Patent Publication Number: US-2015075155-A1

Title: Apparatus including hydrostatic transmission

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This application is a continuation-in-part of U.S. Non-provisional application Ser. No. 14/179,334, filed on Feb. 12, 2014, which claims the benefit of U.S. Provisional Application No. 61/843,165, filed on Jul. 5, 2013, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
     1. Field of the Invention 
     The present invention relates to an apparatus, e.g., a transaxle, including a hydrostatic transmission (hereinafter, “HST”) and a casing incorporating the HST submerged in a fluid sump in the casing. 
     2. Related Art 
     As disclosed by U.S. Pat. No. 6,125,630 B, there is a well-known conventional hydraulic transaxle serving as one of right and left transaxles carrying respective right and left individual axles for driving a zero-turn vehicle, e.g., a lawn mower. The transaxle includes an HST, an axle and a reduction gear train for transmitting power from the HST to the axle. The HST includes a hydraulic pump for receiving power from a prime mover, a hydraulic motor for outputting power to the reduction train, and a center section on which the hydraulic pump and the hydraulic motor are mounted to be fluidly connected to each other via a hydraulic fluid circuit formed in the center section. 
     The conventional transaxle includes a transaxle casing incorporating the HST. In the transaxle casing, a vertical cylindrical filter is interposed between a bottom portion of the center section and a bottom portion of the transaxle casing so as to be submerged in a fluid sump in the transaxle casing. A charge pump is mounted onto the bottom portion of the center section surrounded by the filter. The pump shaft serves as a drive shaft for the charge pump. The hydraulic fluid circuit is fluidly connected to a delivery port of the charge pump via charge check valves so as to be supplied with fluid for operating the HST from the fluid sump. 
     If the hydraulic pressure of the fluid sump is sufficient, it is noticed that charge ports fluidly connected to the hydraulic fluid circuit via charge check valves can be directly open at the bottom portion of the center section to the portion of the fluid sump surrounded by the filter so as to supply the hydraulic fluid circuit with fluid from the fluid sump without the charge pump. However, it should be considered that if the fluid sump is contaminated with air, the air is liable to enter the hydraulic fluid circuit via the charge ports so as to cause cavitation of the hydraulic fluid for the HST influencing operation of the HST. In this regard, since fluid of the fluid sump serves as lubricating fluid for lubricating the HST as well as hydraulic fluid for activating the HST, the lubricating fluid between a movable swash plate and a swash plate retainer, for example, is likely to be invaded by air, and this lubricating fluid contaminated with air is returned to the fluid sump so as to cause the above-mentioned problem of the fluid sump contaminated with air. 
     SUMMARY OF THE INVENTION  
     An object of the present invention is to provide an apparatus including an HST and a casing incorporating the HST submerged in a fluid sump in the casing, wherein the apparatus is improved so as to prevent a closed fluid circuit of the HST from being invaded by air contaminating the fluid sump via a filter. 
     To achieve the object, in a first aspect of the invention, an apparatus comprises an HST and a casing incorporating the HST. The HST includes a hydraulic pump, a hydraulic motor, and a center section. The hydraulic pump and the hydraulic motor are attached to the center section that is formed therein with a fluid sump fluidly connecting the hydraulic pump to the hydraulic motor. At least a bottom portion of the center section is submerged in a fluid sump in the casing. A charge port for fluidly connecting the fluid sump to the fluid passage is open at the bottom portion of the center section in the fluid sump. The bottom portion of the center section is downwardly convex so as to have an apex at the lowest portion. The opening of the charge port is directed to the apex of the convex bottom portion of the center section. 
     Due to the convex bottom portion of the center section of the apparatus in the first aspect, the air contaminated in the fluid sump is prevented from entering the charge port, thereby avoiding cavitation of hydraulic fluid for the HST, and thereby keeping a required activity of the HST. Further, such a technical feature of the center section enables the fluid supply from the fluid sump to the HST without a charge pump, thereby reducing costs and the number of parts and facilitating assembling of the HST in the casing. 
     In the first aspect, preferably, the convex bottom portion of the center section is substantially conical so as to have a slope along which air in the fluid sump is likely to flow without entering the charge port. 
     Due to the substantially conical convex bottom portion of the center section, the air contaminated in the fluid sump flows along the slope of the convex bottom portion of the center section so as to be prevented from entering the charge port. 
     To achieve the object, in a second aspect of the invention, an apparatus comprises an HST, a casing and an air passage. The casing includes a first portion and a second portion that is higher than a top portion of the first portion. The HST is disposed in the first portion a the casing and is submerged in a fluid sump in the casing. Air flows through the air passage from the top portion of the first portion to the second portion. 
     Due to the air passage of the apparatus in the second aspect, even if lubricating fluid for the HST comes to the top portion of the first portion of the casing, e.g., a gap between a movable swish plate and a swash plate retainer, the lubricating fluid is hindered from being contaminated with air that is likely to be collected in the top portion of the first portion of the casing. Therefore, the fluid sump to which the lubricating fluid is drained is hindered from being contaminated with air, thereby preventing cavitation of fluid in the HST. 
     In the second aspect, preferably, the apparatus further comprises a gear mechanism drivingly connected to the HST. The gear mechanism is disposed in the second portion of the casing so as to be submerged in the fluid sump. A portion of the second portion joined to the air passage is defined as an air chamber above the fluid sump. 
     Therefore, the air passage discharges air to the air chamber defined by the second portion of the casing incorporating the gear mechanism distant from the HST so as to keep the lubricating fluid for the HST free from the air in the air chamber of the second portion of the casing. 
     In the second aspect, preferably, the apparatus further comprises a pipe of which both ends are inserted into respective openings formed in the first and second portions of the casing so that the pipe and the openings in the first and second portions of the casing constitute the air passage. 
     Due to the pipe, the casing is simply formed with the opening in the first and second portions to provide the fluid passage, thereby reducing costs for forming the casing. 
     These, further and other objects, features and advantages will appear more fully from the following detailed description of the invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a schematic plan view of a lawn mower  100  serving as a zero-turn vehicle equipped with right and left transaxles  1 R and  1 L and with right and left control levers  101 R and  101 L for controlling respective transaxles  1 R and  1 L. 
         FIG. 2  is a schematic plan view of a lawn mower  300  serving as a zero-turn vehicle equipped with right and left transaxles  1 R and  1 L and with a steering wheel  301  and a speed control pedal  306  for controlling right and left transaxles  1 R and  1 L. 
         FIG. 3  is a plan view of a transaxle  1  representing each of right and left transaxles  1 R and  1 L of a zero-turn vehicle. 
         FIG. 4  is a bottom view of transaxle  1 . 
         FIG. 5  is a side view of transaxle  1  when viewed from a side toward a distal end of an axle  2  projecting outward from a transaxle casing  10  of transaxle  1 . 
         FIG. 6  is a cross sectional side view of transaxle  1  taken along A-A arrows of  FIG. 3 . 
         FIG. 7  is a cross sectional front view of transaxle  1  taken along B-B arrows of  FIG. 3 . 
         FIG. 8  is the same cross sectional front view of transaxle  1  as  FIG. 7  except for an alternative arrangement of a hydrostatic stepless transmission (hereinafter referred to as HST) control arm  46  and a neutral return spring  47 . 
         FIG. 9  is a cross sectional front view of transaxle  1  taken along C-C arrows of  FIG. 3 . 
         FIG. 10  is a cross sectional rear view of transaxle  1  taken along D-D arrows of  FIG. 3 . 
         FIG. 11  is a cross sectional rear view of transaxle  1  taken along E-E arrows of  FIG. 3  wherein an HST  20  and relevant arms are omitted. 
         FIG. 12  is a cross sectional rear view of transaxle  1  taken along F-F arrows of  FIG. 3 . 
         FIG. 13  is the same cross sectional rear view of transaxle  1  as  FIG. 12  except for an alternative reduction gear train  70 B. 
         FIG. 14(   a ) is a sectional side view of a trunnion  45  and an eccentric pin  49  showing neutral return spring  47  when a movable swash plate  44  of HST  20  is disposed at a neutral position. 
         FIG. 14(   b ) is a sectional side view of trunnion  45  and eccentric pin  49  showing neutral return spring  47  when movable swash plate  44  of HST  20  is not disposed at the neutral position. 
         FIG. 15  is a perspective view of a center section  30  for HST  20 . 
         FIG. 16  is a plan view of center section  30 . 
         FIG. 17  is a bottom view of center section  30 . 
         FIG. 18  is a rear view of center section  30 . 
         FIG. 19  is a sectional plan view of center section  30 . 
         FIG. 20  is a sectional side view of center section  30 . 
         FIG. 21  is a side view of an alternative transaxle  1 A serving as right transaxle  1 R that represents the pair of right and left transaxles  1 R and  1 L, including a sectional side view of an upper portion of a reduction gear train  170  taken along N-N arrows of  FIG. 28 . 
         FIG. 22  is a sectional bottom view of transaxle  1 A taken along G-G arrows of  FIG. 21 , showing bottom views of axle  2 , an HST  120 , reduction gear train  170  and a gear locking system  180  in a transaxle casing  110  from which a lower transaxle housing  112  has been removed. 
         FIG. 23  is a sectional side view of transaxle  1 A taken along H-H arrows of  FIG. 22 , showing sectional side views of axle  2 , HST  120 , reduction gear train  170  and a gear locking system  180 A as an embodiment of gear locking system  180 . 
         FIG. 24  is a sectional front view of transaxle  1 A taken along I-I arrows of  FIG. 22 , showing a sectional font view of HST  120 . 
         FIG. 25  is a fragmentary sectional side view of transaxle  1 A taken along J-J arrows of  FIG. 22 , showing a sectional side view of HST  120 . 
         FIG. 26  is a fragmentary sectional front view of transaxle  1 A taken along K-K arrows of  FIG. 22 , showing front views of charge check and bypass valves  121  having function as bypass valves in HST  120  and a bypass valve operation mechanism  127 . 
         FIG. 27  is a fragmentary sectional plan view of transaxle  1 A taken along L-L arrows of  FIG. 23 , showing front views of charge check  121  having the function as bypass valves in HST  120  and bypass valve operation mechanism  127 . 
         FIG. 28  is a plan view of reduction gear train  170  in transaxle casing  110  of transaxle  1 A from which a gear top cover  113  has been removed, showing a gear locking system  180 B serving as another embodiment of gear locking system  180 . 
         FIG. 29(   a ) is a sectional side view of gear locking system  180 A when a locking pawl  181 A engages with bevel pinion  71 ,  FIG. 29(   b ) is a sectional side view of gear locking system  180 A when locking pawl  181 A disengages from bevel pinion  71 , and  FIG. 29(   c ) illustrates gear locking system  180 A when viewed in the axial direction of a motor shaft  51 . 
         FIG. 30(   a ) is a sectional side view of gear locking system  180 B when a locking pawl  181 B engages with bevel pinion  71  via an engagement plate  182 ,  FIG. 30(   b ) is a sectional side view of gear locking system  180 B when locking pawl  181 B disengages from bevel pinion  71  and engagement plate  182 , and  FIG. 30(   c ) illustrates gear locking system  180 B When viewed in the axial direction of motor shaft  51 . 
         FIG. 31  is a fragmentary sectional rear view of transaxle  1 A taken along M-M arrows of  FIG. 28 , showing a sectional rear view of reduction gear train  170  provided with a bevel gear shaft support structure shown in  FIGS. 21 and 28 . 
         FIG. 32  is a fragmentary sectional rear view of transaxle  1 A taken along M-M arrows of  FIG. 28 , showing a sectional rear view of reduction gear train  170  provided with an alternative bevel gear shaft support structure. 
         FIG. 33  is a sectional side view of a transaxle  1 Aa including HST  120 , showing a sectional side view of a center section  30 A and charge check valve  121  Laving the function of bypass valves. 
         FIG. 34  is a perspective bottom view of transaxle  1 Aa from which a lower transaxle housing  112  has been removed. 
         FIG. 35  is another perspective bottom view of transaxle  1 Aa from which lower transaxle housing  112  and a filter  26  have been removed. 
         FIG. 36  is a perspective plan view of a front portion of transaxle  1 Aa with a pump shaft  41  from which an input pulley  14  and a cooling fan  15  have been removed. 
         FIG. 37  is a perspective front view of center section  30 A. 
         FIG. 38(   a ) is a schematic plan view of a bypass operation arm  128 A and stopper edges  111   a - 111   c.    
         FIG. 38(   b ) is another schematic plan view of a bypass operation aim  128 A and stopper edges  111   a - 111   c.    
         FIG. 38(   c ) is another schematic plan view of a bypass operation arm  128 A and stopper edges  111   a - 111   c.    
         FIG. 38(   d ) is another schematic plan view of a bypass operation arm  128 A and stopper edges  111   a - 111   c.    
         FIG. 39  is a fragmentary sectional side view of transaxle  1 Aa, showing a sectional side view of a breathing pipe  115 . 
         FIG. 40  is a sectional side view of an alternative transaxle  1 Ab including HST  120  provided with a charge pump mechanism. 
         FIG. 41  is a sectional plan view of a lower transaxle housing  112 A of transaxle  1 Ab, showing a plan view partly in section of the charge pump mechanism. 
         FIG. 42  is a bottom view of a center section  30 B used for transaxle  1 Ab, showing the charge pump mechanism drawn in phantom lines as fitted to center section  30 B. 
