JP2010047983A5 - - Google Patents

Description

Hydraulic circuit of excavator

  The present invention relates to a hydraulic circuit of a hydraulic excavator, and more particularly to a hydraulic circuit of a hydraulic excavator that can improve work efficiency during combined operation of a boom and an arm.

  The hydraulic excavator is equipped with a boom, an arm and a bucket, and a hydraulic circuit including a boom cylinder, an arm cylinder and a bucket cylinder for driving them. In a negative control (hereinafter referred to as “negative control”) type hydraulic circuit, two hydraulic pumps are provided, and a negative control throttle is provided at the most downstream of the center bypass oil passage of each hydraulic pump to generate negative control pressure. By applying the negative control pressure to the pump regulator, the discharge flow rate of the hydraulic pump can be negatively controlled.

  FIG. 5 shows a conventional example of a hydraulic circuit of this type of excavator. Reference numerals 1 and 2 denote variable displacement hydraulic pumps 1 and 2, respectively. The upstream sides of the center bypass oil passages 10 and 11 of the hydraulic pumps 1 and 2 are connected to an oil tank 3. Further, parallel oil passages 12 and 13 are branched from the center bypass oil passages 10 and 11, respectively, and a throttle 27 is provided in the middle of the parallel oil passage 13.

  Further, a plurality of directional control valves are connected to the center bypass oil passages 10 and 11 respectively. That is, the boom switching valve 17 and the arm first speed switching valve 18 are connected in tandem to the center bypass oil path 10, and the bucket switching valve 14, boom switching valve 15, and arm second speed switching valve 16 are connected to the center bypass oil path 11. Are connected in tandem.

Further, the arm switching valve (first speed, second speed) in the center bypass oil path 10, 11 on the lower stream side of the 18, 16 together with the negative control throttle 8 and 9 are respectively provided, resulting in the negative control throttle 8,9 The negative control pressure is fed back to the pump regulators 4 and 5 via the negative control pressure circuits 6 and 7. The pump regulators 4 and 5 are configured to control the discharge amounts of the hydraulic pumps 1 and 2 by adjusting the tilt angle based on the negative control pressure.

  Thus, when the bucket operating lever 28 is operated to the bucket closing side, the operation signal is input to the bucket closing pilot port of the bucket switching valve 14 via the bucket operation pilot line 29, and the bucket switching valve 14 is closed. Switch to position (A). For this reason, each outlet port of the bucket switching valve 14 is connected to the bucket cylinder 19.

  Further, when the boom operation lever 22 is operated to the boom raising side, the operation signal is input to the boom raising pilot ports of the boom switching valves 15 and 17 via the boom operation pilot line 24, and the boom switching valves 15 and 17. To the boom up position (A). Therefore, the outlet ports of the boom switching valves 15 and 17 are connected to the boom cylinder 20.

Furthermore, by operating the arm control lever 23 to the side can arm pull, is input to the arm Technology of pilot ports of the arm directional control valve 16 via the operation signal arm operating pilot line 26 and 25, the arm the switching valve 16, 18-out arm pull switch to the position (a). For this reason, each outlet port of the arm switching valves 16 and 18 is connected to the arm cylinder 21 (see, for example, Patent Documents 1 and 2).
JP 58-146632 A Japanese Patent No. 3142864

In the prior art described in Patent Document 1, as shown in FIG. 6, at the same time it performs a combined operation of boom-up and-out arm pull in order to perform the horizontal pulling by the hydraulic excavator 40, the switching valve for the cylinder drive 15-18 Are all switched to the operating position (A). In this case, the arm 45 falls by its own weight, but the boom 44 is subjected to the load of the entire work machine.

For this reason, the load pressure of the boom 44 is higher than the load pressure of the arm 45, and most of the discharge oil of the hydraulic pump 1 flows to the arm cylinder 21 side having a lower load than the boom cylinder 20 through the parallel oil passage 12. Supplied. On the other hand, most of the oil discharged from the hydraulic pump 2, since the aperture 27 is provided in parallel fluid passage 13 is subjected fed to the boom cylinder 20.

Here, parts having the greatest need boom-up speed is an A point where the bucket 46 is flat on the ground (see FIG. 6), in water Hirahiki operation, the bucket 46 is substantially perpendicular to the ground As the point B is approached, hydraulic oil for maintaining the boom raising speed gradually becomes unnecessary. When the bucket 46 passes through the point B, the boom 44 shifts from the raising operation to the lowering operation.

