Title of Invention

HYDRAULIC APPARATUS FOR WORK VEHICLE

Abstract

A hydraulic apparatus for a work vehicle for controlling an implement, comprises: a hydraulic cylinder adapted to lift or lower an implement when the implement is attached to the work vehicle; a selector valve mechanism for switching an oil feeding/discharging passage for the hydraulic cylinder among an oil feeding condit~on for feeding pressure oil to the hydraulic cylinder, an oil discharging condition for discharging the oil from the hydraulic cylinder and an oil stopped condition for stopping the feeding or discharging of the oil to the hydraulic cylinder; an operational oil passage incorporating a lift valve for switching the selector valve mechanism by controlling a back pressure thereof, thereby operating the hydraulic cylinder, the lift valve having a position corresponding to each of lowering operation, lifting operation, and stopping of the implement.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10; rule 13) TITLE HYDRAULIC APPARATUS FOR WORK VEHICLE APPLICANT KUBOTA CORPORATION of 2-47, Shikitsuhigashi 1-chome, Naniwa-ku, Osaka-shi, Osaka, Japan; Nationality: a Japanese corporation and THE SIAM KUBOTA INDUSTRY CO., LTD. of 101/19-24, Moo20, Navanakorn, Khlongneung Khlongluang, Pathumtani 12120, Thailand; Nationality: a Thai company and PEDRO ROQUET S.A. of E-08551, Tona, Barcelona, Spain; Nationality: a Spanish company The following specification particularly describes the nature of this invention and the manner in which it is to be performed Hydraulic Apparatus for Work Vehicle BACKGROUND OF THE INVENTION The present invention relates to a hydraulic apparatus for a work vehicle for controlling an implement. With the above-described hydraulic apparatus, pressure oil is fed to the hydraulic cylinder to drive this cylinder with the oil, thereby to lift up an implement or the pressure oil is discharged from the hydraulic cylinder, thus allowing the cylinder to be operated with the load of the implement, thus -l- lowering the implement. As a hydraulic apparatus of the above-noted type, an apparatus is known from e.g. Japanese Patent Application “Kokai” No. 10-304711 (paragraphs [0024H0027], Fig. 4), wherein the implement is lifted up/down by a lift cylinder 6 (corresponding to “hydraulic cylinder”) via a link mechanism 7 having lift arms 11. This apparatus includes an oil discharging passage 44 connected to an oil feeding/discharging passage 22 of the lift cylinder 6 and incorporating a throttle valve 43, and an oil feeding passage 46 connected to the oil feeding/discharging passage 22 and incorporating a check valve 46, wherein the throttle valve 43 provides a flowing resistance to the oil discharged by the lift cylinder 6, thereby adjusting a lowering speed of the lift arms 11. If this type of hydraulic apparatus is adapted, by employing the above-described conventional technique, for allowing the implement being lowered to be stopped under a condition with less stopping shock, during the period from the start of the lowering movement to the stop of the implement, the throttle valve applies a significant flowing resistance which is preset for effectively restricting occurrence of stopping shock. As a result, it takes a long period of time for the lowering implement to reach a target lowered operational position. SUMMARY OF THE INVENTION An object of the present invention is to provide a hydraulic apparatus for a work vehicle which can not only allow the implement to be stopped at the target lowered operational position with less stopping shock, but can also allow the implement to be lowered speedily. A further object of the invention is to realize such hydraulic apparatus with a simple construction. A hydraulic apparatus for a work vehicle for controlling an implement in accordance with the present invention comprises: a hydraulic cylinder -2- adapted to lift or lower an implement when the implement is attached to the work vehicle; a selector valve mechanism for switching an oil feeding/discharging passage for the hydraulic cylinder among an oil feeding condition for feeding pressure oil to the hydraulic cylinder, an oil discharging condition for discharging the oil from the hydraulic cylinder and an oil stopped condition for stopping the feeding or discharging of the oil to the hydraulic cylinder; an operational oil passage incorporating a lift valve for switching the selector valve mechanism by controlling a back pressure thereof, thereby operating the hydraulic cylinder, the lift valve having a position corresponding to each of lowering operation, lifting operation, and stopping of the implement. Between a position of the lift valve corresponding to the implement lowering operation and a position of the lift valve corresponding to the stopping of the implement, a decelerating operational position is provided for throttling said operational oil passage such that said selector