Patent Publication Number: US-6698196-B2

Title: Drive circuit for fluid motor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a drive circuit for a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation. 
     A related drive circuit for a fluid motor is shown in FIG.  10 . This drive circuit is comprised of a fluid motor  12  having a swash plate  11  which is tiltable between positions of high-speed and low-speed rotation; a reduction gear  10  coupled to the fluid motor  12  and adapted to reduce the output rotational speed of the fluid motor  12  before outputting it; a tilting piston  13  which, when a high-pressure fluid is introduced to it, pushes and tilts the swash plate  11  to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate  11  to tilt to the position of low-speed rotation; a changeover valve  18  which is interposed midway in a connecting passage  17  for connecting the tilting piston  13  and a selector valve  16  for selectively obtaining the high-pressure fluid from main circuits  14  and  15  of the fluid motor  12 , and which is changed over between a high-speed position K for introducing to the tilting piston  13  the high-pressure fluid obtained by the selector valve  16  and a low-speed position L for discharging the fluid acting on the tilting piston  13  to a drain passage  22 ; a first pilot passage  19  for introducing into the changeover valve  18  a constant-pressure pilot fluid for a direction in which the changeover valve  18  is changed over to the high-speed position K; and a second pilot passage  21  for introducing into the changeover valve  18  the high-pressure fluid selectively obtained from the main circuits  14  and  15  by a counterbalance valve  20 , for a direction in which the changeover valve  18  is changed over to the low-speed position L. 
     In the above-described drive circuit for the fluid motor  12 , during a normal load, the fluid force based on the constant-pressure pilot fluid in the first pilot passage  19  is higher than the fluid force based on the high-pressure fluid (the high-pressure fluid selectively obtained from the high pressure-side main circuits  14  and  15 ) in the second pilot passage  21 , so that the changeover valve  18  has been changed over to the high-speed position K. Consequently, the high-pressure fluid from the main circuits  14  and  15  obtained by the selector valve  16  causes the tilting piston  13  to project through the connecting passage  17 , thereby holding the swash plate  11  at the position of high-speed rotation. 
     Next, when the load acting on the above-described fluid motor  12  increases, the pressure in the main circuits  14  and  15  on the high-pressure side increases, so that the fluid force based on the high-pressure fluid in the second pilot passage  21  becomes higher than the fluid force based on the constant-pressure pilot fluid in the first pilot passage  19 . Consequently, the changeover valve  18  is changed over to the low-speed position L, so that the fluid acting on the tilting piston  13  is discharged to the drain passage  22 , and the swash plate  11  is tilted to the position of low-speed rotation while pushing the tilting piston  13 . As a result, the fluid motor  12  rotates at low speed with a large torque, and the pressure within the high pressure-side main circuits  14  and  15  declines. 
     However, with such a drive circuit for a fluid motor, since the fluid acting on the tilting piston  13  is instantly discharged into the drain passage  22  simultaneously as the changeover valve  18  is changed over to the low-speed position L, the swash plate  11  is suddenly tilted to the position of low-speed rotation, and the pressure within the high pressure-side main circuits  14  and  15  declines. Consequently, the output torque of the fluid motor  12  instantly changes from a small torque with high-speed rotation to a large torque with low-speed rotation, with the result that there are problems in that a shock can be imparted to an operator to deteriorate an operational feeling, and a large load can be imparted to the drive circuit itself and shorten its life. 
     In addition, if the swash plate  11  is suddenly tilted to the position of low-speed rotation and the pressure within the high pressure-side main circuits  14  and  15  declines substantially as described above, the fluid force based on the constant-pressure pilot fluid in the first pilot passage  19  becomes higher than the fluid force based on the high-pressure fluid in the second pilot passage  21 , and the changeover valve  18  is changed over again to the high-speed position K. However, when the changeover valve  18  is thus changed over to the high-speed position K, the pressure within the high pressure-side main circuits  14  and  15  rises again, so that the changeover valve  18  is changed over again to the low-speed position L. Hence, there is a problem in that the changeover valve  18  is repeatedly changed over between the high-speed position K and the low-speed position within a short time, i.e., hunting due to pressure fluctuations of the fluid motor  12  can occur. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a drive circuit for a fluid motor which is capable of improving the operational feeling and prolong the life of the drive circuit while preventing hunting at the time of changeover of the changeover valve to the low-speed position. 
