JPH0159068B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a flow rate regulating valve suitable for regulating the flow rate of a hydraulic circuit of a die-casting machine or the like.

[Conventional technology]

Generally, a die casting machine performs injection at a low speed at the initial stage of injection of molten metal by an injection cylinder, and switches to high speed injection during injection to supply the molten metal into a mold cavity. The hydraulic circuit of a die casting machine that performs such an operation uses a switching valve that switches from low-speed injection to high-speed injection, and a flow rate adjustment valve for low-speed injection and high-speed injection that adjusts the flow rate of hydraulic oil.

Conventionally, four injection circuits have been used: an on-off valve and a flow rate regulating valve for low-speed injection, and an on-off valve and a flow rate regulating valve for high-speed injection.

[Problems to be solved by the present invention]

However, in conventional flow rate regulating valves, the valve spool slides in direct contact with the inside of the manifold, which requires drilling extremely precise holes in the manifold, which is difficult and time-consuming. There was once a time. In addition, misalignment between the hole in the manifold and the valve spool could not be accommodated, putting stress on the valve spool and the driving part, and the movement of the valve spool, which should open and close quickly, sometimes lacked smoothness.

[Means for solving problems]

The present invention is aimed at eliminating these drawbacks, and includes a valve chamber having an inlet at one end in the axial direction, sealed at the other end in the axial direction, and having an outlet in a direction perpendicular to the axial direction. A manifold is provided, a valve spool that moves back and forth in the axial direction is provided in the valve chamber, and the valve chamber is divided into two parts, a first chamber on the inlet side and a second chamber on the opposite inlet side, by the valve spool. A sleeve that slidably holds the valve spool is installed on the inner circumferential surface of the manifold via packing, which prevents fluid flow between the inner circumferential surface of the manifold and the sleeve, and also prevents fluid flow between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold. The machining between the circumferential surfaces is made rough so that the sleeve can absorb misalignment, and a through hole is provided in the sleeve near the front end of the valve spool that communicates with the circulation hole that forms the outlet provided in the manifold. A through hole is provided so that it can be opened and closed by a valve spool, an annular groove is provided in the through hole portion of the sleeve, and a number of through holes are provided inside the valve spool to communicate the first and second chambers of the valve chamber. On the second chamber side of the valve chamber, a screw mechanism that changes rotational movement into axial movement is connected to the valve shaft of a valve spool provided through the valve chamber wall, and a variable speed screw mechanism is connected to the other end of the screw mechanism. This is a combination of motors.

[Effect]

In this flow rate regulating valve, when the variable speed motor is rotated, the valve spool moves inside the sleeve provided on the inner peripheral surface of the manifold through the action of the screw mechanism and valve shaft that converts rotational movement into axial movement. Move in the axial direction. By changing the rotation direction of the variable speed motor between forward and reverse, the valve spool moves in two directions along the axis, opening and closing the through hole provided in the sleeve at the tip of the valve spool, allowing the hydraulic oil to flow out. or block it. At that time, by changing the rotation amount of the variable speed motor, the opening/closing amount of the valve spool, that is, the flow rate can be changed, and by changing the rotation speed of the variable speed motor, the opening/closing speed of the valve spool, that is, the flow rate can be changed. The degree of increase/decrease can be changed.

At this time, the inlet of the hydraulic oil is provided at one end of the valve spool in the axial direction, and the outlet including the through hole provided in the sleeve is provided in a direction perpendicular to the axis, and the tip of the valve spool blocks the outflow of the hydraulic oil. In addition to regulating the valve spool, there are many through holes in the valve spool, so near the point where the valve spool starts to open, a force in the direction of opening the valve spool acts on the valve spool, and the opening degree of the valve spool increases to a certain extent. When the valve spool is about to stop, a force in the direction of closing the valve spool acts on the valve spool. The valve spool therefore opens smoothly with relatively little force and in a very short time.

In addition, since the sleeve is installed with rough machining between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold, even if there is a slight misalignment between the sleeve and the valve spool, the misalignment will is easily absorbed, no undue force is applied to the valve spool or the drive unit, and the valve spool always slides smoothly. Further, even if the sleeve becomes slightly distorted together with the valve spool due to thermal deformation or mounting conditions, the valve spool will slide smoothly.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 and the following explain one embodiment of the present invention. As shown in FIG. 1, a flow rate regulating valve 1 is roughly divided into a driving section 2 located at one end and a flow regulating section that also serves as an on-off valve. It is composed of 3.