Therefore, the operator performs a full stroke operation of the boom raising operation at the point A, but immediately after returning to the point A, the operator starts to return the boom operation lever 22 to the original operation position. Would otherwise hydraulic oil which becomes unnecessary in boom-up operation, in order to efficiently perform operations-out arm pull, it should be much supplied to the arm cylinder 21 side.

However, the spool opening characteristics of the boom switching valve 15, as shown in FIG. 7, is set to narrow the center bypass opening PT than the stroke initial (initial lever operation). Thereby, when performing the suspension work with the boom, the bleed amount of the discharged oil from the hydraulic pump 2 can be suppressed as much as possible, and the suspension work can be performed with high accuracy even in the fine operation region of the boom operation lever 22.

Therefore, even if the boom control lever 22 is immediately returned at the point A, the center bypass oil passage 11 remains restricted, and the parallel oil passage 13 is provided with the restrictor 27, so that it is not necessary for the boom raising operation. It is not possible to immediately cause the hydraulic oil to flow into the arm cylinder 21.

As a result, either the arm speed during horizontal pulling work is just not up, the with heat balance by the heat generation of the pressure oil in the diaphragm 27 collapses, the engine horse power consumption is disadvantageously worsen the fuel efficiency increases.

Here, it is conceivable to remove the throttle 27 provided in the parallel oil passage 13 so that the hydraulic oil that is no longer necessary when the boom is raised flows to the arm cylinder 21 side. However, when removing the diaphragm 27, oil discharged from the hydraulic pump 2 becomes to flow more towards the arm cylinder 21 to horizontal pull early. As a result, it is impossible to work in the water flat argument is difficult to perform the boom-up.

In order to solve the above problem, the throttle 27 is changed to a variable throttle, the pump discharge pressure and the like are detected, and the electromagnetic proportional valve is operated based on the detection result, thereby controlling the variable throttle and raising the boom. A technique for stably performing the above has also been proposed (see Patent Document 2). However, in this case, the number of parts increases and an expensive electromagnetic proportional valve is required, which increases the cost and is not practical.

  Therefore, there is a technical problem to be solved in order to improve the horizontal pulling work efficiency while ensuring stable boom raising, and to improve the heat balance and fuel consumption, and the present invention solves this problem. The purpose is to do.

  The present invention has been proposed to achieve the above object, and the invention according to claim 1 is characterized in that a boom switching valve and an arm second speed switching valve are connected in tandem to a center bypass oil passage communicating with a hydraulic pump. Further, in the hydraulic circuit of a hydraulic excavator in which the boom switching valve and the arm second speed switching valve are connected in parallel to the parallel oil path branched from the center bypass oil path, the arm second speed switching is performed based on the boom raising operation amount. Stroke control means for controlling the spool stroke of the valve is provided to limit the spool stroke when the boom raising operation amount is equal to or greater than a set value, and to limit the spool stroke when the boom raising operation amount is equal to or less than the set value. A hydraulic circuit for a hydraulic excavator is provided that is configured to release

According to this configuration, by providing the stroke control means for controlling the spool stroke of the arm second speed selector valve on the basis of the boom raising operation amount, the time of operating the-out boom raising and arm pulling simultaneously, the boom raising operation amount Is equal to or greater than the set value, the spool stroke of the arm 2-speed switching valve is limited, so that the opening area of the arm 2-speed switching valve is reduced. When the boom raising operation amount by the boom operating lever is returned is below the set value, the stroke limit of the arm second speed selector valve is released, the arm second speed selector valve corresponds to an operation amount-out arm pull spool Switch to stroke.

  The invention according to claim 2 is the pilot type pressure reducing valve in which the stroke control means is operated by the boom raising operation amount, and the secondary pressure of the pilot pressure reducing valve is applied to the pilot port of the arm second speed switching valve. 2. The hydraulic circuit for a hydraulic excavator according to claim 1, wherein the hydraulic stroke circuit is configured to operate and control a spool stroke of the second speed switching valve of the arm.

According to this arrangement, when the manipulating-out boom raising and arm pulling simultaneously to when the boom raising operation amount is equal to or greater than a set value, pressure reducing action of the pilot pressure reducing valve is inputted to the pilot port of the arm second speed selector valve Thus, the spool stroke of the arm second speed switching valve is limited. When the boom raising operation amount becomes equal to or less than the set value, the pressure reducing action by the pilot type pressure reducing valve is reduced, so that the restriction of the spool stroke is released.

