Transport vehicle

Abstract

A transport vehicle is provided with a steering circuit including a steering cylinder steered by hydraulic oil from a hydraulic pressure source, a steering valve for controlling the flow of the hydraulic oil to the steering cylinder, and an accumulator for accumulating the pressure of the hydraulic oil from the hydraulic pressure source and capable of supplying pressure to the steering cylinder via the steering valve. The steering circuit is connected with the hydraulic source and the working oil tank through the composite control valve. The composite control valve is provided with a first position where the steering loop communicates with the hydraulic source and communication between the steering loop and the working oil tank is cut off, and a second position where the steering loop communicates with the hydraulic source and communication between the steering loop and the working oil tank. The compound control valve has a throttle portion in an oil passage at the second position, the oil passage communicating the steering circuit and the hydraulic oil tank.

Description

Technical Field

[0001] The present invention relates to a transport vehicle such as a dump truck for use in mines, and more particularly to a transport vehicle including an accumulator in a hydraulic circuit for steering steerable wheels. Background Art

[0002] Transport vehicles such as dump trucks use a hydraulic circuit driven by a hydraulic actuator using hydraulic oil supplied from a hydraulic pump to steer the steering wheels. The hydraulic circuit (steering circuit) used to steer the steering wheels is equipped with an accumulator as a safety device (emergency hydraulic source) in the event of a loss of function of the hydraulic pump. The accumulator accumulates the pressure of the hydraulic oil supplied from the hydraulic pump and can supply the accumulated pressure to the hydraulic actuator in the event of a loss of function of the hydraulic pump. Therefore, even without the supply of hydraulic oil from the hydraulic pump, the hydraulic actuator can operate by supplying the pressure accumulated in the accumulator, thereby steering the steering wheels.

[0003] The hydraulic oil accumulated in the accumulator needs to be released to the hydraulic tank when the transport vehicle's operation is completely terminated. Therefore, a discharge control valve is provided on the pipeline connecting the accumulator and the hydraulic tank (see, for example, Patent Document 1). The discharge control valve is configured to switch between an open position, which allows communication between the accumulator and the hydraulic tank, and a closed position, which cuts off this communication. By switching the discharge control valve from the closed position to the open position, the hydraulic oil accumulated in the accumulator is discharged to the hydraulic tank.

[0004] In addition to the steering hydraulic circuit, a transport vehicle also includes a lifting hydraulic circuit for raising and lowering the cargo box. In such a transport vehicle, a control valve is sometimes used to switch the supply destination of hydraulic oil discharged from the hydraulic pump between the steering hydraulic circuit and the lifting hydraulic circuit (see, for example, Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-264456 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] In a hydraulic circuit that includes two control valves, a relief control valve that controls the release of hydraulic oil from an accumulator to a hydraulic oil tank, and a control valve (priority valve) that controls the supply destination of hydraulic oil from a hydraulic pump, as described in Patent Document 1, it is preferable to implement a failsafe measure to address the possibility that both the relief control valve and the priority valve may simultaneously fail to switch to an undesired switching position. Specifically, if the priority valve fails to switch to the switching position for lifting operations and the relief control valve fails to switch to the open position (the switching position that connects the accumulator to the hydraulic oil tank), the supply of hydraulic oil from the hydraulic pump to the steering hydraulic circuit will be cut off, and hydraulic oil from the accumulator to the hydraulic oil tank will be released simultaneously.

[0010] If these problems occur simultaneously, the function of the pressure accumulator as a safety device will rapidly decrease, and the steering wheel may become unable to be turned. Therefore, it is preferable to assume that the undesirable situation of the discharge control valve and the priority valve being in the undesirable switching position at the same time will occur and take countermeasures.

[0011] The present invention is made to solve the above-mentioned problems, and its purpose is to provide a transport vehicle that can avoid the simultaneous release of hydraulic oil from the accumulator of the steering circuit to the working oil tank and the interruption of the supply of hydraulic oil from the hydraulic source to the steering circuit.

[0012] Means for solving problems

[0013] The present application includes a plurality of means for solving the above-mentioned problems. As an example, a transport vehicle is provided, which comprises: a working oil tank for storing working oil; a hydraulic source for converting the working oil in the working oil tank into high-pressure hydraulic oil to supply hydraulic oil; and a steering circuit, which comprises: a steering cylinder for steering the steering wheel by extending and contracting the hydraulic oil supplied from the hydraulic source; a steering valve for controlling the flow of the hydraulic oil supplied from the hydraulic source to the steering cylinder; and an accumulator for accumulating the pressure of the hydraulic oil supplied from the hydraulic source and being able to supply the accumulated pressure as the hydraulic source of the steering cylinder to the steering wheel via the steering valve. Cylinder, the transport vehicle is characterized in that the steering circuit is configured to be connected to the hydraulic source and the working oil tank via a compound control valve, the compound control valve having: a first position that enables the steering circuit to be connected to the hydraulic source and cuts off the connection between the steering circuit and the working oil tank; and a second position that enables the steering circuit to be connected to the hydraulic source and to the working oil tank and discharges the hydraulic oil of the accumulator into the working oil tank, and when the compound control valve is in the second position, a throttling portion is provided in the oil circuit that connects the steering circuit and the working oil tank.

[0014] Effects of the Invention

[0015] According to the present invention, in the second position of the compound control valve, which enables the release (bleeding) of hydraulic oil from the steering circuit's accumulator to the hydraulic tank, the steering circuit and the hydraulic source are in communication, enabling the supply of hydraulic oil from the hydraulic source to the steering circuit via the compound control valve. Furthermore, the throttle in the second position acts as a fluid resistance to the flow from the hydraulic source to the hydraulic tank via the compound control valve. This prevents the simultaneous release of hydraulic oil from the steering circuit's accumulator to the hydraulic tank and the interruption of hydraulic oil supply from the hydraulic source to the steering circuit.

[0016] The problems, structures, and effects other than those described above will become clear through the following description of the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a side view of a dump truck serving as a transport vehicle showing the first embodiment of the present invention.

[0018] Figure 2 Yes Figure 1 FIG. 1 is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to the first embodiment.

[0019] Figure 3 This is a hydraulic circuit diagram showing the configuration of a hydraulic system of a comparative example to the hydraulic system of the transport vehicle of the first embodiment.

[0020] Figure 4 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to a modified example of the first embodiment.

[0021] Figure 5 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to a second embodiment of the present invention.

[0022] Figure 6 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to a third embodiment of the present invention.

[0023] Figure 7 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to a fourth embodiment of the present invention.

[0024] Figure 8 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to another embodiment of the present invention. DETAILED DESCRIPTION

[0025] The following describes an embodiment of a transport vehicle according to the present invention using the accompanying drawings. In this embodiment, a dump truck is used as an example of a transport vehicle. The front, back, left, and right directions described in this specification refer to directions viewed from the perspective of an operator aboard the transport vehicle.

[0026] [First embodiment]

[0027] First, use Figure 1 The structure of a dump truck as a transport vehicle according to the first embodiment will be described. Figure 1 It is a side view of a dump truck serving as a transport vehicle showing the first embodiment of the present invention.

[0028] exist Figure 1 In the present invention, a dump truck 1 is a large transport vehicle used in mines and other places to transport mined ore, sand, and other cargo (transported objects). The dump truck 1 includes front wheels 2 and rear wheels 3 rotatably disposed on the left and right sides of the front and rear portions of the vehicle, respectively; a vehicle body 4 that is movable by the front and rear wheels 2 and 3; and a cargo box 5 mounted on the vehicle body 4 in a manner that allows it to rise and fall (tilt).

[0029] The front wheel 2 is a steering wheel that is steered by the operator. The front wheel 2 as a steering wheel is configured to be steered by the left and right steering cylinders 51 and 52 described later (see the Figure 2 The rear wheel 3 is a driving wheel that is rotationally driven by a travel drive device (not shown), for example.

[0030] The vehicle body 4 has a Figure 1 The vehicle frame 11 is a supporting structure extending in the left and right directions of the vehicle frame, a cabin 12 arranged in the front part of the vehicle frame 11, and a cab 13 arranged on the cabin 12. A hydraulic oil tank 23 (also referred to below) is mounted on the vehicle frame 11. Figure 2 The cabin 12 houses the prime mover 15, hydraulic pumps 21, 22 (see the following) Figure 2 ) and other devices. The cab 13 is a cab where the operator rides and operates the vehicle. Inside the cab 13 are arranged driving pedals (not shown) for driving the dump truck 1, a steering wheel 16 (see below) for steering the front wheels 2 (steering wheels) and a steering wheel 16 (see below) for steering the front wheels 2 (steering wheels). Figure 2 ), the operating device 17 described later for operating the ups and downs (posture) of the cargo box 5 (see the Figure 2 ) and other various operating devices used by the operator to operate the dump truck 1.

[0031] The cargo box 5 is a container for loading ore, sand and soil and other loads (transport objects). The rear side of the cargo box 5 is rotatably mounted on the rear end of the frame 11, so that the front side can rotate in the vertical direction with the rear side as a fulcrum. A pair of left and right lifting cylinders 41 ( Figure 1 Only one is shown in the figure.) The lift cylinder 41 is a hydraulic actuator that expands and contracts through the supply and discharge of hydraulic oil. The cargo box 5 is configured to rotate relative to the vehicle frame 11 between a collapsed state (seated state) when the lift cylinder 41 contracts, thereby achieving a transport posture in which objects can be loaded onto the cargo box 5, and an upright state (tilted state) when the lift cylinder 41 extends, thereby achieving a release posture in which objects can be released from the cargo box 5. Figure 1 In the figure, the container 5 shown in the solid line is in the transport posture, and the container 5 shown in the two-dot chain line is in the release posture. In addition, the lifting cylinder 41 shown in the solid line is in the contracted state, and the lifting cylinder 41 shown in the two-dot chain line is in the extended state.

