JP2000310204A - Automatically reciprocating mechanism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydraulic mechanism to automatically continue reciprocal motion with a certain period. SOLUTION: This mechanism comprises a spool 5 switching between an extending position and a contracting position, according to which a piston 4 is driven in the extending direction by communicating a first piston pressure chamber 11 with a low pressure passage 29 as well as communicating a second piston pressure chamber 21 with a high pressure passage 28, and is driven in a contracting direction by communicating a first spool pressure chamber 12 with the high pressure passage 28, as well as communicating a second spool pressure chamber 22 with the low pressure passage 29. By hydraulic pressures in a first spool driving pressure chamber 13 and a second spool driving pressure chamber 23, which are generated according to the movement of the piston 4, the position of the spool 5 is switched so as to automatically reciprocate the piston 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic reciprocating mechanism in which reciprocating motion of a piston is automatically repeated by hydraulic pressure or the like.

[0002]

2. Description of the Related Art When high pressure is selectively supplied to left and right oil chambers of a cylinder by a direction switching valve for supplying and discharging hydraulic oil from a hydraulic source, a piston moves in accordance with the hydraulic pressure. The directional control valve performs a switching operation at a constant cycle according to a signal, and repeats and continues the operation of introducing high pressure into one oil chamber and opening the other oil chamber to the tank side. Thereby, the cylinder repeats the expansion and contraction operation at a constant cycle.

[0003]

However, even if a proportional solenoid valve or an electro-hydraulic servo valve is used as such a directional control valve, it is not suitable for high flow rate and high pressure control, and the price of the whole system is low. Will be expensive.

[0004] The present invention has been made in view of the above problems, and has as its object to provide an automatic reciprocating mechanism that automatically reciprocates at a certain cycle by a hydraulic mechanism.

[0005]

According to a first aspect of the present invention, there is provided a first piston pressure chamber in which a pressure for driving a piston in a contracting direction is guided, and a second piston pressure chamber in which a pressure for driving a piston is extended. An extension position for communicating the first piston pressure chamber with the low-pressure passage and communicating the second piston pressure chamber with the high-pressure passage to drive the piston in the extension direction; and connecting the first piston pressure chamber with the high-pressure passage and the second piston. A spool that switches to a compression position in which the pressure chamber communicates with the low-pressure passage to drive the piston in the compression direction, a second spool drive pressure chamber that contracts in the process of moving the piston in the compression direction, and a second spool drive pressure chamber A second spool pressure chamber into which the pressure for driving the spool to the extension position is guided from the first spool, and a first spool which contracts in the process of moving the piston in the extension direction. And Lumpur drive pressure chamber, and a first spool pressure chamber pressure for driving the first spool driving pressure chamber in the position shrinkage spool is guided, it was assumed to automatically reciprocate the piston.

According to a second invention, in the first invention, a first throttle which communicates the first piston pressure chamber with the first spool driving pressure chamber, and a first throttle bypassing the first throttle from the first piston pressure chamber side. A first check valve that opens for hydraulic oil flowing into the first spool drive pressure chamber, a second throttle that communicates the second piston pressure chamber with the second spool drive pressure chamber, and a bypass around the second throttle And a second check valve that opens for hydraulic oil flowing into the second spool drive pressure chamber from the second piston pressure chamber side.

In a third aspect based on the first or second aspect, the cross-sectional area of at least one of the first throttle and the second throttle is adjustable.

According to a fourth aspect, in any one of the first to third aspects, a spring is provided for urging the spool to an extended position or a contracted position.

In a fifth aspect based on the fourth aspect, a spring for urging the spool to the extended position, a switching valve for increasing the pressure of the low pressure passage from the high pressure passage, and a pressure for driving the spool to the extended position are guided. A second spool pressure chamber; and a shuttle valve for connecting the low pressure passage to the second spool pressure chamber when the pressure of the low pressure passage rises.

In a sixth aspect based on the fourth aspect, a spring for urging the spool to the contracted position, a switching valve for increasing the pressure of the low pressure passage from the high pressure passage, and a pressure for driving the spool to the contracted position are guided. A first spool pressure chamber and a shuttle valve for connecting the low pressure passage to the first spool pressure chamber when the pressure of the low pressure passage rises are provided.

According to a seventh aspect of the present invention, in any one of the first to sixth aspects, a flow regulating valve is interposed in the high pressure passage.

