JPH05346101A - Hydraulic transmission device for construction equipment - Google Patents

Abstract

PURPOSE:To vary metering characteristics of changeover valve groups in a hydraulic transmission device for a construction equipment by setting the pressure compensation valves installed on center bypass passages variably through operation from outside. CONSTITUTION:Pressure compensation valves 19 and 20 are provided on center bypass passages 1a and 2a on downstreams of changeover valve groups 51 and 52. Hydraulic driving parts 32, 33 are provided on the pressure compensation valves 19 and 20 as a target pressure difference setting means. The target set pressure difference is fluctuated according to a control pressure output from a solenoid proportional control valve 35, for varying metering characteristics of the changeover valves. A setting variation operation device 36 is arranged in an operation chamber of a hydraulic shovel. The solenoid proportional control valve 35 is driven according to a command signal from the setting variation operation device 36, for varying the target set pressure difference of the pressure compensation valves.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic drive system for a construction machine such as a hydraulic excavator, and more particularly to a variable bypass valve bleed-off valve in a center bypass passage for connecting the switching valves of a switching valve group in series. The present invention relates to a hydraulic drive system for a construction machine equipped with a pressure compensation valve that controls a differential pressure across the throttle.

[0002]

2. Description of the Related Art A hydraulic drive system for a construction machine in which a pressure compensating valve for controlling a differential pressure across a variable throttle for bleed-off of each switching valve is installed in a center bypass passage connecting each switching valve of a switching valve group in series. As a conventional technique,
There is one described in Japanese Patent No. 2 publication. This prior art is based on a variable displacement hydraulic pump, a plurality of hydraulic actuators driven by pressure oil from the hydraulic pump, and a switching valve for controlling the flow of pressure oil supplied from the hydraulic pump to the plurality of hydraulic actuators. Group, a center bypass passage connecting each switching valve of the switching valve group in series, a variable throttle for bleed-off that is provided in each switching valve of the switching valve group and adjusts the opening degree of the center bypass passage, and a center bypass. A pressure compensating valve installed in the passage to control the differential pressure across the variable throttle for bleed-off, a fixed throttle installed in the center bypass passage at a position downstream of the pressure compensating valve to generate a signal pressure, and the signal pressure And a pump regulator for changing the displacement of the hydraulic pump.

In this conventional hydraulic drive system, the pump regulator for controlling the displacement of the hydraulic pump performs well-known negative control by the signal pressure generated by the fixed throttle. That is, the opening area of the bleed-off variable throttle gradually becomes smaller according to the stroke amount of the directional control valve, and finally becomes fully closed, but in this process the flow rate through the center bypass passage decreases. The signal pressure generated at the fixed throttle becomes smaller, and the pump regulator operates accordingly to increase the discharge flow rate of the hydraulic pump.
The bleed-off variable throttle characteristic and the metering characteristic of the pressure oil supplied to the actuator are determined.

That is, when one of the plurality of directional control valves is operated, the discharge flow rate of the hydraulic pump increases as the spool stroke increases as described above, but at the same time, as the spool stroke increases, As the opening areas of the meter-in variable throttle and meter-out variable throttle of the directional control valve increase, and the opening area of the bleed-off variable throttle decreases, the hydraulic pump flows out to the tank through the center bypass passage. The discharged flow rate is decreasing, and the discharge pressure of the hydraulic pump is increasing. Then, when the pressure of the pump port of the directional control valve becomes larger than the load pressure applied to the actuator, pressure oil from the hydraulic pump begins to flow into the actuator side, and then flows out from the pump to the tank through the center bypass passage. The flow rate further decreases, and accordingly, the flow rate flowing into the actuator side, that is, the flow rate obtained by subtracting the flow rate flowing out to the tank through the center bypass passage from the pump discharge flow rate increases. This is generally called bleed-off control.

Here, since the pressure compensating valve installed in the center bypass passage controls the differential pressure across the bleed-off variable throttle of each directional control valve so as to be constant, it passes through the bleed-off variable throttle. The flow rate flowing out to the tank has a magnitude determined by the opening area (stroke amount of the directional control valve) of the bleed-off variable throttle regardless of the magnitude of the pump discharge pressure, that is, the magnitude of the load pressure. That is, the flow rate flowing into the actuator side is not affected by the load pressure,
It will have so-called load compensation characteristics.

