Patent Publication Number: US-2002005302-A1

Title: Working vehicle

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a working vehicle such as a combine for the continuous reaping and threshing of grain stalks in a farm, such as a cultivating tractor or any other mobile agricultural machine used for the care of farms.  
       BACKGROUND OF THE INVENTION  
       [0002] It has been known to drive the agricultural vehicle at any selected speed by means of a speed change lever, with a possibility that said vehicle could be controlled to change its path of advance by means of a steering wheel. In the prior art of this type, the right and left crawlers receive traction forces through respective belt- or hydraulic-reduction apparatuses controlled by the speed change lever in a step-less manner. A differential gearing is also involved to be controlled by a step-less speed reducer that in turn is operated by the steering wheel so as to smoothly change the difference in rotational speed between the right and left crawlers. If only the steering wheel is operated, movement speed of the vehicle would remain almost constant during its straight advance and its turning motion. Thus, it has been necessary for drivers of those vehicles to lower the running speed thereof when causing them to make a ‘spinning turn’, of a relatively small radius. In other words, those drivers have had to quickly maneuver both the steering wheel and speed change lever in order to make a turn at an end bare area in any farm. It has also been known to automatically lower the travelling speed of the vehicle in response to the maneuvering of its steering wheel. In this case, the speed will be raised also automatically when said steering wheel is operated to resume the vehicle&#39;s straight running. Any troublesome operations for speed change of the vehicle will no longer be required even when the steering wheel is operated to cause the vehicle to make a spinning turn. However, such a system has proved problematic or disadvantageous in that the vehicle speed would unintentionally be reduced even during the vehicle&#39;s works free from any spinning motion. In this event, operation efficiency would be impaired and such an unwanted speed reduction in a wet farm would cause therein a non-smooth or unreliable advance of the vehicle. The steering mechanism may be designed such that rotation of the steering wheel brings about a lesser change in vehicle speed. Such a modification also is unsatisfactory because the vehicle will not be able to make a quick turn on road on one hand and a spinning turn in dry farms.  
       DISCLOSURE OF THE INVENTION  
       [0003] The present invention relates to a working vehicle such as a mobile agricultural machine comprising a pair of left-side and right-side traction crawlers  2  that are driven through a speed change apparatus  25  at any traveling speed selected in a step-less manner by means of a speed change lever  73 , and the working vehicle further comprising a steering apparatus  28  that is operated by means of a steering wheel  19  so as to differentiate the speeds of left-side and right-side crawlers  2  relative to each other also in a step-less manner. The working vehicle provided herein is characterized in that the traveling speed controlled by means of the speed change lever  73  is reduced in proportion to an angular displacement of the steering wheel  19 , wherein the ratio of speed reduction to a steered angle, i.e., the angular displacement of said wheel  19  can be latered freely at any time. By virtue of this feature, the ratio of speed reduction to the steered angle may be lowered to cause the vehicle to make a gentle turn within wet farms and, to thus move thereon smoothly for efficient performance of various works therein.  
       [0004] In the invention, the speed change lever  73  is operatively connected to the speed change apparatus  25  by a cooperative interconnect member  178  designed such that the traveling speed will be reduced in proportion to operation degree of the steering wheel  19 . A third or stand-by interconnect member  179  does also intervene between these lever  73  and apparatus  25  so as to operatively and occasionally connect them to each other. It is possible to select any desired one of said interconnect members  178  and  179  so as to alter traveling behavior and turning behavior of the vehicle, without modifying control mechanism in the speed change apparatus  25 . Thus, the vehicle can selectively be driven to make slow and gentle turns or, alternatively quick and acute turns.  
       [0005] Also in the invention, the so-called spinning turn mode in which one crawler  2  located ‘inside’ the other with respect to radius of turn locus is driven in reverse direction, can be selected on demand to substitute for the non-spinning turn mode in which both the ‘inside’ and ‘outside’ crawlers  2  are rotated in the same direction even at a maximum angular operation of the steering wheel  19 . If the spinning turn is selected, then the vehicle will sharply spin on road or in dry farms. If contrarily the non-spinning turn is selected, then it will make sure turns in wet farms or on mud. By virtue of such a changeover from the former mode to the latter, or vice versa, the vehicle is improved in its steering performance and maneuverability.  
       [0006] Further, the steering apparatus  25  is constructed such that a maximum output torque will be maintained in the vehicle at lowered travel speeds, if and when the steering wheel  19  is operated beyond a certain predetermined limit. The vehicle can now make surer and easier turns at lower travel speeds, because variation in its turning torque is diminished and not seriously affected by any change in resistance against its travel.  
       [0007] It also is ensured herein that the ‘outside’ and ‘inside’ crawlers  2  be driven at different speeds when making a turn controlled by the steering apparatus  28 , such that the ‘outside’ crawler  2  is kept substantially at the same speed as that at which it will move straightly under control by the speed change apparatus  25 . This is enabled by automatically controlling the speed change apparatus  25  in response to the steering apparatus  28 , and more in detail, by spontaneously lowering the speeds of those ‘outside’ and ‘inside’ crawlers  2  if and when the latter apparatus is operated beyond the given limit. Thus, the ‘outside’ crawler  2  will not move faster when making a turn than when moving straight, although the steering apparatus  28  commands the right-side and left-side crawlers to gradually change their speeds in harmony but in opposite directions, viz., accelerating one and decelerating the other. In other words, the ‘outside’ crawler  2  will no longer operate undesirably at any speed remarkably higher than a normal speed of straight advance, whereby the crawlers do not slide sideways at all even when making a turn of small radius at a bare end area of a farm. The vehicle turning in this way will now take easily its next position for a subsequent travel course, without causing an operator of this vehicle to slow down it so as to decrease speed of its center as compared with the speed of straight running. An engine  21  of the vehicle is now protected well from slowing down due to any overload caused by an increased resistance which the crawlers making a turn would encounter. By virtue of these features, the vehicle can make easier turns at the bare end areas and can be more simply operated to do works making reciprocal travels between those bare ends. Both the crawlers  2  are decelerated in response to the steering action beyond the limit, as discussed above, so that traction force for turn will not show any greater change than ever, even though a turn radius being reduced by steering the crawlers  2  would increase resistance against traction of them. Smoother spinning turns are now possible at those bare end areas in any farm yard. Steering operation can now smoothly decrease travel speed whilst maintaining turning force necessary for the crawlers to make a turn such as a spinning turn, thereby affording smaller radius turns at lowered travel speeds.  
       [0008] The vehicle may comprise a steering input shaft  87  rotated by the steering wheel or the like member  19 , a speed change input shaft  91  rotated by the speed change lever or the like member  73 , a speed change mechanism  124  for operatively connecting the latter input shaft  91  with the speed change apparatus  25 , and a steering mechanism  118  for operatively connecting the former input shaft  87  with the steering apparatus  28 . Degree to which the steering mechanism  118  can be operated is made herein be proportional to such an extent to which the speed change mechanism  124  will be operated. In other words, the degree of operation of the former mechanism  118  will be elevated to follow an increased operation extent of the latter one  124  in such a manner that the faster the vehicle advances, the more intensively it will be steered. Thus, irrespective of degrees of angular displacement which the steering wheel  19  will make from time to time, radius of turning locus of the right-side and left-side crawlers  2  will be kept almost constant. This will assist the vehicle to more readily change in its speed for agricultural works or alter its running path to be in parallel with any row of crops.  
       [0009] A steering input member  95  and a speed change input member  96  may be disposed on the steering input shaft  87  so as to rotate about the axis of the speed change input shaft  91 . A speed change output member  120  secured to a speed change shaft  119  is operatively connected by a speed changing link  121  to the speed change input member  96 , thereby forming the speed change mechanism  124 . A steering output member  114  secured to a steering shaft  113  is operatively connected by a steering link  115  to the steering input member  95 , thereby forming the steering mechanism  118 . Thus, the steering input shaft  87  may rotate to actuate both the steering and speed change input members  95  and  96  in unison so that the turning of the vehicle will be synchronized with the slowing down thereof. In addition, the speed change input member may rotate to actuate both the speed change and steering input members  96  and  95  in unison so that the radius of turn can easily be increased or decreased in harmony with speed change, with a neutral position of the speed change mechanism rendering idle the steering mechanism.  
       [0010] One and the same closed box-shaped casing  71  houses the steering input shaft  87  and steering mechanism  118  for linking the steering wheel  19  to the steering apparatus  28 , together with the speed change mechanism  124  for linking the step-less reducer  25  to the speed change lever  73 . The mechanisms  118  and  124  will thus constitute a unit within such a common casing  71 , enhancing precision and simplicity of assembling works. Further, those mechanisms  118  and  124  can easily be associated with each other within said casing  71  in such a manner that travel speed is lowered in unison with steering operation. Any raised load to steer the crawlers  2  will surely protect the engine  21  from being overloaded, because each crawler is simultaneously geared down at that time to a lower velocity. Therefore, farm works will now be improved in smoothness, efficiently using power from the engine  21  and by virtue of a higher steerability.  
       [0011] The speed change input shaft  91  caused to rotatingly swing by the speed change lever  73  or the like is journaled in the steering column or casing  71  by means of bearings  92 . This structure is advantageous in that assembly accuracy is enhanced to diminish rickety motion of the input shaft and relevant members, improving dynamic strength thereof and decreasing frictional resistance applied thereto. Reliability and adjustability of operation are improved for the speed change mechanism, also saving labor in maintenance thereof.  
