Abstract:
A drive gear ( 37 ) having integral drive teeth ( 37   a ) is rotatably mounted on a transmission shaft ( 36 ) for transmitting power to front wheels, and a movable clutch member ( 52 ) having integral driven teeth ( 52   a ) engageable with the drive teeth is slidably but non-rotatably mounted on the transmission shaft. A spring ( 54; 54 A) for biasing the movable clutch member into a direction of causing engagement of the driven teeth with the drive teeth is inserted at a part thereof into an axial bore ( 52   b ) in the clutch member. A piston ( 54 ) is slidably fitted in a cylinder member ( 55 ) which is fixedly mounted on the transmission shaft at a radially inward location of the drive and driven teeth. The transmission shaft includes a fluid passage ( 60 ) which applies fluid pressure to the piston such that the piston is moved so as to move the movable clutch member into a direction of causing disengagement of the driven teeth from the drive teeth. The front wheel drive clutch of this structure is small in the number of parts, is compact in its axial length, and is rich in durability.

Description:
FIELD OF THE INVENTION 
     This invention relates to a front wheel drive clutch for tractors. More particularly, the present invention relates to a front wheel drive clutch provided in a front wheel drive system which is branched from an intermediate position of a rear wheel drive system including left and right brakes for braking left and right rear wheels. 
     BACKGROUND OF THE INVENTION 
     When the above-referenced front wheel drive clutch is structured such that it is engaged by the action of a spring while it is disengaged by the action of fluid pressure, the clutch is normally engaged so as to drive the front wheels together with the rear wheels so that the braking force by the left and right brakes is applied also to the front wheels. Thus, even when an engine shutdown or a trouble with respect to the fluid supply system is caused during the running of tractor, free running distance of the vehicle can be shortened by actuating the brakes. 
     JP, A No. 59-192619 discloses a front wheel drive clutch of the above-referenced type together with a front wheel drive clutch which is engaged by the action of fluid pressure and is disengaged by the action of a spring. The front wheel drive clutch engaged by the action of a spring and disengaged by the action of fluid pressure comprises a clutch housing which is fixedly mounted on a transmission shaft for transmitting power to the front wheels, a drive gear which is rotatably mounted on the transmission shaft and has a boss portion extending into the clutch housing, a plurality of one and the other frictional elements which are slidably but non-rotatably supported respectively by the clutch housing and by the boss portion of drive gear, a slidable cylinder member which is disposed within the clutch housing at one side of the frictional elements, plural leaf springs which are disposed within the clutch housing in series with the slidable cylinder member so as to bias the cylinder member to move in a direction of causing the engagement of frictional elements, a non-slidable piston which is disposed within the slidable cylinder member, and a fluid passage which is perforated in the transmission shaft so as to open into a fluid chamber defined between the slidable cylinder member and non-slidable piston such that the cylinder member is moved away from the frictional elements by fluid pressure applied to the fluid chamber. Thus, the front wheel drive clutch is engaged by the force of leaf springs, which causes the engagement of frictional elements through the movable cylinder member, and is disengaged by fluid pressure which acts upon the cylinder member so as to move away from the frictional elements. This clutch is very lengthy because the plurality of frictional elements, the slidable cylinder member and the leaf springs are arranged in series within the clutch housing. The clutch is also expensive because number of parts including the plurality of frictional elements is large. 
