Patent Abstract:
A flow control valve includes a pressure compensating valve and a first switching valve. The pressure compensating valve is configured to enlarge an opening area of a variable orifice between a load pressure line and a compensating pressure line when a pressure of working fluid of the compensating pressure line is smaller than a first set pressure, and narrow the opening area of the variable orifice when the pressure of the working fluid of the compensating pressure line is larger than the first set pressure. The first switching valve is configured to switch between a meter-out operation and a neutral operation by an external operation, wherein the working fluid of the compensating pressure line is drained in the meter-out operation, the working fluid of the compensating pressure line is not drained in the neutral operation. The load pressure line guides the working fluid to be supplied to an actuator.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a flow control valve. More particularly, the present invention relates to a flow control valve for controlling the oil pressure applied to a hydraulic actuator.  
         [0003]     2. Description of the Related Art  
         [0004]     A forklift for driving a fork that holds a load by using oil pressure has been well known. The forklift includes a lift cylinder for driving the fork that holds a load along with a flow control valve.  FIG. 1  is a schematic view showing the conventional flow control valve. The flow control valve  101  includes a direction switching valve  102  and a check valve  103 . The flow control valve  101  further includes a plurality of lines for guiding hydraulic oil to transmit oil pressure. The plurality of lines is composed of a pump pressure line  111 , a pump pressure line  112 , a load pressure line  113  and a drain line  115 .  
         [0005]     The pump pressure line  111  connects the direction switching valve  102  to a pump not shown and leads hydraulic oil supplied by the pump. The pump pressure line  112  connects a check valve  103  to the direction switching valve  102 . The load pressure line  113  connects the check valve  103 , the lift cylinder  104  and the direction switching valve  102 . The drain line  115  connects the direction switching valve  102  to a tank  106  and the oil pressure of the drain line  115  is substantially zero (0).  
         [0006]     The check valve  103  prevents hydraulic oil from flowing from the load pressure line  113  to the pump pressure line  112 . That is, the check valve  103  connects the pump pressure line  112  to the load pressure line  113  when the oil pressure of the pump pressure line  112  is larger than that of the load pressure line  113 , and does not connect the pump pressure line  112  to the load pressure line  113  when the oil pressure of the load pressure line  113  is larger than that of the pump pressure line  112 .  
         [0007]     The lift cylinder  104  is an actuator for lifting and lowering the fork of the forklift. That is, the lift cylinder  104  lifts the fork of the forklift when hydraulic oil is supplied from the load pressure line  113  and lowers the fork of the forklift when hydraulic oil is discharged into the load pressure line  113 . At this time, the oil pressure of the load pressure line  113  varies depending on the weight of a load held by the fork of the forklift and becomes larger as the load is heavier.  
         [0008]     The direction switching valve  102  can occupy one of a neutral position, a meter-in position and a meter-out position. That is, operated by the user, the direction switching valve  102  is switched from the neutral position to the meter-in position, from the neutral position to the meter-out position, from the meter-in position to the neutral position and from the meter-out position to the neutral position.  
         [0009]     At the meter-in position, the direction switching valve  102  connects the pump pressure line  111  to the pump pressure line  112 , closes the load pressure line  113  and closes the drain line  115 . At the meter-out position, the direction switching valve  102  closes the pump pressure line  111 , closes the pump pressure line  112  and connects the load pressure line  113  to the drain line  115 . At the neutral position, the direction switching valve  102  closes the pump pressure line  111 , closes the pump pressure line  112 , closes the load pressure line  113  and closes the drain line  115 .  
         [0010]     The tank  106  stores hydraulic oil flowing through the drain line  115  therein. The hydraulic oil stored in the tank  106  is supplied to the pump pressure line  111  by a pump not shown.  
         [0011]     Operations of the flow control valve  101  include a meter-in operation, a neutral operation and a meter-out operation. The meter-in operation is an operation performed when the direction switching valve  102  is switched from the neutral position to the meter-in position by means of the user&#39;s operation. The neutral operation is an operation performed when the direction switching valve  102  is switched from the meter-in position or the meter-out position to the neutral position by means of the user&#39;s operation. The meter-out operation is an operation performed when the direction switching valve  102  is switched from the neutral position to the meter-out position by means of the user&#39;s operation.  
         [0012]     In the meter-in operation, hydraulic oil is supplied from the pump pressure line  111  to the lift cylinder  104  through the direction switching valve  102 , the pump pressure line  112 , the check valve  103  and the load pressure line  113 . When the hydraulic oil is supplied, the lift cylinder  104  lifts the fork.  
         [0013]     In the neutral operation, since the switching valve  102  closes connection between the pump pressure line  111  and the pump pressure line  112  and between the load pressure line  113  and the drain line  115 , no hydraulic oil of the lift cylinder  104  is supplied or discharged and thus lifting or lowering of the fork is stopped. At this time, the load pressure varies depending on a load held by the fork of the forklift and becomes larger as the load is heavier.  
         [0014]     In the meter-out operation, hydraulic oil is discharged from the lift cylinder  104  to the drain line  115  through the load pressure line  113  and the direction switching line  102 . When the hydraulic oil is discharged, the lift cylinder  104  lowers the fork.  
         [0015]     Even when the operation quantity of the direction switching valve  102  is identical, the higher the oil pressure of the load pressure line  113  is, the higher the hydraulic oil flows from the load pressure line  113  to the drain line  115 . That is, in the forklift to which the flow control valve  101  is applied, even with the same operation quantity, the heavier the held load is, the faster the folk is lowered. A forklift with a fork having high operability has been desired.  
