Abstract:
A transaxle is adaptable to a vehicle equipped with a longitudinal engine including an engine output shaft to be oriented in a longitudinal direction of a vehicle. The transaxle comprises a transmission output shaft to be oriented in a lateral direction of the vehicle so as to be drivingly connected to a drive wheel of the vehicle, a hydrostatic transmission and a mechanical transmission for transmitting power from the engine output shaft to the transmission output shaft, and first and second power take-off shafts for taking off power from the mechanical transmission. The engine is joined to the transaxle so that the engine, the hydrostatic transmission and the mechanical transmission are assembled together. The first and second power take-off shafts are distributed rightward and leftward from the engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Application No. 2014-092768, filed on Apr. 28, 2014. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a configuration of an apparatus serving as a transaxle for a working vehicle equipped with a longitudinal engine, and relates to a working vehicle equipped with such an apparatus. 
     2. Related Art 
     An engine having a horizontal engine output shaft extended in the longitudinal (i.e., fore-and-aft) direction of a vehicle is referred to as a “longitudinal engine.” There is a well-known conventional working vehicle (e.g., a utility vehicle) equipped with a longitudinal engine and a transmission assembly including a hydrostatic transmission (hereinafter, “HST”) and a mechanical transmission (e.g., a gear transmission), as disclosed by JP 2007-22379 A. In this vehicle, the HST and the mechanical transmission are configured to transmit power from the longitudinal engine output shaft to a transmission output shaft (e.g., right and left differential output shafts) extended in the lateral direction of the vehicle and drivingly connected to an axle of a drive wheel. The mechanical transmission is arranged to have its transmission shafts extended parallel to the transmission output shaft (i.e., in the lateral direction of the vehicle), thereby being minimized in the longitudinal direction of the vehicle. The HST is disposed on one of right and left sides of the mechanical transmission so as to extend laterally from the mechanical transmission instead of extending rearward from the mechanical transmission, thereby improving the turning performance of the vehicle, and improving the protection of the HST. 
     The vehicle has some problems. First, the vehicle is wrongly balanced so as to spoil its stability in traveling because the heavy engine is disposed laterally eccentrically in the vehicle. Second, the vehicle has to increase its size and the number of its component parts because the engine is separated from the transmission assembly and needs its own fixture member and a space separated from the transmission assembly. Third, the working vehicle also has to increase its size, especially in the longitudinal direction, because it has several power take-off (hereinafter, “PTO”) shafts including a front wheel driving PTO shaft and a working device driving PTO shaft, the front wheel driving PTO shaft being offset laterally from the engine, while the working device driving PTO shaft being extended rearward from the transmission assembly. Fourth, the transmission assembly has to have a portion where plural PTO shafts are collected and supported, thereby reducing durability of component parts in the portion and tending to cause noise from the portion. 
     SUMMARY OF THE INVENTION 
     A first object of the invention is to provide a transaxle appropriately configured for a working vehicle that should have a longitudinal engine and at least two PTO shafts. 
     To achieve the object, a transaxle according to the invention is adaptable to a vehicle equipped with a longitudinal engine including an engine output shaft to be oriented in a longitudinal direction of the vehicle. The transaxle comprises a transmission output shaft to be oriented in a lateral direction of the vehicle so as to be drivingly connected to a drive wheel of the vehicle, an HST and a mechanical transmission for transmitting power from the engine output shaft to the transmission output shaft, and first and second PTO shafts for taking off power from the mechanical transmission. The engine is joined to the transaxle so that the engine, the HST and the mechanical transmission are assembled together. The first and second PTO shafts are distributed rightward and leftward from the engine. 
     Due to the configuration of the transaxle, the heavy longitudinal engine can be disposed in the lateral middle portion of the vehicle so as to laterally balance the vehicle. Further, due to the assembling of the engine together with the HST and the mechanical transmission, the engine approaches the HST and the mechanical transmission, so that a common support member can be used to support the engine, the HST and the mechanical transmission, and a space for mounting the engine can be reduced so as to reduce a size of a fixture member for the engine, thereby minimizing the vehicle and reducing costs for supplying component parts. Further due to the distribution of the first and second PTO shafts, the vehicle is further minimized, and the mechanical transmission is released from a load caused by collecting PTO shafts at only one place, thereby enhancing the durability of component parts and reducing noise. 
     Preferably, the transaxle further comprises a transmission casing of the mechanical transmission, a common input shaft for the HST and the mechanical transmission, and first and second drive trains disposed in the transmission casing. The transmission casing includes upper and lower portions. The input shaft is drivingly connected to the engine output shaft. The HST is attached to a right or left outer side of the upper portion of the transmission casing so as to extend laterally outward from the upper portion of the transmission casing. The input shaft and the transmission output shaft are disposed at an equal level in the lower portion of the transmission casing. The first drive train is extended upward from the input shaft so as to transmit power from the input shaft to the HST. The second drive train is extended downward from the HST to the transmission output shaft so as to transmit power from the HST to the transmission output shaft. 
     Due to the configuration of the transaxle, a level of a fluid sump in the transmission casing can be set so that only the input shaft, the transmission output shaft, and gears on the input shaft and the transmission output shaft are submerged in a fluid sump in the transmission casing. Remaining components for the mechanical transmission are disposed above the level of the fluid sump so as to be lubricated directly or via the first or second drive train with fluid splashed up from the fluid sump agitated by the gears on the input shaft and the transmission output shaft. Therefore, the fluid sump can be volumetrically reduced so as to reduce agitation resistance of the fluid and heating of the fluid, thereby reducing energy loss and running costs. 
     Preferably, the transaxle further comprises a transmission casing of the mechanical transmission, a flywheel casing, a differential unit and a reservoir tank. The flywheel casing is disposed between the engine and the transmission casing. The transmission casing includes a first portion close to the flywheel casing and a second portion away from the flywheel casing. The differential unit is disposed in the second portion of the transmission casing and is drivingly connected to the transmission output shaft. The reservoir tank is disposed in a space below the first portion of the transmission casing between the flywheel casing and the second portion of the transmission casing and is fluidly connected to an inside of the transmission casing. 
     Therefore, such a dead space is used for arranging the reservoir tank, thereby ensuring the compactness of the transaxle joined to the engine and provided with the reservoir tank. The reservoir tank can be used for a stable fluid supply for hydraulic devices in the vehicle, including the HST, while a fluid sump in the inside of the transmission casing may be unstable for such a fluid supply because of tilt of the vehicle during traveling, for example. Further, due to the reservoir tank, the fluid sump in the inside of the transmission casing can be volumetrically reduced so as to reduce agitation resistance of the fluid and heating of the fluid, thereby reducing energy loss and running costs. 
     A second object of the invention is to provide a working vehicle appropriately configured to have a longitudinal engine and at least two PTO shafts. 
     To achieve the second object, a vehicle according to the invention comprises a longitudinal engine and a transaxle. The engine includes an engine output shaft to be oriented in a longitudinal direction of the vehicle. The transaxle includes a transmission output shaft to be oriented in a lateral direction of the vehicle so as to be drivingly connected to a drive wheel of the vehicle, an HST and a mechanical transmission for transmitting power from the engine output shaft to the transmission output shaft, and first and second PTO shafts for taking off power from the mechanical transmission. The engine is joined to the transaxle so that the engine, the HST and the mechanical transmission are assembled together. The first and second PTO shafts are distributed rightward and leftward from the engine. 
     Due to the configuration of the vehicle, the vehicle can have the heavy longitudinal engine mounted at the lateral middle portion of the vehicle, thereby being laterally balanced. Further, due to the assembling of the engine together with the HST and the mechanical transmission, the engine approaches the HST and the mechanical transmission, so that a common support member can be used to support the engine, the HST and the mechanical transmission, and a space for mounting the engine can be reduced so as to reduce a size of a fixture member for the engine, thereby minimizing the vehicle and reducing costs for supplying component parts. Further due to the distribution of the first and second PTO shafts, the vehicle is further minimized, and the mechanical transmission is released from a load caused by collecting PTO shafts at only a place, thereby enhancing the durability of component parts and reducing noise. 
     Preferably, the transaxle in the vehicle has the above-mentioned preferable configuration so as to bring the above-mentioned effects on the vehicle. 
     These and other objects, features and advantages of the invention 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 an entire working vehicle equipped with an engine-transaxle assembly including a transaxle. 
         FIG. 2  is a schematic plan view of the engine-transaxle assembly. 
         FIG. 3  is a sectional side view of the engine-transaxle assembly. 
         FIG. 4  is a cross sectional view of the engine-transaxle assembly taken along the X-X line of  FIG. 3 , showing a first drive train. 
