Patent Publication Number: US-8109198-B2

Title: Hydraulic control apparatus

Description:
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. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view showing a hydraulic control apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view explaining the operation of the hydraulic control apparatus of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view explaining the operation of the hydraulic control apparatus of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view explaining the operation of the hydraulic control apparatus of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view showing a hydraulic control apparatus according to a second embodiment of the present invention; 
         FIG. 6  is a cross-sectional view explaining the operation of the hydraulic control apparatus of  FIG. 5 ; and 
         FIG. 7  is a cross-sectional view showing a hydraulic control apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross-sectional view showing a hydraulic control apparatus  1  according to a first embodiment of the invention. The hydraulic control apparatus  1  is employed for actuating a lift cylinder  50  of a forklift, which selectively raises and lowers a fork. The lift cylinder  50  is formed by a single-acting cylinder. The forklift has a lift cylinder control circuit, or a hydraulic circuit in which the lift cylinder  50  is arranged. The hydraulic control apparatus  1  defines a part of the lift cylinder control circuit. The forklift further includes a hydraulic pump  51  and different hydraulic circuits (not shown) including a tilt cylinder control circuit and a power steering system hydraulic circuit. The hydraulic pump  51  supplies hydraulic oil (fluid) to different circuits including the lift cylinder control circuit. The hydraulic oil is then returned from the circuits to a tank  52 , which is provided in the forklift, re-pressurized by the hydraulic pump  51 , and then recirculated to the circuits. 
     As shown in  FIG. 1 , the hydraulic control apparatus  1  includes a valve housing  10 , a switch valve  11 , n flow control valve  12 , an on-off valve  13 , and a valve control device  14 . Different ports and lines are defined in the valve housing  10 , and the switch valve  11 , the flow control valve  12 , the on-off valve  13 , and the valve control device  14  are incorporated in the valve housing  10 . 
     A cylinder port  31  is defined in the valve housing  10  and connected to the lift cylinder  50 , thus defining a supply-drainage port for selectively supplying the hydraulic oil to the lift cylinder  50  and draining the hydraulic oil from the lift cylinder  50 . The valve housing  10  includes a supply line  36 , a first tank line  37 , and a second tank line  38 . The supply line  36  communicates with the hydraulic pump  51  and is supplied with the hydraulic oil from the hydraulic pump  51 . The first and second tank lines  37 ,  38  communicate with the tank  52 . The valve housing  10  further includes a cylinder line  32 , a switch valve line  33 , and a connection passage  34 . The cylinder line  32  is defined continuously from the cylinder port  31  and communicates with the lift cylinder  50  through the cylinder port  31 . The switch valve line  33  communicates with the switch valve  11 . 
     The flow control valve  12  is located in a valve support chamber  35  formed between the cylinder line  32  and the switch valve line  33 , and can be moved along walls defining the valve support chamber  35 . The walls defining the valve support chamber  35  include a cylinder side opening  35   a  and a switch valve side opening  35   b . The cylinder side opening  35   a  opens to the cylinder line  32  and the switch valve side opening  35   b  opens to the switch valve line  33 . A communication path chamber  12   a  is formed in the flow control valve  12 . The communication path chamber  12   a  is a cylindrical space for accommodating the on-off valve  13 . The flow control valve  12  has a cylinder side through hole  12   b  and a switch valve side through hole  12   c . The cylinder side through hole  12   b  selectively connects the communication path chamber  12   a  with the cylinder side opening  35   a . The switch valve side through hole  12   c  selectively connects the communication path chamber  12   a  with the switch valve side opening  35   b . Accordingly, the cylinder line  32  can be connected to the switch valve line  33  through the communication path chamber  12   a  in the flow control valve  12 . 
     In this manner, the flow control valve  12  and the valve support chamber  35  defines a restrictor between the cylinder side through hole  12   b  and the cylinder side opening  35   a . The restrictor changes the opening degree between the cylinder line  32  and the communication path chamber  12   a  in accordance with movement of the flow control valve  12 . The flow control valve  12  has a spring  17  serving as an urging member and a spring support member  18  at an end in the longitudinal direction. The spring  17  urges the flow control valve  12  through the spring support member  18  in a direction to increase the opening degree of the flow control valve  12  (rightward as viewed in the drawing). 