     The above object can be attained by a drive circuit for a fluid motor including a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation; a tilting piston which, when a high-pressure fluid is introduced thereto, pushes and tilts the swash plate and tilts it to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate to tilt to the position of low-speed rotation; a changeover valve which is interposed midway in a connecting passage for connecting the tilting piston and a selector valve for selectively obtaining the high-pressure fluid from two main circuits of the fluid motor, and which is changed over between a high-speed position for introducing to the tilting piston the high-pressure fluid obtained by the selector valve and a low-speed position for discharging the fluid acting on the tilting piston to a drain passage; a first pilot passage for introducing into the changeover valve a constant-pressure pilot fluid for a direction in which the changeover valve is changed over to the high-speed position; and a second pilot passage for introducing into the changeover valve the high-pressure fluid selectively obtained from the main circuits, for a direction in which the changeover valve is changed over to the low-speed position, the drive circuit for a fluid motor characterized in that: 
     an additional pilot passage is provided for introducing from the connecting passage between the changeover valve and the tilting piston into the changeover valve an additional pilot fluid for the direction in which the changeover valve is changed over to the low-speed position, and that a first narrow passage for allowing the drain passage and the connecting passage extending from the changeover valve on a tilting piston side to communicate with each other and a second narrow passage for allowing the connecting passage extending from the changeover valve on the tilting piston side and the communicating passage extending from the changeover valve on the selector valve side to communicate with each other are provided midway in a stroke in which the changeover valve is changed over from the high-speed position to the low-speed position. 
     When the two main circuits are under low pressure and the rotation of the fluid motor is stopped, the constant-pressure pilot fluid is only being introduced into the changeover valve through the first pilot passage, so that the changeover valve remains changed over to the high-speed position. 
     Next, when the high-pressure fluid is supplied to either one of the main circuits, the fluid motor rotates. At this time, the high-pressure fluid in the high pressure-side main circuit obtained by the selector valve is introduced to the tilting piston through the connecting passage to tilt the swash plate to the position of high-speed rotation. In addition, the high-pressure fluid flowing through this connecting passage is introduced into the changeover valve as an additional pilot fluid through the additional pilot passage. 
     Consequently, the fluid force based on the constant-pressure pilot fluid in the first pilot passage is applied to the changeover valve as the changing-over force for directing the changeover valve toward the high-speed position. Meanwhile, applied as the changing-over force for directing the changeover valve toward the low-speed position is the resultant force of the fluid force based on the high-pressure fluid obtained from the high pressure-side main circuit and introduced through the second pilot passage and the fluid force based on the additional pilot fluid (under the same pressure as the pressure within the second pilot passage) in the additional pilot passage obtained by the selector valve. 
     Here, when a normal load is being applied to the fluid motor, the aforementioned changing-over force for directing the changeover valve toward the high-speed position is larger than the aforementioned changing-over force for directing the changeover valve toward the low-speed position, the changeover valve is held at the high-speed position. 
     Next, when the load acting on the fluid motor increases, the pressure within the high pressure-side main circuit rises, and the resultant force of the fluid force based on the high-pressure fluid in the second pilot passage and the fluid force based on the additional pilot fluid in the additional pilot passage becomes larger than the fluid force based on the constant-pressure pilot fluid in the first pilot passage, so that the changeover valve starts to be changed over from the high-speed position to the low-speed position. 
     Midway in this process of changeover to the low-speed position, the connecting passage extending from the changeover valve on the tilting piston side communicates with the drain passage through the first narrow passage, so that a small amount of the fluid in the connecting passage at in that region is discharged to the drain passage, and the pressure drops. As a result, the pressing force applied to the swash plate by the tilting piston becomes small, and the swash plate starts to tilt from the position of high-speed rotation toward the position of low-speed rotation. 
     Here, when the pressure within the connecting passage extending from the changeover valve on the tilting piston side, i.e., the pressure of the additional pilot fluid, drops as described above, the fluid force being applied to the changeover valve by the additional pilot fluid also becomes small, so that the changing-over force (resultant force) for directing the changeover valve toward the low-speed position L becomes small. Further, when this resultant force becomes lower than the fluid force based on the constant-pressure pilot fluid, the changeover valve is pushed back toward the high-speed position, whereas when the resultant force becomes higher than the fluid force based on the constant-pressure pilot fluid, the changeover valve is pushed back toward the low-speed position. 
     The changeover valve thus moves to the position where a balance is established between the resultant force for directing the changeover valve toward the low-speed position and the fluid force based on the constant-pressure pilot fluid for directing the changeover valve toward the high-speed position. Of this resultant force, the fluid force based on the additional pilot fluid is determined by the amount of fluid flowing out from the connecting passage extending from the changeover valve on the tilting piston side into the drain passage through the first narrow passage and the amount of fluid flowing from connecting passage extending from the changeover valve on the selector valve side into the connecting passage extending from the changeover valve on the tilting piston side through the second narrow passage. The fluid force which is thus imparted to the changeover valve by the additional pilot fluid functions as a pressure regulator for the changeover valve. At this time, the swash plate also tilts to an intermediate position between the position of high-speed rotation and the position of low-speed rotation in correspondence with the pressure within the connecting passage extending from the changeover valve on the tilting piston side. 