         FIG. 5  is a cross sectional view of the engine-transaxle assembly taken along the Y-Y line of  FIG. 3 . 
         FIG. 6  is a cross sectional view of the engine-transaxle assembly taken along the Z-Z line of  FIG. 3 , showing a second drive train. 
         FIG. 7  is a cross sectional view of the engine-transaxle assembly taken along the U-U line of  FIG. 3 . 
         FIG. 8  is a cross sectional view of the engine-transaxle assembly taken along the V-V line of  FIG. 3 . 
         FIG. 9  is a hydraulic circuit diagram for the engine-transaxle assembly. 
         FIG. 10  is a sectional plan view of a valve unit. 
         FIG. 11  is a sectional side view of an alternative engine-transaxle assembly. 
         FIG. 12  is a cross sectional view of the alternative engine-transaxle assembly taken along the X 1 -X 1  line of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, descriptions will be given on an assumption that a utility vehicle  1  equipped with an engine-transaxle assembly  3  faces forward in a direction designated by an arrow F as shown in  FIG. 1 , and words “longitudinal” and “longitudinally” will be used on an assumption that they are only defined as meaning the fore-and-aft direction of utility vehicle  1 . 
     Referring to  FIGS. 1 and 2 , entire utility vehicle  1  will be described. Utility vehicle  1  is provided with a vehicle body frame  8  including a front frame  8   a  and a rear frame  8   b  joined to each other. A front portion of front frame  8   a  is laterally narrowed so as to serve as a front wheel support portion  8   a   1  carrying right and left front wheels  12  on right and left outer sides thereof. Rear frame  8   b  having a constant lateral width carries right and left rear wheels  13  on right and left outer sides of a rear portion thereof serving as a rear wheel supporting portion  8   b   1 . A horizontal platform  9  which is substantially rectangular when viewed in plan is spread inside rear frame  8   b . Vertical side plates  10  are extended upright from front, rear, right and left ends of platform  9 . A dump bed  11  is vertically rotatably mounted on the tops of side plates  10 . 
     An engine-transaxle assembly  3  is settled on platform  9 . A front transaxle  2  is disposed inside of front wheel supporting portion  8   a   1 . Engine-transaxle assembly  3  includes a later-discussed PTO section from which power is transmitted to front transaxle  2 . Engine-transaxle assembly  3  is a combination of a longitudinal engine  4  and a rear transaxle  7 . Rear transaxle  7  includes a mechanical transmission  5  and an HST  6 . HST  6  serves as a main transmission that transmits power from engine  4  to a sub transmission  43  of mechanical transmission  5 , and sub transmission  43  transmits power from HST  6  to right and left rear wheels  13 . In other words, longitudinal engine  4 , HST  6 , and mechanical transmission  5  are assembled together so as to constitute engine-transaxle assembly  3 . 
     Engine-transaxle assembly  3 , i.e., rear transaxle  7 , carries right and left differential output shafts  41 R and  41 L coupled to coaxial axles  13   a  of respective right and left rear wheels  13  via respective coupling sleeves  42 . Front transaxle  2  carries right and left differential output shafts  14  drivingly connected to axles  12   a  of respective right and left steerable front wheels  12  via respective propeller shafts  16  with universal joints  15 . Front wheel supporting portion  8   a   1  of front frame  8  suspends right and left front wheels  12  via ordinary suspensions having coiled springs or shock absorbers. 
     Front transaxle  2  includes a transaxle casing  17  incorporating a front differential unit  24  that differentially connects proximal ends of right and left differential output shafts  14  to each other. Transaxle casing  17  also incorporates a differential locking mechanism  25  for front differential unit  24 , and a drive mode selection clutch  26  on the drive train to front differential unit  24 . 
     Front differential unit  24  includes a differential casing  19 , a differential ring gear  20 , a pinion shaft  21 , differential pinions  22 , and right and left differential side gears  23 . Differential casing  19  fittingly supports right and left differential output shafts  14  coaxial to each other so as to allow right and left differential output shafts  14  to rotate relative to differential casing  19 . In differential casing  19 , right and left bevel gears serving as right and left differential side gears  23  are fixed on respective proximal end portions of differential output shafts  14 . In differential casing  19 , pinion shaft  21  having bevel pinions serving as differential pinions  22  thereon is disposed between right and left differential side gears  23 , and is extended perpendicular to differential output shafts  14  so that right and left differential side gears  23  mesh with each differential pinion  22  therebetween. A bevel ring gear serving as differential ring gear  20  is fixed on an outer peripheral surface of differential casing  19 . 
     Differential locking mechanism  25  includes alternately layered friction elements  18   a  and  18   b . Friction elements  18   a  engage with differential output shaft  14  unrotatably relative to differential output shaft  14 . Friction elements  18   b  engage with an inner peripheral portion of differential casing  19  unrotatably relative to differential casing  19 . A pressure member (not shown) for pressing friction elements  18   a  and  18   b , e.g., a slidable ring and a fork, is interlockingly connected to a differential locking manipulator, e.g., a lever or a pedal. Friction elements  18   a  and  18   b  can be pressed against one another by operating the differential locking manipulator so as to lock right and left differential output shafts  14  to each other via differential casing  19 . Further, differential locking mechanism  25  may be configured so that friction elements  18   a  and friction elements  18   b  have adjustable frictional pressure therebetween so as to provide front differential unit  24  as a limited slip differential unit. 
     An input shaft  27  is extended in the fore-and-aft direction rearward from front differential unit  24  and is journalled by a rear portion of transaxle casing  17 . Input shaft  27  is divided into coaxial front and rear shafts  27   a  and  27   b . A bevel gear  28  is fixed on a front end portion of front shaft  27   a , and meshes with differential ring gear  20 . A rear end portion of rear shaft  27   b  projects rearward from transaxle casing  17 , and is coupled to a front end of a propeller shaft  29  via a universal joint  30  so as to receive power from a later-discussed PTO shaft  106  of rear transaxle  7  of engine-transaxle assembly  3 . 
     In transaxle casing  17 , drive mode selection clutch  26  is interposed between front and rear shaft  27   a  and  27   b . Drive mode selection clutch  26  includes a clutch slider  31 , clutch teeth  32  formed on a rear end portion of clutch slider  31 , and clutch teeth  33  formed on a front end portion of rear shaft  27   b . Clutch slider  31  is fitted on front shaft  27   a  so that clutch slider  31  is axially slidable along front shaft  27   a , however, is unrotatable relative to front shaft  27   a . Clutch slider  31  is operatively connected to a drive mode selection manipulator (not shown), e.g., a lever. Due to manipulation of the drive mode selection manipulator, clutch slider  31  selectively slides rearward to engage clutch teeth  32  with clutch teeth  33  so as to engage drive mode selection clutch  26 , or forward to disengage clutch teeth  32  from clutch teeth  33  so as to disengage drive mode selection clutch  26 . By engaging drive mode selection clutch  26 , utility vehicle  1  is set in 4WD mode where traveling of utility vehicle  1  relies on driving of all front wheels  12  and rear wheels  13 . By disengaging drive mode selection clutch  26 , utility vehicle  1  is set in 2WD mode where traveling of utility vehicle  1  relies on driving of only rear wheels  13 . 
     Referring to  FIGS. 1 to 9 , engine-transaxle assembly  3 , including longitudinal engine  4  and rear transaxle  7 , will be described in detail. As shown in  FIGS. 1 to 3 and 8 , engine  4  includes a crankcase  4   b  incorporating a crankshaft  4   a  extended longitudinally, i.e., in the fore-and-aft direction of utility vehicle  1 . A flywheel  40  is fixed on a rear end portion of crankshaft  4   a . An engine output shaft  39  is extended coaxially rearward from crankshaft  4   a , and is spline-fitted at a front end portion into a coupling sleeve  38  coupled via a damper to flywheel  40 . A flywheel casing  35  is fixed to a rear end of crankcase  4   b  of engine  4  so as to incorporate flywheel  40 , coupling sleeve  38  and the front end portion of engine output shaft  39  spline-fitted into coupling sleeve  38 . 
     As shown in  FIGS. 3 and 8 , flywheel casing  35  is formed at a rear portion thereof with a boss  35   a  having a through hole  35   a   1 . Engine output shaft  39  is journalled at an axial intermediate portion thereof by a bearing  45  fitted in through hole  35   a   1 . Boss  35   a  is also formed as a front portion of a side housing  36   a  of a gear transmission casing  36 . In other words, flywheel casing  35  and side housing  36   a  of gear transmission casing  36  are formed integrally with each other so as to have boss  35   a  therebetween. Alternatively, flywheel casing  35  may be separate from gear transmission casing  36 , and may be fastened to gear transmission casing  36  via a bolt. In this case, boss  35   a  may be formed integrally with either flywheel casing  35  or gear transmission casing  36 , or may be separate from both flywheel casing  35  and gear transmission casing  36 . 
