Patent Publication Number: US-8109355-B2

Title: Hydrostatic transaxle and hydraulically driven vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/214,056, filed Aug. 30, 2005, now U.S. Pat. No. 7,588,103, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydrostatic transaxle, which supports a pair of left and right steerable drive wheels, and incorporates a pair of hydraulic motors for driving the respective drive wheels. Further, the present invention relates to a hydraulically driven vehicle, which comprises a common hydraulic pressure source and at least two hydraulic motors connected in series to the common hydraulic pressure source, wherein one of the at least two hydraulic motors drives front drive wheels (or a front drive wheel), and the other drives rear drive wheels (or a rear drive wheel). 
     2. Related Art 
     Conventionally, as disclosed in International Publication WO 2004/062956, there is a well-known hydraulically driven vehicle having a main transaxle and an auxiliary transaxle. The main transaxle supports main drive wheels, and comprises a hydraulic motor for driving the main drive wheels. The auxiliary transaxle supports a pair of auxiliary drive wheels, and comprises a pair of hydraulic motors for driving the respective auxiliary drive wheels. The vehicle has a hydraulic circuit fluidly connecting the hydraulic motors of the main and auxiliary transaxles to a hydraulic pump. In the hydraulic circuit, the hydraulic motor of the main transaxle and the pair of hydraulic motors of the auxiliary transaxle are connected in series to the hydraulic pump, and the hydraulic motors of the auxiliary transaxle are connected to the hydraulic pump in parallel to each other. 
     The left and right auxiliary drive wheels are steerable wheels. Both of the hydraulic motors of the auxiliary transaxle are variable in displacement so as to prevent drag of the steerable wheels or the main drive wheels. The hydraulic motors of the auxiliary transaxle are provided with respective movable swash plates moved according to steering operation of the vehicle, or according to change of left or right turning angle of the steerable wheels, so as to change rotary speed of the steerable wheels, i.e., generate difference between the main drive wheels and the auxiliary drive wheels, according to change of the left or right turning angle of the vehicle. However, the link mechanism for moving the two movable swash plates according to the turning of the steerable wheels is complicated, and expanded so as to reduce spaces for other parts. Further, the link mechanism requires a large operation force because of its small operational efficiency. 
     Further, the conventional vehicle is set so that peripheral speed of the front drive wheels becomes equal to peripheral speed of the rear drive wheels during straight traveling. However, there is a problem that the peripheral speed of the drive wheel may be unexpectedly reduced because of bad road condition so as to be slower than that corresponding to the amount of fluid flowing through the corresponding hydraulic motor, thereby causing stiff traveling of the vehicle. 
     Further, with respect to the conventional hydraulically driven vehicle, as disclosed in International Publication WO 2004/062956, the main transaxle is expanded because it incorporates the hydraulic pump together with the hydraulic motor for driving the main drive wheels. Such a large-size transaxle may reduce variation of adaptable vehicles in size. Further, a pump shaft of the hydraulic pump projects outward from the transaxle so as to be provided thereon with a cooling fan. The expansion of the transaxle means that the area of the transaxle blown by cooling wind generated from the cooling fan becomes small relative to the transaxle. Besides, to efficiently cool fluid circulating in the HST fluid circuit having the series connection of the hydraulic motor for the main drive wheels and the pair of hydraulic motors for the auxiliary drive wheels to the hydraulic pump, it is advantageous that pipes for circulating fluid are blown by cooling wind generated from the cooling fan. However, it is difficult to concentrating the pipes adjacent to the cooling fan of the large-size transaxle. In this way, the conventional combination of the main transaxle incorporating the hydraulic pump and the hydraulic motor for driving the main drive wheels with the auxiliary transaxle incorporating the pair of hydraulic motors for driving the auxiliary drive wheels cannot ensure sufficient cooling efficiency for the HST fluid circuit. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a hydrostatic transaxle supporting a pair of left and right steerable drive wheels and comprising a pair of hydraulic motors for driving the respective drive wheels, wherein the hydrostatic transaxle is provided with a simple and compact link mechanism for controlling displacement of the hydraulic motors during turning of a vehicle. 
     To achieve the first object, a hydrostatic transaxle according to a first aspect of the present invention comprises: a transaxle casing; a pair of left and right axles disposed in the transaxle casing; a pair of hydraulic motors for driving the respective axles; and a pair of left and right steerable drive wheels disposed on left and right outsides of the transaxle casing so as to be drivingly connected to the respective axles. The hydraulic motors are disposed in the housing so as to be fluidly connected to a common hydraulic pressure source in parallel to each other. One of the hydraulic motors is a fixed displacement hydraulic motor, and the other is a variable displacement hydraulic motor, so that displacement of the variable displacement hydraulic motor is changed according to change of steered angles of the drive wheels. 
     Due to the construction, the change of steered angles of the drive wheels does not have to be transmitted to a fixed swash plate or another fixed displacement-setting member of the fixed displacement hydraulic motor. Therefore, a link mechanism for moving a movable swash plate or another variable displacement-setting member of the variable displacement hydraulic motor according to change of turning angle of the vehicle can be simple and compact in comparison with the conventional type transaxle having a pair of variable displacement hydraulic motors, thereby expanding a space around the hydrostatic transaxle for arrangement of other members, and increasing variation of layout in the vehicle. Further, when the displacement of the variable displacement hydraulic motor is reduced during turning of the vehicle so as to increase peripheral speed of the drive wheel driven by the variable displacement hydraulic motor, peripheral speed of the drive wheel driven by the fixed displacement hydraulic motor is also increased according to the increase of peripheral speed of the drive wheel driven by the variable displacement hydraulic motor so as to ensure smooth turning of the vehicle without drag of the drive wheels while keeping proper driving force. 
     In the first aspect, preferably, a pair of left and right steerable casings are relatively horizontally rotatably attached onto respective left and right ends of the transaxle casing so as to support the respective drive wheels, thereby making the drive wheels steerable. The variable displacement hydraulic motor is provided with a displacement control section for controlling displacement of the variable displacement hydraulic motor. The displacement control section is connected to a rod extended from one of the steerable casings so as to enable the change of displacement of the variable displacement hydraulic motor according to change of steered angles of the drive wheels. 
     Since the displacement control section changes the displacement of the variable displacement hydraulic motor correspondingly to the change of steered angles of the drive wheels, the rod does not have to be extended adjacent to the steerable casing supporting the drive wheel driven by the fixed displacement hydraulic motor, thereby ensuring a large space for arrangement of another member, such as a hydraulic fluid port for fluidly connecting the pair of hydraulic motors to the hydraulic pressure source. 
     Therefore, preferably, the displacement control section of the variable displacement hydraulic motor and a hydraulic fluid port for fluidly connecting the pair of hydraulic motors to the hydraulic pressure source are juxtaposed on an outer surface of the transaxle casing incorporating the pair of hydraulic motors. Due to this layout, the hydrostatic transaxle can be entirely minimized and the hydraulic fluid port can be disposed at a suitable position for piping. 
     The variable displacement hydraulic motor with the movable swash plate requires a cam mechanism for converting axial (push-and-pull) movement of the rod to the rotary movement of the movable swash plate. If both the hydraulic motors in the transaxle were variable displacement hydraulic motors with respective movable swash plates, the hydraulic motors would require a complicated cam mechanism for converting the axial movement of the rod extended from one of the steerable casings to the rotary movements of the two movable swash plates. However, in the transaxle according to the first aspect of the present invention, the variable displacement hydraulic motor and the fixed displacement hydraulic motor are provided, and only the variable displacement hydraulic motor has the displacement control section requiring such a cam mechanism. Therefore, the cam mechanism can be compact and reduce a space for arrangement thereof. Preferably, the cam mechanism of the displacement control section can be simply constituted by a cam member rotated by axial movement of the rod, and a rotary member contacting the cam member so as to be rotated integrally with a movable swash plate of the variable displacement hydraulic motor according to rotation of the cam member. 
     Preferably, a pin is provided on the steerable casing from which the rod is extended, and a tie rod connected to the other steerable casing and the rod connected to the displacement control section are pivoted on the pin, thereby reducing the number of component parts, and preventing the rod and the tie rod from crossing so as to hinder each other and hinder movement of the steerable casings. 
     In the first aspect, preferably, the transaxle casing comprises: a pair of left and right axle casings incorporating the respective axles; and a motor casing incorporating the pair of hydraulic motors. The motor casing is removably interposed between the left and right axle casings. Therefore, the motor casing with the pair of hydraulic motors therein can be removed from the axle casings for easy maintenance of the pair of hydraulic motors. Further, a spacer can be easily disposed between the axle casing and the motor casing so as to ensure a required tread between left and right drive wheels. 
     In the first aspect, preferably, a center pin bracket is removably attached to the transaxle casing so as to swingably suspend the transaxle casing from a vehicle frame. Therefore, different center pin brackets are prepared so that, even if a positional relation of the transaxle casing to a center pin changes due to change of a vehicle, a suitable center pin bracket can be selected so as to suitably equip the transaxle casing on the target vehicle. In this way, the transaxle can be standardized for various vehicles. Similarly, an arm for operatively connecting the steerable casing to a steering operation device on a vehicle may be removably attached to the steerable casing. 
     To achieve the first object, a hydrostatic transaxle according to a second aspect of the present invention comprises: a transaxle casing; a pair of left and right axles disposed in the transaxle casing; a pair of hydraulic motors for driving the respective axles being disposed in the housing so as to be fluidly connected to a common hydraulic pressure source in parallel to each other; and a pair of left and right steerable drive wheels disposed on left and right outsides of the transaxle casing so as to be drivingly connected to the respective axles. Both the hydraulic motors are fixed displacement hydraulic motors. 
     Therefore, the hydrostatic transaxle requires no link mechanism for moving swash plates of the hydraulic motors according to steering rotation of the steerable casings, thereby being compact and economical, and ensuring a large freedom degree of piping for supplying fluid to the hydraulic motors. If a vehicle does not require difference of peripheral speed between front wheels and rear wheels during turning of the vehicle because the front wheels and the rear wheels are equally distant from the turning center of the vehicle or for another reason, this hydrostatic transaxle is thoroughly applicable. Even if a vehicle employing this hydrostatic transaxle requires difference of peripheral speed between front wheels and rear wheels during turning of the vehicle, the vehicle can turn smoothly so as to give a feeling of two-wheel drive by employing later-discussed setting of peripheral speeds of front and rear wheels. 
     A second object of the present invention is to provide a hydraulically driven vehicle, comprising: a first drive wheel disposed at one of front and rear portions of the vehicle; a second drive wheel disposed at the other rear or front portion of the vehicle; first and second hydraulic motors for driving the respective first and second drive wheels; a common hydraulic pump; and a hydraulic circuit fluidly connecting the first and second hydraulic motors in series to the hydraulic pump, smoothened in straight-traveling thereof. 
     To achieve the second object, a hydraulically driven vehicle according to a third aspect of the present invention comprises: a first drive wheel disposed at one of front and rear portions of the vehicle; a second drive wheel disposed at the other rear or front portion of the vehicle; a first hydraulic motor for driving the first drive wheel; a second hydraulic motor for driving the second drive wheel; a common hydraulic pump for supplying fluid to the first and second hydraulic motors; and a hydraulic circuit fluidly connecting the first and second hydraulic motors in series to the hydraulic pump. Peripheral speed of the first drive wheel is set to be equal to peripheral speed of the second drive wheel, or to be slightly larger than peripheral speed of the second drive wheel, during straight traveling of the vehicle. The hydraulic circuit includes a supply portion from which fluid is supplied to the second hydraulic motor. A check valve is disposed in the supply portion so as to introduce fluid into the hydraulic circuit from outside of the hydraulic circuit. 
