Abstract:
A change speed apparatus for a tractor includes a main change speed device ( 24 ) having a combination of gear trains mounted on a first shaft and a second shaft extending parallel to each other, for driving a change speed output shaft ( 58 ) at a plurality of speeds, and a backward and forward drive switching device ( 21 ) cooperable with the main change speed device ( 24 ) to produces forward drive and backward drive in an equal number of speeds. Between the main change speed device ( 24 ) and the backward and forward drive switching device ( 21 ), the forward drive is transmitted through the first shaft ( 45 ) and the backward drive is transmitted through the second shaft ( 44 ). With this construction, a group of gears for returning reversed drive to a forward drive transmission shaft is omitted from the backward and forward drive switching device ( 21 ). Consequently, the backward and forward drive switching device has a reduced fore and aft length, and thus the entire change speed apparatus also has a reduced fore and aft length.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    This invention relates to a change speed apparatus for a tractor. More particularly, the invention relates to a change speed apparatus for a tractor having a main change speed device with a combination of gear trains mounted on a first shaft and a second shaft extending parallel to each other for producing a plurality of speeds on an output shaft, and a backward and forward drive switching device cooperable with the main change speed device to produce forward drive and backward drive in the same number of speeds.  
           [0003]    2. Prior Art of the Invention  
           [0004]    When carrying out a front loading operation with a tractor, switching between forward driving and backward driving has to be made frequently. To perform this operation efficiently, it is desirable that the tractor is driven forward and backward substantially at the same speed. A known change speed apparatus for a tractor that meets the above requirement includes a backward and forward drive switching device as described in Japanese Patent Laying-Open Publication H10-6792 or H11-78560, for example.  
           [0005]    The conventional change speed apparatus for a tractor includes a main change speed device disposed in a transmission system for transmitting power from an engine to wheels. The main change speed device produces a plurality of drive speeds. A backward and forward drive switching device is disposed at an input side (upstream side) or an output side (downstream side) of the main change speed device to provide forward drive and backward drive at the same speed.  
           [0006]    Where, as shown in FIG. 8, for example, a backward and forward drive switching device  152  is disposed at an input side of a main change speed device  151 , the power of an engine  153  is inputted to the backward and forward drive switching device  152  through a main clutch  154 , to be switched to forward drive or backward drive. The drive is then inputted to an input shaft  155  of main change speed device  151  to be changed to one of four speeds, and outputted to an output shaft  156 .  
           [0007]    That is, the drive, whether forward or backward, is inputted to the main change speed device  151  through the same input shaft  155 . For this purpose, the backward and forward drive switching device  152  includes a pair of gear trains  158  and  159  disposed forwardly and rearwardly of a backward and forward drive switching clutch  157 . One of the gear trains is a reversing gear train  158  for providing backward rotation, and the other a back return gear train  159  for returning the backward rotation to the input shaft  155  of main change speed device  151 .  
           [0008]    The same concept of one input shaft and one output shaft has been applied also to the construction having the backward and forward drive switching device disposed at the output side (downstream side) of the main change speed device. That is, to provide backward drive, power received from the output shaft of the main change speed device is reversed by a reversing gear train disposed forwardly of a backward and forward drive switching clutch, and thereafter returned through a back return gear train disposed rearward of the drive switching clutch, to the same shaft used for transmitting forward drive.  
           [0009]    However, each of the conventional change speed apparatus noted above has an increased fore and aft length since the backward and forward drive switching device includes two gear trains, i.e. the reversing gear train and return gear train, forwardly and rearwardly of the switching clutch. The increased fore and aft length of the change speed apparatus results in an increased distance between front wheels and rear wheels. This impairs the small turn performance of the tractor.  
           [0010]    A primary object of this invention, therefore, is to provide a change speed apparatus for a tractor which has a reduced fore and aft length.  
         SUMMARY OF THE INVENTION  
         [0011]    The above object is fulfilled, according to this invention, by a change speed apparatus for a tractor wherein, between the main change speed device and the backward and forward drive switching device, the forward drive is transmitted through the first shaft and the backward drive is transmitted through the second shaft.  
