Abstract:
A tiller for cultivating soil has a power source and a tilling shaft mounted for undergoing rotation by a driving force supplied from the power source. The tilling shaft has a hollow outer shaft and an inner shaft extending through the outer shaft. The inner shaft has a variable rotating speed and/or direction of rotation relative to the outer shaft. A power transmission mechanism transmits a driving force from the power source to the tilling shaft. The power transmission mechanism has a first power transmission system for transmitting the driving force from the power source to the outer shaft and a second power transmission system transmitting the driving force from the power source to the inner shaft. The second power transmission system has a hydrostatic transmission comprised of a hydraulic pump and a hydraulic motor for effecting a stepless change of the rotating speed of the inner shaft as well as a selective change of its direction of rotation. Tilling claws are disposed on the tilling shaft for tilling soil.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tiller which is operable with a variable driving force and under variable tilling conditions, depending on the soil. 
     2. Description of the Related Art 
     A small tiller as disclosed in, for example, Japanese Utility Model Laid-Open Publication No. SHO-57-86502 is known as a controlled machine having tilling claws attached to a tilling shaft rotatably for cultivating the soil with the forward movement of the machine, as well as allowing it to run on a road. The machine has a plurality of appropriately spaced apart tilling claws attached to the tilling shaft extending transversely under its main body, a rearwardly extending operating handlebar, and a resistance bar extending rearwardly and downwardly from its main body. 
     As the tilling claws serve also as traveling wheels, however, the machine requires a great deal of labor and skill for its operation, since the nature of the soil may disable it to keep a good balance between its driving force and tillage, and call for a change of the tilling conditions. If the soil is hard, the machine suffers from a serious lowering of its operability due to a dashing phenomenon, since the tilling claws do not cut into the ground, but roll thereon and cause the machine to move forward uselessly. If the soil is soft, the machine has a lower working efficiency, as it is likely to work on the soil to an unnecessary extent and have a lower driving force. 
     A small tiller as disclosed in, for example, Japanese Utility Model Laid-Open Publication No. HEI-6-3002 is known as having been devised to solve those problems. The tiller has a connecting shaft connected to a tilling shaft, which is the output shaft of a transmission, and carrying tilling claws on its portion close to the transmission. The connecting shaft also carries thereon a planetary gear mechanism composed of a sun gear formed on its middle portion, a plurality of planet gears meshing with the sun gear and gear shafts each attached rotatably to the center of one of the planet gears. Traveling wheels are attached to the gear shafts of the planetary gear mechanism by bosses. A ring gear is rotatably fitted to the connecting shaft. The ring gear has a toothed inner periphery meshing with the planet gears. The ring gear is secured to a fender fixed to the transmission. The rotation of the tilling shaft is transmitted to the traveling wheels by the planetary gear mechanism, so that the traveling wheels may be rotated at a reduced speed relative to the tilling claws rotating with the tilling shaft. As the wheels have a fixed reduction ratio relative to the tilling shaft, however, the wheels have a fixed driving force for moving the machine forward, and under certain soil conditions, therefore, it is impossible to obtain the desired driving force for achieving any adequate tilling work. The tiller is so designed that a part of the planetary gear mechanism may be altered in structure to reverse the rotation of the traveling wheels relative to the tilling claws, but its structural alteration is a large-scaled and complicated job. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of this invention to provide a tiller which can always maintain stability in operation to achieve an improved finish of tilling, a reduction of dashing and an improved ability to move forward irrespective of the conditions of the soil. 
     According to an aspect of this invention, there is provided a tiller for cultivating the soil, having a power source, a tilling shaft rotatable by a driving force supplied to it from the power source through a power transmission, and a plurality of tilling claws carried on the tilling shaft, the tilling shaft being a concentric dual-shaft structure having a hollow outer shaft and an inner shaft extending through the outer shaft, the inner shaft having its rotating speed and/or its direction of rotation variable relative to the outer shaft. 
     If the rotating speed of the inner shaft or its direction of rotation is altered relative to the outer shaft, it is easily possible to alter the tilling conditions as required to suit the nature of the soil of a field and thereby obtain the desired tillage and tilling speed, so that the tiller of this invention can maintain stability in operation despite any change in the nature of the soil. The alteration of the rotating speed of the inner shaft is particularly useful, as it makes it possible to select any tillage and tilling speed from a finely divided range to thereby obtain the soil which is suitable for growing any of various kinds of crops. 
     The power transmission may be composed of a first power transmission system for transmitting a driving force from the power source to the outer shaft and a second power transmission system for transmitting a driving force from the power source to the inner shaft, the second power transmission system including a hydrostatic transmission composed of a hydraulic pump and a hydraulic motor, as will be described more specifically. The hydrostatic transmission makes it possible to change the rotating speed of the inner shaft in a stepless way and control its direction of rotation selectively as desired. 
     In a preferred form, the outer and inner shafts are fitted with a plurality of tilling claws. The tiller can easily be moved backward on the ground if the inner shaft is rotated at an increased speed in the opposite direction to the outer shaft. The dashing of the tiller can be prevented during the tilling of hard soil by the rotation of the outer and inner shafts in the same direction if the inner shaft is rotated at a lower speed than the outer shaft, since the force for driving the tiller by the tilling claws fitted on the outer shaft is restrained by the claws on the inner shaft. 
