Relief valve

A relief valve configured such that a plunger configured to be pushed forward by an elastic member to block communication between an inflow opening and an outflow opening moves rearward by an increase in pressure of the inflow opening against an elastic force of the elastic member to cause the inflow opening and the outflow opening to communicate with each other; and a piston configured to push a rear end of the elastic member forward moves forward by the increase in the pressure of the inflow opening to compress the elastic member, the elastic member includes a first elastic member and a second elastic member arranged in series, and the second elastic member has a smaller elastic constant than the first elastic member.

TECHNICAL FIELD

The present invention relates to a relief valve used for liquid-pressure control of a liquid-pressure circuit.

BACKGROUND ART

A liquid-pressure motor is typically used as a revolving motor or traveling motor for a revolving super structure of a construction machinery vehicle or the like, and is driven by a liquid-pressure circuit. A relief valve is used for liquid-pressure control of the liquid-pressure circuit.

One example of conventional relief valves is shown inFIG. 6(see PTL 1, for example). A relief valve R10will be explained in reference toFIGS. 6 and 2. A dashed line inFIG. 2shows that liquid pressure (relief pressure) of an inflow opening102aof the relief valve R10changes with time.

Currently, the relief valve R10is in a state shown inFIG. 6, and each of the pressure of the inflow opening102aformed at a valve seat102and the pressure of an outflow opening101ais tank pressure PT (state (1) shown inFIG. 2).

Next, for example, when the inflow opening102ais rapidly pressurized (state (5) shown inFIG. 2), the liquid pressure of the inflow opening102ais introduced to a third liquid chamber140through a restrictor103b. Then, when the liquid pressure of the inflow opening102aincreases, and an upward force (rearward acting force) acting on a plunger103by the liquid pressure of the inflow opening102abecomes higher than a downward force (forward acting force) acting on the plunger103by the liquid pressure of the third liquid chamber140and an elastic force (spring force) of a spring105, the plunger103moves upward, that is, rearward, so that the inflow opening102aand the outflow opening101acommunicate with each other. With this, a pressure liquid of the inflow opening102acan be released through the outflow opening101aat a predetermined flow rate from a state of initial set pressure P2.

Next, the liquid pressure of the third liquid chamber140is introduced to a first liquid chamber122and a second liquid chamber132through a communication hole111and restrictors111aand112formed at the piston104. Then, when the forward acting force applied to a forward acting surface131by the pressure liquid becomes higher than the total of a rearward acting force applied to bottom surfaces123and124of the third liquid chamber140and a rearward acting surface121by the pressure liquid and the elastic force of the spring105, the piston104starts moving downward, that is, forward.

As above, when the piston104moves downward, that is, forward, the pressure liquid in the first liquid chamber122is discharged to the third liquid chamber140through the communication hole111and restrictor111aof the piston104. Therefore, the piston104slowly moves forward. Until the piston104moves forward as above to contact a step portion101bof a case101, the spring105is gradually compressed, and a repulsive force of the spring105gradually increases. Therefore, the relief pressure of the inflow opening102asmoothly increases (state (6) shown inFIG. 2).

When the piston104moves forward to contact the step portion101bof the case101, the piston104cannot move downward, that is, forward any more. Therefore, the relief pressure is maintained at a constant maximum relief pressure PS (state (4) shown inFIG. 2). At the time of the states (6) and (4) shown inFIG. 2, the pressure liquid of the inflow opening102ahas pushed the relief valve open and is flowing out through the outflow opening101a. Therefore, the pressure increase from the initial set pressure P2(t1) to the maximum relief pressure PS(t3) can be smoothly performed.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

As shown inFIG. 2, the initial set pressure is set to P2 in the conventional relief valve R10shown inFIG. 6. Therefore, until the liquid pressure of the inflow opening102areaches P2, the pressure liquid of the inflow opening102acannot be released to the outflow opening101aat a predetermined flow rate. In this case, when a speed reduction operation of the revolving motor is started to stop the revolution of the revolving super structure of the construction machinery vehicle or the like using the relief valve R10, a rapid pressure change (torque change) may occur in the liquid-pressure circuit to which the revolving motor is connected, and a big impact may be applied to the liquid-pressure circuit, the revolving motor, and the revolving super structure.

Here, as a method of reducing the set pressure P2 of the relief valve R10shown inFIG. 6, there is a method of reducing the spring constant of the spring105such that the force of pushing down the plunger103by the spring105becomes small in a state where the plunger103closes the inflow opening102a.

