Abstract:
A hydrostatic transmission including a closed fluid circuit is contained together with a deceleration gear mechanism in a housing filled with fluid so as to serve as a fluid sump. The hydrostatic transmission includes a check valve assembly in connection with the closed fluid circuit. The check valve assembly includes a vertically movable valve member, a valve seat, and a draining operation member being movable perpendicularly to the movement of the valve member. The check valve assembly has three functions. A first function of the check valve assembly is to lower the valve member by the gravity and hydraulic pressure in the closed fluid circuit so as to fit the valve member on the valve seat, thereby sealing the closed fluid circuit. A second function of the check valve assembly is to raise the valve member apart from the valve seat because the hydraulic pressure in the closed fluid circuit becomes lower than pressure in the fluid sump, thereby supplying fluid from the fluid sump to the closed fluid circuit. A third function of the check valve assembly is to raise the valve member apart from the valve seat by movement of the draining operation member, thereby draining fluid from the closed fluid circuit to the fluid sump.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hydrostatic transmission (hereinafter, an “HST”), which is applicable to a transaxle apparatus. 
   2. Related Art 
   An integrated hydrostatic transaxle apparatus (hereinafter, “IHT”) comprises an HST, an axle, and a drive train interposed between the HST and the axle disposed together in a common housing which is filled therein with fluid so as to serve as a fluid sump. The HST includes a hydraulic pump and motor fluidly connected through a closed fluid circuit formed in a center section onto which at least one of the hydraulic pump and motor is attached. A fluid charge valve assembly may be attached to the center section so as to supply fluid from the fluid sump to the closed fluid circuit. The fluid charge valve assembly includes a check valve allowing only a fluid flow from the fluid sump to the closed circuit. 
   It is sometimes necessary to drain fluid in the closed circuit to the fluid sump or partly bypass the fluid into a short circuit. For example, if a proper amount of fluid is filled in the closed fluid circuit and a vehicle having the IHT is hauled, the axle of the IHT is necessarily rotated together with the hydraulic motor and the fluid in the closed circuit transmits the rotation of the hydraulic motor to the hydraulic pump. Thus, the hydraulic pump drivingly connected to a prime mover is rotated so as to transmit an opposite force to the prime mover. The drainage of fluid from the closed fluid circuit or the bypass circulation of part of fluid taken from the closed fluid circuit solves the problem. 
   There are some conventional IHTs that permit drainage of fluid from the closed circuit. In these conventional IHTs, the fluid draining means is separated from the fluid charge valve assembly. 
   Some of various IHTs are used for a vehicle having a vertical crankshaft engine, wherein the rotary axis of the hydraulic pump of the HST must be disposed vertically, i.e., in perpendicular to the horizontal axle. Thus, if the rotary axis of the hydraulic motor of the HST is disposed horizontally, the drive train between the HST and the axle can be simplified. However, this arrangement of hydraulic motor complicates the center section having the closed fluid circuit for fluid connection of the hydraulic pump and motor. Further, output means such as a motor shaft of the hydraulic motor must be extended horizontally, thereby expanding the IHT in the horizontal direction. 
   If the hydraulic motor is an axial piston type hydraulic motor, arranging a vertical motor shaft serving as a rotary axis of the hydraulic motor is available to reduce the horizontal size of the IHT for a vertical crankshaft engine and to simplify the center section of the HST. However, the vertical motor shaft projects from a cylinder block of the hydraulic motor through a swash plate abutting heads of pistons fitted in the cylinder block so as to expand the IHT vertically. 
   Usually, the swash plate of the hydraulic motor is a fixed swash plate and a retainer for the swash plate is fixed to the IHT housing. Preferably, the retainer is attached to the IHT housing without bolts or the like so as to facilitate assembly of the IHT and reduce the number of parts. Furthermore, a horizontal transmission shaft may be interposed between the vertical motor shaft and the axle (or a differential gearing). If the IHT housing is divisible into upper and lower halves, at least one of the upper and lower halves is usually provided with an integral vertically extending portion, or with a separate member fitted thereto, so as to support a bearing for the horizontal transmission shaft. However, if the swash plate retainer is also used for supporting a bearing for the transmission shaft, the housing (or the upper and lower housing halves) can be simplified or the number of members for supporting the bearing for the transmission shaft can be reduced. 
   Further, if an IHT is required to be vertically small, a vertically small structure for supporting an axle without a vertically expanded additional member is desired to be provided in a housing containing an HST, especially, between a wall portion of the housing and a center section of the HST. 
   Moreover, the vertical motor shaft is usually fitted at one end thereof into the center section. If the hydraulic motor is provided below the center section, an upper portion of the motor shaft is inserted into the center section. Thus, assembly of the hydraulic motor with the center section can be facilitated if the motor shaft is prevented from falling from the center section after the upper portion of the motor shaft is inserted into the center section and before the hydraulic motor is completely assembled with the center section. This may also be true of the hydraulic pump. 
   SUMMARY OF THE INVENTION 
   A first object of the present invention is to provide an HST having a check valve assembly for fluid charge to a closed fluid circuit of the HST, wherein the check valve can be used for drainage of fluid in the closed fluid circuit. 
