Patent Publication Number: US-2022225556-A1

Title: Tractor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a tractor. 
     2. Description of the Related Art 
     Conventionally, there has been known a tractor which includes a three-point linkage to which a tiller can be mounted, a hydraulic lifting/lowering driver for lifting and lowering the three-point linkage, and a mechanical linkage unit for transmitting the lifting/lowering operation amount of the three-point linkage to the lifting/lowering driver (see, e.g., JP 2006-109802 A). 
     The mechanical linkage unit has a load detector for detecting an amount of change in traction load via the three-point linkage, and a mechanical linkage for transmitting a lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver. Thus, the lifting/lowering driver lifts and lowers the three-point linkage to enable the tiller mounted to the three-point linkage to perform draft control which automatically lifts up and down according to the traction load. 
     SUMMARY OF THE INVENTION 
     In general, since there is a distance from the load detector to the lifting/lowering driver, the mechanical linkage transmits an amount of change in traction load to the lifting/lowering driver by interlocking and connecting a plurality of plate-shaped or rod-shaped link members. 
     Since a plurality of link members is used in the mechanical linkage, the structure of the mechanical linkage is easily complicated. As the mechanical linkage becomes more complicated, the maintenance takes more time and chattering is more likely to occur due to the large number of link members. 
     Accordingly, preferred embodiments of the present invention provide tractors in each of which a structure of a mechanical linkage is simple. 
     A tractor according to a preferred embodiment includes a three-point linkage to which a tiller is mountable, a hydraulic lifting/lowering driver to lift and lower the three-point linkage, and a mechanical linkage to transmit a lifting/lowering operation amount of the three-point linkage to the lifting/lowering driver. The mechanical linkage includes a load detector to detect an amount of change in traction load via the three-point linkage, a cable to interlock according to the amount of change in the traction load detected by the load detector, and a mechanical linkage interlocked and connected to the cable and the lifting/lowering driver, the mechanical linkage being capable of transmitting the lifting/lowering operation amount according to the amount of change in the traction load to the lifting/lowering driver. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a right side view of a tractor according to a first preferred embodiment of the present invention. 
         FIG. 2  is a right side view of a hydraulic lifting/lowering device according to the first preferred embodiment of the present invention. 
         FIG. 3  is a hydraulic circuit diagram for lifting/lowering according to the first preferred embodiment of the present invention. 
         FIG. 4  is a perspective view of a load detector according to the first preferred embodiment of the present invention as viewed from the upper left front. 
         FIG. 5  is a cross-sectional view of  5 A- 5 A in  FIG. 4 . 
         FIG. 6  is a right side view for explaining an intermediate linkage according to the first preferred embodiment of the present invention. 
         FIG. 7  is a right side view of a hydraulic lifting/lowering device according to a modified example of a preferred embodiment of the present invention. 
         FIG. 8  is a perspective view of a load detector according to the modified example as viewed from the upper right rear. 
         FIG. 9  is a rear view of the load detector according to the modified example as viewed from the rear. 
         FIG. 10  is a right side view of the load detector according to the modified example. 
         FIG. 11  is a right side view of the load detector according to the modified example. 
         FIG. 12  is a right side view of a tractor according to a second preferred embodiment of the present invention. 
         FIG. 13  is a right side view of a hydraulic lifting/lowering device according to the second preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A tractor according to a preferred embodiment of the present invention includes a three-point linkage to which a tiller is mountable, a hydraulic lifting/lowering driver to lift and lower the three-point linkage, and a mechanical linkage to transmit a lifting/lowering operation amount of the three-point linkage to the lifting/lowering driver. The mechanical linkage includes a load detector to detect an amount of change in traction load via the three-point linkage, a cable to interlock according to the amount of change in the traction load detected by the load detector, and a mechanical linkage interlocked and connected to the cable and the lifting/lowering driver, the mechanical linkage being capable of transmitting the lifting/lowering operation amount according to the amount of change in the traction load to the lifting/lowering driver. 
     Thus, in the mechanical linkage, the cable can carry a portion of the transmission path to transmit the amount of change in traction load from the load detector to the lifting/lowering driver. Therefore, the amount of change in traction load can be transmitted to the lifting/lowering driver without increasing the number of plate-like or rod-like link members constituting the mechanical linkage. Thus, increasing the number of the link members is prevented, and the structure of the mechanical linkage can be made less complicated. 
     According to a preferred embodiment of the present invention, the cable may include a first connecting portion connected on the mechanical linkage side and a second connecting portion connected on the load detector side, and a portion extending from the second connecting portion toward a backward direction of the tractor. One end of the cable can be connected to the mechanical linkage toward a forward direction of the tractor, and another end of the cable can be connected to the load detector toward a forward direction of the tractor. 
     Thus, an operator positioned in the rear of the tractor can recognize each connecting portion by tracing the cable from the rear of the tractor. Therefore, the operator can easily perform the maintenance work such as cable inspection and replacement work. 
     According to a preferred embodiment of the present invention, the cable may include a first connecting portion connected on the mechanical linkage side, and a second connecting portion connected on the load detector side. The tractor may include a mounting portion to which the second connecting portion is mounted, and a swing that swings back and forth according to a back and forth swing of the load detector. The cable can extend curvedly so that when the second connecting portion swings forward together with the swing, the first connecting portion is pulled backward, and when the second connecting portion swings backward together with the swing, the first connecting portion is pulled forward, in a side view of the tractor. The second connecting portion can be mounted to the mounting portion at a position higher than the first connecting portion. 
     The operator can carry the second connecting portion to the mounting portion without causing the operator to move significantly by bending the cable in the maintenance work. In addition, since the second connecting portion is positioned at a position higher than the first connecting portion, the second connecting portion can be mounted at a position close to the height of the operator&#39;s eye line. The operator can easily perform the maintenance work. 
     According to a preferred embodiment of the present invention, the cable may include an inner cable that swings according to a back and forth swing of the swing, a cover portion that covers an outside of the inner cable, and a cable fixing portion that fixes the cover portion. 
     Fixing the cover portion by the cable fixing portion allows the swinging path of the inner cable to be fixed, and the inner cable to be stably swung according to the back and forth swing of the swing. Thus, the mechanical linkage unit can accurately transmit the lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver, so that a high quality draft control can be executed. 
     According to a preferred embodiment of the present invention, the tractor may include a swing that swings back and forth according to a back and forth swing of the load detector. The mechanical linkage may include a sensitivity adjuster to adjust an operation sensitivity when the lifting/lowering driver interlocks with the swing. The cable may be connected to the swing and the sensitivity adjuster. 
     Since the cable directly connects the swing and the sensitivity adjuster, the number of links connecting the swing and the sensitivity adjuster can be reduced, and the structure of the hydraulic lifting/lowering device is simplified. 
     According to a preferred embodiment of the present invention, at least a portion of the mechanical linkage can be positioned farther rearward than the front end of the lower link of the three-point linkage. The mechanical linkage can be easily confirmed from the rear of the tractor, so that the maintainability can be improved. 
     According to a preferred embodiment of the present invention, the cable may include a first connecting portion connected on the mechanical linkage side, and a second connecting portion connected on the load detector side. A mounting portion to which the second connecting portion can be mounted is positioned above the load detector. 
     Thus, the second connecting portion and the mounting portion are easily confirmed from the rear of the tractor, and the second connecting portion is easily mounted and removed. Therefore, the maintenance work can be easily performed. A space including the mounting portion of the second connecting portion is not required in front of the load detector. 
     According to a preferred embodiment of the present invention, the cable may include a first connecting portion connected on the mechanical linkage side, and a second connecting portion connected on the load detector side. The mechanical linkage may include a first mounting portion to which the first connecting portion is mounted, and a second mounting portion to which the second connecting portion is mounted. The second mounting portion can be positioned farther rearward than the first mounting portion in a state where the traction load is not applied. 
     Thus, since the distance from the rear of the tractor to the second mounting portion is closer than that to the first mounting portion, the second connecting portion is easily mounted and removed from the rear of the tractor. Therefore, the maintenance work can be easily performed. 
     According to a preferred embodiment of the present invention, the mechanical linkage may include an amplification swing that is supported by the load detector so as to be swingable and swings back and forth according to a back and forth swing of the load detector. The amount of change in the traction load can be amplified by the back and forth swing of the amplification swing. 
     Thus, the tiller can be quickly lifted and lowered according to the change of the traction load. Consequently, even when the traction load rapidly increases, the engine stall caused by the increase of the traction load can be avoided. 
     According to a preferred embodiment of the present invention, a state in which the lifting/lowering operation amount according to the amount of change in the traction load amplified by the back and forth swing of the amplification swing is transmitted to the lifting/lowering driver and a state in which the lifting/lowering operation amount according to the amount of change in the traction load not amplified by the back and forth swing of the amplification swing is transmitted to the lifting/lowering driver can be switched. 