[0006]

By the way, in the above-mentioned prior art, since the setting of the pressure compensating valve is constant, as long as the pump flow rate characteristic is constant, the metering characteristic is also constant. There is a problem in that the metering characteristics cannot be changed according to operator preference.

That is, in the work performed by the hydraulic excavator,
There are times when you want to change the metering characteristics according to the work content, work environment, or operator preference.For example, when performing a fine operation, you want to reduce the actuator speed change even with the same stroke of the operating lever, and In large loading work, we want to increase the speed change of the actuator with respect to the stroke amount to improve work efficiency. However, since the metering characteristic is constant in the above-mentioned conventional technique, it is difficult to perform such fine operation or loading operation with desired characteristics.

It is an object of the present invention to make it possible to change the metering characteristic of the switching valve group by changing the setting of the pressure compensating valve provided on the center bypass passage by an external operation. A drive device is provided.

[0009]

In order to achieve the above object, according to the present invention, a hydraulic pump, a plurality of hydraulic actuators driven by pressure oil from the hydraulic pump, and a plurality of hydraulic pumps from the hydraulic pump. A switching valve group that controls the flow of pressure oil supplied to the hydraulic actuator, a center bypass passage that connects the switching valves of the switching valve group in series, and a switching valve of the switching valve group that are provided in the switching valve group. Hydraulic drive device for a construction machine including a bleed-off variable throttle for adjusting the opening of a bypass passage, and a pressure compensation valve installed in the center bypass passage for controlling the differential pressure across the bleed-off variable throttle. In (a) setting change operating means operated by an operator to instruct to change the metering characteristic of the switching valve group; and (b) from the setting change operating means. Decree signal and setting changing drive means for changing the target set differential pressure of the pressure compensating valve according to; hydraulic drive system for a construction machine characterized in that it comprises a are provided.

The setting change driving means preferably drives an electromagnetic proportional pressure reducing valve for generating a control pressure according to the command signal and a control force for generating a control force according to the control pressure to drive the pressure compensating valve. Including and

In the hydraulic drive system, the hydraulic pump is preferably a variable displacement hydraulic pump,
In this case, the hydraulic drive device further includes a flow resistance unit installed in the center bypass passage for generating a signal pressure, and a pump regulator for changing the displacement of the hydraulic pump according to the signal pressure.

The flow resistance means preferably also includes a fixed throttle.

[0013]

In the above structure, the pressure compensating valve controls the differential pressure across the bleed-off variable throttle built in each switching valve to be constant, so that it flows out to the tank through the bleed-off variable throttle. The flow rate is determined by the opening area of the bleed-off variable throttle (stroke amount of the switching valve) regardless of the pump discharge pressure, that is, the load pressure. Therefore, the flow rate flowing into the actuator side is not affected by the load pressure and has a load compensation characteristic, and the metering characteristic is determined by the pump flow rate characteristic and the opening characteristic of the bleed-off variable throttle.

In the hydraulic drive system for performing such bleed-off control, according to the present invention, by providing the setting change operating means and the setting change driving means as described above, when the operator operates the setting change operating means, a corresponding command signal is issued. Is output to the setting change driving means, and the setting change driving means changes the target set differential pressure of the pressure compensation valve according to the command signal. As a result, the bleed-off flow rate characteristic changes even if the opening characteristic of the variable throttle for bleed-off is constant.
Correspondingly, the metering characteristic of the switching valve group is changed. That is, the metering characteristics can be changed according to the work content, work environment, and operator preference.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. In these embodiments, the present invention is applied to a hydraulic drive system for a hydraulic excavator.

First, a first embodiment of the present invention will be described with reference to FIGS.
Will be explained.

Referring to FIG. 1, the hydraulic drive system of this embodiment comprises variable displacement hydraulic pumps 1 and 2, pump regulators 3 and 4 for controlling displacement of the hydraulic pumps 1 and 2, and hydraulic pumps 1 and 2. A plurality of hydraulic actuators 40, 41, 42, 43, which are driven by pressure oil from 2.
44, 45, 46, 47, 48, a tank 49 that constitutes the low-pressure circuit 29, a valve device 50 installed between the hydraulic pumps 1 and 2, the actuators 40 to 48, and the tank 49.
It has and.