valve mechanism progressively switches the oil feeding/discharging passage from the oil discharging condition to the oil stopped condition. As the lift valve is adapted for switching over the selector valve mechanism by controlling a back pressure of the selector valve mechanism, the selector valve mechanism can be operated to progressively switch over the oil feeding/discharging passage from the oil discharging condition to the oil feeding/discharging stopped condition, without requiring any special pilot pump for that purpose. Therefore, with the first characterizing feature of the present invention, as the lift valve operates or is operated to be switched over at an appropriate switching speed, the implement can be lowered speedily without deceleration until the implement reaches a position before the target operational position and can then be lowered with deceleration from that position before the target operational position to be stopped eventually at the target operational position with less stopping shock. Hence, it is possible -3- carry out a work with good finish, with high precision positioning of the implement at the target operational position. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of an entire tractor, Fig. 2 is a plan view of the entire tractor, Fig. 3 is a side view of a transmission line and a link mechanism, Fig. 4 is a side view of a plowing device, Fig. 5 (A) is a section view showing a lift valve, a selector valve mechanism, under a lift arms elevating condition, Fig. 5 (B) a hydraulic circuit diagram under the lift arms elevating condition, Fig. 6 (A) is a section view showing the lift valve, the selector valve mechanism, under a lift arms lowering condition, Fig. 6 (B) a hydraulic circuit diagram under the lift arms lowering condition, Fig. 7 (A) is a section view showing the lift valve, the selector valve mechanism, under a lift arms stopping condition, Fig. 7 (B) is a hydraulic circuit diagram under the lift arms stopping condition, Fig. 8 is a section view of a check valve, Fig. 9 is a side view of a feedback mechanism under a lowering operational condition of a lift lever, Fig. 10 is a side view of the feedback mechanism under an elevating operational condition of the lift lever, Fig. 11 is a front view showing an arranging portion of the lift lever, a hydraulic block and a lift cylinder, Fig. 12 is a plan view showing the arranging portion of the lift lever, the hydraulic block and the lift cylinder, and Fig. 13 is a side view illustrating a function of a balance link. -4- DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described in details with reference to the accompanying drawings. As shown in Figs. 1, 2 and 3, a tractor includes a self-propelled vehicle body frame 3 having a pair of right and left front wheels 1 which are steerable and freely rotatable and a pair of right and left rear wheels 2 which are drivable. Tb a front portion of the frame 2, there is mounted an engine section having an engine 4. Driving power from an output shaft of the engine 4 is transmitted through a transmission mechanism using a transmission belt 5 to a transmission case 6 forming a rear portion of the vehicle body frame 3. Then, the power is transmitted from this transmission case 6 to the right and left rear wheels 2. At a portion of the vehicle body frame 3 rearwardly of the engine section, there is provided a driver's section having a driver's seat 7 and a steering wheel 8. At rear portions of the vehicle body frame 3, there are provided a link mechanism 10 having a pair of right and left lift arms 11, etc., and a power takeoff (PTO) shaft 9 projecting from a rear wall portion of the transmission case 6. With this tractor 4, as shown in Fig. 4, for example, a plowing device 20 can be connected via the link mechanism 10 to a rear portion of the vehicle body frame 3, thereby forming a riding type self-propelled plowing machine. In this way, various implements can be connected to the rear portion of the vehicle body frame 3 via the link mechanism 10, thus forming various types of riding work vehicles. When the tractor is constructed as a riding plowing machine, as shown in e.g. Fig.3, in association of an operation of a lift cylinder 30 operably connected to the lift arms 11 of the link mechanism 10, this lift cylinder 30 pivots the link mechanism 10 up/down relative to the vehicle body frame 3, thereby to lift the plowing device 20 down to a lowered operational condition where a plurality of plowing discs 21 thereof are placed in contact with the -5- ground surface or up to an elevated non-operational condition where the plowing discs 21 are elevated off the ground surface. As the self-propelled vehicle body travels with setting the plowing device 20 under the lowered operational condition, the plowing discs 21 thereof plow a field with the plowing discs 21. Next, the construction for lifting up/down the plowing device 20 (referred to as “implement 20” hereinafter) connected via the link mechanism 10 will be described in details, As shown in e.g. Figs. 2, 3 and 11, the link mechanism 11 includes the right and left lift arms 11 vertically pivotally supported via a rotary support shaft 13 to a pair of right and left support frames 12 attached to an upper portion of the transmission case 6, a pair of right and left lower links 14 vertically pivotally supported to opposed lateral lower portions of the transmission case 6, a lift rod 15 for operably connecting the left lift arm 11 to the lower link 14, another lift rod 15 for operable connecting the right lift arm 11 to the other lower link 14, and a top link 16 vertically pivotally supported to a rear end portion of the vehicle body frame 3. As shown in Fig. 4, free ends of the right and left lower links 14 and the top link 16 are pivotally connected to a frame 22 of the implement 20. As shown in Figs. 9 and 11, each of the pair of right and left lift arms 11 includes an operational arm portion lla projecting from a base portion of the lift arm 11 to be pivotable therewith. And, this operational arm portion lla of each lift arm 11 is a connected via a single connecting shaft 31 to a piston rod of the lift cylinder 30. The base portion of the cylinder tube of the lift cylinder 30 is pivotally supported to the pair of right and left support frames 12 via a connecting shaft 32. The lift cylinder 30 is constructed as a single acting hydraulic cylinder. And, a hydraulic apparatus for operating this lift cylinder 30 includes a hydraulic pump 33 disposed rearwardly of the engine 4 of the -6- engine section as shown in Fig. 3, a hydraulic block 34 supported to one support frame 12 as shown in Figs. 11, 12, and so on. As shown in Fig. 3, the hydraulic pump 33 is driven as the drive power from the output shaft of the engine 4 is transmitted by the transmission belt 35. As shown in e.g. Fig. 5, the hydraulic block 34 includes a pump port P connected via a hydraulic hose (not shown) to a discharging portion of the hydraulic pump 33, a cylinder port C connected via a hydraulic hose (not shown) to the lift cylinder 30, a tank port T connected via a hydraulic hose (not shown) to the transmission case 6 acting also as a working oil tank, an oil feeding/discharging passage 40 having one end thereof connected to the cylinder port C, a relief valve 41 incorporated within this oil feeding/discharging passage 40, and an operational oil passage 50 connected to a cylinder-side opening/closing valve 42 provided at an end of the oil feeding/discharging passage 40 communicated with the cylinder port C. The oil feeding/discharging passage 40 connects the lift cylinder 30 to the hydraulic pump 33 and the transmission case 6 by connecting the cylinder port C to the pump port P and the tank port T. This oil feeding/discharging passage 40 is connected to the lift cylinder 30 via an oil feeding passage 40 incorporating a check valve 43 provided in a member forming the cylinder port C and an oil discharging passage 46 incorporating a throttling portion 45 provided in a member forming the cylinder port C. With functioning of the check valve 43, the oil feeding passage 40 allows introduction of pressure oil from the oil feeding/discharging passage 40 into the lift cylinder 30 or prevents outflow of the pressure oil from the lift cylinder 30 to the oil feeding/discharging passage 40. The oil discharging passage 46 allows outflow of the pressure oil from the lift cylinder 30 to the oil feeding/discharging passage 40 and applies, through the throttling portion 45, flowing resistance to the pressure oil discharged from the lift cylinder 30, thereby to adjust the lowering operational speed of the lift cylinder 30 to -7- allow the lift arms 11 to be lowered at a relatively low and stable lowering speed, regardless of the load of the implement 20. The cylinder-side opening/closing valve 42 provided at the end of the oil feeding/discharging passage 40 communicated with the cylinder port C and a tank-side opening/closing valve 47 provided at a further end of the oil feeding/discharging passage 40 communicated with the tank port T, together constitute a selector valve mechanism 48. The cylinder-side opening/closing valve 42 is switchable between an opened condition in which a cylinder-side portion thereof communicated with the cylinder port C of the oil feeding/discharging passage 40 is communicated with a pump/tank-side portion thereof communicated with the pump port P and the tank port T and a closed condition in which the communications of the cylinder-side portion and the pump/tank-side portion are inhibited. Further, the cylinder-side opening/closing valve 42 is urged to be switched over to the closed condition by a spring 42a. The tank-side opening/closing valve 47 is switchable between an opened condition in which a cylinder/pump-side portion thereof communicated with