     JP, U No. 52-103252 discloses a claw clutch which is disposed between coaxially arranged two transmission shafts and which is engaged by the action of a spring and is disengaged by the action of fluid pressure. The claw clutch comprises, within a clutch casing, a fixed clutch member which is fixedly mounted on a transmission shaft of the driven side and has integral driven teeth, a movable clutch member which is slidably but non-rotatably mounted on a transmission shaft of the drive side and has integral drive teeth engageable with the driven teeth, a coil spring which is arranged in series with the movable clutch member and acts upon the movable clutch member to move into a direction of causing the engagement of drive teeth with the driven teeth, a cylinder member which is secured to the clutch casing and by which the movable clutch member is slidably received in a fluid-tight manner through a plurality of seal rings, and a pipe which extends through the clutch casing and opens into the cylinder member such that the movable clutch member is moved into a direction of causing the disengagement of drive teeth from the driven teeth by fluid pressure applied to the movable clutch member through the pipe. This claw clutch is lengthy because the movable clutch member and the coil spring are arranged in series. The claw clutch is particularly poor in durability because the movable clutch member is rotated in contact with the fixed cylinder member so that the seal rings between these members will be damaged with the use of claw clutch. 
     A primary object of the present invention is to provide a novel front wheel drive clutch of the type engaged by the action of a spring and disengaged by the action of fluid pressure which is small in the number of parts so as to be inexpensive, which is compactly designed in the axial length of clutch, and which is rich in durability because of the absence of relatively rotated two members damaging the fluid tightness. 
     An attendant object is to provide a novel front wheel drive clutch which is easy in assembling. 
     SUMMARY OF THE INVENTION 
     In a tractor comprising a rear wheel drive system which includes left and right brakes ( 28 ) for braking left and right rear wheels, the present invention relates to a front wheel drive clutch ( 38 ) provided in a front wheel drive system which is branched from an intermediate position of the rear wheel drive system. According to the present invention, the front wheel drive clutch comprises: 
     drive teeth ( 37   a ) integral with a drive gear ( 37 ) which is rotatably mounted on a transmission shaft ( 36 ) for transmitting power to the front wheels; 
     driven teeth ( 52   a ) integral with a movable clutch member ( 52 ) which is slidably but non-rotatably mounted on the transmission shaft ( 36 ), the driven teeth being engageable with the drive teeth ( 37   a ); 
     a spring ( 54 ;  54 A) inserted at a portion thereof into a bore ( 52   b ) which is formed in the movable clutch member ( 52 ) along the axial length thereof, the spring abutting against the movable clutch member to bias the clutch member to move into a direction of causing the engagement of the driven teeth ( 52   a ) with the drive teeth ( 37   a ); 
     a cylinder member ( 55 ) fixedly mounted on the transmission shaft ( 36 ), the cylinder member being disposed radially inwardly of the drive teeth ( 37   a ) and the driven teeth ( 52   a ); 
     a piston ( 57 ) slidably fitted in the cylinder member ( 55 ), the piston abutting against the movable clutch member ( 52 ) so as to move the clutch member into a direction of causing the disengagement of the driven teeth ( 52   a ) from the drive teeth ( 37   a ); and 
     a fluid passage ( 60 ) formed in the transmission shaft ( 36 ) so as to open into an inside of the cylinder member ( 55 ), the fluid passage being operable to apply fluid pressure to the piston ( 57 ) and to thereby move the piston into the direction of causing the disengagement of the driven teeth ( 52   a ) from the drive teeth ( 37   a ). 
     Because the front wheel drive clutch according to the present invention is structured such that the driven teeth ( 52   a ) is engaged with the drive teeth ( 37   a ) by the biasing force of spring ( 54 ;  54 A) and is disengaged from the drive teeth by the sliding movement of movable clutch member ( 52 ) caused by the action of fluid pressure applied to the piston ( 57 ), the clutch is engaged by the action of a spring and is disengaged by the action of fluid pressure. The front wheel drive clutch, which is engaged and disengaged by the engagement and disengagement between the drive teeth ( 37   a ) and the driven teeth ( 52   a ), is small in the number of parts because of the non-employment of a plurality of frictional elements, so that it can be manufactured inexpensively in spite of the fact that it can be operated lightly by means of a fluid supply system in the same manner as the clutch having a plurality of frictional elements. Because a plurality of frictional elements are not employed, because the cylinder member ( 55 ) is fixedly mounted on the transmission shaft ( 36 ) at a radially inward location of the drive teeth ( 37   a ) and driven teeth ( 52   a ), and because the spring ( 54 ;  54 A) is inserted at a portion thereof into the axial bore ( 52   b ) formed in the movable clutch member ( 52 ), the front wheel drive clutch according to the present invention can be designed in a compact manner in the axial direction of clutch. Further, because the cylinder member ( 55 ) fixedly mounted on the transmission shaft ( 36 ) is rotated together with the movable clutch member ( 52 ) which is non-rotatably mounted on the transmission shaft and because the piston ( 57 ) fitted in the cylinder member ( 55 ) is, of course, rotated together with the cylinder member so that relative rotation is not caused between the piston and the cylinder member, there are no members which may cause a damage of seal rings or fluid tightness, so that the durability of clutch is enhanced. 
