Hydraulic control apparatus

A hydraulic control apparatus 1 includes a switch valve 11, a valve support chamber 35, a flow control valve 12 movable within the valve support chamber 35, an on-off valve 13 movable within the communication path chamber 12a, and a valve control device 14. The flow control valve 12 has a communication path chamber 12a and a back pressure chamber 12d. The on-off valve 13 is capable of opening and shutting off a communication path X between a cylinder line 32 and a switch valve line 33. A restrictor is formed between the flow control valve 12 and a wall defining the valve support chamber 35. The restrictor connects the cylinder line 32 and the communication path chamber 12a to each other. The opening degree of the restrictor is changed in correspondence with movement of the flow control valve 12. When the switch valve 11 is located at the neutral position or the supply position, the valve control device 14 applies a fluid pressure in the cylinder line 32 to the back pressure chamber 12d for urging the on-off valve 13 in a direction for shutting off the communication path 12a. When the switch valve 11 is located at the drainage position, the valve control device 14 applies a pilot pressure lower than the fluid pressure in the cylinder line 32 to the back pressure chamber 12d, thereby moving the on-off valve 13 in a direction for opening the communication path X.

FIELD OF THE INVENTION

The present invention relates to hydraulic control apparatuses having switch valves for controlling supply and drainage of fluid to cylinders.

BACKGROUND OF THE INVENTION

As a hydraulic control apparatus having a switch valve for controlling supply and drainage of fluid to and from a cylinder, a hydraulic control apparatus used in, for example, a forklift is known. The hydraulic control apparatus may be employed for actuating a lift cylinder of the forklift, which selectively raises and lowers a fork, as described in Japanese Laid-Open Patent Publication No. 2002-327706.

The hydraulic control apparatus of the publication includes an operated check valve and a flow regulator provided in a main passage. The main passage connects a lift control valve, which is operated by means of a lift lever, to the lift cylinder. The lift control valve has a spool that includes a variable restrictor and is switched among a raising position, a neutral position, and a lowering position. More specifically, when the spool is located at the neutral position or the raising position, the lift control valve seals a back pressure chamber of the operated check valve. The operated check valve is thus urged in a direction for blocking the main passage. Meanwhile, a pump operates to apply hydraulic pressure to a second pressure chamber of the flow regulator and a valve body of the flow regulator is maintained at a fully open position.

In contrast, when the spool is located at the lowering position, a tank operates to apply hydraulic pressure to the back pressure chamber of the operated check valve. The operated check valve thus opens the main passage using the hydraulic pressure generated by the lift cylinder. Meanwhile, the hydraulic pressure in the tank is supplied to the second pressure chamber of the flow regulator. This causes the valve body of the flow regulator to move in such a manner that the difference between the pressure in a portion upstream from the variable restrictor and the pressure in a downstream portion is maintained equal to or lower than a predetermined value. The flow rate of the hydraulic oil flowing from the lift cylinder is thus adjusted.

However, in the hydraulic control apparatus, the operated check valve and the flow regulator are formed separately. Besides, the hydraulic control apparatus includes a large number of components and thus has a relatively complicated configuration. Further, since the operated check valve and the flow regulator must be accommodated separately in two different spaces, the hydraulic control apparatus becomes relatively large.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide a compact hydraulic control apparatus that stably performs shutting operation.

To achieve the foregoing objective and in accordance with one aspect of the present invention, a hydraulic control apparatus for a cylinder is provided. The apparatus includes a switch valve, a cylinder line, a switch valve line, a valve support chamber, a flow control valve, an on-off valve and a valve control device. The switch valve controls supply and drainage of a fluid with respect to the cylinder. The switch valve is switched among a supply position for supplying the fluid to the cylinder, a drainage position for draining the fluid from the cylinder, and a neutral position for preventing the supply and the drainage of the fluid with respect to the cylinder. The cylinder line is connected to the cylinder. The switch valve line is connected to the switch valve. The valve support chamber is arranged between the cylinder line and the switch valve line. The valve support chamber has a cylinder side opening communicating with the cylinder line and a switch valve side opening communicating with the switch valve line. The flow control valve is movably located in the valve support chamber. The flow control valve selectively connects and disconnects the cylinder line and the switch valve line with respect to each other. The flow control valve includes a communication path chamber. The flow control valve has a cylinder side through hole that connects the communication path chamber with the cylinder side opening and a switch valve side through hole that connects the communication path chamber with the switch valve side opening. The on-off valve is movably located in the communication path chamber. The on-off valve defines a back pressure chamber in the communication path chamber. A fluid pressure acting on the on-off valve is introduced into the back pressure chamber. The on-off valve selectively opens and shuts off a communication path between the cylinder line and the switch valve line. The valve control device controls operation of the flow control valve and the on-off valve. A restrictor is formed between the flow control valve and a wall defining the valve support chamber. The restrictor connects the cylinder line and the communication path chamber to each other. An opening degree of the restrictor is changed in correspondence with movement of the flow control valve. When the switch valve is located at the neutral position or the supply position, the valve control device applies a fluid pressure in the cylinder line to the back pressure chamber for urging the on-off valve in a direction for shutting off the communication path. When the switch valve is located at the drainage position, the valve control device applies a pilot pressure lower than the fluid pressure in the cylinder line to the back pressure chamber, thereby moving the on-off valve in a direction for opening the communication path.