     Therefore, during straight traveling of the vehicle, even if the peripheral speed of the first drive wheel is reduced relative to a set output of the first hydraulic motor due to bad road condition, the peripheral speed of the first drive wheel can be kept to be equal to the peripheral speed of the second drive wheel, or larger than the peripheral speed of the second drive wheel, so that the second drive wheel may be rotated according to rotation of the first drive wheel, however, the first drive wheel is hardly rotated according to rotation of the second drive wheel, thereby canceling stiffness in straight traveling caused by the phenomenon that peripheral speeds of the first and second drive wheels alternately exceed each other. However, due to this peripheral speed setting of the first and second drive wheels, a suction port of the second hydraulic motor tends to be hydraulically depressed so as to reduce drive efficiency of the second hydraulic motor. This problem is solved by the check valve, which is disposed in the supply portion of the hydraulic circuit for supplying fluid to the second hydraulic motor so as to introduce fluid into the hydraulic circuit from outside of the hydraulic circuit, thereby ensuring a proper drive condition of the second hydraulic motor. Further, by using the configuration of the third aspect, a drive mode of the vehicle can be automatically switched between a normal drive mode of the vehicle by substantially driving only the first drive wheel and an emergency drive mode of the vehicle by driving both the first and second drive wheels when slipping of the first drive wheel is detected. That is, if the vehicle has a pair of left and right first drive wheels and a pair of left and right second drive wheels, the vehicle can be configured so that a two-wheel drive mode depending on driving the first drive wheels is set for normal traveling of the vehicle, and a four-wheel drive mode depending on driving the first and second drive wheels is automatically set when slipping of the first wheel or wheels is detected. Therefore, the vehicle can save energy cost at its work of light-load traction and reduce wear of tires of the first and second drive wheels. 
     In the third aspect, preferably, the hydraulic circuit is configured so that fluid delivered from the hydraulic pump circulates in the hydraulic circuit so as to pass the second hydraulic motor after passing the first hydraulic motor. Therefore, if the second drive wheel is stuck (in a ditch), or if the vehicle weight applied on the first drive wheel exceeds the vehicle weight applied on the second drive wheel because of traveling of the vehicle on a slope, the first hydraulic motor supplied with fluid from the hydraulic pump prior to the second hydraulic motor can be effectively driven so that, by driving the first drive wheel, the vehicle can travel on the slop smoothly or escape from being stuck. 
     Alternatively, in the third aspect, preferably, the hydraulic circuit is configured so that fluid delivered from the hydraulic pump circulates in the hydraulic circuit so as to pass the first hydraulic motor after passing the second hydraulic motor. Therefore, if the first drive wheel is stuck (in a ditch), or if the vehicle weight applied on the second drive wheel exceeds the vehicle weight applied on the first drive wheel because of traveling of the vehicle on a slope, the second hydraulic motor supplied with fluid from the hydraulic pump prior to the first hydraulic motor can be effectively driven so that, by driving the second drive wheel, the vehicle can travel on the slop smoothly or escape from being stuck. 
     To achieve the second object, a hydraulically driven vehicle according to a fourth aspect of the present invention comprises: a first drive wheel disposed at one of front and rear portions of the vehicle; a pair of second steerable drive wheels disposed at the other rear or front portion of the vehicle; a first fixed displacement hydraulic motor for driving the first drive wheel; a pair of second fixed displacement hydraulic motors for driving the respective second drive wheels; a common hydraulic pump for supplying fluid to the first hydraulic motor and the pair of second hydraulic motors; and a hydraulic circuit fluidly connecting the first hydraulic motor and the pair of second hydraulic motors in series to the hydraulic pump, and fluidly connecting the second hydraulic motors to the hydraulic pump in parallel to each other. Peripheral speed of the first drive wheel is set to be equal to peripheral speeds of the second drive wheels, or to be slightly larger than peripheral speeds of the second drive wheels, during straight traveling of the vehicle. The hydraulic circuit includes a supply portion from which fluid is supplied to the pair of second hydraulic motors. A check valve is disposed in the supply portion so as to introduce fluid into the hydraulic circuit from outside of the hydraulic circuit. 
     The second fixed displacement hydraulic motors fluidly connected to the hydraulic pump in parallel to each other require no link mechanism for moving swash plates (or other displacement-setting members) of the second hydraulic motors according to change of steered angles of the second steerable drive wheels. Therefore, a drive system for driving the second drive wheels can be simple, compact, and economical. Further, due to the peripheral speed setting of the first and second drive wheels, the stiffness in straight traveling can be canceled. In this peripheral speed setting condition, the check valve can supply fluid to suction ports of the second hydraulic motors which tend to be hydraulically depressed, thereby ensuring proper drive condition of the second hydraulic motors. Even while both the second hydraulic motors are fixed in displacement, the peripheral speed setting of the first and second drive wheels during straight traveling of the vehicle causes the second steerable drive wheels to be rotated according to rotation of the first drive wheel so as to prevent drag of the second steerable drive wheels during turning of the vehicle, whereby the vehicle can turn as smoothly as a four-wheel drive vehicle set in a two-wheel drive mode. Since both the second hydraulic motors are fixed in displacement, the vehicle in turning can obtain drive force which is substantially equal to that ensured by a three-wheel drive mode, because the second drive wheel disposed on the distal side of turning circle of the vehicle is driven by pressure from the contacting ground surface. 
     A third object of the present invention is to provide a hydraulically driven vehicle comprising two front and rear transaxles incorporating respective hydraulic motors for driving respective axles, wherein one of the front and rear transaxles, which conventionally incorporates a hydraulic pump together with the hydraulic motor, is minimized, and wherein the transaxles and a hydraulic circuit for driving the hydraulic motors is effectively cooled. 
     To achieve the third object, a hydraulically driven vehicle according to a fifth aspect of the present invention comprises: front and rear transaxles incorporating respective hydraulic motors for driving respective axles; and a hydraulic pump unit for supplying the front and rear transaxles. The hydraulic pump unit is disposed separately from the front and rear transaxles. Therefore, one of the front and rear transaxles, which conventionally incorporates a hydraulic pump together with the hydraulic motor, is minimized so as to enhance its applicability range for variable vehicles. 
     In the fifth aspect, preferably, the vehicle further comprises: a vehicle frame; and an engine for driving the hydraulic pump unit. Both the engine and the hydraulic pump unit are vibro-isolatingly mounted on the vehicle frame. Vibro-isolation of the hydraulic pump unit can be ensured easily together with the engine so as to silence the vehicle. 
     In the fifth aspect, preferably, the hydraulic pump unit comprises a cooling fan. In comparison with a cooling fan provided on the conventional large transaxle incorporating both hydraulic pump and motor, the cooling fan of the hydraulic pump unit can be combined with the following configuration so as to improve efficiency of cooling the hydraulic pump unit, the front and rear transaxles and a hydraulic circuit comprising the hydraulic pump and motors, thereby prolonging lives of the hydraulic pump and motors, and improving the mechanical efficiency. 
     In the situation that the hydraulic pump unit comprises the cooling fan, preferably, at least one of the front and rear transaxles is disposed in an area blown by cooling wind generated from the cooling fan of the hydraulic pump unit, thereby being sufficiently cooled by the cooling fan. 
     Since pipes for circulating fluid supplied to the hydraulic motors can be concentrated around the compact hydraulic pump unit, preferably, a pipe for circulating fluid supplied to the first and second hydraulic motors is partly disposed in an area blown by cooling wind generated from the cooling fan of the hydraulic pump unit, thereby effectively cooling hydraulic fluid and improving operational reliability and endurance of the hydraulic pump and motors. 
     Further preferably, a reservoir tank is disposed adjacent to the hydraulic pump unit, and a pipe connecting the reservoir tank to at least one of the hydraulic pump unit and the first and second transaxles is disposed in an area blown by cooling wind generated from the cooling fan of the hydraulic pump unit. Therefore, fluid in the reservoir tank can also be effectively cooled so as to further improve efficiency of cooling fluid circulating between the hydraulic pump and motors. 
     In the fifth aspect, preferably, the hydraulic pump unit is disposed between the front and rear transaxles. Alternatively, preferably, one of the front and rear transaxles is disposed between the hydraulic pump unit and the other transaxle. Therefore, for example, pipes extended from both the transaxles to the hydraulic pump unit can be efficiently concentrated so as to be effectively cooled by the cooling fan of the hydraulic pump unit. Further, due to the arrangement of one of the front and rear transaxles between the hydraulic pump unit and the other rear or front transaxle, the front and rear transaxles can approach each other so as to shorten a pipe or pipes therebetween. 
     In the fifth aspect, preferably, in the hydraulically driven vehicle further comprising: a mower; and a duct for transferring grass mowed by the mower, one of the front and rear transaxles is disposed laterally eccentrically toward one of left and right sides of the vehicle, and the duct is disposed laterally eccentrically toward the other right or left side of the vehicle. Alternatively, preferably, one of the front and rear transaxles is disposed above the duct, and a vertical power train is disposed on lateral outside of the duct so as to be interposed between the axle of the transaxle above the duct and an axial center shaft of a wheel driven by the axle of the transaxle above the duct. Therefore, enough capacity of the duct can be ensured so as to prevent the duct from being clogged with mowed wet grass. 
     These, further and other objects, features and advantages will appear more fully from the following description with reference to accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a four-wheel drive lawn tractor serving as a hydraulically driven vehicle according to a first embodiment of the present invention. 
         FIG. 2  is a skeleton diagram showing a hydraulic circuit and a structure for driving front and rear transaxles equipped on the vehicle of  FIG. 1 , wherein the front transaxle incorporates a combination of a variable displacement hydraulic motor and a fixed displacement hydraulic motor. 
         FIG. 3  is a skeleton diagram showing a hydraulic circuit and a structure for driving front and rear transaxles equipped on the vehicle of  FIG. 1 , wherein the front transaxle incorporates a pair of fixed displacement hydraulic motors. 
         FIG. 4  is a side view of a four-wheel drive lawn tractor serving as a hydraulically driven vehicle according to a second embodiment of the present invention. 
         FIG. 5  is a plan view of the vehicle of  FIG. 4 . 
         FIG. 6  is a skeleton diagram showing a hydraulic circuit and a structure for driving front and rear transaxles of the vehicle of  FIGS. 4 and 5 . 
         FIG. 7  is a side view of a four-wheel drive lawn tractor serving as a hydraulically driven vehicle according to a third embodiment of the present invention. 
         FIG. 8  is a plan view of the vehicle of  FIG. 7 . 
         FIG. 9  is a sectional rear view of the vehicle of  FIGS. 7 and 8 . 
         FIG. 10  is a front view of the front transaxle for the vehicles according to the first to third embodiments, wherein the front transaxle incorporates the combination of the variable displacement hydraulic motor and the fixed displacement hydraulic motor. 
         FIG. 11  is a plan view of the front transaxle. 
         FIG. 12  is an enlarged sectional front view of a portion of the front transaxle incorporating a motor assembly and axles. 
