Pilot check valve, hydraulic circuit, and tractor

Abstract

[Problem] To achieve a pilot check valve structure in which a flow passage can be opened as needed while preventing the inadvertent backflow of hydraulic fluid. [Solution] When hydraulic fluid is introduced into a pilot cylinder space 23 through a pilot pressure introduction port 24, a first valve element 17a is pressed against the biasing force of a first biasing member 18a by a first pilot piston 19a and is moved to a position for opening a first flow passage 22a, and a second valve element 17b is pressed against the biasing force of a second biasing member 18b by a second pilot piston 19b and is moved to a position for opening a second flow passage 22b.

Description

Pilot Check Valve, Hydraulic Circuit, and Tractor

[0001] The present disclosure relates to a pilot check valve whose flow path can be switched between open and closed by pilot pressure, a hydraulic circuit including the pilot check valve, and a tractor including the hydraulic circuit.

[0002] The tractor includes a lifting device for lifting and lowering an attachment for performing agricultural work, civil engineering work, etc. In the lifting device, the supply and discharge of pressure oil to each of two hydraulic chambers present on both sides sandwiching the piston of the piston cylinder device are controlled, and by swinging the link arm in the vertical direction, the attachment supported at the tip of the link arm is lifted and lowered.

[0003] Japanese Utility Model Publication No. 3-67505 describes a tractor equipped with a hydraulic circuit having a floating mode for easily performing leveling work for making the unevenness of the ground substantially flat. In the floating mode, both of the two hydraulic chambers of the piston cylinder device communicate with the tank. As a result, the attachment is in a state of being pressed against the ground based only on the gravity acting on the attachment and the lift arm. Therefore, when foreign objects such as rocks and tree roots protrude from the ground, the attachment can be pushed upward to overcome the foreign objects.

[0004] Japanese Utility Model Publication No. 3-67505

[0005] In a tractor equipped with the hydraulic circuit described in Japanese Utility Model Publication No. 3-67505, a floating mode is realized by connecting both of the two hydraulic chambers present on both sides sandwiching the piston to the tank. Therefore, a check valve for preventing the reverse flow of pressure oil cannot be provided between the two hydraulic chambers and the direction switching valve for switching the oil path. For this reason, when the engine of the tractor is stopped with the attachment lifted, a problem occurs in that the attachment gradually descends due to inevitable internal leakage of pressure oil occurring in the direction switching valve.

[0006] This disclosure aims to realize a pilot check valve structure that can open the flow path as needed while preventing unintentional backflow of pressurized oil.

[0007] A pilot check valve according to one aspect of the present disclosure comprises a pilot body, a first valve element, a second valve element, a first biasing member, a second biasing member, a first pilot piston, and a second pilot piston.

[0008] The pilot body includes a first input port, a first output port, a first flow path connecting the first input port and the first output port, a second input port, a second output port, a second flow path connecting the second input port and the second output port, a pilot cylinder space, and a pilot pressure introduction port opening into the pilot cylinder space.

[0009] The first valve body is movable between a position that closes the first flow path and a position that opens the first flow path.

[0010] The second valve body is movable between a position that closes the second flow path and a position that opens the second flow path.

[0011] The first biasing member biases the first valve body toward a position that closes the first flow path.

[0012] The second biasing member biases the second valve body toward a position that closes the second flow path.

[0013] The first pilot piston is fitted into the pilot cylinder space so as to be movable in the same direction as the movement of the first valve body.

[0014] The second pilot piston is fitted into the pilot cylinder space so as to be movable in the same direction as the movement of the second valve body.

[0015] In particular, in a pilot check valve according to one aspect of the present disclosure, when pressurized oil is introduced into the pilot cylinder space through the pilot pressure introduction port, the first valve body is pressed by the first pilot piston against the biasing force of the first biasing member and moves to a position that opens the first passage, and the second valve body is pressed by the second pilot piston against the biasing force of the second biasing member and moves to a position that opens the second passage. That is, in a pilot check valve according to one aspect of the present disclosure, the first passage and the second passage are opened simultaneously when pressurized oil is supplied to the pilot pressure introduction port.

[0016] In a pilot check valve according to one aspect of the present disclosure, the first input port, the first output port, and the first flow path and the second input port, the second output port, and the second flow path can be arranged on opposite sides of the pilot cylinder space. In this case, the first pilot piston has a first piston body that is fitted into the pilot cylinder space so as to be movable in the same direction as the movement of the first valve body, and a first pressing portion that is positioned opposite the first valve body in the direction of movement of the first valve body. The second pilot piston has a second piston body that is fitted into the pilot cylinder space so as to be movable in the same direction as the movement of the second valve body, and a second pressing portion that is positioned opposite the second valve body in the direction of movement of the second valve body. The first pilot piston and the second pilot piston are arranged coaxially with their first pressing portion and second pressing portion facing opposite directions.

[0017] A hydraulic circuit according to one aspect of the present disclosure comprises a pilot check valve according to one aspect of the present disclosure, a piston cylinder device, and a directional control valve.

[0018] The piston-cylinder device comprises a cylinder including a cylinder space and a piston movably fitted within the cylinder space. Of the two hydraulic chambers located on either side of the piston within the cylinder space, one hydraulic chamber is connected to the first output port, and the other hydraulic chamber is connected to the second output port.

[0019] The directional control valve has a body including a pump port connected to a pump, a tank port connected to a tank, a first directional control valve output port connected to the first input port, and a second directional control valve output port connected to the second input port, and a spool movably fitted to the body. The directional control valve switches the first directional control valve output port to communicate with either the pump port or the tank port, or to close it, and switches the second directional control valve output port to communicate with either the pump port or the tank port, or to close it, by moving the spool relative to the body.

[0020] In particular, in a hydraulic circuit according to one aspect of the present disclosure, the directional control valve is switched to a state in which the first control valve output port is in communication with the tank port and the second control valve output port is in communication with the tank port by moving the spool relative to the body, and pressurized oil is introduced into the pilot cylinder space through the pilot pressure introduction port, thereby switching the pilot check valve to a state in which the first passage is open and the second passage is open.

[0021] In a hydraulic circuit according to one aspect of the present disclosure, the supply of pressurized oil to the pilot pressure introduction port can be carried out via a route different from the route from the tank through the directional control valve.

[0022] In a hydraulic circuit according to one aspect of the present disclosure, the body may include an N port connected to another hydraulic mechanism. In this case, the directional control valve can be connected to the pump port while the first directional control valve output port is connected to the tank port and the second directional control valve output port is connected to the tank port, by moving the spool relative to the body.

[0023] A tractor according to one aspect of the present disclosure comprises a vehicle body and a lifting device for raising and lowering attachments.

[0024] The lifting device comprises a mounting portion to which the attachment is detachably attached, and a hydraulic actuator for raising and lowering the mounting portion.