         FIG. 43  is a hydraulic circuit diagram of a zero-turn vehicle in which transaxle  1 Aa having no charge pump mechanism serves as the left transaxle and transaxle  1 Ab having the charge pump mechanism serves as the right transaxle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Referring to  FIGS. 1 and 2 , each of lawn mowers (hereinafter simply referred to as “vehicles”)  100  and  200  serving as typical zero-turn vehicles is equipped with right and left transaxle  1 R and  1 L carrying respective right and left axles  2 R and  2 L. Each of vehicles  100  and  300  includes a vehicle body frame  4  having right and left parallel sideboards  4   a  extended in the fore-and-aft horizontal direction thereof. Vehicle body frame  4  supports right and left transaxles  1 R and  1 L so that transaxle casings  10  of right and left transaxles  1 R and  1 L are disposed in the inside of vehicle body frame  4 , i.e., between right and left sideboards  4   a  when viewed in plan. Right axle  2 R is extended rightward from transaxle casing  10  of right transaxle  1 R to the outside of right sideboard  4   a  of vehicle body frame  4  so as to be fixed at a distal end thereof to right drive wheel  3 R. Left axle  2 L is extended leftward from transaxle casing  10  of left transaxle  1 L to the outside of left sideboard  4   a  of vehicle body frame  4  so as to be fixed at a distal end thereof to left drive wheel  3 L. 
     Right and left drive wheels  3 R and  3 L serve as rear wheels of each of vehicles  100  and  300  so as to be disposed on the right and left outsides of a rear portion of vehicle body frame  4 . A front end portion of vehicle body frame  4  supports castors (or castor)  5  serving as front wheels (or a front wheel) of vehicle  100  or  300 . A mower unit  6  is disposed below a fore-and-aft middle portion of vehicle body frame  4  between front wheels  5  and rear wheels  3 R and  3 L. Further, each of vehicles  100  and  300  is equipped with an unshown prime mover such as an engine whose output power is transmitted via an unshown belt or the like to input pulleys  14  of respective right and left transaxles  1 R and  1 L. 
     Referring to  FIG. 1 , vehicle  100  is provided with right and left control levers  101 R and  101 L. Link rods  102  are extended rearward from respective right and left control levers  101 R and  101 L to respective transaxle casings  10  of right and left transaxles  1 R and  1 L. Therefore, right and left link rods  102  extended from respective control levers  101 R and  101 L are disposed along respective right and left sideboards  4   a  of vehicle body frame  4 . When viewed in plan, right and left link rods  102  are extended in the fore-and-aft direction along respective right and left sideboards  4   a.  As discussed later, transaxle casing  10  of each of right and left transaxles  1 R and  1 L can be selectively provided with HST control arm  46  on either the right or left proximal side surface thereof or the right or left distal side surface thereof. Therefore, in the present embodiment, transaxles  1 R and  1 L in vehicle  100  are provided with respective HST control arms  46  on the distal side surfaces of respective transaxle casings  10  thereof because right and left link rods  102  are rather close to the distal sides of vehicle  100 . Further, a later-discussed neutral return spring  47  is provided on the right or left side surface of transaxle casing  10  of each of right and left transaxles  1 R and  1 L opposite HST control arm  46 . Therefore, whether neutral returning spring  47  is provided on the proximal or distal side surface of transaxle casing  10  depends on whether HST control arm  46  is provided on the proximal or distal side surface of transaxle casing  10 . 
     Right control lever  101 R is operable to change the tilt angle and direction of a later-discussed movable swash plate  44  of an HST  20  of right transaxle  1 R so as to change the rotary speed and direction of right rear wheel  3 R. Left control lever  101 L is operable to change the tilt angle and direction of movable swash plate  44  of HST  20  of left transaxle  1 L so as to change the rotary speed and direction of left rear wheel  3 L. When right and left control levers  101 R and  101 L are synchronously manipulated, movable swash plates  44  of HSTs  20  of right and left transaxles  1 R and  1 L are synchronously controlled to change the straight travel speed or direction of vehicle  100 . When right and left control levers  101 R and  101 L are independently manipulated, movable swash plates  44  of HSTs  20  of right and left transaxles  1 R and  1 L are differentially controlled so as to control the turn direction, angle, and speed of vehicle  100 . 
     Alternatively, referring to  FIG. 2 , vehicle  300  is provided with a steering wheel  301  that is connected to a sector gear  303  via a stem  302 . Right and left link rods  304  are extended from sector gear  303  to a zero-turn control unit  305 , and right and left rods  307  are extended from zero-turn control unit  305  to respective transaxle casings  10  of right and left transaxles  1 R and  1 L. A speed control pedal  306  is operatively connected to zero-turn control unit  305 . Due to the depression of speed control pedal  306 , right and left link rods  307  are pulled or pushed synchronously in the same direction and to the same degree to rotate respective HST control arms  46 , thereby controlling the travel speed of vehicle  300  in either the forward or backward direction. Due to the rotation of steering wheel  301 , right and left link rods  307  are independently pushed or pulled so as to differentially rotate right and left HST control arms  46 , thereby turning vehicle  300 . Typical zero-turn control unit  305  is configured as disclosed by U.S. Pat. No. 4,875,536 A, for example. 
     In this regard, zero-turn control unit  305  is disposed at the lateral middle portion of vehicle  300  between right and left sideboards  4   a  of vehicle body frame  4  so that right and left link rods  307  extended from zero-turn control unit  305  are disposed inward from respective right and left sideboards  4   a  so as to be rather distantly inward from respective right and left sideboards  4   a.  Therefore, transaxles  1 R and  1 L in vehicle  300  are provided with respective HST control arms  46  on the proximal side surfaces of respective transaxle casings  10  thereof. 
     In each of vehicles  100  and  300 , right and left transaxles  1 R and  1 L are arranged to have respective axles  2 R and  2 L being coaxial to each other. Incidentally, right and left transaxles  1 R and  1 L are arranged so that later-discussed HSTs  20  operatively connected to respective HST control arms  46  are disposed in respective front portions of transaxle casings  10  of transaxles  1 R and  1 L and so that axles  2 R and  2 L are supported by respective rear portions of transaxle casings  10  of transaxles  1 R and  1 L rearward front HSTs  20 . Alternatively, either or both of transaxles  1 R and  1 L may be disposed to have HST  20  rearward from axle  2 R or  2 L. 
     Referring to  FIGS. 3 to 20 , a transaxle  1  carrying an axle  2  will be described on the assumption that it represents right transaxle  1 R carrying right axle  2 R and left transaxle  1 L carrying left axle  2 L such as those of zero-turn vehicle  100  or  300 . Transaxle  1  includes a transaxle casing  10 , an HST  20  disposed in a front portion of transaxle casing  10 , an axle  2  (serving as either right axle  2 R or left axle  2 L of vehicle  100  or  300 ) journalled by a rear portion of transaxle casing  10 , and a reduction gear train  70  disposed in the rear portion of transaxle casing  10  so as to drivingly connect HST  20  to axle  2 . 
     Transaxle casing  10  includes a transaxle housing  11 , a lower cover  12  made of a metal plate, and a top cover  13 . Transaxle housing  11  serving as a main body of transaxle casing  10  is entirely formed by casting. A downwardly open chamber  10   a  is formed in transaxle housing  11  so as to extend from a front end of transaxle housing  11  to a rear end of transaxle housing  11 . HST  20  is disposed in a front half portion of chamber  10   a  in transaxle housing  11 , and reduction gear train  70  is disposed in a rear half portion of chamber  10   a  in transaxle housing  11 . 
     Transaxle housing  11  is formed with bosses  11   a,    11   m,  and  11   n  having respective vertical through holes through which respective bolts are passed to fasten transaxle housing  11  to vehicle body frame  4  of vehicle  100  or  300 . Front and rear bosses  11   a  are formed on front and rear ends of transaxle housing  11 . A substantially laterally symmetric main body of transaxle housing  11  is formed by casting with a main mold (not shown), and an axle support portion  11   i  of transaxle housing  11  for journaling axle  2  is formed by casting with a sub mold (not shown) combined with the main mold so as to be joined to the main body of transaxle housing  11 . Whether the sub mold is disposed rightward or leftward of the main mold can be selected so that axle support portion  11   i  can be selectively formed on either the right rear portion of the main body or the left rear portion of the main body. In this regard, transaxle  1  illustrated in  FIGS. 3 to 13  serves as right transaxle  1 R carrying right axle  2 R so that axle support portion  11   i  extends rightward to support right axle  2 R as drawn in solid lines in  FIGS. 3 and 4  and others. Alternatively, transaxle housing  11  may have axle support portion  11   i  extended leftward to support left axle  2 L as drawn in phantom lines in  FIGS. 3 and 4  and others so as to constitute left transaxle  1 L carrying left axle  2 L. Whether axle support portion  11   i  extends rightward or leftward, front and rear bosses  11   n  are formed on axle support portion  11   i,  and front and rear bosses  11   m  are formed on a right or left side of transaxle housing  11  opposite axle support portion  11   i  in the lateral direction. 
     Lower cover  12  is fastened by bolts  16  to a bottom edge of transaxle housing  11  so as to cover the bottom opening of chamber  10   a  in transaxle housing  11  surrounded by the bottom edge of transaxle housing  11 . A ceiling wall of transaxle housing  11  has a top opening  11   g  in a rear top portion thereof defining the rear half portion of chamber  10   a  incorporating reduction gear train  70 . Top cover  13  is fastened by bolts  17  to the edge portion of transaxle housing  11  surrounding top opening  11   g  so as to cover an upper portion of reduction gear train  70  in transaxle housing  11 . 
     HST  20  includes a center section  30 , a hydraulic pump  40 , and a hydraulic motor  50 . Referring to  FIGS. 15 to 20 , center section  30  is entirely formed by casting so as to have a horizontal discoid pump port block  31 , an upwardly slant discoid motor port block  32 , a bypass valve block  33 , and a plurality (in this embodiment, four) of bolt bosses  34  and  35 . Center section  30  will be described on the assumption that motor port block  32  is joined to pump port block  31  so as to extend rearwardly upward from a rear end of pump port block  31 . 
     As shown in  FIGS. 15 ,  16 ,  19 , and  20 , pump port block  31  is formed therein with right and left pump kidney ports  31   b  and  31   c  and a pump shaft hole  31   d  between pump kidney ports  31   b  and  31   c.  A horizontal pump mounting surface  31   a  is formed on a top end of pump port block  31 . Pump kidney ports  31   b  and  31   c  and pump shaft hole  31   d  are extended vertically upward so as to be open at horizontal pump mounting surface  31   a.    
     As shown in  FIG. 17  and others, a horizontal filter mounting surface  31   e  is formed on a bottom end of pump port block  31 . A circumferential area of filter mounting surface  31   e  is provided to contact a top edge of a cylindrical filter  26  (see  FIG. 6  and others). Plural (in this embodiment, three) filter retainers  31   i  project downward from filter mounting surface  31   e  and are formed and aligned along the circumferential area of filter mounting surface  31   e  so as to fit an upper portion of a peripheral side surface of filter  26  whose top edge contacts the circumferential area of filter mounting surface  31   e.  Pump shaft hole  31   d  is extended vertically downward so as to be open at a central portion of filter mounting surface  31   e.  A charge port gallery  31   h  is open at a front portion of filter mounting surface  31   e  between pump shaft hole  31   d  and the circumferential area of filter mounting surface  31   e.  Right and left charge ports  31   f  and  31   g  are open at respective bottom ends thereof to charge port gallery  31   h.    
     In this regard, when HST  20  is disposed in transaxle casing  10 , filter  26  contacting filter mounting surface  31   e  of center section  30  is submerged in a fluid sump in chamber  10   a  so that fluid is filtered by filter  26  when the fluid penetrates filter  26  from the fluid sump in chamber  10   a  to the inside space of filter  26 . The fluid in the inside of filter  26  constantly enters charge port gallery  31   h  and charge ports  31   f  and  31   g.    
     As shown in  FIGS. 19 ,  20 , and others, right and left main fluid passages  36  and  37  are formed in pump port block  31  so as to extend in the horizontal fore-and-aft direction. Front portions of main fluid passages  36  and  37  are diametrically expanded to serve as charge valve chambers  36   a  and  37   a  that are open outward at the front end of pump port block  31 . Right kidney port  31   b  is joined at a bottom thereof to a top portion of right main fluid passage  36  and right charge port  31   f  is joined at a top thereof to a bottom portion of right charge valve chamber  36   a.  Similarly, left kidney port  31   c  is joined to left main fluid passage  37 , and left charge port  31  is joined to left charge valve chamber  37   a.    
     As shown in  FIG. 19 , right and left charge check valves  21  are disposed in respective right and left charge valve chambers  36   a  and  37   a.  Each charge check valve  21  includes a valve casing  22 , a ball valve member  23 , a pressure pin  24 , and a spring  25 . Valve casing  22  is fitted in each of charge valve chambers  36   a  and  37   a  so as to serve as a plug closing the outward opening of each of charge valve chambers  36   a  and  37   a.  Valve casing  22  is formed therein with a valve port  22   a  that is constantly open to each of charge ports  31   f  and  31   g.  Ball valve member  23  is biased by spring  25  so as to block valve port  22   a  from each of fluid passages  36  and  37 . Charge check valve  21  is configured so that when either main fluid passage  36  or  37  is hydraulically depressed and the hydraulic pressure therein is lower than that in valve port  22   a,  the differential pressure of fluid between valve port  22   a  and hydraulically depressed main fluid passage  36  or  37  thrusts ball valve member  23  against spring  25  so as to open valve port  22   a  to hydraulically depressed main fluid passage  36  or  37 , whereby fluid passed through a later-discussed filter  26  is supplied to main fluid passage  36  or  37 . 
