JPH0758081B2 - Hydraulic drive system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hydraulic drive system used for a work machine having a plurality of driven members such as a hydraulic excavator, and particularly to a hydraulic actuator having a plurality of hydraulic pumps having a plurality of discharge pressures. The present invention relates to a hydraulic drive system including a load sensing control type pump regulator device that controls a pump discharge amount so as to be higher than a maximum load pressure by a certain value and supplies hydraulic pressure to an actuator.

[Conventional technology]

In recent years, in hydraulic drive systems including a plurality of hydraulic actuators that drive a plurality of driven bodies such as hydraulic excavators and hydraulic cranes, the discharge pressure of a hydraulic pump is controlled in conjunction with load pressure or required flow rate, and A pressure compensation valve is arranged in association with the control valve, and the pressure compensation valve controls the differential pressure across the flow control valve to stably control the supply flow rate during combined drive. Among them, DE-A1-3535771 (US Pat. No. 4,738,102) is a representative example of controlling the discharge pressure of a hydraulic pump in conjunction with the load pressure.
No. 62-88803). Load sensing control is to control the pump discharge rate so that the discharge pressure of the hydraulic pump is higher than the maximum load pressure of multiple actuators by a certain value. A pump regulator device is provided for controlling the swash plate position of the hydraulic pump in response to the pressure difference between the pressure and the maximum load pressure of the plurality of hydraulic actuators.

The pump regulator device has a displacement volume variable member of a hydraulic pump, for example, a cylinder that drives a swash plate, and a switching valve that controls the movement of this cylinder. The switching valve is provided with pump discharge pressure and maximum load pressure. It is fed back so that when the pump discharge pressure is higher than the maximum load pressure plus the value set by the spring, the displacement of the hydraulic pump, that is, the pump displacement, is decreased, and when it is low, it is driven to increase. Move the cylinder.

Also, the pump regulator device keeps the swash plate of the hydraulic pump at the almost minimum tilt position to secure the minimum discharge amount in order to radiate heat from the hydraulic pump when all the directional valves are in the neutral position. It is configured to do. An unload valve that operates in response to the pressure difference between the pump discharge pressure and the maximum load pressure is connected to the discharge line of the hydraulic pump, and when the pressure difference reaches the value set by the unload valve spring, the minimum inclination Almost all of the discharge amount of the hydraulic pump in the inversion position flows out to the tank, and the rise of pump discharge pressure is regulated. The set value of the spring of the unload valve is higher than the set value of the spring of the switching valve of the pump regulator device in order to prevent the unload valve from operating while the actuator is being driven.

[Problems to be Solved by the Invention]

However, such a conventional hydraulic drive system has the following problems.

Now, consider the case where one of the directional control valves is operated to move the corresponding actuator at a constant speed. At this time,
The switching valve of the pump regulator device is located at a substantially central switching position by balancing the pump discharge pressure, the maximum load pressure, and the spring, and the pump capacity is controlled to be constant at this position. Here, if one of the other directional control valves is operated to move the corresponding actuator, the pump discharge amount becomes insufficient,
Pump discharge pressure drops. As a result, the switching valve of the pump regulator device switches in the biasing direction of the spring to increase the displacement of the hydraulic pump. When the pump discharge rate increases and reaches the required flow rate, the pump discharge pressure rises. At this time, when the switching valve of the pump regulator device is switched to the original center position, the pressure transmission delay of the pilot line that guides the pump discharge pressure to the switching valve and the response delay of the pump regulator device including the swash plate are mainly caused. Due to the response delay of the swash plate, the pump discharge amount exceeds the required flow rate. Therefore, the pump discharge pressure is further increased. This pressure increase is proportional to the difference between the required flow rate and the actual discharge amount, and a slight difference in flow rate causes a large pressure change. Therefore, at the next moment, the switching valve of the pump regulator device switches in the urging direction of the pump discharge pressure, and the pump capacity is rapidly reduced. Here, since the pump discharge amount decreases too much again, the pump discharge pressure decreases, and as a result, the pump capacity rapidly increases, causing a so-called hunting of pump capacity control in which the increase and decrease of the pump capacity are violently repeated. .

The above phenomenon is also caused by a sudden change in load when driving one actuator or a change in the operation amount of the directional control valve.

For the purpose of using the unload valve described above, it is necessary to flow almost all of the pump discharge amount at the time of minimum tilt, so a large opening area must be secured, and therefore the valve spool cannot be made small. It is also necessary to set a large initial load on the spring. Therefore, a delay in operation cannot be avoided with respect to a sudden change in pressure, a sudden change in pressure due to a response delay in the pump displacement control described above cannot be accommodated, and hunting cannot be prevented.

An object of the present invention is to provide a hydraulic drive system including a pump regulator device of a load sensing control system, which can prevent hunting in pump displacement control.

[Means for Solving the Problems]

According to the present invention, for the above purpose, a variable displacement hydraulic pump, a plurality of actuators driven by pressure oil discharged from the hydraulic pump, and pressures supplied from the hydraulic pump to the plurality of actuators. A plurality of directional control valves that respectively control the flow of oil, and hold the hydraulic pump at a minimum capacity when all the directional control valves are in the neutral position, and set the discharge pressure of the hydraulic pump to the maximum load pressure of the actuator. Is connected to the pump regulator means for controlling the displacement of the hydraulic pump so as to keep it high only by the first set value, and is responsive to the differential pressure between the pump discharge pressure and the maximum load pressure. Of the hydraulic pump having the minimum capacity when the differential pressure reaches a second set value that is larger than the first set value. In a hydraulic drive system provided with a pressure increase regulating means for discharging almost all of the amount, the pressure increase regulating means is configured to cause the pressure difference to be higher than a value near the first set value where the differential pressure is equal to or lower than the second set value. Bleed-off means for opening the unload valve prior to opening the unload valve when exceeding a small third set value and for discharging a small flow rate as compared with the flow rate when the unload valve reaches the second set value is provided. A hydraulic drive system is provided.

