Patent Publication Number: US-2021190222-A1

Title: Relief valve

Description:
TECHNICAL FIELD 
     The present invention relates to a relief valve. 
     BACKGROUND ART 
     JP2001-343082A discloses a relief valve in which a poppet  10  is opened as a check valve cv is opened to allow working fluid to flow from a high-pressure passage  3  to a low-pressure passage  4 , and thereby, a pressure on the high-pressure passage  3  side is maintained at a predetermined pressure. 
     In a relief valve described in JP2001-343082A, in a case in which negative pressure is generated on the high-pressure passage  3  side, in other words, in a case in which the pressure on the high-pressure passage  3  side has become lower than a pressure on the low-pressure passage  4  side, a seat portion  12  is opened as a poppet  7  is moved against a spring  51 . With such a configuration, the working fluid on the low-pressure passage  4  side is supplied to the high-pressure passage  3  side, and therefore, occurrence of a cavitation is prevented. 
     With the relief valve described in JP2001-343082A, the larger the pressure difference between the pressure acting on the outer side of the poppet  7  and the pressure acting on the inner side of the poppet  7  is, the larger the opening degree of the poppet  7  becomes, and thereby, the suction capacity is improved. A pilot chamber  11  inside the poppet  7  is communicated with the high-pressure passage  3  through a communication hole  14  of a control piston P. 
     SUMMARY OF INVENTION 
     However, when the poppet  7  is opened and the working fluid starts to flow from the low-pressure passage  4  to the high-pressure passage  3 , in a region close to the low-pressure passage  4  in the high-pressure passage  3 , the pressure is increased relative to a region distant from the low-pressure passage  4 . Therefore, if an opening portion of the communication hole  14  is located in the region close to the low-pressure passage  4  in the high-pressure passage  3 , the pressure difference between the pressure acting on the outer side of the poppet  7  and the pressure acting on the inner side of the poppet  7  is decreased, and the opening degree of the poppet  7  is not increased sufficiently. Thus, there is a risk in that it may become impossible to ensure a sufficient suction capacity. 
     An object of the present invention is to improve a suction capacity of a relief valve. 
     According to one aspect of the present invention, a relief valve includes a valve case attached to an apparatus main body, the apparatus main body being provided with a high-pressure passage and a low-pressure passage; a suction poppet provided in the valve case, the suction poppet being configured to allow flow of working fluid from the low-pressure passage to the high-pressure passage in a valve-opened state in which the suction poppet is separated from a first seat portion provided in the apparatus main body, and the suction poppet being configured to shut off communication between the high-pressure passage and the low-pressure passage in a valve-closed state in which suction poppet is seated onto the first seat portion; a main poppet provided in the suction poppet, the main poppet being configured to allow flow of the working fluid from the high-pressure passage to the low-pressure passage in a valve-opened state in which the main poppet is separated from a second seat portion provided in the suction poppet, and the main poppet being configured to shut off communication between the high-pressure passage and the low-pressure passage in a valve-closed state in which the main poppet is seated onto the second seat portion; a guide plug configured to define a back-pressure chamber in the suction poppet between the guide plug and the main poppet; a pilot passage provided in the main poppet, the pilot passage being configured such that the high-pressure passage and the back-pressure chamber are communicated through the pilot passage; and a pilot poppet configured to open/close a passage through which the back-pressure chamber and a drain chamber provided in the guide plug are communicated, wherein when a pressure in the high-pressure passage reaches a set pressure, the main poppet is opened as the pilot poppet is opened, and thereby, the working fluid is guided from the high-pressure passage to the low-pressure passage; when the pressure in the high-pressure passage becomes lower than a pressure in the low-pressure passage, the suction poppet is opened and the working fluid is guided from the low-pressure passage to the high-pressure passage; the suction poppet has a first seating portion configured to be seated onto the first seat portion; the main poppet has a second seating portion and a projecting portion, the second seating portion being seated onto the second seat portion, and the projecting portion projecting out from the second seating portion towards the high-pressure passage; the projecting portion is provided with a tip-end-side opening portion of the pilot passage, the tip-end-side opening portion facing the high-pressure passage; and a length from the first seating portion to the tip-end-side opening portion is set so as to be equal to or longer than an outer diameter of the projecting portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a relief valve. 
         FIG. 2A  is a front view of a pilot piston viewed from the IIa direction in  FIG. 3A . 
         FIG. 2B  is a sectional view taken along a line IIb-IIb in  FIG. 2A . 
         FIG. 3A  is a plan view of the pilot piston viewed from the IIIa direction in  FIG. 2A . 