         FIG. 13  is an enlarged sectional front view of the front transaxle, showing a steerable casing thereof. 
         FIG. 14  is a sectional front view of a motor casing fitted into the front transaxle. 
         FIG. 15  is a sectional plan view of the motor casing. 
         FIG. 16  is a rear view of a portion of the front transaxle, showing the motor casing. 
         FIG. 17  is a rearwardly perspective view of the motor assembly to be disposed in the motor casing. 
         FIG. 18  is a front view partly in section of an inside portion of a motor cover covering a rear opening of the motor casing. 
         FIG. 19  is a fragmentary sectional side view of the motor casing and the motor cover supporting shafts for controlling displacement of the hydraulic motor. 
         FIG. 20  is a sectional side view of the motor casing, the motor cover and a pipe connector block, showing hydraulic passages for driving the hydraulic motor therein. 
         FIG. 21  is plan view of the four-wheel lawn tractor according to the second embodiment of the present invention, employing a system for steering rear wheels. 
         FIG. 22  is a rear view of the rear wheel steering system of  FIG. 21 . 
         FIG. 23  is a rear view of another rear wheel steering system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2 , description will be given of a lawn tractor employing an Ackerman type front wheel steering system, serving as a hydraulically driven four-wheel drive vehicle according to a first embodiment of the present invention. The lawn tractor has a vehicle frame  3 , which supports a rear transaxle  1  below a front portion thereof. A bracket  4  is fixed onto a front bottom portion of vehicle frame  3 . A front transaxle  2  is suspended from bracket  4  via a fore-and-aft horizontal center pin  5  so that left and right ends of front transaxle  2  are vertically swingable. Rear transaxle  1  journals left and right opposite axles  6 , which project laterally outward from rear transaxle  1  so as to be fixed at outer ends thereof to respective center portions of rear wheels  7 . Front transaxle  2  journals left and right opposite axles  8 , which project laterally outward from front transaxle  2  so as to be steerably and drivingly connected at outer ends thereof to respective center portions of front wheels  9 . 
     An engine  10  is vibro-isolatingly supported on a front upper portion of vehicle frame  3  via vibro-isolating rubbers or the like, and covered with a bonnet  11 . A steering wheel  12  is extended upwardly rearward at a rear end of bonnet  11 , and a speed change pedal  13  is disposed at the bottom portion of the rear end of bonnet  11 . Speed change pedal  13  is shaped like a seesaw such as to have a front portion depressed for setting forward traveling speed and a rear portion depressed for setting backward traveling speed. Speed change pedal  13  is linked to a speed change lever  14  pivoted on a casing of rear transaxle  1 . A rear cover  15  is mounted on a rear upper portion of vehicle frame  3 . A reservoir tank  28  is disposed in rear cover  15 . A driver&#39;s seat  16  is mounted on rear cover  15 . 
     Engine  10  has a downwardly vertical output shaft  10   a  on which pulleys  10   b  and  10   c  are fixed. An input shaft  17  (serving as a pump shaft of a later-discussed hydraulic pump P) projects upward from the casing of rear transaxle  1 . A belt  18  is interposed between a pulley  17   a  fixed on input shaft  17  and pulley  10   b  on engine output shaft  10   a , so as to drivingly connect hydraulic pump P in rear transaxle  1  to engine  10 . Further, a cooling fan  17   b  is fixed on input shaft  17  so as to blow cooling air to the casing of rear transaxle  1 . 
     A mower  20  is vertically movably suspended below vehicle frame  3  between front wheels  7  and rear wheels  9 . An input pulley  20   b  is disposed on a top of mower  20 . A belt  19  is interposed between pulley  10   c  on engine output shaft  10   a  and input pulley  20   b  so as to drivingly connect a rotary blade  20   a  in mower  20  to engine  10 . 
     Hydraulic pump P, and hydraulic motor M 1  for driving rear wheels  7  (axles  6 ) are disposed in the casing of rear transaxle  1 . On the other hand, a pair of hydraulic motors M 2  and M 3  for driving respective left and right front wheels  9  (axles  8 ) are disposed in a casing of front transaxle  2 . HST fluid pipes  23  and  26  including respective intermediate flexible hydraulic fluid hoses  23   a  and  26   a  are interposed between the casings of front and rear transaxles  1  and  2  so as to constitute an HST circuit HC 1  including hydraulic pump P and hydraulic motors M 1 , M 2  and M 3 . 
     HST circuit HC 1  will be described with reference to  FIG. 2 . The casing of rear transaxle  1  has outwardly opened HST fluid ports  1   a  and  1   b , and the casing of front transaxle  2  has outwardly opened HST fluid ports  2   a  and  2   b . Pipe  23  including hose  23   a  is interposed between ports  1   a  and  2   a , and pipe  26  including hose  26   a  is interposed between ports  1   b  and  2   b.    
     In the casing of rear transaxle  1 , a fluid passage  21  is interposed between hydraulic pump P and hydraulic motor M 1 , a fluid passage  27  is interposed between hydraulic pump P and port  1   b , and a fluid passage  22  is interposed between hydraulic motor M 1  and port  1   a.    
     In the casing of front transaxle  2 , hydraulic motor M 2  is fixed in displacement, and hydraulic motor M 3  is variable in displacement. In the casing of front transaxle  2 , a fluid passage  24  is extended from port  2   a  toward hydraulic motors M 2  and M 3 , and a fluid passage  25  is extended from port  2   b  toward hydraulic motors M 2  and M 3 . Fluid passage  24  bifurcates into a fluid passage  24   a  connected to hydraulic motor M 2  and a fluid passage  24   b  connected to hydraulic motor M 3 . Fluid passage  25  bifurcates into a fluid passage  25   a  connected to hydraulic motor M 2  and a fluid passage  25   b  connected to hydraulic motor M 3 . 
     In HST circuit HC 1 , hydraulic motor M 1  of rear transaxle  1  and the pair of hydraulic motors M 2  and M 3  of front transaxle  2  are connected in series to hydraulic pump P. Hydraulic motors M 2  and M 3  in front transaxle  2  are fluidly connected to hydraulic pump P in parallel to each other so as to be rotatable relative to each other according to conditions of load or the like. 
     In HST circuit HC 1 , during forward traveling of the vehicle, fluid delivered from hydraulic pump P flows through fluid passage  21 , hydraulic motor M 1 , hydraulic passage  22 , port  1   a , pipe  23 , port  2   a , fluid passage  24 , the pair of hydraulic motors M 2  and M 3 , fluid passage  25 , port  2   b , pipe  26 , port  1   b , and fluid passage  27 , so as to return to hydraulic pump P. In other words, during forward traveling of the vehicle, pressurized fluid from hydraulic pump P is supplied to hydraulic motor M 1  of rear transaxle  1  before the pair of hydraulic motors M 2  and M 3  of front transaxle  2 . During backward traveling of the vehicle, the fluid circulation route in HST circuit HC 1  is reversed, that is, pressurized fluid from hydraulic pump P is supplied to the pair of hydraulic motors M 2  and M 3  of front transaxle  2  before hydraulic motor M 1  of rear transaxle  1 . 
     Of course, forward traveling is more frequent than backward traveling. It is now assumed that the circulation route in HST circuit HC 1  during each of forward and backward travels is opposite to the above-mentioned route. On the assumption, when the vehicle climbing over a step is caught on the step and front wheels  9  are stuck, hydraulic motor M 1  of rear transaxle  1  cannot be supplied with fluid through hydraulic motors M 2  and M 3  of front transaxle  2 , whereby rear wheels  7  also stop. For this reason, the vehicle may stop every time it climbs over a step. Considering this problem, actual HST circuit HC 1  is constructed as mentioned above so that fluid from hydraulic pump P is supplied to hydraulic motor M 1  before hydraulic motors M 2  and M 3 . Therefore, even if front wheels are stuck, hydraulic motor M 1  is supplied with fluid from hydraulic pump P so as to drive rear wheels  7 , thereby canceling the stuck condition of front wheels  9 . 
     Additionally, a drive-mode setting valve may be interposed across pipes  23  and  26  between transaxles  1  and  2 . The drive-mode setting valve is opened so as to complete connection between ports  1   a  and  2   a  and between ports  1   b  and  2   b , thereby putting the vehicle into a four-wheel drive mode where fluid from hydraulic pump P circulates through hydraulic motors M 1 , M 2  and M 3 . The drive-mode setting valve is closed so as to shut off ports  2   a  and  2   b  from ports  1   a  and  1   b , and make a closed fluid circuit between hydraulic pump P and hydraulic motor M 1  isolated from hydraulic motors M 2  and M 3 , thereby putting the vehicle into a two-wheel drive mode where only hydraulic motor M 1  is driven by fluid supplied from hydraulic pump P. 
     The casings of transaxles  1  and  2  are filled with fluid so as to form respective fluid sumps therein. The casing of rear transaxle  1  is provided with a drain port  1   c  from which a drainpipe  29  is connected to reservoir tank  28 . The casing of front transaxle  2  is provided with a drain port  2   c  from which a drainpipe  30  is connected to reservoir tank  28 . Therefore, reservoir tank  28  absorbs excessively expanded fluid in the respective fluid sumps of transaxles  1  and  2 . 
     Rear transaxle  1  incorporates a charge pump  33  for supplying fluid to HST circuit HC 1 . As shown in  FIG. 2 , charge pump  33  is preferably driven together with hydraulic pump P by input shaft  17  serving as the pump shaft of hydraulic pump P. Charge pump  33  sucks fluid from the fluid sump in the casing of rear transaxle  1  through a suction line  31  (having an oil filter  32  at its start end). Alternatively, fluid to charge pump  33  may be introduced from reservoir tank  28  out of rear transaxle  1 . 
     In the casing of rear transaxle  1 , a charge fluid passage  34  is interposed between fluid passages  21  and  27  with hydraulic pump P therebetween, so as to charge fluid from charge pump  33  to hydraulically depressed one of fluid passages  21  and  27  via corresponding one of check valves  35 . Charge fluid passage  34  is connected at a portion thereof between check valves  35  and  35  to a hydraulic pressure control valve  36 , so as to drain excessive pressurized fluid via hydraulic pressure control valve  36  into the fluid sump in the casing of rear transaxle  1 . Charge fluid passage  34  is also connected at the portion thereof between check valves  35  to suction line  31  via a check valve  39 , in parallel to charge pump  33 . Check valve  39  is opened when either fluid passage  21  or  27  is hydraulically depressed to open corresponding check valve  35  so as to absorb the suction port of charge pump  33  (suction line  31 ), thereby preventing cavitation in HST circuit HC 1  during straight travel of the vehicle. 
     In the casing of rear transaxle  1 , a differential gear unit  38  is disposed so as to differentially connect axles  6  to each other. Distal ends of axles  6  project oppositely laterally outward from the casing of rear transaxle  1  so as to be fixed to respective rear wheels  7  (alternatively, rear wheels  7  may be steerably connected to the distal ends of axles  6 , as discussed later with reference to  FIG. 23 ). In the casing of rear transaxle  1 , a deceleration gear train  37  is interposed between a motor shaft M 1   a  of hydraulic motor M 1  and differential gear unit  38 . In this way, rear transaxle  1  drives hydraulic motor M 1  for driving both rear drive wheels  7  by pressurized fluid supplied from hydraulic pump P. Hydraulic pump P is provided with a movable swash plate Pa (see  FIG. 2 ) linked with a speed change lever  14  (see  FIG. 1 ) pivoted on the casing of rear transaxle  1 . By depression of speed change pedal  13 , movable swash plate Pa is set in direction and angle so as to set the fluid delivery direction and amount of hydraulic pump P, thereby setting the rotary direction and speed of rear wheels  7 . 
