Abstract:
Embodiments of the invention provide a first piston with a smaller diameter and a second piston with a larger diameter that switch a spool between a first switching position at one end and a second switching position at the other end, and a return spring that moves the spool to a neutral switching position are provided, at the neutral switching position, some of a plurality of ports communicate with one another and remaining ports are closed; at the first switching position, the communication-and-closure relationship of the plurality of ports is the reverse of that in the neutral switching position; and at the second switching position, all the plurality of ports are closed, and when the spool is switched to the neutral switching position and the first switching position, only the first piston is operated, and when the spool is switched to the second switching position, the second piston is operated.

Description:
BACKGROUND OF THE INVENTION 
     [1] Field of the Invention 
     The present invention relates to a pilot-operated three-position switching valve in which a spool is switched between three switching positions by a pilot fluid pressure and a return spring. 
     [2] Description of the Related Art 
     Pilot-operated three-position switching valves of this type have been known, as disclosed in, for example, Patent Literatures, Japanese Unexamined Patent Application Publication No. 59-190586 and Japanese Unexamined Patent Application Publication No. 11-182728. A known three-position switching valve is configured such that pistons are provided at both ends of a spool, which is slidably accommodated in a valve hole, a spring force of a return spring is applied the spool, the spool is moved in one direction and switched to a first switching position when a pilot fluid pressure is applied to one piston through one pilot valve, the spool is moved in the opposite direction and switched to a second switching position when a pilot fluid pressure is applied to the other piston through the other pilot valve, and the spool is switched to a neutral switching position by the return spring when both pilot valves are turned off and the pilot fluid pressure is removed from both pistons. 
     When the spool is switched to the first switching position, the switching valve goes into a first communication state in which some of a plurality of ports communicate with one another and the remaining ports are closed, when the spool is switched to the second switching position, the switching valve goes into a second communication state in which the communication-and-closure relationship of the plurality of ports is different from that in the first communication state, and when the spool is switched to the neutral switching position, the switching valve goes into a non-communication state in which all the ports are closed. 
     This is a so-called closed-center three-position switching valve, and its symbol is shown in  FIG. 5 . The three-position switching valve shown in  FIG. 5  is a five-port switching valve. 
     BRIEF SUMMARY OF THE INVENTION 
     However, when the spool is switched between the first communication state and the second communication state in the above-mentioned conventional three-position switching valve, a control program for the pilot valves becomes complicated because a pilot fluid pressure needs to be applied alternately to the two pistons by alternately performing on-off control on the two solenoid-operated pilot valves. The control program becomes complicated particularly when a plurality of switching valves are simultaneously controlled, because the number of control points increases in proportion to the number of switching valves. 
     Furthermore, the power consumption is high because one of the pilot valves needs to be powered no matter what state, i.e., the first communication state or the second communication state, the spool is in. 
     Furthermore, when the spool is switched from the first communication state or the second communication state to the neutral switching position, where all the ports are closed for emergency stop, the response speed is a problem because the spool is moved to the neutral switching position only by an urging force of the return spring. 
     Hence, objects of the present invention are to simplify the control program for the pilot valves by enabling the operation to switch the spool between the first communication state and the second communication state to be performed by applying a pilot fluid pressure to or removing the pilot fluid pressure from only one piston, to reduce the power consumption, and to improve the responsiveness by performing an emergency stop operation by applying a pilot fluid pressure to the other piston. 
     In order to achieve the above-described object, the present invention provides a pilot-operated three-position switching valve including a valve body having a plurality of ports; a valve hole provided inside the valve body so as to communicate with the ports; a spool accommodated in the valve hole so as to be slidable in an axial direction and having three switching positions, namely, a first switching position at one end of the valve hole, a second switching position at the other end, and a neutral switching position between the first switching position and the second switching position; a first piston and a second piston that are disposed at one end and the other end of the spool and switch the spool between the first switching position and the second switching position when subjected to a pilot fluid pressure; and a return spring that returns the spool to the neutral switching position. 
