JPH0141836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in variable displacement vane or piston pumps with pressure compensation.

Such a variable discharge rate pump operates a pump discharge rate variable device when the self-discharge pressure rises to a preset pressure so that the pump has a large capacity when the pressure is low and a small capacity when the pressure is high, that is, the deadhead pressure. The discharge amount is made small.

In a hydraulic circuit using a variable discharge pump with this structure, compared to a hydraulic circuit that combines a constant discharge pump and a relief valve, relief valve surge pressure is not absorbed, so inside the variable discharge pump, For example, surge pressure is likely to occur when an actuator suddenly stops or a valve is suddenly closed. This surge pressure is said to be approximately 1.5 to 3 times the set pressure of the variable discharge pump, that is, the deadhead pressure.For example, when the deadhead pressure is set to a maximum of 70Kgf/ ^cm2 , the surge pressure is
It can reach up to 200Kgf/cm ² , and as a result, the sliding surfaces and other parts of the pump are subjected to excessive loads, rapidly shortening the life of the pump. The life of equipment such as valves and actuators that are affected by surge pressure will be shortened. In addition, this surge pressure caused excessive strain in the constituent members of the pumps and equipment, resulting in a decrease in the performance of the pumps and equipment.

In order to absorb the surge pressure of such a variable displacement pump, it has been proposed, for example, to install a relief valve in the variable displacement pump to relieve a portion of the pump discharge pressure or the variable device control pressure oil. However, in this case, when changing the set pressure of the pump, it is also necessary to change the set pressure of the relief valve. Otherwise, surge pressure will occur when the set pressure of the pump is lowered, and conversely, when the set pressure of the pump is increased, A portion of the pump discharge escapes into the tank through the relief valve, causing a loss in discharge and horsepower, and an increase in oil temperature. As a result, it is difficult to handle, the spring of the relief valve must be large, and the valve is difficult to open. Furthermore, as a method for absorbing surge pressure, it has been proposed to install, for example, an accumulator that can communicate with the discharge oil of a variable discharge pump, or to install such an accumulator in combination with a relief valve. However, the accumulator is not only expensive, but also has drawbacks such as the capacity of the accumulator must be changed in response to the flow rate and pressure of the hydraulic circuit, and adjustment and other handling are difficult.

SUMMARY OF THE INVENTION An object of the present invention is to provide a variable displacement pump that eliminates surge pressure and does not require adjustment even if the set pressure or deadhead pressure changes.

Another object of the present invention is to release excess pump discharge oil in an amount equivalent to the response delay of the variable mechanism that generates surge pressure in a pressure compensated variable discharge pump to a tank via a surge absorption valve disposed in conjunction with the pump. Another object of the present invention is to provide a variable discharge amount pump that does not generate surge pressure.

The above-mentioned and other objects and advantages of the present invention include: a main piston that presses a variable device in a direction to reduce the discharge volume of a variable discharge volume pump;
A variable discharge pump having an oil passage that guides discharge pressure oil of the variable discharge amount pump to a pressure chamber of a main piston via a pressure compensator, and a second oil passage that guides pressure oil in the pressure chamber to a tank via a throttle. In the variable displacement pump, a small piston to which pressure oil in the pressure chamber is guided, a valve device opened and closed by the small piston, a bypass oil passage guiding discharge oil of the variable discharge amount pump to the valve device, and the valve device. This can be achieved by a variable discharge amount pump characterized in that a surge absorbing valve consisting of a drain oil passage that leads the discharged oil from the valve device to the tank when the pump is opened is arranged in conjunction with the surge absorption valve.

