JPS5997302A - Oil pressure control circuit - Google Patents

Abstract

PURPOSE:To reduce a rise up time in starting operation of an actuator and further enable its control even in case of operation of negative load by supplying a flow rate and pressure required for an actuator and maintaining the pressure of the circuit concerned at a constant level when it is neutral, thereby securing a predetermined flow rate. CONSTITUTION:Meter-in valve mechanisms a, a' are same in composition and connected to meter-out valves b, b' in the downstream side. When a spool 45 of a flow rate adjustment selector valve 7 is changed over rightward, opening of a pilot passage 71 is determined correspondingly to the change over quantity, and a flow corresponding to the opening is generated, and a pilot pressure corresponding to the flow rate is generated due to the function of an orifice 73. The distance of movement of a main poppet 78 is also determined in proportion to the pilot pressure, and a seat part 88 maintains an opening area in proportion to the pilot pressure, and meter-out control can be performed correspondingly to the opening area. A cylinder 64 can be also controlled even when negative load operates. Even if the pressure on the actuator port 62 side is changed, the opening area of the seat part 88 is automatically increased or decreased to maintain self-adjustment function which can prevent any change of the flow rate on the return side.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention supplies the required flow rate and pressure to the actuator when the actuator is activated, and the neutral special number when the actuator is not activated is a hydraulic pressure that reduces the discharge amount of the variable pump. Regarding control circuits.

As this type of circuit, a negative control circuit and a load sensing control circuit are conventionally known.

However, the conventional negative control circuit described above naturally cannot control the actuation of the actuator, and therefore has the disadvantage that it cannot achieve a sufficient energy saving effect.

In addition, the load sensing control circuit can achieve an energy saving effect when the actuator is operated, but the circuit pressure and flow rate in the neutral state when the actuator is not operating are almost zero, so when the actuator starts operating, the circuit pressure and flow rate are almost zero. There is a drawback that it takes time to raise the circuit pressure and flow rate to the required levels.

Furthermore, in the conventional control circuit, when a so-called negative load that moves the actuator in the direction of its own weight is applied, there is a problem that it cannot be controlled.

This invention supplies the required flow rate and pressure to the actuator when the actuator is in operation, while maintaining the circuit pressure at a certain constant pressure and ensuring a predetermined flow rate when the actuator is in neutral state. The object of the present invention is to provide a hydraulic control circuit which shortens the rise time at the start of operation of the hydraulic pressure control circuit and also enables control even when the negative load is applied.

Embodiments of the present invention will be described below based on the drawings.

In the first embodiment shown in FIGS. 1 and 2, a main passage 2 directly connected to a variable pump l is branched into a first neutral passage 3, a flow passage 4, and a parallel passage 5, and the flow passage 4 is A pressure control valve 6 is connected.

A flow rate adjustment switching valve 7 is connected to the downstream side of the pressure control valve 6, and this flow rate adjustment switching valve 7 is a main element of the meter-in valve mechanism a.

Further, a meter-in valve mechanism a' connected to the parallel passage 5 is provided downstream of the make-in valve mechanism a, and this meter-in valve mga' also has a flow rate adjustment switching valve 9 downstream of the pressure control valve 8. Connected.

Both meter-in valve mechanisms a and a' have the same configuration, and a meter-out valve mechanism b' is connected to the downstream side thereof.

The flow rate adjustment switching valve 7.9 as described above is configured to open the neutral port 10.11 in its neutral position, and when both flow rate adjustment switching valves 7.9 are in the neutral position, the variable pump l The oil discharged from the tank returns to the tank 14 via the first neutral passage 3 → neutral port 10 → second neutral passage 12 → neutral port 11 → third neutral passage 13.

A first throttle 15 and a second throttle 16 are arranged in series in the flow process of the third neutral passage 13, and a low pressure relief valve 17 is connected to the upstream side of the first throttle 15. There is.

Furthermore, a pilot passage 18 is connected between the first restrictor 5 and the second restrictor 16, and the differential pressure generated when the oil from the variable pump 1 passes through the second restrictor 18 in the neutral state is controlled as a pilot. The pressure is introduced into the pilot passage 18 as pressure. The pilot pressure guided to the pilot passage 18 in this way flows into the tilting angle control device 21 via the shuttle valves 19 and 20. The main elements are:

Further, pressure from an output detection passage 24 branched from the main passage 2 is also introduced into the tilt angle control device 21. .