     In the above-described three-position switching valve, the first piston has a smaller diameter than the second piston; the neutral switching position is such a switching position where a first communication state is established, in which some of the plurality of ports communicate with one another and the remaining ports are closed; the first switching position is such a switching position where a second communication state is established, in which the spool has been moved toward the second piston with a larger diameter by the first piston with a smaller diameter, and the communication-and-closure relationship of the plurality of ports is different from that in the first communication state; and the second switching position is such a switching position where the spool has been moved toward the first piston with a smaller diameter by the second piston with a larger diameter, and all the plurality of ports are closed and in a mutually non-communication state. 
     In the present invention, when no pilot fluid pressure is applied to the first piston or the second piston, the spool occupies the neutral switching position by the return spring; when a pilot fluid pressure is applied to the first piston with a smaller diameter and no pilot fluid pressure is applied to the second piston with a larger diameter, the spool is switched to the first switching position by the first piston; and when a pilot fluid pressure is applied to the second piston with a larger diameter, the spool is switched to the second switching position by the second piston, regardless of whether or not the pilot fluid pressure is applied to the first piston with a smaller diameter. 
     In the present invention, it is preferable that the plurality of ports include a supply port at the center, a first output port and a second output port on both sides of the supply port, and a first discharge port and a second discharge port on the outer sides of the first output port and the second output port; the spool include two sealing seal members that are provided at positions near the ends of the spool to always close the ends of the valve hole and six (from first to sixth) open/close seal members that open or close flow paths that connect adjacent ports; and the six open/close seal members be disposed so as to occupy asymmetric positions between the two sealing seal members. 
     In this case, the first and second open/close seal members open or close a first discharge flow path connecting the first output port and the first discharge port, the third open/close seal member opens or closes a first output flow path connecting the supply port and the first output port, the fourth and fifth open/close seal members open or close a second output flow path connecting the supply port and the second output port, and the sixth open/close seal member opens or closes a second discharge flow path connecting the second output port and the second discharge port. 
     Furthermore, the present invention may be configured such that a small-diameter portion is formed at a part of the spool, a pair of ring-shaped spring seats are fitted to the small-diameter portion so as to be movable in an axial direction of the spool, the return spring having a coil shape is provided between the pair of spring seats, a fitting groove having the same axial length as the axial length of the small-diameter portion is formed in the inner periphery of the valve body so as to surround the small-diameter portion, the spring seats are fitted to the fitting groove, and, when the spool is located at the neutral switching position, one of the spring seats engages with end walls of the small-diameter portion and the fitting groove at one side, and the other of the spring seats engages with the end walls of the small-diameter portion and the fitting groove at the other side. 
     The small-diameter portion, the pair of spring seats, the return spring, and the fitting groove are disposed at positions close to the second piston with a larger diameter or at positions close to the first piston with a smaller diameter. 
     Furthermore, the present invention provides a method of operating the above-described three-position switching valve. This method includes providing a first pilot valve that drives the first piston, and a second pilot valve that drives the second piston; operating only the first piston with a smaller diameter by the first pilot valve when the plurality of ports are switched to the first communication state and the second communication state by moving the spool to the neutral switching position and to the first switching position; and operating the second piston with a larger diameter when all the plurality of ports are switched to the non-communication state, or a closed state, by moving the spool to the second switching position by the second pilot valve. 
     As has been described, with the present invention, because the operation to switch the spool between the first communication state and the second communication state can be performed by applying a pilot fluid pressure to or removing the pilot fluid pressure from only one piston, the control program for the pilot valves can be simplified, and the power consumption can also be reduced. Furthermore, because the emergency stop operation is performed by applying a pilot fluid pressure to the second piston with a larger diameter to switch the spool, the responsiveness is improved compared with the case where the spool is switched by the spring force of the return spring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an embodiment of a pilot-operated three-position switching valve of the present invention, showing a state in which a spool is located at a neutral switching position. 
         FIG. 2  is a cross-sectional view of the pilot-operated three-position switching valve in  FIG. 1 , showing a state in which the spool is located at a first switching position. 