     Thus, the operator can lift and lower the three-point linkage according to the condition of the soil in which the traction work is to be performed (e.g., hardness of soil, and content ratio of stone, rock and wood). 
     According to a preferred embodiment of the present invention, a first connecting portion which is a connecting portion on one side of the cable can be connected to the mechanical linkage. The cable can selectively transmit the amount of change in the traction load and an amount of change in tilling depth A connection destination of a second connecting portion which is a connecting portion on another side of the cable is switched, whereby either one of a first lifting/lowering operation amount according to the amount of change in the traction load and a second lifting/lowering operation amount according to the amount of change in the tilling depth is selected as the lifting/lowering operation amount transmitted to the lifting/lowering driver. 
     In order to switch the connection destination of the second connecting portion of the cable, the mechanical linkage connected to the first connecting portion of the cable is a linkage common to both draft control and automatic tilling depth control. Therefore, since similar functions of the draft control and the automatic tilling depth control can be combined into one linkage, the structure of the hydraulic lifting/lowering device can be made less complicated. The cable is more flexible than a plate-like or rod-like link member used in a general mechanical linkage. Therefore, the degree of freedom in the disposition of the connection destination of the cable is increased, and the structure of the hydraulic lifting/lowering device can be made less complicated. In addition, switching the connection destination of the cable having a wide movable range by the flexibility allows the switching work between the draft control and the automatic tilling depth control to be easily performed. 
     According to a preferred embodiment of the present invention, the tractor may include a mounting portion for draft control to which the second connecting portion is mounted when the first lifting/lowering operation amount is selected as the lifting/lowering operation amount, and a mounting portion for automatic tilling depth control to which the second connecting portion is mounted when the second lifting/lowering operation amount is selected as the lifting/lowering operation amount. The mounting portion for draft control can be positioned more forward than a front end of a top link of the three-point linkage. The mounting portion for automatic tilling depth control can be positioned farther rearward than a rear end of the top link. 
     When the operator performs the switching work between the draft control and the automatic tilling depth control, the operator can work between the front end and the rear end of the top link in a side view of the tractor, and can easily perform the switching work between the draft control and the automatic tilling depth control. 
     According to a preferred embodiment of the present invention, the tractor may include a mounting portion to which the second connecting portion is mounted when the first lifting/lowering operation amount is selected as the lifting/lowering operation amount. The cable may include a portion extending from the second connecting portion toward a backward direction of the tractor. The second connecting portion can be mounted to the mounting portion toward a forward direction of the tractor. 
     When selecting the first lifting/lowering operation amount as the lifting/lowering operation amount, that is, when switching to the draft control, the operator positioned at the rear of the tractor can easily carry the second connecting portion to the mounting portion by gripping the portion (extension portion) extending toward the backward direction of the tractor so that the second connecting portion is positioned more to the front side of the operator than the extension portion. Therefore, the operator can easily perform the switching work to the draft control. 
     According to a preferred embodiment of the present invention, the tractor may include a load detector to swing back and forth according to the traction load transmitted through a top link of the three-point linkage, a mounting portion to which the second connecting portion is mounted, and a swing to swing back and forth according to a back and forth swing of the load detector. The cable can extend curvedly so that when the second connecting portion swings forward together with the swing, the first connecting portion is pulled backward, and when the second connecting portion swings backward together with the swing, the first connecting portion is pulled forward, in a side view of the tractor. The second connecting portion can be mounted to the mounting portion at a position higher than the first connecting portion. 
     When switching to the draft control, the operator can carry the second connecting portion to the mounting portion without causing the operator to move significantly by bending the cable. In addition, since the second connecting portion is positioned at a position higher than the first connecting portion, the second connecting portion can be mounted at a position close to the height of the operator&#39;s eye line. The operator can easily perform the switching work to the draft control. 
     According to a preferred embodiment of the present invention, the cable may include an inner cable to swing according to a back and forth swing of the swing, a cover portion that covers an outside of the inner cable, and a cable fixing portion that fixes the cover portion. 
     Fixing the cover portion by the cable fixing portion allows the swinging path of the inner cable to be fixed, and the inner cable to be stably swung according to the back and forth swing of the swing. Thus, the mechanical linkage can accurately transmit the lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver, so that a high quality draft control can be executed. 
     According to a preferred embodiment of the present invention, the tractor may include a load detector to swing back and forth according to the traction load transmitted through a top link of the three-point linkage, and a swing that swings back and forth according to a back and forth swing of the load detector. The mechanical linkage may include a sensitivity adjuster to adjust an operation sensitivity when the lifting/lowering driver interlocks with the swing. The cable can be connected to the swing and the sensitivity adjuster. 
     Since the cable directly connects the swing and the sensitivity adjuster, the number of link members connecting the swing and the sensitivity adjuster can be reduced, and the structure of the hydraulic lifting/lowering device is simplified. 
     According to a preferred embodiment, at least a portion of the mechanical linkage can be positioned farther rearward than a front end of a lower link of the three-point linkage. 
     The mechanical linkage can be easily confirmed from the rear of the tractor, so that the maintainability can be improved. 
     According to a preferred embodiment of the present invention, the tractor may include a load detector to swing back and forth according to the traction load transmitted through a top link of the three-point linkage, and a mounting portion to which the second connecting portion is mounted when the first lifting/lowering operation amount is selected as the lifting/lowering operation amount. The mounting portion can be positioned above the load detector. 
     Thus, when the automatic tilling depth control is switched to the draft control, the second connecting portion and the mounting portion is easily confirmed from the rear of the tractor, and the connection destination of the second connecting portion is easily switched. Therefore, the switching work can be easily performed. 
     According to a preferred embodiment of the present invention, the tractor may include a hitch to connect the tiller at a rear of the three-point linkage. The hitch may include a linkage that swings back and forth according to the amount of change in tilling depth. The linkage may include a mounting portion to which the second connecting portion is mounted when the second lifting/lowering operation amount is selected as the lifting/lowering operation amount. 
     Thus, since the second connecting portion of the cable is directly mounted to the linkage of the hitch, the number of members for transmitting the amount of change in tilling depth to the cable can be reduced on the front side of the three-point linkage as compared with the case where the second connecting portion of the cable is mounted. 
     Conventionally, a tractor including a hydraulic lifting/lowering device is widely known. As such a tractor, JP 2006-109802 A discloses a tractor including a mechanical linkage for draft control. A mechanical linkage (draft feedback linkage) for draft control in JP 2006-109802 A transmits a lifting/lowering operation amount according to an amount of change in traction load detected via a top link of a three-point linkage to a lifting/lowering driver (e.g., control valve). The lifting/lowering driver performs lifting/lowering drive of the three-point linkage according to the lifting/lowering operation amount, whereby a traction type tiller (e.g., plow) mounted to the three-point linkage automatically lifts up and down according to the traction load. As described above, the tractor in JP 2006-109802 A can perform draft control when carrying out the tilling work by the traction type tiller, so that the traction load can be kept constant, and the occurrence of engine stall caused by the increase of the traction load can be avoided. 
     As an example of a tractor including a hydraulic lifting/lowering device, JP 2003-102208 A discloses a tractor including a mechanical linkage for automatic tilling depth control. The mechanical linkage (intermediate linkage) for automatic tilling depth control in JP 2003-102208 A transmits a lifting/lowering operation amount according to an amount of change in tilling depth detected via a linkage (e.g., swing arm, interlocking link, linkage link, and sensor wire) provided in a rotary tiller to a lifting/lowering driver (e.g., position control valve). The lifting/lowering driver performs lifting/lowering drive of the three-point linkage according to the lifting/lowering operation amount, whereby the rotary tiller mounted to the three-point linkage automatically lifts up and down according to the tilling depth. As described above, the tractor in JP 2003-102208 A can perform the automatic tilling depth control when carrying out the tilling work by the rotary tiller, so that tilling work with high accuracy can be performed with the tilling depth kept constant. 
     The tractor in JP 2006-109802 A does not include a mechanical linkage for automatic tilling depth control. Therefore, when the tilling work is performed by the rotary tiller, the automatic tilling depth control cannot be performed, and it has been difficult to perform tilling work with high accuracy in which the tilling depth is kept constant. On the other hand, the tractor in JP 2003-102208 A does not include a mechanical linkage for draft control. Therefore, when the tilling work is performed by the traction type tiller, the draft control cannot be performed, and an engine stall may occur due to the increase of the traction load. In order to accommodate both of the tilling works, it is envisioned that the hydraulic lifting/lowering device includes both a mechanical linkage for automatic tilling depth control and a mechanical linkage for draft control. However, there is a problem that the structure of the hydraulic lifting/lowering device is made much more complicated by providing both mechanical linkages. As the structure becomes more complicated, the maintenance takes more time and chattering is more likely to occur due to the number of link members constituting the mechanical linkage. Therefore, in order to provide a tractor capable of performing draft control and automatic tilling depth control, in which the structure of the hydraulic lifting/lowering device is simplified, the tractor may have the following structure. 