The drive unit 2 has a motor 6 connected to a gear 14 etc. via a coupling 5 provided in a part of the casing 4 . The motor 6 is preferably a pulse motor that can be quickly and accurately automatically controlled and can accurately maintain the position of the valve spool 31, which will be described later, after being controlled and changed. Normally, a pulse motor does not move even if you try to move it, and only when a signal is received to turn off the power and start it, will it turn by an amount equivalent to the number of commanded pulses, and if there is no rotation command, it will not move at its current position. If there is a force trying to maintain it and there is a rotation command,
Control the amount of rotation accurately at high speed.

The catch 10 which is the driven side of the coupling ring 5 is
In order to downsize the coupling ring 5 itself, it is integrated with the driven shaft 13, and the tip of this shaft 13, which is supported by a bearing 13a inside the casing 4, has a
A gear 14 is fixed. Gear 14 is gear 18
The gear 18 meshes with the thrust bearing 15, ball bearing 15a, and tapered roller bearing 15 in the center of the casing 4.
It is fixed to a shaft 17 which is rotatably parallel to the shaft 13 and supported so as not to move in the axial direction through a bearing such as b or a shaft holding member 16, etc., and the rotation of the motor 6 is It is transmitted to the shaft 17 via gears 14 and 18.

A nut 19 is screwed onto the outer periphery of the shaft 17 on the valve spool 31 side via a ball screw portion 17a. The nut 19 has a key 20 in a part thereof, and the key 20 is formed in a key groove 21 formed on the casing 4 side.
The nut 19 is slidably fitted therein, and the nut 19 can freely move forward and backward in the axial direction when the shaft 17 rotates. The shaft 17 and the nut 19 are screwed together by the nut 19 being guided in the axial direction by a key 20.
An ordinary screw may be used as long as the screw is moved in the axial direction by the rotation of the shaft 17, but in order to operate efficiently, it is more ideal to use a ball screw with high transmission efficiency. In addition, in a ball screw that uses a ball screw, if a force in the axial direction is applied to the nut 19, the shaft 17 tries to turn, but in this case, the rotation of the shaft 17 is blocked by the pulse motor 6. , the shaft 17 does not rotate. The shaft 17, the ball screw shaft 17a, the nut 19, and the like constitute a screw mechanism that converts rotational motion into axial motion.

A permanent magnet 22 is fixed to a part of the key 20 attached to the nut 19, and a magnetic position detector 23 called a zero cross sensor is mounted on a part of the casing 4 facing the permanent magnet 22. installed. The position detector 23 is composed of a proximity switch that is sensitive to the movement of the permanent magnet 22, and can accurately detect the movement distance of the nut 19 and the valve spool 31 in the axial direction, and provide feedback to the control device. ing.

Further, the nut 19 can maintain its horizontal center by slidably contacting the lower surface of the key 20 with the upper surface of a pedestal 24 that is attached to the casing 4 so as to be able to move up and down. It's summery. The pedestal 24 is supported by a cover 4b attached to the casing 4 via a spring 25 whose elastic force is calculated in advance and placed, and is configured to support the entire shaft 17 in a well-balanced manner. has been done.

A thin inner cylindrical portion 27 larger than the outer diameter of the nut 19 is integrally attached to the nut 19, and a valve shaft 28 is integrally provided at the tip of the inner cylindrical portion 27. The center axis of the valve stem 28 is aligned with the axis 17. 26 is a space part. In the inner cylinder part 27 surrounding this space part 26, a plurality of through holes 27a are formed at predetermined pitches in a staggered manner in the circumferential direction. The reason why the hollow inner cylinder part 27 is provided and a plurality of staggered holes 27a are formed in this part is that the force transmitted through the valve shaft 28 escapes along the holes 27a, is dispersed, and is partially This is to prevent power from being concentrated only in one area. The inner cylindrical portion 27 may be made thinner to increase the stress, but if such a hole 27
When a is present, an even better cushioning effect is exhibited.
Then, impact to the ball screw portion 17a is avoided.