The pressure from the output detection passage 24 flows into one pilot chamber 25 of the differential pressure sensing control valve 22, so that the pilot pressure in the pilot passage 18 is guided to the pilot chamber 26. A spring 27 is interposed in the other pilot chamber 26.

The differential pressure sensing control valve 22 configured as described above is held at the right position in the figure by the action of the spring 27 when no pressure is introduced into the pilot chambers 25, 26. In this state, the passage 28 connected to the tilt angle control cylinder 28 communicates with the tank 31 via the orifice 30, so that the discharge amount of the variable pump 1 increases.

When the differential pressure sensing control valve 22 moves to the left position in the figure, the output detection passage 24 and the passage 29 communicate with each other, and the discharge pressure from the variable pump l flows into the tilting angle control cylinder 28, causing the variable pump Decrease the discharge amount of l.

However, when the flow rate adjustment switching valve 7.9 is in the neutral position,
This control circuit performs negative control as follows.

That is, when the flow rate adjustment switching valve 7.9 is in the neutral position, the discharge pressure from the variable pump 1 flows into one pilot chamber 25 of the differential pressure sensing control valve 22 via the output detection passage 24, and the As shown, the first aperture 15 and the second
The water returns to the tank 14 via the aperture 18. The differential pressure generated when passing through the second throttle lθ is guided as pilot pressure to the pilot passage 18 and flows into the other pilot chamber 26 of the differential pressure sensing control valve 22.

At this time, the differential pressure sensing control valve 22 is balanced when the pressure P1 in the pilot chamber 25, the pilot pressure P2 in the pilot chamber 28, and the pressure P3 corresponding to the spring force of the spring 27 become P1=P2+P3.

In other words, the differential pressure sensing control valve 22 functions so that the differential pressure between the pump discharge pressure and the pilot pressure becomes a pressure P3 corresponding to the spring force of the spring 2''.

As described above, the discharge pressure of the variable pump 1 is maintained at the pressure obtained by adding the spring force of the spring 27 to the pilot pressure of the pilot chamber 26, and by the action of the first throttle 15 described above, the discharge pressure within the circuit is Ensure a predetermined flow rate.

In this way, when the flow rate adjustment switching valve 7.9 is held in the neutral position, the hole negative control is performed to achieve the purpose of energy saving, and the current circuit pressure and flow rate are secured to a certain extent, so the flow rate adjustment switching is performed. Faster rise time when switching valve 7.9.

The neutral passage, the first throttle 15, and the second throttle 16
and the tilt angle control device 21 together constitute a negative control mechanism of the present invention.

Note that the second throttle 16 is for generating pilot pressure, but in order to generate pilot pressure,
For example, a low pressure relief valve may be used in place of the throttle 16.

The specific configurations of the meter-in valve mechanism a and the meter-out valve mechanism A are as shown in FIG. 2.

That is, each element of the valve mechanism is housed inside the valve body 32, and the pressure control valve 6 is provided at the most upstream side thereof.

The pressure control valve 6 has a spool 34 slidably installed in a pore 33 formed in the valve body 32.

The pore 33 has a first annular groove 35 and a second annular groove 3.
8, the first annular groove 35 always communicates with the inflow passage 37 of the flow rate adjustment switching valve 7, and the second annular groove 3
6 is in constant communication with the variable pump 1 via the main passage 38.

Further, the spool 34 has one end facing one pilot chamber 39, and the other end facing the other pilot chamber 4.
-0, and an annular recess 41 is formed in the center. A spring 42 is interposed in the other pilot chamber 40, and the spool 34 is normally maintained in the illustrated state by the action of the spring 42.

That is, in the above state, the working fluid from the variable pump l flows from the main passage 38 to the second annular groove 36 → to the annular recess 41 →
It is in a relationship that it flows into the inflow passage 37 via the first annular groove 35 . At this time, the pressure on the inflow passage 37 side flows into the one pilot chamber 39 via the orifice 43.

Further, the flow rate adjustment switching valve 7 has a spool 45 slidably installed in a pore 44 formed in the valve body 32. The spool 45 has one end protruding outside the valve body 32, and the other end thereof. facing the spring chamber 46. Due to the action of the spring 47 inserted in this spring chamber 4B,
The spool 45 is normally held in the neutral position shown.