the cylinder pump C and the pump port P of the oil feeding/discharging passage 40 is communicated with a tank-side portion thereof communicated with the tank port T and a closed condition in which the communications of the cylinder/pump-side portion and the tank-side portion are inhibited. Further, the tank-side opening/closing valve 47 is urged to be switched over to the closed condition by a spring 47a. In operation, in association with opening/closing operations of the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47, the selector valve mechanism 48 switches over the oil feeding/discharging passage 40 among an oil feeding condition for feeding pressure oil to the lift cylinder 30, an oil discharging condition for discharging the pressure oil from the lift cylinder 30 and a feeding/discharging stopped condition for stopping feeding/discharging of the -8- oil from/to the hydraulic lift cylinder 30. That is to say, as illustrated in Fig. 5(A) and 5 (B), when the cylinder-side opening/closing valve 42 is switched over to the opened condition and the tank-side opening/closing valve 47 is switched over to the closed condition, the oil feeding/discharging passage 40 is rendered into the oil feeding condition in which the pressure oil fed from the hydraulic pump 33 to the pump port P is allowed to enter the cylinder port C to be fed to the lift cylinder 30. As illustrated in Fig. 6(A) and 6 (B), when both the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47 are switched over to the opened condition, the oil feeding/discharging passage 40 is rendered into the oil discharging condition in which the pressure oil of the lift cylinder 30 is allowed to flow from the cylinder port C to the tank port T to be discharged to the transmission case 6. In this, the pressure oil from the hydraulic pump 33 is caused to flow from the pump port P to the cylinder port C to be returned to the transmission case 6. Further, as illustrated in Fig. 7 (A) and 7 (B), when the cylinder-side opening/closing valve 42 is switched over to the closed condition, the oil feeding/discharging passage 40 is rendered into the oil feeding/discharging stopped condition in which the pressure oil of the lift cylinder 30 is inhibited from flowing from the cylinder port C to the tank port T and also the pressure oil fed from the hydraulic pump 33 to the pump port P is inhibited from flowing into the cylinder port C. As shown in Fig. 5 and some other figures, the operational oil passage 50 includes a cylinder-side operational oil passage 51 connected to a rear side of the cylinderside opening/closing valve 42 of the selector valve mechanism 48, an oil discharging passage 52 connected to the tank port T via a portion of the oil feeding/discharging passage 40 and the tank-side opening/closing valve 47, and a tank-side operational oil passage 53 connected to a rear side of the tank-side opening/closing valve 47 of the selector valve mechanism 48. This operational oil passage 50 incorporates a lift valve 60 interposed -9- between the cylinder-side operational oil passage 51, and the oil discharging passage 52 and the tank-side operational oil passage 53, and a check valve 70 interposed between the lift valve 60 of the cylinder-side operational oil passage 51 and the cylinder-side opening/closing valve 42. As clearly shown in Fig. 8, the check valve 70 includes a valve seat 71, a spherical valve member 72, a spring 73 for pressing/urging the valve member 72 to the valve seat 71, and an operational member 74 for pressing/operating the valve member 72 against the spring 74. In operation, when the valve member 72 is pressed by the spring 73 to the valve seat 71, thus closing an oil passage hole 75 of the valve member 71, the check valve 70 is switched over to closed condition for checking flow of the oil from the cylinder-side opening/closing valve 42 to the lift valve 60. Whereas, when the valve member 72 is pressed by a leading end 74a of the operational member 72, thus opening the oil passage hole 75 of the valve seat 71, the check valve 70 is switched over to an opened condition for allowing the flow of the oil from the cylinder-side opening/closing valve 42 to the lift valve 60. When the check valve 70 has been switched to the opened condition, if the pressing operational stroke of the operational member 74 is varied, a distance from the valve member 72 to the valve seat 71 is changed correspondingly, thereby changing an opening amount of the oil passage hole 75 of the valve seat 71, so that the check valve 70 throttles the cylinder-side operational oil passage 51 by the valve member 72 and varies the throttling degree thereof so as to adjustably increase/decrease the amount of oil flowing from the cylinder-side opening/closing valve 42 to the lift valve 60. As shown in e.g. Fig. 5, the lift valve 60 includes a valve case 64 having a cylinder port 61 communicated with the cylinder-side operational oil