[0025] In particular, in a tractor according to one aspect of the present disclosure, the hydraulic actuator comprises a hydraulic circuit according to one aspect of the present disclosure, a pump connected to the pump port, and a tank connected to the tank port.

[0026] A pilot check valve according to one aspect of the present disclosure can open the flow path as needed while preventing unintentional backflow of pressurized oil.

[0027] Figure 1 is a schematic side view showing a tractor according to one embodiment of the present disclosure. Figure 2 is a schematic diagram showing the hydraulic circuit of a hydraulic actuator constituting the lifting device of the tractor. Figure 3(A) is a cross-sectional view showing the pilot check valve with the first and second passages closed, and Figure 3(B) is a cross-sectional view showing the pilot check valve with the first and second passages open. Figure 4(A) is a schematic diagram showing the directional control valve when raising the mounting portion, Figure 4(B) is a schematic diagram showing the directional control valve when maintaining the vertical position of the mounting portion, Figure 4(C) is a schematic diagram showing the directional control valve when lowering the mounting portion, and Figure 4(D) is a schematic diagram showing the directional control valve in floating mode. Figures 5(A) and 5(B) are cross-sectional views showing the body and spool that constitute the directional control valve, with Figure 5(A) showing the state when the upper and lower positions of the mounting portion are maintained, and Figure 5(B) showing the state in the floating mode.

[0028] A tractor according to one aspect of the present disclosure comprises a vehicle body and a lifting device for raising and lowering attachments. The tractor includes not only agricultural tractors but also civil engineering tractors. The lifting device is located at the front or rear of the vehicle body. Hereinafter, an example of a tractor, hydraulic circuit, and pilot check valve according to an embodiment of the present disclosure will be described with reference to Figures 1 to 5(B).

[0029] Attachments can be changed as appropriate depending on the type of work being performed. Examples of attachments for agricultural tractors, though not limited to those listed, include harrows for leveling uneven fields, plows and rotary tillers for tilling fields, and shredders for mowing grass. Examples of attachments for civil engineering tractors, though not limited to those listed, include dozer blades for leveling uneven ground, and shovels for excavation and transport.

[0030] [Overall structure of the tractor] In this example, one embodiment of the tractor of the present disclosure is applied to an agricultural tractor. The tractor 1 comprises a body 2 and a lifting device 3.

[0031] The lifting device 3 is a device for raising and lowering the attachment 4. The lifting device 3 is installed at the front or rear of the vehicle body 2. In this example, the lifting device 3 is installed at the front of the vehicle body 2.

[0032] The lifting device 3 comprises a mounting portion 5 to which an attachment 4 is detachably attached, and a hydraulic actuator 6 for raising and lowering the mounting portion 5. The hydraulic actuator 6 comprises a piston cylinder device 14 having two hydraulic chambers 50a and 50b. The lifting device 3 raises and lowers the mounting portion 5 by controlling the supply and discharge of pressurized oil into the two hydraulic chambers 50a and 50b of the piston cylinder device 14 of the hydraulic actuator 6, thereby moving the piston 49 and raising and lowering the mounting portion 5, and thus raising and lowering the attachment 4 attached to the mounting portion 5.

[0033] The structure of the mounting portion 5 is not particularly limited, as long as the attachment 4 can be detachably attached to it. For example, the mounting portion 5 is composed of a fastener (hitch) that engages with a fastener provided on the attachment 4, a notch that engages with a shaft member provided on the attachment 4, and a shaft portion that engages with a notch provided on the attachment 4. In this example, the mounting portion 5 is composed of a notch that engages with a shaft member 7 provided on the attachment 4.

[0034] The structure for supporting the mounting portion 5 so that it can be raised and lowered is not particularly limited, and any structure can be adopted. For example, the support structure for the mounting portion 5 may include one or more link arms 9 whose base ends are supported so as to be able to swing around a pivot axis 8 that extends in the width direction of the vehicle body 2. Alternatively, the support structure for the mounting portion 5 may be constructed by combining multiple links.

[0035] The lifting device 3 in this example includes a link arm 9 whose base end (rear end) is supported so as to be able to swing around a pivot axis 8 that extends in the width direction of the vehicle body 2. The mounting portion 5 is provided at the tip end (front end) of the link arm 9.

[0036] The hydraulic actuator 6 comprises a hydraulic circuit 10, a pump 11, and a tank 12.

[0037] <Hydraulic Circuit> The hydraulic circuit 10 includes a pilot check valve 13, a piston cylinder device 14, and a directional control valve 15.

[0038] The piston cylinder device 14 can consist of one or more piston cylinder devices 14. Specifically, the hydraulic circuit 10 can be configured to raise and lower the mounting portion 5 and the attachment 4 using one piston cylinder device 14, or it can be configured to raise and lower the mounting portion 5 and the attachment 4 using multiple piston cylinder devices 14 arranged in parallel.

[0039] When the piston cylinder device 14 is composed of multiple piston cylinder devices 14, all of the piston cylinder devices 14 can be operated by one pilot check valve 13, or each of the multiple pilot check valves 13 can be operated by each of the multiple piston cylinder devices 14.

[0040] In this example, the hydraulic circuit 10 is configured to control the supply and discharge of pressurized oil to one pilot check valve 13 by a directional control valve 15, and to operate one piston cylinder device 14 by the pilot check valve 13.

[0041] ≪Pilot Check Valve≫ When no pilot pressure is introduced, the pilot check valve 13 allows the passage of pressurized oil in only one direction, and when pilot pressure is introduced, it allows the passage of pressurized oil regardless of direction. The pilot check valve 13 comprises a pilot body 16, a first valve body 17a, a second valve body 17b, a first biasing member 18a, a second biasing member 18b, a first pilot piston 19a, and a second pilot piston 19b.

[0042] In this example, with respect to the pilot check valve 13, the axial, radial, and circumferential directions refer to the axial, radial, and circumferential directions of the pilot body 16 unless otherwise specified. The axial, radial, and circumferential directions of the pilot body 16 coincide with the axial, radial, and circumferential directions of the first pilot piston 19a and also coincide with the axial, radial, and circumferential directions of the second pilot piston 19b. One axial side is the left side of Figures 3(A) and 3(B), where the first valve body 17a, the first biasing member 18a, and the first pilot piston 19a are located, and the other axial side is the right side of Figures 3(A) and 3(B), where the second valve body 17b, the second biasing member 18b, and the second pilot piston 19b are located.

[0043] The pilot body 16 has a first input port 20a, a first output port 21a, a first flow path 22a connecting the first input port 20a and the first output port 21a, a second input port 20b, a second output port 21b, a second flow path 22b connecting the second input port 20b and the second output port 21b, a pilot cylinder space 23 in which a first pilot piston 19a and a second pilot piston 19b are movably fitted, and a pilot pressure introduction port 24 opening into the pilot cylinder space 23.

