JP2721384B2 - Hydraulic circuit of work machine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hydraulic circuit of a working machine such as a hydraulic shovel or a hydraulic crane.

[Conventional technology]

Some work machines are provided with a plurality of work members required to perform a desired work. A typical example is a hydraulic shovel. That is, the hydraulic shovel includes a lower traveling body for moving the hydraulic shovel, an upper revolving body pivotally mounted on the lower traveling body, and a front mechanism including a boom, an arm, and a bucket. Various equipment such as a driver's cab, a prime mover, and a hydraulic pump are mounted on the upper swing body, and a front mechanism is attached.

By the way, in recent years, in various work machines, a hydraulic actuator is controlled by maintaining a constant pressure difference (differential pressure) between an upstream side and a downstream side of a directional control valve for controlling a flow rate of pressure oil supplied to a hydraulic actuator. An excellent speed control system called a load sensing system for controlling a driving speed has been proposed. This load sensing system will be described with reference to FIG.

FIG. 6 is a hydraulic circuit diagram showing a part of the hydraulic circuit of the hydraulic shovel. In the drawing, reference numeral 1 denotes a variable displacement hydraulic pump (hereinafter, referred to as a hydraulic pump), 1a denotes a variable displacement capacity mechanism (hereinafter, represented by a swash plate) of the hydraulic pump 1, and 2 denotes a regulator for driving and controlling the swash plate 1a. . The regulator 2 includes a hydraulic cylinder 2a for driving the swash plate 1a, a horsepower control mechanism 2b composed of a discharge capacity switching valve, and a control valve 2c driven by the differential pressure. Reference numeral 3 denotes a swing motor that drives the upper swing body, and 4 denotes a direction switching valve that controls the drive of the swing motor 3. Reference numerals 4p ₁ and 4p ₂ denote pilot pipelines of the directional control valve 4, and when a turning lever (not shown) is operated, a pilot pressure corresponding to the operation amount is introduced. Reference numeral 5 denotes a pressure controller interposed downstream of the direction switching valve 4. The pressure controller 5 comprises a piston 5a and a spring 5b for lightly pressing the piston 5a. Further, the piston 5a has a first pressure receiving surface 5a ₁ and the second pressure receiving surface 5a _2. The first pressure receiving surface 5a ₁ area ratio of the second pressure receiving surface 5a ₂ is, for example, 1.
Reference numerals 6a and 6b denote relief valves provided in the main circuit of the swing motor 3, which regulate the maximum load pressure of the swing motor 3. Reference numeral 7 denotes a detection pipe for guiding the load pressure of the swing motor 3, and reference numeral 8 denotes a shuttle valve for selecting a higher one of the load pressure of the detection pipe and the load pressure of a boom described later. 9 is a tank, 10
Is a maximum load pressure detection pipeline, and 11 is a throttle.

13 is a boom cylinder that drives the boom of the hydraulic shovel. 14 is a directional control valve for controlling the boom cylinder 13,
14p ₁ and 14p ₂ are the pilot pipeline, 15 is the pressure controller, 15a
Is the piston, 15a ₁ and 15a ₂ are the pressure receiving surfaces, 15b is a spring,
Reference numeral 17 denotes a detection pipe for guiding the load pressure of the boom cylinder 13 to the shuttle valve 8, and these correspond to each element of the hydraulic circuit of the swing motor 3. The area ratio between the pressure receiving surfaces 15a ₁ and 15a _{2 is the same as} the area ratio of the pressure controller 5. Further, the direction switching valve 4 and the pressure controller 5, and the direction switching valve 14 and the pressure controller 15 may be integrally formed.

