, vehicle  100 D shown in  FIG. 19  is equipped with steering transaxle  1  (shown also in  FIG. 1 ) and an alternative unsteering transaxle  10 C. Unsteering transaxle  10 C is provided with HST  280  including hydraulic pump  81  and motor  282 , and incorporates a mechanical auxiliary transmission  95   a  including deceleration gear shaft  93  and a clutch shaft  98 . On deceleration gear shaft  93  are fixed bevel gear  93   a  meshing with bevel motor gear  82   c  fixed on motor shaft  82   a , a small low-speed gear  93   c  and a large high-speed gear  93   d . Clutch shaft  98  is disposed in parallel to deceleration gear shaft  93 . A large low-speed clutch gear  98   b  and a small high-speed clutch gear  98   c  are fixed together on a slider  98   a  slidably fitted on clutch shaft  98 . One of clutch gears  98   b  and  98   c  selectively mesh with corresponding gear  93   c  or  93   d . When clutch gear  98   b  meshes with gear  93   c , auxiliary transmission  95   a  is set at a low speed stage. When clutch gear  98   c  meshes with gear  93   d , auxiliary transmission  95   a  is set at a high speed stage. A gear  98   d  is fixed on clutch shaft  98  and meshes with bull gear  84   a  of differential gear unit  84 . 
         [0145]    In vehicle  100 D of  FIG. 19 , other parts and members designated by the same reference numerals of  FIGS. 1 ,  14  and  17  are identical with those of vehicles  100 ,  100 B and  100 C, and so description of them is omitted.