         [0016]     In conjunction with the above description, Japanese Laid-Open Patent Application JP-A-Heisei, 08-100804 discloses a pressure compensating valve which only varies a set pressure of a relief valve without exchanging a piston, etc. The pressure compensating valve is characterized by including: a valve for opening and closing an inlet port and an outlet port; a piston for pressing the valve in the closing direction with a load pressure within a pressure chamber; an intermediate pressure chamber connected to the inlet port through a small cavity for pressing the valve in the closing direction; and a variable set pressure relief valve for relieving pressure oil in the intermediate pressure chamber to the outlet port through the small cavity.  
       SUMMARY OF THE INVENTION  
       [0017]     An object of the present invention is to provide a flow control valve which improves operability of a hydraulic actuator.  
         [0018]     Another object of the present invention is to provide a flow control valve which reduces the influence of a load of a hydraulic actuator.  
         [0019]     Still another object of the present invention is to provide a flow control valve which reduces hunting of the operation of a hydraulic actuator.  
         [0020]     Yet still another object of the present invention is to provide a flow control valve which reduces shock of the operation of a hydraulic actuator.  
         [0021]     It is also an object of the present invention to provide a forklift which improves operability of a fork.  
         [0022]     This and other objects, features and advantages of the present invention will be readily ascertained by referring to the following description and drawings.  
         [0023]     In order to achieve an aspect of the present invention, the present invention provides a flow control valve comprising a pressure compensating valve and a first switching valve. The pressure compensating valve is configured to enlarge an opening area of a variable orifice between a load pressure line and a compensating pressure line when a pressure of working fluid of said compensating pressure line is smaller than a first set pressure, and narrow said opening area of said variable orifice when said pressure of said working fluid of said compensating pressure line is larger than said first set pressure. The first switching valve is configured to switch between a meter-out operation and a neutral operation by an external operation, wherein said working fluid of said compensating pressure line is drained in said meter-out operation, said working fluid of said compensating pressure line is not drained in said neutral operation. Said load pressure line guides said working fluid to be supplied to an actuator.  
         [0024]     The flow control valve may further comprises a relief valve configured to drain said working fluid of said compensating pressure line when said pressure of said working fluid of said compensating pressure line is larger than a second set pressure, and configured not to drain said working fluid of said compensating pressure line when said pressure of said working fluid of said compensating pressure line is smaller than said second set pressure.  
         [0025]     In the flow control valve, said first switching valve may switch among a meter-in operation, a meter-out operation and a neutral operation by an external operation. Working fluid may be supplied to said load pressure line in said meter-in operation for operating said actuator.  
         [0026]     In the flow control valve, said relief valve may not be connected to said compensating pressure line when said first switching valve is in said meter-in operation.  
         [0027]     In the flow control valve, said first switching valve may include a first spool chamber and a first spool configured to be slidably inserted into said first spool chamber. Said relief valve may include a second spool chamber configured to be formed in said first spool and a second spool configured to be slidably inserted into said second spool chamber.  
         [0028]     The flow control valve may further comprise a second switching valve configured to connect said compensating pressure line to said relief valve when said first switching valve is in said neutral operation and said meter-out operation, and configured not to connect said compensating pressure line to said relief valve when said first switching valve is in said meter-in operation.  
         [0029]     In the flow control valve, said first switching valve may switch among a meter-in operation, a meter-out operation and a neutral operation by an external operation. Working fluid may be supplied to said load pressure line in said meter-in operation for operating said actuator.  
         [0030]     In order to achieve another aspect of the present invention, the present invention provides a forklift comprising a flow control valve, a fork configured to lift a load and an actuator configured to be connected between said flow control valve and said fork. Said flow control valve includes a pressure compensating valve and a first switching valve. The pressure compensating valve is configured to enlarge an opening area of a variable orifice between a load pressure line and a compensating pressure line when a pressure of working fluid of said compensating pressure line is smaller than a first set pressure, and narrow said opening area of said variable orifice when said pressure of said working fluid of said compensating pressure line is larger than said first set pressure. The first switching valve is configured to switch between a meter-out operation and a neutral operation by an external operation, wherein said working fluid of said compensating pressure line is drained in said meter-out operation, said working fluid of said compensating pressure line is not drained in said neutral operation. The load pressure line guides said working fluid to be supplied to said actuator.  
         [0031]     In the forklift, said flow control valve may further include a relief valve configured to drain said working fluid of said compensating pressure line when said pressure of said working fluid of said compensating pressure line is larger than a second set pressure, and configured not to drain said working fluid of said compensating pressure line when said pressure of said working fluid of said compensating pressure line is smaller than said second set pressure.  
         [0032]     In the forklift, said first switching valve may switch among a meter-in operation, a meter-out operation and a neutral operation by an external operation. Working fluid is supplied to said load pressure line in said meter-in operation for operating said actuator.  
         [0033]     In the forklift, said relief valve may not be connected to said compensating pressure line when said first switching valve is in said meter-in operation.  
         [0034]     In the forklift, said first switching valve includes a first spool chamber and a first spool configured to be slidably inserted into said first spool chamber. Said relief valve includes a second spool chamber configured to be formed in said first spool and a second spool configured to be slidably inserted into said second spool chamber.  
         [0035]     In the forklift, said flow control valve further includes a second switching valve configured to connect said compensating pressure line to said relief valve when said first switching valve is in said neutral operation and said meter-out operation, and configured not to connect said compensating pressure line to said relief valve when said first switching valve is in said meter-in operation.  