However, in order to obtain the maximum relief pressure PS ofFIG. 2by using a spring having a small spring constant, a compression stroke (length) of the spring needs to be increased by increasing the length of the spring. In this case, the relief valve R10increases in volume, and therefore, increases in size.

The present invention was made to solve the above problems, and an object of the present invention is to provide a relief valve that is compact and capable of in a case where liquid pressure of an inflow opening rapidly increases, releasing a pressure liquid of the inflow opening from a state where the liquid pressure is lower than those of conventional cases, and also capable of preventing a rapid pressure change (torque change) from occurring by smoothly increasing the liquid pressure of the inflow opening from a sate where the liquid pressure is low as above.

Solution to Problem

A relief valve according to the present invention is a relief valve configured to adjust relief pressure in such a manner that: a plunger configured to be pushed forward by an elastic unit to block communication between an inflow opening and an outflow opening moves rearward by an increase in pressure of the inflow opening against an elastic force of the elastic unit to cause the inflow opening and the outflow opening to communicate with each other; and a piston configured to push a rear end of the elastic unit forward moves forward by the increase in the pressure of the inflow opening to compress the elastic unit, wherein: the elastic unit includes a first elastic member and a second elastic member arranged in series; and the second elastic member has a smaller elastic constant than the first elastic member.

According to the relief valve of the present invention, in a case where the liquid pressure of the inflow opening rapidly increases, and a rearward acting force applied to the plunger by the liquid pressure in a rearward direction becomes higher than a forward acting force applied to the plunger by the elastic forces (for example, spring forces) of the first and second elastic members in a forward direction, the plunger moves rearward. Here, the distance of the rearward movement of the plunger corresponds to the total of compression lengths of the first and second elastic members. Since the elastic constant of the second elastic member is set to be smaller than the elastic constant of the first elastic member, the second elastic member is compressed more significantly than the first elastic member with the liquid pressure of the inflow opening comparatively low (with the set pressure comparatively low), and the plunger is caused to move rearward by a predetermined distance corresponding to the liquid pressure. Thus, the inflow opening and the outflow opening can be caused to communicate with each other. With this, the pressure liquid of the inflow opening can be released through the outflow opening

Next, in a state where the plunger has moved rearward by a distance corresponding to the liquid pressure of the inflow opening, and the pressure liquid of the inflow opening is being released through the outflow opening, the piston configured to push forward the rear ends of the first and second elastic members arranged in series starts moving forward by the liquid pressure of the inflow opening. With this, the first and second elastic members can be gradually compressed, and the relief pressure can be smoothly increased. Thus, the liquid pressure of the inflow opening can be prevented from rapidly increasing.

Then, the pressure increase damping time until the liquid pressure (relief pressure) reaches the maximum relief pressure by the forward movement of the piston can be extended.

According to the relief valve of the present invention, by utilizing the elastic force of the first elastic member having the large elastic constant, a desired magnitude of maximum relief pressure can be set with respect to the relief valve.

The relief valve according to the present invention may be configured such that: a maximum relief pressure is mainly set by the first elastic member; and the relief pressure before the piston moves forward is mainly set by the second elastic member.

With this, the maximum relief pressure can be set mainly by the first elastic member, and pressure increase damping start pressure can be set mainly by the second elastic member.

The relief valve according to the present invention may be configured such that a stopper configured to stop the rearward movement of the plunger at a predetermined most rearward position is provided at a case in which the plunger is housed.

With this, when the second elastic member having the small elastic constant is compressed by the increase in the liquid pressure of the inflow opening, the rearward movement of the plunger can be stopped at the predetermined most rearward position. With this, a maximum opening degree of the relief valve can be set to a predetermined opening degree.

The relief valve according to the present invention may include a cup-shaped spring seat configured to restrict a compression length of the second elastic member such that the compression length of the second elastic member becomes equal to or smaller than a predetermined maximum compression length.

With this, when the plunger or the piston moves in such a direction that the first and second elastic members are compressed by the increase in the pressure of the inflow opening, the second elastic member is compressed more significantly than the first elastic member. In this case, when the compression length of the second elastic member becomes the predetermined maximum compression length, further compression of the second elastic member can be restricted by the cup-shaped spring seat. When the plunger or the piston further moves in such a direction that the first and second elastic members are compressed, the second elastic member is not compressed, but the first elastic member is compressed. Thus, the relief pressure can be maintained at the predetermined maximum relief pressure by the first elastic member.

The relief valve according to the present invention may be configured such that a ratio K1/K2 that is a ratio of an elastic constant K1 of the first elastic member to an elastic constant K2 of the second elastic member is from 5 to 20.