The bleed-off means can take various forms. For example, the bleed-off means may be a bleed-off valve separate from the unload valve, which is connected to the discharge line of the hydraulic pump and operates in response to the pressure difference between the pump discharge pressure and the maximum load pressure. Further, the bleed-off means may be a bleed-off valve which is coaxially built in the unload valve and which operates in response to a pressure difference between the pump discharge pressure and the maximum load pressure.

Further, the bleed-off means may have a bleed-off function added to the unload valve so that a small flow rate is discharged when the differential pressure exceeds the third set value.

As a specific example, in an unloading valve having a valve spool provided with a first groove means for giving a relatively large opening area for discharging almost all of the pump discharge amount, in the valve spool, the first groove means of the first groove means is provided. Second groove means are further provided that open prior to the opening and provide a relatively small opening area for outflowing a small flow rate.

As another specific example, a valve spool in which the differential pressure acts in the valve opening direction and a first spring means for biasing the valve spool against the differential pressure are provided, and an initial load of the first spring means is provided. When the differential pressure reaches the second set value, the valve spool is allowed to move in the valve opening direction, and in the unload valve set so that almost all of the pump discharge amount is discharged, the valve is opposed to the differential pressure. The second spring means for urging the spool and the first spring means and the valve spool are separated from each other before the valve spool moves by a predetermined stroke, and the first spring means and the valve spool are brought into contact with each other when the stroke moves by a predetermined stroke. A stopper for transmitting the biasing force of the first spring means to the valve spool, and the initial load of the second spring means
When the differential pressure exceeds the third set value, it is set smaller than the initial load of the first spring means so as to allow the valve spool to move in the valve opening direction, until the differential pressure exceeds the second set value. May be held at the position of the predetermined stroke by a stopper so that a small flow rate may flow out.

The third set value of the differential pressure may be larger or smaller than the first set value.

[Action]

In the present invention configured as described above, by providing the above-mentioned bleed-off means, the pump discharge amount increases more than the required flow rate due to the response delay of the pump regulator means, and the differential pressure between the pump discharge pressure and the maximum load pressure is increased. When exceeds the third set value, the surplus flow rate flows out, which prevents an extreme rise or fall of the differential pressure and suppresses the occurrence of hunting in pump displacement control.

Here, when the third set value of the differential pressure is set higher than the first set value, the pump regulator device keeps the pump discharge pressure higher than the maximum load pressure by the first set value. In the controlled normal operation, the bleed-off means does not function, an unnecessary leakage of the flow rate from the bleed-off means does not occur, and a loss of the pump discharge flow rate does not occur.

When the third set value of the differential pressure is set lower than the first set value, there is some flow rate leakage during normal operation, but the outflow rate is smaller than the outflow rate of the unload valve. By setting it, the loss can be suppressed to the minimum. On the other hand, in this case, since the appropriate flow rate flows out before the differential pressure exceeds the first set value, excessive rise of the differential pressure is more effectively prevented, and hunting for pump displacement control is more reliably prevented. This enables stable load sensing control.

If the bleed-off means is composed of a bleed-off valve coaxially built into the unload valve, or if the bleed-off function is added to the unload valve, the unload valve and bleed-off means are integrated. As a result, the valve structure becomes compact, and the conventional unload valve can be replaced with the unload valve without changing the circuit.

Furthermore, when the bleed-off means is configured by a bleed-off valve that is separate from the unload valve, or when it is configured by a bleed-off valve that is coaxially built in the unload valve,
Since the spool of the bleed-off valve can be made smaller in diameter and lighter in weight, the responsiveness of the bleed-off means can be improved, and the unload pressure and the bleed-off pressure can be set independently, increasing the degree of freedom in control.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS.

In FIG. 1, reference numeral 1 is a variable displacement hydraulic pump,
The displacement is controlled by the displacement variable mechanism, that is, the swash plate 2. The pressure oil discharged from the hydraulic pump 1 is supplied from the discharge pipe line 3 to the hydraulic cylinders 6A and 6B, which are loads, through the pressure compensating valves 4A and 4B and the direction switching valves 5A and 5B, and drives them. The maximum pressure of the circuit is regulated by the relief valve 7.

The pressure upstream of the directional control valve 5A, that is, the inlet pressure is fed back to the pressure compensating valve 4A in the valve closing direction by the pilot line 8, and at the same time, the load pressure of the hydraulic cylinder 6A, that is, the directional control valve 5A is controlled by the pilot line 9. The outlet pressure is fed back in the valve opening direction. Further, a spring 10 for urging the valve opening direction is provided. With this configuration, when the differential pressure across the directional control valve 5A is larger than the value set by the spring 10 during the operation of the hydraulic cylinder 6A, the flow passage area is reduced, and when it is smaller, the flow passage area is increased. By this action,
The differential pressure across the directional control valve 5A is maintained at a constant value set by the spring 10. Therefore, even if the pump discharge pressure or the load pressure fluctuates, the differential pressure across the directional control valve 5A becomes constant, so the drive speed of the hydraulic cylinder 6A does not change as long as the passage area of the directional control valve 5A is constant. . The same applies to the pressure compensation valve 4B.