Next, the operation of the hydraulic circuit constituting the load sensing system will be described. To swing the upper swing body of the hydraulic shovel, the operator operates a swing lever (not shown). In response to this, hydraulic pressure is generated in _one pilot line of the direction switching valve 4, for example, the pilot line 4p1, and the direction switching valve 4 is switched to the left position with a throttle according to the operation amount of the turning lever. For this reason, the pressure oil of the hydraulic pump 1 passes through the throttle of the directional control valve 4 and passes through the piston of the pressure controller 5.
A first pressure receiving surface 5a ₁ of 5a presses, through the pressure controller 5 of the piston 5a Te pushed up, it is supplied to the swing motor 3 from the left side main line of the swing motor 3 after a directional control valve 4 again. As a result, the turning motor 3 starts turning in one direction. In this case, since the inertia of the upper swing body is extremely large, most of the oil to be supplied to the swing motor 3 is discharged to the tank 9 via the relief valve 6a, and the load pressure appearing in the detection pipe 7 is reduced by the relief valve 6a. Set pressure. This load pressure is introduced to one side of the control valve 2c of the regulator 2 through the maximum load pressure detection pipe 10 to increase the amount of tilt of the swash plate 1a. However, since the load pressure of the swing motor 3 is high,
By the horsepower control mechanism 2b of the regulator 2, an increase in the amount of tilt of the swash plate 1a is suppressed, and thus the discharge flow rate of the hydraulic pump 1 is also suppressed.

When the swing motor 3 is gradually accelerated in this manner, the amount of oil relieved from the relief valve 6a also gradually decreases accordingly, and the swing motor 3 is driven in accordance with the opening area of the direction switching valve 4. After reaching the vicinity of the normal rotation speed, the load pressure rapidly decreases to a value far lower than the set pressure of the relief valve 6a. Then, the regulator 2 controls the discharge flow rate of the hydraulic pump 1 according to the load pressure having such a low value.

Now, in the above state, if the load pressure increases due to an external load or the like, the downstream pressure of the directional control valve 4 increases, so that the differential pressure decreases. At the same time pressure controller 5
The increased pressure in the pressure receiving surface 5a _1, the pressure controller 5 tries to return to the specified value a differential pressure by reducing the downstream pressure of the directional control valve 4 to increase the throttle amount. Further, the increased load pressure is introduced into the regulator 2, whereby the regulator 2 is driven so as to increase the discharge flow rate of the hydraulic pump 1, so that the upstream pressure of the directional control valve 4 increases, and the differential pressure increases. The pressure returns to the specified value. That is, even if the load pressure rises for some reason such as an external load, the differential pressure of the directional control valve 4 is maintained at the specified value, and the swing motor 3 operates the swing lever despite the increase in the load pressure. A flow rate according to the operation amount is supplied. The operation when the load pressure decreases is the reverse of the above operation, and similarly, a flow rate corresponding to the operation amount of the turning lever is supplied.

As a result, when the swing motor 3 is driven, in a normal state in which the relief valve 6a does not operate, the load sensing system that keeps the differential pressure of the direction switching valve 4 operates, and excellent speed control can be performed. The driving operation of the boom follows this.

Further, an operation in the case of a combined operation of simultaneously driving the swing motor 3 and the boom cylinder 13 will be described. When the swing lever and the boom lever are operated at the same time, the direction switching valves 4 and 14 are opened with the throttles corresponding to the amounts of operation, and pressurized oil is supplied to the swing motor 3 and the boom cylinder 13. , 14 has a predetermined pressure difference between both sides. These differential pressures are finally kept constant as in the operation of the swing motor 3. By the way, in a hydraulic circuit, when a combined operation is performed on a plurality of loads to which the pressure oil is supplied from a common hydraulic pump, the pressure oil is supplied to the light load without any care. It is known that a heavy load becomes difficult to drive. However,
Pressure controller 5 and 15 in the present hydraulic circuit that both the maximum load pressure is applied to the second pressure receiving surface 5a _2, the phenomenon because it is restricted equal can be prevented accordingly.

Here, a hydraulic circuit diagram of a part of another hydraulic circuit of the hydraulic shovel using the load sensing system is shown in FIG. In FIG. 7, parts that are the same as or equivalent to the parts shown in FIG. 6 are given the same reference numerals, and descriptions thereof will be omitted. This load sensing system is different only in that pressure compensating valves 5 'and 15' are provided upstream of the directional control valves 4 and 14 in place of the pressure controllers 5 and 15 of the previous load sensing system. It is the same as the previous load sensing system. The discharge pressure of the hydraulic pump 1 and the load pressure of the hydraulic actuator to which it belongs belong to one side of the pressure compensating valves 5 ', 15', and the other side receives the maximum load from the maximum load pressure detecting line 10. Pressure and its own discharge pressure are introduced.

The operation of this hydraulic circuit is the same as the operation of the previous hydraulic circuit. However, the pressure controllers 5 and 15 define the pressure on the downstream side of the directional control valves 4 and 14, whereas the pressure compensating valves 5 ′ and 15 ′ define the pressure on the upstream side. But,
The function of pressure oil distribution during combined operation is the same for both.