     A tractor according to one aspect of a preferred embodiment of the present invention includes a three-point linkage to which a tiller can be mounted, a hydraulic lifting/lowering driver to lift and lower the three-point linkage, and a mechanical linkage to transmit a lifting/lowering operation amount of the three-point linkage to the lifting/lowering driver. The mechanical linkage includes a cable to selectively transmit an amount of change in traction load and an amount of change in tilling depth, and a mechanical linkage interlocked and connected to the cable and the lifting/lowering driver. A first connecting portion which is a connecting portion on one side of the cable is connected to the mechanical linkage. A connection destination of a second connecting portion which is a connecting portion on an other side of the cable is switched, such that either one of a first lifting/lowering operation amount according to the amount of change in traction load and a second lifting/lowering operation amount according to the amount of change in tilling depth is selected as the lifting/lowering operation amount transmitted to the lifting/lowering driver. 
     Preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following drawings, the same or similar reference numerals denote the same or similar portions. However, it should be noted that the drawings are schematic, and the proportions of each dimension are different from the actual figures. Therefore, specific dimensions should be determined in consideration of the following explanation. Further, the drawings may include portions having different dimensional relationships and ratios between the drawings. In the present description and drawings, elements having substantially the same functions and structures are denoted by the same reference numerals, and redundant description thereof is omitted, and elements not directly related to the present disclosure are omitted from illustration. 
     First Preferred Embodiment 
     A schematic configuration of a tractor will be described with reference to  FIG. 1 .  FIG. 1  is a right side view of the tractor according to the present preferred embodiment. In the drawing, an arrow F indicates the forward direction (forward) of the tractor, an arrow B indicates the backward direction (backward) of the tractor, an arrow U indicates the upward direction (upward) of the tractor, an arrow D indicates the downward direction (downward) of the tractor, an arrow R indicates the rightward direction (rightward) of the tractor, and an arrow L indicates the leftward direction (leftward) of the tractor. As illustrated in  FIG. 1 , the tractor  1  illustrated in the present preferred embodiment includes a vehicle body  2 , a three-point linkage  3 , and a hydraulic lifting/lowering device  5 . The vehicle body  2  includes a frame  21 , an engine  22 , a clutch housing  23 , a transmission case (hereinafter referred to as a T/M case)  24 , a wheel  25 , a rear fender  26 , a riding type operation section  27 , and a cylinder case  28 , for example. 
     The frame  21  includes a front frame  21 F positioned at the front of the vehicle body  2  and an intermediate frame  21 M connected to the rear end of the clutch housing  23 . The engine  22  is connected to the rear of the front frame  21 F. The clutch housing  23  is connected to the rear end lower portion of the engine  22 . The T/M case  24  is connected to the rear end of the intermediate frame  21 M. The T/M case  24  is also used as a rear frame. The wheel  25  includes left and right front wheels  25 F disposed on the right and left sides of the front frame  21 F, and right and left rear wheels  25 B disposed on the left and right sides of the T/M case  24 . The rear fender  26  covers the right and left rear wheels  25 B. The operation section  27  has a steering wheel  271  for front wheel steering and a driver seat  272  positioned between right and left rear fenders  26 . The cylinder case  28  houses, for example, a control valve  63  to be described below. The driver seat is disposed above the cylinder case  28 . 
     Although not illustrated, the power from the engine  22  is transmitted to the main transmission included in the T/M case  24  via, for example, the main clutch included in the clutch housing  23  and a transmission shaft covered with the intermediate frame  21 M. The power after the shift by the main transmission is transmitted to the wheels  25  via, for example, an auxiliary transmission included in the T/M case  24 . 
     The three-point linkage  3  can mount a tiller  4 . The three-point linkage  3  is connected to the rear of the T/M case  24  so as to be vertically swingable. The three-point linkage  3  has a single top link  3 A and right and left lower links  3 B. When the tilling work is performed by the tractor  1 , a traction type tiller  4 A can be mounted to the three-point linkage  3 . The present preferred embodiment illustrates the state where a plow is mounted to the three-point linkage  3  as an example of the traction type tiller  4 A. A disc harrow, a cultivator, or a subsoiler, for example, can be mounted to the three-point linkage  3  as the traction type tiller  4 A. 
     As illustrated in  FIG. 1 , the front end of the top link  3 A is connected to a load detector  71  (load detector  711 ) to be described below via a connection pin. The front ends of the right and left lower links  3 B are connected to right and left brackets provided on the rear end of the T/M case  24  via right and left connection pins. This connection structure allows the traction load during tilling work to act on the load detector  711  via the top link  3 A. 
     The hydraulic lifting/lowering device  5  is of a mechanical linkage type. The hydraulic lifting/lowering device  5  is disposed on the rear side of the tractor. The configuration of the hydraulic lifting/lowering device  5  is illustrated below. 
     The configuration of the hydraulic lifting/lowering device according to the present preferred embodiment will be described with reference to  FIGS. 1 to 6 .  FIG. 2  is a right side view of the hydraulic lifting/lowering device according to the preferred embodiment.  FIG. 3  is a hydraulic circuit diagram for lifting/lowering according to the present preferred embodiment.  FIG. 4  is a perspective view of the load detector according to the present preferred embodiment as viewed from the upper left front.  FIG. 5  is a cross-sectional view of  5 A- 5 A in  FIG. 4 .  FIG. 6  is a right side view for explaining the intermediate linkage according to the present preferred embodiment. 
     As illustrated in  FIG. 2 , the hydraulic lifting/lowering device  5  (tractor  1 ) includes a hydraulic lifting/lowering driver  6  and a mechanical linkage  7 . The lifting/lowering driver  6  can lift and lower the three-point linkage  3 . The tiller  4  mounted to the three-point linkage  3  is lifted up and down according to the lifting/lowering drive of the three-point linkage  3 . The mechanical linkage  7  transmits the lifting/lowering operation amount of the three-point linkage  3  to the lifting/lowering driver  6 . Details of the mechanical linkage  7  will be described below. The lifting/lowering driver  6  includes right and left lift arms  61 , a lift cylinder  62 , a control valve  63 , a balance arm  64 , a position arm  65 , a position arm shaft  65 A, a feedback arm shaft  66 A, a position lever  67 , and a position plate  68 . 
     As illustrated in  FIG. 1 , the right and left lift arms  61  suspend and support the right and left lower links  3 B via the right and left supports. The lift arm  61  is hydraulically driven by the lift cylinder  62  so as to be vertically swingable. The lift cylinder  62  lifts and lowers the three-point linkage  3  via the lift arm  61 . The lift cylinder  62  is housed in the cylinder case  28 . 
     The control valve  63  controls the lift cylinder  62 . As illustrated in  FIG. 3 , the control valve  63  may include a spool  631 , a descending valve  632 , a relief valve  633 , and an unload valve  634 , for example. The control valve  63  may be connected to the lift cylinder  62  via a drop speed adjusting valve. The control valve  63  is housed in the cylinder case  28 . The spool  631  is always biased in the downward direction D by a biasing means (e.g., a spring). 
     The balance arm  64  is swingably connected to an end of the spool  631 . The position arm shaft  65 A is locked and connected to one end of the balance arm  64  via an operation pin, and the feedback arm shaft  66 A is locked and connected to the other end of the balance arm  64  via the operation pin. The feedback arm shaft  66 A is interlocked and connected to the lift arm  61  via a feedback linkage (not illustrated). 
     The position arm  65  is fixed to the outer end of the position arm shaft  65 A. When the position arm  65  is rotated, the balance arm  64  swings about a locking point of the feedback arm shaft  66 A with the operation pin. Thus, the spool  631  is shifted to the ascending side or the descending side, and the lift arm  61  performs a driving ascending operation or a dead weight descending operation. The actuation of the lift arm  61  is transmitted via the feedback linkage and converted into rotation of the feedback arm shaft  66 A. When the feedback arm shaft  66 A is rotated, the spool  631  is shifted in the opposite direction via the balance arm  64 . The spool  631  returns to the neutral position when the feedback arm shaft  66 A is rotated to a predetermined rotational position corresponding to the rotational operation position of the position arm  65 . As described above, arbitrarily rotating the position arm  65  allows the lift arm  61  to be lifted and lowered to a position corresponding to the position arm  65 . 