     Motor port block  32  is formed therein with right and left motor kidney ports  32   b  and  32   c  and a motor shaft hole  32   d  between motor kidney ports  32   b  and  32   c.  A slant upper end of motor port block  32  is formed as a rearwardly downward slant motor mounting surface  32   a.  Right and left motor kidney ports  32   b  and  32   c  and motor shaft hole  32   d  are extended rearwardly upward so as to be open at motor mounting surface  32   a.  Right and left main fluid passages  36  and  37  are extended rearward into motor port block  32  so as to be joined at rear ends thereof to respective right and left motor kidney ports  32   b  and  32   c.    
     Bypass valve block  33  projects downward from motor port block  32  so as to extend in the lateral horizontal portion. A lateral horizontal bypass valve hole  33   a  is found through bypass valve block  33 . Right and left ends of bypass valve holes  33   a  are open outward at right and left vertical end surfaces of bypass valve block  33 . Right and left vertical bypass ports  33   b  and  33   c  are extended downward from respective right and left motor kidney ports  32   b  and  32   c  so as to cross bypass valve hole  33   a  and are further extended downward from bypass valve hole  33   a  so as to be open. outward at a horizontal bottom surface of bypass valve block  33 . 
     In this regard, as shown in  FIG. 9 , a rotary bypass valve  27  having right and left diametric valve passages  27   a  and  27   b  is fitted in bypass valve hole  33   a.  Bypass valve  27  is rotatably centered on its laterally horizontal axis so as to be shiftable between a valve closing position and a valve opening position. To release fluid from main fluid passages  36  and  37  to the fluid sump in chamber  10   a,  bypass valve  27  is disposed at the valve opening position so as to orient valve passages  27   a  and  27   b  vertically to fluidly connect the upper portions of bypass ports  33   b  and  33   c  above bypass valve hole  33   a  to the lower portions of bypass ports  33   b  and  33   c  below bypass valve hole  33   a  via respective bypass passages  27   a  and  27   b,  thereby releasing fluid from motor kidney ports  32   b  and  32   c  to the fluid sump in chamber  10   a  via the open bottom ends of bypass ports  33   b  and  33   c.  Bypass valve  27  is normally disposed at the valve closing position so that valve passages  27   a  and  27   b  deviate from respective bypass ports  33   b  and  33   c,  thereby blocking bypass ports  33   b  and  33   c.  Bypass valve  27  can be selectively inserted into bypass valve hole  33   a  from either the right or left vertical end surface of bypass valve block  33 . 
     Right and left front bosses  34  having respective vertical through holes  34   a  are expanded rightwardly and leftwardly forward from pump port block  31 . Right and left rear bosses  35  having respective vertical through holes  35   a  are expanded rightward and leftward from motor port block  32 . Referring to  FIG. 10 , bolts  38  are passed upward through respective vertical through holes  34   a  in right and left front bosses  34  and are screwed upward into transaxle housing  11 , and bolts  39  are passed upward through respective vertical through holes  35   a  in right and left rear bosses  35  and are screwed upward into transaxle housing  11 , thereby fastening center section  30  with four-pointed bosses  34  and  35  to transaxle housing  11 . 
     Center section  30 , configured as mentioned above, is advantageous in its lateral symmetry. In this regard, for example, right and left main fluid passages  36  and  37 , right and left charge ports  31   f  and  31   g,  right kidney ports  31   b  and  32   b  and left kidney ports  31   c  and  32   c,  right and left charge check valve chambers  36   a  and  37   a  and right and left bypass ports  33   b  and  33   c  are laterally symmetric so that hydraulic pump  40  and hydraulic motor  50  mounted on center section  30  are aligned in the fore-and-aft direction of transaxle  1  without lateral eccentricity or deviation. Charge check valves  121  are also laterally symmetrically arranged. Such a lateral symmetry of center section  30  is advantageous to standardize transaxle  1  serving as either of right and left transaxles  1 R and  1 L. 
     Referring to  FIGS. 6 to 9 , hydraulic pump  40  includes a pump shaft  41 , a pump cylinder block  42 , plungers  43 , and a movable swash plate  44 . Pump shaft  41  is fittingly passed through pump shaft hole  31   d  of center section  30  rotatably relative to center section  30 . Pump cylinder block  42  is formed with a center through hole serving as a pump shaft hole  42   a  and with cylinder bores  42   b  aligned radially around pump shaft hole  42   a.  Pump shaft  41  is fittingly passed through pump shaft hole  42   a  unrotatably relative to pump cylinder block  42 . Pump cylinder block  42  is slidably rotatably fitted onto pump mounting surface  31   a  so as to fluidly connect cylinder bores  42   b  therein to pump kidney ports  31   b  and  31   c.  Plungers  43  are fitted into respective cylinder bores  42   b  reciprocally in the axial direction of pump shaft  41 , thereby constituting axial piston type hydraulic pump  40 . Movable swash plate  44  has a bearing  44   a  abutting against heads of plungers  43  projecting from pump cylinder block  42 . 
     A front top portion of the ceiling wall of transaxle housing  11  is formed as a pump support portion  11   b.  Referring to  FIG. 7 , movable swash plate  44  of hydraulic pump  40  is fitted to a ceiling surface of pump support portion  11   b  slidably rotatably relative to transaxle housing  11 . Transaxle housing  11  is formed through right and left side walls of pump support portion  11   b  with right and left symmetric trunnion holes  11   c.  Movable swash plate  44  is formed with right and left symmetric feet  44   b  having respective right and left symmetric trunnion holes  44   c.  While movable swash plate  44  is fitted to pump support portion  11   b,  right and left trunnions  45  are fittingly passed through respective right and left trunnion holes  44   c  of movable swash plate  44  and respective right and left trunnion holes  11   c  of transaxle housing  11  so as to project at respective distal end portions thereof rightwardly and leftwardly outward from right and left side walls of transaxle housing  11 . Right and left fixture pins  45   a  fasten right and left feet  44   b  of movable swash plate  44  to fix right and left trunnions  45  listed in trunnion holes  44   c.    
     Pump support portion lib is formed with a vertical through hole  11   b   1 . An upper portion of pump shaft  41  projecting upward from pump cylinder block  42  fitted on pump mounting surface  31   a  of center section  30  is freely passed through movable swash plate  44  fitted to pump support portion  11   b  and through vertical through hole  11   b   1  of pump support portion  11   b  so as to project upward from pump support portion  11   b  of transaxle housing  11 . A bearing  41   a  and a fluid seal  41   b  are fitted in through hole  11   b   1  of pump support portion  11   b  so as to be interposed between pump shaft  41  and pump support portion  11   b  of transaxle housing  11 . An input pulley  14  and a cooling fan  15  are fixed on the upper portion of pump shaft  41  projecting upward from pump support portion  11   b  of transaxle housing  11 . Vehicle  100  or  300  is equipped with an unshown prime mover such as an engine whose output power is transmitted to input pulley  14  via a belt and so on. 
     Referring to  FIGS. 6 and 9 , hydraulic motor  50  includes a motor shaft  51 , a motor cylinder block  52 , plungers  53 , and a fixed swash plate  54 . Motor shaft  51  is fitted into motor shaft hole  32   d  of center section  30  rotatably relative to center section  30 . Motor cylinder block  52  is formed with a center through hole serving as a motor shaft hole  52   a  and with cylinder bores  52   b  aligned radially around motor shaft hole  52   a.  Motor shaft  51  is fittingly passed through motor shaft hole  52   a  unrotatably relative to motor cylinder block  52 . Motor cylinder block  52  is slidably rotatably fitted onto motor mounting surface  32   a  of center section  30  so as to fluidly connect cylinder bores  52   b  therein to motor kidney ports  32   b  and  32   c.  Plungers  53  are fitted into respective cylinder bores  52   b  reciprocally in the axial direction of motor shaft  51 , thereby constituting axial piston type hydraulic motor  50 . A bearing abuts against heads of plungers  53  projecting from motor cylinder block  52  so as to serve as fixed awash plate  54 . 
     Referring to  FIG. 6  an inner surface of the ceiling wall of transaxle housing  11  at the fore-and-aft middle portion thereof between pimp support portion  11   b  and top opening  11   g  is formed to protrude rear Hardly downward slantwise so as to face upward slant motor port block  32  a center section  30 , thereby serving as a motor support portion  11   f.  Motor support portion  11   f  is formed with a recess  11   f   1  that is open forwardly downward to face the front portion of chamber  10   a  in transaxle housing  11  incorporating HST  20 . The thrust bearing serving as fixed swash plate  54  is fitted into forwardly downward open recess  11   f   1  to abut against the heads of plungers  53  projecting from motor cylinder block  52 . On the other hand, motor support portion  11   f  is formed with another recess  11   f   3  that is open rearwardly upward to face the rear portion of chamber  10   a  in transaxle housing  11  incorporating reduction gear train  70 , and a bearing  55  for journaling motor shaft  51  is fitted into rearwardly upward open recess  11   f   3 . A through hole  11   f   2  is formed in motor support portion  11   f  between recesses  11   f   1  and  11   f   3 . Motor shaft  51  is extended rearwardly upward from motor cylinder block  52  and is freely passed through fixed swash plate  54  and through hole  11   f   2  in motor support portion  11   f.  Further, motor shaft  51  is fittingly passed through bearing  55 , and a tip portion of motor shaft  51  projects rearwardly upward from bearing  55  so as to be fixedly provided thereon with a bevel pinion  71 . 
     The advantage of hydraulic motor  50  having the slant axis with regard to the vertical axis of hydraulic pump  40  is to horizontally and vertically minimize a portion of transaxle  1  incorporating hydraulic motor  50  while ensuring the sufficient length of motor shaft  51  and the sufficient capacity of motor cylinder block  52 . Therefore, the dimension of transaxle  1  in the fore-and-aft direction between pump shaft  41  serving as the input shaft of transaxle  1  and axle  2  serving as the output shaft of transaxle  1  and the dimension in the lateral direction of transaxle casing  10  required to incorporate both hydraulic pump  40  and hydraulic motor  50  are shortened to facilitate the mounting of transaxle  1  on a small vehicle. In this embodiment, the fore-and-aft middle portion of transaxle casing  10  incorporating hydraulic motor  50  between the front portion of transaxle casing  10  incorporating hydraulic pump  40  and the rear portion of transaxle casing  10  incorporating reduction gear train  70  is shortened in the fore-and-aft direction so as to reduce the entire fore-and-aft length of transaxle  1 , and is lowered at the top portion thereof so as to reduce the vertical dimension of the fore-and-aft middle portion of transaxle  1 . 
     More specifically, to enhance the effect of reducing the fore-and-aft dimension and vertical dimension of transaxle  1  from pump shaft  41  to the rear upper end of motor shaft  51  that is more distant from pump shaft  41  than the front lower end of motor shaft  51 , it is preferable that the vertical axis of pump shaft  41  and the vertically slant axis at motor shaft  51  have an acute angle therebetween. In this regard, preferably, center section  30  is configured so that pump mounting surface  31   a  and motor mounting surface  32   a  have an obtuse angle therebetween to ensure such an acute angle arrangement of pump shaft  41  and motor shaft  51 . 
     HST  20  is installed in chamber  10   a  in transaxle housing  11  while the bottom opening of chamber  10   a  is opened by removing lower cover  12  from transaxle housing  11  and top opening  11   g  is opened by removing top cover  13  from transaxle housing  11 . In this regard, movable swash plate  44 , trunnions  45 , and fixed swash plate  54  are inserted into chamber  10   a  in transaxle housing  11  through the bottom opening of chamber  10   a,  so that movable swash plate  44  is fitted to pump support portion  11   b,  right and left trunnions  45  are fitted through trunnion holes  11   c  and are fixed to movable swash plate  44 , and fixed swash plate  54  is fitted to motor support portion  11   f.    
     Then, center section  30  having pump cylinder block  42  on pump mounting surface  31   a  and having motor cylinder block  52  on motor mounting surface  32   a  is inserted into chamber  10   a  in transaxle housing  11  through the bottom opening of chamber  10   a  so as to make the heads of plungers  43  abut against movable swash plate  44  fitted to pump support portion  11   b  and so as to make the heads of plungers  53  abut against fixed swash plate  54 . Bolts  38  and  39  are passed through holes  34   a  and  35   a  of bosses  34  and  35  and are screwed upward into transaxle housing  11 , thereby fixing center section  30  to transaxle housing  11 . 
     Cylindrical filter  26  of HST  20  is mounted on an inner surface of lower cover  12 . After HST  20  except for filter  26  is assembled in chamber  10   a  in transaxle housing  11  as mentioned above, lower cover  12  is fitted to the bottom edge of transaxle housing  11  so as to cover the bottom opening of chamber  10   a  in transaxle housing  11 , thereby fitting filter  26  on lower cover  12  to filter mounting surface  31   e  of center section  30 . Then, bolts  16  are screwed into the bottom edge of transaxle housing  11  so as to fasten lower cover  12  to transaxle housing  11 . Incidentally, a charge pump such as a trochoidal pump may be disposed in filter  26  and fitted to filter mounting surface  31   e  of center section  30 . The bottom end portion of pump shaft  41  projecting downward in filter  26  from filter mounting surface  31   e  of center section  30  can be used as a drive shaft for the charge pump disposed in filter  26 . Preferably, center section  30  has an inner fluid passage configured to distribute the fluid delivered from the charge pump to valve ports  22   a.    