According to an eighth aspect of the present invention, in the seventh aspect, a third check valve is provided for opening the hydraulic oil flowing back through the high-pressure passage bypassing the flow regulating valve.

[0013]

In the first aspect of the present invention, the second spool driving pressure chamber contracts while the piston moves in the contracting direction, and the hydraulic oil in the second spool driving pressure chamber passes through the second spool driving pressure passage. Then, the spool flows into the second spool pressure chamber, the spool is switched to the extension position, and the piston moves in the extension direction.

While the piston moves in the extension direction, the first spool driving pressure chamber contracts, and the hydraulic oil in the first spool driving pressure chamber flows into the first spool pressure chamber through the first spool driving pressure passage. The spool switches to the retracted position,
The piston moves in the contraction direction.

In this manner, the movement direction of the piston and the position of the spool are switched in conjunction with each other, so that the piston can automatically repeat reciprocation. The piston can reciprocate with a substantially constant stroke regardless of the level of the response frequency.

In the second aspect, the second spool driving pressure chamber contracts as the piston moves in the contracting direction,
Since the hydraulic oil flows to the second piston pressure chamber through the second throttle, the pressure of the second spool drive pressure chamber increases due to the resistance of the second throttle, and the spool is switched to the extended position.

As the piston moves in the extension direction, the volume of the second spool driving pressure chamber increases and the pressure decreases. However, the second check valve opens and hydraulic oil flows into the second spool driving pressure chamber. I do.

As the piston moves in the extension direction, the first spool driving pressure chamber contracts, and the hydraulic oil flows through the first throttle to the first piston pressure chamber side. The pressure in the first spool drive pressure chamber rises, and the spool switches to the contracted position.

As the piston moves in the contracting direction, the volume of the first spool driving pressure chamber increases and the pressure decreases. However, the first check valve opens and hydraulic oil flows into the first spool driving pressure chamber. I do.

In the third invention, the pressure rise in the first and second spool drive pressure chambers due to the movement of the piston is more rapid as the movement speed of the piston is higher. Open the two diaphragms and close the first diaphragm if there is no need to relax.

Further, by adjusting the flow rate by the first and second variable throttles, the stroke of the reciprocating movement of the piston can be changed.

In the fourth aspect, the spool moves to the extended position or the contracted position by the urging force of the spring while the operation is stopped, and the moving direction of the piston when the operation is resumed is determined.

In the fifth aspect, the spool moves to the extension position by the urging force of the spring while the operation is stopped, and the piston moves in the extension direction when the operation is resumed.

However, when the piston is fully extended,
Operation cannot be resumed. To solve this problem, the pressure in the low-pressure passage is increased from the high-pressure passage via the switching valve, and the pressure in the low-pressure passage is guided to the second spool pressure chamber via the shuttle valve, thereby holding the spool in the extended position. The high pressure in the passage is guided to the first piston pressure chamber to move the piston in the contracting direction.

In the sixth aspect of the present invention, the spool moves to the contracted position by the urging force of the spring while the operation is stopped, and the piston moves in the contracted direction when the operation is resumed.

However, when the piston is fully contracted,
Operation cannot be resumed. In order to solve this problem, the pressure of the low pressure passage is increased from the high pressure passage via the switching valve, and the high pressure of the low pressure passage is guided to the first spool pressure chamber via the shuttle valve, thereby holding the spool in the contracted position. The piston is moved in the extension direction by guiding the high pressure in the low pressure passage to the second piston pressure chamber.

In the seventh aspect, the speed at which the piston reciprocates can be arbitrarily set via the flow control valve.

In the eighth aspect, when the operation is resumed from a state in which the piston is fully contracted, the pressure in the high pressure passage is made lower than that in the low pressure passage through the switching valve, and the hydraulic oil in the first and second piston pressure chambers is discharged. The piston is moved in the contraction direction or the extension direction by escaping from the high-pressure passage through the third check valve.

[0029]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, reference numeral 8 denotes an automatic switching valve which automatically switches in response to oil pressure, and 10 denotes a reciprocating operation of the piston 4 at a constant stroke by supplying and discharging hydraulic pressure performed through the automatic switching valve 8. The reciprocating cylinder 40 is a load driven by the piston 4. The reciprocating cylinder 10 is used, for example, as a vibration generator such as a hydraulic vibrator or a sieve device.