In accordance with another aspect of the present invention, another hydraulic control apparatus for a cylinder is provided. The hydraulic control apparatus includes a switch valve, a cylinder line, a switch valve line, a valve support chamber, a flow control valve, and an on-off valve and a valve device. The switch valve controls supply and drainage of a fluid with respect to the cylinder. The switch valve is switched among a supply position for supplying the fluid to the cylinder, a drainage position for draining the fluid from the cylinder, and a neutral position for preventing the supply and the drainage of the fluid with respect to the cylinder. The cylinder line is connected to the cylinder. The switch valve line is connected to the switch valve. The valve support chamber is arranged between the cylinder line and the switch valve line. The flow control valve is movably located in the valve support chamber. The flow control valve selectively connects and disconnects the cylinder line and the switch valve line with respect to each other. The flow control valve includes a communication path chamber. The on-off valve is movably located in the communication path chamber. The on-off valve defines a back pressure chamber in the communication path chamber. A fluid pressure acting on the on-off valve is introduced into the back pressure chamber. The on-off valve selectively opens and shuts off a communication path between the cylinder line and the switch valve line. The valve control device controls operation of the flow control valve and the on-off valve. A restrictor is formed between the flow control valve and a wall defining the valve support chamber. The restrictor connects the cylinder line and the communication path chamber to each other. An opening degree of the restrictor is changed in correspondence with movement of the flow control valve. When the switch valve is located at the neutral position or the supply position, the valve control device applies a fluid pressure in the cylinder line to the back pressure chamber for urging the on-off valve in a direction for shutting off the communication path. When the switch valve is located at the drainage position, the valve control device applies a pilot pressure lower than the fluid pressure in the cylinder line to the back pressure chamber, thereby moving the on-off valve in a direction for opening the communication path.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a cross-sectional view showing a hydraulic control apparatus1according to a first embodiment of the invention. The hydraulic control apparatus1is employed for actuating a lift cylinder50of a forklift, which selectively raises and lowers a fork. The lift cylinder50is formed by a single-acting cylinder. The forklift has a lift cylinder control circuit, or a hydraulic circuit in which the lift cylinder50is arranged. The hydraulic control apparatus1defines a part of the lift cylinder control circuit. The forklift further includes a hydraulic pump51and different hydraulic circuits (not shown) including a tilt cylinder control circuit and a power steering system hydraulic circuit. The hydraulic pump51supplies hydraulic oil (fluid) to different circuits including the lift cylinder control circuit. The hydraulic oil is then returned from the circuits to a tank52, which is provided in the forklift, re-pressurized by the hydraulic pump51, and then recirculated to the circuits.

As shown inFIG. 1, the hydraulic control apparatus1includes a valve housing10, a switch valve11, n flow control valve12, an on-off valve13, and a valve control device14. Different ports and lines are defined in the valve housing10, and the switch valve11, the flow control valve12, the on-off valve13, and the valve control device14are incorporated in the valve housing10.

A cylinder port31is defined in the valve housing10and connected to the lift cylinder50, thus defining a supply-drainage port for selectively supplying the hydraulic oil to the lift cylinder50and draining the hydraulic oil from the lift cylinder50. The valve housing10includes a supply line36, a first tank line37, and a second tank line38. The supply line36communicates with the hydraulic pump51and is supplied with the hydraulic oil from the hydraulic pump51. The first and second tank lines37,38communicate with the tank52. The valve housing10further includes a cylinder line32, a switch valve line33, and a connection passage34. The cylinder line32is defined continuously from the cylinder port31and communicates with the lift cylinder50through the cylinder port31. The switch valve line33communicates with the switch valve11.

The flow control valve12is located in a valve support chamber35formed between the cylinder line32and the switch valve line33, and can be moved along walls defining the valve support chamber35. The walls defining the valve support chamber35include a cylinder side opening35aand a switch valve side opening35b. The cylinder side opening35aopens to the cylinder line32and the switch valve side opening35bopens to the switch valve line33. A communication path chamber12ais formed in the flow control valve12. The communication path chamber12ais a cylindrical space for accommodating the on-off valve13. The flow control valve12has a cylinder side through hole12band a switch valve side through hole12c. The cylinder side through hole12bselectively connects the communication path chamber12awith the cylinder side opening35a. The switch valve side through hole12cselectively connects the communication path chamber12awith the switch valve side opening35b. Accordingly, the cylinder line32can be connected to the switch valve line33through the communication path chamber12ain the flow control valve12.

In this manner, the flow control valve12and the valve support chamber35defines a restrictor between the cylinder side through hole12band the cylinder side opening35a. The restrictor changes the opening degree between the cylinder line32and the communication path chamber12ain accordance with movement of the flow control valve12. The flow control valve12has a spring17serving as an urging member and a spring support member18at an end in the longitudinal direction. The spring17urges the flow control valve12through the spring support member18in a direction to increase the opening degree of the flow control valve12(rightward as viewed in the drawing).