[0032] Next, use Figure 2 The structure of the hydraulic system in the transport vehicle of the first embodiment will be described. Figure 2 Yes Figure 1 FIG. 1 is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to the first embodiment.

[0033] exist Figure 2 In the embodiment, the dump truck 1 is equipped with a hydraulic system 20 for raising and lowering the cargo box 5 and steering the front wheels 2. The hydraulic system 20 includes a first hydraulic pump 21 and a second hydraulic pump 22 as hydraulic sources, which convert the hydraulic oil into high-pressure hydraulic oil for supply; a hydraulic oil tank 23, which stores the hydraulic oil; and a lifting circuit 40, which includes a pair of lifting cylinders 41 ( Figure 2 The hoist circuit 40 is connected to the hydraulic source via a compound control valve 30. The steering circuit 50 is connected to the hydraulic source and the hydraulic oil tank 23 via the compound control valve 30.

[0034] The first hydraulic pump 21 and the second hydraulic pump 22 are, for example, variable displacement pumps and have regulators 21a and 22a, respectively. The regulators 21a and 22a adjust the pump volumes in response to control commands from the control device 80. The first hydraulic pump 21 and the second hydraulic pump 22 are driven by, for example, a prime mover 15. The prime mover 15 is, for example, an engine or an electric motor. The prime mover 15 is driven in response to control commands from the control device 80. The first hydraulic pump 21 and the second hydraulic pump 22 are connected to the compound control valve 30 via a first discharge line 25 and a second discharge line 26, respectively.

[0035] The lifting cylinder 41 of the lifting circuit 40 is used to make the cargo box 5 (see Figure 1 ) Fluctuating single-stage or multi-stage (in Figure 1 and Figure 2 The lifting cylinder 41 has a cylinder bottom side oil chamber (hereinafter referred to as the cylinder bottom chamber) 41a and a rod side oil chamber (hereinafter referred to as the rod chamber) 41b. The lifting cylinder 41 is extended by supplying hydraulic oil to the cylinder bottom chamber 41a and discharging return oil from the rod chamber 41b. By extending the lifting cylinder 41, the cargo box 5 is in the released position ( Figure 1 The lifting cylinder 41 contracts by supplying hydraulic oil to the rod chamber 41b and discharging return oil from the cylinder bottom chamber 41a. By contracting the lifting cylinder 41, the cargo box 5 becomes the transport posture ( Figure 1 middle, solid line).

[0036] The lift circuit 40 also includes a lift control valve 42. The lift control valve 42 and the compound control valve 30 are located on the center bypass line 31, the oil circuit connecting the first hydraulic pump 21 and the hydraulic tank 23. The lift control valve 42 is connected in series, located downstream of the compound control valve 30. The lift control valve 42 controls the flow of hydraulic oil supplied from the hydraulic sources 21 and 22 to the lift cylinder 41 via the compound control valve 30, and controls the flow of return oil discharged from the lift cylinder 41 to the hydraulic tank 23. A supply oil line 43, which branches from the center bypass line 31, is connected to the lift control valve 42. The lift control valve 42 is connected to the cylinder bottom chamber 41a and rod chamber 41b of the lift cylinder 41 via a pair of actuator lines 44a and 44b. Furthermore, the lift control valve 42 is connected to the hydraulic tank 23 via a return oil line 45.

[0037] The lift control valve 42 is constituted by, for example, a 6-port, 4-position hydraulic pilot type directional control valve. The lift control valve 42 is constituted by a single directional control valve and has left and right pressure receiving portions 42a and 42b to which pilot pressure is input.

[0038] The lift control valve 42 has four switch positions: a hold position (a neutral position N), a raise position R, a lower position L, and a float position F. The valve is switched to one of these positions in response to operation of the operating device 17 for the cargo box 5 within the cab 13. The hold position (neutral position N) cuts off the supply and discharge of hydraulic oil to the lift cylinder 41, maintaining the position of the cargo box 5. The raise position R extends the lift cylinder 41 by supplying and discharging hydraulic oil, causing the cargo box 5 to rotate upward. The lower position L retracts the lift cylinder 41 by supplying and discharging hydraulic oil, causing the cargo box 5 to rotate downward. The float position F retracts the lift cylinder 41 by its own weight, allowing the cargo box 5 to descend. Normally, both the pressure-receiving portions 42a and 42b of the lift control valve 42 are connected to the hydraulic oil tank 23 and are maintained in the neutral position N by a centering spring.

[0039] When the lift control valve 42 is in the neutral position N, the supply oil line 43 and the return oil line 45 are disconnected from the actuator lines 44a and 44b. Consequently, the supply and discharge of hydraulic oil to and from the lift cylinder 41 are disabled, and extension and retraction of the lift cylinder 41 cease. In this state, the upstream and downstream sides of the lift control valve 42 of the center bypass line 31 are in communication.

[0040] When the lift control valve 42 is in the raised position R, the supply oil passage 43 communicates with the actuator line 44a, and the actuator line 44b communicates with the return oil passage 45. The communication between the upstream and downstream sides of the lift control valve 42 in the center bypass line 31 is cut off. Consequently, hydraulic oil from the hydraulic sources 21 and 22 is supplied to the cylinder bottom chamber 41a of the lift cylinder 41, while return oil is discharged from the rod chamber 41b to the hydraulic oil tank 23, causing the lift cylinder 41 to extend.

[0041] When the lift control valve 42 is in the lowered position L, the supply oil passage 43 communicates with the actuator line 44b, and the actuator line 44a communicates with the return oil passage 45. Furthermore, the communication between the upstream and downstream sides of the lift control valve 42 in the center bypass line 31 is cut off. Consequently, hydraulic oil from the hydraulic sources 21 and 22 is supplied to the rod chamber 41b of the lift cylinder 41, while return oil is discharged from the cylinder bottom chamber 41a to the hydraulic oil tank 23, causing the lift cylinder 41 to retract.

[0042] When the lift control valve 42 is in the floating position F, the supply oil passage 43 communicates with the center bypass line 31 downstream of the lift control valve 42. Furthermore, the actuator line 44a communicates with the return oil passage 45. This allows return oil to be discharged from the cylinder bottom chamber 41a of the lift cylinder 41 into the hydraulic oil tank 23, while hydraulic oil is replenished from the hydraulic oil tank 23 to the rod chamber 41b of the lift cylinder 41 via a non-illustrated check valve. Consequently, the lift cylinder 41 can be retracted by the weight of the cargo box 5.

[0043] The steering circuit 50 includes a pair of left and right steering cylinders 51 and 52 and a steering valve 53, and is connected to the compound control valve 30 via a connecting line 32. The steering valve 53 is connected to the connecting line 32 (compound control valve 30) via a high-pressure line 54 and to the hydraulic oil tank 23 via a low-pressure line 57. The steering valve 53 is connected to the steering cylinders 51 and 52 via steering lines 55 and 56.

[0044] The left and right steering cylinders 51 and 52 are hydraulic actuators that expand and contract using hydraulic oil supplied from the second hydraulic pump 22, serving as a hydraulic source. They steer the left and right front wheels 2. The left and right steering cylinders 51 and 52 have cylinder-bottom-side oil chambers (hereinafter referred to as cylinder bottom chambers) 51a and 52a, respectively, and rod-side oil chambers (hereinafter referred to as rod chambers) 51b and 52b, respectively. The cylinder bottom chamber 51a of the left steering cylinder 51 and the rod chamber 52b of the right steering cylinder 52 are connected via a steering line 55. The rod chamber 51b of the left steering cylinder 51 and the cylinder bottom chamber 52a of the right steering cylinder 52 are connected via a steering line 56.

[0045] The steering valve 53 controls the flow of hydraulic oil supplied from the second hydraulic pump 22 to the steering cylinders 51 and 52, as well as the flow of return oil discharged from the steering cylinders 51 and 52 to the hydraulic oil tank 23. The steering valve 53 includes a direction control unit 53a, which controls the flow direction of hydraulic oil supplied from the second hydraulic pump 22 to the left and right steering cylinders 51 and 52, and the flow direction of return oil discharged from the left and right steering cylinders 51 and 52 to the hydraulic oil tank 23; and a flow adjustment unit 53b, which adjusts the flow rate of hydraulic oil supplied from the second hydraulic pump 22 to the left and right steering cylinders 51 and 52. The direction control unit 53a is configured to switch from a neutral position N to left and right steering positions L and R in response to rotation of the steering wheel 16. The flow adjustment unit 53b is configured to adjust the flow rate of hydraulic oil (hydraulic oil supplied to the left and right steering cylinders 51 and 52) flowing through the flow adjustment unit 53b in response to rotation of the steering wheel 16.