     According to a preferred embodiment of the present invention, the transmission shaft ( 36 ) extends axially of the tractor and is rotatably supported by a clutch casing ( 50 ) which is secured to a bottom wall of a transmission case ( 48 ) of the tractor with an opening in the bottom wall being closed by the clutch casing, and the clutch casing ( 50 ) is provided with an integral support portion ( 50   a ), which is inserted into the transmission casing ( 48 ) through the opening ( 49 ) and which supports gears ( 32 ,  33 ) for transmitting power to the drive gear ( 37 ), and a cover plate ( 50   c ) which closes a front end opening ( 50   b ) of the clutch casing ( 50 ) and which supports a front end portion of the transmission shaft ( 36 ). Further, the front end opening ( 50   b ) is sized such that the drive gear ( 37 ) and the movable clutch member ( 52 ) can pass through the front end opening. 
     According to this structure, the gears ( 32 ,  33 ) for transmitting power to the drive gear ( 37 ) need not be supported by a separate support member supported by the transmission case ( 48 ) and the gears ( 32 ,  33 ) can be assembled into the transmission case at a same time when the clutch casing ( 50 ) is secured to the bottom wall of the transmission case. Further, the front wheel drive clutch can be assembled very easily. That is, the front wheel drive clutch can be assembled onto the transmission shaft ( 36 ) at an outside of the clutch casing ( 50 ) and, then, the transmission shaft having thereon the front wheel drive clutch is inserted into the clutch casing ( 50 ), having been secured to the bottom wall of the transmission case ( 48 ) with the gears ( 32 ,  33 ) supported by the support portion ( 50   a ), through the front end opening ( 50   b ) which permits passing of the drive gear ( 37 ) and movable clutch member ( 52 ). The assembly of front wheel drive clutch is then completed by securing the cover plate ( 50   c ) to the clutch casing so as to close the front end opening ( 50   b ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention and its attendant advantages will become more readily apparent as the specification is considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagram showing a transmission system employed in a tractor in which a front wheel drive clutch according to an embodiment of the present invention is employed; 
     FIG. 2 is a sectional side view of the front wheel drive clutch; 
     FIG. 3 is an enlarged sectional side view of a part of FIG. 2; 
     FIG. 4 is a diagram showing a disengaging mechanism for the front wheel drive clutch; and 
     FIG. 5 is a sectional side view of a front wheel drive clutch according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 depicts the transmission mechanism of a tractor in which a preferred embodiment of the present invention is employed. There is provided a hollow drive shaft  3  which is connected to an engine  1  through a main clutch  2 . An output shaft  4  is disposed below the drive shaft  3 , and a direction-reversing mechanism  5  for driving the output shaft  4  to rotate selectively into a forward or backward direction is arranged between the drive shaft  3  and the output shaft  4 . The reversing mechanism  5  comprises a forward directional synchronizer clutch  6  and a backward directional synchronizer clutch  7  which are disposed on the drive shaft  4 . The output shaft  4  extends rearwardly, and a hollow transmission shaft  8  is disposed behind and coaxially with the drive shaft  3 . A high/low speed-selecting mechanism  9  for providing the transmission shaft  8  selectively with a high or low rotational speed is arranged between the output shaft  4  and the transmission shaft  8 . The selecting mechanism  9  comprises a high speed fluid-operated clutch  10  and a low speed fluid-operated clutch  11  which are disposed on the output shaft  4 . 
