JP2966224B2 - Hydraulic drive for construction machinery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic drive provided in a construction machine such as a hydraulic excavator, and more particularly to a hydraulic drive capable of changing a discharge pressure of a variable displacement hydraulic pump defined by a relief valve to two stages. About.

[0002]

2. Description of the Related Art FIG. 11 is a hydraulic circuit diagram schematically showing one example of this type of conventional hydraulic drive system for construction machines, and FIG. 12 is a diagram showing processing in control means provided in the conventional hydraulic drive system shown in FIG. FIG. 13 is a flowchart showing the procedure, and FIG.
1 is a diagram showing characteristics of a two-stage relief valve provided in the conventional hydraulic drive device shown in FIG. 1, and FIG. 14 is a diagram showing a minimum flow rate of a variable displacement hydraulic pump set in the conventional hydraulic drive device shown in FIG.

[0003] A conventional hydraulic drive device shown in FIG. 11 includes a prime mover 1, a variable displacement hydraulic pump 2 driven by the prime mover 1, an actuator 3 driven by pressure oil discharged from the hydraulic pump 2, and a hydraulic pump. A flow control valve 4 for controlling the flow of pressure oil supplied from 2 to the actuator 3, a discharge control means 5 for controlling the discharge of the hydraulic pump 2 driven according to the discharge pressure P of the hydraulic pump 2, A two-stage relief valve 6 for regulating the discharge pressure of the hydraulic pump 2 to one of a first pressure P0 and a second pressure P1 larger than the first pressure P0; 2
Of pressure P1 selectively, ON-OFF
And a changeover switch 7 for automatically outputting an instruction signal. When the changeover switch 7 is OFF, it outputs an instruction signal corresponding to the first pressure P0,
When it is ON, it is configured to output an instruction signal corresponding to the second pressure P1 that is higher than the first pressure P0. In addition to these devices, there is provided control means 8 having a logic judgment and calculation function and outputting a drive signal corresponding to an instruction signal output from the changeover switch 7 to the drive section of the two-stage relief valve 6. I have. The maximum force of the actuator 3 described above is determined by the set pressure of the two-stage relief valve 6.

[0004] The operation of the conventional hydraulic drive device configured as described above is as follows. When the operating means for operating the flow control valve 4, for example, a neutral operating lever (not shown), or when the discharge pressure of the hydraulic pump 2 is maximized by the two-stage relief valve 6, the discharge pressure of the hydraulic pump 2 is controlled by the discharge amount control means. 5 and is controlled so that the displacement of the hydraulic pump 2 is minimized.
Is controlled so as to be the minimum flow rate Q0 shown in FIG. Now, for example, the changeover switch 7 is OFF
It is assumed that the first pressure P0 has been selected.
The control means 8 first inputs an instruction signal of the changeover switch 7 as shown in step S31 of FIG. Then step S
The process proceeds to 32, where it is determined whether or not the instruction signal output from the changeover switch 7 is ON. Since it is now OFF, this determination is not satisfied, and the routine goes to step S33. In step S33, a signal for lowering the set pressure of the two-stage relief valve 6 is output. Thereby, the set pressure of the two-stage relief valve 6 is maintained at the relatively small first pressure P0.

[0005] From the situation where the maximum discharge pressure of the hydraulic pump 2 is set to the first pressure P0, the type of the two-stage relief valve 6 is considered in consideration of the type of work performed through the operation of the actuator 3. To increase the set pressure, the changeover switch 7 is turned on. This satisfies the determination of step S32 in FIG. 12 by the control means 8, and outputs a signal for increasing the set pressure of the two-stage relief valve 6 in step 34. Accordingly, the set pressure of the two-stage relief valve 6 becomes equal to the second pressure P larger than the first pressure P0.
Can be changed to 1. FIG. 13 shows the characteristics of the two-stage relief valve 6 described above. In FIG. 13, reference numeral 9 denotes the characteristics when the changeover switch 7 is OFF, and 10 denotes the characteristics when the changeover switch 7 is ON. Q0 indicates the minimum flow rate described above.

When the flow control valve 4 is switched by operating an operation lever (not shown), the pressure oil of the hydraulic pump 2 is sent to the actuator 3 via the flow control valve 4, and the actuator 3 is driven, and When the drive device is provided in, for example, a hydraulic excavator, a desired excavation operation or the like is performed.

