Abstract:
A pilot check valve includes: a fluid channel body; a moveable body that moves in an axial direction under effects of pilot pressure; a packing including a lip portion that is displaceable, integrally with the moveable body, between a valve close position and a valve open position; and a detector to detect position of the moveable body. The packing, in the valve close position, allows flow of an operating liquid from a first port side of the fluid channel body to a second port side and prevents the flow of the operating liquid from the second port side to the first port side, and in the valve open position, connects the first port and the second port.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a pilot check valve that utilizes a pilot pressure for controlling flow of a working fluid. 
       BACKGROUND ART 
       [0002]    Heretofore, it is well known that, in a fluid pressure circuit for operating a fluid pressure cylinder, a pilot check valve is provided in order to appropriately control the flow of a working fluid (for example, see Japanese Laid-Open Utility Model Publication No. 03-025080). For example, in a fluid pressure circuit in which the fluid pressure cylinder is used as a lift cylinder, a head-side pressure chamber of the cylinder and a rod-side pressure chamber thereof are connected to a pressure supply source through a switching valve, and a pilot check valve is arranged in a flow channel between the rod-side pressure chamber and the switching valve. 
         [0003]    The pilot check valve is equipped with a poppet valve plug that is elastically urged toward a valve seat by a spring, and a pilot piston facing the poppet valve plug and which is disposed slidably in an axial direction thereof. The pilot piston presses the poppet valve plug when a pilot pressure is applied. When a supply pressure is applied to the head-side pressure chamber of the cylinder, the pilot pressure is applied to the pilot check valve. 
         [0004]    In the fluid pressure circuit as above constructed, when the working fluid is supplied from the pressure supply source to the pilot check valve through the switching valve, under pressure based on the working fluid, the poppet valve plug is separated away from the valve seat against the elastic force of the spring, whereby the working fluid flows into the rod-side pressure chamber of the cylinder, to thereby press the piston of the cylinder upwardly. When the piston reaches an upper end position, the pressure difference between the upstream side and the downstream side of the poppet valve plug becomes zero, and as a result, the poppet valve plug is seated on the valve seat by the elastic force of the spring. Thus, even if supply of the pressure from the pressure supply source is stopped, the poppet valve plug remains closed, and therefore the cylinder is prevented from dropping down. 
         [0005]    On the other hand, when the working fluid is supplied from the pressure supply source to the head-side pressure chamber of the cylinder through the switching valve, the pilot piston is advanced by the pilot pressure, and presses the poppet valve plug, whereby the poppet valve plug is separated away from the valve seat. Consequently, the fluid in the rod-side pressure chamber of the cylinder is discharged from the switching valve through the pilot check valve, so that the cylinder is lowered. 
         [0006]    In the above fluid pressure circuit to which the conventional pilot check valve is applied, in order to prevent the cylinder from dropping down, it is necessary to stop the pressure supply after the valve has been placed in a valve closed state. However, the conventional pilot check valve is not equipped with a function for detecting the closing of the valve. Further, the poppet valve plug and the pilot piston are provided separately from each other, and the position of the pilot piston does not always correspond to the state of the poppet valve plug. Thus, even if the position of the pilot piston is detected, it is difficult to determine whether the poppet valve plug is seated or not, based on the detected position. Therefore, in the conventional pilot check valve, it is impossible to suitably detect whether the valve is in a valve closed state or not. 
       SUMMARY OF INVENTION 
       [0007]    The present invention has been devised taking into consideration the aforementioned problems, and has the object of providing a pilot check valve which is capable of suitably detecting whether the valve is placed in a valve closed state or not. 
         [0008]    In order to achieve the above object, the present invention is characterized by a pilot check valve including a flow passage body including a first port and a second port, a movable body which is at least partly disposed slidably in the flow passage body and configured to move in an axial direction thereof under action of a pilot pressure, a packing mounted on the movable body and configured to be displaced between a valve closed position and a valve open position integrally with the movable body, the packing including an inclined lip which is elastically deformable radially, the lip being configured to make sliding contact with an inner circumferential surface of the flow passage body, and a detector configured to detect whether or not the movable body is placed at a position that causes the packing to be positioned at the valve closed position, wherein, in the valve closed position, the packing allows a working fluid to flow from the first port toward the second port, and blocks flow of the working fluid from the second port toward the first port, and in the valve open position, the packing allows the first port and the second port to communicate with each other. 