The pressure of the pressure oil in the pressure chamber is throttled by a pressure compensator, and by appropriately selecting the throttle, even if the set pressure of the pump is changed, the set pressure (dead head pressure) will not go up or down with the same pump. Even if the set pressure is different between pumps with different maximum pressures, approximately 20Kgf/cm ² to 30Kgf/
It can be maintained within a relatively narrow pressure range such as cm ² . That is, the pressure in the pressure chamber is such that when the pump pressure, which is the pressure of the discharge pressure oil of the pump guided to the oil passage, is reduced by the pressure loss passing through the pressure compensator, and the pump pressure is further increased from the throttle. As more pressure oil escapes to the tank through the second oil passage, as the deadhead pressure increases, more pressure oil escapes to the tank, increasing the set pressure (deadhead pressure).
Even if the pressure increases or decreases, the pressure can be maintained within a narrow range of, for example, 20 Kgf/cm ² to 30 Kgf/cm ² .

Since the pressure in this pressure chamber acts on the small piston to open and close the valve device, that is, the surge absorption valve,
If the set pressure of the valve device is set to Ppr (Fig. 1), which is slightly higher than the above-mentioned 30 Kgf/cm ² , even if the set pressure of the pump, that is, the deadhead pressure set by the pressure compensator is changed, the surge absorption valve There is no need to change the pressure settings of the system, and therefore surge pressure can be efficiently removed without the need for additional adjustments.

The surge pressure absorption valve may be disposed inside or outside the pump, but preferably the oil passage and the bypass oil passage may be connected to each other in order to simplify piping and downsize the device. When the pump capacity is large and it is necessary to release a large amount of excess pump pressure oil to prevent the generation of surge pressure, preferably a relief valve operated by the valve device is provided in the bypass oil passage. You may go through it. Also,
In order to make the surge absorption performance more effective, the pressure oil in the pressure chamber is guided to the small piston through a throttle valve that is throttled only in the backward direction, and the return of the valve device is slightly controlled by the throttle valve. The amount of delay pump pressure oil released into the tank may be increased.

The valve device is preferably a poppet valve, but may also be a spool valve. More preferably, the valve device can be provided with a stopper that limits the degree of opening.

Although the invention can preferably be used in pressure compensated variable displacement vane pumps, the invention can equally be used in pressure compensated variable displacement piston pumps, especially variable displacement axial pistons.

When the variable discharge amount pump of the present invention is used in a hydraulic circuit, for example, suppose that the cylinder suddenly reaches the end of its stroke in actual use. Then, the variable mechanism of the variable pump comes into play, and the pump delivery rate is reduced to an amount close to zero, ie, to the deadhead delivery rate. In the conventional known variable discharge amount pump, a surge pressure P _S is generated in the pump pressure P as shown by the solid line in FIG. 1 due to the response delay of the variable mechanism. After repeating some oscillatory waves, the dead head pressure stabilizes at _PDH . At this time, the control pressure P _P of the main piston also generates a surge pressure ≒ P _S that is almost the same as the pump pressure P.
Thereafter, the pressure stabilizes at the dead head pressure P _PDH , which is the control pressure corresponding to the dead head pressure P _DH of the pump. In the present invention, the pump pressure _P is applied to the small piston to open the valve device and release a part of the pump discharge pressure oil to the tank.
This eliminates the surge pressure P _S at the pump and the surge pressure P _S at the control pressure P _P inside the pump. To be more precise, the set pressure at which the surge absorption valve operates is a pressure slightly higher than the dead head pressure (P _PDH , for example 20Kgf/cm ² to 30Kgf/cm ² ) of the control pressure P _P (P _PR , 30Kgf/cm 2 above). cm ² ) and the corresponding pump pressure is P _R , i.e. the highest deadhead pressure (slightly higher than
Corresponds to a slightly higher pressure than P _DH . Then the surge pressure P _S exceeding this P _R at the pump pressure P is removed, and at the control pressure P _P it corresponds to P _R
Surge pressure exceeding P _PR ≒ P _S is removed.

The surge pressure P _S is the pump head pressure
Since it is sufficiently high at 1.5 to 3 times compared to P _DH , the surge pressure at the control pressure P _P entering the pressure chamber ≒ P _S is
Even if the pump head pressure P _DH goes up or down, the accuracy of the signal pressure that operates the small piston is not affected and is very small, so the surge valve set pressure P _PR
does not require readjustment even if the pump head pressure _PDH goes up or down. Note that the pump discharge oil led from the bypass oil passage to the valve device does not act to open the surge absorption valve.