       [0012] The speed change link  121  connected to the speed change apparatus  25  as well as the steering link  115  connected to the steering apparatus  28  are in turn operatively connected to the interconnect or speed change input member  96 , which is removably fixed on the steering input member  95  operated with the steering wheel  19 . One of these members  96  is easy to adjust in position relative to the other  95 , thereby making it possible to maintain substantially constant the controlling relationship between speed change and steering apparatuses  25  and  28 . This feature facilitates change in travel speed and improves steerability, further enabling the mounting and dismounting of the interconnect  96  together with the links  121  and  115  fixed thereon so that assembly and maintenance of them are rendered easier. In addition, position of the steering input member  95  can be altered relative to the speed change and steering links  121  and  115 . Positional relationship between these links will no way be adversely affected, but rendering easier the works for assembly, maintenance and/or adjustment of said member and links.  
       [0013] Further, the steering input member  95  and speed change input member  96  may operatively be connected to the steering input shaft  87  so as to freely rotate about the axis of the speed change input shaft  91 . The speed change output member  120  operatively born by the speed change output shaft  119  is in turn connected operatively by the speed change link  121  to the speed change input member  96 , thereby constituting the speed change mechanism  124 . The steering output member  114  operatively born on the steering output shaft  113  is in turn connected operatively by the steering link  115  to the steering input member  95 , thus forming the steering mechanism  118 . A speed change joint  123  serves as a hinge between the speed change output member  120  and link  121 , and a steering joint  117  serves as a hinge between the steering output member  114  and link  115 . Both the joints  123  and  115  are disposed in alignment with the axis ‘D’ of the steering input shaft  87 . The speed change input member  96  and link  121  swing along a reversed-conical locus, with the steering input member  95  and link  115  likewise swinging along another reversed-conical locus. By virtue of these features, the vehicle can easily and smoothly be accelerated or decelerated and readily brought into parallel with a row of corps during agricultural works. Its travel speed can be lowered while being steered to make a turn, and can also readily be changed to increase or decrease radius of its turning or to cease it. The steering wheel  19  or the like, the speed change and steering mechanisms  124  and  118  are now integrated to form a compact assembly. The so-called “reversed steering” will never take place when forward travel is switched over to rearward travel, or vice versa. Further, the speed change and steering output members  120  and  114  are now easily designed and assembled, simplified in structure and improved in function. In addition, both the speed change and steering mechanisms  124  and  118  are arranged around the axis of the steering input shaft  87  so as to provide a highly compacted and functional assembly that will improve the vehicle in its running properties and its steerability. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is a side elevation of a combine shown in its entirety;  
     [0015]FIG. 2 is a plan view of the combine also shown in its entirety;  
     [0016]FIG. 3 is a scheme of a transmission of the combine;  
     [0017]FIG. 4 is a perspective view of a system for operating a speed changing mechanism in connection with a steering mechanism;  
     [0018]FIG. 5 is a scheme of operation of the mechanisms;  
     [0019]FIG. 6 is a side elevation of a steering column;  
     [0020]FIG. 7 is an enlarged side elevation of an upper part of the column;  
     [0021]FIG. 8 is an enlarged side elevation of a lower part of the column;  
     [0022]FIG. 9 is a front elevation of the steering column;  
     [0023]FIG. 10 is an enlarged front elevation of an upper part of the column;  
     [0024]FIG. 11 is an enlarged front elevation of a lower part of the column;  
     [0025]FIG. 12 is a plan view of the system shown in FIG. 4;  
     [0026]FIG. 13 is an enlarged plan view of a part of the system;  
     [0027]FIG. 14 is a horizontal cross section of he steering column;  
     [0028]FIG. 15 is a scheme showing a setting operation made in the column;  
     [0029]FIG. 16 is a partial plan view of an upper part of the column;  
     [0030]FIG. 17 is another partial plan view;  
     [0031]FIG. 18 is a scheme of operation of the part shown in FIG. 16;  
     [0032]FIG. 19 is a scheme of operation of a speed change apparatus;  
     [0033]FIG. 20 also is a scheme of a main speed changer;  
     [0034]FIG. 21 is a scheme of operation of a steering apparatus;  
     [0035]FIG. 22 is a diagram of relationship between the main speed changer and a steering wheel;  
     [0036]FIG. 23 is a front elevation of a switching-over mechanism for alteration of output level in response to speed change;  
     [0037]FIG. 24 is a fragmentary scheme of the switching-over mechanism;  
     [0038]FIG. 25 is a scheme showing actuation of the speed change and steering mechanisms;  
     [0039]FIG. 26 is a plan view of parts relevant to the steering wheel;  
     [0040]FIG. 27 shows a circuit for controlling the speed change and steering mechanisms;  
     [0041]FIG. 28 is a diagram of relationship between the steering wheel and an output power for steering the vehicle;  
     [0042]FIG. 29 is a diagram of relationship between the steering wheel and an output power for speed change;  
     [0043]FIG. 30 is another diagram of relationship between the steering wheel and the output power for speed change;  
     [0044]FIG. 31 is a diagram of relationship between a main speed change lever and the output power for steering the vehicle;  
     [0045]FIG. 32 is another diagram of the relationship between the steering wheel and the output power for steering the vehicle;  
     [0046]FIG. 33 is a flow chart of a program for controlling the speed change and steering mechanisms; and  
     [0047]FIG. 34 is a diagram of output power applied to traction crawlers that are being steered.  
    
    
     BEST MODES OF CARRYING OUT THE INVENTION  
     [0048] Now some embodiments of the invention will be described in detail referring to the drawings. FIG. 1 is an overall side elevation of a combine and FIG. 2 is a plan view thereof, wherein the reference numeral  1  denotes a truck frame supporting a pair of right- and left-side crawlers  2 . A chassis  3  is mounted on the frame  1 . A threshing section  4  comprises a feed chain spanned along the left side of the combine, and a threshing drum  6  and a treatment drum  7  incorporated in said part. A reaping section  8  comprises a reaping blade  9  and a stalks-conveying assembly  10 . The reference numeral  11  denotes an oil-hydraulic cylinder for lifting and lowering the reaping section  8 , through a reaper frame, with the further numeral  13  denoting a waste stalks disposal section located to face the end of a waste stalks chain  14 . A crops tank  15  continuously receives crops conveyed thereto through a crops lift  16 . A discharging auger  17  discharges the crops from the tank  15  and out of the combine. The numeral  18  denotes an operator&#39;s station where a round steering handle (wheel)  19  and an operator&#39;s seat  20  are disposed. An engine  21  is located below the operator&#39;s seat  20 . This combine is constructed so that grain stalks are continuously reaped and then threshed.  
     [0049] As shown in FIG. 3, incorporated in a transmission case  22  driving the crawlers  2  are a pair of first oil-hydraulic pump  23  and first oil-hydraulic motor  24  that constitute a main travel speed change apparatus  25 . This apparatus is thus of an oil-hydraulic and step-less type. Further incorporated in said case  22  are a pair of second oil-hydraulic pump  26  and second oil-hydraulic motor  27  that constitute a steering apparatus also of an oil-hydraulic and step-less type. An output shaft  21   a  from the engine  21  is connected by transmission belts  30   a  and  30   b  to respective input shafts  29   a  and  29   b  of the first and second hydraulic pumps  23  and  26 , respectively, so as to drive them.  
     [0050] A subsidiary speed change mechanism  32  and a differential mechanism  33  do operatively connect an output shaft  31  of the first hydraulic motor  24  to respective wheels  34  driving the crawlers  2 . The differential mechanism  33  comprises a pair of right- and left-side planetary gear trains  35  and  35 . Each planetary gear train  35  substantially consists of a sun gear  36 , a group of three planetary gears  37  in mesh with outer periphery of the sun gear  36 , a ring gear  38  that stands in turn in mesh with the planetary gears  37 .  
     [0051] Those planetary gears  37  are rotatingly held on a carrier  41  whose shaft  40  is aligned with the shaft  39  of the sun gear. The right- and left-side carriers  41  face one another, with the sun gears  36  intervening between them. Each ring gear  38  having inner peripheral teeth  38   a  in mesh with the planetary gears  37  is disposed coaxial with the sun gear shaft  39 . Each ring gear  38  is journaled on and freely rotatable about the carrier shaft  40 , that extends outwards to form an axle for supporting and driving the drive wheel  34 .  
     [0052] The hydraulic step-less speed changer  25  comprises an inclined rotary plate installed in the first oil-hydraulic pump  23 . This plate can be controlled to alter its angle of inclination so that the first hydraulic motor  24  can be driven selectively in one of normal and reversed directions and also at any selected speed. A torque from the first motor  24  will be transmitted at first to a gear  42  fixed on the output shaft  31 . The torque is then transmitted through further gears  43 ,  44  and  45  and the subsidiary speed change mechanism  32  to a central gear  46  fixed on the sun gear shaft  39 , thereby driving the sun gear  36 . The subsidiary speed changer  32  has a subsidiary speed change shaft  47  on which the gear  44  is fixed and a parking brake shaft  49 . This shaft  49  carries a gear  48  fixed thereon and indirectly engaging with the central gear  46  via the gear  45  described above. A couple of gears  50  and  51  respectively born on the subsidiary shaft  47  and braking shaft  49  are for a range of low speeds. A further couple of gears  52  and  53  on those shafts  47  and  49  are for a range of intermediate speeds, with a still further couple of a gear  54  and the above-mentioned gear  48  being for a range of high speeds. Either a slider  55  for lower speeds or another slider  56  for higher speeds will be operated to take any selected position giving a desired speed between the lowest and highest speeds. There is provided a neutral zone between the low speed range and the intermediate speed range, and another neutral zone between the intermediate speed range and the high speed range as well. A parking brake  57  is connected to the brake shaft  49 . A ‘PTO’ shaft  58  for transmitting torque to the reaping section  8  is operatively connected to the subsidiary speed change shaft  47 , via another couple of gears  59  and  60  and a one-way clutch  61 .  