     As shown in  FIGS. 4 to 8 , side housing  36   a  and a main housing  36   b  are joined to each other at a longitudinal and vertical joint plane so as to constitute gear transmission casing  36 . Further, as shown in  FIGS. 4 to 7 , a center section  34  of HST  6  having an HST casing  37  attached thereon is joined to side housing  36   a  of gear transmission casing  36  opposite main housing  36   b , so that HST casing  37 , center section  34 , and gear transmission casing  36  are joined to one another to constitute a transaxle casing  46  of rear transaxle  7 . Therefore, crankcase  4   b  of engine  4  and transaxle casing  46  are joined to each other so as to constitute an engine-transaxle assembly casing  47  of entire engine-transaxle assembly  3 . 
     As shown in  FIG. 1 , utility vehicle  1  has a lateral width W, and engine-transaxle assembly  3  is located in utility vehicle  1  so that heavy engine  4  is located at the laterally middle position in the direction of lateral width W so as to balance utility vehicle  1  in the lateral direction. Gear transmission casing  36  of transaxle casing  46  is extended rearward from engine  4 , so that, as discussed later, HST  6  (including center section  34  and HST casing  37 ) and a first PTO casing  64  are disposed on one of right and left sides (in this embodiment, the left side) of gear transmission casing  36 , and a second PTO casing  94  is disposed on the other of right and left sides (in this embodiment, the right side) of gear transmission casing  36 . 
     Hereinafter, engine-transaxle assembly  3  will be described on the assumption that HST  6  and first PTO casing  64  are disposed on the left side of gear transmission casing  36 , and second PTO casing  94  is disposed on the right side of gear transmission casing  36 . On this assumption, side housing  36   a  is joined to a left open end of main housing  36   b.    
     Side housing  36   a  and main housing  36   b  are joined to each other to constitute gear transmission casing  36  defining a main chamber  36   c  therein. In this regard, side housing  36   a  is formed with a bearing wall  36   a   1  that is extended substantially vertically and longitudinally so as to define the left end of main chamber  36   c . A right end wall portion of main housing  36   b  is extended substantially vertically and longitudinally so as to define the right end of main chamber  36   c.    
     Mechanical transmission  5  includes transmission shafts  50 ,  51 ,  52 ,  53  and  54 , i.e., input shaft  50 , counter shaft  51 , main transmission input shaft  52 , sub transmission input shaft (or main transmission output shaft)  53 , and sub transmission output shaft  54 , which are journalled in gear transmission casing  36  to transmit power from engine output shaft  39  to right and left differential output shafts  41 R and  41 L. In main chamber  36   c , transmission shafts  50 ,  51 ,  52 ,  53  and  54  are extended in the lateral direction of utility vehicle  1  in parallel to each other and perpendicular to engine output shaft  39 . 
     Referring to  FIGS. 1 to 4 and 8 , input shaft  50  is spanned in a lower front portion of main chamber  36   c  between bearing wall  36   a   1  of side housing  36   a  and the right end wall portion of main housing  36   b . A bevel gear  56  is fixed on a left end portion of input shaft  50 , and is fitted into a bearing  57  disposed in a lower front portion of bearing wall  36   a   1 , so that the left end portion of input shaft  50  with bevel gear  56  is journalled by the lower front portion of bearing wall  36   a   1  via bearing  57 . A bevel gear  55  is formed (or fixed) on a rear end portion of engine output shaft  39 , and meshes with bevel gear  56  so as to transmit power from engine output shaft  39  to input shaft  50 . In this regard, engine output shaft  39  is disposed at or close to the lateral center position of main chamber  36   c , and bevel gear  56  is disposed leftward from the axis of engine output shaft  39  so as to mesh with a left portion of bevel gear  55 . 
     Referring to  FIGS. 1, 2 and 4 , a PTO clutch shaft  58  is disposed coaxially leftward from input shaft  50 . The left end portion of input shaft  50  is further extended leftward from bearing wall  36   a   1 , and a projection  50   a  is formed so as to project leftward from the left end of input shaft  50 , and is fitted into a recess  58   a  formed in a right end portion of PTO clutch shaft  58  so as to be allowed to rotate relative to PTO clutch shaft  58 . A wet multi-disc type PTO clutch  59  is provided on input shaft  50  and PTO clutch shaft  58  so as to be interposed between input shaft  50  and PTO clutch shaft  58 . The lower front portion of side housing  36   a  is expanded leftward from the portion of bearing wall  36   a   1  with bearing  57  journaling input shaft  50  so as to form a lower side chamber  36   e  incorporating PTO clutch  59 . 
     PTO clutch  59  includes an inner drum  60 , an outer drum  61 , friction elements  60   a  and  61   a , a clutch piston  62 , and a spring  63 . Inner drum  60  is expanded radially from input shaft  50 , and leftward so as to surround the end portions of input shaft  50  and PTO clutch shaft  58  fitted to each other with projection  50   a  in recess  58   a . Friction elements  60   a  engage at inner peripheral edges thereof with inner drum  60  unrotatably relative to inner drum  60  and input shaft  50 . Outer drum  61  is fixed on PTO clutch shaft  58  and is extended rightward so as to surround inner drum  60 . Friction elements  61   a  engage at outer peripheral edges thereof with outer drum  61  unrotatably relative to outer drum  61  and PTO clutch shaft  58 . Friction elements  60   a  and  61   a  are alternately layered in the axial direction of input shaft  50  and PTO clutch shaft  58 . 
     Clutch piston  62  is disposed inside of outer drum  61 , and is formed with a boss  62   a  that is axially slidably fitted on PTO clutch shaft  58 . Spring  63  is wound around boss  62   a  of clutch piston  62  so as to bias clutch piston  62  leftward. A space between clutch piston  62  and outer drum  61  serves as a hydraulic fluid chamber  190 . When fluid is supplied into hydraulic fluid chamber  190 , the fluid pushes clutch piston  62  rightward against spring  63  so as to press friction elements  60   a  and  61   a  against each other, thereby engaging PTO clutch  59 . When fluid is released from hydraulic fluid chamber  190 , spring  63  pushes clutch piston  62  leftward so as to separate friction elements  60   a  and  61   a  from each other, thereby disengaging PTO clutch  59 . 
     The lower front portion of side housing  36   a  expanded leftward from bearing wall  36   a   1  to define lower side chamber  36   e  incorporating PTO clutch  59  has a vertical left end surface  36   a   2 , to which first PTO casing  64  is fixed. More specifically, a right casing part  64   b  contacts vertical left end surface  36   a   2  of side housing  36   a  at a right end thereof so as to be fixed to side housing  36   a . Left and right casing parts  64   a  and  64   b  are joined to each other to constitute first PTO casing  64  defining a PTO gear chamber  64   d  therein. PTO clutch shaft  58  is journalled by right casing part  64   b  via right and left bearings  66  and  67 . A left end portion of PTO clutch shaft  58  is extended leftward from left bearing  67  into PTO gear chamber  64   d  so as to be fixedly provided thereon with a bevel gear  68  in PTO gear chamber  64   d.    
     A first PTO shaft  105  is extended in the longitudinal direction of utility vehicle  1  perpendicular to PTO clutch shaft  58 , and is clamped between left and right casing parts  64   a  and  64   b  of first PTO casing  64  via a bearing. A front end portion of first PTO shaft  105  projects forward from first PTO casing  64  so as to be drivingly connected to a working device, e.g., a snow blower, attached to a front portion of utility vehicle  1 . In PTO gear chamber  64   d  of first PTO casing  64 , a bevel gear  69  is fixed on a rear end portion of first PTO shaft  105  and meshes with bevel gear  68 . 
     Outer drum  61  is formed with a leftwardly extended boss  61   b  fitted on PTO clutch shaft  58 . Right casing part  64   b  is expanded rightward into lower side chamber  36   e  of side housing  36   a  incorporating PTO clutch  59  so as to surround boss  61   b . A PTO brake  71  is provided in lower side chamber  36   e  so as to be interposed between boss  61   b  of outer drum  61  and right casing part  64   b  of first PTO casing  64 . 