     The on-off valve  13  has a columnar shape so that it can be moved along the inner circumference of the communication path chamber  12   a . The on-off valve  13  divides the communication path chamber  12   a  into a fluid chamber  12   h  and a back pressure chamber  12   d . The switch valve side through holes  12   c  are located in the fluid chamber  12   h . Further, the on-off valve  13  selectively shuts off a communication path X (indicated by arrow X in  FIG. 1 ) between the cylinder side through hole  12   b  and the switch valve side through hole  12   c.    
     As described above, the back pressure chamber  12   d  is a space formed by a valve support chamber  35  and a zone in which the communication path chamber  12   a . The back pressure chamber  12   d  serves as a back pressure chamber of the on-off valve, and also as a back pressure chamber of the flow control valve  12 . 
     A pressure introduction line  13   b  is a through hole formed in the on-off valve  13 . The pressure introduction line  13   b  selectively connects the back pressure chamber  12   d  with the cylinder side through hole  12   b  and the cylinder line  32 , and expose the back pressure chamber  12   d  to the pressure of fluid in the cylinder line  32 . The hydraulic pressure in the back pressure chamber  12   d  is controlled by the valve control device  14  as shown below. 
     Further, the on-off valve  13  has a space defined in it for accommodating a spring  16 , which serves as an urging member. In the back pressure chamber  12   d , the spring  16  is located between the on-off valve  13  and the spring support member  18 . The on-off valve  13  is urged in a direction to shut off the communication path X (rightward as viewed in the drawing) by the spring  16 . A distal portion  13   a  of the on-off valve  13  contacts a valve seat  12   e , which is a step formed in the wall defining the communication path chamber  12   a , so that the communication path X is shut off. 
     The connection passage  34  is defined in such a manner as to permit communication between the cylinder line  32  and the switch valve line  33 . The connection passage  34  is defined separately from a hydraulic oil path (a first line) including the communication path X between the cylinder side through hole  12   b  and the switch valve side through hole  12   c , and serves as a second line connecting the cylinder line  32  to the switch valve line  33 . A check valve  39  is provided between the connection passage  34  and the switch valve line  33 . 
     The switch valve  11  controls supply and drainage of the hydraulic oil with respect to the lift cylinder  50 . The switch valve  11  is formed as a spool valve having a spool  22 , a spool bore  23 , and a spring mechanism  24 . The spool  22  is arranged in the spool bore  23  in an axially movable manner. The spring mechanism  24  maintains the spool  22  at a neutral position. The spool  22  is caused to move axially through manipulation of a non-illustrated lift lever, thus switching the switch valve  11  (more specifically, the spool  22 ) among a supply position, the neutral position, and a drainage position. 
     In  FIG. 1 , the switch valve  11  is held at the neutral position at which the switch valve  11  does not permit either supply or drainage of the hydraulic oil with respect to the lift cylinder  50 . If the spool  22  moves from the neutral position in a direction indicated by arrow A of  FIG. 1 , the switch valve  11  is switched to the supply position. In this state, as will be described later, the hydraulic pump  51  supplies the hydraulic oil to the lift cylinder  50 , that is, a bottom chamber  54  of the lift cylinder  50  (see  FIG. 2 ). Contrastingly, if the spool  22  moves from the neutral position of  FIG. 1  in a direction indicated by arrow B of the drawing, the switch valve  11  is switched to the drainage position. In this state, the hydraulic oil is drained from the lift cylinder  50  to the tank  52  (see  FIG. 3 ). The spool  22  includes a first land portion  22   a  having a relatively small diameter and a second land portion  22   b , which are formed in two axial portions of the spool  22 . 
     The on-off valve  13 , 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 valve  13  that faces the back pressure chamber  12   d  due to the force of the spring  16  and the hydraulic pressure acting on the back pressure chamber  12   d . The second urging force is generated due to hydraulic pressure acting on an end face  13   c  of the on-off valve  13  that faces the fluid chamber  12   h . If the first urging force is greater than the second urging force, the on-off valve  13  is maintained in contact with the valve seat  12   e . In contrast, if the second urging force is greater than the first urging force, the on-off valve  13  is shifted to an open state. 
     Since the fluid chamber  12   h , in which the end face  13   c  of the on-off valve  13  is located, communicates with the switch valve line  33  through the switch valve side through hole  12   c , the end face  13   c  of the on-off valve  13  is exposed to a hydraulic pressure that is substantially the same as the hydraulic pressure of the switch valve line  33 . 
