JPH10102546A - Hydraulic circuit for construction machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate no fluctuation of travel speed even in case of operaing a dozer cylinder while making a construction machine travel by providing a flow dividing valve having selector valves in a traveling motor feed oil passage position of a hydraulic circuit of a construction machine provided with a dozer cylinder and in a flow dividing oil passage position to traveling motors and the dozer cylinder. SOLUTION: In case of performing dozer work, a switch is operated on in advance. When a hydraulic remote-controlled valve 49 for a dozer is operated at the time of a travel skate, a dozer selector valve 19' is put in switching operation, and a relatively small flow quantity of pressure oil led out of a dozer cylinder output port 62 of a flow dividing valve 44 flows out to a dozer cylinder 5' to actuate an earth discharge plate 26. In this case, since oil pressure is supplied to traveling motors 6L, 6R from traveling motor output ports 56L, 56R of the flow dividing valve 44, fluctuation of travel speed is not generated even with actuation/stop operation of the dozer cylinder 5' during the travel of a hydraulic shovel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic circuit of an upper swing type construction machine such as a hydraulic excavator having a lower traveling body equipped with a dozer cylinder for driving a vertically rotating operation of an earth discharging plate.

FIG. 3 is a circuit diagram of a dozer according to an embodiment described in Japanese Utility Model Publication No. 7-30777. In FIG, 5 is a dozer apparatus 1 for dozer cylinder, 6 _L, 6 _R left and right traveling motors, the 7 first pump, the 8 second pump,
9 is a swing motor, 10 is a boom cylinder, 11 is an arm cylinder, and 12 is a bucket cylinder. In this dozer circuit, various hydraulic actuators (left and right traveling motors 6 _L and 6 _R , a swing motor 9, a boom cylinder 10, an arm cylinder 11 and a bucket cylinder 12) provided on a hydraulic excavator are controlled direction switching valves 13 _L and 13 _R. , 14,1
5, 16, and 17 are divided into two groups A (13 _L , 14, 1).
6), B (13 _R , 15, 17)
_L and directional control valve 14, 16 in parallel, and is provided with a right travel valve 13 _R and the direction switching valve 15, 17 in parallel. When the hydraulic excavator travels independently, the left and right traveling motors 6 _L and 6 _R are driven by independent first and second pumps 7 and 8, respectively, and the traveling is performed when the traveling and one of the other actuators are simultaneously operated. By the switching operation of the straight valve 18, the pressure oil from the first pump 7 is supplied in parallel to the left and right traveling motors 6 _L and 6 _R , and the pressure oil from the second pump 8 is supplied to another hydraulic actuator. I have to. As a result, even if another actuator is operated while the hydraulic excavator is traveling straight, the vehicle may be bent,
It does not meander. Further, in this dozer circuit, a dozer switching valve 19 is connected in parallel with the direction switching valves 15 and 17.

When a hydraulic excavator having a dozer circuit shown in FIG. 3 runs, for example, the switch 27 of the switches 27 and 28 is turned on, the dozer switching valve 19 is turned on.
It switched to Guy position, and because the movement of the left and right travel valve 13 _L, 13 _R is the detection signal is detected by the detector 31 is input to the controller 32, the straight traveling valve 18 is switched to the b position. At the same time, the solenoid 23 of the on-off valve 22
, The opening / closing valve 22 switches from the shutoff oil passage position C to the communication oil passage position d. Thereby, the pilot pressure from the pilot pump 24 acts on the regulator 25 through the communication oil passage position d of the on-off valve 22. Therefore, the swash plate tilt angle of the second pump 8 is adjusted, and the pump discharge amount decreases.
The discharge pressure oil having the reduced flow rate is supplied from the second pump 8 to the oil passages 38 and 39, the low position of the traveling linear valve 18, the oil passage 40, the check valve 41, the oil passage 42, and the a position of the dozer switching valve 19,
The oil is sent to the dozer cylinder 5 via the oil passage 43. Therefore, when the dozer operation is performed while the hydraulic excavator is traveling, the operating speed of the dozer cylinder 5 can be set to the optimum slow speed.

