JP2002323001A - Hydraulic drive unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydraulic drive unit to be mounted on a construction machine such as a hydraulic shovel or the like, capable of preventing a shock from occurring at an actuator which is not stopped while another actuator is stopped, when two and more actuators are simultaneously in operation. SOLUTION: The target differential pressure of a pressure compensation valve 21a of a direction changeover valve 20a connected to a swivel motor 4a of a plurality of actuators 4a, 4b is set as target differential pressure (PGR) for load sensing control depending on a revolutionary speed of an engine. A differential pressure reducing valve 34 is operative to detect secondary pressure (Pc) equal to the differential pressure between the discharging pressure of an oil pressure pump 10 and the maximum load pressure (Plmax) of the actuators 20a, 20b, and the secondary pressure (Pc) is set as the target differential pressure of the pressure compensation valve 21b of the direction switching valve 20b to the other actuator 4b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a load compensating valve which performs load sensing control so that the discharge pressure of a hydraulic pump is higher than the maximum load pressure of a plurality of actuators by a target differential pressure, and controls the differential pressure across a plurality of direction switching valves. In particular, the target compensation differential pressure of the pressure compensating valve is set by the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure of a plurality of actuators, and load sensing is performed. The present invention relates to a hydraulic drive device in which a target differential pressure for control is set as a variable value depending on the engine speed.

[0002]

2. Description of the Related Art The applicant has filed a patent application and has not yet published a patent application 2000
FIG. 1 of JP-A-004074 proposes a hydraulic drive as shown in FIG. In FIG. 3, the engine 1, a variable displacement hydraulic pump 10 driven by the engine 1, a plurality of actuators 4a, 4b driven by pressure oil discharged from the hydraulic pump 10, and a hydraulic pump 10 A plurality of directional control valves 20a and 20b for controlling the flow rate of the pressure oil supplied to the plurality of actuators 4a and 4b, and a plurality of pressure compensating valves 2 for controlling the differential pressure between the front and rear of the directional control valves 2
1a, 21b, and a main relief valve 30 that regulates the upper limit of the discharge pressure of the hydraulic pump 10. The target pumping differential pressure of each of the plurality of pressure compensating valves 21a, 21b is adjusted by a hydraulic pump via a differential pressure reducing valve 34. Secondary pressure (Pc) equal to the differential pressure between the discharge pressure (Ps) of 10 and the maximum load pressure (Plmax) of the actuators 20a and 20b
And the load sensing target differential pressure and the secondary pressure are guided to oppose each other so that the discharge pressure of the hydraulic pump 10 becomes higher than the maximum load pressure of the plurality of actuators 4a and 4b by the load sensing target differential pressure. A pump control means 12 'for load sensing control including a pump tilt angle control valve 12 is provided, and a target differential pressure (PGR) for load sensing control, which is disclosed in JP-A-11-196604, depends on the rotation speed of the engine 1. And a pressure generating valve 51 for extracting a differential pressure around the throttle portion 50a of the flow rate detection valve 50 provided in the discharge line 9 of the fixed displacement type pilot hydraulic pump 11, and load sensing. In order to improve the control function, a hydraulic pump 10 disclosed in JP-A-10-205501 is disclosed.
The pressure differential line (PGR) for load sensing control is connected to the pressure oil line 7
a, 20b have operation drive units 31a, 31d to which the maximum load pressures (Plmax) are respectively guided. The target differential pressure (ΔPun) of the own is also changed according to the change of the target differential pressure (PGR) of the load sensing control. Having a variable unload valve 31,
2 shows a hydraulic circuit of a hydraulic drive device.

[0003] In FIG. 3, similarly, in order to improve the load sensing control function, when the discharge pressure of the hydraulic pump 10 increases to the set pressure (Pr) of the main relief valve 30 as proposed in Japanese Patent Application No. 2000-004074, a plurality of times are set. A correction value (Plmax) different from the differential pressure (Pc) between the discharge pressure (Ps) of the hydraulic pump 10 and the maximum load pressure (Plmax) of the plurality of actuators 20a and 20b as the target compensation differential pressure of the pressure compensating valves 21a and 21b ') Is provided, and the target compensation differential pressure correction means is connected to the maximum load pressure line 35 via the fixed throttle 32, and is detected by the maximum load pressure line 35. Load pressure (Plmax)
The upper limit of the correction value (P) is set based on the set pressure (Pr) of the main relief valve 30.
lmax ') (set pressure Plmax' = Pr-PGR +
α: α is smaller than PGR) The signal pressure variable relief valve 33 is provided.

