JPH073041Y2 - Pressure control valve - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a pressure control valve which is preferably used as, for example, a counterbalance valve, a brake valve, etc., and particularly when a hydraulic motor etc. in a hydraulic circuit is suddenly stopped. The present invention relates to a pressure control valve that can alleviate shocks and prevent cavitation.

[Conventional technology]

Generally, in a machine such as a hydraulic excavator or a hydraulic crane that drives an inertial body by using a hydraulic actuator such as a hydraulic motor or a hydraulic cylinder, a large inertial load is applied from the inertial body when the hydraulic actuator is stopped. Therefore, a brake valve is provided in the middle of the hydraulic circuit, and the inertia load is absorbed by the brake valve. And, in this type of brake valve, the
As shown, a counterbalance valve having a pressure control valve is used.

That is, FIG. 8 shows a hydraulic circuit provided with a brake valve, in which 1 is a hydraulic motor, and the hydraulic motor 1 is a pair of main pipelines 2.
A hydraulic pump 3 serving as a hydraulic source and a tank 4 are connected via A and 2B, and an inertial body (not shown) is rotationally driven by pressure oil from the hydraulic pump 3. 5 is the main pipeline 2
A directional control valve provided in the middle of A and 2B is shown. The directional control valve 5 is manually operated by a driver or the like, and the neutral position (a)
Then, stop the hydraulic motor 1 and switch positions (b) and (c).
When the switching operation is switched to, the hydraulic motor 1 is rotated together with the inertial body in one direction and the other direction.

Reference numeral 6 denotes a brake valve located between the hydraulic motor 1 and the direction switching valve 5 and provided in the middle of the main pipelines 2A, 2B. The brake valve 6 is provided between the main pipelines 2A, 2B. It is composed of a pair of overload relief valves 7A and 7B and a counter balance valve 8. The counter balance valve 8 is provided in the middle of the main pipelines 2A and 2B to distribute the pressure oil only from the hydraulic pump 3 to the hydraulic motor 1 side. A pair of check valves 9A and 9B, and a branch line 1 on the actuator side before and after the check valves 9A and 9B, respectively.
Main line 2 through 0A, 10B and hydraulic source side branch line 11A, 11B
It is composed of a pressure control valve 12 connected to A and 2B.
The pressure control valve 12 is constantly biased to the neutral position (a) by the left and right springs 13A and 13B, and the pilot pressure from the hydraulic pump 3 is supplied through the left and right throttle passages 14A and 14B. At the time of turning, the pilot pressure is automatically switched to either of the switching positions (b) and (c), and the return oil from the hydraulic motor 1 is returned to the tank 4 side.

Reference numeral 15 denotes a shuttle valve, which is located between the direction switching valve 5 and the brake valve 6 and is provided between the main pipelines 2A and 2B as a high pressure selection valve. The shuttle valve 15 is one of the main pipelines 2A and 2B. The pressure oil on the high pressure side is selected, and this pressure oil is led out into the brake pipe line 16. Further, reference numeral 17 denotes a brake device as a negative brake provided on the output shaft 1A side of the hydraulic motor 1, and the brake device 17 constantly applies a braking force to the output shaft 1A by a spring 17A so as to apply a pressure from the brake line 16. When the oil is supplied, the braking of the output shaft 1A is released. A pressure reducing valve (not shown) may be provided in the brake pipe 16 to reduce the pressure of the pressure oil supplied to the brake device 17 to a predetermined pressure.

In the conventional technology configured as described above, first, the directional control valve 5
When is switched from the neutral position (a) to the switching position (b), the pressure oil from the hydraulic pump 3 is supplied to the main pipe line 2A, the check valve 9A is opened, and the throttle passage 14A is connected to the pressure control valve 12.
Pilot pressure is supplied via the switch to switch the pressure control valve 12 to the switching position (B) against the spring 13B. As a result, the hydraulic motor 1 is rotated in one direction by the pressure oil from the main pipeline 2A to drive the inertial body, and the return oil from the hydraulic motor 1 passes through the main pipeline 2B, the branch pipelines 10B, 11B, etc. to the tank. Four
Returned to.