           [0012]    By employing this construction, the conventional backward return gear train may be dispensed with. This not only reduces the number of components but reduces the fore and aft length of the change speed apparatus. The fore and aft length of the transmission case may be reduced correspondingly, which in turn reduces the wheelbase to improve the small turn performance of the tractor.  
           [0013]    In this way, this invention provides a compact change speed apparatus for a tractor.  
           [0014]    Where the backward and forward drive switching device is disposed upstream of the main change speed device, the forward drive is transmitted from the backward and forward drive switching device to the main change speed device through the first shaft, and the backward drive is transmitted from the backward and forward drive switching device to the main change speed device through the second shaft.  
           [0015]    Some conventional tractors have no backward and forward drive switching device. The tractors with and those without the backward and forward drive switching device are regarded as different in specification and design, and thus as different models. This complicates control and management in designing, manufacture and maintenance. According to this invention, the backward and forward drive switching device and the main change speed device are disposed in a transmission case, the backward and forward drive switching device is pre-assembled in a gear case fixable to the transmission case. With this construction, only by assembling to the transmission case the gear case pre-assembled with the backward and forward drive switching device, a tractor without the backward and forward drive switching device may advantageously be converted into one having the backward and forward drive switching device.  
           [0016]    According to this invention, the transmission case may be divided by a partition into a clutch chamber for housing a main clutch for connecting and disconnecting power from an engine, and a transmission chamber for housing the main change speed device, the gear case being disposed in the clutch chamber, the main clutch being operable by a release fork disposed between the main clutch and the gear case, the release fork being curved to bulge toward the main clutch. This construction reduces the distance between the main clutch and gear case, to make the change speed apparatus compact.  
           [0017]    A conventional backward and forward drive switching device is disposed in the transmission chamber storing a lubricant for the transmission case, is therefore remote from a control lever disposed adjacent the steering wheel. It has been difficult to connect the switching device and the control lever, and a complicated connecting mechanism has been required. It is conceivable to place the backward and forward drive switching device in the clutch chamber close to the control lever. However, the clutch chamber is a dry chamber, and it is difficult to place, in the clutch chamber, the backward and forward drive switching device which needs lubrication. In this invention, the backward and forward drive switching device and the main change speed device may be disposed in a transmission case, and the transmission case is divided by a partition into a clutch chamber for housing a main clutch for connecting and disconnecting power from an engine, and a transmission chamber for housing the main change speed device, the backward and forward drive switching device having a control lever thereof disposed on a steering wheel unit, the control lever being interlocked to the backward and forward drive switching device through an operating shaft extending vertically in the clutch chamber. By employing such a construction, the control lever and the operating shaft are disposed close to each other, and may advantageously be interconnected by a simple connecting mechanism. 
       
    
    
       [0018]    Other features, functions, effects and advantages of the present invention will be appreciated upon reading the following description with reference to the drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a side view of a tractor having a change speed apparatus according to this invention.  
         [0020]    [0020]FIG. 2 is a plan view of the tractor shown in FIG. 1.  
         [0021]    [0021]FIG. 3 is a sectional view of a change speed apparatus in one embodiment of this invention.  
         [0022]    [0022]FIG. 4 is a sectional view taken on line IV-IV of FIG. 3.  
         [0023]    [0023]FIG. 5 is a sectional view taken on line V-V of FIG. 3.  
         [0024]    [0024]FIG. 6 is a plan view of a transmission case.  
         [0025]    [0025]FIG. 7 is a schematic view of gears mounted in the change speed apparatus shown in FIG. 3.  
         [0026]    [0026]FIG. 8 is a schematic view of gears mounted in a conventional change speed apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    A preferred embodiment of this invention will be described hereinafter with reference to the drawings.  