In the neutral position shown above, the annular recess 48 formed at the center of the spool 45 corresponds to the inflow passage 37, and the inflow passage 37 is closed by lands 499.50 formed on both sides of the annular recess 48. . The lands 49 and 50 are formed with tapered surfaces or notches that taper toward the annular recess 48.

Then, from the above state, the spool 45 is connected to the spring 4.
7 and move it to the left or right, the load boat 5
One of the annular grooves 51 and 52 communicating with the inflow passage 37 communicates with the inflow passage 37.

Furthermore, a load detection boat 55.5B is formed outside the annular groove 51.52 and at a position substantially adjacent thereto, and this load detection boat 55.58 is connected to the inlet passageway as described above. The pressure in the load boat 53 or 54 communicating with the load boat 37 is introduced as a pilot pressure to the other pilot chamber 40 of the pressure control valve 6, and at the same time, the pilot pressure is introduced into the shuttle valve 20. However, when the spool 45 is in the neutral position shown in the figure, the load detection port 55,56 is communicated with the tank 59 through the passage formed in the spool 45, and the other pilot chamber 40 is in the neutral position. to prevent pressure from building up.

The load ports 53 and 54 as described above are connected to the actuator port 6 via the load check valves 80 and 61.
2.63, but both actuator ports 62.83 are connected to the bottom side chamber 65 and rod side chamber 6B of the cylinder 64.

The spool 45 also has a pilot pressure switching notch 67.
is formed. This pilot pressure switching notch 67 is always in communication with a branch passage 68 branched from the main passage 38, but a pressure reducing valve 89 is installed in the flow process of this branch passage 68.
has been established.

When the spool 45 is in the neutral position, the pilot pressure switching notch 87 is closed, and when the spool 45 is switched to the left or right, it communicates with either the pilot passage 70 or 71, and the amount of movement of the spool 45 The opening area for the pilot passage 70 or 71 is determined in proportion to.

The pilot passage 70.71 is an orifice 72.73.
Since it communicates with the tank 58, 74 via the pilot passage 70 or 71, a pilot pressure is generated in the pilot passage 70 or 71 according to the flow rate proportional to the opening area.

The pilot pressure generated as described above flows into the meter-out valve mechanism, and the configuration of this meter-out valve mechanism is as follows.

That is, the pores 75 and 76 are located on the most downstream side of the valve body 32.
These pores 75 and 76 are opposed to each other with the tank passage 77 in between.

A main poppet 78.78 is provided inside the pore 75.76, and this main poppet 78
．． A first guide member 80.81 is fixed to the outside of 78, and a second guide member 82.83 is further fixed to the outside of this first guide member 80.81.

A spring 86.87 is interposed in a pressure chamber 84.85 formed between the main poppet 78.79 and the first guide member 80.81.
7 and the pressure inside the pressure chamber 84.85, the main poppet 78.79 is normally pressed against the seat portion 88.89 formed on the actuator port 62.63 side, and the tank passage 77 and the actuator are connected to each other. Ports 62 and 63 are closed.

In addition, the first guide member 80.81 and the second guide member 8
2.83 is connected to the pressure chamber 84.85 through an orifice 82.83, and also communicates with the actuator port 62.83 through a passage 94.95. ing. Furthermore, a pilot chamber 88.87 formed in the second guide member 82.83 communicates with the pilot passage 70.71 via damper orifices 115, 118.

A hole is formed in the center of each of the main poppet 78.78, the first guide member 80.81 and the second guide member 82.83 as described above, and a rod 98.98 is inserted into the hole.
are slidably passed through the rods 98, and both rods 98
．． The inner ends of 88 abut against each other within the tank passage 77.

In this way, both ends of the rods 98, 98 are in contact with the spring guides 100, 101, and the spring guides 100, 101 are
02 and 103 push the rods 98 and 89,
Normally, rods 98, 89 are maintained in the neutral position shown.

When the rods 88.88 are held in the neutral position as described above, communication between the tank passage 77 and the pressure chamber 84.85 is cut off, but when the rods 88.88 are moved against the spring force of the springs 102.103. At this time, the pressure chamber 84.85 is opened through the notch 104.105 formed in this rod 98.89.
The tank passage 77 and the tank passage 77 are in communication with each other. Further, the rods 98.98 are formed with stepped portions 1013 and 107 on which the pressure introduced into the relay chamber 90.81 acts, and the inner diameter D1 is set as the boundary of the stepped portions. It is made larger than the diameter D2.