The valve device 50 includes a plurality of center bypass type directional control valves 5 for controlling the flow of pressure oil supplied from the hydraulic pump 1 to the plurality of hydraulic actuators 40 to 43.
First switching valve group 51 including 6, 7, and 8 and hydraulic pump 2
Second switching valve group 52 including a plurality of center bypass type directional switching valves 9, 10, 11, 12, 13 for controlling the flow of pressure oil supplied to the plurality of hydraulic actuators 44 to 48 from the hydraulic pump. 1 and a center bypass passage 1a that connects the center bypasses of the directional switching valves 5 to 8 of the first switching valve group 51 in series to the tank 49, and the second bypass switching valve group that is connected to the hydraulic pump 2. A center bypass passage 2a for connecting the center bypasses of the directional control valves 9 to 13 of 52 to the low pressure circuit 29 including the tank 49 in series.
A pressure compensation valve 19 installed in the center bypass passage 1a at a position downstream of the first switching valve group 51, and a downstream position of the second switching valve group 52 adjacent to the most downstream directional switching valve 13. Of the pressure compensating valve 20 installed in the center bypass passage 2a, and the fixed throttle 15 installed in the center bypass passage 1a at a position further downstream of the pressure compensating valve 19 and generating the signal pressure Pc1. Relief valve 17 that controls so that the signal pressure that does not exceed the specified pressure
And a fixed throttle 16 installed in the center bypass passage 2a at a position further downstream of the pressure compensating valve 20 to generate the signal pressure Pc2, and the signal pressure generated by the fixed throttle 16 is controlled so as not to exceed the specified pressure. A relief valve 18,
Relief valves 30a, which are connected to the center bypass passages 1a, 2a at positions upstream of the first and second switching valve groups 51, 52, respectively, and control so that the discharge pressures of the hydraulic pumps 1, 2 do not exceed specified values. 30b and. The pump regulators 3 and 4 are fixed throttles 15 and 16 respectively.
The displacements of the hydraulic pumps 1 and 2 are changed according to the signal pressure generated in step 1 to control the discharge flow rate of the hydraulic pump 1.

Hydraulic actuators 40, 41, 42, 4
3, 44, 45, 46, and 48 are provided as actuators for a traveling right motor, a bucket cylinder, a boom cylinder, an arm cylinder (merging), a swing motor, an arm cylinder, a boom cylinder (merging), and a traveling left motor, respectively. Has been. The hydraulic actuator 47 is a removable attachment hydraulic motor, and thus the directional control valve 12 is a spare for that attachment.

The directional control valves 5 to 13 are respectively metered-in variable throttles 54a and 54b (hereinafter referred to as 54) as shown in FIG.
And a meter-out variable diaphragm 55a, 55
b (hereinafter represented by 55) is formed, and a bleed-off variable diaphragm 56 is provided in the center bypass thereof. The relationship of the opening area A with respect to the spool stroke (operation amount of the directional control valve) S of the directional control valve in the meter-in variable throttle 54, the meter-out variable throttle 55, and the bleed-off variable throttle 56 is as shown in FIG. is there. In the figure, 57 and 58 are the characteristics of the opening area of the meter-in variable diaphragm 54 and the meter-out variable diaphragm 55, and 59 is the characteristics of the opening area of the bleed-off variable diaphragm 56. The meter-out variable throttle 55 is fully closed when the spool stroke is 0 (when the directional control valve is in the neutral position), and the opening area is increased as the spool stroke is increased. The variable throttle 56 is fully opened when the spool stroke is 0, and the opening area is reduced as the spool stroke increases.

As described above, the variable aperture 56 for bleed-off is provided.
By setting the opening characteristic of the directional control valve 5,
Is in the neutral position, the flow rate flowing through the center bypass passage 1a (center bypass flow rate) is maximum, the signal pressure Pc1 generated by the fixed throttle 15 is also maximum, and the operation amount of the directional control valve 5 is increased. Decreases and the signal pressure Pc1 also decreases.
In response to the signal pressure Pc1, the pump regulator 3 minimizes the displacement of the hydraulic pump 1 when the signal pressure Pc1 is maximum, and increases the displacement of the hydraulic pump 1 as the signal pressure Pc1 decreases. Control. As a result, the discharge flow rate Q of the hydraulic pump 1 is controlled so as to increase according to the stroke amount S of the directional control valve 5, as shown by the characteristic line 70 in FIG.