With this, for example, in a case where the relief valve of the present invention is adopted in a liquid-pressure circuit of a revolving motor for a revolving super structure of a construction machinery vehicle or the like, at the time of the speed reduction of the revolving motor, the rapid pressure change (torque change) of the liquid-pressure circuit can be effectively prevented from occurring. Thus, the big impact can be prevented from being applied to the liquid-pressure circuit, the revolving motor, and the revolving super structure, and the revolving motor can be reduced in speed with an appropriate negative acceleration to be stopped. To be specific, if the elastic constant ratio K1/K2 is lower than 5, the impact applied to the revolving super structure and the like may not be appropriately reduced. Then, if the elastic constant ratio K1/K2 exceeds 20, the length of the second elastic member needs to be increased in order to set predetermined set pressure, and this increases the volume and cost.

The relief valve according to the present invention may be configured such that: a liquid chamber in which the elastic unit is housed communicates with the outflow opening; the piston includes a first slide portion that liquid-tightly slides on an inner hole of a case; a rear portion of the plunger is slidably, fittingly inserted in a slide hole formed at the piston along a central axis of the piston; the plunger includes a through hole through which pressure liquid is introduced from the inflow opening to a rear side of the plunger; a third liquid chamber is formed by the through hole and a space which is located at a rear portion of the slide hole and to which the plunger does not reach; the piston includes a rearward acting surface and a forward acting surface located at a rear side of the first slide portion; the third liquid chamber and a first liquid chamber to which the rearward acting surface faces communicate with each other through a communication hole formed at the piston; the third liquid chamber and a second liquid chamber to which the forward acting surface faces communicate with each other through a communication hole formed at the piston; and the piston moves forward by a difference between a forward acting force applied to the forward acting surface by the pressure liquid and a rearward acting force applied by the pressure liquid to a bottom surface of the slide hole of the piston and the rearward acting surface.

With this, the difference between the area of the forward acting surface and the total of the area of the bottom surface of the slide hole of the piston and the area of the rearward acting surface becomes the effective pressure receiving area of the piston. On this account, for example, independently of the thickness of the first slide portion of the piston, the effective pressure receiving area for causing the piston to move forward can be set. Thus, the piston having predetermined stiffness can be manufactured.

The relief valve according to the present invention may be configured such that: an area of the forward acting surface is larger than a total of an area of the bottom surface of the slide hole and an area of the rearward acting surface; and the difference between the forward acting force and the rearward acting force is generated by a difference between the area of the forward acting surface and the total of the area of the bottom surface of the slide hole and the area of the rearward acting surface.

With this, by reducing the effective pressure receiving area for causing the piston to move forward, the piston slowly moves forward. Thus, the pressure increase damping time can be extended. By increasing the effective pressure receiving area, the piston quickly moves forward. Thus, the pressure increase damping time can be shortened. As above, the pressure increase damping time can be set freely.

The relief valve according to the present invention may be configured such that: a pressure difference is generated between front and rear sides of each of the communication holes by the difference between the forward acting force and the rearward acting force; and the piston moves forward while the pressure liquid flows through the communication holes.

With this, by thinly forming the communication hole, the piston slowly moves forward. Thus, the pressure increase damping time can be extended. By thickly forming the communication hole, the piston quickly moves forward. Thus, the pressure increase damping time can be shortened. As above, the pressure increase damping time can be set freely.

Advantageous Effects of Invention

The relief valve according to the present invention is configured such that the first and second elastic members that are different in the elastic constant from each other are arranged in series. Therefore, the pressure increase damping start pressure can be reduced. In addition, in a case where the liquid pressure of the inflow opening rapidly increases, the pressure liquid of the inflow opening can be released through the outflow opening from a state where the liquid pressure is comparatively low. With this, in a case where the relief valve of the present invention is adopted in a liquid-pressure circuit of a revolving motor for a revolving super structure of a construction machinery vehicle or the like, at the time of the start of the speed reduction of the revolving motor, the rapid pressure change (torque change) of the liquid-pressure circuit can be prevented from occurring. Thus, the big impact can be prevented from being applied to the liquid-pressure circuit, the revolving motor, and the revolving super structure.

Since the liquid pressure of the inflow opening can be released to the outflow opening from a state where the liquid pressure is comparatively low, the pressure increase damping time until the relief pressure of the inflow opening reaches the maximum relief pressure can be extended. With this, the revolving motor can be appropriately, slowly stopped such that the big impact is not applied to the liquid-pressure circuit, the revolving motor, and the revolving super structure.