         [0028]    [0028]FIGS. 1 and 2 show a side view and a plan view of a tractor to which this invention is applied. The tractor includes a vehicle body having an engine  1  disposed in a forward portion thereof and a transmission case  2  fixed to the rear of engine  1 . The vehicle body is supported by front wheels  3  and rear wheels  4 . The engine  1  is covered by a hood  5 . An instrument panel  6  and a steering wheel unit  7  are disposed rearwardly of the hood  5 . The rear wheels  4  are covered by fenders  8 . A driver&#39;s seat  9  is disposed between the right and left fenders  8 . A PTO shaft (power takeoff shaft)  10  projects from the rear end of the vehicle body.  
         [0029]    The transmission case  2  housing a transmission for transmitting power from the engine  1  to the rear wheels  4 , front wheels  3  and PTO shaft  10 . The transmission includes a change speed apparatus.  
         [0030]    A clutch pedal  11  is disposed below the instrument panel  6  for engaging and disengaging a main clutch described later. The clutch pedal  11  is suspended to be pivotable forward and backward. Amain shift lever  12  is disposed on an upper surface of transmission case  2  between the steering wheel unit  7  and driver&#39;s seat  9 . The steering wheel unit  7  includes a backward and forward drive switching lever  13 . An auxiliary shift lever  14  is disposed laterally of the driver&#39;s seat  9 .  
         [0031]    [0031]FIGS. 3 through 6 show the transmission case  2  and the transmission mounted therein. The transmission case  2  is divided by a first partition  15  into a clutch chamber  16  at the front and a transmission chamber  17  at the rear. The transmission chamber  17  has oil stored at a predetermined level therein for use as lubricant and hydraulic oil. A second partition  18  is disposed rearwardly of the transmission chamber  17 .  
         [0032]    The clutch chamber  16  includes a main clutch  19  and a gear case  20 . The gear case  20  is fixed to the first partition  15 . The gear case  20  contains a backward and forward drive switching device  21 . The gear case  20  is divided into a front case  22  and a rear case  23 .  
         [0033]    The transmission chamber  17  contains a main change speed device  24  for providing four speeds, an auxiliary change speed device  25  for providing a high speed and a low speed, a creep change speed device  26  for providing a superlow speed, and a front wheel driving device  27 . A rear wheel driving device and a PTO change speed device, not shown, are arranged rearwardly of the transmission case  2 .  
         [0034]    The main clutch  19  includes a flywheel  28  fixed to the rear end of a crankshaft of engine  1 , a clutch disk  30  pressed by a pressure plate  29  to an end surface of flywheel  28 , a diaphragm spring  31  for applying a pressing force to the pressure plate  29 , and a release hub  32  for releasing the pressing force of spring  31 .  
         [0035]    The release hub  32  is mounted to be sidable fore and aft on a boss projecting from a front surface of the front case  22 .  
         [0036]    A PTO drive shaft  33  is directly connected to an axial position of the flywheel  28 . The PTO drive shaft  33  extends rearward through the gear case  20  to be coupled to the PTO shaft  10  through the change speed device (not shown).  
         [0037]    The main clutch  19  has a tubular driven shaft  34  extending rearward and acting also as a shuttle input shaft. The driven shaft  34  is coaxially mounted on the PTO drive shaft  33  to be rotatable relative thereto. The clutch disk  30  is fixed to the driven shaft  34 . The driven shaft  34  is relatively rotatably supported at a forward end thereof by the flywheel  28  through a bearing. The driven shaft  34  is relatively rotatably supported in a rearward position thereof by the front case  22  through a bearing. The rear end of the driven shaft  34  is located in the gear case  20 .  
         [0038]    A clutch operating shaft  35  is supported by side walls of the clutch chamber  16  to be rotatable about an axis extending transversely of the vehicle body. A main clutch release fork  36  is fixed to the clutch operating shaft  35 . Bifurcated ends of the fork  36  are engaged with the release hub  32 . The fork  36  is disposed between the main clutch  19  and gear case  20 , and is curved to bulge toward the main clutch  19 . One end portion of the clutch operating shaft  35  projects from the clutch chamber  16 , and the clutch pedal  11  (FIG. 2) is connected to the projecting end portion through a clutch lever  37  and a clutch rod  38 .  