[0004]

In the dozer circuit of the prior art shown in FIG. 3, when the hydraulic excavator is running alone without operating the dozer cylinder 5,
The pressure oil is left travel motor 6 _L from the first pump 7, and pressurized oil from the second pump 8 is supplied independently to the right traveling motor 6 _R, can be performed considerably faster travel. When the actuating the dozer cylinder 5 during the running, the straight traveling valve 18 is switching operation, with pressure oil from the first pump 7 is supplied to a parallel with respect to the left and right traveling motors 6 _L and 6 _R, the Pressure oil from the two pumps 8 with a limited flow rate is supplied to the dozer cylinder 5. That is, during the combined operation of the left and right traveling motors 6 _L, 6 _R and dozer cylinder 5, only the pressure oil from the first pump 7 is supplied to the left and right traveling motors 6 _L, 6 _R, left and right traveling motors 6 _L, rotational speed of 6 _R decreases. Then, when only the operation of the dozer cylinder 5 is stopped at the time of the combined operation,
The traveling speed of the excavator returns to the high speed again. Therefore, every time the operation of the dozer cylinder 5 is stopped during the dozer work, the traveling speed of the hydraulic cylinder fluctuates, so that not only the ground finishing surface becomes rough, but also the work efficiency cannot be sufficiently increased. The present invention provides a hydraulic circuit of a construction machine having a dozer cylinder with a flow dividing valve (flow divider) provided in the hydraulic circuit so that the traveling speed does not fluctuate even when the dozer cylinder is operated while the construction machine is traveling. The purpose is to provide.

[0005]

According to the present invention, an upper revolving unit is connected to an upper portion of a lower traveling unit, and a plurality of directional control valves for controlling a hydraulic actuator are divided into two groups A and B. , Each of which is driven by the first and second pumps, and the left and right traveling motor control direction switching valves are arranged in each of the above groups, respectively, and the other hydraulic actuator control direction switching valves are connected in parallel with each other. Also, the lower traveling body is provided with a dozer work earthing plate, and pressure oil from the first and second pumps disposed on the upper rotating body is disposed on the lower traveling body via a swivel joint that is a rotating joint. Left and right traveling motors, a left and right traveling motor control direction switching valve disposed on the upper rotating body, and A line connecting the pressure oil supply ports of the left and right traveling motors at the time of the body discharging travel, a supply oil passage position for the traveling motor that directly communicates only with each of the pressure oil supply ports, a left and right traveling motor and a dozer cylinder. A diversion valve provided with a selection switching valve capable of switching to a diversion oil passage position with respect to. In the above case, a flow dividing valve is provided in a pipe connecting the swivel joint and the left and right traveling motors, and a dozer cylinder control direction switching valve is provided in a pipe leading from the flow dividing valve to the dozer cylinder. did. A command switching valve for outputting a switching pilot pressure signal to the selection switching valve in the flow dividing valve is disposed on the upper revolving unit, and the command switching valve is connected to a pilot hydraulic source and the selection switching valve. The command switching valve is interposed in a pipeline communicating with the pilot port, and is switched from the tank communication oil passage position to the open oil passage position before the start of the dozer operation.

According to the present invention, in the first case, when the hydraulic excavator does not perform the dozer work, the switching valve for command is set to the tank communication oil path position (the switching valve for command is switched by, for example, a switch operation). In this case, the switch is turned off.) In this case, when the sole travel is performed by only the left and right travel motors, the pressure oil from the first and second pumps is fully supplied to the left and right travel motors via the travel motor supply oil passage position, and the so-called 100
% Running speed can be set. Then, in the above state, when at least one of the other hydraulic actuator control directional switching valves (excluding the dozer cylinder control directional switching valve) is operated, the traveling straight traveling valve is moved from the one open oil passage position to the traveling straight traveling position. The position is switched to the opening 2 oil passage position. Therefore, even if another actuator (except for the earth discharging plate) is operated during the straight traveling of the hydraulic shovel, the undercarriage does not bend or meander as in the case of the prior art.

Next, when performing a dozer operation as a second case, a switch operation (including lever operation, etc.) is performed in advance to switch the command switching valve to the open oil passage position. The pilot pressure derived from the command switching valve acts on the selection switching valve in the flow dividing valve, and the selection switching valve switches from the position of the supply oil passage for the traveling motor to the position of the distribution oil passage. At this time, if all of the left and right traveling motor control direction switching valves, dozer cylinder control direction switching valves, and other hydraulic actuator control direction switching valves are in the neutral position, the hydraulic oil from the first and second pumps will be It is simply returned to the oil tank through the center bypass oil passages of groups A and B. Next, when only the left and right traveling motor control direction switching valves are switched without operating the dozer cylinder, the pressure oil from the first and second pumps passes through the selection switching valve in the flow dividing valve, and the right and left of the flow dividing valve are switched. It is diverted from the traveling motor output port and the dozer cylinder output port and is derived. As a result, the left and right traveling motors rotate, and the excavator travels straight. In this case, the pressure oil derived from the output port for the dozer cylinder is
Since the directional control valve for dozer cylinder control remains in the neutral position, when the specified relief set pressure of the relief valve connected to the supply line is reached, the relief valve is returned to the oil tank by the relief operation of the relief valve. The flow rate of the hydraulic oil derived from the output port for the dozer cylinder is a relatively small flow rate required for the operation of the dozer cylinder. Therefore, the traveling speed in the above case is slightly lower than the traveling speed in the first case by a small amount of the pressurized oil which is diverted from the output port for the dozer cylinder and returned to the oil tank.