With respect to the load pressures of the plurality of cylinders 6A and 6B, the maximum load pressure PLmax is selected by the high pressure selection valve 11 and sent to the pilot line 12.

The capacity of the hydraulic pump 1, that is, the tilted position of the swash plate 2 is controlled by the pump regulator device 13 of the load sensing control system. The pump regulator device 13 has a cylinder 14 that drives the swash plate 2 of the hydraulic pump 1 and a switching valve 15 that determines the movement of the cylinder 14. The pump discharge pressure Ps is fed back to the switching valve 15 through the pilot line 16, and the maximum load pressure PLm is fed through the pilot line 17.
ax is fed back. Further, the spring 18 is provided on the side where the maximum load pressure is fed back. With this configuration, when the pump discharge pressure is higher than the pressure obtained by adding the value set by the spring 18 to the maximum load pressure, the cylinder 14 is driven so as to decrease the pump capacity and increase it when the pressure is low. By this action, the pump displacement is controlled so that the discharge pressure Ps of the hydraulic pump is kept higher than the maximum load pressure PLmax by the set value of the spring 18.

Further, the pump regulator device 13 includes the direction switching valves 5A and 5B.
Are all in the neutral position, that is, when the load pressure is zero, the swash plate 2 is held at the substantially minimum tilt position to ensure the minimum discharge amount for the purpose of radiating heat from the hydraulic pump 1 and the like. In addition, the positional relationship between the cylinder 14 and the swash plate 2 is defined.

An unload valve 20 is provided in the discharge pipe line 3. The unload valve 20 cooperates with a bleed-off valve 24, which will be described later, when almost all the directional control valves 5A, 5B are in the neutral position, and almost all of the above-mentioned minimum discharge amount of the hydraulic pump 1 flows to the return circuit. That is, for unloading, the pump discharge pressure is fed back to the unload valve 20 by the pilot line 21 and the pilot line 22.
The maximum load pressure is fed back. Further, the spring 23 is provided on the side where the maximum load pressure is fed back. The unloading valve 20 in which the pressure difference between the pump discharge pressure and the maximum load pressure tends to become higher than the value set by the spring 23.
Opens the valve and discharges the pump discharge amount. By the action of this unloading valve, the pump discharge pressure is maintained at the value set by the spring 23 with substantially no load when the directional control valves 5A and 5B are in the neutral position.

The set value of the spring 18 of the pump regulator device 13 is equal to or larger than the value of the set value of the spring 10 of the pressure compensation valves 4A and 4B in consideration of the pressure loss of each part, and is larger than that.
The set value of the spring 23 of 20 is higher than the set value of the spring 18 in order to prevent the unload valve 20 from operating while the hydraulic cylinders 6A and 6B are being driven.

As an example, when the set pressure of the relief valve 7 is 280 kg / cm ² , the set value of the spring 23 is 25 kg / cm ² , and the set value of the spring 18 is 15.
kg / cm ² , the set value of the spring 10 is 8 kg / cm ² .

Further, in the present embodiment, the discharge line 3 is further provided with a bleed-off valve 24. This bleed-off valve
While the hydraulic cylinders 6A and 6B are being driven, when the pump discharge pressure becomes higher than the maximum load pressure by a target value or more, a part of the pump discharge amount is released from the discharge pipe line 3 to the return circuit, that is, to bleed off. Of the bleed-off valve 24
The pump discharge pressure is fed back by the pilot line 25, and the maximum load pressure is fed back by the pilot line 26. Further, the spring 27 for setting the above-mentioned target value is provided on the side where the maximum load pressure is fed back. When the differential pressure between the pump discharge pressure and the maximum load pressure becomes higher than the value set by the spring 27, the bleed-off valve 24 opens prior to the opening of the unload valve 20, and the bleed-off valve 20 is Small flow rate to the tank.

The structure of the bleed-off valve 24 is shown in FIG. A spool 31 is slidably incorporated in the body 30, and pressure chambers 32 and 33 are provided at both ends of the spool 31. The body 30 is provided with three ports 34, 35, 36, the pressure chamber 32 communicates with the port 34 through an internal passage 37 provided in the spool 31, and the pressure chamber 33 communicates directly with the port 35. . The port 34 is connected to the discharge pipe line 3 in FIG. 1 to guide the pump discharge pressure Ps,
The port 35 is connected to the pilot line 26 of FIG. 1, the maximum load pressure PLmax is introduced, and the port 39 is connected to the tank. Further, the internal passage 37 is the pilot line 25 shown in FIG.
Fulfills the function of. Groove 39 on the outer circumference of spool 31
Is provided, and when the spool 31 moves to the left from the illustrated position, the port 34 and the port 36 communicate with each other through this groove.
In the pressure chamber 33, the above-mentioned spring 27 that biases the spool 31 to the right in the drawing is arranged.

FIG. 3a is an enlarged view of part III of FIG. 2, showing port 3 of groove 39.
A second groove 40, which opens prior to the groove 39, is provided at the end on the 6 side, and the groove 40 comprises a large number of notches arranged in the circumferential direction as shown in FIG. 3b, and these notches are deep. It is V-shaped in both the length and length directions.