In the figure, numerals 108 and 1013 are anti-void valves, and 110 and 112 are relief valves.

The meter-in valve mechanism a and the meter-out valve mechanism a' and the meter-out valve mechanism b' have the same construction.

However, if only the upstream cylinder 64 is operated and the spool 45 is switched to the right in FIG. 2, the neutral port 10 gradually closes during the switching process. The inflow passage 37 and the load port 54 communicate with each other, but the opening area in the communication process is limited to the spool 4.
It is determined according to the amount of movement of 5.

As described above, the inflow passage 37 has a predetermined opening area.
When the and load port 54 communicate with each other, the working fluid from the variable pump 1 flows from the pressure control valve 6 to the inflow passage 37 to the annular recess 48.
→load boat 54→load check valve θ] and flows into the rod side chamber 68 of the cylinder θ4.

When the spool 45 is switched to the right as described above, the pilot pressure switching notch 67 and the pilot passage 71 are communicated with each other, and the amount of opening thereof is determined according to the amount of movement of the spool 45. Once the opening amount is determined, the pilot flow rate that flows in via the pressure reducing valve 68 is determined, and the determined pilot flow rate flows from the pilot passage 71 through the orifice 73 to the tank 74. Therefore, a pressure difference is generated before and after the orifice 73 depending on the pilot flow rate.

The pilot pressure generated before and after the orifice 73 in this manner flows into the pilot chamber 97 via the damper orifice 116 and acts on the outer end surface of the rod 99, that is, the contact surface with the spring guide 101.

When pilot pressure is introduced into the pilot chamber 87 as described above, the rod 99 is pushed leftward in the drawing by the action of the pilot pressure. Along with this, the rod 88 is also pushed, and the notch 104 of the rod 88 moves into the pressure chamber 84 .

When the notch 104 opens into the pressure chamber 84, the pressure chamber 84 communicates with the tank passage 77 and its pressure decreases, so the high pressure on the actuator port 62 side opens the main poppet 78, and the actuator port 62 opens into the tank passage 77. It communicates with passage 77. Therefore, the fluid on the return side of the bottom side chamber 65 returns to the tank 112, and the cylinder 64 contracts.

When the cylinder 64 operates as described above, the load pressure on the load port 54 side is introduced from the load detection port 5B into the other pilot chamber 40 of the pressure control valve 6, and at the same time, the load pressure on the load port 54 side is introduced into the other pilot chamber 40 of the pressure control valve 6. has orifice 4
3, the pressure on the inflow passage 37 side is introduced.

At this time, the pressure control valve 6 is within the range where the pressure P1 on one pilot chamber 39 side, the pressure P2 on the other pilot chamber 40 side, and the pressure P3 corresponding to the spring force of the spring 42 maintain the relationship of P1=P2+P3. is balanced, and the annular recess 41, inflow passage 37, and @-opening amount are adjusted to control the differential pressure before and after the spool 45 to be constant. Therefore, when the spool 45 is switched, a flow rate proportional to its opening area is obtained, and as a result, a predetermined flow rate is supplied to the cylinder 84 regardless of its load fluctuation.

At this time, the discharge pressure of the variable pump 1 is transferred to one viroft chamber 25 of the differential pressure sensing control valve 22 via the output detection passage 24.
At the same time, the load pressure on the load port 54 side flows into the other pilot chamber 28 via the shuttle valve 20.

Therefore, in the differential pressure sensing control valve 22, the pressure P1 in one pilot chamber 25, the pressure P2 in the other pilot chamber 2B, and the pressure P3 corresponding to the spring force of the spring 27 are in a relationship such that P1=P2+P3. There will be a balance between.

For example, when the pilot pressure P1 on the pilot chamber 25 side is higher than the pilot pressure P2 on the pilot chamber 28 side,
The differential pressure sensing control valve 22 moves against the spring 27,
It stops at a position balanced to P2+P3, and the output detection passage 24 and the passage 28 are communicated with each other. Therefore, the discharge pressure of the variable pump 1 flows into the tilt angle control cylinder 28,
Decrease the discharge amount of the variable pump l.

