Abstract:
This invention provides a pressure control valve  20  including a valve plug  30  facing a seat As at a relief port A that communicates with a pressure circuit  46 , and a spring  32  pressing the valve plug  30  against the seat, and when the fluid pressure in the pressure circuit surpasses the force of the spring  32  for the valve plug, the pressure control valve discharges part of the fluid to keep the interior of the pressure circuit at a specified setting pressure. In the pressure control valve, at one end of the spring for the valve plug which is remote from the valve plug there is provided a pilot piston  40  freely movable in the expansion and compression direction of the spring for the valve plug. There is also provided a spring for adjustment  38   c  urging the pilot piston  40  toward the valve plug side. The pilot piston  40  is so structured as to move in a direction of going away from the valve plug when supplied with a pilot fluid under a specified pressure P pi .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressure control valve for keeping the interior of a pressure circuit at a specified setting pressure. 
     2. Description of the Related Art 
     As a pressure control valve for keeping the interior of a pressure circuit at a specified setting pressure, a direct-operated relief valve  2 , for example, is known which is illustrated in FIG.  6 . 
     The direct-operated relief valve  2  keeps a relief port  3 , which is open to the pressure circuit, closed with a valve plug  6  which is pressed by a pressure adjusting spring  4  against the relief port  3 . When the pressure in the pressure circuit rises above a specified setting pressure, the valve plug  6  is pushed up and the relief port  3  communicates with the pressure circuit and part of the fluid flows out of the pressure circuit into the drain port, so that the pressure circuit can be held at a specified setting pressure. Note that the setting pressure is adjusted by changing the pressure of the spring by turning the adjust screw  8 . 
     In the direct-operated relief valve  2 , the pressure adjusting spring  4  also presses the valve plug  6  and therefore it is necessary to use a strong spring for the pressure adjusting spring  4  when the direct-operated relief valve  2  is used in a pressure circuit at high pressure. However, the use of a large, strong spring makes the relief valve itself large and there is a limit to the strength of the spring used. For this reason, normally, a pilot-operated relief valve  10  shown in FIG. 7 is adopted in pressure circuits for high pressures. 
     The pilot-operated relief valve  10  is structured such that the relief port  3  communicates through a throttle  12  with an upper chamber  14   a  of a main valve  14  and also with a direct-operated relief valve  18  that constitutes a pilot section as illustrated in FIG.  7 . When the pressure of the pressure circuit rises above a specified setting pressure and surpasses the pressure of the spring  18   b , the valve plug  18   a  goes down and the direct-operated relief valve  18  opens. Consequently, the upper chamber  14   a  communicates with the drain port  5 , the pressure of the upper chamber  14   a  decreases, with the result that the main valve  14  opens and the fluid of the pressure circuit flows to the side of the drain port  5 . Thus, the pressure circuit at high pressure can be kept at the specified setting pressure. 
     For use as an alternate pilot section to replace the direct-operated relief valve  18  of the pilot-operated relief valve  10  shown in FIG. 7, if a proportional electromagnetic pressure control section for controlling the pressure of the upper chamber  14   a  by an electric pilot signal is incorporated, it is possible to implement automatic control of the setting pressure of the pressure circuit within a specified range. 
     However, if the proportional electromagnetic pressure control section mentioned above is used, there is a possibility that the resultant pressure control valve is a type of a complicated internal structure, which increases production cost and poses a problem of safety, such as a fire caused by the use of electricity. 
     The present invention has been made with the above problem in mind and has as its object to provide a pressure control valve capable of stepless automatic control of the setting value of a pressure circuit, which is made possible by the adoption of a pilot-operated simple-structure without use of electric signals. 