passage 51, a tank port 62 communicated with the oil discharging passage 52, and a tank valve port 63 communicated with the tank-side operational oil passage 53, and a spool 65 slidably fitted within a spool hole defined in the -10- valve case 64. As the spool 65 is slidably operated relative to the valve case 64, the spool 65 can be switched over to a stopping position N shown in Fig. 7 (A), an elevating position U shown in Fig, 5 (A) or to a lowering position D shown in Fig. 6 (A). More particularly, as shown in Fig. 5 (A) and (B), when the spool 65 is operated to the elevating position U, a large diameter portion 65a of the spool 65 is brought into opposition to the tank valve port 63 of the valve case 64 thus closing this valve port 63 and a small diameter end portion 65b of the spool 65 is brought into opposition to the operational member 74 of the check valve 70, thus operating the check valve 70 into the full closed condition by the spring 73. With this, the cylinder-side operational oil passage 42 is subjected to a back pressure from the pressure oil which has entered the cylinder-side opening/closing valve 74 from an oil passage hole 42b formed in the peripheral wall of the cylinder-side opening/closing valve 42. In the course of this, the cylinder-side operational oil passage 51 of the operational oil passage 50 adjusts the back pressure to the cylinder-side opening/closing valve 42 so that the cylinder-side opening/closing valve 42 may be switched over, with the pressure oil from the pump port P, to the opened condition against the urging force of the spring 42a. The tank-side opening/closing valve 47 is subjected to a back pressure from the pressure oil which has entered the tank-side opening/closing valve 47 from an oil passage hole 47b formed in an end wall of the tank-side opening/closing valve 47. In the course of this, the tank-side operational oil passage 53 of the operational oil passage 50 adjusts the back pressure to the tank-side opening/closing valve 47 so that the tank-side opening/closing valve 47 may be switched over, with the force of the spring 47a, to the closed condition. With these, the lift valve 60 controls the back pressures of the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47, with the operational oil passage 50, thereby operating the selector valve mechanism 48, so that the selector -11- valve mechanism 48 switches over the oil feeding/discharging passage 40 to the oil feeding condition and the lift cylinder 30 is operated into an implement elevating condition for elevating the lift arms. As shown in Fig. 6 (A) and (B), when the spool 65 is operated to the lowering position D, a small diameter portion 65a of the spool 65 is brought into opposition to the cylinder valve port 61, the tank port 62 and the tank valve port 63 of the valve case 64, thus communicating the cylinder valve port 61 and the tank valve port 63 with the tank port 62, and a large diameter portion of an inclined outer peripheral portion 65d of the spool 65 is pressed to the operational member 74 of the check valve 70, thus opening the check valve 70. With this, the cylinder-side operational oil passage 51 of the operational oil passage 50 is communicated with the tank port T via the oil discharging passage 52 and the cylinder-side operational oil passage 51 and the oil discharging passage 52 decreases the back pressure to the cylinder-side opening/closing valve 42 so that the cylinder-side opening/closing valve 42 may be switched over, with the pressure oil from the pump port P, to the opened condition against the urging force of the spring 42a. The tank-side operational oil passage 53 of the operational oil passage 50 is communicated with the oil discharging passage 52. The tank-side operational oil passage 53 of the operational oil passage 50 and the oil discharging passage 52 decrease the back pressure to the tank-side opening/closing valve 47, so that the lift valve 60 controls the back pressures to the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47 with the operational oil passage 60, thus operating the selector valve mechanism 48. Hence, the selector valve mechanism 48 switches over the oil feeding/discharging passage 40 to the oil discharging condition and the lift cylinder 30 is operated into an implement lowering condition for lowering the lift arms. As shown in Fig. 7 (A) and (B), when the spool 65 is operated to the -12- stopping position N, the small diameter portion 65c of the spool 65 is brought into opposition to the cylinder valve port 61, the tank port 63 and the tank valve port 63 of the valve case 64, thus communicating the cylinder valve port 61 and the tank valve port 63 with the tank port 62. The small diameter portion of the inclined outer peripheral portion 65d of the spool 65 is brought into opposition to the operational member 74 of the check valve 70, thus operating the check valve 70 by the spring 73, With