The invention will be explained in more detail on exemplary preferred embodiments and with reference to the drawings, in which: FIG. Second
The portion of the variable discharge amount vane pump shown in the figure, excluding the surge absorption valve A, is the variable discharge amount vane pump disclosed in Japanese Patent Application Laid-Open No. 57-5585, and this portion itself does not constitute the present invention, so please refer to the details below. do not explain.

FIG. 2 shows a sectional view of a hydraulic variable displacement vane pump used in machine tools, industrial equipment, etc. taken along a plane perpendicular to the axes of the sub-piston and the main piston. FIG. 3 schematically shows FIG. 2 in a hydraulic circuit diagram. Inside the pump body 1,
A rotor 8 into which a plurality of vanes 9 are inserted so as to be freely removable in the radial direction, a ring 7 that is a variable device that surrounds the rotor 8, and a side plate (not shown) that is connected to the rotor 8 and the ring 7.
Served on both sides. The sub-piston 5 has one end in contact with the ring 7, and the other end is urged by a spring 10 so as to press the ring 7 toward the center _O1 of the rotor 8. Furthermore, the pump discharge pressure is applied to the oil passage 20 in the pressure chamber 22 on which the sub-piston 5 acts.
(including an oil passage (not shown) that communicates with the pump discharge oil chamber 15), and acts on the surface 5' of the sub-piston 5 that receives the pump discharge pressure, causing the sub-piston 5 to move as shown in FIGS. 2 and 3. The ring 7 is biased to be pressed in the right direction when viewed from the front. The main piston 4 is provided on the opposite side of the sub-piston 5 with respect to the center O ₁ of the rotor 8 and substantially coaxially with the sub-piston 5 . The main piston 4 has one end in contact with a ring 7 and the other end in contact with a discharge rate adjusting screw 13.
The discharge amount adjusting screw 13 limits the maximum eccentricity of the ring 7 via the main piston 4. Pump discharge oil is guided to a pressure chamber 23 that operates the main piston 4 through an oil passage 21 (including an oil passage (not shown) that communicates with the pump discharge oil chamber 15) via a pressure compensator 3. The main piston 4 is urged to press the ring 7 to the left as viewed in FIGS. 2 and 3 by acting on the pressure-receiving surface 4'. The pressure compensator 3 controls the hydraulic pressure acting on the main piston 4, and includes a spool 6 urged in the closing direction by a spring 11, a pressure adjustment screw 14, and ports 17, 1.
8 and 19, and when the pump discharge pressure is lower than the pressure set by the pressure adjustment screw 14, the pressure receiving surface of the spool 6 connects the port 17, which communicates with the oil passage 21, to the port 18.
Closed against. When changing the pump dead head pressure _PDH , the setting can be changed arbitrarily by adjusting the pressure adjusting screw 14 of the pressure compensator 3 and changing the force of the spring 11. That is, if the force of the spring 11 is increased, a greater pressure in the oil passage 21 is required to push the spool 6 open, and as a result, the set pressure, that is, the pump head pressure _PDH becomes higher. When the pump discharge pressure exceeds the set pressure, the pressure oil flows to port 1.
7 to the port 18 and flows into the pressure chamber 23. Port 19 is a drain port that communicates with the tank or pump drain section. In this embodiment, a second oil passage 24 is located approximately in the center of the main piston 4 and has one end open to the pressure chamber and thus the surface 4', and the other end communicates with the tank (pump drain part) via the throttle 12.
is installed internally in the axial direction. The state shown in Figs. 2 and 3 is also the position where the pump is stopped, and the ring 7 is pushed to the right by the spring 10 via the sub-piston 5, and the center of the ring 7 and the center e of the rotor 8 are at the discharge position. It is eccentric by the maximum eccentricity amount set by the adjustment screw 13. When the rotor 8 is now rotated in the direction of the arrow (counterclockwise), the pump sucks oil from the lower suction chamber 26 through a port (not shown), and pump discharge oil at the discharge pressure P from the upper discharge chamber 15. is discharged through a port not shown. This pump discharge pressure P is passed through the oil passage 20 to the sub piston 5.
At the same time, it acts on the pressure receiving surface of the spool 6 of the pressure compensator 3 through the oil passage 21.