     Then, when the pressure within the high pressure-side main circuit gradually increases in correspondence with the increase in the load acting on the fluid motor, of the resultant force which is balanced with the fluid force of a fixed value based on the constant-pressure pilot fluid, the fluid force based on the high-pressure fluid in the second pilot passage becomes gradually large. Hence, the remaining fluid force, i.e., the fluid force imparted to the changeover valve by the additional pilot fluid, becomes gradually small; namely, the pressure within the connecting passage extending from the changeover valve on the tilting piston side gradually drops, thereby causing the swash plate to tilt gradually toward the position of low-speed rotation. When the pressure within the connecting passage extending from the changeover valve on the tilting piston side is thus caused to drop to the level of the pressure within the drain passage (tank pressure), the changeover valve is changed over to the low-speed position, and the swash plate tilts to the position of low-speed rotation. 
     While the swash plate tilts from the position of high-speed rotation to the position of low-speed rotation owing to the changeover of the changeover valve to the low-speed position, the pressure within the high pressure-side main circuit of the fluid motor does not undergo a sharp drop and only rises gradually in correspondence with the load under constant-pressure control. Therefore, no shock occurs in the drive circuit, the operational feeling improves, a long life is obtained, and the occurrence of hunting is prevented. 
     In addition, if the arrangement according to claim 2 is provided, it is possible to control the above-described operation with high accuracy. 
     Furthermore, if the arrangements according to claims 3 and 5 are provided, the first and second narrow passages can be provided in the spool or the spool chamber of the changeover valve simply and at low cost. 
     In addition, if the arrangement according to claim 5 is provided, a counterbalance valve, for example, for obtaining from the main circuits the high-pressure fluid (pilot fluid) to be introduced into the changeover valve becomes unnecessary, so that the structure becomes simple, and the fabrication cost can be lowered. 
     Two or more of the arrangements of claims 2 to 5 can be combined with the arrangement of claim 1 to provided the combined effects. 
     The present disclosure relates to the subject matter contained in Japanese patent application Nos. 2001-73446 (filed on Mar. 15, 2001) and 2002-5319 (filed on Jan. 11, 2002), which are expressly incorporated herein by reference in their entireties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating a first embodiment of the invention; 
     FIG. 2 is a side cross-sectional view of a changeover valve and its vicinities when the changeover valve is changed over to a low-speed position L; 
     FIG. 3 is an enlarged side cross-sectional view of an additional pilot path and its vicinities; 
     FIG. 4 is a side cross-sectional view of the changeover valve and its vicinities when the changeover valve is changed over to a high-speed position K; 
     FIG. 5 is a graph illustrating the relationship between the pressure applied to a tilting piston and high pressure-side main circuit pressure; 
     FIG. 6 is a side cross-sectional view of the changeover valve and its vicinities when the changeover valve is changed over to the low-speed position L, and illustrates a second embodiment of the invention; 
     FIG. 7 is a circuit diagram illustrating a third embodiment of the invention; 
     FIG. 8 is a side cross-sectional view of the changeover valve and its vicinities when the changeover valve is changed over to the low-speed position L; 
     FIG. 9 is a circuit diagram illustrating a fourth embodiment of the invention 
     FIG. 10 is a circuit diagram illustrating an example of a related drive circuit for a fluid motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, a description will be given of a first embodiment of the invention. 
     In FIG. 1, reference numeral  25  denotes a fluid motor for imparting a driving force for traveling to a civil engineering and construction machine such as a power shovel. This fluid motor  25  is a two-speed motor and has a swash plate  26  which is capable of tilting between two tilting positions of the position of high-speed rotation and the position of low-speed rotation. When this swash plate  26  is set to the position of high-speed rotation, the suction capacity of the fluid motor  25  is changed over to a small level, and the fluid motor  25  undergoes high-speed rotation. Meanwhile, when the swash plate  26  is set to the position of low-speed rotation, the suction capacity of the fluid motor  25  is changed over to a high level, and the fluid motor  25  undergoes low-speed rotation. A reduction gear  100  is coupled to the fluid motor  25 , and this reduction gear  100  reduces the output rotational speed of the fluid motor  25  before outputting it. 
     The fluid motor  25  and an unillustrated manual changeover valve are connected by a pair of main circuits  28  and  29 , and a counterbalance valve  30  which is capable of selectively obtaining a high-pressure fluid from the high-pressure side of the main circuits  28  and  29  is interposed midway in the main circuits  28  and  29 . This counterbalance valve  30  has a valve body  33  which is urged so as to return to a neutral position by springs  31  and  32 , and has check valves  36  and  37  which are respectively provided in passages  34  and  35  bypassing the valve body  33 . 