Casing 4 housing shaft 17 and nut 19
The root portion of the valve body is attached to a manifold 29 forming the valve body by bolts (not shown). A valve spool 31 is fitted into a sleeve 40 disposed in the bore 30 of the manifold 29 so as to be slidable in the axial direction with extremely high fitting precision.The valve spool 31 is fixed to the valve shaft 28. ing.

The sleeve 40 itself is integrally formed as one piece, and the entire or most of the outer circumferential surface of the valve spool 31 can slide against the inner circumferential surface of the sleeve 40. Further, even when the valve spool 31 is at its forward limit and the valve is closed, the tip of the valve spool 31 is attached to the sleeve 40.
It is located in a part of the uniform inner diameter portion of the valve, so that it will not be stopped by being pressed against, for example, a valve seat.

The valve spool 31 is provided with a large number of communication holes 31a penetrating in the axial direction, and communicates with a hydraulic pump and an accumulator (not shown). The chamber 30a and a second chamber 30b on the opposite side of the casing 4 are communicated with each other. This is because the valve shaft 28 and nut 1 are connected via the valve spool 31.
This is to make the pressing force in the axial direction by the hydraulic oil acting on the valve spool 9 relatively small, and also to reduce the weight of the valve spool 31 itself, so that the shaft 17 can be easily rotated by the pulse motor 6. .

The valve spool 31 has a circulation hole 3 on its outer peripheral surface at the center.
An annular groove 32 is formed in communication with 1a.

Further, on the manifold 29 side, a constriction portion 29a is provided.
By providing the hydraulic oil circulation holes 33 and 3 communicating with the secondary side, that is, the injection cylinder side.
4 is formed, and as the valve spool 31 moves, the state of communication between the groove 32 and the tip of the valve spool 31 changes through the through holes 40a, 40b of the sleeve 40, and the flow rate can be changed. It's becoming like that. The sleeve 40 is provided with a plurality of through holes 40a and 40b in the circumferential direction, and an annular groove having a depth of approximately half the thickness of the sleeve 40 is provided on the inner surface of the sleeve at the through hole portion. You can also do that.

A groove 32 communicating with the first chamber 30a is provided in the middle of the valve spool 31, and a communication hole 33 is provided in the manifold 29.
34 and the holes 40a and 40b in the sleeve 40 are so that more oil can pass through with a small opening of the valve spool 31. That is, in the state shown in FIG. 1, oil not only flows from the first chamber 30a through the tip of the valve spool 31 to the through hole 40a and the circulation hole 33, but also at the same time, oil flows from the groove 32 through the notch 32a. This is because the oil in the first chamber 30a is made to flow into the through hole 40b and the circulation hole 34 as well. Of course, two or more grooves 32 are provided,
Corresponding through holes 40b and communication holes 34 can also be provided at two or more locations. In this way, the direct diameter of the valve spool can be reduced,
The entire valve device can also be made smaller. In addition, valve spool 3
If the opening degrees of the through holes 40a and 40b when 1 is opened to the maximum are b and c, respectively, b=c may be set so that the amount of oil flowing from both gaps is the same, Also, when c>b and when performing low-speed injection with a small opening degree, the through hole 40a section is blocked and oil flows only from the groove 32 to the through hole 40b section, and when the opening degree is increased and high-speed injection is performed. Alternatively, the oil may flow from both the through holes 40a and 40b.

The part of the manifold 29 in contact with the valve spool 31 has a double structure.
A sleeve 40 with which the valve spool 31 is in direct contact is fitted inside. This sleeve 40 is formed with through holes 40a and 40b that communicate with the aforementioned communication holes 33 and 34, respectively. The machining accuracy between the inner peripheral surface of the sleeve 40 and the valve spool 31 is as follows:
For example, make it relatively severe like 0.03mm,
Even when a large oil pressure is applied, hydraulic oil is prevented from leaking from the gap between the valve spool 31 and flowing into the flow holes 33 and 34 side.