     [0053] As discussed above, power of the first hydraulic motor  24  will be applied to the sun gear shaft  39  through the central gear  46 . Torque received by this shaft  39  will then be directed to the right- and left-side carrier shafts  40  through the respective planetary gear trains  35 . Those carrier shafts will thus rotate the respective drive wheels  34  for driving the right- and left-side crawlers  2 .  
     [0054] The steering apparatus  28  for turning or swiveling the vehicle is also of the oil-hydraulic and step-less type. An inclined rotary plate installed in the second hydraulic pump  26  is controllable to alter its angle of inclination. The second hydraulic motor  27  can thus be driven selectively in one of normal and reversed directions and also at any selected speed. Associated with the hydraulic members are a brake shaft  63  and a clutch shaft  65 . An anti-steering brake  62  is connected to the brake shaft, with a steering clutch  64  being fixed on the clutching shaft  65 . Outer peripheral teeth  38   b  of the ring gears  38  are always in mesh with right- and left-side input gears  66  and  67 , respectively. An output shaft  68  of the second hydraulic motor  27  is operatively connected to the clutch shaft  65  through the brake shaft  63  and the steering clutch  64 . This clutch shaft  65  is connected in turn to the right-side input gear  67  through a normal direction gear  69  (rotating only in a normal direction), and also connected to the left-side input gear  66  through the normal-direction gear  69  and a reversed gear  70  (rotating only in a reversed direction). Both the sliders  55  and  56  in the subsidiary speed changer may be shifted to their neutral zones to actuate the brake  62  and to render idle the clutch  64 , in one operational state of the vehicle. In the other case wherein either slider in subsidiary speed changer is shifted to its operative position, the brake  62  remains idle and the clutch  64  stands ready for shift into one of its operative positions. Thus, the outer peripheral teeth  38   b  of the right-side ring gear  38  will be given the output from the motor  27  through normal direction gear  69 , on one hand. Simultaneously, the outer peripheral teeth  38   b  of the left-side ring gear  38  will be given the output from the motor  27  through normal direction gear  69  and the reversed direction gear  70 , on the other hand. If the second oil-hydraulic motor  27  is occasionally brought into normal rotation, then the left-side ring gear  38  will be caused to rotate reversedly, with the right-side one  38  being caused to rotate normally at the same speed as the former gear. If contrarily that motor  27  is brought into reversed rotation, then the left-side ring gear  38  will be driven normally, with the right-side one  38  rotating reversedly also at the same speed.  
     [0055] However, the second motor  27  for turning the vehicle may be held not operative in any direction so that both the right- and left-side ring gears  38  remain stationary. In this case, the first oil-hydraulic motor  24  for driving the vehicle will operate as follows. Torque output from this motor  24  will be led to the central gear  46  so as to drive both the right- and left-side sun gears  36  to spin in harmony at the same speed. Consequently, the planetary gears  37  and carriers  41  in the right- and left-side planetary gear trains  35  will transmit torque to the right- and left-side crawlers  2  to rotate at the same speed and in the same direction, causing the vehicle to straightly advance forwards or backwards. In another case wherein the first hydraulic motor  24  for driving the vehicle to travel is switched off to render inoperative the right and left sun gears  36 , the second motor  27  for turning the vehicle may be switched on to activate the planetary gear trains  35  in opposite directions. If the left-side gear train is driven in normal direction, with the right-side one reversed, then right- and left-side crawlers  2  will rotate in such opposite directions as causing the vehicle to spin ‘anti-clockwise’. If contrarily the left-side gear train is driven in reversed direction, with the right-side one in normal direction, then those crawlers  2  will rotate also in opposite directions but causing the vehicle to spin ‘clockwise’. In still another case wherein both the first and second oil-hydraulic motors  24  and  27  concurrently operate, respectively causing travel and turn of the vehicle, it will curve itself leftwards or rightwards. Radius of such a curved locus in this case depends upon rotational speed of the second oil-hydraulic motor  27 .  
     [0056] As shown in FIGS. 2 and 4 to  13 , a steering column  71  disposed in the operator station  18  stands upright from a frontal end thereof. The steering wheel  19  located above this column  71  is capable of operated to rotate about a vertical axis. A side column  72  overlying the transmission  22  is mounted leftwards and beside the operator station  18 . A main speed change lever  73 , a subsidiary speed change lever  74 , a reaping clutch lever  75  and a threshing clutch lever  76  are operatively attached to the side column  72 . The steering column  71  consists of split and vertical halves that are made each by processing an aluminum alloy cast. Those halves are consolidated together using bolts  77  to give the steering column a box-like appearance.  
     [0057] A tilt base  78  is formed integral with an upper end portion of the steering column  71 . Tilt brackets  80  connected by pivot bolts  79  to the tilt base  78  are capable of angular displacement relative thereto. A tilt lever  81  serves to set the tilt brackets  80  at any desired angular position. A tubular casing  82  extending upwardly from a top cover  83  of the column  71  has a lower end secured to the tilt brackets  80 . An upper steering shaft  84  rotatingly supported in the tubular casing  82  has an upper end to which the steering wheel  19  is attached. By operation of the tilt lever  81 , the steering wheel  19  can be tilted fore and aft when adjusting and fixing position thereof to be convenient to a driver.  
     [0058] A lower steering shaft  86  has an upper end connected by a universal joint  85  to a lower end of the upper steering shaft  84 . The lower steering shaft  86  and an upper end of the steering input shaft  87  are rotatingly supported in the upper end portion of the steering column  71 . A gear  88  fixed on the lower steering shaft  86  is in mesh with a sector gear  89  fixed on the steering input shaft  87 , so that these shafts  86  and  87  are always in an operative engagement with each other. The steering input shaft  87  accommodated in the steering column  71  does stand generally upright therein and extend almost coaxially with this column.  
     [0059] A bearing member  90  that is located in the left-hand half of the steering column  71  is removably attached to a middle height thereof. Bearings  92  fitted in this member  90  support in a cantilever fashion an end of the speed change input shaft  91 . This shaft  91  is thus extends generally transversely and horizontally to rotate relative to the bearing member. A further universal joint  93  connects a lower end of the steering input shaft  87  to the upper end of a fulcrum shaft  94  to which a steering input member  95  is fixed. The speed change input shaft  91  holds this steering input member  95  to be rotatable therein and relative thereto. An input connector  96  is removably attached to the steering input member  95 , with set-screws  97  serving to fix position of the former  96  relative to the latter  95 . A further bearing  95   a  enables the speed change input shaft  91  to support thereon the steering input member  95 , allowing same to rotate around the steering input shaft  87 . Forward or reverse rotation of this steering input shaft  87  will cause the input member  95  to rotate in a forward or reverse direction around the generally vertical axis of said input shaft  87 . Forward or reverse rotation of the speed change input shaft  91  will cause both the fulcrum shaft  94  and steering input member  95  to tilt forwards or backwards around the generally horizontal and transverse axis of said input shaft  91 . The universal joint  93  described above is located at a position where the vertical axis of the steering input shaft  87  intersects at a right angle the horizontal axis of the sped change input shaft  91 . Thus, as the steering wheel  19  is operated to rotate the steering input shaft  87  clockwise or anti-clockwise, both the steering input member  95  and the input connector  96  will rotate in unison and clockwise or anti-clockwise.  
     [0060] A main speed change shaft  99  is supported in a lower forward region in the steering column  71  so as to be rotatable therein. This main speed change shaft  99  that is generally horizontal and transverse has a left-hand end protruding leftwards and outwardly from the left side wall of the steering column  71 . Links  101  and  102  and a rod  104  having a turn buckle  103  for length adjustment thereof do connect an interconnect shaft  100  to the main speed change shaft  99 . This interconnect shaft  100  is rotatably supported on the chassis  3  beneath the side column  72 . As shown in FIG. 4, a fulcrum plate  106  has a fulcrum shaft  105  also rotatably supported by the chassis  3 . A cylindrical shaft  107  fixed on the fulcrum plate  106  fits on the basal end of the main speed change lever  73 , allowing same to rock sideways. Further links  108  and  109  connect this fulcrum plate  106  to the interconnect shaft  100 , so that as the main speed change lever  73  rocks fore and aft around the fulcrum shaft  105 , the main speed change shaft  99  will rotate forwards or reversedly to thereby effect main speed change operation. On the other hand, the speed change input shaft  91  is connected to the main speed change shaft  99  through a tie-rod  110 , an upper plate  111  and a lower link  112 . Therefore, as the main speed change lever  73  operates the main speed change shaft  99  to rotate forwards or reversedly, the steering input member  95  will tilt fore and aft about the axis of the speed change input shaft  91 .  
     [0061] A steering output shaft  113  of a cylindrical shape rotatingly fits on the main speed change shaft  99 , with a link-shaped steering output member  114  fixed on the output shaft  113 . A rod-shaped steering link  115  has its upper end connected by a universal joint-shaped member  116  to the input connector  96 , and its lower end similarly connected by a ball joint-shaped member  117  to the steering output member  114 . A steering mechanism  118  constructed in this manner will function when the vehicle is steered to change or adjust its running course.  
     [0062] A speed change output shaft  119  located above and in parallel with the steering output shaft  113  is also incorporated rotatably in the steering column  71 , with a link-shaped speed change output member  120  fixed on the output shaft  119 . A rod-shaped speed change link  121  has its upper end connected by a universal joint-shaped member  122  to the input connector  96 , and its lower end similarly connected by a ball joint-shaped member  123  to the speed change output member  120 . A change speed mechanism  124  constructed in this manner will function when the vehicle is accelerated or decelerated or switched over from forward travel to rearward travel, or vice versa.  