[0007]

In the above-mentioned prior art, the maximum discharge pressure of the hydraulic pump 2 is changed by providing a two-stage relief valve 6, which is a normal relief valve. Compared to the relief valve, the configuration is more complicated and the number of components is large.In other words, in addition to the normal relief valve configuration, special springs, pistons, pipes, pressure chambers, sliding parts, etc. for switching the set pressure are required. However, there is a problem that the manufacturing cost of the hydraulic drive device is increased.
In addition, during long-term use, dust such as iron powder generated in the hydraulic circuit accumulates in the pipeline or pressure chamber constituting the above-described two-stage relief valve 6, or accumulates in the sliding portion of the piston. There is also a problem that the operation of the piston or the like cannot realize the operation originally set, which causes deterioration of the operation accuracy and the reliability of the relief valve is likely to be reduced.

The present invention has been made in view of the above-mentioned circumstances in the prior art, and an object thereof is to provide a normal relief valve and change the discharge pressure of a variable displacement hydraulic pump to two stages. An object of the present invention is to provide a hydraulic drive device for a construction machine.

[0009]

To SUMMARY OF THE INVENTION To achieve this object, the present invention includes a prime mover, a variable displacement hydraulic pump driven by the prime mover, thereby defining a discharge pressure of the variable displacement hydraulic pump, the variable Capacity of hydraulic pump
Override feature that increases the discharge pressure as the pressure increases
Relief valve having a characteristic, an actuator driven by pressure oil discharged from the variable displacement hydraulic pump, a control valve for controlling a flow of pressure oil supplied from the variable displacement hydraulic pump to the actuator, and the control valve And a discharge amount control means for controlling a discharge amount of the variable displacement hydraulic pump .
Discharge pressure of the variable displacement hydraulic pump
And a predetermined pressure greater than the first pressure.
In the hydraulic drive system of modifiable construction machines in two stages of second pressure, the variable defined by the relief valve
Determining the first pressure of the discharge pressure of the displacement hydraulic pump;
First limit value defining the minimum flow of the variable displacement hydraulic pump that, and a value greater than the first limit value
The minimum of the variable displacement hydraulic pump that determines the second pressure
Either one of the second limit values defining the flow rate is provided with a flow rate instruction means for selectively outputting as an instruction signal,
When an instruction signal from the flow rate instructing means which corresponds to the first limit value is output, a driving signal corresponding to the first limit value the output to the discharge amount control means, said first from said flow rate instructing means when an instruction signal corresponding to the second limit value is output, a control means for outputting a driving signal corresponding to the second limit value to the discharge amount control means, said relief
The variable volume using the override characteristic of the valve
The discharge pressure of the hydraulic pump corresponds to the first limit value.
The first pressure and the second pressure corresponding to the second limit value;
The pressure can be changed in two stages .

[0010]

According to the present invention having the above-described structure,
When the first limit value is selected by the flow rate instruction means, the control means outputs a drive signal corresponding to the first limit value to the discharge amount control means. Thereby, the discharge amount control means operates,
The displacement of the hydraulic pump is controlled, and the minimum flow rate, which is the first limit value, is discharged from the hydraulic pump. Further, when the second limit value is selected by the flow rate instruction means, the control means outputs a drive signal corresponding to the second limit value to the discharge amount control means. In response to this, the discharge amount control means operates, and the hydraulic pump discharges the minimum flow rate that is the second limit value larger than the above-described first limit value. Here, the override characteristic of the normal relief valve is such that the discharge pressure of the hydraulic pump increases as the flow rate discharged from the hydraulic pump increases. Therefore, from the override characteristic of the relief valve, the discharge pressure of the hydraulic pump at the minimum flow rate as the first limit value is set to the first pressure P0, and the discharge pressure of the hydraulic pump at the minimum flow rate as the second limit value is reduced. When the discharge pressure is the second pressure P1, the second pressure P1 is larger than the first pressure P0 because the second limit value is larger than the first limit value. In this way, by selecting the first limit value and the second limit value by the flow rate instructing means, the discharge pressure of the hydraulic pump can be reduced to two stages by utilizing the inherent override characteristic of a normal relief valve. Can be switched.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a hydraulic drive device for a construction machine according to the present invention. FIG. 1 is a hydraulic circuit diagram schematically showing a first embodiment of a hydraulic drive device for a construction machine according to the present invention, and FIG. 2 shows a processing procedure in control means provided in the first embodiment shown in FIG. FIG. 3 is a flowchart showing the operation of the changeover switch and the first limit value which is the minimum flow rate of the variable displacement hydraulic pump in the first embodiment shown in FIG.
FIG. 4 is a diagram showing the override characteristic of the relief valve provided in the first embodiment shown in FIG. 1, and FIG. 5 is set in the first embodiment shown in FIG. FIG. 6 is a view showing a first limit value and a second limit value which are minimum flow rates of the variable displacement hydraulic pump. FIG. 6 is a view showing an example of an arrangement of the flow rate indicating means shown in FIG.