     Further, HST control arm  46  is fixed on the distal end portion Of one of right and left trunnions  45  projecting outward from transaxle housing  11  so that the tilt angle and direction of movable swash plate  44  of hydraulic pump  40  is controlled by rotating HST control arm  46 . On the other hand, a neutral return spring  47  and a spring retainer  48  are provided on the distal end portion of one of right and left trunnions  45  opposite HST control arm  46 . In this regard, as shown in  FIGS. 3 ,  14 ( a ) and  14 ( b ), neutral return spring  47  is coiled on the distal end portion of trunnion  45  and both end portions of neutral return spring  47  are twisted to cross each other and are extended rearward. Spring retainer  48  is fixed on the distal end portion of trunnion  45  by a nut so as to cover the coiled neutral return spring  47 . Spring retainer  48  is formed with a stopper  48   a  that is disposed between the rearwardly extended end portions of neutral return spring  47 . 
     As shown in  FIG. 9 , a right or left side wall of transaxle housing  11  adjacently rearward from trunnion  45  having neutral returning spring  47  and spring retainer  48  thereon is formed with an eccentric pin hole  11   d,  and a laterally horizontal axis shaft  49   a  of an eccentric pin  49  is fitted into eccentric pin hole  11   d.  In this regard, the right and left side walls of transaxle housing  11  adjacently rearward from right and left trunnion holes  11   c  are formed to have sufficient thickness before boring eccentric pin hole  11   d  so that either the right or left wall can be selectively bored with eccentric pin hole  11   d.    
     Eccentric pin  49  is fitted on a portion of axis shaft  49   a  projecting outward from transaxle housing  11  and a nut is screwed on a distal end of axis shaft  49   a  so as to fasten eccentric pin  49  to axis shaft  49   a.  The biasing force of spring  47  functions to move both rearwardly extended end portions of spring  47  toward each other when viewed in side. Therefore, movable swash plate  44  and HST control arm  46  are set at their neutral positions when both end portions of spring  47  pinch eccentric pin  49  therebetween as shown in  FIG. 14(   a ). When HST control arm  46  fixed on one trunnion  45  is rotated to move movable swash plate  44  from the neutral position, spring retainer  48  fixed on the other trunnion  45  also rotates so that stopper  48   a  pushes one end portion of spring  47  away from the other end portion of spring  47  as shown in  FIG. 14(   b ), thereby generating the above-mentioned biasing force of spring  47 . When HST control arm  46  having been moved from the neutral position is released, both ends of spring  47  are returned by their own biasing force to pinch eccentric pin  49 , thereby returning movable swash plate  44  and HST control arm  46  to their neutral positions. 
     Due to the eccentricity of eccentric pin  49  relative to axis shaft  49   a,  the neutral position of HST control arm  46  defined as the position where both end portions of spring  47  pinch eccentric pin  49  can be adjusted relative to the real neutral position of movable swash plate  44  for stopping the fluid delivery from hydraulic pump  40 . In this regard, by loosening the nut, the rotational position of eccentric pin  49  pinched by neutral returning spring  47  relative to axis shaft  49   a  can be changed to adjust the neutral position of HST control arm  46  relative to the neutral position of movable swash plate  44 . 
     Further, as shown in  FIG. 9 , right and left symmetric bypass valve holes  11   e  are formed through right and left side walls of transaxle housing  11  facing respective right and left end surfaces of bypass valve block  33  of center section  30 . As mentioned above, laterally horizontal axial bypass valve  27  is fitted in bypass valve hole  33   a  rotatably relative to center section  30 . One of right and left ends of bypass valve  27  is disposed in bypass valve hole  33   a  and the other of right and left ends of bypass valve  27  projects outward from the corresponding right or left end of bypass valve block  33  and is passed through corresponding right or left bypass valve hole  11   e  rotatably relative to transaxle housing  11  so as to project outward from transaxle housing  11 . A bypass valve arm  28  is fixed on the end portion of bypass valve  27  projecting from transaxle housing  11 . In this way, either right or left bypass valve hole  11   e  can be selectively used for passing bypass valve  27 . 
     As shown in  FIGS. 6 and 11 , a laterally horizontal gear shaft  72  is disposed in chamber  10   a  adjacent to the fore-and-aft middle portion of top opening  11   g  and is supported at right and left ends thereof by right and left side walls  11   j.  In this regard, top edges of right and left side walls  11   j  facing top opening  11   g  are formed with respective recesses  11   j   1  facing each other. A bearing  74  is fitted in recess  11   j   1  of one side wall  11   j  so as to journal one end of gear shaft  72 . A bearing  75  is fitted in recess  11   j   1  of the other side wall  11   j  so as to journal the other end of gear shaft  72 . Right and left bearings  74  and  75  project upward from the top edges of side walls  11   j,  and right and left bearing covers  76  are provided on the top edges of side walls  11   j  so as to cover respective bearings  74  and  75 . 
     Referring to  FIGS. 6 and 10  to  13 , a bevel gear  73  formed with a spur pinion  73   a  is fixed on gear shaft  72  between right and left bearings  74  and  75 . Bevel gear  73  meshes with bevel pinion  71  fixed on the tip portion of motor shaft  51 . An axial boss of bevel gear  73  extended along gear shaft  72  serves as spur pinion  73   a.  A spur bull gear  77  is fixed on axle  2  supported by transaxle housing  11  and meshes with spur pinion  73   a.  Therefore, pinions  71  and  73   a  and gears  73  and  77  constitute reduction gear train  70  for transmitting power from motor shaft  51  of HST  20  to axle  2 . 
     Transaxle  1  includes a gear locking mechanism for applying a parking brake to reduction gear train  70  and axle  2 , the gear locking mechanism including a locking pawl  80  adapted to engage with bevel gear  73  meshing with bevel pinion  71  fixed on motor shaft  51 . Referring to  FIG. 10 , right and left symmetric shaft holes  11   h  are formed through right and left side walls of transaxle housing  11 , and a laterally horizontal locking shaft  79  is supported through right and left shaft holes  11   h  rotatably relative to transaxle housing  11 . Either the right or left end of locking shaft  79  is selected to have a locking arm  78  fixed thereon. Referring to  FIGS. 6 ,  10  to  13 , bevel gear  73  is peripherally formed on a back surface thereof with plural recesses  73   b.  A locking pawl  80  is fixed on locking shaft  79 . By rotating locking arm  78  and locking shaft  79 , locking pawl  80  is shiftable between a locking position where locking pawl  80  is hooked into any one of recesses  73   b  to lock gear  73  and an unlocking position where locking pawl  80  is not hooked into any recess  73   b.  By setting locking pawl  80  at the locking position, gear  73  is locked, thereby stopping axle  2  for parking vehicle  100  or  300 . 
     In this regard, referring to  FIG. 10 , locking shaft  79  is formed with right and left pin holes  79   a.  One of pin holes  79   a  is selected and a lock pin  80   a  is inserted into selected pin hole  79   a  to fasten locking pawl  80  to locking shaft  79 . Whether right or left pin hole  79   a  is selected depends on whether bevel gear  73  is disposed rightward or leftward of pinion  73   a.  In the embodiment of  FIG. 10 , right pin hole  79   a  is selected to fix locking pawl  80  to locking shaft  79  because bevel gear  73  is disposed rightward of pinion  73   a.    
     More specifically, the rotary direction of axle  2  depends on the rotary direction of bevel gear  73  and pinion  73   a,  and the rotary direction of bevel gear  73  depends on whether bevel pinion  71  meshes with bevel gear  73  at the right or left side thereof, i.e., whether bevel gear  73  is disposed rightward or leftward of pinion  73   a.  Therefore, if the rotary direction of pump shaft  41  relative to an engine mismatches with the rotary direction of axle  2  for ensuring the desired travel direction of the vehicle, bevel gear  73  with pinion  73   a  can be reversed to change its position relative to bevel pinion  71  so as to correctly match the rotary direction of axle  2  with the rotary direction of pump shaft  41  relative to the engine for ensuring the desired travel direction of the vehicle. 
     Referring to  FIGS. 3 ,  5  and  11  to  13 , top cover  13  is formed with a central cover portion  13   a,  a flange  13   c,  and right and left side cover portions  13   b  between central cover portion  13   a  and flange  13   c.  Flange  13   c  is fastened onto a top edge of transaxle housing  11  surrounding top opening  11   g  by screwing bolts  17  downward into the ceiling wall of transaxle hosing  11 , thereby fixing top cover  13  on transaxle housing  11  so that central cover portion  13   a  and surrounding cover portion  13   b  cover top opening  11   g.  Central cover portion  13   a  is arc-shaped in the side view along the periphery of the upper portion of bevel gear  73  projecting upward from top edges of side walls  11   j  of transaxle housing  11 . Right and left side cover portions  13   b  have respective horizontal flat surfaces that are lower than the major part of central cover portion  13   a.  The horizontal flat surfaces of right and left horizontal side cover portions  13   b  are fitted onto the top portions of right and left bearing covers  76  projecting upward from the top edges of side walls  11   j  of transaxle housing  11 . 
     Incidentally, the top of central cover portion  13   a  is lower than input pulley  14  fixed on the top portion of pump shaft  41  projecting upward from pump support portion  11   b.  Therefore, even if an engine is disposed rearward from transaxles  1 R and  1 L arranged so as to have HSTs  20  forward from reduction gear trains  70  as mentioned above, or even if an engine is disposed forward from transaxles  1 R and  1 L arranged so as to have HSTs  20  rearward from reduction gear trains  70 , a belt looped over an output pulley of the engine and input pulleys  14  of transaxles  1 R and  1 L is prevented from interfering with central cover portion  13   a,  thereby enhancing the freedom of layout of the right and left transaxles  1 R and  1 L in the vehicle relative to the engine. 
     Referring to  FIGS. 12 and 13 , axle support portion  11   i  journals axle  2  via a bearing  86 . Axle  2  supported by axle support portion  11   i  is extended at a distal portion thereof outward from a distal end of axle support portion  11   i,  and is extended at a proximal portion thereof into chamber  10   a  in transaxle housing  11  between right and left side walls  11   j  so as to be fixedly provided thereon with hull gear  77  meshing with pinion  73   a.    
     In this regard, as shown in  FIG. 6 , each of right and left support side walls  11   j  is formed to have a reverse U-shaped edge  11   k  defining top, front and rear ends of an opening that laterally penetrates each side wall  11   j  and is downwardly open. A bearing  84  is disposed in the opening defined by one of right and left edges  11   k  or bearings  84  or  85  are disposed in the openings defined by respective right and left edges  11   k  so as to journal axle  2 . Each reverse U-shaped edge  11   k  has front and rear bottom ends to which a support plate  82  can be fastened by front and rear bolts  83  so as to close the downward open end of the opening, thereby supporting bearing  84  or  85  in the opening defined by edge  11   k.    
     Regardless of whether transaxle  1  serves as right transaxle  1 R or left transaxle  1 L, two types are prepared as reduction gear train  70  of transaxle  1  as mentioned above. One type referred to as a reduction gear train  70 A has bevel gear  73  being closer to axle support portion  11   i  than pinion  73   a,  as shown in  FIG. 12 . The other type referred to as a reduction gear train  70 B has bevel gear  73  being more distant from axle support portion  11   i  than pinion  73   a,  as shown in  FIG. 13 . 
     When reduction gear train  70 A having bevel gear  73  rightward of pinion  73   a  in  FIG. 12  is selected as reduction gear train  70  of transaxle  1 , bull gear  77  meshing with pinion  73   a  approaches the proximal end of axle  2  so as to sufficiently receive the support force from bearing  84  fitted to one reverse U-shaped edge  11   k.  Therefore, it is unnecessary to use the opening defined by the other reverse U-shaped edge  11   k  closer to axle support portion  11   i  for journaling axle  2  so that axle  2  is journalled at two points by bearings  84  and  86 . 
     When reduction gear train  70 B having bevel gear  73  leftward of pinion  73   a  in  FIG. 13  is selected as reduction gear train  70  of transaxle  1 , bull gear  77  meshing with pinion  73   a  comes distant from bearing  84  so as to insufficiently receive the support force from bearing  84 . Therefore, the opening defined by reverse U-shaped edge  11   k  closer to axle support portion  11   i  has bearing  85  journaling an axial intermediate portion of axle  2  therein. In this regard, bearing  85  journaling axle  2  is fitted to edge  11   k,  and another support plate  82  is fastened to front and rear bottom ends of edge  11   k  fitting bearing  85  by front and rear bolts  83  so as to close the downward open end of the opening, thereby supporting bearing  85 . Therefore, axle  2  is journalled at three points by bearings  84 ,  85  and  86 . 
     Referring to  FIGS. 21 to 30 , description will be given of an alternative transaxle  1 A including axle  2 , an HST  120  and a reduction gear train  170  for transmitting power from HST  120  to axle  2 . Transaxle  1 A has members and portions designated by the same reference numerals as those used for transaxle  1 . Description of these members and portions of transaxle  1 A will be omitted unless otherwise specifically noted, because they are identical or similar to the corresponding members and portions of transaxle  1  with regard to arrangement, structure, function and the like. Further, some reference numerals to be used to designate corresponding members and portions of transaxle  1  are omitted from the drawings illustrating transaxle  1 A on the premise that the members and portions of transaxle  1 A illustrated as being identical or similar to the corresponding members and portions of transaxle  1  should have been marked with the same reference numerals as those designating the corresponding members and portions of transaxle  1 . Further, on the assumption that HST  120  is disposed forward from axle  2  and reduction gear train  170 , illustrated transaxle  1 A serves as right transaxle  1 R carrying right axle  2 R, however, transaxle  1 A may be adapted as left transaxle  1 L carrying left axle  2 L. 