this, the cylinder-side operational oil passage 51 is closed by the check valve 780, and the cylinder-side operational oil passage 51 of the operational oil passage 50 increases the back pressure to the cylinder-side opening/closing valve 42 so that the cylinder-side opening/closing valve 42 may be switched over to the closed condition by the spring 42a. The tank-side operational oil passage 53 of the operational oil passage 50 is communicated with the oil discharging passage 52. The tank-side operational oil passage 53 of the operational oil passage 50 and the oil discharging passage 52 decrease the back pressure to the tank-side opening/closing valve 47, so that the lift valve 60 controls the back pressures to the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47 with the operational oil passage 50, thus operating the selector valve mechanism 48. Hence, the selector valve mechanism 48 switches over the oil feeding/discharging passage 40 to the oil feeding/discharging stopped condition and the lift cylinder 30 is operated into an implement stopping condition for stopping the lift arms. In association with the sliding operation of the spool 65 from the lowering position D to the stopping position N, the inclined outer peripheral portion 65d of the spool 65 slides against and presses the operational member 74 of the check valve 70 and the outer diameter of the inclined outer peripheral portion 65d is progressively decreased, whereby the check valve 70 is varied in its condition progressively from the opened side to the closed side, thus into the throttling condition. Also, in association with the sliding -13- operation of the spool 65 from the lowering position D to the stopping position N, the pressure oil flowing from the cylinder-side opening/closing valve 42 to the cylinder-side operational oil passage 51 and from the oil discharging passage 52 to the tank port T is gradually decreased and the cylinder-side opening/closing valve 42 is progressively closed. And, when the spool 65 reaches the stopping position N, the cylinder-side opening/closing valve 42 is set to the closed condition. With the above-described construction, between the implement lowering condition and the implement stopping condition, the lift valve 60 is set under a decelerating operational condition. In the course of transition from this decelerating operational condition toward the implement stopping condition, the inclined outer peripheral portion 65d of the spool 65 progressively closes the check valve 70 to its throttling condition. With this, the selector valve mechanism 48 progressively throttles the cylinder-side operational oil passage 51 of the operational oil passage 60 and closes the passage 51 after the throttling thereof, thus progressively switching over the oil feeding/discharging passage 40 from the oil discharging condition to the oil feeding/discharging stopped condition. As shown in Fig. 2, a lift lever 80 operably coupled with the lift valve 60 is disposed on a lateral side of the driver's seat 7 at the driver's section. As shown in Figs. 9 and 11 etc., a base portion 80a of this lift lever 80 is pivotally supported to a lever support shaft 82 supported by the one support frame 12 via a bracket 81, whereby the lift lever 80 is pivoted about an axis 82a oriented laterally of the vehicle body of the lever support shaft 82 along the fore and aft direction of the vehicle body. As shown in e.g. Fig. 11 and Fig. 13, as the base portion 80a of the lift lever 80 is operably coupled, via a spherical coupling 85, with one end of a balance link 84 having an intermediate portion thereof pivotally connected via a connecting pin 83 to an end of the spool 65 of the lift valve 60, the lift lever 80 and the lift valve 60 -14- are operably coupled with each other. As shown in e.g. Fig. 9, Fig. 12 and Fig. 13, the balance link 84, a pivot link 87 having one end thereof connected via a spherical coupling 86 to the opposite end of the balance link 84 to the end connected to the lift lever 80, a connecting rod 88 having a front end thereof connected to the other end of the pivot link 87, and a detecting member 89 to which the rear end of the connecting rod 88 is connected, together constitute a feedback mechanism 90. The detecting member 89 is connected to the end of the pivot shaft 13 of the lift arms 11 to be pivotable therewith. An intermediate portion of the pivot link 87 is pivotally supported to a support shaft 92 supported by the bracket 81 via a support member 91. In operation, the pivot link 87 is pivoted about an axis 92a (see Fig. 13) of the support shaft 92, which axis 92a is oriented laterally of the vehicle body. As shown in Fig. 9, when the lift lever 80 is pivotally operated about the axis 82a to the lowering side (“Down”) (to the front side of the vehicle body), the balance link 84 is pivoted by the lever base portion 80a to the rear side of the vehicle body about the