[0044] The arrangements of the ports 20a, 20b, 21a, 21b, 24 of the pilot body 16, the flow paths 22a, 22b, and the pilot cylinder space 23 are not particularly limited as long as, when pressure oil is introduced into the pilot cylinder space 23 through the pilot pressure introduction port 24, the first pilot piston 19a can move the first valve body 17a until the first flow path 22a is opened, and the second pilot piston 19b can move the second valve body 17b until the second flow path 22b is opened, and can be arbitrarily arranged.

[0045] In this example, the first input port 20a, the first output port 21a, and the first flow path 22a, and the second input port 20b, the second output port 21b, and the second flow path 22b are arranged on opposite sides of each other with the pilot cylinder space 23 interposed therebetween. Specifically, in this example, the first output port 21a, and the first flow path 22a, and the second input port 20b, the second output port 21b, and the second flow path 22b are symmetrically arranged with the pilot cylinder space 23 interposed therebetween.

[0046] The pilot body 16 further has a first valve seat 25a against which the first valve body 17a abuts when the first valve body 17a is in a position closing the first flow path 22a, and a second valve seat 25b against which the second valve body 17b abuts when the second valve body 17b is in a position closing the second flow path 22b.

[0047] The pilot body 16 can be integrally formed as a whole, or can be formed by combining a plurality of members. In this example, the pilot body 16 is formed by combining a plurality of members. Specifically, the pilot body 16 has a body main body 26, a first outer sleeve 27a, a first inner poppet 28a, and a first plug 29a, and a second outer sleeve 27b, a second inner poppet 28b, and a second plug 29b.

[0048] The body main body 26 is configured to be substantially cylindrical as a whole and has a through hole 30 penetrating in the axial direction.

[0049] The pilot pressure introduction port 24 is located in the axial center of the main body 26 and connects the outer and inner surfaces of the main body 26.

[0050] The first input port 20a is located on one axial side of the main body 26 and connects the outer circumferential surface and the inner circumferential surface of the main body 26.

[0051] The first output port 21a is located in a portion of the body 26 that is axially offset from the first input port 20a, and connects the outer and inner surfaces of the body 26. In this example, the first output port 21a is located in a portion of the body 26 that is axially offset from the first input port 20a.

[0052] The second input port 20b is located on the other axial side of the main body 26 and connects the outer circumferential surface and the inner circumferential surface of the main body 26.

[0053] The second output port 21b is located in the portion of the body 26 that is axially away from the second input port 20b, and connects the outer and inner surfaces of the body 26. In this example, the second output port 21b is located in the portion of the body 26 that is axially away from the second input port 20b.

[0054] The circumferential positions of the ports 20a, 20b, 21a, 21b, and 24 of the pilot body 16 are not particularly limited and are determined as appropriate according to the routing of the piping connected to each port 20a, 20b, 21a, 21b, and 24. Specifically, the circumferential positions of each port 20a, 20b, 21a, 21b, and 24 can all be the same, or some or all of them can be different. In this example, the circumferential positions of the input ports 20a, 20b and the pilot pressure introduction port 24 are the same, and the output ports 21a, 21b are located on the radially opposite sides of the input ports 20a, 20b and the pilot pressure introduction port 24.

[0055] The pilot cylinder space 23 is formed by the axial intermediate portion of the space located inside the through hole 30.

[0056] The first outer sleeve 27a is generally cylindrical in shape and is fitted and held inside the axial side portion of the body 26 in a state where axial displacement is prevented. In this state, most of the first outer sleeve 27a (the other axial side portion) is inserted into the through hole 30.

[0057] The first outer sleeve 27a has a first outer through-hole 31a that penetrates radially in a portion located radially inward of the first output port 21a. The first outer through-hole 31a is provided at one or more locations in the circumferential direction of the first outer sleeve 27a. In this example, the first outer through-hole 31a is provided at multiple locations in the circumferential direction of the first outer sleeve 27a.

[0058] In the portion between the outer circumferential surface of the first outer sleeve 27a and the inner circumferential surface of the body 26, sealing members 32 are provided at two axial positions on either side of the first outer through hole 31a, and this portion is oil-tight.

[0059] The first inner poppet 28a is generally constructed in a substantially bottomed cylindrical shape and has a bottom plate portion 33a and a cylindrical portion 34a extending axially from the radially outer end of the bottom plate portion 33a. The first inner poppet 28a is held inside the first outer sleeve 27a in a state that prevents displacement to the other axial side.

[0060] The bottom plate portion 33a has an oil passage hole 35a that penetrates axially in its central part. The first valve seat 25a is formed from the portion of the side surface on one axial side of the bottom plate portion 33a that is located around the oil passage hole 35a.

[0061] The cylindrical portion 34a has a first inner through hole 36a that penetrates radially in the portion located radially inward of the first outer through hole 31a. The first inner through hole 36a is provided at one or more locations in the circumferential direction of the first inner poppet 28a. In this example, the first inner through hole 36a is provided at one location in the circumferential direction of the first inner poppet 28a.

[0062] The first plug 29a has a stepped shape. Specifically, the first plug 29a has a shaft portion 37a located on the other axial side and a head portion 38a located on the one axial side.

[0063] The shaft portion 37a is configured in a substantially cylindrical shape. The shaft portion 37a has a recess 39a on the other end face in the axial direction and a female thread on its outer circumferential surface. The shaft portion 37a is screwed onto one end in the axial direction of the first outer sleeve 27a. A sealing member 32 is provided in the space between one end in the axial direction of the outer circumferential surface of the shaft portion 37a and one end in the axial direction of the inner circumferential surface of the first outer sleeve 27a, making that portion oil-tight.

[0064] The head portion 38a has a circumscribed diameter that is larger than the outer diameter of the shaft portion 37a. The shape of the head portion 38a is not particularly limited as long as it has a circumscribed diameter that is larger than the outer diameter of the shaft portion 37a, and various shapes such as cylindrical or prismatic can be adopted. In this example, the head portion 38a has a hexagonal prism shape.

[0065] The first input port 20a and the first output port 21a are in communication when the first valve body 17a is in a position that opens the first flow path 22a, that is, when the first valve body 17a is not in contact with the first valve seat 25a, via the portion of the through hole 30 located radially inward of the first input port 20a, the inner axial end of the first outer sleeve 27a, the oil passage hole 35a, the inner axial end of the first inner poppet 28a, the first inner through hole 36a, the portion between the inner circumferential surface of the first outer sleeve 27a and the outer circumferential surface of the axial end of the first inner poppet 28a, and the first outer through hole 31a.