     The position lever  67  sets a control target height of the tiller  4 . The position lever  67  is disposed on the right side of the driver seat. The position lever  67  is disposed so as to be swingable back and forth about a fulcrum a positioned near the rear side of the position arm  65 . The fulcrum a is a fulcrum common to the control adjustment lever  751  to be described below. The position lever  67  is disposed more to the driver seat  272  side (left side) than the control adjustment lever  751 . The position lever  67  is interlocked and connected to the position arm  65  via an elongated hole  65 h and the position plate  68 . Since the spool  631  is biased in the downward direction D by the biasing means, the position arm  65  is rotated and biased clockwise in a side view from the right side (see, e.g.,  FIG. 2 ). Thus, the rear end of the elongated hole  65 h is engaged with and in contact with the position plate  68 . 
     As the position lever  67  is swung toward the backward direction (counterclockwise direction in  FIG. 2 ) about the fulcrum a, the position arm  65  engaged and supported by the position plate is rotated backward (counterclockwise direction in  FIG. 2 ) against the biasing force. Thus, the lift arm  61  is raised to a position corresponding to the position arm  65 . On the other hand, as the position lever  67  is swung toward the forward direction F (clockwise direction in  FIG. 2 ) about the fulcrum a, the position arm  65 , which is rotated and biased, rotates following the position lever  67 . Thus, the lift arm  61  is lowered to a position corresponding to the position arm  65 . 
     The mechanical linkage  7  will then be described in detail. The mechanical linkage  7  includes the load detector  71 , a biasing mechanism  72 , a limiting mechanism  73 , an operation tool  74 , an intermediate linkage  75 , and a cable  78 . As illustrated in  FIG. 2 , the vehicle body  2  may include a first cable fixing portion  241  to which a cover portion  78 B to be described below is fixed on the intermediate linkage  75  side of the rear end of the vehicle body  2  (T/M case  24 ). As illustrated in  FIGS. 2, 4, and 5 , a support  91  is fixed to the rear end of the vehicle body  2  (cylinder case  28 ). The support  91  includes a vertical wall portion  911  connected to the rear portion of the vehicle body  2  (cylinder case  28 ), a linkage portion  912  which is U-shaped in a top view, an outer wall portion  913  which extends in the upward and downward direction outside the lateral direction of the vertical wall portion  911 , a swing support portion  914  which supports a swing  715  to be described below, and a second cable fixing portion  915  to which the cover portion  78 B is fixed. The linkage portion  912  includes right and left side portions  912 A which extend backward from the vertical wall portion  911 , and a rear portion  912 B which extends in the lateral direction so as to connect the right and left side portions  912 A. 
     The load detector  71  detects the amount of change in traction load via the three-point linkage  3 , and transmits the amount of change in traction load to the intermediate linkage  75  via the cable  78 . The load detector  71  includes the load detector  711  and the swing  715 . 
     The load detector  711  detects the amount of change in traction load via the three-point linkage  3 . The load detector  711  is supported by the support  91  so as to be swingable and displaceable in the forward and backward direction via a first support shaft  92 . The load detector  711  swings back and forth according to a traction load transmitted through the top link  3 A. The load detector  711  includes right and left side wall portions  711 A, a vertical wall portion  711 B to which the side wall portions  711 A are fixed, connection holes  711 C in the side wall portions  711 A, an extension portion  711 E which extends forward from the vertical wall portion  711 B, and a contact portion  711 F which contacts the swing  715 . 
     The vertical wall portion  711 B is provided on the free end side (upper side) of the load detector  711 . The linkage portion  912  of the support  91  enters between the right and left vertical wall portions  711 B. The top link  3 A or a top link bracket supporting the top link  3 A can be connected to the connection hole  711 C. The extension portion  711 E extends from the vertical wall portion  711 B toward the forward direction. The extension portion  711 E is disposed more to the right side than the side wall portion  711 A. The contact portion  711 F extends rightward from the extension portion  711 E at the front of the extension portion  711 E. Thus, when the load detector  711  swings forward in response to an increase of the traction load, the contact portion  711 F moves forward and comes into contact with the swing  715 . 
     The swing  715  swings back and forth according to the back and forth swing of the load detector  711 . The swing  715  is supported by the swing support portion  914  via a second support shaft  914 A extending in the lateral direction. The swing  715  is swingable about a fulcrum b passing through the axial center of the second support shaft  914 A. The swing  715  swings forward by being pushed forward from the contact portion  711 F which moves forward. When there is no contact with the contact portion  711 F (i.e., when no forward force is exerted from the contact portion  711 F), the swing  715  swings backward by a biasing mechanism (not illustrated). For example, the swing  715  is pulled backward to a predetermined position by the spring force of the torsion spring mounted to the second support shaft  914 A. 
     The swing  715  includes a second mounting portion  715 A to which a cable  78  (a second connecting portion  782  to be described below) is mounted. The second mounting portion  715 A is disposed on the free end side. The second mounting portion  715 A is positioned above the load detector  711 . The second mounting portion  715 A is positioned farther rearward than the front end of the lower link  3 B. The second mounting portion  715 A is positioned farther rearward than a first mounting portion  752 A to be described below in a state where no traction load is applied. The second mounting portion  715 A is positioned more forward than the front end of the top link  3 A. The second mounting portion  715 A is a mounting portion for draft control. When the first lifting/lowering operation amount according to the amount of change in traction load is selected as the lifting/lowering operation amount transmitted to the lifting/lowering driver  6 , the second connecting portion  782  is mounted to the second mounting portion  715 A toward the forward direction. 
     The biasing mechanism  72  swings and biases the load detector  711  in a direction (backward) resisting a traction load applied to the load detector  711 . As illustrated in  FIG. 5 , the biasing mechanism  72  may include a spring having elasticity in the forward and backward direction. 
     The limiting mechanism  73  limits the back and forth swing range of the load detector  711 . The limiting mechanism  73  may include a through hole  711   h , an elongated hole  912   h , a rubber block  73   b , and a linkage pin  73   p . The through hole  711   h  is located in the load detector  711 . Specifically, the through holes  711   h  face the right and left side wall portions  711 A, respectively. 
     The elongated hole  912   h  is provided in the support  91  and is elongated in the forward and backward direction. Specifically, the elongated hole  912   h  faces the through hole  711   h  in each of the right and left side portions  912 A of the linkage portion  912 . The rubber block  73   b  is fitted in a space between the vertical wall portion  911  of the support  91  and the linkage portion  912  (side portion  912 A and rear portion  912 B). In the rubber block  73   b , an elongated hole  73   h  having a forward and backward length shorter than that of the elongated hole  912   h  of the support  91  faces the elongated hole  912   h  of the support  91 . The linkage pin  73   p  is inserted into each through hole  711   h , each elongated hole  912   h , and each elongated hole  73   h.    
     The back and forth swing range of the load detector  711  is limited by the forward and backward length of each elongated hole  912   h  of the support  91 . When the limiting mechanism  73  limits the back and forth swing of the load detector  711 , the linkage pin  73   p  of the limiting mechanism  73  collides with the rubber block  73   b.    
     The load detector  711  is held in a reference posture extending vertically upward from the first support shaft  92  by the action of the biasing mechanism  72  and the limiting mechanism  73 . When the traction load exceeds a predetermined value, the load detector  711  swings and displaces forward from the reference posture against the action of the biasing mechanism  72  in conjunction with the increase of the traction load. The load detector  711  swings and displaces backward by the action of the biasing mechanism  72  in conjunction with the decrease of the traction load, and returns to the reference posture. 
     The operation tool  74  switches a first state in which the load detector  711  detects the traction load and a second state in which the load detector  711  does not detect the traction load. The operation tool  74  includes a swing plate  741  supported to be swingable in the lateral direction on the support  91 , an operation handle  742  extending in an upward direction U from the swing plate  741 , and a contact member  743  extending backward from the swing plate  741 . The contact member  743  moves between the non-contact position and the contact position. In the non-contact position (first operation position P 1 ), the contact member  743  does not contact the load detector  711  (vertical wall portion  711 B), and the back and forth swing of the load detector  711  is allowed. On the other hand, at the contact position, the contact member  743  comes into contact with the load detector  711  to prevent the load detector  711  from swinging back and forth. At the contact position (second operation position P 2 ), the contact member  743  is sandwiched between the vertical wall portion  711 B and the swing plate  741 . 
     The operation tool  74  swings in the lateral direction about an axis extending in the forward and backward direction. The operation tool  74  is selectively positioned and held in either the first operation position P 1  (left side) or the second operation position P 2  (right side) by a detent mechanism. When the operation tool  74  is positioned and held at the first operation position P 1 , the contact member  743  is positioned at a non-contact position, and the load detector  71  enters the first state. On the other hand, when the operation tool  74  is positioned and held at the second operation position P 2 , the contact member  743  is positioned at the contact position, and the load detector  71  enters the second state. The operator can perform draft control by moving the operation tool  74  to the first operation position P 1 . 