[Problems to be solved by the invention]

In general, in the case of a working machine, there are cases where the work is performed while the drive speed of the hydraulic actuator is greatly reduced.
For example, in the case of a hydraulic shovel, a work for thinning the ground, a work for leveling, a work for making a slope, etc. (hereinafter collectively referred to as a fine operation work) correspond to this. When performing the work in this manner, it is clear that the work can be performed more easily when the change in the flow rate supplied to the hydraulic actuator is smaller than the operation amount of the operation lever of the hydraulic actuator. 8th
The figure is a characteristic diagram of the supply flow rate with respect to the lever operation amount of the operation lever. The horizontal axis represents the lever operation amount, and the vertical axis represents the supply flow rate. In a normal operation, the supply flow rate has a characteristic actually shown, but in a fine operation operation, it is desirable to have a characteristic shown by a broken line.

By the way, in the fine operation work in the hydraulic circuit in which the load sensing system is not used, the characteristic shown by the broken line in FIG. 9 is obtained by lowering the rotation speed of the prime mover, whereby the operator easily performs the fine operation work. I was able to. However, in a hydraulic circuit using a load sensing system, the direction switching valve 4,
Since the control for keeping the differential pressure at 14 constant is performed, the supply flow rate is determined in accordance with the operation amount of the operation lever even when the rotation speed of the prime mover is reduced, and the desired characteristics shown by the broken line in FIG. 8 are obtained. Therefore, it is difficult to control the hydraulic actuator at a low speed by the operation lever, and it is difficult to perform a fine operation work.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the related art, and to provide a hydraulic circuit of a working machine that can easily perform a fine operation even when a load sensing system is used.

[Means for solving the problem]

To achieve the above object, the present invention provides a hydraulic pump, a regulator for controlling the hydraulic pump, a hydraulic actuator driven by pressure oil of the hydraulic pump, and a directional control valve for controlling the hydraulic actuator.
A pressure regulator that regulates the differential pressure between the upstream side and the downstream side of the direction switching valve in accordance with the pressure difference between the supply pressure of the hydraulic pump and the load pressure of the actuator, and the pump supply pressure and the load pressure of the actuator. In a hydraulic circuit of a work machine that controls the regulator so that the differential pressure of the regulator maintains a specified value, the load pressure introduced into the regulator may be set so that the differential pressure acting on the regulator becomes larger than the specified value. The control pressure adjusting means for adjusting the pressure is provided.

(Operation)

During normal work, the control pressure adjusting means is not operated, and control is performed such that the differential pressure at the direction switching valve becomes a predetermined value (prescribed value). On the other hand, at the time of fine operation, the control pressure adjusting means is operated, and the regulated pressure from the operated control pressure adjusting means is applied to the regulator. Thereby, the regulator controls the hydraulic pump such that the differential pressure at the directional control valve becomes lower than the specified value. As a result, the supply flow rate through the directional control valve with respect to the operation amount of the operation lever is reduced, and it becomes possible to easily perform the fine operation.

〔Example〕

 Hereinafter, the present invention will be described with reference to the illustrated embodiments.

FIG. 1 is a circuit diagram of a part of a hydraulic circuit of a hydraulic shovel according to a first embodiment of the present invention. In the figure, the same parts as those shown in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted. 20
Is a pressure reducing valve interposed in the maximum load pressure detecting line 10 connected to the regulator 2, and 21 is an adjusting spring of the pressure reducing valve 20. Output pressure of the pressure reducing valve 20 (command pressure) on the adjustment spring 21 side
P _S is applied, and the opposite side is applied with the maximum load pressure P _L.MAX .

Next, the operation of this embodiment will be described. During normal operation, the spring force F of the adjustment spring 21 is set to 0 or a value close to 0. As a result, the maximum load pressure P
_{When L.MAX} occurs, the pressure on the output side of the pressure reducing valve 20 is almost equal to the value P.
_L.MAX and this pressure is applied to the regulator.
This is an operation mode similar to that of the conventional hydraulic circuit shown in FIG. 6, in which control is performed to maintain the pressure difference between the upstream side and the downstream side of the directional control valve at a specified value (this is ΔP). It will be.