The flow rate characteristic of the bleed-off valve 24 thus configured is shown in FIG. 5 in comparison with the flow rate characteristic of the unload valve 20 when the bleed-off valve 24 is not used together, that is, the flow rate characteristic of the conventional unload valve. .

FIG. 5a shows the flow rate characteristic of the conventional unloading valve, the horizontal axis is the differential pressure ΔP between the pump discharge pressure and the maximum load pressure, and the vertical axis is the opening area A. Further, ΔP1 on the horizontal axis is the set value of the spring 10, that is, the target differential pressure across the directional control valves 5A, 5B, and ΔP2 is the set value of the spring 18, that is, load sensing between the pump discharge pressure and the maximum load pressure. It is a control compensation differential pressure, and ΔP3 is a set value of the spring 23, that is, an unloading differential pressure. The conventional unload valve opens when the differential pressure ΔP exceeds the set value ΔP2 and approaches the set value ΔP3, and secures an opening area A0 at which the entire minimum discharge amount of the hydraulic pump 1 can flow out at the set value ΔP3.

FIG. 5b shows the flow rate characteristic of the bleed-off valve 24 of this embodiment. When the differential pressure ΔP reaches the set value ΔP4 of the spring 27 slightly exceeding the set value ΔP2, the above-mentioned V-shaped groove 40 starts to open,
The opening area increases with the increase of the differential pressure ΔP with a gradient smaller than the characteristic line of the conventional unload valve. The opening area A1 when intersecting the characteristic line of the unload valve is about 1/3 of the above-mentioned opening area A0 of the unload valve, which causes the bleed-off valve 24 to open the unload valve 20 as described above. The valve opens prior to the valve, and a small flow rate outflows to the tank compared to the outflow rate.

As an example, as described above, the set value ΔP3 of the spring 23 is 25
kg / cm ^2, the spring 18 is set value ΔP2 is 15 kg / cm ^2, when the set value ΔP1 spring 10 is 8 kg / cm ^2, the set value ΔP4 number than the set value ΔP2 is kg / cm ² of the spring 27 High value, for example 18 kg / cm ² .

Another shape of the second groove of the bleed-off valve 24 is shown in FIGS. 4a and 4b. In this example, the groove 41 comprises a number of substantially rectangular notches of substantially constant depth and width. The flow rate characteristic of the bleed-off valve 27 provided with this groove 41 is shown in FIG. 5c. Since the groove 41 has a substantially rectangular shape, unlike the V-shaped groove 40, after opening at the set value ΔP4,
The opening area A1 at the beginning is secured, and the same opening area is maintained even if the differential pressure ΔP increases further.

6 and 7, the flow rate characteristics of the unload valve 20 of this embodiment, and the flow rate characteristics thereof and the bleed-off valve 27 described above.
The result of synthesizing the flow rate characteristics of is shown. FIG. 6 shows a case where the bleed-off valve 24 has the groove 40 shown in FIGS. 3a and 3b and has the flow rate characteristic shown in FIG. 5b.
This is the case where 24 has the groove 41 shown in FIGS. 4a and 4b and has the flow rate characteristic shown in FIG. 5c.

Unlike the conventional unloading valve, the unloading valve 20 can cooperate with the bleed-off valve 24 so that the minimum discharge amount of the hydraulic pump 1 can be completely discharged, so that the opening area for the same ΔP is the same as that of the conventional unloading valve. Is getting smaller. The opening area A0 is set when the sum of the opening areas of the unload valve 20 and the bleed-off valve 24 is the set value ΔP3.

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

Now, consider a case where one of the directional control valves 5A and 5B, for example, the directional control valve 5A is operated to move the hydraulic cylinder 6A at a constant speed. At this time, the switching valve of the pump regulator device 13
15 is located at a substantially central switching position due to the balance between the pump discharge pressure, the maximum load pressure and the spring 18, and at this position the tilted position of the swash plate 2, that is, the pump capacity is controlled to be constant.
Here, if the other directional control valve 5B is operated to move the hydraulic cylinder 6B, the pump discharge amount is momentarily insufficient and the pump discharge pressure is reduced. As a result, the switching valve 15 has the swash plate 2
Is switched to the right side in the figure so as to increase the tilt position.
When the pump discharge rate increases and reaches the required flow rate, the pump discharge pressure rises. At this time, when the switching valve 13 is switched to the original center position, the pump discharge amount is equal to or more than the required flow amount due to the transmission delay of the pressure of the pilot line 16 and the response delay of the pump regulator device 13 and the swash plate 2. Try to increase.

Here, the purpose of the bleed-off valve 24 is to prevent a pressure increase due to the flow rate difference, and the flow rate to be bleed-off is small. Therefore, the valve size can be reduced, resulting in high responsiveness.

Therefore, when the actual discharge amount is too large with respect to the required flow rate, the pump discharge pressure tries to further increase, but the differential pressure ΔP increases accordingly, and the set value ΔP2 of the spring 18 is slightly exceeded. When the set value ΔP4 of the spring 27 is reached, the bleed-off valve 24 immediately operates, and a part of the pump discharge amount is released to the tank to prevent an excessive increase in the differential pressure ΔP.
As a result, the feedback differential pressure to the switching valve 15 of the pump regulator device 13 does not exceed a certain constant value, too much control of the pump displacement does not easily occur, and hunting of the pump displacement control can be suppressed.