     In a state where the on-off valve  13  opens the communication path X, the flow control valve  12 , 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 spring  17  through the spring support member  18  and the urging force due to the hydraulic pressure acting on the end face of the flow control valve  12  in the back pressure chamber  12   d . Also, the flow control valve  12  receives, 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 chamber  12   h . Further, the spring support member  18  receives an urging force that corresponds to the difference in hydraulic pressure between the zones defined by the on-off valve  13 , that is, the difference in hydraulic pressure between the back pressure chamber  12   d  and the fluid chamber  12   h . The flow control valve  12  is maintained at a position where these urging forces are in equilibrium. 
     In a state where the on-off valve  13  opens the communication path X, when the hydraulic pressure of the fluid chamber  12   h  and the switch valve line  33  is increased, the urging force that acts on the flow control valve  12  and the on-off valve  13 , or back pressure chamber  12   d  is increased. The urging force acting on the on-off valve  13  is transmitted to the spring support member  18  through the spring  16 . Alternatively, when the on-off valve  13  contacts the spring support member  18 , the urging force is transmitted to the spring support member  18  through the spring  16  and the on-off valve  13 . Also, the urging force acting on the flow control valve  12  is transmitted to the spring support member  18 . Accordingly, the spring  17  is contracted by the spring support member  18 , and the flow control valve  12  is moved toward the back pressure chamber  12   d  (leftward as viewed in the drawing) until the elastic force of the spring  17  and the above described urging force are in equilibrium. This reduces the opening degree of the restrictor between the cylinder side through hole  12   b  and the cylinder side opening  35   a . In this manner, the flow control valve  12  is moved in accordance with the hydraulic pressure of the switch valve line  33 . 
     The valve control device  14  controls operation of the flow control valve  12  and the on-off valve  13 , and, as shown in  FIG. 1 , includes a pilot line  20  and an electromagnetic switch valve  21 . 
     The pilot line  20  is defined in the valve housing  10  as a passage that connects the back pressure chamber  12   d  of the flow control valve  12  and the on-off valve  13  to the tank  52  in correspondence with switching of the electromagnetic switch valve  21 . The pilot line  20  defines a pilot pressure generating portion that generates pilot pressure lower than the hydraulic pressure in the cylinder line  32  and applies the hydraulic pressure to the back pressure chamber  12   d . The pilot line  20  has an opening  20   a  communicating with the spool bore  23  of the switch valve  11 . If the spool  22  is moved in the direction indicated by arrow B of  FIG. 1 , the switch valve  11  is switched to the drainage position of  FIG. 3 . In this state, a second land portion  22   b  of the spool  22  corresponds to the opening  20   a  and thus the pilot line  20  is connected to a second tank line  38  through the spool bore  23 . 
     In the opening  20   a  of the pilot line  20 , only the portion corresponding to the second land portion  22   b  functions as a portion that is permitted to communicate with the second tank line  38 . In other words, as the spool  22  moves in the direction indicated by arrow B of  FIG. 1 , the area of the portion of the opening  20   a  corresponding to the second land portion  22   b  gradually increases. The communication area (the opening degree) of the passage between the pilot line  20  and the second tank line  38  thus gradually increases, correspondingly. 
     The electromagnetic switch valve  21  is formed by an electromagnetic valve that is switched for selectively connecting and disconnecting the back pressure chamber  12   d  of the flow control valve  12  and the on-off valve  13  to and from the pilot line  20 . The electromagnetic switch valve  21  is excited or de-excited by a non-illustrated controller that detects the operational state of a limit switch  25  incorporated in the valve housing  10 . When the switch valve  11  is held at the neutral position or the supply position, the electromagnetic switch valve  21  disconnects the back pressure chamber  12   d  from the pilot line  20  (see  FIGS. 1 and 2 ). Contrastingly, if the switch valve  11  is held at the drainage position, the electromagnetic switch valve  21  connects the back pressure chamber  12   d  to the pilot line  20  (see  FIGS. 3 and 4 ). When the back pressure chamber  12   d  is disconnected from the pilot line  20 , the hydraulic pressure in the cylinder line  32 , which is introduced through the pressure introduction line  13   b  of the on-off valve  13 , is applied to the back pressure chamber  12   d  through the pressure introduction line  14   c  of the valve body  14 . In contrast, when the back pressure chamber  12   d  is connected to the pilot line  20 , the hydraulic pressure in the second tank line  38 , which is the aforementioned pilot pressure lower than the hydraulic pressure in the cylinder line  32 , is applied to the back pressure chamber  12   d  through the pilot line  20 . That is, the electromagnetic switch valve  21  serving as a switch portion operates to apply the hydraulic pressure in the cylinder line  32  to the back pressure chamber  12   d  when the switch valve  11  is held at the neutral or supply positions. The electromagnetic switch valve  21  operates to apply the pilot pressure to the back pressure chamber  12   d  when the switch valve  11  is maintained at the drainage position. 