[0066] In other words, the first flow path 22a is composed of the portion of the through hole 30 located radially inward of the first input port 20a, the inner axial end of the first outer sleeve 27a, the oil passage hole 35a, the inner axial end of the first inner poppet 28a, the first inner through hole 36a, the portion between the inner circumferential surface of the first outer sleeve 27a and the outer circumferential surface of the axial end of the first inner poppet 28a, and the first outer through hole 31a.

[0067] The shapes of the second outer sleeve 27b, the second inner poppet 28b, and the second plug 29b, and how they are assembled to the body 26, can be symmetrical or asymmetrical with respect to the axial direction with respect to the shapes of the first outer sleeve 27a, the first inner poppet 28a, and the first plug 29a, and how they are assembled to the body 26.

[0068] In this example, the shapes of the second outer sleeve 27b, the second inner poppet 28b, and the second plug 29b, and how they are assembled to the body 26, are symmetrical in the axial direction with respect to the shapes of the first outer sleeve 27a, the first inner poppet 28a, and the first plug 29a, and how they are assembled to the body 26. That is, in this example, the portion of the pilot body 16 to which the first valve body 17a, the first biasing member 18a, and the first pilot piston 19a are assembled, and the portion to which the second valve body 17b, the second biasing member 18b, and the second pilot piston 19b are assembled, have shapes that are symmetrical in the axial direction, and the pilot cylinder space 23 between these portions also has a shape that is symmetrical in the axial direction.

[0069] The second outer sleeve 27b is generally cylindrical in shape and is fitted and held inside the other axial side portion of the body 26 in a state where axial displacement is prevented. In this state, most of the second outer sleeve 27b (the axial side portion) is inserted into the through hole 30. The second outer sleeve 27b has a second outer through hole 31b that penetrates radially in the portion located radially inward of the second output port 21b. In this example, the second outer through hole 31b is provided at multiple locations in the circumferential direction of the second outer sleeve 27b.

[0070] In the portion between the outer circumferential surface of the second outer sleeve 27b and the inner circumferential surface of the body 26, sealing members 32 are provided at two axial positions on either side of the second outer through hole 31b, making that portion oil-tight.

[0071] The second inner poppet 28b is generally constructed in a bottomed cylindrical shape and has a bottom plate portion 33b and a cylindrical portion 34b extending from the radially outer end of the bottom plate portion 33b to the other axial direction. The second inner poppet 28b is held inside the second outer sleeve 27b in a state where displacement to one axial side is prevented.

[0072] The bottom plate portion 33b has an oil passage hole 35b that penetrates axially in its central part. The second valve seat 25b is formed from the portion of the other axial side surface of the bottom plate portion 33b that is located around the oil passage hole 35b.

[0073] The cylindrical portion 34b has a second inner through hole 36b that penetrates radially in the portion located radially inward of the second outer through hole 31b. In this example, the second inner through hole 36b is provided at one location in the circumferential direction of the second inner poppet 28b.

[0074] The second plug 29b has a stepped shape. Specifically, the second plug 29b has a shaft b 37b located on one axial side and a head 38b located on the other axial side.

[0075] The shaft portion 37b is configured in a substantially cylindrical shape. The shaft portion 37b has a recess 39b on one end face on the axial side and a female thread on its outer circumferential surface. The shaft portion 37b is screwed onto the other end on the axial side of the second outer sleeve 27b. A sealing member 32 is provided in the space between the other end on the axial side of the outer circumferential surface of the shaft portion 37b and the other end on the axial side of the inner circumferential surface of the second outer sleeve 27b, making this portion oil-tight.

[0076] The head portion 38b has a circumscribed diameter that is larger than the outer diameter of the shaft portion 37b. The shape of the head portion 38b is not particularly limited as long as it has a circumscribed diameter that is larger than the outer diameter of the shaft portion 37b, and various shapes such as cylindrical or prismatic can be adopted. In this example, the head portion 38b has a hexagonal prism shape.

[0077] The second input port 20b and the second output port 21b are in communication when the second valve body 17b is in a position that opens the second flow path 22b, that is, when the second valve body 17b is not in contact with the second valve seat 25b, via the portion of the through hole 30 located radially inward of the second input port 20b, the inner axial end of the second outer sleeve 27b, the oil passage hole 35b, the inner axial end of the second inner poppet 28b, the second inner through hole 36b, the portion between the inner circumferential surface of the second outer sleeve 27b and the outer circumferential surface of the axial end of the second inner poppet 28b, and the second outer through hole 31b.

[0078] In other words, the second flow path 22b is composed of the portion of the through hole 30 located radially inward of the second input port 20b, the inner axial end of the second outer sleeve 27b, the oil passage hole 35b, the inner axial end of the second inner poppet 28b, the second inner through hole 36b, the portion between the inner circumferential surface of the second outer sleeve 27b and the outer circumferential surface of the axial end of the second inner poppet 28b, and the second outer through hole 31b.

[0079] The first valve body 17a is movable between a position that closes the first passage 22a and a position that opens the first passage 22a. In this example, the first valve body 17a closes the first passage 22a by bringing a part of it into contact with the first valve seat 25a, and opens the first passage 22a by not bringing a part of it into contact with the first valve seat 25a.

[0080] The first valve body 17a may be composed of a single component or by combining multiple components. The first valve body 17a may include components having shapes such as a spherical or truncated cone.

[0081] In this example, the first valve body 17a is constructed by combining two members. Specifically, the first valve body 17a has a guide 40a and a sphere 41a. The guide 40a has a substantially cylindrical shape and is positioned inside the first inner poppet 28a so as to be free from radial play and movable in the axial direction. The sphere 41a is held at the other end of the guide 40a on the axial side.

[0082] The shape of the second valve body 17b and the way it is assembled to the pilot body 16 (second inner poppet 28b) can be symmetrical or asymmetrical with respect to the shape of the first valve body 17a and the way it is assembled to the pilot body 16 (first inner poppet 28a) with respect to the axial direction. In this example, the shape of the second valve body 17b and the way it is assembled to the pilot body 16 (second inner poppet 28b) are symmetrical with respect to the shape of the first valve body 17a and the way it is assembled to the pilot body 16 (first inner poppet 28a).

[0083] The second valve body 17b is movable between a position that closes the second passage 22b and a position that opens the second passage 22b. In this example, the second valve body 17b closes the second passage 22b by bringing a part of it into contact with the second valve seat 25b, and opens the second passage 22b by not bringing a part of it into contact with the second valve seat 25b.

[0084] In this example, the second valve body 17b is constructed by combining two components. Specifically, the second valve body 17b has a guide 40b and a sphere 41b. The guide 40b has a substantially cylindrical shape and is positioned inside the second inner poppet 28b so as to be free from radial play and capable of axial movement. The sphere 41b is held at one end of the guide 40b on the axial side.