The first embodiment shown in FIG.
1 illustrates a hydraulic drive device provided in a hydraulic excavator, for example. In this embodiment, similarly to the embodiment shown in FIG.
A hydraulic pump 2 driven by the actuator, an actuator 3 such as a boom cylinder or an arm cylinder, and a flow control valve 4. Further, in this embodiment, a pilot pump 11 driven by the prime mover 1 in synchronization with the variable displacement hydraulic pump 2, and the pilot pump 11 is driven as a hydraulic pressure source, and a discharge amount of the hydraulic pump 2, that is,
Discharge amount control means 12 for controlling the displacement, pressure sensor 13 for detecting the discharge pressure of the hydraulic pump 2, tilt sensor 14 for detecting the tilt angle of the swash plate of the hydraulic pump 2, and discharge of the hydraulic pump 2. A normal relief valve 15 for regulating the pressure. As shown in FIG. 4, the override characteristic of the relief valve 15 is such that the discharge pressure P increases as the flow rate Q of the hydraulic pump 2 increases.

In particular, in the first embodiment, one of the first limit value Q0 that defines the minimum flow rate of the hydraulic pump 2 and the second limit value Q1 that is larger than the first limit value Q0 is set. And a flow rate instructing means for selectively instructing and outputting ON-OFF as an instruction signal, that is, a changeover switch 16. When the changeover switch 16 is OFF, it outputs an instruction signal corresponding to the first limit value Q0,
When ON, an instruction signal corresponding to a second limit value Q1 larger than the first limit value Q0 is output. It also has logical judgment, calculation, and storage functions, and has an instruction signal output from the changeover switch 16 and a pressure sensor 13.
And a control means 17 for outputting a drive signal for driving the discharge amount control means 12 in accordance with the pressure signal output from the sensor and the tilt signal output from the tilt sensor 14. The relationship between the flow rate Q of the hydraulic pump 2 and the discharge pressure P shown in FIG. 5 is stored in advance in the control means 17, and the tilt of the swash plate corresponding to the flow rate of the hydraulic pump 2 is stored in advance. ing.

The discharge amount control means 12 includes a control spool 19 in which the small-diameter chamber 18 is connected to the pilot pump 11 to control the displacement of the hydraulic pump 2, a small-diameter chamber 18 of the control spool 19, and a large-diameter chamber. An electromagnetic control valve 22 disposed in a conduit 21 communicating with the chamber 20; and a conduit 2 communicating the large diameter chamber 20 of the control spool 19 with the tank 23.
4 and an electromagnetic control valve 25 disposed in the control valve 4. Such a discharge amount control means 12 is publicly known. 6 is an operation means for switching the flow control valve 4 shown in FIG. 1, for example, an operation lever, and 31 is an operation lever for switching another flow control valve (not shown). As shown, for example, it is arranged on the operation lever 30.

The operation of the first embodiment configured as described above is as follows. When the operation lever 30 for operating the flow control valve 4 shown in FIG. 6 is neutral, or when the discharge pressure P of the hydraulic pump 2 reaches the maximum pressure by the relief valve 15, the discharge pressure P is detected by the pressure sensor 13 and the control is performed. In the means 17, the minimum flow rate is obtained as the pump flow rate from the pressure signal and the PQ curve shown in FIG. 5 stored in advance, and the electromagnetic control valve 22 is moved so that the displacement of the hydraulic pump 2 is minimized. A drive signal for switching or holding the electromagnetic control valve 25 in the direction of the closed position, which is the upper position of FIG. As a result, the pilot pressure of the pilot pump 11 is reduced via the pipe 21 to the small-diameter chamber 1 of the control spool 19.
8, the control spool 19 is moved leftward in FIG. 1 due to the area difference between the small-diameter chamber 18 and the large-diameter chamber 20, and the displacement is minimized.
Discharges the minimum flow rate.