[0004]

The circuit configuration of the hydraulic drive device as shown in FIG. 3 has the following problems. That is,
In a construction machine such as a hydraulic shovel, when one or more actuators are stopped from a state in which two or more actuators are simultaneously driven, the discharge flow rate of the hydraulic pump 10 that has been consumed by the two or more actuators until then is reduced. Since one side stops, an excess flow occurs in the discharge flow rate of the hydraulic pump 10, so that the discharge pressure of the hydraulic pump 10 sharply increases, and the discharge pressure of the hydraulic pump 10 (Ps) and the maximum load pressure of the actuator ( Plmax) and secondary pressure (Pc) equal to the differential pressure
The secondary pressure generated by the differential pressure reducing valve that detects
The suddenly increased secondary pressure acts on the pressure compensating valve that is simultaneously driven and does not stop, increases the outlet flow rate, and causes the surplus flow rate of the hydraulic pump 10 to flow into the actuator on the side that did not stop, causing a sudden flow. There was a problem that the vehicle accelerated and a shock was generated. That is, when the two actuators that are simultaneously driven are the swing motor 4a and the boom cylinder 4b, and the operation of the boom cylinder 4b is stopped, the excess flow of the hydraulic pump 10 flows into the swing motor 4a, and the swing motor 4a is rapidly accelerated. And caused a shock.

[0005] The secondary pressure which originally rises sharply also acts on the pump tilt angle control valve 12 of the pump control means 12 'for load sensing control, thereby reducing the discharge flow rate of the hydraulic pump 10,
Although control should be performed so as not to generate an excess flow rate, the discharge flow rate of the pump does not actually suddenly decrease because the hydraulic pump 10 has a response delay. Further, in such a case, the surplus flow rate should escape from the variable unload valve 31, but the variable unload valve 31 also has a response delay, and the problem has not been solved. However, in the circuit shown in FIG. 3, since the suddenly increased secondary pressure acts on the pressure compensating valve of the other actuator which is simultaneously driven and does not stop, the actuator is rapidly accelerated. This sudden acceleration generates a shock. This shock becomes a problem particularly in driving the swing motor, and there is a problem that soil and the like mounted on the bucket collapse and that the operator feels a problem.

An object of the present invention is to provide a construction machine, such as a hydraulic shovel, in which, when two or more actuators are driven simultaneously, even when one of the actuators is stopped, the other actuator which has not stopped is stopped. An object of the present invention is to provide a hydraulic drive device having a circuit configuration that does not generate a shock.

[0007]

According to the present invention, there is provided an engine, a variable displacement hydraulic pump driven by the engine, and a hydraulic pump driven by the hydraulic pump. A plurality of actuators; a plurality of directional control valves that respectively control flow rates of pressure oil supplied from the hydraulic pump to the plurality of actuators; and a plurality of pressure compensations that respectively control differential pressures before and after the directional control valves. And a main relief valve that regulates an upper limit of the discharge pressure of the hydraulic pump. The target compensation differential pressure of each of the plurality of pressure compensating valves is determined by the discharge pressure of the hydraulic pump and the maximum load of the plurality of actuators. The pressure is set by a secondary pressure equal to the pressure difference from the pressure, and the discharge pressure of the hydraulic pump is higher than the maximum load pressure of the plurality of actuators. Pump control means for performing load sensing control including a pump tilt angle control valve in which the load sensing target differential pressure and the secondary pressure are guided in opposition to each other so as to be higher by the single target differential pressure; The differential pressure is set as a variable value depending on the number of revolutions of the engine, and is connected to a pressure oil line discharged from the hydraulic pump, the target differential pressure of the load sensing control and the maximum load pressure of the plurality of actuators. The operation of the hydraulic drive device having a variable unload valve, wherein the hydraulic drive device has a variable unload valve that has an operation drive unit that is respectively guided, and the target differential pressure of itself is also changed according to the change of the target differential pressure of the load sensing control. The load compensating pressure dependent on the rotation speed of the engine, the target compensating differential pressure of the pressure compensating valve to the swing motor among the actuators. The target differential pressure of the sing control and the target compensation differential pressure of the pressure compensating valve to an actuator other than the swing motor is a secondary pressure equal to the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators. The above-described problem of the present invention has been solved by providing a hydraulic drive device characterized by setting according to (1).