[0085] The first biasing member 18a biases the first valve body 17a toward a position that closes the first flow path 22a. The first biasing member 18a is not limited to but can be made of a compression coil spring, a leaf spring, or an elastomer such as rubber. In this example, the first biasing member 18a is made of a compression coil spring and is elastically compressed and sandwiched between the bottom surface of the recess 39a of the first plug 29a and the axial end of the guide 40a. As a result, the first valve body 17a is biased toward the other axial direction, which is the direction in which a part of the sphere 41a comes into contact with the first valve seat 25a, by the force of the first biasing member 18a trying to return to its original shape elastically.

[0086] Therefore, when no pressurized oil is supplied to either the first input port 20a or the pilot pressure introduction port 24, a portion of the sphere 41a comes into contact with the first valve seat 25a. This prevents the flow of pressurized oil from the first output port 21a toward the first input port 20a.

[0087] The second biasing member 18b biases the second valve body 17b toward a position that closes the second flow path 22b. The second biasing member 18b is not limited to but can be made of a compression coil spring, a leaf spring, or an elastomer such as rubber. In this example, the second biasing member 18b is made of a compression coil spring and is elastically compressed and sandwiched between the bottom surface of the recess 39b of the second plug 29b and the other axial end of the guide 40b. As a result, the second valve body 17b is biased toward one axial direction, which is the direction in which a part of the sphere 41b comes into contact with the second valve seat 25b, by the force of the second biasing member 18b trying to return to its original shape elastically.

[0088] Therefore, when no pressurized oil is supplied to either the second input port 20b or the pilot pressure introduction port 24, a portion of the sphere 41b comes into contact with the second valve seat 25b. This prevents the flow of pressurized oil from the second output port 21b toward the second input port 20b.

[0089] The first pilot piston 19a is fitted into the pilot cylinder space 23 so as to be movable in the same direction as the movement of the first valve body 17a. The first pilot piston 19a has a first piston body portion 42a fitted into the pilot cylinder space 23 so as to be movable in the same direction as the movement of the first valve body 17a, and a first pressing portion 43a arranged opposite to the first valve body 17a in the direction of movement of the first valve body 17a.

[0090] The first pilot piston 19a has a first piston body portion 42a and a first pressing portion 43a, as well as two small-diameter portions 44a and 44b, and is configured as a stepped cylindrical shape overall. In this example, the first pilot piston 19a is constructed as a single unit, but in implementing this disclosure, the first pilot piston can also be constructed by combining multiple parts.

[0091] The first piston body 42a is fitted into one axial side portion of the pilot cylinder space 23 so as to be able to move in the axial direction, which is the direction of movement of the first valve body 17a.

[0092] Each of the small-diameter portions 44a and 44b has an outer diameter smaller than the outer diameter of the first piston body portion 42a and is arranged on both sides of the first piston body portion 42a in the axial direction. The outer circumferential surface of the small-diameter portion 44a on the axial side of the two small-diameter portions 44a and 44b is connected to the outer circumferential surface of the first piston body portion 42a by a first pressed surface 45a facing the other side in the axial direction.

[0093] The first pressing portion 43a is shaped like a pin and is provided to protrude from the axial end face of the axially-oriented small-diameter portion 44b of the two small-diameter portions 44a and 44b. The first pressing portion 43a is axially opposite the spherical body 41a of the first valve body 17a.

[0094] The shape of the second pilot piston 19b and the way it is assembled to the pilot body 16 (pilot cylinder space 23) can be symmetrical or asymmetrical with respect to the shape of the first pilot piston 19a and the way it is assembled to the pilot body 16. In this example, the shape of the second pilot piston 19b and the way it is assembled to the pilot body 16 are symmetrical with respect to the shape of the first pilot piston 19a and the way it is assembled to the pilot body 16.

[0095] The second pilot piston 19b is fitted into the pilot cylinder space 23 so as to be movable in the same direction as the movement of the second valve body 17b. The second pilot piston 19b has a second piston body 42b fitted into the pilot cylinder space 23 so as to be movable in the same direction as the movement of the second valve body 17b, and a second pressing portion 43b arranged opposite to the second valve body 17b in the direction of movement of the second valve body 17b.

[0096] The second pilot piston 19b has a second piston body 42b and a second pressing portion 43b, as well as two small-diameter portions 46a, 46b, and is configured as a stepped cylindrical shape overall. In this example, the second pilot piston 19b is constructed as a single unit, but in implementing this disclosure, the second pilot piston can also be constructed by combining multiple parts.

[0097] The second piston body 42b is fitted into the other axial portion of the pilot cylinder space 23 so as to be able to move in the axial direction, which is the direction of movement of the second valve body 17b.

[0098] Each of the small-diameter portions 46a and 46b has an outer diameter smaller than the outer diameter of the second piston body portion 42b and is arranged on both sides of the second piston body portion 42b in the axial direction. The outer circumferential surface of the small-diameter portion 46a on one side of the two small-diameter portions 46a and 46b in the axial direction is connected to the outer circumferential surface of the second piston body portion 42b by a second pressed surface 45b facing in the axial direction.

[0099] The second pressing portion 43b is pin-shaped and is provided to protrude from the axial end face of the axially opposite small-diameter portion 46b of the two small-diameter portions 46a and 46b. The second pressing portion 43b is axially opposite the sphere 41b of the second valve body 17b.

[0100] In this example, the first pilot piston 19a and the second pilot piston 19b are arranged coaxially with their first pressing portion 43a and second pressing portion 43b facing in opposite directions. However, when implementing the pilot check valve of this disclosure, the central axes of the first pilot piston and the second pilot piston can be offset slightly (by a few millimeters), as long as their proximal ends can face each other.

[0101] In the pilot check valve 13, when pressurized oil is introduced into the through-hole 30 from the first input port 20a while pressurized oil is not introduced into the pilot pressure introduction port 24, the first valve body 17a (sphere 41a) is pressed axially in one direction against the elasticity of the first biasing member 18a, and a part of the first valve body 17a separates from the first valve seat 25a. As a result, the first flow path 22a is opened, and the flow of pressurized oil in the direction from the first input port 20a to the first output port 21a is permitted. In contrast, when pressurized oil is not introduced into either the first input port 20a or the pilot pressure introduction port 24, the first valve body 17a is in a position that closes the first flow path 22a, and in this example, the sphere 41a is in contact with the first valve seat 25a, thus preventing the flow of pressurized oil in the direction from the first output port 21a to the first input port 20a.