Another embodiment of the present invention will be described with reference to FIG. In this embodiment, a bleed-off valve is coaxially built in the unload valve, and both are integrated.

Referring to FIG. 8, in the bleed-off valve / unload valve 49 of this embodiment, a spool 51 for an unload valve is slidably incorporated in a body 50, and a spool 52 for a bleed-off valve slides in the spool 51. Freely integrated, spool 51,5
Common pressure chambers 53 and 54 are provided at both ends of 2. The body 50 is provided with three ports 55, 56 and 57, the pressure chamber 53 communicates with the port 55 through a passage 58 provided in the body 50, and the pressure chamber 54 directly communicates with the port 50. Port 55
Is connected to the discharge line 3 in FIG. 1, the pump discharge force Ps is introduced, the port 56 is connected to the pilot line 26 in FIG. 1, the maximum load pressure PLmax is introduced, and the port 57 is connected to the tank. It Further, a groove 59 is provided on the outer periphery of the spool 51, and when the spool 51 moves to the left from the position shown in the figure, the port 55 and the port 57 communicate with each other via this groove. The passage 58 may be provided in the spool 51.

Passage 60 that communicates with port 55 on spool 52 and port 57
And a passage 61 communicating with the
62 is provided, and when the spool 52 is moved to the left from the position shown in the drawing, the passage 60 and the passage 61, and thus the port 55 and the port 57 communicate with each other through this groove.

A spring 63 for urging the spool 51 to the right in the drawing is arranged in the pressure chamber 54 in the body 50, and a spring 64 for urging the spool 52 to the right in the drawing is arranged in the pressure chamber 54 in the spool 51. Further, inside the spool 51, a stopper 65 that restricts the stroke of the spool 52 to the right in the figure and a stopper 66 that receives the load of the spring 64 are provided.

A groove similar to the groove 40 or 41 shown in FIGS. 3a and 4a is formed at the end of the groove 62 of the spool 52 for the bleed-off valve on the port 57 side. Now, the differential pressure ΔP (= Ps-PLma
x) increases and exceeds the set value ΔP2 of the spring 18 (see FIG. 1) and further exceeds the set value ΔP4 of the spring 64 (see FIGS. 5b and 5c), the spool 52 compresses the spring 64. Then, the inside of the spool 51 is moved to the left in the drawing. As a result, port 55
The pressure oil guided to the passage passes through the passage 60 and the groove 62, flows through the passage 61 to the port 57, and flows out to the return circuit. Further differential pressure Δ
When P increases and approaches the set value ΔP3 of the spring 63, the spool
51 compresses the spring 63 and moves it in the body 50 to the left in the drawing, so that the pressure oil from the port 55 flows into the port 57 through the groove 59. Therefore, also in the present embodiment, as in the first embodiment, when the pump discharge pressure suddenly rises due to a control error, the spool 52 for the bleed-off valve, which is light in weight, is immediately released.
Acts to regulate the pressure rise and prevent hunting.

Further, according to the present embodiment, since the unload valve and the bleed-off valve are integrated, a compact valve structure can be obtained, and the conventional unload valve can be replaced with this, without changing the circuit. Can be implemented.

Still another embodiment of the present invention will be described with reference to FIGS. In the figure, the same members as those shown in FIG. 1 are designated by the same reference numerals. In this embodiment, a bleed-off function is incorporated in the spool of the unload valve.

That is, in FIG. 9, the discharge pipe line 3 of the hydraulic pump 1 is shown.
The unload valve 70 with a bleed-off function is arranged in place of the unload valve 20 of FIG. 1, and the bleed-off valve 24 of FIG. 1 is not arranged. A spring 71 is provided on the side where the maximum load pressure of the unload valve 70 is fed back. When the pressure difference between the pump discharge pressure and the maximum load pressure becomes higher than the value set by the spring 71, the unload valve 70 opens and the pump discharge amount flows out.

The overall structure of the unload valve 70 with bleed-off function is the second
It is substantially the same as the bleed-off valve 24 of the first embodiment shown. However, since it is basically an unloading valve, the valve dimension, that is, the diameter of the spool, is the diameter required for the unloading valve, that is, almost the entire minimum discharge amount of the hydraulic pump 1 can flow out at the set value ΔP3. It has a diameter.

FIG. 10 shows an enlarged view of a portion corresponding to the enlarged view of the part III in FIG. Reference numeral 72 is a body, 73 is a spool slidable in the body 72, 74 is an unloading groove provided around the spool, and 75 is an outlet port. A second groove that opens prior to the groove 74, that is, a bleed-off groove 76, is provided at the end of the groove 74 on the outlet port 75 side.
Similar to the groove 40 shown in FIG. 3b, it comprises a number of notches arranged in the circumferential direction, and these notches are V-shaped both in the depth direction and the length direction. However, the length of this groove is shorter than the groove 40 of the bleed-off valve 24, as can be seen from the comparison between FIG. 3a and FIG. This bleed-off groove is a rectangular groove 77 having a substantially constant depth and width as shown in FIG. 11 like the groove 41 shown in FIGS. 4a and 4b of the first embodiment. Good.