On the other hand, if the above P1 becomes lower than P2+P3, the differential pressure sensing control valve 22 is switched, the fluid from the second cylinder flows into the tank 31, and the discharge amount of the variable pump 1 is increased accordingly.

The discharge amount of the variable pump 1 is determined as described above, and the discharge amount is determined within a range that satisfies the condition of P□=P2+P3.

Eventually, the pump discharge amount is controlled so that the differential pressure between the pump discharge pressure and the load pressure becomes equal to the pressure corresponding to the spring force.

On the other hand, when a load is applied to the cylinder 84 in the direction of arrow 117 and pressure is supplied to the rod side chamber 6θ side, that is, when a negative load is applied, the cylinder 84 cannot be controlled by the meter-in valve mechanism a.

This is because when a negative load is acting as described above, the pump moves with the load pressure rather than the supply pressure from the pump, so the cylinder cannot be controlled even if the flow rate on the supply side is controlled. .

In such a case, the meter-out valve mechanism functions, and its specific operation is as follows.

That is, as described above, the spool 4 of the flow rate adjustment switching valve 7
5 to the right in the drawing, the opening degree of the pilot passage 71 is determined according to the switching amount.

Once the opening degree of the pilot passage 71 is determined, a flow rate corresponding to the opening degree flows, and the function of the orifice 73 generates a pilot pressure corresponding to the flow rate.

This pilot pressure flows into the pilot chamber 87 through the damper orifice 116 and acts on the rod 98, pushing the rods 89 and 98 to the left in the drawing.

At this time, pressure from the actuator port 62 side is flowing into the relay chamber 90 and the pressure chamber 84;
The pressure within acts on step m 106, which force acts as an opposing force on said rods 99 and 88.

Therefore, the rod 8 pushed by the pilot pressure
8 and 9i will move to a position where the pressing force due to the pilot pressure and the force acting on the spring 102 and the stepped portion 10fl are balanced.

In other words, the rods 89 and 88 move in proportion to the pilot pressure, that is, the switching amount of the spool 45.

When the rod 88 moves as described above, the notch @ 1
04 opens toward the pressure chamber 84 and communicates the pressure chamber 84 with the tank passage 77. When the pressure chamber 84 communicates with the tank passage 77, the pressure in the pressure chamber 84 is reduced by the action of the orifice 82. When the pressure inside the pressure chamber 84 decreases in this manner, the main poppet 78 moves in proportion to the amount of movement of the rod 88 due to the action of the high pressure on the actuator port 82 side, thereby determining the opening area of the seat portion 88.

When the seat portion 88 is opened as described above, the actuator port 82 and the tank passage 77 communicate with each other, and a flow rate corresponding to the opening degree of the seat portion 8 flows into the tank passage 77.

In other words, the pilot pressure is determined according to the amount of movement of the spool 45 of the flow rate adjustment switching valve 7, the amount of movement of the main poppet 78 is also determined in proportion to the pilot pressure, and the seat portion 88 has an opening area proportional to the pilot pressure. will be maintained.

If the opening area of the seat portion 88 is thus obtained in proportion to the pilot pressure, meter-out control can be performed according to the opening area, and the cylinder 64 can be controlled even when the above-mentioned negative load is applied.

Then, in the above condition, there is a load fluctuation on the cylinder in question,
When the pressure on the return side changes, the rods 88 , 88 sense the pressure change as a change in the force acting on the step 106 .

Therefore, when the pressure on the actuator port 62 side increases while the opening area of the seat portion 88 is constant, the flow rate flowing into the tank passage 77 tends to increase in accordance with the pressure increase. However, at this time, the pressure inside the relay chamber 90 also increases,
Furthermore, since the force acting on the stepped portion 106 also increases, the rods 88, 99 move to the right in the drawing. As the rod moves, the main poppet 78 follows, and the seat part 8
8 is self-adjusted to cancel the increase in the outflow amount by reducing the opening area.

On the contrary, when the pressure on the actuator port 62 side decreases,
The amount of outflow flowing into the tank passage 77 tends to decrease in accordance with the pressure drop. However, at this time, the pressure within the relay chamber 80 also decreases, and the force acting on the stepped portion 10B also decreases, so the rods 98 and 89 move to the left in the drawing.