         [0036]     In the forklift, said first switching valve switches among a meter-in operation, a meter-out operation and a neutral operation by an external operation, wherein working fluid is supplied to said load pressure line in said meter-in operation for operating said actuator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]      FIG. 1  is a schematic view showing the conventional flow control valve;  
         [0038]      FIG. 2  is a schematic view showing the flow control valve of the present invention;  
         [0039]      FIG. 3  is a cross sectional view showing the flow control valve main unit including the flow control valve  1 ;  
         [0040]      FIG. 4  is a schematic view showing another embodiment of a flow control valve according to the present invention;  
         [0041]      FIG. 5  is a schematic view showing still another embodiment of a flow control valve according to the present invention; and  
         [0042]      FIG. 6  is a schematic perspective view showing the forklift with the flow control valve of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]     An embodiment of a forklift according to the present invention will be described below with reference to attached drawings. The forklift includes a lift cylinder for driving a folk that holds a load along with a flow control valve.  FIG. 2  is a schematic view showing the flow control valve of the present invention. As shown in  FIG. 2 , the flow control valve  1  includes a direction switching valve  2 , a check valve  3  and a pressure compensating valve  5 . The flow control valve  1  further includes a plurality of lines for guiding hydraulic oil to transmit oil pressure. The plurality of lines is composed of a pump pressure line  11 , a pump pressure line  12 , a load pressure line  13 , a compensating pressure line  14  and a drain line  15 .  
         [0044]     The pump pressure line  11  connects the direction switching line  2  to a pump not shown and guides hydraulic oil supplied by the pump. The pump pressure line  12  connects the direction switching valve  2  to the check valve  3 . The load pressure line  13  connects between the check valve  3 , the lift cylinder  4  and the pressure compensating valve  5 . The compensating pressure line  14  connects the pressure compensating valve  5  to the direction switching valve  2 . The drain line  15  connects the direction switching valve  2  to a tank  6  and the oil pressure of the drain line  15  is substantially zero (0). The check valve  3  prevents hydraulic oil from flowing from the load pressure line  13  to the pump pressure line  12 . That is, the check valve  3  connects the pump pressure line  12  to the load pressure line  13  when the oil pressure of the pump pressure line  12  is larger than that of the load pressure line  13 , and the check valve  3  does not connect the pump pressure line  12  to the load pressure line  13  when the oil pressure of the load pressure line  13  is larger than that of the pump pressure line  12 . The check valve  3  may be omitted from the flow control valve  1 .  
         [0045]     The lift cylinder  4  is an actuator for lifting and lowering the fork of the forklift according to the present invention. That is, the lift cylinder  4  lifts the fork of the forklift when hydraulic oil is supplied from the load pressure line  13  and lowers the fork of the forklift when hydraulic oil is discharged into the load pressure line  13 . At this time, the oil pressure of the load pressure line  13  varies depending on the weight of a load held by the fork of the forklift and becomes larger as the load is heavier.  
         [0046]     The pressure compensating valve  5  controls the oil pressure of the compensating pressure line  14  so as to become a set pressure. That is, the pressure compensating valve  5  enlarges the opening area of a variable orifice between the load pressure line  13  and the compensating pressure line  14  when the oil pressure of the compensating pressure line  14  is smaller than the set pressure, and narrows the opening area of the variable orifice when the oil pressure of the compensating pressure line  14  is larger than the set pressure.  
         [0047]     The direction switching valve  2  includes a relief valve  21 , an inlet side line  22  and an outlet side line  23 . The relief valve  21  prevents the oil pressure of the inlet side line  22  from exceeding a set pressure by providing the set pressure. The set pressure of the relief valve  21  is larger than that of the pressure compensating valve  5 . That is, the relief valve  21  connects the line  22  to the outlet side line  23  when the oil pressure of the inlet side line  22  is larger than that of the set pressure, and does not connect the line  22  to the outlet side line  23  when the oil pressure of the inlet side line  22  is smaller than that of the set pressure.  
         [0048]     The direction switching valve  2  can occupy one of a neutral position, a meter-in position and a meter-out position. That is, operated by the user, the direction switching valve  2  is switched from the neutral position to the meter-in position, from the neutral position to the meter-out position, from the meter-in position to the neutral position and from the meter-out position to the neutral position.  
         [0049]     At the meter-in position, the direction switching valve  2  connects the pump pressure line  11  to the pump pressure line  12 , closes the compensating pressure line  14  and closes the drain line  15 . At the meter-out position, the direction switching valve  2  closes the pump pressure line  11 , closes the pump pressure line  12  and connects the compensating pressure line  14  to the drain line  15 .  
         [0050]     At the neutral position, the direction switching valve  2  closes the pump pressure line  11 , closes the pump pressure line  12 , connects the compensating pressure line  14  to the inlet side line  22  and connects the line  23  to the drain line  15 . That is, at the neutral position, the direction switching valve  2  performs control such that the oil pressure of the compensating pressure line  14  does not exceed the set pressure set for the relief valve  21 .  
         [0051]     At the meter-out position, the direction switching valve  2  may connect the compensating pressure line  14  to the inlet line  22  and the line  23  to the drain line  15 . That is, at the meter-out position, the direction switching valve  2  may perform control such that the oil pressure of the compensating pressure line  14  does not exceed the set pressure set for the relief valve  21 .  
         [0052]     The tank  6  stores hydraulic oil flowing through the drain line  15  therein. The hydraulic oil stored in the tank  6  is supplied to the pump pressure line  11  by a pump not shown.  