     When the hydraulic pressure in the cylinder line  32  is applied to the back pressure chamber  12   d , the on-off valve  13  is urged toward the valve seat  12   e  in such a manner as to disconnect the cylinder line  32  from the switch valve line  33 . In contrast, if the pilot pressure, which is lower than the hydraulic pressure in the cylinder line  32 , is applied to the back pressure chamber  12   d , the on-off valve  13  is spaced from the valve seat  12   e  in such a manner as to connect the cylinder line  32  to the switch valve line  33 . In this state, the flow control valve  12  moves in correspondence with the hydraulic pressure in the switch valve line  33 , thus adjusting the opening degree of the restrictor between the cylinder side through hole  12   b  and the cylinder side opening  35   a.    
     Next, the operation of the hydraulic control apparatus  1  will be explained. If the switch valve  11  is held at the neutral position as shown in  FIG. 1 , the spool  22  is located in such a manner as to disconnect the supply line  36  and the first tank line  37  from the switch valve line  33 . Therefore, the hydraulic oil is neither supplied to nor drained from the switch valve line  33 . Further, in this state, the electromagnetic switch valve  21  operates to disconnect the back pressure chamber  12   d  of the on-off valve  13  from the pilot line  20 . The hydraulic pressure in the cylinder line  32  is thus introduced into the back pressure chamber  12   d  via the pressure introduction line  13   b . At this stage, the first urging force generated by the hydraulic pressure in the cylinder line  32  and the spring  16  is greater than the second urging force generated by the hydraulic pressure in the switch valve line  33 , the distal portion  13   a  of the on-off valve  13  is caused to contact the valve seat  12   e . This maintains the cylinder line  32  in a state disconnected from the switch valve line  33 . Likewise, the flow control valve  12  is maintained in a state where its stepped portion  12   f  contacts a projection  35   f  on the wall defining the valve support chamber  35 . In other words, the on-off valve  13  blocks the flow of the hydraulic oil in a direction in which the hydraulic oil is drained from the lift cylinder  50 . This prevents the lift cylinder  50  from retracting (i.e., from lowering due to the own weight) and thus maintains the fork at a predetermined height. Further, the connection passage  34  extending from the cylinder line  32  to the switch valve line  33  is blocked by the check valve  39 . 
     When the switch valve  11  is switched from the neutral position to the supply position, the hydraulic control apparatus  1  operates in the following manner.  FIG. 2  shows the hydraulic control apparatus  1  in which the switch valve  11  is held at the supply position. If the switch valve  11  is switched from the neutral position to the supply position, the spool  22  moves in the direction indicated by arrow A of  FIG. 1 . Thus, after having been supplied from the pump  51  to the supply line  36 , the hydraulic oil is introduced into the switch valve line  33  via a communication passage  36   a  and a passage defined between the first land portion  22   a  of the spool  22  and a corresponding wall of the spool bore  23  as indicated by the corresponding arrows of  FIG. 2 . In this state, the first tank line  37  is held in a state disconnected from the switch valve line  33 . This raises the hydraulic pressure in the switch valve line  33 , thus applying a correspondingly increased urging force to the check valve  39 . When this urging force exceeds the urging force acting on the check valve  39  generated by the spring and the hydraulic pressure in the cylinder line  32 , the check valve  39  becomes open. This connects the switch valve line  33  to the cylinder line  32  through the connection passage  34 , thus sending the hydraulic oil to the cylinder line  32 . The hydraulic oil is then supplied to the lift cylinder  50  and thus raises the fork. In this state, the electromagnetic switch valve  21  maintains the pilot line  20  in a state disconnected from the back pressure chamber  12   d . Therefore, the first urging force generated by the hydraulic pressure in the back pressure chamber  12   d  and the spring  16  is greater than the second urging force generated by the hydraulic pressure in the switch valve line  33 . The on-off valve  13  is thus maintained closed. Likewise, the flow control valve  12  is maintained in a state where its stepped portion  12   f  contacts a projection  35   f  on the wall defining the valve support chamber  35 . 