[0102] When pressurized oil is introduced into the through-hole 30 from the second input port 20b while pressurized oil is not introduced into the pilot pressure introduction port 24, the second valve body 17b (sphere 41b) is pressed axially toward the other side against the elasticity of the second biasing member 18b, and a part of the second valve body 17b separates from the second valve seat 25b. As a result, the second flow path 22b is opened, allowing the flow of pressurized oil in the direction from the second input port 20b toward the second output port 21b. In contrast, when pressurized oil is not introduced into either the second input port 20b or the pilot pressure introduction port 24, the second valve body 17b is in a position that closes the second flow path 22b, and in this example, the sphere 41b is in contact with the second valve seat 25b, thus preventing the flow of pressurized oil in the direction from the second output port 21b toward the second input port 20b.

[0103] Furthermore, in the pilot check valve 13 of this example, by introducing pressurized oil into the pilot cylinder space 23 through the pilot pressure introduction port 24, the first passage 22a and the second passage 22b can be opened simultaneously, regardless of whether or not hydraulic pressure is applied to the first output port 21a and / or the second output port 21b.

[0104] When pressurized oil is introduced into the pilot cylinder space 23 through the pilot pressure introduction port 24, the first pressed surface 45a of the first pilot piston 19a is pressed in one axial direction, causing the first pilot piston 19a to move in one axial direction. As a result, the first valve body 17a (sphere 41a) is pressed in one axial direction against the elasticity of the first biasing member 18a, and a part of the first valve body 17a separates from the first valve seat 25a, opening the first flow path 22a. Consequently, the flow of pressurized oil between the first input port 20a and the first output port 21a is permitted regardless of the direction of flow.

[0105] Simultaneously, the second pressed surface 45b of the second pilot piston 19b is pressed in the other axial direction, causing the second pilot piston 19b to move in the other axial direction. As a result, the second valve body 17b (sphere 41b) is pressed in the other axial direction against the elasticity of the second biasing member 18b, and a part of the second valve body 17b separates from the second valve seat 25b, opening the second flow path 22b. Consequently, the flow of pressurized oil between the second input port 20b and the second output port 21b is permitted regardless of its direction of flow.

[0106] The supply of pressurized oil to the pilot pressure introduction port 24 in the hydraulic circuit 10 can be carried out via any route, as long as the supply and discharge of pressurized oil to the pilot pressure introduction port 24 can be controlled at the appropriate timing. Specifically, the supply of pressurized oil to the pilot pressure introduction port 24 can be carried out via a route from the tank 12 through the directional control valve 15, or via a route different from the route from the tank 12 through the directional control valve 15.

[0107] In this example, the hydraulic circuit 10 is configured such that the supply of pressurized oil to the pilot pressure inlet port 24 is via a route different from the route from the pump 11 through the directional control valve 15. More specifically, the supply and discharge of pressurized oil to the pilot pressure inlet port 24 can be controlled by adjusting the flow rate of pressurized oil flowing from the pump 11 to the pilot pressure inlet port 24 using another directional control valve 62 and a pressure reducing valve 63 provided between the pump 11 and the pilot pressure inlet port 24.

[0108] In the pilot check valve 13 of this example, the portion of the pilot body 16 to which the first valve body 17a, the first biasing member 18a, and the first pilot piston 19a are assembled and the portion to which the second valve body 17b, the second biasing member 18b, and the second pilot piston 19b are assembled have shapes that are symmetrical with respect to the axial direction. Therefore, the shape of the first valve body 17a and the shape of the second valve body 17b can be made symmetrical with respect to the axial direction, the shape of the first biasing member 18a and the shape of the second biasing member 18b can be made symmetrical with respect to the axial direction, and the shape of the first pilot piston 19a and the shape of the second pilot piston 19b can be made symmetrical with respect to the axial direction. Therefore, parts can be shared between the first valve body 17a and the second valve body 17b, parts can be shared between the first biasing member 18a and the second biasing member 18b, and parts can be shared between the first pilot piston 19a and the second pilot piston 19b.

[0109] In the pilot check valve 13 of this example, the first pilot piston 19a and the second pilot piston 19b are arranged coaxially, and the assembly of the first pilot piston 19a to the pilot body 16 and the assembly of the second pilot piston 19b are symmetrical with respect to the axial direction. Therefore, it is easy to miniaturize the pilot check valve 13 and increase the degree of freedom in its arrangement.

[0110] ≪Piston Cylinder Device≫ The piston cylinder device 14 raises and lowers the mounting portion 5 and the attachment 4 based on the movement of the piston 49. The piston cylinder device 14 comprises a cylinder 48 having a cylinder space 47 and a piston 49 movably fitted within the cylinder space 47. One of the two hydraulic chambers 50a and 50b located on either side of the piston 49 within the cylinder space 47, the first hydraulic chamber 50a, is connected to the first output port 21a of the pilot check valve 13, and the other, the second hydraulic chamber 50b, is connected to the second output port 21b of the pilot check valve 13.

[0111] With respect to the piston-cylinder device 14, unless otherwise specified, the axial direction refers to the axial direction of the cylinder space 47. The axial direction of the cylinder space 47 coincides with the axial direction of the piston 49. One axial side refers to the left side of Figure 2, and the other axial side refers to the right side of Figure 2.

[0112] In this example, the piston 49 has a main body 51 fitted into the cylinder space 47 so as to be movable in the axial direction, and a rod 53 that protrudes axially from the other axial side of the main body 51 and is inserted through the opening 52 of the cylinder 48 so as to be movable relative in the axial direction. In other words, in this example, the piston cylinder device 14 is a so-called single-rod type cylinder device. However, in this disclosure, the piston cylinder device may also be configured as a so-called double-rod type cylinder device having a pair of rods that protrude axially from both axial sides of the main body fitted into the cylinder space.

[0113] The piston cylinder device 14 extends or retracts its overall length, that is, the length from the other axial end of the cylinder 48 to the one axial end of the rod 53 of the piston 49, by controlling the supply and discharge of pressurized oil to the two hydraulic chambers 50a and 50b. Specifically, when pressurized oil is supplied to the first hydraulic chamber 50a and pressurized oil is discharged from the second hydraulic chamber 50b, the overall length of the piston cylinder device 14 extends. When pressurized oil is discharged from the first hydraulic chamber 50a and pressurized oil is supplied to the second hydraulic chamber 50b, the overall length of the piston cylinder device 14 contracts.

[0114] In this example, the piston cylinder device 14 is provided so as to span between the vehicle body 2 and the link arm 9. Specifically, one end of the cylinder 48 on one axial side and the other end of the rod 53 of the piston 49 is supported so as to be able to swing about a pivot that extends in the width direction of the vehicle body 2 relative to the vehicle body 2, and the other end of the cylinder 48 on one axial side and the other end of the rod 53 of the piston 49 is supported so as to be able to swing about a pivot that extends in the width direction of the vehicle body 2 relative to the middle portion of the link arm 9 in the extension direction.