[0046] exist Figure 2In the steering valve 53 shown, for example, when the steering wheel 16 is rotated counterclockwise, the direction control unit 53a is switched to the left steering position L. In this case, hydraulic oil from the second hydraulic pump 22 flows from the left to the right through the direction control unit 53a in the left steering position L and passes through the flow control unit 53b. The flow rate of the hydraulic oil is regulated by the flow control unit 53b and the hydraulic oil returns to the direction control unit 53a. The hydraulic oil returned to the direction control unit 53a is supplied to the rod chamber 51b of the left steering cylinder 51 and the cylinder bottom chamber 52a of the right steering cylinder 52 via the steering line 56. Meanwhile, the hydraulic oil in the cylinder bottom chamber 51a of the left steering cylinder 51 and the rod chamber 52b of the right steering cylinder 52 is discharged from the steering line 55 through the direction control unit 53a of the steering valve 53 to the hydraulic oil tank 23. This causes the left steering cylinder 51 to contract while the right steering cylinder 52 to extend, thereby performing left steering in response to the rotation of the steering wheel 16.

[0047] When the steering wheel 16 is turned clockwise, the direction control unit 53a is switched to the right steering position R. In this case, hydraulic oil from the second hydraulic pump 22 flows from the right to the left through the direction control unit 53a in the right steering position R. The flow rate of the hydraulic oil is regulated by the flow control unit 53b and then returned to the direction control unit 53a. The hydraulic oil returned to the direction control unit 53a is supplied to the cylinder bottom chamber 51a of the left steering cylinder 51 and the rod chamber 52b of the right steering cylinder 52 via the steering line 55. Meanwhile, the hydraulic oil in the rod chamber 51b of the left steering cylinder 51 and the cylinder bottom chamber 52a of the right steering cylinder 52 is discharged from the steering line 56 through the direction control unit 53a of the steering valve 53 to the hydraulic oil tank 23. This causes the left steering cylinder 51 to extend while the right steering cylinder 52 to contract, thereby performing right steering in response to the rotation of the steering wheel 16.

[0048] An accumulator 58 is connected to the high-pressure pipeline 54. The accumulator 58 accumulates the pressure of the hydraulic oil supplied from the second hydraulic pump 22. The high-pressure pipeline 54 is connected to the working oil tank 23 via a relief valve 59. The relief valve 59 specifies the maximum pressure of the high-pressure pipeline 54. Therefore, the high-pressure pipeline 54 is maintained at a predetermined pressure by the accumulator 58 and the relief valve 59. As a result, the driving of the steering cylinders 51 and 52 corresponding to the operation of the steering wheel 16 can be ensured. The accumulator 58 is configured to supply the accumulated pressure to the steering cylinders 51 and 52 via the steering valve 53 as a hydraulic source for the steering cylinders 51 and 52 when the hydraulic oil from the second hydraulic pump 22 is not supplied to the steering cylinders 51 and 52.

[0049] The high-pressure line 54 is provided with a pressure sensor 61 for detecting the pressure of the high-pressure line 54 . The pressure sensor 61 outputs a detection signal corresponding to the detected pressure value of the high-pressure line 54 to the control device 80 .

[0050] A check valve 33 is disposed between the steering circuit 50 and the second hydraulic pump 22. The check valve 33 allows the flow of hydraulic oil from the second hydraulic pump 22 to the steering circuit 50 while preventing reverse flow. The check valve 33 is disposed, for example, upstream of the compound control valve 30.

[0051] The compound control valve 30 is a single control valve that combines two functions: a circuit switching valve that switches the supply destinations of the hydraulic oil discharged from the hydraulic sources 21 and 22 between the circuits 40 and 50, and a relief control valve that releases (drains) the hydraulic oil accumulated in the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. The compound control valve 30 is, for example, a six-port, three-position hydraulic pilot-operated control valve. The compound control valve 30 has left and right pressure-receiving portions 30a and 30b to which pilot pressure is input.

[0052] The compound control valve 30 has six ports: a first pump port p1 connected to the first hydraulic pump 21 via the first discharge line 25 and the center bypass line 31; a second pump port p2 connected to the second hydraulic pump 22 via the second discharge line 26; a tank port t connected to the hydraulic oil tank 23 via the return oil line 34; a lift port h connected to the lift circuit 40 via the center bypass line 31; a first steering port s1 connected to the steering circuit 50 via the connecting line 32 and enabling communication between the steering circuit 50 and the second hydraulic pump 22; and a second steering port s2 connected to the steering circuit 50 via the connecting line 32 and enabling communication between the steering circuit 50 and the hydraulic oil tank 23. The compound control valve 30 has three switch positions: a first position N serving as a neutral position (non-merging position); a second position D serving as a bleed position; and a third position C serving as a merging position. The compound control valve 30 is configured to be switchable to any of the three switch positions. Normally, both the pressure-receiving parts 30 a and 30 b of the compound control valve 30 are connected to the hydraulic oil tank 23 and are held in the neutral position N by a centering spring.

[0053] The neutral position N of the compound control valve 30 is a non-merging position, directing the hydraulic oil discharged from the first hydraulic pump 21 to the lift circuit 40 and the hydraulic oil discharged from the second hydraulic pump 22 to the steering circuit 50. Specifically, the hydraulic oil discharged from the first hydraulic pump 21 and the hydraulic oil discharged from the second hydraulic pump 22 are not merged, but are supplied to the lift circuit 40 and the steering circuit 50, respectively. Specifically, the neutral position N (non-merging position) allows only communication between the steering circuit 50 and the second hydraulic pump 22, of the hydraulic sources 21 and 22, while disconnecting the steering circuit 50 from the hydraulic oil tank 23. Furthermore, communication between the lift circuit 40 and the first hydraulic pump 21, of the hydraulic sources 21 and 22, is maintained.

[0054] The second position D of the compound control valve 30 is a drain position, which is switched to when the hydraulic oil accumulated in the accumulator 58 is released to the hydraulic tank 23 (draining) upon complete termination of operation of the dump truck 1. Drain position D directs the hydraulic oil in the accumulator 58 (steering circuit 50) to the hydraulic tank 23, enabling hydraulic oil to be supplied from the first hydraulic pump 21 to the lift circuit 40, and from the second hydraulic pump 22 to the steering circuit 50. Specifically, drain position D connects the steering circuit 50 only to the second hydraulic pump 22 of the hydraulic sources 21 and 22, connects the steering circuit 50 to the hydraulic tank 23, and connects the lift circuit 40 only to the first hydraulic pump 21 of the hydraulic sources 21 and 22.

[0055] In the second position D (drain position), the compound control valve 30 includes a throttle 30c in the oil passage connecting the steering circuit 50 and the hydraulic oil tank 23. The throttle 30c functions as a fluid resistance when the hydraulic oil from the second hydraulic pump 22 flows into the hydraulic oil tank 23 through the compound control valve 30.

[0056] The third position C of the compound control valve 30 directs the hydraulic oil discharged from the first hydraulic pump 21 to the lift circuit 40, and also directs the hydraulic oil discharged from the second hydraulic pump 22 to the lift circuit 40. Specifically, the merging position allows the hydraulic oil discharged from the second hydraulic pump 22 and the hydraulic oil discharged from the first hydraulic pump 21 to merge and be directed to the lift circuit 40. Specifically, the merging position C disconnects the steering circuit 50 from both the first and second hydraulic pumps 21, 22, and disconnects the steering circuit 50 from the hydraulic oil tank 23, while maintaining communication between the lift circuit 40 and both the first and second hydraulic pumps 21, 22.

[0057] The hydraulic system 20 includes a pilot circuit 70. This pilot circuit 70 generates a pilot pressure from a pilot hydraulic source for driving the lift control valve 42 and inputs the pressure to the pressure-receiving portions 42a and 42b of the lift control valve 42. Furthermore, this pilot pressure is generated from a pilot hydraulic source for driving the compound control valve 30 and inputs the pressure to the pressure-receiving portions 30a and 30b of the compound control valve 30. The pilot circuit 70 generates the pilot pressure for the lift control valve 42 in response to operation of the operating device 17 for the cargo box 5.

[0058] The pilot circuit 70 includes a pilot pump 71 driven by the prime mover 15 and four solenoid valves 72, 73, 74, and 75 connected to the pilot pump 71 via a pilot line 76. The portion of the pilot line 76 between the pilot pump 71 and the solenoid valves 72, 73, 74, and 75 is connected to the hydraulic oil tank 23 via a pilot relief valve 77 and is also connected to an accumulator 78. The pilot pump 71 is a pilot hydraulic source and is, for example, a fixed-displacement pump. The pilot relief valve 77 regulates the maximum pressure of the pilot line 76 (the maximum discharge pressure of the pilot pump 71). The accumulator 78 accumulates the pressure of the hydraulic oil discharged from the pilot pump 71. Therefore, the pilot line 76 is maintained at a predetermined pressure by the pilot relief valve 77 and the accumulator 78. This ensures the initial pressure for generating the pilot pressure for driving the lift control valve 42 and the compound control valve 30.

[0059] Two of the four solenoid valves 72, 73, 74, and 75 generate pilot pressure for the lift control valve 42 in response to operation of the operating device 17 for the cargo box 5, thereby switching the lift control valve 42 to one of four switching positions (neutral position N, raising position R, lowering position L, and floating position F). The remaining solenoid valves 74 and 75 generate pilot pressure for the compound control valve 30, switching the compound control valve 30 to one of three switching positions (non-merging position N, bleed position D, and merging position C). Each solenoid valve 72, 73, 74, and 75 is a pressure reducing valve that reduces the pressure (primary pressure) in the pilot line 76 in response to a control signal (excitation current) from the control device 80 and outputs the reduced pressure (secondary pressure) as a pilot pressure.