         [0009]    With the above-constructed pilot check valve, since the movable body and the packing are integrally displaced in the axial direction, it is possible to appropriately detect whether or not the valve is in a valve closed state (whether or not the packing is placed at the valve closed position) by detecting the position of the movable body. Further, when the packing is placed at the valve closed position, the packing allows the working fluid to flow from the first port toward the second port, while blocks the flow of the working fluid from the second port toward the first port. Thus, also in a state that a pilot pressure is released, a check valve function can be fulfilled effectively. 
         [0010]    In the above pilot check valve, when the lip of the packing is moved from the valve open position toward the valve closed position, the lip preferably starts to make contact with the inner circumferential surface of the flow passage body at a sealing start position located between the valve closed position and the valve open position. In this case, when the packing is moved from the valve open position toward the valve closed position, the detector preferably outputs a signal after the packing has been moved beyond the sealing start position. 
         [0011]    With the structure, when the lip of the packing is moved from the valve open position toward the valve closed position, the lip is placed in sliding contact with the inner circumferential surface of the flow passage body within a predetermined range in the axial direction. In this manner, the sealing mechanism formed by contact of the lip with the inner circumferential surface has an overlap region in the axial direction. Owing to the overlap region, since detection error by the detector and influence of hysteresis are eliminated, the detector can be prevented from outputting a signal in a state that the valve is not fully closed. Stated otherwise, it is secured that the valve is fully closed when the detector outputs a signal. Thus, reliability of the position detecting function can be enhanced. 
         [0012]    In the above pilot check valve, preferably, the inner circumferential surface of the flow passage body includes a sealing region configured to make pressing contact with the lip when the packing is placed at the valve closed position, and a non-sealing region configured to be separated away from the lip when the packing is placed at the valve open position, and an inner diameter of the non-sealing region is larger than that of the sealing region. 
         [0013]    With the above structure, depending on the position of the packing in the axial direction within the flow passage body, it is possible to easily and reliably switch between a communication state of the first port and the second port, and a non-communication state thereof. 
         [0014]    In the above pilot check valve, preferably, a packing support member configured to surround the packing and prevent the lip from being deformed radially outwardly by a predetermined amount or more is further provided. 
         [0015]    With the structure, it is possible to prevent excessive deformation of the lip and enhance the durability of the packing. 
         [0016]    In the above pilot check valve, preferably, the packing support member includes a passage configured to allow the working fluid to flow from the first port toward the lip. 
         [0017]    With the structure, a function for protecting the lip can be suitably fulfilled without inhibiting the check valve function of the packing. 
         [0018]    With the pilot check valve according to the present invention, it is possible to appropriately detect whether or not the valve is in a valve closed state. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a cross sectional view showing a valve closed state of a pilot check valve according to an embodiment of the present invention; 
           [0020]      FIG. 2  is a cross sectional view showing an intermediate state of the pilot check valve shown in  FIG. 1 ; 
           [0021]      FIG. 3  is a cross sectional view showing a valve open state of the pilot check valve shown in  FIG. 1 ; 
           [0022]      FIG. 4  is a perspective view showing a packing support member; and 
           [0023]      FIG. 5  is a schematic diagram of an example of a fluid pressure circuit to which the pilot check valve shown in  FIG. 1  is applied. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Hereinafter, a preferred embodiment of a pilot check valve according to the present invention will be described with reference to the accompanying drawings. 