     A transaxle casing  110  of transaxle  1 A includes an upper transaxle housing  111 , a lower transaxle housing  112 , and a gear top cover  113 . Bolts  116  are screwed upward to fasten a flanged top edge of lower transaxle housing  112  to a bottom edge of upper transaxle housing  111  fringing a bottom opening of upper transaxle housing  111 . Bolts  117  are screwed downward to fasten a flanged bottom edge of gear top cover  113  to a top edge of a rear half portion of upper transaxle housing  111  fringing a top opening of the rear half portion of upper transaxle housing  111 . 
     In comparison with substantially horizontally flat thin plate-shaped bottom cover  12  fixed to the bottom edge of transaxle housing  11  defining the bottom end of transaxle casing  10 , lower transaxle housing  112  has an inner space whose vertical dimension defines a depth for accommodating a lower portion of bull gear  77  and a lower portion of HST  120 . Especially, center section  30  fastened to upper transaxle housing  111  via bolts  38  and  39  screwed through bosses  34  and  35  has filter mounting surface  31   e  (see  FIGS. 17 and 20 ) that is level with the bottom edge of upper transaxle housing  111 , and the top edge of cylindrical filter  26  accommodated in lower transaxle housing  112  is level with the top edge of lower transaxle housing  112 , so that the top edge of filter  26  in lower transaxle housing  112  can be easily fitted to filter mounting surface  31   e,  and the top outer peripheral surface of filter  26  to filter retainers  31   i,  when the bottom edge of upper transaxle housing  111  is joined to the top edge of lower transaxle housing  112 . 
     In this regard, a space in transaxle casing  10  between slanted hydraulic motor  50  and flat bottom cover  12  serving as the bottom end of transaxle casing  10  below hydraulic motor  50  has a considerably great vertical length, and right and left vertical walls of transaxle housing  11  are extended downward to ensure this space in transaxle casing  10 . Fluid in this space serves as a part of the fluid sump in transaxle casing  10 , thereby increasing the volume of the fluid sump so as to increase the weight of transaxle  1 . On the contrary, the space in transaxle casing  110  between slanted hydraulic motor  50  and the bottom portion of transaxle casing  110  below hydraulic motor  50  is defined by lower transaxle housing  112 . The bottom of the fore-and-aft middle portion of lower transaxle housing  112  below hydraulic motor  50  is formed higher than the bottom of the front portion of lower transaxle housing  112  incorporating the bottom portion of center section  30  and filter  26  and is slanted to have a shape corresponding to the lower side portion of hydraulic motor  50 . Further, the bottom of the rear portion of lower transaxle housing  112  is formed higher than the bottom of the front portion of lower transaxle housing  112  and is curved so as to have a shape corresponding to the lower end portion of bull gear  77 . Therefore, the space between hydraulic motor  50  and bull gear  77  and lower transaxle housing  112  is reduced so as to reduce the volume of the fluid sump in transaxle casing  110 , thereby lightening transaxle  1 A. 
     As discussed later, transaxle  1 A is provided with a gear locking system  180  for applying a parking brake to reduction gear train  170  and axle  2 . In this regard, referring to  FIG. 22 , gear top cover  113  of transaxle casing  110  for accommodating an upper portion of reduction gear train  170  supports locking shaft  79  of gear locking system  180 , in comparison with transaxle housing  11  of transaxle casing  10  that supports locking shaft  79 . Therefore, gear top cover  113  of transaxle casing  110  for transaxle  1 A is a thick cover made by die-casting or the like so as to ensure a sufficient rigidity for supporting locking shaft  79 , in comparison with thin plate-shaped gear top cover  13  of transaxle casing  10  for transaxle  1 , which does not support locking shaft  79 . 
     Features of HST  120  of transaxle  1 A distinguished from corresponding features of HST  20  of transaxle  1  will be described. Referring to  FIGS. 26 and 27 , right and left charge check valves  121  having function as bypass valves are fitted in respective right and left charge check valve chambers  36   a  and  37   a  formed in center section  30 . Right and left charge check valves  121  correspond to right and left charge check valves  21 , excluding that charge check and bypass valves  121  are additionally provided with respective pressure rods  131  that are disposed parallel to each other mechanically operably for simultaneous opening of charge check valves  121 . Therefore, HST  120  does not have an additional bypass valve corresponding to bypass valve  27  as shown in  FIG. 9  to be disposed in center section  30 , so that center section  30  can be formed without bypass valve hole  33   a  for accommodating bypass valve  27  and/or without bypass ports  33   b  and  33   c  for fluidly connecting bypass valve  27  to a fluid sump in transaxle casing  110 , thereby reducing processes to form center section  30 . 
     Referring to  FIGS. 22 ,  23 ,  26  and  27 , transaxle  1 A is provided with a bypass valve operation mechanism  127  for operating right and left charge check valves  121  to function as bypass valves. Bypass valve operation mechanism  127  includes a bypass valve operation arm  128 , a vertical camshaft  129  formed with a cam  129   a,  a pressure member  130 , right and left pressure rods  131  and springs  132  wound around respective pressure rods  131 . Upper transaxle housing  111  journals vertical camshaft  129 . Bypass valve operation arm  128  is fixed on a top end of camshaft  129  projecting upward from upper transaxle housing  111  of transaxle casing  110 . Referring to  FIG. 27 , bypass valve operation arm  128  is rotatable to be shifted between a valve-close position P 1  as drawn in phantom lines and a valve-open position P 2 . 
     Referring to  FIGS. 22 and 26 , a bottom plate  34   b  is interposed between bottom surfaces of right and left front bosses  34  and bottom heads of bolts  38 . Pressure member  130  is disposed in a space between right and left front bosses  34  and slidably abuts at a bottom horizontal surface thereof against a top horizontal surface of bottom plate  34   b.  Pressure member  130  is formed with right and left vertical surface  130   b  and a rear vertical surface  130   c  defining an upwardly open recess  130   a.  Cam  129   a  formed on a bottom end of camshaft  129  is disposed in recess  130   a.  Right and left pressure rods  131  are extended rearward from right and left portions of pressure member  130  and are inserted into respective valve casings  22 . Springs  132  wound around respective pressure rods  131  are interposed between the right and left portions of pressure member  130  and front end surfaces of respective valve casings  22  so as to bias pressure member  130  forward. 
     When bypass valve operation arm  128  is disposed at valve-close position P 1  as drawn in phantom lines, cam  129   a  is directed to right or left vertical surface  130   b  so that pressure member  130  is located at its forward limit position by the forward biasing forces of springs  132 . In this state, rear ends of pressure rods  131  are spaced from respective ball valve members  23  so as to allow ball valve members  23  forwardly biased by respective springs  25  to function as charge check valves. On the other hand, when bypass valve operation arm  128  is disposed at valve-open position P 2 , cam  129   a  is directed rearward so as to contact rear vertical surface  130   c,  thereby pushing pressure member  130  and pressure rods  131  rearward against springs  132 . In this state, the rear ends of pressure rods  131  push respective ball valve members  23  rearward against springs  25  so as to open respective valve ports  22   a  (see  FIG. 19 ) to respective main fluid passages  36  and  37 , thereby draining pressurized fluid from main fluid passages  36  and  37  to the fluid sump in transaxle casing  110 . 
     A motor swash plate support member  114  is fixedly disposed in transaxle casing  110  so as to support the thrust bearing serving as fixed swash plate  54  of hydraulic motor  50  mounted on motor port block  32  of center section  30  and so as to journal slant motor shaft  51  of hydraulic motor  50 , in comparison with motor support portion  11   f  that is formed on transaxle housing  11  so as to support fixed swash plate  54  and motor shaft  51 . Referring to  FIGS. 22 ,  23 , and  25 , upper transaxle housing  111  is formed to extend under gear top cover  113  so as to cover an upper surface of motor swash plate support member  114 , and motor swash plate support member  114  is fastened to the extended portion of upper transaxle housing  111  under gear top cover  113  via bolts  118 . 
     The advantage of motor swash plate support member  114  separated from transaxle casing  110  is that motor swash plate support member  114  is reversible so as to enable fixed swash plate  54  to be reversed in correspondence to which rotational direction of pump shaft  41  relative to motor shaft  51  is designed. Therefore, it is unnecessary to design a later-discussed bevel gear  173  of reduction gear train  170  as being reversible in correspondence to the rotational direction of pump shaft  41  relative to motor shaft  51 . On the contrary, as mentioned above, reduction gear train  70  of transaxle  1  has bevel gear  73  designed to be reversible in accordance to the rotational direction of pump shaft  41  relative to motor shaft  51  because fixed swash plate  54  supported by motor support portion  11   f  of transaxle housing  11  is riot reversible. 
     An upper rear end of slant motor shaft  51  projects upwardly rearward from motor swash plate support member  114  so as to be fixedly provided thereon with bevel pinion  71 . Bevel pinion  71  meshes with bevel gear  173  corresponding to bevel gear  73  of transaxle  1 . Referring to  FIG. 22 , representative gear locking system  180  for transaxle  1 A has a representative locking pawl  181  engageable with bevel pinion  71  in comparison with the gear locking system for transaxle  1  having locking pawl  80  engageable with bevel gear  73 . In this regard, as discussed later, gear locking system  180  is representative of gear locking systems  180 A and  180 B, and locking pawl  181  is representative of locking pawls  181 A and  181 B. Therefore, bevel gear  173  is not formed with recesses for engagement with locking pawl  181 . The use of bevel pinion  71  to engage with locking pawl  181  is advantageous in consideration of a case that large bevel gear  73  meshing with bevel pinion  71  may be hard to be formed with recesses  73   b  by sintering or the like. 
     Either a gear locking system  180 A or a gear locking system  180 B serves as gear locking system  180 . Referring to  FIGS. 23 ,  29 ( a ),  29 ( b ), and  29 ( c ), gear locking system  180 A is configured to have a locking pawl  181 A adapted to be directly engaged into one of recesses among gear teeth of bevel pinion  71 .  FIG. 29(   a ) illustrates locking pawl  181 A rotated forward to be engaged in one of the recesses among gear teeth of bevel pinion  71  so as to engage with bevel pinion  71 .  FIG. 29(   b ) illustrates locking pawl  181 A rotated rearward to be removed from the recess of bevel pinion  71  so as to disengage from bevel pinion  71 . 
     Referring to  FIGS. 28 ,  30 ( a ),  30 ( b ), and  30 ( c ), gear locking system  180 B is configured to have a locking pawl  181 B adapted to engage with bevel pinion  71  via an engagement plate  182 . Engagement plate  182  is slidably fitted onto a slant surface of motor swash plate support member  114  so as to be annularly fitted onto a maximum diametric end portion of bevel pinion  71  adjacent to the slant surface of motor swash plate support member  114 . In other words, engagement plate  182  serves as a flange fixed on bevel pinion  71 . To engage with bevel pinion  71 , as shown in  FIG. 30(   a ), locking pawl  181 B is rotated forward to be hooked or engagement plate  182  so as to stop engagement plate  182  rotatably integrated with bevel pinion  71 .  FIG. 30(   b ) illustrates locking pawl  181 B rotated rearward to be separated from engagement plate  182  so as to disengage from bevel pinion  71 . 
     Bevel pinion  71  is disposed at the laterally middle position in transaxle casing  110  so that locking pawl  181  adapted to engage with bevel pinion  71  (directly or via engagement plate  182 ) is fixed on a laterally middle portion of locking shaft  79  in comparison with locking pawl  80  that is fixed on a rightward or leftward portion of locking shaft  79  so as to correspond to bevel gear  73  disposed rightward or leftward from bevel pinion  71 . Therefore, to fasten locking pawl  181  to locking shaft  79 , locking shaft  79  has to be provided with only a single fastening means such as a fastener pin hole at the lateral middle portion thereof so as to reduce the number of processes and costs, in comparison with locking shaft  79  for transaxle  1 , which is formed with two right and left holes  79   a  so that one of holes  79   a  is selected to fasten locking pawl  80  to locking shaft  79  in correspondence to whether bevel gear  73  is disposed rightward or leftward in transaxle casing  10 . 
     Bevel pinion  71  has the engagement portion to engage with locking pawl  181  (directly or via engagement plate  182 ) at or below the lateral middle bottom end of bevel pinion  71  so that the movement direction of rotating bevel pinion  71  at the engagement portion is substantially laterally horizontal parallel to the axial direction of locking shaft  79  while the rotation direction of locking pawl  181  with locking shaft  79  for engaging with bevel pinion  71  is perpendicular to the axial direction of locking shaft  79 . Therefore, an operation force required to rotate locking pawl  181  for engaging with bevel pinion  71  is constant regardless of whether motor shaft  51  rotates clockwise or counterclockwise. On the contrary, if a right or left portion of bevel pinion  71  were adapted to engage with locking pawl  181 , the operation force required to rotate locking pawl  181  for engaging with bevel pinion  71  would be changed depending on whether bevel pinion  71  rotates clockwise or counterclockwise because the right or left portion of bevel pinion  71  moves either upward or downward perpendicularly to the axis of locking shaft  79  and parallel to the rotation direction of locking pawl  181  depending on whether bevel pinion  71  rotates clockwise or counterclockwise. 