         [0039]    A conventional release fork is curved to bulge rearward. With the release fork  36  curved to bulge forward, the release fork  36  does not interfere with the gear case  20 . This construction realizes a reduction in the distance between the main clutch  19  and gear case  20 . As a result, the transmission has a reduced fore and aft length.  
         [0040]    The main clutch  19  normally transmits the power of engine  1  to the driven shaft  34  through the clutch disk  30 . To break this power transmission, the driver depresses the clutch pedal  11 . A depression of the clutch pedal  11  rotates the clutch operating shaft  35  through the clutch rod  38  and clutch lever  37 . This rotation swings the main clutch release fork  36 , which in turn moves the release hub  32  forward to push the diaphragm spring  31 , thereby canceling its pressing force. Thus, the power transmission to the clutch disk  30  is broken to place the driven shaft  34  in a free state.  
         [0041]    The backward and forward drive switching device  21  is placed, by way of sub-assembly, inside the gear case  20  consisting of the front case  22  and rear case  23 , and then the two cases are provisionally integrated by bonding the mating surfaces thereof with an adhesive. Subsequently, the gear case  20  is fixed to the first partition  15  with common bolts  39 . This construction realizes an improved efficiency of assembly.  
         [0042]    The backward and forward drive switching device  21  in the gear case  20  has a backward and forward drive switching clutch  40  mounted on the rear end of the driven shaft  34  of main clutch  19 . A backward drive gear  41  is freely rotatably mounted on the driven shaft  34  forwardly of the clutch  40 . A reversing intermediate gear  42  is meshed with the backward drive gear  41 . A backward drive input gear  43  is meshed with the intermediate gear  42 . The backward drive input gear  43  has a boss projecting forward. The boss is rotatably supported at an outer circumference thereof by the front case  22  through a bearing. The backward drive input gear  43  defines a splined axial bore. A backward drive input shaft  44  is removably inserted into the splined bore for spline engagement. The backward drive input shaft  44  extends parallel to the driven shaft  34 . The backward drive input shaft  44  is rotatably supported by the rear case  23  through a bearing to extend rearward and have a rear end thereof supported by the second partition  18  through a bearing.  
         [0043]    With the backward drive input shaft  44  removably inserted into the backward drive input gear  43 , the backward and forward drive switching device  21  may be placed in the gear case  20  by way of sub-assembly.  
         [0044]    Aforward drive input shaft  45  is disposed rearwardly of the switching clutch  40  and coaxially with the driven shaft  34  and a coaxial core. The forward drive input shaft  45  is rotatably supported by the rear case  23  through a bearing. The forward drive input shaft  45  is a tubular shaft mounted on the PTO drive shaft  33  to be rotatable relative thereto.  
         [0045]    The backward and forward drive switching clutch  40  is a synchromesh clutch as disclosed in Japanese Patent Publication H8-1221, for example. The switching clutch  40  has a hub  46  mounted on the rear end of driven shaft  34  to be rotatable therewith, a shifter  47  mounted on the hub  46  not to be rotatable but axially movable relative thereto, and synchronizing rings (not shown) disposed forwardly and rearwardly of the hub  46 . The backward drive gear  41  has an engaging portion  48  for engagement with the shifter  47  through one of the synchronizing rings. The forward drive input shaft  45  also has an engaging portion  49  for engagement with the shifter  47  through the other synchronizing ring.  
         [0046]    The shifter  47  is engaged with the shift fork  50 . The shift fork  50  is attached, to be pivotable fore and aft, to a rod  51  fixed between the front case  22  and rear case  23  of the gear case  20 .  