         FIG. 3  is a cross-sectional view of the pilot-operated three-position switching valve in  FIG. 1 , showing a state in which the spool is located at a second switching position. 
         FIG. 4  is a diagram showing a symbol of the pilot-operated three-position switching valve in  FIG. 1 . 
         FIG. 5  is a diagram showing a symbol of the conventional pilot-operated three-position switching valve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 3  show the specific configuration of a pilot-operated three-position switching valve of the present invention, and  FIG. 4  shows the symbol of this three-position switching valve. This three-position switching valve has a five-port valve configuration. 
     In  FIGS. 1 to 3 , a valve body  10  of the three-position switching valve includes a main body  11  having a plurality of ports P, A, B, EA, and EB; a first piston cover  12  connected to an end of the main body  11 ; and a spring cover  13  and a second piston cover  14 , which are serially connected to the other end of the main body  11 . 
     The plurality of ports include the supply port P at the center, the first output port A and the second output port B located on both sides of the supply port P, the first discharge port EA located on the first piston cover  12  side of the first output port A, and the second discharge port EB located on the second piston cover  14  side of the second output port B. These five ports are located at asymmetric positions with respect to the supply port P and communicate with a valve hole  15 . 
     The circular valve hole  15  communicating with the ports is formed in the main body  11  and the spring cover  13  so as to continuously penetrate the main body  11  and the spring cover  13 , and a spool  16  having a through-hole  16   a  at the center is inserted into the valve hole  15  so as to be slidable in an axial direction L of the valve hole  15 . One end and the other end of the spool  16  extend to piston chambers  12   a  and  14   a  inside the first piston cover  12  and the second piston cover  14  and are in contact with or connected to a first piston  17  and a second piston  18  slidably accommodated in the piston chambers  12   a  and  14   a , respectively. 
     The first piston  17  and the second piston  18  switch the spool  16  between a first switching position in  FIG. 2  and a second switching position in  FIG. 3  when subjected to pilot fluid pressure. The first piston  17  has a smaller diameter than the second piston  18 , and a pressure-receiving surface of the first piston  17  with a smaller diameter faces a first pilot chamber  21  with a smaller diameter, and a pressure-receiving surface of the second piston  18  with a larger diameter faces a second pilot chamber  22  with a larger diameter. 
     The first pilot chamber  21  is connected to a pilot output port PA of a first pilot valve  19 , which is composed of a three-port solenoid valve, through a first pilot output path  23 , and the second pilot chamber  22  is connected to a pilot output port PA of a second pilot valve  20 , which is composed of a three-port solenoid valve, through a second pilot output path  24 , and pilot supply ports PP of the pilot valves  19  and  20  are connected to the supply port P through a pilot supply path  25 . Furthermore, pilot discharge ports PE of the pilot valves  19  and  20  are open to the atmosphere. 
     The first pilot valve  19  and the second pilot valve  20  are attached to the end surfaces of the first piston cover  12  and the second piston cover  14 , and the first pilot output path  23 , the second pilot output path  24 , and the pilot supply path  25  are formed inside the valve body  11 . 
     Furthermore, a rear chamber  26  facing the back surface of the first piston  17  and a rear chamber  27  facing the back surface of the second piston  18  communicate with each other through the through-hole  16   a  in the spool  16 , and gaps and grooves formed between the end surfaces of the spool  16  and the back surfaces of the pistons, and are open to the atmosphere through open paths  28 . 
     As shown in  FIG. 2 , when the first pilot valve  19  is turned on and the pilot fluid is supplied from the supply port P to the first pilot chamber  21 , and when the second pilot valve  20  is turned off and the second pilot chamber  22  is opened to the atmosphere, because the first piston  17  is pressed toward the second piston  18  by the pilot fluid pressure, the spool  16  is moved toward the second piston  18  through the valve hole  15  and switched to the first switching position. 