Instead, the machining between the outer peripheral surface of the sleeve 40 and the inner peripheral surface of the manifold 29 is, for example, 0.1 to 0.3
mm, and is relatively rough, and a gasket 41 such as an O-ring is used to prevent oil leakage between the manifold 29 and the manifold 29. If such a structure is adopted, it is only necessary to increase the machining accuracy of only the inner circumferential surface of the sleeve 40, making the machining operation easy and extremely economical. That is, when the valve spool 31 is directly slidably assembled into the manifold 29, it is necessary to machine a highly precise hole in the manifold 29, which is difficult and time-consuming. 31, the hole drilling of the manifold 29, which is difficult to machine, can be made several times as rough, and the sleeve 4, which is easy to machine, can be used.
It is only necessary to process the inner peripheral surface of 0 more severely. Further, by providing the sleeve 40, even a slight misalignment can be sufficiently absorbed, and the center can be maintained accurately to match the movement of the valve spool 31. 31
It is also possible to prevent excessive force from acting on the parts.

In addition, the outer peripheral surface of the sleeve 40 and the manifold 2
By interposing the packing 41 between the inner peripheral surface of the sleeve 40 and the inner peripheral surface of the manifold 29, it is possible to prevent the flow of fluid therebetween and to make the machining between the outer peripheral surface of the sleeve 40 and the inner peripheral surface of the manifold 29 rough. Since the sleeve 40 is provided in such a way that it can absorb even a slight misalignment, this is similar to providing the sleeve 40 so that it can move by a very small amount in a direction perpendicular to the axis of the sleeve 40. Even if the sleeve 40 is slightly distorted together with the valve spool 31 due to thermal deformation or mounting conditions, the valve spool 31 will slide smoothly.

37 is the casing 4 and chamber 30 around the valve stem 28
This is a shaft seal member provided between the shaft seal member and the shaft member b, which has a drain discharge function.

Next, the operation of this embodiment configured as above will be explained.

First, when a die-casting machine or the like to which this flow rate adjustment valve is applied is in a non-operating state, the valve spool 31 is advanced leftward in FIG. , the first chamber 30a and the groove 3 of the valve spool 31
Communication with 2 has been cut off.

In this state, when a command to start low-speed injection is issued and a low-speed injection speed is commanded, the command is applied to the pulse motor 6, and the pulse motor 6 is rotated by a predetermined angle. Axis 17
Rotation is transmitted to. Rotation of shaft 17 is done by nut 19
is transmitted to the nut 19 via a ball screw device provided between the two, and the nut 19 retreats. The retreat of the nut 19 is transmitted to the valve spool 31 side via the inner cylinder portion 27 and the valve shaft 28, and for example, the groove 32 first becomes in communication with the through hole 40b. Then, the primary hydraulic oil guided to the bore 30 at the tip end of the valve spool 31 flows into the through hole 40b through the groove 32, is supplied to the injection cylinder (not shown), and low-speed injection is started. Ru. At this time, the degree of communication between the groove 32 and the through hole 40b, that is, the flow rate of the hydraulic oil, is accurately set according to the injection cylinder of the die-casting machine.

In this way, the low-speed injection state is set and maintained for a predetermined time, but when a high-speed injection command comes soon, the shaft 17 is rotated by the pulse motor 6 in accordance with the command, and the nut 19 is rotated. Step back further. Since the retraction of the nut 19 via the ball screw is extremely smooth and at high speed, the valve spool 3
1 also retreats at high speed.

When the valve spool 31 retreats to a predetermined position,
The communication area between the groove 32 and the through hole 40b further increases, and the bore 3 on the tip side of the valve spool 31
0 and the through hole 40a expand all at once.
4, a large amount of hydraulic fluid is sent to the injection cylinder, and high-speed injection begins almost instantaneously. When the high-speed injection is finished, the pulse motor 6 is operated again to close the valve spool 31.

In addition, in the embodiment, the valve spool 31
Although one groove 32 is provided in the axial direction, two or more grooves 32 may be provided in the axial direction. At that time, the through hole 40b of the sleeve 40 and the communication hole 34 of the manifold 29 are also provided depending on the position and number of the plurality of grooves 32. The present invention can also be used for valves that are manually opened and closed.