When the hydraulic pressure due to the pump discharge pressure P acting on this pressure receiving surface is smaller than the spring force of the spring 11 set by the pressure adjustment screw 14, the ring 7 The pump is pushed toward the right by the sub-piston 5 on which the resultant force acts, and the pump discharges a discharge amount corresponding to the maximum eccentricity set by the discharge amount adjustment screw 13. The pump discharge pressure P increases, and the spool 6
When the hydraulic pressure acting on the pressure receiving surface becomes greater than the spring force of the spring 11, the spool 6 is moved to the right. Then, the pump discharge oil flows through the oil passage 21.
From there, it passes through ports 17 and 18 and enters the pressure chamber 23 of the main piston 4. A portion of this pressure oil passes through the throttle 12 and the second oil passage 24 and escapes from the outlet 25 to the pump drain section (not shown) (and eventually to the tank), but the pressure oil squeezed by the throttle 12 increases the pressure in the pressure chamber 23 and increases the pressure on the surface 4. ′. As the pump discharge pressure P increases, the rightward movement of the spool 6 increases, and the control pressure P _P in the pressure chamber 23 increases, and finally the hydraulic pressure due to the control pressure P _P acting on the surface 4' of the main piston 4 increases. overcomes the resultant force of the hydraulic pressure acting on the surface 5' of the sub-piston 5 and the spring 10, and moves the ring 7 to the left. For this reason, the discharge amount of the pump decreases, and the pump discharge pressure P is set by the pressure adjusting screw 1 in advance.
When the pressure rises to the pressure set in step 4, that is, the dead head pressure _PDH , the dead head no longer discharges pressure oil, and the center _O2 of the ring 7 is connected to the rotor 8.
It is almost equal to the center of O ₁ .

Next, when the pump discharge pressure P decreases below the set pressure _PDH , the spool 6 moves to the left, the amount of pressure oil flowing into the pressure chamber 23 decreases, and as a result, the pressure in the pressure chamber 23 decreases. So, ring 7
are the hydraulic pressure acting on the sub-piston 5 and the spring 1
It is moved to the right by the resultant force with 0. Therefore, the pump discharge amount increases and the pump returns to the state shown in FIGS. 2 and 3. Such a ring 7 forms a variable device for increasing or decreasing the pump discharge in a variable discharge vane pump.

A shown in FIGS. 2 and 3 shows the surge absorption valve according to the present invention as a whole. The surge absorption valve A is a housing 101 that is integrated with the pressure compensator 3 (it may be a separate unit for ease of processing).
, a small piston 102 slidably fitted into a hollow hole bored in the housing 101 , and a passage 103 that guides control pressure oil in the pressure chamber 23 of the main piston 4 to an oil chamber 110 of the small piston 102 . , a valve device B having a small piston 102 and a poppet 107 connected by a small diameter connecting rod 105. The valve device B is pressed by a spring 108 so as to close the poppet 107 to the tank port T with respect to the pump discharge pressure port P, and the strength of this spring is adjustable by a screw 111. Figures 2 and 3
As can be seen in the figure, the pump discharge oil entering from the bypass oil passage 104 acts on the inside of the small piston 102 and the opposite-to-spring side of the poppet 107 with the same area and pressurizes them so as to separate them. and poppet 107 are small diameter connecting rods 10
5, the poppet 107 will not move toward the spring side, and therefore, the pump discharge pressure and surge pressure will not move at all in the direction of opening the poppet 107, that is, in the direction of opening the surge pressure absorption valve A. Doesn't work. In the embodiment, a stopper 109 is provided to limit the opening degree of the poppet 107, but this prevents the poppet 107 from moving too far when the force pressing against the small piston 102 due to surge pressure is very large compared to the spring force of the spring 108. This valve is provided for safety purposes so as not to cause malfunctions such as inoperability, and to arbitrarily adjust the maximum opening degree based on the magnitude of surge pressure in the hydraulic circuit. Pump discharge oil is guided to the discharge oil port P by a bypass oil passage 104 that includes a pump internal passage (not shown) that is separate from the oil passage 21, and to the tank port T, a drain oil passage 106 that includes a pump internal passage (not shown) is introduced.
It communicates with the pump's drain passage.