Here, temporarily, the changeover switch 16 is turned off.
It is assumed that the first limit value Q0 is selected as the minimum flow rate. The control means 17 first inputs an instruction signal of the changeover switch 16 as shown in step S1 of FIG. Next, the procedure proceeds to step S2, where it is determined whether or not the instruction signal output from the changeover switch 16 is ON. Since it is now OFF, this determination is not satisfied, and the routine goes to step S3.
In step S3, the flow rate Q of the hydraulic pump 2 is set to the first limit value Q
The minimum flow rate is set to 0, and a drive signal corresponding to the first limit value Q0 is output to each of the electromagnetic control valves 22 and 25. As a result, for example, the electromagnetic control valve 22 is switched to the communication position and the electromagnetic control valve 25 is switched to the closed position, respectively, and held, the control spool 19 is sufficiently moved to the left to control the displacement, and the hydraulic pump 2 Limit value Q of 1
A minimum flow of 0 is delivered. At this time, the set pressure of the relief valve 15 is a relatively small first pressure as shown in FIG.
Is maintained at the pressure P0.

As described above, the changeover switch 16 is turned off, the minimum flow rate of the first limit value Q0 is discharged from the hydraulic pump 2, and the discharge pressure P of the hydraulic pump 2 is reduced to the first pressure P0 by the relief valve 15. When it is desired to increase the set pressure of the relief valve 15 in consideration of the type of work performed via the operation of the actuator 3 from the set situation,
The changeover switch 16 is turned on. Thereby, the determination of the procedure S2 in FIG. 2 by the control means 17 is satisfied, and in the procedure 4, the flow rate Q of the hydraulic pump 2 is set to the minimum flow rate which is the second limit value Q1 larger than the first limit value Q0. A drive signal corresponding to the second limit value Q1 is output to each of the electromagnetic control valves 22 and 25. At this time, for example, the electromagnetic control valve 22 is switched to the communication position, and the electromagnetic control valve 25 is switched to the vicinity of the closed position, and a small amount of pressure oil flows through the tank 23 via the electromagnetic control valve 25. As a result, the control spool 19 moves slightly to the right in FIG. 1 as compared with the case where the above-described changeover switch 16 is turned off, and a relatively large minimum flow rate (second restriction) from the hydraulic pump 2 as shown in FIG. The value Q1) is discharged. At this time, from the override characteristic of the relief valve 15, the set pressure of the relief valve 15 is changed to the second pressure P1 which is larger than the first pressure P0 as shown in FIG.

Therefore, when the operation lever 30 is operated to switch the flow control valve 4 and the hydraulic oil of the hydraulic pump 2 is supplied to the actuator 3 to drive the actuator 3,
A desired excavation operation or the like in which the maximum force of the actuator 3 is increased can be performed.

When controlling the flow rate of the hydraulic pump 2 described above, the actual displacement of the swash plate of the hydraulic pump 2 is detected by the displacement sensor 14. The control means 17 compares the target displacement corresponding to the flow rate Q obtained based on the pressure signal with the target displacement. If there is a difference between the two, the drive signal corresponding to the difference is transmitted to the electromagnetic control valves 22 and 23. The actual displacement is accurately controlled so as to coincide with the target displacement.

In the first embodiment, as shown in FIG. 3, the minimum flow rate of the hydraulic pump 2 is set to the first limit value Q0, that is, the hydraulic pump 2 Is controlled so that the discharge pressure P of the hydraulic pump 2 becomes the first pressure P0, and the minimum flow rate of the hydraulic pump 2 becomes the second limit value Q1 only while the changeover switch 16 is ON.
That is, the discharge pressure P of the hydraulic pump 2 is controlled to be the second pressure P1 larger than the first pressure P0, and the set pressure of the relief valve 15 is changed according to the intention of the operator who operates the changeover switch 16. Can be made.

In the first embodiment constructed as described above, a two-stage relief valve is not provided as a relief valve for regulating the discharge pressure P of the hydraulic pump 2, and a normal one-stage relief valve 1 is provided.
5, the discharge pressure P can be changed to two stages by utilizing the override characteristic of the normal relief valve 15, so that the structure relating to the relief valve is simple, the number of members can be reduced, and the entire embodiment can be reduced. Production costs can be reduced. In addition, since the accumulation of dust such as iron powder due to the complicated structure can be prevented, the operation accuracy of the relief valve 15 can be kept high over a long period of time, and high reliability in operation performance can be secured.