     The transmission shaft  8  extends rearwardly, and a driven shaft  12  is disposed behind and coaxially with the output shaft  4 . A mechanical speed change mechanism  13  for driving the driven shaft  12  to rotate selectively at any one of four speed-change ratios is arranged between the transmission shaft  8  and the driven shaft  12 . The mechanical speed change mechanism  13  comprises two double-acting synchronizer clutches  14  and  15  which are disposed on the transmission shaft  8 . A propeller shaft  16  is disposed behind and coaxially with the driven shaft  12 , and another mechanical speed change mechanism  17  for driving the propeller shaft  16  to rotate selectively at any one of four speed-change ratios is arranged between the driven shaft  12  and the propeller shaft  16 . The mechanical speed change mechanism  17  includes a hollow counter shaft  18 , which is disposed behind and coaxially with the transmission shaft  8  and is driven to rotate by the driven shaft  12 , and a speed-changing gear which is connected to the counter shaft  18  through a speed-reduction gearing  19 . On the propeller shaft  16 , there are mounted a shift gear  20  which is meshed selectively with the above-referenced speed-changing gear and with another speed-changing gear on the counter shaft  18 , and a double-acting clutch  22  which is shifted selectively to a position where a rotatable gear  21  meshing with another speed-changing gear on the counter shaft  18  is coupled to the propeller shaft  16  and to another position where the propeller shaft  16  is coupled directly to the driven shaft  12 . 
     The propeller shaft  16  has, at its rear end, a bevel pinion  23  which is meshed with a larger input bevel gear  26  of a differential gearing  25  for left and right rear wheels  24 . Left and right output shafts  27  of the differential gearing  25  are associated with left and right brakes  28  which are operable to brake the left and right rear wheels  24 , and the left and right output shafts  27  are connected to the left and right rear wheel axles  24 a through left and right speed-reduction devices  29  of a planetary gear type. A differential locking clutch  30  for selectively disabling the differential gearing  25  is mounted on one of the output shafts  27 . 
     On the propeller shaft  16  which constitutes a final shaft of the speed change assembly for the rear wheel drive system, there is fixedly mounted a gear  31  for taking-off the driving power for front wheels. The front wheel drive system comprises integral larger and smaller gears  32  and  33  which are rotatably mounted on a support shaft  34  with the larger gear  32  being meshed with the gear  31  on the propeller shaft, a drive gear  37  which is rotatably mounted on a transmission shaft  36  for transmitting power to the left and right front wheels  35  and which is meshed with the smaller gear  33  on the support shaft, and a front wheel drive clutch  38  which is mounted on the transmission shaft  36 . The transmission shaft  36  is coupled to another transmission shaft  39  which is lengthy in the longitudinal direction of the tractor, and the transmission shaft  39  is coupled to a further transmission shaft  41  having a bevel pinion  40  at its front end. The bevel pinion  40  is meshed with a larger input bevel gear  43  of a differential gearing  42  for the left and right front wheels  35 , and left and right output shafts  44  of the differential gearing  42  are connected to left and right front wheel axles  35   a  through left and right speed-reduction gearings  45  of a planetary gear type. A differential locking clutch  46  for selectively disabling the differential gearing  42  is mounted on one of the output shafts  44 . 