[0008]

In the circuit of the hydraulic drive device shown in FIG. 3, the secondary pressure equal to the differential pressure (Pc) between the discharge pressure of the pump obtained by the differential pressure reducing valve and the maximum load pressure is applied to all the pressure compensating valves. By applying pressure, the function of distributing the discharge flow rate of the pump at the same ratio to all actuators was obtained, but in the present invention, the pressure compensation valve for the swing motor, which is the most problematic in the circuit of FIG. Instead of applying the secondary pressure obtained by the differential pressure reducing valve, the target load sensing differential pressure (PGR) is applied as it is, and the circuit shown in FIG. Similarly, the secondary pressure obtained by the differential pressure reducing valve is applied. With such a configuration, it is assumed that the two actuators that are simultaneously driven are the turning motor and the boom cylinder, and the operation of the boom cylinder is stopped. Even in the circuit of the hydraulic drive device of the present invention, the discharge flow rate of the pump becomes excessive due to the stop of the boom cylinder. Therefore, immediately after the stop of the boom cylinder, the discharge pressure of the pump rapidly rises, thereby causing the secondary pressure generated by the differential pressure reducing valve. Although the pressure also rises sharply, the secondary pressure does not act on the pressure compensating valve for the swing motor, and the sudden rise in the secondary pressure acts only to reduce the pump discharge flow rate. Actually, due to a response delay, the pump discharge flow rate does not suddenly decrease, but during this time, the variable unload valve opens to act to release the surplus flow rate. The unloading action of the variable unloading valve itself is the same as that of the circuit of FIG. 3, but unlike the circuit of FIG. 3, the secondary pressure does not act on the pressure compensating valve for the swing motor that is continuously driven. ,
The excess flow rate of the hydraulic pump 10 flows into the swing motor that did not stop by increasing the outlet flow rate by acting on the pressure compensation valve for the swing motor and did not stop. Since there is no pressure drop
The variable unload valve becomes easier to operate. Therefore, there was no acceleration of the turning motor which did not stop, which was a problem in the case of the circuit of FIG. 3, and no shock was generated. In the circuit of the present invention, the fact that the secondary pressure obtained by the differential pressure reducing valve does not act on the pressure compensating valve for the swing motor and the target load sensing differential pressure (PGR) acts means that the maximum flow rate of the pump On the other hand, even in the so-called saturation state where the required flow rate has increased, the swing motor is driven according to the target load sensing differential pressure (PGR), so that the swing motor is given priority over other actuators. However, in the case of a construction machine such as a hydraulic shovel in which it is necessary to maintain the independence of the swing motor, this characteristic is advantageous.

Preferably, at least one of the plurality of pressure compensating valves has a self-load pressure that rises sharply due to a decrease in its own discharge rate when the self-load increases, thereby reducing the flow rate to the actuator. The required discharge rate is reduced to prevent a sudden rise in pump pressure when the other actuator stops, thereby preventing a sudden rise in the secondary pressure generated by the differential pressure reducing valve and a sudden increase in the pump discharge quantity accompanying a sudden rise in the secondary pressure. It is possible to prevent a decrease or the like and improve the composite operability at the time of the composite operation of simultaneously operating a plurality of actuators of the hydraulic drive device.

More preferably, when the discharge pressure of the hydraulic pump rises to a set pressure of the main relief valve,
A target compensation differential pressure correcting means for setting a correction value different from a differential pressure between a discharge pressure of the hydraulic pump and a maximum load pressure of the plurality of actuators as a target compensation differential pressure of the plurality of pressure compensating valves; The compensation differential pressure correction means is connected via a fixed throttle to a maximum load pressure line for detecting the maximum load pressure, and sets an upper limit of the maximum load pressure detected on the maximum load pressure line based on a set pressure of the main relief valve. Even if the load pressure of any one of the actuators reaches the set pressure of the main relief valve during the combined operation of simultaneously operating a plurality of actuators, the pressure The compensating valve does not close and the other actuators do not increase in speed, so that the combined operability of the hydraulic drive device can be further improved.