[0060] The solenoid valve 72 is used to switch the lift control valve 42 to the raised position R. When a standby control signal (off signal) is input, one pressure-receiving portion 42a of the lift control valve 42 is connected to the hydraulic oil tank 23. When a driving control signal is input, a pilot pressure is output to one pressure-receiving portion 42a of the lift control valve 42. The solenoid valve 73 is used to switch the lift control valve 42 between the floating position F and the lowered position L. When a standby control signal (off signal) is input, the other pressure-receiving portion 42b of the lift control valve 42 is connected to the hydraulic oil tank 23. When a driving control signal is input, a pilot pressure corresponding to the floating position F or the lowered position L is output to the other pressure-receiving portion 42b of the lift control valve 42. Furthermore, when both solenoid valves 72 and 73 cut off the input of pilot pressure to the lift control valve 42, the lift control valve 42 is in the neutral position N.

[0061] The solenoid valve 74 is used to switch the compound control valve 30 to the relief position D. When a standby control signal (off signal) is input, the other pressure-receiving portion 30b of the compound control valve 30 is connected to the hydraulic oil tank 23. When a driving control signal (on signal) is input, a pilot pressure is output to the other pressure-receiving portion 30b of the compound control valve 30. The solenoid valve 75 is used to switch the compound control valve 30 to the merging position C. When a standby control signal (off signal) is input, one pressure-receiving portion 30a of the compound control valve 30 is connected to the hydraulic oil tank 23. When a driving control signal is input, a pilot pressure is output to one pressure-receiving portion 30a of the compound control valve 30. When both the solenoid valves 74 and 75 cut off the input of the pilot pressure to the compound control valve 30, the compound control valve 30 is in the neutral position N, which is a non-merging position.

[0062] The hydraulic system 20 is configured to drive the lift control valve 42 in response to operation of the operating device 17 for the cargo box 5. The operating device 17 is comprised of, for example, an electric lever device and includes an operating lever 17a that is manually tilted by the operator. The operating device 17 can be operated to any of four operating positions: a holding position, a raising position, a floating position, and a lowering position, corresponding to the respective switching positions of the lift control valve 42 (holding position N, raising position R, floating position F, and lowering position L). The operating lever 17a is normally positioned in the holding position. The operating device 17 outputs an operating signal corresponding to the operating position to the control device 80.

[0063] The control device 80 is electrically connected to the operating device 17 and receives an operating signal from the operating device 17. The control device 80 controls the two solenoid valves 72 and 73 of the pilot hydraulic circuit 70 based on the operating position (operation signal) of the operating device 17, thereby indirectly controlling the switching position of the hydraulic pilot lift control valve 42. Furthermore, the control device 80 indirectly controls the switching position of the hydraulic pilot compound control valve 30 by controlling the two solenoid valves 74 and 75 of the pilot hydraulic circuit 70. Furthermore, the control device 80 controls the pump capacities of the first and second hydraulic pumps 21 and 22 via the regulators 21a and 22a of the first and second hydraulic pumps 21 and 22, respectively.

[0064] The control device 80 is comprised of, for example, a computer including a storage device 81 comprised of RAM, ROM, or the like, and a processing device 82 comprised of a CPU, MPU, or the like. The storage device 81 pre-stores programs and various information necessary for controlling the switching positions of the lift control valve 42 and the compound control valve 30. The processing device 82 appropriately reads the programs and various information from the storage device 81 and executes processing according to the programs, thereby realizing various functions. The control device 80 may be comprised of a single computer or multiple computers.

[0065] When the operating lever 17a of the operating device 17 is operated to the raising operating position, the control device 80 outputs a control signal (excitation current) to the solenoid valve 72 and a control signal for a cutoff signal to the solenoid valve 73. As a result, the pilot pressure from the solenoid valve 72 is input to the pressure-receiving portion 42a of the lift control valve 42, causing the lift control valve 42 to switch to the raising position R.

[0066] In this case, the control device 80 switches the compound control valve 30 to the merging position C, for example. Specifically, the control device 80 outputs a control signal (excitation current) to the solenoid valve 75 and a control signal for a cutoff signal to the solenoid valve 74. Consequently, the pilot pressure from the solenoid valve 75 is input to the pressure-receiving portion 30a of the compound control valve 30, and the compound control valve 30 switches from the neutral position N to the merging position C.

[0067] Therefore, the hydraulic oil from the first hydraulic pump 21 and the second hydraulic pump 22 merges through the compound control valve 30 at the merging position C and is guided to the lift control valve 42. Since the lift control valve 42 is switched to the raising position R, the hydraulic oil from the first hydraulic pump 21 and the second hydraulic pump 22 is supplied to the cylinder bottom chamber 41a of the lift cylinder 41 via the lift control valve 42 at the raising position R, thereby changing the cargo box 5 loaded with the transport object from the transport position to the deploying position.

[0068] When the operating lever 17a of the operating device 17 is operated to the lowering position, the control device 80 outputs a control signal (excitation current) corresponding to the lowering operation to the solenoid valve 73 and a control signal to disconnect the solenoid valve 72. This causes the pilot pressure from the solenoid valve 73 to be input to the other pressure-receiving portion 42b of the lift control valve 42, switching the lift control valve 42 to the lowering position L. In this state, the compound control valve 30 is maintained in the neutral position N. When the operating device 17 is in the lowering position, the lift cylinder 41 can be retracted solely by the hydraulic oil supplied from the first hydraulic pump 21.

[0069] When the operating lever 17a of the operating device 17 is operated to the float operation position, the control device 80 outputs a control signal (excitation current) corresponding to the float operation to the solenoid valve 73 and a control signal to disconnect the solenoid valve 72. This causes the pilot pressure from the solenoid valve 73 to be input to the other pressure-receiving portion 42b of the lift control valve 42, switching the lift control valve 42 to the float position F. In this state, the compound control valve 30 is maintained in the neutral position N. When the operating device 17 is in the float operation, the lift cylinder 41 can be retracted using the weight of the cargo box 5.

[0070] Furthermore, when the operation of the dump truck 1 is completely terminated, the control device 80 discharges the hydraulic oil accumulated in the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. For example, after a certain period of time has passed since the prime mover 15 stopped, the control device 80 switches the compound control valve 30 to the discharge position D. This causes the pressure accumulated in the accumulator 58 of the steering circuit 50 to be gradually discharged from the return line 34 to the hydraulic oil tank 23 via the throttle portion 30c of the compound control valve 30 in the discharge position D. At this time, the flow from the steering circuit 50 to the second hydraulic pump 22 via the compound control valve 30 in the discharge position D is blocked by the check valve 33. Furthermore, the control device 80 can determine the completion of the discharge based on the detection value of the pressure sensor 61 that detects the pressure in the accumulator 58.

[0071] Next, the effects of the transport vehicle of the first embodiment will be compared with the hydraulic system of the transport vehicle of the comparative example. Figure 2 and Figure 3 Provide explanation. Figure 3 1 is a hydraulic circuit diagram showing the structure of a hydraulic system of a comparative example relative to the hydraulic system of the transport vehicle of the first embodiment. Figure 3 In, with Figure 2 The parts denoted by the same reference numerals are the same parts, and thus detailed description thereof will be omitted.

[0072] In the hydraulic system 120 of the comparative example, the steering circuit 150 includes a relief control valve 162 that has the sole function of discharging the hydraulic oil from the accumulator 58 to the hydraulic oil tank 23. The relief control valve 162 is provided on the line connecting the high-pressure line 54 and the hydraulic oil tank 23. The relief control valve 162 comprises a two-port, two-position electromagnetic control valve that selectively switches between a closed position C and an open position D in response to a control signal (excitation current) from the control device 180. When the relief control valve 162 is in the closed position C, the hydraulic oil is retained in the accumulator 58. On the other hand, when the relief control valve 162 is switched to the open position D, the hydraulic oil in the accumulator 58 is discharged to the hydraulic oil tank 23 via the high-pressure line 54 and the relief control valve 162.

[0073] Furthermore, the hydraulic system 120 of the comparative example includes a switching valve 130, which switches the supply destination of the hydraulic oil from the second hydraulic pump 22 between the lift circuit 40 and the steering circuit 150, in place of the compound control valve 30 of the hydraulic system 20 of the present embodiment. The switching valve 130 is connected to the second hydraulic pump 22 via a second discharge line 26. The output side of the switching valve 130 is connected to the steering circuit 150 via a connecting line 32, and is also connected to the lift circuit 40 via a connecting line 135 branching from the center bypass line 31. Furthermore, the first hydraulic pump 21 of the hydraulic system 120 is connected to the lift circuit 40 via the first discharge line 25, without using a valve mechanism.

[0074] The switching valve 130 is a hydraulically pilot-operated valve with two switching positions. In the first position S, the switching valve 130 directs the hydraulic oil from the second hydraulic pump 22 to the steering circuit 150, thereby connecting the steering circuit 150 and the second hydraulic pump 22. In the second position H, the switching valve 130 directs the hydraulic oil from the second hydraulic pump 22 to the lift circuit 40, thereby connecting the lift circuit 40 and the second hydraulic pump 22. The switching valve 130 is configured to be switched by a pilot pressure generated by the solenoid valve 175 of the pilot circuit 170. Switching between the first position S and the second position H is performed in response to a control signal from the control device 180.