         [0053]      FIG. 6  is a schematic perspective view showing the forklift with the flow control valve of the present invention. The forklift  7  includes the flow control valve  1 , the fork  8  and the lift cylinder  4 . The flow control valve  1  is included in a hydraulic circuit (not shown) mounted on the forklift  7 . The lift cylinder  4  is connected between the flow control valve  1  and the fork  8 . The fork  8  lifts and lowers a load. The lift cylinder  4  drives the folk  8  along with the flow control valve  1 . The fork  8 , for example, is composed of an outer mast  8   c , an inner mast  8   b  and a fork body  8 . The inner mast  8   b  is lifted up and down to the vertical direction guided by the outer mast  8   c . The fork body  8   a  is lifted up and down supported by the inner mast  8   b  in an integrated manner to the inner mast  8   b . The inner mast  8   b  is driven to lift up and down by the lift cylinder  4 .  
         [0054]      FIG. 3  is a cross sectional view showing the flow control valve main unit including the flow control valve  1 . The flow control valve main unit  30  includes a spool chamber  31  and a spool  32  which constitute the direction switching valve  2 . That is, the spool chamber  31  has a cylindrical sliding surface therein. The spool  32  is provided so as to internally touch the sliding surface of the spool chamber  31  and be slidably inserted thereinto in the direction parallel to a direction A. In the flow control valve main unit  30 , a pump pressure chamber  33 , a load pressure chamber  34 , a compensating pressure chamber  35  and a drain chamber  36  are provided in the spool chamber  31 . The pump pressure chamber  33  is connected to the pump pressure line  11 . The drain chamber  36  is connected to the drain line  15 .  
         [0055]     By sliding in the direction parallel to the direction A, the spool  32  is set at any of the neutral position, the meter-in position and the meter-out position. That is, the spool  32  is set at the meter-in position by moving from the neutral position in the direction A, and is set at the meter-out position by moving from the neutral position in the direction opposite to the direction A. The spool  32  is mechanically connected to a lever operated by the operator through a link mechanism and moves in the direction parallel to the direction A in proportion to an operation quantity of the lever.  
         [0056]     The spool  32  may be replaced with the other spool moved by the other moving mechanism. An electric hydraulic pilot mechanism is exemplified as the moving mechanism of the spool. The electric hydraulic pilot mechanism further includes a potentiometer and a solenoid valve. The potentiometer detects an operation quantity of the lever operated by the operator and outputs a current corresponding to the operation quantity to the solenoid valve directly or through a control device not shown. The solenoid valve applies a pressure to the hydraulic oil such that the hydraulic oil has a pilot pressure corresponding to the current. The spool  32  of the direction switching valve  2  is pressed by the hydraulic oil with the pilot pressure to be directly operated.  
         [0057]     The spool chamber  31  and the spool  32  include a variable orifice  38  and a variable orifice  37 . The variable orifice  37  closes connection between the pump pressure chamber  33  and the load pressure chamber  34  when the spool  32  is set at the neutral position or the meter-out position, and connects the pump pressure chamber  33  to the load pressure chamber  34  when the spool  32  is set at the meter-in position. When the spool  32  is set at the meter-in position, the orifice area of the variable orifice  37  becomes larger as the spool  32  moves toward the direction A.  
         [0058]     The variable orifice  38  closes connection between the compensating pressure chamber  35  and the drain chamber  36  when the spool  32  is set at the neutral position or the meter-in position, and connects the compensating pressure chamber  35  to the drain chamber  36  when the spool  32  is set at the meter-out position. When the spool  32  is set at the meter-out position, the orifice area of the variable orifice  38  becomes larger as the spool  32  moves toward the direction opposite to the direction A.  
         [0059]     The spool  32  includes a spool chamber  41 , a spool  42  and a spring  43  which constitutes the relief valve  21 . The spool chamber  41  has a cylindrical sliding surface. The spool  42  is provided so as to internally touch the sliding surface of the spool chamber  41  and be slidably inserted thereinto in the direction parallel to a direction A. The spring  43  presses the spool  42  in the direction opposite to the direction A. In the spool  32 , a pressure chamber  44  is provided between the spool  42  and the spool chamber  41 .  
         [0060]     The hydraulic oil of the pressure chamber  44  presses the spool  42  by its oil pressure in the direction A. That is, the spool  42  moves in the direction A when the oil pressure of the pressure chamber  44  is larger than the set pressure set by the spring  43 .  
         [0061]     The spool  32  further includes a hole  45  and a hole  46 . The hole  45  is connected to the pressure chamber  44 . The hole  45  is not connected to the compensating pressure chamber  35  when the spool  32  is set at the meter-in position and is connected to the compensating pressure  35  when the spool  32  is set at the neutral position or the meter-out position.  
         [0062]     The hole  46  is connected to the drain chamber  36 . The hole  46  is connected to the pressure chamber  44  when the spool  42  moves in the direction A, that is, when the oil pressure of the pressure chamber  44  is larger than the set pressure and is not connected to the pressure chamber  44  when the spool  42  does not move, that is, when the oil pressure of the pressure chamber  44  is smaller than the set pressure.  
         [0063]     The flow control valve main unit  30  further includes a spool chamber  52 , a spool  51  and a spring  53  which constitute the pressure compensating valve  5 . That is, the spool chamber  52  has a cylindrical sliding surface. The spool  51  is provided so as to internally touch the sliding surface of the spool chamber  52  and be slidably inserted thereinto in the direction parallel to a direction A. The spring  53  presses the spool  52  in the direction opposite to the direction A.  
