Hydraulic circuit for construction machine

Abstract

Provided is a hydraulic circuit (10) for a construction machine that reduces shocks resulting from switching of work switching valves (3C, 3D, 3E). The hydraulic circuit (10) for a construction machine includes: a straight-travel valve (11) for switching between the connection of the first and second pumps (4, 5) to the first and second unloading passages (14, 15) and the connection of the first and second pumps (4, 5) to the first and second parallel passages (12, 13), in accordance with the operation of the travel motors (2A, 2B) and the work actuators (2C, 2D, 2E); a first signal passage (18) for supplying control pressure from the third pump (6) to the straight-travel valve (11), the first signal passage (18) running to a tank (7) via signal passage sections (21A, 22A) of the travel switching valves (3A, 3B); and a second signal passage (19) branching off from the first signal passage (18) and running to the tank (7) via signal passage sections (25A, 24A, 23A) of the work switching valves (3E, 3D, 3C). The signal passage sections (25A, 24A, 23A) of the work switching valves (3E, 3D, 3C) are switched between a communicating position and a closed position, and when the signal passage sections (25A, 24A, 23A) of the work switching valves (3E, 3D, 3C) are switched in conjunction with switching of the work switching valves (3E, 3D, 3C), the signal passage sections (25A, 24A, 23A) of the work switching valves (3E, 3D, 3C) are switched after a predetermined amount of time has elapsed.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a hydraulic circuit for a construction machine.BACKGROUND

[0002] The hydraulic circuit for a construction machine disclosed in Japanese Patent Application Publication No. 2010-156135 includes travel switching valves for controlling the supply of hydraulic fluid that drives hydraulic actuators for traveling, work switching valves for controlling the supply of hydraulic fluid that drives hydraulic actuators for working, and a straight-travel valve that switches flow paths of the hydraulic fluid to the travel switching valves and the work switching valves.SUMMARY

[0003] In the hydraulic circuit for a construction machine disclosed in Japanese Patent Application Publication No. 2010-156135, upon switching of the work switching valves, shocks may occur as a result of the switching. Therefore, there is a need to reduce shocks resulting from the switching of the work switching valves.

[0004] One aspect provides a hydraulic circuit for a construction machine provided with a travel motor and a work actuator. The hydraulic circuit comprises: a hydraulic fluid pump for supplying hydraulic fluid to the travel motor and the work actuator; a travel switching valve for controlling supply of the hydraulic fluid to drive the travel motor; a work switching valve for controlling supply of the hydraulic fluid to drive the work actuator; a first passage running from the hydraulic fluid pump to a tank via the travel switching valve and the work switching valve; a second passage running from the hydraulic fluid pump to a tank via the work switching valve; a straight-travel valve for switching between connection of the hydraulic fluid pump to the first passage and connection of the hydraulic fluid pump to the second passage, in accordance with operation of the travel motor and the work actuator; a control pressure pump for supplying control pressure to control the straight-travel valve; a first signal passage for supplying control pressure from the control pressure pump to the straight-travel valve, the first signal passage running to a tank via a signal passage section of the travel switching valve; and a second signal passage branching off from the first signal passage and running to a tank via a signal passage section of the work switching valve, wherein the signal passage section of the work switching valve is switched between a communicating position and a closed position, and when the signal passage section of the work switching valve is switched in conjunction with switching of the work switching valve, the signal passage section of the work switching valve is switched after a predetermined amount of time has elapsed.

[0005] In one embodiment, the signal passage section of the work switching valve may have an orifice in a portion between the communicating position and the closed position, and when the signal passage section of the work switching valve is switched, the signal passage section of the work switching valve may be switched after a predetermined amount of time has elapsed, by means of the orifice.

[0006] In one embodiment, the signal passage section of the travel switching valve may be switched between a communicating position and a closed position. In one embodiment, the signal passage section of the travel switching valve may have an orifice in a portion between the communicating position and the closed position, and when the signal passage section of the travel switching valve is switched, the signal passage section of the travel switching valve may be switched after a predetermined amount of time has elapsed, by means of the orifice.