     A side disk is fitted on each of the opposite ends of the inner shaft, and a plurality of tilling claws are fitted on the outer shaft. Each side disk is provided on its inner surface with a plurality of upstanding plates each lying at an angle to the radius of the disk for producing a greater amount of friction with the soil. The friction force produced in the soil by the upstanding plates on the side disks enables the tiller to remain stable on both sides throughout its operation to thereby achieve an improved straight drive. If the rotating speed of the side disks on the inner shaft or their direction of rotation is altered relative to the tilling claws on the outer shaft, it is possible to vary the driving force of the side disks as desired, so that the tilling conditions can easily be altered to suit the nature of the soil to realize any desired tillage and tilling speed. The alteration of the rotating speed of the side disks is particularly useful, since it makes it possible to select any tilling speed from a finely divided range and thereby control tillage as desired. Thus, this invention makes it possible to realize an adequate tilling speed for achieving an improved operating efficiency and the desired control of tillage for making the soil suitable for growing any of various kinds of crops. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of this invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a side elevational view of a small tiller embodying this invention; 
     FIG. 2 is a circuit diagram showing the transmission of power in the tiller shown in FIG. 1; 
     FIG. 3 is a front elevational view of a lower portion of the tiller; 
     FIG. 4 is a view similar to FIG. 3, but showing a different form of side disks; 
     FIG. 5 is an enlarged sectional view of the upper casing of the tiller shown in FIG. 3; 
     FIG. 6 is an enlarged sectional view of the lower casing of the tiller shown in FIG. 3; 
     FIG. 7 is a horizontal sectional view of the upper casing of the tiller shown in FIG. 3; 
     FIG. 8 is a horizontal sectional view of the hydrostatic transmission shown in FIG. 1; 
     FIG. 9 is a view showing an oil passage in the hydrostatic transmission shown in FIG. 8; 
     FIG. 10 is a front elevational view of one of the side disks shown in FIG. 3; 
     FIG. 11 is a sectional view taken along the line  11 — 11  of FIG. 10; 
     FIG. 12 is a view showing a mechanism for adjusting the hydrostatic transmission; 
     FIG. 13 is an enlarged sectional view taken along the line  13 — 13  of FIG. 12; 
     FIGS. 14A and 14B are a set of views illustrating the adjustment of inclination of an inclined plate by the lever shown in FIG. 12; 
     FIG. 15 is a view showing an arrangement of parts for power transmission; 
     FIG. 16 is a diagram showing a first pattern of operation for the power transmission circuit shown in FIG. 2; 
     FIGS. 17A to  17 C are a set of views showing the operation of the hydrostatic transmission; 
     FIG. 18 is a diagram similar to FIG. 16, but showing a second pattern of operation; and 
     FIGS. 19A to  19 C are a set of views for explaining the conditions which are suitable for the soil to be cultivated by the tiller embodying this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is merely exemplary in nature and is in no way intended to limit the invention, its application or uses. 
     Referring to FIG. 1, a small tiller  10  has an engine  12  as a power source, a gear casing  15  mounted under the engine  12  for transmitting power from the engine  12  to a plurality of tilling claws  13  and  14 , a hydrostatic transmission  16  mounted in front of the gear casing  15 , a handle post  17  extending rearwardly and upwardly from the gear casing  15 , a handlebar  18  attached to the top of the handle post  17  and a clutch lever  19  attached to the handlebar  18 . A fuel tank is shown at  22 , an engine cover at  23 , an air strainer at  24 , and a pair of side disks at  26  (only one of which is shown), and a fan shown at  28  has a cover not shown. 
     FIG. 2 is a diagram showing the transmission of power in the tiller. A power transmission  29  for transmitting power from the engine  12  to a tilling shaft (to be described), to which the tilling claws  13  and  14  (FIG. 1) are attached, includes a crank gear  32  connected to the distal end of a crankshaft  31  extending from the engine  12 . A plurality of planet gears  33  mesh with the crank gear  32 . The planet gears  33  are rotatably supported by a planet carrier  34 . The planet gears  33  mesh with a ring gear  35 . A plurality of brake shoes  36  are engageable with the inner periphery of the ring gear  35 . A first bevel gear  37  is attached to the planet carrier  34 . A second bevel gear  38  meshes with the first bevel gear  37 . The second bevel gear  38  has a first supporting shaft  41 . 
     The first supporting shaft  41  carries an outer drive sprocket  42  thereon. An outer driven sprocket  44  is connected to the outer drive sprocket  42  by an outer drive chain  43 . The outer driven sprocket  44  has a second supporting shaft  45 . A pair of transversely spaced apart outer drive gears  46  are carried on the second supporting shaft  45 . A pair of transversely spaced apart outer driven gears  48  mesh with the outer drive gears  46 , respectively. The outer driven gears  48  have outer shafts  47  which are rotatable with the tilling claws  13  and  14 . A system for transmitting power from the engine  12  to the outer shafts  47  is a first power transmission system  49  (which excludes the engine  12  and the outer shafts  47 ). 
     A third bevel gear  51  is carried on the first supporting shaft  41 . A fourth bevel gear  52  meshes with the third bevel gear  51 . The fourth bevel gear  52  has a third supporting shaft  53 . A pump drive gear  54  is carried on the third supporting shaft  53 . A pump driven gear  55  meshes with the pump drive gear  54 . The pump driven gear  55  has a pump axle  56 . The pump axle  56  is connected to the hydrostatic transmission (HST)  16 . The HST  16  effects a stepless change of the rotating speed of the pump axle  56  and rotates a motor axle  57  by varying its direction of rotation as desired. 