[0115] When pressurized oil is supplied to the first hydraulic chamber 50a and pressurized oil is discharged from the second hydraulic chamber 50b, and the total length of the piston cylinder device 14 is extended, the link arm 9 swings clockwise around the pivot axis 8 in the direction shown in Figure 1. As a result, the mounted portion 5 and the attachment 4 rise. When pressurized oil is discharged from the first hydraulic chamber 50a and pressurized oil is supplied to the second hydraulic chamber 50b, and the total length of the piston cylinder device 14 is retracted, the link arm 9 swings counterclockwise around the pivot axis 8 in the direction shown in Figure 1. As a result, the mounted portion 5 and the attachment 4 descend.

[0116] <Directional Control Valve> The directional control valve 15 switches the oil passage through which pressurized oil flows. The directional control valve 15 comprises a body 54 and a spool 55.

[0117] The body 54 includes a pump port 56 connected to the pump 11, a tank port 57 connected to the tank 12, a first switching valve output port 58 connected to the first input port 20a, and a second switching valve output port 59 connected to the second input port 20b.

[0118] The body 54 may be configured with one or more pump ports for the pump port 56 and / or with one or more tank ports for the tank port 57, in order to secure the necessary oil passages.

[0119] In this example, the body 54 has an optional N port 60 that is connected to another hydraulic mechanism different from the hydraulic circuit 10. The N port 60 is not limited to, but may be connected to, for example, the opening and closing of a wing harrow, the retraction of an adjustment cylinder for tilling or subsoil crushing operations, or a control valve for controlling the raising and lowering of a link section to which an attachment is mounted. These control valves are also connected to the tank 12.

[0120] The spool 55 is movably fitted inside the body 54. The spool 55 is constructed in a roughly cylindrical shape overall by repeatedly arranging large diameter sections and small diameter sections alternately.

[0121] The spool 55 can be configured to be mechanically movable based on a drive such as an electric motor, electrically movable by energizing a solenoid, or manually movable. In this example, the spool 55 is configured to be electrically movable. Specifically, the position of the spool 55 relative to the body 54 can be adjusted by controlling the amount of current supplied to the solenoid 61. In other words, in this example, the directional control valve 15 is configured as a so-called electromagnetic proportional control valve.

[0122] The directional control valve 15 switches between connecting the first control valve output port 58, the second control valve output port 59, and the N port 60 to either the pump port 56 or the tank port 57, or closing them, by adjusting the position of the spool 55 relative to the body 54.

[0123] Specifically, in the position shown in Figure 4(A), the first switching valve output port 58 communicates with the pump port 56, the second switching valve output port 59 communicates with the tank port 57, and the N port 60 is closed. In the position shown in Figure 4(B), both the first switching valve output port 58 and the second switching valve output port 59 are closed, and the N port 60 communicates with the pump port 56. In the position shown in Figure 4(C), the first switching valve output port 58 communicates with the tank port 57, the second switching valve output port 59 communicates with the pump port 56, and the N port 60 is closed. In the position shown in Figure 4(D), both the first switching valve output port 58 and the second switching valve output port 59 communicate with the tank port 57, and the N port 60 communicates with the pump port 56.

[0124] In addition to the pilot check valve 13, directional control valve 15, directional control valve 62, and pressure reducing valve 63, the hydraulic circuit 10 may be equipped with components such as pressure control valves, flow control valves, and directional control valves at appropriate locations as needed.

[0125] Pump 11 compresses the pressurized oil stored in tank 12 and sends it to the hydraulic circuit 10. Any type of pump 11 can be used as long as it can secure the required pressurized oil pressure (hydraulic pressure). Specifically, pump 11 can consist of, but is not limited to, a plunger pump, turbine pump, gear pump, screw pump, etc.

[0126] The tank 12 can have any shape as long as it can secure the required capacity.

[0127] In the tractor 1 of this example, in order to lower the mounting portion 5 and the attachment 4, the directional control valve 15 is switched to the position shown in Figure 4(A) based on the energization of the solenoid 61, and the directional control valve 62 is switched so that pressurized oil can be introduced into the pilot pressure introduction port 24, thereby opening the first passage 22a and the second passage 22b of the pilot check valve 13 as shown in Figure 3(B).

[0128] This supplies pressurized oil to the first hydraulic chamber 50a of the piston cylinder device 14 and discharges pressurized oil from the second hydraulic chamber 50b, thereby shortening the overall length of the piston cylinder device 14. When the overall length of the piston cylinder device 14 is shortened, the link arm 9 swings counterclockwise around the pivot axis 8 in Figure 1 so that it approaches the ground, causing the mounting portion 5 and the attachment 4 to descend.

[0129] To maintain the vertical position of the mounting portion 5 and the attachment 4, the directional control valve 15 is switched to the position shown in Figure 4(B), and the directional control valve 62 is switched so that the supply of pressurized oil to the pilot pressure introduction port 24 is stopped, thereby closing the first passage 22a and the second passage 22b of the pilot check valve 13 as shown in Figure 3(A). This stops the supply and discharge of pressurized oil to the first hydraulic chamber 50a and the second hydraulic chamber 50b of the piston cylinder device 14, thereby maintaining a constant pressure (hydraulic) in the first hydraulic chamber 50a and the pressure in the second hydraulic chamber 50b, and locking the extension and retraction of the piston cylinder device 14. As a result, the vertical position of the mounting portion 5 and the attachment 4 is maintained.

[0130] To raise the mounting portion 5 and the attachment 4, the system is switched to the position shown in Figure 4(C), and the directional control valve 62 is switched to allow pressurized oil to be introduced into the pilot pressure introduction port 24, opening the first passage 22a and the second passage 22b of the pilot check valve 13 as shown in Figure 3(B). This discharges pressurized oil from the first hydraulic chamber 50a of the piston cylinder device 14 and supplies pressurized oil to the second hydraulic chamber 50b, thereby extending the overall length of the piston cylinder device 14. When the overall length of the piston cylinder device 14 is extended, the link arm 9 swings clockwise around the pivot axis 8 in Figure 1 so that it leaves the ground, and the mounting portion 5 and the attachment 4 rise.

[0131] In the tractor 1 of this example, to switch the attachment 4 to a floating mode in which it is pressed to the ground based solely on gravity acting on the attachment 4 and the link arm 9, the directional control valve 15 is switched to the position shown in Figure 4(D), and the directional control valve 62 is switched to introduce pressurized oil into the pilot pressure introduction port 24, thereby opening the first passage 22a and the second passage 22b of the pilot check valve 13, as shown in Figure 3(B).

[0132] As a result, the first hydraulic chamber 50a of the piston cylinder device 14 communicates with the tank 12 via the first passage 22a of the pilot check valve 13 and the directional control valve 15, and the second hydraulic chamber 50b communicates with the tank 12 via the second passage 22b of the pilot check valve 13 and the directional control valve 15. Consequently, the resistance of the pressurized oil to the movement of the piston 49 relative to the cylinder 48 becomes substantially zero, and the attachment 4 is pressed to the ground based solely on gravity acting on the attachment 4 and the link arm 9.