   To achieve the first object, an HST according to the present invention is contained together with a deceleration gear mechanism in a housing which is filled therein with fluid so as to serve as a fluid sump, wherein the HST comprises a closed fluid circuit and a check valve assembly in connection with the closed fluid circuit. The check valve assembly includes a vertically movable valve member, a valve seat, and a draining operation member being movable perpendicularly to the movement of the valve member. The check valve assembly has three functions. A first function of the check valve assembly is to lower the valve member by the gravity and hydraulic pressure in the closed fluid circuit so as to fit the valve member on the valve seat, thereby sealing the closed fluid circuit. A second function of the check valve assembly is to raise the valve member apart from the valve seat because the hydraulic pressure in the closed fluid circuit becomes lower than pressure in the fluid sump, thereby supplying fluid from the fluid sump to the closed fluid circuit. A third function of the check valve assembly is to raise the valve member apart from the valve seat by movement of the draining operation member, thereby draining fluid from the closed fluid circuit to the fluid sump. 
   A second object of the present invention is to provide an HST having a short motor shaft so as to reduce the size of the HST along the motor shaft. 
   To achieve the second object, an HST according to the present invention includes an axial piston hydraulic motor. The hydraulic motor comprises a cylinder block, a plurality of pistons fitted in the cylinder block, a thrust bearing serving as a swash plate abutting against heads of the pistons, and a motor shaft fitting with said cylinder block and extended from said cylinder block. A bearing for the motor shaft is disposed in the thrust bearing at the substantially same position with the thrust bearing in an axial direction of the motor shaft. 
   A third object of the present invention is to provide an HST contained together with a transmission shaft, which receives output power of the HST, in a housing, wherein an arrangement for supporting the transmission shaft is minimized, simplified and reduced in number of parts. 
   To achieve the third object, an HST according to the present invention comprises a hydraulic motor. The hydraulic motor includes an output shaft whose rotational force is transmitted to the transmission shaft, a cylinder block, a plurality of pistons fitted in the cylinder block, a swash plate abutting against heads of the pistons, and a retainer disposed in the housing so as to retain the swash plate. The retainer has a main cylindrical portion and a pair of feet. The main cylindrical portion is fitted to an inside portion of the housing, and one of the feet is selected to support the bearing. 
   Preferably, the inside portion of the housing is formed with a recess into which the main cylindrical portion of the retainer is fitted, thereby facilitating the retainer to be fixed to the housing without a bolt or another member. 
   Further preferably, a slant direction of the swash plate is reversed by reversing the retainer so as to select which of the pair of feet is used to support the bearing. Therefore, if a rotational direction of an axle driven by the HST is set opposite, only arranging the retainer so as to exchange the feet is required to change the slant direction of the swash plate of the hydraulic motor in correspondence to the rotational direction of the axle. 
   A fourth object of the present invention is to provide an appropriate structure for an axle in a narrowed space between a center section of an HST and a wall portion of a housing containing the HST. 
   To achieve the fourth object, an HST according to the present invention is contained together with an axle in a housing. The HST comprises a hydraulic pump including a cylinder block, a hydraulic motor including a cylinder block, and a center section forming therein a closed fluid circuit through which the hydraulic pump and the hydraulic motor are fluidly connected with each other. The axle is disposed between the cylinder blocks of both the hydraulic pump and the hydraulic motor perpendicularly to the cylinder blocks. An extended portion of the center section and an extended portion of the housing constitute a bearing for the axle. 
   If the housing comprises two divided portions and each of the hydraulic pump and the hydraulic motor comprises a swash plate abutting against pistons fitted in the cylinder block, one of the two divided portions of the housing may support both the swash plates, thereby reducing the size of HST in the direction perpendicular to the dividing line of the housing. 
   Both the cylinder blocks of the hydraulic pump and the hydraulic motor may be disposed substantially axially parallel to each other below the center section in the housing. In this case, a cylindrical fluid filter may be disposed substantially axially parallel to the cylinder blocks of the hydraulic pump and the hydraulic motor. 
   A fifth object of the present invention is to provide an HST including a center section and an axial piston type hydraulic unit serving as a hydraulic pump or motor, wherein, even if an axial (input or output) shaft of the hydraulic unit is disposed vertically and rotatably fitted into the center section from below, the shaft is prevented from falling from a center section before the hydraulic unit is completely assembled with the center section. 
   To achieve the fifth object, an HST according to the present invention comprises an axial piston type hydraulic unit assembled with a center section. The hydraulic unit includes a vertical axial shaft rotatably inserted into the center section. The shaft is fixedly provided with a pin projecting from an outer peripheral surface of the shaft. The center section includes a fluid circuit for supplying fluid to the hydraulic unit, a bottom surface onto which the hydraulic unit is fitted, and a shaft hole which is open at the bottom surface. A pin-passing groove is bored in the center section along the shaft hole and is open at the bottom surface. A pin-retaining groove, which is wider than the pin-passing groove in a peripheral direction of the shaft hole, is bored on a top end of the pin-passing groove in the center section around the shaft hole. 
   The shaft is inserted into the center section through the shaft hole by passing the pin in the pin-passing groove. The shaft is axially fixed in place in the center section by locating the pin in the pin-retaining groove. The shaft is rotated so as to offset the pin in the pin-retaining groove from the pin-passing groove, thereby preventing the shaft from falling down from the center section before the hydraulic unit is completely assembled with the center section. 
   These, other and further objects, features and advantages of the present invention will appear more fully from the following description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       FIG. 1  is a plan view partly in section of a first transaxle apparatus T 1  serving as an IHT incorporating an HST according to the present invention, from which an upper housing half is removed while a part thereof remains. 
       FIG. 2  is a sectional front view of first transaxle apparatus T 1 . 
       FIG. 3  is a sectional side view of first transaxle apparatus T 1  showing the HST and a fluid charge valve assembly for the HST. 