     The motor axle  57  carries a motor drive gear  61  thereon. A motor driven gear  62  meshes with the motor drive gear  61 . The motor driven gear  62  has a fourth supporting shaft  63 . A fifth bevel gear  64  is carried on the fourth supporting shaft  63 . A sixth bevel gear  65 meshes with the fifth bevel gear  64 . The sixth bevel gear  65  has a fifth supporting shaft  66 . The fifth supporting shaft  66  is connected about the first supporting shaft  41  rotatably relative to it. An inner drive sprocket  67  is carried on the fifth supporting shaft  66 . An inner driven sprocket  72  is connected to the inner drive sprocket  67  by an inner drive chain  68 . The inner driven sprocket  72  is connected to an inner shaft  71  extending through the outer shafts  47  which are hollow. Ball bearings are shown at  74   a  to  74   g , and needle bearings at  75   a  and  75   b . A system for transmitting power from the engine  12  to the inner shaft  71  is a second power transmission system  76  (which excludes the engine  12  and the inner shaft  71 ). 
     FIG. 3 shows examples of tilling claws and side disks on the tiller  10 . The tiller  10  has the gear casing  15  situated in its central portion. The gear casing  15  has a lower casing portion  15   a  from which the hollow outer shafts  47  project laterally in the opposite directions. A plurality of tilling claws  13  each curved inwardly at both ends and a plurality of tilling claws  14  each curved outwardly at both ends are attached to the outer shafts  47  by brackets  77 . The inner shaft  71  extends transversely through the gear casing  15  and the outer shafts  47 . Each side disk  26  has a boss  83  into which the inner shaft  71  is connected at one end. An upper portion of the gear casing  15  is shown at  15   b , and a clutch casing at  15   c . The construction of the side disks  26  will be described later with reference to FIGS. 10A and 10B. 
     FIG. 4 shows other examples of tilling claws and side disks on the tiller  10 , the side disks being of the same construction with known side disks. Two outermost tilling claws  14  are attached to the inner shaft  71  by two brackets  81 , respectively. In the other aspects of construction, the tiller  10  shown in FIG. 4 is equal to that shown in FIG.  3 . 
     FIG. 5 is a sectional view showing the arrangement of gears in the upper casing portion and clutch casing shown in FIG.  3 . Each of the two transversely spaced apart planet gears  33  in the clutch casing  15   c  is attached to the planet carrier  34  by a rotary shaft  85 . The planet carrier  34  is composed of a disk portion  86  and a shaft portion  87  fitted to the center of the disk portion  86  and having an end splined to the first bevel gear  37 . The shaft portion  87  is supported rotatably by the ball bearing  74   b  on the clutch casing  15   c . The ring gear  35  is composed of a disk portion  88  having an inner periphery engaging with the planet gears  33  and a cylindrical portion  91  extending from the outer periphery of the disk portion  88 . The brake shoes  36  are engageable with the inner peripheral surface of the cylindrical portion  91  of the ring gear  35  for holding the ring gear  35  against rotation in the clutch casing  15   c . The planet gears  33 , rotary shafts  85 , planet carrier  34 , ring gear  35 , and brake shoes  36  form a clutch mechanism  92 . 
     The clutch mechanism  92  is so operated that when the brake shoes  36  stay away from the cylindrical portion  91  of the ring gear  35 , the rotation of the crankshaft  31  is transmitted to the ring gear  35  by the planet gears  33 , but not to the planet carrier  34 . If the brake shoes  36  are held against the inner surface of the cylindrical portion  91 , the rotation of the ring gear  35  is stopped, and the rotation of the crankshaft  31  is transmitted to the planet carrier  34  by the planet gears  33 , whereby the first bevel gear  37  is rotated. A semiclutched situation occurs if the rotation of the ring gear  35  is not completely stopped by the brake shoes  36 . 
     Description will now be made of the arrangement of gears, etc. in the upper casing portion  15   b . The second bevel gear  38 , outer drive sprocket  42 , and third bevel gear  51  are splined to the large diameter portion  41   a  of the first supporting shaft  41 . The first supporting shaft  41  has at both ends thereof small diameter portions  41   b  supported rotatably by the ball bearings  74   c  on the upper casing portion  15   b . The sixth bevel gear  65  is splined to the fifth supporting shaft  66  and has its opposite ends secured to the fifth supporting shaft  66  by retaining rings  93 . The fifth supporting shaft  66  is supported rotatably by the needle bearings  75   a  on the medium diameter portion  41   c  of the first supporting shaft  41 . The fifth supporting shaft  66  has the inner drive sprocket  67  as an integral part thereof. A thrust bearing is shown at  94 , and collars at  95  and  96 . 