Then, when the direction switching valve 5 is returned to the neutral position (a) again in order to stop the inertial body, the supply of the pressure oil to the main pipeline 2A side is cut off, so that the check valve 9A closes and The pressure control valve 12 is suddenly returned to the neutral position (a) by the spring 13B to disconnect the branch pipe lines 10B and 11B from each other and interrupt the return oil from the hydraulic motor 1 to the tank 4 side. As a result, the return oil from the hydraulic motor 1 causes the main pipe line 2B side to have a high pressure, and this pressure (brake pressure) opens the relief valve 7B, so that the high pressure on the main pipe line 2B side becomes the main pipe line 2A.
By relieving to the side, the inertial rotation (load) of the hydraulic motor 1 by the inertial body is gradually absorbed, and the hydraulic motor 1 is stopped together with the inertial body.

When the directional control valve 5 is switched to the switching position (C),
Also, when it is returned to the neutral position (a), it operates almost in the same manner as the above-mentioned case, and therefore further explanation is omitted.

By the way, in the above-mentioned conventional technique, since the pressure control valve 12 having only the left and right switching positions (b) and (c) is used as the counter balance valve 8, for example, the directional switching valve 5 is switched to the switching position (b). When the pressure control valve 12 returns to the neutral position (a) when the pressure control valve 12 returns to the neutral position (a), the pressure (brake pressure) in the main pipe line 2B rapidly increases due to the return oil from the hydraulic motor 1 due to inertial rotation. from the delayed response of the relief valve 7B (opening) is even slightly, the surge pressure P _a as shown in FIG. 9 is generated main conduit 2B, the damage causes such as a hydraulic motor 1 and the brake valve 6 Not only hydraulic motor 1
However, there is a drawback in that a large shock is given to each device in the hydraulic circuit.

On the other hand, in order to solve the above-mentioned drawbacks, as shown in FIG. 10, the neutral position (a) and the switching positions (b), (c) of the pressure control valve 12 are
Aperture area (d), (e) as an intermediate switching position between and
Other prior arts are known which provide a. Here, as the throttle regions (d) and (e), notches are formed in the left and right switching lands sandwiching the central land of the spool (not shown) that constitutes the pressure control valve 12. Will be realized.

In this case, for example, when the pressure control valve 12 returns from the switching position (b) to the neutral position (a), the throttle passage
By moving the throttle area (d) relatively slowly by the throttle action of 14A, 14B, etc., the flow path between the branch pipelines 10B, 11B is secured in the throttle area (d) until just before the neutral position (b). Then, it becomes possible to gradually reduce the flow rate and gradually increase the pressure (brake pressure) in the main pipeline 2B. As a result,
As shown in FIG. 11, it is possible to suppress the sudden increase in pressure in the main pipeline 2B and the generation of the surge pressure P _a to some extent.

[Problems to be solved by the device]

However, in another conventional technique shown in FIG. 10, in order to make the pressure rise in the main pipeline 2B gentle as shown by the dotted line in FIG. 11, the passage area of the throttle region (d) is increased, or By reducing the passage area of the throttle passages 14A, 14B,
When the pressure control valve 12 is caused to move more slowly in the throttle region (d), a relatively large amount of return oil in the main pipe line 2B flows into the tank 4 through the branch pipe lines 10B and 11B in the throttle region (d). It will be leaked. And in this case, the hydraulic motor 1
Due to the inertial rotation, the pressure oil in the main pipeline 2A is sucked into the hydraulic motor 1 and is sequentially returned to the main pipeline 2B side, so that the main pipeline 2A on the suction side has a negative pressure, causing so-called cavitation. This is apt to occur, and the pressure (brake pressure) in the main pipeline 2B pulsates as shown in FIG. 12 to generate noise, which causes damage to the hydraulic motor 1 and the brake valve 6 and the like.

The present invention has been made in view of the above-mentioned drawbacks of the prior art. The present invention can prevent surge pressure or cavitation from occurring when stopping an actuator such as a hydraulic motor, and reliably reduce shock at the time of stop. The pressure control valve is made possible.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, the present invention provides a valve main body having a pair of hydraulic pressure source side ports and a pair of actuator side ports, and a slidably provided inside the valve main body, and the respective hydraulic pressure source side ports. A spool that opens and closes between each actuator-side port, a pair of oil chambers formed at both ends of the spool and formed in the valve body, each oil chamber and each hydraulic-power-source-side port, respectively. The present invention is applied to a pressure control valve including a throttle passage that communicates with each other and a pair of springs that are provided in the oil chambers and always bias the spool to the neutral position.