         [0047]    The front case  22  has a forwardly bulging projection  52  formed in a lower portion thereof. A shift fork operating shaft  53  is pivotably supported in vertical posture by the projection  52  and the upper wall of transmission case  2 . This operating shaft  53  is disposed rearwardly of the release fork  36  of main clutch  19  and forwardly of the shifter  47  of backward and forward drive switching device  21 . The operating shaft  53  is vertically divided into an upper shaft portion  54  and a lower shaft portion  55  separably interconnected through a coupling  56 . The lower shaft portion  55  has a yoke  57  fixed thereto in the projection  52 . The distal end of yoke  57  is engaged with the shift fork  50 . The upper shaft portion  54  has an upper end thereof projecting upward from the upper surface of transmission case  2 . The backward and forward drive switching lever  13  is connected to the projecting end through a link (not shown). An operation in the fore and aft direction of the backward and forward drive switching lever  13  attached to the steering wheel unit  7  rotates the operating shaft  53 , which forwardly or rearwardly swings the yoke  57  attached to the lower end thereof. The swing of the yoke  57  moves the shift fork  50  forward or backward, which in turn moves the shifter  47  forward or backward.  
         [0048]    The shifter  47  has a neutral position intermediate in the fore and aft direction. When the shifter  47  is moved forward from the neutral position to engage the engaging portion  48  of backward drive gear  41 , power is transmitted from the driven shaft  34  to the backward drive input shaft  44  through the backward drive gear  41 , intermediate gear  42  and backward drive input gear  43 . Conversely when the shifter  47  is moved backward from the neutral position to engage the engaging portion  49  of forward drive input shaft  45 , power is transmitted from the driven shaft  34  to the forward drive input shaft  45 .  
         [0049]    Since the shift fork operating shaft  53  is disposed in vertical posture, with the upper end connected to the backward and forward drive switching lever  13 , the two components may be connected through a minimum distance. Further, since the operating shaft  53  is vertically divided into two parts, the lower shaft portion  55  may be incorporated into the gear case  20 . This allows the backward and forward drive switching device  21  to form a sub-assembly with ease.  
         [0050]    A tubular change speed output shaft  58  is relatively rotatably and coaxially mounted on the PTO drive shaft  33  in the transmission chamber  17 . The change speed output shaft  58  is rotatably supported at a forward end thereof by an inner surface of the forward drive input shaft  45  through a needle bearing. The rear end of change speed output shaft  58  is rotatably supported by the second partition  18  of transmission case  2  through a bearing.  
         [0051]    The main change speed device  24  for providing four speeds is disposed on the forward drive input shaft  45 , backward drive input shaft  44  and change speed output shaft  58  in the transmission chamber  17 .  
         [0052]    Specifically, the backward drive input shaft  44  has, arranged from rear to front, a first speed gear  60  and a second speed gear  61  freely rotatably mounted thereon, and a third speed gear  62  and a fourth speed gear  63  fixed to the shaft  44 . A first and second speed switching clutch  64  is disposed between the first speed gear  60  and second speed gear  61 . This clutch  64  is a synchromesh clutch as is the backward and forward drive switching clutch  40 .  
         [0053]    The first and second speed switching clutch  64  has a hub  65  fixed to the backward drive input shaft  44 , a shifter  66  axially movably mounted on the hub  65 , and synchronizing ring (not shown) disposed forwardly and rearwardly of the hub  65 . The first and second speed gears  60  and  61  have engaging portions  67  and  68  formed on inward ends thereof, respectively, for engagement with the shifter  66 .  
         [0054]    The change speed output shaft  24  has gears  69  and  70  fixed thereto and in constant mesh with the first and second speed gears  60  and  61 . Further, the output shaft  24  has a gear  71  relatively rotatably mounted thereon and in constant mesh with the third speed gear  62 .  
         [0055]    The forward drive input shaft  45  has a gear  72  formed on a rear portion thereof and in constant mesh with the fourth speed gear  63 . A third and fourth speed switching clutch  73 , which is a synchromesh clutch, is disposed between the rear end of forward drive input shaft  45  and the third speed gear  71  on the change speed output shaft  58 .  