         FIG. 3B  is a sectional view of a main poppet and shows a cross-section of the pilot piston taken along a line IIIb-IIIb in  FIG. 3A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A relief valve  100  according to an embodiment of the present invention will be described with reference to the drawings. 
     When a pressure of working oil in a high-pressure passage  4 H reaches a set pressure, the relief valve  100  is opened to allow the working oil to escape from the high-pressure passage  4 H to a low-pressure passage  4 L, and thereby, the pressure of the working oil in the high-pressure passage  4 H is prevented from becoming abnormally high. In addition, the relief valve  100  has an antivoid function and prevents occurrence of a cavitation by being opened when a negative pressure is generated in the high-pressure passage  4 H to supply the working oil from the low-pressure passage  4 L to the high-pressure passage  4 H. In the present embodiment, although the working oil is used as the working fluid, other fluid such as working water, compressed air, or the like may also be used as the working fluid. 
     The relief valve  100  is attached to an apparatus main body  1  by being screw-fastened. The apparatus main body  1  is a main body of a hydraulic apparatus such as a hydraulic cylinder, a hydraulic pump, a hydraulic motor, a valve block having a plurality of valves, and so forth. The apparatus main body  1  is provided with the high-pressure passage  4 H and the low-pressure passage  4 L with the relief valve  100  provided therebetween as a boundary. In the apparatus main body  1 , a first seat portion  1   a  is provided between the high-pressure passage  4 H and the low-pressure passage  4 L such that a suction poppet  3  of the relief valve  100 , which will be described later, seats thereon. In the above, the apparatus main body  1  is not limited to the main body of the hydraulic apparatus, and the apparatus main body  1  also includes a block body provided between the respective hydraulic apparatus. 
     As shown in  FIG. 1 , the relief valve  100  has a cylindrical valve case  2 , a cylindrical suction poppet  3 , a main poppet  5 , a guide plug  7 , a pilot poppet  9 , and a case plug  13 . The left end side in the figure of the cylindrical valve case  2  is attached to the apparatus main body  1 . The cylindrical suction poppet  3  is provided in the valve case  2 . The main poppet  5  is provided in the suction poppet  3 . The guide plug  7  forms, together with the main poppet  5 , a back-pressure chamber  8  in the suction poppet  3 . The pilot poppet  9  opens/closes a passage  21  through which the back-pressure chamber  8  and a drain chamber  12  provided in the guide plug  7  are communicated with each other. The case plug  13  closes an opening of the valve case  2  on the right end side in the figure. The case plug  13  has a base portion  14  that is screw-fastened to the valve case  2  and a spring bearing portion  15  that is screw-fastened to the base portion  14 . 
     The suction poppet  3  is provided so as to be movable in the valve case  2  in the axial direction, and a part thereof is projected out from an opening of the valve case  2  on the left end side in the figure. In this description, when simply referred to as “the axial direction”, it means the center axial direction of the relief valve  100 , in other words, the operating directions of the respective components of the relief valve  100 . 
     The suction poppet  3  is a bottomed cylindrical member having a cylindrical portion and a bottom portion. The suction poppet  3  has an accommodating hole  30  that accommodates the main poppet  5 . In the suction poppet  3 , the bottom portion is provided with a high-pressure port  3 H that communicates with the high-pressure passage  4 H, and the cylindrical portion is provided with a low-pressure port  3 L that communicates with the low-pressure passage  4 L. 
     A tip end portion of the suction poppet  3  is formed to have a gradually narrowing tapered shape the outer diameter and the inner diameter of which are reduced towards the end in the axial direction. The tapered shaped tip end portion of the suction poppet  3  is configured such that an outer circumferential side thereof is formed as a first seating portion  3   a  that is seated onto the first seat portion  1   a  of the apparatus main body  1  and such that an inner circumferential side thereof is formed as a second seat portion  3   b  onto which the main poppet  5 , which will be described later, is seated. 
     As described later, the suction poppet  3  is a valve body that operates in the opening direction when the negative pressure is generated in the high-pressure passage  4 H. In a valve-opened state in which the first seating portion  3   a  is separated from the first seat portion  1   a  provided on the apparatus main body  1 , the suction poppet  3  allows the flow of the working oil from the low-pressure passage  4 L to the high-pressure passage  4 H. In a valve-closed state in which the first seating portion  3   a  is seated onto the first seat portion  1   a  of the apparatus main body  1 , the suction poppet  3  shuts off the communication between the high-pressure passage  4 H and the low-pressure passage  4 L. 