A feature of the configuration adopted by the present invention is that the spool has a central land that blocks the actuator-side ports from each other, and extends in the axial direction with the central land sandwiched therebetween so that the actuator-side ports communicate with each other. An oil passage, and a passage portion formed in the central land for communicating a return port of the actuator-side ports with the oil passage when the spool returns from the fully open position to the neutral position, and In the oil passage, pressure oil is allowed to flow from the return side port of each of the actuator side ports to the suction side port with the passage portion sandwiched therebetween in the axial direction, and the reverse direction flow is allowed. There is a pair of check valves to prevent this.

[Action]

With the above configuration, when the spool returns from the fully open position to the neutral position, part of the pressure oil (return oil) from the return side port of each actuator side port is directed to the suction side port through the check valve. It can be distributed (divided). As a result, the pressure oil (return oil) at this time can be gradually circulated (shunted) from the main pipe line on the return side to the main pipe line on the suction side, a rapid pressure increase on the return side can be prevented, and the suction side It is possible to reliably prevent a negative pressure.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. In the embodiment, the same components as those of the prior art shown in FIG. 8 described above are designated by the same reference numerals, and the description thereof will be omitted.

1 to 4 show the first embodiment of the present invention.

In the figure, reference numeral 21 denotes a pressure control valve that constitutes a counterbalance valve 8 together with a pair of check valves 9A and 9B.
The valve body 22 has a spool sliding hole 23 extending in the axial direction as shown in FIGS.
In the peripheral wall of 23, annular oil grooves 24A, 24 are provided on the left and right in the figure
Via a pair of hydraulic power source side ports 25A, 25B communicating with the spool sliding hole 23 via B and other annular oil grooves 26A, 26B located between the oil grooves 24A, 24B and spaced left and right. Spool sliding hole 2
A pair of actuator-side ports 27A and 27B communicating with 3 are formed. The hydraulic pressure source side ports 25A, 25B are connected to the branch pipe lines 11A, 11B, and the actuator side port 26
A and 26B are connected to the branch lines 10A and 10B (see FIG. 1). Further, the valve main body 22 is covered with lids 28A and 28B on both axial ends of the spool sliding hole 23, and the lids 28A and 28B are located on both end sides of a spool 30 which will be described later. 29
A and 29B are formed.

Reference numeral 30 denotes a spool slidably fitted in the spool sliding hole 23, and the spool 30 is interposed between the oil grooves 26A and 26B,
A central land 32 having an annular oil passage 31 formed on the outer peripheral surface, and large and small notches 33 located on both left and right sides of the central land 32.
Left and right switching lands 34A and 34B, which communicate and block oil grooves 24A and 26A on one side through A and 33B, and communicate and block oil grooves 24B and 26B on the other The oil grooves 24A and 24B and the oil chamber
Left and right end lands 35A and 35B for blocking between 29A and 29B are provided, and a central land 32 is provided at a central portion of the spool 30.
An oil passage 37 is formed which extends in the axial direction with the left and right sides sandwiched between the oil grooves 26A and 26B, that is, the ports 27A and 27B, which are in constant communication with each other via left and right oil holes 36A and 36B.

Further, the spool 30 has an oil passage 31 and an oil passage 31 of the central land 32.
A throttle 38 as a small-diameter oil hole communicating with 37 is bored in the radial direction, and the throttle 38 constitutes a passage portion together with the oil passage 31, and when the oil passage 31 communicates with either of the ports 27A, 27B. The pressure oil is circulated in the oil passage 37 while a throttle action is applied to the check valve 42A or 42B, which will be described later.

On the other hand, the spool 30 is located between the switching lands 34A, 34B and the end lands 35A, 35B, and the ports 25A, 25B are connected to the oil chambers 29A, 29B via the oil grooves 24A, 24B. Small-diameter throttle passages 39A, 39B are formed to communicate with each other. Pilot pressure is applied to the end face of the spool 30 when the directional control valve 5 is switched to the switching position (b) or (c). Is supplied and the spool 30 is slid to the right or left in the figure. The throttle passages 39A, 39B do not necessarily have to be formed in the spool 30, and the ports 25A, 25B and the oil chambers 29A, 2A as shown by the two-dot chain line in FIG.
You may form between 9B. Further, the throttle passages 39A, 39
The sliding speed of the spool 30 becomes slower when the passage area of B is made smaller, and becomes faster when it is made larger.