Next, as a third case, during the traveling state in the second case (when the switch is turned on and the selection switching valve in the flow dividing valve is at the position of the flow dividing oil passage), The directional control valve for cylinder control is switched from the neutral position to the switching position. A relatively small amount of pressure oil derived from the dozer cylinder output port of the flow dividing valve is supplied to the dozer cylinder through the switching position of the dozer cylinder control direction switching valve, so that the earth discharging plate is operated at an optimum speed. be able to. In this case, the hydraulic oil is supplied to the left and right traveling motors from the left and right traveling motor output ports of the flow dividing valve in the same manner as in the second case. -The running speed does not fluctuate even if the stop operation is performed. Further, in the hydraulic circuit of the present invention, the flow dividing valve is disposed on the lower traveling body, and the command switching valve for outputting a switching pilot pressure signal to the selection switching valve in the flow dividing valve is disposed on the upper revolving structure.
The piping for the two valves can be minimized and facilitated, and the layout can be rationally made in the space of the vehicle body.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a perspective view of a hydraulic shovel equipped with the hydraulic circuit of the present invention. For convenience of description, the same reference numerals are given to components using the same components as the hydraulic devices and the like used in the conventional technology shown in FIG.
Reference numeral 20 denotes a lower traveling body of the excavator, and 6 _L and 6 _R denote left and right traveling motors (left traveling motor 6) mounted on the lower traveling body 20.
_L is not shown in the figure), 21 is a dozer device mounted on the lower traveling unit 20, 26 is a dumping plate (also called a dozer blade) of the dozer unit 21, 5 'is a dozer cylinder, and 29 is an upper portion of the lower traveling unit 20 , A work attachment attached to the front portion of the upper swing body 29, 33 is a boom of the work attachment 30, 34 is an arm, 35 is a bucket, 10 is a boom cylinder, 11 is an arm cylinder, and 12 is Bucket cylinder. FIG.
1 is a hydraulic circuit diagram of the present invention. In the figure, components using the same components as the prior art shown in FIG. 36 _L , 36 _R are traveling motors 6 _L , 6
_{R is} a brake valve, and 37 is a swivel joint which is a swivel joint for exclusively connecting the hydraulic passage on the upper revolving unit 29 side and the hydraulic passage on the lower traveling unit 20 side to each other.
44 is a flow dividing valve (commonly called a flow divider), 45 is a switching valve for selection in the flow dividing valve 44, 19 'is a switching valve for dozer which is a directional switching valve for controlling the dozer cylinder (5'), 46, 47 and 48 are relief valves attached to the dozer device 21, respectively, and 49 is a dozer switching valve 1
A hydraulic remote control valve for dozer which derives a switching command pilot pressure to the left and right pilot ports 50 _L and 50 _{R of} 9 ′ and operates the dozer cylinder 5 ′, 51 performs the traveling of the hydraulic shovel and the simultaneous operation of the dozer device 21. Time switch 5
An electromagnetic switching valve for dozer operation mode command, which is a command switching valve for setting the hydraulic circuit to the dozer operation mode by the ON operation of 2, a power supply 53, an oil tank 54.