     FIG. 6 is a vertical sectional view of the lower portion  15   a  of the gear casing  15  shown in FIG.  3 . The outer driven sprocket  44  and the outer drive gears  46  are splined to the large diameter portion  45   a  of the second supporting shaft  45 , as shown in FIG. 6. A collar for positioning the outer driven sprocket  44  is shown at  44   a . The second supporting shaft  45  has at both ends thereof small diameter portions  45   b  at which it is supported rotatably by the ball bearings  74   d  on the lower casing portion  15   a . The outer shafts  47  are mounted rotatably by the ball bearings  74   e  on the lower casing portion  15   a . Each outer shaft  47  is a hollow shaft held against rotation in a bracket  77  by a key  97  (only the key for one of the shafts is shown), and held against axial displacement by a bolt  98  (only the bolt for one of the shafts is shown). Oil seals are shown at  47   a , and each bracket  77  has a key groove  101  for the insertion of the key  97 . Each bolt  98  is locked by a nut  102  (only the lock nut for one of the bolts is shown), and oil seals are shown at  103 . The inner shaft  71  is supported in the outer shafts  47  rotatably by the needle bearings  75   b  provided on the inner surfaces of the outer shafts  47 . The inner driven sprocket  72  is splined to the middle portion of the inner shaft  71 . A stop ring  104  is provided at one end of the inner driven sprocket  72  for restraining its movement in one axial direction. A thrust bearing  105  is interposed between each outer shaft  47  and the middle portion of the inner shaft  71 . The outer shafts  47 , inner shaft  71 , and needle bearings  75   b  form a tilling shaft  106 . 
     FIG. 7 is a top plan view, partly in section, of the upper portion of the gear casing  15 . The third supporting shaft  53  lies at right angles to the first supporting shaft  41  and is connected thereto by the third and fourth bevel gears  51  and  52 . The third supporting shaft  53  is supported by the ball bearings  74   f  on the upper casing portion  15   b . The third supporting shaft  53  is splined at one end to the pump drive gear  54 . The fourth supporting shaft  63  lies at right angles to the fifth supporting shaft  66  fitted about the first supporting shaft  41  and is connected to the fifth supporting shaft by the fifth and sixth bevel gears  64  and  65 . The fourth supporting shaft  63  is supported rotatably by the ball bearings  74   g  on the upper casing portion  15   b , and is splined at one end to the motor driven gear  62 . 
     FIG. 8 is an enlarged top plan view, partly in section, of the HST  16  in the tiller. The HST  16  has a base  107  mounted to the gear casing  15  (see FIG.  7 ), a casing  108  attached to the base  107 , and a hydraulic pump  110  and a hydraulic motor  120  having their principal parts located within the base  107  and the casing  108 , as shown in FIG.  8 . The base  107  and the casing  108  support the pump axle  56  and the motor axle  57  rotatably. The hydraulic pump  110  is a device for generating a hydraulic pressure by the rotation of the pump axle  56 . The hydraulic pump  110  is composed of the pump axle  56 , a cylinder block  112  splined to the pump axle  56  and having a plurality of cylinders  111 , a plurality of plungers  113  each fitted slidably in one of the cylinders  111 , an inclined plate  114  contacting the ends of the plungers  113 , an inclined plate shaft  151  supporting the inclined plate  114  (as will be described), springs  116  urging the plungers  113  against the inclined plate  114 , and a handle  117  attached to the inclined plate shaft  151  for altering the inclination of the inclined plate  114 . Each cylinder  111  has a port  118  through which oil is allowed to flow between the cylinder and an oil passage formed in the base  107 , but not shown. The inclined plate  114  is a thrust bearing having one of its track disks secured to the inclined plate shaft  151 , while the other contacts the ends of the plungers  113 . 
     The hydraulic motor  120  is a device for rotating the motor axle  57  by the hydraulic pressure generated by the hydraulic pump  110 . The hydraulic motor  120  is composed of the motor axle  57 , a cylinder block  122  splined to the motor axle  57  and having a plurality of cylinders  121 , a plurality of plungers  123  each fitted slidably in one of the cylinders  121 , an inclined plate  124  contacting the ends of the plungers  123 , and springs  125  urging the plungers  123  against the inclined plate  124 . Each cylinder  121  has a port  128  through which oil is allowed to flow between the cylinder and an oil passage formed in the base  107 , but not shown. The inclined plate  124  is a thrust bearing having one of its track disks secured to the casing  108 , while the other contacts the ends of the plungers  123 . 
     FIG. 9 is a diagram showing the oil passages in the HST of the tiller. The hydraulic pump  110  has the cylinders  111  formed along the circumference of the cylinder block  112 . The base  107  (see FIG. 8) has a first arcuate groove  131  lying over some of the ports  118  of the cylinders  111 . The base  107  also has a second arcuate groove  132  lying over some of the remaining ports  118 . The hydraulic motor  120  has the cylinders  121  formed along the circumference of the cylinder block  122 . The base  107  (see FIG.  9 ) has a first arcuate groove  133  lying over some of the ports  128  of the cylinders  121 . The base  107  also has a second arcuate groove  134  lying over some of the remaining ports  128 . The first arcuate groove  131  above the pump and the first arcuate groove  133  above the motor are connected to each other by a first oil passage  135 . The second arcuate groove  132  above the pump and the second arcuate groove  134  above the motor are connected to each other by a second oil passage  136 . 