40A and 40B indicate stoppers provided in the oil chambers 29A and 29B,
The stoppers 40A and 40B are configured to regulate the fully opened position of the spool 30 with a predetermined stroke when the spool 30 slides to the left or right. 31A, 41B shows a spring for centering arranged between the stopper 40A, 40B and the bottom of the oil chamber 29A, 29B, the spring 41A, 41B always biases the spool 30 to the neutral position, When the directional control valve 5 is in the neutral position (a), the pressure control valve 21 is returned to the neutral position (a) as shown in FIG.

Here, in the pressure control valve 21, as shown in FIG. 1, the direction switching valve 5 is changed from the neutral position (a) to the switching position (b) or (c).
41B by the pilot pressure when switched to
Alternatively, the spool 30 is switched to the switching position (b) or (c) against 41A, at which time the spool 30 is in the fully open position, and the return oil from the hydraulic motor 1 is transferred from the port 27B or 27A side to the port 25B or 25A side. It is made to flow out to and returned to the tank 4. Further, throttle regions (d) and (e) as intermediate switching positions are provided between the neutral position (a) and the switching positions (b) and (c) of the pressure control valve 21. D) and (e), the notch 33 of the switching land 34B, 34A
The outflow of the return oil to the tank 4 side is controlled by B and 33A by a throttling action or the like.

When the directional control valve 5 is returned from the switching position (b) to the neutral position (a), for example, the pilot pressure decreases, so that the pressure control valve 21 operates by the action of the spring 41B and the throttle passages 39A, 39B. When returning from (b) to the neutral position (a) relatively slowly through the throttle area (d) and passing through the throttle area (d), the spool 30 passes through the main pipeline 2B and the branch pipeline 10B. The return oil from the hydraulic motor 1 flowing into the port 27B side through the notch 33B and the oil groove 24B and the port 25B.
While flowing out to the B side, oil passage 31, throttle 38, oil passage 37
And it circulates to the port 27A side through the check valve 42A. At this time, the hydraulic motor 1 is rotating by inertia,
The port 27B side is the return side and the port 27A side is the suction side. Further, when the direction switching valve 5 is returned from the switching position (C) to the neutral position (A), the pressure control valve 21 is compared from the switching position (C) to the neutral position (A) via the throttle area (E). The port 27A side becomes the return side, and the port 27B side becomes the suction side.

Further, 42A and 42B represent a pair of check valves provided in the oil passage 37 so as to be spaced apart left and right, and the check valves 42A and 42B are provided.
Is until the pressure control valve 21 returns to the neutral position (a) via the throttle regions (d) and (e) while the oil passage 31 of the spool 30 communicates with the port 27B or 27A. The port 27
Of the return oil (pressure oil) that is the return side of A, 27B, for example, only from the port 27B to the suction side port 27A or from the port 27A to the suction side port 27B, through the oil passage 37 and the like. Main line 2A or 2B that allows circulation and is on the suction side
It is designed to prevent negative pressure inside.

The present embodiment has the configuration as described above, and the basic operation thereof is not different from that of the prior art.

Therefore, in the present embodiment, the spool 30 of the pressure control valve 21 and the oil passage 37 that extends in the axial direction with the central land 32 sandwiched between the ports 27A and 27B to communicate with each other through the oil holes 36A and 36B, and the central land 3 are provided.
An annular oil passage 31 is formed on the outer peripheral surface of the second oil passage 37 and communicates with the oil passage 37 via a throttle 38.
A pair of checks that allow the return oil (pressure oil) to flow only from the throttle 38 side toward the oil holes 36A, 36B side and flow the pressure oil from the return side to the suction side between each port 27A, 27B. Valve 42A, 42
Since B is provided, the direction switching valve 5 is set to the switching position (b),
While returning from (c) to the neutral position (a) and the hydraulic motor 1 is rotating inertially, the pressure control valve 21 is throttled to return to the switching position (b) and from (c) to the neutral position (a). While passing through the areas (d) and (e), the main pipelines 2A, 2
B, the return oil flowing from the branch pipelines 10A, 10B to the branch pipelines 11A, 11B side is shunted to flow from the return side of the ports 27A, 27B to the intake side via the check valves 42A, 42B. It is possible to prevent the inside of the main pipeline 2A or 2B on the return side from becoming negative pressure.