       FIG. 4  is a sectional side view of first transaxle apparatus T 1  showing a supporter supporting a horizontal transmission shaft while a brake is removed from the transmission shaft. 
       FIG. 5  is a bottom view partly in section of a second transaxle apparatus T 2  serving as an IHT incorporating an HST according to the present invention, from which a lower housing half is removed while a part thereof remains. 
       FIG. 6  is a sectional side view of second transaxle apparatus T 2 . 
       FIG. 7  is a sectional front view of second transaxle apparatus T 2  showing a hydraulic pump. 
       FIG. 8  is a sectional rear view of second transaxle apparatus T 2  showing a hydraulic motor. 
       FIG. 9  is a fragmentary sectional side view of first transaxle apparatus T 1  showing the fluid charge valve assembly for the HST set in a charge mode. 
       FIG. 10  is a fragmentary sectional side view of first transaxle apparatus T 1  showing the fluid charge valve assembly set in a drain mode. 
       FIG. 11  is an exploded perspective view of the fluid charge valve assembly attached to a center section of the HST of first transaxle apparatus T 1 . 
       FIG. 12  is a bottom view of a motor-swash-plate retainer, a supporter, and the horizontal transmission shaft supported by the motor-swash-plate retainer and the supporter when the motor-swash-plate retainer is arranged so as to slant a motor swash plate perpendicularly to the transmission shaft when viewed in plan and drive the transmission shaft in a first direction. 
       FIG. 13  is a bottom view of the motor-swash-plate retainer, the supporter, and the transmission shaft supported by the motor-swash-plate retainer and the supporter when the motor-swash-plate retainer is arranged so as to reverse the slant direction of the motor swash plate perpendicularly to the transmission shaft when viewed in plan and drive the transmission shaft in a second direction opposite to the first direction. 
       FIG. 14  is a fragmentary sectional side view of the first or second transaxle apparatus T 1  or T 2  showing a brake provided on the transmission shaft. 
       FIG. 15  is a sectional plan view of an HST center section showing a structure for preventing a motor shaft from falling from the HST center section. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A first transaxle apparatus T 1  shown in  FIGS. 1 to 4  and a second transaxle apparatus T 2  shown in  FIGS. 5 to 8  will be described on the assumption that a later-discussed fluid charge valve assembly for an HST is disposed at a front end of each of transaxle apparatuses T 1  and T 2  while axles  6  of each apparatus are disposed laterally. 
   First transaxle apparatus T 1  shown in  FIGS. 1 to 4  is provided with a housing  11  consisting of an upper housing half  12  and a lower housing half  13 . Second transaxle apparatus T 2  shown in  FIGS. 5 to 8  is provided with a housing  21  consisting of an upper housing half  22  and a lower housing half  23 . Vertical bolts  9  join upper and lower housing halves  12  and  13  to each other, and upper and lower housing halves  22  and  23  to each other. 
   Each of transaxle apparatuses T 1  and T 2  is provided with coaxial left and right axles  6  extended laterally from each of housings  11  and  21 . Axles  6  of first transaxle apparatus T 1  are housed in lower housing half  13  so that a horizontal surrounding joint surface between upper and lower housing halves  12  and  13  is disposed just above axles  6 . Axles  6  of second transaxle apparatus T 2  are housed in upper housing half  22  so that a horizontal surrounding joint surface between upper and lower housing halves  22  and  23  is disposed just below axles  6 . 
   In each of housings  11  and  21 , a differential gearing  7  is disposed between left and right axles  6  so as to differentially connect axles  6 . A pair of bearings  8  are disposed adjacent to left and right ends of differential gearing  7 , respectively, so as to journal proximal portions of axles  6 . Upper and lower housing halves  12  and  13  are integrally formed with respective vertically extending portions so as to pinch bearings  8 , as shown in  FIGS. 2 to 4 . One of bearings  8  in second transaxle apparatus T 2  is pinched by upper and lower housing halves  22  and  23  formed similarly. The other of bearings  8  in second transaxle apparatus T 2  is pinched between lower housing half  23  and a later-discussed center section  24 , as best shown in  FIG. 6 . In each of transaxle apparatuses T 1  and T 2 , a pair of bearings  8   a  are disposed at respective distal ends of left and right laterally extended portions of either lower housing half  13  or upper housing half  22  so as to journal distal portions of axles  6 . 
   In housing  11  of first transaxle apparatus T 1  is disposed a center section  14 , and in housing  21  of second transaxle apparatus T 2  is disposed center section  24 . Vertical bolts  10  fasten each of center sections  14  and  24  to one or both housing halves  12  and  13 , as shown in  FIG. 2 , or one or both housing halves  22  and  23 , as shown in  FIGS. 1 and 5 . 
   Each of center sections  14  and  24  is a rather thin plate-like block having horizontal flat top and bottom surfaces. While center section  14  of first transaxle apparatus T 1  is disposed in front of axles  6 , center section  24  of second transaxle apparatus T 2  is elongated in the fore-and-aft direction perpendicularly to axles  6  so as to be disposed across one of the axles  6 . 
   As for first transaxle apparatus T 1 , as best shown in  FIGS. 2 and 3 , a hydraulic pump  1  is mounted upward on the top surface of center section  14 , and a hydraulic motor  2  is mounted downward on the bottom surface of center section  14  vertically oppositely to hydraulic pump  1 , thereby constituting an HST of first transaxle apparatus T 1 . As for second transaxle apparatus T 2 , as best shown in  FIG. 6 , both hydraulic pump  1  and hydraulic motor  2  are mounted downward onto the bottom surface of center section  24  so that hydraulic pump  1  is disposed in front of axles  6 , and hydraulic motor  2  is disposed behind axles  6 , thereby constituting an HST of second transaxle apparatus T 2 . 