     When the switch valve  11  is switched from the neutral position of  FIG. 1  to the drainage position, the hydraulic control apparatus  1  operates as follows.  FIG. 3  shows the hydraulic control apparatus  1  in which the switch valve  11  is held at the drainage position, that is, the on-off valve  13  is moved.  FIG. 4  shows the hydraulic control apparatus  1  in which the flow control valve  12  is moved together with the movement of the on-off valve  13 . If the switch valve  11  is switched from the neutral position to the drainage position, the spool  22  moves in the direction indicated by arrow B of  FIG. 1 . The switch valve line  33  is thus connected to the first tank line  37  through a passage defined between the first land portion  22   a  of the spool  22  and the corresponding wall of the spool bore  23 . 
     Further, if the switch valve  11  is switched to the drainage position, the limit switch  25  generates a detection signal. In response to the detection signal, the controller (not shown) switches the electromagnetic switch valve  21  in such a manner as to connect the pilot line  20  to the back pressure chamber  12   d . The hydraulic oil is thus sent from the back pressure chamber  12   d  to the pilot line  20 . 
     Meanwhile, in correspondence with the movement of the spool  22 , the second land portion  22   b  reaches a position corresponding to the opening  20   a  of the pilot line  20 . As the spool  22  further moves, the portion of the opening  20   a  blocked by the spool  22  becomes gradually smaller and, in contrast, the portion of the opening  20   a  corresponding to the second land portion  22   b  becomes gradually larger. Accordingly, the communication area (the opening degree) of the passage between the pilot line  20  and the second tank line  38  gradually increases, thus increasing the flow rate of the hydraulic oil from the pilot line  20  to the second tank line  38 , correspondingly. Once the opening  20   a  entirely corresponds to the second land portion  22   b , the communication state of the pilot line  20  with respect to the second tank line  38  is maintained without changing. 
     When the switch valve  11  is switched to the drainage position, the hydraulic oil flows from the back pressure chamber  12   d  to the second tank line  38  through the pilot line  20  as indicated by the corresponding arrows of  FIG. 3 . This lowers the pressure in the back pressure chamber  12   d . In other words, the pilot pressure lower than the hydraulic pressure in the cylinder line  32  acts in the back pressure chamber  12   d . Therefore, the second urging force generated by the hydraulic pressure in the fluid chamber  12   h  becomes greater than the first urging force generated by the hydraulic pressure in the back pressure chamber  12   d  and the spring  16 . This causes the on-off valve  13  to separate from the valve seat  12   e , thus opening the communication path X between the cylinder side through hole  12   b  and the switch valve side through hole  12   c . The hydraulic oil thus flows from the lift cylinder  50  to the switch valve line  33  via the cylinder line  32  and the communication path X. The hydraulic fluid is then sent from the first tank line  37  to the tank  52 , thus lowering the fork. 
     Further, if the hydraulic pressure in the switch valve line  33  changes when the switch valve  11  is held at the drainage position and the hydraulic fluid flows out of the lift cylinder  50  as shown in  FIG. 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 chamber  12   d  and the spring  17 , and the second urging force, which is generated by the hydraulic pressure in the fluid chamber  12   h , is quickly cancelled, which displaces the flow control valve  12 . This changes the opening degree α of the restrictor between the cylinder side through hole  12   b  and the cylinder side opening  35   a.    
     As a result, the flow rate of the hydraulic oil from the cylinder line  32  to the fluid chamber  12   h  is changed, so that the hydraulic pressure of oil flowing from the switch valve side through hole  12   c  to the switch valve line  33  is adjusted. In this manner, the lowering speed of the fork is adjusted (pressure compensation function). 
     As has been described, when the switch valve  11  is held at the neutral position in the hydraulic control apparatus  1  of the first embodiment, the hydraulic pressure in the cylinder line  32  is applied to the back pressure chamber  12   d  of the on-off valve  13  for urging the on-off valve  13  in such a manner as to disconnect the cylinder line  32  from the switch valve line  33 . Therefore, with the switch valve  11  held at the neutral position, the on-off valve  13  is maintained in a state in which the cylinder line  32  is disconnected from the switch valve line  33 . This restricts the drainage of the hydraulic oil from the lift cylinder  50  and thus retracting motion of the lift cylinder  50 . That is, as long as the switch valve  11  is maintained at the neutral position, the flow control valve  12 , in which the on-off valve  13  is provided, functions as an operated check valve. 