That is, as described in the other prior art shown in FIG. 10, for example, the passage areas of the notches 33A and 33B are increased, or the passage areas of the throttle passages 39A and 39B are decreased to make the spool 30 in the fully open position. Even if, for example, the switching position (b) is relatively slowly returned to the neutral position (a) through the throttle region (d), a part of the oil returned from the hydraulic motor 1 in the throttle region (d) Can be circulated from the return side port 27B to the suction side port 27A side via the check valve 42A, and it is possible to prevent cavitation from occurring due to negative pressure on the main pipeline 2A side. The return oil is gradually throttled by the notch 33B in the throttle region (d) and flows from the port 27B side to the port 27A side through the throttle 38. Therefore, the pressure in the main pipe line 2B on the return side (brake) is reduced. Pressure) can be prevented from rising relatively slowly as shown by the characteristic 43 in FIG. 4 and surge pressure can be prevented from occurring, and the inertial rotation of the hydraulic motor 1 can be gradually absorbed together with the relief valve 7B etc. The shock at the time can be reliably reduced.

Further, even if the passage areas of the notches 33A, 33B are further reduced, or even if the notches 33A, 33B are eliminated, the passage areas of the throttle passages 14A, 14B are reduced to slow the sliding speed of the spool 30, and If the passage area of 38 is set to an appropriate size, the increase speed of the brake pressure can be moderated only by the throttle 38, and the occurrence of cavitation and surge pressure can be prevented.

Further, since each check valve 42A, 42B is configured to be provided in the oil passage 37 formed in the spool 30, the check valve 42A, 42B
Pressure control valve 2 without the need for a separate space for 42B
1 can be formed compactly and the whole can be miniaturized, and various effects are exhibited.

Next, FIGS. 5 to 7 show an embodiment of FIG. 2 of the present invention. In this embodiment, the same components as those of the first embodiment are designated by the same reference numerals and the description thereof will be omitted. However, the feature of this embodiment resides in that a plurality of (or one) shallow-bottomed recesses 51 as diaphragms are formed on the outer peripheral surface of the central land 52 on the spool 50.

Here, the spool 50 is formed in the same manner as the spool 30 described in the first embodiment except for the recess 51 and the oil hole 58 described later, and the notches 53A and 53B and the switching land 54 are formed in the spool 50.
A, 54B, end lands 55A, 55B, oil holes 56A, 56B, oil passage 57,
An oil hole 58 and a throttle passage 59 that form a passage together with the recess 51.
A and 59B are provided.

Further, the oil passage 57 communicates with each recess 51 through the large-diameter oil holes 58, 58, and the oil passage 57 is provided with a pair of check valves 42A, 42A as in the first embodiment. ing. Each of the recesses 51 has a large size at the center of the center land 52 and is formed, for example, in an oval shape so as to have a passage area that gradually decreases toward the left and right in the drawing. As a result, in the recessed portion 51, the spool 50 is in the neutral position from the fully open position, that is, from the switching position (b) and (c) in FIG. 5 to the neutral position (a) via the throttle inclination region (d) and (e). When returning to, the passage area of the recess 51 is continuously changed from the maximum to the zero state in which the passage is gradually closed.

Thus, in this embodiment configured as described above, substantially the same effects as those of the first embodiment can be obtained. However, particularly in this embodiment, since the concave portion 51 is formed in an elliptical shape, the hydraulic pressure is reduced. When the motor 1 is stopped, the amount of return oil discharged from the hydraulic motor 1 into the main pipe line 2B or 2A on the return side gradually decreases.
The passage area of 51 can be reduced, and the shock at the time of stop can be effectively mitigated.

In the second embodiment, the shallow-bottomed recess 51 is described as being formed in an oval shape. However, instead of this, the shallow-bottomed recess 51 has a passage area from the center to the left and right. For example, it may be formed in a rhombus shape so as to gradually shrink toward it.