         [0056]    The third and fourth speed switching clutch  73  has a hub  74  fixed to the change speed output shaft  58 , a shifter  75  axially movably mounted on the hub  74 , and synchronizing ring (not shown) disposed forwardly and rearwardly of the hub  74 . The rear end of forward drive input shaft  45  and the third speed gear  71  have engaging portions  76  and  77  formed on inward ends thereof, respectively, for engagement with the shifter  75 .  
         [0057]    [0057]FIG. 7 shows a schematic view of the gears of backward and forward drive switching device  21  and main change speed device  24  described above.  
         [0058]    As shown in FIG. 5, the first and second speed switching shifter  66  and third and fourth speed switching shifter  75  are engaged with a first and second speed switching shift fork  78  and a third and fourth speed switching shift fork  79 , respectively. The shift forks  78  and  79  are fixed, respectively, to a first and second speed switching shift rod  80  and a third and fourth speed switching shift rod  81  extending parallel to each other and on the same horizontal plane. The shift rods  80  and  81  are parallel to the PTO drive shaft  33 . The front and rear ends of shift rods  80  and  81  are supported by the first and second partitions  15  and  18  of transmission cases  2 . The lower end of main shift lever  12  is selectively engageable with the shift forks  78  and  79 . The shift fork  78  or  79  engaged by the shift lever  12  is moved forward or rearward.  
         [0059]    The first and second speed switching shifter  66  has an inside diameter larger than the outside diameter of first speed gear  60 . Thus, the shifter  66 , when shifted for the first speed, could move past the engaging portion  67  of first speed gear  60  (i.e. to be overshifted).  
         [0060]    To prevent such an overshift of the first and second speed switching shifter  66 , as shown in FIG. 6, the first and second speed switching shift rod  80  has a cutout  82  formed in an end region thereof, and a stopper pin  83  projects from the transmission case  2  for engaging a shoulder of the cutout  82 . The stopper pin  83  is attached to the transmission case  2  as screwed into a ball receiving bore  84  formed to receive interlocking balls between the two shift rods  80  and  81 .  
         [0061]    The change speed output shaft  58  has, arranged from rear to front, a low speed gear  85  and a high speed gear  86  rotatable with the change speed output shaft  58 .  
         [0062]    The transmission case  2  has a propelling shaft  87  extending parallel to the change speed output shaft  58 . The propelling shaft  87  is rotatably supported at a forward end thereof by the rear case  23  of gear case  20  through a bearing, and rotatably supported in an intermediate position by the second partition  18  through a bearing. The rear end of the propelling shaft  87  is connected to a rear differential not shown).  
         [0063]    The propelling shaft  87  has, arranged from adjacent the second partition  18  to front, a superlow speed input gear  88 , a low speed gear  89 , a high speed gear  90  and a superlow speed output gear  91  which are rotatable relative to the propelling shaft  87 . In addition, a front wheel drive gear  92  is fixed to the propelling shaft  87  forwardly of the above gears. The low speed gear  89  and high speed gear  90  on the propelling shaft  87  are in constant mesh with the low speed gear  85  and high speed gear  86  on the change speed output shaft  58 , respectively. The superlow speed input gear  88  and low speed gear  89  on the propelling shaft  87  are twin gears.  
         [0064]    The transmission case  2  further includes an intermediate shaft  93  supported to be rotatable through bearings and extending parallel to the propelling shaft  87 . The intermediate shaft  93  has gears  94  and  95  mounted to be rotatable therewith and in constant mesh with the superlow speed input gear  88  and superlow speed output gear  91 .  
         [0065]    The propelling shaft  87  has a high and low speed switching clutch  96  mounted thereon between the low speed gear  89  and high speed gear  90 . The high and low speed switching clutch  96  is a claw clutch having a hub  97  fixed to the propelling shaft  87 , and a shifter  98  mounted on the hub  97  not to be rotatable but axially movable relative thereto. The low speed gear  89  and high speed gear  90  have engaging portions formed on inward ends thereof for engagement with the shifter  98 .  