[0007] In one embodiment, orifices may be provided on an upstream side and a downstream side of a junction at which the first signal passage branches off from the second signal passage.ADVANTAGEOUS EFFECTS

[0008] The present disclosure makes it possible to reduce shocks resulting from the switching of the work switching valve.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 shows one embodiment of a hydraulic circuit for a construction machine.DESCRIPTION OF THE PREFERRED EMBODIMENTS<Embodiment>

[0010] With reference to Fig. 1, the following describes one embodiment of a hydraulic circuit for a construction machine. The hydraulic circuit is installed in a hydraulic drive device for driving the construction machine.<Hydraulic Drive Device 1>

[0011] Fig. 1 shows a hydraulic drive device 1. The hydraulic drive device 1 is installed in a construction machine such as a hydraulic excavator. The hydraulic drive device 1 includes a plurality of hydraulic actuators 2. The hydraulic actuator 2 is driven by a hydraulic circuit 10. The hydraulic circuit 10 includes a plurality of switching valves 3 and a straight-travel valve 11. The plurality of hydraulic actuators 2 includes, for example, a first hydraulic motor 2A for driving the right crawler (traveling unit), a second hydraulic motor 2B for driving the left crawler (traveling unit), a first hydraulic cylinder 2C for driving an arm, a second hydraulic cylinder 2D for driving a boom, and a third hydraulic cylinder 2E for driving an attachment such as a bucket. The first hydraulic motor 2A and the second hydraulic motor 2B correspond to travel motors. The first hydraulic cylinder 2C, the second hydraulic cylinder 2D, and the third hydraulic cylinder 2E correspond to work actuators.

[0012] The switching valves 3 are supplied with hydraulic fluid from a first pump 4 and a second pumps 5, and control the supply of the hydraulic fluid to the hydraulic actuators 2. The straight-travel valve 11 is a hydraulic pilot-operated spool valve in which the spool position is switched by hydraulic fluid supplied from a third pump 6. The straight-travel valve 11 has a straight-travel spool 26. The first pump 4 and the second pump 5 correspond to hydraulic fluid pumps. The third pump 6 corresponds to a control pressure pump.<Switching Valves 3>

[0013] Each of the switching valves 3 can be switched to multiple positions by sliding the spool. Fig. 1 shows the spool in the switching valve 3. The switching valve 3 is a hydraulic pilot-operated spool valve in which the spool position is switched by the control pressure output from an electromagnetic proportional valve (not shown). The switching valve 3 is provided for each hydraulic actuator 2. For the first hydraulic motor 2A, the supply of hydraulic fluid is controlled by the first switching valve 3A. For the second hydraulic motor 2B, the supply of hydraulic fluid is controlled by the second switching valve 3B. For the first hydraulic cylinder 2C, the supply of hydraulic fluid is controlled by the third switching valve 3C. For the second hydraulic cylinder 2D, the supply of hydraulic fluid is controlled by the fourth switching valve 3D. For the third hydraulic cylinder 2E, the supply of hydraulic fluid is controlled by the fifth switching valve 3E. The first switching valve 3A and the second switching valve 3B correspond to travel switching valves. The third switching valve 3C, the fourth switching valve 3D, and the fifth switching valve 3E correspond to work switching valves.

[0014] The first switching valve 3A has a first spool 21 and a first signal passage section 21A. The second switching valve 3B has a second spool 22 and a second signal passage section 22A. The third switching valve 3C has a third spool 23 and a third signal passage section 23A. The fourth switching valve 3D has a fourth spool 24 and a fourth signal passage section 24A. The fifth switching valve 3E has a fifth spool 25 and a fifth signal passage section 25A.<Hydraulic Circuit 10>

[0015] The hydraulic circuit 10 includes the straight-travel valve 11, the plurality of switching valves 3, a first parallel passage 12, a second parallel passage 13, a first unloading passage 14, a second unloading passage 15, a first signal passage 18, and a second signal passage 19. The first spool 21 and the second spool 22 are traveling section spools. The third spool 23, the fourth spool 24, and the fifth spool 25 are working section spools. The traveling section spools are supplied with hydraulic fluid preferentially over the working section spools. The straight-travel valve 11 switches between the connection of the hydraulic fluid pumps to the first and second unloading passages 14 and 15 and the connection of the hydraulic fluid pumps to the first and second parallel passages 12 and 13, in accordance with the operation of the travel motors and the work actuators. The first parallel passage 12 and the second parallel passage 13 correspond to a second passage running from a hydraulic fluid pump to a tank via a work switching valve. The first unloading passage 14 and the second unloading passage 15 correspond to a first passage running from the hydraulic fluid pump to a tank via a travel switching valve and the work switching valve.