[0133] In this example, the tractor 1 is configured to close the first passage 22a and the second passage 22b of the pilot check valve 13, which is located between the piston cylinder device 14 and the directional control valve 15, by stopping the supply of pressurized oil to the pilot pressure introduction port 24 when the vertical position of the attachment 4 is maintained. Therefore, when the engine is stopped while the vertical position of the attachment 4 is maintained, internal leakage of pressurized oil in the directional control valve 15 can be effectively prevented, and the attachment 4 can be prevented from gradually descending.

[0134] In this particular example, the supply of pressurized oil to the pilot pressure introduction port 24 is via a route different from the route from the pump 11 through the directional control valve 15. This makes it possible to more effectively prevent internal leakage of pressurized oil in the directional control valve 15 while maintaining the vertical position of the attachment 4.

[0135] When raising or lowering the attachment 4, or when switching to floating mode, pressurized oil is introduced into the pilot pressure introduction port 24, which allows the first passage 22a and the second passage 22b of the pilot check valve 13 to open simultaneously. This allows for quick establishment of communication between the first hydraulic chamber 50a of the piston cylinder device 14 and the pump 11 or tank 12, and between the second hydraulic chamber 50b and the pump 11 or tank 12.

[0136] 1 Tractor 2 Body 3 Lifting device 4 Attachment 5 Mounted part 6 Hydraulic actuator 7 Shaft member 8 Pivot shaft 9 Link arm 10 Hydraulic circuit 11 Pump 12 Tank 13 Pilot check valve 14 Piston cylinder device 15 Directional control valve 16 Pilot body 17a First valve body 17b Second valve body 18a First biasing member 18b Second biasing member 19a First pilot piston 19b Second pilot piston 20a First input port 20b Second input port 21a First output port 21b Second output port 22a First flow path 22b Second flow path 23 Pilot cylinder space 24 Pilot pressure introduction port 25a First valve seat 25b Second valve seat 26 Body 27a First outer sleeve 27b Second outer sleeve 28a First inner poppet 28b Second inner poppet 29a First plug 29b Second plug 30 Through hole 31a First outer hole 31b Second outer hole 32 Seal member 33a, 33b Bottom plate portion 34a, 34b Cylindrical portion 35a, 35b Oil hole 36a First inner hole 36b Second inner hole 37a, 37b Shaft portion 38a, 38b Head 39a, 39b Recessed portion 40a, 40b Guide 41a, 41b Sphere 42a First piston body portion 42b Second piston body portion 43a First pressing portion 43b Second pressing portion 44a, 44b Small diameter portion 45a First pressed surface 45b Second pressed surface 46a, 46b Small diameter portion 47 Cylinder space 48 Cylinder 49 Piston 50a First hydraulic chamber 50b Second hydraulic chamber 51 Main body 52 Opening 53 Rod 54 Body 55 Spool 56 Pump port 57 Tank port 58 First switching valve output port 59 Second switching valve output port 60 N port 61 Solenoid 62 Directional control valve 63 Pressure reducing valve

Claims

1. A pilot body having a first input port, a first output port, a first flow path connecting the first input port and the first output port, a second input port, a second output port, a second flow path connecting the second input port and the second output port, a pilot cylinder space, and a pilot pressure introduction port opening into the pilot cylinder space; a first valve body movable between a position closing the first flow path and a position opening the first flow path; a second valve body movable between a position closing the second flow path and a position opening the second flow path; a first biasing member that biases the first valve body toward the position closing the first flow path; a second biasing member that biases the second valve body toward the position closing the second flow path; a first pilot piston fitted in the pilot cylinder space so as to be movable in the same direction as the movement direction of the first valve body; and a second pilot piston fitted in the pilot cylinder space so as to be movable in the same direction as the movement direction of the second valve body. A pilot check valve, wherein when pressurized oil is introduced into the pilot cylinder space through the pilot pressure introduction port, the first valve body is pressed by the first pilot piston against the biasing force of the first biasing member and moves to a position that opens the first passage, and the second valve body is pressed by the second pilot piston against the biasing force of the second biasing member and moves to a position that opens the second passage.

2. The pilot check valve according to claim 1, wherein the first input port, the first output port, and the first flow path and the second input port, the second output port, and the second flow path are arranged on opposite sides of the pilot cylinder space, the first pilot piston has a first piston body that is fitted in the pilot cylinder space so as to be movable in the same direction as the movement direction of the first valve body, and a first pressing portion that is positioned opposite the first valve body in the direction of movement, the second pilot piston has a second piston body that is fitted in the pilot cylinder space so as to be movable in the same direction as the movement direction of the second valve body, and a second pressing portion that is positioned opposite the second valve body in the direction of movement, and the first pilot piston and the second pilot piston are arranged coaxially with their first pressing portion and second pressing portion facing opposite directions.

3. A pilot check valve according to claim 1 or 2; a piston cylinder device comprising a cylinder including a cylinder space and a piston movably fitted within the cylinder space, wherein one of two hydraulic chambers located on either side of the piston within the cylinder space is connected to the first output port and the other hydraulic chamber is connected to the second output port; a directional control valve comprising a body including a pump port connected to a pump, a tank port connected to a tank, a first switching valve output port connected to the first input port, and a second switching valve output port connected to the second input port, and a spool movably fitted into the body, wherein by moving the spool relative to the body, the first switching valve output port is switched to communicate with either the pump port or the tank port, or to be closed, and the second switching valve output port is switched to communicate with either the pump port or the tank port, or to be closed; A hydraulic circuit that switches the directional control valve to a state in which the first control valve output port communicates with the tank port and the second control valve output port communicates with the tank port by moving the spool relative to the body, and then introduces pressurized oil into the pilot cylinder space through the pilot pressure introduction port, thereby switching the pilot check valve to a state in which the first passage and the second passage are open.

4. The hydraulic circuit according to claim 3, wherein the supply of pressurized oil to the pilot pressure introduction port is carried out via a route different from the route from the tank through the directional control valve.

5. The hydraulic circuit according to claim 3 or 4, wherein the body includes an N port connected to another hydraulic mechanism, and the directional control valve is connected to the pump port by moving the spool relative to the body, thereby connecting the first control valve output port to the tank port and the second control valve output port to the tank port.

6. A tractor comprising a vehicle body and a lifting device for raising and lowering an attachment, wherein the lifting device has a mounting portion to which the attachment is detachably attached and a hydraulic actuator for raising and lowering the mounting portion, and the hydraulic actuator has a hydraulic circuit according to any one of claims 3 to 5, a pump connected to the pump port, and a tank connected to the tank port.

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