         [0066]    The propelling shaft  87  has a superlow speed switching clutch  99  mounted thereon forwardly of the superlow speed output gear  91 . The superlow speed switching clutch  99  is a claw clutch having a hub  100  fixed to the propelling shaft  87 , and a shifter  101  mounted on the hub  100  not to be rotatable but axially movable relative thereto. The superlow speed output gear  91  has an engaging portion formed on an inward end thereof for engagement with the shifter  101 .  
         [0067]    As shown in FIG. 3, the high and low speed switching shifter  98  and superlow speed switching shifter  101  are engaged with shift forks  102  and  103 , respectively. The high and low speed switching shift fork  102  is fixed to a shift rod  102   a . The shift rod  102   a  has a yoke  102   b  (FIG. 1) engaged with the rear end thereof and pivotable by the auxiliary shift lever  14  as shown in FIG. 5, whereby the shift fork  102  is moved fore and aft. The superlow speed switching fork  103  is fixed to a shift rod  103   a . The rod  103   a  also is movable fore and aft by operating a the control lever not shown.  
         [0068]    The low speed gears  85  and  89 , high speed gears  86  and  90  and high and low speed switching clutch  96  constitute the auxiliary change speed device  25 . The superlow speed gears  88 ,  91 ,  94  and  95  and superlow speed switching clutch  99  constitute the creep change speed device  26 .  
         [0069]    The front wheel drive gear  92  is meshed with an output gear  105  through an intermediate gear  104 . The output gear  105  is engageable with and disengageable from an output shaft  106  through a front wheel drive clutch  107 . The output shaft  106  is connected at a forward end thereof to a front wheel driving differential not shown.  
         [0070]    The front wheel drive gear  92  and front wheel drive clutch  107  constitute the front wheel driving device  27 .  
         [0071]    The lubricant stored in the transmission chamber  17  is picked up by the gears in rotation to lubricate the change speed devices in the transmission chamber  17 . The lubricant is also sucked up by a hydraulic pump to be supplied as hydraulic oil to various actuators.  
         [0072]    Returning hydraulic oil is supplied from a rearward position of change speed output shaft  58  to a gap between PTO drive shaft  33  and change speed output shaft  58 . The oil flows forward to lubricate the synchromesh clutches and the like. Then, the oil flows out from the forward end of the change speed output shaft  58  to lubricate the backward and forward drive switching clutch  40 , and is thereafter stored in the gear case  20 .  
         [0073]    As shown in FIG. 5, the rear case  23  of gear case  20  defines an oil communicating bore  108 . Through the oil communicating bore  108 , the oil in the gear case  20  returns to the transmission chamber  17 . However, the oil communicating bore  108  is located in a higher position than the lubricant level in the transmission chamber  17 . The lubricant level in the gear case  20  is always maintained higher than the lubricant level in the transmission chamber  17 .  
         [0074]    According to the embodiment having the above construction, the backward and forward drive switching device  21  no longer requires a backward return gear train as required in the prior art. Thus, the change speed apparatus a correspondingly reduced fore and aft length.  
         [0075]    The change speed apparatus for a tractor according to this invention is not limited to the foregoing embodiment. The invention is applicable also to an engine power transmitting system with the backward and forward drive switching device disposed downstream, and not upstream, of the main change speed device.  
         [0076]    According to this invention, the backward and forward drive switching device no longer requires a backward return gear train as required in the prior art. Thus, the change speed apparatus a correspondingly reduced fore and aft length. This construction provides for a reduction in the distance between the front and rear wheels, to achieve an improved small turn performance.  
         [0077]    Further, since the backward and forward drive switching device may be sub-assembled in the gear case, an assembly operation may carried out with increased efficiency.  
         [0078]    The backward and forward drive switching device has an operating shaft and a control lever disposed close to each other. This arrangement results in a simplified connecting mechanism between the shaft and lever.