[0016] The first pump 4 is connected to the straight-travel valve 11 and also connected to the third spool 23, fourth spool 24, and fifth spool 25 via the first parallel passage 12. The first unloading passage 14 branches off from a junction between the first pump 4 and the straight-travel valve 11. The first unloading passage 14 extends through the first spool 21, second spool 22, third spool 23, fourth spool 24, and fifth spool 25 to a tank 7A. In other words, the first pump 4 is connected to the first spool 21, second spool 22, third spool 23, fourth spool 24, fifth spool 25, and tank 7A via the first unloading passage 14.

[0017] The second pump 5 is connected to the straight-travel valve 11 and also connected to the first spool 21, second spool 22, third spool 23, fourth spool 24, fifth spool 25, and tank 7A via the second unloading passage 15. The second parallel passage 13 branches off from a junction between the second pump 5 and the straight-travel valve 11. The second pump 5 is connected to the third spool 23, fourth spool 24, and fifth spool 25 via the second parallel passage 13.

[0018] The third pump 6 is connected to the first signal passage 18 and the second signal passage 19 that branches off from the first signal passage 18. The first signal passage 18 has a first orifice 16 and a second orifice 17 and is connected to the pressure receiving surface of the straight-travel spool 26 serving as a pressure receiving section, and the first signal passage 18 also extends through the first signal passage section 21A of the first spool 21 and the second signal passage section 22A of the second spool 22 and is connected to a tank 7B. The first signal passage 18 has a junction in the first signal passage section 21A, and this junction is connected to a tank 7C. The second signal passage 19 branches off from the first signal passage 18 at a junction between the first and second orifices 16 and 17, and extends through the third signal passage section 23A of the third spool 23, the fourth signal passage section 24A of the fourth spool 24, and the fifth signal passage section 25A of the fifth spool 25 to a tank 7D. The hydraulic fluid from the third pump 6 is supplied through the first and second orifices 16 and 17, which reduce abrupt pressure changes. The tanks 7A, 7B, 7C, and 7D may be the same tank or different tanks.

[0019] The straight-travel valve 11 is switched among three positions. Specifically, the straight-travel spool 26 of the straight-travel valve 11 is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the straight-travel spool 26 is in the neutral position, the hydraulic fluid passes through the straight-travel valve 11 and is supplied to the first switching valve 3A and to the working section spools, i.e., the third spool 23, fourth spool 24, and fifth spool 25, at a flow rate reduced by the orifices. When the straight-travel spool 26 is in the first operating position, the hydraulic fluid from the first pump 4 is supplied to the first switching valve 3A, and the hydraulic fluid from the second pump 5 is supplied to the working section spools. When the straight-travel spool 26 is in the second operating position, the hydraulic fluid from the first pump 4 is supplied to the working section spools, and the hydraulic fluid from the second pump 5 is supplied to the first switching valve 3A. The neutral position corresponds to a communicating position.

[0020] The first switching valve 3A is switched among three positions. Specifically, the first spool 21 of the first switching valve 3A is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the first spool 21 is in the neutral position, the hydraulic fluid passes through the first switching valve 3A and is supplied to the second switching valve 3B. When the first spool 21 is in the first operating position, the hydraulic fluid is supplied to the first hydraulic motor 2A in a first direction. When the first spool 21 is in the second operating position, the hydraulic fluid is supplied to the first hydraulic motor 2A in a second direction, opposite to the first direction. The first signal passage section 21A is switched among three positions in conjunction with the movement of the first spool 21. When the first signal passage section 21A is in the neutral position, the hydraulic fluid is supplied to the second signal passage section 22A and also discharged to the tank 7C. When the first signal passage section 21A is in the left or right operating position, the hydraulic fluid is supplied to the second signal passage section 22A. The neutral position corresponds to the communicating position.