Although the directional control valve 5 has been described above, the same applies to the other directional control valves 6 to 8, and the same applies to the directional control valves 9 to 13 of the second switching valve group 52. ..

As shown in FIG. 5, the pump regulator 3 is a displacement volume variable member of the hydraulic pump 1, for example, a swash plate 6.
A piston / cylinder device 61 for driving 0, and a first for adjusting the flow rate of the pressure oil supplied to the piston / cylinder device 61 in response to the signal pressure Pc1 to control the swash plate tilt amount of the hydraulic pump 1. With the servo valve 62, the tilt amount of the swash plate 60 is increased so that the displacement of the hydraulic pump 1 is increased as the signal pressure Pc1 is decreased from the maximum as described above by the operation of the first servo valve 62. Is controlled. Further, the pump regulator 3 adjusts the flow rate of the pressure oil supplied to the piston / cylinder device 61 in response to the pump discharge pressure, and controls the swash plate tilting amount of the hydraulic pump 1 by the second input torque limiter. The servo valve 63 is provided. The pump regulator 4 is similarly configured.

The pressure compensating valve 19 is configured to give a load compensating function to the directional switching valves 5 to 8 of the first switching valve group 51. That is, the first differential pressure detection line 21 that introduces the hydraulic pressure into the hydraulic drive unit for operating the pressure compensating valve 19 in the valve closing direction, that is, the pressure receiving chamber is connected to the center bypass passage 1a at a position upstream of the first switching valve group 51. The second differential pressure detection line 23 for introducing the hydraulic pressure into the hydraulic drive unit for operating the pressure compensating valve 19 in the valve opening direction, that is, the pressure receiving chamber is connected to the center bypass passage 1a at a position downstream of the first switching valve group 51. Therefore, regardless of which one of the directional control valves 5 to 8 is operated, the differential pressure generated at the corresponding bleed-off variable throttle 56 in accordance with the operation is the first and second differential pressure detection lines 2
1 and 2 to each driving portion of the pressure compensation valve 19,
The pressure difference between the front and rear is controlled to be constant.

The pressure compensating valve 20 is also the second switching valve group 5
The second direction switching valves 9 to 13 are configured to have a load compensation function. That is, the first differential pressure detection line 22 that introduces the hydraulic pressure into the hydraulic drive unit for operating the pressure compensation valve 20 in the valve closing direction, that is, the pressure receiving chamber is connected to the center bypass passage 2a at a position upstream of the second switching valve group 52. And pressure compensation valve 2
The second differential pressure detection line 24 that introduces the hydraulic pressure into the hydraulic drive unit that operates in the valve opening direction of 0, that is, the pressure receiving chamber is the second switching valve group 5
Even if any of the directional control valves 9 to 13 is operated by being connected to the center bypass passage 2a at a position downstream of 2, the differential pressure generated in the corresponding bleed-off variable throttle 56 in accordance with the operation is changed. It is guided to each drive section of the pressure compensating valve 20 via the first and second differential pressure detection lines 22 and 24, and is controlled so that the front-back differential pressure becomes constant.

The pressure compensating valves 19 and 20 serve as target differential pressure setting means, respectively.
And hydraulic drive units, that is, pressure receiving chambers 32 and 33. The drive units 32 and 33 are connected to the solenoid proportional control valve 3 via a control line 34.
5 and generates a control force according to the control pressure output from the electromagnetic proportional control valve 35. In the operator's cab of the hydraulic excavator, a setting change operating device 3 operated by an operator to instruct to change the metering characteristics of the switching valve groups 51 and 52.
6, a command signal from the setting change operating device 36 is input to the controller 37, and the controller 37 converts this command signal into a command signal for the electromagnetic proportional control valve 35 and outputs it. The electromagnetic proportional control valve 35 receives the command signal, generates a corresponding control pressure, and outputs it to the drive units 32 and 33.