     As understood from the above-mentioned features, the setting of relative rotational direction between pump shaft  41  and motor shaft  51  depends on the setting of motor swash plate support member  114  to define the slant angle of fixed swash plate  54  relative to motor shaft  51  or the setting of bevel gear  173  to define whether bevel gear  173  meshes with the right or left side portion of bevel pinion  71 . Therefore, the constant operation force for engagement of locking pawl  181  with bevel pinion  71  regardless of the rotational direction of motor shaft  51  relative to pump shaft  41  means that the operation force is constant regardless of the setting of motor swash plate support member  114  or regardless of the setting of bevel gear  173 . 
     Right and left ends of locking shaft  79  project outward from right and left side surfaces of gear top cover  113 . Locking arm  78  is selectively fixed on either the right or left end of locking shaft  79 . The rotation of locking shaft  79  with locking pawl  181  for engaging with bevel pinion  71  is constant regardless of whether locking arm  78  is fixed on the right or left end of locking shaft  79 , thereby meaning that an operation force to be applied to locking arm  78  for rotating locking shaft  79  for the engagement of locking pawl  181  with bevel pinion  71  is constant regardless of whether locking arm  78  is fixed on the right or left end of locking shaft  79 . 
     For example, the rotation direction of locking shaft  79  for engaging locking pawl  181  with bevel pinion  71  corresponds to the depression direction of a brake pedal for forwardly pulling locking arm  78  via a wire or a rod. Therefore, when a pair of transaxles  1 A are equipped on a vehicle to serve as right and left transaxles  1 R and  1 L, both transaxles  1 A serving as right and left transaxles  1 R and  1 L have respective locking arms  78  that are operatively connected to the brake pedal so that both locking arms  78  are pulled forward together by depressing the brake pedal regardless of whether each transaxle  1 A has locking arm  78  on the right or left side of transaxle casing  110 , regardless of the setting of motor swash plate support member  114  of each transaxle  1 A to define the slant angle of fixed swash plate  54  relative to motor shaft  51 , or regardless of the setting of bevel gear  173  to define whether bevel gear  173  meshes with the right or left side portion of bevel pinion  71 . 
     Referring to  FIGS. 21 ,  23 , and  31 , a structure for supporting gear shaft  72  will be described. Gear shaft  72  has a ball bearing  176  on one axial end portion (in this embodiment, a right end portion) thereof adjacent to bevel gear  173  fixed on gear shaft  72 , so that ball bearing  176  surely journals gear shaft  72  against the weight of bevel gear  173 . On the other hand, gear shaft  72  has a bush  174  on the other axial end portion (in this embodiment, a left end portion) thereof laterally opposite bevel gear  173 . Between bush  174  and ball bearing  176 , bevel gear  173  is fixed on gear shaft  72  so as to mesh with a right or left (in this embodiment, right) portion of bevel pinion  71 , and a pinion  173   a  is fixed on gear shaft  72  so as to mesh with the upper portion of bull gear  77 . 
     As shown in  FIG. 21 , upper transaxle housing  111  has edges along right and left bottom edges of gear for cover  113 . Each of these edges of upper transaxle housing  111  is formed with front and rear horizontal surfaces and is formed with a downward semicircular recess between the front and rear horizontal surfaces. Right and left bearing covers  178  are disposed along right and left vertical portions of gear top cover  113  so as to cover bush  174  and ball bearing  176  and the axial ends of gear shaft  72 , respectively. In this regard, each bearing cover  178  has front and rear horizontal extended portions and a downward semicircular portion between the front and rear horizontal extended portions. The front and rear horizontal extended potions of bearing covers  178  are fitted to the respective front and rear horizontal surfaces of the edges of upper transaxle housing  111 , and the downward semicircular portions of bearing covers  178  are fitted to the respective downward semicircular recesses of the edges of upper transaxle housing  111 . Right and left horizontal upper bearing covers  177  are extended so as to cover the top ends of bush  174  and ball bearing  176  and are fastened by respective bolts  179  at front and rear portions thereof to upper transaxle housing  111  via the front and rear horizontal extended portions of respective bearing covers  178 , thereby journaling the right and left ends of gear shaft  72  with bush  174  and ball bearing  176 . 
     Referring to  FIG. 32 , an alternative simple structure for supporting gear shaft  72  will be described. Upper transaxle housing  111  is formed at an upper surface thereof integrally with right and left bracket portions  111   d  extended upright along the right and left vertical portions of gear top cover  113 . Bracket portions  111   d  of upper transaxle housing  111  are formed with respective sectionally circular bearing holes that laterally penetrate respective bracket portions  111   d,  thereby accommodating bush  174  and ball bearing  175  on the axial end portions of gear shaft  72 . Further, to support ball bearing  176  in the corresponding hearing hole of bracket portion  111   d,  the corresponding bearing hole is formed to have a step for defining the axial position of ball bearing  176 . 
     Such a boring of bearing holes in bracket portions  111   d  for accommodating bush  174  and ball bearing  176  is advantageous to reduce the number of component parts and to accurately and surely journal bush  174  and ball bearing  176  so as to flexibly respond the diametric and/or axial dimensional errors of bush  174  and ball bearing  176  provided on gear shaft  72 , in comparison with the foregoing structure using bearing covers  177  and  178 , which increases the number of component parts and which is hard to flexibly correspond to diametric and/or axial dimensional errors of bush  174  and ball bearing  176  on gear shaft  72 . 
     A transaxle  1 Aa serving as a modification of transaxle  1 A will be described with reference to  FIGS. 33 to 39 . Transaxle  1 Aa has members and portions designated by the same reference numerals as those used for transaxle  1 A. Omission or adaptation of description and reference numerals are employed as those for transaxle  1 A in comparison with transaxle  1  for the same reasons. Further, HST  120  is regarded as being disposed forward from axle  2  as that of transaxle  1 A. In this regard, as understood from  FIG. 35 , transaxle  1 Aa shown in  FIGS. 33 to 39  serves as a left transaxle having left axle  2 L, however, this is just a representative of right and left transaxles having respective right and left axles  2 R and  2 L, similar to representative transaxle  1 A while transaxle  1 A shown in  FIGS. 21 to 32  serves as a right transaxle having right axle  2 R. 
     Referring to  FIG. 33 , transaxle  1 Aa includes a transaxle casing  110 A incorporating HST  120 , reduction gear train  170 , axle  2  (in this embodiment, left axle  2 L) and gear locking system  180  (in this embodiment, gear locking system  180 A), similar to transaxle  1 A. Transaxle casing  110 A includes an upper transaxle housing  111 A and lower transaxle housing  112  joined to each other, and includes a gear top cover  113  mounted on upper transaxle housing  111 A. 
     Referring to  FIGS. 33 to 35 , a center section  30 A used for HST  120  of transaxle  1 Aa is the same as center section  30  except that a bottom portion of pump port block  31  of center section  30 A is different from that of center section  30 . Pump port block  31  of center section  30 A is formed with downwardly projecting filter retainers  31   i  defining an annular horizontal filter mounting surface  31   j  to which the top surface of vertical cylindrical filter  26  is fitted. In this regard, as understood from  FIG. 34 , filter  26  includes a cylindrical main filter member  26   a  having meshes and a filter frame  26   b  surrounding main filter member  26   a.  A horizontal flat top surface of filter frame  26   b  has a width in the radial direction of cylindrical filter  26 , and this width of the top surface of filter frame  26   b  corresponds to the radial width of annular horizontal filter mounting surface  31   j.    
     The bottom potion of pump port block  31  of center section  30 A is formed as a convex bottom  31   k  that projects downward in an irregularly semispherical hill-like shape. Annular horizontal filter mounting surface  31   j  is defined as a circumferential edge surrounding convex bottom  31   k.  Therefore, convex bottom  31   k  projects downward in the vertical columnar space surrounded by filter  26 . Convex bottom  31   k  has a maximum vertical length at a forward portion between right and left charge ports  31   f  and  31   g.  This portion is referred to as maximum vertical length portion  31   k   1 . In other words, maximum vertical length portion  31   k   1  of convex bottom  31   k  of center section  30 A is serves as an apex of the downward convex shape of bottom  31   k  at the lowest end of convex bottom  31   k,  and bottom openings of charge ports  31   f  and  31   g  are directed to maximum vertical length portion  31   k   1 . Convex bottom  31   k  is rather steeply sloped upward as it goes forward from maximum vertical length portion  31   k   1  to a forward portion of annular horizontal filter mounting surface  31   j.  On the other hand, convex bottom  31   k  is rather gently sloped upward as it goes rearward from maximum vertical length portion  31   k   1  to a rearward portion of annular horizontal filter mounting surface  31   j  so as to pass the bottom opening of pump shaft hole  31   d.    
     The upwardly sloped surface of convex bottom  31   k  toward annular horizontal filter mounting surface  31   j  is advantageous to prevent air bubbles from entering charge ports  31   f  and  31   g.  In this regard, if air bubbles entered the fluid in the closed circuit of HST  120  including main fluid passages  36  and  37 , they would cause cavitation of the closed circuit so as to be harmful for operation of HST  120 . Actually, most of all the air bubbles in the fluid in the columnar space surrounded by filter  26  are dispersed at maximum vertical length portion  31   k   1  of convex bottom  31   k  and flow along the upwardly sloped forward surface of convex bottom  31   k  (as an arrow “a” in  FIG. 33 ) and the upwardly sloped rearward surface of convex bottom  31   k  (as an arrow “b” in  FIG. 33 ) away from charge ports  31   f  and  31   g  toward annular horizontal filter mounting surface  31   j,  thereby reducing air bubbles to enter the closed circuit of HST  120  via charge port  31   f  or  31   g.    
     Vertical camshaft  129  is disposed forward from center section  30 A so that cam  129   a  formed on the bottom end portion of camshaft  129  is associated with pressure member  130  for forcibly opening charge check valves  21  similar to that of camshaft  129  in transaxle  1 A. A top end portion of camshaft  129  projects upward from an upper transaxle housing  111 A, and a bypass operation arm  128 A is fixed on the top end portion of camshaft  129 . Bypass operation arm  128 A is formed with a round base portion  128   a,  an arm portion  128   b  extended from round base portion  128   a,  and a square corner portion  128   c  projecting from round base portion  128   a  perpendicular to arm portion  128   b.  Round base portion  128   a  is fitted on the top end portion of camshaft  129  so as to surround the top end portion of camshaft  129 . Arm portion  128   b  is adapted to be joined to a link or the like operative connected to a bypass manipulator provided in a vehicle for forcibly opening both charge check valves  21 . Bypass operation arm  128  of transaxle  1 A also has round base portion  128   a  and arm portion  128   b  although it has not literally described as having such portions. Square corner portion  128   c  is a characteristic portion of bypass operation arm  128 A in comparison with bypass operation am  128  that does not have such a shaped portion. 
     Square corner portion  128   c  of bypass operation arm  128 A projects either rightward or leftward on the assumption that arm portion  128   b  is extended forward from round base portion  128   a.  In this regard, bypass operation arm  128 A has two horizontal surfaces opposite each other so that bypass operation arm  128 A is reversibly fitted onto the top end portion of camshaft  129  to selectively have each horizontal surface as either the top or bottom surface of bypass operation arm  128 A in correspondence to the decision of whether square corner portion  128   c  should project rightward or leftward from round base portion  128   a.    
     Upper transaxle housing  111 A of transaxle  1 Aa is the same as upper transaxle housing  111  of transaxle  1 A except that upper transaxle housing  111 A is formed with stopper edges  111   a,    111   b  and  111   c  in correspondence to bypass operation arm  128 A and is formed with a later-discussed opening  111   f  for a breathing pipe  115 . Stopper edge  111   a  is a laterally vertical surface adapted to constantly contact a rear end of bypass operation arm  128 A regardless of rotation of bypass operation arm  128 A together with camshaft  129 . Right stopper edge  111   b  and left stopper edge  111   c  are formed as vertical surfaces extended rightward and leftward from right and left ends of stopper edge  111   a,  and stepped slightly forward from stopper edge  111   a,  so that one of right and left stopper edges  111   b  and  111   c  is adapted to contact arm portion  128   b  of bypass operation arm  128 A. 
     Referring to  FIG. 38(   a ) and  38 ( c ), regardless of whether bypass operation arm  128 A is fitted on camshaft  129  so as to have square corner portion  128   c  projecting rightward or leftward, square corner portion  129   c  comes to the rear end of bypass operation arm  128 A so as to contact stopper edge  111   a  when arm portion  128   b  is extended forward. In this state, bypass operation arm  128 A is unrotatable to further move arm portion  128   b  rightward or leftward toward square corner port  128   c  contacting stopper edge  111   a,  but is rotatable to move arm portion  128   b  rightward or leftward opposite square corner portion  128   c.  Referring to  FIG. 38(   a ), when square corner portion  128   c  projects rightward to contact stopper edge  111   a,  bypass operation arm  128 A is unrotatable further rightward but is rotatable leftward. Referring to  FIG. 38(   c ), when square corner portion  128   c  projects leftward to contact stopper edge  111   a,  bypass operation arm  128 A is unrotatable further leftward but is rotatable rightward. 