Unload valve with bleed-off function configured in this way
The flow rate characteristic of 70 will be described with reference to FIGS. 12a to 12c. 12th
Similar to FIG. 5a of the first embodiment, FIG. a shows the flow characteristic of the conventional unload valve, ΔP1 on the horizontal axis is the set value of the spring 10,
ΔP2 is the set value of the spring 18, and ΔP3 is the set value of the spring 23.
A0 on the vertical axis is an opening area capable of flowing out the entire minimum discharge amount of the hydraulic pump 1 when the differential pressure ΔP is at the set value ΔP3.

FIG. 12b shows a flow rate characteristic of the unload valve 70 with the bleed-off function having the groove 76 shown in FIG. When the differential pressure ΔP reaches the set value ΔP4 of the spring 71, which slightly exceeds the set value ΔP2, the V-shaped groove 76 for bleed-off starts to open, and the opening area increases with the increase of the differential pressure ΔP and the characteristics of the conventional unload valve. Increase with a gradient smaller than the line, the differential pressure ΔP further increases,
When approaching the set value ΔP3, the groove 74 for unloading opens, and the opening area thereof increases with the increase of the differential pressure ΔP with the same gradient as the characteristic line of the conventional unloading valve. The opening area A1 when the characteristic line of the groove 76 intersects the characteristic line of the groove 74 is about 1/3 of the above-mentioned opening area A0 of the unload valve.

FIG. 12c shows a flow rate characteristic of the unload valve 70 with a bleed-off function having the groove 77 shown in FIG. For groove 77,
Similar to FIG. 5c, after opening the valve at the set value ΔP4, the initial opening area A1 is secured.

Unload valve with bleed-off function configured in this way
The bleed-off groove 76 is opened in the 70 prior to the opening of the unloading groove 74, and a small flow rate outflows to the tank compared to the outflow rate at the time of unloading.

Therefore, also in the present embodiment, when the actual discharge amount is too large with respect to the required flow rate of the hydraulic pump 1, the pump discharge pressure increases, the differential pressure ΔP exceeds the set value ΔP2 of the spring 18, and the spring 27 If the set value ΔP3 is exceeded, the bleed-off groove
76 or 77 opens to prevent the differential pressure ΔP from rising excessively. As a result, the feedback differential pressure to the switching valve 15 of the pump regulator device 13 does not exceed a certain constant value, too much control of the pump displacement does not easily occur, and hunting of the pump displacement control can be suppressed.

Still another embodiment of the present invention will be described with reference to FIGS. 13 and 14. In the figure, the same members as those shown in FIG. 1 are designated by the same reference numerals. In this embodiment, springs for urging the valve spool are provided in two stages to provide an unload valve with a bleed-off function.

In FIG. 13, an unload valve 80 with a bleed-off function is installed in the discharge line 3 of the hydraulic pump 1 as in the above-described embodiment shown in FIG. Springs 81 and 82 are provided in two stages on the side where the maximum load pressure of the unload valve 80 is fed back, and one spring 82 comes into contact with the valve spool after the unload valve 80 has moved a predetermined stroke, and acts. It is like this.

The structure of the unload valve 80 with a bleed-off function is shown in FIG. The unload valve 80 is basically the same as the bleed-off valve 24 shown in FIG. 2, and includes a body 90, a spool 91,
It has pressure chambers 92, 93, three ports 94, 95, 96, an internal passage 97, and a groove 98 on the outer circumference of the spool. However, like the unload valve 70 in the embodiment of FIG. 9, the unload valve 90 has a valve size that allows almost all of the minimum discharge amount of the hydraulic pump to flow out. Further, no groove is formed at the end of the groove 98 prior to this. The above-mentioned two springs 81 and 82 are arranged in the pressure chamber 93, and the spring 81 is always in contact with the spool and urges it to the right in the drawing, and the spring 82 is a stopper located at the stepped portion 99 of the body 90. It is held at 100. A step portion 101 is provided at the same side end portion of the spool 91, and can come into contact with the inner peripheral portion of the stopper 100.

Initially, the spool 91 moves in balance with the spring 81, the pump discharge pressure Ps, and the maximum load pressure PLmax. Therefore,
When the differential pressure ΔP exceeds the set value ΔP4 of the spring 81, the spool 91 moves until it contacts the stopper 100, and the port 94 and the port 96.
Communicate with a relatively small opening area A1. The differential pressure ΔP is a spring
The pre-compression force of 82, that is, the opening area does not change even if the differential pressure ΔP increases until the initial load is exceeded, and the opening area is constant.
Hold A1. When the differential pressure ΔP exceeds the initial load of the spring 82, the spool 91 moves again and the opening area increases. This flow rate characteristic is almost the same as in FIG. 12c described above.

Therefore, according to the present embodiment as well, similar to the above-described embodiment of FIG. 9, it is possible to prevent an excessive increase in the differential pressure ΔP and prevent hunting in pump displacement control.

Still another embodiment of the present invention will be described with reference to FIGS. 15 and 16. In this embodiment, the set value of the differential pressure when the pump discharge amount is bleed off is changed.

That is, FIGS. 15a and 15b correspond to FIGS. 5b and 5c of the embodiment shown in FIG. 1, respectively.
In Figures and 5c, the bleed-off valve 25 (see Figure 1)
Is a V-shaped groove 40 (see FIG. 3a) or a rectangular groove when the differential pressure ΔP reaches a set value ΔP4 which slightly exceeds the set value ΔP2.
41 (see FIG. 4a) starts to open, and a small flow rate is allowed to flow into the tank prior to opening the unload valve 20 (see FIG. 1). In this embodiment, the spring 27 (see FIG. 1) is used. (See FIG. 2), the groove 40 or 41 starts to open when the differential pressure ΔP reaches a set value ΔP5 smaller than the set value ΔP2.