In FIG. 1, 1 is a hydraulic pressure source, 2 is a tank,
Reference numeral 6 denotes a switching valve for switching between operation and stop of the reciprocating cylinder 10. The switching valve 6 communicates the high-pressure passage 28 with the discharge side of the hydraulic pressure source 1 and the low-pressure passage 29 with the tank 2 to operate the reciprocating cylinder 10.
And a stop position B for closing the high-pressure passage 28 and the low-pressure passage 29 and stopping the reciprocating cylinder 10.
Switching operation is performed manually or via electromagnetic force.

In FIG. 1, reference numeral 7 denotes a flow control valve for controlling the speed of the reciprocating cylinder 10. The flow control valve 7 is provided in the middle of the high-pressure passage 28, and is constituted by a variable throttle that changes the passage cross-sectional area manually or via an electromagnetic force.

The automatic switching valve 8 accommodates the spool 5 inside the housing 39 so as to be slidable, and the housing 39 has one pump port 30 and two tank ports 31 and 32 and the first and second pressure regulating ports 33. , 34 are formed at predetermined intervals. The high pressure passage 28 is connected to the pump port 30, and the low pressure passage 29 is connected to the tank ports 31 and 32.

When the spool 5 is in the contracted position (left position in the figure), the first pressure adjusting port 33 communicates with the pump port 30 to guide the discharge pressure Ps, and the second pressure adjusting port 34 communicates with the tank port 32. The tank pressure Pt is led through the communication.

When the spool 5 is in the extended position (the right position in the figure), the first pressure adjusting port 33 communicates with the tank port 31 to guide the tank pressure Pt, and the second pressure adjusting port 34 connects to the pump port 30. The discharge pressure Ps is led through the communication.

In the reciprocating cylinder 10, a piston 4 is housed inside the body 3 so as to be freely slidable, and a rod 9 integrally connected to the piston 4 projects outside from an end of the body 3. The first piston pressure chamber 11 and the second piston pressure chamber 21 are defined by the piston 4, and the first piston pressure chamber 11 communicates with the first pressure adjustment port 33 via the first piston pressure passage 35, Two piston pressure chamber 21
Communicates with the pressure adjustment port 34 via the second piston pressure passage 36.

A first spool pressure chamber 12 and a second spool pressure chamber 22 are defined at both ends of the spool 5. The pressure receiving area e of the spool 5 with respect to the first spool pressure chamber 12 and the pressure receiving area f with respect to the second spool pressure chamber 22 are set to be substantially equal, but are not necessarily limited thereto.

A first spool driving pressure chamber 13 which contracts while the piston 4 moves in the direction of extension (right) in the drawing;
In the drawing, a second spool driving pressure chamber 23 is provided which contracts in the process of moving in the contracting (left) direction.

As shown in FIG. 2, the outer peripheral surface 47 of the piston 4 has a small diameter portion 3 of the body 3.
7 is defined with respect to the first piston pressure chamber 11.

As shown in FIG. 3, the outer peripheral surface 48 of the piston 4 is
8, the second piston pressure chamber 21 is defined.

The first spool drive pressure chamber 13 communicates with the first spool pressure chamber 12 via the first spool drive pressure passage 15 and communicates with the first piston pressure chamber 11 via the first variable throttle 17. When the first spool drive pressure chamber 13 contracts, the hydraulic oil in the first spool drive pressure chamber 13 flows into the first spool pressure chamber 12 through the first spool drive pressure passage 15 and passes through the first variable throttle 17. It flows into the first piston pressure chamber 11.

A first check valve 16 is arranged between the first spool drive pressure chamber 13 and the first piston pressure chamber 11 in parallel with the first variable throttle 17. When the first spool drive pressure chamber 13 is expanded, the operating oil in the first piston pressure chamber 11 is supplied to the first check valve 16.
And flows into the first spool drive pressure chamber 13.

The second spool drive pressure chamber 23 communicates with the second spool pressure chamber 22 via a second spool drive pressure passage 25 and communicates with the second piston pressure chamber 21 via a second variable throttle 27. When the second spool drive pressure chamber 23 contracts, the hydraulic oil in the second spool drive pressure chamber 23 flows into the second spool pressure chamber 22 through the second spool drive pressure passage 25 and passes through the second variable throttle 27. It flows into the second piston pressure chamber 21.