The control pressure P _P in the oil chamber 23 of the main piston 4 guides the pump discharge oil through the pressure compensator 3,
And by appropriately selecting the restrictor 12 according to the pump rated discharge pressure and flow rate, normally 20Kgf/cm ² ~ 30
It can be set within a very narrow range such as Kgf/cm ² . This control pressure P _P can be maintained approximately within the above pressure range even if the set pressure, that is, the deadhead pressure changes, or even if the set pressures differ between pumps with different maximum pressures. The strength of the spring 108 that presses the poppet 107 is set to a pressure P _PR that is somewhat higher than the pressure P _PDH at the time of the dead head as explained earlier in FIG. 1, and the corresponding pump discharge pressure is _PR .

When the load on the hydraulic circuit for the pump, that is, the load applied by starting and stopping the actuator or opening and closing the valve, increases gradually, the pump pressure P generates a surge pressure P _S as shown in Figure 1. The deadhead pressure P _DH rises statically without any pressure.
At this time, the control pressure P _P also statically rises to the pressure P _PDH . At this time, small piston 10 of surge absorption valve A
Since the hydraulic pressure acting on the spring 108 does not exceed the set pressure P _PR of the spring 108, the poppet 107 remains closed and the valve device B is inactive, as shown in FIGS. 2 and 3. .

On the other hand, if the pump load is high, for example when the cylinder suddenly reaches the end of its stroke and stops, or when the valve is completely suddenly closed, a surge of pressure will occur in the hydraulic circuit and therefore inside the pump.
_PS occurs. Then, the control pressure P _P also exhibits a similar peak pressure as explained in FIG. When the surge pressure of the control pressure P _P becomes larger than the set pressure P _PR , the poppet 107 is moved to the right in FIGS. runs away. As a result, the surge pressure P _S exceeding the pump pressure _PR corresponding to the pressure P _PR as seen in FIG. 1 is removed. When the control pressure P _P becomes smaller than the set pressure P _PR of the spring 108, the pot 107 is pressed by the spring 108 and closed, so that the pump discharge oil does not escape. Since this control pressure P _P is within an almost narrow pressure range as mentioned above, compared to the case where a conventional relief valve is provided to release the pump discharge pressure oil, the set pressure P There is no need to change _{the PR} , and the best surge absorption capacity can always be maintained regardless of the pump setting pressure. Furthermore, since the poppet is not directly subjected to pump discharge pressure as is the case with conventional direct relief valves, the spring 108 can be made smaller and the poppet 107 can be opened larger than in conventional products, effectively suppressing surges. A large amount of relief can be taken to remove pressure, and surge pressure can be removed extremely effectively.

In this way, the variable displacement pump using the present invention can effectively remove surge pressure at all times without requiring troublesome adjustment once the spring 108 is set, thereby extending the life of the pump. Furthermore, it has become possible to prevent a decrease in mechanical precision, a decrease in performance, and damage to equipment due to distortion caused by excessive pressure loads.