[0075] In the comparative example hydraulic system 120, there is a very low probability that the switching valve 130 may erroneously operate to the second position H (a position where the hydraulic oil is supplied to the lift circuit 40) or become stuck in the second position H, and that the relief control valve 162 may erroneously operate to the open position D or become stuck in the open position D. In this case, the hydraulic oil from the second hydraulic pump 22 is supplied to the lift circuit 40 and therefore not to the steering circuit 150. Furthermore, the hydraulic oil accumulated in the accumulator 58 of the steering circuit 150 is released to the hydraulic oil tank 23 via the relief control valve 162 in the open position D. Therefore, if both the switching valve 130 and the relief control valve 162 are in unexpected positions simultaneously, the vehicle will more quickly enter a state where steering is impossible due to a pressure drop in the steering circuit 150 than if only one of the switching valves 130 and 162 fails.

[0076] In contrast, in the hydraulic system 20 of this embodiment, as Figure 2 As shown, the compound control valve 30 has a bleed function for releasing hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, and also has a switching function for the supply of hydraulic oil from the hydraulic source 22 to the steering circuit 50. In other words, the single compound control valve 30 combines the functions of both the switching valve 130 and the bleed control valve 162 in the hydraulic system 120 of the comparative example. In the bleed position D, where the compound control valve 30 allows hydraulic oil to be released from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, the hydraulic oil supply from the hydraulic source 22 to the steering circuit 50 is maintained. This avoids the simultaneous release of hydraulic oil from the accumulator 58 to the hydraulic tank 23 and the interruption of the supply of hydraulic oil from the hydraulic source 22 to the steering circuit 50.

[0077] In this embodiment, if the compound control valve 30 unexpectedly malfunctions or becomes stuck in the relief position D during operation of the dump truck 1, the steering circuit 50 becomes connected to the hydraulic oil tank 23. At this time, the first and second hydraulic pumps 21 and 22 are driven. Hydraulic oil from the second hydraulic pump 22 flows through the compound control valve 30 in the relief position D toward the steering circuit 50 (arrow in the center of position D). It is then supplied to the steering circuit 50 via the connecting line 32 and flows out of the hydraulic oil tank 23 through the throttle 30c in the relief position D of the compound control valve 30 (arrow on the right side of position D). The flow of hydraulic oil from the second hydraulic pump 22 out of the hydraulic oil tank 23 through the compound control valve 30 in the relief position D is limited by the fluid resistance of the throttle 30c. Consequently, the flow rate of hydraulic oil supplied from the second hydraulic pump 22 to the steering circuit 50 exceeds the flow rate of hydraulic oil flowing out of the second hydraulic pump 22 to the hydraulic oil tank 23 through the throttle 30c in the relief position D. Therefore, the pressure of the steering circuit 50 (accumulator 58) is maintained, so the steering function of the steering circuit 50 can be maintained. In addition, the hydraulic oil from the first hydraulic pump 21 is supplied to the lift circuit 40. In this case, the lift cylinder 41 can be operated.

[0078] As described above, the dump truck 1 (transport vehicle) of the first embodiment includes: a hydraulic oil tank 23 storing hydraulic oil; a first hydraulic pump 21 and a second hydraulic pump 22 serving as hydraulic sources, which compress the hydraulic oil in the hydraulic oil tank 23 into high-pressure hydraulic oil for supply; a steering circuit 50 including: steering cylinders 51 and 52 that steer the front wheels 2 (steerable wheels) by expanding and contracting the hydraulic oil supplied from the hydraulic source 22; a steering valve 53 that controls the flow of hydraulic oil supplied from the hydraulic source 22 to the steering cylinders 51 and 52; and an accumulator 58 that accumulates the pressure of the hydraulic oil supplied from the hydraulic source 22 and can supply the accumulated pressure to the steering cylinders 51 and 52 via the steering valve 53 as a hydraulic source for the steering cylinders 51 and 52. The steering circuit 50 is connected to the hydraulic source 22 and the hydraulic oil tank 23 via the compound control valve 30. The compound control valve 30 has a neutral position N (first position) in which the steering circuit 50 communicates with the hydraulic source 22 and blocks communication with the hydraulic oil tank 23; and a relief position D (second position) in which the steering circuit 50 communicates with the hydraulic source 22 and the hydraulic oil tank 23, thereby discharging the hydraulic oil in the accumulator 58 into the hydraulic oil tank 23. In the relief position D (second position), the compound control valve 30 has a throttle portion 30 c in the oil passage connecting the steering circuit 50 and the hydraulic oil tank 23.

[0079] According to this configuration, when the compound control valve 30 is in the bleed position D (second position), which enables the release (drain) of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, the steering circuit 50 and the second hydraulic pump 22 (hydraulic source) are in communication. This allows the supply of hydraulic oil from the second hydraulic pump 22 (hydraulic source) to the steering circuit 50 via the compound control valve 30. Furthermore, the throttle 30c in the bleed position D (second position) acts as a fluid resistance to the flow from the second hydraulic pump 22 (hydraulic source) to the hydraulic tank 23 via the compound control valve 30. This prevents the simultaneous release of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23 and the interruption of the supply of hydraulic oil from the hydraulic source 22 to the steering circuit 50.

[0080] The dump truck 1 (transport vehicle) of this embodiment also includes a lift circuit 40 including a lift cylinder 41. This lift cylinder 41 is connected to the hydraulic sources 21 and 22 via a compound control valve 30, and extends and contracts using hydraulic oil supplied from the hydraulic sources 21 and 22, thereby raising and lowering the cargo bed. In addition to the aforementioned neutral position N (first position) and relief position D (second position), the compound control valve 30 also has a third position C. In this third position C, the steering circuit 50 is disconnected from the hydraulic sources 21 and 22, the steering circuit 50 is disconnected from the hydraulic oil tank 23, and the lift circuit 40 is connected to the hydraulic sources 21 and 22.

[0081] With this configuration, by switching the compound control valve 30 between the first position N and the third position C, the supply destination of the hydraulic oil from the hydraulic sources 21 and 22 can be switched to the lift circuit 40 or the steering circuit 50. Furthermore, by switching the compound control valve 30 to the drain position D (second position), the accumulator 58 can be drained. Furthermore, in the first position N and the third position C of the compound control valve 30, the hydraulic oil is not released from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23.

[0082] In this embodiment, the hydraulic source is composed of the first hydraulic pump 21 and the second hydraulic pump 22. The neutral position N (first position) of the compound control valve 30 allows communication between the steering circuit 50 and the second hydraulic pump 22 of the hydraulic sources 21 and 22, cuts off communication between the steering circuit 50 and the hydraulic oil tank 23, and allows communication between the lift circuit 40 and the first hydraulic pump 21 of the hydraulic sources 21 and 22. The bleed position D (second position) of the compound control valve 30 allows communication between the steering circuit 50 and at least the second hydraulic pump 22 of the hydraulic sources 21 and 22, and cuts off communication between the lift circuit 40 and at least the second hydraulic pump 22 of the hydraulic sources 21 and 22. The merging position C (third position) of the compound control valve 30 is a position in which the connection between the steering circuit 50 and both the first hydraulic pump 21 and the second hydraulic pump 22 is cut off, the connection between the steering circuit 50 and the hydraulic oil tank 23 is cut off, and the lifting circuit 40 is connected to both the first hydraulic pump 21 and the second hydraulic pump 22.

[0083] According to this configuration, when the hydraulic source comprises the first hydraulic pump 21 and the second hydraulic pump 22, when the compound control valve 30 is in the non-merging position N (first position), the hydraulic oil from the first hydraulic pump 21 and the hydraulic oil from the second hydraulic pump 22 can be supplied separately to the lift circuit 40 and the steering circuit 50 without merging. Furthermore, when the compound control valve 30 is in the merging position C (third position), the hydraulic oil from the first hydraulic pump 21 and the hydraulic oil from the second hydraulic pump 22 can be combined and supplied to the lift circuit 40.

[0084] In addition, in this embodiment, the relief position D (second position) of the compound control valve 30 is a position in which only the steering circuit 50 is connected to the second hydraulic pump 22 of the hydraulic sources 21 and 22, the steering circuit 50 is connected to the working oil tank 23, and only the lifting circuit 40 is connected to the first hydraulic pump 21 of the hydraulic sources 21 and 22.

[0085] With this configuration, when the compound control valve 30 is in the relief position D (second position), the hydraulic oil can be supplied from the second hydraulic pump 22 to the steering circuit 50. This prevents the hydraulic oil from the accumulator 58 from being released into the hydraulic oil tank 23 and the hydraulic oil supply from the hydraulic source 22 from being cut off simultaneously. Furthermore, the hydraulic oil from the first hydraulic pump 21 is supplied to the lift circuit 40, allowing the lift cylinder 41 to be operated.

[0086] [Modification of the First Embodiment]

[0087] Next, use Figure 4 A transport vehicle according to a modified example of the first embodiment will be described. Figure 41 is a hydraulic circuit diagram showing the structure of a hydraulic system of a transport vehicle according to a modified example of the first embodiment. Figure 4 In, with Figures 1 to 3 The parts denoted by the same reference numerals are the same parts, and thus their detailed description is omitted.

[0088] The transport vehicle of the modified example of the first embodiment differs from the first embodiment in the structure for connecting the steering circuit 50 and the compound control valve 30 in the hydraulic system 20A. The other structures of the hydraulic system 20A of the transport vehicle of the modified example of the first embodiment are the same as the hydraulic system 20 of the transport vehicle of the first embodiment (see Figure 2 ) are the same, and their descriptions are omitted.