Next, the configuration of the hydraulic circuit of the present invention will be described with reference to FIGS. In the present invention, the left and right traveling valves 1 which are the direction switching valves for controlling the left and right traveling motors disposed on the upper swing body 29 (not shown in FIG. 1 but shown in the hydraulic circuit of FIG. 2).
3 _L, 13 _R and the traveling motor 6 of the left and right in dumping after traveling of the lower traveling body 20 _L, 6 _R of the pressurized oil supply port a, b
And a traveling motor supply oil passage position e which directly communicates only with the above-mentioned pressure oil supply ports a and b, and a diversion oil passage for the left and right traveling motors 6 _L and 6 _R and the dozer cylinder 5 ′. A diversion valve 44 having a selection switching valve 45 switchable to a position is provided. And in the above case, the diversion valve 44
And the swivel joint 37 and the left and right traveling motors 6
_L, and the 6 _R while interposed conduit communicating, is interposed a "switching valve 19 for the dozer to the conduit leading to the 'dozer cylinder 5 from the diverter valve 44. A dozer work mode commanding electromagnetic switching valve 51 for outputting a switching pilot pressure signal to the pilot port c of the selection switching valve 45 in the flow dividing valve 44 is disposed on the upper revolving unit 29, and the dozer work mode command is provided. Solenoid switching valve 51 for pilot pump 24
And the pilot switching port 45 of the selection switching valve 45 are interposed in a pipe communicating the same. The electromagnetic switching valve 51 for dozer work mode command is opened from the tank communication oil passage position G before the start of the dozer work. Switch to position H.

Next, the operation of the hydraulic circuit of the present invention will be described. In the first embodiment of the present invention, when the excavator does not perform the dozer work, the switch 52 is kept in the off state when the dozer work is not performed.
1 is the position of the oil passage in the tank communication, so that the diversion valve 4
No pilot pressure acts on the pilot port c of the selection switching valve 45 in FIG. In this case, when the individual traveling only by the left and right traveling motors 6 _L and 6 _R is performed, the first traveling is performed.
Pressurized oil from the pump 7, dumping progression for the switching position of the left travel valve 13 _L, supply oil passage position E for driving the motor of the selection switch valve 45, the bypass oil passage 55 _L, left travel motor for the output ports 56 _L It is supplied to the pressure oil supply port a of the left travel motor 6 _L via conduit 57 _L, and simultaneously the pressure oil from the second pump 8, 1 open oil passage position Li of the straight traveling valve 18, right travel valve 13 _R pressure of the dumping progression for the switching position, the oil supply passage position e for driving the motor of the selection switch valve 45, the bypass oil passage 55 _R, the right travel motor for the output ports 56 _R, the right traveling motor 6 _R through conduit 57 _R The oil is supplied to the oil supply port b. That is, when the vehicle travels independently using only the left and right traveling motors 6 _L and 6 _R , the pressure oil from the first and second pumps 7 and 8 is supplied to the left and right traveling motors 6 via the traveling motor supply oil path position E. _L, is full oil feeding to 6 _R, it is possible to set the running speed of the so-called 100%. In the above state, when at least one of the other hydraulic actuator control direction switching valves (excluding the dozer switching valve 19 ') is operated, the traveling straight valve 18 moves straight from the position of the one open oil passage. To the second opening oil passage position b. Therefore, while the excavator is running straight, other actuators (excluding the earth discharging plate 26)
Is operated, the undercarriage 20 does not bend or meander as in the case of the prior art.

Next, as a second case, dozer work is performed.
In this case, the switch 52 is turned on in advance. Dozer work
The solenoid 58 of the mode command electromagnetic switching valve 51 is energized
The dozer work mode command electromagnetic switching valve 51 is connected to the tank.
The position is switched from the oil passage position G to the open oil passage position H. Pyro
The pilot pressure from the pump 24 is
Through the position of the solenoid switch valve 51
Acting on the pilot port c of the selection switching valve 45
The switching valve 45 for selection is located at the position of the supply oil passage for the traveling motor.
The position is switched to the diversion oil passage position. At this time, the left and right traveling valves
13_L , 13_R And dozer switching valve 19 ′, other hydraulic
Set all directional control valves for the tutor control to the neutral position.
The pressure oil from the first and second pumps 7 and 8
Each of the center bypass oil passages 60 of the groups A and B_L , 60
_R Through the oil tank 54. Then do
The left and right travel valves 13 without operating the user cylinder 5 '._L ,
13_RIs switched to the unloading progress position.
The pressure oil from the second pumps 7 and 8 is diverted to the selection switching valve 45.
Through the diversion control unit 61 inside the diversion valve 44 to the oil passage position.
The output ports 56 for the left and right traveling motors of the flow dividing valve 44
_L , 56_R And output port 62 for dozer cylinder
It is derived by flowing. Thus, the left and right traveling motors 6 _L ,
6_R Rotates and the excavator runs. And this
Output from the dozer cylinder output port 62
The pressure oil remains in the neutral position with the dozer switching valve 19 '.
The relief valve 46 connected to its supply line 63
Is returned to the oil tank 54 by the relief operation of. Above
Pressure derived from the dozer cylinder output port 62
The oil flow is relatively low for the operation of the dozer cylinder 5 '.
The volume flow rate. Therefore, the traveling speed in the above case is
Diverted from dozer cylinder output port 62
A small amount of pressurized oil flow returned to the oil tank 54
Is slightly lower than the traveling speed in the case of.