     On the other hand, referring to  FIGS. 38(   b ) and  38 ( d ), regardless of whether bypass operation arm  128 A is fitted on camshaft  129  so as to have square corner portion  128   c  projecting rightward or leftward, arm portion  128   b  contacts one of right and left stopper edges  111   b  and  111   c  when arm portion  128   b  is extended rightward or leftward. In this state, bypass operation arm  128 A is rotatable rightward or leftward freely from stopper edge  111   a  because square corner portion  128   c  does not contact stopper edge  111   a  so that only the round edge of round base portion  128   a  contacts stopper edge  111   a.  However, referring to  FIG. 38(   b ), arm portion  128   b  contacting left stopper edge  111   c  prevents bypass operation arm  128 A from further rotating leftward but allows bypass operation arm  128 A to rotate only rightward. Referring to  FIG. 38(   d ), arm portion  128   b  contacting right stopper edge  111   c  prevents bypass operation arm  128 A from further rotating rightward but allows bypass operation arm  128 A to rotate only leftward. 
     Any one of the four states of bypass operation arm  128 A shown in  FIGS. 38(   a ) to  38 ( d ) may be selectively defined as a non-bypass position (a normal position) of bypass operation arm  128 A to allow operation of charge check valves  21  free from pressure member  130 . This selection of non-bypass position of bypass operation arm  128 A depends on whether bypass operation arm  128 A and camshaft  129  should be rotated rightward (clockwise) or leftward (counterclockwise) to reach a bypass position for forcibly opening charge check valves  21 , and depends on which of the forward, rightward and leftward directions bypass operation arm  128 A should be extended when it is located at its non-bypass (or bypass) position, in consideration of a design of transaxle  1 A. For example, if bypass operation arm  128 A has to be extended forward at the non-bypass position and has to be rotated rightward to reach the bypass position, the state of bypass operation arm  128 A shown in  FIG. 38(   c ) is set as that at the non-bypass position, and the state of bypass operation arm  128 A shown in  FIG. 38(   d ) is sot as that at the bypass position. 
     In this regard, cam  129   a  formed on the bottom portion of camshaft  129  extends leftward or rightward from camshaft  129  at the non-bypass position, and extends rearward from camshaft  129  at the bypass position. If cam  129   a  extends leftward at the non-bypass position, the rotation of camshaft  129  from the non-bypass position to the bypass position by 90 degrees should be leftward (counterclockwise). Therefore, the state of bypass operation arm  128 A shown in  FIG. 38(   a ) should be selected to correspond to cam  129   a  extended leftward at the non-bypass position, so that the state of bypass operation arm  128 A shown in  FIG. 38(   b ) corresponds to cam  129   a  extended rearward at the bypass position. Alternatively, the state of bypass operation arm  128 A shown  FIG. 38(   d ) may be selected to correspond to cam  129   a  extended leftward at the non-bypass position, so that the state of bypass operation arm  128 A shown in  FIG. 38(   c ) corresponds to cam  129   a  extended rearward at the bypass position. 
     On the contrary, if cam  129   a  extends rightward at the non-bypass position, the rotation of camshaft  129  from the non-bypass position to the bypass position by 90 degrees should be rightward (clockwise). Therefore, the state of bypass operation arm  128 A shown in  FIG. 38(   c ) should be selected to correspond to cam  129   a  extended rightward at the non-bypass position, so that the state of bypass operation arm  128 A shown in  FIG. 38(   d ) corresponds to cam  129   a  extended rightward at the bypass position. Alternatively, the state of bypass operation arm  128 A shown in  FIG. 38(   b ) may be selected to correspond to cam  129   a  extended rightward at the non-bypass position, so that the state of bypass operation arm  128 A shown in  FIG. 38(   a ) corresponds to cam  129   a  extended rearward at the bypass position. 
     Breathing pipe  115  will be described with reference to  FIGS. 33 ,  36  and  39 . In this regard, cradle-type movable swash plate  44  is slidably rotatably fitted to a dome-shaped ceiling  111   e  of upper transaxle housing  111 A (corresponding to pump support portion  11   b  of transaxle housing  11 ) covering the upper portion of hydraulic pump  40  of HST  120 . In this regard, a gap between movable swash plate  44  and ceiling  111   e  of upper transaxle housing  111 A tends to catch air in lubricating fluid in upper transaxle housing  111  so as to act as an air pocket wrongly influencing flow of fluid for operating HST  120  because the lubricating fluid is also used as operation fluid of HST  120 . Accurate design of ceiling  111   e  for reducing the gap space between ceiling  111   e  and movable swash plate  44  may contribute to reduction of air in the gap space, however, it is costly. Increase of fluid for transaxle  1 Aa may contribute to increase of lubricating fluid in the gap space between ceiling  111   e  and movable swash plate  44  so as to reduce air in the gap space, however, it is also costly, and increase of lubricating fluid may increase agitation resistance of gears of reduction gear train  170 . 
     Therefore, breathing pipe  115  for breathing air from the gap space between ceiling  111   e  and movable swash plate  44  is interposed between upper transaxle housing  111  and a gear top cover  113 A. In this regard, a rear portion of ceiling  111   e  is formed with a vertical opening  111   f  that is open to the gap space between ceiling  111   f  and movable swash plate  44 . An L-shaped breathing pipe  115  is fitted at a vertical end portion thereof into opening  111   f.  On the other hand, gear top cover  113 A is the same as gear top cover  113  of transaxle casings  110  of transaxle  1 A except that gear top cover  113 A is formed with a fore-and-aft horizontal opening  113   b  that is open to an upper space in gear top cover  113 A above reduction gear train  170 . Air in this upper space in gear top cover  113 A hardly influences to be contaminated in lubricating fluid in transaxle casing  110 A of transaxle  1 Aa. Another horizontal end portion of breathing pipe  115  is fitted into opening  113   b.  Therefore, air contaminated in the gap space in upper transaxle housing  111 A rises into the vertical end portion of breathing pipe  115  in opening  111   f,  flows through breathing pipe  115 , and is released to the upper space in gear top cover  113 A via the horizontal end portion of breathing pipe  115  in opening  113   b,  thereby reducing the contamination of lubricating fluid with air, and thereby making the circulation of fluid for operating HST  120  free from the air contamination of fluid in the gap space between swash plate  44  and ceiling  111   e  of upper transaxle housing  111 A. 
     Incidentally, breathing pipe  115  may be formed in another shape than the L-shape, and openings  111   f  in upper transaxle housing  111 A and opening  113   a  of gear top cover  113 A may be designed in shape or arrangement in another way, as far as the required function of breathing pipe  115  for releasing air from the gap space between ceiling  111   e  and movable swash plate  44  to an appreciate space like the upper space in top cover  113 A is ensured. Further, the name “breathing pipe” of the pipe designated by the reference numeral “ 115 ” does not merely mean a pipe for passage of only air but also means a pipe for passage of fluid contaminated with air. In other words, pipe  115  serves as an air passage, that also means a fluid passage for flow of fluid contaminated with air. This is also adapted to a later-discussed “breathing pipe  115 A” of a transaxle  1 Ab shown in  FIGS. 40 and 43 . 
     An alternative transaxle  1 Ab will be described with reference to  FIGS. 40 to 42  on the same assumption as adapted to the description of transaxle  1 Aa based on the description of transaxle  1 A (and transaxle  1 ). For example, transaxle  1 Ab is assumed to be arranged so as to have HST  120  disposed forward from reduction gear train  170  and axle  2 . 
     Transaxle  1 Ab includes HST  120  including hydraulic pump  40  and hydraulic motor  50 , however, HST  120  of transaxle  1 Ab is provided with a charge pump mechanism in comparison with HST  120  of transaxle  1 Aa (or  1 A) which includes center section  30 A (or  30 ) having charge pots  31   f  and  31   g  directly open to the fluid sump in transaxle casing  110 A (or  110 ) so as to supply fluid to the closed fluid circuit of HST  120  without a charge pump. 
     A transaxle casing  110 B of transaxle  1 Ab includes an upper transaxle housing  111 Aa, a lower transaxle housing  112 A and gear top cover  113 A. Gear top cover  113 A is the same as gear top cover  113 A of transaxle casing  110 A of transaxle  1 Aa. Upper transaxle housing  111 Aa is discussed later. Similar to lower transaxle housing  112  (not literally described in the above description), lower transaxle housing  112 A is fainted so that a front portion of lower transaxle housing  112 A for housing a lower portion of HST  120  (e.g., a center section  30 B of HST  120 ) is deeper than a rear portion of lower transaxle housing  112 A so as to have a lateral vertical wall at a fore-and-aft intermediate portion between the front and rear portions. Especially, the front portion of lower transaxle housing  112 A (or  112 ) will be referred to as a “deeper front portion” hereinafter. The lateral vertical wall of lower transaxle housing  112 A at the fore-and-aft intermediate portion, i.e., the rear end of the deeper front portion, is formed with an outwardly open filter hole  112   a  for inserting or releasing a later-discussed filter  126  into and from transaxle casing  110 B in comparison with the rear end of the deeper front portion of lower transaxle housing  112  that has no hole for inserting or releasing a filter into and from transaxle casing  110 A. Further, a front end portion of transaxle housing  111 Aa is formed with right and left outwardly open port holes  112   b  and  112   c  for fitting later-discussed outwardly open port members  205  and  206  therein in comparison with the front end portion of lower transaxle housing  112  that does not have such an outwardly open hole. 
     HST  120  of transaxle  1 Ab includes a center section  30 B in which pump port block  31  has a bottom surface that is not convex but is flat in comparison with pump port block  31  of center section  30 A having convex bottom  31   k.  The charge pump mechanism of transaxle  1 Ab includes a charge pump housing  200  and a charge pump  201  fitted in charge pump housing  200 . Charge pump housing  200  has a flat top surface contacting the flat bottom surface of pump port block  31  of center section  30 B. Vertical bolts  208  are passed through bolt holes of center section  30 B so as to fasten center section  30 B to transaxle casing  110 B of transaxle  1 Ab. 
     Charge pump housing  200  is formed at the top surface thereof with an upwardly open discoid recess serving as a charge pump chamber  200   a  in which charge pump  201  is fitted. Charge pump  201  is a trochoid pump including an outer rotor  201   a  and an inner rotor  201   b  surrounded by outer ring  201   a.  Outer rotor  201   a  is fitted at an outer peripheral surface thereof to an inner peripheral surface of charge pump chamber  200   a  slidably and rotatably relative to charge pump housing  200 . Inner rotor  201   b  is fixed on a bottom portion of pump shaft  41  and engages with outer rotor  201   a  so that outer and inner rotors  201   a  and  201   b  rotate according to rotating pump shaft  41 . In this regard, a bottom portion of vertical hydraulic pump  40  of transaxle  1 Ab disposed in vertical pump shah hole  31   d  formed through pump port block  31  of center section  30 B is further extended downward into pump chamber  200   a  of pump housing  200  below center section  30 B so as to be fixedly provided thereon with inner rotor  201   b.  Top surfaces of outer and inner rotors  201   a  and  201   b  are leveled with the top surface of pump housing  200  contacting the flat bottom surface of pump port block  31  of center section  30 B. 
     A bottom portion of charge pump housing  200  is fixed to a bottom portion of the deeper front portion of lower transaxle housing  112 A. A filter retaining recess  200   b  is open at a rear end portion of charge pump housing  200  in correspondence to filter hole  112   a  of lower transaxle housing  112 A. A cylindrical mesh filter  126  is fitted at a rear end portion thereof into filter retaining recess  200   b  and is extended rearward from charge pump housing  200  so as to have a fore-and-aft horizontal axis of its cylindrical shape. A cap  126   a  is fitted into filter hole  112   a  so as to close filter hole  112   a,  and a front end of filter  126  is fitted to cap  126   a  fitted in filter hole  112   a.  In this way, filter  126  is disposed in the deeper front portion of lower transaxle housing  112 A so as to be submerged in a fluid sump ins transaxle casing  110 B. 
     When filter  126  has to be exchanged for a new one, cap  126   a  is removed from filter hole  112   a  so as to open filter hole  112   a,  and filter  126  is moved rearward away from charge pump housing  200  and is released from transaxle casing  110 B via filter hole  112   a.  Alternatively, filter  126  may engage with cap  126   a  so as to be movable together with cap  126   a,  so that filter  126  can be released together with cap  126   a  from transaxle casing  110 B when cap  126   a  is moved away from filter hole  112   a.  When filter  126  is inserted into transaxle casing  110 B via filter hole  112   a  of tower transaxle housing  112 A, vice versa. 
     Charge pump housing  200  is formed therein with a suction port  200   d.  Suction port  200   d  is arcuate when viewed in plan, and a top end of suction port  200   d  is open at a bottom surface of a rear portion of charge pump chamber  200   a.  Charge pump housing  200  is formed therein with a suction hole  200   c  extended fore-and-aft horizontally between filter retaining recess  200   b  and suction port  200   d.  Therefore, charge pump  201  sucks fluid from the fluid sump in transaxle casing  110 B via filter  126 , filter retaining recess  200   b,  suction hole  200   c  and suction port  200   d.  In other words, filter retaining recess  200   b,  suction hole  200   c  and suction port  200   d  constitute a suction fluid passage  210  (see  FIG. 43 ) in transaxle casing  110 B to supply fluid from filter  126  to charge pump  201 . 