     Structure of the front wheel drive clutch  38  is shown in great detail in FIGS. 2 and 3. In these figures, numeral  48  designates a transmission case which houses the mechanical speed change mechanisms  13  and  17  shown in FIG.  1 . The transmission case  48  has, at its bottom wall, an opening  49  which is closed by a clutch casing  50  secured to the bottom wall using bolts  90 . The clutch casing  50  has an integral support portion  50   a  which extends into the transmission case  48  through the opening  49 , and the above-referenced support shaft  34  is fixedly supported by the support portion  50   a.  The above-referenced transmission shaft  36  is rotatably supported by the clutch casing  50  and by a cover plate  50   c,  which closes a front end opening  50   b  of the clutch casing, and extends forwardly of the cover plate  50   c.  The above-referenced drive gear  37  is rotatably mounted on the transmission shaft  36  through a needle bearing and is meshed with the smaller gear  33  on the support shaft  34 . The cover plate  50   c  is detachably secured to the clutch casing  50  using bolts  91 . 
     A movable clutch member  52  is slidably but non-rotatably mounted on the drive shaft  36  by means of a splined connection  51  using an enlarged splined portion  36   a  of the drive shaft. The front wheel drive clutch  38  is composed of a claw clutch comprising drive teeth  37   a,  which are formed integrally in a boss portion of the drive gear  37 , and driven teeth  52   a  which are formed integrally in the movable clutch member  52  and can be engaged with the drive teeth  37   a.  The movable clutch member  52  includes a plurality (five, for example) of bores  52   b  which extend along the axial direction of drive shaft  36  and open at the opposite side of drive gear  37 . A plurality of coil springs  54 , which are received at their base ends by a ring  53  mounted non-slidably on the drive shaft  36 , extend into the bores  52   b  and abut at their extreme ends against the movable clutch member  52  so as to bias the movable clutch member to slide into a position where the driven teeth  52   a  are engaged with the drive teeth  37   a,  as shown in an upper side of FIG.  2  and in FIG.  3 . 
     At a rear side of the enlarged splined portion  36   a,  there is fixedly mounted on the drive shaft  36 , using a key  56 , a cylinder member  55  which is positioned radially inwardly of the drive and driven teeth  37   a  and  52   a.  In the cylinder member  55  having an open front end, a piston  57  is slidably fitted in a fluid-tight manner using seal rings  58  and  59  in the outer and inner circumferential surfaces of piston  57 . The piston  57  abuts against an annular shoulder  52   c  of the movable clutch member  52 , formed at an inner circumferential surface of the clutch member, such that when the piston  57  is actuated the movable clutch member  52  is displaced into a direction of disengaging the driven teeth  52   a  from the drive teeth  37   a,  as shown in a lower side of FIG.  2 . 
     The transmission shaft  36  includes in it a fluid passage  60  which opens at its rear end into a fluid chamber  61  formed within the clutch casing  50  and at its front end into the cylinder member  55  through a fluid passage bore  55   a  in the cylinder member  55 . The piston  57  is displaced forwardly by fluid pressure applied through the fluid passage  60  whereby the driven teeth  52   a  are disengaged from the drive teeth  37   a.  The clutch casing  50  includes a fluid passage  62  communicating with the fluid chamber  61 , and a fitting  63  is secured to the clutch casing  50  for connecting a fluid piping to the fluid passage  62 . In a bottom wall of the clutch casing  50 , an opening for getting lubricant into and out of the clutch casing  50  is provided and is closed by a threaded plug  64 . 
     Support portion  50   a  of the clutch casing  50  is adapted to be placed into the transmission case  48  through the opening  49  in a condition where the support shaft  34  and gears  32  and  33  are supported by the support portion  50   a,  and then the clutch casing  50  is secured to the bottom wall of transmission casing  48 . A bearing  65  for receiving a rear portion of the transmission shaft  36  has previously been assembled into the clutch casing  50 . The transmission shaft  36  having thereon the drive gear  37 , movable clutch member  52 , coil springs  54  and cylinder member  55  having therein the piston  57  is then assembled into the clutch casing  50  through the opening  50   b  in a condition where the transmission shaft  36  is supported by the cover plate  50   c  through a bearing  66 , and then the cover plate  50   c  is secured to the clutch casing  50 . For permitting such assembly, the diameter of opening  50   b  at the front end of clutch casing  50  is made somewhat larger than the outer diameters of drive gear  37  and movable clutch member  52  so that the drive gear  37  and movable clutch member  52  can pass through the opening  50   b.  Thus, the clutch elements including the drive gear  37  and movable clutch member  52  can be preassembled onto the transmission shaft  36  at an outside of the clutch casing  50 . Of course, the inspection and maintenance of the front wheel drive clutch  38  can be made very easily by getting away the bolts  91  and by getting the transmission shaft  36  having thereon the front wheel drive clutch  38  out of the clutch casing  50  together with the cover plate  50   c.    