According to a third aspect of the invention, the stroke control means, and the electromagnetic proportional reducing valve secondary pressure to a pilot port of the arm second speed selector valve acts, the arm pull-out pilot pressure and the boom raising Paitto pressure respectively detected And a controller that controls a secondary pressure of the electromagnetic proportional pressure reducing valve based on a detection signal from the pressure sensor, and the controller is configured to control the arm when the boom raising pit pressure is equal to or higher than a set value. The secondary pressure of the electromagnetic proportional pressure reducing valve is controlled so as to limit the spool stroke of the 2-speed switching valve, and the secondary pressure of the electromagnetic proportional pressure reducing valve is reduced when the boom raising pit pressure becomes lower than the set value. to provide a hydraulic circuit of a hydraulic excavator according to claim 1, wherein the controller controls so as to equal to the beam pull-out pilot pressure.

According to this arrangement, when the manipulating-out boom raising and arm pulling simultaneously, arm pull-out pilot pressure and the boom raising Paitto pressure is respectively detected two pressure sensors, the detection signal is input to the controller. The controller adjusts the secondary pressure of the electromagnetic proportional pressure reducing valve based on the detection signal to control the spool stroke of the arm second speed switching valve.

That is, when the boom raising pit pressure is equal to or higher than the set value, the secondary pressure of the electromagnetic proportional pressure reducing valve is controlled so as to limit the spool stroke of the arm second speed switching valve, and the boom raising pit pressure becomes lower than the set value. secondary pressure of the electromagnetic proportional pressure reducing valve is controlled to be equal to the arm Technology of pilot pressure when the. Therefore, the boom raising and arm pulling Kino during the combined operation, the boom-raising Paitto pressure falls below a specified value, corresponding to the operation amount-out arm pull spool stroke of the arm second speed selector valve is increased.

Claim invention of claim 4, when arm pulling Kino load pressure is equal to or greater than the set value, characterized by being configured so that the control spool stroke of the arm second speed selector valve is not carried out A hydraulic circuit for a hydraulic excavator as described in 1, 2 or 3 is provided.

According to this arrangement, when the arm pulling Kino load pressure is equal to or greater than the set value, the stroke control means (pilot type pressure reducing valve, electromagnetic proportional pressure reducing valve) controls the spool stroke of the arm second speed selector valve according to is stopped The arm is operated by normal hydraulic control.

First aspect of the present invention, when operating the-out boom raising and arm pulling simultaneously and begins returning to its original position the operating lever of the boom-up, since the opening area of the arm second speed selector valve is increased, the arm hydraulic oil increases flowing into the cylinder, at the same time arm pull-outs working speed is up, the heat generated by such aperture conventional improves heat balance performance not occur. Furthermore, engine power consumption is reduced and fuel efficiency is improved.

  In the invention according to claim 2, in addition to the effect of the invention according to claim 1, since the spool stroke of the arm second speed switching valve is controlled by the pilot type pressure reducing valve operated by directly inputting the boom raising operation amount. Although the configuration is simple, the switching control of the spool stroke of the arm second speed switching valve can be performed more quickly. Further, since the number of parts is smaller than that of the conventional example and an expensive proportional solenoid valve is not required, the manufacturing cost can be reduced.

In the invention of claim 3, wherein the controller adjusts the secondary pressure of the electromagnetic proportional pressure reducing valve based on a signal arm Technology of the pilot pressure and the boom raising Paitto pressure, and controls the spool stroke of the arm second speed selector valve In addition to the effect of the first aspect of the invention, the secondary pressure of the electromagnetic proportional pressure reducing valve can be accurately adjusted by an electric signal output from the controller to the electromagnetic proportional pressure reducing valve. Control can be performed with higher accuracy.

The invention of claim 4, wherein, when the arm pulling Kino load pressure rises above the set value, the spool stroke control arm second speed selector valve according to the stroke control means is stopped, according to claim 1, wherein in addition to the effect of the invention, it is possible to efficiently perform the work-out arm pull according to the normal hydraulic control.

