Dump truck

Abstract

To provide a dump truck capable of shortening the undulating time of a loading platform while preventing the occurrence of cavitation of a fan motor.SOLUTION: A dump truck includes: a first hydraulic pump and a second hydraulic pump; an accumulator; a control valve capable of switching between supplying hydraulic fluid pressure-fed from one or both of the first hydraulic pump and the second hydraulic pump to a hoist cylinder, accumulating pressure in the accumulator, or returning the hydraulic fluid to a hydraulic fluid tank; a switching valve capable of switching between supplying hydraulic fluid pressure-fed from the second hydraulic pump to a fan motor through a supply line and supplying the hydraulic fluid to the hoist cylinder; a first make-up line extending from the accumulator to the supply line; and a first switching valve capable of switching between opening the first make-up line and supplying hydraulic fluid from the accumulator to the fan motor or blocking it.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a dump truck in which a hydraulic pump is shared by a hoist cylinder and a fan motor. [Background technology]

[0002] Conventionally, a dump truck has been known that includes a vehicle body, a loading platform supported on the vehicle body so that it can be raised and lowered, a hoist cylinder that is supplied with hydraulic oil to raise and lower the loading platform, and a fan motor that is supplied with hydraulic oil to generate a driving force for rotating a cooling fan.

[0003] As an example of such a dump truck, Patent Document 1 discloses a configuration in which a hydraulic pump that discharges pressure oil for driving a hoist cylinder and a fan motor is shared. More specifically, the dump truck of Patent Document 1 includes a switching valve that can switch between a state in which hydraulic oil pumped from the hydraulic pump is supplied to the hoist cylinder and a state in which it is supplied to the fan motor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7253672 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the dump truck of Patent Document 1, when the switching valve is switched from the fan motor side to the hoist cylinder side to raise and lower the bed, it is necessary to prevent the inlet of the fan motor from becoming negative pressure and causing cavitation. Therefore, the pump discharge flow rate is slowly reduced and the rotation speed of the fan motor is sufficiently reduced before the switching valve is switched. As a result, it takes time to raise and lower the bed, which may worsen the cycle time at the customer's site. Therefore, the challenge is to prevent cavitation of the fan motor and shorten the time it takes to raise and lower the bed.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a dump truck that can reduce the time it takes to raise and lower the bed while preventing cavitation from occurring in the fan motor when the supply destination of the hydraulic oil pumped from the hydraulic pump is switched from the fan motor side to the hoist cylinder side. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides a dump truck including a vehicle body, a loading platform supported on the vehicle body so as to be able to be raised and lowered, a hoist cylinder that is supplied with hydraulic oil to raise and lower the loading platform, a fan motor that is supplied with hydraulic oil through a supply line to generate a driving force for rotating a cooling fan, and a hydraulic circuit that supplies hydraulic oil to the hoist cylinder and the fan motor. The hydraulic circuit includes a hydraulic oil tank that stores hydraulic oil, a first hydraulic pump and a second hydraulic pump that pump out the hydraulic oil stored in the hydraulic oil tank, an accumulator that accumulates hydraulic oil, and a hydraulic circuit that supplies the hydraulic oil pressurized from one or both of the first hydraulic pump and the second hydraulic pump to the accumulator. The hydraulic system is characterized by comprising a control valve that can be switched between a hoist position where hydraulic oil is supplied to a hoist cylinder, a pressure accumulation position where hydraulic oil is accumulated in the accumulator, and a center bypass position where hydraulic oil is returned to the hydraulic oil tank; a switching valve that can be switched between a fan position where hydraulic oil pressurized from the second hydraulic pump is supplied to the fan motor through the supply line, and a hoist position where hydraulic oil is supplied to the hoist cylinder; a first makeup line extending from the accumulator to the supply line; and a first opening / closing valve that can be switched between an open position where the first makeup line is opened to supply hydraulic oil from the accumulator to the fan motor, and a closed position where the first makeup line is closed. Effect of the Invention