Figure 4 shows a surge absorption valve different from Figure 2,
FIG. 5 is a schematic diagram showing a hydraulic circuit diagram when the surge absorption valve of FIG. 4 is applied to a variable discharge amount vane pump as shown in FIG. 2. Components equivalent to those in FIGS. 2 and 3 are designated by the same reference numerals. In the embodiment shown in FIG. 4, when the pump capacity becomes large and it is necessary to increase the relief amount, the pump is discharged to the relief inlet 113 communicating with the bypass oil passage 104 via the relief valve 112 inside the pump or in the hydraulic circuit. The oil is led to the discharge oil port P of the surge absorption valve A.
is communicated with a relief oil chamber 119 through a passage 114. When the small piston 102 is actuated and the poppet 107 is opened, the pressure in the oil chamber 119 drops, the relief valve body 115 opens, and a large amount of pump pressure oil in the bypass oil passage 104 is transferred to the drain oil passage 117.
Being made to miss.

In all of the above embodiments, the bypass oil passage 104 is provided separately from the oil passage 21 for the pump discharge pressure leading to the pressure compensator 3, but as will be explained in detail later in FIG. By communicating with the bypass oil passage 104, the piping may be simplified and the device may be miniaturized.

In FIG. 6, which shows yet another embodiment, the valve device is a spool valve having a small piston 101 and a spool 120 connected by a small diameter rod 105'.
, the discharge oil port P is closed to the tank port T, and the movement of the spool 120 is stopped by the stopper 10.
9'. This operation is shown in Figure 1.
It is almost the same as the poppet valve described in FIG. 3, and its explanation will be omitted. FIG. 6 shows a throttle valve 1 which is restricted only in the backward direction to a passage 103 that guides the pressure oil in the pressure chamber 23 of the main piston 4 to the oil chamber 110 of the small piston 101 in order to make the surge absorption performance even more effective.
21, the surge pressure decreases and the valve device, that is, the spool 120 in FIG.
8' throttles the return of the valve device closed by valve 1
21, the amount of relief of pump pressure oil to the tank is increased.

FIG. 7 shows an example in which the present invention is applied to a pressure compensated variable piston pump. The parts of the piston pump shown in FIG. 7 except for the surge absorption valve are known as shown in, for example, Japanese Utility Model Application Laid-open No. 57-61185, and do not constitute the present invention, so a detailed description thereof will not be given. In this pump, as an input shaft 31 rotates, a piston 33 whose head is connected to a swash plate 35 through a spherical joint 34 reciprocates within a cylinder barrel 32, and pump discharge oil is discharged from a discharge port. ing. The discharge amount of the pump is determined by the main piston 38 being connected to the spring 41.
By being pushed to the right against the pressure, the swash plate 35 becomes almost perpendicular to the input shaft 31 via the connecting member 42, so that the stroke of the piston 33 becomes 0 and the pump discharge amount is reduced to 0. It is being done. An oil passage 43 is provided in the pressure chamber 37 of the main piston 38.
The pump's own pressure is introduced via the pressure compensator 3 and the pressure compensator 3. Pressure chamber 37 of main piston 38
The pressure oil is led to the drain passage through the throttle 39.
A part of the oil is guided to the tank through an oil passage 40. Note that the pressure compensator 3 and the surge absorption valve A are shown enlarged for convenience of explanation. These constituent members and members equivalent to those shown in FIG. 2 are designated by the same reference numerals, and explanations of their structures, operations, and effects will be omitted. Furthermore, in the variable discharge amount axial piston pump shown in FIG.
Apart from the fact that 5 is moved, the variable device operates in the same manner as described for the variable displacement vane pump of FIG. 2, so a detailed explanation will be omitted.

In this embodiment, in particular, the bypass oil passage 104 of the surge absorption valve A is communicated with the oil passage 43 that guides the pump discharge pressure oil to the pressure compensator 3 through the communication passage 122. It has become possible to downsize. Apart from this, it will be apparent that the device of FIG. 7 operates in a manner similar to that detailed in FIG. 2 and absorbs pump pressure surges very effectively.

It will be clear to those skilled in the art that the present invention can also be used with pressure compensated piston pumps having a construction different from that shown in FIG.

Also, for those skilled in the art, regarding the pump shown in FIG. 7, the relief valve 11 shown in FIGS.
2 through the communication path 122. It will also be apparent that the structure of FIG. 2 can be used without the communication passage 122. Similarly, the valve device may be a spool valve as shown in FIG.
It will also be clear that 03 may be used instead. In the embodiment, the surge absorption valve A is arranged integrally with the pump, but it may be arranged outside the pump and connected to it through piping.