     Description will now be given on peripheral speed setting of rear wheels  7  and front wheels  9  depending on the output rotary speed setting of hydraulic motor M 1  and the pair of hydraulic motors M 2  and M 3 , and design of HST circuit HC 1  in correspondence to the peripheral speed setting of wheels  7  and  9 . In a general manner, hydraulic motor M 1  and hydraulic motors M 2  and M 3  would be set in output rotary speed so as to equalize peripheral speeds of rear wheels  7  to those of front wheels  9  during straight travel of the vehicle. However, if the setting is excessively strict, peripheral speed difference between rear wheels  7  and front wheels  9  due to road conditions causes stiff travel of the vehicle. That is, on one occasion, the peripheral speeds of rear wheels  7  exceed those of front wheels  9  (i.e., the peripheral speeds of front wheels  9  become lower than those corresponding to the set output of hydraulic motors M 2  and M 3 ), so as to cause a condition where front wheels  9  rotate following rotation of rear wheels  7 . On another occasion, the peripheral speeds of front wheels  9  exceed those of rear wheels  7  (i.e., the peripheral speeds of rear wheels  7  become lower than those corresponding to the set output of hydraulic motor M 1 ), so as to cause a condition where rear wheels  7  rotate following rotation of front wheels  9 . 
     Therefore, the output rotary speeds of hydraulic motors M 1 , M 2  and M 3  are actually set so that, during straight travel of the vehicle, peripheral speeds of rear wheels  7  strictly exceed those of front wheels  9 . Due to this setting, front wheels  9  almost constantly rotate following rotation of rear wheels  7 . Even if the slowing-down degree of rear wheels  7  is considerably large (the peripheral speeds of rear wheels  7  is considerably reduced to be lower than those corresponding to the set output of hydraulic motor M 1 ), peripheral speeds of rear wheels  7  is still larger than peripheral speeds of front wheels  9 , or equal to peripheral speeds of front wheels  9 . Even if the reduced peripheral speeds of rear wheels  7  become lower than the peripheral speeds of front wheels  9 , the major case is that the peripheral speed difference between rear wheels  7  and front wheels  9  is still small so that rear wheels  7  are rotated with a little assistance of front wheels  9 , and the difference of reduced peripheral speeds of rear wheels  7  from peripheral speeds of front wheels  9  hardly becomes sincerely large. Consequently, the vehicle is substantially prevented from stiffly traveling. 
     However, due to the output speed setting of hydraulic motors M 1 , M 2  and M 3  for making peripheral speeds of rear wheels  7  exceed peripheral speeds of front wheels  9 , i.e., for rotating front wheels  9  following rotation of rear wheels  7 , hydraulic motors M 2  and M 3  receive opposite torque from respective axles  8  so as to act as pumps. Consequently, primary (suction) ports of hydraulic motors M 2  and M 3  are hydraulically depressed. This phenomenon remarkably happens during turning of the vehicle, especially during sharp turning of the vehicle, thereby causing cavitation in the HST circuit, and causing hunting of the vehicle. Therefore, HST circuit HC 1  includes a check valve disposed on the suction side fluid passage of the pair of hydraulic motors M 2  and M 3  so as to introduce fluid from the outside of HST circuit HC 1  into HST circuit HC 1 . During forward travel of the vehicle, the suction side fluid passage comprises fluid passages  22  to  24   a  and  24   b  between hydraulic motor M 1  and the pair of hydraulic motors M 2  and M 3 . During backward travel of the vehicle, the suction side fluid passage comprises fluid passages  21  to  25   a  and  25   b  between hydraulic motor M 1  and the pair of hydraulic motors M 2  and M 3  through hydraulic pump P. In the embodiment of  FIG. 2 , in front transaxle  2 , a check valve  40  is connected to fluid passage  24   a  so as to introduce fluid from the fluid sump in the casing of front transaxle  2  to the suction side fluid passage of the pair of hydraulic motors M 2  and M 3  during forward travel of the vehicle. An oil filter  40   a  is interposed between check valve  40  and fluid passage  24   a . On the other hand, in rear transaxle  1 , the above-mentioned check valve  39  is provided for introducing fluid from the fluid sump in the casing of rear transaxle  1  to the suction side fluid passage of the pair of hydraulic motors M 2  and M 3  during backward travel of the vehicle. 
     Alternatively, check valve  40  to be opened during forward travel of the vehicle may be connected to fluid passage  24  or  24   b  in front transaxle  2 . Alternatively, a check valve for absorbing fluid from the fluid sump in the casing of rear transaxle  1  may be connected to fluid passage  22  in rear transaxle  1  so as to have the same effect as check valve  40 . On the other hand, in stead of check valve  39  in rear transaxle  1 , a check valve may be connected to fluid passage  25 ,  25   a  or  25   b  in front transaxle  2  so as to introduce fluid from the fluid sump in the casing of front transaxle  2  to the suction side fluid passage of the pair of hydraulic motors M 2  and M 3  during backward travel of the vehicle. 
     In association with the peripheral speed setting such that peripheral speeds of rear wheels  7  exceed peripheral speeds of front wheels  9  during straight travel of the vehicle, fluid circulates in HST circuit HC  1  during forward travel of the vehicle so as to flow from hydraulic pump P to the pair of hydraulic motors M 2  and M 3  through hydraulic motor M 1 . Therefore, during forward climbing a slope, rear wheels  9  can be effectively rotated for smooth climbing of the vehicle over a slope, because hydraulic motor M 1  for driving rear wheels  7  weighed more than front wheels  9  is effectively supplied with fluid from hydraulic pump P before the pair of hydraulic motors M 2  and M 3  is supplied. 
     However, during forward descending a slope, front wheels  9  weighed more than rear wheels  7  require larger driving force than that for rear wheels  7 . 
     If the forward descending on a slope is more important than the forward ascending on a slope, the circulation route in HST circuit HC 1  may be reversed so that, during forward travel of the vehicle, fluid from hydraulic pump P is supplied to hydraulic motor M 1  through the pair of hydraulic motors M 2  and M 3 . 
     Due to the above output setting of hydraulic motors M 1 , M 2  and M 3  and construction of HST circuit HC 1 , front wheels  9  receive driving forces from hydraulic motors M 2  and M 3  with the assistance of rotation of rear wheels  7 . Therefore, the vehicle can travel straight without stiff movement, and the operation force required for steering front wheels  9  can be lightened so as to enable the vehicle to turn smoothly with the feeling of almost two-wheel drive mode travel. It may be said that acceleration of the forward wheels during turning of the vehicle is unnecessary. Due to this theory, as shown in  FIG. 3 , both of the hydraulic motors in front transaxle  2  for driving respective axles  8  may be fixed displacement hydraulic motors M 2 . In this case, fixed displacement hydraulic motors M 2  of front transaxle  2  are fluidly connected to common hydraulic pump P in parallel to each other, and in series with hydraulic motor M 1 , so as to make peripheral speeds of rear wheels  7  slightly exceed or equal to peripheral speeds of front wheels  9  during straight travel of the vehicle. Check valve  40  is provided for introducing fluid from the fluid sump in front transaxle  2  to the suction side passage (in  FIG. 3 , fluid passage  24   a ) of the pair of hydraulic motors M 2  in HST circuit HC 1 . Due to this construction, a later-discussed link mechanism for changing the displacement of the hydraulic motor is unnecessary. 
     Referring to  FIGS. 4 ,  5  and  6 , an Ackerman-type steering lawn tractor serving as a four-wheel drive vehicle equipped with a hydraulic transaxle according to a second embodiment of the present invention will be described. The lawn tractor has vehicle frame  3  supporting a rear transaxle  101  at a rear portion thereof, and front transaxle  2  at a front portion thereof. A pump unit  50  and reservoir tank  28  are laterally juxtaposed and supported by vehicle frame  3  just in front of rear transaxle  101 . Preferably, similar to the vehicle of  FIG. 1 , the present vehicle includes bracket  4  fixed on a front end portion of vehicle frame  3 , and fore-and-aft horizontal center pin  5  pivoted on bracket  4  for rotatably suspending front transaxle  2  therefrom. Rear transaxle  101  supports left and right axles  6  whose outer ends are fixed to the center portions of respective rear wheels  7  (alternatively, steerably connected to respective rear wheels  7 , as discussed later). Front transaxle  2  supports left and right axles  8  to which respective front wheels  9  (serving as steerable second drive wheels) are vertically swingably and drivingly connected. 
     Engine  10  is vibro-isolatingly supported on vehicle frame  3  via vibro-isolating rubbers or the like. Pulley  10   b  is fixed on output shaft  10   a  of engine  10 . Input shaft  17  serves as the pump shaft of hydraulic pump P in the casing of pump unit  50 . Input shaft  17  projects upward from a casing of pump unit  50  so as to be fixedly provided thereon with pulley  17   a . Belt  18  is interposed between pulleys  10   b  and  17   a  so as to drivingly connect hydraulic pump P to engine  10 . Cooling fan  17   b  is fixed on input shaft  17  above the casing of pump unit  50  so as to blow cooling air to the casing of pump unit  50  and reservoir tank  28  adjacent to the casing of pump unit  50 . Pipes  51 ,  123  and  126 , constituting a later-discussed HST circuit HC 2 , and drainpipes  52 ,  129  and  30  connected to reservoir tank  28  are partly disposed in an area blown by cooling wind generated from cooling fan  17   b  so as to enhance the effect of cooling fluid therein. 
     Pump unit  50  also obtains the vibro-isolating effect of engine  10  against vehicle frame  3 . More specifically, pump unit  50  is vibro-isolatingly supported together with engine  10  onto vehicle frame  3 . To get such vibro-isolating effect, engine output shaft  10   a  may be coaxially and integrally rotatably connected to input shaft  17 , or single engine output shaft  10   a  may be extended so as to serve as input shaft  17  of pump unit  50 . 
     Variable displacement hydraulic pump P in pump unit  50  is provided with movable swash plate Pa linked to speed change lever  14  pivoted on the casing of pump unit  50 , and speed change lever  14  is linked to speed change pedal  13 . By depression of speed change pedal  13 , movable swash plate Pa is set in direction and angle so as to set the fluid delivery direction and amount of hydraulic pump P. 
     Mower  20  is disposed below vehicle frame  3  between front wheels  9  and rear wheels  7 . Rotary blade  20   a  in mower  20  is drivingly connected to engine  10  via power transmission means, e.g., a belt transmission, such as the belt transmission of the vehicle shown in  FIG. 1  including pulleys  10   c  and  20   b  and belt  19 . 
     A duct  92  is extended rearward from mower  20  so as to send grass mowed by rotary blade  20   a . Rear transaxle  101  having no hydraulic pump is vertically and laterally slimed. Such rear transaxle  101  is laterally eccentrically disposed toward one of left and right sides of the vehicle (in this embodiment, leftward) so as to ensure a large space for duct  92  on the other right or left side of the vehicle (in this embodiment, right side). Therefore, duct  92  can have such a large capacity as to prevent wet grass from clogging therein. 