     The intermediate linkage  75  will then be described. The intermediate linkage  75  is a mechanical linkage. As illustrated in  FIG. 2 , the intermediate linkage  75  is interlocked and connected to the cable  78  and the lifting/lowering driver  6 . The intermediate linkage  75  transmits a lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver  6 . The intermediate linkage  75  includes a control adjustment lever  751 , a cable connecting arm  752 , a linkage plate  753 , an adjustment arm  754 , and a linkage rod  755 . 
     The control adjustment lever  751  is a lever to adjust draft control. The control adjustment lever  751  is swingable back and forth about the fulcrum a. 
     The cable connecting arm  752  is swingable back and forth about a fulcrum c. The cable connecting arm  752  defines and functions as a connection destination on one side of the cable  78 . The cable connecting arm  752  includes the first mounting portion  752 A to which the cable  78  is mounted. Specifically, a first connecting portion  781  to be described below is mounted to the first mounting portion  752 A. The cable connecting arm  752  is provided with a tension spring (not illustrated) to swinging and bias forward. The cable connecting arm  752  includes a cam surface S with a convex shape on a rear end side (a side on the roller  754   r  side to be described below) of the cable connecting arm  752 . 
     The linkage plate  753  extends backward from the proximal end (a support shaft extending from the right side surface of the cylinder case  28  in the right direction R) of the control adjustment lever  751 . The linkage plate  753  is swingable about the fulcrum a. The relative positions of the control adjustment lever  751  and the linkage plate  753  are fixed. Therefore, the linkage plate  753  also rotates about the fulcrum a according to the operation of the control adjustment lever  751 . 
     The adjustment arm  754  is mounted to the tip (rear end) of the linkage plate  753  so as to be swingable back and forth about the fulcrum d. A roller  754   r  as a cam follower is mounted to the adjustment arm  754 . The adjustment arm  754  is provided with a torsion spring to swing and bias the adjustment arm  754  forward. The roller  754   r  abuts on the cam surface S of the cable connecting arm  752  from the rear by the forward bias and swing of the adjustment arm  754  and follows the cam surface S. 
     The linkage rod  755  is bridged over the position arm  65  and the cable connecting arm  752 . The linkage rod  755  transmits the swing and displacement of the adjustment arm  754  according to the following of the cam surface S of the roller  754   r  to the position arm  65 . 
     As illustrated in  FIG. 1 , at least a portion of the intermediate linkage  75  is positioned farther rearward than the front end of the lower link  3 B of the three-point linkage  3 . In the present preferred embodiment, at least a portion of the cable connecting arm  752  and at least a portion of the adjustment arm  754  are positioned farther rearward than the front end of the lower link  3 B. The intermediate linkage  75  (mechanical linkage  7 ) has a sensitivity adjuster to adjust operation sensitivity when the lifting/lowering driver  6  is interlocked with the swing  715 . In the present preferred embodiment, the sensitivity adjuster includes the control adjustment lever  751 , the cable connecting arm  752 , the linkage plate  753 , the adjustment arm  754 , and the linkage rod  755 . 
     Since the relative position of the control adjustment lever  751  and the linkage plate  753  is fixed, the linkage plate  753  rotates downward about the fulcrum a (counterclockwise direction in  FIG. 6 ) and the fulcrum d displaces downward as the operator operates the control adjustment lever  751  to position the control adjustment lever  751  backward (counterclockwise in  FIG. 6 ). Thus, the distance between the contact point of the roller  754   r  with respect to the cable connecting arm  752  and the fulcrum c of the cable connecting arm  752  is reduced, and the lever ratio to bring the adjustment arm  754  into contact with the cable connecting arm  752  by the control adjustment lever  751  is reduced. Therefore, the rotation of the adjustment arm  754  becomes sensitive to the rotation of the cable connecting arm  752 . In other words, the operation sensitivity of the lifting/lowering driver  6  becomes sensitive to the swing of the swing  715 , and the responsiveness when the tiller  4 A raises is improved when the traction load exceeds a predetermined value. 
     On the other hand, as the control adjustment lever  751  is positioned forward (clockwise direction in  FIG. 6 ), the linkage plate  753  rotates upward about the fulcrum a (clockwise in  FIG. 6 ), and the fulcrum d displaces upward. Thus, the distance between the contact point of the roller  754   r  with respect to the cable connecting arm  752  and the fulcrum c of the cable connecting arm  752  is increased, and the lever ratio to bring the adjustment arm  754  into contact with the cable connecting arm  752  by the control adjustment lever  751  is increased. Therefore, the rotation of the adjustment arm  754  becomes insensitive to the rotation of the cable connecting arm  752 . In other words, the operation sensitivity of the lifting/lowering driver  6  becomes insensitive to the swing of the swing  715 , and the responsiveness when the tiller  4 A raises becomes gradual when the traction load exceeds a predetermined value. 
     The cable  78  will then be described. The cable  78  is interlocked according to the amount of change in traction load detected by the load detector  711 . The cable  78  is interlocked and connected to the load detector  71  and the lifting/lowering driver  6 . Specifically, the cable  78  is connected to the swing  715  and the sensitivity adjuster. One end of the cable  78  is connected to the intermediate linkage  75  toward the forward direction of the tractor  1 . The other end of the cable  78  is connected to the load detector  711  toward the forward direction of the tractor  1 . The cable  78  includes a first connecting portion  781  which is a connecting portion on one side of the cable  78  and a second connecting portion  782  which is a connecting portion on the other side of the cable  78 . 
     The first connecting portion  781  is connected on the intermediate linkage  75  side. Specifically, the first connecting portion  781  is connected to the cable connecting arm  752 . The second connecting portion  782  is connected on the load detector  711  (load detector  71 ) side. The second connecting portion  782  is connected to the swing  715 . The second connecting portion  782  is mounted to the second mounting portion  715 A at a position higher than the first connecting portion  781 . 
     The cable  78  is a push-pull cable. The cable  78  includes an inner cable  78 A and a cover portion  78 B. The inner cable  78 A swings according to the swing of the load detector  711 . The inner cable  78 A can move inside an outer cable  78 B 3  to be described below. The first connecting portion  781  is disposed at one end of the inner cable  78 A, and the second connecting portion  782  is disposed at the other end of the inner cable  78 A. Therefore, the inner cable  78 A is directly connected to the swing  715 . The inner cable  78 A is directly connected to the cable connecting arm  752 . 
     The cover portion  78 B covers the outside of the inner cable  78 A. The cover portion  78 B has a first fixing portion  78 B 1  fixed to the first cable fixing portion  241 , a second fixing portion  78 B 2  fixed to the second cable fixing portion  915 , and an outer cable  78 B 3 . The first fixing portion  78 B 1  is positioned at the end of the outer cable  78 B 3  on the intermediate linkage  75  side, and the second fixing portion  78 B 2  is positioned at the end of the outer cable  78 B 3  on the load detector  711  side. The second fixing portion  78 B 2  is fixed to the second cable fixing portion  915  at a position higher than the first connecting portion  781 . 
     As illustrated in  FIGS. 2 and 4 , the cable  78  includes a portion (extension portion E) extending from the second connecting portion  782  toward the backward direction of the tractor  1 . The cable  78  extends curvedly. Specifically, the cable  78  extends in a C shape or a U shape, in a side view of the tractor  1 . Therefore, when the second connecting portion  782  swings forward together with the swing  715 , the first connecting portion  781  is pulled backward, in a side view of the tractor  1 . When the second connecting portion  782  swings backward together with the swing  715 , the first connecting portion  781  is pulled forward. In other words, when a force in the forward direction F is applied to the second connecting portion  782 , a force in the backward direction B is applied to the first connecting portion  781 , and when a force in the backward direction B is applied to the second connecting portion  782 , a force in the forward direction F is applied to the first connecting portion  781 . 
     The draft control will then be described. When a traction load of the traction type tiller  4 A becomes larger than a predetermined value in the tilling work using the traction type tiller  4 A, the load detector  711  is swung forward, and the swing  715  is pushed by the contact portion  711 F and swings forward. Thus, when the second connecting portion  782  of the cable  78  swings forward together with the swing  715 , the first connecting portion  781  is pulled backward. Thus, the cable connecting arm  752  rotates backward about the fulcrum c, and the adjustment arm  754  swings backward following the cable connecting arm  752 . The linkage rod  755  is pulled backward together with the adjustment arm  754 , and the position arm  65  rotates backward about the position arm shaft  65 A. Thus, the lift arm  61  is raised to a position corresponding to the position after the rotation of the position arm  65 . The displacement of the position arm  65  corresponds to a lifting/lowering operation amount. Therefore, the control valve  63  controls the lift cylinder  62  according to the lifting/lowering operation amount (amount of displacement in the position arm  65 ). 