FIGS. 16a and 16b correspond to FIGS. 12b and 12c of the embodiment shown in FIG. 9, respectively.
In Fig. 12 and Fig. 12c, a V-shaped groove 76 for bleed-off of the unload valve 70 with a bleed-off function (see Fig. 9) is shown.
(See FIG. 10) or rectangular groove 77 (see FIG. 11) starts to open when the differential pressure ΔP reaches a set value ΔP4 slightly exceeding the set value ΔP2, and opens for unloading groove 74 (see FIG. 10 and 11th
Although the small flow rate is made to flow out to the tank prior to the valve opening (see FIG. 9), in this embodiment, the setting of the spring 71 (see FIG. 9) is changed to obtain the flow rate characteristics shown in FIGS. 16a and 16b. Similarly, when the differential pressure ΔP reaches a set value ΔP5 smaller than the set value ΔP2, the groove 76 or 77 starts to open.

As an example, as described above, the set value ΔP1 is 8 kg / cm ² ,
When the set value ΔP2 is 15 kg / cm ² and the set value ΔP3 is 25 kg / cm ² , the set value ΔP5 is a few kg / cm ² lower than the set value ΔP2,
For example, 12 kg / cm ² .

The other structure is the same as that of the embodiment shown in FIG. 1 or 9.

In the embodiment described above, the set value Δ for starting the bleed-off is set.
Since P4 is set higher than the set value ΔP2 to start unloading, bleed-off is performed in normal operation in which the pump regulator device 13 is controlled so that the pump discharge pressure is kept higher than the maximum load pressure by the set value ΔP2. The valve 25 or the bleed-off groove 76 or 77 does not function, unnecessary flow rate leakage from the bleed-off means does not occur, and pump discharge flow rate loss does not occur.

On the other hand, in the present embodiment, since the set value ΔP5 is set lower than the set value ΔP2, the bleed-off means functions during normal operation and some flow rate leakage occurs, but the opening area A1
By adjusting the setting of, the leakage of the flow rate can be minimized, and in this embodiment, since the excessive flow rate flows out before the differential pressure exceeds the set value ΔP2, the excessive rise of the differential pressure is more effective. Therefore, hunting for pump capacity control can be prevented more reliably, and stable load sensing control can be performed.

In the above embodiments, the pressure for operating each valve is a constant value determined by each spring, but the present invention can be applied even when the pressure is changed by applying an external pressure or a pressing force. Would be obvious.

〔The invention's effect〕

According to the present invention, it is possible to prevent hunting of a hydraulic pump subjected to load sensing control due to a sudden change in pressure due to a response delay in displacement control, and to perform stable load sensing control.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a hydraulic drive system according to an embodiment of the present invention, FIG. 2 is a sectional view of the bleed-off valve of FIG. 1, and FIG. 3 (a) is a portion III of FIG. FIG. 3 (b) is an expanded plan view of the same portion, and FIGS. 4 (a) and 4 (b) are enlarged views and plan views showing another configuration example of the III portion, respectively. FIG. 5a is an exploded view, FIG. 5a is a flow characteristic diagram of a conventional unload valve, and FIG.
FIG. 5c is a diagram showing a flow rate characteristic of the bleed-off valve of the present embodiment corresponding to FIGS. 4a and 4b, respectively.
FIG. 7 and FIG. 7 are views showing the combined characteristics of the flow rate characteristics of the unload valve and the bleed-off valve of this embodiment corresponding to FIG. 5b and FIG. 5c, respectively, and FIG. FIG. 9 is a sectional view of a valve structure in which an unload valve and a bleed-off valve according to another embodiment are integrated, and FIG. 9 is a circuit diagram of a hydraulic drive system according to still another embodiment of the present invention.
FIG. 10 and FIG. 11 are enlarged views of the essential parts of the unload valve with a bleed-off function in the embodiment of FIG. 9, corresponding to FIG. 3a and FIG. 4a, respectively.
FIG. 12 is a flow rate characteristic diagram of a conventional unload valve, and FIGS. 12b and 12c are diagrams showing the flow rate characteristic of the bleed-off valve of this embodiment corresponding to FIGS. 10 and 11, respectively. FIG. 13 is a circuit diagram of a hydraulic drive system according to still another embodiment of the present invention, FIG. 14 is a cross-sectional view of the bleed-off unloading valve of FIG. 13, and FIGS. 15a and 15b. Figures are diagrams showing flow characteristics according to yet another embodiment of the present invention, which correspond to Figures 5b and 5c respectively, and Figures 16a and 16b show Figures 12b and 12c, respectively. FIG. 9 is a diagram showing a corresponding flow rate characteristic according to still another embodiment of the present invention. Explanation of code 1 …… hydraulic pump 2 …… swash plate (variable capacity mechanism) 3 …… Discharge pipe 5A, 5B …… Direction switching valve 6A, 6B …… Actuator 13 …… Pump regulator device 20 …… Unload valve (Pressure rise regulation means) 24 …… Bleed-off valve (bleed-off means) 49 …… Bleed-off valve and unload valve (pressure rise regulation means) 51 …… Unload spool (unload valve) 52 …… Bleed-off Spool (bleed-off valve) 70,80 …… Unload valve with bleed-off function 73 …… Valve spool 74 …… Groove (first groove means) 76,77 …… Bleed-off groove (second groove) 81) Spring (first spring means) 82 ... Spring (second spring means) 91 ... Valve spool 100 ... Stopper ΔP ... Differential pressure ΔP2 ... First set value ΔP3 ... Set value of 2 ΔP4, ΔP5 …… The third set value