     Further, as shown in  FIG. 5 , pump unit  50  and reservoir tank  28  are aligned in the fore-and-aft direction on one of left and right sides of the vehicle along one side surface of duct  92 , so as to increase the capacity of duct  92 , and to compactly collect fluid pipes including pipe  126 . 
     Cooling fan  17   b  fixed on a bottom end of input shaft  17  of pump unit  50  blows upward cooling wind to adjoining pump unit  50  and reservoir tank  28  and fluid pipes so as to cool fluid therein. Additionally, as drawn in phantom lines, input shaft  17  may be extended upward from pump unit  50  so as to be provided thereon with another cooling fan  17   b  for blowing downward cooling wind. In this case, vehicle frame  3  is opened to lead the downward cooling wind to pipes. 
     Hydraulic motor M 1  for driving rear wheels  7  (axles  6 ) are disposed in the casing of rear transaxle  101 . On the other hand, a pair of hydraulic motors M 2  and M 3  for driving respective left and right front wheels  9  (axles  8 ) are disposed in the casing of front transaxle  2 . An HST fluid pipe  51  is interposed between pump unit  50  and rear transaxle  101 , an HST fluid pipe  123  between rear transaxle  101  and front transaxle  2 , and an HST fluid pipe  126  between front transaxle  2  and pump unit  50 , thereby constituting HST circuit HC 2 . Pipes  123  and  126  may be provided at intermediate portions thereof with flexible hoses, respectively, similar to fluid pipes  23  and  26  including flexible hoses  23   a  and  26   a  in the vehicle according to the first embodiment as shown in  FIG. 1 . 
     HST circuit HC 2  will be described with reference to  FIG. 6 . The casing of pump unit  50  has outwardly opened HST fluid ports  50   a  and  50   b , the casing of rear transaxle  101  has outwardly opened HST fluid ports  101   a  and  101   b , and the casing of front transaxle  2  has outwardly opened HST fluid ports  2   a  and  2   b . Pipe  51  is interposed between ports  50   a  and  101   a , pipe  123  is interposed between ports  101   b  and  2   a , and pipe  126  is interposed between ports  2   b  and  50   b.    
     In the casing of pump unit  50  incorporates common hydraulic pump P for supplying fluid to hydraulic motors M 1 , M 2  and M 3 . In the casing of pump unit  50 , a fluid passage  121   a  is extended from hydraulic pump P to port  50   a , and a fluid passage  127  from hydraulic pump P to port  50   b . In the casing of rear transaxle  101 , a fluid passage  121   b  is extended from hydraulic motor M 1  to port  101   a , and a fluid passage  122  from hydraulic motor M 1  to port  10   b . Fluid passages  121   a  and  121   b  and pipe  51  constitute a fluid passage  121  between hydraulic pump P and hydraulic motor M 1 . 
     Similar to front transaxle  2  of the vehicle shown in  FIG. 1 , in the casing of the present front transaxle  2 , fixed displacement hydraulic motor M 2  for driving one front wheel  9  and variable displacement hydraulic motor M 3  for driving the other front wheel  9  are disposed, fluid passages  24 ,  24   a  and  24   b  are interposed between port  2   a  and the pair of hydraulic motors M 2  and M 3 , and fluid passages  25 ,  25   a  and  25   b  between port  2   b  and the pair of hydraulic motors M 2  and M 3 . 
     In HST circuit HC 2 , hydraulic motor M 1  of rear transaxle  101  and the pair of hydraulic motors M 2  and M 3  of front transaxle  2  are connected in series to hydraulic pump P. Hydraulic motors M 2  and M 3  in front transaxle  2  are fluidly connected to hydraulic pump P in parallel to each other. 
     In HST circuit HC 1 , during forward traveling of the vehicle, fluid delivered from hydraulic pump P flows through fluid passage  121  (i.e., fluid passage  121   a , pipe  51  and fluid passage  121   b ), hydraulic motor M 1 , hydraulic passage  122 , outwardly opened port  101   b , pipe  123 , outwardly opened port  2   a , fluid passage  24 , the pair of hydraulic motors M 2  and M 3 , fluid passage  25 , outwardly opened port  2   b , pipe  126 , outwardly opened port  50   b , and fluid passage  127 , so as to return to hydraulic pump P. In other words, during forward travel of the vehicle, pressurized fluid from hydraulic pump P is supplied to hydraulic motor M 1  of rear transaxle  1  before the pair of hydraulic motors M 2  and M 3  of front transaxle  2 . During backward travel of the vehicle, the fluid circulation route in HST circuit HC 2  is reversed, that is, pressurized fluid from hydraulic pump P is supplied to the pair of hydraulic motors M 2  and M 3  of front transaxle  2  before hydraulic motor M 1  of rear transaxle  1 . 
     Such circulation route of HST circuit HC 2  is set for the same reason as the circulation route setting of HST circuit HC 1  of the vehicle shown in  FIGS. 1 and 2  (and  FIG. 3 ). 
     Additionally, a drive-mode setting valve for switching drive mode of the vehicle between four-wheel drive mode and two-wheel drive mode may be interposed between hydraulic motor M 1  and the pair of hydraulic motors M 2  and M 3  in HST circuit HC 2 . 
     The casings of pump unit  50  and transaxles  1  and  2  are filled with fluid so as to form respective fluid sumps therein. As shown in  FIG. 6 , the casing of pump unit  50  is provided with a drain port  50   c  from which a drainpipe  52  is connected to reservoir tank  28 . The casing of rear transaxle  101  is provided with a drain port  101   c  from which a drainpipe  129  is connected to reservoir tank  28 . The casing of front transaxle  2  is provided with drain port  2   c  from which drainpipe  30  is connected to reservoir tank  28 . Therefore, reservoir tank  28  absorbs excessively expanded fluid in the respective fluid sumps of pump unit  50  and transaxles  101  and  2 . 
     Pump unit  50  incorporates charge pump  33  for supplying fluid to HST circuit HC 2 . As shown in  FIG. 6 , charge pump  33  is preferably driven together with hydraulic pump P by input shaft  17  serving as the pump shaft of hydraulic pump P. Pump unit  50  is provided with a charge pump port  50   d . A charge pipe  131  including an intermediate oil filter  132  is interposed between reservoir tank  28  and charge pump port  50   d , so as to introduce fluid from reservoir tank  28  to charge pump  33 . 
     In the casing of pump unit  50 , charge fluid passage  34  is interposed between fluid passages  121   a  and  127  with hydraulic pump P therebetween, so as to charge fluid from charge pump  33  to hydraulically depressed one of fluid passages  121  and  127  via corresponding one of check valves  35 . Charge fluid passage  34  is connected at a portion thereof between check valves  35  and  35  to hydraulic pressure control valve  36 , so as to drain excessive pressurized fluid via hydraulic pressure control valve  36  into the fluid sump in the casing of pump unit  50 . Check valve  39  for preventing cavitation is interposed between charge fluid passage  34  and hydraulic pressure control valve  36  so as to bypass one of check valves  35 . 
     Similar to rear transaxle  1  of the vehicle shown in  FIGS. 1 and 2 , rear transaxle  101  incorporates differential gear unit  38  for mutually differentially connecting axles  6 , and deceleration gear train  37  interposed between motor shaft M 1   a  of hydraulic motor M 1  and differential gear unit  38 . 
     As the vehicle shown in  FIGS. 1 and 2 , the present vehicle shown in  FIGS. 4 ,  5  and  6  has the same relative peripheral speed setting of rear wheels  7  and front wheels  9 , and the same output rotary speed of hydraulic motor M 1 , M 2  and M 3  for ensuring the relative peripheral speed setting of rear wheels  7  and front wheels  9 . Further, similar to HST circuit HC 1 , HST circuit HC 2  is provided with check valves for compensation of fluid reduced by the output rotary speed setting of hydraulic motors M 1 , M 2  and M 3 . More specifically, peripheral speeds of rear wheels  7  exceed or are equal to peripheral speeds of front wheels  9  during straight travel of the vehicle. A check valve for introducing fluid into HST circuit HC 2 , e.g., check valve  40  in front transaxle  2  as shown in  FIG. 6 , is connected to the suction side fluid passage of the pair of hydraulic motors M 2  and M 3 . Alternatively, the vehicle of  FIGS. 4 ,  5  and  6  with HST circuit HC 2  may be provided with fixed displacement hydraulic motors M 2  serving as the pair of hydraulic motors in front transaxle  2 , similar to that shown in  FIG. 3 . 
     Referring to  FIGS. 7 ,  8  and  9 , an Ackerman-type steering lawn tractor serving as a four-wheel drive vehicle equipped with a hydraulic transaxle according to a third embodiment of the present invention will be described. In the present lawn tractor, pump unit  50  incorporating hydraulic pump P is separated from rear transaxle  101  incorporating hydraulic motor M 1 , similar to those in the lawn tractor of the second embodiment. The different point of the present vehicle from the vehicle of the second embodiment is that a duct  192  is extended rearward from mower  20  at the lateral center of the vehicle. 
     Separated pump unit  50  and rear transaxle  101  are so compact as to be distributed on vehicle frame  3  while ensuring sufficient capacity in duct  192 . In the present embodiment, pump unit  50  and reservoir tank  28  are laterally juxtaposed on vehicle frame  3  just in front of duct  192 , and rear transaxle  101  is supported on vehicle frame  3  just above a fore-and-aft intermediate portion of duct  192 . Cooling fan  17   b  fixed on a bottom end of input shaft  17  of pump unit  50  blows upward cooling wind to adjoining pump unit  50  and reservoir tank  28  and fluid pipes so as to cool fluid therein. Additionally, as drawn in phantom lines, input shaft  17  may be extended upward from pump unit  50  so as to be provided thereon with another cooling fan  17   b  for blowing downward cooling wind. In this case, vehicle frame  3  is opened to lead the downward cooling wind to pipes. Rear transaxle  101  may be disposed on vehicle frame  3  in front or rear of pump unit  50 . Especially, as shown in  FIG. 7 , rear transaxle  101  may be disposed so as to have a bottom surface thereof sloped along a sloped upper surface of duct  192 , thereby approaching pump unit  50  and reducing a dead space. This is appropriate for a small size vehicle. Another effect of rear transaxle  101  disposed at such a high position on vehicle frame  3  is that a space in rear cover  15  can be used for locating a portion of rear transaxle  101  projecting upward from vehicle frame  3 . 
     Rear transaxle  101  has left and right rear-wheel-driving output shafts  106  which are differentially connected to each other by differential gear unit  38  and extended oppositely outward from the casing of rear transaxle  1 . As shown in  FIGS. 8 and 9 , left and right chain casings  108  are disposed at left and right outsides of duct  192 . Outer ends of output shafts  106  are inserted into upper portions of respective chain casings  108 , and sprockets  106   a  are fixed on the outer ends of respective output shafts  106  in respective chain casings  108 . When viewed in front or rear, left and right output shafts  106  and chain casings  108  look like a gate. 
     Left and right rear wheels  7  has respective axles  107  extended laterally proximally of the vehicle. Axles  107  are inserted into lower portions of respective chain casings  108 , and sprockets  107   a  are fixed on the proximal ends of respective axles  107  in respective chain casings  108 . In each of chain casings  108 , a chain  109  is interposed between sprockets  106   a  and  107   a  so as to transmit the driving force of output shaft  106  to axle  107 , thereby driving left and right rear wheels  7 . 