     If the switch valve  11  is switched from the neutral position to the drainage position, the pilot pressure lower than the hydraulic pressure in the cylinder line  32  is applied to the back pressure chamber  12   d  of the on-off valve  13 . This reduces the urging force applied from the back pressure chamber  12   d  to the on-off valve  13 , thus switching the on-off valve  13  from a closed state to an open state, or to a state allowing the cylinder line  32  and the communication path X to communicate with each other. The hydraulic oil is thus drained from the lift cylinder  50  to the tank  52 . With the switch valve  11  held at the drainage position, the flow control valve  12  is permitted to move in the valve support chamber  35  in correspondence with change of the hydraulic pressure in the switch valve line  33 . In correspondence with the movement of the flow control valve  12 , the opening degree of the restrictor provided between the cylinder line  32  and the fluid chamber  12   h  changes. Accordingly, the flow control valve  12 , in which the on-off valve  13  is provided, functions also as a flow regulator for adjusting the flow rate of the fluid drained from the lift cylinder  50 . 
     That is, since the on-off valve  13  serving as a flow regulator is located inside the flow control valve  12  serving as an operated check valve, the flow control valve  12  serves 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 apparatus  1 . 
     Further, the on-off valve  13  can shut off communication path X independently of movement of the flow control valve  12 . That is, the shutting off operation is hardly influenced by changes in the opening degree of the flow control valve  12 . Therefore, in the case where the communication path X stops drainage while being narrowed by the flow control valve  12 , the lowering motion of the fork by the lift cylinder  50  can be stopped by shutting off the communication path X by the on-off valve  13  without maximizing the opening degree of the flow control valve  12 . Thus, when stopping the drainage, the flow rate of fluid is prevented from being instantly increased, and the lift cylinder  50  is stopped in a stable manner. 
     If the hydraulic pressure in the fluid chamber  12   h , which is part of the communication path X, rises when the switch valve  11  is held at the drainage position and the hydraulic fluid is drained from the lift cylinder  50 , the opening degree of the restrictor of the flow control valve  12  decreases and the hydraulic pressure in the switch valve line  33  drops. The flow rate of the hydraulic oil drained from the lift cylinder  50  is thus adjusted in a predetermined range. That is, the lowering speed of the fork is adjusted correspondingly (the pressure compensation function). 
     Since the valve seat  12   e  with which the on-off valve  13  is held in contact is integrally formed with the communication path chamber  12   a , the configuration of the on-off valve  13 , which is used for shutting off and opening the communication path X becomes further simple. 
     The pressure introduction line  13   b  is defined in the on-off valve  13 . Therefore, when the switch valve  11  is held at the neutral or supply positions, the hydraulic pressure is supplied from the cylinder line  32  to the back pressure chamber  12   d  by means of a relatively simple structure. 
     The valve control device  14  is formed by the pilot line (the pilot pressure generating portion)  20  and the electromagnetic switch valve (the switch portion)  21 , which cooperates with each other. By operating the electromagnetic switch valve  21  with the pilot line  20  maintained in a state generating the pilot pressure, the pilot pressure is quickly supplied to the back pressure chamber  12   d  in response to such operation. This improves the response of the on-off valve  13 . 
     Further, the pilot pressure generating portion for generating the pilot pressure lower than the hydraulic pressure in the cylinder line  32  is relatively easily provided simply by defining the pilot line  20 , which connects the back pressure chamber  12   d  to the tank  52 . This permits the flow control valve  12  to operate in such a manner that the difference between the hydraulic pressure in the switch valve line  33  upstream from the switch valve  11  and the hydraulic pressure in the second tank line  38  (the tank  52 ) downstream from the switch valve  11  is 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 valve  11  (the pressure compensation function). 
     When the switch valve  11  is switched to the drainage position, the portion of the opening  20   a  corresponding to the second land portion  22   b  becomes gradually larger in correspondence with the movement of the spool  22  in the spool bore  23 . This gradually changes the communication state of the back pressure chamber  12   d  with respect to the tank  52 . Therefore, at an initial stage of switching of the switch valve  11  to the drainage position, the opening degree of the on-off valve  13  gradually increases, thus permitting the fork to be finely controlled when being lowered. These advantages are brought about simply by forming the second land portion  22   b  in the spool  22  and connecting the pilot line  20  to the spool bore  23  through the opening  20   a.    
     Further, since the hydraulic oil leaking from the electromagnetic switch valve  21 , which is arranged between the back pressure chamber  12   d  and the pilot line  20 , is extremely small, leakage of the hydraulic oil from the electromagnetic switch valve  21  to the tank  52  is suppressed. Therefore, when the switch valve  11  is held at the neutral position, the retraction of the lift cylinder  50  is suppressed, thus preventing the fork from lowering due to the weight of the fork. 