     SUMMARY OF THE INVENTION 
     The pressure control valve of the present invention comprises a valve plug placed against a valve seat at a relief port connected to a pressure circuit; and a spring for the valve plug to press the valve plug against the valve seat, the pressure control valve being capable of keeping the interior of the pressure circuit at a specified setting pressure by discharging part of the fluid when the fluid pressure in the pressure circuit surpasses the force of the spring for the valve plug, wherein the force of the spring for the valve plug is increased or decreased by the movement of a pilot piston according to an increase or decrease in the pressure of a pilot liquid, the pilot piston freely movable in the direction of expansion and compression of the spring for the valve plug being disposed at one end of the spring for the valve plug away from the valve plug, the pilot piston being provided with an adjusting spring at the other end of the pilot piston opposite the one end where there is the spring for the valve plug, and the pilot piston being so structured as to move in a direction of going away from the valve plug when the pilot piston receives a pilot fluid at a specified pressure. 
     According to the pressure control valve when there is no influx of the pilot fluid, the valve plug is being pressed against the valve seat by the spring for the valve plug and the adjusting spring and therefore the interior of the pressure circuit is held at a specified setting pressure. When the pressure of the pressure circuit rises above the setting pressure and surmounts the force of the spring for the valve plug, the valve plug recedes to open the relief port and allow the fluid from the pressure circuit to enter and flow out of the drain port. And, when the pressure of the pressure circuit goes down and reaches the setting pressure, the valve plug is brought into contact with the valve seat at the relief port by the force of the spring for the valve plug and the interior of the pressure circuit is held at the setting pressure. 
     Because, when the pilot fluid flows in, the pilot piston moves in the direction of going away from the valve plug, the spring for the valve plug is increased in length and the pressure of the spring is decreased. As the pressure of the spring for the valve plug is decreased, the force pressing the valve plug against the valve seat decreases, and thus the pressure control valve works as a pressure control valve with a lower setting pressure. 
     The pressure control valve further comprises a first piston movement limiting means for preventing the pilot piston, which has moved to a position closer to the valve plug, from moving the pilot piston beyond the above-mentioned closer position on the side of the valve plug even if the pressing force of the adjusting spring is increased. The pilot piston does not move in the direction of going away from the valve plug before the pressure of the pilot fluid rises to a specified value and a fixed large spring force presses the valve plug against the valve seat. Therefore, this pressure control valve can be used as a pressure control valve with an upper limit of setting pressure. 
     The pressure control valve furthermore comprises a second piston movement limiting means for preventing the pilot piston, which has moved to a position remote from the valve plug, from moving to a remoter position beyond the above-mentioned remote position even if the pressing force of the adjusting spring is decreased. When the pressure of the pilot fluid rises above a specified value, a fixed small spring force of the spring for the valve plug presses the valve plug against the valve seat. Therefore, this pressure control valve can be used as a pressure control valve with a lower limit of setting pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a pressure control valve according to the present invention. 
     FIG. 2 is a diagram of setting pressure control when the pressure control valve according to the present invention is used. 
     FIGS.  3 ( a ) and  3 ( b ) are diagrams showing a first example of arrangement of a pilot piston used in the pressure control valve according to the present invention. 
     FIGS.  4 ( a ) and  4 ( b ) are diagrams showing a second example of arrangement of the pilot piston used in the pressure control valve according to the present invention. 
     FIGS.  5 ( a ) and  5 ( b ) are diagrams showing a third example of arrangement of the pilot piston used in the pressure control valve according to the present invention. 
     FIG. 6 is a diagram showing a direct-operated relief valve. 
     FIG. 7 is a diagram showing a pilot-operated relief valve. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 shows a relief valve  20  according to a first embodiment of the present invention. 
     A valve main body  22  of the relief valve  20  has formed therein a valve chamber  24  as an internal cylindrical space with its central axis extending on the axis S 1  and a cylinder chamber  26  as an internal cylindrical space with a larger diameter than the valve chamber  24  located adjacent to and above the valve chamber  24  with its central axis extending on the same axis S 1 . 