[0008] According to the present invention, when the supply destination of the hydraulic oil pumped from the hydraulic pump is switched from the fan motor side to the hoist cylinder side, it is possible to prevent cavitation from occurring in the fan motor and shorten the time required for raising and lowering the platform. Note that the problems, configurations and effects other than those described above will become clear from the following description of the embodiments. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a side view of the dump truck according to the embodiment. [Diagram 2] FIG. 2 is a circuit diagram of a drive circuit mounted on the dump truck. [Diagram 3]FIG. 2 is a layout diagram of hydraulic components constituting the control valve. [Figure 4] FIG. 2 is a hardware configuration diagram of the dump truck. [Diagram 5] 13 is a flowchart of a loading platform raising and lowering process. [Figure 6] FIG. 13 is a diagram showing combinations of control valve positions during bed raising and lowering processing. [Figure 7] 4 is a time chart of the bed raising and lowering process in a conventional hydraulic circuit. [Figure 8] 5 is a time chart of the bed raising and lowering process in the hydraulic circuit according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An embodiment of a dump truck according to the present invention will be described with reference to the drawings. Fig. 1 is a side view of a dump truck 1 according to this embodiment. In this specification, front, rear, left and right are based on the viewpoint of an operator who rides on and operates the dump truck 1, unless otherwise specified.

[0011] As shown in FIG. 1, the dump truck 1 according to this embodiment mainly comprises a body frame 2 (body), a pair of front tires 3L, 3R rotatably connected to the left and right vehicle body axles with respect to suspension arms supported swingably on the body frame 2 and the left and right vehicle body axles, a pair of rear tires 4L, 4R rotatably supported via rear axles 60 at both the left and right ends of the rear of the body frame 2, a cargo bed 5 supported on the body frame 2 so as to be able to rise and fall, and a cab 6 in which an operator who operates the dump truck 1 rides.

[0012] The pair of front tires 3L, 3R are steered wheels whose steering angle changes according to the steering operation by the operator. Meanwhile, the pair of rear tires 4L, 4R are drive wheels that rotate by transmitting the driving force of a travel motor (not shown). The dump truck 1 is equipped with a pair of left and right travel motors in order to transmit the driving force independently to each of the pair of left and right rear tires 4L, 4R.

[0013] The rear axle 60 is connected to the center of the rear of the body frame 2 via a spherical bearing. The rear axle 60 rotatably supports the rear tires 4L, 4R. The rear axle 60 also houses components for rotating the rear tires 4L, 4R (e.g., a travel motor, a travel clutch, a reduction gear, a hydraulic brake, etc.).

[0014] The loading platform 5 rises and falls (changes its posture) in the vertical direction around a hinge pin 8 at the rear of the vehicle body frame 2 as the hoist cylinder 7 expands and contracts. One end of the hoist cylinder 7 is connected to the vehicle body frame 2, and the other end is connected to the loading platform 5, and the hoist cylinder 7 expands and contracts by receiving a supply of hydraulic oil from the first hydraulic pump 22 or the second hydraulic pump 23 (see FIG. 2). When the hoist cylinder 7 expands, the loading platform 5 stands up and assumes a release posture, and when the hoist cylinder 7 contracts, the loading platform 5 falls down and assumes a seated posture.

[0015] The earth-releasing position is the position of the loading platform 5 when the loaded load is released to the rear of the dump truck 1. The seating position is the position of the loading platform 5 when the loading platform 5 is seated on the vehicle body frame 2 and a load is loaded. Note that the load loaded on the loading platform 5 is not limited to soil and sand, but since soil and sand is a typical example of a load, the following description will be given assuming that the loading platform 5 is loaded with soil and sand.

[0016] The cab 6 is disposed at the left end on the deck at the front end of the body frame 2. The cab 6 forms a driver's cab in which an operator who operates the dump truck 1 sits. An operation device 9 (see FIG. 4) for operating the dump truck 1 is disposed inside the cab 6. The operation device 9 outputs an operation signal corresponding to an operation by the operator in the cab 6 to a controller 50 (see FIG. 4) described later. When the operator operates the operation device 9, the dump truck 1 travels (accelerates, brakes, turns) and the bed 5 is raised and lowered. The operation device 9 includes, for example, an accelerator pedal, a brake pedal, a steering wheel, a travel lever, and a hoist lever.

[0017] The accelerator pedal is an operating device operated to instruct acceleration of the dump truck 1. The brake pedal is an operating device operated to instruct braking of the dump truck 1. The brake pedal may include, for example, a retard brake pedal that causes the travel motor to function as a regenerative brake, and a hydraulic brake pedal that drives a hydraulic brake. The steering wheel is an operating device operated to instruct the turning direction of the dump truck 1. The travel lever is an operating device operated to instruct the traveling direction (forward position, reverse position, neutral position) of the dump truck 1 when the accelerator pedal is depressed. The hoist lever is an operating device operated to instruct the raising and lowering of the loading platform 5 (seating position, soil discharge position).