On the other hand, at the time the fine operation work, to adjust the spring force of the adjusting spring 21, the command pressure P _S is set to be the following relation with respect to the maximum load pressure P _L.MAX and differential pressure of the specified value [Delta] P.

_{P L.MAX -ΔP <P S ≦ P} L.MAX ...... (1) is described in equation (1), below P _L.MAX -ΔP <P _S. Now, when the supply pressure from the hydraulic pump 1 and P _P, the specified value ΔP of the differential pressure is expressed by the following equation.

ΔP = P _P −P _S (2) Accordingly, the supply pressure P _P is P _P = P _S + ΔP (3). By the way, since the condition for driving the hydraulic actuator is P _P > P _L.MAX , if this is expressed using the equation (3), P _S + ΔP> P _L.MAX (4) 4) the above equation (1) P _L.MAX -ΔP <P _S ...... from the equation becomes (5). That, in order to drive one or more hydraulic actuators to be operated, command pressure P _S from the pressure reducing valve 20 must be a value satisfying the expression (5).

By the way, the pressure reducing valve 20 has a maximum load pressure P _{L.MAX on one side.}
However, since the command pressure P _S and the spring force F are applied to the other, the balance equation is as follows.

P _L.MAX = P _S + F (6) Accordingly, the command pressure P _s becomes P _S = P _L.MAX -F (7). Therefore, if the spring force F is selected so that 0 ≦ F <ΔP, the expression (1) can be satisfied. Note that the case where F = 0 is selected corresponds to the above-described normal operation.

In the fine operation, the differential pressure to be kept constant is reduced by appropriately selecting the spring force F of the adjusting spring 21. Now, by selecting an adjustment spring 21 as appropriate, and to obtain a pressure in the following equation as a command pressure P _S.

P _S = P _L.MAX −ΔP / 2 (8) Then, the supply pressure P _P becomes P _P = P _S + ΔP = P _L.MAX + ΔP / 2 (9). Therefore, the differential pressure in this case is P _P -P _L.MAX = ΔP / 2 (10). That is, the differential pressure in this case is reduced to half of the specified value. In other words, by appropriately selecting the spring force of the adjusting spring 21 of the pressure reducing valve 20, the prescribed value of the differential pressure of the direction switching valve can be changed to a small value. in this way,
By setting the differential pressure to a small value, even if the operation amount of the lever of the hydraulic actuator is the same as that during the normal operation, the supply amount of the pressure oil supplied to the hydraulic actuator is reduced, and is indicated by a broken line in FIG. Properties can be obtained.

As described above, in the present embodiment, the command pressure to the regulator is reduced by the pressure reducing valve inserted into the maximum load pressure detection line during the fine operation work, so that the hydraulic actuator is operated with respect to the operation amount of the operation lever. The supply flow rate of the pressurized oil can be reduced, and fine operation can be easily performed.

FIG. 2 is a circuit diagram of a part of a hydraulic circuit of a hydraulic shovel according to a second embodiment of the present invention. In the figure, the same parts as those shown in FIG. twenty three
Is a pressure reducing valve, and 24 is an adjusting spring of the pressure reducing valve 23. The pressure reducing valve 23 and the adjusting spring 24 correspond to the pressure reducing valve 20 and the adjusting spring 21 in the above embodiment, respectively. The operation of this embodiment is similar to the operation of the previous embodiment, and the effect is the same as that of the previous embodiment.

FIG. 3 is a circuit diagram of a part of a hydraulic circuit of a hydraulic shovel according to a third embodiment of the present invention. In the figure, the same parts as those shown in FIG. 1 are denoted by the same reference numerals. It should be noted that the detailed illustration of the regulator 2 is omitted and is simply shown by a block (hereinafter the same in FIGS. 4 and 5). In this figure, illustrations of a hydraulic actuator, a direction switching valve, a pressure controller, and the like are omitted. E is a prime mover for driving the hydraulic pump 1, E _G is the prime mover E governor lever, E _L is the prime mover lever for operating the governor lever E _G, E _R is the governor lever E _G
As a rod that connects the prime mover lever E _L. 26 is a pressure reducing valve inserted into the maximum load pressure detecting line 10, and 27 is an adjusting spring thereof. Adjusting spring 27 is connected to the rod E _R, the spring force is caused to vary in accordance with the shift rod E _P.
Structure of this embodiment, except for the portion where adjusting spring 27 is coupled to the rod E _R, is the same as the configuration of the first embodiment.