In the first embodiment, the changeover switch 16 serving as the flow rate indicating means is provided on the operation lever 30 as shown in FIG. 6, but this changeover switch 16 is operated by a foot pedal. May be arranged as follows.

FIGS. 7 and 8 are explanatory views showing the main parts of a second embodiment of the present invention. FIG. 7 is a flowchart showing the processing procedure in the control means. FIG. 8 is a changeover switch in the second embodiment. 6 is a time chart showing a relationship between the operation of FIG. 1 and a first limit value and a second limit value which are minimum flow rates of the variable displacement hydraulic pump.

In the second embodiment, the control means 17 shown in FIG. 1 sends an instruction signal for changing the first limit value Q0 to the second limit value Q1 from the changeover switch 16 which is a flow rate instruction means. A first drive signal corresponding to the first limit value Q0 is forcibly output to the discharge amount control means 2 to return to the original state after a predetermined time T1 has elapsed based on the input time point.
Of the present invention. Other basic configurations are the same as those shown in FIG. Therefore, the following description will be made using the reference numerals shown in FIG.

The operation of the second embodiment is as follows. It is now assumed that the changeover switch 16 is OFF and the first limit value Q0 is selected as the minimum flow rate. The control means 17 first inputs an instruction signal of the changeover switch 16 as shown in step S11 of FIG. Next, the procedure proceeds to step S12, where it is determined whether or not the instruction signal output from the changeover switch 16 is ON. Since it is now OFF, this determination is not satisfied, and the routine goes to step S13. In step S13, the flow rate Q of the hydraulic pump 2 is set to the first limit value Q0.
Is set to the minimum flow rate, and a drive signal corresponding to the first limit value Q0 is output to each of the electromagnetic control valves 22 and 25. As a result, for example, the electromagnetic control valve 22 is switched to the communication position and the electromagnetic control valve 25 is switched to the closed position, respectively, and held, the control spool 19 is sufficiently moved to the left to control the displacement, and the hydraulic pump 2 Limit value Q of 1
A minimum flow of 0 is delivered. At this time, the set pressure of the relief valve 15 is a relatively small first pressure as shown in FIG.
Is maintained at the pressure P0.

As described above, the changeover switch 16 is turned off, the minimum flow rate of the first limit value Q0 is discharged from the hydraulic pump 2, and the discharge pressure P of the hydraulic pump 2 is reduced to the first pressure P0 by the relief valve 15. When it is desired to increase the set pressure of the relief valve 15 in consideration of the type of work performed via the operation of the actuator 3 from the set situation,
The changeover switch 16 is turned on. Thereby, the determination of the procedure S12 in FIG. 7 by the control means 17 is satisfied, and the procedure goes to the procedure S14. In this step S14,
It is determined whether or not T1 seconds have elapsed since the switch 16 was turned on. If this determination is satisfied, the process moves to the above-described step S13. In this step S13, as described above, the flow rate Q of the hydraulic pump 2 is set to the minimum flow rate which is the first limit value Q0, and a drive signal corresponding to the first limit value Q0 is transmitted to the electromagnetic control valves 22 and 25. Output to each.
As a result, for example, the electromagnetic control valve 22 is switched to the communication position and the electromagnetic control valve 25 is switched to the closed position, respectively, and held, the control spool 19 is sufficiently moved to the left to control the displacement, and the hydraulic pump 2 The minimum flow rate which is the limit value Q0 of 1 is discharged.

If the determination in the step S14 is not satisfied, that is, if it is within the time from when the changeover switch 16 is turned on until T1 second elapses, the process proceeds to the step S15. In step 15, the flow rate Q of the hydraulic pump 2 is set to the first
Is set to the minimum flow rate which is the second limit value Q1 larger than the limit value Q0, and a drive signal corresponding to the second limit value Q1 is output to each of the electromagnetic control valves 22 and 25. At this time, for example, the electromagnetic control valve 22 is switched to the communication position, and the electromagnetic control valve 25 is switched to the vicinity of the closed position, and a small amount of pressure oil flows through the tank 23 via the electromagnetic control valve 25. As a result, the control spool 19 moves slightly to the right in FIG. 1 as compared with the case where the above-described changeover switch 16 is turned off, and a relatively large minimum flow rate (second restriction) from the hydraulic pump 2 as shown in FIG. The value Q1) is discharged. At this time, from the override characteristic of the relief valve 15, the set pressure of the relief valve 15 is changed to the second pressure P1 which is larger than the first pressure P0 as shown in FIG. After step S15, the process returns to step S14.
That is, after the changeover switch 16 is once switched from OFF to ON, even if the changeover switch 16 is turned off before the predetermined time T1 seconds elapse, the flow rate Q of the hydraulic pump 2 is changed until the predetermined time elapses. The minimum flow rate is set as the second limit value Q1 larger than the first limit value Q0. Therefore, similarly to the first embodiment, when the operation lever 30 is operated to switch the flow control valve 4 and the hydraulic oil of the hydraulic pump 2 is supplied to the actuator 3 to drive the actuator 3, the actuator 3 It is possible to carry out a desired excavation work or the like in which the maximum force of Step 3 is increased.