     [0063] A double cylinder assembly that consists of an inner steering maneuver shaft  125  and an outer speed change maneuver shaft  126  hollow and rotating relative thereto is disposed in a lower and rear portion of the steering column  71 . A bearing assembly  127  holding the double cylinder assembly in place and allowing it to rotate is located intermediate a left and right lateral sides of said column  71 . The speed change output shaft  119  is connected to an upper end of the speed change maneuver shaft  126 , through speed change links  129  and  130  having their ends connected by ball joints to a common variable length rod  128 . The steering output shaft  113  is likewise connected to an upper end of the steering change maneuver shaft  125 , through steering links  132  and  133  having their ends connected by ball joints to another common variable length rod  131 .  
     [0064] Both the maneuver shafts  125  and  126  extending coaxially with each other stand upright on the bottom of the steering column  71 . Upper portions of those shafts  125  and  126  are disposed in said column and operatively connected to the speed change output shaft  113  and steering output shaft  119 . Lower ends of those maneuver shafts  125  and  126  protrude down from the column&#39;s  71  bottom so as to extend below an operator step  134  for the operator seat  20 . An output control shaft  135  of the speed change apparatus  25  is fixed on one end of a control arm  136 . The other end of this speed change control arm  136  is connected to the lower end of the speed change maneuver shaft  126 , by a link  139  and an adjustable length rod  138  having a turn buckle  137  integral therewith. With the output shaft  135  being rotated forward or backward, the inclined rotary plate in the first hydraulic pump  23  will thus be changed in its angle. The first hydraulic motor  24  controlled in this way with respect to its rotational speed and direction, forward or backward, will function to accelerate or decelerate the vehicle in a step-less manner and also to alter its travel direction from forward to backward, or vice versa. Similarly, an output control shaft  140  of the steering apparatus  28  is fixed on one end of a steering control arm  141 . The other end of this steering control arm  141  is connected to the lower end of the steering maneuver shaft  125 , by a link  144  and an adjustable length rod  143  having a built-in turn buckle  142 . With the output shaft  140  being rotated forward or backward, the inclined rotary plate in the second hydraulic pump  26  will thus be changed in its inclination angle. The second hydraulic motor  27  controlled in this way with respect to its rotational speed and direction, forward or backward, will function to the steered angle of the vehicle also in a step-less manner and also to alter its turning direction from clockwise to anti-clockwise, or vice versa.  
     [0065] An acceleration lever  145  pivoted on a right side wall of the steering column  71  protrudes outwards to be caused to swivel fore and aft. An acceleration wire  146  connecting this lever  145  to the engine  21  extends inside and along the frontal face of said column and protrudes down from its base. This structure enables the acceleration lever  145  to manually control rotational speed of the engine. A maintenance hand-hole  147  opened in the rear wall of the steering column  71  is normally closed with a removable lid  148 .  
     [0066] It will now be apparent that actual degree of operating the steering mechanism  118  depends proportionally upon actual extent to which the speed change mechanism  124  is operated. This means that any acceleration of the vehicle will automatically increase more or less a steering factor or ratio, whereas deceleration will decrease it. Thus, any given angle of the steering wheel  19  will give almost the same turning radius, that is intrinsic to that angle and irrespective of actual speed of the vehicle advancing at that time. The vehicle can now easily change its travel speed and readily adjust its advancing path into parallel with any desired row of crops or the like. By virtue of the reversed-conical configuration of the speed change and steering mechanisms  124  and  118 , any steering operation will rotate the input shaft  87  to actuate the steering input member  95 , resulting in deceleration of the vehicle synchronously with the steered motion thereof. Any speed-changing operation will rotate the other input shaft  91  to also actuate the same steering input member  95 , thereby enabling any speed change to increase or decrease the turning radius. If however the neutral position is taken during such a speed-changing operation, then the turning power will be interrupted not to applied to the vehicle.  
     [0067] The steering joint member  116  connecting the input member  95  to the steering link  115  is aligned with the speed change input shaft  91 . Speed change joint member  122  connecting the other input member  96  to the speed change link  121  is aligned with a line ‘A’ intersecting the axis of speed change input shaft  91 . This structure renders easier the determination of relative motion of the speed change input member  95  rotating around and relative to the steering and speed change input shafts  87  and  91 , whereby design, assembly and structure of these members are simplified and motion thereof is rendered more reliable. Further, there is formed an imaginary circle ‘C’ having a center ‘B’ at which the axes of those speed change and steering input shafts  91  and  87  intersect one another. The speed change and steering joint members  122  and  116  are arranged on this circle ‘C’ so that the steering input member  95  and the like are more simplified and compacted. Arranged on the axis of the steering input shaft  87  is the ball joint-shaped member  123  connecting the speed change output member  120  and link  121  to each other, on one hand. Also arranged on said axis of the shaft  87  is the other ball joint-shaped member  117  connecting the steering output member  114  and link  115  to each other, on the other hand. Thanks to this feature, the so-called ‘reversed steering’ phenomenon will not occur when the vehicle is changed over from forward travel to backward travel, or vice versa. Design, assembly and structure of the speed change and steering output members  120  and  114  are further simplified and motion thereof is rendered more reliable. A distance measured between the point ‘B’ of mutual intersection of inputs shafts  91  and  87  and the ball joint-shaped member  123  does remarkably differ herein from another distance measured between the point ‘B’ and the other ball joint-shaped member  117 . Thus, those joint members  123  and  117  are spaced noticeably from each other on a common line ‘D’. By virtue of this feature, those joint members  117  and  123  are inhibited from interfering one another, scope or range in which they move is designed easier, and those speed change and steering links  121  and  115  can now be installed in a narrower space.  
     [0068] The speed change and steering joint members  116  and  122  are arranged on the circle ‘C’ having the center ‘B’ at which the axes of speed change and steering input shafts  91  and  87  intersect one another. These members  116  and  122  are spaced about 90 (ninety) degrees from each other. Thus, revolving motion of the speed change input shaft  91  will not displace the steering joint member  116 , whilst displacing the speed change joint member  122  to a maximum extent to accelerate or decelerate the vehicle. Also due to the speed change input shaft  91  disposed in a plane on which the joint members  116  and  122  move, the latter can enjoy a larger area for their movement and the steering input member  95  can be incorporated in a compacted and highly functional manner. Both the steering and speed change joint members  116  and  122  are capable of rotating about an angle of about 90 (ninety) degrees or less around the steering input shaft  87 . This feature affording a sufficient range in which each of joint members  116  and  122  moves, is also effective to prevent the so-called ‘reversed steering’ phenomenon from occurring when the vehicle is changed over from forward travel to backward travel, or vice versa. Dependently upon steered angles which the steering input shaft  87  does command, the speed change joint member  122  will be displaced to decelerate the vehicle so as to assist the vehicle to make a ‘spinning turn’ around its inside crawler  2 . The structure affording this effect is constructed herein in such a fashion that its function is improved despite its compactness. In the spinning turn effected by the differential mechanism  33  in response to an output from the steering apparatus  28 , one of the right and left crawlers  2  is driven forwards and the other rearwards. In this mode, the vehicle will swivel about a center that is disposed intermediate the right and left ends and also intermediate the fore and aft ends of said crawlers as a whole. In other words, forward or backward advance takes place simultaneously with the turning motion of the vehicle at a turning radius that is determined on the basis of a ratio of an operated degree of the speed change apparatus  25  to that of the steering apparatus  28 . Further, both the speed change output and steering output shafts  119  and  113  located in parallel with the speed change input shaft  91  are accurately supported in and rotate within the column  71 , which is composed of discrete segments separable from each other. Since all of these input and output shafts  91 ,  119  and  113  extend sideways, relative connection between relevant members is facilitated longitudinally of the vehicle. In detail, the main speed change lever  73  is easily connected to the output shaft  119 , with the speed change and steering apparatuses  25  and  28  being also connected easily to the corresponding output shafts  113  and  119 . In this way, a simpler operative structure as well as an improved maneuverability are afforded herein.  
     [0069] As will be seen in FIG. 14, the set-screws or linkage bolts  97  are freely fitted in phase-adjusting elongated holes  149 , and the latter are formed in the input interconnect member  96 . Those elongated holes  149  are arranged on a common circle having a center coincide with the axis of the steering input shaft  87 . The speed change input shaft  91  may be kept at its neutral position by means of a bolt-shaped standardizing member  98  engaged with a neutralizing hole  150 . In this state, the steering input member  95  can be shifted clockwise or anti-clockwise so as to take a selected position relative to the input shaft  91 . In this way, the neutral position in the vehicle&#39;s turning motion given by operation of the steering wheel  19  will thus be adjusted, while allowing determination of the neutral position for speed change operation. The bolt-shaped standardizing member  98  for fixing the input shaft  91  at its standard position is detachable, and a bolt stopper  98   a  shown in FIG. 14 is removable. Thus, it is possible to bring a free end of the standardizing member  98  into the neutralizing hole  150  as shown in FIG. 15, in order to select a desired neutral position for the range of speed change and to select that for the range of steering. Joint nuts  121   a  and  115   a  respectively shown in FIGS. 8 and 11 may be fastened or alternatively loosened so as to alter overall effective length of each of speed change and steering links  121  and  115 , facilitating adjustment and other works when assembling the structure.  