The present invention, while ensuring a stable boom raising was up arm pull-outs work efficiency, and, in order to achieve the object of improving the heat balance and fuel consumption, the center bypass oil passage communicating the hydraulic pump Hydraulic pressure of a hydraulic excavator in which a boom switching valve and an arm second speed switching valve are connected in tandem, and the boom switching valve and the arm second speed switching valve are connected in parallel to a parallel oil path branched from the center bypass oil path. The circuit is provided with stroke control means for controlling the spool stroke of the second speed switching valve of the arm on the basis of the boom raising operation amount, the spool stroke is limited when the boom raising operation amount is a set value or more, and the boom raising is performed. Realized by releasing the restriction of the spool stroke when the manipulated variable falls below the set value. It was.

  A preferred embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a side view showing a hydraulic excavator 40 according to this embodiment. As shown in the figure, an upper swing body 43 is mounted on the lower traveling body 41 via a swing mechanism 42. The upper swing body 43 drives a boom 44, an arm 45, a bucket 46, and these. A hydraulic cylinder such as a boom cylinder 20, an arm cylinder 21 and a bucket cylinder 19, and a cabin 47 are mounted.

  FIG. 2 shows a hydraulic circuit of the excavator 40. The present embodiment is applied to a negative control type hydraulic circuit, and includes two variable displacement type hydraulic pumps 1 and 2, and is provided at the most downstream of the center bypass oil passages 10 and 11 of the hydraulic pumps 1 and 2. Negative control throttles 8 and 9 are provided to generate a negative control pressure, and the negative control pressure is applied to pump regulators 4 and 5 to negatively control the discharge flow rates of the hydraulic pumps 1 and 2. It should be noted that the same components as those in the conventional example shown in FIG.

In this embodiment, removing the diaphragm installed in the hydraulic circuit of the conventional example (reference numeral 27 in FIG. 5) from the parallel fluid passage 13,-out arm pull arm second speed selector valve 16 a pilot type pressure reducing valve 30 and instead the pilot A pilot port of the pilot type pressure reducing valve 30 is interposed in the line 26 and connected to a branch path 24A separated from the boom raising pilot line 24. The pilot pressure reducing valve 30 functions as a stroke control means for controlling the spool stroke of the arm second speed switching valve 16.

Further, the spool opening characteristics of the arm second speed selector valve 16 in the time-out arm pull is set as shown in FIG. That, PC port opening area Q of the arm second speed selector valve 16 is identical to the opening area of the conventional diaphragm 27 to the spool of the arm second speed selector valve 16 reaches the arm Technology of initial stroke position B Is set to have an area A. And in the area | region where the arm spool passed the said position B, the increase degree of PC port opening area Q of the arm 2 speed switching valve 16 is set so that it may expand gradually.

Therefore, the secondary pressure of the pilot type pressure reducing valve 30, even if the operation-out arm pull in the arm second speed selector valve 16, if the boom raising pilot pressure less than a predetermined value is applied, the arm second speed The stroke of the spool of the switching valve 16 is restricted at the position B. The stroke restriction is released only when the boom raising pilot pressure falls below a predetermined value, and the spool of the arm second speed switching valve 16 can stroke beyond the position B.

By configuring as described above, since the stroke of the spool operation of the arm second speed switching valve 16 is restricted at the position B at the initial stage of water drawing, the PC port of the arm second speed switching valve 16 is connected to the parallel oil passage 13. to demonstrate the aperture control function for both the boom-up operation is carried out smoothly and-out arm pull.

As shown in FIG. 6, when the operator starts to return the boom operation lever 22 to the original operation position after the bucket 46 has landed on the ground during the combined operation, the second speed switching by the pilot type pressure reducing valve 30 is performed. The pressure reducing action on the valve 16 gradually decreases. Therefore, the spool of the horizontal pull arm second speed selector valve 16 that initially the stroke has been regulated, move beyond the position B, and finally will be able to full stroke.

Therefore, when the boom operation lever 22 starts to be returned, the PC port opening area Q of the arm second speed switching valve 16 starts to increase gradually, so that the flow rate of pressure oil toward the boom cylinder 20 decreases and at the same time toward the arm cylinder 21. Pressure oil flow increases. Therefore, the operating speed of the horizontal pull is up, heat balance eliminates heat generated by such squeezing the prior art are favorably maintained, and fuel consumption performance is significantly improved.
Further, the number of parts is smaller than that of the conventional example, and the manufacturing cost can be reduced. Further, the spool stroke switching control of the arm second speed switching valve 16 can be performed more quickly while the configuration is simple.