     The valve main body  22  has further formed therein a relief port A communicating with the valve chamber  24  with its central axis extending on the axis S 1 . The valve main body  22  has additionally formed therein a drain port D opening at a portion, near the relief port A, of the internal circumferential surface of the valve chamber  24  and a pilot port B opening at another portion, near the cylinder chamber  26 , of the internal circumferential surface of the valve chamber  24 . The inner diameter of the valve chamber  24  is formed such that the side where there is the cylinder chamber  26  has a large diameter  24   a   1  and the other side where there is the relief port A has a small diameter  24   a   2 . The diameter of the valve chamber  24  changes at the pilot port B. The working fluid of the pressure circuit  46  flows into the relief port A. 
     In the valve chamber  24 , there is disposed a valve plug  30  whose conical end portion is placed against the valve seat As of the relief port A, and also there is disposed a spring  32  for the valve plug pressing the valve plug  30  against the seat As. Formed at the basal end of the valve plug  30  is a cylindrical guide portion  30   a  for guiding the spring  32  for the valve plug by having the spring  32  for the valve plug slide along the surface thereof. 
     Disposed through the valve chamber  24  and the cylinder chamber  26  is a pilot piston  40  which is freely slidable in the direction of the axis S 1 . The pilot piston  40  includes a piston barrel  40   a  of circular cross-section inserted in the valve chamber  24 , and a piston head  40   b , which is larger in diameter than the piston barrel  40   a  and located in the cylinder chamber  26 . The pilot piston  40  has an oil hole extending along the axis S 1 . The bottom end of the piston barrel  40   a  engages against the end of the spring  32  for the valve plug away from the valve plug  30 . 
     The piston barrel  40   a  consists of a larger-diameter portion  40   a   1  that slides on the large-diameter wall  24   a   1  of the valve chamber  24 , and a small-diameter portion  40   a   2  that slides on the small-diameter wall  24   a   2  of the valve chamber  24 . Those two different-diameter portions form a pressure receptor  40   a   3 . 
     The pilot port B is connected to a pilot oil pressure setter  44 , and a pilot pressure P pi , set at a specified pressure by the pilot oil pressure setter  44 , is supplied through the pilot port B to the valve chamber  24 . The pilot oil that has flowed into the valve chamber  24  acts on the pressure receptor  40   a   3  of the pilot piston  40 , causing the pilot piston  40  to move up. 
     Meanwhile, internal threads are formed on the inner surface of the cylinder chamber  26 . Engaging in the internal threads are the external threads on the outer surface of the connector portion  38   a   1  of the cylinder  38   a  as a component member of an initial value adjuster  38 , and thus the initial value adjuster  38  is connected to the valve main body  22 . 
     The initial value adjuster  38  includes a cylinder  38   a  mentioned above, a piston position setting member  38   b  disposed in the cylinder  38   a  and being in contact with a piston head  40   b  of the pilot piston  40 , an initial value adjusting spring  38   c , provided with a spring constant larger than that of the spring  32  for the valve plug, for giving a specified downward spring force to the pilot piston  40  through a piston position setting member  38   b , an adjust screw  38   d , engaging in the internal threads formed at the top portion of the cylinder  38   a , for varying the spring force of the initial value adjusting spring  38   c , and an end face  38   e , formed at the bottom end of the connector portion  38   a   1 , for engaging with the piston head  40   b  to stop the upward movement of the pilot piston  40 . Therefore, by adjusting the adjust screw  38   d , it is possible to set the initial value of the pilot piston  40  (the position of the pilot piston when P pi =0) in a range from a position of the pilot piston  40  at which the piston head  40   b  contacts the bottom face  26   a  of the cylinder chamber  26  (C 1 &gt;0 and C 2 =0 in FIG. 3, where C 1  denotes the gap between the piston head  40   b  and the end face  38   e  and C 2  denotes the gap between the piston head  40   b  and the bottom face  26   a ) to another position of the pilot piston  40  at which the piston head  40   b  contacts the end face  38   e  (C 1 =0 and C 2 =max). 