A second check valve 26 is disposed between the second spool drive pressure chamber 23 and the second piston pressure chamber 21 in parallel with the second variable throttle 27. When the second spool drive pressure chamber 23 is expanded, the operating oil in the second piston pressure chamber 21 is supplied to the second check valve 26.
And flows into the second spool drive pressure chamber 23.

The first variable throttle 17 and the first check valve 1
6. The second variable throttle 27 and the second check valve 26 may be housed in the body 3.

The operation of the embodiment of the present invention configured as described above will now be described with reference to FIGS.

The switching valve 6 is switched from the stop position B to the operation position A, and the spool 5 is set to e × Ps−f × P
Under the force of t, the first pressure adjustment port 33 communicates with the pump port 30 to discharge the discharge pressure Ps, and is held in the contracted (left) position as shown in FIG. The port 34 communicates with the tank port 32 and communicates with the tank pressure Pt.
Is led. Accordingly, assuming that the pressure receiving area c + d of the piston 4 with respect to the second piston pressure chamber 21 and the pressure receiving area a + b with respect to the first piston pressure chamber 11, the piston 4 has a
Under the force of × P1 + b × P1- (c + d) × Pt, it starts to move in the contraction (left) direction.

As the piston 4 moves in the contracting (left) direction, the outer peripheral surface 48 of the piston 4 fits into the small diameter portion 38 of the body 3 as shown in FIG.
And the communication of the second spool drive pressure chamber 23 is closed. From here, the second spool drive pressure chamber 23 contracts as the piston 4 further moves in the contracted (left) direction, and the hydraulic oil flows through the second variable throttle 27 to the second piston pressure chamber 21. The pressure P2 in the second spool drive pressure chamber 23 increases due to the resistance of the second variable throttle 27.

Here, the pressure P2 due to the movement of the piston 4
Rises rapidly as the moving speed of the piston 4 increases, and if it is necessary to reduce the speed, the second variable throttle 27 may be opened, and if it is not necessary, the second variable throttle 27 may be closed.

When the pressure P2 in the second spool driving pressure chamber 23 rises until f × P2> e × Ps, FIG.
As shown in the figure, the spool 5 moves rightward and switches to the extended (right) position, the first pressure adjustment port 33 communicates with the tank port 31 to guide the tank pressure Pt, and the second pressure adjustment port 34. Is connected to the pump port 30 and discharge pressure P
s is derived. Thereby, the piston 4 is c × Ps + d
Under the force of × P2− (a + b) × Pt, it starts to expand and move in the right direction.

When the volume of the second spool driving pressure chamber 23 increases while the piston 4 moves in the extending (right) direction, the pressure P2
However, when the pressure P2 becomes lower than the pressure Ps guided to the second piston pressure chamber 21, the second check valve 26 opens and the hydraulic oil flows into the second spool drive pressure chamber 23, The pressure in the spool drive pressure chamber 23 and the pressure in the second piston pressure chamber 21 are substantially equal to Ps.

As the piston 4 moves in the extending (right) direction, the outer peripheral surface 47 of the piston 4 fits into the small diameter portion 37 of the body 3 as shown in FIG.
And the first spool drive pressure chamber 13 is closed. Since the first spool drive pressure chamber 13 contracts as the piston 4 further extends (moves in the right direction) from here, the hydraulic oil flows through the first variable throttle 17 to the first piston pressure chamber 11. The pressure P1 of the first spool drive pressure chamber 13 increases due to the resistance of the first variable throttle 17.

Here, the pressure P1 due to the movement of the piston 4
Rises rapidly as the moving speed of the piston 4 increases. If it is necessary to alleviate this, the first variable throttle 17 is opened, and if it is not necessary, the first variable throttle 17 may be closed.

When the pressure P1 in the first spool driving pressure chamber 13 rises until e × P1> f × Ps, FIG.
As shown in (5), the spool 5 is switched to the contracted (left) position, and the piston 4 is a × Ps + b × P1- (c + d).
Start moving in the contraction (left) direction under the force of × Pt.

As long as the pressure oil is supplied to the pump port 30, the piston 4 repeats the reciprocating motion with a substantially constant stroke regardless of the response frequency.

By providing the first variable throttle 17 and the second variable throttle 27, the pressure rise in the first spool drive pressure chamber 13 and the second drive pressure chamber 23 is reduced, and smooth operability is ensured.

Further, by adjusting the flow rate by the first variable throttle 17 or the second variable throttle 27, the stroke of the reciprocating movement of the piston 4 can be changed to some extent.