[0021] The second switching valve 3B is switched among three positions. Specifically, the second spool 22 of the second switching valve 3B is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the second spool 22 is in the neutral position, the hydraulic fluid passes through the second switching valve 3B and is supplied to the fifth switching valve 3E. When the second spool 22 is in the first operating position, the hydraulic fluid is supplied to the second hydraulic motor 2B in the first direction. When the second spool 22 is in the second operating position, the hydraulic fluid is supplied to the second hydraulic motor 2B in the second direction. The second signal passage section 22A is switched among three positions in conjunction with the movement of the second spool 22. When the second signal passage section 22A is in the neutral position, the hydraulic fluid is discharged to the tank 7B. When the second signal passage section 22A is in the left or right closed position, the supply of the hydraulic fluid is blocked. The neutral position corresponds to the communicating position.

[0022] The fifth switching valve 3E is switched among three positions. Specifically, the fifth spool 25 of the fifth switching valve 3E is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the fifth spool 25 is in the neutral position, the hydraulic fluid passes through the fifth switching valve 3E and is supplied to the fourth switching valve 3D. When the fifth spool 25 is in the first operating position, the hydraulic fluid is supplied to the third hydraulic cylinder 2E in the first direction. When the fifth spool 25 is in the second operating position, the hydraulic fluid is supplied to the third hydraulic cylinder 2E in the second direction. The fifth signal passage section 25A is switched among three positions in conjunction with the movement of the fifth spool 25. The fifth signal passage section 25A has orifices 25B, one provided in the portion between the neutral position and the left closed position, another provided in the portion between the neutral position and the right closed position. The orifices 25B are shown schematically. When the fifth signal passage section 25A is in the neutral position, the hydraulic fluid is supplied to the fourth signal passage section 24A. When the fifth signal passage section 25A is in the left or right closed position, the supply of the hydraulic fluid is blocked. When the fifth signal passage section 25A is switched from the neutral position to the closed position, the fifth signal passage section 25A is switched after a predetermined amount of time has elapsed. Specifically, when the fifth signal passage section 25A moves in conjunction with the movement of the fifth spool 25, the fifth signal passage section 25A is switched from the neutral position to the portion having the orifice 25B and then switched to the closed position. Thus, the fifth signal passage section 25A is gently switched by means of the orifice 25B. The neutral position corresponds to the communicating position.

[0023] The fourth switching valve 3D is switched among three positions. Specifically, the fourth spool 24 of the fourth switching valve 3D is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the fourth spool 24 is in the neutral position, the hydraulic fluid passes through the fourth switching valve 3D and is supplied to the third switching valve 3C. When the fourth spool 24 is in the first operating position, the hydraulic fluid is supplied to the second hydraulic cylinder 2D in the first direction. When the fourth spool 24 is in the second operating position, the hydraulic fluid is supplied to the second hydraulic cylinder 2D in the second direction. The fourth signal passage section 24A is switched among three positions in conjunction with the movement of the fourth spool 24. The fourth signal passage section 24A has orifices 24B, one provided in the portion between the neutral position and the left closed position, another provided in the portion between the neutral position and the right closed position. The orifices 24B are shown schematically. When the fourth signal passage section 24A is in the neutral position, the hydraulic fluid is supplied to the third signal passage section 23A. When the fourth signal passage section 24A is in the left or right closed position, the supply of the hydraulic fluid is blocked. When the fourth signal passage section 24A is switched from the neutral position to the closed position, the fourth signal passage section 24A is switched after a predetermined amount of time has elapsed. Specifically, when the fourth signal passage section 24A moves in conjunction with the movement of the fourth spool 24, the fourth signal passage section 24A is switched from the neutral position to the portion having the orifice 24B and then switched to the closed position. Thus, the fourth signal passage section 24A is gently switched by means of the orifice 24B. The neutral position corresponds to the communicating position.

[0024] The third switching valve 3C is switched among three positions. Specifically, the third spool 23 of the third switching valve 3C is switched among the first operating position (left side in the drawing), the neutral position, and the second operating position (right side in the drawing). When the third spool 23 is in the neutral position, the hydraulic fluid passes through the third switching valve 3C and is discharged to the tank 7A. When the third spool 23 is in the first operating position, the hydraulic fluid is supplied to the first hydraulic cylinder 2C in the first direction. When the third spool 23 is in the second operating position, the hydraulic fluid is supplied to the first hydraulic cylinder 2C in the second direction. The third signal passage section 23A is switched among three positions in conjunction with the movement of the third spool 23. The third signal passage section 23A has orifices 23B, one provided in the portion between the neutral position and the left closed position, another provided in the portion between the neutral position and the right closed position. The orifices 23B are shown schematically. When the third signal passage section 23A is in the neutral position, the hydraulic fluid is discharged to the tank 7A. When the third signal passage section 23A is in the left or right closed position, the supply of the hydraulic fluid is blocked. When the third signal passage section 23A is switched from the neutral position to the closed position, the third signal passage section 23A is switched after a predetermined amount of time has elapsed. Specifically, when the third signal passage section 23A moves in conjunction with the movement of the third spool 23, the third signal passage section 23A is switched from the neutral position to the portion having the orifice 23B and then switched to the closed position. Thus, the third signal passage section 23A is gently switched by means of the orifice 23B. The neutral position corresponds to the communicating position.<How Embodiment Works>