     When the load of the tiller  4  falls below a predetermined value with the lift arm  61  being raised, the lifting operation of the lift arm  61  is stopped. Since draft control in which the tiller  4 A automatically lifts up and down according to the traction load can be performed in the tilling work, the occurrence of an engine stall caused by the increase of the traction load can be avoided. 
     In the mechanical linkage  7 , the configuration described above enables the cable  78  to carry a portion of the transmission path to transmit the amount of change in traction load from the load detector  711  to the lifting/lowering driver  6 . Therefore, the amount of change in the traction load can be transmitted to the lifting/lowering driver  6  without increasing the number of plate-like or rod-like link members constituting the mechanical intermediate linkage  75 . Thus, increasing the number of the link members can be reduced or prevented, and the structure of the mechanical linkage can be made less complicated. 
     The cable  78  includes an extension portion E extending from the second connecting portion  782  toward the backward direction of the tractor  1 . One end of the cable  78  is connected to the intermediate linkage  75  toward the forward direction, and the other end of the cable  78  is connected to the load detector  711  toward the forward direction. Thus, an operator positioned in the rear of the tractor  1  can recognize the connecting portions (the first connecting portion  781  and the second connecting portion  782 ) by tracing the cable  78  from the rear of the tractor. Thus, the operator can easily perform the maintenance work such as inspection and replacement work of the cable  78 . In addition, the operator can easily carry the second connecting portion  782  to the second mounting portion  715 A by gripping the extension portion E so that the second connecting portion  782  is positioned more to the front side of the operator than the extension portion E. Therefore, the operator can easily mount the second connecting portion  782  and easily perform the maintenance work. 
     The cable  78  extends curvedly so that when the second connecting portion  782  swings forward with the swing  715 , the first connecting portion  781  is pulled backward, and when the second connecting portion  782  swings backward with the swing  715 , the first connecting portion  781  is pulled forward, in a side view of the tractor  1 . In addition, the second connecting portion  782  is mounted to the first mounting portion  752 A at a position higher than the first connecting portion  781 . The operator can carry the second connecting portion  782  to the mounting portion without causing the operator to move significantly by bending the cable in the maintenance work. In addition, since the second connecting portion  782  is positioned at a position higher than the first connecting portion  781 , the second connecting portion can be mounted at a position close to the height of the operator&#39;s eye line. The operator can easily perform the maintenance work. 
     The tractor  1  includes cable fixing portions (a first cable fixing portion  241  and a second cable fixing portion  915 ). Fixing the cover portion  78 B by the cable fixing portions allows the inner cable  78 A to be stably swung. Thus, the mechanical linkage  7  can accurately transmit the lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver  6 , so that a high quality draft control can be executed. 
     The cable  78  is connected to the swing  715  and the sensitivity adjuster. Thus, the number of link members connecting the swing  715  and the sensitivity adjuster can be reduced, and the structure of the hydraulic lifting/lowering device  5  is simplified. 
     At least a portion of the intermediate linkage  75  is positioned farther rearward than the front end of the lower link  3 B. Thus, the intermediate linkage  75  can be easily confirmed from the rear of the tractor  1 , so that the maintainability can be improved. 
     The second mounting portion  715 A is positioned above the load detector  711 . Thus, when the second mounting portion  715 A is visually recognized from the rear of the tractor  1 , the second mounting portion  715 A can be visually recognized above the load detector  711 , and the load detector  711  is hardly overlapped with the second mounting portion  715 A. The second connecting portion and the mounting portion are easily confirmed from the rear of the tractor  1 , and the second connecting portion  782  is easily mounted and removed. Therefore, the switching work can be easily performed. A space including the second mounting portion  715 A is not required in front of the load detector  711 . 
     The second mounting portion  715 A is positioned farther rearward than the front end of the lower link  3 B. Since the distance from the rear of the tractor  1  to the second mounting portion  715 A is closer than that to the front end of the lower link  3 B, the second connecting portion  782  is easily mounted and removed from the rear of the tractor  1 . Therefore, the maintenance work can be easily performed. 
     The second mounting portion  715 A is positioned farther rearward than the first mounting portion  752 A in a state where no traction load is applied. Since the distance from the rear of the tractor  1  to the second mounting portion  715 A is closer than that to the first mounting portion  752 A, the second connecting portion  782  is easily mounted and removed from the rear of the tractor  1 . Therefore, the maintenance work can be easily performed. 
     Modified Example 
     A modified example of the first preferred embodiment will then be described mainly with reference to  FIGS. 7 to 11 .  FIG. 7  is a right side view of a hydraulic lifting/lowering device according to the modified example.  FIG. 8  is a perspective view of a load detector according to the modified example as viewed from the upper right rear.  FIG. 9  is a rear view of the load detector according to the modified example as viewed from the rear.  FIG. 10  is a right side view of the load detector according to the modified example.  FIG. 11  is a right side view of the load detector according to the modified example. Note that the same structure as described above will not be explained. 
     The mechanical linkage  7  in the modified example includes an amplification swing  77 . The amplification swing  77  is supported by the load detector  711  so as to be swingable. Specifically, the amplification swing  77  is supported by the load detector  711  via a third support shaft  773  extending in the lateral direction. Thus, the amplification swing  77  is swingable about a fulcrum f. The upper portion of the amplification swing  77  is positioned above the fulcrum f, and the lower portion of the amplification swing  77  is positioned below the fulcrum f. 
     The amplification swing  77  swings back and forth according to the back and forth swing of the load detector  711 . The amount of change in traction load is amplified by the back and forth swing of the amplification swing  77 . A contact roller  775  rotatable about a fulcrum of a fourth support shaft  774  is mounted to the amplification swing  77  via the fourth support shaft  774  extending in the lateral direction. Specifically, the contact roller  775  is mounted to a lower portion of the amplification swing  77 . The contact roller  775  contacts the swing  715 . 
     The load detector  71  includes an amplification contact portion  711 G in contact with the amplification swing  77 . The amplification contact portion  711 G extends outward in the lateral direction (in the present modification, from the right side wall portion  711 A in the right direction L) from the side wall portion  711 A. In the present modification, the load detector  71  does not have an extension portion  711 E and a contact portion  711 F, which differs from the first preferred embodiment. 
     The operation tool  74  includes a receiving portion  745  to receive the amplification swing  77 . The receiving portion  745  includes a receiving roller  745 A to receive the amplification swing  77  and a support portion  745 B to support the receiving roller  745 A. The operation tool  74  switches a first state, a second state and a third state. As described above, the first state is a state in which the load detector  711  detects a traction load. In the first state, the lifting/lowering operation amount according to the amount of change in traction load which is not amplified by the back and forth swing of the amplification swing  77  is transmitted to the lifting/lowering driver  6 . The second state is a state in which the load detector  711  does not detect a traction load. The third state is a state in which the lifting/lowering operation amount according to the amount of change in traction load amplified by the back and forth swing of the amplification swing  77  is transmitted to the lifting/lowering driver. Therefore, in the third state, the amount of change in traction load detected by the load detector  711  is amplified. 
     As illustrated in  FIG. 9 , the operation tool  74  is selectively positioned and held in either the first operation position P 1  (right side), the second operation position P 2  (left side), and a third operation position (center). When the operation tool  74  is positioned and held at the first operation position P 1 , the contact member  743  is positioned at a non-contact position. In addition, the receiving portion  745  does not contact the amplification swing  77 . The load detector  71  enters the first state. When the operation tool  74  is positioned and held at the second operation position P 2 , the contact member  743  is positioned at a contact position, and the load detector  71  enters the second state. When the operation tool  74  is positioned and held at the third operation position P 3 , the contact member  743  is positioned at a non-contact position. In addition, the receiving portion  745  contacts the amplification swing  77 . The load detector  71  enters the third state. 
     As illustrated in  FIG. 10 , in the first state, when the load detector  711  swings forward in response to an increase of the traction load, the amplification contact portion  711 G moves forward. The forward movement of the amplification contact portion  711 G allows the amplification swing  77  to be pushed forward. Since the receiving portion  745  (the receiving roller  745 A) does not contact the amplification swing  77 , the amplification swing  77  is pushed forward only by an amount according to the swing of the load detector  711 . The contact roller  775  coming into contact with the swing  715  moves forward by the amount that the amplification swing  77  is pushed forward, and the swing  715  swings forward according to the moving amount of the contact roller  775 . Thus, when the second connecting portion  782  of the cable  78  swings forward together with the swing  715 , the first connecting portion  781  is pulled backward. 