[0089] In a hydraulic system 20A according to a modified example of the first embodiment, the steering circuit 50 is connected to the compound control valve 30 via a first connecting line 35 and a second connecting line 36 connected in parallel. The first connecting line 35 is connected to the first steering port s1, a port in the compound control valve 30 that allows communication between the steering circuit 50 and the second hydraulic pump 22. The second connecting line 36 is connected to the second steering port s2, a port in the compound control valve 30 that allows communication between the steering circuit 50 and the hydraulic oil tank 23. Of the first and second connecting lines 35 and 36, only the first connecting line 35 is provided with a filter 37 for removing foreign matter from the hydraulic oil.

[0090] According to this configuration, foreign matter can be removed from the hydraulic oil flowing from the second hydraulic pump 22 to the steering circuit 50 via the first connecting line 35 by the filter 37. This improves the contamination resistance of the steering circuit 50.

[0091] [Second embodiment]

[0092] Next, use Figure 5 A transport vehicle according to a second embodiment of the present invention will be described. Figure 5 : is a hydraulic circuit diagram showing the structure of the hydraulic system of the transport vehicle of the second embodiment. Figure 5 In, with Figures 1 to 4 The parts denoted by the same reference numerals are the same parts, and thus their detailed description is omitted.

[0093] The transport vehicle of the second embodiment differs from the modified example of the first embodiment in that the structure of the compound control valve 30B in the hydraulic system 20B is different and a check valve 38 is added. The other structures of the hydraulic system 20B of the transport vehicle of the second embodiment are the same as those of the hydraulic system 20A of the transport vehicle of the modified example of the first embodiment (see FIG. Figure 4 ) are the same, and their descriptions are omitted.

[0094] Specifically, the first non-merging position N and the third merging position C of the compound control valve 30B are identical to the non-merging position N and merging position C of the compound control valve 30 of the first embodiment. Meanwhile, the second position, or bleed position D, of the compound control valve 30B directs hydraulic oil from the accumulator 58 (steering circuit 50) to the hydraulic tank 23, enabling hydraulic oil to be supplied from the first and second hydraulic pumps 21, 22 to the steering circuit 50. Specifically, the bleed position D of the compound control valve 30B establishes communication between the steering circuit 50 and both the first and second hydraulic pumps 21, 22, and between the steering circuit 50 and the hydraulic tank 23, while blocking communication between the lift circuit 40 and both the first and second hydraulic pumps 21, 22.

[0095] A check valve 38 is provided between the first hydraulic pump 21 and the compound control valve 30B. The check valve 38 allows hydraulic oil to flow from the first hydraulic pump 21 to the compound control valve 30B while preventing reverse flow. When the compound control valve 30B is in the bleed position D, the check valve 38 prevents flow from the steering circuit 50 through the compound control valve 30B to the first hydraulic pump 21.

[0096] In this embodiment, when the compound control valve 30B malfunctions to or becomes stuck in the relief position D, the hydraulic oil from the first hydraulic pump 21 and the hydraulic oil from the second hydraulic pump 22 merge and flow toward the steering circuit 50. Consequently, the flow rate of hydraulic oil flowing toward the steering circuit 50 increases compared to the compound control valve 30 of the modified example of the first embodiment, in which only the hydraulic oil from the second hydraulic pump 22 flows toward the steering circuit 50. Consequently, the flow rate of hydraulic oil supplied to the steering circuit 50 via the compound control valve 30B in the relief position D is reliably higher than the flow rate of hydraulic oil flowing out of the hydraulic oil tank 23 via the throttle 30c in the relief position D. Consequently, the pressure within the steering circuit 50 is maintained, maintaining the steering function of the steering circuit 50. However, since the supply of hydraulic oil from the first and second hydraulic pumps 21, 22 to the lift circuit 40 is cut off, the lift cylinder 41 cannot be operated.

[0097] According to the transport vehicle of the second embodiment described above, similar to the first embodiment (modification), when the compound control valve 30B is in the drain position D (second position), which enables the release (drain) of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, the steering circuit 50 and the second hydraulic pump 22 (hydraulic source) are in communication. This allows the supply of hydraulic oil from the second hydraulic pump 22 (hydraulic source) to the steering circuit 50 via the compound control valve 30B. Furthermore, the throttle 30c in the drain position D (second position) acts as a fluid resistance to the flow from the second hydraulic pump 22 (hydraulic source) to the hydraulic tank 23 via the compound control valve 30B. This prevents the simultaneous release of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23 and the interruption of the supply of hydraulic oil from the hydraulic source 22 to the steering circuit 50.

[0098] In addition, the relief position D (second position) of the compound control valve 30B of this embodiment is a position in which the steering circuit 50 is connected to both the first hydraulic pump 21 and the second hydraulic pump 22 and the steering circuit 50 is connected to the working oil tank 23, and the connection between the lifting circuit 40 and both the first hydraulic pump 21 and the second hydraulic pump 22 is cut off.

[0099] According to this structure, when the compound control valve 30B is in the relief position D (second position), hydraulic oil can be supplied from the first hydraulic pump 21 and the second hydraulic pump 22 to the steering circuit 50 side, and the flow rate of the hydraulic oil flowing to the steering circuit 50 side can be increased compared with the compound control valve 30 of the first embodiment.

[0100] [Third embodiment]

[0101] Next, use Figure 6 A transport vehicle according to a third embodiment of the present invention will be described. Figure 6 : is a hydraulic circuit diagram showing the structure of the hydraulic system of the transport vehicle of the third embodiment. Figure 6 In, with Figures 1 to 5 The parts denoted by the same reference numerals are the same parts, and thus detailed description thereof will be omitted.

[0102] The transport vehicle of the third embodiment differs from the modified example of the first embodiment in that the hydraulic source of the hydraulic system 20C is only the first hydraulic pump 21, and the structure of the compound control valve 30C is modified accordingly. The other structures of the hydraulic system 20C of the transport vehicle of the third embodiment are the same as the hydraulic system 20A of the transport vehicle of the modified example of the first embodiment (see FIG. Figure 4 ) are the same, and their descriptions are omitted.

[0103] The hydraulic system 20C of the third embodiment comprises a single first hydraulic pump 21 as the hydraulic source, eliminating the second hydraulic pump 22, which was a hydraulic source in the hydraulic system 20A of the modified example of the first embodiment. The first pump port p1 and the second pump port p2 of the compound control valve 30C are connected to the center bypass line 31 and the oil passage branching from the center bypass line 31. A check valve 33 is located on the side of the second pump port p2.

[0104] The neutral position N, which is the first position of the compound control valve 30C, is a position in which the hydraulic oil discharged from the first hydraulic pump 21 is directed to the steering circuit 50. Specifically, the neutral position N is a position in which the steering circuit 50 is connected to the first hydraulic pump 21, the connection between the steering circuit 50 and the hydraulic oil tank 23 is cut off, and the connection between the lift circuit 40 and the first hydraulic pump 21 is cut off.

[0105] The drain position D, which serves as the second position of the compound control valve 30C, directs the hydraulic oil in the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23 and enables the supply of hydraulic oil from the first hydraulic pump 21 to the steering circuit 50. Specifically, the drain position D establishes communication between the steering circuit 50 and the first hydraulic pump 21, and between the steering circuit 50 and the hydraulic oil tank 23, while disconnecting the hoist circuit 40 from the first hydraulic pump 21.

[0106] The third position C of the compound control valve 30C directs the hydraulic oil discharged from the first hydraulic pump 21 to the lift circuit 40. Specifically, the third position C disconnects the steering circuit 50 from the first hydraulic pump 21 and the hydraulic oil tank 23, while connecting the lift circuit 40 to the first hydraulic pump 21.

[0107] In this embodiment, by switching the compound control valve 30C to the first position N, the hydraulic oil of the first hydraulic pump 21 can be supplied to the steering circuit 50. On the other hand, by switching the compound control valve 30C to the third position C, the hydraulic oil of the first hydraulic pump 21 can be supplied to the lifting circuit 40. In other words, by switching the compound control valve 30C between the first position N and the third position C, the supply destination of the hydraulic oil of the first hydraulic pump 21 can be switched.

[0108] In this embodiment, when the compound control valve 30C malfunctions to or becomes stuck in the relief position D, hydraulic oil from the first hydraulic pump 21 flows toward the steering circuit 50. Therefore, the flow rate of hydraulic oil supplied to the steering circuit 50 via the compound control valve 30C in the relief position D can be greater than the flow rate of hydraulic oil flowing out of the hydraulic oil tank 23 via the throttle 30c in the relief position D. Consequently, the pressure within the steering circuit 50 can be maintained, maintaining the steering function of the steering circuit 50. However, since the supply of hydraulic oil from the first hydraulic pump 21 to the lift circuit 40 is cut off, the lift cylinder 41 cannot be operated.

[0109] According to the transport vehicle of the third embodiment described above, similar to the first embodiment described above, when the compound control valve 30C is in the drain position D (second position), which enables the release (drain) of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, the steering circuit 50 and the first hydraulic pump 21 are in communication. This allows the supply of hydraulic oil from the first hydraulic pump 21 to the steering circuit 50 via the compound control valve 30C. Furthermore, the throttle 30c in the drain position D (second position) acts as a fluid resistance to the flow from the first hydraulic pump 21 to the hydraulic tank 23 via the compound control valve 30C. This prevents the simultaneous release of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23 and the interruption of the supply of hydraulic oil from the hydraulic source 21 to the steering circuit 50.