pivotal center of the spherical coupling 86 which connects the balance link 84 to the pivot link 87, thus pressing the spool 65 of the lift valve 60 from the stopping position N to the lowering position D. With this, when the lift valve 60 is set to the implement lowering condition and the lift arms 11 are then lowered, in association therewith, the connecting rod 88 is pulled to the rear side of the vehicle body by the detecting member 89, thus pivoting the pivot link 87 about the axis 92a in the direction of pivoting the lower end of the pivot link 87 to the front side of the vehicle body and the pivot link 87 pivots the balance link 84, about the pivotal center of the spherical coupling 85 connecting the balance link 84 with the lift lever 80, toward the front side of the vehicle body. As the pivot link 84 pulls the spool 65 of the lift valve 60 and when the lowered angle of the lift arms 11 has reached an angle corresponding to the operational stroke -15- of the lift lever 80, the balance link 84 returns the spool 65 of the lift valve 60 to the stopping position N. As shown in Fig. 10, when the lift lever 80 is pivotally operated to the elevating side (“Up”) (the rear side of the vehicle body), the balance link 84 is pivoted by the lever base portion 80a to the front side of the vehicle body about the pivotal center of the spherical coupling 86 which connects the balance link 84 to the pivot link 87, thus pulling the spool 65 of the lift valve 60 from the stopping position N to the elevating position U. With this, when the lift valve 60 is set to the implement elevating condition and the lift arms 11 are then elevated, in association therewith, the connecting rod 88 is pushed to the front side of the vehicle body by the detecting member 89, thus pivoting the pivot link 87 about the axis 92a in the direction of pivoting the lower end of the pivot link 87 to the rear side of the vehicle body and the pivot link 87 pivots the balance link 84, about the pivotal center of the spherical coupling 85 connecting the balance link 84 with the lift lever 80, toward the rear side of the vehicle body. As the pivot link 84 pushes the spool 65 of the lift valve 60 and when the elevated angle of the lift arms 11 has reached an angle corresponding to the operated stroke of the lift lever 80, the balance link 84 returns the spool 65 of the lift valve 60 to the stopping position N. With the above-described construction of the feedback mechanism 90 in operation, when the implement 20 is elevated, the pivoting direction and the pivoting angle of the lift arms 11 relative to the support frame 12 is detected as the lift and the lift stroke of the implement 20. And, the result of this detection is fed back to the lift valve 60. And, when the implement 20 has been elevated or lowered by the operated stroke of the lift lever 80 to reach a connecting height and the lift lever 80 corresponding to the operated stroke of the lift lever 80, the lift valve 60 is automatically switched over to the implement stopping condition. The feedback mechanism 90 switches over the lift valve 60 through the decelerating operational condition to the -16- implement stopping condition, when the implement 20 is lowered. Incidentally, a spring 65 shown in Fig. 5 and some other figures is provided for urging the lift valve 60 toward the implement lowering condition. When the lift valve 60 is switched over to the implement elevating condition or the implement stopping condition by the lift lever 80 or the feedback mechanism 90, the lift valve 60 is maintained under the implement elevating condition or the implement stopping condition, against the spring 66, by the operational resistance of the lift lever 80 or the lift arm 11. That is, when the lift lever 80 is pivoted about the axis 82a in the fore and aft direction of the vehicle body, the implement 20 is lifted up/down to reach a connecting height (referred to as “target operational position” hereinafter) corresponding to the operated position of the lift lever 80. When the implement 20 is lowered, the implement will be stopped gently at the target operational position with less stopping shock. Namely, suppose the lift lever 80 is pivoted to the elevating side “Up” about the axis 82a. Then, the balance link 84 is pivoted to the front side of the vehicle body about the pivotal center of the spherical coupling 86, thus switching over the spool 65 of the lift valve 60 from the stopping position N to the elevating position U. With this, the lift valve 60 is switched over from the implement stopping condition to the implement elevating condition and the operational oil passage 60 is operated to adjust the back pressures of the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47of the selector valve mechanism 48. Then, the selector valve mechanism 47 switches over the oil feeding/discharging passage 40 to the oil feeding condition, whereby the pressure oil from