The on-off valve13has a columnar shape so that it can be moved along the inner circumference of the communication path chamber12a. The on-off valve13divides the communication path chamber12ainto a fluid chamber12hand a back pressure chamber12d. The switch valve side through holes12care located in the fluid chamber12h. Further, the on-off valve13selectively shuts off a communication path X (indicated by arrow X inFIG. 1) between the cylinder side through hole12band the switch valve side through hole12c.

As described above, the back pressure chamber12dis a space formed by a valve support chamber35and a zone in which the communication path chamber12a. The back pressure chamber12dserves as a back pressure chamber of the on-off valve, and also as a back pressure chamber of the flow control valve12.

A pressure introduction line13bis a through hole formed in the on-off valve13. The pressure introduction line13bselectively connects the back pressure chamber12dwith the cylinder side through hole12band the cylinder line32, and expose the back pressure chamber12dto the pressure of fluid in the cylinder line32. The hydraulic pressure in the back pressure chamber12dis controlled by the valve control device14as shown below.

Further, the on-off valve13has a space defined in it for accommodating a spring16, which serves as an urging member. In the back pressure chamber12d, the spring16is located between the on-off valve13and the spring support member18. The on-off valve13is urged in a direction to shut off the communication path X (rightward as viewed in the drawing) by the spring16. A distal portion13aof the on-off valve13contacts a valve seat12e, which is a step formed in the wall defining the communication path chamber12a, so that the communication path X is shut off.

The connection passage34is defined in such a manner as to permit communication between the cylinder line32and the switch valve line33. The connection passage34is defined separately from a hydraulic oil path (a first line) including the communication path X between the cylinder side through hole12band the switch valve side through hole12c, and serves as a second line connecting the cylinder line32to the switch valve line33. A check valve39is provided between the connection passage34and the switch valve line33.

The switch valve11controls supply and drainage of the hydraulic oil with respect to the lift cylinder50. The switch valve11is formed as a spool valve having a spool22, a spool bore23, and a spring mechanism24. The spool22is arranged in the spool bore23in an axially movable manner. The spring mechanism24maintains the spool22at a neutral position. The spool22is caused to move axially through manipulation of a non-illustrated lift lever, thus switching the switch valve11(more specifically, the spool22) among a supply position, the neutral position, and a drainage position.

InFIG. 1, the switch valve11is held at the neutral position at which the switch valve11does not permit either supply or drainage of the hydraulic oil with respect to the lift cylinder50. If the spool22moves from the neutral position in a direction indicated by arrow A ofFIG. 1, the switch valve11is switched to the supply position. In this state, as will be described later, the hydraulic pump51supplies the hydraulic oil to the lift cylinder50, that is, a bottom chamber54of the lift cylinder50(seeFIG. 2). Contrastingly, if the spool22moves from the neutral position ofFIG. 1in a direction indicated by arrow B of the drawing, the switch valve11is switched to the drainage position. In this state, the hydraulic oil is drained from the lift cylinder50to the tank52(seeFIG. 3). The spool22includes a first land portion22ahaving a relatively small diameter and a second land portion22b, which are formed in two axial portions of the spool22.

The on-off valve13, which is constructed as described above, operates based on a first urging force and a second urging force. Specifically, the first urging force is generated at an end face of the on-off valve13that faces the back pressure chamber12ddue to the force of the spring16and the hydraulic pressure acting on the back pressure chamber12d. The second urging force is generated due to hydraulic pressure acting on an end face13cof the on-off valve13that faces the fluid chamber12h. If the first urging force is greater than the second urging force, the on-off valve13is maintained in contact with the valve seat12e. In contrast, if the second urging force is greater than the first urging force, the on-off valve13is shifted to an open state.

Since the fluid chamber12h, in which the end face13cof the on-off valve13is located, communicates with the switch valve line33through the switch valve side through hole12c, the end face13cof the on-off valve13is exposed to a hydraulic pressure that is substantially the same as the hydraulic pressure of the switch valve line33.

In a state where the on-off valve13opens the communication path X, the flow control valve12, which is constructed as described above, receives, along a direction to increase the opening degree (rightward as viewed in the drawing), the urging force of the spring17through the spring support member18and the urging force due to the hydraulic pressure acting on the end face of the flow control valve12in the back pressure chamber12d. Also, the flow control valve12receives, along a direction to decrease the opening degree (leftward as viewed in the drawing), the urging force due to the hydraulic pressure acting on the end face corresponding to the fluid chamber12h. Further, the spring support member18receives an urging force that corresponds to the difference in hydraulic pressure between the zones defined by the on-off valve13, that is, the difference in hydraulic pressure between the back pressure chamber12dand the fluid chamber12h. The flow control valve12is maintained at a position where these urging forces are in equilibrium.