Here, the setting change operating device 36 has three push button type switches 36a, 36b, as shown in FIG.
36a, and the switch 36a is a switch for setting a metering characteristic suitable for fine operation.
The voltage V1 is output when is pressed, the switch 36b is a switch for setting a normal metering characteristic, the command signal of the voltage V2 is output when the switch 36b is pressed, and the switch 36c is a metering suitable for a large amount of work. It is a switch for setting the characteristics. When switch 36c is pressed, voltage V3
Command signal is output. The voltages V1, V2 and V3 are shown in FIG.
As shown in, the relationship of V1 <V2 <V3 is established. In response to this command signal, the electromagnetic proportional solenoid valve 35 controls the pressures P1, P2 and
A control pressure P3 (P1 <P2 <P3) is generated.

In the above, the setting change operation device 35 constitutes a setting change operation means operated by the operator to instruct the change of the metering characteristic of the switching valve group, and includes the controller 37, the electromagnetic proportional control valve 35 and the hydraulic drive unit 32. ，
Reference numeral 33 constitutes setting change driving means for changing the target set differential pressure of the pressure compensating valves 19 and 20 in response to a command signal from the setting change operating means.

In the hydraulic drive system configured as described above, for example, in the first switching valve group 51, the directional switching valve 5
1 to 8, for example, when the direction switching valve 5 is operated, the discharge flow rate of the hydraulic pump 1 increases as the spool stroke S increases as described above, and at the same time, as the spool stroke S increases. As shown in FIG. 3, as the opening area A of the meter-in variable throttle 54 and the meter-out variable throttle 55 of the directional control valve 5 increases, the opening area A of the bleed-off variable throttle 56 decreases. Therefore, the discharge pressure of the hydraulic pump 1 increases. Then, when the pressure of the pump port of the direction switching valve 5 becomes larger than the load pressure applied to the actuator 40, the pressure oil from the hydraulic pump 1 begins to flow into the actuator 40 side, and thereafter the pump 1 passes through the center bypass passage 1a. The flow rate that has flowed out to the tank 49 gradually decreases, and accordingly, the flow rate that flows into the actuator 40 side, that is, the flow rate that is obtained by subtracting the flow rate that flows out to the tank 49 through the center bypass passage 1a from the pump discharge flow rate. Increase. This is generally called bleed-off control.

Here, in the present embodiment, the pressure compensating valve 19 controls so that the differential pressure across the bleed-off variable throttle 56 built into each switching valve becomes constant, so the bleed-off variable throttle 56 is controlled. The flow rate through which it flows out to the tank has a magnitude determined by the opening area of the bleed-off variable throttle 56 (stroke amount of the directional control valve) regardless of the magnitude of the pump discharge pressure, that is, the magnitude of the load pressure. Therefore, the flow rate flowing into the actuator side is not affected by the load pressure and has a load compensation characteristic.

FIG. 8 shows the metering characteristics of the directional control valve during bleed-off control. That is, the switch 36 for setting the normal metering characteristic with the setting change operation device 36.
If b is pressed, the characteristic of the center bypass flow rate that can flow out through the bleed-off variable throttle 56 with respect to the spool stroke S is the opening characteristic 5 shown in FIG.
Corresponding to No. 9, it becomes as shown by 59B in FIG. The discharge flow rate Q of the hydraulic pump 1 is as shown by the characteristic line 70 in FIG. 8 corresponding to FIG. 4, so the metering characteristic of the flow rate of the pressure oil supplied to the actuator 40 is as shown by 71B in FIG. become. As described above, in the bleed-off control, when the pressure compensating valve 19 has the load compensating characteristic, the metering characteristic is determined by the pump flow rate characteristic and the opening characteristic of the bleed-off variable throttle.