In the second embodiment constructed as described above, the discharge pressure P can be changed to two stages by utilizing the override characteristic of the normal relief valve 15, as in the first embodiment. Therefore, the structure relating to the relief valve is simple, the number of members is small, the manufacturing cost can be reduced, and
Since the accumulation of dust such as iron powder due to the complicated structure can be prevented, high reliability in operation performance can be secured. In particular, the time during which the set pressure of the relief valve 15 is maintained at the relatively large second pressure P1 is limited to a predetermined time T1 in advance in consideration of the durability of components of the hydraulic circuit and the like. The durability of the device can be ensured.

FIGS. 9 and 10 are explanatory views showing the main parts of a third embodiment of the present invention. FIG. 9 is a flowchart showing the processing procedure in the control means. FIG. 10 is a changeover switch in the third embodiment. 6 is a time chart showing a relationship between the operation of FIG. 1 and a first limit value and a second limit value which are minimum flow rates of the variable displacement hydraulic pump.

In the third embodiment, the control means 17 shown in FIG. 1 sends an instruction signal for changing the first limit value Q0 to the second limit value Q1 from the changeover switch 16 which is a flow rate instruction means. Based on the input time point, after a predetermined time T2 has elapsed, a drive signal corresponding to the first limit value Q0 is forcibly output to the discharge amount control means 2 to return to the original state, and the predetermined time T2 has elapsed. When an instruction signal for changing to the first limit value Q0 is previously output from the changeover switch 16, a second drive signal forcibly outputting a drive signal corresponding to the first limit value Q0 to the discharge amount control means 12 is provided. It is configured to include a return means. Other basic configurations are the same as those shown in FIG. Therefore, the following description will be made using the reference numerals shown in FIG.

The operation of the third embodiment is as follows. It is now assumed that the changeover switch 16 is OFF and the first limit value Q0 is selected as the minimum flow rate. The control unit 17 first inputs an instruction signal of the changeover switch 16 as shown in step S21 of FIG. Next, the procedure proceeds to step S22, where it is determined whether or not the instruction signal output from the changeover switch 16 is ON. Since it is now OFF, this determination is not satisfied, and the routine goes to step S23. In step S23, the flow rate Q of the hydraulic pump 2 is set to the first limit value Q0.
Is set to the minimum flow rate, and a drive signal corresponding to the first limit value Q0 is output to each of the electromagnetic control valves 22 and 25. As a result, for example, the electromagnetic control valve 22 is switched to the communication position and the electromagnetic control valve 25 is switched to the closed position, respectively, and held, the control spool 19 is sufficiently moved to the left to control the displacement, and the hydraulic pump 2 Limit value Q of 1
A minimum flow of 0 is delivered. At this time, the set pressure of the relief valve 15 is a relatively small first pressure as shown in FIG.
Is maintained at the pressure P0.

As described above, the changeover switch 16 is turned off, the minimum flow rate of the first limit value Q0 is discharged from the hydraulic pump 2, and the discharge pressure P of the hydraulic pump 2 is reduced to the first pressure P0 by the relief valve 15. When it is desired to increase the set pressure of the relief valve 15 in consideration of the type of work performed via the operation of the actuator 3 from the set situation,
The changeover switch 16 is turned on. Thereby, the determination of the procedure S22 in FIG. 9 by the control means 17 is satisfied, and the procedure goes to the procedure S24. In this step S24,
It is determined whether T2 seconds have elapsed since the changeover switch 16 was turned ON. If this determination is satisfied, the procedure moves to the above-described procedure S23. In this step S23, as described above, the flow rate Q of the hydraulic pump 2 is set to the minimum flow rate that is the first limit value Q0, and a drive signal corresponding to the first limit value Q0 is transmitted to the electromagnetic control valves 22 and 25. Output to each.
As a result, for example, the electromagnetic control valve 22 is switched to the communication position and the electromagnetic control valve 25 is switched to the closed position, respectively, and held, the control spool 19 is sufficiently moved to the left to control the displacement, and the hydraulic pump 2 The minimum flow rate which is the limit value Q0 of 1 is discharged.