   As shown in  FIGS. 2 and 3  or  FIGS. 6 and 7 , hydraulic pump  1  of each of transaxle apparatuses T 1  and T 2  has a pump cylinder block  31  slidably rotatably fitted onto a pump-mounting surface which is formed on the top surface of center section  14  or on the front bottom surface of center section  24 . A pump shaft  4  is disposed axially in pump cylinder block  31  and not-relatively rotatably fitted with pump cylinder block  31  through splines. A plurality of pistons  32  are reciprocally fitted into pump cylinder block  31  and disposed so as to surround pump shaft  4  in parallel. 
   As shown in  FIGS. 2 and 3  or  FIGS. 6 and 8 , hydraulic motor  2  of each of transaxle apparatuses T 1  and T 2  has a motor cylinder block  33  slidably rotatably fitted onto a motor mounting surface which is formed on the bottom surface of center section  14  or on the rear bottom surface of center section  24 . A motor shaft  5  is disposed axially in motor cylinder block  33  and not-relatively rotatably fitted with motor cylinder block  33  through splines. A plurality of pistons  34  are reciprocally fitted into motor cylinder block  33  and disposed so as to surround motor shaft  5  in parallel. 
   As shown in  FIGS. 1 and 2 , center section  14  of first transaxle apparatus T 1  is vertically pierced between the upper pump-mounting surface and the lower motor-mounting surface by a pair of left and right kidney ports  14   c  so as to fluidly connect pump and motor cylinder blocks  31  and  33  to each other. That is, only the pair of kidney ports  14   c  simply formed in center section  14  serve as a closed hydraulic fluid circuit between hydraulic pump  1  and hydraulic motor  2  which are aligned vertically. Moreover, a pair of left and right horizontal bores  14   b  are bored in center section  14  from a front end surface of center section  14  and connected at rear ends thereof to respective kidney ports  14   c.    
   In center section  24  of second transaxle apparatus T 2 , a pair of left and right kidney ports  24   c  are recessed vertically upward and open at the pump-mounting surface as shown in  FIGS. 5 and 7 , and a pair of left and right kidney ports  24   d  are recessed vertically upward and open at the motor-mounting surface as shown in  FIGS. 5 and 8 . A pair of left and right horizontal bores  24   b  are bored in center section  24  from a front end surface of center section  24 , so that left kidney ports  24   c  and  24   d  are connected to each other through left horizontal bore  24   b , and right kidney ports  24   c  and  24   d  through right horizontal bore  24   b , thereby constituting a closed hydraulic fluid circuit between hydraulic pump  1  and hydraulic motor  2  which are juxtaposed horizontally. 
   As shown in  FIGS. 1 and 3 , in transaxle apparatus T 1 , upper housing half  12  includes a vertically downward extending upper partition wall  12   a , and lower housing half  13  includes a vertically upward extending lower partition wall  13   a . Upper and lower vertical partition walls  12   a  and  13   a  abut against each other with center section  14  passing horizontally therebetween, thereby substantially separating a front fluid charge chamber  11   a  from a rear main chamber  11   b  in which hydraulic pump  1  and motor  2  and differential gearing  7  are disposed. 
   As shown in  FIGS. 5 and 6 , in transaxle apparatus T 2 , lower housing half  23  includes a vertically upward partition wall  23   a  so as to substantially separate a front fluid charge chamber  21   a  from a rear main chamber  21   b  in which hydraulic pump  1  and motor  2  and differential gearing  7  are disposed. Center section  24  horizontally passes just above partition wall  23   a  and is disposed between both chambers  21   a  and  21   b.    
   As shown in  FIGS. 1 ,  3 ,  5  and  6 , in each of fluid charge chambers  11   a  and  21   a  of transaxle apparatuses T 1  and T 2 , an elliptical cylindrical filter  41  is vertically disposed between the bottom surface of center section  14  or  24  and an upper bottom surface of lower housing half  13  or  23 , and a fluid charge valve assembly is attached to each of center sections  14  and  24  so as to supply the closed hydraulic fluid circuit in center section  14  or  24  with fluid from the fluid sump of housing  11  or  21  through filter  41 . 
   As shown in  FIG. 1 , in fluid charge chamber  11   a , a vertically discoid magnet  60  is mounted upright on the upper bottom surface of lower housing half  13  adjacent to cylindrical filter  41 . Also, in main chamber  11   b , another magnet  60  is disposed upright on the upper bottom surface of lower housing half  13  adjacent to differential gearing  7 . Iron particles generating from meshing gears or the like are stuck onto magnets  60  so as to clean the fluid sump in housing  11 . Particularly, magnet  60  in fluid charge chamber  11   a  prevents such iron particles from invading the closed hydraulic fluid circuit of the HST. Although such magnets  60  are not shown in  FIG. 5 , they may be provided in housing  21  of second transaxle apparatus T 2  (in both or one of chambers  21   a  and  21   b ) similarly. 
   The fluid charge valve assembly of first transaxle apparatus T 1  will be described in accordance with  FIGS. 1 ,  3  and  9  to  11 . At the front end surface of center section  14 , left and right horizontal bores  14   b  are open and plugged by respective valve casings  42 . As shown in  FIGS. 1 and 3 , in center section  14 , a pair of left and right vertical suction ports  14   a  branch downward from respective horizontal bores  14   b  just under respective valve casings  42  and are open at the bottom surface of center section  14  surrounded by a top edge of filter  41 . A valve retaining portion  13   b  is formed of lower housing half  13  so as to abut against front end surfaces of valve casings  42 , as shown in  FIGS. 1 and 11 . 