[0011]

FIG. 1 shows a hydraulic circuit of a hydraulic drive unit according to an embodiment of the present invention. In the hydraulic circuit according to the embodiment of the present invention, the engine 1, the variable displacement hydraulic pump 10 driven by the engine 1, and the hydraulic pump 10 discharged from the hydraulic pump 10, as described in the circuit of FIG. 3. A plurality of actuators 4a, 4b driven by pressure oil (including
A plurality of directional control valves 20a and 20b (only two of which are shown) for controlling the flow rate of the hydraulic oil supplied from the hydraulic pump 10 to the plurality of actuators 4a and 4b, respectively,
A plurality of pressure compensating valves 21a and 21b (only two of which are shown) for controlling the pressure difference between the front and rear of the plurality of directional control valves, and a main relief valve 30 for regulating the upper limit of the discharge pressure of the hydraulic pump 10
And

The discharge pressure of the hydraulic pump 10 disclosed in Japanese Patent Application Laid-Open No. 11-196604 is controlled by a plurality of actuators 4a,
The pump tilt angle control valve 12 in which the load sensing target differential pressure and the secondary pressure are guided to be higher than the maximum load pressure (Plmax) of 4b by the load sensing target differential pressure.
A pump control means 12 'for load sensing control including a target differential pressure (PGR) generation circuit for load sensing control
With 5. That is, the target differential pressure of the load sensing control
In order to set (PGR) as a variable value that depends on the number of revolutions of the engine 1, the differential pressure (PGR) across the throttle 50a of the flow rate detection valve 50 provided in the discharge line 9 of the fixed displacement pilot hydraulic pump 11 And a pressure generating valve 51 to be taken out.
In order to improve the load sensing control function,
Connected to the pressure oil line 7 discharged from the hydraulic pump 10 disclosed in Japanese Patent No. 05501, the target differential pressure (PGR) of load sensing control and the maximum load pressure (Plmax) of the plurality of actuators 20a, 20b are respectively guided. A variable unload valve 31 having operation drive units 31a and 31d and having its own target differential pressure (ΔPun) changed in accordance with a change in the target differential pressure (PGR) of the load sensing control. The details of each configuration and operation of the target differential pressure (PGR) generation circuit 5 and the variable unload valve 31 for the load sensing control have been described in the above-mentioned publication, and will not be described repeatedly.

In the present invention, a plurality of actuators 4a, 4b
Direction switching valve to swing motor 4a
The target compensation differential pressure of 21a is the target differential pressure (PGR) of load sensing control depending on the engine speed, and the discharge pressure (Ps) of the hydraulic pump 10 and the maximum load pressure (Plmax) of the plurality of actuators 4a and 4b. The differential pressure reducing valve 34 detects a secondary pressure equal to the differential pressure (Pc) with the pressure compensation valve 21b of the direction switching valve 20b to the actuator 4b other than the swing motor 4a. Is the target compensation differential pressure.