     The passage  34  and the valve body  33  are connected by a pilot passage  40  having a throttle  39  provided midway therein, while the passage  35  and the valve body  33  are connected by a pilot passage  42  having a throttle  41  provided midway therein. These pilot passages  40  and  42  introduce the fluid in the passage  34  (main circuit  28 ) or the passage  35  (main circuit  29 ) into the valve body  33  of the counterbalance valve  30  so as to impart a pressing force for counteracting the springs  32  and  31  to the valve body  33 . 
     Reference numeral  45  denotes a negative brake for imparting a braking force when the rotation of the fluid motor  25  is stopped. This negative brake  45  and a high-pressure obtaining port of the counterbalance valve  30  are connected by a fluid path  47  having a throttle  46  provided midway therein. When the high-pressure fluid selectively obtained from the high-pressure side of the main circuits  28  and  29  by the counterbalance valve  30  is supplied to the negative brake  45  through the fluid path  47 , the negative brake  45  releases the braking force with respect to the fluid motor  25 . Meanwhile, when the high-pressure fluid is not being obtained by the counterbalance valve  30 , the fluid is discharged from the negative brake  45  to a drain passage  49  by the urging force of a spring  48 , and a braking force is applied to the fluid motor  25 . 
     Reference numeral  51  denotes a tilting cylinder which is capable of imparting a tilting force to the swash plate  26 , and a tilting piston  53  is slidably accommodated in a cylinder casing  52  of this tilting cylinder  51 . This tilting piston  53  has a piston rod  54  whose distal end abuts against the swash plate  26 . consequently, when the high-pressure fluid is introduced to the tilting piston  53  of the tilting cylinder  51 , the tilting piston  53  and the piston rod  54  project to press the swash plate  26 , so that the swash plate  26  is tilted from the position of low-speed rotation to the position of high-speed rotation. Meanwhile, when the interior of the cylinder casing  52  is set under the tank pressure, and the high-pressure fluid ceases to be introduced to the tilting piston  53 , the swash plate  26  is tiled from the position of high-speed rotation to the position of low-speed rotation by the high-pressure fluid which flows into the fluid motor  25  from the high-pressure side of the main circuits  28  and  29 . At this time, the tilting piston  53  and the piston rod  54  of the tilting cylinder  51  retract while allowing the tilting. 
     Reference numeral  56  denotes a selector valve which is interposed midway in a connecting passage  57  for connecting the main circuit  28  and the main circuit  29 , and this selector valve  56  selectively obtaines the high-pressure fluid from the high pressure-side main circuit  28  or main circuit  29 . Reference numeral  58  denotes a connecting passage which connects the selector valve  56  and the tilting piston  53  of the tilting cylinder  51  and has a throttle  59  interposed midway therein. This connecting passage  58  introduces to the tilting piston  53  the high-pressure fluid obtained by the selector valve  56 . 
     In FIGS. 1,  2 ,  3 , and  4 , a changeover valve  61  is interposed midway in the connecting passage  58 , specifically between the throttle  59  and the selector valve  56 . This changeover valve  61  is fixed to the fluid motor  25 , and has a casing  62  which is jointly used for the counterbalance valve  30 . Reference numeral  63  denotes a spool chamber which is penetratingly formed in the casing  62 . One end of this spool chamber  63  is sealed by a plug  64  making up a portion of the casing  62 , while the other end thereof is connected to the drain passage  49 . 
     A substantially cylindrical spool  65  is accommodated inside the spool chamber  63  in such a manner as to be axially movable. This spool  65  has a large-diameter portion  65   a  provided in one axial side portion, a small-diameter portion  65   b  provided in the other axial side portion, and an intermediate-diameter portion  65   c  provided between the large-diameter portion  65   a  and the small-diameter portion  65   b . An annular groove  65   e  having a pressure receiving surface  65   d  is provided between the large-diameter portion  65   a  and the intermediate-diameter portion as  65   c . Here, the diameters of the large-diameter portion  65   a , the intermediate-diameter portion  65   c , and the small-diameter portion  65   b  are smaller in order. 
     Reference numeral  67  denotes a first passage formed in the casing  62 , and one end of the first passage  67  is connected to the selector valve  56 , while the other end thereof is open in the spool chamber  63 . In addition, reference numeral  68  denotes a second passage which is similarly formed in the casing  62 , and one end of the second passage  68  is open in the spool chamber  63 , while the aforementioned throttle  59  is provided the other end thereof. 