Further, by adjusting the flow rate by the flow rate adjusting valve 7, the speed at which the piston 4 reciprocates can be arbitrarily set.

When the switching valve 6 is switched from the operating position A to the stop position B, the high pressure passage 28 and the low pressure passage 29 are closed, and the movement of the spool 5 and the piston 4 is stopped.

However, the stop position of the spool 5 changes depending on the timing at which the switching valve 6 is switched to the stop position B, and the moving direction of the piston 4 is not constant when the operation is resumed.

In order to solve this problem, as another embodiment, as shown in FIG. 4, the spool 5 is extended (right).
A spring 42 biasing the position may be interposed.

In this case, while the operation is stopped, the spool 5 moves to the extended (right) position by the urging force of the spring 42, and when the operation is resumed, the reciprocating cylinder 10 starts operating from the extending direction. As shown in FIG. Alternatively, a spring 41 for urging the spool 5 to the contracted (left) position may be interposed.

In this case, the spool 5 is moved to the contracted (left) position by the urging force of the spring 41 while the operation is stopped, and the operation of the reciprocating cylinder 10 is started from the contracted direction when the operation is resumed.

However, in the case of the device shown in FIG. 4, the operation cannot be resumed when the piston 4 is fully extended, which is located to the right in the drawing. Further, in the case of the device shown in FIG. 5, the operation cannot be resumed when the piston 4 is located at the left side in the figure and is in a contracted state.

In order to solve the problem of the device shown in FIG.
As another embodiment, as shown in FIG. 6, the switching valve 6 is changed from a two-position directional switching valve to a three-position directional switching valve. A shuttle valve 52 may be provided for selectively communicating the higher pressure of the pressure passage 25.

The switching valve 6 connects the high pressure passage 28 to the discharge side of the hydraulic pressure source 1 and the low pressure passage 29 to the tank 2 to operate the reciprocating cylinder 10.
And a stop position B in which the high-pressure passage 28 and the low-pressure passage 29 are closed to stop the reciprocating cylinder 10, and the high-pressure passage 28 communicates with the tank 2 and the low-pressure passage 29 communicates with the discharge side of the hydraulic power source 1. A reverse operation position C for operating the dynamic cylinder 10 on the extension side.

The flow control valve 7 is interposed in the middle of the high-pressure passage 28, and bypasses the flow control valve 7 to bypass the high-pressure passage 28.
A third check valve 53 is provided for opening the hydraulic oil flowing back through the valve. FIG. 7 shows a state in which the switching valve 6 is switched to the reverse operation position C with the reciprocating cylinder 10 fully extended, and the discharge pressure Ps of the hydraulic power source 1 is reduced via the low pressure passage 29 and the shuttle valve 52 to the second position. It is guided to the spool pressure chamber 22, is guided to the first piston pressure chamber 11 via the automatic switching valve 8, and is guided to the first spool drive pressure chamber 13 via the first check valve 16.

At this time, the urging force of the spring 42 is changed to F1
Then, the spool 5 is extended by receiving the force of fxPs + F1-exPs, and is held at the right position.

The piston 4 is (a + b) × Ps−
Under the force of (c + d) × Pt, it starts moving in the contraction (left) direction, and stops in the contracted state as shown in FIG.
If the switching valve 6 is switched to the stop position B before the piston 4 stops in the contracted state, the piston 4 stops at that point.

When the switching valve 6 is switched to the operating position A with the piston 4 stopped in this way, the reciprocating cylinder 10 reciprocates via the automatic switching valve 8, and starts reciprocating from the position where the piston 4 is fully contracted. It is possible to start.

In order to solve the problem of the device shown in FIG.
As another embodiment, as shown in FIG. 9, the switching valve 6 is changed from a two-position directional switching valve to a three-position directional switching valve. A shuttle valve 51 may be provided for selectively communicating the higher pressure of the pressure passages 15.

The flow control valve 7 is interposed in the middle of the high pressure passage 28, and bypasses the flow control valve 7 to bypass the high pressure passage 28.
A third check valve 53 is provided for opening the hydraulic oil flowing back through the valve. FIG. 9 shows a state in which the reciprocating cylinder 10 has contracted and stopped, and when the switching valve 6 is switched to the reverse operation position C, the piston 4 moves to the fully extended state. Then, the switching valve 6 is in the operating position A.
, The reciprocating cylinder 10 reciprocates via the automatic switching valve 8, and the reciprocating operation can be started from the position where the piston 4 is fully extended.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the same operating state.