[0110] In this embodiment, the hydraulic source is comprised of only one first hydraulic pump 21. The neutral position N, which serves as the first position of the compound control valve 30C, connects the steering circuit 50 to the first hydraulic pump 21, disconnects the steering circuit 50 from the hydraulic oil tank 23, and disconnects the lift circuit 40 from the first hydraulic pump 21. The drain position D, which serves as the second position, connects the steering circuit 50 to the first hydraulic pump 21 and the hydraulic oil tank 23, while disconnecting the lift circuit 40 from the first hydraulic pump 21. The third position C disconnects the steering circuit 50 from the first hydraulic pump 21 and the hydraulic oil tank 23, while connecting the lift circuit 40 to the first hydraulic pump 21.

[0111] With this configuration, even if the hydraulic source is only the first hydraulic pump 21, when the compound control valve 30C is in the relief position D (second position), hydraulic oil can be supplied from the first hydraulic pump 21 to the steering circuit 50. Furthermore, by switching the compound control valve 30C between the first position N and the third position C, the supply destination of the hydraulic oil from the first hydraulic pump 21 can be switched.

[0112] [Fourth embodiment]

[0113] Next, use Figure 7 A transport vehicle according to a fourth embodiment of the present invention will be described. Figure 7 : is a hydraulic circuit diagram showing the structure of the hydraulic system of the transport vehicle of the fourth embodiment. Figure 7 In, with Figures 1 to 6 The parts denoted by the same reference numerals are the same parts, and thus their detailed description is omitted.

[0114] The transport vehicle of the fourth embodiment differs from the modified example of the first embodiment in that a cooling water system 18 for the prime mover 15 is added, and that a fan circuit 90 is added to the hydraulic system 20D in conjunction with the addition of the cooling water system 18. The remaining structure of the transport vehicle of the fourth embodiment is similar to that of the hydraulic system 20A of the transport vehicle of the modified example of the first embodiment (see FIG. Figure 4 ) are the same, and their descriptions are omitted.

[0115] The cooling water system 18 of the fourth embodiment includes a cooling water tank 18a that stores cooling water; a cooling water circulation pump 18b that draws and discharges cooling water from the cooling water tank 18a; and a radiator 18c that cools the cooling water using cooling air. The cooling water system 18 is a circulation system that circulates cooling water within the system via the cooling water circulation pump 18b. The cooling air supplied to the radiator 18c is generated by a cooling fan 18d.

[0116] The hydraulic system 20D of the fourth embodiment includes a fan circuit 90 that drives the cooling fan 18d. The fan circuit 90 includes a hydraulic motor 91, which is rotated by hydraulic oil supplied from the first hydraulic pump 21, thereby driving the cooling fan 18d; and a fan control valve 92, which controls the flow of hydraulic oil supplied from the first hydraulic pump 21 to the hydraulic motor 91 and the flow of hydraulic oil discharged from the hydraulic motor 91 to the hydraulic oil tank 23. The fan control valve 92, the compound control valve 30, and the lift control valve 42 are connected in series along the center bypass line 31. The fan control valve 92 is located upstream of the compound control valve 30.

[0117] Return oil from the hydraulic motor 91 is discharged into the hydraulic oil tank 23 via the return oil line 93. The inlet and outlet (suction port and discharge port) of the hydraulic motor 91 are connected to the fan control valve 92 via a pair of motor oil lines 94 and 95. A pair of check valves 97a and 97b are provided between the motor oil lines 94 and 95 and the return oil line 93 for replenishment. When the hydraulic motor 91 inertially rotates or when the hydraulic motor 91 rotates due to wind, and negative pressure is achieved within the motor oil lines 94 and 95, the check valves 97a and 97b allow the hydraulic oil in the hydraulic oil tank 23 to be replenished into the motor oil lines 94 and 95 via the return oil line 93. A pair of relief valves 98a and 98b are provided between the motor oil lines 94 and 95 and the return oil line 93. The pair of relief valves 98 a and 98 b release the hydraulic oil to the hydraulic oil tank 23 when the pressure in the pair of motor oil passages 94 and 95 exceeds a predetermined value, thereby protecting the hydraulic equipment of the fan circuit 90 .

[0118] Fan control valve 92 is, for example, a six-port, three-position hydraulic pilot-operated directional control valve. It utilizes a single directional control valve with left and right pressure-receiving sections 92a and 92b for pilot pressure input. Fan control valve 92 is a selector valve capable of switching between a forward position F, a reverse position R, and a neutral position N. Normally, both pressure-receiving sections 92a and 92b of fan control valve 92 are connected to the hydraulic oil tank 23 and maintained in the neutral position N by a centering spring.

[0119] When the fan control valve 92 is in the neutral position N, the first hydraulic pump 21 is connected to the compound control valve 30, and the connection between the first hydraulic pump 21 and the hydraulic motor 91 is cut off. As a result, hydraulic oil discharged from the first hydraulic pump 21 is supplied to the compound control valve 30 through the fan control valve 92. Therefore, hydraulic oil can be supplied from the first hydraulic pump 21 to the lift circuit 40 via the compound control valve 30.

[0120] When the fan control valve 92 is in the forward rotation position F, hydraulic oil discharged from the first hydraulic pump 21 is supplied to the hydraulic motor 91 via the motor oil passage 94, causing the hydraulic motor 91 to rotate in the forward direction. The hydraulic oil discharged from the hydraulic motor 91 is discharged into the hydraulic oil tank 23 via the motor oil passage 95 and the fan control valve 92. Furthermore, when the fan control valve 92 is in the reverse rotation position R, hydraulic oil discharged from the first hydraulic pump 21 is supplied to the hydraulic motor 91 via the motor oil passage 95, causing the hydraulic motor 91 to rotate in the reverse direction. The hydraulic oil discharged from the hydraulic motor 91 is discharged into the hydraulic oil tank 23 via the motor oil passage 94 and the fan control valve 92.

[0121] Thus, the forward rotation position F and the reverse rotation position R are rotational positions in which the first hydraulic pump 21 communicates with the hydraulic motor 91, and the hydraulic oil discharged from the first hydraulic pump 21 rotates the hydraulic motor 91. When the fan control valve 92 is in the rotational positions F and R, the communication between the first hydraulic pump 21 and the compound control valve 30 via the center bypass line 31 is cut off. Therefore, the hydraulic oil cannot be supplied from the first hydraulic pump 21 to the lift circuit 40.

[0122] On the other hand, the second hydraulic pump 22 is connected to the compound control valve 30, bypassing the fan control valve 92. Therefore, the flow of hydraulic oil supplied from the second hydraulic pump 22 to the compound control valve 30 is not affected by the actuation of the fan control valve 92. In other words, the supply of hydraulic oil from the second hydraulic pump 22 to the steering circuit 50 is not affected by the addition of the fan circuit 90.

[0123] Fan control valve 92 is switched by pilot pressure generated by solenoid valves 79 a and 79 b of pilot circuit 70D and is switched among three switching positions: forward position F, reverse position R, and neutral position N according to a control signal from control device 80 .

[0124] According to the fourth embodiment, similar to the modified example of the first embodiment, simultaneous release of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and interruption of hydraulic oil supply from the hydraulic source 21 to the steering circuit 50 can be avoided.

[0125] The dump truck 1 (transport vehicle) of this embodiment also includes a hydraulic motor 91 driven by hydraulic oil supplied from the first hydraulic pump 21, thereby driving the cooling fan 18d; and a fan control valve 92 that controls the flow of hydraulic oil supplied from the first hydraulic pump 21 to the hydraulic motor 91. The fan control valve 92 and the compound control valve 30 are connected in series on a bypass line 31 connecting the first hydraulic pump 21 to the hydraulic oil tank 23, with the compound control valve 30 positioned downstream of the fan control valve 92. The second hydraulic pump 22 is connected to the compound control valve 30, bypassing the fan control valve 92.

[0126] According to this configuration, the fan control valve 92 for controlling the hydraulic motor 91 for driving the cooling fan 18 d can be located between the first hydraulic pump 21 and the compound control valve 30 without affecting the supply of hydraulic oil from the second hydraulic pump 22 to the steering circuit 50 via the compound control valve 30 .

[0127] [Other embodiments]

[0128] In addition, the present invention is not limited to the first to fourth embodiments described above, and includes various modifications. The above embodiments are embodiments described in detail to easily explain the present invention, and are not limited to all the structures described. For example, a part of the structure of a certain embodiment can be replaced with the structure of another embodiment, and the structure of another embodiment can be added to the structure of a certain embodiment. In addition, for a part of the structure of each embodiment, other structures can be added, deleted, or replaced.

[0129] For example, in the first to fourth embodiments described above, an example is shown in which the lift control valve 42 is composed of a single directional control valve. However, the lift control valve may also be a structure composed of a first control valve and a second control valve. For example, the first control valve and the second control valve are composed of a 6-port 3-position directional control valve, and the first control valve and the second control valve are connected in parallel with each other. The first control valve is configured to be switchable between three switching positions: a neutral position N, a rising position R, and a floating position F. The second control valve is configured to be switchable between three switching positions: a neutral position N, a rising position R, and a lowering position L. According to this structure, the drive of the lift cylinder 41 can be controlled in the same manner as with a single lift control valve 42.

[0130] In addition, in the above embodiment, the hydraulic system 20, 20A, 20B, 20C, 20D includes the lifting circuit 40. However, Figure 8 As shown, the hydraulic system 20E may also be without the lifting circuit 40 (see Figure 2 、 Figures 4 to 7 ) structure. Figure 8 This is a hydraulic circuit diagram showing the configuration of a hydraulic system included in a transport vehicle according to another embodiment.