     Reference numeral  70  denotes a spring interposed between a flange  71  provided at one end of the spool  65  and a stepped portion  72  formed in the spool chamber  63 . This spring  70  urges the spool  65  of the changeover valve  61  toward one axial side, i.e., toward a low-speed position L, with a relatively small force. Reference numeral  73  denotes a second pilot passage formed in the casing  62 , and one end of this second pilot passage  73  is connected to a high-pressure obtaining port of the counterbalance valve  30  incorporated in the casing  62 , while the other end thereof is open in the spool chamber  63  opposing the aforementioned pressure receiving surface  65   d . Consequently, when the high-pressure fluid (pilot fluid) selectively obtained from the high-pressure side of the main circuits  28  and  29  by the counterbalance valve  30  is introduced to the pressure receiving surface  65   d  of the spool  65  through the second pilot passage  73 , a fluid force directed toward one axial side, i.e., a fluid force acting in the direction for effecting a changeover to the low-speed position L, is imparted to the spool  65  of the changeover valve  61 . 
     Reference numeral  74  denotes an annular groove serving as an additional pilot passage formed on an outer peripheral surface of the spool  65  between the small-diameter portion  65   b  and the intermediate-diameter portion  65   c , and the fluid (additional pilot fluid) in the connecting passage  58  (second passage  68 ) between the changeover valve  61  and the tilting piston  53  is introduced through this annular groove  74  to one side surface, i.e., a pressure receiving surface  74   a , of the annular groove  74 . Here, although the pressure receiving area of the pressure receiving surface  74   a  is set to 50% of the pressure receiving area of the aforementioned pressure receiving surface  65   d , the pressure receiving area of the pressure receiving surface  74   a  is preferably set in the range of 40 to 60%. The reason for this is that if it is less than 40%, the feedback acting force with respect to the pressure receiving surface  74   a  becomes small, and the pressure range of the fluid when the fluid motor  25  undergoes a change from the high-speed rotation to the low-speed rotation becomes small, so that the fluid motor  25  comes to respond too sensitively to pressure fluctuations of the load, whereas, if that pressure receiving area exceeds 60%, the fluid motor  25  undergoes a change from the high-speed rotation to the low-speed rotation under a low pressure, so that the holding capability in high-speed rotation declines. 
     When the additional pilot fluid is thus introduced to the pressure receiving surface  74   a  of the changeover valve  61  through the annular groove  74 , the fluid force directed to one axial side, i.e., the fluid force acting in the direction for effecting a changeover to the low-speed position L, is imparted to the spool  65  of the changeover valve  61 . Thus, the force acting in the direction for effecting a changeover to the low-speed position L, i.e., the resultant force of the urging force of the spring  70 , the fluid force based on the high-pressure fluid in the second pilot passage  73 , and the fluid force based on the additional pilot fluid in the annular groove  74  (additional pilot passage), is imparted to the spool  65  of the changeover valve  61 . 
     Here, when the spool  65  is located in the low-speed position L as shown in FIGS. 1 and 2, one end of a passage  75  formed in the spool  65  communicates with the first passage  67 , but the other end thereof is sealed by an inner periphery of the spool chamber  63  and is completely cut off from the second passage  68 . The aforementioned first passage  67 , second passage  68 , and passage  75  as a whole make up the connecting passage  58 , and this connecting passage  58  is cut off midway by the changeover valve  61  when the changeover valve  61  is changed over to the low-speed position L. In addition, when the spool is changed over to the low-speed position L as described above, the second passage  68  of the connecting passage  58  communicates with the drain passage  49  with a maximum flow passage area, and discharges the fluid acting on the tilting piston  53  as drainage. 
     Reference numeral  79  denotes a first pilot passage which is formed in the casing  62  and has one end connected to an unillustrated constant-pressure pilot source and the other end open in one end portion of the spool chamber  63 . This first pilot passage  79  introduces a constant-pressure pilot fluid held under a constant pressure to one end face of the spool  65  of the changeover valve  61  so as to impart to the spool  65  the fluid force directed toward the other axial side, i.e., the fluid force acting in the direction for effecting a changeover to a high-speed position K. 
     Reference numeral  81  denotes a first notch which is formed in an outer periphery of the small-diameter portion  65   b  of the spool  65  and serves as a first narrow passage extending in the axial direction. This first notch  81  allows the drain passage  49  and the connecting passage  58  (second passage  68 ) extending from the changeover valve  61  on the tilting piston  53  side to communicate with each other with a narrow flow passage area midway in the stroke in which the spool  65  of the changeover valve  61  is changed over (moved) from the high-speed position K (the other axial side limit) to the low-speed position L (one axial side limit). Here, the cross-sectional area of the first notch  81  is made larger toward the other axial side, with the result that the flow passage area of the first notch  81  becomes larger as the spool  65  of the changeover valve  61  approaches the low-speed position L. 
     Reference numeral  82  denotes a second notch which is formed in an outer periphery of the intermediate-diameter portion  65   c  of the spool  65  and serves as a second narrow passage extending in the axial direction. This second notch  82  allows the connecting passage  58  (second passage  68 ) extending from the changeover valve  61  on the tilting piston  53  side and the connecting passage  58  extending from the changeover valve  61  on the selector valve  56  side, i.e., the first passage  67 , to communicate with each other with a narrow flow passage area midway in the stroke in which the spool  65  of the changeover valve  61  is changed over (moved) from the high-speed position K (the other axial side limit) to the low-speed position L (one axial side limit). Here, the cross-sectional area of the second notch  82  is made larger toward one axial side, with the result that the flow passage area of the second notch  82  becomes smaller as the spool  65  of the changeover valve  61  approaches the low-speed position L. 