     When the switch valve  11  is switched to the supply position, the hydraulic oil is supplied from the switch valve line  33  to the cylinder line  32  through the connection passage  34 , which is different from the communication path X. This simplifies the configuration of the connection passage  34 , thus decreasing the pressure loss caused through the supply of the hydraulic oil to the lift cylinder  50 . 
       FIG. 5  is a cross-sectional view showing a hydraulic control apparatus  2  according to a second embodiment of the present invention. 
     The hydraulic control apparatus  2  shown in  FIG. 5  is different from the hydraulic control apparatus  1  of the first embodiment in that an auxiliary communication path Y is formed between the wall defining the valve support chamber  35  and the outer circumferential surface of the flow control valve  12 . The auxiliary communication path Y includes a groove formed in the wall defining the valve support chamber  35  and a groove formed in the outer circumferential surface of the flow control valve  12 . 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 apparatus  2  will now be described. If the switch valve  11  is held at the neutral position as shown in  FIG. 5 , the on-off valve  13  is held at a closed state with its distal portion  13   a  held in contact with the valve seat  12   e  as in the case of the first embodiment. A step-like auxiliary valve portion  12   g  is formed on the outer circumferential surface of the flow control valve  12  and an auxiliary valve seat  35   g  is formed on the wall defining the valve support chamber  35 . The flow control valve  12  is urged by the spring  17  so that the auxiliary valve portion  12   g  contacts and seated on the auxiliary valve seat  35   g . In this state, the auxiliary communication path Y is blocked. That is, the flow of hydraulic oil exiting the lift cylinder  50  is blocked by the contacting portions of on-off valve  13  and the auxiliary valve portion  12   g  with the auxiliary valve seat  35   g . This prevents the lift cylinder  50  from retracting and thus maintains the fork at a predetermined height. 
     Switching of the switch valve  11  from the neutral position to the supply position is the same as that of the first embodiment. 
     When the switch valve  11  is switched from the neutral position of  FIG. 5  to the drainage position, the hydraulic control apparatus  2  operates as follows.  FIG. 6  is a cross-sectional view showing the hydraulic control apparatus  2 , when the switch valve  11  is at the drainage position. If the switch valve  11  is switched from the neutral position to the drainage position, the on-off valve  13  separates from the valve seat  12   e , thus opening the communication path X connecting the cylinder side through hole  12   b  with the switch valve side through hole  12   c . If the hydraulic pressure in the fluid chamber  12   h , which is part of the communication path X, rises when the switch valve  11  is held at the drainage position and the hydraulic fluid is being drained, the urging force acting on the flow control valve  12  from the fluid chamber  12   h  is increased, so that the flow control valve  12  is moved in a direction contracting the spring  17  (leftward as viewed in the drawing). This reduces the opening degree α of the restrictor between the cylinder line  32  and the fluid chamber  12   h . At this time, the auxiliary valve portion  12   g  is moved together with the flow control valve  12 , so as to be shifted from the seated state on the auxiliary valve seat  35   g  to a separated state. This opens the auxiliary communication path Y from the shut off state. 
     When the movement of the flow control valve  12  is 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 chamber  12   h  through the cylinder side through hole  12   b . The flow through the auxiliary communication path Y is substantially maintained to a constant level. Thus, when the movement of the flow control valve  12  is 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 chamber  12   h  through the cylinder side through hole  12   b . Therefore, even if an excessive displacement of the flow control valve  12  causes the path through the cylinder side through hole  12   b  to be completely blocked, hydraulic oil is drained from the cylinder line  32  to the switch valve line  33  through the auxiliary communication path Y at a certain flow rate. 
     Thus, while the fork is being lowered, drainage from the cylinder line  32  to the switch valve line  33  is not stopped. This permits the fork to be smoothly lowered. Further, since the auxiliary valve seat  35   g  is integrally formed with the valve support chamber  35 , the structure for shutting off the auxiliary communication path Y with the auxiliary valve portion  12   g  is simplified. The structure is thus easily formed. 
       FIG. 7  is a cross-sectional view showing a hydraulic control apparatus  3  according to a third embodiment of the present invention. The hydraulic control apparatus  3  shown in  FIG. 7  is different from the first embodiment in that a damper  40  is provided at an end of the flow control valve  12 . Also, an on-off valve  43 , which has a shape different from that of the on-off valve  13  of 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 apparatus  3 , the damper  40  is located at an end of the flow control valve  12  that is opposite to the back pressure chamber  12   d , and defines the valve support chamber  35 . The damper  40  has an oil chamber  35   h . The damper  40  is attached to the flow control valve  12  so as to be moved as the flow control valve  12  is moved, and has a first passage  40   a  and a second passage  40   b , which connect the interior of the oil chamber  35   h  with the outside. A check valve  40   c  is located in the first passage  40   a . The check valve  40   c  only permits flow of fluid from the communication path chamber  12   a  toward the oil chamber  35   h . The second passage  40   b  is an orifice that connects the oil chamber  35   h  with the switch valve line  33  and has a great flow resistance. 