[0131] The hydraulic system 20E without a lift circuit does not require a first hydraulic pump (see Figure 2 The compound control valve 30E has a supply position N for supplying hydraulic oil from the second hydraulic pump 22 to the steering circuit 50, and a drain position D for discharging hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. In other words, the compound control valve 30E is configured as a single control valve that combines the functions of supplying hydraulic oil from the second hydraulic pump 22 to the steering circuit 50 and draining hydraulic oil from the accumulator 58.

[0132] The compound control valve 30E is, for example, a four-port, two-position hydraulic pilot-operated directional control valve, with a pressure-receiving portion 30b on one side receiving a pilot pressure. The compound control valve 30E has four ports: a second pump port p2 connected to the second hydraulic pump 22 via the second discharge line 26; a tank port t connected to the hydraulic oil tank 23 via the return oil line 34; a first steering port s1 connected to the steering circuit 50 via the first connecting line 35 and enabling communication between the steering circuit 50 and the second hydraulic pump 22; and a second steering port s2 connected to the steering circuit 50 via the second connecting line 36 and enabling communication between the steering circuit 50 and the hydraulic oil tank 23. Normally, the compound control valve 30E is held in the supply position N by a spring. The compound control valve 30E is configured to switch to the relief position D by pilot pressure generated by the solenoid valve 74E of the pilot circuit 70E.

[0133] The supply position N of the compound control valve 30E guides the hydraulic oil discharged from the second hydraulic pump 22 to the steering circuit 50. Specifically, the supply position N connects the steering circuit 50 to the second hydraulic pump 22 and blocks the connection between the steering circuit 50 and the hydraulic oil tank 23.

[0134] The compound control valve 30E's bleed position D directs the hydraulic oil from the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23, allowing hydraulic oil to be supplied from the second hydraulic pump 22 to the steering circuit 50. Specifically, the bleed position D connects the steering circuit 50 with the second hydraulic pump 22 and the hydraulic oil tank 23. In the bleed position D, the compound control valve 30E includes a throttle 30c in the oil path connecting the steering circuit 50 and the hydraulic oil tank 23. The throttle 30c functions as a flow resistance when the hydraulic oil from the second hydraulic pump 22 flows through the compound control valve 30E to the hydraulic oil tank 23.

[0135] Even in this hydraulic system 20E without a lift circuit, the steering circuit 50 is connected to the second hydraulic pump 22 (hydraulic source) and the hydraulic oil tank 23 via the compound control valve 30E. The compound control valve 30E has a supply position N (first position), which connects the steering circuit 50 to the second hydraulic pump 22 (hydraulic source) and blocks communication with the hydraulic oil tank 23; and a drain position D (second position), which connects the steering circuit 50 to the second hydraulic pump 22 (hydraulic source) and the hydraulic oil tank 23, thereby draining the hydraulic oil from the accumulator 58 into the hydraulic oil tank 23. In the drain position D (second position), the compound control valve 30E has a throttle 30c in the oil path connecting the steering circuit 50 to the hydraulic oil tank 23.

[0136] With this configuration, when the compound control valve 30E is in the drain position D (second position), which enables the release (drain) of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23, the steering circuit 50 and the second hydraulic pump 22 are in communication. This allows the supply of hydraulic oil from the second hydraulic pump 22 to the steering circuit 50 via the compound control valve 30E. Furthermore, the throttle 30c in the drain position D (second position) acts as a fluid resistance to the flow from the second hydraulic pump 22 to the hydraulic tank 23 via the compound control valve 30E. This prevents the simultaneous release of hydraulic oil from the accumulator 58 of the steering circuit 50 to the hydraulic tank 23 and the interruption of the supply of hydraulic oil from the hydraulic source 22 to the steering circuit 50.

[0137] Description of Reference Signs

[0138] 1…dump truck (transport vehicle), 2…front wheels (steering wheels), 5…cargo box, 18d…cooling fan, 21…first hydraulic pump (hydraulic source), 22…second hydraulic pump (hydraulic source), 23…hydraulic oil tank, 30, 30B, 30C, 30E…compound control valve, 31…center bypass line (bypass line), 35…first connecting line (first line), 36…second connecting line (second line), 37…filter, 40…lift circuit, 41…lift cylinder, 50…steering circuit, 51, 52…steering cylinder, 53…steering valve, 58…accumulator, 91…hydraulic motor, 92…fan control valve, s1…first steering port, s2…second steering port, N…neutral position, non-merging position, supply position (first position), D…drain position (second position), C…merging position (third position).

Claims

1. A transport vehicle comprising: a working oil tank, which stores working oil; a hydraulic source that supplies the hydraulic oil in the hydraulic oil tank as high-pressure hydraulic oil; and Steering circuit, The steering circuit comprises: a steering cylinder that steers the steering wheel by expanding and contracting with the hydraulic oil supplied from the hydraulic source; a steering valve that controls the flow of hydraulic oil supplied from the hydraulic source to the steering cylinder; and an accumulator that accumulates the pressure of the hydraulic oil supplied from the hydraulic source and is capable of supplying the accumulated pressure to the steering cylinder via the steering valve as a hydraulic source of the steering cylinder, It is characterized in that The steering circuit is connected to the hydraulic source and the hydraulic oil tank via a composite control valve. The compound control valve has: a first position that connects the steering circuit to the hydraulic source and blocks communication between the steering circuit and the hydraulic oil tank; and a second position in which the steering circuit is connected to the hydraulic source and the hydraulic oil tank is connected to discharge the hydraulic oil of the accumulator into the hydraulic oil tank; When the compound control valve is in the second position, the compound control valve has a throttle portion in the oil passage that connects the steering circuit and the hydraulic oil tank.

2. The transport vehicle according to claim 1, characterized in that: The steering circuit is connected to the compound control valve via a first pipeline and a second pipeline connected in parallel. The first pipeline is connected to a port in the composite control valve that enables communication between the steering circuit and the hydraulic source, that is, a first steering port. The second pipe is connected to a port in the composite control valve that enables communication between the steering circuit and the hydraulic oil tank, that is, a second steering port. A filter for removing foreign matter in the hydraulic oil is provided only in the first pipeline among the first pipeline and the second pipeline.

3. The transport vehicle according to claim 1, characterized in that: The transport vehicle has a lifting circuit including a lifting cylinder, which is connected to the hydraulic source via the composite control valve and is extended and retracted by the hydraulic oil supplied from the hydraulic source, thereby raising and lowering the cargo box. The compound control valve has a third position in addition to the first and second positions. The third position cuts off the connection between the steering circuit and the hydraulic source, cuts off the connection between the steering circuit and the hydraulic oil tank, and connects the lifting circuit to the hydraulic source.

4. The transport vehicle according to claim 3, characterized in that: The hydraulic source is composed of a hydraulic pump, The first position of the compound control valve is a position in which the steering circuit is connected to the hydraulic pump, the steering circuit is disconnected from the hydraulic oil tank, and the lifting circuit is disconnected from the hydraulic pump. The second position of the compound control valve is a position in which the steering circuit is connected to the hydraulic pump and the steering circuit is connected to the hydraulic oil tank, and the connection between the lifting circuit and the hydraulic pump is cut off. The third position of the compound control valve is a position that cuts off communication between the steering circuit and the hydraulic pump, cuts off communication between the steering circuit and the hydraulic oil tank, and connects the lift circuit to the hydraulic pump.

5. The transport vehicle according to claim 3, characterized in that: The hydraulic source is composed of a first hydraulic pump and a second hydraulic pump. The first position of the compound control valve is a position in which the steering circuit is connected to only the second hydraulic pump of the hydraulic source, the steering circuit is disconnected from the hydraulic oil tank, and the lifting circuit is connected to only the first hydraulic pump of the hydraulic source. The second position of the compound control valve is a position in which the steering circuit is connected to at least the second hydraulic pump in the hydraulic source and the steering circuit is connected to the hydraulic oil tank, while the lifting circuit is disconnected from the at least the second hydraulic pump in the hydraulic source. The third position of the compound control valve is a position that cuts off communication between the steering circuit and both the first hydraulic pump and the second hydraulic pump, cuts off communication between the steering circuit and the hydraulic oil tank, and connects the lift circuit to both the first hydraulic pump and the second hydraulic pump.

6. The transport vehicle according to claim 5, characterized in that: The second position of the compound control valve is a position in which the steering circuit communicates with only the second hydraulic pump of the hydraulic source, the steering circuit communicates with the hydraulic oil tank, and the lift circuit communicates with only the first hydraulic pump of the hydraulic source.

7. The transport vehicle according to claim 5, characterized in that: The second position of the compound control valve is a position that connects the steering circuit to both the first and second hydraulic pumps and the hydraulic oil tank, and blocks communication between the lift circuit and both the first and second hydraulic pumps.

8. The transport vehicle according to claim 5, characterized in that: The transport vehicle has: a hydraulic motor driven by hydraulic oil supplied from the first hydraulic pump to thereby drive a cooling fan; and a fan control valve that controls the flow of hydraulic oil supplied from the first hydraulic pump to the hydraulic motor, The fan control valve and the composite control valve are connected in series to a bypass line connecting the first hydraulic pump and the hydraulic oil tank. The composite control valve is arranged on the downstream side of the fan control valve, The second hydraulic pump is connected to the compound control valve bypassing the fan control valve.

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