         [0064]     In the flow control valve main unit  30 , the spool chamber  52  includes a load pressure chamber  54 , a compensating pressure chamber  55  and a pressure chamber  56 . The load pressure chamber  54  is connected to a load pressure line  13 . The compensating pressure chamber  55  is connected to the compensating pressure chamber  35 . A hole  57  is formed on the spool  51 . The hole  57  connects the compensating pressure chamber  55  to the pressure chamber  56 . The hydraulic oil of the pressure chamber  56  presses the spool  52  by its oil pressure toward the direction A.  
         [0065]     The spool chamber  52  and the spool  51  include a variable orifice  58 . The variable orifice  58  narrows or closes the opening area between the load pressure chamber  54  and the compensating pressure chamber  55  when the spool  52  moves toward the direction A and enlarges the opening area when the spool  52  moves toward the direction opposite to the direction A.  
         [0066]     Operations of the flow control valve  1  include the meter-in operation, the neutral operation and the meter-out operation. The meter-in operation is the operation performed when the direction switching valve  2  is switched from the neutral position to the meter-in position by the user. The neutral operation is the operation performed when the direction switching valve  2  is switched from the meter-in position or the meter-out position to the neutral position by the user. The meter-out operation is the operation performed when the direction switching valve  2  is switched from the neutral position to the meter-out position by the user.  
         [0067]     In the meter-in operation, hydraulic oil is supplied from the pump pressure line  11  to the lift cylinder  4  through the direction switching valve  2 , the pump switching line  12 , the check valve  3  and the load pressure line  13 . The lift cylinder  4  lifts the fork when the hydraulic oil is supplied.  
         [0068]     In the neutral operation, since no hydraulic oil is supplied or discharged to the lift cylinder  4 , lifting and lowering of the fork is stopped. The load pressure varies according to the weight of the load held by the fork of the forklift and becomes larger as the load is heavier. The hydraulic oil of the load pressure line  13  is supplied to the compensating pressure line  14  through the pressure compensating valve  5 . The pressure compensating valve  5  prevents the oil pressure of the compensating pressure line  14  from becoming the set pressure or more by closing connection between the load pressure line  13  and the compensating pressure line  14  when the oil pressure of the compensating pressure line  14  is raised to the set pressure of the pressure compensating valve  5 .  
         [0069]     When the load pressure is larger than the set pressure, the pressure compensating valve  5  gradually leaks the hydraulic oil from the load pressure line  13  to the compensating pressure line  14  through a gap between the spool chamber  52  and the spool  51  with time even when connection between the load pressure line  13  and the compensating pressure line  14  is closed, and raises the oil pressure of the compensating pressure line  14 . When the oil pressure of the compensating pressure line  14  is raised to the set pressure of the relief valve  22 , the relief valve  22  connects the compensating pressure line  14  to the drain line  15  to flow the hydraulic oil of the compensating pressure line  14  to the drain line  15  and lowers the oil pressure of the compensating pressure line  14  to the set pressure.  
         [0070]     In the meter-out operation, the hydraulic oil is discharged from the lift cylinder  4  to the drain line  15  through the load pressure line  13 , the pressure compensating valve  5  and the direction switching valve  2 . When the hydraulic oil is discharged, the lift cylinder  4  lowers the fork. At this time, the oil pressure of the compensating pressure line  14  is controlled to be the set pressure through the pressure compensating valve  5  irrespective of the weight of the load held by the fork. For this reason, in the meter-out operation, irrespective of the weight of the load held by the fork, the flow control valve  1  can associate the flow of the hydraulic oil discharged from the lift cylinder  4  to the drain line  15  with the operation quantity of the direction switching valve  2  on one-to-one basis. In other words, the forklift according to the present invention can associate the lowering speed of the fork with the operation quantity of the direction switching valve  2  on one-to-one basis, thereby improving operability of the fork.  
         [0071]     In the case that the pressure of the compensating pressure line  14  is much higher than the set pressure, when the compensating pressure line  14  is connected to the drain line  15 , the hydraulic oil rapidly flows from the compensating pressure line  14  to the drain line  15 . The rapid flow generates shock or hunting in the operation of the lift cylinder  4 . The flow control valve  1  controls the oil pressure of the compensating pressure line  14  in the neutral operation such that the oil pressure of the compensating pressure line  14  may not exceed the set pressure of the relief valve  21 . Thus, the flow control valve  1  can prevent the hydraulic oil from rapidly flowing from the compensating pressure line  14  to the drain line  15  when the direction switching valve  2  is switched from the neutral position to the meter-out position. Therefore, the flow control valve  1  can prevent shock or hunting from occurring in the operation of the lift cylinder  4 . That is, the forklift according to the present invention can prevent shock or hunting in the fork from occurring when the fork is lowered.  
         [0072]      FIG. 4  is a schematic view showing another embodiment of a flow control valve according to the present invention. The flow control valve  61  includes a direction switching valve  62 , a check valve  63 , a pressure compensating valve  65 , a direction switching valve  67  and a relief valve  68 . The flow control valve  61  further includes a plurality of lines for guiding hydraulic oil and transmitting oil pressure. The plurality of lines is composed of a pump pressure line  71 , a pump pressure line  72 , a load pressure line  73 , a compensating pressure line  74 , a drain line  75 , a compensating pressure line  77  and a drain line  78 .  
         [0073]     The pump pressure line  71  connects the direction switching valve  62  to a pump not shown and guides the hydraulic oil supplied by the pump. The pump pressure line  72  connects the direction switching valve  62  to the check valve  63 . The load pressure line  73  connects between the check valve  63 , the lift cylinder  64  and the pressure compensating valve  65 . The compensating pressure line  74  connects between the pressure compensating valve  65 , the direction switching valve  62  and the direction switching valve  67 . The compensating pressure line  77  connects the direction switching valve  67  to the relief valve  68 . The drain line  75  connects the direction switching valve  62  to the tank  66 . The oil pressure of the drain line  75  is substantially zero (0). The drain line  78  connects the relief valve  68  to the tank  66 . The oil pressure of the drain line  78  is substantially zero (0).  