If the determination in the step S24 is not satisfied, that is, within the time from when the changeover switch 16 is turned on until T2 seconds elapse, the process proceeds to the step S25. In this step 25, the flow rate Q of the hydraulic pump 2 is set to the first
Is set to the minimum flow rate which is the second limit value Q1 larger than the limit value Q0, and a drive signal corresponding to the second limit value Q1 is output to each of the electromagnetic control valves 22 and 25. At this time, for example, the electromagnetic control valve 22 is switched to the communication position, and the electromagnetic control valve 25 is switched to the vicinity of the closed position, and a small amount of pressure oil flows through the tank 23 via the electromagnetic control valve 25. As a result, the control spool 19 moves slightly to the right in FIG. 1 as compared with the case where the above-described changeover switch 16 is turned off, and a relatively large minimum flow rate (second restriction) from the hydraulic pump 2 as shown in FIG. The value Q1) is discharged. At this time, from the override characteristic of the relief valve 15, the set pressure of the relief valve 15 is changed to the second pressure P1 which is larger than the first pressure P0 as shown in FIG.

Therefore, as in the first embodiment, the operation lever 30 is operated to switch the flow control valve 4, and
When driving the actuator 3 by supplying the pressure oil of the hydraulic pump 2 to the actuator 3, the actuator 3
It is possible to carry out a desired excavation work or the like in which the maximum force is increased.

Further, the processing of the above-described step S25, the procedure S
After the process of 23, the process returns to the beginning. Accordingly, in the third embodiment, as shown in the time chart of FIG. 10, even before the predetermined time T2 has elapsed, when the changeover switch 16 is turned off, the procedure S2 of FIG.
If the determination in step S2 is not satisfied, the flow proceeds to step S23 to set the flow rate Q of the hydraulic pump 2 to the minimum flow rate that is the second limit value Q1.

In the third embodiment constructed as described above, similarly to the second embodiment, the structure relating to the relief valve is simple, the number of members is small, and the manufacturing cost can be reduced. High reliability of the operation performance can be ensured, and the set pressure of the relief valve 15
Is limited within the predetermined time T2, the durability of the hydraulic drive device can be ensured.

[0037]

According to the present invention since the are in the above configuration, without providing a two-stage relief valve such as that kicked slave come set as a relief valve for defining the discharge pressure of the variable displacement hydraulic pump, usually 1 A relief valve having two stages is provided, and the discharge pressure of the hydraulic pump can be changed to two stages by utilizing the override characteristic of the normal relief valve. Therefore, the structure relating to the relief valve is simple, and the number of members can be reduced. There is an effect that the manufacturing cost of the entire driving device can be reduced. In addition, since the accumulation of dust such as iron powder due to the complicated structure can be prevented, the operation accuracy of the relief valve can be maintained at high accuracy for a long period of time, and the effect of ensuring high reliability of the operation performance can be secured. is there.

[Brief description of the drawings]

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

FIG. 2 is a flowchart showing a processing procedure in a control unit provided in the first embodiment shown in FIG.

FIG. 3 is a time chart showing the relationship between the operation of a changeover switch and a first limit value and a second limit value which are minimum flow rates of a variable displacement hydraulic pump in the first embodiment shown in FIG. 1;

FIG. 4 is a diagram showing override characteristics of a relief valve provided in the first embodiment shown in FIG.

FIG. 5 is a diagram showing a first limit value and a second limit value which are minimum flow rates of the variable displacement hydraulic pump set in the first embodiment shown in FIG. 1;

FIG. 6 is a diagram showing an example of an arrangement of the flow rate indicating means shown in FIG.

FIG. 7 is an explanatory view showing a main part of a second embodiment of the present invention;
It is a flowchart which shows the processing procedure in a control means.

FIG. 8 shows the operation of the changeover switch and the first limit value which is the minimum flow rate of the variable displacement hydraulic pump in the second embodiment,
6 is a time chart showing the relationship between the limit values.