In a state where the on-off valve13opens the communication path X, when the hydraulic pressure of the fluid chamber12hand the switch valve line33is increased, the urging force that acts on the flow control valve12and the on-off valve13, or back pressure chamber12dis increased. The urging force acting on the on-off valve13is transmitted to the spring support member18through the spring16. Alternatively, when the on-off valve13contacts the spring support member18, the urging force is transmitted to the spring support member18through the spring16and the on-off valve13. Also, the urging force acting on the flow control valve12is transmitted to the spring support member18. Accordingly, the spring17is contracted by the spring support member18, and the flow control valve12is moved toward the back pressure chamber12d(leftward as viewed in the drawing) until the elastic force of the spring17and the above described urging force are in equilibrium. This reduces the opening degree of the restrictor between the cylinder side through hole12band the cylinder side opening35a. In this manner, the flow control valve12is moved in accordance with the hydraulic pressure of the switch valve line33.

The valve control device14controls operation of the flow control valve12and the on-off valve13, and, as shown inFIG. 1, includes a pilot line20and an electromagnetic switch valve21.

The pilot line20is defined in the valve housing10as a passage that connects the back pressure chamber12dof the flow control valve12and the on-off valve13to the tank52in correspondence with switching of the electromagnetic switch valve21. The pilot line20defines a pilot pressure generating portion that generates pilot pressure lower than the hydraulic pressure in the cylinder line32and applies the hydraulic pressure to the back pressure chamber12d. The pilot line20has an opening20acommunicating with the spool bore23of the switch valve11. If the spool22is moved in the direction indicated by arrow B ofFIG. 1, the switch valve11is switched to the drainage position ofFIG. 3. In this state, a second land portion22bof the spool22corresponds to the opening20aand thus the pilot line20is connected to a second tank line38through the spool bore23.

In the opening20aof the pilot line20, only the portion corresponding to the second land portion22bfunctions as a portion that is permitted to communicate with the second tank line38. In other words, as the spool22moves in the direction indicated by arrow B ofFIG. 1, the area of the portion of the opening20acorresponding to the second land portion22bgradually increases. The communication area (the opening degree) of the passage between the pilot line20and the second tank line38thus gradually increases, correspondingly.

The electromagnetic switch valve21is formed by an electromagnetic valve that is switched for selectively connecting and disconnecting the back pressure chamber12dof the flow control valve12and the on-off valve13to and from the pilot line20. The electromagnetic switch valve21is excited or de-excited by a non-illustrated controller that detects the operational state of a limit switch25incorporated in the valve housing10. When the switch valve11is held at the neutral position or the supply position, the electromagnetic switch valve21disconnects the back pressure chamber12dfrom the pilot line20(seeFIGS. 1 and 2). Contrastingly, if the switch valve11is held at the drainage position, the electromagnetic switch valve21connects the back pressure chamber12dto the pilot line20(seeFIGS. 3 and 4). When the back pressure chamber12dis disconnected from the pilot line20, the hydraulic pressure in the cylinder line32, which is introduced through the pressure introduction line13bof the on-off valve13, is applied to the back pressure chamber12dthrough the pressure introduction line14cof the valve body14. In contrast, when the back pressure chamber12dis connected to the pilot line20, the hydraulic pressure in the second tank line38, which is the aforementioned pilot pressure lower than the hydraulic pressure in the cylinder line32, is applied to the back pressure chamber12dthrough the pilot line20. That is, the electromagnetic switch valve21serving as a switch portion operates to apply the hydraulic pressure in the cylinder line32to the back pressure chamber12dwhen the switch valve11is held at the neutral or supply positions. The electromagnetic switch valve21operates to apply the pilot pressure to the back pressure chamber12dwhen the switch valve11is maintained at the drainage position.

When the hydraulic pressure in the cylinder line32is applied to the back pressure chamber12d, the on-off valve13is urged toward the valve seat12ein such a manner as to disconnect the cylinder line32from the switch valve line33. In contrast, if the pilot pressure, which is lower than the hydraulic pressure in the cylinder line32, is applied to the back pressure chamber12d, the on-off valve13is spaced from the valve seat12ein such a manner as to connect the cylinder line32to the switch valve line33. In this state, the flow control valve12moves in correspondence with the hydraulic pressure in the switch valve line33, thus adjusting the opening degree of the restrictor between the cylinder side through hole12band the cylinder side opening35a.

Next, the operation of the hydraulic control apparatus1will be explained. If the switch valve11is held at the neutral position as shown inFIG. 1, the spool22is located in such a manner as to disconnect the supply line36and the first tank line37from the switch valve line33. Therefore, the hydraulic oil is neither supplied to nor drained from the switch valve line33. Further, in this state, the electromagnetic switch valve21operates to disconnect the back pressure chamber12dof the on-off valve13from the pilot line20. The hydraulic pressure in the cylinder line32is thus introduced into the back pressure chamber12dvia the pressure introduction line13b. At this stage, the first urging force generated by the hydraulic pressure in the cylinder line32and the spring16is greater than the second urging force generated by the hydraulic pressure in the switch valve line33, the distal portion13aof the on-off valve13is caused to contact the valve seat12e. This maintains the cylinder line32in a state disconnected from the switch valve line33. Likewise, the flow control valve12is maintained in a state where its stepped portion12fcontacts a projection35fon the wall defining the valve support chamber35. In other words, the on-off valve13blocks the flow of the hydraulic oil in a direction in which the hydraulic oil is drained from the lift cylinder50. This prevents the lift cylinder50from retracting (i.e., from lowering due to the own weight) and thus maintains the fork at a predetermined height. Further, the connection passage34extending from the cylinder line32to the switch valve line33is blocked by the check valve39.