     When fluid flows into the oil chamber  35   h , fluid flows in through the first passage  40   a  at a low flow resistance. When fluid is drained from the oil chamber  35   h , the fluid flows out through the second passage  40   b  having a great flow resistance since the check valve  40   c  in the first passage  40   a  blocks the flow of the fluid. 
     When the switch valve  11  is switched to the drainage position, the flow control valve  12  is moved, based on the operation of the valve control device  14 , in a direction to increase the volume of the oil chamber  35   h , 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 chamber  35   h  through the first passage  40   a , which has a small flow resistance. Thus, when the flow control valve  12  is moved in a direction to reduce the opening degree, the damper  40  receives a small movement resistance. 
     In contrast, when the flow control valve  12  is moved in a direction to reduce the volume of the oil chamber  35   h , that is, in a direction to increase the opening degree (rightward as viewed in the drawing), the hydraulic oil in the oil chamber  35   h  flows out, at a reduced flow rate, to the switch valve line  33  through the second passage  40   b . Thus, when the flow control valve  12  is moved in a direction to increase the opening degree, the damper  40  receives a great movement resistance. The movement rate of the flow control valve  12  is reduced, accordingly. 
     In this manner, the damper  40  damps hydraulic pulsation that may be generated through movement of the flow control valve  12 . 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 chamber  35   h  is made greater than the flow resistance of fluid flowing into the oil chamber  35   h  by a simple and easy-to-form configuration of the first passage  40   a , in which the check valve  40   c  is located, and the second passage  40   b  including an orifice. 
     A groove  43   a  is formed in the outer circumferential surface of the on-off valve  43 . The groove  43   a  communicates with the cylinder side through hole  12   b  when the communication path X is shut off. The groove  43   a  is defined by a first surface  43   b , which is perpendicular to the moving direction of the on-off valve  43 , a second surface  43   c , which faces and is parallel to the first surface  43   b , and a bottom  43   d  connecting the first surface  43   b  and the second surface  43   c  to each other. The first surface  43   b  receives a force that urges the on-off valve  43  in a direction to shut off the communication path X (rightward as viewed in the drawing). The second surface  43   c  receives a force that urges the on-off valve  43  in a direction to open the communication path X (leftward as viewed in the drawing). The area of the first surface  43   b  is smaller than the area of the second surface  43   c . A pressure introduction line  43   e  is formed through the groove bottom  43   d . The pressure introduction line  43   e  connects the cylinder line  32  to the back pressure chamber  12   d , thereby exposing the back pressure chamber  12   d  to the pressure of the fluid in the cylinder line  32 . 
     In the present embodiment, the first surface  43   b  and the second surface  43   c  are perpendicular to the movement direction of the on-off valve  43 . However, the surfaces  43   b ,  43   c  do not need to be perpendicular to the movement direction as long as the projected area of the first surface  43   b  on a plane the normal line of which agrees with the movement direction of the on-off valve  43  is smaller than the projected area of the second surface  43   c  on the same plane. 
     Accordingly, in the groove  43   a , the difference of pressure receiving area in the movement direction of the on-off valve  43  increases the urging force in a direction to open the communication path X. This urging force acts as resistance against movement when the on-off valve  43  is moved in a direction to shut off the communication path X. 
     Also, compared to the case where the on-off valve  43  is moved in an opening direction, the second surface  43   c , which projects further outward in the radial direction of the on-off valve  43  than the first surface  43   b  receives a greater flow resistance in the case where the on-off valve  43  is moved in the shutting off direction. Accordingly, the on-off valve  43  can 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 cylinder  50  of 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 cylinder  50 . 
     The shapes of the valve support chamber  35 , the flow control valve  12 , and the on-off valve  13  do 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 line  20  that introduces the pressure in the tank  52  into the back pressure chamber  12   d . 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 line  32  is generated and applied to the back pressure chamber  12   d . Also, the switch portion does not necessarily have to be formed by the electromagnetic switch valve  21 . 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 valve  11  is not limited to a manually operated type but may be formed by an electromagnetic proportional control valve. In this case, the hydraulic control apparatus  1  is formed as an electromagnetic hydraulic control system.