[0018] Fig. 2 is a circuit diagram of a drive circuit mounted on the dump truck 1. As shown in Fig. 2, the dump truck 1 mainly includes an engine 10, a radiator 11, an oil cooler 12, a cooling fan 13, a fan motor 14, hydraulic accessories 15, and a hydraulic circuit 20.

[0019] The engine 10 generates driving force for operating the dump truck 1. Electricity generated by a generator (not shown) by the driving force of the engine 10 is supplied to a traveling motor, thereby causing the dump truck 1 to travel. In addition, a first hydraulic pump 22 and a second hydraulic pump 23, which will be described later, are rotated by the driving force of the engine 10.

[0020] The radiator 11 exchanges heat between the coolant that cools the engine 10 and the cooling air generated by the cooling fan 13. The oil cooler 12 exchanges heat between the hydraulic oil circulating in the hydraulic circuit 20 and the cooling air generated by the cooling fan 13. The cooling fan 13 generates cooling air toward the radiator 11 and the oil cooler 12 by being rotated by the driving force of the fan motor 14. The fan motor 14 is supplied with hydraulic oil pumped from the second hydraulic pump 23 and generates a driving force for rotating the cooling fan 13. The hydraulic accessories 15 are a general term for hydraulic devices that are operated by being supplied with hydraulic oil.

[0021] The hydraulic circuit 20 supplies hydraulic oil to the hoist cylinder 7, the fan motor 14, and the hydraulic accessories 15, thereby raising and lowering the platform 5, rotating the cooling fan 13, and operating the hydraulic accessories 15. The hydraulic circuit 20 includes, for example, a hydraulic oil tank 21, a first hydraulic pump 22, a second hydraulic pump 23, an accumulator 24, a control valve 25, a switching valve 26, a first back pressure retention valve 27, a second back pressure retention valve 28, a first on-off valve 29, a second on-off valve 30, a first check valve 31, a second check valve 32, a third check valve 33, a pressure reducing valve 34, relief valves 35 and 36, and a pressure sensor 37.

[0022] The hydraulic circuit 20 includes, as main oil passages through which the hydraulic oil flows, a first pump line L1, a second pump line L2, a supply line L3, a discharge line L4, a loop line L5, a CV line L6, a bottom line L7, a rod line L8, a pressure accumulation line L9, a first makeup line L10, a return line L11, a bypass line L12, and a second makeup line L13. The lines L1 to L13 are formed by combining, for example, metal piping and flexible hoses.

[0023] The first pump line L1 is an oil passage leading from the first hydraulic pump 22 to the control valve 25. In other words, the first pump line L1 is an oil passage that supplies the hydraulic oil pressure-fed from the first hydraulic pump 22 to the control valve 25. The second pump line L2 is an oil passage that leads from the second hydraulic pump 23 to the switching valve 26. In other words, the second pump line L2 is an oil passage that supplies the hydraulic oil pressure-fed from the second hydraulic pump 23 to the switching valve 26.

[0024] The supply line L3 is an oil passage that extends from the switching valve 26 to the inlet side of the fan motor 14. In other words, the supply line L3 is an oil passage that supplies the hydraulic oil pumped from the second hydraulic pump 23 to the fan motor 14. The discharge line L4 is an oil passage that extends from the outlet side of the fan motor 14 to the hydraulic oil tank 21 via the second back pressure maintenance valve 28 and the oil cooler 12. In other words, the discharge line L4 is an oil passage that returns the hydraulic oil discharged from the fan motor 14 to the hydraulic oil tank 21. The loop line L5 is an oil passage that branches off from the discharge line L4 from the second back pressure maintenance valve 28 upstream of the hydraulic oil flow, passes through the second check valve 32, and extends to the supply line L3.

[0025] The CV line L6 is an oil passage that extends from the switching valve 26 to the control valve 25. In other words, the CV line L6 is an oil passage that supplies the hydraulic oil pumped from the second hydraulic pump 23 to the control valve 25.

[0026] The bottom line L7 is an oil passage leading from the control valve 25 to the bottom chamber of the hoist cylinder 7. In other words, the bottom line L7 is an oil passage for supplying and discharging hydraulic oil between the control valve 25 and the bottom chamber of the hoist cylinder 7. The rod line L8 is an oil passage leading from the control valve 25 to the rod chamber of the hoist cylinder 7. In other words, the rod line L8 is an oil passage for supplying and discharging hydraulic oil between the control valve 25 and the rod chamber of the hoist cylinder 7.

[0027] The pressure accumulation line L9 is an oil passage leading from the control valve 25 to the accumulator 24. In other words, the pressure accumulation line L9 is an oil passage for accumulating the hydraulic oil supplied from the control valve 25 in the accumulator. The first makeup line L10 is an oil passage leading from the accumulator 24 to the supply line L3 via the pressure reducing valve 34, the first on-off valve 29, and the third check valve 33. In other words, the first makeup line L10 is an oil passage for supplying the hydraulic oil accumulated in the accumulator 24 to the supply line L3.

[0028] The return line L11 is an oil passage that leads from the control valve 25 to the hydraulic oil tank 21 via the first back pressure retention valve 27. In other words, the return line L11 is an oil passage that returns the hydraulic oil discharged from the control valve 25 to the hydraulic oil tank 21. The bypass line L12 is an oil passage that connects the upstream side and downstream side of the flow of hydraulic oil from the first back pressure retention valve 27 of the return line L11. In other words, the bypass line L12 is an oil passage that returns the hydraulic oil discharged from the control valve 25 to the hydraulic oil tank 21, bypassing the first back pressure retention valve 27. The second make-up line L13 is an oil passage that branches from the upstream side of the flow of hydraulic oil from the first back pressure retention valve 27 of the return line L11, and leads to the supply line L3 via the first check valve 31. In other words, the second make-up line L13 is an oil passage that supplies the hydraulic oil passing through the return line L11 to the supply line L3.

[0029] The hydraulic oil tank 21 stores hydraulic oil to be supplied to and discharged from hydraulic equipment (e.g., the hoist cylinder 7, the fan motor 14, the hydraulic accessories 15, the accumulator 24, etc.). The hydraulic oil tank 21 is preferably disposed so that the oil level of the stored hydraulic oil is always located vertically above the first hydraulic pump 22 and the second hydraulic pump 23.

[0030] The first hydraulic pump 22 and the second hydraulic pump 23 are connected to the output shaft of the engine 10. The first hydraulic pump 22 is rotated by the driving force of the engine 10, thereby pumping the hydraulic oil stored in the hydraulic oil tank 21 to the first pump line L1. The second hydraulic pump 23 is rotated by the driving force of the engine 10, thereby pumping the hydraulic oil stored in the hydraulic oil tank 21 to the second pump line L2. The first hydraulic pump 22 and the second hydraulic pump 23 are variable displacement pumps that can change the pump displacement per rotation under the control of the controller 50.

[0031] The accumulator 24 is connected to the pressure accumulation line L9. The accumulator 24 compresses and accumulates (i.e., accumulates pressure) the hydraulic oil pumped from the first hydraulic pump 22. The hydraulic oil accumulated in the accumulator 24 is supplied to, for example, a brake device that brakes the dump truck 1 and a steering cylinder that changes the steering angle of the front tires 3L, 3R, and is also used as pilot pressure for various hydraulic devices. The accumulator 24 supplies the accumulated hydraulic oil to the supply line L3 through the first make-up line L10.

[0032] The control valve 25 is connected to the first pump line L1, the CV line L6, the bottom line L7, the rod line L8, the pressure accumulation line L9, and the return line L11. The control valve 25 supplies the hydraulic oil pumped from the first hydraulic pump 22 to the hoist cylinder 7, the accumulator 24, and the hydraulic oil tank 21. More specifically, the control valve 25 is an electromagnetic valve configured to be switchable between a hoist position, a pressure accumulation position, a center bypass position, and a pressure accumulation & hoist position under the control of the controller 50. The first pump line L1 and the CV line L6 may be joined and then connected to the control valve 25, and the hydraulic oil pumped from one or both of the first hydraulic pump 22 and the second hydraulic pump 23 may be supplied to the hoist cylinder 7, the accumulator 24, and the hydraulic oil tank 21.

[0033] The hoist position is a spool position that supplies hydraulic oil pressure-fed from the first hydraulic pump 22 and the second hydraulic pump 23 to the hoist cylinder 7. The pressure-accumulation position is a spool position that accumulates hydraulic oil pressure-fed from the first hydraulic pump 22 in the accumulator 24. The center bypass position is a spool position that returns hydraulic oil pressure-fed from the first hydraulic pump 22 and the second hydraulic pump 23 to the hydraulic oil tank 21. The pressure-accumulation & hoist position is a spool position that accumulates hydraulic oil pressure-fed from the first hydraulic pump 22 in the accumulator 24 and supplies hydraulic oil pressure-fed from the second hydraulic pump 23 to the hoist cylinder 7. A detailed configuration of the control valve 25 will be described later with reference to FIG. 3.