When the switch valve11is switched from the neutral position to the supply position, the hydraulic control apparatus1operates in the following manner.FIG. 2shows the hydraulic control apparatus1in which the switch valve11is held at the supply position. If the switch valve11is switched from the neutral position to the supply position, the spool22moves in the direction indicated by arrow A ofFIG. 1. Thus, after having been supplied from the pump51to the supply line36, the hydraulic oil is introduced into the switch valve line33via a communication passage36aand a passage defined between the first land portion22aof the spool22and a corresponding wall of the spool bore23as indicated by the corresponding arrows ofFIG. 2. In this state, the first tank line37is held in a state disconnected from the switch valve line33. This raises the hydraulic pressure in the switch valve line33, thus applying a correspondingly increased urging force to the check valve39. When this urging force exceeds the urging force acting on the check valve39generated by the spring and the hydraulic pressure in the cylinder line32, the check valve39becomes open. This connects the switch valve line33to the cylinder line32through the connection passage34, thus sending the hydraulic oil to the cylinder line32. The hydraulic oil is then supplied to the lift cylinder50and thus raises the fork. In this state, the electromagnetic switch valve21maintains the pilot line20in a state disconnected from the back pressure chamber12d. Therefore, the first urging force generated by the hydraulic pressure in the back pressure chamber12dand the spring16is greater than the second urging force generated by the hydraulic pressure in the switch valve line33. The on-off valve13is thus maintained closed. Likewise, the flow control valve12is maintained in a state where its stepped portion12fcontacts a projection35fon the wall defining the valve support chamber35.

When the switch valve11is switched from the neutral position ofFIG. 1to the drainage position, the hydraulic control apparatus1operates as follows.FIG. 3shows the hydraulic control apparatus1in which the switch valve11is held at the drainage position, that is, the on-off valve13is moved.FIG. 4shows the hydraulic control apparatus1in which the flow control valve12is moved together with the movement of the on-off valve13. If the switch valve11is switched from the neutral position to the drainage position, the spool22moves in the direction indicated by arrow B ofFIG. 1. The switch valve line33is thus connected to the first tank line37through a passage defined between the first land portion22aof the spool22and the corresponding wall of the spool bore23.

Further, if the switch valve11is switched to the drainage position, the limit switch25generates a detection signal. In response to the detection signal, the controller (not shown) switches the electromagnetic switch valve21in such a manner as to connect the pilot line20to the back pressure chamber12d. The hydraulic oil is thus sent from the back pressure chamber12dto the pilot line20.

Meanwhile, in correspondence with the movement of the spool22, the second land portion22breaches a position corresponding to the opening20aof the pilot line20. As the spool22further moves, the portion of the opening20ablocked by the spool22becomes gradually smaller and, in contrast, the portion of the opening20acorresponding to the second land portion22bbecomes gradually larger. Accordingly, the communication area (the opening degree) of the passage between the pilot line20and the second tank line38gradually increases, thus increasing the flow rate of the hydraulic oil from the pilot line20to the second tank line38, correspondingly. Once the opening20aentirely corresponds to the second land portion22b, the communication state of the pilot line20with respect to the second tank line38is maintained without changing.

When the switch valve11is switched to the drainage position, the hydraulic oil flows from the back pressure chamber12dto the second tank line38through the pilot line20as indicated by the corresponding arrows ofFIG. 3. This lowers the pressure in the back pressure chamber12d. In other words, the pilot pressure lower than the hydraulic pressure in the cylinder line32acts in the back pressure chamber12d. Therefore, the second urging force generated by the hydraulic pressure in the fluid chamber12hbecomes greater than the first urging force generated by the hydraulic pressure in the back pressure chamber12dand the spring16. This causes the on-off valve13to separate from the valve seat12e, thus opening the communication path X between the cylinder side through hole12band the switch valve side through hole12c. The hydraulic oil thus flows from the lift cylinder50to the switch valve line33via the cylinder line32and the communication path X. The hydraulic fluid is then sent from the first tank line37to the tank52, thus lowering the fork.

Further, if the hydraulic pressure in the switch valve line33changes when the switch valve11is held at the drainage position and the hydraulic fluid flows out of the lift cylinder50as shown inFIG. 4, or when the fork is being lowered, the equilibrium between the first urging force, which is generated by the hydraulic pressure in the back pressure chamber12dand the spring17, and the second urging force, which is generated by the hydraulic pressure in the fluid chamber12h, is quickly cancelled, which displaces the flow control valve12. This changes the opening degree α of the restrictor between the cylinder side through hole12band the cylinder side opening35a.