     PTO brake  71  includes friction elements  61   c  and  64   c , a brake pin  65  and a pressure disc  70 . Brake pin  65  is extended leftward from clutch piston  62 , and is passed through a left wall portion of outer drum  61 . Friction elements  61   c  engage at inner peripheral edges thereof with boss  61   b  of outer drum  61  so as to be unrotatable relative to outer drum  61  and PTO clutch shaft  58 . Friction elements  64   c  engage at outer peripheral edges thereof with the rightward expanded portion of right casing part  64   b  of first PTO casing  64  so as to be unrotatable relative to first PTO casing  64 . Friction elements  61   c  and  64   c  are alternately layered in the axial direction of PTO clutch shaft  58 . Pressure disc  70  is disposed rightward from all friction elements  61   c  and  64   c , and a left end of brake pin  65  contacts pressure disc  70 . 
     When PTO clutch  59  is engaged, clutch piston  62  with brake pin  65  is disposed at the right end position of its slidable range so as to separate friction elements  61   c  and  64   c  from one another, whereby PTO brake  71  does not act to brake first PTO shaft  105  rotated by the rotary power of input shaft  50  via engaged PTO clutch  59 . When PTO clutch  59  is disengaged, clutch piston  62  with brake pin  65  is disposed at the left end position of its slidable range so as to press friction elements  61   c  and  64   c  against one another via pressure disc  70 , whereby PTO brake  71  acts to brake first PTO shaft  105  isolated from the rotary power of input shaft  50  via disengaged PTO clutch  59 , thereby preventing first PTO shaft  105  from rotating inertially. 
     Referring to  FIGS. 1 to 4 , counter shaft  51  is journalled at a left end thereof by bearing wall  36   a   1  of side housing  36   a  via a bearing, and at a right end thereof by the right end wall portion of main housing  36   b  via another bearing, thereby being spanned in a vertically middle front portion of main chamber  36   c  between bearing wall  36   a   1  of side housing  36   a  and the right end wall portion of main housing  36   b . Gears  72  and  73  are disposed in a right portion of main chamber  36   c , rightward from engine output shaft  39  when viewed in rear, are fixed on right portions of input shaft  50  and counter shaft  51 , respectively, and mesh with each other so as to transmit power from input shaft  50  to counter shaft  51 . 
     Referring to  FIGS. 1 to 4 and 7 , main transmission input shaft  52  is journalled at a left end portion thereof by bearing wall  36   a   1  of side housing  36   a  via a bearing, and at a right end portion thereof by the right end wall portion of main housing  36   b  via another bearing, thereby being spanned in an upper front portion of main chamber  36   c  between bearing wall  36   a   1  of side housing  36   a  and the right end wall portion of main housing  36   b . A gear  74  is fixed on a right portion of main transmission input shaft  52 , and meshes with gear  73  on counter shaft  51 , so that gears  72 ,  73  and  74  transmit power from input shaft  50  to main transmission input shaft  52  via counter shaft  51 . 
     Referring to  FIGS. 4 to 7 , an upper portion of side housing  36   a  is expanded leftward from bearing wall  36   a   1  so as to define an upper side chamber  36   d  above lower side chamber  36   e  incorporating PTO clutch  59  and PTO brake  71 . The upper portion of side housing  36   a  defining upper side chamber  36   d  has vertical left end surface  36   a   2  expanded continuously from vertical left end surface  36   a   2  of the lower portion of side housing  36   a  defining lower side chamber  36   e . In other words, vertical left end surface  36   a   2  of side housing  36   a  defines the left open ends of upper and lower side chambers  36   d  and  36   e.    
     Referring to  FIGS. 1 to 4 and 7 , the left end portion of main transmission input shaft  52  is further extended rearward from bearing wall  36   a   1 , and is spline-fitted into a coupling sleeve  77  in a front portion of upper side chamber  36   d . A pump shaft  75  of HST  6  is extended coaxially leftward from main transmission input shaft  52 , and is spline-fitted at a right end portion thereof into coupling sleeve  77  in the front portion of upper chamber  36   d  so as to be coupled to main transmission input shaft  52  via coupling sleeve  77  rotatably integrally with main transmission input shaft  52 . 
     On the other hand, a pump housing  82   a  of a hydraulic device drive pump  82  is attached onto the right outer side of the right end wall portion of main housing  36   b  of gear transmission casing  36 . Pump housing  82   a  journals a pump drive shaft  81  extended coaxially rightward from main transmission input shaft  52 . A projection projects rightward from the right end of main transmission input shaft  52  and is spline-fitted into a left end portion of pump drive shaft  81 . Pump housing  82   a  incorporates a drive gear  79  fixed on pump drive shaft  81  and a driven gear  80  meshing with drive gear  79 . Therefore, hydraulic device drive pump  82  for supplying hydraulic fluid to later-discussed hydraulic devices (actuators) is configured as a gear pump including drive gear  79  and driven gear  80 . 
     A first drive train  83  includes engine output shaft  39 , bevel gear  55  and  56 , input shaft  50 , gears  72 ,  73  and  74 , and main transmission input shaft  52 . Therefore, the engine power as the rotary power of engine output shaft  39  is transmitted upward to main transmission input shaft  52  via first drive train  83 , and is distributed between pump shaft  75  of HST  6  disposed leftward from main transmission input shaft  52  and pump drive shaft  81  of hydraulic device drive pump  82  disposed rightward from main transmission input shaft  52 , thereby simultaneously driving HST  6  and hydraulic device drive pump  82 . 
     Referring to  FIGS. 1 to 3 and 5 to 7 , sub transmission input shaft (or main transmission output shaft)  53  is disposed rearward from main transmission input shaft  52 , and has an axis disposed on a horizontal plane  87  on which an axis of main transmission input shaft  52  is also disposed. Sub transmission input shaft  53  is journalled at a left end portion thereof by bearing wall  36   a   1  of side housing  36   a  via a bearing, and at a right end portion thereof by the right end wall portion of main housing  36   b  via another bearing, thereby being spanned in a longitudinally middle upper portion of main chamber  36   c  between bearing wall  36   a   1  and the right end wall portion of main housing  36   b.    
     The left end portion of sub transmission input shaft  53  is further extended rearward from bearing wall  36   a   1 , and is spline-fitted into a coupling sleeve  78  in a rear portion of upper side chamber  36   d . A motor shaft  76  of HST  6  is extended coaxially leftward from sub transmission input shaft  53 , and is spline-fitted at a right end portion thereof into coupling sleeve  78  in the rear portion of upper side chamber  36   d  so as to be coupled to sub transmission input shaft  53  via coupling sleeve  78  rotatably integrally with sub transmission input shaft  53 . Therefore, axes of main transmission input shaft  52 , pump shaft  75 , motor shaft  76  and sub transmission input shaft  53  are disposed to have their axes on common horizontal plane  87 , i.e., at an even level. 
     Referring to  FIGS. 1 to 4, 6, 7 and 9 , HST  6  includes center section  34 , HST casing  37 , a hydraulic pump  48  and a hydraulic motor  49 . Vertical rectangular plate-shaped center section  34  contacts left end surface  36   a   2  of side housing  36   a  at a right end surface thereof, thereby being attached on the left outer side of gear transmission casing  36 . Hydraulic pump  48  and hydraulic motor  49  are mounted onto a left end surface  34   a  of center section  34 , and HST casing  37  is attached to left end surface  34   a  of center section  34  so as to incorporate hydraulic pump  48  and hydraulic motor  49 . A pair of main fluid passages  153  and  154  are formed in center section  34  so as to fluidly connect hydraulic pump  48  and hydraulic motor  49  to each other. 
     Axial piston type hydraulic pump  48  having a variable displacement includes pump shaft  75 , a valve plate  151 , a cylinder block  133 , plungers  134 , and a movable swash plate  135 . Horizontal pump shaft  75  joined at the right end thereof to main transmission input shaft  52  as mentioned above is passed through a front portion of center section  34 . In HST casing  37 , valve plate  151  is fixed onto left end surface  34   a  at the front portion of center section  34 , and cylinder block  133  is slidably rotatably fitted onto valve plate  151 , and is fixed on pump shaft  75  extended leftward from center section  34  via valve plate  151 . Cylinders  133   a  are bored in cylinder block  133  around pump shaft  75 , and plungers  134  are fitted in respective cylinders  133   a  reciprocally parallel to pump shaft  75 . Movable swash plate  135  is pivotally supported in HST casing  37  and abuts against heads, i.e., left ends, of plungers  134  projecting from cylinder block  133 . 