     An opening of the accommodating hole  30  of the suction poppet  3  on the right end side in the figure is closed by the guide plug  7 . The guide plug  7  has a tip end portion  7   a  that is accommodated in the accommodating hole  30  of the suction poppet  3  and a base end portion  7   b  that is held by being sandwiched between a tip end portion of the base portion  14  and a step portion formed in the valve case  2 . As the base end portion  7   b  is held by being sandwiched between the base portion  14  and the valve case  2 , the guide plug  7  is fixed in the valve case  2 . 
     The suction poppet  3  slides along an outer circumferential surface of the tip end portion  7   a  of the guide plug  7 . A seal member that seals a gap between the guide plug  7  and the suction poppet  3  is provided between the outer circumferential surface of the tip end portion  7   a  of the guide plug  7  and an inner circumferential surface of the suction poppet  3 . 
     In the accommodating hole  30  of the suction poppet  3 , the main poppet  5  is provided so as to be slidable in the axial direction. The main poppet  5  has a main body portion  50  that is provided with a sliding hole  50   a  penetrating through the main body portion  50  in the axial direction and a pilot piston  51  that is provided in the sliding hole  50   a  so as to be slidable. The main poppet  5  is provided with a pilot passage  60  through which the high-pressure passage  4 H and the back-pressure chamber  8  are communicated. A detail configuration of the pilot passage  60  will be described later. 
     The main poppet  5  slides along the inner circumferential surface of the accommodating hole  30  of the suction poppet  3 . A seal member that seals a gap between the main body portion  50  and the suction poppet  3  is provided between an outer circumferential surface of the main body portion  50  and the inner circumferential surface of the suction poppet  3 . An outer circumference edge of the main body portion  50  on the tip end side is formed as a second seating portion  5   b  that is seated onto the second seat portion  3   b  of the suction poppet  3 . 
     The back-pressure chamber  8  is a space that is defined by the inner circumferential surface of the suction poppet  3 , a back surface of the main poppet  5 , and a tip end surface of the guide plug  7 . The back-pressure chamber  8  is communicated with the high-pressure passage  4 H via the pilot passage  60 . 
     The pilot piston  51  has a flange portion  52  that is arranged in a recessed portion  55  formed in a surface of the main body portion  50  facing the back-pressure chamber  8  and a columnar shaft portion  53  that extends from the flange portion  52  in the axial direction. A tip end portion of the pilot piston  51 , in other words, a tip end portion of the shaft portion  53  is formed as a projecting portion  53   a  that projects out from a tip end surface of the main body portion  50  facing the high-pressure passage  4 H. In other words, the main poppet  5  has the projecting portion  53   a  that projects out from the second seating portion  5   b  into the high-pressure passage  4 H. 
     A spring  81  is provided between the main body portion  50  of the main poppet  5  and the guide plug  7 , and a spring  82  is provided between the flange portion  52  of the pilot piston  51  and the guide plug  7 . The spring  81  is a biasing member that pushes the main body portion  50  so as to bias the second seating portion  5   b  of the main poppet  5  in the direction in which the second seating portion  5   b  is seated onto the second seat portion  3   b . The spring  82  is a biasing member that pushes the flange portion  52  so as to bias the flange portion  52  in the direction in which the flange portion  52  is brought into contact with a bottom surface of the recessed portion  55  of the main body portion  50 . 
     The main poppet  5  is a valve body that is operated in the opening direction when the pressure in the high-pressure passage  4 H reached the set pressure. In a valve-opened state in which the second seating portion  5   b  is separated from the second seat portion  3   b  provided on the suction poppet  3 , the main poppet  5  allows the flow of the working oil from the high-pressure passage  4 H to the low-pressure passage  4 L. In the valve-closed state in which the second seating portion  5   b  is seated onto the second seat portion  3   b  of the suction poppet  3 , the main poppet  5  shuts off the communication between the high-pressure passage  4 H and the low-pressure passage  4 L. 
     The pilot poppet  9  is assembled into the guide plug  7 . The guide plug  7  is formed with the drain chamber  12  that communicates with the low-pressure passage  4 L through a gap  2   a  between an outer circumferential surface of the suction poppet  3  and an inner circumferential surface of the valve case  2 . The drain chamber  12  communicates with the back-pressure chamber  8  through the passage  21  provided in the guide plug  7 . The passage  21  is provided with a seat portion  21   s  onto which a seating portion  9   a  of the pilot poppet  9  is seated. A restrictor  21   a  that imparts resistance to the flow of the working oil passing therethrough is provided between the seat portion  21   s  and an opening surface facing the back-pressure chamber  8  in the passage  21 . 