   As shown in  FIGS. 3 and 9  to  11 , each of valve casings  42  is formed therein with a vertical inlet port  42   a , a horizontal outlet port  42   d  and a valve chamber  42   c  interposed between ports  42   a  and  42   d . Inlet port  42   a  is open at a bottom surface of valve casing  42  for free passage to each of suction ports  14   a  of center section  14 . Outlet port  42   d  is open for free passage to each of horizontal bores  14   b  of center section  14 . A valve ball  43  serving as a main valve member is disposed in valve chamber  42   c . Between inlet port  42   a  and valve chamber  42   c  is formed a valve seat  42   b  corresponding to valve ball  43 . 
   As shown in  FIGS. 1 ,  3 ,  9  to  11 , a horizontal rod hole  42   e  is bored in valve casing  42  between inlet port  42   a  and the front end surface of valve casing  42 . An outer member  45  and a pair of rods  44  serving as inner members are fixed together so as to constitute a draining operation member of the fluid charge valve assembly. Outer member  45  is disposed in front of valve casings  42  and between rods  44 . The pair of rods  44  are L-like bent so as to be joined to a rear end portion  45   a  of outer member  45 . Rods  44  are extended horizontally backward from outer member  45  and inserted into valve casings  42  through rod holes  42   e , respectively, so that a rear end of each of rods  44  is disposed in inlet port  42   a . Above-mentioned valve retaining portion  13   b  is shaped so as to allow rods  44  and rear end portion  45   a  of outer member  45  to pass therethrough. 
   Outer member  45  is extended axially horizontally so as to project forward from a front end of housing  11 . As shown in  FIG. 11 , each of upper and lower housing halves  12  and  13  is formed at the front end thereof with a semicircular recess so as to fit an upper or lower half intermediate portion of outer member  45 . Therefore, outer member  45  is sandwiched between upper and lower housing halves  12  and  13  while outer member  45  is allowed to move axially (in a fore-and-aft direction or perpendicular to axles  6 ). 
   Rear end portion  45   a  of outer member  45  is diametrically larger than the intermediate portion thereof held by housing  11  so as to prevent outer member  45  from escaping forward from housing  11 . In other words, a position of outer member  45  and rods  44 , where rear end portion  45   a  of outer member  45  abuts against the inner front side surface of housing  11  as shown in  FIG. 9 , is defined as a front limit position of the movement thereof. 
   Rear end of portion  45  of axially backwardly moved outer member  45  finally comes to abut against the front end surfaces of valve casings  42 , as shown in  FIG. 10 . This position of outer member  45  and rods  44  is defined as a rear limit position of movement of outer member  45 . 
   When outer member  45  and rods  44  are located at the front limit position, as shown in  FIG. 9 , the rear end of each of rods  44  is disposed in inlet port  42   a , however, rod  44  is separated from valve ball  43 . This state of the fluid charge valve assembly is defined as a charge mode. In this mode, valve ball  43  sits on valve seat  42   b  so as to prevent fluid from being drained from horizontal bore  14   b  to the fluid sump of housing  11 , however, if the hydraulic pressure of fluid in horizontal bore  14   b  is reduced lower than the pressure of fluid in the fluid sump, valve ball  43  is pushed up by pressure of fluid from the fluid sump and separated from valve seat  42   b , thereby supplying fluid to the closed hydraulic fluid circuit in center section  14 . 
   When outer member  45  and rods  44  are located at the rear limit position, as shown in  FIG. 10 , the rear end of each of rods  44  is extended backward in inlet port  42   a  so as to raise valve ball  43 . This state of the fluid charge valve assembly is defined as a drain mode. In this mode, valve ball  43  is constantly separated from valve seat  42   b  so as to allow fluid to flow freely between horizontal bore  14   b  and the fluid sump of housing  11  in two opposite directions, thereby allowing fluid in the closed fluid circuit in center section  14  to be drained to the fluid sump of housing  11 . 
   Usually, outer member  45  and rods  44  are located at the front limit position so as to keep the charge mode of the fluid charge valve assembly. If the HST is required to be free from rotating axles  6 , e.g., in a case of hauling a vehicle equipped with first transaxle apparatus T 1 , outer member  45  is pushed backward and located at the rear limit position so as to drain fluid from the HST. 
   If an HST is perfectly prevented from draining fluid, it is difficult to set the HST in neutral because a slight difference in the amount of flowing fluid between twin fluid passages (corresponding to the pair of horizontal bores  14   b ) as a closed fluid circuit of the HST often occurs. Therefore, in each of valve casings  42  is formed an orifice  42   f  extended from inlet port  42   a  so as to be open for free passage to horizontal bore  14   b , whereby a little amount of fluid is allowed to be drained from the closed fluid circuit in center section  14  even if the fluid charge valve assembly is set in the charge mode. 
   As shown in  FIGS. 1 ,  9  and  10 , a diametrical hole  45   b  penetrates a portion of outer member  45  in front of housing  11 . Hole  45   b  may be used to connect outer member  45  to a manipulator so as to facilitate the movement of outer member  45 . 