     A charge pump  150  for supplying fluid to main fluid passages  153  and  154  in center section  34  and to a later-discussed hydraulic servomechanism  136  in HST casing  37  is attached on a left outer side of HST casing  37 . Charge pump  150  is a gear pump, e.g., a trochoid pump, including a pump housing  150   a  fixed to HST casing  37  and a gear (or rotor)  150   b  disposed in pump housing  150   a . Pump shaft  75  is freely passed through movable swash plate  135 , is journalled by HST casing  37 , and is extended leftward from HST casing  37  into pump housing  150   a  so as to serve as a drive shaft for driving gear  150   b  of charge pump  150 . 
     Referring to  FIG. 4 , hydraulic servomechanism  136 , including a piston  137  and a proportional directive control valve  138 , is assembled in HST casing  37 . HST casing  37  is formed with a longitudinal cylinder  37   a . Piston  137  is longitudinally slidably fitted in cylinder  37   a , and is coupled to movable swash plate  135  via a connection pin  140 , so that movable swash plate  135  is pivotally moved according to the longitudinal sliding of piston  137  in cylinder  37   a . Proportional directive control valve  138  is configured in piston  137  so as to hydraulically control the position of piston  137  in cylinder  37   a , as shown in  FIG. 9 . In this regard, proportional directive control valve  138  includes a spool  138   a  longitudinally slidably disposed in piston  137  so that the position change of spool  138   a  in piston  137  is defined as the shift of proportional directive control valve  138  for controlling the position of piston  137 . 
     Referring to  FIG. 4 , a top cover  155  is fixed on a top portion of HST casing  37  so as to cover hydraulic servomechanism  136 . A vertical HST control shaft  144  is journalled by top cover  155  and is coupled at a bottom end thereof to spool  138   a  in piston  137 . An HST control lever  156  is fixed on a top portion of HST control shaft  144  projecting upward from top cover  155 . By manipulating HST control lever  156 , HST control shaft  144  rotates centered on its own vertical axis so as to slide spool  138   a , i.e., shift proportional directive control valve  138 , thereby changing the position of piston  137  in cylinder  37   a , and thereby changing the tilt direction and angle of movable swash plate  135  so as to change the fluid delivery direction and amount of hydraulic pump  48 . 
     Further, referring to  FIG. 4 , top cover  155  is formed with a longitudinal cylinder  155   a , and a spool  141  is longitudinally slidably fitted in cylinder  155   a , and is interlockingly connected with spool  138   a  and HST control shaft  144 . In this regard, as shown in  FIG. 9 , a spring  143   a  is disposed in top cover  155  and HST casing  37  so as to bias spool  141  to locate spool  138   a  at a position to set piston  137  and movable swash plate  135  at a neutral position. Therefore, spool  141  and spring  143   a  constitute a neutral retaining mechanism  143  for returning piston  137 , when released from an operation force, to its neutral position and for retaining piston  137  at the neutral position while it is free from an operation force. 
     In this regard, HST  6  is provided with a pilot pressure control mechanism  200  as shown in  FIG. 9  (not shown in  FIG. 4 ) for controlling the position of spool  141  of neutral retaining mechanism  143 , thereby controlling the position of spool  138   a , i.e., the state of proportional directive control valve  138  for hydraulically controlling the position of piston  137 . Pilot pressure control mechanism  200  includes a proportional electromagnetic directive control valve  199  for hydraulically controlling the position of spool  141  in cylinder  155   a . A controller (not shown) controls electricity applied on solenoids of proportional electromagnetic directive control valve  199  in correspondence to operation of a later-discussed sub speed control lever  100 , a reverser (i.e., a manipulator for determining a traveling direction of utility vehicle  1 ), an accelerator (i.e., a manipulator for controlling a throttle degree of engine  4  for determining an output rotary speed of engine  4 ) and so on, so as to automatically control the position of spool  141 , the position of spool  138   a , i.e., the state of proportional directive control valve  138 , and the position of piston  137 , thereby automatically determining the tilt direction and angle of movable swash plate  135 , i.e., the rotary direction and speed of motor shaft  76  of HST  6  so as to realize a set traveling direction and speed of utility vehicle  1 . 
     Axial piston type hydraulic motor  49  having a fixed displacement includes motor shaft  76 , a valve plate  152 , a cylinder block  147 , plungers  148 , and a fixed swash plate  149 . Horizontal motor shaft  76  joined at the right end thereof to sub transmission input shaft  53  as mentioned above is passed through a rear portion of center section  34 . In HST casing  37 , valve plate  152  is fixed onto left end surface  34   a  at the rear portion of center section  34 , and cylinder block  147  is slidably rotatably fitted onto valve plate  152 , and is fixed on motor shaft  76  extended leftward from center section  34  via valve plate  152 . Cylinders  147   a  are bored in cylinder block  147  around motor shaft  76 , and plungers  148  are fitted in respective cylinders  147   a  reciprocally parallel to motor shaft  76 . Fixed swash plate  149  is settled in HST casing  37  and abuts against heads, i.e., left ends, of plungers  148  projecting from cylinder block  147 . 
     Main fluid passages  153  and  154  fluidly connect cylinders  133   a  in cylinder block  133  of hydraulic pump  48  mounted on center section  34  to cylinders  147   a  in cylinder block  147  of hydraulic motor  49  mounted on center section  34 . Therefore, pump shaft  75  is rotated together with main transmission input shaft  52  receiving power from engine  4  via first drive train  83 , so as to drive hydraulic pump  48 , and hydraulic motor  49  is driven by hydraulic fluid delivered from hydraulic pump  48  via main fluid passage  153  or  154 , so as to rotate motor shaft  76  in the direction and speed corresponding to the set tilt direction and angle of movable swash plate  135  of hydraulic pump  48 . In this way, HST  6  transmits the rotary power of main transmission input shaft  52  driven by engine  4  via first drive train  83  to sub transmission input shaft  53  so as to determine a rotary direction and speed of sub transmission input shaft  53 . 
     Referring to  FIGS. 1 to 3 and 5 to 7 , a gear transmission including gears  85 ,  86 ,  89  and  90  is configured in a lateral middle portion of main chamber  36   c  so as to serve as sub transmission  43  driven by HST  6  serving as the main transmission. High speed drive gear  85  and low speed drive gear  86  that is diametrically smaller than high speed drive gear  85  are disposed in a laterally middle upper portion of main chamber  36   c , and are fixed on sub transmission input shaft  53 . 
     Sub transmission output shaft  54  is disposed below sub transmission input shaft  53 , and is journalled at a left end portion thereof by bearing wall  36   a   1  of side housing  36   a  via a bearing, and at a right end portion thereof by the right end wall portion of main housing  36   b  via another bearing, thereby being spanned in a longitudinally and vertically middle portion of main chamber  36   c  between bearing wall  36   a   1  of side housing  36   a  and the right end wall portion of main housing  36   b . High speed driven gear  89  and low speed driven gear  90  that is diametrically larger than high speed driven gear  89  are disposed in a laterally and vertically middle portion of main chamber  36   c . High speed driven gear  89  is fitted at an axial boss thereof on sub transmission output shaft  54  rotatably relative to sub transmission output shaft  54 , and low speed driven gear  90  is fitted on the axial boss of high speed driven gear  89  rotatably relative to high speed driven gear  89  and sub transmission output shaft  54 . High speed drive and driven gears  85  and  89  mesh with each other so as to serve as a high speed gear train of sub transmission  43 . Low speed drive and driven gears  86  and  90  mesh with each other so as to serve as a low speed gear train of sub transmission  43 . 
     Sub transmission  43  also includes a spline hub  91  and a clutch slider  92 . Spline hub  91  is fixed on a right portion of sub transmission output shaft  54  rightward from high and low speed driven gears  89  and  90 . Clutch slider  92  is spline-fitted on spline hub  91  so as to be slidable on spline hub  91  in the axial direction of sub transmission output shaft  54  and unrotatable relative to spline hub  91  and sub transmission output shaft  54 . Low speed driven gear  90  is formed on a right end portion thereof with clutch teeth  90   a , and high speed driven gear  89  is formed on a right end portion thereof with clutch teeth  89   a  disposed between clutch teeth  90   a  and spline hub  91  in the axial direction of sub transmission output shaft  54 . 
     A fork shaft  96  is extended laterally in parallel to transmission shafts  50 ,  51 ,  52 ,  53  and  54 , and is disposed rearward from sub transmission input shaft  53 . Fork shaft  96  is supported at a left end portion thereof by bearing wall  36   a   1  of side housing  36   a , and at a right end portion thereof by the right end wall portion of main housing  36   b , thereby being spanned in an upper rear portion of main chamber  36   c  between bearing wall  36   a   1  and the right end wall portion of main housing  36   b . A fork  95  is engaged at an end portion thereof on clutch slider  92 , and is engaged at another end portion thereof to a slide member  99  axially slidably fitted on fork shaft  96 . 