With this configuration, it is assumed that the two actuators 4a and 4b that are being driven at the same time are the turning motor 4a and the boom cylinder 4b, and the operation of the boom cylinder 4b is stopped. The circuit of the hydraulic drive device according to the present invention also includes the boom cylinder 4b.
As the discharge flow of the pump 10 becomes excessive due to the stop of the pump, the discharge pressure of the pump 10 rises immediately after the stop of the boom cylinder 4b, and the secondary pressure (Pc) generated by the differential pressure reducing valve 34 also rises sharply. However, the secondary pressure does not act on the pressure compensating valve 21a for the swing motor 4a, and
The secondary pressure acts only to reduce the pump discharge flow. Actually, since there is a response delay, the pump discharge flow rate does not suddenly decrease, but during this time, the variable unload valve 31 is opened to act to release the surplus flow rate. The unloading action of the variable unloading valve 31 is the same as that of the circuit of FIG. 3, but is different from the circuit of FIG. 3, and the secondary pressure acts on the pressure compensating valve 21a for the swing motor 4a that is continuously driven. Therefore, the excess flow rate of the hydraulic pump 10 flows into the swing motor 4a that did not stop by acting on the pressure compensating valve 21a for the swing motor 4a and increasing its outlet flow rate, so that rapid acceleration was not performed.
As a result, the excess flow rate is reduced and the discharge pressure of the pump 10 is not reduced, so that the variable unload valve 31 is easily operated. Therefore, there was no acceleration of the turning motor 4a which did not stop, which was a problem in the case of the circuit of FIG. 3, and no shock was generated. In the circuit of the present invention, the secondary pressure (Pc) obtained by the differential pressure reducing valve 34 does not act on the pressure compensating valve 21a for the swing motor 4a, but the target load sensing differential pressure (PGR) acts. Is the maximum flow rate of the pump 10,
Even in the so-called saturation state where the required consumption flow rate has increased, the swing motor 4a operates at the target load sensing differential pressure.
(PGR) driven, so the swing motor has a higher priority circuit than other actuators, but in the case of construction equipment such as a hydraulic excavator that needs to maintain the independence of the swing motor , This property is favorable on the contrary.

FIG. 2 is a sectional view of the pressure compensating valves 21A and 21B showing a schematic structure of at least one preferred embodiment of the pressure compensating valves 21a and 21b of FIG. 3 shows a detailed configuration and operation, and duplicate description will be omitted. The pressure Pz at the outlet 105 is led to the pressure receiving surface A1 of the chamber 121 of the pressure compensating valves 21A and 21B, and the secondary pressure Pc obtained by the differential pressure reducing valve 34 is led to the pressure receiving surface A2 of the chamber 113. The self-loading pressure Pl is guided to the pressure receiving surface A3 of FIG. So far, Figure 1
2 is the same as the pressure compensating valves 21a and 21b, but the diameter d1 of the spool 117 of the pressure compensating valves 21A and 21B of FIG.
The diameter of the small diameter part in the room 119 of 12 is made larger than d3, and d3 <d1
A3 <A1, A3 = kA1 (where k <1). The spool 112 is balanced by receiving the following forces from the left and right. (A3 · Pl) + (A2 · Pc) = (A1 · Pz) Thus, the differential pressure ΔP around each directional control valve 20a, 20b compensated by each pressure compensating valve 21A, 21B can be expressed by the following equation. it can. ΔP = [(k · A2) / A3] · Pc-(1-k) · Pl For this reason, when the self-load pressure Pl of each of the plurality of pressure compensating valves 21a and 21b increases, the actuator to which the self-load pressure sharply increases is used. The differential pressure ΔP before and after the directional control valve decreases, and the discharge amount at each outlet of the pressure compensating valve and the directional switch valve decreases. To prevent a sudden increase in the pump pressure when the other actuator stops, thereby preventing a sudden increase in the secondary pressure generated by the differential pressure reducing valve.
Prevents a sudden decrease in pump discharge due to a sudden rise in the next pressure,
The composite operability at the time of the composite operation of simultaneously operating a plurality of actuators of the hydraulic drive device can be improved.

Similarly, in the embodiment of the present invention shown in FIG. 1, in order to improve the load sensing control function, Japanese Patent Application No. 2000-004074.
When the discharge pressure of the hydraulic pump 10 rises to the set pressure (Pr) of the main relief valve 30, the pressure
The discharge pressure of the hydraulic pump 11 (P
s) and the maximum load pressure (Plm
ax) is provided with a target compensation differential pressure correction means for setting a correction value (Plmax ′) different from the differential pressure (Pc). This target compensation differential pressure correction means is provided on the maximum load pressure line 35. , The maximum load pressure (Plma
x) is lower than the set pressure (Pr) of the main relief valve 30 by the correction value (Plmax ') (set pressure Plmax' = Pr-PG
R + α: α is smaller than PGR) Variable signal pressure relief valve
33, the pressure compensating valve does not close even if the load pressure of one of the actuators reaches the set pressure of the main relief valve during the combined operation of simultaneously operating a plurality of actuators, and the other actuators Speed does not increase, and the combined operability of the hydraulic drive device can be further improved.

[Brief description of the drawings]

FIG. 1 shows a hydraulic circuit of a hydraulic drive device according to an embodiment of the present invention.