Next, as a third case, the above-mentioned second case (when the switch 52 is turned on and the selection switching valve 45 in the flow dividing valve 44 is at the position of the flow dividing oil passage).
When the dozer hydraulic remote control valve 49 is operated during the traveling state of the vehicle, when the dozer hydraulic remote control valve 49 is operated, the pilot pressure derived from the dozer hydraulic remote control valve 49 is changed to the pilot port 50 _{L of the} dozer switching valve 19 '.
Or acting on the 50 _R. The switching oil of the dozer switching valve 19 ′ is operated, and the relatively small amount of pressure oil discharged from the dozer cylinder output port 62 of the flow dividing valve 44 is
Since the dozer cylinder 5 'is supplied through the switching position of the dozer switching valve 19', the earth discharging plate 26 can be operated at an optimum speed. And in this case, the diversion valve 44
Since pressure oil to the traveling motor 6 _L, 6 _R of the left and right as in the case than the running motor for the output ports 56 _L, 56 _R of the second right and left are supplied, dozer while the hydraulic excavator is traveling Even if the cylinder 5 'is operated / stopped, the traveling speed does not fluctuate.

In the hydraulic circuit of the present invention, the flow dividing valve 44 (disposed at a position downstream of the swing joint 37 as shown in FIG. 2) is provided on the lower traveling body 20 and the electromagnetic switching valve for dozer work mode command. Since the 51 is disposed on the upper revolving unit 29, the hydraulic piping for the two valves 44 and 51 can be minimized and facilitated, and the layout can be rationally made in the space of the vehicle body. In this embodiment, the dozer operation mode commanding electromagnetic switching valve 51 for performing the switching operation by operating the switch 52 is used, but the invention is not limited thereto, and a mechanical or hydraulic (not shown) or the like switching command valve may be used. You may. Hydraulic remote control valve for dozer 4
Although the switching operation of the dozer switching valve 19 ′ is performed by the pilot pressure derived from the valve 9, an electric or mechanical (not shown) dozer switching valve may be used. Although not shown, the left and right traveling valves 13 _L , 13 _R , other hydraulic actuator control direction switches 14, 15, 16, 17 and the traveling straight valve 18 may be valves that are switched by pilot pressure. .

In place of the switch 52 shown in FIG. 2, although not shown, operation detecting means (for example, a limit switch) for the dozer hydraulic remote control valve (49) is provided. Automatic switching operation of the electromagnetic switching valve (51) for dozer work mode command is also commonly practiced, but in this case, when the hydraulic remote control valve (49) for the dozer is operated while traveling, the inside of the flow dividing valve (44) is changed. Selection switching valve (45) is switched to the diversion oil passage position (F), and the dozer hydraulic remote control valve (4
When the operating lever of (9) is returned to the neutral position, the selector switching valve (45) returns to the position of the traveling motor supply oil passage (e). That is, when the earth removal plate is moved while traveling, the traveling speed is slightly lower than when the earth removal plate is not moved, so that a sufficiently smooth ground level cannot be obtained.

[0016]