The second elastic member having the small elastic constant is adopted to reduce the pressure increase damping start pressure, and the first elastic member having the large elastic constant is adopted to maintain the relief pressure at the predetermined maximum relief pressure. With this, the volume of the relief valve can be made smaller than that of a valve configured to act in the same manner as above and using one elastic member, such as a spring.

DESCRIPTION OF EMBODIMENTS

Hereinafter, Embodiment 1 of a relief valve according to the present invention will be explained in reference toFIGS. 1 to 4. According to a relief valve R1shown inFIG. 1, when a speed reduction operation of a revolving motor is started to stop the revolution of a revolving super structure of, for example, a construction machinery vehicle using the relief valve R1, a rapid pressure change can be prevented from occurring in a liquid-pressure circuit to which the revolving motor is connected, and the revolution of the revolving super structure can be stopped such that a big impact is prevented from being applied to the liquid-pressure circuit, the revolving motor, and the revolving super structure.

As shown in a longitudinal sectional view ofFIG. 1, the relief valve R1includes: a substantially cylindrical outer case1; an inner case54that is threadedly engaged with the outer case1; a valve seat2provided so as to be fixed to a tip end portion (front side portion) of the outer case1; a plunger3arranged in the outer case1; a piston4arranged in the inner case54; and a first elastic member5aand a second elastic member5b, each of which is a coil-shaped compression spring (elastic unit) interposed between the plunger3and the piston4.

A force (for example, set pressure) of compressing the first and second elastic members5aand5bcan be adjusted by changing the thickness of a spring seat7a.

Regarding an inner hole of the outer case1, the inner diameter of a portion1cto which a large-diameter portion (second slide portion4b) of the piston4is attached is d1, and the inner diameter of a portion1dto which a medium-diameter portion (first slide portion4a), located at a front side of the large-diameter portion, of the piston4is attached is d4. An outflow opening1aand a passage1bcommunicating with the outflow opening are formed on a side surface of the outer case1. The valve seat2is a circular member, and a pressure liquid inflow opening2ais formed at a center portion of the valve seat2. The plunger3includes a tip end portion having a tapered shape, that is, a substantially truncated cone shape. A through hole3ais formed at the center of the plunger3.

The through hole3ais formed so as to penetrate the plunger3from a tip end of the plunger3to a rear end thereof and introduce the pressure liquid from the inflow opening2ato a rear side of the plunger3. A restrictor3bis formed at a portion of the through hole3a. The restrictor3bis formed so as to give a damping force to the operation of the plunger3to prevent hunting.

The first slide portion4athat is the medium-diameter portion having the outer diameter d4 is formed at a front portion of the piston4, and the second slide portion4bthat is the large-diameter portion having the outer diameter d1 is formed at a rear side of the first slide portion4a. Further, a third slide portion4cthat is a small-diameter portion is formed at a rear end portion of the piston4. A recess51that opens in a front direction along a central axis is formed at the inner case54.

The third slide portion4cis fittingly inserted in the recess51of the inner case54so as to slide liquid-tightly, and a liquid chamber52is formed between the rear end portion of the third slide portion4cand the recess51. Then, the first slide portion4ais fittingly inserted in the portion, having the inner diameter d4, of the inner hole1dof the outer case1so as to be slidable liquid-tightly, and the second slide portion4bis fittingly inserted in the portion of the inner hole1cof the outer case1so as to be slidable liquid-tightly. An outer diameter of the third slide portion4cis d2.

A slide hole4fthat opens at a front surface of the piston4is formed at the piston4along a central axis thereof. An inner diameter of the slide hole4fis d3. A rear portion of the plunger3is fittingly inserted in the slide hole4fso as to be slidabe. A rear space4dof the slide hole4fand the through hole3aof the plunger3constitute a third liquid chamber40. The rear space4dof the slide hole4fis formed such that even if the plunger3slides to a rearmost position of a slidable range of the plunger3, the rear end of the plunger3does not reach the rear space4dof the slide hole4f. The piston4includes a passage4econfigured to penetrate the first slide portion4a, the second slide portion4b, and the third slide portion4cin an axial direction and cause a spring chamber8in which the first and second elastic members5aand5bare housed and the liquid chamber52to communicate with each other.