As a result, the flow rate of the hydraulic oil from the cylinder line32to the fluid chamber12his changed, so that the hydraulic pressure of oil flowing from the switch valve side through hole12cto the switch valve line33is adjusted. In this manner, the lowering speed of the fork is adjusted (pressure compensation function).

As has been described, when the switch valve11is held at the neutral position in the hydraulic control apparatus1of the first embodiment, the hydraulic pressure in the cylinder line32is applied to the back pressure chamber12dof the on-off valve13for urging the on-off valve13in such a manner as to disconnect the cylinder line32from the switch valve line33. Therefore, with the switch valve11held at the neutral position, the on-off valve13is maintained in a state in which the cylinder line32is disconnected from the switch valve line33. This restricts the drainage of the hydraulic oil from the lift cylinder50and thus retracting motion of the lift cylinder50. That is, as long as the switch valve11is maintained at the neutral position, the flow control valve12, in which the on-off valve13is provided, functions as an operated check valve.

If the switch valve11is switched from the neutral position to the drainage position, the pilot pressure lower than the hydraulic pressure in the cylinder line32is applied to the back pressure chamber12dof the on-off valve13. This reduces the urging force applied from the back pressure chamber12dto the on-off valve13, thus switching the on-off valve13from a closed state to an open state, or to a state allowing the cylinder line32and the communication path X to communicate with each other. The hydraulic oil is thus drained from the lift cylinder50to the tank52. With the switch valve11held at the drainage position, the flow control valve12is permitted to move in the valve support chamber35in correspondence with change of the hydraulic pressure in the switch valve line33. In correspondence with the movement of the flow control valve12, the opening degree of the restrictor provided between the cylinder line32and the fluid chamber12hchanges. Accordingly, the flow control valve12, in which the on-off valve13is provided, functions also as a flow regulator for adjusting the flow rate of the fluid drained from the lift cylinder50.

That is, since the on-off valve13serving as a flow regulator is located inside the flow control valve12serving as an operated check valve, the flow control valve12serves both as an operated check valve and a flow regulator. This makes it unnecessary to provide an operated check valve and a flow regulator separately from each other, simplifying the configuration of the hydraulic control apparatus1.

Further, the on-off valve13can shut off communication path X independently of movement of the flow control valve12. That is, the shutting off operation is hardly influenced by changes in the opening degree of the flow control valve12. Therefore, in the case where the communication path X stops drainage while being narrowed by the flow control valve12, the lowering motion of the fork by the lift cylinder50can be stopped by shutting off the communication path X by the on-off valve13without maximizing the opening degree of the flow control valve12. Thus, when stopping the drainage, the flow rate of fluid is prevented from being instantly increased, and the lift cylinder50is stopped in a stable manner.

If the hydraulic pressure in the fluid chamber12h, which is part of the communication path X, rises when the switch valve11is held at the drainage position and the hydraulic fluid is drained from the lift cylinder50, the opening degree of the restrictor of the flow control valve12decreases and the hydraulic pressure in the switch valve line33drops. The flow rate of the hydraulic oil drained from the lift cylinder50is thus adjusted in a predetermined range. That is, the lowering speed of the fork is adjusted correspondingly (the pressure compensation function).

Since the valve seat12ewith which the on-off valve13is held in contact is integrally formed with the communication path chamber12a, the configuration of the on-off valve13, which is used for shutting off and opening the communication path X becomes further simple.

The pressure introduction line13bis defined in the on-off valve13. Therefore, when the switch valve11is held at the neutral or supply positions, the hydraulic pressure is supplied from the cylinder line32to the back pressure chamber12dby means of a relatively simple structure.

The valve control device14is formed by the pilot line (the pilot pressure generating portion)20and the electromagnetic switch valve (the switch portion)21, which cooperates with each other. By operating the electromagnetic switch valve21with the pilot line20maintained in a state generating the pilot pressure, the pilot pressure is quickly supplied to the back pressure chamber12din response to such operation. This improves the response of the on-off valve13.

Further, the pilot pressure generating portion for generating the pilot pressure lower than the hydraulic pressure in the cylinder line32is relatively easily provided simply by defining the pilot line20, which connects the back pressure chamber12dto the tank52. This permits the flow control valve12to operate in such a manner that the difference between the hydraulic pressure in the switch valve line33upstream from the switch valve11and the hydraulic pressure in the second tank line38(the tank52) downstream from the switch valve11is maintained in a predetermined range. Accordingly, regardless of the load pressure acting on the fork, the fork lowering speed is adjusted in accordance with the operational amount of the switch valve11(the pressure compensation function).

When the switch valve11is switched to the drainage position, the portion of the opening20acorresponding to the second land portion22bbecomes gradually larger in correspondence with the movement of the spool22in the spool bore23. This gradually changes the communication state of the back pressure chamber12dwith respect to the tank52. Therefore, at an initial stage of switching of the switch valve11to the drainage position, the opening degree of the on-off valve13gradually increases, thus permitting the fork to be finely controlled when being lowered. These advantages are brought about simply by forming the second land portion22bin the spool22and connecting the pilot line20to the spool bore23through the opening20a.