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideyo Kato 650 Kazutachi-cho, Tsuchiura-shi, Ibaraki Hitachi Construction Machinery Co., Ltd. Tsuchiura factory (72) Inventor Toichi Hirata 650 Kintate-cho, Tsuchiura-shi, Ibaraki Hitachi Construction Machinery Inside the Tsuchiura Plant of the stock company (72) Hideaki Tanaka, 650 Jinrachi-cho, Tsuchiura City, Ibaraki Prefecture Hitachi Construction Machinery Inside the Tsuchiura Plant of the stock company (72) Genroku Sugiyama, 650 Kintate-cho, Tsuchiura City, Ibaraki Hitachi Construction Machinery Co., Ltd. Ceremony Company Tsuchiura Plant (72) Inventor Yusuke Kajita 650 Kintatecho, Tsuchiura City, Ibaraki Prefecture Hitachi Construction Machinery Co., Ltd.Tsuchiura Factory (72) Inventor Kazunori Nakamura 650 Kintatecho, Tsuchiura City, Ibaraki Hitachi Construction Machinery Co., Ltd. Tsuchiura Factory (56) References JP-A-56-20803 (JP, A) JP-A-1-247805 (JP, A) JP-A 59-226702 (JP, A) Actually developed JP-A-56-54301 (JP, A) U)

Claims (9)

[Claims] 1. A variable displacement hydraulic pump, a plurality of actuators driven by pressure oil discharged from the hydraulic pump, and a flow of pressure oil supplied from the hydraulic pump to the plurality of actuators, respectively. A plurality of directional control valves, and when the directional control valves are all in the neutral position, the hydraulic pump is held at a minimum capacity, and the discharge pressure of the hydraulic pump is set to a first set value higher than the maximum load pressure of the actuator. Pump regulator means for controlling the displacement of the hydraulic pump so as to keep it as high as possible, and an unload valve that is connected to the discharge pipe line of the hydraulic pump and operates in response to the differential pressure between the pump discharge pressure and the maximum load pressure. And when the differential pressure reaches a second set value that is larger than the first set value, almost all of the discharge amount of the hydraulic pump in the minimum capacity is discharged. In the hydraulic drive system including a pressure increase regulating means, a third set value in which the differential pressure is smaller than a value near the first set value, which is equal to or less than the second set value, is set in the pressure increase regulating means. If the bleed-off means is exceeded, the unload valve is opened prior to opening the unload valve, and a bleed-off means is provided to allow a small flow rate to flow out compared to the outflow flow rate when the second set value is reached. Drive system. 2. The hydraulic drive system according to claim 1, wherein the bleed-off means is connected to a discharge line of the hydraulic pump and operates in response to a pressure difference between the pump discharge pressure and the maximum load pressure. The hydraulic drive system is a bleed-off valve that is separate from the unload valve. 3. The hydraulic drive system according to claim 1, wherein the bleed-off means is coaxially built in the unload valve, and is responsive to a differential pressure between the pump discharge pressure and the maximum load pressure. A hydraulic drive system, which is a bleed-off valve that operates. 4. The hydraulic drive system according to claim 1, wherein the unload valve is provided with a bleed-off function for discharging the small flow rate when the differential pressure exceeds a third set value. A hydraulic drive system characterized in that 5. The hydraulic drive system according to claim 4, wherein the unload valve has a valve spool provided with a first groove means for providing a relatively large opening area for flowing substantially all of the pump discharge amount. The valve spool of the unload valve is further provided with second groove means which is opened prior to the opening of the first groove means and which provides a relatively small opening area for outflowing the small flow rate. Characteristic hydraulic drive system. 6. The unload valve includes a valve spool on which the differential pressure acts in a valve opening direction, and a first spring means for urging the valve spool against the differential pressure. The initial load of the spring means of No. 1 is set so that when the differential pressure reaches the second set value, the valve spool is allowed to move in the valve opening direction and almost the entire pump discharge amount is discharged. Four
In the hydraulic drive system described above, the unload valve includes a second spring unit that biases the valve spool against the differential pressure, and the first spring unit before the valve spool moves a predetermined stroke. Means for separating the valve spool from the valve spool and contacting the valve spool when a predetermined stroke is moved to transmit the biasing force of the first spring means to the valve spool. The first load is adapted to allow the valve spool to move in the valve opening direction when the differential pressure exceeds the third set value.
Is set to be smaller than the initial load of the spring means, so that until the differential pressure exceeds the second set value, it is held at the position of the predetermined stroke by the stopper and the small flow rate flows out. Hydraulic drive system. 7. The hydraulic drive system according to claim 1, wherein the third set value of the differential pressure is larger than the first set value. 8. The hydraulic drive system according to claim 1, wherein the third set value of the differential pressure is smaller than the first set value. 9. The hydraulic drive system according to claim 1, wherein the opening area when the bleed-off means opens the valve and the small flow rate flows out, the pressure increase regulating means flows out almost all of the pump discharge rate. About 1/3 of the opening area when
A hydraulic drive system characterized in that: