Transport vehicles

Abstract

This transport vehicle comprises a steering circuit including: a steering cylinder steered by hydraulic oil of a hydraulic pressure source; a steering valve that controls the flow of the hydraulic oil to the steering cylinder; and an accumulator that stores the hydraulic-oil pressure from the hydraulic pressure source and that can supply the pressure to the steering cylinder via the steering valve. The steering circuit is connected to the hydraulic pressure source and a hydraulic oil tank via a composite control valve. The composite control valve has: a first position at which communication is enabled between the steering circuit and the hydraulic pressure source and at which communication is shut-off between the steering circuit and the hydraulic oil tank; and a second position at which communication is enabled between the steering circuit and the hydraulic pressure source and at which communication is enabled between the steering circuit and the hydraulic oil tank. The composite control valve includes a narrowed section in an oil path that enables communication between the steering circuit and the hydraulic oil tank, at the second position.

Description

Technical Field

[0001] The present invention relates to a transport vehicle such as a dump truck for a mine, and more particularly to a transport vehicle in which a hydraulic circuit for steering a steering wheel includes an accumulator.

Background Art

[0002] A transport vehicle such as a dump truck steers a steering wheel using a hydraulic circuit in which a hydraulic actuator is driven by pressure oil supplied from a hydraulic pump. Some hydraulic circuits (steering circuits) for steering a steering wheel include an accumulator as a safety device (emergency hydraulic source) when the function of the hydraulic pump is lost. An accumulator stores the pressure of the pressure oil supplied from the hydraulic pump and can supply the stored pressure to the hydraulic actuator when the function of the hydraulic pump is lost. Therefore, the hydraulic actuator can operate by supplying the pressure stored in the accumulator even without the supply of pressure oil from the hydraulic pump, and the steering wheel can be steered.

[0003] The pressure oil stored in the accumulator needs to be discharged to the hydraulic oil tank when the operation of the transport vehicle is completely terminated. Therefore, a bleed-down control valve is provided on the line connecting the accumulator and the hydraulic oil tank (see, for example, Patent Document 1). The bleed-down control valve is configured to be switchable between an open valve position that allows the accumulator and the hydraulic oil tank to communicate and a closed valve position that blocks their communication. The bleed-down control valve performs a bleed-down operation to discharge the pressure oil stored in the accumulator to the hydraulic oil tank by switching from the closed valve position to the open valve position.

[0004] In addition, some transport vehicles include a hydraulic circuit for hoist operation for raising and lowering a loading platform in addition to the hydraulic circuit for steering. Among transport vehicles having such a configuration, there are those configured to switch the supply destination of the pressure oil discharged from the hydraulic pump between the hydraulic circuit for steering and the hydraulic circuit for hoist operation by a control valve (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-264456 [Overview of the project] [Problems that the invention aims to solve]

[0006] In a hydraulic circuit including two control valves, such as the hydraulic circuit described in Patent Document 1, which includes a bleed-down control valve that controls the release of pressurized oil from the accumulator to the hydraulic oil tank and a control valve (priority valve) that controls the destination of the pressurized oil supplied to the hydraulic pump, it is preferable from a fail-safe perspective to take measures to address the case in which the bleed-down control valve and the priority valve simultaneously become unable to switch at an unintended switching position. Specifically, if the priority valve becomes unable to switch at the switching position for hoist operation, and the bleed-down control valve becomes unable to switch at the open position (the switching position that connects the accumulator and the hydraulic oil tank), the supply of pressurized oil from the hydraulic pump to the steering hydraulic circuit is cut off and pressurized oil is released from the accumulator to the hydraulic oil tank simultaneously.

[0007] If these problems occur simultaneously, there is a concern that the accumulator's function as a safety device will rapidly deteriorate, leading to a loss of steering control for the steering wheels. Therefore, it is preferable to take measures to address the possibility of a malfunction in which the bleed-down control valve and the priority valve are simultaneously in an unintended switching position.

[0008] The present invention was made to solve the above-mentioned problems, and its objective is to provide a transport vehicle that can avoid the simultaneous occurrence of pressurized oil discharge from the steering circuit accumulator to the hydraulic oil tank and the interruption of pressurized oil supply from the hydraulic source to the steering circuit. [Means for solving the problem]

[0009] The present invention includes multiple means for solving the above problems. To give one example, in a transport vehicle comprising a hydraulic oil tank for storing hydraulic oil, a hydraulic source for supplying hydraulic oil from the hydraulic oil tank as high-pressure pressurized oil, a steering cylinder that extends and retracts by the pressurized oil supplied from the hydraulic source to steer the steering wheels, a steering valve for controlling the flow of pressurized oil supplied from the hydraulic source to the steering cylinder, and a steering circuit having an accumulator that stores the pressure of the pressurized oil supplied from the hydraulic source and can supply the stored pressure to the steering cylinder via the steering valve as the hydraulic source for the steering cylinder, the steering... The steering circuit is configured to be connected to the hydraulic power source and the hydraulic oil tank via a composite control valve, and the composite control valve has a first position in which it connects the steering circuit and the hydraulic power source and blocks the connection between the steering circuit and the hydraulic oil tank, and a second position in which it connects the steering circuit and the hydraulic power source and connects the steering circuit and the hydraulic oil tank to discharge the pressurized oil of the accumulator to the hydraulic oil tank, and the composite control valve is characterized in that, in the second position, it has a throttling in the oil passage connecting the steering circuit and the hydraulic oil tank. [Effects of the Invention]

[0010] According to the present invention, the second position of the composite control valve, which enables pressurized oil discharge (bleed-down) from the steering circuit accumulator to the hydraulic oil tank, results in a state of communication between the steering circuit and the hydraulic power source, thus enabling pressurized oil supply from the hydraulic power source to the steering circuit via the composite control valve. Furthermore, the throttling at the second position creates fluid resistance to the flow from the hydraulic power source to the hydraulic oil tank via the composite control valve. Therefore, it is possible to avoid the simultaneous occurrence of pressurized oil discharge from the steering circuit accumulator to the hydraulic oil tank and the interruption of pressurized oil supply from the hydraulic power source to the steering circuit. Other issues, configurations, and effects will be clarified by the following description of the embodiments. [Brief explanation of the drawing]

[0011] [Figure 1] This is a side view showing a dump truck as a transport vehicle according to the first embodiment of the present invention. [Figure 2] Figure 1 is a hydraulic circuit diagram showing the configuration of the hydraulic system installed in the transport vehicle according to the first embodiment. [Figure 3] This is a hydraulic circuit diagram showing the configuration of a comparative hydraulic system to the hydraulic system of a transport vehicle according to the first embodiment. [Figure 4] This is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a transport vehicle according to a modified example of the first embodiment. [Figure 5] This is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a transport vehicle according to a second embodiment of the present invention. [Figure 6] This is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a transport vehicle according to a third embodiment of the present invention. [Figure 7] This is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a transport vehicle according to a fourth embodiment of the present invention. [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. [Modes for carrying out the invention]

[0012] Hereinafter, embodiments of the transport vehicle of the present invention will be described with reference to the drawings. In this embodiment, a dump truck will be used as an example of a transport vehicle. In this specification, the directions of front, rear, left, and right refer to the directions as seen from the perspective of the operator riding in the transport vehicle.

[0013] [First Embodiment] First, the configuration of the dump truck as a transport vehicle according to the first embodiment will be explained using Figure 1. Figure 1 is a side view showing the dump truck as a transport vehicle according to the first embodiment of the present invention.

[0014] In Fig. 1, the dump truck 1 is a large transport vehicle operating in a mine or the like, and is configured to transport loads (objects to be transported) such as mined ore and earth and sand. The dump truck 1 includes front wheels 2 and rear wheels 3 that are rotatably arranged on both left and right sides at the front and rear of the vehicle, a vehicle body 4 that can travel by means of the front wheels 2 and rear wheels 3, and a loading platform 5 that is mounted on the vehicle body 4 so as to be able to rise and fall (tilt).

[0015] The front wheels 2 are, for example, steering wheels steered by an operator. The front wheels 2 as steering wheels are configured to be steered by the expansion and contraction of left and right steering cylinders 51 and 52 (see Fig. 2 described later). The rear wheels 3 are, for example, drive wheels rotationally driven by a travel drive device (not shown).

[0016] The vehicle body 4 includes a frame 11 that is a support structure extending in the front-rear direction (the left-right direction in Fig. 1), a cabin 12 arranged at the front part of the frame 11, and a cab 13 arranged on the cabin 12. Various devices such as a hydraulic oil tank 23 (also see Fig. 2 described later) are attached to the frame 11. The cabin 12 houses various devices such as a prime mover 15 and hydraulic pumps 21 and 22 (see Fig. 2 described later). The cab 13 is a driver's cab in which an operator rides and operates the vehicle. Inside the cab 13, various operating devices for the operator to operate the dump truck 1 are arranged, such as a travel pedal (not shown) for driving the dump truck 1, a steering wheel 16 (see Fig. 2 described later) for steering the front wheels 2 (steering wheels), and an operating device 17 (see Fig. 2 described later) for operating the rise and fall (posture) of the loading platform 5.

[0017] The loading platform 5 is a container for loading cargo (objects to be transported) such as ore and earth and sand. The loading platform 5 is rotatably attached to the rear end of the frame 11 such that the front side rotates vertically with the rear side as a fulcrum. A pair of left and right hoist cylinders 41 (only one is shown in FIG. 1) are interposed between the frame 11 and the loading platform 5. The hoist cylinder 41 is a hydraulic actuator that expands and contracts by supplying and discharging pressure oil. The loading platform 5 is configured to rotate with respect to the frame 11 between a transport posture in which it is in a reclined state (seated state) due to the contraction of the hoist cylinder 41 and the transport object can be loaded onto the loading platform 5, and a discharge posture in which it is in a standing state (tipped state) due to the extension of the hoist cylinder 41 and the transport object can be discharged from the loading platform 5. In FIG. 1, the loading platform 5 shown by the solid line is in the transport posture, and the loading platform 5 shown by the two-dot chain line is in the discharge posture. Also, the hoist cylinder 41 shown by the solid line is in the contracted state, and the hoist cylinder 41 shown by the two-dot chain line is in the extended state.

[0018] Next, the configuration of the hydraulic system in the transport vehicle according to the first embodiment will be described with reference to FIG. 2. FIG. 2 is a hydraulic circuit diagram showing the configuration of the hydraulic system provided in the transport vehicle according to the first embodiment shown in FIG. 1.

[0019] In FIG. 2, the dump truck 1 includes a hydraulic system 20 for raising and lowering the loading platform 5 and steering the front wheels 2. The hydraulic system 20 includes a first hydraulic pump 21 and a second hydraulic pump 22 which are hydraulic sources for supplying working oil as high-pressure pressure oil, a working oil tank 23 for storing the working oil, a hoist circuit 40 including a pair of hoist cylinders 41 (only one is shown in FIG. 2) driven by the pressure oil supplied from the hydraulic source, and a steering circuit 50 including a pair of left and right steering cylinders 51, 52 driven by the pressure oil supplied from the hydraulic source. The hoist circuit 40 is connected to the hydraulic source via a composite control valve 30. The steering circuit 50 is connected to the hydraulic source and the working oil tank 23 via the composite control valve 30.

[0020] The first hydraulic pump 21 and the second hydraulic pump 22 are, for example, variable displacement pumps and each has regulators 21a and 22a. The regulators 21a and 22a adjust the pump volume in accordance with 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 accordance with control commands from the control device 80. The first hydraulic pump 21 and the second hydraulic pump 22 are connected to the composite control valve 30 via the first discharge line 25 and the second discharge line 26, respectively.

[0021] The hoist cylinder 41 of the hoist circuit 40 is a single-stage or multi-stage (three-stage in Figures 1 and 2) hydraulic cylinder that raises and lowers the loading platform 5 (see Figure 1). The hoist cylinder 41 has a bottom-side oil chamber (hereinafter referred to as the bottom chamber) 41a and a rod-side oil chamber (hereinafter referred to as the rod chamber) 41b. The hoist cylinder 41 extends when pressurized oil is supplied to the bottom chamber 41a and return oil is discharged from the rod chamber 41b. As the hoist cylinder 41 extends, the loading platform 5 assumes the release position (dotted line in Figure 1). The hoist cylinder 41 retracts when pressurized oil is supplied to the rod chamber 41b and return oil is discharged from the bottom chamber 41a. As the hoist cylinder 41 retracts, the loading platform 5 assumes the transport position (solid line in Figure 1).

[0022] The hoist circuit 40 further includes a hoist control valve 42. The hoist control valve 42 and the composite control valve 30 are located on the center bypass line 31, which is an oil passage connecting the first hydraulic pump 21 and the hydraulic oil tank 23, and are connected in tandem with the hoist control valve 42 located downstream of the composite control valve 30. The hoist control valve 42 controls the flow of pressurized oil supplied from the hydraulic sources 21 and 22 to the hoist cylinder 41 via the composite control valve 30, and also controls the flow of return oil discharged from the hoist cylinder 41 to the hydraulic oil tank 23. A supply oil passage 43 branching from the center bypass line 31 is connected to the hoist control valve 42. The hoist control valve 42 is connected to the bottom chamber 41a and rod chamber 41b of the hoist cylinder 41 via a pair of actuator lines 44a and 44b. The hoist control valve 42 is also connected to the hydraulic oil tank 23 via a return oil passage 45.

[0023] The hoist control valve 42 is configured, for example, as a 6-port, 4-position hydraulic pilot-operated directional control valve. The hoist control valve 42 is configured using a single directional control valve and has pressure receiving sections 42a and 42b on both the left and right sides to which pilot pressure is input.

[0024] The hoist control valve 42 has four switching positions: a holding position (neutral position N), a lifting position R, a lowering position L, and a floating position F. It is configured to switch to one of the four switching positions in response to the operation of the operating device 17 for the cargo bed 5 inside the cab 13. The holding position (neutral position N) is the position in which the supply and discharge of pressurized oil to the hoist cylinder 41 is shut off, and the posture of the cargo bed 5 is maintained. The lifting position R is the position in which the supply and discharge of pressurized oil to the hoist cylinder 41 extends the hoist cylinder 41, causing the cargo bed 5 to rotate upward. The lowering position L is the position in which the supply and discharge of pressurized oil to the hoist cylinder 41 retracts the hoist cylinder 41, causing the cargo bed 5 to rotate downward. The floating position F is the position in which the hoist cylinder 41 is retracted by the weight of the cargo bed 5, allowing the cargo bed 5 to fall under its own weight. Under normal circumstances, both pressure-receiving sections 42a and 42b of the hoist control valve 42 are connected to the hydraulic oil tank 23, and the valve is held in a neutral position N by a centering spring.

[0025] When the hoist control valve 42 is in the neutral position N, communication between the supply oil passage 43 and the return oil passage 45 and the actuator lines 44a and 44b is cut off. As a result, it becomes impossible to supply pressurized oil to the hoist cylinder 41 and to discharge pressurized oil from the hoist cylinder 41, and the extension and retraction of the hoist cylinder 41 stops. In this case, the upstream and downstream sides of the hoist control valve 42 of the center bypass line 31 are in communication.

[0026] When the hoist control valve 42 is in the raised position R, the supply oil passage 43 and the actuator line 44a are connected, and the actuator line 44b and the return oil passage 45 are connected. The connection between the upstream and downstream sides of the hoist control valve 42 in the center bypass line 31 is blocked. As a result, pressurized oil from the hydraulic sources 21 and 22 is supplied to the bottom chamber 41a of the hoist cylinder 41, and return oil is discharged from the rod chamber 41b to the hydraulic oil tank 23, causing the hoist cylinder 41 to extend.

[0027] When the hoist control valve 42 is in the lowered position L, the supply oil passage 43 and the actuator line 44b are connected, and the actuator line 44a and the return oil passage 45 are connected. However, the connection between the upstream and downstream sides of the hoist control valve 42 in the center bypass line 31 is blocked. As a result, pressurized oil from the hydraulic sources 21 and 22 is supplied to the rod chamber 41b of the hoist cylinder 41, and return oil is discharged from the bottom chamber 41a to the hydraulic oil tank 23, causing the hoist cylinder 41 to retract.

[0028] When the hoist control valve 42 is in the floating position F, the supply oil passage 43 and the center bypass line 31 downstream of the hoist control valve 42 are in communication. Also, the actuator line 44a and the return oil passage 45 are in communication. As a result, return oil can be discharged from the bottom chamber 41a of the hoist cylinder 41 to the hydraulic oil tank 23, and hydraulic oil is supplied to the rod chamber 41b of the hoist cylinder 41 from the hydraulic oil tank 23 via a check valve (not shown). Therefore, the hoist cylinder 41 can be retracted by the weight of the loading platform 5.

[0029] The steering circuit 50 has a pair of left and right steering cylinders 51 and 52 and a steering valve 53, and is connected to a composite control valve 30 via a connecting pipeline 32. The steering valve 53 is connected to the connecting pipeline 32 (composite control valve 30) via a high-pressure line 54 and is also connected to the hydraulic fluid 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.

[0030] The left and right steering cylinders 51 and 52 are hydraulic actuators that extend and retract using pressurized oil supplied from the second hydraulic pump 22, which is the hydraulic power source, and steer the left and right front wheels 2. The left and right steering cylinders 51 and 52 each have bottom-side oil chambers (hereinafter referred to as bottom chambers) 51a and 52a and rod-side oil chambers (hereinafter referred to as rod chambers) 51b and 52b, respectively. The 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 bottom chamber 52a of the right steering cylinder 52 are connected via a steering line 56.

[0031] The steering valve 53 controls the flow of pressurized oil supplied from the second hydraulic pump 22 to the steering cylinders 51 and 52, and the flow of return oil discharged from the steering cylinders 51 and 52 to the hydraulic oil tank 23. The steering valve 53 has a direction control unit 53a that controls the flow direction of pressurized 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 rate adjustment unit 53b that adjusts the flow rate of pressurized 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 the rotation operation of the steering wheel 16. The flow rate adjustment unit 53b is configured to adjust the flow rate of pressurized oil (pressurized oil supplied to the left and right steering cylinders 51 and 52) flowing through the flow rate adjustment unit 53b in response to the rotation operation of the steering wheel 16.

[0032] In the steering valve 53 shown in Figure 2, 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, the pressurized oil from the second hydraulic pump 22 passes from left to right through the flow rate adjustment unit 53b via the direction control unit 53a at the left steering position L. At this time, the flow rate of the pressurized oil is adjusted by the flow rate adjustment unit 53b and returned to the direction control unit 53a. The pressurized oil that has returned to the direction control unit 53a is supplied via the steering line 56 to the rod chamber 51b of the left steering cylinder 51 and the bottom chamber 52a of the right steering cylinder 52. On the other hand, the pressurized oil in the 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 via the direction control unit 53a of the steering valve 53 to the hydraulic oil tank 23. As a result, the left steering cylinder 51 contracts while the right steering cylinder 52 extends, performing left steering operation in response to the rotation of the steering wheel 16.

[0033] Furthermore, when the steering wheel 16 is rotated clockwise, the direction control unit 53a is switched to the right steering position R. In this case, the pressurized oil from the second hydraulic pump 22 passes through the flow rate adjustment unit 53b from right to left via the direction control unit 53a at the right steering position R. At this time, the flow rate of the pressurized oil is adjusted by the flow rate adjustment unit 53b and returned to the direction control unit 53a. The pressurized oil that has returned to the direction control unit 53a is supplied via the steering line 55 to the bottom chamber 51a of the left steering cylinder 51 and the rod chamber 52b of the right steering cylinder 52. Meanwhile, the pressurized oil in the rod chamber 51b of the left steering cylinder 51 and the bottom chamber 52a of the right steering cylinder 52 is discharged from the steering line 56 to the hydraulic fluid tank 23 via the direction control unit 53a of the steering valve 53. As a result, the left steering cylinder 51 extends while the right steering cylinder 52 contracts, performing right steering operation in response to the rotation of the steering wheel 16.

[0034] An accumulator 58 is connected to the high-pressure line 54. The accumulator 58 stores the pressure of the pressurized oil supplied from the second hydraulic pump 22. The high-pressure line 54 is connected to the hydraulic oil tank 23 via a relief valve 59. The relief valve 59 defines the maximum pressure of the high-pressure line 54. Therefore, the high-pressure line 54 is maintained at a predetermined pressure by the accumulator 58 and the relief valve 59. This ensures that the steering cylinders 51 and 52 are driven in response to the operation of the steering wheel 16. The accumulator 58 is configured to supply the stored pressure to the steering cylinders 51 and 52 via the steering valve 53 as a hydraulic power source for the steering cylinders 51 and 52 when pressurized oil from the second hydraulic pump 22 is not supplied to the steering cylinders 51 and 52.

[0035] The high-pressure line 54 is equipped with a pressure sensor 61 that detects the pressure in the high-pressure line 54. The pressure sensor 61 outputs a detection signal to the control device 80 corresponding to the detected pressure value of the high-pressure line 54.

[0036] A check valve 33 is positioned between the steering circuit 50 and the second hydraulic pump 22. The check valve 33 allows pressurized oil to flow from the second hydraulic pump 22 to the steering circuit 50, while preventing reverse flow. The check valve 33 is positioned, for example, upstream of the composite control valve 30.

[0037] The composite control valve 30 is a single control valve that combines two functions: a circuit switching valve that switches between circuits 40 and 50 to which pressurized oil discharged from hydraulic power sources 21 and 22 is supplied, and a bleed-down control valve that enables the release (bleed-down) of pressurized oil stored in the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. The composite control valve 30 is composed of, for example, a 6-port, 3-position hydraulic pilot-operated control valve. The composite control valve 30 has pressure receiving sections 30a and 30b on both the left and right sides to which pilot pressure is input.

[0038] The composite control valve 30 has six ports, for example: a first pump port p1 connected to the first hydraulic pump 21 via a first discharge line 25 and a center bypass line 31; a second pump port p2 connected to the second hydraulic pump 22 via a second discharge line 26; a tank port t connected to the hydraulic oil tank 23 via a return oil passage 34; a hoist port h connected to the hoist circuit 40 via a center bypass line 31; a first steering port s1 connected to the steering circuit 50 via a connecting pipe 32, 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 a connecting pipe 32, enabling communication between the steering circuit 50 and the hydraulic oil tank 23. The composite control valve 30 has three switching positions: a first neutral position N as a non-merging position, a second position D as a bleed-down position, and a third position C as a merging position, and is configured to be switched to any of the three switching positions. Under normal circumstances, both pressure-receiving sections 30a and 30b of the composite control valve 30 are connected to the hydraulic fluid tank 23 and are held in a neutral position N by a centering spring.

[0039] The neutral position N of the composite control valve 30 is a non-merging position in which pressurized oil discharged from the first hydraulic pump 21 is directed to the hoist circuit 40, and pressurized oil discharged from the second hydraulic pump 22 is directed to the steering circuit 50. In other words, it is a position in which the pressurized oil discharged from the first hydraulic pump 21 and the pressurized oil discharged from the second hydraulic pump 22 are supplied to the hoist circuit 40 and the steering circuit 50 respectively without being merged. More specifically, the neutral position N (non-merging position) is a position in which the steering circuit 50 is connected to only the second hydraulic pump 22 of the hydraulic power sources 21 and 22, while the connection between the steering circuit 50 and the hydraulic oil tank 23 is blocked, and the hoist circuit 40 is connected to only the first hydraulic pump 21 of the hydraulic power sources 21 and 22.

[0040] The second position D of the composite control valve 30 is a bleed-down position that is switched to when the operation of the dump truck 1 is completely finished and the pressurized oil stored in the accumulator 58 is released to the hydraulic oil tank 23 (bleed-down). The bleed-down position D is a position that guides the pressurized oil from the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23, and is also a position that allows pressurized oil to be supplied from the first hydraulic pump 21 to the hoist circuit 40 and from the second hydraulic pump 22 to the steering circuit 50. In detail, the bleed-down position D is a position that connects the steering circuit 50 to only the second hydraulic pump 22 of the hydraulic power sources 21 and 22, connects the steering circuit 50 to the hydraulic oil tank 23, and connects the hoist circuit 40 to only the first hydraulic pump 21 of the hydraulic power sources 21 and 22.

[0041] The composite control valve 30 has a throttle 30c in the oil passage connecting the steering circuit 50 and the hydraulic fluid tank 23 when in the second position D (bleed-down position). The throttle 30c functions as fluid resistance to the flow of pressurized oil from the second hydraulic pump 22 out to the hydraulic fluid tank 23 via the composite control valve 30.

[0042] The third position C of the composite control valve 30 is the position that guides the pressurized oil discharged from the first hydraulic pump 21 to the hoist circuit 40, and also guides the pressurized oil discharged from the second hydraulic pump 22 to the hoist circuit 40. In other words, it is the merging position where the pressurized oil discharged from the second hydraulic pump 22 is combined with the pressurized oil discharged from the first hydraulic pump 21 and guided to the hoist circuit 40. More specifically, the merging position C is the position that blocks communication between the steering circuit 50 and both the first hydraulic pump 21 and the second hydraulic pump 22, blocks communication between the steering circuit 50 and the hydraulic oil tank 23, and connects the hoist circuit 40 to both the first hydraulic pump 21 and the second hydraulic pump 22.

[0043] The hydraulic system 20 includes a pilot circuit 70 that generates pilot pressure from a pilot hydraulic power source to drive the hoist control valve 42 and inputs it to the pressure receiving sections 42a and 42b of the hoist control valve 42, and also generates pilot pressure from a pilot hydraulic power source to drive the composite control valve 30 and inputs it to the pressure receiving sections 30a and 30b of the composite control valve 30. The pilot circuit 70 generates pilot pressure for the hoist control valve 42 in response to the operation of the operating device 17 for the loading platform 5.

[0044] 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 fluid tank 23 via a pilot relief valve 77, and an accumulator 78 is also connected to it. The pilot pump 71 is a pilot hydraulic source, and is, for example, a fixed-displacement pump. The pilot relief valve 77 defines the maximum pressure of the pilot line 76 (the maximum discharge pressure of the pilot pump 71). The accumulator 78 stores the pressure of the pressurized oil discharged from the pilot pump 71. Thus, the pilot line 76 is maintained at a predetermined pressure by the pilot relief valve 77 and the accumulator 78. This ensures the source pressure necessary to generate the pilot pressure that drives the hoist control valve 42 and the composite control valve 30.

[0045] Two of the four solenoid valves 72, 73, 74, and 75, solenoid valves 72 and 73, generate pilot pressure for the hoist control valve 42 in response to the operation of the operating device 17 for the loading platform 5, thereby switching the hoist control valve 42 to one of four switching positions (neutral position N, raised position R, lowered position L, floating position F). The remaining solenoid valves 74 and 75 generate pilot pressure for the composite control valve 30, thereby switching the composite control valve 30 to one of three switching positions (non-merging position N, bleed-down position D, merging position C). Each of the solenoid valves 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 pilot pressure.

[0046] Solenoid valve 72 is used to switch the hoist control valve 42 to the raised position R. When a standby control signal (off signal) is input, one pressure-receiving part 42a of the hoist control valve 42 is connected to the hydraulic oil tank 23. When a drive control signal is input, pilot pressure is output to one pressure-receiving part 42a of the hoist control valve 42. Solenoid valve 73 is used to switch the hoist control valve 42 to the floating position F and the lowered position L. When a standby control signal (off signal) is input, the other pressure-receiving part 42b of the hoist control valve 42 is connected to the hydraulic oil tank 23. When a drive control signal is input, pilot pressure corresponding to the floating position F or the lowered position L is output to the other pressure-receiving part 42b of the hoist control valve 42. When both solenoid valves 72 and 73 block the input of pilot pressure to the hoist control valve 42, the hoist control valve 42 returns to the neutral position N.

[0047] Solenoid valve 74 is used to switch the composite control valve 30 to the bleed-down position D. When a standby control signal (off signal) is input, the other pressure-receiving section 30b of the composite control valve 30 is connected to the hydraulic oil tank 23. When a drive control signal (on signal) is input, pilot pressure is output to the other pressure-receiving section 30b of the composite control valve 30. Solenoid valve 75 is used to switch the composite control valve 30 to the merging position C. When a standby control signal (off signal) is input, one pressure-receiving section 30a of the composite control valve 30 is connected to the hydraulic oil tank 23. When a drive control signal is input, pilot pressure is output to one pressure-receiving section 30a of the composite control valve 30. When both solenoid valves 74 and 75 block the input of pilot pressure to the composite control valve 30, the composite control valve 30 is in the neutral position N, which is the non-merging position.

[0048] The hydraulic system 20 is configured such that the hoist control valve 42 is driven in response to the operation of the operating device 17 for the loading platform 5. The operating device 17 is, for example, an electric lever device and has an operating lever 17a that is tilted manually by the operator. The operating device 17 can be operated to one of four operating positions: holding position, lifting position, lifting position, and lowering position, which correspond to each switching position of the hoist control valve 42 (holding position N, lifting position R, floating position F, lowering position L). The operating lever 17a is normally positioned in the holding position. The operating device 17 outputs an operation signal to the control device 80 according to the operating position.

[0049] The control device 80 is electrically connected to the operating device 17 and receives the operating signal from the operating device 17. The control device 80 indirectly controls the switching position of the hydraulic pilot-operated hoist control valve 42 by controlling two solenoid valves 72 and 73 of the pilot hydraulic circuit 70 according to the operating position (operating signal) of the operating device 17. It also indirectly controls the switching position of the hydraulic pilot-operated composite control valve 30 by controlling two solenoid valves 74 and 75 of the pilot hydraulic circuit 70. Furthermore, the control device 80 controls the pump volume of the first hydraulic pump 21 and the second hydraulic pump 22 via the regulator 21a of the first hydraulic pump 21 and the regulator 22a of the second hydraulic pump 22.

[0050] The control device 80 is composed of a computer that includes, for example, a storage device 81 consisting of RAM or ROM, and a processing device 82 consisting of a CPU or MPU. The storage device 81 has programs and various information necessary for controlling the switching positions of the hoist control valve 42 and the composite control valve 30 stored in advance. The processing device 82 reads programs and various information from the storage device 81 as appropriate and performs various functions by executing processing according to the programs. The control device 80 may be composed of one computer or multiple computers.

[0051] When the operating lever 17a of the operating device 17 is operated to the raised position, the control device 80 outputs a control signal (excitation current) to the solenoid valve 72 and an off signal control signal to the solenoid valve 73. As a result, the pilot pressure from the solenoid valve 72 is input to one of the pressure receiving sections 42a of the hoist control valve 42, and the hoist control valve 42 switches to the raised position R.

[0052] In this case, the control device 80 switches the composite control valve 30 to the merging position C, for example. Specifically, it outputs a control signal (excitation current) to the solenoid valve 75 and an off signal control signal to the solenoid valve 74. As a result, the pilot pressure from the solenoid valve 75 is input to one of the pressure receiving sections 30a of the composite control valve 30, and the composite control valve 30 switches from the neutral position N to the merging position C.

[0053] Therefore, the pressurized oil from the first hydraulic pump 21 and the second hydraulic pump 22 are combined by the combined control valve 30 at the confluence position C and guided to the hoist control valve 42. Since the hoist control valve 42 is switched to the lifting position R, the pressurized oil from the first hydraulic pump 21 and the second hydraulic pump 22 is supplied to the bottom chamber 41a of the hoist cylinder 41 via the hoist control valve 42 at the lifting position R, making it possible to change the loading platform 5, which is loaded with the object to be transported, from the transporting position to the release position.

[0054] When the operating lever 17a of the operating device 17 is operated to the lower position, the control device 80 outputs a control signal (excitation current) corresponding to the lowering operation to the solenoid valve 73, and also outputs an off signal control signal to the solenoid valve 72. As a result, the pilot pressure from the solenoid valve 73 is input to the other pressure receiving section 42b of the hoist control valve 42, and the hoist control valve 42 is switched to the lowered position L. In this case, the composite control valve 30 is maintained in the neutral position N. When the operating device 17 is in the lowering position, the hoist cylinder 41 can be retracted by pressurized oil supplied from the first hydraulic pump 21 alone.

[0055] When the operating lever 17a of the operating device 17 is operated to the floating position, the control device 80 outputs a control signal (excitation current) corresponding to the floating operation to the solenoid valve 73 and an off signal control signal to the solenoid valve 72. As a result, the pilot pressure from the solenoid valve 73 is input to the other pressure receiving part 42b of the hoist control valve 42, and the hoist control valve 42 switches to the floating position F. In this case, the composite control valve 30 is maintained in the neutral position N. When the operating device 17 is in the floating position, the hoist cylinder 41 can be retracted by the weight of the loading platform 5.

[0056] Furthermore, when the operation of the dump truck 1 is completely finished, the control device 80 performs a bleed-down, which discharges the pressurized oil stored in the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. For example, after a certain period of time has elapsed since the prime mover 15 stopped, the combined control valve 30 is switched to the bleed-down position D. As a result, the pressure stored in the accumulator 58 of the steering circuit 50 is gradually discharged from the return line 34 to the hydraulic oil tank 23 via the throttle 30c of the combined control valve 30 in the bleed-down position D. At this time, the flow from the steering circuit 50 to the second hydraulic pump 22 via the combined control valve 30 in the bleed-down position D is blocked by the check valve 33. The control device 80 can also determine the completion of the bleed-down based on the detected value of the pressure sensor 61 that detects the pressure in the accumulator 58.

[0057] Next, the effects of the transport vehicle according to the first embodiment will be explained using Figures 2 and 3, comparing them with the hydraulic system of the comparative transport vehicle. Figure 3 is a hydraulic circuit diagram showing the configuration of the comparative hydraulic system with respect to the hydraulic system of the transport vehicle according to the first embodiment. In Figure 3, parts with the same reference numerals as those in Figure 2 are similar, so their detailed explanation will be omitted.

[0058] In the comparative example hydraulic system 120, the steering circuit 150 is equipped with a bleed-down control valve 162 that has only the function of discharging pressurized oil from the accumulator 58 to the hydraulic oil tank 23. The bleed-down control valve 162 is located on the line connecting the high-pressure line 54 and the hydraulic oil tank 23. The bleed-down control valve 162 is composed of a 2-port, 2-position electromagnetic control valve and is selectively switched 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 bleed-down control valve 162 is in the closed position C, pressurized oil is retained in the accumulator 58. On the other hand, when the bleed-down control valve 162 is switched to the open position D, the pressurized oil in the accumulator 58 is discharged to the hydraulic oil tank 23 via the high-pressure line 54 and the bleed-down control valve 162.

[0059] Furthermore, the comparative example hydraulic system 120 has a switching valve 130 that switches the supply destination of pressurized oil from the second hydraulic pump 22 between the hoist circuit 40 and the steering circuit 150, instead of the composite control valve 30 of the hydraulic system 20 of this embodiment. The switching valve 130 is connected to the second hydraulic pump 22 via the second discharge line 26. The output side of the switching valve 130 is connected to the steering circuit 150 via the connecting pipe 32 and also to the hoist circuit 40 via the connecting line 135 that branches off from the center bypass line 31. The first hydraulic pump 21 of the hydraulic system 120 is connected to the hoist circuit 40 via the first discharge line 25 without going through a valve mechanism.

[0060] The switching valve 130 is a hydraulic pilot-operated valve having two switching positions. The first position S of the switching valve 130 is the position where the supply destination of the pressurized oil from the second hydraulic pump 22 is the steering circuit 150, and the steering circuit 150 and the second hydraulic pump 22 are connected. The second position H of the switching valve 130 is the position where the supply destination of the pressurized oil from the second hydraulic pump 22 is the hoist circuit 40, and the hoist circuit 40 and the second hydraulic pump 22 are connected. The switching valve 130 is configured to be switched by the pilot pressure generated by the solenoid valve 175 of the pilot circuit 170, and is switched between the first position S and the second position H according to the control signal of the control device 180.

[0061] In the comparative example hydraulic system 120, there is a very low probability that the switching valve 130 may malfunction or become stuck in the second position H (the position where the pressurized oil is supplied to the hoist circuit 40), and that the bleed-down control valve 162 may malfunction or become stuck in the open position D. In this case, the pressurized oil from the second hydraulic pump 22 is supplied to the hoist circuit 40 and not to the steering circuit 150. Furthermore, the pressurized oil stored in the accumulator 58 of the steering circuit 150 is released to the hydraulic oil tank 23 via the bleed-down control valve 162 in the open position D. Therefore, if both the switching valve 130 and the bleed-down control valve 162 are simultaneously in unintended positions, the steering circuit 150 will become uncontrollable due to a pressure drop more quickly than if only one of the valves 130 or 162 malfunctions.

[0062] In contrast, in the hydraulic system 20 of this embodiment, as shown in Figure 2, the composite control valve 30 has a bleed-down function that releases pressurized oil from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, and also has a switching function for supplying pressurized oil from the hydraulic source 22 to the steering circuit 50. In other words, the single composite control valve 30 combines the functions of two valves, the switching valve 130 and the bleed-down control valve 162, in the comparative example hydraulic system 120. The composite control valve 30 enables the supply of pressurized oil from the hydraulic source 22 to the steering circuit 50 at the bleed-down position D, which allows pressurized oil to be released from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, thereby avoiding the simultaneous occurrence of pressurized oil release from the accumulator 58 to the hydraulic oil tank 23 and the interruption of pressurized oil supply from the hydraulic source 22 to the steering circuit 50.

[0063] In this embodiment, if the composite control valve 30 malfunctions or gets stuck in the bleed-down position D unintentionally while the dump truck 1 is in operation, the steering circuit 50 becomes connected to the hydraulic oil tank 23. At this time, the first hydraulic pump 21 and the second hydraulic pump 22 are driven. Pressurized oil from the second hydraulic pump 22 flows towards the steering circuit 50 side via the composite control valve 30 at the bleed-down position D (arrow on the center side of position D), and is then supplied to the steering circuit 50 via the connecting pipe 32 and flows out to the hydraulic oil tank 23 via the throttle 30c of the composite control valve 30 at the bleed-down position D (arrow on the right side of position D). The flow from the second hydraulic pump 22 to the hydraulic oil tank 23 via the composite control valve 30 at the bleed-down position D is limited by the fluid resistance of the throttle 30c. Therefore, the flow rate of pressurized oil supplied from the second hydraulic pump 22 to the steering circuit 50 exceeds the flow rate of oil flowing from the second hydraulic pump 22 to the hydraulic oil tank 23 via the throttle 30c at the bleed-down position D. Therefore, the pressure in the steering circuit 50 (accumulator 58) is maintained, and the steering function of the steering circuit 50 can be maintained. Pressurized oil from the first hydraulic pump 21 is supplied to the hoist circuit 40. In this case, the hoist cylinder 41 can be operated.

[0064] As described above, the dump truck 1 (transport vehicle) according to the first embodiment includes a hydraulic oil tank 23 for storing hydraulic oil, a first hydraulic pump 21 and a second hydraulic pump 22 as a hydraulic source that supply high-pressure hydraulic oil from the hydraulic oil tank 23, steering cylinders 51 and 52 that extend and contract with the hydraulic oil supplied from the hydraulic source 22 to steer the front wheels 2 (steering wheels), 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 a steering circuit 50 having an accumulator 58 that stores the pressure of the hydraulic oil supplied from the hydraulic source 22 and can supply the stored 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 configured to be connected to the hydraulic source 22 and the hydraulic oil tank 23 via a composite control valve 30. The composite control valve 30 has a neutral position N (first position) that connects the steering circuit 50 to the hydraulic power source 22 and shuts off the connection between the steering circuit 50 and the hydraulic oil tank 23, and a bleed-down position D (second position) that connects the steering circuit 50 to the hydraulic power source 22 and the steering circuit 50 to the hydraulic oil tank 23, thereby discharging the pressurized oil from the accumulator 58 to the hydraulic oil tank 23. In the bleed-down position D (second position), the composite control valve 30 has a throttle 30c in the oil passage that connects the steering circuit 50 and the hydraulic oil tank 23.

[0065] With this configuration, the bleed-down position D (second position) of the composite control valve 30, which enables pressurized oil discharge (bleed-down) from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, is in communication with the second hydraulic pump 22 (hydraulic source), thus enabling pressurized oil supply from the second hydraulic pump 22 (hydraulic source) to the steering circuit 50 via the composite control valve 30. Furthermore, the throttle 30c at the bleed-down position D (second position) acts as fluid resistance to the flow from the second hydraulic pump 22 (hydraulic source) to the hydraulic oil tank 23 via the composite control valve 30. Therefore, simultaneous occurrence of pressurized oil discharge from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and interruption of pressurized oil supply from the hydraulic source 22 to the steering circuit 50 can be avoided.

[0066] Furthermore, the dump truck 1 (transport vehicle) according to this embodiment is connected to hydraulic power sources 21 and 22 via a composite control valve 30 and includes a hoist circuit 40 which includes a hoist cylinder 41 that expands and contracts with pressurized oil supplied from the hydraulic power sources 21 and 22 to raise and lower the cargo bed. In addition to the neutral position N (first position) and bleed-down position D (second position) described above, the composite control valve 30 has a third position C which shuts off communication between the steering circuit 50 and the hydraulic power sources 21 and 22, shuts off communication between the steering circuit 50 and the hydraulic oil tank 23, and allows communication between the hoist circuit 40 and the hydraulic power sources 21 and 22.

[0067] With this configuration, the supply destination of pressurized oil from the hydraulic sources 21 and 22 can be switched to the hoist circuit 40 or the steering circuit 50 by switching the composite control valve 30 between the first position N and the third position C, and the bleed-down of the accumulator 58 can be performed by switching the composite control valve 30 to the bleed-down position D (second position). Furthermore, in the first position N and the third position C of the composite control valve 30, pressurized oil is not released from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23.

[0068] In this embodiment, the hydraulic power source is composed of a first hydraulic pump 21 and a second hydraulic pump 22. The neutral position N (first position) of the composite control valve 30 is a position in which the steering circuit 50 is connected to only the second hydraulic pump 22 of the hydraulic power sources 21 and 22, while the connection between the steering circuit 50 and the hydraulic oil tank 23 is blocked, and the hoist circuit 40 is connected to only the first hydraulic pump 21 of the hydraulic power sources 21 and 22. The bleed-down position D (second position) of the composite control valve 30 is a position in which the steering circuit 50 is connected to at least the second hydraulic pump 22 of the hydraulic power sources 21 and 22, while the steering circuit 50 is connected to the hydraulic oil tank 23, and the connection between the hoist circuit 40 and at least the second hydraulic pump 22 of the hydraulic power sources 21 and 22 is blocked. The junction position C (third position) of the composite control valve 30 is a position that blocks communication between the steering circuit 50 and both the first hydraulic pump 21 and the second hydraulic pump 22, blocks communication between the steering circuit 50 and the hydraulic fluid tank 23, and allows communication between the hoist circuit 40 and both the first hydraulic pump 21 and the second hydraulic pump 22.

[0069] With this configuration, when the hydraulic power source consists of a first hydraulic pump 21 and a second hydraulic pump 22, when the combined control valve 30 is in the non-merging position N (first position), the pressurized oil from the first hydraulic pump 21 and the pressurized oil from the second hydraulic pump 22 can be supplied separately to the hoist circuit 40 and the steering circuit 50 without being merged. Also, when the combined control valve 30 is in the merging position C (third position), the pressurized oil from the first hydraulic pump 21 and the pressurized oil from the second hydraulic pump 22 can be merged and supplied to the hoist circuit 40.

[0070] Furthermore, in this embodiment, the bleed-down position D (second position) of the composite control valve 30 is a position in which the steering circuit 50 is connected to only the second hydraulic pump 22 of the hydraulic power sources 21 and 22, the steering circuit 50 is connected to the hydraulic oil tank 23, and the hoist circuit 40 is connected to only the first hydraulic pump 21 of the hydraulic power sources 21 and 22.

[0071] With this configuration, when the composite control valve 30 is in the bleed-down position D (second position), pressurized oil can be supplied from the second hydraulic pump 22 to the steering circuit 50. This avoids the simultaneous occurrence of pressurized oil discharge from the accumulator 58 to the hydraulic oil tank 23 and the interruption of pressurized oil supply from the hydraulic source 22 to the steering circuit 50. Furthermore, since pressurized oil from the first hydraulic pump 21 is supplied to the hoist circuit 40, the hoist cylinder 41 can be operated.

[0072] [Modified version of the first embodiment] Next, a modified transport vehicle according to the first embodiment will be described with reference to Figure 4. Figure 4 is a hydraulic circuit diagram showing the configuration of the hydraulic system installed in the transport vehicle according to the modified embodiment. In Figure 4, parts with the same reference numerals as those in Figures 1 to 3 are the same parts, so a detailed explanation of them will be omitted.

[0073] The difference between the modified transport vehicle of the first embodiment and the first embodiment is that the configuration connecting the steering circuit 50 and the composite control valve 30 in the hydraulic system 20A is different. The other configurations of the hydraulic system 20A of the modified transport vehicle of the first embodiment are the same as those of the hydraulic system 20 of the transport vehicle of the first embodiment (see Figure 2), and their explanation will be omitted.

[0074] In the hydraulic system 20A according to a modified version of the first embodiment, the steering circuit 50 is connected to the composite control valve 30 via a first connecting pipe 35 and a second connecting pipe 36 that are in parallel. The first connecting pipe 35 is connected to the first steering port s1 side, which is a port that enables communication between the steering circuit 50 and the second hydraulic pump 22 in the composite control valve 30. The second connecting pipe 36 is connected to the second steering port s2 side, which is a port that enables communication between the steering circuit 50 and the hydraulic oil tank 23 in the composite control valve 30. Of the first connecting pipe 35 and the second connecting pipe 36, only the first connecting pipe 35 is provided with a filter 37 to remove foreign matter from the pressurized oil.

[0075] With this configuration, foreign matter in the pressurized oil flowing from the second hydraulic pump 22 to the steering circuit 50 via the first connecting pipe 35 can be removed by the filter 37. This improves the contamination resistance of the steering circuit 50.

[0076] [Second Embodiment] Next, a transport vehicle according to a second embodiment of the present invention will be described with reference to Figure 5. Figure 5 is a hydraulic circuit diagram showing the configuration of the hydraulic system provided in the transport vehicle according to the second embodiment. In Figure 5, parts with the same reference numerals as those shown in Figures 1 to 4 are the same parts, so a detailed explanation of them will be omitted.

[0077] The transport vehicle according to the second embodiment differs from a modified example of the first embodiment in that the configuration of the composite control valve 30B in the hydraulic system 20B is different, and a check valve 38 is added accordingly. The other configurations of the hydraulic system 20B of the transport vehicle according to the second embodiment are the same as those of the hydraulic system 20A of the transport vehicle according to a modified example of the first embodiment (see Figure 4), and their description is omitted.

[0078] Specifically, of the three switching positions of the composite control valve 30B, the first position, the non-merging position N, and the third position, the merging position C, are the same as the non-merging position N and merging position C of the composite control valve 30 in the first embodiment. On the other hand, the second position of the composite control valve 30B, the bleed-down position D, is a position that guides the pressurized oil from the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23, and is also a position that allows pressurized oil to be supplied from the first hydraulic pump 21 and the second hydraulic pump 22 to the steering circuit 50. More specifically, the bleed-down position D of the composite control valve 30B is a position that connects the steering circuit 50 to both the first hydraulic pump 21 and the second hydraulic pump 22, connects the steering circuit 50 to the hydraulic oil tank 23, and disconnects the hoist circuit 40 from both the first hydraulic pump 21 and the second hydraulic pump 22.

[0079] A check valve 38 is provided between the first hydraulic pump 21 and the composite control valve 30B. The check valve 38 allows the flow of pressurized oil from the first hydraulic pump 21 to the composite control valve 30B, while blocking the flow in the reverse direction. The check valve 38 also blocks the flow from the steering circuit 50 to the first hydraulic pump 21 via the composite control valve 30B when the composite control valve 30B is in the bleed-down position D.

[0080] In this embodiment, when the composite control valve 30B malfunctions or gets stuck in the bleed-down position D, the pressurized oil from the first hydraulic pump 21 merges with the pressurized oil from the second hydraulic pump 22 before flowing to the steering circuit 50. Therefore, compared to the case of the composite control valve 30 in the modified version of the first embodiment, where only the pressurized oil from the second hydraulic pump 22 flows to the steering circuit 50, the flow rate of pressurized oil flowing to the steering circuit 50 increases. Consequently, the flow rate of pressurized oil supplied to the steering circuit 50 via the composite control valve 30B in the bleed-down position D is reliably greater than the flow rate of pressurized oil flowing out to the hydraulic oil tank 23 via the throttle 30c in the bleed-down position D. Therefore, the pressure in the steering circuit 50 is maintained, and the steering function of the steering circuit 50 can be maintained. However, since the supply of pressurized oil from the first hydraulic pump 21 and the second hydraulic pump 22 to the hoist circuit 40 is cut off, the hoist cylinder 41 becomes inoperable.

[0081] According to the transport vehicle of the second embodiment described above, similar to the first embodiment (modified version) described above, the bleed-down position D (second position) of the composite control valve 30B, which enables pressurized oil discharge (bleed-down) from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, is such that the steering circuit 50 and the second hydraulic pump 22 (hydraulic source) are in communication, thus enabling pressurized oil supply from the second hydraulic pump 22 (hydraulic source) to the steering circuit 50 via the composite control valve 30B. Furthermore, the throttle 30c at the bleed-down position D (second position) acts as fluid resistance to the flow from the second hydraulic pump 22 (hydraulic source) to the hydraulic oil tank 23 via the composite control valve 30B. Therefore, simultaneous occurrence of pressurized oil discharge from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and interruption of pressurized oil supply from the hydraulic source 22 to the steering circuit 50 can be avoided.

[0082] Furthermore, the bleed-down position D (second position) of the composite control valve 30B according to this embodiment is a position that connects the steering circuit 50 to both the first hydraulic pump 21 and the second hydraulic pump 22, connects the steering circuit 50 to the hydraulic fluid tank 23, and disconnects the hoist circuit 40 from both the first hydraulic pump 21 and the second hydraulic pump 22.

[0083] With this configuration, when the composite control valve 30B is in the bleed-down position D (second position), pressurized oil can be supplied from the first hydraulic pump 21 and the second hydraulic pump 22 to the steering circuit 50, and the flow rate of pressurized oil flowing to the steering circuit 50 can be increased compared to the composite control valve 30 of the first embodiment.

[0084] [Third Embodiment] Next, a transport vehicle according to a third embodiment of the present invention will be described with reference to Figure 6. Figure 6 is a hydraulic circuit diagram showing the configuration of the hydraulic system provided in the transport vehicle according to the third embodiment. In Figure 6, parts with the same reference numerals as those shown in Figures 1 to 5 are the same parts, so a detailed explanation of them will be omitted.

[0085] The difference between the transport vehicle according to the third embodiment and the modified version of the first embodiment is that the hydraulic system 20C has only one hydraulic power source, the first hydraulic pump 21, and consequently, the configuration of the composite control valve 30C has been changed. The other configurations of the hydraulic system 20C of the transport vehicle according to the third embodiment are the same as those of the hydraulic system 20A of the transport vehicle modified from the first embodiment (see Figure 4), and their description is omitted.

[0086] The hydraulic system 20C according to the third embodiment consists of only one hydraulic power source, the first hydraulic pump 21, and the second hydraulic pump 22, which is one of the hydraulic power sources in the hydraulic system 20A according to a modification of the first embodiment, has been removed. The first pump port p1 and the second pump port p2 of the composite control valve 30C are connected to the center bypass line 31 and the oil passages branched from the center bypass line 31. The check valve 33 is located on the side of the second pump port p2.

[0087] The neutral position N, which is the first position of the composite control valve 30C, is the position in which the pressurized oil discharged from the first hydraulic pump 21 is guided to the steering circuit 50. More specifically, the neutral position N is the position in which the steering circuit 50 and the first hydraulic pump 21 are connected, the connection between the steering circuit 50 and the hydraulic oil tank 23 is blocked, and the connection between the hoist circuit 40 and the first hydraulic pump 21 is blocked.

[0088] The second position of the composite control valve 30C, the bleed-down position D, is the position that guides the pressurized oil from the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23, and is also the position that allows pressurized oil to be supplied from the first hydraulic pump 21 to the steering circuit 50. In detail, the bleed-down position D is the position that connects the steering circuit 50 to the first hydraulic pump 21 and the steering circuit 50 to the hydraulic oil tank 23, while blocking the connection between the hoist circuit 40 and the first hydraulic pump 21.

[0089] The third position C of the composite control valve 30C is the position that directs the pressurized oil discharged from the first hydraulic pump 21 to the hoist circuit 40. More specifically, the third position C is the position that blocks communication between the steering circuit 50 and the first hydraulic pump 21, blocks communication between the steering circuit 50 and the hydraulic oil tank 23, and connects the hoist circuit 40 and the first hydraulic pump 21.

[0090] In this embodiment, by switching the composite control valve 30C to the first position N, pressurized oil from the first hydraulic pump 21 can be supplied to the steering circuit 50. On the other hand, by switching the composite control valve 30C to the third position C, pressurized oil from the first hydraulic pump 21 can be supplied to the hoist circuit 40. In other words, by switching the composite control valve 30C between the first position N and the third position C, the destination of the pressurized oil supplied by the first hydraulic pump 21 can be switched.

[0091] In this embodiment, when the composite control valve 30C malfunctions or gets stuck in the bleed-down position D, pressurized oil from the first hydraulic pump 21 flows to the steering circuit 50. Therefore, the flow rate of pressurized oil supplied to the steering circuit 50 via the composite control valve 30C in the bleed-down position D can exceed the flow rate of pressurized oil flowing out to the hydraulic oil tank 23 via the throttle 30c in the bleed-down position D. Consequently, the pressure in the steering circuit 50 is maintained, and the steering function of the steering circuit 50 can be maintained. However, since the supply of pressurized oil from the first hydraulic pump 21 to the hoist circuit 40 is cut off, the hoist cylinder 41 becomes inoperable.

[0092] According to the transport vehicle of the third embodiment described above, similar to the first embodiment described above, the bleed-down position D (second position) of the composite control valve 30C, which enables pressurized oil discharge (bleed-down) from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, is such that the steering circuit 50 and the first hydraulic pump 21 are in communication, thus enabling pressurized oil supply from the first hydraulic pump 21 to the steering circuit 50 via the composite control valve 30C. Furthermore, the throttle 30c at the bleed-down position D (second position) acts as fluid resistance to the flow from the first hydraulic pump 21 to the hydraulic oil tank 23 via the composite control valve 30C. Therefore, simultaneous occurrence of pressurized oil discharge from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and interruption of pressurized oil supply from the hydraulic source 21 to the steering circuit 50 can be avoided.

[0093] Furthermore, in this embodiment, the hydraulic power source consists of only one first hydraulic pump 21. In addition, the neutral position N, which is the first position of the composite control valve 30C, is a position that connects the steering circuit 50 and the first hydraulic pump 21, while blocking the connection between the steering circuit 50 and the hydraulic oil tank 23, and also blocking the connection between the hoist circuit 40 and the first hydraulic pump 21. The bleed-down position D, which is the second position, is a position that connects the steering circuit 50 and the first hydraulic pump 21, while also connecting the steering circuit 50 and the hydraulic oil tank 23, and also blocking the connection between the hoist circuit 40 and the first hydraulic pump 21. The third position C is a position that blocks the connection between the steering circuit 50 and the first hydraulic pump 21, while also blocking the connection between the steering circuit 50 and the hydraulic oil tank 23, and also connecting the hoist circuit 40 and the first hydraulic pump 21.

[0094] With this configuration, even if there is only one hydraulic power source, the first hydraulic pump 21, pressurized oil can be supplied from the first hydraulic pump 21 to the steering circuit 50 when the composite control valve 30C is in the bleed-down position D (second position). Furthermore, by switching the composite control valve 30C between the first position N and the third position C, it is possible to switch the destination of the pressurized oil supplied by the first hydraulic pump 21.

[0095] [Fourth Embodiment] Next, a transport vehicle according to the fourth embodiment of the present invention will be described with reference to Figure 7. Figure 7 is a hydraulic circuit diagram showing the configuration of the hydraulic system provided in the transport vehicle according to the fourth embodiment. In Figure 7, parts with the same reference numerals as those shown in Figures 1 to 6 are the same parts, so a detailed explanation of them will be omitted.

[0096] The transport vehicle according to the fourth embodiment differs from the modified version of the first embodiment in that a cooling water system 18 for the prime mover 15 has been added, and a fan circuit 90 has been added to the hydraulic system 20D in conjunction with the addition of the cooling water system 18. The other configurations of the transport vehicle according to the fourth embodiment are the same as those of the hydraulic system 20A (see Figure 4) of the transport vehicle according to the modified version of the first embodiment, and their description will be omitted.

[0097] The cooling water system 18 according to the fourth embodiment includes a cooling water tank 18a for storing cooling water, a cooling water circulation pump 18b for drawing in and discharging cooling water from the cooling water tank 18a, and a radiator 18c for cooling the cooling water with cooling air. The cooling water system 18 is a circulation system capable of circulating cooling water within the system by the cooling water circulation pump 18b. The cooling air supplied to the radiator 18c is generated by a cooling fan 18d.

[0098] The hydraulic system 20D according to the fourth embodiment includes a fan circuit 90 for driving a cooling fan 18d. The fan circuit 90 includes a hydraulic motor 91 that drives the cooling fan 18d by being rotated by pressurized oil supplied from a first hydraulic pump 21, and a fan control valve 92 that controls the flow of pressurized oil supplied from the first hydraulic pump 21 to the hydraulic motor 91 and the flow of pressurized oil discharged from the hydraulic motor 91 to the hydraulic oil tank 23. The fan control valve 92, the composite control valve 30, and the hoist control valve 42 are connected in tandem along the center bypass line 31. The fan control valve 92 is located upstream of the composite control valve 30.

[0099] The return oil from the hydraulic motor 91 is discharged to the hydraulic oil tank 23 through the return oil passage 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 passages 94 and 95. Between the motor oil passages 94 and 95 and the return oil passage 93, a pair of check valves 97a and 97b for makeup are provided. The pair of check valves 97a and 97b supply hydraulic oil from the hydraulic oil tank 23 to the motor oil passages 94 and 95 via the return oil passage 93 when the hydraulic motor 91 rotates due to inertia or when the hydraulic motor 91 is rotated by the airflow during driving, creating negative pressure in the motor oil passages 94 and 95. Between the motor oil passages 94 and 95 and the return oil passage 93, a pair of relief valves 98a and 98b are provided. The pair of relief valves 98a and 98b protect the hydraulic equipment of the fan circuit 90 by releasing the hydraulic fluid into the hydraulic fluid tank 23 when the pressure in the pair of motor oil passages 94 and 95 exceeds a predetermined value.

[0100] The fan control valve 92 is configured, for example, as a 6-port, 3-position hydraulic pilot-operated directional control valve. The fan control valve 92 is configured using a single directional control valve and has pressure receiving sections 92a and 92b on both the left and right sides to which pilot pressure is input. The fan control valve 92 is a switchable valve that can be switched between a forward rotation position F, a reverse rotation position R, and a neutral position N. Normally, both pressure receiving sections 92a and 92b of the fan control valve 92 are connected to the hydraulic fluid tank 23 and are held in the neutral position N by a centering spring.

[0101] When the fan control valve 92 is in the neutral position N, the first hydraulic pump 21 and the composite control valve 30 are in communication, and the communication between the first hydraulic pump 21 and the hydraulic motor 91 is cut off. As a result, the hydraulic fluid discharged from the first hydraulic pump 21 is supplied to the composite control valve 30 through the fan control valve 92. Therefore, pressurized oil can be supplied from the first hydraulic pump 21 to the hoist circuit 40 via the composite control valve 30.

[0102] When the fan control valve 92 is in the forward rotation position F, the pressurized oil discharged from the first hydraulic pump 21 is supplied to the hydraulic motor 91 through the motor oil passage 94, causing the hydraulic motor 91 to rotate in the forward direction. The pressurized oil discharged from the hydraulic motor 91 is discharged to the hydraulic oil tank 23 through the motor oil passage 95 and the fan control valve 92. When the fan control valve 92 is in the reverse rotation position R, the hydraulic oil discharged from the first hydraulic pump 21 is supplied to the hydraulic motor 91 through 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 to the hydraulic oil tank 23 through the motor oil passage 94 and the fan control valve 92.

[0103] Thus, the forward rotation position F and the reverse rotation position R are rotational positions in which the first hydraulic pump 21 and the hydraulic motor 91 are in communication, and the hydraulic motor 91 is rotated by the pressurized oil discharged from the first hydraulic pump 21. When the fan control valve 92 is in rotational positions F or R, the communication between the first hydraulic pump 21 and the composite control valve 30 via the center bypass line 31 is cut off. As a result, pressurized oil cannot be supplied from the first hydraulic pump 21 to the hoist circuit 40.

[0104] On the other hand, the second hydraulic pump 22 is connected to the composite control valve 30 by bypassing the fan control valve 92. Therefore, the flow of pressurized oil supplied from the second hydraulic pump 22 to the composite control valve 30 is not affected by the operation of the fan control valve 92. In other words, the supply of pressurized oil from the second hydraulic pump 22 to the steering circuit 50 is not affected by the addition of the fan circuit 90.

[0105] The fan control valve 92 is configured to be switched by pilot pressure generated by solenoid valves 79a and 79b of the pilot circuit 70D, and is switched between three switching positions: forward position F, reverse position R, and neutral position N, according to the control signal of the control device 80.

[0106] According to the fourth embodiment described above, similar to the modification of the first embodiment described above, it is possible to avoid the simultaneous occurrence of pressurized oil discharge from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and the interruption of pressurized oil supply from the hydraulic source 21 to the steering circuit 50.

[0107] Furthermore, the dump truck 1 (transport vehicle) according to this embodiment includes a hydraulic motor 91 that drives a cooling fan 18d by being driven by pressurized oil supplied from a first hydraulic pump 21, and a fan control valve 92 that controls the flow of pressurized oil supplied from the first hydraulic pump 21 to the hydraulic motor 91. The fan control valve 92 and the composite control valve 30 are connected in tandem on a bypass line 31 that connects the first hydraulic pump 21 and the hydraulic oil tank 23, with the composite control valve 30 positioned downstream of the fan control valve 92. The second hydraulic pump 22 is connected to the composite control valve 30 by bypassing the fan control valve 92.

[0108] With this configuration, a fan control valve 92 that controls the hydraulic motor 91 that drives the cooling fan 18d can be interposed between the first hydraulic pump 21 and the composite control valve 30 without affecting the supply of pressurized oil from the second hydraulic pump 22 to the steering circuit 50 via the composite control valve 30.

[0109] [Other embodiments] It should be noted that the present invention is not limited to the first to fourth embodiments described above, and includes various modifications. The embodiments described above are explained in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described. For example, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace parts of the configuration of each embodiment with other configurations.

[0110] For example, in the first to fourth embodiments described above, an example was shown in which the hoist control valve 42 is configured with a single directional control valve. However, the hoist control valve can also be configured by combining a first control valve and a second control valve. For example, the first and second control valves can be configured with 6-port, 3-position directional control valves, and the first and second control valves can be connected in parallel to each other. The first control valve can be switched between three switching positions: neutral position N, raised position R, and floating position F. The second control valve can be switched between three switching positions: neutral position N, raised position R, and lowered position L. With this configuration, the drive of the hoist cylinder 41 can be controlled in the same way as with a single hoist control valve 42.

[0111] Furthermore, in the embodiments described above, examples were shown in which the hydraulic systems 20, 20A, 20B, 20C, and 20D included a hoist circuit 40. However, as shown in Figure 8, the hydraulic system 20E can also be configured without a hoist circuit 40 (see Figures 2, 4 to 7). Figure 8 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a transport vehicle according to another embodiment.

[0112] The hydraulic system 20E, which lacks a hoist circuit, also eliminates the need for a first hydraulic pump (see Figure 2) that supplies pressurized oil to the hoist circuit 40. The composite control valve 30E has a supply position N for supplying pressurized oil from the second hydraulic pump 22 to the steering circuit 50, and a bleed-down position D for discharging pressurized oil from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23. In other words, the composite control valve 30E is configured as a single control valve that combines two functions: supplying pressurized oil from the second hydraulic pump 22 to the steering circuit 50 and bleed-down the accumulator 58.

[0113] The composite control valve 30E is configured, for example, as a 4-port, 2-position hydraulic pilot-operated directional control valve and has a pressure receiving section 30b on one side to which pilot pressure is input. The composite control valve 30E has four ports, for example, a second pump port p2 connected to the second hydraulic pump 22 via a second discharge line 26, a tank port t connected to the hydraulic oil tank 23 via a return oil passage 34, a first steering port s1 connected to the steering circuit 50 via a first connecting pipe 35, 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 a second connecting pipe 36, enabling communication between the steering circuit 50 and the hydraulic oil tank 23. The composite control valve 30E is normally held in the supply position N by a spring. The composite control valve 30E is configured to be switched to the bleed-down position D by the pilot pressure generated by the solenoid valve 74E of the pilot circuit 70E.

[0114] The supply position N of the composite control valve 30E is the position that guides the pressurized oil discharged from the second hydraulic pump 22 to the steering circuit 50. More specifically, the supply position N is the position that connects the steering circuit 50 to the second hydraulic pump 22 while blocking the connection between the steering circuit 50 and the hydraulic oil tank 23.

[0115] The bleed-down position D of the composite control valve 30E is the position that guides the pressurized oil from the accumulator 58 (steering circuit 50) to the hydraulic oil tank 23, and is also the position that allows pressurized oil to be supplied from the second hydraulic pump 22 to the steering circuit 50. More specifically, the bleed-down position D is the position that connects the steering circuit 50 to the second hydraulic pump 22 and also connects the steering circuit 50 to the hydraulic oil tank 23. In the bleed-down position D, the composite control valve 30E has a throttle 30c in the oil passage that connects the steering circuit 50 to the hydraulic oil tank 23. The throttle 30c functions as a fluid resistance to the flow of pressurized oil from the second hydraulic pump 22 out through the composite control valve 30E to the hydraulic oil tank 23.

[0116] Even in a hydraulic system 20E without such a hoist circuit, the steering circuit 50 is configured to be connected to the second hydraulic pump 22 (hydraulic source) and the hydraulic oil tank 23 via a composite control valve 30E. The composite control valve 30E has a supply position N (first position) that connects the steering circuit 50 to the second hydraulic pump 22 (hydraulic source) and shuts off the connection between the steering circuit 50 and the hydraulic oil tank 23, and a bleed-down position D (second position) that connects the steering circuit 50 to the second hydraulic pump 22 (hydraulic source) and connects the steering circuit 50 to the hydraulic oil tank 23, allowing the pressurized oil from the accumulator 58 to be discharged into the hydraulic oil tank 23. In the bleed-down position D (second position), the composite control valve 30E has a throttle 30c in the oil passage that connects the steering circuit 50 and the hydraulic oil tank 23.

[0117] With this configuration, the bleed-down position D (second position) of the composite control valve 30E, which enables pressurized oil discharge (bleed-down) from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23, is in communication with the steering circuit 50 and the second hydraulic pump 22. This allows pressurized oil to be supplied from the second hydraulic pump 22 to the steering circuit 50 via the composite control valve 30E. Furthermore, the throttle 30c at the bleed-down position D (second position) acts as fluid resistance to the flow from the second hydraulic pump 22 to the hydraulic oil tank 23 via the composite control valve 30E. Therefore, simultaneous occurrence of pressurized oil discharge from the accumulator 58 of the steering circuit 50 to the hydraulic oil tank 23 and interruption of pressurized oil supply from the hydraulic source 22 to the steering circuit 50 can be avoided. [Explanation of Symbols]

[0118] 1…Dump truck (transport vehicle), 2…Front wheels (steering wheels), 5…Cargo bed, 18d…Cooling fan, 21…First hydraulic pump (hydraulic source), 22…Second hydraulic pump (hydraulic source), 23…Hydraulic oil tank, 30, 30B, 30C, 30E…Combined control valve, 31…Center bypass line (bypass line), 35…First connecting pipeline (first pipeline), 36…Second connecting pipeline (second pipeline), 37…Filter, 40…Hoist circuit, 41…Hoist 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...Breed-down position (second position), C...Confluence position (third position)

Claims

1. A hydraulic oil tank for storing hydraulic fluid, A hydraulic power source that supplies the hydraulic fluid in the hydraulic fluid tank as high-pressure pressurized oil, A transport vehicle comprising: a steering cylinder that expands and contracts to steer the steering wheels by pressurized oil supplied from the hydraulic source; a steering valve that controls the flow of pressurized oil supplied from the hydraulic source to the steering cylinder; and a steering circuit having an accumulator that stores the pressure of the pressurized oil supplied from the hydraulic source and can supply the stored pressure to the steering cylinder via the steering valve as the hydraulic source for the steering cylinder, The steering circuit is configured to be connected to the hydraulic power source and the hydraulic fluid tank via a composite control valve. The aforementioned composite control valve is A first position that connects the steering circuit and the hydraulic power source, while blocking communication between the steering circuit and the hydraulic fluid tank, It has a second position that connects the steering circuit and the hydraulic power source, and also connects the steering circuit and the hydraulic fluid tank to discharge the pressurized oil of the accumulator into the hydraulic fluid tank, The composite control valve, in the second position, has a restriction in the oil passage connecting the steering circuit and the hydraulic fluid tank. A transport vehicle characterized by the following features.

2. In the transport vehicle described in claim 1, The steering circuit is connected to the composite control valve via a first and second parallel pipeline. The first pipeline is connected to the first steering port side, which is a port that enables communication between the steering circuit and the hydraulic power source in the composite control valve. The second pipeline is connected to the second steering port side, which is a port that enables communication between the steering circuit and the hydraulic fluid tank in the composite control valve. A filter for removing foreign matter from the pressurized oil is provided only in the first of the two pipelines. A transport vehicle characterized by the following features.

3. In the transport vehicle described in claim 1, The hoist circuit includes a hoist cylinder connected to the hydraulic power source via the composite control valve, which extends and retracts by pressurized oil supplied from the hydraulic power source to raise and lower the loading platform. In addition to the first and second positions, the composite control valve has a third position that blocks communication between the steering circuit and the hydraulic power source, blocks communication between the steering circuit and the hydraulic fluid tank, and connects the hoist circuit and the hydraulic power source. A transport vehicle characterized by the following features.

4. In the transport vehicle described in claim 3, The aforementioned hydraulic power source consists of one hydraulic pump, The first position of the composite control valve is a position that connects the steering circuit and the hydraulic pump, blocks communication between the steering circuit and the hydraulic fluid tank, and blocks communication between the hoist circuit and the hydraulic pump. The second position of the composite control valve is a position that connects the steering circuit and the hydraulic pump, connects the steering circuit and the hydraulic fluid tank, and blocks the connection between the hoist circuit and the hydraulic pump. The third position of the composite control valve is a position that blocks communication between the steering circuit and the hydraulic pump, blocks communication between the steering circuit and the hydraulic fluid tank, and allows communication between the hoist circuit and the hydraulic pump. A transport vehicle characterized by the following features.

5. In the transport vehicle described in claim 3, The hydraulic power source consists of a first hydraulic pump and a second hydraulic pump. The first position of the composite control valve is a position that connects the steering circuit to only the second hydraulic pump of the hydraulic power source, while blocking communication between the steering circuit and the hydraulic fluid tank, and connecting the hoist circuit to only the first hydraulic pump of the hydraulic power source. The second position of the composite control valve is a position that connects the steering circuit to at least the second hydraulic pump of the hydraulic power source, connects the steering circuit to the hydraulic fluid tank, and blocks communication between the hoist circuit and at least the second hydraulic pump of the hydraulic power source. The third position of the composite control valve is a position that blocks communication between the steering circuit and both the first and second hydraulic pumps, blocks communication between the steering circuit and the hydraulic fluid tank, and allows communication between the hoist circuit and both the first and second hydraulic pumps. A transport vehicle characterized by the following features.

6. In the transport vehicle described in claim 5, The second position of the composite control valve is a position that connects the steering circuit to only the second hydraulic pump of the hydraulic power source, connects the steering circuit to the hydraulic fluid tank, and connects the hoist circuit to only the first hydraulic pump of the hydraulic power source. A transport vehicle characterized by the following features.

7. In the transport vehicle described in claim 5, The second position of the composite control valve is a position that connects the steering circuit to both the first and second hydraulic pumps, connects the steering circuit to the hydraulic fluid tank, and blocks the connection between the hoist circuit to both the first and second hydraulic pumps. A transport vehicle characterized by the following features.

8. In the transport vehicle described in claim 5, A hydraulic motor that drives a cooling fan by being driven by pressurized oil supplied from the first hydraulic pump, The system includes a fan control valve that controls the flow of pressurized oil supplied from the first hydraulic pump to the hydraulic motor, The fan control valve and the combined control valve are connected in tandem on a bypass line connecting the first hydraulic pump and the hydraulic fluid tank. The composite control valve is located downstream of the fan control valve. The second hydraulic pump is connected to the composite control valve, bypassing the fan control valve. A transport vehicle characterized by the following features.

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