         [0025]      FIG. 1  is a cross sectional view showing a valve closed state of a pilot check valve  10  according to an embodiment of the present invention.  FIG. 3  is a cross sectional view showing a valve open state of the pilot check valve  10 . The pilot check valve  10  comprises a flow passage body  16  having a first port  12  and a second port  14 , a working rod  18  disposed in the flow passage body  16  so as to be slidable in an axial direction thereof (in the direction indicated by arrow X), a packing  20  mounted on the working rod  18 , a packing support member  22  mounted on the working rod  18 , a pilot mechanism  24  joined to the flow passage body  16 , and a detector  25  provided on the pilot mechanism  24 . 
         [0026]    The flow passage body  16  is a member that forms a flow passage for a working fluid (e.g., air). In the present embodiment, the flow passage body  16  comprises a hollow first body  26  having the first port  12 , and a hollow second body  28  having the second port  14 . The first body  26  includes an inserted portion  30  on an opposite side of the first port  12 . The second body  28  is inserted in the inserted portion  30 . The first body  26  surrounds the second body  28  in the inserted portion  30 . An annular space  32 , which communicates with the first port  12 , is formed between the inserted portion  30  and the second body  28 . 
         [0027]    The second body  28  includes a coupling portion  34  which is coupled to the pilot mechanism  24 , and a hollow cylindrical sleeve portion  36  which protrudes from the coupling portion  34  in a direction opposite to the pilot mechanism  24 . The coupling portion  34  may be in the form of a nut having a hexagonal shape in transverse section, for example. The sleeve portion  36  is a portion that is inserted in the above-mentioned inserted portion  30  of the first body  26 . 
         [0028]    On an outer circumference of the sleeve portion  36 , annular sealing members  38 ,  40  (for example, o-rings) are arranged in the axial direction at an interval. One sealing member  38  is installed between the sleeve portion  36  and the inserted portion  30 . Between the inserted portion  30  and the sleeve portion  36 , a hollow cylindrical spacer member  42  is provided. The other sealing member  40  is installed between the sleeve portion  36  and the spacer member  42 . Between the spacer member  42  and the inserted portion  30 , another sealing member  44  is provided. 
         [0029]    In the sleeve portion  36 , one or more (two in the illustrated example) side holes  46  are provided between the sealing member  38  and the sealing member  40 . A hollow portion of the first body  26  and a hollow portion of the second body  28  communicate with each other through the side hole  46 . Further, in the sleeve portion  36 , an enlarged-diameter portion  48  is provided on an inner circumferential surface thereof located on a region closer to the second port  14  than the side hole  46 , i.e., between the side hole  46  and the second port  14 . The inner diameter of the enlarged-diameter portion  48  is larger than the inner diameter of a portion of the sleeve portion  36  between the side hole  46  and the enlarged-diameter portion  48 , which will be hereinafter referred to as “a reduced-diameter portion  50 ”. The reduced-diameter portion  50  has a constant inner diameter along the axial direction. 
         [0030]    The working rod  18  is disposed so as to be slidable in the axial direction (X direction) within the sleeve portion  36  of the second body  28 . More specifically, on a middle portion of the working rod  18  in the axial direction, an annular protrusion  52  which protrudes radially outwardly is provided. In the annular protrusion  52 , a seal installation groove  54  is formed, and a sealing member  56  is installed in the seal installation groove  54 . The sealing member  56  prevents leakage of fluid from the interior of the second body  28  toward the pilot mechanism  24 . 
         [0031]    On an end portion of the working rod  18  on a distal end side thereof (i.e., an opposite side to the pilot mechanism  24  or a side in the X1 direction), the packing  20  and the packing support member  22  are mounted. More specifically, a bush  58  is fixed to a distal end portion of the working rod  18  (in the illustrated example, the bush is screw-engaged with the distal end portion), whereby the packing  20  and the packing support member  22  are fixed to the distal end portion of the working rod  18 . Thus, when the working rod  18  is moved in the axial direction, the packing  20  and the packing support member  22  are also moved in the axial direction integrally with the working rod  18 . 
         [0032]    The packing  20  is made up of an elastic body of a rubber or the like (for example, butyl rubber, isoprene rubber, butadiene rubber, silicone rubber, etc.), and has a hollow cylindrical shape as a whole. More specifically, the packing  20  includes a tubular base  60  which extends in parallel to the axial direction, and an annular lip  62  which is inclined and protrudes from an outer circumference of the tubular base  60  and which is elastically deformable radially. 
         [0033]    In a valve closed position shown in  FIG. 1 , the packing  20  allows the working fluid to flow from the first port  12  toward the second port  14 , while blocks flow of the working fluid from the second port  14  toward the first port  12 . In a valve open position shown in  FIG. 3 , the packing  20  allows the first port  12  and the second port  14  to communicate with each other. 
         [0034]    The lip  62  is inclined toward the distal end direction of the working rod  18  (X1 direction). The lip  62  circumferentially extends around an outer circumference of the tubular base  60  over the entire outer circumferential length. The lip  62  has an inner surface of a distal end portion which faces and is separated away from the outer circumferential surface of the tubular base  60 . An annular groove  63 , which is recessed in the X2 direction, is formed between the lip  62  and the tubular base  60 . In a natural state of the lip  62 , the outer diameter thereof is larger than the inner diameter of the reduced-diameter portion  50  of the sleeve portion  36 , and smaller than the inner diameter of the enlarged-diameter portion  48 . 
         [0035]    As shown in  FIG. 1 , when the packing  20  is positioned within the reduced-diameter portion  50 , the lip  62  is in contact with the inner circumferential surface of the sleeve portion  36  over the entire circumferential length in a state of being elastically compressively deformed slightly in a radial inward direction (in a state that the diameter of the lip  62  becomes slightly smaller than that in the natural state). Thus, the reduced-diameter portion  50  forms a sealing region that makes pressing contact with the lip  62  when the packing  20  is placed at the valve closed position. 
         [0036]    On the other hand, as shown in  FIG. 3 , when the packing  20  is positioned within the enlarged-diameter portion  48 , since the lip  62  is separated away from the inner circumferential surface of the sleeve portion  36 , the packing  20  does not offer a sealing function. Thus, the enlarged-diameter portion  48  forms a non-sealing region that is separated away from the lip  62  when the packing  20  is placed at the valve open position. 
         [0037]    The packing support member  22  is a member that surrounds the packing  20  and prevents the lip  62  from being deformed radially outwardly by a predetermined amount or more. In order for the packing support member  22  to have a higher rigidity than the packing  20 , the packing support member  22  is made of hard resin material, metal material, etc., for example. The outer diameter of the packing support member  22  is substantially equal to or slightly smaller than the inner diameter of the inner circumferential surface of the sleeve portion  36  (more specifically, the inner diameter of the reduced-diameter portion  50 ). 
         [0038]    As shown in  FIG. 4 , the packing support member  22  includes a base  66  having a hole  64 , and a plurality of support pieces  68  which protrude from the outer circumference of the base  66  in a thickness direction of the base  66  (X1 direction in  FIG. 1 , etc.). The support pieces  68  are circumferentially arranged at angular intervals, and passages  70  are formed in the axial direction between the adjacent support pieces  68 , respectively. On an inner surface of each of the support pieces  68  on a protruding end side (distal end side), there is formed a tapered portion  72 , by which an inner diameter formed by the support pieces  68  is gradually expanded toward the protruding end. 
         [0039]    As shown in  FIG. 1 , etc., the packing  20  is disposed inside the support pieces  68  of the packing support member  22 , and is held between a flange  58   a  of the bush  58  and an engagement protrusion  84  provided on the working rod  18 . The support pieces  68  of the packing support member  22  are located at a position closer to a proximal end side of the working rod  18  (a side in the X2 direction) than the distal end of the lip  62 , and support the lip  62  from the outside. The packing support member  22  is held between the packing  20  and the engagement protrusion  84 , and also held between an end surface of the bush  58  and the engagement protrusion  84 . 
         [0040]    The pilot mechanism  24  includes a cylinder body  76  with a sliding hole  74  being formed therein, a piston  78  which is slidable in the axial direction within the sliding hole  74  and has a piston packing  77  installed on an outer circumference thereof, a drive rod  80  to which the piston  78  is joined, and a magnet  82  for position detection. The working rod  18  is joined to an end of the drive rod  80  on the side of the flow passage body  16 , for example, by screw-engagement. The piston  78 , the magnet  82 , the drive rod  80 , and the working rod  18  can be integrally displaced in the axial direction. The drive rod  80  and the working rod  18  jointly form a movable body  85 . 
         [0041]    The interior of the sliding hole  74  is partitioned into a first pressure chamber  92   a  on the piston  78  side, and a second pressure chamber  92   b  on the drive rod  80  side by the piston  78 . The cylinder body  76  is provided with a pilot port  86  which communicates with the first pressure chamber  92   a  , and an atmosphere port  88  which allows the second pressure chamber  92   b  to communicate with the atmosphere. In the atmosphere port  88 , it is preferable that, for example, a filter  90  having gas permeability may be disposed. 
         [0042]    When the drive rod  80  and the piston  78  are displaced in the axial direction, the magnet  82  is also displaced in the axial direction integrally therewith. More specifically, a magnet holder  95  is fixed to the drive rod  80  at a position adjacent to the piston  78 . The magnet  82  is retained between the magnet holder  95  and the piston  78 . Alternatively, the magnet  82  may be attached to the piston  78 . 
         [0043]    The piston  78  is urged toward a side opposite to the flow passage body  16  by a spring  94  as an elastic urging means which is disposed in the cylinder body  76 . In the illustrated example, one end of the spring  94  (the end in the X1 direction) abuts against a rod-side cover  96  joined to an end of the cylinder body  76  on the flow passage body  16  side, while another end of the spring  94  (the end in the X2 direction) abuts against an outward protrusion  97  provided on the magnet holder  95 . 
         [0044]    In the pilot mechanism  24  as constructed above, when a pilot pressure is applied to the first pressure chamber  92   a  through the pilot port  86 , under the action of the pilot pressure, the piston  78  is moved in the X1 direction against the elastic force of the spring  94 , and is stopped at a position shown in  FIG. 3 . At this time, a fluid in the second pressure chamber  92   b  is discharged to the atmosphere through the atmosphere port  88 . On the other hand, when the pilot pressure is released, under the action of the elastic force of the spring  94 , the piston  78  is moved in the X2 direction, and is returned to a position shown in  FIG. 1 . 
         [0045]    The detector  25  detects whether or not the movable body  85  is located at a position which causes the packing  20  to be positioned at the valve closed position. For example, the detector  25  is attached to an attachment groove  100  provided on a side portion of the cylinder body  76 . In the present embodiment, when the packing  20  reaches the valve closed position (fully closed position) shown in  FIG. 1 , and accordingly the magnet  82  reaches a predetermined position, the detector  25  detects a magnetic field of the magnet  82  and then outputs a signal. That is, the detector  25  is configured as a switch that is turned on when the pilot check valve  10  is in the valve closed state, and is turned off when the pilot check valve  10  is not in the valve closed state. 
         [0046]    When the lip  62  of the packing  20  is moved from the valve open position ( FIG. 3 ) toward the valve closed position ( FIG. 1 ), the lip  62  starts to make contact with (seal) the inner circumferential surface of the flow passage body  16  at a position located between the valve closed position and the valve open position, as shown in  FIG. 2 . Hereinafter, the position of the packing  20  shown in  FIG. 2  is referred to as “a sealing start position”. In the present configuration, when the lip  62  is moved from the valve open position to the valve closed position, the lip  62  is placed in sliding contact with the inner circumferential surface of the flow passage body  16  within a predetermined range in the axial direction (the range from the sealing start position to the valve closed position). In this way, a sealing mechanism formed by contact of the lip  62  with the inner circumferential surface contains an overlap region in the axial direction. 
         [0047]    In the case that the packing  20  is moved from the valve open position to the valve closed position, the detector  25  detects a magnetic field of the magnet  82  and outputs a signal after the packing  20  has been moved beyond the sealing start position. More specifically, in the case that the packing  20  is moved from the valve open position to the valve closed position, if the packing  20  only reaches the sealing start position, then the detector  25  neither detects a magnetic field of the magnet  82  nor outputs a signal. Thus, in the pilot check valve  10 , when the detector  25  is outputting a signal, the valve is in a fully closed state (the packing  20  is at the valve closed position). 
         [0048]    When the pilot check valve  10  as constructed above is applied, for example, to a fluid pressure circuit  102  shown in  FIG. 5 , a cylinder  104  can be prevented from dropping down. 
         [0049]    The fluid pressure circuit  102  is equipped with a cylinder  104  for moving up and down a heavy load W, a first supply/discharge passage  108  connected to a rod chamber  106  of the cylinder  104 , a second supply/discharge passage  110  connected to a head chamber  105  of the cylinder  104 , a first speed controller  112  provided in the first supply/discharge passage  108 , a second speed controller  114  provided in the second supply/discharge passage  110 , and the pilot check valve  10  provided in the first supply/discharge passage  108 . 
         [0050]    The fluid pressure circuit  102  is further equipped with a solenoid switching valve  116  connected to the first and second supply/discharge passages  108 ,  110 , a pressure supply source  118  connected to the solenoid switching valve  116 , and a pilot flow passage  120  branching from the second supply/discharge passage  110  and which is connected to the pilot port  86  of the pilot check valve  10 . In this case, the first port  12  of the pilot check valve  10  is connected to the side of the solenoid switching valve  116 , and the second port  14  thereof is connected to the side of the rod chamber  106  of the cylinder  104 . 
         [0051]    In the above-constructed fluid pressure circuit  102 , as shown in  FIG. 5 , when the solenoid switching valve  116  is operated so as to establish communication between the first supply/discharge passage  108  and the pressure supply source  118 , a working fluid from the pressure supply source  118  flows into the flow passage body  16  through the first port  12  of the pilot check valve  10 . In this case, since a pilot pressure is not applied to the pilot check valve  10 , as shown in  FIG. 1 , under the action of the elastic force of the spring  94 , the packing  20  is placed at the valve closed position. 
         [0052]    In the meanwhile, the working fluid flows into the second body  28  through the side hole  46  provided on the sleeve portion  36  of the second body  28 , and passes through the packing  20  while deforming the packing  20  radially inwardly. More specifically, under the action of the working fluid, the lip  62  is pressed radially inwardly, whereby the lip  62  is separated away from the inner circumferential surface of the sleeve portion  36  (reduced-diameter portion  50 ) to thereby form a gap, and the working fluid then flows to the second port  14  through the gap. In this case, since the passages  70  (see  FIG. 4 ) are formed in the axial direction on the outer circumference of the packing support member  22 , flow of the working fluid from the first port  12  toward the second port  14  is not blocked. 
         [0053]    Further, in  FIG. 5 , the working fluid flowing out of the second port  14  flows into the rod chamber  106  of the cylinder  104 , whereas the fluid in the head chamber  105  of the cylinder  104  is discharged to the atmosphere through the second supply/discharge passage  110  and the solenoid switching valve  116 , so that the cylinder  104  is moved up. When the cylinder  104  reaches an upper end position, the pressure difference between the upstream side and the downstream side of the packing  20  (pressure difference between the first port  12  side and the second port  14  side) becomes zero, and as a result, the lip  62  comes into close contact with the inner circumferential surface of the sleeve portion  36  again. In this state, since the packing  20  that is placed at the valve closed position blocks flow of the working fluid from the second port  14  toward the first port  12 , the cylinder  104  is kept in position. Thus, the cylinder  104  can be prevented from dropping down. 
         [0054]    On the other hand, when the solenoid switching valve  116  is operated so as to establish communication between the second supply/discharge passage  110  and the pressure supply source  118 , the working fluid from the pressure supply source  118  is supplied to the head chamber  105  of the cylinder  104  through the second supply/discharge passage  110 , while the working fluid is introduced into the first pressure chamber  92   a  of the pilot check valve  10  through the pilot flow passage  120 . Accordingly, the piston  78  receives an acting force based on the pilot pressure, and the drive rod  80  joined to the piston  78  and the working rod  18  joined to the drive rod  80  are moved in the axial direction (X1 direction). 