     Furthermore, as shown in  FIG. 3 , when the second pilot valve  20  is turned on and the pilot fluid is supplied from the supply port P to the second pilot chamber  22  and when the first pilot valve  19  is turned off and the first pilot chamber  21  is opened to the atmosphere, because the second piston  18  is pressed toward the first piston  17  by the pilot fluid pressure, the spool  16  is moved toward the first piston  17  through the valve hole  15  and switched to the second switching position. However, because the pressure-receiving area of the second piston  18  is larger than the pressure-receiving area of the first piston  17 , even when a pilot fluid pressure is applied to the first piston  17  while the first pilot valve  19  is turned on, the spool  16  is switched to the second switching position because of the difference in force of the fluid pressure applied to the pistons  17  and  18 . 
     The spool  16  is provided with a small-diameter portion  31  at a part penetrating the spring cover  13 , and a pair of ring-shaped spring seats  32   a  and  32   b  is fitted to the small-diameter portion  31  so as to be movable in the axial direction L of the spool  16 . A compressed coil-shaped return spring  33  is provided between the pair of spring seats  32   a  and  32   b.    
     On the other hand, a fitting groove  34  having the same axial length (groove width) as the axial length of the small-diameter portion  31  is formed in the inner periphery of the spring cover  13  so as to surround the small-diameter portion  31 , and part of the outer peripheries of the spring seats  32   a  and  32   b  fits into the fitting groove  34 . When the spool  16  is located at the neutral switching position in  FIG. 1 , one of the spring seats, namely, the first spring seat  32   a , engages with end walls  31   a  and  34   a  of the small-diameter portion  31  and the fitting groove  34  at one side, and the other of the spring seats, namely, the second spring seat  32   b , engages with the end walls  31   b  and  34   b  of the small-diameter portion  31  and the fitting groove  34  at the other side. In other words, when no pilot fluid pressure is applied to the first piston  17  or the second piston  18 , the two spring seats  32   a  and  32   b  are urged by the urging force of the return spring  33  and engage with the end walls  31   a  and  34   a  at one side and the end walls  31   b  and  34   b  at the other side of the small-diameter portion  31  and the fitting groove  34 , thereby switching the spool  16  to the neutral switching position shown in  FIG. 1 . 
     As shown in  FIG. 1 , when the spool  16  is switched to the neutral switching position by the return spring  33 , the three-position switching valve goes into the first communication state, in which the supply port P communicates with the second output port B, the first output port A communicates with the first discharge port EA, and the second discharge port EB is closed. 
     Furthermore, as shown in  FIG. 2 , when the spool  16  is switched to the first switching position by the first piston  17 , the three-position switching valve goes into the second communication state, in which the supply port P communicates with the first output port A, the second output port B communicates with the second discharge port EB, and the first discharge port EA is closed. 
     Furthermore, as shown in  FIG. 3 , when the spool  16  is switched to the second switching position by the second piston  18 , all the flow paths that connect adjacent ports in the three-position switching valve are closed, and the ports are in mutually non-communication state. 
     Therefore, as can be seen from  FIG. 3 , the spool  16  is provided with six (from first to sixth) open/close seal members  35   a  to  35   f  that open or close the flow paths that connect adjacent ports, and two sealing seal members  36   a  and  36   b  that always seal the ends of the valve hole  15 . 
     The six open/close seal members (hereinbelow, simply, “seal members”)  35   a  to  35   f  are disposed between the two sealing seal members  36   a  and  36   b  in sequence, such that the seal member located at a position closest to the first piston  17  is the first seal member  35   a  and the seal member located at a position closest to the second piston  18  is the sixth seal member  35   f , and such that they occupy asymmetric positions with respect to the middle position between the two sealing seal members  36   a  and  36   b . The first, third, fourth, and sixth seal members  35   a ,  35   c ,  35   d , and  35   f  open or close the flow paths between adjacent ports when the spool  16  is alternately switched between the neutral switching position and the first switching position, and the second to sixth seal members  35   b  to  35   f  close all the flow paths between adjacent ports when the spool  16  is switched to the second switching position. 