The piston4includes communication holes11and12. A restrictor11ais formed at a portion of the communication hole11. The communication hole12does not include a restrictor. A front end surface of the large-diameter portion (second slide portion4b) serves as a rearward acting surface21on which the pressure liquid acts, and a first liquid chamber22to which the rearward acting surface21faces communicates with the third liquid chamber40through the communication hole11. A rear end surface of the large-diameter portion (second slide portion4b) serves as a forward acting surface31on which the pressure liquid acts, and a second liquid chamber32to which the forward acting surface31faces communicates with the third liquid chamber40through the communication hole12.

The first elastic member5aand the second elastic member5bare housed in the spring chamber8(fifth liquid chamber) so as to be arranged in series. The spring chamber8(fifth liquid chamber) is formed between an inner peripheral surface1dof the outer case1and an outer peripheral surface of the plunger3. The first elastic member5ais arranged in a compressed state such that a tip end thereof contacts a rear end of the second elastic member5bvia a cup-shaped spring seat7b, and a rear end thereof presses a front end surface of the first slide portion4aof the piston4rearward via the spring seat7a. The second elastic member5bis arranged in a compressed state such that a tip end thereof presses a large portion3cof the plunger3forward from behind, and a rear end thereof presses the cup-shaped spring seat7brearward.

An elastic constant K2 of the second elastic member5bis set to be smaller than an elastic constant K1 of the first elastic member5a. An elastic constant ratio K1/K2 is, for example, from 5 to 20, and preferably from 5 to 15.

For reference, if the elastic constant ratio K1/K2 is smaller than 5, the impact applied to the revolving super structure and the like may not be appropriately reduced. Then, if the elastic constant ratio K1/K2 exceeds 20, the length of the second elastic member5bneeds to be increased in order to set predetermined set pressure P1, and this increases the volume and cost.

Then, a stopper55configured to stop a rearward movement of the plunger3at a predetermined most rearward position is formed at an inner peripheral surface of a tip end portion of the outer case1. The stopper55is formed by an annular-shaped step portion projecting toward an inner side of the outer case1. The rearward movement of the plunger3can be stopped at the predetermined most rearward position by causing the large portion3cof the plunger3to contact the stopper55.

By forming the stopper55as above, the rearward movement of the plunger3can be stopped at the predetermined most rearward position when the second elastic member5bhaving the small elastic constant is compressed by an increase in the liquid pressure of the inflow opening2a. With this, a maximum opening degree of the relief valve R1can be set to a predetermined opening degree. It should be noted that “a” shown inFIG. 1denotes a stroke of the plunger3that can move forward and backward.

The cup-shaped spring seat7bcan restrict a maximum compression length b of the second elastic member5b.

To be specific, when the plunger3or the piston4moves in such a direction that the first and second elastic members5aand5bare compressed by the increase in the pressure of the inflow opening2a, the second elastic member5bis compressed more significantly than the first elastic member5a. In this case, when the compression length of the second elastic member5bbecomes the predetermined maximum compression length b, a rear end surface of the large portion3cof the plunger3and a front surface of a cylindrical portion of the spring seat7bcontact each other. Thus, further compression of the second elastic member5bcan be restricted. When the piston4further moves in such a direction that the first and second elastic members5aand5bare compressed, the second elastic member5bis not compressed, but the first elastic member5ais compressed. Thus, the set pressure increases. When the front surface (rearward acting surface)21of the second slide portion4bof the piston4contacts a step portion50of the inner hole1c, the first elastic member5ais not compressed any more, and the set pressure becomes maximum. Therefore, the relief pressure can be maintained at the predetermined maximum relief pressure PS.

Next, the actions of the relief valve R1configured as shown inFIG. 1will be explained in reference toFIGS. 1 and 2. A solid line shown inFIG. 2denotes a state where the liquid pressure (relief pressure) of the inflow opening2aof the relief valve R1changes with time.

Currently, the relief valve R1is in a state shown inFIG. 1, and each of the pressure of the inflow opening2aand the pressure of the outflow opening1ais tank pressure PT (state (1) shown inFIG. 2).

Next, for example, when the inflow opening2ais rapidly pressurized (state (2) shown inFIG. 2), the liquid pressure of the inflow opening2ais introduced to the third liquid chamber40through the restrictor3b. Then, when an upward force (rearward acting force) acting on the plunger3by the increased liquid pressure of the inflow opening2abecomes higher than a downward force (forward acting force) acting on the plunger3by the liquid pressure of the third liquid chamber40and the elastic forces (spring forces) of the first and second elastic members5aand5b, the plunger3moves upward, that is, rearward. Then, the valve opens, and the pressure liquid flows out to the outflow opening1a(relief pressure P1).