Next, the operation of this embodiment will be described. During normal work requiring a high drive speed, the prime mover lever _EL is operated to a position where the rotational speed of the prime mover E is increased. By this operation, the spring force of the adjustment spring 27 becomes zero or a value close to zero, and the maximum load pressure is applied to the regulator 2. Therefore, in this case, control for maintaining the differential pressure at the specified value is performed. On the other hand, during the fine operation, the prime mover lever E _L
Is operated to a position where the rotational speed of the motor is reduced. By this operation, the spring force of the adjustment spring 27 becomes an appropriate value, and a command pressure satisfying the above equation (1) determined by the spring force is applied to the regulator 2. Therefore, in this case, control for maintaining the differential pressure at a value smaller than the specified value is performed, and the supply flow rate is reduced. This embodiment also provides the same effects as the previous embodiments.

FIG. 4 is a circuit diagram of a part of a hydraulic circuit of a hydraulic shovel according to a fourth embodiment of the present invention. In the figure, the same parts as those shown in FIG. 30
Is a pilot pump driven by the prime mover E together with the hydraulic pump 1, 31 is a relief valve for regulating the maximum pressure of the pilot line, 32 is a pressure reducing valve, 33 is a pilot port at one end of the pressure reducing valve 32, and 34 is a pilot port 33. This is an electromagnetic proportional valve inserted in the pilot line leading to. 35 is a lever position detector for outputting an electrical signal corresponding to the detected the operating position an operating position of the prime mover lever E _L. A controller 36 controls the electromagnetic proportional valve 34 based on the signal of the lever position detector 35. The pressure reducing valve 32 is different from the first and third embodiments only in that the pressure reducing valves 20, 26 are adjusted by the adjusting springs 21, 27, whereas they are adjusted by the pilot pressure.

Next, the operation of this embodiment will be described. During normal operation, the prime mover lever _EL is operated to the high speed position, and the lever position detector 35
Outputs a signal corresponding to this. Controller 36
Motor lever E _L is determined to be in the high speed position, controls the electromagnetic proportional valve 34 as pilot pressure applied to the pilot port 33 becomes a value close to 0 or 0 by the signal. Thereby, control for maintaining the differential pressure at the specified value is performed. During fine operation work, the prime mover lever E _L is operated in the low speed position, the controller 36 determines this by the output signal of the lever position detector 35, the pilot pressure applied to the pilot port 33 by controlling the solenoid proportional valve 34 The value shall be a predetermined appropriate value or a value corresponding to the motor lever position.
As a result, the command pressure to the _{regulator 2} is smaller than the value P _{L.MAX and} is smaller than the value (P
_{L.MAX -ΔP)} is caused to drop to a pressure corresponding to a defined position of the pressure or the prime mover lever E _L within a larger range, as a result, control for maintaining the value smaller than the specified value a differential pressure is carried out . This embodiment also provides the same effects as the previous embodiments.

FIG. 5 is a circuit diagram of a part of a hydraulic circuit of a hydraulic shovel according to a fifth embodiment of the present invention. In the figure, the same portions as those shown in FIGS. 1 and 4 are denoted by the same reference numerals, and description thereof will be omitted. 37 is a maximum load pressure detector which outputs the maximum load pressure selected by the shuttle valve 8 as an electric signal proportional thereto, and 38 is a lever position detector 35 and a maximum load pressure detector 37.
Is a controller for performing predetermined calculation and control by inputting the signal of the above. 39 is an electromagnetic valve inserted between the discharge side pipeline of the hydraulic pump 1 and the regulator 2. The solenoid valve 39 is controlled by the controller 38.

Next, the operation of this embodiment will be described. During normal operation, with the prime mover lever E _L it is determined that it is operated in the high speed position by the output signal of the controller 38 the fourth embodiment and the same lever position detector 35 for Hasaki, the maximum load pressure detector 37 The maximum load pressure is captured by the output signal. And
By controlling the solenoid valve 39, the discharge pressure of the hydraulic pump 1 is reduced to the maximum load pressure. This maximum load pressure is applied to the regulator 2. As a result, normal control for maintaining the differential pressure at the specified value is performed.