   As shown in  FIGS. 5 and 6 , second transaxle apparatus T 2  is provided with a similar fluid charge valve assembly, including a center section  24  having a pair of vertical suction ports  24   a  similar to ports  14   a . However, a difference between the first charge valve assembly of transaxle apparatus T 2  and the fluid charge valve assembly of first transaxle apparatus T 1  is that outer member  45  is disposed lower than valve casings  42  and axially movably held by only upper housing half  22  while outer member  45  of first transaxle apparatus T 1  is higher than valve casings  42  and axially movably held by upper and lower housing halves  12  and  13 . In correspondence to outer member  45  disposed below valve casings  42 , front portions of rods  44 , which extend upward to be connected to outer member  45  of first transaxle apparatus T 1 , are turned to extend downward and connected to outer member  45 , as shown in  FIG. 6 . 
   Since the fluid charge valve assembly is also used for drainage of fluid from the closed hydraulic fluid circuit to the fluid sump, additional parts for the drainage is unnecessary and the center section size is minimized. The center section is also simplified. Also, in first transaxle apparatus T 1 , center section  14  vertically sandwiched between hydraulic pump  1  and hydraulic motor  2  is close to axle  6  behind center section  14  and reduces the size of first transaxle apparatus T 1  in the fore-and-aft direction (horizontally perpendicular to axles  6 ). 
   Pump and motor shafts  4  and  5  of first transaxle apparatus T 1  are fitted to center section  14  in the following way. As shown in  FIGS. 2 ,  3  and  15 , center section  14  is pierced through both the top and bottom surfaces thereof, i.e., the pump and motor-mounting surfaces by a vertical shaft hole  14   d  surrounded by the pair of kidney ports  14   c . Vertical pump shaft  4  penetrates pump cylinder block  31  so as to be axially slidably but not-relatively rotatably fitted to pump cylinder block  31  through splines. Also, vertical motor shaft  5  penetrates motor cylinder block  33  so as to be axially slidably but not-relatively rotatably fit to motor cylinder block  33  through splines. A bottom portion of pump shaft  4  and a top portion of motor shaft  5  are slidably rotatably inserted together into shaft hole  14   d  so that the horizontal bottom end surface of pump shaft  4  and the horizontal top end surface of motor shaft  5  face each other in shaft hole  14   d  while pump shaft  4  and motor shaft  5  are rotatable independently of each other. 
   Vertical motor shaft  5  inserted upward from below into center section  14  requires to be prevented from falling down from center section  14  before motor cylinder block  33  is fitted to motor shaft  5  and the bottom motor-mounting surface of center section  14 . Therefore, as shown in  FIGS. 2 ,  3  and  15 , pin  5   a  diametrically (horizontally) penetrates the top end portion of motor shaft  5  and projects at both ends thereof outward from an outer peripheral surface of motor shaft  5 . In center section  14 , a pair of pin-passing grooves  14   e  are formed along shaft hole  14   d  and open at the bottom surface of center section  14  so as to allow both the ends of pin  5   a  to pass therein, thereby enabling motor shaft  5  with pin  5   a  to be inserted into center section  14 . In a vertically intermediate portion of center section  14 , a pair of pin-retaining grooves  14   f  are formed on respective top ends of pin-passing grooves  14   e . Pin-retaining grooves  14   f  are horizontally (in the perpendicular direction of shaft hole  14   d ) wider than pin-passing grooves  14   e.    
   Motor shaft  5  is inserted upward into shaft hole  14   d  from below while the ends of pin  5   a  pass in respective pin-passing grooves  14   e . When the ends of pin  5   a  reach respective pin-retaining grooves  14   f , motor shaft  5  is rotated to some degree around its axis so as to offset the ends of pin  5   a  in pin-retaining grooves  14   f  from pin-passing grooves  14   e . Thus, bottom surfaces of pin-retaining grooves  14   f  prevent the ends of pin  5   a  from falling into pin-passing grooves  14   e , thereby preventing motor shaft  5  from falling down from center section  14  and facilitating assembly of hydraulic motor  2 . When hydraulic motor  2  with motor shaft  5  is completely assembled, motor shaft  5  is disposed at an axially proper position where the ends of pin  5   a  in pin-retaining grooves  14   f  are separated from the bottom surfaces of pin-retaining grooves  14   f , thereby enabling motor shaft  5  to rotate freely from center section  14 . 
   Pump and motor shafts  4  and  5  in second transaxle apparatus T 2  are fitted to center section  24  in the following way. As shown in  FIGS. 5 to 8 , center section  24  is formed therein with a vertical motor shaft hole  24   f , which is open at a center of the motor-mounting surface of center section  24  in front of axles  6 . Motor shaft  5  is inserted into motor shaft hole  24   f  from below and located at a top end thereof in a vertically intermediate portion of center section  24 . Further, center section  24  is penetrated by a vertical pump shaft hole  24   e , which is open at a center of the pump-mounting surface of center section  24  behind axles  6 . Pump shaft  4  vertically penetrates center section  14  through pump shaft hole  24   e.    
   As shown in  FIGS. 6 and 8  (and  15 ), center section  24  is formed therein with a pair of pin-passing grooves  24   g  and a pair of pin-retaining grooves  24   h  close to motor shaft hole  24   f , similarly with shaft hole  14   d , pin-passing grooves  14   e  and pin-retaining grooves  14   f  in center section  14  of first transaxle apparatus T 1 . Motor shaft  5  with pin  5   a  is inserted into motor shaft hole  24   f  from below while both ends of pin  5   a  pass in pin-passing grooves  24   g  and are located in pin-retaining grooves  24   h . Thus, motor shaft  5  inserted in motor shaft hole  24   f  is prevented from falling out so as to facilitate assembly of hydraulic motor  2  of second transaxle apparatus T 2 . 