     A drive system of the present vehicle is represented by the description of drive system of the vehicle according to the second embodiment with reference to  FIG. 6 , except that the chain transmissions are interposed between differential gear unit  38  and respective rear wheels  7 . Alternatively, both the hydraulic motors in front transaxle  2  may be fixed displacement hydraulic motors M 2 . 
     Front transaxle  2  incorporating fixed displacement hydraulic motor M 2  and variable displacement hydraulic motor M 3  will be described with reference to  FIGS. 1 ,  2  and  4  to  20 . 
     Referring to a casing structure of front transaxle  2 , as shown in  FIGS. 10 to 12 , a motor casing  201  is disposed between left and right axle casings  203 , and detachably attached to each axle casing  203 . Axle casings  203  incorporate respective axles  8 . A spacer may be interposed between motor casing  201  and any of axle casings  203 , if a tread between front wheels  9  has to be lengthened. Kingpin casings  204  are fixed on distal ends of respective axle casings  203 , extended laterally outwardly downward at kingpin angles, and inserted into respective steerable casings  205 . Steerable casings  205  are rotatable centered on the axes of respective kingpin casings  204 . 
     As shown in  FIGS. 11 and 15 , motor casing  201  is opened at a rear end thereof. A motor cover  202 , onto which a assembly of hydraulic motors M 2  and M 3  (motor assembly) are attached as shown in  FIG. 17 , is fitted to the rear end of motor casing  201  so as to cover the rear opening of motor casing  201 , thereby setting hydraulic motors M 2  and M 3  in motor casing  201 . Motor casing  201  can be removed from motor casing  202  so as to easily take out the assembly of hydraulic motors M 2  and M 3  from motor casing  201 , thereby facilitating maintenance of hydraulic motors M 2  and M 3 . 
     The motor assembly will be described with reference to  FIGS. 14 ,  15 ,  17  and others. Each of hydraulic motors M 2  and M 3  comprises: a horizontal motor shaft  61  to be drivingly connected to corresponding axle  8 ; a cylinder block  62  which is rotatable together with motor shaft  61  centered on the axis of motor shaft  61 ; and horizontal pistons  63  reciprocally slidably fitted in respective cylinder holes of cylinder block  62  around motor shaft  61 . Cylinder blocks  62  of hydraulic motors M 2  and M 3  are slidably rotatably fitted onto a center section  60  disposed between cylinder blocks  62 . 
     As best shown in  FIGS. 12 ,  17  and  20 , a pair of parallel fluid passages  60   a  and  60   b  are bored in center section  60  so as to be extended in the fore-and-aft direction and opened rearward. Fluid passages  60   a  and  60   b  serve as respective fluid passages  24  and  25  in each of HST circuit HC 1  shown in  FIG. 2  and HST circuit HC 2  shown in  FIG. 6 . As best shown in  FIGS. 15 and 20 , a pair of kidney ports  60   c  and  60   d  laterally penetrate center section  60  so as to be opened at left and right ends thereof to the cylinder holes of respective cylinder blocks  62 . Fluid passage  60   a  is connected to kidney port  60   c , and fluid passage  60   b  to kidney port  60   d . Therefore, kidney port  60   c  serves as bifurcating fluid passages  24   a  and  24   b , and kidney port  60   d  serves as bifurcating fluid passages  25   a  and  25   b . Center section  60  is penetrated between kidney ports  60   c  and  60   d  by a laterally horizontal shaft hole  60   e , into which proximal end portions of motor shafts  61  are slidably rotatably fitted. Further, a fluid drain passage  60   f  is bored in center section  60 . Fluid drain passage  60   f  is opened at one end thereof to a portion of shaft hole  60   e  between the proximal ends of motor shafts  61 , and be opened at the other end thereof outward from center section  60  to the fluid sump in motor casing  201 , thereby fluidly connecting shaft hole  60   e  to the fluid sump in motor casing  201 . As shown in  FIG. 14 , in shaft hole  60   e , a bush  61   a  (or a needle bearing) is interposed between center section  60  and motor shafts  61  so as to reduce frictional resistance to motor shafts  61 , thereby improving mechanical efficiency of rotation of motor shafts  61 . 
     As shown in  FIGS. 12 ,  14 ,  15  and  17 , each of hydraulic motors M 2  and M 3  has a thrust bearing  64  pressed against heads of pistons  63 . Thrust bearing  64  of fixed displacement hydraulic motor M 2  is positionally fixed in a fixed swash plate support  65 , thereby serving as a fixed swash plate (hereinafter referred to as “fixed swash plate  64 ”). Some fixed swash plate supports  65  having different slant angles may be prepared for optional setting of slant angle fixed swash plate  64  of hydraulic motor M 2 . Variable displacement hydraulic motor M 3  has a movable swash plate  67 , which is rotatably supported by a movable swash plate support  66  so that its tilt angle relative to movable swash plate support  66  can be changed. Thrust bearing  64  of hydraulic motor M 3  is integrally assembled in movable swash plate  67 . 
     To fix the motor assembly to an inside (front) surface of motor cover  202 , as best shown in  FIG. 17 , bolts  68  project rearward from rear end surfaces of respective swash plate supports  65  and  66 . A bolt hole  60   g  is bored in center section  60  and opened at the rear end surface of center section  60 . Swash plate supports  65  and  66  are fastened to motor cover  202  by bolts  68 , and center section  60  is fastened to motor cover  202  by a bolt  96  (see  FIG. 18 ) screwed into bolt hole  60   g . Additionally, a knock pin  69  for locating motor cover  202  projects rearward from the rear end surface of center section  60 . 
     As best shown in  FIG. 15 , rearwardly opened front joggle holes are bored in a front wall of motor casing  201 . Forwardly opened rear joggle holes are bored in center section  60  and swash plate supports  65  and  66 , respectively. When the motor assembly is inserted into motor casing  201 , the rear joggle holes of center section  60  and swash plate supports  65  and  66  coincide to the respective front joggle holes of motor casing  201  with respective joggles  93  fitted between the coinciding front and rear joggle holes, thereby retaining center section  60  and swash plate supports  65  and  66  to motor casing  201  onto the front wall of motor casing  201 . In this state, motor cover  202  is fitted to motor casing  201  so as to cover the rear opening of motor casing  201 . Then, bolts  95  are screwed forward together with motor cover  202  into motor casing  201 , thereby completing setting of the motor assembly. 
     Motor cover  202  with the motor assembly in motor casing  201  is fixedly provided with a pipe coupling block  70  on the rear surface thereof just in rear of center section  60 . Pipe coupling block  70  is bored by fluid passages  70   c  and  70   d , which are opened to respective fluid passages  60   a  and  60   b  via respective fluid passages  202   a  and  202   b  penetrating motor cover  202 . Pipe couplings  70   a  and  70   b  are disposed at outer ends of respective fluid passages  70   c  and  70   d , and project outward from pipe coupling block  70 . Pipe couplings  70   a  and  70   b  serve as fluid ports  2   a  and  2   b  of each of HST circuits HC 1  and HC 2  shown in  FIGS. 2 and 6 . The series of fluid passages  70   c ,  202   a  and  60   a  serves as fluid passage  24 . The series of fluid passages  70   d ,  202   b  and  60   b  serves as fluid passage  25 . 
     In comparison with the case where pipe couplings  70   a  and  70   b  are directly attached to motor cover  202 , pipe coupling block  70  with pipe couplings  70   a  and  70   b  is advantageous in increasing angle variation of pipe couplings  70   a  and  70   b  relative to fluid passages  60   a  and  60   b  and the like serving as fluid passages  24  and  25 . Therefore, pipe coupling block  70  with pipe couplings  70   a  and  70   b  can be designed so that angles of pipes  23  (or  123 ) and  26  (or  126 ) coupled to respective pipe couplings  70   a  and  70   b  are optimized in relation to a later-discussed linkage for controlling the swash plate of hydraulic motor M 3 . 
     Alternatively, pipe coupling block  70  may be separated from motor cover  202  and supported on vehicle frame  3  or the like, so as to have pipes between pipe coupling block  70  and motor cover  202 . 
     As shown in  FIGS. 11 ,  15  and  16 , a pipe coupling  71 , serving as drain port  2   c  of each of HST circuits HC 1  and HC 2  shown in  FIGS. 2 and 6 , projects from the rear end surface of motor cover  202  adjacent to pipe coupling block  70  so as to be connected to reservoir tank  28  via drainpipe  30 . As shown in  FIG. 18 , a drain hole  202   c  is bored through motor cover  202  between pipe coupling  71  and the fluid sump in motor casing  201 . Therefore, excessively expanded fluid in the fluid sump in motor casing  201  can be drained to reservoir tank  28 . Pipe coupling  71  is disposed in a space behind fixed displacement hydraulic motor M 2 , where the linkage for controlling the displacement of hydraulic motor M 3  is not disposed. 
     As shown in  FIG. 20 , above-mentioned check valve  40  is disposed in center section  60  so as to be connected to kidney port  60   c  serving as a part of fluid passage  24  (and fluid passages  24   a  and  24   b ) which is hydraulically higher-pressurized in each of HST circuits HC 1  and HC 2  during forward travel of the vehicle. If kidney port  60   c , i.e., fluid passage  24  is hydraulically depressed, check valve  40  is opened to introduce fluid from the fluid sump in motor casing  201  into fluid passage  24 . Check valve  40  is provided with an oil filter  40   a  interposed between check valve  40  and kidney port  60   c.    
     The linkage for controlling the displacement of hydraulic motor M 3  will be described. As shown in  FIGS. 15 ,  16  and  17 , a fore-and-aft horizontal swash plate pivot shaft  73  is pivoted onto motor cover  202  in rear of hydraulic motor M 3 . In motor casing  201 , an inner arm  74  is fixed on an inner (front) end of swash plate pivot shaft  73  and engages with movable swash plate  67  of hydraulic motor M 3 . In motor casing  201 , a spring  75  is wound around swash plate pivot shaft  73  so as to return swash plate pivot shaft  73  and inner arm  74  to their initial position (that is, the swash plate angle setting position during straight travel of the vehicle). 
     As shown in  FIGS. 18 and 19 , inner arm  74  has a projection  74   a  fitted to swash plate  67 . A pushpin  74   b  projects from inner arm  74 . One end of spring  75  is constantly pressed against an inner surface of a wall of motor casing  202  so as to be retained. During turning of the vehicle, inner arm  74  is rotated for changing the displacement of hydraulic motor M 3  so that pushpin  74   b  pushes the other end of spring  75  away from the retained end of spring  75 , whereby spring  75  generates biasing force for returning inner arm  74  to the initial position. A stopper pin  76  is planted into a wall of motor casing  202  so as to abut against inner arm  74  disposed at the initial position. Stopper pin  76  is an eccentric pin, which is usually fastened to motor casing  202  by a nut  76   b . By loosening nut  76   b  and revolving stopper pin  76  around its center axis portion  76   a , the relative position of stopper pin  76  to inner arm  74  can be adjusted so as to adjust the initial position of inner arm  74  and swash plate pivot shaft  73 , thereby canceling positional error of inner arm  74  and swash plate pivot shaft  73  relative to the initial slant angle position of swash plate  67 . 