[Brief explanation of drawings]

FIG. 1 shows a graph showing the relationship between pressure and time for a variable displacement pump, FIG. 2 shows a cross-sectional view of a preferred embodiment in which the present invention is applied to a variable displacement vane pump, and FIG. 2 schematically shows a hydraulic circuit diagram, FIGS. 4 and 5 show another preferred embodiment of the invention, FIG. 4 shows a surge absorption valve different from that in FIG. 2, and FIG. The figure shows a schematic hydraulic circuit diagram of a device using this surge absorption valve, FIG. 6 shows a schematic hydraulic circuit diagram showing another preferred embodiment of the present invention, and FIG. Figure 3 is a cross-sectional view of a further preferred embodiment for use in a piston pump, with the pressure compensator and surge absorbing valve shown on an enlarged scale; A... Surge absorption valve, B... Valve device, 3... Pressure compensator, 4, 38... Main piston, 7
...Ring (variable device), 12,39...Aperture,
21, 43... Oil passage, 23, 37... Pressure chamber, 2
4, 40...Second oil passage, 35...Swash plate (variable stroke device), 102...Small piston, 104...
...Bypass oil passage, 106...Drain oil passage, 107
...Poppet, 108...Spring, 108'
...Coil spring, 109,109'...Stopper,
112...Relief valve, 120...Spool, 1
21... Throttle valve.

Claims (1)

[Scope of Claims] 1. A main piston that presses a variable device in a direction to reduce the discharge amount of the variable discharge amount pump, and a main piston that leads the discharge pressure oil of the variable discharge amount pump to the pressure chamber of the main piston via a pressure compensator. A variable discharge amount pump having an oil passage and a second oil passage that guides the pressure oil in the pressure chamber to a tank via a throttle, a small piston to which the pressure oil in the pressure chamber is guided, and a small piston. A surge absorption device comprising a valve device that is opened and closed, a bypass oil path that leads the discharge oil of the variable discharge amount pump to the valve device, and a drain oil path that leads the discharge oil from the valve device to the tank when the valve device is opened. A variable displacement pump characterized by having a valve. 2. The variable discharge amount pump according to claim 1, wherein the oil passage and the bypass oil passage are communicated with each other. 3. The variable discharge amount pump according to claim 1, wherein the bypass oil passage is provided with a relief valve operated by the valve device. 4. The variable discharge amount pump according to claim 1, wherein the pressure oil in the pressure chamber is guided to the small piston via a throttle valve that is throttled only in the backward direction. 5. The valve device is a poppet valve having a poppet that is urged in a closing direction by a spring, and the poppet is opened by the small piston against the spring. variable displacement pump. 6. Claim 1, wherein the valve device is a spool valve having a spool that is biased in a closing direction by a coil spring, and the spool is opened by the small piston against the coil spring. Variable displacement pump as described in section. 7. The variable discharge amount pump according to claim 1, wherein the opening degree of the valve device is limited by a stopper. 8. The variable discharge amount pump includes a main piston that presses the ring in a direction that reduces the amount of eccentricity between the rotor and the ring, and a device located on the opposite side of the main piston that presses the ring in the direction that increases the amount of eccentricity. 2. The variable displacement pump according to claim 1, which is a pressure compensated variable displacement vane pump, and wherein said variable device is said ring. 9. The variable discharge amount pump is a pressure compensated variable piston pump having a piston that reciprocates by rotation of an input shaft and a variable stroke device that changes the reciprocating stroke of the piston, and the variable device changes the variable stroke. The variable displacement pump according to claim 1, which is a device. 10. The pressure compensated variable piston pump is an axial piston pump, and the variable stroke device is a swash plate connected to one end of each of a plurality of pistons slidably housed in a cylinder barrel. The variable discharge amount pump according to item 9.