     [0070] As illustrated in FIGS.  16  to  18 , the gear  88  has teeth  151  formed in and along its partial periphery ranging over 270 (two hundreds and seventy) degrees, with an arc of 90 (ninety) degrees having no teeth. A maximum overall revolution angle of the steering wheel  19  is thus 270 degrees, that is 135 (hundred and thirty-five) degrees of steering to the right plus 135 degress to the left. Drivers of this vehicle can operate with their one hand the steering wheel  19 . The sector gear  89  has teeth  153  over its peripheral portion of 130 (hundred and thirty) degrees of its periphery, its remainder peripheral portion of 230 (two hundreds and thirty) degrees functioning as an arcuate cam  154 . Some teeth  151  of the gear  88  are variably in mesh with some teeth  153  of he sector gear  89 . Maximum revolution of those gears  88  and  89  in either direction will cause one or the other of stoppers  155  of the former gear to abut against the corresponding one of the latter&#39;s stoppers  156 . Revolution angle of the steering wheel  19  is thus delimited, and the steering input member  95  and the input interconnect  96  allowed to rotate 65 (sixty-five) degrees in each of forward and backward directions of revolution. In this way, a space for disposing upper ends of the change speed input shaft  91  and the tie-rod or main speed change member  110  is included in a plane in which the input member  95  rotates. Such a structure further enables arrangement of the steering joint member  116  aligned with the axis of the input shaft  91 , on one hand. It also enables arrangement of the input joint members  116  and  122  on one and the same circle but spaced 90 (ninety) degrees from each other, on the other hand, rendering it easier to design and assemble such a compacted structure.  
     [0071] A notch  157  for causing straight advance to the vehicle is formed at a middle region of the arcuate cam  154  which the sector gear  89  comprises. A detent shaft  158  is rotatably held in position in an upper wall of the steering column  71 . A detent arm  159  is fixed on a lower portion of the detent shaft, and a detent roller  161  is journaled on a roller shaft  160  secured to the arm  159 . The detent roller  161  held in contact with the arcuate cam  154  can disengageably come into engagement with the straight advance notch  157  so as to hold the steering wheel  19  at its neutral position for the vehicle&#39;s straight advance. Fixed on the top of the detent shaft  158  is a detent lever  162 , on which one end of a neutralizing spring  163  surrounding this shaft  158  does act. The other end of this spring  163  bears against a stopper  164  formed integral with the steering column  71  so that the detent roller  161  urged by said spring is kept in a forced contact with the arcuate cam  154  having the straight advance notch  157 . A microswitch  165  fixed on the detent lever  162  and capable of changeover between ON and OFF does serve as an electrical sensor for detecting the straight-advance position of the wheel  19 .  
     [0072] As shown in FIGS. 12 and 13, a retractibly extensible damper  166  is fixed on the steering rod  143  at its middle portion. A casing  167 , that is attached to an end of aligned halves of this rod  143  have, is a constituent of the damper  166  accommodating a coiled spring. The damper  166  further comprises spring seats  168  and  169  that are formed integral with an end of the other end of the other half of the rod  143 . The coiled spring  170  surrounding the other half&#39;s end is compressed between those seats  168  and  169 . The steering wheel  19  will tend to apply to the steering rod  143  either a pushing force or a pulling force, beyond a certain limit. This limit corresponds to a steered angle of 116 (hundred and sixteen) degrees (that is equal to 85% of the maximum angle) of the steering wheel, where the steering control arm  141  will exert a maximum output. If the wheel  19  is rotated to exceed this limit, then the spring  170  will cause the rod  143  either to extend or to shrink. Therefore, it is possible to further rotate the steering wheel from 116 to 135 (hundred and thirty-five) degrees, with the second oil-hydraulic pump  26  being kept at its highest output level.  
     [0073] As shown in FIGS. 12, 19 and  20 , a pin  171  is fixed on the speed change control arm  136 . An elongated or elliptic hole  172  formed in the speed change rod  138  receives the pin  171  to be slidable therein, thereby operatively connecting the arm  136  to the rod  138 . A detent cam  173  fixed on the control arm  136  is in alignment therewith, and a spring  175  urges a detent roller  174  into engagement with this cam. The detent roller  174  cooperates with the detent cam  173  to cause the speed change control arm  136  to automatically return to neutral position. There may be an occasion that this speed change arm  136  remains at its neutral position to keep idle the first oil-hydraulic motor  24 , with the main change speed lever  73  being at neutral position to hold the pin  171  at a middle region of the elongated hole  172 . In this occasion, substantially the same stroking distance will be provided for the pin  171  whether the speed change rod  138  is pulled or pushed causing the vehicle to run forwards or backwards. Thus, forward and backward motions of the control arm  136  will generally be symmetrical with each other.  
     [0074] As illustrated in FIG. 20, the main speed change lever  73  at its neutral position ‘O’ may be operated to either side. Consequently, this lever  73  will thus pass through either insensitive zone ‘Q’, before reaching either change speed initiation point ‘P’. The speed change rod  138  thus pushed or pulled will bring the pin  171  into contact with either end of the elongated hole  172 , and the speed change lever  73  starts to actuate the speed control arm  136 . This lever  73  may further be operated through its acceleration/deceleration zone ‘R’, until stopped at any point behind a maximum output limit ‘S’. Accordingly, the speed control arm  136  will leave its neutral position ‘T’ and then move a distance through its acceleration/de-celeration zone ‘R’, in proportion to displacement of the lever  73 , but behind its maximum output limit ‘S’. The first hydraulic motor  24  in the speed change apparatus  25  will thus output its power at a speed that has been selected in such a step-less manner, so that both the crawlers  2  are driven at the same speed and in the same direction, either forward or backward.  
     [0075] As seen in FIGS. 14, 21 and  22 , a recess  177  is defined in and by a cutout  176  the bearing member  90 . This recess will receive the joint member  122  and the speed change link  121  when the steering wheel  19  is steered to the left up to the maximum angle (135 degrees). As compared with an imaginary case wherein the joint member  122  directly contacts the bearing member  90 , maximum deceleration degree is increased herein to follow a larger steered range of the wheel  19 . Two constant speed zones ‘V’ are provided each beside a neutral position ‘U’ of the steering wheel  19  commanding the vehicle to advance along a straight path. With the said wheel being operated within a range of up to 15 degrees on either side, the running path can be adjusted without altering velocity of vehicle&#39;s center. The wheel  19  may further be operated through a turning zone ‘W’ but up to a maximum output position ‘X’ for the steering apparatus  28 , this position corresponding to rotation of said wheel by 116 degrees. The steering control arm  141  will thus be shifted an angle in proportion to the swiveled angle of the steering wheel  19 , but also up to the maximum output position ‘X’. Accordingly, the second hydraulic motor  27  in the steering apparatus  28  changes its output in a step-less manner, gradually changing difference in speed between the right and left crawlers  2 . Simultaneously with this process, the speed change rod  138  and the speed control arm  136  will be caused to move in a reversed direction towards their neutral position also in proportion to the swiveled angle of the steering wheel  19 , thereby gradually lowering the absolute velocities of the crawlers  2 . In short, a greater angle of the steering wheel  19  will bring about a smaller turning radius and a lower overall speed of the vehicle turning either to the right or to the left. If however the steering wheel  19  is operated beyond 116 degrees but short of a maximum steer limit ‘Z’ so as to enter a spinning turn zone ‘Y’, the damper  166  will extend or shrink to change effective length of the adjustable rod  143 . At this stage, the steering control arm  141  is kept at the maximum output position ‘X’, with both the speed change rod  138  and control arm  136  being further shifted to their neutral position to effect a spinning turn about the vehicle center intermediate the right and left crawlers  2 .  
     [0076] As shown in FIGS.  19  to  21 , the speed change rod  138  is connected to the speed control arm  136  in the elongated hole  172 , to give the main speed change lever  73  an increased apparent stroke ‘L’ larger than an actually-needed stroke. The bearing member  90  has the recess  177  facing the joint member  122  and the latter moving to the left will fit in the recess, so that a maximum speed reduction ratio of 25% is attained at maximum output position of the speed change lever  73 , corresponding to maximum operated angle (135 degrees) of the steering wheel  19 . If the vehicle is lacking in such a feature, its minimum speed would merely be 40% due to the speed change mechanism  124  discussed above. However in this invention having that feature, said ratio will further be lowered to 25% so as to much more reduce the vehicle speed, enabling it to make a spinning turn. Thus, it can spin its body at any bare end area in a farm, changing its position by 180 (hundred and eighty) degrees to transfer to the next row of un-reaped crops. As will be seen in FIG. 22, at the maximum output position of the main speed change lever  73  and at the operated angle of 116 degrees of the steering wheel  19 , the damper  166  will start to function. As a result, speed reduction will be maintained generally at the same ratio for both the right and left crawlers  2  within a range of 116 to 135 degrees of the wheel  19 . This means that a maximum difference will be reached between the crawlers  2  at the operated angle of 116 degrees of the steering wheel, and continue until the maximum angle 135 degrees so that the overall running speed is reduced in proportion to the operated angle of wheel  19 .  
     [0077] The speed control arm  136  is displaced herein a more increased distance to lower vehicle speed as the steering wheel  19  is operated, to an extent that the vehicle can now surely spin at the bare end area in farm. The pin  171  connects the speed change rod  138  to the speed control arm  136 , within the elongated hole  172 . Size or dimension of those hole  172  and pin  171  may be designed to easily select an operable extent of the rod  138  or a reduction ratio given by the arm  136 . Thus, output characteristic of the speed change and steering apparatuses  25  and  25  as well as speed reduction behavior of the steering wheel  19  can readily be taken into account when a simpler structure for mounting the speed change rod  138  is wanted. Such a simpler structure of this rod will improve assembling works and maneuverability thereof and to lower manufacture cost.  