     A vertical speed control shaft  98  is journalled by an upper rear portion of main housing  36   b . A top portion of speed control shaft  98  projects upward from gear transmission casing  36  so as to be interlockingly connected to sub speed control lever  100  via a link mechanism  128 , as shown in  FIG. 3 . In main chamber  36   c , an arm  97  is fixed on a bottom end of speed control shaft  98 , and is extended forward so that an engaging pin  97   a  provided on a front end portion of arm  97  is extended downward so as to engage with slide member  99 . 
     As shown in  FIG. 3 , sub speed control lever  100  is shiftable among a low speed position  101 , a neutral position  102 , and a high speed position  103 . When sub speed control lever  100  is set at low speed position  101 , clutch slider  92  engages with clutch teeth  90   a  and disengages from clutch teeth  89   a  so as to transmit power from sub transmission input shaft (main transmission output shaft)  53  to sub transmission output shaft  54  via the low speed gear train including gears  86  and  90 , thereby realizing a low speed state Lo of sub transmission  43 . When sub speed control lever  100  is set at neutral position  102 , clutch slider  92  disengages from both clutch teeth  89   a  and  90   a  so as to isolate sub transmission output shaft  54  from the rotary power of sub transmission input shaft  53 , thereby realizing a neutral state N of sub transmission  43 . When sub speed control lever  100  is set at high speed position  103 , clutch slider  92  engages with clutch teeth  89   a  and disengages from clutch teeth  90   a  so as to transmit power from sub transmission input shaft  53  to sub transmission output shaft  54  via the high speed gear train including gears  85  and  89 , thereby realizing a high speed state Hi of sub transmission  43 . 
     Referring to  FIGS. 1, 2, and 4 to 7 , second PTO casing  94  is attached onto the right end wall portion of main housing  36   b  of gear transmission casing  36  at a longitudinally and vertically middle portion of gear transmission casing  36 . A coupling sleeve  104  is disposed in a chamber formed by joining the right end wall portion of main housing  36   b  of gear transmission casing  36  and a left end wall portion of second PTO casing  94 . The right end portion of sub transmission output shaft  54  is further extended rightward from the bearing in the right end wall portion of main housing  36   b , and is spline-fitted into coupling sleeve  104 . A PTO transmission shaft  93  is extended coaxially rightward from sub transmission output shaft  54 , and is spline-fitted at a left end portion thereof into coupling sleeve  104 , so that PTO transmission shaft  93  is rotatable integrally with sub transmission output shaft  54 . 
     A right portion of second PTO casing  94  is radially expanded to define a PTO gear chamber  94   a  therein. PTO transmission shaft  93  is extended across PTO gear chamber  94   a . A right end portion of PTO transmission shaft  93  projects rightwardly outward from a right end of second PTO casing  94 . As shown in  FIG. 1 , a parking brake  111  is provided on the right end portion of PTO transmission shaft  93  projecting outward from second PTO casing  94 . In a left portion of PTO gear chamber  94   a , a bevel pinion  107  is fixed on PTO transmission shaft  93 . 
     A second PTO shaft  106  is extended in the longitudinal direction of utility vehicle  1  perpendicular to PTO transmission shaft  93 , and is journalled by a front right portion of second PTO casing  94  via bearings. The front right portion of second PTO casing  94  may be vertically inclined so as to orient second PTO shaft  106  vertically slantwise. A rear end portion of second PTO shaft  106  is disposed in PTO gear chamber  94   a  forward from PTO transmission shaft  93 , and is fixedly provided thereon with a bevel gear  108 . Bevel gear  108  meshes at a left portion thereof with a front end portion of bevel pinion  107 . 
     A front end portion of second PTO shaft  106  projects forward from second PTO casing  94  and is spline-fitted into a coupling sleeve  109 . A propeller shaft  110  is extended coaxially forward from second PTO shaft  106 , and is spline-fitted at a rear end portion thereof into coupling sleeve  109 , so that propeller shaft  110  is rotatable integrally with second PTO shaft  106 . Alternatively, propeller shaft  110  may be flexibly connected to second PTO shaft  106  via a universal joint. Propeller shaft  29  coupled to the rear end of rear shaft  27   b  of input shaft  27  of front transaxle  2  via universal joint  30  as mentioned above is coupled at a rear end thereof to a front end of propeller shaft  110  via another universal joint  30 . Therefore, the rotary power of sub transmission output shaft  54  is distributed to front wheels  12  via PTO transmission shaft  93 , second PTO shaft  106 , propeller shafts  110  and  29 , and front transaxle  2 , while the rotary direction of sub transmission output shaft  54  is determined by HST  6  and the rotary speed of sub transmission output shaft  54  is determined by HST  6  and sub transmission  43 . 
     Referring to  FIGS. 1 to 3, 5, 7 and 8 , right and left differential output shafts  41 R and  41 L are extended coaxially to each other in the lateral direction of utility vehicle  1  so as to be journalled by right and left lower rear portions of gear transmission casing  36  via bearings, and are differentially connected at proximal end portions thereof to each other via a rear differential unit  113  with a differential locking mechanism  112  in a lower rear portion of main chamber  36   c . Rear differential unit  113  includes a differential ring gear  114 , a differential casing  115 , a pinion shaft  116 , differential pinions  117 , and right and left differential side gears  118  and  119 . 
     Differential casing  115  fittingly supports right and left differential output shafts  41 R and  41 L coaxial to each other so as to allow right and left differential output shafts  41 R and  41 L to rotate relative to differential casing  115 . In differential casing  115 , right and left bevel gears serving as right and left differential side gears  118  and  119  are fixed on respective proximal end portions of differential output shafts  41 R and  41 L. In differential casing  115 , pinion shaft  116  having bevel pinions serving as differential pinions  117  thereon is disposed between right and left differential side gears  118  and  119 , and is extended perpendicular to differential output shafts  41 R and  41 L so that right and left differential side gears  118  and  119  mesh with each differential pinion  117  therebetween. 
     Differential side gear  119  is formed with recesses  119   a  corresponding to later-discussed lock pins  121  of differential locking mechanism  112  disposed at either right or left side of differential unit  113 . In this embodiment, differential locking mechanism  112  is disposed at the right side of differential unit  113 . Therefore, differential side gear  119  having recesses  119   a  corresponding to lock pins  121  of differential locking mechanism  112  is fixed on right differential output shaft  41 R, and differential side gear  118  having no recess corresponding to lock pins  121  of differential locking mechanism  112  is fixed on left differential output shaft  41 L. 
     A spur gear serving as a final pinion  88  is formed (or fixed) on a left portion of sub transmission output shaft  54  leftward from high and low speed driven gears  89  and  90 . A spur ring gear serving as differential ring gear  114  serving as an input gear of rear differential unit  113  is fixed on an outer peripheral surface of differential casing  115 , and meshes with final pinion  88  so as to receive the rotary power from sub transmission output shaft  54 . 
     Sub transmission  43  and rear differential unit  113  constitute a second drive train  84  extended downward in main chamber  36   c  of gear transmission casing  36 . In other words, second drive train  84  includes sub transmission input shaft  53 , high speed gears  85  and  89 , low speed gears  86  and  90 , sub transmission output shaft  54 , final pinion  88 , differential ring gear  114 , differential pinions  117  and differential side gears  118  and  119  in differential casing  115 , and right and left differential output shafts  41 R and  41 L. In this regard, sub transmission input shaft  53  also serves as main transmission output shaft  53 , i.e., the output shaft of HST  6  connected coaxially to motor shaft  76  of HST  6 . Therefore, the rotary power of sub transmission input shaft (or main transmission output shaft)  53  as the output power of HST  6  is transmitted downward via either the high or low gear train of sub transmission  43  to differential unit  113 , and is distributed between right and left differential output shafts  41 R and  41 L drivingly connected to right and left rear wheels  13 . 
     Differential locking mechanism  112  includes a differential locking slider  120  and lock pins  121 . In this regard, differential casing  115  is formed with a right end boss portion  115   a  that is extended rightward from a vertical right end wall portion of differential casing  115  so as to be fitted on right differential output shaft  41 R. Differential locking slider  120  is axially slidably fitted on right end boss portion  115   a  of differential casing  115 . Lateral through holes  115   b  are bored in the vertical right end wall portion of differential casing  115 . Differential side gear  119  is formed with recesses  119   a  corresponding to respective through holes  115   b . Lock pins  121  are fixed to differential locking slider  120  and are extended horizontally leftward from differential locking slider  120  into differential casing  115  via respective through holes  115   b . In this regard, lock pins  121  is constantly inserted into through holes  115   b  so as to engage differential locking slider  120  with differential casing  115  unrotatably relative to differential casing  115  regardless of whether differential locking slider  120  is located at a locking position or an unlocking position. 