In this embodiment, the target set differential pressure of the pressure compensating valve can be changed, and when the switch 36b for setting the normal metering characteristic is pushed by the setting change operating device 36, the voltage V2 Command voltage is output and the control pressure of the pressure P2 is output from the electromagnetic proportional control valve 35, the voltage V1 (<V2) changes when the switch 36a that sets the metering characteristic suitable for the fine operation is pressed. A command signal is output, and in response thereto, a control pressure of pressure P1 (<P2) is output by the electromagnetic proportional control valve 35, and the control force for pushing the pressure compensating valve 19 in the valve opening direction is reduced to reduce the target set differential pressure. Function to reduce. As the target set differential pressure decreases, the characteristic of the center bypass flow rate that can flow out through the bleed-off variable throttle 56 with respect to the spool stroke S changes as indicated by 59A in FIG. 8, and the metering characteristic also changes to 71A. To change. Setting change operation device 3
When the switch 36c for setting the metering characteristic suitable for a large amount of work is pressed in 6, the voltage V3 (> V2) is output, and in response to this, the pressure P3 is applied from the electromagnetic proportional control valve 35.
The control pressure of (> P2) is output, and the control force for pushing the pressure compensating valve 19 in the opening direction is increased to increase the target set differential pressure. The characteristic of the center bypass flow rate that can flow out through the bleed-off variable throttle 56 with respect to the spool stroke S as the target set differential pressure increases is 59C in FIG.
And the metering characteristic also changes as in 71C.

That is, in this embodiment, by operating the setting change operation device 36, the metering characteristic of the directional control valve 5 can be changed to the characteristic 71A, 71B, 71C shown in FIG.

When the other directional control valves 6 to 8 are operated in the first directional control valve group 51 and the directional control valves 9 to 13 are operated in the second directional control valve group 52, the meter is operated in the same manner as above. The ring characteristics can be changed.

Therefore, according to the present embodiment, it is possible to improve the operability by changing the metering characteristics according to the work content, work environment and operator's preference. That is, in the case of performing a fine operation with the hydraulic excavator, when the operator presses the push button switch 36a of the setting change operation device 36, the metering characteristic is changed as the characteristic 71A in FIG. With this metering characteristic 71A, since the stroke amount for changing the flow rate from the minimum to the maximum is large, the speed change of the actuator with respect to the same stroke amount of the operating lever is small, and therefore fine operation is facilitated. Further, in the case of carrying out a loading work with a large amount of work, when the operator presses the push button switch 36c of the setting change operation device 36, the metering characteristic is changed as shown by the characteristic 71C in FIG. With this metering characteristic 71C, the stroke amount that changes the flow rate from the minimum to the maximum is small, so a large speed change can be given with a small stroke amount, and efficient loading work can be performed.

A second embodiment of the present invention will be described with reference to FIGS. 9 and 10. In the figure, the same members as those shown in FIG. 1 are designated by the same reference numerals. In this embodiment, a fixed pump is used as the hydraulic pump instead of the variable displacement type.

That is, in FIG. 9, the hydraulic drive system of this embodiment has fixed displacement hydraulic pumps 1A and 2A,
The structure of the valve device 50A for controlling the flow and pressure of the pressure oil from the hydraulic pumps 1A and 2A is the same as that of the embodiment shown in FIG.

FIG. 10 shows fixed displacement hydraulic pumps 1A, 2
The metering characteristic of the directional control valve at the time of bleed-off control when A is used is shown. That is, the setting change operation device 3
Switch 36b for setting normal metering characteristics with 6
If is pressed, the characteristic of the center bypass flow rate that can flow out through the variable throttle for bleed-off with respect to the spool stroke S is as shown by 79B in FIG. 10 corresponding to the opening characteristic of the variable throttle. .. Since the discharge flow rate Q of the hydraulic pump 1 is constant as indicated by the characteristic line 80, the metering characteristic of the flow rate of the pressure oil supplied to the actuator 40 is as indicated by 81B in FIG. In this way, even with bleed-off control when using a fixed pump,
When the pressure compensating valve 19 has a load compensating characteristic, the metering characteristic is determined by the pump flow rate characteristic and the opening characteristic of the bleed-off variable throttle.

When the setting of the setting change operation device 36 is changed and the switch 36a for setting the metering characteristic suitable for the fine operation is pushed, the pressure compensating valve 19 is opened in the same direction as in the first embodiment. Since the control force for pushing to a small value reduces the target set differential pressure, the characteristic of the center bypass flow rate that can flow out through the variable throttle for bleed-off with respect to the spool stroke S changes as shown by 79A in FIG.
The metering characteristic also changes like 81A. When the switch 36c for setting the metering characteristic suitable for a large amount of work is pushed, the control force for pushing the pressure compensating valve 19 in the valve opening direction increases, the target set differential pressure increases, and variable for bleed-off. The characteristic of the center bypass flow rate that can flow through the throttle with respect to the spool stroke S is 79C in FIG.
And the metering characteristic also changes as 81C.