     A spring  83  is provided between the pilot poppet  9  and the spring bearing portion  15 . The spring  83  is a biasing member that biases the pilot poppet  9  in the direction in which the seating portion  9   a  is seated onto the seat portion  21   s.    
     The pilot passage  60  that guides the working oil in the high-pressure passage  4 H to the back-pressure chamber  8  will be described. As shown in  FIGS. 1, 2A, 2B, 3A, and 3B , the pilot passage  60  has a tip-end-side passage  61 , a base-end-side passage  62 , a communicating passage  67 , through holes  63 , and a variable restrictor  66 . The tip-end-side passage  61  is provided in the pilot piston  51  and has a tip-end-side opening portion  61   a  facing the high-pressure passage  4 H. The base-end-side passage  62  is provided in the pilot piston  51  and has a base-end-side opening portion  62   a  facing the back-pressure chamber  8 . The communicating passage  67  is provided between an outer circumferential surface of the pilot piston  51  and an inner circumferential surface of the sliding hole  50   a  of the main body portion  50  and through which the tip-end-side passage  61  and the base-end-side passage  62  are communicated. The through holes  63  through which the tip-end-side passage  61  and the communicating passage  67  are communicated. The variable restrictor  66  an opening area of which is changed as the pilot piston  51  slides with respect to the main body portion  50 . 
     In this embodiment, because the pilot passage  60  is provided with the variable restrictor  66  the opening area of which is changed as the pilot piston  51  slides with respect to the main body portion  50 , it is possible to adjust a valve-opening property of the relief valve  100 . 
     The variable restrictor  66  has first restricting holes  64  and a second restricting hole  65  provided in the pilot piston  51 . The first restricting holes  64  and the second restricting hole  65  are formed between the base-end-side passage  62  and the communicating passage  67  and impart resistance to the flow of the working oil passing therethrough. The second restricting hole  65  has a smaller opening area than the first restricting hole  64 . The second restricting hole  65  is provided at a position closer to the base-end-side opening portion  62   a  than the first restricting hole  64 . 
     As the pressure in the high-pressure passage  4 H is increased, the pilot piston  51  is moved in the axial direction against the biasing force exerted by the spring  82 . When the pressure in the high-pressure passage  4 H is lower than a predetermined pressure P 1 , the pilot piston  51  is in a state in which the flange portion  52  is pushed against the recessed portion  55  of the main body portion  50  by the biasing force exerted by the spring  82 . At this time, a projecting amount of the projecting portion  53   a  of the pilot piston  51  is maximized, and so, this state is also referred to as a maximum-projected state. When the pressure in the high-pressure passage  4 H becomes equal to or higher than the predetermined pressure P 1 , the pilot piston  51  slides inside the sliding hole  50   a  so as to approach the guide plug  7 . 
     The first restricting holes  64  communicate with the communicating passage  67  when the pressure in the high-pressure passage  4 H is lower than the predetermined pressure P 1 , and when the pressure in the high-pressure passage  4 H becomes equal to or higher than the predetermined pressure P 1 , the openings of the first restricting holes  64  are closed by the inner circumferential surface of the sliding hole  50   a , and its communication with the communicating passage  67  is shut off. 
     An annular groove  53   c  is formed on an outer circumferential surface of the shaft portion  53  of the pilot piston  51 , and the above-mentioned through holes  63  are formed in the groove  53   c . In this embodiment, the number of the through holes  63  is the same as the number of the first restricting holes  64 , and the inner diameter of each through hole  63  is set so as to have a dimension that is equal to or larger than that of the inner diameter of the first restricting holes  64 . In other words, the total opening area of the through holes  63  is set so as to be equal to or larger than the total opening area of the first restricting holes  64 . 
     As shown in  FIGS. 2A and 2B , the outer circumferential surface of the shaft portion  53  of the pilot piston  51  is formed with a flat surface portion  53   b  that defines a space with the inner circumferential surface of the sliding hole  50   a , and the above-mentioned second restricting hole  65  is formed in the flat surface portion  53   b . As shown in  FIGS. 1 to 3B , the second restricting hole  65  is communicated with the communicating passage  67  when the pressure in the high-pressure passage  4 H is lower than the predetermined pressure P 1 . Furthermore, when the pressure in the high-pressure passage  4 H is equal to or higher than the predetermined pressure P 1 , in other words, even in a state in which the communication between the first restricting holes  64  and the communicating passage  67  is shut off, the second restricting hole  65  is communicated with the communicating passage  67 . 