In addition, in the embodiment described above, by roughening the space between the outer circumferential surface of the sleeve 40 and the inner circumferential surface of the manifold 29, the sleeve 40 is slightly lifted from the inner circumferential surface of the manifold 29 by the packing 41. I was able to make it look like this, but
This does not necessarily mean that a single member has both a sealing function to prevent leakage and a floating function; for example, floating materials, such as a simple rubber ring, have flexibility but do not have a sealing function. Patsukin 41
Alternatively, as long as it can prevent most of the oil leakage, even a small amount of oil leakage will not affect its operation, so simply use only the floating material. It may be provided.

Although a manifold is usually referred to as a manifold having a plurality of holes through which oil passes, the sleeve 41, valve spool 31, or casing 4 of the present invention can of course be attached directly to the valve body, injection cylinder, etc. Therefore, in the present invention, the structure to which the sleeve 41 and the like can be attached is collectively referred to as a manifold.

〔Effect of the invention〕

As is clear from the above description, in the present invention, as described in the claims, a sleeve is provided on the inner peripheral surface of the manifold on which the valve spool slides, and the sleeve is communicated with the communication hole provided in the manifold. Since a through hole is provided in the sleeve and the through hole can be opened and closed by the valve spool, it is only necessary to increase the machining accuracy of the inner peripheral surface of the sleeve, which is relatively small and cylindrical and easy to lathe. Holes in manifolds, where precision machining of hole surfaces is extremely difficult, can be roughly machined with a slight reduction in precision, making the machining work easier and extremely economical.

In addition, since the sleeve has been provided with a rough machining between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold, even a slight misalignment can be sufficiently absorbed, and it can be adjusted to match the movement of the valve spool. Keeps the core accurate and prevents excessive force from acting on the valve spool, drive unit, etc. Then, the valve spool can be moved smoothly and quickly, and as a result, good die-casting work can be performed.

In addition, by interposing gaskets between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold, fluid flow between them is prevented, and
The machining between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold has been roughened so that the sleeve can absorb even the slightest misalignment, so the sleeve can be moved only a small amount in the direction perpendicular to the axis of the sleeve. This is similar to what is possible, and even if the sleeve becomes slightly distorted together with the valve spool due to thermal deformation or mounting conditions, the valve spool will still slide smoothly.

That is, in the present invention, the sleeve is separated from a large object such as a manifold to prevent thermal deformation due to rise in oil temperature. Even if the sleeve and valve spool extend in the same direction, they escape to the outside in the same state and are not affected by the manifold, and the outer peripheral surface of the valve spool and the inner peripheral surface of the sleeve always maintain an appropriate minute clearance, and the valve spool The movement can be performed smoothly, and the flow rate can also be kept constant.

In addition, in the present invention, since the valve spool is provided with a large number of through holes, the entire valve spool approaches the oil temperature.
Valve spools do not have local thermal expansion changes. Therefore, the diameter of the valve spool can be kept as constant as possible, and the valve spool can always move smoothly.

Furthermore, in the present invention, when the valve is closed and the flow rate through the valve is set to 0, the outer circumference of the tip of the valve spool is not pressed against the valve seat and stopped, and the outer circumference of the tip of the valve spool is uniformly aligned with the sleeve. Since it is in contact with the inner peripheral surface of the diameter in the same manner as when the valve is open, the threaded part of the threaded mechanism provided at the rear of the valve stem of the valve spool will engage and move when the valve is closed. The valve can always be opened and closed smoothly and quickly without becoming difficult.

In the first place, the present invention can be applied within a very short period of time, such as about 0.05 seconds, for example, from 0.1 to 0.2 seconds.
In order to change speed from a low speed of m/sec to a high speed of 1 to 5 m/sec and to obtain a predetermined high speed with high accuracy,
This valve was invented for use in the injection circuit of a die-casting machine, etc., where the valve spool must be moved quickly and stopped quickly and reliably, to achieve greater effectiveness.