According to the present invention, when the excavator does not perform the dozer operation, the command switching valve (the electromagnetic switching valve for dozer operation mode command) is set to the tank communication oil passage position (the command switching valve is operated by a switch operation). If it is to be switched, the switch is turned off). In this case, when the sole travel is performed by only the left and right travel motors, the pressure oil from the first and second pumps is fully supplied to the left and right travel motors via the travel motor supply oil passage position, and the so-called 100 % Running speed can be set. Next, when performing a dozer operation, a switch is turned on in advance. When the command switching valve switches to the open oil passage position,
The switching valve for selection in the flow dividing valve switches from the position of the supply oil passage for the traveling motor to the position of the flow dividing oil passage. In this state, if only the left and right traveling motor control direction switching valves are switched without operating the dozer cylinder, the pressure oil from the first and second pumps passes through the selection switching valve in the flow dividing valve, and flows through the flow dividing valve. Are diverted from the left and right traveling motor output ports and the dozer cylinder output port. As a result, the left and right traveling motors rotate, and the excavator travels straight. In this case, the pressure oil derived from the output port for the dozer cylinder is
Since the directional control valve for dozer cylinder control remains in the neutral position, when the specified relief set pressure of the relief valve connected to the supply line is reached, the relief valve is returned to the oil tank by the relief operation of the relief valve. The flow rate of the hydraulic oil derived from the output port for the dozer cylinder is a relatively small flow rate required for the operation of the dozer cylinder. Therefore, the traveling speed in the above case is slightly lower than the traveling speed in the switch-off state by a small amount of the pressure oil which is diverted from the output port for the dozer cylinder and is returned to the oil tank. Next, during the traveling state in the above case (when the switch is turned on and the selection switching valve in the flow dividing valve is at the flow dividing oil passage position), the dozer cylinder control direction switching valve is switched from the neutral position. Switch to position. A relatively small amount of pressure oil derived from the dozer cylinder output port of the flow dividing valve is supplied to the dozer cylinder through the switching position of the dozer cylinder control direction switching valve, and from the left and right traveling motor output ports of the flow dividing valve. Pressure oil is supplied to the left and right traveling motors, respectively. That is, even if the operation of the dozer cylinder is performed while the hydraulic excavator is traveling, the traveling speed does not change. Therefore, in the construction machine equipped with the hydraulic circuit of the present invention, the running speed can be prevented from fluctuating even if the dozer cylinder is operated while running, so that the ground level can be extremely smoothly finished and the work efficiency can be sufficiently improved. . Further, in the present invention, since the flow dividing valve is disposed on the lower traveling body of the construction machine, and the command switching valve for outputting the switching pilot pressure signal to the selecting switching valve in the flow dividing valve is disposed on the upper revolving structure. The piping for both valves can be minimized and facilitated, and the layout can be rationally made in the space of the vehicle body.

[Brief description of the drawings]

FIG. 1 is a perspective view of a hydraulic shovel equipped with a hydraulic circuit of the present invention.

FIG. 2 is a hydraulic circuit diagram of the present invention.

FIG. 3 is a dozer circuit diagram of one embodiment of the prior art.

[Explanation of symbols]

1,21 dozer device 5, 5 'dozer cylinder 6 _L, 6 _R traveling motors 7 and 8 first, second pump 13 _L, 13 _R travel valve (traveling motor control directional control valve) 18 the straight traveling valve 19, 19 '' Dozer switching valve (Dozer cylinder control direction switching valve) 20 Lower traveling body 26 Drainage plate 29 Upper revolving body 37 Swivel joint 44 Dividing valve 45 Selection switching valve 51 Dozer work mode command electromagnetic switching valve (command switching) Valve) 52, 27, 28 Switch 56 _L , 56 _R Output port for traveling motor 61 Dividing control unit 62 Output port for dozer cylinder

Claims (3)

[Claims] An upper revolving unit is connected to an upper portion of a lower traveling unit, and a plurality of directional control valves for controlling a hydraulic actuator are divided into two groups A and B, each of which is a first group.
It is driven by the second pump, and left and right traveling motor control direction switching valves are arranged in each of the groups,
This and the other directional control valves for controlling the hydraulic actuator are connected in parallel with each other, and the lower traveling body is provided with a dozer work discharge plate, and the hydraulic oil from the first and second pumps disposed on the upper revolving body is connected. Through a swivel joint as a swivel joint, a left and right traveling motor disposed on the lower traveling body and a dozer cylinder for driving the up-down rotation operation of the earth discharging plate. And the pressure switching oil supply ports of the left and right traveling motors at the time of the unloading traveling of the lower traveling body are directly connected only to the respective pressure oil supply ports. Oil for a construction machine, comprising a diverting valve having a switching oil supply position for a traveling motor and a selection switching valve capable of switching between a diverting oil passage position for the left and right traveling motors and a dozer cylinder. Circuit. 2. The hydraulic circuit for a construction machine according to claim 1, wherein said flow dividing valve is interposed in a pipe communicating between a swivel joint and left and right traveling motors. A hydraulic circuit for a construction machine, wherein a directional control valve for controlling a dozer cylinder is provided in a pipe line leading to the dozer cylinder. 3. The hydraulic circuit for a construction machine according to claim 1, wherein a command switching valve for outputting a switching pilot pressure signal to the selection switching valve in the flow dividing valve is provided on the upper revolving unit. The switching valve for command is disposed in a pipe connecting the pilot hydraulic pressure source and the pilot port of the switching valve for selection, and the switching valve for command is positioned in the tank communication oil path before starting the dozer work. A hydraulic circuit for a construction machine, wherein the hydraulic circuit is switched to an open oil passage position.