     FIGS. 10 and 11 show one of the two side disks  26  shown in FIG.  3 . Referring to FIG. 10, the side disk  26  is composed of a disk portion  141  curved outwardly of the tiller  10  (see FIG.  1 ), a plurality of upstanding plates or lugs  142  attached to the inner surface of the disk portion  141  close to its outer edge for producing a greater amount of friction with the soil, and a boss  83  extending inwardly from the center of the disk portion  141 . Each lug  142  has a base  143  attached to the disk portion  141 , and an upstanding portion  144  projecting from the base  143 . The upstanding portion  144  lies at an angle a of, for example, from 30° to 60° to a line RL extending along the radius of the disk. FIG. 10 also includes an arrow showing the direction of normal rotation of the side disk  26  in which the tiller  10  is moved forward. The upstanding portion  144  of each lug  142  is substantially rectangular, as shown in FIG.  11 . The other side disk  26  is similar to the side disk  26  shown in FIG. 10, but the upstanding portion  144  of each of its lugs  142  has an angle of −α to the line RL, so that the inclination of its upstanding portions  144  relative to the direction of its normal rotation may be equal to that of the side disk  26  shown in FIG.  10 . The inclination of the upstanding portions  144  of the lugs  142  on one side disk  26  at an angle of α to the lines RL and the inclination of the upstanding portions  144  of the lugs  142  on the other side disk  26  at an angle of −α to the lines RL as described enable each upstanding portion  144  to have a greater area of contact with the ground to thereby prevent the side disks  26  from sinking undesirably in the ground, while also striking against the ground more effectively to produce a greater traction, when the side disks  26  are rotated in the direction of their normal rotation, than in the event that 0°≦α&lt;30°, or 60°&lt;α≦90°. 
     FIG. 12 is a top plan view of the HST for the tiller embodying this invention and a mechanism for adjusting the inclination of the inclined plate shown in FIG.  8 . The inclined plate shaft  151  is rotatably mounted on the casing  108  of the HST  16 . A sectorial lever  152  has a base end  153  secured to the shaft  151  to which the handle  117  for adjusting the inclination of the inclined plate is also secured. The lever  152  has an arcuate end  154  having an arcuate guide hole  155 . The lever  152  has a side edge  157  to which a coiled tension spring  158  is fastened at one end. A wire  162  is connected at one end to the other side edge  161  of the lever  152 . The other end of the wire  162  is connected to a lever  163  attached to the handlebar  18  for adjusting the inclination of the inclined plate by pulling the wire. The lever  152  is shown in its position in which the inclined plate  114  is not inclined, so that the inner shaft  71  (see FIG. 3) may be out of rotation, as will be explained. The other end of the spring  158  is fastened to the casing  108  by a fitting  164 . The wire  162  has an outer tube  165 , and an inner wire  166  inserted slidably in the outer tube  165 . The outer tube  165  has one end secured to the casing  108  by a bracket  167 . A friction generator  168  extends through the guide hole  155  and contacts the lever  152  on both sides thereof to produce a friction (or resistance) force when the lever  152  is swung. 
     Referring to FIG. 13, the inclined plate shaft  151  is shaped like a crankshaft. It has a crank portion  171  to which the inclined plate  114  is mounted. The crank portion  171  is supported at both ends on the casing  108  by bearings  172 . Stop rings for the bearings  172  are shown at  173 , an oil seal at  174 , and a plug at  175 . A cylindrical member is shown at  176  for attaching the handle  117  for adjusting the inclination of the inclined plate and the lever  152  to the inclined plate shaft  151 . 
     As is obvious from the foregoing, the inclination of the inclined plate  114  can be adjusted by using either the handle  117  or the lever  163  (FIG.  12 ). Description will now be made with reference to FIGS. 14A and 14B of a method in which the lever  163  is used for adjusting the inclination of the inclined plate  114 . 
     If the lever  163  is turned counterclockwise from its position shown in FIG. 12 (as shown by phantom lines in FIG. 14A) to its position shown by solid lines, the wire  162  is loosened. The sectorial lever  152  is caused by the tensile force of the tension spring  158  to swing clockwise. The inclined plate shaft  151  secured to the base end of the lever  152  is rotated in the same direction with the lever  152 , and the handle  117  secured to the shaft  151  is inclined by rotating in the same direction, whereby the inclined plate  114  is inclined into its position in which the inner shaft is rotated in the direction of its normal rotation (as will be described in further detail). If the lever  163  is turned clockwise from its position shown in FIG. 12 (as shown by phantom lines in FIG.  14 B), the lever  152  is caused by the wire  162  to swing counterclockwise by overcoming the tensile force of the tension spring  158 , as shown in FIG.  14 B. The inclined plate shaft  151  is rotated in the same direction with the lever  152 , and the handle  117  is inclined by rotating in the same direction, whereby the inclined shaft  114  is inclined into its position in which the inner shaft is rotated in the reverse direction (as will be described in further detail). 
     FIG. 15 is a view showing the layout of parts for the power transmission in the tiller. The engine  12  in the tiller  10  is so mounted that its output shaft, or crankshaft  31  may be vertical. The shaft portion  87  of the planet carrier  34  and the first bevel gear  37  connected to the shaft portion  87  are positioned below the crankshaft  31  coaxially therewith. The pump and motor axles  56  and  57  extend horizontally toward the fan  28 . The third supporting shaft  53  is connected to the pump axle  56  by the pump drive and driven gears  54  and  55 , and extends horizontally toward the first supporting shaft  41 . The third supporting shaft  53  terminates in the fourth bevel gear  52 . The fourth supporting shaft  63  is connected to the motor axle  57  by the motor drive and driven gears  61  and  62 , and likewise extends horizontally toward the first supporting shaft  41 . The fourth supporting shaft  63  terminates in the fifth bevel gear  64 . The first, fourth and fifth bevel gears  37 ,  52  and  64  are operationally connected to the first supporting shaft  41 . The rotation of the first supporting shaft  41  is transmitted to the outer shafts  47  by the outer drive chain  43 , and to the inner shaft  71  by the inner drive chain  68 . 