     A method for setting the setting pressure Ps for the relief valve  20  according to the first embodiment of the present invention will be described with reference to the setting pressure control diagram in FIG.  2  and the diagrams of FIG. 3 to FIG. 5 showing the interior of the valve. 
     FIG.  3 ( a ) shows the arrangement of the pilot piston  40  when a relief valve  20  provided with an upper limit of the setting pressure is used. In this case, by increasing the screwed-in amount of the adjust screw  38   d  of the initial value adjuster  38  from the condition of C 1 &gt;0 and C 2 =0 (the adjust screw  38   d  is moved downwards), the pilot piston  40  is pushed down through the piston position setting member  38   b  by a large force of the initial value adjusting spring  38   c , to thereby more strongly press the piston head  40   b  against the bottom face  26   a  of the cylinder chamber  26 . In this process, the piston barrel  40   a  of the pilot piston  40  goes down to the side of the valve plug  30 , so that the compressing force of the spring  32  for the valve plug becomes maximum. 
     With the pilot piston  40  located in this position, when the pilot pressure P pi  is a small value, even if a small thrust of the pilot oil acts on the pilot piston  40 , because the pressure of the initial value setting spring  38   c  is greater than this thrust, the pilot piston does not move upwards and the valve plug  30  is pressed against the seat As by a large spring force of the spring  32  for the valve plug, and thus a setting pressure upper limit Ps max  is set. 
     When an increased thrust by a rise in the pilot pressure P pi  surpasses the pressure of the initial value setting spring  38   c , as shown in FIG.  3 ( b ), the pilot piston  40  moves up (C 1 &gt;0 and C 2 &gt;0) and therefore the length of the spring  32  for the valve plug becomes longer. Consequently, the spring force of the spring  32  for the valve plug is reduced and the force pressing the valve plug  30  against the valve seat decreases and as the result, the setting pressure Ps decreases linearly. 
     Therefore, a setting pressure control diagram is obtained in which as indicated by solid lines f 1  and f 2 , when the pilot pressure P pi  is a small value, the setting pressure is at its upper limit Ps max  and when the pilot pressure is higher than a specified value, the setting pressure Ps decreases linearly. 
     FIG.  4 ( a ) shows the arrangement of the pilot piston  40  when a relief valve  20  provided with a lower limit of the setting pressure is used. In this case, the screwed-in amount of the adjust screw  38   d  of the initial value adjuster  38  is reduced (the adjust screw  38   d  is moved upwards), the piston position setting member  38   b  is moved up to a position at which there is provided the gap C 1  between the end face  38   e  of the connector portion  38   a   1  and the piston head  40   b  (C 1 &gt;0, C 2 &gt;0 and C 1 &lt;C 2 ). In this process, the piston barrel  40   a  of the pilot piston  40  moves up, so that the spring force of the spring  32  for the valve plug becomes smaller than in the case shown in FIG.  3 ( a ). 
     With the pilot piston  40  located as mentioned above, when a pilot pressure P pi  is supplied to the pilot port B, the pilot piston  40  is moved up by a thrust generated and the length of the spring  32  for the valve plug is increased. Consequently, the spring force of the spring  32  for the valve plug decreases and the force pressing the valve plug  30  against the seat As decreases and therefore the setting pressure Ps decreases linearly. As the pilot pressure P pi  rises, as shown in FIG.  4 ( b ), the piston head  40   b  contacts the end face  38   e  of the connector portion  38   a   1  (C 1 =0 and C 2 =max), with the result that the spring force of the spring  32  for the valve plug is fixed at the minimum value. Therefore, even if the pilot pressure P pi  rises further, the valve plug  30  is pressed against the seat As by a small spring force of the spring  32  for the valve plug, so that the setting force lower limit Ps min  is set. 
     Accordingly, a setting pressure control diagram can be obtained in which as shown by the solid lines g 1  and g 2  in FIG. 2, when the pilot pressure P pi  is a small value, the setting pressure Ps decreases linearly and when the pilot pressure P pi  is higher than a specified value, the setting pressure stays at its lower limit Ps min . In the case as shown in FIG.  4 ( b ), if the pilot piston  40  is arranged at its full length such that the spring  32  for the valve plug has a natural length, a setting pressure control diagram can be obtained in which the setting pressure lower limit Ps min  is “0” Mpa. 