     [0078] The main speed change lever  73  operated in the insensible zone ‘P’ will cause the input members  95  and  96  to leave their neutral position ‘P’ and to swivel around the input shaft  91  and onto the speed change initiation position ‘Q’. The steering wheel  19  operated in the constant speed zone ‘V’ beside its neutral position for straight running will increase ratio of motion of the steering control arm  141  to an operated angle of the steering wheel  19 . With the speed change lever  73  being subsequently shifted into the acceleration/deceleration zone ‘R’ to cause the vehicle to run gently, the steering wheel  19  operated in the constant speed zone ‘V’ will further increase the motion of the steering control arm  141 . As a result, a ratio of change in rotational speed of the second hydraulic pump  26  in the steering apparatus  28  will be increased to the operated angle of the steering wheel  19 . A time for turning the vehicle will thus be shortened while the second pump  26  is giving a moderately gentle output, so that the second hydraulic motor  27  also operating at a gentle level of output will inhibit the vehicle from turning. In other words, the steering wheel  19  can cause the vehicle to turn only when the output from the second pump  26  is raised above a certain level, insofar as the main speed change lever  73  is at its gentle speed position. It will be apparent that this speed change lever  73  may be operated in such a manner that it starts to effectively change the vehicle speed only after the steering wheel  19  has become able to control the steering apparatus  28  to exert a vehicle-turning power above a certain level. This feature will compensate the relatively weak turning power of the apparatus  28  in the vehicle then gently running, and reduce manufacture cost of said apparatus and relevant parts, improving steerability of the crawlers  2 . In more detail, the speed change lever  73  will rotate at first the steering and speed change input members  95  and  96  over a certain angle, so that these members thus operatively connected to the speed change apparatus  25  will actuate it. After these members  95  and  96  are caused then also to take their steering position, the change speed apparatus  25  will effectively act to drive the crawlers  2 . Even when the vehicle is running slowly with a lower level output from said apparatus  25 , the steering wheel  19  can be operated to intensively control the steering apparatus  28  to ensure a sufficient turning power for such a slowly running vehicle.  
     [0079] With the main speed change lever  73  standing at its neutral position, the steering wheel  19  may be operated in a forward (or reversed) direction. The input members  95  and  96  will thus move together with the links  115  and  121  about the axis of steering input shaft  87  and on and along a conical locus. In this state, all of the output members  114  and  120  and the output shafts  113  and  119  remain still. However, the speed change lever  73  can be tilted forwards (or backwards) to cause a forward advance (or rearward advance) to the vehicle. Consequently, those input members  95  and  96  will tilt forwards (or backwards) about the axis of speed change input shaft  91 , so that the steering joint member  116  remains still at the given position, while raising (or lowering) the change speed joint member  122 . The speed change output member  120  will thus rock upwards (or downwards) to rotate the output shafts  119  in a forward (or reversed) direction. The first hydraulic pump  23  in the speed change apparatus  23  will in this way have its inclined rotary plate changed in angle to operate the first hydraulic motor  24  in a forward (or reversed) direction. Its output shaft  31  thus revolving in a forward (or reversed) direction drives the crawlers forwards (or backwards). In such an operation, rotational speed of the output shaft  31  increases or decreases in proportion to the angle to which the lever  73  is tilted, thereby steplessly changing the forward (or rearward) speed of the crawlers  2 .  
     [0080] In another condition that the speed change lever  73  is tilted forwards (or backwards) to cause a forward advance (or rearward advance) to the vehicle, the steering wheel  19  may be operated to swivel to the left (or to the right). Consequently, the steering input member  95  will tilt forwards (or backwards) about the axis of steering input shaft  87 , so that the steering joint member  116  is lowered (or raised) to rock the steering output member  114  upwards (or downwards). This member  114  will thus rotate the steering output shaft  113  in a forward (or reversed) direction, whereby the second hydraulic pump  26  in the steering apparatus  28  has its inclined rotary plate changed in angle to operate the second hydraulic motor  27  in a forward (or reversed) direction. Its output shaft  68  thus revolving in a forward (or reversed) direction decelerates (or accelerates) the crawlers  2 , simultaneously accelerating (or decelerating) the right-side crawler  2  so as to turn the vehicle to adjust its running path to the left (or to the right). With the steering wheel  19  swiveling to the left (or to the right) to adjust the running path, the speed change input member  96  tilts forwards (or backwards) away from the axis of change speed input shaft  91  and simultaneously rotates around the axis of steering input shaft  87  in a forward (or reversed) direction. As a result, the change speed joint member  122  is lowered (or raised) to rock the output member  120  upwards (or downwards) to rotate the change speed output shaft  119  in a forward (or reversed) direction. This means that the speed change apparatus  25  is forced towards its neutral position so as to speed down the output shaft  31  to decelerate the vehicle.  
     [0081] The swiveling of the steering wheel  19  during advance of the vehicle will give rise to change in its turning radius (i.e., angle) and also in its running speed, both in a proportional manner. In detail, the larger angle by which the steering wheel  19  is rotated, the greater difference will be produced in speed between the crawlers  2  and the smaller radius turn the vehicle will make, while being further decelerated. On the other hand, the universal joint-shaped member  116  in the steering system will operate in one or the other of opposite directions, depending upon whether the vehicle moves forwards or backwards, thus depending upon whether the steering wheel be turned to the right or to the left. This means that the vehicle turns in the same direction as the wheel rotates, whether it is running forwards or backwards. Thanks to this feature, a round steering wheel  19  that may be mounted on a tractor, a rice-planting machine or the like will give a driver an operation feeling similar to that which he or she may experience when driving any four-wheel automobiles.  
     [0082]FIG. 22 illustrates a relationship between the operated angle of the steering wheel  19  and the speeds of the right and left crawlers  2 . As seen in this figure, difference in speed between the crawlers will become greater as the wheel  19  is operated more and more. An average speed of those crawlers  2 , that is the speed at which the center of the vehicle advances, may further be reduced in response to changeover of the subsidiary speed change lever  74  (between the higher, middle and lower ranges of speeds). There may be an exemplary case of adjusting the reaping path by rotating the wheel  19  about 15 (fifteen) degrees or less to the left (or to the right) away from its neutral position. In this case, the joint member  122  in the speed change system will be shifted substantially in a tangential direction following the rotation of input member  95 . The speed change output member  120  is not displaced noticeably from its straight running position. However, the second pump  26  in the steering apparatus  28  is operated to actuate the second motor  27  in a forward (or reversed) direction in order that the vehicle turns to the left (or to the right) so as to follow a curved row of un-reaped crops. In such an event, an extent to which one crawler advancing on an inner course with respect to locus of the vehicle is decelerated will be equal to that to which the other crawler advancing on an outer course is accelerated. Thus, the center of the vehicle will move at almost the same speed as it travels on a straight path. There may be another occasion that the steering wheel  19  is operated more than 15 (fifteen) degrees. On this occasion, the rotating input member  95  will pull or push the change speed link  121  to decelerate the output member  120 , regardless of the direction in which the vehicle is turning either to the right or to the left. Consequently, the first hydraulic pump  23  and motor  24  will reduce their output to drive both the crawlers  2  in the same direction, causing the vehicle to make a ‘braked turn’ to the left (or to the right) due to differential speed between the crawlers. Such a motion of the vehicle may be useful when returning it to the present row of un-reaped crops, if it has deviated therefrom, or when transferring it to the next row. If the steering wheel  19  is rotated about 116 (hundred and sixteen) degrees or more, then the damper  166  will come into effect such that the maximum steering force continues to be output. Until the steering wheel is operated to the maximum angle 135 (hundred and thirty-five) degrees, the speed of the vehicle center will decrease more to become about one fourth of that for straight running. In this state, the inner crawler  2  with respect to the turning radius is caused to rotate in a reversed direction, thus forcing the vehicle to turn about its center located intermediately between its crawlers. This motion is called ‘spinning turn’ herein. The vehicle&#39;s turning of 180 (hundred and eighty) degrees will be accompanied by a transverse displacement thereof by a distance corresponding to transverse width of its crawlers. It will now be apparent that the steering wheel  19  is operable between the range of steered angles from 0 to 135 degrees, wherein operation of the wheel up to 15 degrees offset from neutral position allows the vehicle straightly running at its normal speed to be shifted sideways to advance exactly along the subject crops row in issue. As discussed above, if the wheel  19  is operated an angle from 116 to 135 degrees, then the ‘spinning turn’ will be made with the maximum output from the steering apparatus  28  but at an automatically reduced speed that is one fourth (i.e., 25%) of the normal speed, so as to enable the vehicle to transfer to the next course on the bare end area in any farm.  
     [0083] The subsidiary speed changer may be set at ‘standard’ speed (1.5 m/sec), and at this speed the steering wheel may occasionally be revolved 90 (ninety) degrees. On such an occasion, only the turning speed (that is velocity of the vehicle center) will be changed, with turning radius remaining unchanged. This applies to any exemplary cases wherein the main speed change lever  68  is operated to provide the highest velocity, two thirds thereof or one third thereof. Further, within a range which the elongated holes  149  in engagement with the linkage bolts  97  provides, the first hydraulic pump  23  and motor  24  will continue to drive the vehicle to run straightly. Therefore, the vehicle is protected from any inadvertent and irregular change in speed while it is steered to follow any row of crops or any ridge in a farm. The vehicle can now adjust its running path, without affecting the generally constant speed of the vehicle being steered, but giving a driver a normal driving feeling in harmony with its natural motion. The subsidiary speed change lever  74  causes a shift between target speeds for the main speed change lever  73 . Thus, the subsidiary lever  74  may be operated to take its position presetting lower, normal or higher speeds for the vehicle in order to vary its turning radius from minimum to maximum. By virtue of this feature, it is easy to predetermine the reduction ratio of the first hydraulic pump  23  to its motor  24  and that of the second pump  26  to its motor  27 . A sufficient traction power is also ensured for smaller radius turning motions such as the ‘spinning turn’. In detail, at any position of the subsidiary speed changer, the main speed change lever  73  can be operated to change actual speed of the vehicle making a turning motion of the same radius.  