         [0074]     The check valve  63  prevents the hydraulic oil from flowing from the load pressure line  73  to the pump pressure line  72 . That is, the check valve  63  connects the pump pressure line  72  to the load pressure line  73  when the oil pressure of the pump pressure line  72  is larger than that of the load pressure line  73 , and does not connect the pump pressure line  72  to the load pressure line  73  when the oil pressure of the load pressure line  73  is larger than that of the pump pressure line  72 .  
         [0075]     The lift cylinder  64  is an actuator for lifting and lowering the fork of the forklift according to the present invention. That is, the lift cylinder  64  lifts the fork of the forklift when hydraulic oil is supplied from the load pressure line  73  and lowers the fork of the forklift when hydraulic oil is discharged into the load pressure line  73 . At this time, the oil pressure of the load pressure line  73  varies depending on the weight of a load held by the fork of the forklift and becomes larger as the load is heavier.  
         [0076]     The pressure compensating valve  65  performs control such that the oil pressure of the compensating pressure line  74  is a set pressure. That is, the pressure control valve  65  enlarges the opening area of a variable orifice between the load pressure line  73  and the compensating pressure line  74  when the oil pressure of the compensating pressure line  74  is smaller than the set pressure, and narrows the opening area of the variable orifice when the oil pressure of the compensating pressure line  74  is larger than the set pressure.  
         [0077]     The spool of the direction switching valve  62  can occupy one of the neutral position, the meter-in position and the meter-out position. That is, the direction switching valve  62  includes a potentiometer and a solenoid valve not shown. The potentiometer detects an operation quantity of the lever operated by the operator and outputs a current corresponding to the operation quantity to the solenoid valve directly or through a control device not shown. The solenoid valve applies a pressure such that the hydraulic oil has a pilot pressure corresponding to the current. The hydraulic oil is composed of two hydraulic oils. One is a hydraulic oil for pressing the spool of the direction switching valve  62  from right to left. The other is a hydraulic oil for pressing the spool of the direction switching valve  62  from left to right. The spool of the direction switching valve  62  is moved by being pressed by the hydraulic oil with the pilot pressure to be switched from the neutral position to the meter-in position and from the neutral position to the meter-out position.  
         [0078]     At the meter-in position, the direction switching valve  62  connects the pump pressure line  71  to the pump pressure line  72 , closes the compensating pressure line  74  and closes the drain line  75 . At the meter-out position, the direction switching valve  62  closes the pump pressure line  71 , closes the pump pressure line  72  and connects the compensating pressure line  74  to the drain line  75 . At the neutral position, the direction switching valve  62  closes the pump pressure line  71 , closes the pump pressure line  72 , closes the compensating pressure line  74  and closes the drain line  75 .  
         [0079]     The flow control valve  61  further includes a pilot pressure line  79 . The pilot pressure line  79  presses the spool of the direction switching valve  67  from left to right to transmit the pilot pressure of the hydraulic oil for moving the spool from the neutral position to the meter-in position to the direction switching valve  67 . The pilot pressure is raised when the spool of the direction switching valve  67  is moved from the neutral position to the meter-in position, and is not raised when the spool of the direction switching valve  67  is moved to the neutral position or the meter-out position.  
         [0080]     When the pilot pressure is raised, the spool of the direction switching valve  67  is pressed by the pilot pressure to close connection between the compensating pressure line  74  and the compensating pressure line  77 . When the pilot pressure is not raised, the spool of the direction switching valve  67  is pressed by the pilot pressure to connect the compensating pressure line  74  to the compensating pressure line  77 . That is, the direction switching valve  67  closes connection between the compensating pressure line  74  and the compensating pressure line  77  when the spool of the direction switching valve  67  is set at the meter-in position, and connects the compensating pressure line  74  to the compensating pressure line  77  when the spool of the direction switching valve  67  is set at the neutral position or the meter-out position.  
         [0081]     The relief valve  68  performs control such that the oil pressure of the compensating pressure line  77  does not exceed the set pressure. The set pressure of the relief valve  68  is larger than the set pressure of the pressure compensating valve  65 . That is, the relief valve  68  connects the compensating pressure line  77  to the drain line  78  when the oil pressure of the compensating pressure line  77  is larger than the set pressure, and does not connect the compensating pressure line  77  to the drain line  78  when the oil pressure of the compensating pressure line  77  is smaller than the set pressure.  
         [0082]     The tank  66  stores hydraulic oil flowing through the drain line  75  and the drain line  78  therein. The hydraulic oil stored in the tank  66  is supplied to the pump pressure line  71  by a pump not shown.  
         [0083]     As shown in  FIG. 6 , the flow control valve  61  is mounted on the forklift  7  of the present invention. The forklift  7  includes the flow control valve  61 , the fork  8  and the lift cylinder  64 . The flow control valve  61  is included in a hydraulic circuit (not shown) mounted on the forklift  7 . The lift cylinder  64  is connected between the flow control valve  61  and the fork  8 . The fork  8  lifts and lowers a load. The lift cylinder  64  drives the folk  8  along with the flow control valve  61 .  