     As illustrated in  FIG. 11 , in the third state, as in the first state, when the load detector  711  swings forward in response to an increase of the traction load, the amplification contact portion  711 G moves forward and the amplification swing  77  is pushed forward. Since the receiving portion  745  (the receiving roller  745 A) comes into contact with the amplification swing  77 , the upper portion of the amplification swing  77  is not pushed forward, while the lower portion of the amplification swing  77  is pushed forward. Consequently, in  FIG. 11 , the amplification swing  77  swings counterclockwise about the fulcrum f. Therefore, the amplification swing  77  swings relative to the load detector  711 . Thus, the lower portion of the amplification swing  77 , specifically, the contact roller  775  moves forward by the amount of swing of the amplification swing  77  in addition to the amount of swing of the load detector  711 . Since the contact roller  775  moves forward by the amount that the amplification swing  77  is pushed forward, in the third state, the swing  715  swings further forward than in the first state even under the same traction load as in the first state. Therefore, the swing of the amplification swing  77  increases the amount of change in traction load. 
     As described above, in the modified example, the amount of change in traction load is amplified by the back and forth swing of the amplification swing  77 . Thus, the tiller  4 A can be quickly lifted and lowered according to the change of the traction load. Consequently, even when the traction load rapidly increases, the engine stall caused by the increase of the traction load can be avoided. 
     The third state in which a lifting/lowering operation amount according to the amount of change in traction load amplified by the back and forth swing of the amplification swing  77  is transmitted to the lifting/lowering driver  6 , and the first state in which a lifting/lowering operation amount according to the amount of change in traction load not amplified by the back and forth swing of the amplification swing  77  is transmitted to the lifting/lowering driver  6 , are switched. Thus, the operator can lift and lower the three-point linkage  3  according to the condition of the soil in which the traction work is to be performed (e.g., hardness of soil, and content ratio of stone, rock and wood). 
     The vehicle body  2  may have a fourth cable fixing portion  244  to which the cover portion  78 B is fixed. The front end of the fourth cable fixing portion  244  is supported, and the fourth cable fixing portion  244  may extend backward. The rear end of the fourth cable fixing portion  244  may be provided with a spiral portion extending in a spiral shape. The cover portion  78 B may be able to insert into the spiral portion. The cover portion  78 B may be fixed by inserting the cover portion  78 B into the spiral portion. Thus, the cover portion  78 B is fixed at the three points of the first fixing portion  78 B 1 , the second fixing portion  78 B 2 , and the fourth cable fixing portion  244 , so that the cover portion  78 B can be prevented from swinging due to, for example, vibration caused by traveling. Thus, the vibration of the first fixing portion  78 B 1  and the second fixing portion  78 B 2  is suppressed, the load applied to the first mounting portion  752 A and the second mounting portion  715 A is reduced, and the damage of the first mounting portion  752 A and the second mounting portion  715 A can be reduced. 
     Second Preferred Embodiment 
     A second preferred embodiment of the present invention will be described below. The tractor  1  may be replaceable with a traction type tiller  4 A and a rotary tiller  4 B. In the first preferred embodiment, since the tractor  1  including the traction type tiller  4 A has been described, a tractor  1  including the rotary tiller  4 B will then be described mainly with reference to  FIGS. 12 and 13 .  FIG. 12  is a right side view of the tractor according to the second preferred embodiment.  FIG. 13  is a right side view of a hydraulic lifting/lowering device according to the second preferred embodiment. Note that the same structure as described above will not be described. 
     As illustrated in  FIGS. 12 and 13 , the rotary tiller  4 B can be mounted to the three-point linkage  3 . The rotary tiller  4 B can be mounted to and removed from the tractor  1  (three-point linkage  3 ) by mounting and removing the rotary tiller  4 B via a hitch  8 . The hitch  8  is provided at the rear of the three-point linkage  3 . The hitch  8  is used to connect the tiller  4  (rotary tiller  4 B). 
     The hitch  8  includes a linkage  81  and a third cable fixing portion  83 . The linkage  81  swings back and forth together with a swing arm  45  to be described below. Therefore, the linkage swings back and forth according to the amount of change in tilling depth. The linkage  81  includes a third mounting portion  813  to which the second connecting portion  782  is mounted. The third mounting portion  813  is a mounting portion for automatic tilling depth control. When the second lifting/lowering operation amount according to the amount of change in tilling depth is selected as the lifting/lowering operation amount transmitted to the lifting/lowering driver  6 , the second connecting portion  782  is mounted to the third mounting portion  813 . The third mounting portion  813  is positioned farther rearward than the rear end of the top link  3 A. The third cable fixing portion  83  has the cover portion  78 B fixed thereto. A bracket  35  to support the top link  3 A is mounted to the load detector  711 . The top link  3 A is shorter than the top link  3 A in  FIG. 1 . 
     The rotary tiller  4 B includes a rotor portion  41 , a tilling cover  42  covering the rotor portion  41  from above, a ground body (rear cover)  43  to detect the amount of change in the tilling depth of the rotary tiller  4 B, a transmission case portion  44 , a swing arm  45  provided on the transmission case portion  44  so as to be swingable back and forth, and a linkage link  46  bridged over the lower free end of the swing arm  45  and the ground body  43 . The ground body  43  is a rear cover to prevent scattering of soil during tilling and averaging tilling traces. The ground body is provided at the rear end of the tilling cover  42 . The ground body  43  is vertically swingable about a fulcrum e. 
     The linkage  81  is rotatably provided on the rear of the three-point linkage  3  with the central portion of the linkage  81  as a fulcrum. When the rotary tiller  4 B is connected to the three-point linkage  3  via the hitch  8 , one end of the linkage  81  is engaged and connected to an engaging recess (not illustrated) formed in the swing arm  45 . Thus, the linkage  81  rotates in accordance with the swing of the swing arm  45 . The third mounting portion  813  is provided at the other end of the linkage  81 . The third mounting portion  813  includes a second connecting portion  782  of the cable  78  mounted thereto. Thus, the ground body  43  and the control valve  63  are linked via the cable  78 . 
     The automatic tilling depth control will then be described. When tilling work is being performed by using the rotary tiller  4 B and the rotary tiller  4 B starts to go deeper than the target tilling depth, the ground body  43  is lifted and swung accordingly. Thus, the linkage link  46  moves forward, the swing arm  45  swings forward, and the linkage  81  rotates counterclockwise. The rotation of the linkage  81  pulls the second connecting portion  782  of the cable  78  backward and also pulls the first connecting portion  781  backward. Thus, the cable connecting arm  752  rotates backward about the fulcrum c and the position arm  65  rotates backward about the position arm shaft  65 A, as in the draft control. The lift arm  61  is raised to a position corresponding to the position after the rotation of the position arm  65 , and the rotary tiller  4 B is also raised. When the ground body  43  is restored to the original posture, the lifting operation is stopped and the original tilling depth is returned. 
     On the other hand, when tilling work is being performed by using the rotary tiller  4 B and the rotary tiller  4 B starts to go shallower than the target tilling depth, the ground body  43  is hung down and swung accordingly to loosen the inner cable  78 A of the cable  78 . Since the cable connecting arm  752  is swung and biased forward (clockwise direction in  FIG. 13 ) by a tension spring, when the inner cable  78 A is loosened, the cable connecting arm  752  swings forward (clockwise direction) so as to absorb the slack of the inner wire  36 a. Thus, the adjustment arm  754  swings forward following the cable connecting arm  752 . The linkage rod  755  is pushed forward together with the adjustment arm  754 , and the position arm  65  rotates forward about the position arm shaft  65 A. Thus, the lift arm  61  is lowered to a position corresponding to the position after the rotation of the position arm  65 , and the rotary tiller  4 B also is lowered. When the ground body  43  is restored to the original posture, the lowering operation is stopped and the original tilling depth is returned. 
     When automatic tilling depth control is performed, the operation tool  74  is moved to the second operation position P 2 . Thus, the load detector  71  enters the second state, and the load detector  711  does not swing back and forth. Therefore, the amount of change in tilling depth can be accurately detected. 
     The above configuration switches the connection destination of the second connecting portion  782  of the cable  78 , whereby either one of the first lifting/lowering operation amount according to the amount of change in traction load and the second lifting/lowering operation amount according to the amount of change in tilling depth is selected as the lifting/lowering operation amount transmitted to the lifting/lowering driver  6 . Thus, the cable  78  can selectively transmit the amount of change in traction load and the amount of change in tilling depth. 
     Specifically, when the second mounting portion  715 A of the swing  715  is selected as the connection destination of the second connecting portion  782 , the first lifting/lowering operation amount is transmitted to the lifting/lowering driver  6  as the lifting/lowering operation amount. The lifting/lowering driver  6  controls the lift cylinder  62  according to the first lifting/lowering operation amount (i.e., the amount of change in traction load). Therefore, the traction type tiller  4 A can be automatically lifted and lowered according to the traction load in the tilling work. 