     As the crankshaft  31  and the third and fourth supporting shafts  53  and  63  are all so mounted as to terminate adjacent to the first supporting shaft  41  from which a driving force is transmitted to the outer and inner shafts  47  and  71  mounted therebelow, the power transmission  29  of the tiller  10  is simple in construction, and is operable without causing any substantial mechanical loss. As the power transmission  29  is compact, the tiller  10  is small and light in weight, and is operable with an improved efficiency and a low fuel consumption. 
     Description will now be made of the operation of the power transmission  29  of the tiller  10  with reference to FIGS. 16 to  18 . 
     (1) Description will first be made of the mode in which the outer and inner shafts  47  and  71  are both rotated in the normal direction. In FIG. 16, the direction of rotation of the crankshaft  31  of the engine  12  is shown as direction A, and the direction of normal rotation of the outer shafts  47  as direction B. The rotation of the crankshaft  31  in the direction A is transmitted by the crank gear  32  and the clutch mechanism  92  to rotate the shaft portion  87  of the planet carrier  34  in the direction A if the clutch mechanism  92  is in its engaged position. Its rotation is transmitted by the first and second bevel gears  37  and  38  to rotate the first supporting shaft  41  in direction RB (the reverse of direction B). Its rotation is transmitted by the outer drive sprocket  42 , outer drive chain  43 , and outer driven sprocket  44  to rotate the second supporting shaft  45  in the direction RB. Its rotation is transmitted by the outer drive and driven gears  46  and  48  to rotate the outer shafts  47  in the normal direction B. The rotation of the first supporting shaft  41  is also transmitted to the third supporting shaft  53  by the third and fourth bevel gears  51  and  52  to rotate it in direction RA (the reverse of direction A). Its rotation is transmitted by the pump drive and driven gears  54  and  55  to rotate the pump axle  56  in the direction A. 
     FIGS. 17A to  17 C show the operation of the HST  16  in the power transmission of the tiller. FIG. 17A shows the flow of oil, and FIGS. 17B and 17C show the movements of the plungers  113  and the inclined plate  114  in the hydraulic pump  110  and the corresponding movements of the plungers  123  and the inclined plate  124  in the hydraulic motor  120 . For the convenience of description, only four have been chosen from the cylinders  111 , plungers  113 , ports  118 , cylinders  121 , plungers  123 , or ports  128  shown in FIGS. 8 and 9, and are shown at  111   a  to  111   d ,  113   a  to  113   d  (including  113   c  and  113   d  not shown),  118   a  to  118   d ,  121   a  to  121   d ,  123   a  to  123   d  (including  123   c  and  123   d  not shown), or  128   a  to  128   d.    
     The rotation of the pump axle  56  for the hydraulic pump  110  in the direction A as shown in FIG. 16 causes the cylinder block  112  to rotate therewith in the direction A as shown by a white arrow in FIG.  17 A. If the inclined plate  114  is inclined by the handle  117 , or lever  163  shown in FIGS. 14A and 14B by an angle θ to a line L extending at right angles to the direction of movement of the plungers  113   a  and  113 B as shown in FIG. 17B, the plungers  113   a  and  113   b  in the cylinders  111   a  and  111   b  facing the first arcuate groove  131  (FIG. 17A) move from right to left as shown by an arrow M in FIG. 17B, and retract into the cylinders  111   a  and  111   b , respectively, as shown by arrows P and Q, while remaining in contact with the inclined plate  114 . As a result, the oil in the cylinders  111   a  and  111   b  flows out through the ports  118   a  and  118   b  into the first arcuate groove  131  shown in FIG. 17A, and from the groove  131  into the first arcuate groove  133  above the motor through the first oil passage  135 , as shown by arrows each having a solid line. 
     The oil flows from the first arcuate groove  133  into the cylinders  121   a  and  121   b  of the hydraulic motor  120  through the ports  128   a  and  128   b , as shown in FIG.  17 A. The plungers  123   a  and  123   b  project from the cylinders  121   a  and  121   b , respectively, as shown by arrows R and S, and move from right to left as shown by an arrow T in FIG. 17B, while remaining in contact with the inclined plate  124 . As a result, the cylinder block  122  is rotated in the direction A as shown by a thick solid arrow in FIG. 17A to cause the motor axle  57  to rotate in the same direction. 
     On the other hand, the plungers  113   c  and  113   d  in the cylinders  111   c  and  111   d  facing the second arcuate groove  132  above the hydraulic pump  110  as shown in FIG. 17A move in the opposite direction to the arrow M and project from the cylinders  111   c  and  111   d , while remaining in contact with the inclined plate  114 . The oil in the cylinders  121   c  and  121   d  of the hydraulic motor  120  flows out through the ports  128   c  and  128   d , second arcuate groove  134  above the motor, second oil passage  136 , second arcuate groove  132  above the pump, and ports  118   c  and  118   d , as shown by arrows having a solid line, and is drawn into the cylinders  111   c  and  111   d . As a result, the plungers  123   c  and  123   d  retract into the cylinders  121   c  and  121   d , respectively. 