On the other hand, during the fine operation tasks, it determines that the motor lever E _L is operated to the low speed position by the output signal of the controller 38 the fourth embodiment and the same lever position detector 35 of Hasaki. When the determination is made in this way, the controller 38 controls the solenoid valve 39 so that the discharge pressure of the hydraulic pump 1 is smaller than the value P _{L.MAX and} is smaller than the value (P
_{L.MAX -ΔP)} depressurizing the pressure corresponding to a defined position of the pressure or the prime mover lever E _L within a larger range. The reduced pressure is applied to the regulator 2 to control the differential pressure to be maintained at a certain value smaller than a specified value. The effect of this embodiment is the same as the effect of each embodiment described above.

In the above embodiments, the hydraulic circuit of the hydraulic shovel has been described as an example. However, the present invention is not limited to this, and it is apparent that the present invention is applicable to hydraulic circuits of other work machines.

〔The invention's effect〕

As described above, in the present invention, the control pressure adjusting means can apply a command pressure to the regulator so as to reduce the differential pressure of the directional control valve. The supply flow rate with respect to the operation amount can be reduced, and the fine operation can be easily performed even when the load sensing system is employed.

[Brief description of the drawings]

FIGS. 1, 2, 3, 4, and 5 show one example of a hydraulic circuit of a hydraulic shovel according to first, second, third, fourth, and fifth embodiments of the present invention, respectively. FIG. 6 and FIG. 7 are circuit diagrams of a part of a hydraulic circuit of a conventional hydraulic shovel,
FIG. 8 is a characteristic diagram of the supply flow rate with respect to the lever operation amount. 1 ... Hydraulic pump, 2 ... Regulator, 3 ... Slewing motor, 4,14 ... Directional switching valve, 5,15 ... Pressure controller,
5 ', 15': pressure compensation valve, 8: shuttle valve, 10 ...
Maximum load pressure detection line, 13 …… Boom cylinder, 20,23,2
6, 32 ... pressure reducing valve, 34 ... electromagnetic proportional valve, 35 ... lever position detection, 36, 38 ... controller, 39 ... electromagnetic valve, E ...
Motor, E _L … Motor lever

Claims (8)

(57) [Claims] 1. A hydraulic pump, a regulator for controlling the hydraulic pump, a hydraulic actuator driven by pressure oil of the hydraulic pump, a directional control valve for controlling the hydraulic actuator, and an upstream side of the directional control valve. And a pressure regulator that regulates the differential pressure between the downstream side and the supply pressure of the hydraulic pump and the load pressure of the actuator, wherein the pressure difference between the pump supply pressure and the load pressure of the actuator is a specified value. In the hydraulic circuit of the work machine controlling the regulator to maintain the control pressure, the control pressure for arbitrarily adjusting the load pressure introduced to the regulator so that the differential pressure acting on the regulator becomes larger than the specified value. Hydraulic circuit of a working machine, characterized by having an adjusting means 2. A hydraulic circuit for a working machine according to claim 1, wherein said pressure regulator is a pressure compensating valve interposed upstream of said direction switching valve. 3. A hydraulic circuit for a working machine according to claim 1, wherein said pressure regulator is a pressure controller interposed downstream of said directional control valve. 4. A hydraulic circuit for a working machine according to claim 1, wherein said control pressure adjusting means is a valve device for reducing and outputting a maximum load pressure of said hydraulic actuator. 5. A hydraulic circuit for a working machine according to claim 4, wherein said valve device comprises an adjusting means for adjusting the opening and closing of said valve device. 6. An apparatus according to claim 5, wherein said adjusting means comprises:
A hydraulic circuit for a working machine, wherein the hydraulic circuit is an adjusting spring that is operated in conjunction with the operation of a rotation speed command means of a prime mover that drives the hydraulic pump. 7. The method according to claim 5, wherein the adjusting means comprises:
A hydraulic pressure for a working machine, comprising: an electromagnetic proportional valve driven in response to operation of a rotation speed command means of a prime mover for driving the hydraulic pump; and a pilot port for introducing a pilot pressure output from the electromagnetic proportional valve. circuit 8. The control pressure adjusting means according to claim 1, wherein said control pressure adjusting means reduces the discharge pressure of said hydraulic pump based on a rotation speed of a motor driving said hydraulic pump and a maximum load pressure of said hydraulic actuator. Hydraulic circuit for working machine, characterized by being a valve