[0025] When the first and second spools 21 and 22, or the traveling section spools, are switched from the neutral position to the operating position, the first and second signal passage sections 21A and 22A are switched to the closed position, and the passage from the third pump 6 to the tanks 7B and 7C is blocked. When at least one of the third spool 23, fourth spool 24, and fifth spool 25, or the working section spools, is switched while the passage from the third pump 6 to the tanks 7B and 7C is blocked, the opening area of the signal passage section of the working section spool is reduced in accordance with the switching amount of the working section spool. Thus, the pressure increases in the portion of the second signal passage 19 upstream of the working section spool. However, since the increase in pressure acting on the pressure receiving section of the straight-travel spool 26 is moderated by the first and second orifices 16 and 17, the straight-travel spool 26 can be switched gently.

[0026] When the first signal passage 18 from the third pump 6 to the tanks 7B and 7C is blocked, the pressure increases in the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A, or the signal passage sections of the working section spools. Even when at least one of the third spool 23, the fourth spool 24, and the fifth spool 25, or the working section spools, is switched while the pressure is increased in the signal passage sections of the working section spools, the working section spools can be gently switched by means of the orifices 23B, 24B, and 25B provided in the third signal passage section 23A, the fourth signal passage section 24A, and the fifth signal passage section 25A, respectively.<Advantageous Effects of Embodiment>

[0027] Advantageous effects of the above embodiment will be now described. (1) When at least one of the third switching valve 3C, fourth switching valve 3D, or fifth switching valve 3E is switched, and accordingly corresponding at least one of the third signal passage section 23A, fourth signal passage section 24A, or fifth signal passage section 25A is switched, the corresponding at least one of the third signal passage section 23A, fourth signal passage section 24A, or fifth signal passage section 25A is switched after a predetermined amount of time has elapsed. Thus, when at least one of the third switching valve 3C, fourth switching valve 3D, or fifth switching valve 3E is switched, the corresponding at least one of the third signal passage section 23A, fourth signal passage section 24A, or fifth signal passage section 25A can be switched gently. This reduces shocks resulting from the switching of the third switching valve 3C, fourth switching valve 3D, or fifth switching valve 3E.

[0028] (2) The orifices 23B, 24B, and 25B are provided in the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A of the work switching valves at the portions between the respective communicating position and closed positions. Thus, when the third switching valve 3C, fourth switching valve 3D, and fifth switching valve 3E are switched, corresponding ones of the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A can be switched gently by means of the orifices 23B, 24B, and 25B.

[0029] (3) When the second signal passage section 22A of the second switching valve 3B is in the closed position, the hydraulic fluid in the first signal passage 18 is not discharged to the tank 7B, and thus the signal pressure in the second signal passage 19 becomes high. Even though the signal pressure in the second signal passage 19 becomes high, the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A of the work switching valves can be switched gently by means of the orifices 23B, 24B, and 25B provided in the portions between the respective communicating position and closed positions of the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A.

[0030] (4) The first orifice 16 and the second orifice 17 are provided on the upstream side and the downstream side of the junction at which the second signal passage 19 branches off from the first signal passage 18. Therefore, a moderate increase in control pressure results from the closing of the first signal passage section 21A and second signal passage section 22A of the travel switching valves or the third signal passage section 23A, fourth signal passage section 24A, and fifth signal passage section 25A of the work switching valves.<Other Embodiments>

[0031] The above embodiment can be modified as described below. The above embodiment and the following modification examples can be implemented in combination to the extent where they are technically consistent to each other.