     More specifically, the first seal member  35   a  and the second seal member  35   b  open or close a first discharge flow path A-EA connecting the first output port A and the first discharge port EA, at least the third seal member  35   c  of the third seal member  35   c  and the fourth seal member  35   d  opens or closes a first output flow path P-A connecting the supply port P and the first output port A, the fourth seal member  35   d  and the fifth seal member  35   e  open or close a second output flow path P-B connecting the supply port P and the second output port B, and the sixth seal member  35   f  opens or closes a second discharge flow path B-EB connecting the second output port B and the second discharge port EB. 
     When the spool  16  is located at the neutral switching position, the first output flow path P-A is closed by the third seal member  35   c , the second discharge flow path B-EB is closed by the sixth seal member  35   f , and the second output flow path P-B and the first discharge flow path A-EA are open. 
     Furthermore, when the spool  16  is located at the first switching position, the first discharge flow path A-EA is closed by the first seal member  35   a , the second output flow path P-B is closed by the fourth seal member  35   d , and the first output flow path P-A and the second discharge flow path B-EB are open. 
     Furthermore, when the spool  16  is located at the second switching position, the first and second output flow paths P-A and P-B, and the first and second discharge flow paths A-EA and B-EB are all closed by the second to sixth seal members  35   b  to  35   f.    
     On the other hand, the two sealing seal members  36   a  and  36   b  always seal the ends of the valve hole  15  regardless of which switching position the spool  16  is in. 
     The thus-configured three-position switching valve can switch the flow paths between the ports P, A, B, EA, and EB by applying a pilot fluid pressure to or removing the pilot fluid pressure from the first piston  17  with a smaller diameter by alternately turning on and off only the first pilot valve  19 , while the second pilot valve  20  is turned off so as not to apply a pilot fluid pressure to the second piston  18  with a larger diameter. 
     More specifically, when the second pilot valve  20  is in an off state and the first pilot valve  19  is also in an off state, as shown in  FIG. 1 , the spool  16  is returned by the urging force of the return spring  33  and occupies the neutral switching position, and the ports P, A, B, EA, and EB are in the first communication state. When power is supplied to the first pilot valve  19  from this state, a pilot fluid pressure is applied to the first piston  17 , switching the spool  16  to the first switching position, as shown in  FIG. 2 , and the ports P, A, B, EA, and EB are in the second communication state. When the supply of power to the first pilot valve  19  is stopped, the spool  16  is returned to the neutral switching position, as shown in  FIG. 1 , by the urging force of the return spring  33 , and the ports P, A, B, EA, and EB are in the first communication state. 
     On the other hand, when the fluid pressure device is subjected to an emergency stop during the operation, power is supplied to the second pilot valve  20  to apply a pilot fluid pressure to the second piston  18 . As a result, the spool  16  is switched to the second switching position by the second piston  18 , as shown in  FIG. 3 , and the ports P, A, B, EA, and EB are in the non-communication state. Because the pressure-receiving area of the second piston  18  is larger than the pressure-receiving area of the first piston  17 , this emergency stop operation can be performed when the first pilot valve  19  is in an off state and the spool  16  is in the neutral switching position, as shown in  FIG. 1 , as well as when the first pilot valve  19  is in an on state and the spool  16  occupies the first switching position, as shown in  FIG. 2 . 
     As has been described above, in the three-position switching valve, because the operation to switch the spool  16  between the first communication state and the second communication state can be performed by applying a pilot fluid pressure to or removing the pilot fluid pressure from only the first piston  17  with a smaller diameter, not only can the control program for the pilot valves  19  and  20  be simplified, but also can the power consumption be reduced. Furthermore, because the emergency stop operation is performed by switching the spool  16  by applying a pilot fluid pressure to the second piston  18  with a larger diameter, the responsiveness is improved compared with the case where the spool  16  is switched by the spring force of the return spring  33 . 
     Although the spring cover  13 , the small-diameter portion  31 , the spring seats  32   a  and  32   b , and the return spring  33  are disposed at positions close to the second piston  18  with a larger diameter in the example shown, these components may be disposed at positions close to the first piston  17  with a smaller diameter. 
     Furthermore, although the three-position switching valve shown is configured as a five-port valve, the present invention may also be applied to a three-position switching valve composed of a three-port valve or a four-port valve.