     As shown in FIG. 17B, as the inclined plate  114  has a larger angle θ of inclination, the plungers  113   a  to  113   d  of the hydraulic pump  110  have a higher speed of axial movement, and oil flows into and out of the cylinders  121   a  to  121   d  of the hydraulic motor  120  at a higher speed, so that the motor axle  57  has a gradually increasing speed of rotation in the direction A. As the inclined plate  114  has a smaller angle θ of inclination (θ&gt;0), the plungers  113   a  to  113   d  of the hydraulic pump  110  have a lower speed of axial movement, and oil flows into and out of the cylinders  121   a  to  121   d  of the hydraulic motor  120  at a lower speed, so that the motor axle  57  has a gradually decreasing speed of rotation in the direction A. If the angle θ of inclination of the inclined plate  114  is reduced to zero, the plungers  113   a  to  113   d  cease to move relative to the cylinders  111   a  to  111   d , oil ceases to flow between the hydraulic pump and motor  110  and  120 , and the plungers  123   a  to  123   d  cease to move, so that the motor axle  57  stops its rotation. 
     Referring to FIG. 16, the rotation of the motor axle  57  in the direction A is transmitted by the motor drive and driven gears  61  and  62  to rotate the fourth supporting shaft  63  in the direction RA, and its rotation is transmitted by the fifth and sixth bevel gears  64  and  65  to rotate the fifth supporting shaft  66  in the direction B. Its rotation is transmitted by the inner drive sprocket  67 , inner drive chain  68 , and inner driven sprocket  72  to rotate the inner shaft  71  in the direction B of normal rotation. 
     Thus, as the inclination θ of the inclined plate  114  shown in FIG. 17B is increased by using the handle  117  shown in FIG. 8, the motor axle  57  of the HST  16  shown in FIG. 16 has a higher speed of rotation, and the inner shaft  71  has a gradually increasing speed of normal rotation. As the inclination θ of the inclined plate  114  is decreased (θ&gt;0) by the handle  117 , the motor axle  57  has a lower speed of rotation, and the inner shaft  71  has a gradually decreasing speed of normal rotation. If the inclination θ of the inclined plate  114  is kept at an appropriate angle by the handle  117 , the outer and inner shafts  47  and  71  have an equal speed of normal rotation. Moreover, the inner shaft  71  stops its rotation if the inclination θ of the inclined plate  114  is reduced to zero by the handle  117 . 
     (2) Description will now be made of the mode in which the outer shafts  47  are rotated in the normal direction, while the inner shaft  71  is rotated in the reverse direction. The normal rotation of the outer shafts  47  has already been described at (1) above, and no repeated description thereof is made. With regard to the reverse rotation of the inner shaft  71 , the directions of rotation of the parts of the power transmission from the crankshaft  31  to the pump axle  56  have already been explained at (1) above with reference to FIG. 16, and no repeated description thereof is made, but description will be made of the directions of rotation of the parts after the motor axle  57 . Description will first be made of the operation of the HST  16  with reference to FIGS. 17A and 17B. 
     The rotation of the pump axle  56  of the hydraulic pump  110  in the direction A as shown in FIG. 17A causes the cylinder block  112  to rotate therewith in the same direction. If the inclined plate  114  is inclined by using the handle  117 , or lever  163  shown in FIGS. 14A and 14B by an angle of −θ to a line L as shown in FIG. 17C, the plungers  113   a  and  113   b  of the cylinders  111   a  and  111   b  facing the first arcuate groove  131  (FIG. 17A) above the pump move from right to left as shown by an arrow U in FIG. 17C, while remaining in contact with the inclined plate  114 . As a result, the plungers  113   a  and  113   b  project from the cylinders  111   a  and  111   b , respectively, as shown by arrows V and W. As a result, oil flows from the cylinders  121   a  and  121   b  of the hydraulic motor  120  to the first arcuate groove  131  above the pump through the ports  128   a  and  128   b , the first arcuate groove  133  above the motor, and the first oil passage  135  as shown by broken arrows in FIG.  17 A. The oil is drawn from the first arcuate groove  131  above the pump into the cylinders  111   a  and  111   b  of the hydraulic pump  110  through the ports  118   a  and  118   b . As a result, the plungers  123   a  and  123   b  retract into the cylinders  121   a  and  121   b , respectively, as shown by arrows X and Y, and are urged to move from left to right as shown by an arrow Z, while remaining in contact with the inclined plate  124 . 
     On the other hand, the plungers  113   c  and  113   d  move in the opposite direction to the arrow U (FIG. 17C) and retract into the cylinders  111   c  and  111   d  facing the second arcuate groove  132  above the hydraulic pump  110  as shown in FIG. 17A, while remaining in contact with the inclined plate  114 . As a result, oil flows from the cylinders  111   c  and  111   d  into the cylinders  121   c  and  121   d  through the ports  118   c  and  118   d , the second arcuate groove  132  above the pump, the second oil passage  136 , the second arcuate groove  134  above the motor and the ports  128   c  and  128   d , as shown by broken arrows. As a result, the plungers  123   c  and  123   d  project from the cylinders  121   c  and  121   d , and move from right to left in the opposite direction to the arrow Z (FIG.  17 C), while remaining in contact with the inclined plate  124 . Thus, the cylinder block  122  is rotated in the direction RA as shown by a thick broken arrow to rotate the motor axle  57  in the same direction. 