         [0084]     Operations of the flow control valve  61  include the meter-in operation, the neutral operation and the meter-out operation. The meter-in operation is an operation performed when the direction switching valve  62  is switched from the neutral position to the meter-in position by means of the user&#39;s operation. The neutral operation is an operation performed when the direction switching valve  62  is switched from the meter-in position or the meter-out position to the neutral position by means of the user&#39;s operation. The meter-out operation is an operation performed when the direction switching valve  62  is switched from the neutral position to the meter-out position by means of the user&#39;s operation.  
         [0085]     In the meter-in operation, the hydraulic oil supplied by the pump is supplied from the pump pressure line  71  to the lift cylinder  64  through the direction switching valve  62 , the pump pressure line  72 , the check valve  63  and the load pressure line  73 . When the hydraulic oil is supplied, the lift cylinder  64  lifts the fork.  
         [0086]     In the neutral operation, since no hydraulic oil is supplied or discharged between the lift cylinder  64  and the load pressure line  73 , lifting or lowering of the fork is stopped. The load pressure varies depending on a load held by the fork of the forklift and becomes larger as the load is heavier. The hydraulic oil of the load pressure line  73  is supplied to the compensating pressure line  74  through the pressure compensating valve  65 . The pressure compensating valve  65  closes connection between the load pressure line  73  and the compensating pressure line  74 , when the oil pressure of the compensating pressure line  74  is raised to the set pressure of the pressure compensating valve  65 , thereby preventing the oil pressure of the compensating pressure line  74  from exceeding the set pressure. The direction switching valve  67  connects the compensating pressure line  74  to the compensating pressure line  77 .  
         [0087]     When the load pressure is larger than the set pressure, the pressure compensating valve  65  gradually leaks the hydraulic oil from the load pressure line  73  to the compensating pressure line  74  through a gap between the spool chamber and the spool with time even when connection between the load pressure line  73  and the compensating pressure line  74  is closed, and raises the oil pressure of the compensating pressure line  74 . When the oil pressure of the compensating pressure line  77  is raised to the set pressure of the relief valve  68 , the relief valve  68  connects the compensating pressure line  77  to the drain line  78  to flow the hydraulic oil of the compensating pressure line  77  to the drain line  78  and lowers the oil pressure of the compensating pressure line  77  to the set pressure.  
         [0088]     In the meter-out operation, the hydraulic oil is discharged from the lift cylinder  64  to the drain line  15  through the load pressure line  73 , the pressure compensating valve  65 , the compensating pressure line  74  and the direction switching valve  62 . When the hydraulic oil is discharged, the lift cylinder  64  lowers the fork. At this time, the oil pressure of the compensating pressure line  74  is controlled by the pressure compensating valve  65  to be the set pressure irrespective of the weight of the load held by the fork. For this reason, in the meter-out operation, irrespective of the weight of the load held by the fork, the flow control valve  61  can associate the flow of the hydraulic oil discharged from the lift cylinder  64  to the drain line  75  with the operation quantity of the direction switching valve  62  on one-to-one basis. In other words, the forklift according to the present invention can associate the lowering speed of the fork with the operation quantity of the direction switching valve  62  on one-to-one basis, thereby improving operability of the fork.  
         [0089]     Like the flow control valve  1  in the above-mentioned embodiment, the flow control valve  61  controls the oil pressure of the compensating pressure line  74  in the neutral position is controlled so as to be smaller than the set pressure of the relief valve  68 . The flow control valve  61  has more complicated configuration than the flow control valve  1  in the above-mentioned embodiment since the direction switching valve  67  is provided. However, similarly to the flow control valve  1  in the above-mentioned embodiment, the flow control valve  61  can prevent shock or hunting from occurring in the operation of the lift cylinder  64 . That is, the relief valve performs control such that the oil pressure of the compensating pressure line  74  in the neutral position does not exceed the set pressure. The relief valve can be installed inside or outside of the direction switching valve operated by the operator and thus no attention is paid to the installation position.  
         [0090]      FIG. 5  is a schematic view showing still another embodiment of a flow control valve according to the present invention. The flow control valve  81  includes a direction switching valve  82 , a check valve  83  and a pressure compensating valve  85 . The flow control valve  81  further includes a plurality of lines for guiding hydraulic oil and transmitting oil pressure. The plurality of lines is composed of a pump pressure line  91 , a pump pressure line  92 , a load pressure line  93 , a compensating pressure line  94  and a drain line  95 .  
         [0091]     The pump pressure line  91  connects the direction switching valve  82  to a pump not shown and guides the hydraulic oil supplied by the pump. The pump pressure line  92  connects the direction switching valve  82  to the check valve  83 . The load pressure line  93  connects between the check valve  83 , the lift cylinder  84  and the pressure compensating valve  85 . The compensating pressure line  94  connects the pressure compensating valve  85  to the direction switching valve  82 . The drain line  95  connects the direction switching valve  82  to the tank  86  and the oil pressure of the drain line  95  is substantially zero (0).  
         [0092]     The check valve  83  prevents the hydraulic oil from flowing from the load pressure line  93  to the pump pressure line  92 . That is, the check valve  83  connects the pump pressure line  92  to the load pressure line  93  when the oil pressure of the pump pressure line  92  is larger than that of the load pressure line  93 , and does not connect the pump pressure line  92  to the load pressure line  93  when the oil pressure of the load pressure line  93  is larger than that of the pump pressure line  92 .  
         [0093]     The lift cylinder  84  is an actuator for lifting and lowering the fork of the forklift according to the present invention. That is, the lift cylinder  84  lifts the fork of the forklift when hydraulic oil is supplied from the load pressure line  93  and lowers the fork of the forklift when hydraulic oil is discharged into the load pressure line  93 . At this time, the oil pressure of the load pressure line  93  varies depending on the weight of a load held by the fork of the forklift and becomes larger as the load is heavier.  