FIG. 9 is an explanatory diagram showing a main part of a third embodiment of the present invention;
It is a flowchart which shows the processing procedure in a control means.

FIG. 10 is a time chart showing a relationship between an operation of a changeover switch and a first limit value and a second limit value which are minimum flow rates of a variable displacement hydraulic pump in a third embodiment.

FIG. 11 is a hydraulic circuit diagram schematically showing an example of a conventional hydraulic drive device for a construction machine.

FIG. 12 is a flowchart showing a processing procedure in control means provided in the conventional hydraulic drive device shown in FIG.

13 is a diagram showing characteristics of a two-stage relief valve provided in the conventional hydraulic drive device shown in FIG.

14 is a view showing a minimum flow rate of a variable displacement hydraulic pump set by the conventional hydraulic drive device shown in FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 prime mover 2 variable displacement hydraulic pump 3 actuator 4 flow control valve 11 pilot pump 12 discharge rate control means 13 pressure sensor 14 tilt sensor 15 relief valve 16 changeover switch (flow rate indicating means) 17 control means 30 operating lever (operating means)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toichi Hirata 650, Kandamachi, Tsuchiura-shi, Ibaraki Hitachi Construction Machinery Co., Ltd. Tsuchiura Plant (56) References JP-A-57-91388 (JP, A) 62-167880 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) F15B 11/00-11/22

Claims (5)

(57) [Claims] 1. A prime mover, a variable displacement hydraulic pump driven by the prime mover, a discharge pressure of the variable displacement hydraulic pump, and a flow rate of the variable displacement hydraulic pump.
Override feature that increases the discharge pressure as the pressure increases
Relief valve having a characteristic, an actuator driven by pressure oil discharged from the variable displacement hydraulic pump, a control valve for controlling a flow of pressure oil supplied from the variable displacement hydraulic pump to the actuator, and the control valve Operating means for switching the displacement, and discharge rate control means for controlling the discharge rate of the variable displacement hydraulic pump, wherein the variable displacement hydraulic pump defined by the relief valve is provided .
The discharge pressure of the pump is determined by a predetermined first pressure and the first pressure.
In the hydraulic drive system of modifiable construction machine in two steps of a predetermined second pressure greater Ri, the variable displacement hydraulic port that is defined by the relief valve
First limit value defining the minimum flow of the first said variable displacement hydraulic pump which determines the pressure of the discharge pressure of the pump, and there are the second value larger than the first limit value
Defines the minimum flow rate of the variable displacement hydraulic pump that determines the pressure
Either a second limit value, provided with a flow rate instruction means for outputting a selectively instructing signal, when an instruction signal corresponding to the first limit value is outputted from the flow rate instructing means, the first a driving signal corresponding to the first limit value is output to the discharge amount control means, said when an instruction signal from the flow rate instructing means which corresponds to the second limit value is output, corresponding to the second limit value the drive signal comprises a control means for outputting to the discharge amount control means, said relief
The variable volume using the override characteristic of the valve
The discharge pressure of the hydraulic pump corresponds to the first limit value.
The first pressure and the second pressure corresponding to the second limit value;
A hydraulic drive device for a construction machine, wherein the hydraulic pressure device can be changed to two pressure levels . 2. The method according to claim 1, wherein the control means forcibly sets a predetermined time after a lapse of a predetermined time with reference to a time point when an instruction signal for changing the first limit value to the second limit value is input from the flow rate instruction means. 2. The hydraulic drive device for a construction machine according to claim 1, further comprising a first return unit that outputs a drive signal corresponding to a first limit value to said discharge amount control unit. 3. The control means forcibly sets a predetermined time after a predetermined time has elapsed with reference to a point in time at which an instruction signal for changing the first limit value to the second limit value from the flow rate instruction means is input. A drive signal corresponding to the first limit value is output to the discharge amount control means, and when an instruction signal for changing to the first limit value is output from the flow rate instruction means before the predetermined time has elapsed, 2. The hydraulic drive system for a construction machine according to claim 1, further comprising second return means for forcibly outputting a drive signal corresponding to said first limit value to said discharge amount control means. 4. The hydraulic drive device for a construction machine according to claim 1, wherein said operating means is an operating lever, and said flow rate indicating means is provided on said operating lever. 5. The flow rate indicating means is arranged to be operated by a foot pedal.
A hydraulic drive device for a construction machine according to any one of the above.