     As shown in  FIGS. 11 ,  15 ,  16  and  18 , a camshaft  80  is disposed in parallel to swash plate pivot shaft  73 , and pivoted onto motor cover  202  on one of left and right sides of swash plate pivot shaft  73  (preferably, leftwardly or rightwardly outward from hydraulic motor M 3 ) adjacent to swash plate pivot shaft  73 . A cam plate  81  is fixed on an outer (rear) end of camshaft  80  on the outside of (in rear of) motor cover  202 . Cam plate  81  has a pair of cam profiles  81   a  above camshaft  80 . An upper edge of cam plate  81  between cam profiles  81   a  is disposed horizontally when cam plate  81  is disposed at the initial position. On the outside (in rear) of motor cover  202 , an outer arm  77  is fixed on an outer (rear) end of swash plate pivot shaft  73 . A pressure plate  78  is fixed on outer arm  77  so as to be pressed at a bottom edge thereof against the upper edge of cam plate  81 . 
     Pressure plate  78  is fastened to outer arm  77  so as to be shiftable relative to outer arm  77 , so that, when cam plate  81  is disposed at the initial position, the bottom edge of pressure plate  78  is disposed horizontally to abut against the upper edge of cam plate  81  regardless of the initial position of swash plate pivot shaft  73  with inner and outer arms  74  and  77  adjusted by stopper pin  76 . In this regard, outer arm  77  is bored by a pair of bolt holes. One of the bolt holes is disposed toward a tip of outer arm  77 , and the other toward a basal end of outer arm  77 . As shown in  FIG. 16 , a pair of bolts  79  screwed into the respective bolt holes of outer arm  77  are passed through respective slots  78   a  and  78   b  bored in pressure plate  78 . Nuts are screwed on respective bolts  79  so as to fasten outer arm  77  and pressure plate  78  together. By loosening the nuts and adjusting positions of bolts  79  in slots  78   a  and  78   b , the position of pressure plate  78  relative to outer arm  77  can be adjusted. Due to this construction, the present linkage for controlling the displacement of hydraulic motor M 3  can be adapted to various vehicles having different steering angle settings. 
     Cam plate  81  is extended downward from camshaft  80 . An acceleration rod  82  is pivotally connected at one end thereof to a bottom end of downwardly extended cam plate  81 . Acceleration rod  82  is pivoted at the other end thereof onto one of left and right steerable casings  205 . Preferably, the other end of acceleration rod  82  is pivoted onto a pivot pin  83  (as best shown in  FIGS. 10 and 11 ), which is planted into a wall of steerable casing  205  supporting front wheel  9  drivingly connected to variable displacement hydraulic motor M 3 . In this way, steered steerable casings  205  are rotated leftward or rightward centered on respective kingpin casings  203  so as to push or pull acceleration rod  82 , thereby swinging the bottom end of cam plate  81  leftward or rightward. Accordingly, the upper edge of cam plate  81  is slanted so that one of cam profiles  81   a  rises to push up pressure plate  78 . Consequently, outer arm  77  are rotated upward so as to integrally rotate swash plate pivot shaft  73  and inner arm  74 , thereby reducing the tilt angle of swash plate  67  of hydraulic motor M 3 . 
     As shown in  FIGS. 10 and 11 , acceleration rod  82  has an intermediate spline collar  82   a  so as to be telescoped in correspondence to the above-mentioned tread adjustment by interposition of a spacer between motor casing  201  and any of axle casings  203 . 
     In this way, the cam system including cam plate  81  and pressure plate  78  abutting against each other, and arms  77  and  74  connected to pressure plate  78  so as to rotate integrally with movable swash plate  67  constitute the linkage for moving swash plate  67  of hydraulic motor M 3  according to lateral rotation of steerable casings  205  during turning of the vehicle. The linkage is disposed on the outer surface of motor cover  202  in rear of variable displacement hydraulic motor M 3 . Pipe coupling block  70  is disposed on the outer surface of motor cover  202  in rear of the portion between hydraulic motors M 2  and M 3  adjacent to the linkage. 
     It is now assumed that both the hydraulic motors in front transaxle  2  are variable displacement hydraulic motors M 3 . The linkage must have a cam system for converting the push-and-pull movement of a rod for the lateral rotation of steerable casings  205  into rotation of movable swash plates  67  of both hydraulic motors M 3 . Therefore, the linkage must be complicated and large so as to substantially entirely cover the outer surface of motor cover  202  (the rear surface of front transaxle  2 ) in the left-and-right direction. Accordingly, a port member having ports for fluidly connecting both hydraulic motors M 3  to hydraulic pump P and motor M 1  (such as pipe coupling block  70 ) has to be disposed on a surface of rear transaxle  2 , which is different from the outer surface of motor cover  202 . For instance, the port member has to be attached to the front surface of front transaxle  2  opposite to the outer surface of motor cover  202 . Pipes to be connected to the port member have to be extended along or beyond motor casing  201 . 
     However, in front transaxle  2  of the present embodiment, one of the pair of hydraulic motors therein is fixed displacement hydraulic motor M 2 . The linkage for angle controlling of movable swash plate  67  does not have to be disposed on the outer (rear) surface of motor cover  202  in rear of hydraulic motor M 2  or in rear of the portion between hydraulic motors M 2  and M 3 , thereby ensuring a space in rear of the rear surface of front transaxle  2  for attaching pipe coupling block  70  and drain pipe coupling  71 . Due to pipe coupling block  70  disposed at this position, pipes connected to front transaxle  2  from rear transaxle  1  (or from pump unit  50  and front transaxle  101 ) are streamlined so as to increase fluid circulation efficiency, and to reduce interference thereof with other parts or members. 
     A drive train from each motor shaft  61  to corresponding front wheel  9  will be described. As shown in  FIG. 12 , motor shaft  61  of each of hydraulic motors M 2  and M 3  freely rotatably passes through thrust bearing  62  and swash plate support  65  or  66  so as to be connected to coaxial axle  8  supported in axle casing  203 . Facing ends of motor shaft  61  and axle  8  are spline-fitted into a bush  85  disposed in a proximal end portion of axle casing  203  from opposite openings of bush  85 , whereby coaxial motor shaft  61  and axle  8  are integrally rotatably connected to each other. Alternatively, single motor shaft  61  may be extended into axle casing  203  so as to serve as axle  8 . 
     As best shown in  FIG. 13 , a bevel gear shaft  86  is journalled by a distal end portion of axle casing  203 . A distal end of axle  8  and a proximal end of bevel gear shaft  86  are spline-fitted into bush  85  disposed in the distal end portion of axle casing  203  from opposite openings of bush  85 , wherein coaxial axle  8  and bevel gear shaft  86  are integrally rotatably connected to each other. The distal end portion of axle casing  203  projects into kingpin casing  204 , so that a distal end of bevel gear shaft  86 , formed thereon with a bevel gear  86   a , projects from axle casing  203  into an upper portion of kingpin casing  204 . A kingpin center shaft  87  is rotatably passed through the downwardly extended portion of kingpin casing  204 . A bevel gear  87   a  is fixed on a top of kingpin center shaft  87  in the upper portion of kingpin casing  204 , so as to mesh with bevel gear  86   a.    
     A top opening of kingpin casing  204  facing the inner space of kingpin casing  204 , in which meshing bevel gears  86   a  and  87   a  are disposed, is wide so as to be adapted for economically forming kingpin casing  204  by die casting. Further, the wide opening facilitates easy and accurate forming of bearing grooves in the inner wall surface of kingpin casing  204 . The opening is closed by a grommet after the bevel gears and others are completely disposed in kingpin casing  204 . 
     The downwardly extended portion of kingpin casing  204  is inserted into steerable casing  205 . Steerable casing  205  relatively rotatably supports the downwardly extended portion of kingpin casing  204  therein with bearings. In other words, steerable casing  205  is laterally rotatable around kingpin casing  204 . Kingpin center shaft  87  is extended downward from a bottom end of kingpin casing  204 , and fixedly provided on a bottom end thereof with a bevel gear  87   b  in steerable casing  205  below kingpin casing  204 . Bevel gear  87   b  is journalled by a bottom portion of steerable casing  205 . 
     A bearing cover support portion  205   a  is formed on a lateral distal end of steerable casing  205  so as to journal a proximal end of a center axis shaft  9   a  of front wheel  9  in a center hole thereof. A diametrically large bevel gear  89  is fixed on center axis shaft  9   a  and meshes with bevel gear  87   b . A bearing cover  206  is fastened to bearing cover support portion  205   a  so as to cover bevel gear  89 . Center axis shaft  9   a  is extended distally outward from bevel gear  89  so as to be pivoted onto bearing cover  206  via bearings, and further extended distally outward from bearing cover  206  so as to be connected at a distal end thereof to front wheel  9 . 
     In this way, front wheels  8  can be steered by rotating steerable casing  205  around kingpin casing  204  while receiving the output force of respective hydraulic motors M 2  and M 3  through respective motor shafts  61 , axles  8  and kingpin center shafts  87 . 
     As mentioned above, the entire casing of front transaxle  2  is formed by joining motor casing  201  (with motor cover  202 ), left and right axle casings  203 , kingpin casings  204 , steerable casings  205  and bearing covers  206  to one another. A bearing for journaling a shaft is disposed at a junction between any adjoining casings of rear transaxle  2 . The bearing is provided with no oil seal. In this regard, lube (also used as operation fluid for the HST) filled in motor casing  201 , axle casings  203 , kingpin casings  204 , steerable casings  205  and bearing covers  206  can freely pass among these casings and covers. The costs for oil seals are saved. Oiling ports to fill entire front transaxle  2  with fluid (lube) can be reduced, and even only a single oiling port can be provided. Further, the total amount of fluid and the total surface area of hydraulic devices are increased so as to suppress heating of the hydraulic devices, thereby improving the hydraulic devices in endurance and mechanical efficiency. 
     The linkage from steering wheel  12  to steerable casing  205  will be described with reference to  FIGS. 2 and 21 . A steering link bracket  207  is fixed on one of steerable casings  205  so as to cover the top surface of this steerable casing  205 . Preferably, steerable casing  205  having no acceleration rod  82  connected thereto is provided with steering link bracket  207 , so as to prevent steering link bracket  207  from interfering with acceleration rod  82  and the like. A drag rod  90  is extended in parallel to an outer side surface of vehicle frame  3 , and pivotally connected at a front end thereof to steering link bracket  207 . A gearbox  12   b  is provided on a bottom end of a stem  12   a  (see  FIG. 2 ) of steering wheel  12 , and an arm  12   c  is fore-and-aft rotatably attached onto an output end of gearbox  12   b . Drag rod  90  is pivotally connected at a rear end thereof to a free tip of arm  12   c . Drag rod  90  may be replaced with a hydraulic power steering cylinder. Preferably, drag rod  90  or the hydraulic power steering cylinder is disposed substantially in parallel to vehicle frame  3 . 
     As best shown in  FIGS. 10 and 11 , steering casing  205  on variable displacement hydraulic motor M 3  side and steering casing  205  on fixed displacement hydraulic motor M 2  are connected to each other via tie rod  84  so as to be laterally rotatable substantially integrally with each other. An end of tie rod  84  toward steering casing  205  on variable displacement hydraulic motor M 3  side is pivoted onto pivot pin  83 . Preferably, pivot pin  83  is stepped so that upper and lower portions of pivot pin  83 , onto which accelerator rod  82  and tie rod  84  are pivotally connected respectively, have different diameters. In this way, ends of accelerator rod  82  and tie rod  84  are pivoted on common pivot pin  82 , so as to save the parts count, and to prevent rods  82  and  84  from intercrossing each other hindering their own movement and movement of steerable casing  205 . 