Further, since the hydraulic oil leaking from the electromagnetic switch valve21, which is arranged between the back pressure chamber12dand the pilot line20, is extremely small, leakage of the hydraulic oil from the electromagnetic switch valve21to the tank52is suppressed. Therefore, when the switch valve11is held at the neutral position, the retraction of the lift cylinder50is suppressed, thus preventing the fork from lowering due to the weight of the fork.

When the switch valve11is switched to the supply position, the hydraulic oil is supplied from the switch valve line33to the cylinder line32through the connection passage34, which is different from the communication path X. This simplifies the configuration of the connection passage34, thus decreasing the pressure loss caused through the supply of the hydraulic oil to the lift cylinder50.

FIG. 5is a cross-sectional view showing a hydraulic control apparatus2according to a second embodiment of the present invention.

The hydraulic control apparatus2shown inFIG. 5is different from the hydraulic control apparatus1of the first embodiment in that an auxiliary communication path Y is formed between the wall defining the valve support chamber35and the outer circumferential surface of the flow control valve12. The auxiliary communication path Y includes a groove formed in the wall defining the valve support chamber35and a groove formed in the outer circumferential surface of the flow control valve12. In the second embodiment, like or the same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment.

The operation of the hydraulic control apparatus2will now be described. If the switch valve11is held at the neutral position as shown inFIG. 5, the on-off valve13is held at a closed state with its distal portion13aheld in contact with the valve seat12eas in the case of the first embodiment. A step-like auxiliary valve portion12gis formed on the outer circumferential surface of the flow control valve12and an auxiliary valve seat35gis formed on the wall defining the valve support chamber35. The flow control valve12is urged by the spring17so that the auxiliary valve portion12gcontacts and seated on the auxiliary valve seat35g. In this state, the auxiliary communication path Y is blocked. That is, the flow of hydraulic oil exiting the lift cylinder50is blocked by the contacting portions of on-off valve13and the auxiliary valve portion12gwith the auxiliary valve seat35g. This prevents the lift cylinder50from retracting and thus maintains the fork at a predetermined height.

Switching of the switch valve11from the neutral position to the supply position is the same as that of the first embodiment.

When the switch valve11is switched from the neutral position ofFIG. 5to the drainage position, the hydraulic control apparatus2operates as follows.FIG. 6is a cross-sectional view showing the hydraulic control apparatus2, when the switch valve11is at the drainage position. If the switch valve11is switched from the neutral position to the drainage position, the on-off valve13separates from the valve seat12e, thus opening the communication path X connecting the cylinder side through hole12bwith the switch valve side through hole12c. If the hydraulic pressure in the fluid chamber12h, which is part of the communication path X, rises when the switch valve11is held at the drainage position and the hydraulic fluid is being drained, the urging force acting on the flow control valve12from the fluid chamber12his increased, so that the flow control valve12is moved in a direction contracting the spring17(leftward as viewed in the drawing). This reduces the opening degree α of the restrictor between the cylinder line32and the fluid chamber12h. At this time, the auxiliary valve portion12gis moved together with the flow control valve12, so as to be shifted from the seated state on the auxiliary valve seat35gto a separated state. This opens the auxiliary communication path Y from the shut off state.

When the movement of the flow control valve12is small and the opening degree α of the restrictor is great, the flow rate of fluid flowing through the auxiliary communication path Y is small in comparison with the flow rate of fluid flowing to the fluid chamber12hthrough the cylinder side through hole12b. The flow through the auxiliary communication path Y is substantially maintained to a constant level. Thus, when the movement of the flow control valve12is great and the opening degree α of the restrictor is small, the flow rate of fluid flowing through the auxiliary communication path Y is great in comparison with the flow rate of fluid flowing to the fluid chamber12hthrough the cylinder side through hole12b. Therefore, even if an excessive displacement of the flow control valve12causes the path through the cylinder side through hole12bto be completely blocked, hydraulic oil is drained from the cylinder line32to the switch valve line33through the auxiliary communication path Y at a certain flow rate.

Thus, while the fork is being lowered, drainage from the cylinder line32to the switch valve line33is not stopped. This permits the fork to be smoothly lowered. Further, since the auxiliary valve seat35gis integrally formed with the valve support chamber35, the structure for shutting off the auxiliary communication path Y with the auxiliary valve portion12gis simplified. The structure is thus easily formed.

FIG. 7is a cross-sectional view showing a hydraulic control apparatus3according to a third embodiment of the present invention. The hydraulic control apparatus3shown inFIG. 7is different from the first embodiment in that a damper40is provided at an end of the flow control valve12. Also, an on-off valve43, which has a shape different from that of the on-off valve13of the first embodiment, is provided. Like or the same reference numerals are given to those components that are like or the same as the corresponding components of the first embodiment.