FIG. 3 is a cross-sectional view showing the same operating state.

FIG. 4 is a cross-sectional view illustrating another embodiment.

FIG. 5 is a cross-sectional view showing still another embodiment.

FIG. 6 is a cross-sectional view showing still another embodiment.

FIG. 7 is a cross-sectional view showing the same operating state.

FIG. 8 is a sectional view showing the same operating state.

FIG. 9 is a sectional view showing still another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hydraulic source 2 Tank 3 Body 4 Piston 5 Spool 6 Switching valve 7 Flow control valve 8 Automatic switching valve 10 Reciprocating cylinder 11 First piston pressure chamber 12 First spool pressure chamber 13 First spool driving pressure chamber 15 First driving pressure Passage 16 First check valve 17 First variable throttle 21 Second piston pressure chamber 22 Second spool pressure chamber 23 Second spool drive pressure chamber 25 Second drive pressure passage 26 Second check valve 27 Second variable throttle 30 Pump Port 31 Tank port 32 Tank port 41 Spring 42 Spring 51 Shuttle valve 52 Shuttle valve 53 Third check valve

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3H089 AA62 BB27 CC01 DA02 DB13 DB33 DB46 DB48 DB75 EE15 GG02 JJ20

Claims (8)

[Claims] 1. A first piston pressure chamber into which a pressure for driving a piston in a contracting direction is introduced, a second piston pressure chamber into which a pressure for driving the piston in an extending direction is introduced, and a low pressure in the first piston pressure chamber. The second piston pressure chamber communicates with the high-pressure passage and the first piston pressure chamber communicates with the high-pressure passage while the second piston pressure chamber communicates with the high-pressure passage and the second piston pressure chamber communicates with the high-pressure passage. A spool that is in communication with a low-pressure passage and switches to a contraction position that drives the piston in a contraction direction; a second spool drive pressure chamber that contracts while the piston moves in the contraction direction; and the second spool drive pressure chamber. A second spool pressure chamber into which the pressure for driving the spool to the extended position is guided, and the piston moves in the extended direction. A first spool drive pressure chamber that contracts in the process, and a first spool pressure chamber into which pressure for driving the spool to the contracted position is guided from the first spool drive pressure chamber, and the piston is automatically reciprocated. Automatic reciprocating mechanism. 2. A first throttle which communicates with the first piston pressure chamber and the first spool drive pressure chamber; and a first spool drive pressure chamber which bypasses the first throttle and is provided from the first piston pressure chamber side. A first check valve that opens for the hydraulic oil flowing into the second, a second throttle that communicates with the second piston pressure chamber and the second spool drive pressure chamber, and a second throttle that bypasses the second throttle. 2. The automatic reciprocating mechanism according to claim 1, further comprising: a second check valve that opens for hydraulic oil flowing into the second spool drive pressure chamber from the two-piston pressure chamber side. 3. 3. The automatic reciprocating mechanism according to claim 1, wherein a cross-sectional area of at least one of the first throttle and the second throttle is adjustable. 4. The automatic reciprocating mechanism according to claim 1, further comprising a spring for urging the spool to an extension position or a contraction position. 5. A spring for urging the spool to the extended position, a switching valve for increasing the pressure of the low pressure passage higher than the high pressure passage, and a second spool pressure chamber into which a pressure for driving the spool to the extended position is guided. The automatic reciprocating mechanism according to claim 4, further comprising: a shuttle valve that connects the low-pressure passage to the second spool pressure chamber when the pressure of the low-pressure passage increases. 6. A spring for urging the spool to a contraction position, a switching valve for increasing the pressure of the low-pressure passage above the high-pressure passage, and a first spool pressure chamber to which a pressure for driving the spool to the contraction position is guided. The automatic reciprocating mechanism according to claim 4, further comprising: a shuttle valve that connects the low-pressure passage to the first spool pressure chamber when the pressure of the low-pressure passage increases. 7. The automatic reciprocating mechanism according to claim 1, wherein a flow regulating valve is interposed in the high pressure passage. 8. The automatic reciprocating motion according to claim 7, further comprising a third check valve that opens for hydraulic oil flowing back through the high-pressure passage bypassing the flow control valve. mechanism.