Here, the distance of the rearward movement of the plunger3corresponds to the total of the compression lengths of the first and second elastic members5aand5b. Since the elastic constant K2 of the second elastic member5bis set to be smaller than the elastic constant K1 of the first elastic member5a, the second elastic member5bis compressed more significantly than the first elastic member5awith the liquid pressure of the inflow opening2acomparatively low, and the plunger3is caused to move rearward by a predetermined distance corresponding to the liquid pressure (the opening becomes substantially maximum). Thus, the inflow opening2aand the outflow opening1acan be caused to communicate with each other. With this, the pressure liquid of the inflow opening2acan be released through the outflow opening1a.

Next, in a state where the plunger3has moved rearward by the distance corresponding to the liquid pressure of the inflow opening2a(the opening has become substantially maximum), and the pressure liquid of the inflow opening2ais being released through the outflow opening1a, the liquid pressure of the third liquid chamber40is introduced to the first liquid chamber22and the second liquid chamber32through the restrictor11aand communication holes11and12of the piston4. Then, when the forward acting force applied to the forward acting surface31by the pressure liquid becomes higher than the total of the rearward acting force applied to a bottom surface23of the third liquid chamber40and the rearward acting surface21by the pressure liquid and the elastic forces of the first elastic member5aand the second elastic member5b, the piston4starts moving downward, that is, forward.

As above, when the piston4moves downward, that is, forward, the pressure liquid in the first liquid chamber22is discharged to the third liquid chamber40through the communication hole11and restrictor11aof the piston4. Therefore, the piston4slowly moves forward. By this slow forward movement of the piston4, the first and second elastic members5aand5bcan be gradually compressed, and the relief pressure P can be increased smoothly (state (3) shown inFIG. 2).

As above, from a state where the liquid pressure of the inflow opening2ais the comparatively low liquid pressure (P1), the piston4slowly moves forward, and the liquid pressure of the inflow opening2astarts increasing. Therefore, the liquid pressure of the inflow opening2acan be prevented from rapidly increasing. Then, since the liquid pressure of the inflow opening2astarts increasing from the low pressure (P1), a pressure increase damping time (t2−t1) until the liquid pressure (relief pressure P) reaches the maximum relief pressure PS by the forward movement of the piston4can be extended.

As shown inFIG. 1, according to the relief valve R1, since the elastic force of the first elastic member5ahaving the large elastic constant K1 is being utilized, a desired magnitude of the maximum relief pressure PS can be set to the relief valve R1.

Then, when the piston4moves forward to contact the step portion50of the outer case1, the piston4cannot move downward, that is, forward any more. Therefore, the relief pressure P is maintained at the constant maximum relief pressure PS (state (4) shown inFIG. 2).

At the time of the states (3) and (4) shown inFIG. 2, the pressure liquid of the inflow opening2ais flowing out through the outflow opening1aat a substantially constant flow rate (a present device used flow rate of the liquid-pressure circuit, and for example, 100 L/min as shown inFIG. 3).

As above, according to the relief valve R1shown inFIG. 1, the first and second elastic members5aand5bthat are different in the elastic constant (K1, K2) from each other are arranged in series. Therefore, in a case where the liquid pressure of the inflow opening2arapidly increases, the pressure liquid of the inflow opening2acan be released to the outflow opening1afrom a state where the liquid pressure of the inflow opening2ais the comparatively low liquid pressure (P1 shown inFIG. 2).

With this, as shown in, for example,FIG. 4, in a case where the relief valve R1and a relief valve R2are adopted in a liquid-pressure circuit of a revolving motor M for a revolving super structure of a construction machinery vehicle or the like, the rapid pressure change (torque change) of the liquid-pressure circuit can be prevented from occurring at the time of the start of the speed reduction of the revolving motor M, so that the big impact can be prevented from being applied to the liquid-pressure circuit, the revolving motor M, and the revolving super structure.

InFIG. 4, the relief valves R1and R2(the relief valve R2is the same as the relief valve R1) are respectively connected to a pressure liquid supply port and pressure liquid exhaust port of the revolving motor M. The outflow openings1a(release sides) of the relief valves R1and R2are connected to a liquid tank T through a boost check valve B configured to secure suction pressure of the revolving motor M. It should be noted that “P” denotes a liquid-pressure pump, and “V” denotes a switching valve.