[0032] In the above embodiment, an orifice may be provided in the portion between the communicating position and the closed position of the signal passage section of each travel switching valve. Specifically, the first signal passage section 21A of the first switching valve 3A and the second signal passage section 22A of the second switching valve 3B may have an orifice in the portion between the neutral position and the closed position. In this case, when the first switching valve 3A and the second switching valve 3B are switched, the first signal passage section 21A and the second signal passage section 22A of the travel switching valves can be gently switched by means of the orifices.

[0033] In the above embodiment, the first orifice 16 and the second orifice 17 are provided on the upstream side and the downstream side of the junction at which the second signal passage 19 branches off from the first signal passage 18. However, the first orifice 16 and the second orifice 17 may be omitted.

[0034] In the above embodiment, a third hydraulic motor may be provided as a hydraulic actuator for slewing the main body of an excavator, including the cab, relative to the traveling unit and other components. In this case, a switching valve is provided to control the hydraulic fluid supplied to the third hydraulic motor, and the spool of this switching valve is included in the working section spools.

[0035] The foregoing embodiment describes a plurality of physically separate constituent parts. They may be combined into a single part, and any one of them may be divided into a plurality of physically separate constituent parts. Irrespective of whether or not the constituent parts are integrated, they are acceptable as long as they are configured to solve the problems.

Claims

1. A hydraulic circuit (10) for a construction machine provided with a travel motor (2A, 2B) and a work actuator (2C, 2D, 2E), the hydraulic circuit (10) comprising: a hydraulic fluid pump (4, 5) for supplying hydraulic fluid to the travel motor (2A, 2B) and the work actuator (2C, 2D, 2E); a travel switching valve (3A, 3B) for controlling supply of the hydraulic fluid to drive the travel motor (2A, 2B); a work switching valve (3C, 3D, 3E) for controlling supply of the hydraulic fluid to drive the work actuator (2C, 2D, 2E); a first passage (14, 15) running from the hydraulic fluid pump (4, 5) to a tank (7) via the travel switching valve (3A, 3B) and the work switching valve (3C, 3D, 3E); a second passage (12, 13) running from the hydraulic fluid pump (4, 5) to a tank (7A) via the work switching valve (3C, 3D, 3E); a straight-travel valve (11) for switching between connection of the hydraulic fluid pump (4, 5) to the first passage (14, 15) and connection of the hydraulic fluid pump (4, 5) to the second passage (12, 13), in accordance with operation of the travel motor (2A, 2B) and the work actuator (2C, 2D, 2E); a control pressure pump (6) for supplying control pressure to control the straight-travel valve (11); a first signal passage (18) for supplying control pressure from the control pressure pump (6) to the straight-travel valve (11), the first signal passage (18) running to a tank (7B, 7C) via a signal passage section (21A, 22A) of the travel switching valve (3A, 3B); and a second signal passage (19) branching off from the first signal passage (18) and running to a tank (7D) via a signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E), wherein the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) is switched between a communicating position and a closed position, and when the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) is switched in conjunction with switching of the work switching valve (3C, 3D, 3E), the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) is switched after a predetermined amount of time has elapsed.

2. The hydraulic circuit (10) for a construction machine according to claim 1, wherein the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) has an orifice (23B, 24B, 25B) in a portion between the communicating position and the closed position, and wherein when the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) is switched, the signal passage section (23A, 24A, 25A) of the work switching valve (3C, 3D, 3E) is switched after a predetermined amount of time has elapsed, by means of the orifice (23B, 24B, 25B).

3. The hydraulic circuit (10) for a construction machine according to claim 2, wherein the signal passage section (21A, 22A) of the travel switching valve (3A, 3B) is switched between a communicating position and a closed position.

4. The hydraulic circuit (10) for a construction machine according to claim 3, wherein the signal passage section (21A, 22A) of the travel switching valve (3A, 3B) has an orifice in a portion between the communicating position and the closed position, and wherein when the signal passage section (21A, 22A) of the travel switching valve (3A, 3B) is switched, the signal passage section (21A, 22A) of the travel switching valve (3A, 3B) is switched after a predetermined amount of time has elapsed, by means of the orifice.

5. The hydraulic circuit (10) for a construction machine according to any one of claims 1 to 4, wherein orifices (16, 17) are provided on an upstream side and a downstream side of a junction at which the first signal passage (18) branches off from the second signal passage (19).

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