That is, also in this embodiment, the metering characteristic of the directional control valve 5 can be changed to the characteristic 81A, 81B, 81C of FIG. 10 by operating the setting change operating device 36.

When the other directional control valves 6 to 8 in the first directional control valve group 51 are operated and when the directional control valves 9 to 13 in the second directional control valve group 52 are operated, the meters are operated in the same manner as above. The ring characteristics can be changed.

Therefore, according to this embodiment, the same effect as that of the first embodiment can be obtained.

In each of the above embodiments, the fixed throttles 15 and 16 are used as the flow resistance means for generating the signal pressure, but a relief valve having an override characteristic is used instead of the fixed throttles. Good.

[0044]

According to the present invention, the metering characteristic can be set to a desired characteristic according to the work content, work environment and operator's preference, and the operability can be greatly improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a hydraulic drive system for a construction machine according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a transitional position of the directional control valve shown in FIG.

3 is a diagram showing opening characteristics of a bleed-off variable throttle, a meter-in variable throttle, and a meter-out variable throttle with respect to a stroke amount of the directional control valve shown in FIG.

FIG. 4 is a diagram showing a relationship between a stroke amount of the directional control valve shown in FIG. 1 and a pump discharge flow rate.

5 is a circuit diagram showing details of the pump regulator shown in FIG. 1. FIG.

6 is a diagram showing the structure of the setting change operating device shown in FIG.

7 is a diagram showing a relationship between pushbutton switches and outputs of the setting change operating device shown in FIG.

8 is a diagram showing a metering characteristic of the directional control valve shown in FIG.

FIG. 9 is a circuit diagram of a hydraulic drive system for a construction machine according to a second embodiment of the present invention.

10 is a diagram showing a relationship between a stroke amount of the directional control valve shown in FIG. 9 and a pump discharge flow rate.

[Explanation of symbols]

 1, 2 hydraulic pump 3, 4 pump regulator 5-13 directional switching valve 15, 16 fixed throttle 19, 20 pressure compensation valve 30, 31 spring 32, 33 hydraulic drive unit (setting change drive means) 35 electromagnetic proportional pressure reducing valve (setting Change drive means) 36 Setting change operation device (setting change operation means) 37 Controller (setting change drive means) 40 to 48 Actuator 51, 52 Switching valve group

Claims (4)

[Claims] 1. A hydraulic pump, a plurality of hydraulic actuators driven by pressure oil from the hydraulic pump, and a switching valve group for controlling the flow of pressure oil supplied from the hydraulic pump to the plurality of hydraulic actuators. A center bypass passage connecting each of the switching valves of the switching valve group in series, and a bleed-off variable throttle provided in each of the switching valves of the switching valve group for adjusting the opening of the center bypass passage,
A hydraulic drive system for a construction machine, comprising: a pressure compensation valve installed in the center bypass passage for controlling a differential pressure across a variable throttle for bleed-off; (a) metering of the switching valve group operated by an operator; And (b) setting change driving means for changing a target set differential pressure of the pressure compensation valve in response to a command signal from the setting change operating means. Hydraulic drive for construction machinery. 2. The hydraulic drive system for a construction machine according to claim 1, wherein the setting change drive means is an electromagnetic proportional pressure reducing valve that generates a control pressure according to the command signal, and a control force according to the control pressure. And a drive means for driving the pressure compensation valve. 3. The hydraulic drive system for a construction machine according to claim 1, wherein the hydraulic pump is a variable displacement hydraulic pump, and the flow resistance means is installed in the center bypass passage to generate a signal pressure, and the signal. A hydraulic drive system for a construction machine, further comprising: a pump regulator that changes a displacement of the hydraulic pump according to a pressure. 4. The hydraulic drive system for a construction machine according to claim 3, wherein the flow resistance means includes a fixed throttle.