         [0055]    Accompanying the movement of the working rod  18 , as shown in  FIG. 3 , the packing  20  is moved to the valve open position, and the first port  12  and the second port  14  are brought into communication with each other. That is, since the fluid is allowed to flow from the second port  14  toward the first port  12 , the fluid in the rod chamber  106  of the cylinder  104  is discharged to the atmosphere through the first supply/discharge passage  108  and the solenoid switching valve  116 , so that the cylinder  104  is moved downward. In this case, the dynamic pressure of the fluid that flows from the second port  14  toward the first port  12  acts on the lip  62  of the packing  20 . However, since the lip  62  is held from the outer side by the support pieces  68  of the packing support member  22 , excessive deformation (rolling back) of the lip  62  is prevented from occurring. 
         [0056]    As described above, the pilot check valve  10  according to the present embodiment can detect whether or not the valve is in the valve closed state (whether or not the packing  20  is placed at the valve closed position) based on whether or not the detector  25  is outputting a signal. More specifically, when the valve is in the valve closed state, the detector  25  is turned on responsive to the magnetic field of the magnet  82 , and then outputs a signal. Thus, in the fluid pressure circuit  102 , after the cylinder  104  has reached the upper end position, in a state that the signal is being outputted from the detector  25 , the supply pressure applied to the first port  12  is released. In this manner, the cylinder  104  can be prevented from dropping down. 
         [0057]    Further, when the packing  20  is placed at the valve closed position, the working fluid is allowed to flow from the first port  12  toward the second port  14 , whereas the flow of the working fluid from the second port  14  toward the first port  12  is blocked. Thus, also in a state that a pilot pressure is not applied to the pilot check valve  10 , a check valve function can be fulfilled effectively. 
         [0058]    In the case of the present embodiment, when the packing  20  is moved from the valve open position to the valve closed position, the lip  62  of the packing  20  starts to make contact with the inner circumferential surface of the flow passage body  16  at a position (sealing start position) located between the valve closed position and the valve open position. With the structure, when the lip  62  of the packing  20  is moved from the valve open position toward the valve closed position, the lip  62  is placed in sliding contact with the inner circumferential surface of the flow passage body  16  within a predetermined range in the axial direction. 
         [0059]    In this manner, the sealing mechanism formed by contact of the lip  62  with the inner circumferential surface has an overlap region in the axial direction. Owing to the overlap region, since detection error by the detector  25  and influence of hysteresis are eliminated, the detector  25  can be prevented from outputting a signal in a state that the valve is not fully closed. Stated otherwise, it is secured that the valve is fully closed when the detector  25  is outputting a signal. Thus, reliability of the position detecting function can be enhanced. 
         [0060]    Further, the pilot check valve  10  according to the present embodiment is equipped with the packing support member  22  surrounding the packing  20  and which prevents the lip  62  from being deformed radially outwardly by a predetermined amount or more. Thus, excessive deformation of the lip  62  is prevented when the fluid flows from the second port  14  toward the first port  12 , and durability of the packing  20  can be enhanced. 
         [0061]    Further, the packing support member  22  is provided with the passages  70  that allow the working fluid to flow from the first port  12  toward the lip  62 . Thus, a function for protecting the lip  62  can be suitably fulfilled without Inhibiting the check valve function of the packing  20 . 
         [0062]    The scope of application of the present embodiment is not limited to the fluid pressure circuit  102  as shown in  FIG. 5 . For example, the present invention may be applied to a fluid pressure circuit where two pilot check valves  10  are connected respectively to the head chamber and the rod chamber of the cylinder, and the position of the cylinder is fixed and held (i.e., operation of the cylinder is restricted) by the check valve function of the two pilot check valves  10  at the time of emergency stop. 
         [0063]    Although a preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the present embodiment, and it goes without saying that various design modifications may be made to the embodiment without departing from the scope of the present invention as set forth in the appended claims.