     Referring to  FIGS. 1 ,  16 ,  20  and others, a structure for supporting front transaxle  2  onto vehicle frame  3  will be described. Upright stays  201   a  are disposed at optimal positions (e.g., four stays  201   a  are disposed at the front, rear, right and left) on a top surface of motor casing  201 , and a center pin boss member  200  is mounted over all stays  201   a . Center pin boss member  200  includes a horizontal plate portion  200   a , which is integrally formed at the lateral center portion thereof with a fore-and-aft axial boss portion  200   b . Four corners of plate portion  200   a  are screwed onto respective stays  201   a . Center pin  5  is freely rotatably passed through a boss hole of boss portion  200   b.    
     Some different designed center pin brackets  200  are prepared, and one corresponding to relative pivot positions of front transaxle  2  in a vehicle is selected to be fastened to stays  201   a  of motor casing  201 . Therefore, front transaxle  2  can be standardized for various designed vehicles. 
     Center pin  5  is journalled at front and rear ends thereof by front axle bracket  4  fixed on the front end of vehicle frame  3 . Front axle bracket  4  comprises a pair of left and right vertical side plates  4   a , a vertical rear plate  4   b  connecting side plates  4   a  to each other, a front plate  4   c , and a horizontal plate  4   d  connected between rear and front plates  4   b  and  4   c.    
     Each of left and right side plates  4   a  has bolt holes  4   h  bored in a rear end portion thereof. Bolts are passed through bolt holes  4   h  of left and right side plates  4   a  so as to fasten left and right side plates  4   a  onto front end portions of respective left and right side plate portions of vehicle frame  3 , thereby fixing front axle bracket  4  onto the front end portion of vehicle frame  3 . Center pin  5  passed through center pin bracket  5  is journalled at the rear end portion by rear plate  4   b , and at the front end portion by front plate  4   c . Therefore, front transaxle  2  is swingably supported. Flexibility of hydraulic fluid hoses  23   a  and  26   a  permits the swing of front transaxle  2 . 
     In this state, horizontal plate  4   d  covers the top of motor casing  201  disposed in front of the front end of vehicle frame  3 . A pair of left and right stoppers  4   e  are hung down from the bottom surface of horizontal plate  4   d . The position where one of stoppers  4   e  abuts against the top of plate portion  200   a  of center pin bracket  200  fixed on front transaxle  2  is defined as the limit swing position of front transaxle  2  centered on center pin  5 . Stoppers  4   e  also serve as ribs for reinforcing horizontal plate  4   d.    
     Further, horizontal plate  4   d  is at a lower degree than the top of the front end of vehicle frame  3 . An engine muffler  10   d  is mounted on horizontal plate  4   d , so as to be covered at a rear portion thereof with rear plate  4   b , and at left and right end portions thereof with left and right side plates  4   a . In this way, a suitable space for efficient radiation of engine muffler  10   d  is ensured in front axle bracket  4 . Further, horizontal plate  4   d  protects front transaxle  2  from the heat radiated from engine muffler  10   d.    
     As shown in  FIG. 20 , front ends of left and right side plates  4   a  are extended upwardly forward so as to serve as bonnet brackets  4   f  for supporting a pivot shaft for opening and closing bonnet  11 . The pivot shaft is passed through holes  4   g  formed in bonnet brackets  4   f . A rear end of bonnet  11  can be rotated upward centered on the pivot shaft so as to open engine  10  mounted on vehicle frame  3 . Front axle bracket  4  can be fastened by bolts, or joined by welding, to a front end of a vehicle frame for a two-wheel drive vehicle, thereby platforming the vehicle frame for both two-wheel drive vehicles and four-wheel drive vehicles. 
     A part of each side plate  4   a  adjacent to corresponding bonnet bracket  4   f  is cut off and bent laterally inward of the vehicle so as to form a spring stay  4   j , onto which an end of a spring for biasing bonnet  11  to its initial closed position is connected. 
     As shown in  FIGS. 1 ,  4  and  7 , mower lifting cradles  91  are extended downward from front ends of respective side plates  4   a  so as to guide the vertical movement of mower  20 . Link rods  91   a  are extended rearward from respective cradles  91  and connected to mower  20 . Plurality of bolt holes are bored in each of cradles  91 . On the other hand, as shown in  FIG. 20 , a bolt hole  4   k  is bored in each side plate  4  below spring stay  4   j . Each cradle  91  is disposed so as to selectively coincide one of the bolt holes thereof to bolt hole  4   k , and fastened to each side plate  4   a  by a bolt through the selected bolt hole thereof and bolt hole  4   k . In this way, the vertical position of mower  20  relative to front axle bracket  4  can be adjusted by selecting one of the bolt holes in each cradle  91 . 
     Front axle bracket  4  may be replaced with a simple vertical plate covering the front end of vehicle frame  3 , if the vehicle is a two-wheel drive vehicle having no front transaxle  2  incorporating the pair of hydraulic motors. 
     Referring to  FIGS. 21 to 23 , two type rear-wheel steering systems will be described. These steering systems are applicable to the respective four-wheel drive lawn tractors according to the first embodiment shown in  FIGS. 1 and 2 , the second embodiment shown in  FIGS. 4 and 5 , and the third embodiment shown in  FIGS. 7 and 8 . The rear-wheel steering system shown in  FIGS. 21 and 22  is applied to the lawn tractor as shown in  FIGS. 4 and 5 , having separate pump unit  50  and rear transaxle  101 . However, as shown in  FIGS. 1 and 2 , rear transaxle  1  can be applied to a lawn tractor having rear transaxle  1  incorporating hydraulic pump P, such as the lawn tractor shown in  FIGS. 1 and 2 . 
     Referring to the rear-wheel steering system of  FIGS. 21 and 22 , a connection portion  3   b  is connected between left and right side plates  3   a  of vehicle frame  3 . Connection portion  3   b  is provided at the lateral center portion thereof with a bearing portion  3   c . A vertical pivot shaft  221  is journalled by bearing portion  3   c . A rear axle support frame  222  has a laterally central boss  222   a , into which pivot shaft  221  extended downward from bearing portion  3   c  is inserted and fixed. A horizontal arm  222   b  is extended from boss  222   a  so as to be interlockingly connected to steering wheel  12 . Rear axle support frame  222  has laterally opposite first and second end portions  222   c  and  222   d . First end portion (in this embodiment, the left end portion)  222   c  of rear wheel support frame  222  is extended downward, and fastened at the bottom end portion thereof to one of left and right end portions (in this embodiment, the left end portion) of the casing of rear transaxle  101 . Second end portion (in this embodiment, the right end portion)  222   d  of rear wheel support frame  222  is extended downward, and provided at the bottom thereof with a bearing  222   e  for longer axle  6  (in this embodiment, right axle  6 ) extended outward from rear transaxle  101 . A space for duct  92  extended from mower  20  is ensured below axle  6  between bearing  222   e  and the other right or left end portion (in this embodiment, the right end portion) of rear transaxle  101 . 
     Rear transaxle  101  has longer and shorter axles  6 . Shorter axle  6  is extended laterally outward from the end portion of the casing of rear transaxle  101  fastened to the bottom of first end portion  222   c  of rear wheel support frame  222 , and fixed at the distal end thereof to one rear wheel  7 . Longer axle  6  is further extended laterally outward from bearing  222   e , and fixed at the distal end thereof to the other rear wheel  7 . 
     Therefore, by rotating steering wheel  12 , rear wheel support frame  222  rotates integrally with rear transaxle  101 , left and right axles  6  and rear wheels  7  relative to vehicle frame  3  centered on pivot shaft  221 , thereby steering rear wheels  7 . 
     A structure of rear wheel support frame  222 , and a structure of the casing of rear transaxle  101  with axles  6  to be supported by rear wheel support frame  222  may be different from those shown in  FIGS. 21 and 22 . The only important thing for this embodiment is to make the casing of rear transaxle  101  with axles  6  horizontally rotatable relative to vehicle frame  3  according to operation of steering wheel  12  while ensuring the driving of axles  6 . 
     Referring to the rear wheel steering system of  FIG. 23 , rear wheel support frames  223  are fixed onto respective left and right side plates  3   a  of vehicle frame  3 . Each rear wheel support frame  223  has a portion extended downward from the bottom end of corresponding side plate  3   a . The downwardly extended portion of rear wheel support frame  223  may be fastened to an end portion of the casing of rear transaxle  101  while avoiding interference thereof with corresponding axle  6  (axle  6  may be freely rotatably passed through the downwardly extended portion of rear wheel support frame  223 ), as the downwardly extended portion of left rear wheel support frame  223  in  FIG. 23  is so. Otherwise, the downwardly extended portion of rear wheel support frame  223  may be provided with a bearing portion  223   a  for journaling corresponding axle  6 , as the downwardly extended portion of right rear wheel support frame  223  in  FIG. 23  is so provided. Both left and right rear wheel support frames  223  may be fastened to the casing of rear transaxle  101 . Alternatively, both left and right rear wheel support frames  223  may be provided with respective bearing portions  223   a  for respective axles  6 . The important distinction of this steering system of  FIG. 23  from the above steering system of  FIGS. 21 and 22  is that the casing of rear transaxle  101  is fixed to vehicle frame  3  regardless of steering of rear wheels  7 . 
     An upper kingpin support portion  223   b  and a lower kingpin support portion  223   c  are extended laterally outward from each rear wheel support frame  223 . The distal end of each axle  6  extended outward from the casing of rear transaxle  101  is disposed between upper and lower kingpin support portions  223   b  and  223   c . The distal end of axle  6  is connected to center axis shaft  7   a  of rear wheel  7  via a universal joint  6   a , so that rear wheel  7  with center axis shaft  7   a  can be substantially horizontal relative to axle  6 . 
     Center axis shaft  7   a  is journalled in a bearing block  224 . An upper arm  224   a  is extended from an upper portion of bearing block  224  so as to be pivotally connected to upper kingpin support portion  223   b  via a kingpin  225 . A lower arm  224   b  is extended from a lower portion of bearing block  224  so as to be pivotally connected to lower kingpin support portion  223   c  via a kingpin  226 . Upper and lower kingpins  225  and  226  are disposed coaxially, and the pivot of universal joint  6   a  is coaxially disposed between upper and lower kingpins  225  and  226 . 
     A tie rod (not shown) is interposed between tie rod stays  224   c  extended from respective upper portions of bearing blocks  224 , and connected to steering wheel  12 . By rotating steering wheel  12 , left and right bearing blocks  224  are substantially horizontally rotated around respective kingpins  225  and  226  so as to steer left and right rear wheels  7 . 
     The hydrostatic transaxle and the hydraulically driven vehicle of the present invention are applicable to various vehicles such as working vehicles including disclosed lawn tractors. The illustrated vehicles have Ackerman-type steering systems. However, the peripheral speed setting of front and rear wheels, the structure of the hydraulic pump unit separated from the front and rear transaxles, and the like are applicable to vehicles having different steering systems, such as an articulated vehicle. 
     It is further understood by those skilled in the art that the foregoing description is a preferred embodiment of the disclosed apparatus and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.