Then, as shown inFIG. 2, since the liquid pressure of the inflow opening2acan be released to the outflow opening1afrom a state where the liquid pressure of the inflow opening2ais the comparatively low liquid pressure (P1) (state (3) shown inFIG. 2), the pressure increase damping time (t2−t1) until the relief pressure P of the inflow opening2areaches the maximum relief pressure PS can be extended. With this, the revolving motor M can be appropriately, slowly stopped without applying the big impact to the liquid-pressure circuit, the revolving motor M, and the revolving super structure.

The second elastic member5bhaving the small elastic constant is adopted to reduce the operation start pressure (initial set pressure P1) of the relief valve, and the first elastic member5ahaving the large elastic constant is adopted such that the relief pressure P can be maintained at the predetermined maximum relief pressure PS. With this, the volume of the relief valve can be made smaller than that of a valve configured to act in the same manner as above and using one elastic member having a small elastic constant.

Next, a mechanism of the forward movement of the piston4will be explained in reference toFIG. 1. The spring chamber8(fifth liquid chamber) communicates with the liquid tank T through the boost check valve B, and the liquid pressure of the spring chamber8is a value substantially close to the tank pressure PT. Further, the liquid chamber52constituted by the recess51of the inner case54and the third slide portion4cof the piston4communicates with the tank T through the passage4e, the spring chamber8, and the boost check valve B, and the liquid pressure of the liquid chamber52is also a value substantially close to the tank pressure PT. Therefore, an effective pressure receiving area of the piston4is obtained by subtracting the total of the area of the bottom surface23and the area of the rearward acting surface21from the area of the forward acting surface31. The area of the bottom surface23is represented by (d32×(π/4))=AS, and the area of the rearward acting surface21is represented by ((d12−d42)×(π/4))=AU. In addition, the area of the forward acting surface31is represented by ((d12−d22)×(π/4))=AM. Therefore, an effective pressure receiving area A1 of the piston4is represented by the following formula. In the following formula, the pressure of the spring chamber8is approximately set to 0.
A1=(d42−d32−d22)×(π/4)

As above, the piston4moves forward by the difference between the forward acting force acting on the forward acting surface31(area AM) by the pressure liquid and the rearward acting force acting on the bottom surface23(area AS) of the rear space4dof the slide hole4fof the piston4and the rearward acting surface21(area AU) of the piston4by the pressure liquid.

As is clear from the above formula, by the setting of the values of d2, d3, and d4, the effective pressure receiving area A1 can be set freely. Therefore, even if the difference between d4 and d4 is increased, the value of A1 can be decreased by increasing the value of d2.

On this account, independently of the thickness of the first slide portion4aof the piston4, the effective pressure receiving area A1 for causing the piston4to move forward can be set. Thus, the piston4having predetermined stiffness can be manufactured.

The area AM of the forward acting surface31is larger than the total of the area AS of the bottom surface23and the area AU of the rearward acting surface21, and the difference between the forward acting force and the rearward acting force is generated by this area difference.

Further, the pressure difference between front and rear sides of each of the communication holes11and12is generated by the difference between the forward acting force and the rearward acting force, and the piston4moves forward while the pressure liquid flows through the communication holes11and12.

In this case, by thinly forming the communication holes11and12or by forming the restrictor11aat each of the communication holes11and12, the piston4slowly moves forward. Thus, the pressure increase damping time (t2−t1) can be extended. By thickly forming the communication holes11and12, the piston4quickly moves forward. Thus, the pressure increase damping time (t2−t1) can be shortened. As above, the pressure increase damping time (t2−t1) can be set freely.

Next, a comparison between the override characteristic of the relief valve R1of the present embodiment shown inFIG. 1and the override characteristic of a conventional relief valve R10shown inFIG. 6will be explained in reference toFIG. 3.

A curved line S1inFIG. 1shows the override characteristic when the piston4is located at a rearward position (stroke St=0 mm) in the relief valve R1of the present embodiment shown inFIG. 3. Regarding the curved line S1, cracking pressure is Pa, and the set pressure when the present device used flow rate is, for example, 100 L/mm is P1.

The cracking pressure is the liquid pressure of the inflow opening2awhen the plunger3starts opening. The present device used flow rate is a flow rate of pressure liquid used in the revolving motor M. The set pressure is pressure when the plunger3is pushed upward by the increase in the pressure of the pressure liquid of the inflow opening2a, and the flow rate of the pressure liquid flowing through the relief valve R1becomes the present device used flow rate.

A curved line S2inFIG. 3shows the override characteristic when the piston104is located at the rearward position (stroke St=0 mm) in the conventional relief valve R10shown inFIG. 6. Regarding the curved line S2, the cracking pressure is Pb, and the set pressure when the present device used flow rate is 100 L/mm is P2.