     The tip-end-side opening portion  61   a  of the pilot passage  60  facing the high-pressure passage  4 H is provided on a tip end surface of the projecting portion  53   a . As shown in  FIG. 1 , when the suction poppet  3  is in the valve-closed state and the projecting portion  53   a  is in the maximum-projected state, the tip-end-side opening portion  61   a  is positioned at the outer side of the suction poppet  3  in the axial direction. As described later, as the pilot poppet  9  is opened, the working oil in the high-pressure passage  4 H is introduced into the pilot passage  60  from the tip-end-side opening portion  61   a  and is guided to the back-pressure chamber  8  through the variable restrictor  66 . The working oil that has guided to the back-pressure chamber  8  is then guided to the drain chamber  12  by passing through the restrictor  21   a  and guided to the low-pressure passage  4 L through the gap  2   a . The low-pressure passage  4 L is connected to a tank (not shown) that stores the working oil. 
     An main operation of the relief valve  100  according to this embodiment will be described. 
     &lt;Relief Operation&gt; 
     The pilot poppet  9  is opened when the pressure in the high-pressure passage  4 H reaches the set pressure of the pilot poppet  9  (a cracking pressure of the pilot poppet  9 ) that is set by the spring  83 . As the pilot poppet  9  is opened, the working oil in the back-pressure chamber  8  flows into the drain chamber  12  through the passage  21  and is discharged to the low-pressure passage  4 L. In the above, the restrictor  21   a  formed in the passage  21  imparts resistance to the working oil flowing from the back-pressure chamber  8  to the drain chamber  12 . Therefore, the pressure in the back-pressure chamber  8  is maintained so as to fall within a predetermined pressure range that is higher than the pressure in the low-pressure passage  4 L. 
     The working oil in the high-pressure passage  4 H is guided to the back-pressure chamber  8  through the variable restrictor  66  of the pilot passage  60 . In other words, the pressure in the back-pressure chamber  8  is determined in accordance with a balance between the opening area of the variable restrictor  66  that is an upstream-side restrictor and an opening area of the restrictor  21   a  on the downstream side. 
     In a state in which before the pressure in the high-pressure passage  4 H reaches the cracking pressure of the main poppet  5 , the working oil in the high-pressure passage  4 H flows into the back-pressure chamber  8  through the first restricting holes  64  of the pilot passage  60 , and a differential pressure is caused between the back-pressure chamber  8  and the high-pressure passage  4 H in accordance with the first restricting holes  64 . When the pressure difference between the high-pressure passage  4 H and the back-pressure chamber  8  is increased, the pilot piston  51  is moved against the biasing force exerted by the spring  82 . As the pressure in the high-pressure passage  4 H is increased and the communication between the first restricting holes  64  and the communicating passage  67  is shut off due to the movement of the pilot piston  51 , the working oil in the tip-end-side passage  61  flows into the base-end-side passage  62  through a space between the flat surface portion  53   b  and the inner circumferential surface of the sliding hole  50   a  and the second restricting hole  65 , and the working oil is guided to the back-pressure chamber  8 . 
     Because the opening area of the second restricting hole  65  is smaller than those of the first restricting holes  64 , the pressure difference between the high-pressure passage  4 H and the back-pressure chamber  8  is increased. Thus, the second seating portion  5   b  of the main body portion  50  is separated from the second seat portion  3   b  of the suction poppet  3 , and the main poppet  5  is shifted to the valve-opened state. As the main poppet  5  is opened, the working oil is guided from the high-pressure passage  4 H to the low-pressure passage  4 L, and thereby, it is possible to prevent the pressure in the high-pressure passage  4 H from becoming abnormally high. 
     As described above, soon after the pilot poppet  9  is opened, in other words, in a state in which the pressure in the high-pressure passage  4 H is equal to or higher than the cracking pressure of the pilot poppet  9  but lower than the cracking pressure of the main poppet  5 , the working oil flows from the high-pressure passage  4 H to the back-pressure chamber  8  through the first restricting holes  64  having larger opening area than the second restricting hole  65 . Therefore, it is possible to suppress occurrence of a phenomenon of start-to-leak in which the working oil flows from the high-pressure passage  4 H to the low-pressure passage  4 L as the main poppet  5  is opened slightly before the pressure reaches the cracking pressure of the main poppet  5 . 