     If the first and second narrow passages are formed by the axially extending first and second notches  81  and  82  which are respectively formed in the outer periphery of the changeover valve  61 , these first and second narrow passages can be provided simply at low cost. 
     Next, a description will be given of the operation in accordance with the first embodiment of the invention. 
     It is now assumed that the manual changeover valve has been changed over to a neutral position, and that both main circuits  28  and  29  are set under the tank pressure. At this time, the rotation of the fluid motor  25  has been stopped, the counterbalance valve  30  has been returned to the neutral position, and the negative brake  45  is applying a braking force to the fluid motor  25 . In addition, at this time, since the constant-pressure pilot fluid is only being introduced into the changeover valve  61  through the first pilot passage  79 , the spool  65  of the changeover valve  61  has moved to the other axial side end while compressing the spring  70 , and has been changed over to the high-speed position K, as shown in FIG.  4 . 
     Next, if the manual changeover valve is changed over to supply the high-pressure fluid to either one of the main circuits, e.g., the main circuit  28 , the high-pressure fluid is introduced into the valve body  33  of the counterbalance valve  30  through the pilot passage  40 , so that the valve body  33  moves while compressing the spring  32 , and is changed over to a first position D. At this time, the counterbalance valve  30  selectively obtains the high-pressure fluid from the high pressure-side main circuit  28 , and supplies it to the negative brake  45  through the fluid path  47 , so that the negative brake  45  releases the braking force with respect to the fluid motor  25 , allowing the fluid motor  25  to start rotation. In addition, at this time, the counterbalance valve  30  selectively obtains the high-pressure fluid from the main circuit  28 , and introduces it into the changeover valve  61  through the second pilot passage  73 . 
     In addition, when the high-pressure fluid is supplied to the main circuit  28  as described above, after the high-pressure fluid in the main circuit  28  has been selectively obtained by the selector valve  56 , the high-pressure fluid is supplied to the first passage  67 . At this time, since the changeover valve  61  has been changed over to the high-speed position K as described above, the high-pressure fluid flows into the second passage  68 . When the high-pressure fluid is thus introduced to the tilting piston  53  of the tilting cylinder  51  through the connecting passage  58 , the tilting piston  53  and the piston rod  54  project to press the swash plate  26 , and tilts the swash plate  26  to the position of high-speed rotation. At this time, the high-pressure fluid (additional pilot fluid) flowing through the connecting passage  58  is introduced to the pressure receiving surface  74   a  of the changeover valve  61  through the annular groove  74  (additional pilot passage), thereby imparting to the spool  65  a fluid force directed toward the low-speed position L. 
     Consequently, the fluid force based on the constant-pressure pilot fluid in the first pilot passage  79  is applied to the spool  65  of the changeover valve  61  as the changing-over force for directing the spool  65  toward the high-speed position K. Meanwhile, applied to the spool  65  of the changeover valve  61  as the changing-over force for directing the spool  65  toward the low-speed position L is the resultant force of the fluid force based on the high-pressure fluid introduced through the second pilot passage  73 , the urging force of the spring  70  (a relatively weak force as described above), and the fluid force based on the high-pressure fluid (additional pilot fluid under the same pressure as the pressure within the second pilot passage  73 ) in the annular groove (additional pilot passage)  74 . 
     Here, when a normal load is being applied to the fluid motor  25  (e.g., when the civil engineering and construction machine is traveling on level ground), the aforementioned changing-over force for directing the spool  65  toward the high-speed position K is larger than the aforementioned changing-over force for directing the spool  65  toward the low-speed position L, the changeover valve  61  is held at the high-speed position K. 
     Next, when a hill climbing operation or a steering operation is performed with respect to the above-described civil engineering and construction machine, the load acting on the fluid motor  25  increases, which in turn increases the pressure within the high pressure-side main circuit  28 . However, the pressure acting on the tilting piston  53  (pressure within the connecting passage  58 ) also rises by following this increase, as shown by the solid line in FIG.  5 . Then, when the pressure within the main circuit  28  rises to a low-speed changeover pressure P 1  and reaches the point A, the resultant force of the fluid force based on the high-pressure fluid in the second pilot passage  73 , the urging force of the spring  70 , and the fluid force based on the additional pilot fluid in the annular groove  74  comes to exceed the fluid force based on the constant-pressure pilot fluid in the first pilot passage  79 . Thus, the spool  65  starts to move toward one axial side, and the changeover valve  61  starts to be changed over from the high-speed position K to the low-speed position L. 