Therefore, in the present invention, when the valve starts to open or when it starts to switch from low speed to high speed, a relatively large force acts on the front surface of the valve spool, and a thrust force acts in the direction of opening the valve spool. I tried to make it bigger. Also, when the valve starts to open and oil flows out into the through hole, the first valve that acts on the front surface of the valve spool will increase the flow rate of the oil flowing out.
The pressure in the chamber gradually decreases, but on the other hand, due to the action of the through hole provided in the valve spool, the pressure in the second chamber on the rear side of the valve spool does not decrease immediately as the pressure in the first chamber decreases. Therefore, the thrust force acting on the valve spool acts in the direction of closing the valve spool. When the valve spool moves to a position where the desired high-speed injection can be obtained and it is necessary to quickly stop the opening operation of the valve spool, the thrust force acting on the entire valve spool is combined with the force acting in the direction of closing the valve spool. This allows the valve spool to stop quickly and smoothly in the open position. Also, when the valve spool is stopped at a predetermined position such as in a high-speed range, the pressure acting on the second chamber through the through hole becomes the same as the pressure acting on the first chamber, which acts on the valve spool. The thrust force is made relatively small so as not to put too much stress on mechanical holding devices such as screw mechanisms.

In addition, if you use a pulse motor as the motor and control the amount of rotation of the motor by inputting desired pulses to the pulse motor, you can accurately and quickly open/close the flow rate regulating valve and adjust the opening. , and can be done automatically.

In addition, since we have adopted a structure in which the rotation of the motor is changed to change the stroke of the valve spool to adjust the flow rate, the movement of the valve spool can be automatically performed at extremely high speed and accurately, allowing from low-speed injection to high-speed injection. Switching to injection can be performed accurately and responsively at high speed, and high-speed injection can be reliably controlled.

Further, in the present invention, on the second chamber side of the valve chamber,
A screw mechanism that changes rotational movement into axial movement is connected to the valve shaft of the valve spool that penetrates the valve chamber wall, and a variable speed motor is connected to the other end of the screw mechanism, so the valve can be opened and closed. Not only can this be done automatically, accurately and quickly, but the screw mechanism can receive the force acting from the valve spool side and prevent it from being transmitted to the rear where the variable speed motor is located.

In addition, if one flow rate adjustment valve doubles as the flow rate adjustment valve and on-off valve for low-speed injection, and the flow rate adjustment valve and on-off valve for high-speed injection, each valve can be used in multiple numbers as in the past. There is no need to provide separate units, and the overall structure of the injection device etc. becomes extremely simple and compact, and the hydraulic piping can be shortened, which is extremely advantageous economically.

In addition, by adopting the special structure of each part as described above, it is possible to obtain a flow regulating valve with multiple functions with a single valve, which allows accurate flow control and valve opening/closing that is unaffected by external disturbances. control can be performed reliably.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of the present invention. 1...Flow rate adjustment valve, 2...Drive unit, 3...Flow rate adjustment unit, 4...Casing, 6...Motor, 17
...Axis, 19...Nut, 23...Position detector,
27...Inner cylinder part, 28...Valve shaft, 29...Manifold, 30...Bore, 31...Valve spool, 3
1a, 33, 34... Distribution hole, 32... Groove, 40
...Sleeve, 40a, 40b...Through hole.

Claims (1)

[Claims] 1. Provide a manifold with a valve chamber that has an inlet at one end in the axial direction, is sealed at the other end in the axial direction, and has an outlet in a direction perpendicular to the axial direction, and moves back and forth in the axial direction within the valve chamber. A valve spool is provided, and the valve chamber is divided into two parts, a first chamber on the inlet side and a second chamber on the opposite inlet side, and the valve spool is slidably attached to the inner peripheral surface of the manifold. The retained sleeve is provided through packings to prevent the flow of fluid between the inner circumferential surface of the manifold and the sleeve, and the machining between the outer circumferential surface of the sleeve and the inner circumferential surface of the manifold is made rough so that the sleeve can be inserted into the core. A through hole is provided in the sleeve near the front end of the valve spool so as to be able to absorb the misalignment, and the through hole is in communication with the flow hole forming the outlet provided in the manifold, and the through hole is provided so as to be able to be opened and closed by the valve spool. An annular groove is provided in the through-hole portion of the sleeve, and a large number of through holes are provided inside the valve spool to communicate the first and second chambers of the valve chamber, and the valve chamber wall is formed on the second chamber side of the valve chamber. A flow rate regulating valve in which a screw mechanism that changes rotational movement into axial movement is connected to the valve shaft of a valve spool provided through the valve spool, and a variable speed motor is connected to the other end of the screw mechanism.