     A mechanism for disengaging the front wheel drive clutch  38  is schematically shown in FIG.  4 . An electromagnetic directional control valve  72  is provided between an outlet path  70  of a hydraulic pump  69 , the fluid pressure in which is determined by a relief valve  68 , and a fluid path  71  which includes the above-referenced fluid passage  60  and is connected to the inside of cylinder member  55 . The electromagnetic directional control valve  72  has a neutral position N shown where it connects the fluid path  71  to an oil reservoir so that the front wheel drive clutch  38  is engaged due to the force of coil springs  54 , and a clutch-disengaging position C where it is displaced by the energization of a solenoid  72   a  and where it connects the outlet path  70  to the fluid path  71  so that fluid pressure is applied to the piston  57  whereby the front wheel drive clutch  38  is disengaged. A controller  73  is connected to the solenoid  72   a,  and there is connected to an input side of the controller  73  a switch  75  operated by a front wheel drive lever  74  which is displaced selectively to a four wheel drive position 4WD and to a two wheel drive position 2WD. The switch  75  is turned on at the two wheel drive position 2WD of the front wheel drive lever  74 , and the controller  73  energizes the solenoid  72   a  in the turned-on condition of switch  75  so as to disengage the front wheel drive clutch  38  and to thereby achieve a two wheel drive condition of the tractor. 
     To the input side of controller  73  are also connected switches  77 L and  77 R which are operated respectively by left and right brake pedals  76 L and  76 R for operating the left and right brakes  28  shown in FIG.  1 . Each of the switches  77 L and  77 R is turned on when each brake pedal  76 L,  76 R is operated from a non-braking position OFF to a braking position ON. The controller  73  is adapted such that, when any one of the switches  77 L and  77 R is turned on in the turned-on or turned-off condition of the switch  75 , it keeps the solenoid  72   a  in the deenergized or energized condition so as to maintain the four or two wheel drive condition of tractor, whereas, when both of the switches  77 L and  77 R are turned on, it necessarily deenergizes the solenoid  72   a  so as to achieve the four wheel drive condition of tractor regardless of the turned-on or turned-off condition of the switch  75 . That is, when the left and right brakes  28  are operated so as to brake the tractor, the braking force is necessarily applied also to the left and right front wheels whereby the running distance of vehicle under the braking force is shortened. 
     The structure of PTO (power take-off) drive system of the tractor shown will be described in reference to FIG. 1. A drive shaft  80 , which is connected to the engine  1  through a main clutch  79  of the PTO drive system, extends through the hollow drive shaft  3 , transmission shaft  8  and counter shaft  18  and is coupled, at its rear end, to a transmission shaft  81 . A PTO shaft  82  is provided which extends rearwardly of the tractor, and a speed-reduction gearing  83  is disposed between the transmission shaft  81  and the PTO shaft  82 . 
     FIG. 5 depicts another embodiment of the present invention in which the movable clutch member  52  is provided with an annular bore  52   b  which is coaxial with the transmission shaft  36 . In place of the above-referenced coil springs  54 , plural leaf springs  54 A which are received by the ring  53  extend into the annular bore  52   b  and act upon the movable clutch member  52 . The other parts are same as the corresponding parts of the first embodiment.