FIG. 2 shows a schematic configuration of at least one preferred embodiment of the pressure compensating valves 21a, 21b of FIG. 1;
It is sectional drawing of 21A, 21B.

FIG. 3 is a patent application pending and unpublished Japanese Patent Application No. 2000-00407.
4 shows a hydraulic circuit of the hydraulic drive device shown in FIG.

[Explanation of symbols]

1 Engine 4a, 4b Actuator 7 Pressure oil line 10 Variable displacement hydraulic pump 1 Pilot hydraulic pump 12 Pump tilt angle control valve 12 'Pump control means 20a, 20b ..Directional valves 21a, 21b, 21A, 21BPressure compensating valve 30.Main relief valve 31.Variable unload valve 33.Signal pressure variable relief valve 35.Maximum load pressure line

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koji Okazaki 1-1-1, Fujikoshi Honcho, Toyama, Toyama Pref., Ltd. (72) Inventor Yukiaki Nagao 1-1-1, Fujikoshi Honcho, Toyama, Toyama No. 1 Fujitsu Koshiuchi Co., Ltd. Hitachi Construction Machinery Tierra Shiga Plant, 1-2 Sasakioka (72) Inventor Junya Kawamoto 1-2 Term Sasakioka, Mizuguchi-machi, Koga-gun, Shiga Prefecture F-term in Hitachi Construction Machinery Tierra Shiga Plant (72) Reference) 2D003 AA01 AB02 AB05 AC09 BA01 BA08 CA03 DA03 DB02 3H089 AA27 AA71 BB05 BB14 BB15 BB19 CC01 CC08 CC11 DA03 DA13 DB03 DB12 DB24 DB32 EE02 FF06 FF16 GG02 JJ02

Claims (3)

[Claims] 1. An engine, a variable displacement hydraulic pump driven by the engine, a plurality of actuators driven by pressure oil discharged from the hydraulic pump,
A plurality of directional control valves that respectively control flow rates of pressure oil supplied from the hydraulic pump to the plurality of actuators; a plurality of pressure compensating valves that respectively control differential pressures before and after the plurality of directional change valves; A main relief valve that regulates an upper limit of the discharge pressure of the pump, wherein a target compensation differential pressure of each of the plurality of pressure compensating valves is a differential pressure between a discharge pressure of the hydraulic pump and a maximum load pressure of the plurality of actuators. And the load sensing target differential pressure and the secondary pressure oppose each other so that the discharge pressure of the hydraulic pump is higher than the maximum load pressure of the plurality of actuators by the load sensing target differential pressure. Pump control means for performing load sensing control including a pump tilt angle control valve guided by the It is set as a variable value depending on the engine speed, and is connected to a pressure oil line discharged from the hydraulic pump, and a target differential pressure of the load sensing control and the maximum load pressure of the plurality of actuators are respectively derived. A hydraulic drive device having a variable unload valve, comprising an operation drive unit, wherein the own target differential pressure is also changed in accordance with a change in the target differential pressure of the load sensing control; The target compensation differential pressure of the pressure compensating valve to the swing motor is a target differential pressure of the load sensing control depending on the rotation speed of the engine, and the target compensation differential pressure of the pressure compensating valve to an actuator other than the swing motor is Setting by a secondary pressure equal to the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators. Hydraulic drive system according to symptoms. 2. The hydraulic drive device according to claim 1, wherein at least one of the plurality of pressure compensating valves is configured to decrease its own outlet discharge amount when the self-load pressure increases. 3. When the discharge pressure of the hydraulic pump rises to the set pressure of the main relief valve, the target compensation differential pressure of the plurality of pressure compensating valves is determined by the discharge pressure of the hydraulic pump and the maximum pressure of the plurality of actuators. A target compensation differential pressure correction means for setting a correction value different from the pressure difference from the load pressure is provided, and the target compensation differential pressure correction means is connected to a maximum load pressure line for detecting the maximum load pressure via a fixed throttle. And a signal pressure variable relief valve for lowering the upper limit of the maximum load pressure detected in the maximum load pressure line by the correction value below the set pressure of the main relief valve. Item 3. The hydraulic drive device according to Item 2.