In the hydraulic control apparatus3, the damper40is located at an end of the flow control valve12that is opposite to the back pressure chamber12d, and defines the valve support chamber35. The damper40has an oil chamber35h. The damper40is attached to the flow control valve12so as to be moved as the flow control valve12is moved, and has a first passage40aand a second passage40b, which connect the interior of the oil chamber35hwith the outside. A check valve40cis located in the first passage40a. The check valve40conly permits flow of fluid from the communication path chamber12atoward the oil chamber35h. The second passage40bis an orifice that connects the oil chamber35hwith the switch valve line33and has a great flow resistance.

When fluid flows into the oil chamber35h, fluid flows in through the first passage40aat a low flow resistance. When fluid is drained from the oil chamber35h, the fluid flows out through the second passage40bhaving a great flow resistance since the check valve40cin the first passage40ablocks the flow of the fluid.

When the switch valve11is switched to the drainage position, the flow control valve12is moved, based on the operation of the valve control device14, in a direction to increase the volume of the oil chamber35h, that is, in a direction to reduce the opening degree (leftward as viewed in the drawing). In this case, hydraulic oil flows into the oil chamber35hthrough the first passage40a, which has a small flow resistance. Thus, when the flow control valve12is moved in a direction to reduce the opening degree, the damper40receives a small movement resistance.

In contrast, when the flow control valve12is moved in a direction to reduce the volume of the oil chamber35h, that is, in a direction to increase the opening degree (rightward as viewed in the drawing), the hydraulic oil in the oil chamber35hflows out, at a reduced flow rate, to the switch valve line33through the second passage40b. Thus, when the flow control valve12is moved in a direction to increase the opening degree, the damper40receives a great movement resistance. The movement rate of the flow control valve12is reduced, accordingly.

In this manner, the damper40damps hydraulic pulsation that may be generated through movement of the flow control valve12. Accordingly, when the fork carries an object and is lowered in this state, vibration is prevented from being caused in the object due to the hydraulic pulsation.

The flow resistance of fluid flowing out of the oil chamber35his made greater than the flow resistance of fluid flowing into the oil chamber35hby a simple and easy-to-form configuration of the first passage40a, in which the check valve40cis located, and the second passage40bincluding an orifice.

A groove43ais formed in the outer circumferential surface of the on-off valve43. The groove43acommunicates with the cylinder side through hole12bwhen the communication path X is shut off. The groove43ais defined by a first surface43b, which is perpendicular to the moving direction of the on-off valve43, a second surface43c, which faces and is parallel to the first surface43b, and a bottom43dconnecting the first surface43band the second surface43cto each other. The first surface43breceives a force that urges the on-off valve43in a direction to shut off the communication path X (rightward as viewed in the drawing). The second surface43creceives a force that urges the on-off valve43in a direction to open the communication path X (leftward as viewed in the drawing). The area of the first surface43bis smaller than the area of the second surface43c. A pressure introduction line43eis formed through the groove bottom43d. The pressure introduction line43econnects the cylinder line32to the back pressure chamber12d, thereby exposing the back pressure chamber12dto the pressure of the fluid in the cylinder line32.

In the present embodiment, the first surface43band the second surface43care perpendicular to the movement direction of the on-off valve43. However, the surfaces43b,43cdo not need to be perpendicular to the movement direction as long as the projected area of the first surface43bon a plane the normal line of which agrees with the movement direction of the on-off valve43is smaller than the projected area of the second surface43con the same plane.

Accordingly, in the groove43a, the difference of pressure receiving area in the movement direction of the on-off valve43increases the urging force in a direction to open the communication path X. This urging force acts as resistance against movement when the on-off valve43is moved in a direction to shut off the communication path X.

Also, compared to the case where the on-off valve43is moved in an opening direction, the second surface43c, which projects further outward in the radial direction of the on-off valve43than the first surface43breceives a greater flow resistance in the case where the on-off valve43is moved in the shutting off direction. Accordingly, the on-off valve43can be moved in the shutting off direction at a relatively low speed, which reduces the shock caused by shutting off the communication path X.

The present invention is not limited to the illustrated embodiments, but may be modified in the following forms.

The illustrated embodiments each have been described for a hydraulic control apparatus for actuating the lift cylinder50of the forklift. However, the present invention may be applied to hydraulic control apparatuses for actuating different types of single-acting cylinders other than the lift cylinder50.

The shapes of the valve support chamber35, the flow control valve12, and the on-off valve13do not necessarily have to be those of the illustrated embodiments but may be modified as needed.

The pilot pressure generating portion does not necessarily have to be formed by the pilot line20that introduces the pressure in the tank52into the back pressure chamber12d. The pilot pressure generating portion may be configured in any other suitable manner as long as the pilot pressure lower than the hydraulic pressure in the cylinder line32is generated and applied to the back pressure chamber12d. Also, the switch portion does not necessarily have to be formed by the electromagnetic switch valve21. For example, the pilot pressure generating portion may be formed by a switch valve of a hydraulic pilot type instead of an electromagnetic switch valve. In this case, the valve control apparatus can be switched without using electrical wiring.

The switch valve11is not limited to a manually operated type but may be formed by an electromagnetic proportional control valve. In this case, the hydraulic control apparatus1is formed as an electromagnetic hydraulic control system.