     [0084] As shown or seen in FIGS. 11, 23 and  24 , the change speed output shaft  119  rotatably carries thereon the cylindrical interconnect output shafts  178  and  179 , the former  178  being for a reducing connection and the latter  179  being for a direct connection. The speed change output member  120  is fixed on the reducing interconnect shaft  178 . Further, the speed change link  129  is fixed on the change speed output shaft  119  by means of a boss  180 . Another boss  181  serves to fix a direct input link  182  on the main speed shaft  99 . A direct output link  183  is fixed on the direct interconnect shaft  179 . A roller  185  rotatably mounted on a short shaft  184  of the direct input link  182  does slidably fits in an elongated hole  186  that is formed in the output link  183 . These links  182  and  183  cooperate with each other to operatively connect the direct interconnect shaft  179  to the main speed shaft  99 , so that these shafts  99  and  179  rotate in unison.  
     [0085] The change speed output shaft  119  has a bore in which a clutch shaft  187  slides fore and aft. A clutch pin  188  fixed on an inner end of the clutch shaft  187  is engageable with the output shaft  119 . Notches  189  and  190  engaging with and disengaging from the clutch pin  188  are formed in the interconnect output shafts  178  and  179 , respectively. By shifting the clutch shaft  187  inwards or outwards, the clutch pin  188  will selectively engage with one of the notches  189  or with the other  190 . Either the reducing interconnect shaft  178  or the direct interconnect shaft  179  is brought into a selective connection with the change speed output shaft  119 . A positioning ball  192  is always urged by a spring  191  towards the clutch shaft  178 , which has positioning grooves  193  and  194  brought into a selective engagement with the ball  192 . Thus, if one of the grooves  193  fits on this ball  192 , then the clutch pin  188  will be in engagement with one of the notches  189 . If contrarily the other grooves  194  fits on this ball  192 , then the clutch pin  188  will be in engagement with the other notches  190 . On the other hand, a feeling lever  195  is disposed outside the steering column  71  so as to be maneuvered by a driver whose feet are on the operator step  134 . This lever  195  is connected to the clutch shaft  187  to be operated thereby for changeover between its positions. This feature enables that either the speed change output member  120  or the output link  183  does input power to the change speed link  129 . An acceleration/deceleration commanding power either from the steering wheel  19  or from the main speed change lever  73  will be input to speed change apparatus  25  to be controlled, wherein the operated degree of said wheel  19  proportionally decelerates the vehicle.  
     [0086] Now, it will be apparent from the foregoing that the command input from the main speed change lever  73  is divided into two outputs by the main speed change shaft  99 . One of such divided outputs is used as a deceleration command in proportion to the operated angle of the steering wheel  19 . This output will be transmitted from the output member  120  to the speed change link  129 , through the reducing output shaft  178  fitted on the speed change output shaft  119 . The other divided output from the lever  73  will be transmitted from the output link  183  to the speed change link  129 , through the direct output shaft  179  also fitted on the speed change output shaft  119 . In this manner, either of those two outputs is selected on the speed change output shaft  119  in order to control the first hydraulic pump  23  in a desired mode. This feature will be preferred in any mobile agricultural machine whose right and left crawlers  2  receive traction power from speed change apparatus  25  controlled by main speed change lever  73 , such that output therefrom is adjusted steplessly to alter velocity difference between the crawlers  2  by the steering wheel  19  controlling the steering apparatus  28 . This is because the main speed change lever  73  is operatively connectable to the speed change apparatus  25  through reduction output shaft  178  functioning as a route for decelerating the vehicle proportionally to operated angle of the steering wheel  19 , wherein said lever  73  is connectable also to said apparatus  25  through a third route, that is direct output shaft  179 . Alternative selection of either the former shaft  178  or the latter  179  will alter the vehicle&#39;s turning and/or running performance, enabling a choice between gentler turning and sharper turning, but without necessity of modifying the speed change apparatus  25  in any manner.  
     [0087] Also discussed above, the feeling selection lever  195  is provided to switch over the reducing connection output shaft  178  to the direct connection output shaft  179 , or vice versa. This is for the purpose of selection between a reducing input and a speed changing input, to effect speed control using either one of the reducing and direct connection output shafts  178  and  179 , enhancing steerability and improving maneuverability. Selection between the reducing input from the former output shaft  178  and the speed changing input from the latter  179  is conducted on a common shaft, controlling the speed change apparatus  25 . Thanks to this feature, connection of both the change lever  73  and steering wheel  19  with the speed change apparatus  25  is simplified and compacted to render simpler the assembly and adjustment of relevant members.  
     [0088] As shown in FIGS. 25, 26 and  27 , an electrical speed change motor  196  may be employed and actuated by manual operation of the main speed change lever  73 . This motor  196  will function to change the angle of inclined rotary plate in first hydraulic pump  23 , to thereby steplessly alter rotational speed of output shaft  31  of the first motor  24 , in proportion to operated distance of said lever  73 . That motor will also be actuated to switch over its direction of rotation between forward and reverse. An electrical steering motor  197  and a valve  198  for keeping straight running path of the vehicle may be employed together with a steering clutch cylinder  199 . The steering motor  197  will be actuated by manual operation of the steering wheel  19 , with the straight running valve  198  being actuated in response to neutralization of said wheel  19  and neutralization of the subsidiary change speed mechanism  32 . The steering clutch cylinder  199  is operatively connected to the straight running valve  198 . The latter electrical motor  197  will function to change the angle of inclined rotary plate in second hydraulic pump  26 , to thereby steplessly alter rotational speed of its pump motor  27 , in response to operated angle of steering wheel  19 . That motor  197  will also be actuated to switch over angular direction of the second hydraulic motor  27 , between forward and reverse, when causing the vehicle to the right or to the left to turn at the bare end area or adjust its running course. Accordingly, the steering wheel  19  will produce, corresponding to its operated angle, change in rotational speed of the steering motor  197 . With the steering wheel  19  and subsidiary speed changer  32  being operated to take their neutral positions, the straight running valve  198  will automatically change its state so as to actuate the steering clutch cylinder  199 . As a result, both the steering output clutch  62  and second hydraulic motor  27  will be turned off to cease the steering of vehicle.  
     [0089] A steering output arm  220  also possibly incorporated herein has its one end fixed on the steering input shaft  87 . A pair of right and left coiled springs  221  to urge the steering wheel  19  towards its straight running position may be employed to be in an operative connection with the steering output arm  220 . Resistant absorbers  222  cooperating with this arm will act against those springs  221  in order to retard rotational motion of the steering wheel  19 . By virtue of these members, a driver who has operated the wheel  19  to the right or left does not have to manually restore its neutral position, because said members will automatically return it thereto. A steered-angle sensor  223  of the sliding potentiometer type may be applied to one of the output arms  220  so as to detect the operated angle of the steering wheel  19 . A main speed-change sensor  224  also of the potentiometer type will detect the displaced position of the main speed change lever  73  in either direction, and the neutral position and changeover thereof between forward and backward running directions. A subsidiary speed-change sensor  225  will detect the displaced position of the subsidiary speed change lever  74  and its neutral position. A knob-shaped member  226  disposed on the top of steering column  71  is maneuverable with the driver&#39;s fingers. A turning-mode setting device  227  of the volume controller type is operable by the knob-shaped member  226  to change reduction ratio corresponding to the operated angle of steering wheel  19 . The straight-advance detecting sensor  165  as well as the steered-angle sensor  223 , main speed-change sensor  224 , subsidiary speed-change sensor  225  and turning-mode setting device  227  are all electrically connected to a microcomputer, that constitutes a speed-change/steering controller  228 .  
     [0090] The electrical speed change motor  196  will be actuated in a forward or reverse direction by an acceleration circuit  229  or deceleration circuit  230 , respectively. These circuits  229  and  230  are connected to the controller  228  so that the motor  196  will alter the angle of slanted rotary plate in the first hydraulic pump  23  substantially in proportional to the operated angle of main speed change lever  73 . The vehicle thus will run at a speed corresponding to the operated angle of the main speed change lever  73 . Also connected to the controller  228  are a left-turn circuit  231  and a right-turn circuit  232 , which will energize the electrical steering motor  197  in a forward or reverse direction. The motor  197  will alter the angle of slanted rotary plate in the second hydraulic pump  26  substantially in proportional to the operated angle (to the left or to the right) of the steering wheel  19 . As seen from the diagram of steering output given in FIG. 28, the vehicle will exert a clockwise-swiveling force by turning its steering wheel  19  to the right or to the left while it is advancing forwards or backwards, respectively. This motion will be reversed so that an anti-clockwise-swiveling force is obtained by turning the steering wheel  19  to the left or to the right while advancing forwards or backwards, respectively. Thus, the so-called ‘reversed steering’ is avoided herein which would otherwise inhibit the vehicle from being steered in the same manner as the ordinary four-wheel automobiles. Insofar as the main speed change lever  73  stands at its neutral position, the rotary plate in the second hydraulic pump  26  will remain non-inclined to interrupt output from the second motor  27 . The vehicle whose change lever  73  is temporarily at such a neutral position can not turn itself in any way even if its steering wheel  19  would be operated in any direction. It will be apparent that the rotary plate in the second hydraulic pump  26  will increase its absolute angle of inclination as the operated angle of steering wheel  19  increases. However, the absolute angle varying in this manner is controlled herein to be proportional to the absolute angle to which the main speed change lever  73  will be operated from time to time. This feature enables the present vehicle to make the same steering motion as in the ordinary four-wheel automobiles, because the turning radius does not vary in response to any change in vehicle speed unless the steering wheel is operated further. A straight-run circuit  228  also is connected to the controller  228  so as to actuate a straight-run valve  234 , which in turn will cause operation of the steering clutch cylinder  199 . Thus, both the neutral position of the subsidiary speed changer and that of the steering wheel  19  are effective to automatically cease the steering output.  