     Furthermore, as the first restricting holes  64  are closed along with the increase in the pressure in the high-pressure passage  4 H and the working oil flows form the high-pressure passage  4 H to the back-pressure chamber  8  through the second restricting hole  65 , the pressure in the back-pressure chamber  8  is reduced greatly, and the main poppet  5  is opened responsively in response to the increase in the pressure in the high-pressure passage  4 H. In other words, in a state in which the pressure in the high-pressure passage  4 H is equal to or higher than the cracking pressure of the main poppet  5 , the working oil is guided to the back-pressure chamber  8  through the second restricting hole  65  having smaller opening area than the first restricting holes  64 , and thereby, it is possible to improve an override property of the relief valve  100 . 
     &lt;Suction Operation&gt; 
     On the other hand, when the negative pressure is generated in the high-pressure passage  4 H, in other words, the pressure in the high-pressure passage  4 H becomes lower than the pressure in the low-pressure passage  4 L, the suction poppet  3  is opened to guide the working oil from the low-pressure passage  4 L to the high-pressure passage  4 H. During a suction operation, a force Fs acting on the suction poppet  3  is expressed as Equation (1) below. 
       (Eq. 1) 
         Fs =( P   L   −P   H )×( D   2   2   −D   1   2 )×π/4  (1)
 
     Wherein, P L  is the pressure in the low-pressure passage  4 L, P H  is the pressure in the high-pressure passage  4 H, D 2  is the inner diameter of the inner circumferential surface of the suction poppet  3  that is in sliding contact with an outer circumferential surface of the guide plug  7 , and D 1  is the outer diameter of the first seating portion  3   a  that is seated onto the first seat portion  1   a.    
     The larger the difference between the pressure P L  in the low-pressure passage  4 L and the pressure P H  in the high-pressure passage  4 H is, the larger the force Fs acting on the suction poppet  3  becomes, and the greater the amount of opening of the suction poppet  3  becomes. The greater the amount of opening of the suction poppet  3  is, the higher the suction capacity becomes. 
     Here, when the first seating portion  3   a  of the suction poppet  3  is separated from the first seat portion  1   a  and when the working oil start flow from the low-pressure passage  4 L to the high-pressure passage  4 H, the pressure becomes high in a region A 1  close to the low-pressure passage  4 L in the high-pressure passage  4 H relative to the pressure in a region A 2  distant from the low-pressure passage  4 L. Therefore, if the tip-end-side opening portion  61   a  is located in the region A 1  in the high-pressure passage  4 H that is close to the low-pressure passage  4 L, the difference between the pressure acting on the outer side of the suction poppet  3  and the pressure acting on the inner side of the suction poppet  3  is reduced, and the opening degree of the suction poppet  3  does not become sufficiently large, and thereby, there is a risk in that it may become impossible to ensure a sufficient suction capacity. 
     In contrast, in this embodiment, the tip-end-side opening portion  61   a  is located in the region A 2  distant from the low-pressure passage  4 L in the high-pressure passage  4 H. It is confirmed based on experiments, etc. that the sufficient suction capacity can be achieved, if a length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is equal to or longer than the outer diameter D 0  of a part of the projecting portion  53   a  that slides in the sliding hole  50   a  when the suction poppet  3  is in the valve-closed state. Therefore, when the suction poppet  3  is in the valve-closed state, the length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is set so as to be equal to or longer than the outer diameter D 0  of the projecting portion  53   a.    
     As described above, in this embodiment, by increasing the pressure difference acting between the outer side and the inner side of the suction poppet  3  by locating the tip-end-side opening portion  61   a  in the region A 2  distant from the low-pressure passage  4 L in the high-pressure passage  4 H, it is possible to improve the suction capacity of the relief valve  100 . 
     According to the above-described embodiment, following operational advantages are afforded. 
     The length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is set so as to be equal to or longer than the outer diameter D 0  of the projecting portion  53   a . With such a configuration, because the pressure in the back-pressure chamber  8  is equalized to the pressure in the region A 2  distant from the low-pressure passage  4 L in the high-pressure passage  4 H, the difference between the pressure acting on the outer side of the suction poppet  3  and the pressure acting on the inner side of the suction poppet  3  is increased. With such a configuration, the amount of opening of the suction poppet  3  during the suction operation is increase, and thereby, it is possible to improve the suction capacity of the relief valve  100 . 
     Following modifications are also within the scope of the present invention, and it is also possible to combine the configurations shown in the modification with the configurations described in the above-described embodiment, and to combine the configurations described in the following different modifications. 
     &lt;First Modification&gt; 
     In the above-mentioned embodiment, although a description has been given of an example in which the tip-end-side opening portion  61   a  is formed in the tip end surface of the projecting portion  53   a , the present invention is not limited to this configuration. The tip-end-side opening portion  61   a  may be formed in the outer circumferential surface of the projecting portion  53   a . In this case, the shortest distance from the tip-end-side opening portion  61   a  to the first seating portion  3   a  in the axial direction is set so as to be equal to or longer than the outer diameter D 0  of the projecting portion  53   a.    