Since one spring105having the large spring constant is used in the conventional relief valve R10, the set pressure P2 is higher than the set pressure P1 of the relief valve R1of the present embodiment. Therefore, when stopping the rotation of the revolving motor M, the big impact is applied to the revolving super structure.

A curved line S3inFIG. 3shows the override characteristic when the piston104is located at a forward position (stroke St=3 mm) in the conventional relief valve R10shown inFIG. 6. Regarding the curved line S3, the cracking pressure is Pc, and the set pressure when the present device used flow rate is 100 L/mm is P3.

When the piston104is located at the forward position as above, the spring105is in a compressed state. With this, the set pressure P3 is higher than the set pressure P2 of the curved line S2. Therefore, when stopping the rotation of the revolving motor M, the bigger impact than the case of the curved line S2is applied to the revolving super structure.

A curved line S4inFIG. 3shows the override characteristic when the piston104is located at the rearward position (stroke St=0 mm) in the conventional relief valve R10shown inFIG. 6. Regarding the curved line S4, the spring105is provided to have a natural length such that the cracking pressure becomes zero, and the set pressure when the present device used flow rate is 100 L/mm is P4.

Even in a case where the spring105is provided to have the natural length such that the cracking pressure becomes 0, the set pressure P4 becomes higher than the set pressure P1 of the curved line S1of the present embodiment. Therefore, in a case where a spring having a comparatively large spring constant is used, and even if an initial load of pressing the plunger103against the valve seat102by the spring105is set to zero, the bigger impact than the case of the curved line S2is applied to the revolving super structure when stopping the rotation of the revolving motor M.

FIG. 5is a diagram showing Embodiment 2 of the relief valve of the present invention and is a longitudinal sectional view showing the periphery of the piston4of a relief valve R2. Unlike the relief valve R1shown inFIG. 1, the relief valve R3is configured such that the piston4does not include the communication hole11that causes the third liquid chamber40and the first liquid chamber22to communicate with each other but includes a communication hole13that causes the first liquid chamber22and the second liquid chamber32to communicate with each other. To be specific, the first liquid chamber22indirectly communicates with the third liquid chamber40through the communication hole12and the communication hole13. The communication hole12does not include a restrictor, but the communication hole13includes a restrictor13a. The first liquid chamber22becomes higher in pressure than the second liquid chamber32, so that the first liquid chamber22can contribute to the adjustment of the pressure increase damping time (t2−t1). Since components other than the above are the same as those of the relief valve R1shown inFIG. 1, and the behavior of the relief valve R2is the same as that of the relief valve R1shown inFIG. 1, explanations thereof are omitted.

In the above embodiments, as shown inFIG. 1, the first elastic member5ahaving the larger elastic constant is arranged at the rear end portion side of the plunger3, and the second elastic member5bhaving the smaller elastic constant is arranged between the first elastic member5aand the tip end portion side (the valve seat2side) of the plunger3. However, instead of this, as shown inFIG. 7, the positions of the first elastic member5aand the second elastic member5bmay be reversed. To be specific, the second elastic member5bhaving the smaller elastic constant is arranged at the rear end portion side of the plunger3, and the first elastic member5ahaving the larger elastic constant is arranged between the second elastic member5band the tip end portion side (the valve seat2side) of the plunger3. Even in this case, the same behavior as the above embodiments is realized. In the case of this configuration, the stopper55may be formed at the outer case1so as to act in the same manner as the above embodiments.

In the above embodiments, the configurations shown inFIGS. 1 and 5are adopted, and the piston4presses forward the rear ends of the first and second elastic members5aand5b, arranged in series, to move forward in accordance with the increase in the pressure of the inflow opening2a. With this, the relief pressure P is adjusted by compressing the first and second elastic members5aand5b. However, the piston4may be configured to act in the same manner as the above embodiments while adopting configurations other than the configurations shown inFIGS. 1 and 5.

INDUSTRIAL APPLICABILITY

As above, the relief valve according to the present invention has excellent effects of: in a case where the liquid pressure of the inflow opening rapidly increases, being able to release the pressure liquid of the inflow opening from a state where the liquid pressure is lower than those of conventional cases; being able to prevent the generation of high surge pressure at the inflow opening by starting the increase in the liquid pressure of the inflow opening from a state where the liquid pressure is low as above; and being able to be configured to be small in size. Thus, the present invention is suitably applied to such a relief valve.

REFERENCE SIGNS LIST