     A fork shaft  123  is extended laterally in parallel to transmission shafts  50 ,  51 ,  52 ,  53  and  54 , and is disposed rearward from fork shaft  96  for sub transmission  43 . Fork shaft  123  is journalled at a left end portion thereof by bearing wall  36   a   1  of side housing  36   a , and at a right end portion thereof by the right end wall portion of main housing  36   b , thereby being spanned in the upper rear portion of main chamber  36   c  between bearing wall  36   a   1  and the right end wall portion of main housing  36   b  rearward from fork shaft  96 . A fork  122  is engaged at an end portion thereof on differential locking slider  120 , and is engaged at another end portion thereof to a cam member  124  axially slidably fitted on fork shaft  123 . 
     A retaining ring  125  is fixed on a left portion of fork shaft  123 , and a spring  126  is wound around fork shaft  123  between a left end of cam member  124  and retaining ring  125 . A right end portion of cam member  124  is formed with cam recesses  124   a , and pressure pins  127  are radially extended from an outer peripheral surface of fork shaft  123  so as to correspond to respective cam recesses  124   a . A right end portion of fork shaft  123  projects rightwardly outward from gear transmission casing  36 , and is fixedly provided thereon with a differential locking arm  123   a  that is operatively connected to a differential locking lever  130  via a link mechanism  129  as shown in  FIG. 3 . 
     Differential locking lever  130  is shiftable between an unlocking position  131  and a locking position  132 . When differential locking lever  130  is set at unlocking position  131 , pressure pins  127  are pressed against deepest ends of respective cam recesses  124   a  of cam member  124  biased rightward by spring  126  so as to locate cam member  124  and differential locking slider  120  at the unlocking position that is the right limit position of their slidable range, so that left end portions of lock pins  121  are disposed rightwardly outside of recesses  119   a  of differential side gear  119 , thereby allowing differential rotation of right and left differential output shafts  41 R and  41 L. 
     By rotating differential locking lever  130  from unlocking position  131  to locking position  132 , fork shaft  123  is rotated so that pressure pins  127  come to abut against the right end edge of cam member  124  outside of cam recesses  124   a  so as to push cam member  124  and differential locking slider  120  leftward against spring  126  to the locking position that is the left limit position of their slidable range, whereby the left end portions of lock pins  121  are engaged into recesses  119   a  of differential side gear  119 , thereby locking right and left differential output shafts  41 R and  41 L in rotation to each other. 
     Referring to  FIGS. 3 to 6 and 9 to 12 , a hydraulic fluid circuit  157  of engine-transaxle assembly  3  and a lubrication system for components of engine-transaxle assembly  3  will be described. As shown in  FIGS. 3, 4 and 9 , a lower rear portion of gear transmission casing  36  incorporating differential unit  113  is expanded downward so as to accommodate differential ring gear  114 . In this regard, referring to  FIG. 3 , a reference numeral “ 36   b   1 ” designates a lower rear portion of main housing  36   b  expanded to accommodate differential ring gear  114 .  FIG. 3  does not illustrate side housing  36   a , however, a lower rear portion of side housing  36   a  is also expanded to correspond to expanded lower rear portion  36   b   1  of main housing  36   b . Therefore, a lower portion of flywheel casing  35  and the lower rear portion of gear transmission casing  36  are expanded downward in comparison with the lower front portion of gear transmission casing  36 , so that a space  206  is provided below the lower front portion of gear transmission casing  36  incorporating input shaft  50  between the lower portion of flywheel casing  35  and the lower rear portion of gear transmission casing  36 . A reservoir tank  205  is disposed in space  206 . A vertical pipe  169  is interposed between a top portion of reservoir tank  205  and a bottom wall of the lower rear portion of gear transmission casing  36  so as to allow fluid to flow therethrough, thereby holding a level  158   a  of a fluid sump  158  in main chamber  36   c  of gear transmission casing  36 . 
     A line filter  159  is attached to reservoir tank  205 . In this embodiment, line filter  159  is fixed onto an outer end surface of a right wall portion  205   a  of reservoir tank  205 . A suction port  170  that is upwardly open C-shaped when viewed in side is formed through right wall portion  205   a  so as to be connected to a suction port (not shown) of line filter  159 . A delivery fluid passage  171  is formed in right wall portion  205   a  so as to be surrounded by C-shaped suction port  170 . Delivery fluid passage  171  is L-shaped when viewed in rear so as to have a lateral horizontal portion  171   a  and a vertical portion  171   b . Lateral horizontal portion  171   a  of delivery fluid passage  171  is fluidly connected at a right end thereof to a delivery port  159   a  of line filter  159 . Vertical portion  171   b  of delivery fluid passage  171  is open upward at a top of right wall portion  205   a  of reservoir tank  205 . 
     As best understood from  FIG. 4 , a vertical pipe  172  is interposed between a top of vertical portion  171   b  of delivery fluid passage  171  and a bottom of pump housing  82   a  of hydraulic device drive pump  82  disposed vertically upward from right wall portion  205   a  of reservoir tank  205 . Pipe  172  has a pipe  174  (shown in only the hydraulic circuit diagram of  FIG. 9 ) branching therefrom to a suction port of charge pump  150 . In  FIG. 9 , a portion of pipe  172  between a branching point to pipe  174  and the end of pipe  172  connected to a suction port of hydraulic device drive pump  82  is defined as a branching pipe  173 . 
     Referring to  FIGS. 11 and 12 , gear transmission casing  36  may be formed with an alternative reservoir tank  205 A. In this alternative embodiment, side housing  36   a  and main housing  36   b  are formed with respective divisional parts  36   aa  and  36   bb  of reservoir tank  205  occupying space  206 . By joining side housing  36   a  and main housing  36   b  to each other to constitute gear transmission casing  36 , a chamber serving as reservoir tank  205 A is formed in gear transmission casing  36  so as to be partitioned from main chamber  36   c . In this regard, a bottom wall of the lower front portion of main chamber  36   c  also serves as a top wall of reservoir tank  205 A. A groove  169 A is bored through this wall between main chamber  36   c  and reservoir tank  205 A so as to fluidly connect main chamber  36   c  to reservoir tank  205 A. Groove  169 A is formed on at least one of vertical end surfaces of the corresponding walls of respective housings  36   a  and  36   b  joined to the other.  FIG. 11  illustrates main housing  36   b  formed with groove  169 A, however, side housing  36   a  may be formed with groove  169 A alternatively or additionally. A right end wall portion  205 Aa is formed at a right end portion of divisional part  36   bb  of main housing  36   b , and line filter  159  is fixed onto a right end surface of right end wall portion  205 Aa. Pipe  172  is extended from right end wall portion  205 Aa so as to supply fluid to hydraulic device drive pump  82  and charge pump  150 , similar to pipe  172  extended from right end wall portion  205   a  of reservoir tank  205 . 
     Referring to  FIG. 9 , a pipe  161  is extended from a delivery port of hydraulic device drive pump  82  so as to supply hydraulic fluid to hydraulic devices provided on utility vehicle  1 , e.g., hydraulic actuators  162  and  163 . For example, hydraulic actuator  162  is a power steering cylinder for turning front wheels  12 , and hydraulic actuator  163  is a lift cylinder for rotating dump bed  11 . A pipe  165  having a fluid cooler  164  thereon is extended from hydraulic actuators  162  and  163  to a drain port  165   a  open to reservoir tank  205  (or  205 A). 
     A bypass pipe  166  is extended between pipe  161  and pipe  165  so as to bypass hydraulic actuators  162  and  163 . A pressure regulation valve  167  is provided on bypass pipe  166  so as to regulate the hydraulic pressure of pipe  161  for supplying fluid to hydraulic actuators  162  and  163 . Pressure regulation valve  167  releases a surplus pressure fluid to reservoir tank  205  (or  205 A) via pipe  165 . 