   In each of first and second transaxle apparatuses T 1  and T 2 , a thrust bearing serving as a movable pump swash plate  35  abut against pistons  32  in pump cylinder block  31 . A retainer  36 , which can be tilted while fitting an inner surface of housing  11  or  21 , holds pump swash plate  35 . As for first transaxle apparatus T 1 , pump swash plate  35  and retainer  36  are disposed above pump cylinder block  31  mounted upright on the top surface of center section  14 , and retainer  36  fits an inner ceiling portion of upper housing half  12 , as show in  FIGS. 2 and 3 . As for second transaxle apparatus T 2 , pump swash plate  35  and retainer  36  are disposed below pump cylinder block  31  hung down from the bottom surface of center section  24 , and a substantially vertically cylindrical main portion of retainer  36  is fitted into a recess formed in an inner bottom portion of lower housing half  23 , as shown in  FIGS. 6 and 7 . 
   As shown in  FIGS. 2 ,  3 ,  6  and  7 , in each of first and second transaxle apparatuses T 1  and T 2 , pump shaft  4  extends vertically upward and projects upwardly outward from housing  11  or  21  so as to be fixedly provided thereon with a cooling fan  50  and an input pulley  51  above housing  11  or  21 . 
   As shown in  FIGS. 2 and 3 , as for first transaxle apparatus T 1 , pump swash plate  35  and retainer  36  disposed above pump cylinder block  31  are provided with center holes which pump shaft  4  penetrates freely permitting tilting of retainer  36  with pump swash plate  35 . Above retainer  36 , a bearing  20  is provided in a top portion of housing  11  so as to rotatably hold a vertically intermediate portion of pump shaft  4 . 
   As shown in  FIGS. 6 and 7 , as for second transaxle apparatus T 2 , a bottom end of pump shaft  4  projecting downward from pump cylinder block  31  is disposed above pump swash plate  35  and retainer  36 . In the case of second transaxle apparatus T 2 , a pair of upper and lower bearings  20  are disposed in a top portion of housing  21  so as to stably and rotatably hold a vertically intermediate portion of pump shaft  4 . Such increase of bearings  20  enables the bottom end of pump shaft  4  to be disposed above pump swash plate  35  and retainer  36 , thereby shortening pump shaft  4  and reducing the vertical size of second transaxle apparatus T 2 . 
   As shown in  FIGS. 1 ,  2 ,  5  and  7 , for controlling the tilt angle of pump swash plate  35 , a speed control shaft  25  is disposed horizontally in parallel to axles  6 , and rotatably supported by each of boss portions  12   d  and  22   d  formed of respective upper housing halves  12  and  22 . In each of housings  11  and  21 , an arm  26  is fixed onto an inner end portion of speed control shaft  25  and engages with retainer  36 . An intermediate portion of a speed control lever  27  is fixed onto an outer end portion of speed control shaft  25  outside housing  11  or  21 . Speed control lever  27  is operatively connected at an upper end thereof to a manual speed control operation device such as a lever or a pedal provided on a vehicle. 
   A coiled and twisted spring  28  is wound around each of boss portions  12   d  and  22   d  so as to bias speed control lever  27  toward the neutral position. A retaining pin  29  is disposed horizontally in parallel to speed control shaft  25  and fixed onto a lower end of speed control lever  27 . Below retaining pin  29 , an eccentric bolt  30  is disposed horizontally in parallel to retaining pin  29  and screwed into a sidewall of each of lower housing halves  13  and  23 . Two end portions of spring  28  are extended downward and cross each other so as to sandwich retaining pin  29  and eccentric bolt  30  and press them toward each other. The neutral position of speed control lever  27  can be adjusted by rotating eccentric bolt  30 . 
   When the speed control operation device on the vehicle is operated, speed control lever  27  is rotated together with speed control shaft  25  against the biasing force of spring  28  so as to tilt retainer  36  together with pump swash plate  35 . According to the tilt angle and direction of pump swash plate  35 , the capacity and fluid-discharging direction of hydraulic pump  1  varies, thereby rotating hydraulic motor  2  at various speeds in one selective direction of two opposite directions. 
   In each of first and second transaxle apparatuses T 1  and T 2 , a thrust bearing serving as a fixed motor swash plate  37  is disposed below motor cylinder block  33  so as to abut against pistons  34  in motor cylinder block  33 . A retainer  38  retains motor swash plate  37  and is fitted to a bottom portion of each of housings  11  and  21 . As shown in  FIGS. 2 ,  3 ,  6  and  8 , a bottom portion of retainer  38  serving as a motor-shaft-support portion  38   a  is convexed upward so as to be inserted into a center hole of motor swash plate  37 . A bottom end portion of motor shaft  5  is formed as a bevel gear which is a motor output gear  5   b  meshing with a bevel gear serving as a later-discussed first transmission gear  53 . A portion of motor shaft  5  immediately above motor output gear  5   b  is supported by motor-shaft-support portion  38   a  of retainer  38  through a motor-shaft bearing  39 . Due to this construction, the bottom end of motor shaft  5  serving as motor output gear  5   b  does not project or hardly projects downward from a bottom surface of retainer  38 , thereby shortening motor shaft  5  and reducing the vertical size of each of transaxle apparatuses T 1  and T 2 . 