     &lt;Second Modification&gt; 
     In the above-mentioned embodiment, although a description has been given of an example in which the main poppet  5  is configured with the main body portion  50  and the pilot piston  51 , the present invention is not limited to this configuration. The main poppet  5  may be formed as, for example, a valve body in which the main body portion  50  and the pilot piston  51  described in above-mentioned embodiment are formed integrally. In this case, a part corresponding to the pilot piston  51  does not move relative to the main body portion  50 . Also in such a case, by setting the length from the tip-end-side opening portion formed in the projecting portion projecting out from the main poppet to the first seating portion so as to be equal to or longer than the outer diameter of the projecting portion, it is possible to improve the suction capacity of the relief valve. 
     &lt;Third Modification&gt; 
     In the above-mentioned embodiment, although a description has been given of an example in which the second restricting hole  65  comes to communicate with the communicating passage  67  when the pressure in the high-pressure passage  4 H is equal to or higher than the predetermined pressure P 1 , the present invention is not limited to this configuration. Instead of forming the second restricting hole  65 , it may be possible to provide, between the outer circumferential surface of the shaft portion  53  and the inner circumferential surface of the sliding hole  50   a , an annular gap serving as a restrictor that imparts resistance to the working oil passing therethrough. In this case, when the pressure in the high-pressure passage  4 H becomes equal to or higher than the predetermined pressure P 1 , the above-described annular gap comes to communicate with the communicating passage  67 . In other words, the working oil in the high-pressure passage  4 H is guided to the back-pressure chamber  8  through the tip-end-side passage  61 →the through hole  63 →the communicating passage  67 →the space between the flat surface portion  53   b  and the inner circumferential surface of the sliding hole  50   a →the above-described annular gap between the outer circumferential surface of the shaft portion  53  and the inner circumferential surface of the sliding hole  50   a . In the above, the resistance imparted to the flow of the working oil flowing therethrough may be adjusted by providing both of the second restricting hole  65  and the above-described annular gap in the pilot piston  51 . 
     The configurations, operations, and effects of the embodiment of the present invention configured as described above will be collectively described. 
     The relief valve  100  includes the valve case  2 , the suction poppet  3 , the main poppet  5 , the guide plug  7 , the pilot passage  60 , the pilot poppet  9 , the pilot piston passage  60 , and the pilot poppet  9 . The valve case  2  is attached to the apparatus main body  1 , and the apparatus main body  1  is provided with the high-pressure passage  4 H and the low-pressure passage  4 L. The suction poppet  3  is provided in the valve case  2 , and the suction poppet  3  is configured to allow the working fluid of from the low-pressure passage  4 L to the high-pressure passage  4 H in the valve-opened state in which the suction poppet  3  is separated from the first seat portion  1   a  provided in the apparatus main body  1 , the suction poppet  3  being configured to shut off the communication between the high-pressure passage  4 H and the low-pressure passage  4 L in the valve-closed state in which the suction poppet  3  is seated onto the first seat portion  1   a . The main poppet  5  is provided in the suction poppet  3 , and the main poppet  5  is configured to allow the flow of the working fluid from the high-pressure passage  4 H to the low-pressure passage  4 L in the valve-opened state in which the main poppet  5  is separated from the second seat portion  3   b  provided in the suction poppet  3 , and the main poppet  5  is configured to shut off the communication between the high-pressure passage  4 H and the low-pressure passage  4 L in the valve-closed state in which the main poppet  5  is seated onto the second seat portion  3   b . The guide plug  7  is configured to define, together with the main poppet  5 , the back-pressure chamber  8  in the suction poppet  3 . The pilot passage  60  is provided in the main poppet  5 , and the pilot passage  60  is configured such that the high-pressure passage  4 H and the back-pressure chamber  8  are communicated through the pilot passage  60 . The pilot poppet  9  is configured to open/close the passage  21  through which the back-pressure chamber  8  and the drain chamber  12  provided in the guide plug  7  are communicated. When the pressure in the high-pressure passage  4 H reaches the set pressure, the main poppet  5  is opened as the pilot poppet  9  is opened, and thereby, the working fluid is guided from the high-pressure passage  4 H to the low-pressure passage  4 L. When the pressure in the high-pressure passage  4 H becomes lower than the pressure in the low-pressure passage  4 L, the suction poppet  3  is opened and the working fluid is guided from the low-pressure passage  4 L to the high-pressure passage  4 H. The suction poppet  3  has the first seating portion  3   a  configured to be seated onto the first seat portion  1   a . The main poppet  5  has the second seating portion  5   b  seated onto the second seat portion  3   b  and the projecting portion  53   a  projecting out from the second seating portion  5   b  towards the high-pressure passage  4 H. The projecting portion  53   a  is provided with the tip-end-side opening portion  61   a  of the pilot passage  60 , and the tip-end-side opening portion  61   a  is facing the high-pressure passage  4 H. And the length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is set so as to be equal to or longer than the outer diameter D 0  of the projecting portion  53   a.    