     Referring to  FIGS. 3, 4, 9 and 10  (and  11  and  12 ), a pipe  175  is extended from a delivery port of charge pump  150  to a valve unit  176  having a valve unit housing  182  fixed on a right top surface of a front portion of gear transmission casing  36 . As best understood from  FIG. 10 , valve unit  176  includes a line filter  177 , a priority valve  178 , an electromagnetic switching valve serving as a clutch valve  179 , an accumulator  180 , a relief valve  181 , and valve unit housing  182 . On the above-mentioned assumption that charge pump  150  is disposed on the left outer side of HST casing  37  on the left side of gear transmission casing  36 , an end of pipe  175  is connected to a left end surface of a longitudinally middle portion of valve unit housing  182 , as shown in  FIGS. 3 and 10  (and  11 ). Priority valve  178  is extended longitudinally in a laterally middle portion of valve unit housing  182 . A lateral horizontal fluid passage  183  is extended in the longitudinally middle portion of valve unit housing  182  rightward from the left end surface of valve unit housing  182  to an inlet port of priority valve  178 . Line filter  177  is attached to the end of pipe  175  in a left end portion of fluid passage  183 . 
     As shown in  FIG. 10 , clutch valve  179  is disposed in a front left portion of valve unit housing  182  forward from fluid passage  183 . As shown in  FIG. 10 , a lateral horizontal fluid passage  184  is extended in the front left portion of valve unit housing  182  leftward from a first outlet port  178   a  of priority valve  178  to a suction port  179   a  of clutch valve  179 . As shown in  FIGS. 4, 5, 6 and 10  (and  12 ), a pipe joint serving as a first outlet port  203  is provided on a top surface of a front right portion of valve unit housing  182 . A pipe  189  is extended from first outlet port  203  to hydraulic fluid chamber  190  of PTO clutch  59 , as shown in  FIG. 9 . As shown in  FIG. 10 , a fluid passage  185  is extended longitudinally slantwise in a front portion of valve unit housing  182  from a valve port  179   b  of clutch valve  179 , and a vertical fluid passage  188  is extended in the front right portion of valve unit housing  182  upward from fluid passage  185  to first outlet port  203 . 
     As shown in  FIG. 10 , relief valve  181  is disposed in a front right portion of valve unit housing  182 , and accumulator  180  is extended longitudinally in a right portion of valve unit housing  182  rearward from relief valve  181 . Vertical fluid passage  188  extended upward from fluid passage  185  to first outlet port  203  is disposed between relief valve  181  and accumulator  180 , and is connected to relief valve  181  and accumulator  180 . 
     As understood from  FIGS. 9 and 10 , a vertical fluid passage  191  is extended downward from a drain port of relief valve  181  so as to have a bottom end, serving as a drain port  192  of valve unit  176 , open at a bottom surface of valve unit housing  182 . Further, a drain fluid passage (not shown in  FIG. 10 ) is extended in valve unit housing  182  from a drain port  179   c  of clutch valve  179 , and is joined to drain port  192 . Gear transmission casing  36  disposed under valve unit housing  182  is provided with a fluid passage for leading fluid from drain port  192  to fluid sump  158  in main chamber  36   c.    
     Therefore, the fluid delivered from charge pump  150  is supplied to suction port  179   a  of clutch valve  179  via pipe  175 , line filter  177 , priority valve  178 , and fluid passage  193 . When a solenoid of clutch valve  179  is excited, clutch valve  179  makes a flow of fluid from fluid passage  184  to fluid passage  185 , thereby supplying hydraulic fluid to hydraulic fluid chamber  190  for engaging PTO clutch  59 . In this state, accumulator  180  absorbs an excessive amount of fluid flowing in fluid passage  185  so as to regulate the flow of fluid to hydraulic fluid chamber  190 , and relief valve  181  releases fluid of an excessive pressure to fluid sump  158  via drain port  192  so as to regulate a pressure of fluid supplied to hydraulic fluid chamber  190 , thereby ensuring a proper activation of clutch piston  62  for pressing friction elements  60   a  and  61   a  without shock or damage. When the solenoid of clutch valve  179  is unexcited, clutch valve  179  connects suction port  179   a  and valve port  179   b  to drain port  179   c  so as to absorb fluid from hydraulic fluid chamber  190  via valve port  179   b  and release the fluid from drain port  179   c  to fluid sump  158  in main chamber  36   c  via drain port  192 , thereby disengaging PTO clutch  59 . 
     On the other hand, as shown in  FIG. 10 , a fluid passage  193  is formed in a rear left portion of valve unit housing  182  rearward from fluid passage  183 . Fluid passage  193  is extended leftward from a second outlet port  178   b  of priority valve  178 , and is bent rearward so as to have an open end provided with a pipe joint serving as a second outlet port  204  of valve unit  176  at a rear end surface of valve unit housing  182 , as shown in  FIGS. 4, 5, 6 and 10  (and  11  and  12 ). A pipe  194  is extended from second outlet port  204  so as to supply hydraulic fluid to the closed fluid circuit of HST  6  including main fluid passages  153  and  154 , and to a hydraulic mechanism, including hydraulic servomechanism  136 , for controlling the tilt direction and angle of movable swash plate  135  of hydraulic pump  48 . Therefore, the fluid delivered from charge pump  150  is also supplied to HST  6  via pipe  175 , line filter  177 , priority valve  178 , fluid passage  193 , second outlet port  204 , and pipe  194 . 
     In this regard, priority valve  178  is configured so as to reduce the open area of first outlet port  178   a  in comparison with second outlet port  178   b , thereby causing the fluid flow to hydraulic fluid chamber  190  of PTO clutch  59  prior to the fluid flow for HST  6 , because PTO clutch  59  should be given preference for preventing friction elements  60   a  and  61   a  from seizing. 
     Referring to  FIG. 9 , HST  6  is provided with fluid passages  195 ,  196  and  202  branching from pipe  194 . To supply fluid to the closed fluid circuit of HST  6 , referring to  FIG. 9 , HST  6  is provided with charge fluid passage  195  (in center section  34 , for example) extended from pipe  194 . Charge fluid passage  195  is connected to both main fluid passages  153  and  154  via respective charge check relief valves  197 . A relief valve  198  is connected to charge fluid passage  195  so as to regulate the pressure of fluid supplied to charge check relief valves  197 . When hydraulic pump  48  supplies fluid to hydraulic motor  49  via one of main fluid passages  153  and  154 , the other of main fluid passages  153  and  154  is hydraulically depressed so as to open a charge check valve  197   a  of corresponding charge check relief valve  197 , thereby being supplied with fluid from charge fluid passage  195 . If either main fluid passage  153  or  154  is hydraulically pressurized excessively, a relief valve  197   b  of corresponding charge check relief valve  197  is opened to drain fluid from corresponding main fluid passage  153  or  154  to charge fluid passage  195 . 
     To supply cylinder  37   a  of servomechanism  136  with hydraulic fluid for hydraulically controlling the position of piston  137  via proportional directive control valve  138 , HST  6  is provided with fluid passage  202  (in HST casing  37 , for example) extended from pipe  194 . Further, to supply hydraulic fluid to proportional electromagnetic directive control valve  199  of pilot pressure control mechanism  200  for automatically (electrically) controlling the position of spool  141  of neutral retaining mechanism  143  coupled to spool  138   a  of proportional directive control valve  138 , HST  6  is provided with fluid passage  196  (in top cover  155 , for example) extended from pipe  194 . 
     In this way, referring to  FIG. 4 , reservoir tank  205 , pumps  82  and  150 , hydraulic actuators  162  and  163 , valve unit  176 , PTO clutch  59 , and HST  6  constitute a hydraulic fluid circuit  157  in which pumps  82  and  150  suck fluid from reservoir tank  205 , hydraulic device drive pump  82  delivers hydraulic fluid to hydraulic actuators  162  and  163 , and charge pump  150  delivers hydraulic fluid to PTO clutch  59  and HST  6  via valve unit  176 . 
     Regarding lubrication of components of drive trains  83  and  84 , fluid of fluid sump  158  in main chamber  36   c  of gear transmission casing  36  is used for lubricating them. In this regard, level  158   a  of fluid sump  158  is controlled by reservoir tank  205  so that input shaft  50  and differential output shafts  41 R and  41 L disposed at the same level in the lower portion of main chamber  36   c  of gear transmission casing  36  are submerged in fluid sump  158  so as to submerge the gears thereon in fluid sump  158 , and the other components in main chamber  36   c  are disposed above level  158   a  of fluid sump  158 . Therefore, when the gears in main chamber  36   c  are rotated, rotating gears  56  and  72  on input shaft  50  and rotating differential ring gear  114  on differential casing  115  supporting differential output shafts  41 R and  41 L agitate the fluid of fluid sump  158  and splash up the fluid to the components above level  158   a  of fluid sump  158  directly or via the components of drive trains  83  and  84 , thereby lubricating all the components in main chamber  36   c.    
     It is further understood by those skilled in the art that the foregoing description is given to 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.