     On the other hand, when the third mounting portion  813  of the hitch  8  is selected as the connection destination of the second connecting portion  782 , the second lifting/lowering operation amount is transmitted to the lifting/lowering driver  6  as the lifting/lowering operation amount. The lifting/lowering driver  6  controls the lift cylinder  62  according to the second lifting/lowering operation amount (i.e., the amount of change in tilling depth). Therefore, the rotary tiller  4 B can be automatically lifted and lowered according to the tilling depth in the tilling work. 
     As described above, in order to switch the connection destination of the second connecting portion  782 , the intermediate linkage  75  connected to the first connecting portion  781  is a linkage common to both draft control and automatic tilling depth control. Therefore, since similar functions of the draft control and the automatic tilling depth control can be combined into one linkage, the structure of the hydraulic lifting/lowering device  5  can be much less complicated. 
     The second mounting portion  715 A is positioned more forward than the front end of the top link  3 A. The third mounting portion  813  is positioned farther rearward than the rear end of the top link. When the operator performs the switching work between the draft control and the automatic tilling depth control, the operator can work between the front end and the rear end of the top link  3 A in a side view of the tractor  1 , and can easily perform the switching work between the draft control and the automatic tilling depth control. 
     The cable  78  is more flexible than a plate-like or rod-like link member used in a general mechanical linkage. Therefore, the degree of freedom in the disposition of the connection destination of the cable  78  is increased, and the structure of the hydraulic lifting/lowering device  5  can be made less complicated. 
     In addition, switching the connection destination of the cable  78  having a wide movable range by the flexibility allows the switching work between the draft control and the automatic tilling depth control to be easily performed. 
     The cable  78  includes an extension portion E extending from the second connecting portion  782  toward the backward direction of the tractor  1 . The second connecting portion  782  is mounted to the second mounting portion  715 A toward the forward direction. Thus, an operator positioned in the rear of the tractor  1  can easily carry the second connecting portion  782  to the second mounting portion  715 A by gripping the extension portion E so that the second connecting portion  782  is positioned more to the front side of the operator than the extension portion E. Therefore, the operator can easily mount the second connecting portion  782 , and can easily perform the switching work to the draft control. 
     The cable  78  extends curvedly so that when the second connecting portion  782  swings forward with the swing  715 , the first connecting portion  781  is pulled backward, and when the second connecting portion  782  swings backward with the swing  715 , the first connecting portion  781  is pulled forward, in a side view of the tractor  1 . In addition, the second connecting portion  782  is mounted to the first mounting portion  752 A at a position higher than the first connecting portion  781 . The operator can carry the second connecting portion  782  to the mounting portion without causing the operator to move significantly by bending the cable in the maintenance work. In addition, since the second connecting portion  782  is positioned at a position higher than the first connecting portion  781 , the second connecting portion can be mounted at a position close to the height of the operator&#39;s eye line. The operator can easily perform the switching work to the draft control. 
     The tractor  1  includes cable fixing portions (first cable fixing portion  241 , second cable fixing portion  915 , and third cable fixing portion  83 ). Fixing the cover portion  78 B by the cable fixing portions allows the inner cable  78 A to be stably swung. Thus, the mechanical linkage  7  can accurately transmit the lifting/lowering operation amount according to the amount of change in traction load to the lifting/lowering driver  6 , so that a high quality draft control can be executed. 
     The cable  78  is connected to the swing  715  and the sensitivity adjuster. Thus, the number of link members connecting the swing  715  and the sensitivity adjuster can be reduced, and the structure of the hydraulic lifting/lowering device  5  is simplified. 
     At least a portion of the intermediate linkage  75  is positioned farther rearward than the front end of the lower link  3 B. Thus, the intermediate linkage  75  can be easily confirmed from the rear of the tractor  1 , so that the maintainability can be improved. 
     The second mounting portion  715 A is positioned above the load detector  711 . Thus, when the second mounting portion  715 A is visually recognized from the rear of the tractor  1 , the second mounting portion  715 A can be visually recognized above the load detector  711 , and the load detector  711  is hardly overlapped with the second mounting portion  715 A. The second connecting portion and the mounting portion are easily confirmed from the rear of the tractor  1 , and the second connecting portion  782  is easily mounted and removed. Therefore, the switching work can be easily performed. A space including the second mounting portion  715 A is not required in front of the load detector  711 . 
     The second mounting portion  715 A is positioned farther rearward than the front end of the lower link  3 B. Since the distance from the rear of the tractor  1  to the second mounting portion  715 A is closer than that to the front end of the lower link  3 B, the second connecting portion  782  is easily mounted and removed from the rear of the tractor  1 . Therefore, the switching work can be easily performed. 
     The hitch  8  includes a linkage  81  to swing back and forth according to the amount of change in tilling depth. The linkage  81  includes a third mounting portion  813  to which the second connecting portion  782  is mounted. Thus, since the second connecting portion  782  of the cable  78  is directly mounted to the linkage  81  of the hitch  8 , the number of members for transmitting the amount of change in tilling depth to the cable  78  can be reduced on the front side of the three-point linkage  3  as compared with the case where the second connecting portion  782  of the cable  78  is mounted. 
     The second mounting portion  715 A is positioned farther rearward than the first mounting portion  752 A in a state where no traction load is applied. Since the distance from the rear of the tractor  1  to the second mounting portion  715 A is closer than that to the first mounting portion  752 A, the second connecting portion  782  is easily mounted and removed from the rear of the tractor  1 . Therefore, the switching work can be easily performed. 
     Other Preferred Embodiments 
     Although the present disclosure has been described in detail with reference to the preferred embodiments described above, it will be apparent to those skilled in the art that the present disclosure is not limited to the preferred embodiments described herein. The present disclosure may be practiced as modifications and variations without departing from the spirit and scope of the present disclosure as defined by the claims. Accordingly, the description herein is intended to be illustrative and has no restrictive meaning with respect to the present disclosure. 
     The configuration of the tractor  1  can be variously changed. For example, the tractor  1  may be of a semi-crawler specification including right and left crawlers in place of the right and left rear wheels  25 B. For example, the tractor  1  may be of a full-crawler specification including right and left crawlers in place of the right and left wheels  25 . For example, the tractor  1  may be of an electric specification including an electric motor in place of the engine  22 . For example, the tractor  1  may be of a hybrid specification including the engine  22  and an electric motor. 
     The configuration of the lifting/lowering driver  6  and the configuration of the mechanical linkage  7  can be variously changed. For example, the cable  78  may include a portion extending laterally of the tractor  1 . In other words, the extension portion E may extend laterally in a back view of the tractor  1  (as viewed from the rear of the tractor  1 ). An operator positioned on the side of the tractor  1  can easily carry the second connecting portion  782  to the second mounting portion  715 A by gripping the extension portion E so that the second connecting portion  782  is positioned more to the front side of the operator than the extension portion E. Therefore, the operator can easily mount the second connecting portion  782  and can easily perform the maintenance work and/or the switching work to the draft control. 
     In the above preferred embodiments, the swing is provided to swing back and forth according to the back and forth swing of the load detector  711 , and the swing  715  is provided with a second mounting portion  715 A to which the cable  78  is mounted, but the preferred embodiments are not limited thereto. For example, the load detector  711  may be provided with the second mounting portion  715 A. 
     In the above preferred embodiments, when the second connecting portion  782  swings forward together with the swing  715 , the first connecting portion  781  is pulled backward, and when the second connecting portion  782  swings backward together with the swing  715 , the first connecting portion  781  is pulled forward, in a side view of the tractor  1 , but the preferred embodiments are not limited thereto. For example, the first connecting portion  781  may be pulled forward when the second connecting portion  782  swings forward, and the first connecting portion  781  may be pulled backward when the second connecting portion  782  swings backward, by a configuration for reversing the movement of the swing  715 . 
     The linkage  81  is engaged with and connected to the engaging recess of the swing arm  45 , and is rotatable, but is not limited thereto. For example, the linkage link  46  may be connected to the lower free end of the linkage  81 . Thus, the linkage  81  may be swung back and forth by the linkage link  46  without the swing arm  45 . 
     The tractor  1  in the second preferred embodiment may include the load detector  71  in the modified example of the first preferred embodiment. 
     The tractor  1  may include a member other than the member described above, or may include only a portion of the member described above. Therefore, for example, the lifting/lowering driver  6  and the mechanical linkage  7  may include members other than the members described above, or may have only a portion of the members described above. 
     The entire contents of Japanese Patent Application No. 2019-100766 (filed May 29, 2019) and Japanese Patent Application No. 2019-100767 (filed May 29, 2019) are incorporated herein by reference. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.