FIG. 4 shows another embodiment of the present invention. This example, stroke control means for controlling the spool stroke of the arm second speed selector valve 16, the solenoid proportional pressure reducing valve 31 acting secondary pressure to the pilot port of the arm second speed selector valve 16,-out arm pull pilot Two pressure sensors 34 and 35 for detecting the pressure and the boom raising pit pressure, respectively, and a controller 33 for controlling the secondary pressure of the electromagnetic proportional pressure reducing valve 31 based on the detection signals from the pressure sensors 34 and 35, The secondary pressure of the electromagnetic proportional pressure reducing valve 31 is controlled so as to limit the spool stroke of the arm second speed switching valve 16 when the boom raising pit pressure is equal to or higher than the set value, and the boom raising pit pressure is less than the set value. sometimes secondary pressure of the electromagnetic proportional pressure reducing valve 31 and controls to be equivalent to the arm Technology of pilot pressure.
In this embodiment, an electromagnetic proportional pressure reducing valve 31 is provided instead of the pilot pressure reducing valve 30 shown in FIG. Then, the inlet side port of the solenoid proportional pressure reducing valve 31 is connected to the hydraulic pump 36, electric磁比example pressure reducing valve 31 exerts a pressure reducing action in response to a command signal from the controller 33 (current signals), The secondary pressure of the electromagnetic proportional pressure reducing valve 31 is configured to act on the pilot port of the arm second speed switching valve 16.

Further, an arm operating lever pressure sensor 34 and a boom operating lever pressure sensor 35 are connected to the controller 33. The arm control lever for the pressure sensor 34 detects the pressure of the arm pull-out pilot line 25 which occurs during the horizontal pulling operation of the arm operation lever 23, also, the boom operating lever for the pressure sensor 35, the boom operation lever 22 The pressure of the boom raising pilot line 24 generated during the raising operation is detected. As a result, the controller 33 controls the secondary pressure of the electromagnetic proportional pressure reducing valve 31 based on the detection signals from the pressure sensors 34 and 35 as follows.

That is, when the boom raising pilot pressure is set pressure, the arm at the operating lever 23 a-out arm pull and full stroke operation, the spool of the arm second speed selector valve 16 is higher stroke position B in FIG. 3 The secondary pressure of the electromagnetic proportional pressure reducing valve 31 is controlled so as not to move. When the boom raising pilot pressure becomes lower set pressure, the secondary pressure of the electromagnetic proportional pressure reducing valve 31 is controlled to be equal to the arm Technology of pilot pressure.

Therefore, in this embodiment, the electromagnetic proportional pressure reducing valve 31 whose operation is controlled by the controller 33 has a function equivalent to that of the pilot pressure reducing valve 30 according to the above embodiment. obtain. That is, the time of operating the-out boom raising and arm pulling simultaneously, the spool stroke of the early horizontal pulling lever arm second speed selector valve 16 when the boom raising operation amount (boom raising pilot pressure) is equal to or greater than a set value The amount is regulated.

Depending, even by operating the spool of the arm second speed selector valve 16, can not be obtained stroke corresponding to the operation amount-out arm pull, as a result, the opening area of the PC port of the arm second speed selector valve 16 is throttled . When the boom raising operation amount by the boom operation lever 22 is returned it is below the set value, the stroke restricting arm second speed selector valve 16 is released, the stroke can be obtained which corresponds to the arm pull-out operation amount.

Thus, when operating the-out boom raising and arm pulling At the same time, the boom operating lever 22 starts to return the original operating position, equivalent to the secondary pressure arm pull-out pilot pressure proportional solenoid pressure reducing valve 31 It is controlled to become. Thus, as the spool can move stroke amount corresponding to an operation amount-out arm pull, because switching the arm second speed selector valve 16, the opening area of the PC port of the arm second speed selector valve 16 in Figure 3 the spool opening Increases according to characteristics.

Therefore, inflow of hydraulic fluid to the arm cylinder 21 increases abruptly, arm pull-outs working speed is up. In addition, since heat generation as in the conventional case does not occur, the heat balance performance is improved, and the engine consumption horsepower is reduced and the fuel consumption performance is also improved.

  In this embodiment, the secondary pressure of the electromagnetic proportional pressure reducing valve 31 can be accurately adjusted by an electric signal output from the controller 33 to the electromagnetic proportional pressure reducing valve 31, so that the spool stroke of the arm second speed switching valve 16 can be controlled with higher accuracy. can do.