     With such a configuration, the length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is set so as to be equal to or longer than the outer diameter D 0  of the projecting portion  53   a , and therefore, the difference between the pressure acting on the outer side of the suction poppet  3  and the pressure acting on the inner side of the suction poppet  3  is increased. As a result, it is possible to improve the suction capacity of the relief valve  100 . 
     In the relief valve  100 , the main poppet  5  has the main body portion  50  having the second seating portion  5   b  and the pilot piston  51  provided in the sliding hole  50   a  so as to be slidable, and the sliding hole  50   a  is provided in the main body portion  50 . In the relief valve  100  the pilot passage  60  has the variable restrictor  66 , and the variable restrictor  66  is configured such that the opening area of the variable restrictor  66  is changed as the pilot piston  51  slide with respect to the main body portion  50 . In the relief valve  100  the projecting portion  53   a  is the tip end portion of the pilot piston  51 , and the pilot piston  51  projects from the tip end surface of the main body portion  50  facing the high-pressure passage  4 H. And in the relief valve  100  the length L from the first seating portion  3   a  to the tip-end-side opening portion  61   a  is set so as to be equal to or longer than the outer diameter D 0  of the part, and the part is configured to slide in the sliding hole  50   a  in the pilot piston  51 . 
     With such a configuration, because the pilot passage  60  has the variable restrictor  66  the opening area of which is changed as the pilot piston  51  slides with respect to the main body portion  50 , it is possible to adjust the valve-opening property of the relief valve  100 . 
     The relief valve  100  comprise the pilot passage  60 , the tip-end-side passage  61 , the base-end-side passage  62 , and the communicating passage  67 . The pilot passage  60  has the tip-end-side passage  61  provided in the pilot piston  51 . The tip-end-side passage  61  having the tip-end-side opening portion  61   a  facing the high-pressure passage  4 H. The base-end-side passage  62  provided in the pilot piston  51 , the base-end-side passage  62  having the base-end-side opening portion  62   a  facing the back-pressure chamber  8 . The communicating passage  67  provided between the outer circumferential surface of the pilot piston  51  and the inner circumferential surface of the sliding hole  50   a  of the main body portion  50 , and the communicating passage  67  is configured such that the tip-end-side passage  61  and the base-end-side passage  62  are communicated through the communicating passage  67 . In the relief valve  100 , the variable restrictor  66  has the first restricting hole  64  and the second restricting hole  65  provided between the base-end-side passage  62  and the communicating passage  67 , which the first restricting hole  64  and the second restricting hole  65  are configured to impart resistance to the flow of the working fluid passing through the first restricting hole  64  and the second restricting hole  65 . The first restricting hole  64  is communicated with the communicating passage  67  when the pressure in the high-pressure passage  4 H is lower than the predetermined pressure P 1 , and the communication between the first restricting hole  64  and the communicating passage  67  is shut off when the pressure in the high-pressure passage  4 H is equal to or higher than the predetermined pressure P 1 . And the second restricting hole  65  has a smaller opening area than the first restricting hole  64 , and the second restricting hole  65  is communicated with the communicating passage  67  when the pressure in the high-pressure passage  4 H is equal to or higher than the predetermined pressure P 1 . 
     With such a configuration, when the pressure in the high-pressure passage  4 H is lower than the predetermined pressure P 1 , the pressure in the high-pressure passage  4 H is guided to the back-pressure chamber  8  through the first restricting hole  64 , and when the pressure in the high-pressure passage  4 H is equal to or higher than the predetermined pressure P 1 , the pressure in the high-pressure passage  4 H is guided to the back-pressure chamber  8  through the second restricting hole  65  having a smaller opening area than the first restricting hole  64 . With such a configuration, it is possible to suppress the start-to-leak of the working fluid from the relief valve  100  and to improve the override property of the relief valve  100 . 
     Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments. 
     This application claims priority based on Japanese Patent Application No. 2018-049249 filed with the Japan Patent Office on Mar. 16, 2018, the entire contents of which are incorporated into this specification.