     As the inclined plate  114  shown in FIG. 17C has a smaller angle of −θ (or a larger degree of inclination to the negative side), the plungers  113   a  to  113   d  of the hydraulic pump  110  have a higher speed of axial movement and oil flows into and out of the cylinders  121   a  to  121   d  of the hydraulic motor  120  at a higher speed, so that the motor axle  57  has a gradually increasing speed of rotation in the direction RA (FIG.  17 A). As the inclined plate  114  has a larger angle of −θ (θ&gt;0) (or a smaller degree of inclination to the negative side), the plungers  113   a  to  113   d  of the hydraulic pump  110  have a lower speed of axial movement and oil flows into and out of the cylinders  121   a  to  121   d  of the hydraulic motor  120  at a lower speed, so that the motor axle  57  has a gradually decreasing speed of rotation in the direction RA. 
     Referring to FIG. 18, the rotation of the motor axle  57  in the direction RA is transmitted by the motor drive and driven gears  61  and  62  to rotate the fourth supporting shaft  63  in the direction A. Its rotation is transmitted by the fifth and sixth bevel gears  64  and  65  to rotate the fifth supporting shaft  66  in the direction RB. Its rotation is transmitted by the inner drive sprocket  67 , inner drive chain  68 , and inner driven sprocket  72  to rotate the inner shaft  71  in the direction RB opposite to the direction of rotation of the outer shafts  47 . 
     Thus, as the inclination −θ of the inclined plate  114  shown in FIG. 17C is decreased, the motor axle  57  of the HST  16  has a gradually increasing speed of reverse rotation, and the inner shaft  71  also has a gradually increasing speed of reverse rotation. As the inclination −θ of the inclined plate  114  is increased (−θ&lt;0) the motor axle  57  has a gradually decreasing speed of reverse rotation, and the inner shaft  71  has, therefore, a gradually decreasing speed of reverse rotation. 
     Description will now be made with reference to FIGS. 19A to  19 C of the operating conditions which are suitable for the soil to be cultivated by the tiller  10 . If the soil is soft as shown in FIG. 19A, the outer and inner shafts are both rotated in the direction of normal rotation, and the inner shaft is rotated at a higher speed. This mode is obtained if the inclined plate is inclined by the handle, or lever over the angle at which the outer and inner shafts have an equal speed of rotation, as described before at (1) with reference to FIGS. 16,  17 A and  17 B. If the inner shaft has a higher speed of normal rotation, the tilling laws  13  and  14  attached to the outer shafts produce a smaller driving force on the soft soil. The side disks  26  attached to the inner shaft, however, produces a larger driving force, and the tilling claws  13  and  14  and the side disks  26  or  27  produce a larger total driving force F 1  (as shown by a white arrow), so that the tilling claws  13  and  14  are moved forward at a higher speed without working the soil to any undesirably large depth. Thus, the tiller  10  has a higher tilling rate and a higher working efficiency. 
     If the soil is hard as shown in FIG. 19B, the outer and inner shafts are both rotated in the direction of normal rotation, and the inner shaft is rotated at a lower speed. This mode is obtained if the inclined plate is inclined by an angle smaller than that at which the outer and inner shafts have an equal speed of rotation, as described before at (1) with reference to FIGS. 16,  17 A and  17 B. If the inner shaft has a lower speed of normal rotation, the tilling claws  13  and  14  produce a larger driving force on the hard soil. The side disks  26 , however, produce a smaller driving force and resist the driving force of the claws  13  and  14 . Thus, the claws  13  and  14  and the side disks  26  produce a smaller total driving force F 2  (as shown by a white arrow), so that no dashing of the tiller  10  may occur. When the soil is hard, it is alternatively possible to hold the inner shaft against rotation, or place it in reverse rotation, so that the side disks  26  or  27  may produce a still greater resistance, depending on the nature of the field to be cultivated. In either event, the tiller  10  can do an adequate tilling job with a higher efficiency without any fear of dashing. 
     The side disks  26  or  27  are also placed in reverse rotation for moving the tiller  10  backward. The tiller  10  can be moved backward if the inner shaft is rotated in reverse direction, and sometimes at a higher speed, while the outer shafts are rotated in normal direction. When the tiller  10  has reached an edge of a rectangular field after working the soil along one ridge, for example, the lever for adjusting the inclination of the inclined plate is operated to rotate the side disks  26  or  27  in reverse direction to move back the tiller  10  to a position in which the tiller  10  can make a turn, and the lever is operated again to rotate the side disks  26  or  27  in normal direction, so that the tiller can work the soil along a neighboring ridge. The backward movement of the tiller  10  by the reverse rotation of the inner shaft as described ensures an improved working efficiency, as it facilitates the cultivation of the soil even along any edge or corner of a field which has hitherto been difficult. 
     When the tiller  10  is, for example, transferred from one field to another as shown in FIG. 19C, the outer and inner shafts are both rotated in normal direction at a substantially equal speed. The tilling claws  13  and  14  and the side disks  26  or  27  are rotated at substantially the same speed to enable the tiller  10  to travel easily. 
     Although the foregoing description has been directed to the cases in which the soil is soft, or hard, and in which the tiller is transferred, it is not intended for limiting the scope of this invention, but it is alternatively possible to alter the rotating speed of the inner shaft and its direction of rotation in any other appropriate way depending on the nature of the soil to be cultivated. It is also possible to employ, for example, a throttle lever for varying the rotating speed of the outer shafts so that it may suit the nature of the soil. Although the hydrostatic transmission composed of a hydraulic pump and a hydraulic motor has been employed for changing the rotating speeds of the shafts, it is alternatively possible to employ a belt or traction drive type CVT for that purpose. 
     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.