     Midway in this process of changeover to the low-speed position L (midway in the movement of the spool  65 ), the connecting passage  58  (second passage  68 ) extending from the changeover valve  61  on the tilting piston  53  side communicates with the drain passage  49  through the narrow flow passage area of the first notch (first narrow passage)  81 , as shown in FIG.  3 . Consequently, a small amount of the fluid in the second passage  68  is discharged to the drain passage  49 , and the pressure within the second passage  68  drops. At this time, the pressure within the high pressure-side main circuit  28  gradually rises along the low-speed/high-speed changeover balance formula S of the changeover valve  61  in conjunction with the increase in the load acting on the fluid motor  25 . 
     Then, when the pressure in the second passage  68  acting on the tilting piston  53  drops to the tilting start pressure P 2  and reaches the point B, the pressing force being applied to the swash plate  26  by the tilting piston  53  becomes smaller than the tilting force being applied to the swash plate  26  by the high-pressure fluid in the fluid motor  25 , so that the swash plate  26  starts to tilt from the position of high-speed rotation toward the position of low-speed rotation. 
     Here, when the pressure within the second passage  68  (the pressure of the additional pilot fluid) drops as described above, the fluid force being applied to the spool  65  by the additional pilot fluid also becomes small, so that the changing-over force (resultant force) for directing the changeover valve  61  toward the low-speed position L becomes small. Further, when this resultant force becomes lower than the fluid force based on the constant-pressure pilot fluid, the changeover valve  61  is pushed back toward the high-speed position K, whereas when the resultant force becomes higher than the fluid force based on the constant-pressure pilot fluid, the changeover valve  61  is pushed back toward the low-speed position L. 
     The spool  65  of the changeover valve  61  thus moves to the position where a balance is established between the resultant force for directing the spool  65  toward the low-speed position L and the fluid force based on the constant-pressure pilot fluid for directing the spool  65  toward the high-speed position K. Of this resultant force, the fluid force based on the additional pilot fluid in the annular groove  74  is determined by the amount of fluid flowing out from the second passage  68  into the drain passage  49  through the first notch  81  and the amount of fluid flowing from the first passage  67  into the second passage  68  through the second notch  82 , i.e., by the position of the spool  65 . The fluid force which is thus imparted to the spool  65  by the additional pilot fluid functions as a pressure regulator for the changeover valve  61 . At this time, the swash plate  26  also tilts to an intermediate position between the position of high-speed rotation and the position of low-speed rotation in correspondence with the pressure within the annular groove  74  (second passage  68 ). 
     Then, when the pressure within the high pressure-side main circuit  28  gradually increases in correspondence with the increase in the load acting on the fluid motor  25 , of the resultant force which is balanced with the fluid force of a fixed value based on the constant-pressure pilot fluid, the fluid force based on the high-pressure fluid in the second pilot passage  73  becomes gradually large. Hence, the remaining fluid force, i.e., the fluid force imparted to the changeover valve  61  by the additional pilot fluid, becomes gradually small; namely, the pressure within the second passage  68  gradually drops along the balance formula from the point B toward the point C in FIG. 5, thereby causing the swash plate  26  to tilt gradually toward the position of low-speed rotation. When the pressure within the connecting passage  58  extending from the changeover valve  61  on the tilting piston  53  side is thus caused to drop to the level of the pressure within the drain passage  49  (tank pressure), the changeover valve  61  is changed over to the low-speed position L, and the swash plate  26  tilts to the position of low-speed rotation. 
     While the swash plate  26  tilts from the position of high-speed rotation to the position of low-speed rotation in the above-described manner, the pressure within the high pressure-side main circuit  28  of the fluid motor  25  does not undergo a sharp drop and only rises gradually in correspondence with the load under constant-pressure control. Therefore, no shock occurs in the drive circuit, the operational feeling improves, a long life is obtained, and the occurrence of hunting is prevented. At this time, the above-described operation can be controlled with high accuracy if an arrangement is provided such that the flow passage area of the first notch  81  becomes larger as the spool  65  of the changeover valve  61  approaches the low-speed position L, and such that, meanwhile, the flow passage area of the second notch  82  becomes smaller as the spool  65  of the changeover valve  61  approaches the low-speed position L. 
     FIG. 6 is a diagram illustrating a second embodiment of the invention, portions identical to those of the above-described first embodiment are denoted by the same reference numerals, and a detailed description thereof will be omitted. In this embodiment, the other end of the connecting passage be  58  (first passage  67 ) extending from the changeover valve  61  on the selector valve  56  side is made open in the spool chamber  63  opposing an axially central portion of the intermediate-diameter portion of the spool  65 , and instead of the above-described second notch  82  only one second narrow groove  65   g  serving as the second  95  narrow passage extending in the axial direction and having a fixed width is formed in an outer periphery of an axially central portion of the intermediate-diameter portion  65   c . Consequently, the second passage  68  and the first passage  67  are made to communicate with each other with the narrow flow passage area of the second narrow groove  65   g  midway in the stroke in which the spool  65  of the changeover valve  61  is changed over (moved) from the high-speed position K (the other axial side limit) to the low-speed position L (one axial side limit). 