In the above two embodiments, the spool stroke control by pilot type pressure reducing valve 30 or the solenoid proportional pressure reducing valve 31 or the like is configured to stop when arm pulling Kino load pressure is equal to or greater than the set value. Therefore, when the arm pulling Kino load pressure rises above the set value, the spool stroke control arm second speed selector valve 16 is not performed, arm according to the normal hydraulic control (control the spool stroke is not restricted) it is possible to perform the pull-out work efficiently.

  In the above embodiment, the negative control type hydraulic circuit is adopted, but the same effect as described above can be obtained even when applied to the positive control type hydraulic circuit.

  It should be noted that the present invention can be variously modified without departing from the spirit of the present invention, and the present invention naturally extends to the modified ones.

The side view of the hydraulic shovel which shows one Example which concerns on this invention. The block diagram which shows the hydraulic circuit of the hydraulic shovel which concerns on one Example. The graph which shows the arm spool opening characteristic which concerns on one Example. The block diagram which shows the hydraulic circuit of the hydraulic shovel which concerns on the other Example which concerns on this invention. The block diagram which shows the hydraulic circuit of the hydraulic shovel which concerns on a prior art example. Explanatory drawing which shows the state when performing horizontal pulling with a hydraulic excavator. The graph which shows the boom spool opening characteristic which concerns on a prior art example.

DESCRIPTION OF SYMBOLS 1 Hydraulic pump 2 Hydraulic pump 10 Center bypass oil path 11 Center bypass oil path 12 Parallel oil path 13 Parallel oil path 14 Bucket switching valve 15 Boom switching valve 16 Arm 2 speed switching valve 17 Boom switching valve 18 Arm 1 speed switching valve 19 Bucket Cylinder 20 Boom cylinder 21 Arm cylinder 22 Boom operation lever 23 Arm operation lever 28 Bucket operation lever 30 Pilot pressure reducing valve (stroke control means)
31 Electromagnetic proportional pressure reducing valve (stroke control means)
34 Pressure sensor (stroke control means)
35 Pressure sensor (stroke control means)
33 Controller (Stroke control means)
40 Excavator 44 Boom 45 Arm 46 Bucket

Claims (4)

A boom switching valve and an arm second speed switching valve are connected in tandem to a center bypass oil path communicating with the hydraulic pump, and the boom switching valve and the arm second speed switching valve are connected to a parallel oil path branched from the center bypass oil path. In the hydraulic circuit of a hydraulic excavator that is connected in parallel,
Stroke control means for controlling a spool stroke of the arm second speed switching valve based on a boom raising operation amount is provided, the spool stroke is limited when the boom raising operation amount is a set value or more, and the boom raising operation amount is provided. A hydraulic circuit for a hydraulic excavator, characterized in that the restriction of the spool stroke is released when becomes below a set value.   The stroke control means is a pilot type pressure reducing valve that operates according to a boom raising operation amount, and the secondary pressure of the pilot type pressure reducing valve acts on the pilot port of the arm second speed switching valve to switch the second speed of the arm. 2. A hydraulic circuit for a hydraulic excavator according to claim 1, wherein the hydraulic circuit is configured to control a spool stroke of the valve. The stroke control means, and the electromagnetic proportional pressure reducing valve acting secondary pressure to the pilot port of the arm second speed selector valve, and two pressure sensors which respectively detect the arm Technology of pilot pressure and the boom raising Paitto pressure, the A controller that controls the secondary pressure of the electromagnetic proportional pressure reducing valve based on a detection signal from the pressure sensor, and the controller controls the spool stroke of the arm second speed switching valve when the boom raising pit pressure is equal to or higher than a set value. the so limited that controls the secondary pressure of the electromagnetic proportional pressure reducing valve, and, equal to the secondary pressure arm pull-out pilot pressure of the solenoid proportional pressure reducing valve when the boom-up Paitto pressure becomes less than the set value The hydraulic circuit for a hydraulic excavator according to claim 1, wherein the hydraulic circuit is controlled to be When arm pulling Kino load pressure is equal to or greater than the set value, the hydraulic pressure according to claim 1, 2 or 3, wherein the constructed as control spool stroke of the arm second speed selector valve is not carried out Excavator hydraulic circuit.