         [0094]     The pressure compensating valve  85  performs control such that the oil pressure of the compensating pressure line  94  is a set pressure. That is, the pressure control valve  85  enlarges the opening area of a variable orifice between the load pressure line  93  and the compensating pressure line  94  when the oil pressure of the compensating pressure line  94  is smaller than the set pressure, and narrows the opening area of the variable orifice when the oil pressure of the compensating pressure line  94  is larger than the set pressure.  
         [0095]     The direction switching valve  82  can occupy one of the neutral position, the meter-in position and the meter-out position. That is, the direction switching valve  82  is switched from the neutral position to the meter-in position, from the neutral position to the meter-out position, from the meter-in position to the neutral position and from the meter-out position to the neutral position by the user&#39;s operation.  
         [0096]     At the meter-in position, the direction switching valve  82  connects the pump pressure line  91  to the pump pressure line  92 , closes the compensating pressure line  94  and closes the drain line  95 . At the meter-out position, the direction switching valve  82  closes the pump pressure line  91 , closes the pump pressure line  92  and connects the compensating pressure line  94  to the drain line  95 . At the neutral position, the direction switching valve  82  closes the pump pressure line  91 , closes the pump pressure line  92 , closes the compensating pressure line  94  and closes the drain line  95 .  
         [0097]     The tank  86  stores hydraulic oil flowing through the drain line  95  therein. The hydraulic oil stored in the tank  86  is supplied to the pump pressure line  91  by the pump not shown.  
         [0098]     As shown in  FIG. 6 , the flow control valve  81  is mounted on the forklift  7  of the present invention. The forklift  7  includes the flow control valve  81 , the fork  8  and the lift cylinder  84 . The flow control valve  81  is included in a hydraulic circuit (not shown) mounted on the forklift  7 . The lift cylinder  84  is connected between the flow control valve  81  and the fork  8 . The fork  8  lifts and lowers a load. The lift cylinder  84  drives the folk  8  along with the flow control valve  81 .  
         [0099]     Operations of the flow control valve  81  include the meter-in operation, the neutral operation and the meter-out operation. The meter-in operation is an operation performed when the direction switching valve  82  is switched from the neutral position to the meter-in position by means of the user&#39;s operation. The neutral operation is an operation performed when the direction switching valve  82  is switched from the meter-in position or the meter-out position to the neutral position by means of the user&#39;s operation. The meter-out operation is an operation performed when the direction switching valve  82  is switched from the neutral position to the meter-out position by means of the user&#39;s operation.  
         [0100]     In the meter-in operation, the hydraulic oil is supplied from the pump pressure line  91  to the lift cylinder  84  through the direction switching valve  82 , the pump pressure line  92 , the check valve  83  and the load pressure line  93 . When the hydraulic oil is supplied, the lift cylinder  84  lifts the fork.  
         [0101]     In the neutral operation, since no hydraulic oil is supplied or discharged between the lift cylinder  84  and the load pressure line  93 , lifting or lowering of the fork is stopped. The load pressure of the hydraulic oil of the load pressure line  93  varies depending on a load held by the fork of the forklift and becomes larger as the load is heavier. The hydraulic oil of the load pressure line  93  is supplied to the compensating pressure line  94  through the pressure compensating valve  85 . The pressure compensating valve  85  closes connection between the load pressure line  93  and the compensating pressure line  94 , when the oil pressure of the compensating pressure line  94  is raised to the set pressure of the pressure compensating valve  85 , thereby preventing the oil pressure of the compensating pressure line  94  from becoming the set pressure or more.  
         [0102]     In the meter-out operation, the hydraulic oil is discharged from the lift cylinder  84  to the drain line  95  through the load pressure line  93 , the pressure compensating valve  85 , the compensating pressure line  94  and the direction switching valve  82 . When the hydraulic oil is discharged, the lift cylinder  84  lowers the fork. At this time, the oil pressure of the compensating pressure line  94  is controlled by the pressure compensating valve  85  to be the set pressure irrespective of the weight of the load held by the fork. At this time, in the meter-out operation, irrespective of the weight of the load held by the fork, the flow control valve  81  can associate the flow of the hydraulic oil discharged from the lift cylinder  84  to the drain line  95  with the operation quantity of the direction switching valve  82  on one-to-one basis. In other words, the forklift according to the present invention can associate the lowering speed of the fork with the operation quantity of the direction switching valve  82  on one-to-one basis, thereby improving operability of the fork.  
         [0103]     In the case of high pressure of the compensating pressure line  94 , when the compensating pressure line  94  is connected to the drain line  95 , the hydraulic oil rapidly flows from the compensating pressure line  94  to the drain line  95 . The rapid flow generates shock or hunting in the operation of the lift cylinder  84 . Since the flow control valve does not control the oil pressure of the compensating pressure line  94  in the neutral operation, when the direction switching valve  82  is switched from the neutral position to the meter-out position, the hydraulic oil cannot be prevented from rapidly flowing from the compensating pressure line  94  to the drain line  95 . Although the forklift to which the flow control valve  81  is applied cannot prevent shock or hunting from generating in the fork when the fork is lowered, it is better than that the lowering speed of the fork cannot be associated with the operation quantity of the direction switching valve  82  on one-to-one basis.  
         [0104]     A flow control valve according to the present invention can improve operability of a hydraulic actuator.  
         [0105]     It is apparent that the present invention is not limited to the above embodiment, that may be modified and changed without departing form the scope and spirit of the invention.

Technology Classification (CPC): 5