     FIG.  5 ( a ) shows the arrangement of the pilot piston  40  when a relief valve is used which has been set such that the setting pressure Ps decreases as the pilot pressure P pi  rises. 
     In this case, the relief valve  20  is set by adjusting the screwed-in amount of the initial value adjuster  38  such that C 2 &gt;0 and C 1 &gt;0 when the pilot pressure P pi =0 and that C 2 &gt;0 and C 1 &gt;0 when the pilot pressure is at maximum P pimax . 
     If the pilot piston  40  is arranged as mentioned, when a pilot pressure P pi  of a small value is supplied and a small thrust acts on the pilot piston  40 , as shown in FIG.  4 ( b ), the pilot piston  40  moves up and the spring  32  for the valve plug is increased in length, thus reducing the force pressing the valve plug  30  against the seat As. 
     Thus, it is possible to obtain a setting pressure control diagram in which as the pilot pressure P pi  increases, the setting pressure Ps decreases linearly as shown by a solid line h in FIG.  2 . 
     The setting pressure Ps can be changed freely in the range as indicated by oblique lines by adjusting the spring force of the initial value adjusting spring  38   c  that acts on the pilot piston  40  by operating the initial value adjuster  38 . As has been described, with this relief valve  20  according to this embodiment, by increasing or decreasing the spring force of the spring  32  for the valve plug by changing the amount of movement of the pilot piston  40  in axial direction S 1  by operating the initial value adjuster  38  and inputting a pilot pressure P pi  to the pilot port B, an upper-limit setting pressure Ps max  or a lower-limit setting pressure Ps min  can be set, and a setting pressure Ps can be set freely in a wide range. Therefore, the oil pressure of the pressure circuit  46  can be controlled automatically and steplessly. 
     Because the setting pressure Ps can be set freely by a simple mechanism by which to move the pilot piston  40  in the axial direction S 1 , the relief valve  20  can be manufactured at reduced cost. 
     The use of oil pressure as a pilot signal ensures safety against a fire compared with a prior-art proportional electromagnetic pressure controller using an electric pilot signal. 
     In this embodiment, the bottom face  26   a  of the cylinder chamber  26  which is contacted by the piston head  40   b  when it moves downwards corresponds to the first piston movement limiting means in the present invention, and the end face  38   e  formed at the bottom end of the connector portion  38   a   1  which is contacted by the piston head  40   b  when it moves upwards corresponds to the second piston movement limiting means in the present invention. 
     As described above, in the pressure control valve, when a pilot fluid flows into the pilot port, the pilot fluid moves the pilot piston in the direction of its going away from the valve plug side, and the spring force of the spring for the valve plug, which has been prolonged, is decreased, thus reducing the force pressing the valve plug against the valve seat, with the result that the pressure control valve changes its capacity to become a pressure control valve with a reduced setting pressure. Because, according to the present invention, the setting pressure can be changed freely, the fluid pressure in the pressure circuit can be controlled automatically and steplessly. 
     Because a fluid is used as means to give a pilot signal, the safety of the device against a fire is ensured sufficiently as compared with the conventional proportional magnetic type of pressure controller using an electric pilot signal, and its simple structure makes for the reduction of production cost. 
     According to the invention, the pilot piston does not move in the direction of going away from the valve plug before the pressure of the pilot fluid rises to a specified value, and a fixed large spring force presses the valve plug against the valve seat, and therefore a pressure control valve with an upper-limit setting pressure can be provided. 
     According to the invention, when the pressure of the pilot fluid rises above a specified value, the force pressing the valve plug against the valve seat by the spring for the valve plug becomes a fixed small force, and therefore a pressure control valve with a lower-limit setting pressure can be provided.