     [0091]FIGS. 29 and 30 are diagrams showing traction force imparted to the vehicle. As will be seen there, an increased angle of the steering wheel  19  renders the vehicle&#39;s actual speed lower than a target velocity that corresponds to a position which the main speed change lever  73  takes then temporarily. This means that the vehicle will be decelerated every time when the steering wheel  19  is operated, irrespective of the lever&#39;s  73  position. If however this wheel is returned to its neutral position for straight running of the vehicle, then it will consequently restore its target velocity. If, on the other occasion, the wheel  19  is operated to one of its opposite maximum limits, then the vehicle will be decelerated to its lowest speed for a spinning turn. There may be another occasion on which said wheel  19  stays in the insensitive zone (that is, up to about 15 degrees of steering in either direction). In this case, the target velocity given depending on the position of the speed change lever  73  will not be disturbed, lest any irregular or unwanted acceleration and/or deceleration should take place during a harvesting work of the vehicle. Now, a driver of the vehicle, for instance a harvesting combine, can properly steer it, without feeling any incongruity that would otherwise be caused by irregular motions of the combine. The driver can also alter reduction ratio and prefer any of acute, medium and gentle steering modes, by manually choosing one of corresponding positions of the knob-shaped member  226 , taking into account the type of agricultural works, the condition of farms and the nature of crops.  
     [0092] As seen in FIG. 31 showing a diagram of the steering outputs, a curve of secondary degree represents the relationship between an input from the main speed-change sensor  224  and an output to and controlling the steering motor  197 . This input to the controller  228  corresponds to the operated angular position of the main speed change lever  73 , which position is detected by that sensor  224 , as described above. That output from the controller  228  is produced taking into account a signal from the steered-angle sensor  223 . Due to such a relationship, even if the rotary plate in the second pump  26  is at a gentle angle to provide a low volume coefficient, a little angular displacement of the steering wheel  19  will remarkably increase the said angle while the vehicle is running slowly. In other words, characteristics of the second hydraulic pump  26  and motor  27  are modified by such an electronic means that the steering motor  197  can sensitively control the second pump  26  to sharply steer the vehicle running at a low speed. Thus, over the whole range of operable main speed change lever  73 , the turning radius of crawlers  2  will remain almost constant regardless of actually operated angles of the steering wheel  19 . Rather at lower speeds than at higher speeds commanded with the main speed change lever  73 , here is provided a higher ratio of the controlled extent of second pump  26  to the actually operated angle of wheel  19 . In still other words, even when the vehicle runs slowly with the second pump  26  less efficiently operating, a slight extent of rotating the steering wheel  19  is enough for the vehicle to make a proper turning motion. In this manner, operated angles of that wheel  19  correspond well to the thus controlled turning radii of crawlers  2 , thereby improving maneuverability and function of the vehicle steering mechanism. However, if and when the main speed-change sensor  224  detects the speed change lever  73  standing then at its neutral position, the second hydraulic pump  26  will accordingly return to its own neutral position. This means that those crawlers  2  standing still are inhibited from making any turn, without adversely affecting such an improved easiness and exactness in using the wheel  19  to drive the vehicle.  
     [0093] As seen also in FIG. 32 showing another diagram of the steering outputs, an input from the steered-angle sensor  223  to the controller  228  does correspond to the operated angular position of the steering wheel  19 , as described. Outputs from the controller  228  are not strictly proportional to steered angles, but it will give a more intensive output to the steering motor  197  to be controlled if the wheel  19  is rotated smaller angles (from about 0to 10 degrees from neutral). If contrarily this wheel  19  is rotated larger angles (from about 10 to 70 degrees), then the output from the controller  228  to the steering motor  197  will be weakened, as compared with a case of simple proportional control. In other words, characteristics of the second hydraulic pump  26  and motor  27  are modified by such an electronic means for controlling the steering motor  197  more sensitively and intensively, within a range of relatively smaller operated angles of the steering wheel  19 . Thus, even when the vehicle runs slowly with the second pump  26  and motor  27  less efficiently operating, they will work to sharply steer the crawlers  2 . In other words, rather in a range of slight extents of rotating the steering wheel  19 , the steering control output varies at a higher rate to sharply turn the vehicle. The vehicle can now perform a delicate correction of its running path to exactly follow any desired ridge or row of crops in a farm, thus improving steerability of the vehicle being generally straightly running for agricultural works. Further, any unnoticeably erroneous or improper handling of the wheel  19  will never cause any excessive adjustment of the running path, and the vehicle can now make a more reliable turning while running fast. The described non-linear relationship between the operated angles of said wheel  19  and the output from second pump  26  is also important herein. Such a design will contribute to adoption of the most proper steering performance in view of the characteristics of pump  26  and/or the kinds of agricultural works, further enhancing steerabilty and controllability of the vehicle.  
     [0094] It will now be apparent that the mobile agricultural machine provided herein comprises a pair of the left-side and right-side traction crawlers  2  that are driven through the speed change apparatus  25  at any traveling speed selected in a step-less manner using the speed change lever  73 . The mobile agricultural machine further comprises the steering apparatus  28  that is operated using the steering wheel  19  so as to steplessly differentiate the relative speeds of left-side and right-side crawlers  2 . The mobile is characterized in that the traveling speed controlled by means of the speed change lever  73  is reduced in proportion to an angular displacement of the steering wheel  19 , wherein the ratio of speed reduction to a steered angle, i.e., the angular displacement of said wheel  19  can be altered freely at any time. By virtue of this feature, the ratio of speed reduction to the steered angle may be lowered to cause the mobile machine to make a gentle turn in wet farms and, to thus move thereon smoothly for efficient performance of various works therein. The manually operable member  226  serves to change the ratio of operated angles of steering wheel  19  to the extent of reduction of the mobile machine, any manner of steering performance can be selected readily to match the personality of a driver and the nature or kinds of works to be done, particularly in a wet farm, so as to lower loss of power and afford an improved steerability.  
     [0095] In this invention, the electrical speed change motor  196  is employed as an actuator to control the speed change apparatus  25  by detecting current position of the main speed change lever  73 . This feature renders easier control, mounting, maintenance, adjustment and maneuverability of these apparatus  25  and lever  73 . Further in this invention, the electrical steering motor  197  is employed as another actuator to control the steering apparatus  28  by detecting current position of the steering wheel  19 . This further feature renders easier control, mounting, maintenance, adjustment and maneuverability of these apparatus  28  and wheel  19 .  
     [0096] As also seen from the flow chart given in FIG. 32, input to the controller  228  are outputs from: the main speed change sensor  224 , the subsidiary speed change sensor  225 , the steered-angle sensor  223 , the turning-mode setting device  227  and the straight-advance detecting sensor  165 . With the subsidiary speed change lever  74  being detected to be at its neutral position, power to turn the vehicle will be intercepted by turning off the steering output clutch  62  to make idle the second oil-hydraulic motor  27 . With the main speed change lever  73  detected at its neutral position, power to turn the vehicle will be intercepted likewise, and with the steering wheel  19  sanding at its neutral position, the steering output clutch  62  will be turned off to also make idle the second oil-hydraulic motor  27 . If contrarily the subsidiary speed changer is detected to be at its position commanding middle or lower speeds, with the main speed changer being not at its neutral position when the steering wheel  19  is operated out of its straight-advance position, then the outputs from the main speed change sensor  224  and steered-angle sensor  223  will be used to calculate and determine a desirable reduction ratio given by the main speed changer together with a desirable degrees and direction of the steering motion. The electrical speed change motor  196  and the electrical steering motor  197  will thus be actuated to perform respective actions for main speed change and steering. In this way, the right and left crawlers  2  will be driven at different speeds to adjust the running path of the vehicle along any row of crops, or to make a spin at the bare end area in a farm, so that a continuous reaping and threshing work can be done therein. The manually operable member  226  may also be operated, if necessary, to control the turning-mode setting device  22  so as to gradually alter the target speeds that are desired corresponding to current operated angles of the steering wheel  19 . Thus in one case of the vehicle running on road or in dry farms, the target speeds may be varied greatly relative to said operation of steering wheel  19  so as to enable shaper turnings of the vehicle. In another case of the vehicle running in wet and muddy farms, the target speeds may be varied mildly relative to the operation of steering wheel  19  so as to enable gentle turnings of the vehicle. This means that any unwanted spinning turn is avoided regardless of any possible sharp steering operation made on the vehicle running on such a wet place.  
     [0097]FIG. 34 shows that the feeling selection lever  195  or the manually operable knob-shaped member  226  can be used to make choice between two modes, in one of which actual speed is reduced in response to operation of the steering wheel  19 . In the other mode, the actual speed will not be reduced in response to any operation of the steering wheel  19 . The former mode enables a reversed driving of the one crawler  2  temporarily located inwardly of a current turning radius so as to make a spinning turn, whilst the latter mode driving both the crawlers  2  in the same direction not to make any spinning turn. On the former occasion, the vehicle can do shaper turnings on road or in dry farms, whereas on the latter occasion it can do gentle but surer turnings in wet and muddy farms, thus further enhancing steerability and maneuverability of the working vehicle such as a mobile agricultural machine.