As the rod moves, the main poppet 78 follows and increases the opening area of its seat portion 88 to self-adjust to cancel the decrease in the outflow amount.

In other words, even if the pressure on the actuator boat 62 side changes,
It maintains a self-adjustment function that automatically increases or decreases the opening area of the seat portion 88 to prevent changes in the flow rate on the return side.

Further, the stepped portions 1013 and 107 do not need to have equal pressure receiving areas; for example, if the direction of the negative load is always determined depending on the usage situation of the device,
The pressure receiving area of one step may be increased.

A motor 11 connected to the meter-in valve mechanism a' side
3 and the cylinder 84 at the same time is substantially the same as when only the cylinder 84 is operated, except that the pressure control valve 6 on the meter-in valve mechanism a side and the meter-in valve mechanism a' The side pressure control valve 8 functions to prevent oil discharged from the variable pump l from being supplied only to the actuator with the lower load.

In this case, the higher load pressure of the actuators is selected and introduced by the shuttle valve 20, and the discharge amount of the variable pump 1 is controlled in conjunction with that load pressure.

The second embodiment shown in FIG. 3 uses an auxiliary pump 114 different from the variable pump 1 to generate pilot pressure when the flow rate adjustment switching valve 7.8 is in the neutral position. All other configurations are the same as in the first embodiment.

Furthermore, in each of the above embodiments, when controlling one actuator, the pressure control valves 6 and 8 do not need to be provided.

Further, in the second embodiment described above, the pilot passage 18 is guided to the pilot chamber 2θ independently of the flow rate adjustment switching valve 7.9, but the following arrangement may be made, for example.

That is, the flow rate adjustment switching valve may be formed with a boat that opens when it is in the neutral position, and the pilot passage 18 may be guided to the pilot chamber 26 via this boat.

As is clear from the above description, the configuration of the present invention is that in a hydraulic control circuit using a valve mechanism that can control the flow rate according to the amount of movement of the spool, C. and a meter-in valve mechanism that supplies a constant flow rate, a meter-out valve mechanism operated by the switching operation of the meter-in valve mechanism, and a neutral passage that runs through a neutral boat of the valve mechanism,
A restriction is provided in the flow process of this neutral passage, and a pressure generation source for generating pilot pressure for negative control is provided downstream of this restriction, and the discharge pressure of the variable pump is adjusted according to the pressure of this pressure generation source. It is characterized in that it consists of a negative control mechanism equipped with a tilt angle control device for adjustment.

With the above configuration, in this invention, the negative control mechanism functions when the actuator is in neutral, reduces the discharge amount of the variable pump, and exhibits an energy saving effect, and when the actuator is operated, the load is detected and the discharge amount of the pump is maintained at an optimal state.

Furthermore, when a negative load is acting on the actuator as described above, the meter-out valve mechanism functions to control the actuator.

[Brief explanation of the drawing]

Drawings 1 and 2 show a first embodiment of this invention,
FIG. 1 is a circuit diagram, FIG. 2 is a sectional view of an integrated meter-in valve mechanism and meter-out valve mechanism, and FIG. 3 is a circuit diagram of a second embodiment. a, a'...-meter-in valve mechanism, b, b'Φ...
Meter-out valve mechanism, 10.11...neutral boat, 18 fist--throttle which is the pressure generation source, 21...head rotation angle control device, 45...φ spool, 64・φ・actuator cylinder, 113・φ・A motor that acts as an actuator. Representative Patent Attorney Nobuyuki Shima

Claims (1)

[Claims] In a hydraulic control circuit using a valve mechanism that can control the flow rate according to the amount of movement of the spool, there is a meter-in valve mechanism that always supplies a constant flow rate according to the opening degree, regardless of actuator load fluctuations, and a meter-in valve mechanism that always supplies a constant flow rate according to the opening degree, regardless of actuator load fluctuations. A meter-out valve mechanism that operates by switching the valve mechanism;
A neutral passage passing through the neutral port of the valve mechanism is provided, a restriction is provided in the flow process of this neutral passage, and a pressure generation source for generating pilot pressure for negative control is provided downstream of this restriction. A hydraulic control circuit consisting of a negative control mechanism equipped with a tilt angle control device that adjusts the discharge pressure of a variable pump according to the pressure of the pressure source.