[Effect of device]

As described in detail above, according to the present invention, the spool has an oil passage that cuts off between the actuator-side ports and an oil passage that extends in the axial direction with the central land interposed therebetween and that connects between the actuator-side ports. And a passage portion which is formed in the central land and which communicates the return side port of the actuator side ports with the oil passage when the spool returns from the fully opened position to the neutral position, and the oil passage Allows the pressure oil to flow from the return side port of each of the actuator side ports to the suction side port with the passage portion interposed therebetween in the axial direction,
Since a pair of check valves that prevent reverse flow are provided, when the spool returns from the fully open position to the neutral position, the return side port and the suction side port of each actuator side port By connecting the two via a check valve, it is possible to prevent the suction side main pipe line from becoming negative pressure due to inertial rotation of the hydraulic motor, for example.
It is possible to suppress the occurrence of cavitation and also suppress the occurrence of surge pressure, and it is possible to reliably mitigate the shock at the time of stop.

Further, since the oil passage and the passage portion are formed in the spool and the pair of check valves are provided in the oil passage, the check valves and the like can be housed in the spool in a compact manner. The control valve can be made compact and the overall size can be reduced.

[Brief description of drawings]

1 to 4 show a first embodiment of the present invention, FIG. 1 is a hydraulic circuit diagram, FIG. 2 is a longitudinal sectional view of a pressure control valve, and FIG. 3 is III- in FIG. III is a sectional view in the direction of the arrow, FIG. 4 is a characteristic diagram of brake pressure, FIGS. 5 to 7 show a second embodiment, FIG. 5 is a hydraulic circuit diagram, and FIG. 6 is a pressure control valve. FIG. 7 is a longitudinal sectional view, FIG. 7 is a sectional view in the direction of arrow VII-VII in FIG. 6, FIGS. 8 and 9 show a prior art, FIG. 8 is a hydraulic circuit diagram, and FIG. Characteristic diagram, FIGS. 10 to 12 show another prior art, FIG. 10 is a hydraulic circuit diagram,
FIG. 11 is a characteristic diagram of brake pressure, and FIG. 12 is a characteristic diagram of brake pressure different from FIG. 11. 1 ... Hydraulic motor, 2A, 2B ... Main pipeline, 3 ... Hydraulic pump, 4
... tank, 5 ... direction switching valve, 6 ... brake valve, 7A, 7B ...
Relief valve, 8 ... Counter balance valve, 9A, 9B ... Check valve, 10A, 10B, 11A, 11B ... Branch line, 21 ... Pressure control valve, 2
2 ... Valve body, 23 ... Spool sliding hole, 24A, 24B, 26A, 26B ... Oil groove, 25A, 25B ... Hydraulic pressure source side port, 27A, 27B ... Actuator side port, 29A, 29B ... Oil chamber, 30, 50 … Spools, 31… Oil passages (passages), 32,52… Central lands, 33A, 33B, 53A, 53B…
Notch, 34A, 34B, 54A, 54B ... Switching land, 37, 57 ... Oil passage, 38 ... Throttle, 39A, 39B, 59A, 59B ... Throttle passage, 40A, 40B ...
Stopper, 41A, 41B ... Spring, 42A, 42B ... Check valve, 51 ...
Recess (passage), 58 ... Oil hole.

Claims (1)

[Scope of utility model registration request] 1. A valve main body having a pair of hydraulic pressure source side ports and a pair of actuator side ports, and a slidably provided inside the valve main body, between the hydraulic pressure source side ports and the actuator side ports. A spool for opening and closing, a pair of oil chambers formed in the valve body at both ends of the spool, and throttle passages for communicating the oil chambers with the hydraulic pressure source side ports, respectively. In a pressure control valve that is provided in each oil chamber and includes a pair of springs that normally urge the spool to a neutral position, the spool includes a central land that blocks between the actuator-side ports and the central land. It is formed in the central land and an oil passage that extends in the axial direction by sandwiching it and connects the actuator-side ports with each other.
And a passage portion that communicates a return port of the actuator-side ports with the oil passage when the spool returns from the fully opened position to the neutral position, and the oil passage sandwiches the passage portion. A pair of check valves, which are axially separated from each other, allow pressure oil to flow from the return side port of the actuator side ports toward the suction side port, and prevent the flow in the opposite direction. Pressure control valve characterized by.