     In addition, instead of the first notch  81  only one first narrow groove  65   h  serving as the first narrow passage extending in the axial direction and having a fixed width is formed in an outer periphery of the other axial end portion of the small-diameter portion  65   b  of the spool  65 . Consequently, the second passage  68  and the drain passage  49  are made to communicate with each other with the narrow flow passage area of the first narrow groove  65   h  midway in the stroke in which the spool  65  of the changeover valve  61  is changed over (moved) from the high-speed position K (the other axial side limit) to the low-speed position L (one axial side limit). If the arrangement is provided ad in this second embodiment, the passage  75  having a complicated structure need not be formed in the spool  65 , so that the fabrication cost can be lowered. It should be noted that the other arrangements and operation are similar to those of the above-described first embodiment. 
     FIGS. 7 and 8 are diagrams illustrating a third embodiment of the invention, portions identical to those of the above-described first embodiment are denoted by the same reference numerals, and a detailed description thereof will be omitted. In this embodiment, the counterbalance valve  30  and the second pilot passage  73  formed in the casing  62  in the first embodiment are omitted, and a second pilot passage  67   a  branching off from the first passage  67 , i.e., midway from the connecting passage  58  extending from the changeover valve  61  on the selector valve  56  side, is provided, so that the high-pressure fluid selectively obtained from the main circuits  28  and  29  by the selector valve  56  is introduced to the spool  65  of the changeover valve  61  as the pilot pressure. 
     Specifically, the width of a circumferential groove provided at the other end of the first passage  67  which is open in the spool chamber  63  is made wide, the first passage  67  is constantly made to communicate with the annular groove  65   e  formed between the large-diameter portion  65   a  and the intermediate-diameter portion  65   c , and a gap formed between this circumferential groove and the large-diameter portion  65   a  is formed as the aforementioned second pilot passage  67   a . Consequently, the high-pressure fluid selectively obtained from the high-pressure side of the main circuits  28  and  29  by the selector valve  56  is introduced to pressure receiving surface  65   d  of the spool  65  through the first passage  67  and the second pilot passage  67   a.    
     In addition, instead of the first notch  81  only one first narrow groove  65   h  serving as the first narrow passage extending in the axial direction is formed in the outer periphery of the other axial end portion of the small-diameter portion  65   b  of the spool  65  in the same way as the above-described second embodiment. Further, since the counterbalance valve  30  is omitted in the above-described manner, the fluid path  47  for supplying to the negative brake  45  the high-pressure fluid for releasing the braking force is connected to an external circuit outside this circuit. If the arrangement is provided as in this embodiment, the counterbalance valve  30 , for example, for obtaining from the main circuits  28  and  29  the high-pressure fluid (pilot fluid) to be introduced into the changeover valve  61  becomes unnecessary, so that the structure becomes simple, and the fabrication cost can be lowered. It should be noted that the other arrangements and operation are similar to those of the above-described first embodiment. In addition, the changeover valve described in the above-described second embodiment may be used instead of the changeover valve of this third embodiment. 
     FIG. 9 is a diagram illustrating a fourth embodiment of the invention, portions identical to those of the above-described first embodiment are denoted by the same reference numerals, and a detailed description thereof will be omitted. In this embodiment, the manual changeover valve and the tank are not connected to the main circuits  28  and  29 , and a fluid pump is directly connected to these main circuits  28  and  29  so as to form the fluid circuit as a closed circuit. It should be noted that the other arrangements and operation are similar to those of the above-described third embodiment. 
     It should be noted that although, in the foregoing embodiments, the selector valve  56  is used as the selector valve for obtaining the high-pressure fluid from the main circuits  28  and  29  into the connecting passage  58 , in the invention, the counterbalance valve  30  may be used as the selector valve. In addition, although, in the foregoing embodiments, a description has been given of the case where the high-pressure fluid is supplied to the main circuit  28 , also in the case where the high-pressure fluid is supplied to the main circuit  29  the drive circuit operates in the same way as described above. Further, the first and second narrow grooves  65   h  and  65   g  serving as the first and second narrow passages described in the above-described second embodiment may be formed not in the outer periphery of the spool  65  but in the inner periphery of the spool chamber  63 . 
     As described above, in accordance with the invention, it is possible to improve the operational feeling and prolong the life of the drive circuit while preventing hunting at the time of changeover of the changeover valve to the low-speed position.