the hydraulic pump 33 is fed to the lift cylinder 30, then the lift cylinder 30 is driven to the extending side thereof to pivot the pair of right and left lift arms 11 to the elevating side and the lift mechanism 10 is elevated to lift up the implement 20. When the elevating -17- stroke of the implement 20 has reached a stroke corresponding to the operated stroke of the lift lever 80, the feedback mechanism 90 automatically switches over the lift valve 60 to the implement stopping condition. Hence, the lift valve 60 operates the operational oil passage 50 to adjust the back pressures of the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47 of the selector valve mechanism 48, whereby the selector valve mechanism 48 switches over the oil feeding/discharging passage 40 to the feeding/discharging stopped condition. With this, the lift cylinder 30 is stopped, thereby stopping the right and left lift arms 11 and maintains them at their operated positions, whereby the link mechanism 10 is stopped and maintained at its operated position, so that the implement 20 is maintained at the target operational position. Suppose next the lift lever 80 is pivoted to the lowering side “Down” about the axis 82a. Then, the balance link 84 is pivoted to the rear side of the vehicle body about the pivotal center of the spherical coupling 86, thus switching over the spool 65 of the lift valve 60 from the stopping position N to the lowering position D. With this, the lift valve 60 is switched over from the implement stopping condition to the implement lowering condition and the operational oil passage 60 is operated to adjust the back pressures of the cylinder-side opening/closing valve 42 and the tank-side opening/closing valve 47of the selector valve mechanism 48. Then, the selector valve mechanism 47 switches over the oil feeding/discharging passages 40 to the oil discharging condition, whereby the lift cylinder 30 discharges the oil due to the load of the implement 20 and is operated to the contracting side thereof to pivot the pair of right and left lift arms 11 to the lowering side and the lift mechanism 10 is lowered to lower the implement 20. In the course of this, the feedback mechanism 90 feeds back the lowering of the implement 20 in operating the lift valve 60. Hence, the implement 20 is lowered to a connecting height slightly before the target operational position, the feedback mechanism 90 -18- operates the lift valve 60 to the decelerating operational condition. Thereafter, in association with arrival of the implement 20 at the target operational position, the feedback mechanism 90 returns the lift valve 60 to the implement stopping condition. Wit this, during the period between the arrival of the implement 20 at the position slightly before the target operational position and the arrival thereof at the target operational position, the selector valve mechanism 48 progressively switches over the oil feeding/discharging passage 40 from the oil discharging condition to the oil feeding/discharging stopped condition, whereby the lift cylinder 30 is operated to the contracting side to progressively decrease the amount of oil discharged therefrom and the pair of right and left lift arms 11 are pivoted to the lowering side, thus reducing the lowering speed of the implement 20 (lowering speed adjusted by the throttling portion 45), with the rate of this deceleration being progressively increased as approaching the target operational position. When the implement is lowered to the target operational position, with the operation of the operational oil passage 50 by the lift valve 60, the selector valve mechanism 48 switches over the oil feeding/discharging passage 40 to the oil feeding/discharging stopped condition. With this, the lift cylinder 30 is stopped, thereby stopping the right and left lift arms 11 and maintains them at their operated positions, whereby the link mechanism 10 is stopped and maintained at its operated position, so that the implement 20 is maintained at the target operational position. [Other Embodiments] The present invention may be applied not only to the hydraulic apparatus mounted on a tractor, but also to a hydraulic apparatus mounted on a combined harvester for lifting up/down a harvesting pre-processing unit -19- thereof or a hydraulic apparatus mounted on a grass mower for lifting up/down a mower unit thereof. Therefore, various work vehicles such as a tractor, a combined harvester, a mower, etc. are generically referred to as “work vehicles”herein. -20-

Documents:

513-CHE-2006 POWER OF ATTORNEY.pdf

513-che-2006 claims granted.pdf

513-che-2006 decription (complete) granted.pdf

513-che-2006 drawings granted.pdf

513-che-2006-abstract.pdf

513-che-2006-claims.pdf

513-che-2006-correspondance -others.pdf

513-che-2006-description(complete).pdf

513-che-2006-diagrams.pdf

513-che-2006-form 1.pdf

513-che-2006-form 3.pdf

513-che-2006-form 5.pdf