   Retainer  38  extends a pair of left and right feet  38   b  downward from the bottom thereof. Each foot  38   b  includes an upper-semicircular surface so as to fit an upper half-peripheral outer surface of a transmission-shaft bearing  55  for supporting a later-discussed transmission shaft  52 . Each of bottom portions  13   c  and  23   c  of lower housing halves  13  and  23  is convexed upward in a sectionally upper-half semicircular shape so as to fit the upper-semicircular surface of foot  38   b . Thus, retainer  38  is disposed in each of transaxle apparatuses T 1  and T 2  so that one of feet  38   b  is fitted to transmission-shaft bearing  55 . 
   In each of transaxle apparatuses T 1  and T 2 , as shown in  FIGS. 3 and 6 , motor swash plate  37  in retainer  38  is disposed vertically slantwise in the fore-and-aft direction, i.e., perpendicular to axles  6 , thereby being disposed eccentrically forward or rearward, as shown in  FIGS. 12 and 13 . Consequently, one slant direction of motor swash plate  37  in the fore-and-aft direction is selected between two mutually symmetric directions with respect to the vertical axis of motor shaft  5  depending upon which of feet  38   b  is fitted to transmission-shaft bearing  55 , thereby deciding the rotational direction of motor shaft  5  between two opposite directions. 
   Retainer  38  is provided on left and right ends thereof with upwardly extending portions  38   c , which contact with the bottom surface of center section  14  or  24 . Thus, retainer  38  is vertically located between center section  14  or  24  and the bottom portion of housing  11  or  21  without bolts or the like. 
   As shown in  FIGS. 1 ,  2 ,  4 ,  5 ,  6  and  8 , transmission shaft  52  is disposed horizontally in parallel to axles  6  along the bottom of housing  11  or  21  (i.e., lower housing half  13  or  23 ). A bevel gear serving as first transmission gear  53  is fixed on a first axial end portion of transmission shaft  52  so as to mesh with motor output gear  5   b . Transmission-shaft bearing  55  fitted to retainer  38  is disposed adjacent to first transmission gear  53 . A second end portion of transmission shaft  52  is opposite to the first end portion thereof and provided thereon with later-discussed brake discs  62 . Another transmission-shaft bearing  56  is provided on a portion of transmission shaft  52  adjacent to the second end portion thereof. 
   As shown in  FIGS. 1 ,  2 ,  4 ,  5  and  8 , a vertically elongated supporter  61  is disposed between a ceiling portion of housing  11  or  21  (i.e., upper housing half  12  or  22 ) and transmission-shaft bearing  56  adjacent to the second end portion of transmission shaft  52 . As best shown in  FIGS. 4 and 14 , a bottom portion of supporter  61  is semicircularly curved so as to fit transmission-shaft bearing  56 . The bottom portion of lower housing half  13  or  23  is formed so as to retain supporter  61  and the lower halves of the pair of transmission-shaft bearings  55 ,  56 . 
   As shown in  FIGS. 1 ,  2 ,  5  and  8 , an outer peripheral portion of transmission shaft  52  between the pair of transmission-shaft bearings  55 ,  56  is formed into a second transmission gear  54 , which meshes with an input gear  7   a  of conventionally-constructed differential gearing  7  so as to transmit power to axles  6 . 
   As shown in  FIGS. 1 ,  2 ,  5 ,  8  and  14 , a pair (or a larger number) of vertical brake discs  62  are not-relatively-rotatably but axially-slidably fitted onto the second end portion of transmission shaft  52 . A ring-like brake pad (or pads)  63  is disposed between brake discs  62  and not-relatively-rotatably but axially-slidably fitted to supporter  61  and the bottom portion of housing  11  or  21 . 
   As shown in  FIGS. 1 ,  2 ,  5  and  8 , a vertical brake control shaft  64  is supported rotatably around its axis by a ceiling portion of housing  11  or  21  (i.e., upper housing half  12  or  22 ) and extended downward along supporter  61 . A brake arm  66  is fixed onto a top of brake control shaft  64  above the ceiling portion of housing  11  or  21  so as to be operatively connected to a brake operation member, such as a lever or a pedal, provided on a vehicle. As shown in  FIGS. 2 and 8 , a spring  67  is wound around the ceiling portion of housing  11  or  21  so as to bias brake arm  66  (and brake control shaft  64 ) to a brake-release position. A lower portion of brake control shaft  64  is partly cut away and fittingly provided thereon with a cam  65 , as shown in  FIGS. 1 ,  2 ,  5 ,  8  and  14 . A portion  61   a  of supporter  61  immediately above cam  65  is horizontally extended so as to fit brake pad  63  and cam  65 . 
   When brake arm  66  is set in the brake-release position, the cut-away portion of brake control shaft  64  fits cam  65  so that cam  65  is disposed apart from an outermost brake disc  62 , thereby separating brake discs  62  and brake pad  63  from one another so as to allow transmission shaft  52  to rotate. 
   When the brake operation member is handled so as to rotate brake arm  66  and brake control shaft  64  to a brake position against the force of spring  67 , a cam portion of brake control shaft  64  which is each of edges of the cut-away portion thereof comes to confront and push cam  65  so that cam  65  abuts against the outermost brake disc  62 , thereby pressing brake discs  62  and brake pad  63  against one another so as to brake transmission shaft  52 . If an operator release the handling force from the brake operation member, brake arm  66  and brake control shaft  64  return to the brake-release position by the biasing force of spring  67 . 
   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. For example, transmission shaft  52  and differential gear unit  7 , which are arranged rightward of the HST in each of the above-mentioned two embodiments, may be arranged leftward of the HST.