     Charge pump housing  200  is formed therein with a delivery port  200   e.  Delivery port  200   e  is arcuate when viewed in plan, and a top end of delivery port  200   e  is open at the bottom surface of a front portion of charge pump chamber  200   a.  In the plan view, arcuate suction port  200   d  and arcuate delivery port  200   e  are symmetric with respect to the vertical axis of pump shaft  41 . Charge pump housing  200  is formed therein with a port hole  200   f  extended forward from delivery port  200   e  so as to open at a front end surface of pump housing  200  in correspondence to port hole  112   b  of lower transaxle housing  112 A. Port member  205  is passed through port hole  112   b  and is fitted at a rear end thereof into the open front end of port hole  200   f.  Therefore, charge pump  201  delivers fluid from delivery port  200   e  to pump member  205  via port hole  200   f.  In other words, delivery port  200   e  and port hole  200   f  constitute a delivery fluid passage  211  (see  FIG. 4 ) in transaxle casing  110 B to supply fluid delivered from charge pump  201  to port member  205 . 
     An outwardly open front end of port member  205  is disposed forward from the front end surface of lower transaxle housing  112 A so as to serve as a fluid extraction port for supplying fluid delivered from charge pump  201  via delivery port  200   e  and port hole  200   f  to an external hydraulic implement  220  disposed outside of transaxle casing  110 B of transaxle  1 Ab. External hydraulic implement  220  is fluidly connected to port member  205  via a fluid pipe  225  joined to port member  205 . 
     Port hole  200   f  is extended from one of right and left end portions (in this embodiment, the right end portion) of delivery port  200   e.  A relief valve  202  is fitted into a side portion of charge pump housing  200  laterally opposite port hole  200   f  and is joined to the other of right and left end portions (in this embodiment, the left end portion) of delivery port  200   e.  Therefore, relief valve  202  serves as an implement pressure regulation valve  202  for regulating the pressure of fluid supplied to external hydraulic implement  220  via port member  205 . Fluid released from implement pressure regulation valve  202  is recovered to fluid in a later-discussed charge fluid passage  212  (see  FIG. 43 ) to be charged to HST  120 . Incidentally, relief valve  202  is extended horizontally, however, slantwise in the fore-and-aft and lateral directions so as to be prevented from interfering with the left side portion of lower transaxle housing  110 B and so as to ensure a space for a junction of a later-discussed charge port  200   i  to a later-discussed vertical fluid hole  200   h.    
     Charge pump housing  200  is formed with an upwardly open groove serving as a charge port  200   i  at the top surface thereof. Charge port  200   i  is substantially C-shaped when viewed in plan so as to surround front and rear portions and one of right and left portions (in this embodiment, the left portion) of charge pump chamber  200   a.  A vertical fluid hole  200   h  is formed in charge pump housing  200  so as to extend downward from a leftward front portion of charge port  200   i.  A fore-and-aft horizontal port hole  200   g  is formed in charge pump housing  200  so as to extend forward from vertical fluid hole  200   h  and is open at the front end surface of charge pump housing  200  in correspondence to port hole  112   c  of lower transaxle housing  112 A 
     Port member  206  is passed through port hole  112   c  and is fitted at a rear end thereof into the open front end of port hole  200   g.  An outwardly open front end of port member  206  is disposed forward from the front end surface of lower transaxle housing  112 A, and is fluidly connected to external hydraulic implement  220  via a fluid pipe  226 . Therefore the opening of port member  206  serves as a fluid introduction port for introducing fluid discharged from external hydraulic implement  220  to charge port  200   i  via port hole  200   g  and vertical fluid hole  200   h.  In other words, port hole  200   g,  vertical fluid hole  200   h  and charge port  200   i  constitute a charge fluid passage  212  (see  FIG. 43 ) in transaxle casing  110 B to supply fluid discharged from external hydraulic implement  220  to the closed fluid circuit of HST  120 . 
     Port holes  200   e  and  200   f  in the front end portion of charge pump housing  200  are juxtaposed right and left, and port holes  112   b  and  112   c  in the front end portion of lower transaxle housing  112 A are juxtaposed right and left so as to correspond to right and left port holes  200   e  and  200   f,  whereby port member  205  serving as the fluid extraction port and port member  206  serving as the fluid introduction port are juxtaposed right and left, thereby preventing lower transaxle housing  112 A and charge pump housing  200  from being expanded vertically. 
     Pump port block  31  of center section  30 B is formed therein with right and left charge ports  31   f   1  and  31   g   1 . Right and left charge ports  31   f   1  and  31   g   1  are joined at top ends thereof to right and left main fluid passages  36  and  37  in center section  30 B, respectively, and are joined at bottom ends thereof to right and left portions of the front portion of charge port  200   i  of charge pump housing  200 . In this regard, the junctions of main fluid passages  36  and  37  to the top ends of charge ports  31   f   1  and  31   g   1  have to be considerably distant rearward from the front ends of main fluid passages  36  and  37 , i.e., the front end of center section  30 B, so as to correspond to charge check valves  121  disposed in front portions of main fluid passages  36  and  37 . On the other hand, as noticed from  FIG. 41  that an imaginary circle drawn along the C-shape of charge port  200   i  in the plan view has a great diameter so that the front portion of charge port  200   i  approaches the front end of center section  30 B in the fore-and-aft direction of transaxle  1 Ab. Therefore, charge ports  31   f   1  and  31   g   1  are bent in side view as understood from  FIGS. 40 and 42  so that the bottom ends of charge ports  31   f   1  and  31   g   1  joined to the front portion of charge port  200   i  are disposed forward from the top ends of charge ports  31   f   1  and  31   g   1  joined to fluid passages  36  and  37 . 
     Further, a relief valve  203  is fitted into charge pump housing  200  and is joined to the rear portion of charge port  200   i  so as to serve as a charge pressure regulation valve for regulating the pressure of fluid in charge port  200   i  before supplied to main fluid passage  36  or  37 . An auxiliary charge check valve  204  is disposed in charge pump housing  200  so as to be interposed between suction hole  200   c  and the rear portion of charge port  200   i.  Auxiliary charge check valve  204  is adapted to only allow the flow fluid from the fluid sump in transaxle casing  110 B to charge port  200   i,  thereby preventing the dosed fluid circuit of HST  120  from lacking fluid when pump shaft  41  and charge pump  201  are not driven. 
     Features of transaxle  1 Ab other than the charge pump mechanism will be described. As shown in  FIG. 40 , transaxle  1 Ab uses a horizontal straight breathing pipe  115 A in comparison with L-like bent breathing pipe  115  of transaxle  1 Aa. In this regard, a front end portion of breathing pipe  115 A is not bent vertically but is extended horizontally as a rear end portion of breathing pipe  115 A fitted into opening  113   b  of gear top cover  113 A. Upper transaxle housing  111 Aa is the same as upper transaxle housing  111 A except that upper transaxle housing  111 Aa is formed at a rear portion of dome-shaped ceiling  111   e  with a horizontal opening  111   h  instead of a vertical opening such as vertical opening  111   f.  The horizontal front end portion of breathing pipe  115 A is fitted into horizontal opening  111   h  so that horizontal breathing pipe  115 A is interposed between upper transaxle housing  111 Aa and gear top cover  113 A. 
     A front end of horizontal opening  111   h  open into transaxle casing  110 B is higher than the bottom end of vertical opening  111   f  open into transaxle casing  110 A so that the front end of horizontal opening  111   h  is disposed rearwardly adjacent to bearing  41   a  journaling pump shaft  41  in ceiling  111   e  of upper transaxle housing  111 . Therefore, an air guide hole  111   g  is formed in ceiling  111   e  along a roar portion of bearing  41   a  so as to be open at a bottom end thereof to the gap between ceiling  111   e  and movable swash plate  44  and so as to be joined to the front end of horizontal opening  111   h.  Accordingly, air in the gap between ceiling  111   e  and movable swash plate  44  is introduced into the front end portion of breathing pipe  115 A via air guided hole  111   g  and is expelled into the space surrounded by gear top cover  113 A via breathing pipe  115 A. Straight breathing pipe  115 A is advantageous in reduction of manufacturing processes and costs and in facilitation of inventory control. 
     Locking pawl  181  (in this embodiment,  181 A, however, it may be  181 B) is provided with a weight  181   a  for balancing locking pawl  181  at its unlocking position. In other words, weight  181   a  directs locking pawl  181  to disengage from bevel pinion  71 . Therefore, locking pawl  181  can disengage from bevel pinion  71  as soon as a manipulator having been operated to locking bevel pinion  71  with locking pawl  181  is operated to unlock bevel pinion  71 . Alternatively, a torque spring may be coiled around locking shaft  79  if the torque spring has the function equivalent to weight  181   a  for locking pawl  181 . 
     The illustration of straight breathing pipe  115 A and locking pawl  181  with weight  181   a  in this embodiment of transaxle  1 Ab does not mean that straight breathing pipe  115 A and locking pawl  181  with weight  181   a  have to be employed by a transaxle having the charge pump mechanism. In other words, the transaxle ( 1 ,  1 A or  1 Aa) having no charge pump for supplying fluid to the HST ( 20  or  120 ) may employ a straight breather pipe, such as pipe  115 A, for breathing air from the air pocket along movable swash plate  44  of hydraulic pump  40  and/or a locking pawl with a weight, such as locking pawl  181  with weight  181   a,  for balancing the locking pawl at the unlocking position. 
     An embodiment of a zero-turn vehicle shown in  FIG. 43  will be described. The zero-turn vehicle includes right and left transaxles carrying respective right and left axles  2 R and  2 L of respective right and left drive wheels  3 R and  3 L. Each of the right and left transaxles includes HST  120  (or  20 ) having hydraulic pump  40 , hydraulic motor  50  and main fluid passages  36  and  37  fluidly connecting hydraulic pump  40  to hydraulic motor  50 . Transaxle  1 Aa represents one of the right and left transaxles (in this embodiment, the left transaxle carrying left axle  2 L), which includes no charge pump for supplying fluid to HST  120 , and transaxle  1 Ab represents the other of the right and left transaxles (in this embodiment, the right transaxle carrying right axle  2 R), which includes charge pump  201  for supplying fluid to HST  120 . 
     This zero-turn vehicle includes an engine  230  having an engine output pulley  231  that is drivingly connected to input pulleys  14  on pump shafts  41  of hydraulic pumps  40  of left and right transaxles  1 Aa and  1 Ab via a belt  232 , so that pump shafts  41  are rotated by the output power of engine  230 . In transaxle  1 Ab, charge pump  201  is driven together with hydraulic pump  40  by rotating pump shaft  41 . 
     In this embodiment, a hydraulic cylinder  221  serves as typical external hydraulic implement  220 . A piston  222  serving as a hydraulic actuator is disposed in hydraulic cylinder  221  so that piston  222  divides an inside space of hydraulic cylinder  221  into fluid chambers  221   a  and  221   b.  Fluid chamber  221   a  is fluidly connected via fluid pipe  225  to fluid extraction port (port member)  205 . Fluid chamber  221   b  is fluidly connected via fluid pipe  226  with a line filter  224  to fluid introduction port (port member)  206  for introducing fluid from fluid chamber  221   b  of hydraulic cylinder  221  into transaxle casing  110 B. 
     A switching valve  223  is interposed between fluid pipes  225  and  226 . Switching valve  223  is manipulatable to be shifted between an opening position and a closing position. During the rotation of pump shaft  41 , when switching valve  223  is set at the closing position so as to separate fluid pipes  225  and  226  from each other, fluid delivered from charge pump  201  is supplied to fluid chamber  221   a  of hydraulic cylinder  221  so as to push piston  222  in one direction to thrust out a piston rod extended from piston  222 . During the movement of piston  222 , fluid is discharged from fluid chamber  221   b  to fluid introduction port  206 , and flows into charge fluid passage  212  via a charge check valve  207 . After piston  222  reaches its maximum stroke, fluid delivered from charge pump  201  is released from implement pressure regulation valve  202  to charge fluid passage  212 . 
     During the rotation of pump shaft  41 , when switching valve  223  is set at the opening position so as to fluidly connect fluid pipes  225  and  226  to each other, fluid delivered from charge pump  201  flows through switching valve  223  so as to bypass hydraulic cylinder  221 . A part of fluid having passed switching valve  223  flows to fluid introduction port  206  via fluid pipe  226  and line filter  224 . The remaining part of fluid having passed switching valve  223  flows to fluid chamber  221   b  of hydraulic cylinder  221  via fluid pipe  226  so as to push piston  222  in the other direction to withdraw the piston rod. Therefore, fluid is discharged from fluid chamber  221   a  into fluid pipe  225  against the fluid flow from fluid extraction port  205 , however, this fluid is introduced to fluid pipe  226  via switching valve  223 . 
     In this way, HST  120  of transaxle  1 Ab has charge pump  201  for supplying fluid to main fluid passages  36  and  37  via external hydraulic implement  220  outside of transaxle casing  110 B. On the contrary, HST  120  of transaxle  1 Aa does not have such a charge pump, however, charge ports  31   f  and  31   g  directly open to the fluid sump in transaxle casing  110 A so as to naturally suck fluid from the fluid sump for sufficient fluid supply to the closed fluid circuit of HST  120 . 
     Incidentally, referring to  FIG. 43 , this zero-turn vehicle is provided with a reservoir tank  233  that is fluidly connected to the fluid sump in transaxle casing  110 B of transaxle  1 Ab via a fluid pipe  234  and that is fluidly connected to the fluid sump in transaxle casing  110 A of transaxle  1 Aa via a fluid pipe  235 , thereby regulating the fluid sumps in transaxle casings  110 A and  110 B of transaxles  1 Aa and  1 Ab in volume and pressure. 
     It is further understood by those skilled in the art that the foregoing description is given of preferred embodiments of the disclosed apparatus and that various changes and modifications may be made in the invention without departing from the scope thereof defined by the following claims.