[0034] The switching valve 26 is connected to the second pump line L2, the supply line L3, and the CV line L6. The switching valve 26 supplies the hydraulic oil pumped from the second hydraulic pump 23 to one or both of the fan motor 14 and the control valve 25. The switching valve 26 is an electromagnetic valve configured to be switchable between a fan position A, a hoist position B, and a distribution position C under the control of the controller 50.

[0035] The fan position A is a spool position that supplies hydraulic oil pumped from the second hydraulic pump 23 to the hoist cylinder 7 and only the fan motor 14 of the fan motor 14. The hoist position B is a spool position that supplies hydraulic oil pumped from the second hydraulic pump 23 to only the hoist cylinder 7 of the hoist cylinder 7 and fan motor 14. The distribution position C is a spool position that supplies hydraulic oil pumped from the second hydraulic pump 23 to both the fan motor 14 and the hoist cylinder 7.

[0036] The first back pressure retention valve 27 is disposed in the return line L11. The first back pressure retention valve 27 returns the hydraulic oil in the return line L11 to the hydraulic oil tank 21 while maintaining the hydraulic pressure of the hydraulic oil discharged from the control valve 25 (i.e., the upstream side of the flow of the hydraulic oil from the first back pressure retention valve 27 in the return line L11 and the second make-up line L13) at a first pressure (e.g., 0.3 MPa) that is higher than the minimum pressure (e.g., 0.1 MPa) required for the fan motor 14 to function as a hydraulic motor.

[0037] The second on-off valve 30 is disposed in the bypass line L12. The second on-off valve 30 is an electromagnetic valve configured to be switchable between a closed position D and an open position E under the control of the controller 50. The closed position D is a spool position that closes the bypass line L12. The open position E is a spool position that opens the bypass line L12.

[0038] The first check valve 31 is disposed in the second makeup line L13. The first check valve 31 allows the flow of hydraulic oil from the return line L11 to the supply line L3 and blocks the flow of hydraulic oil from the supply line L3 to the return line L11.

[0039] When the second on-off valve 30 is switched to the closed position D, the hydraulic pressure in the upstream side of the flow of hydraulic oil from the first back pressure retention valve 27 of the return line L11 and in the second makeup line L13 becomes a first pressure. Then, when the hydraulic pressure in the supply line L3 falls below the first pressure, the hydraulic oil in the return line L11 maintained at the first pressure flows into the supply line L3 through the second makeup line L13. On the other hand, even if the hydraulic pressure in the supply line L3 exceeds the first pressure, the hydraulic oil in the supply line L3 does not flow into the return line L11 through the second makeup line L13.

[0040] Furthermore, when the second on-off valve 30 is switched to the open position E, the hydraulic oil in the return line L11 returns to the hydraulic oil tank 21 without passing through the first back pressure maintenance valve 27. In other words, no hydraulic pressure is established upstream of the first back pressure maintenance valve 27 in the return line L11 in the hydraulic oil flow, and the hydraulic oil discharged from the control valve 25 returns to the hydraulic oil tank 21 without resistance.

[0041] The second back pressure retention valve 28 is disposed in the discharge line L4. The second back pressure retention valve 28 returns the hydraulic oil in the discharge line L4 to the hydraulic oil tank 21 while maintaining the hydraulic pressure of the hydraulic oil discharged from the fan motor 14 (i.e., the upstream side of the second back pressure retention valve 28 in the discharge line L4 in the flow of the hydraulic oil and the loop line L5) at a second pressure (e.g., 0.3 MPa) higher than the minimum pressure (e.g., 0.1 MPa) required for the fan motor 14 to function as a hydraulic motor. Note that the first pressure and the second pressure may be the same value or different values ​​as long as they are higher than the minimum pressure.

[0042] The second check valve 32 is disposed in the loop line L5. The second check valve 32 allows the flow of hydraulic oil from the discharge line L4 to the supply line L3 and blocks the flow of hydraulic oil from the supply line L3 to the discharge line L4.

[0043] As a result, when the hydraulic pressure in the supply line L3 falls below the second pressure, the hydraulic oil in the discharge line L4 maintained at the second pressure flows into the supply line L3 through the loop line L5. On the other hand, even if the hydraulic pressure in the supply line L3 exceeds the second pressure, the hydraulic oil in the supply line L3 does not flow into the discharge line L4 through the loop line L5.

[0044] The first on-off valve 29 is disposed in the first makeup line L10. The first on-off valve 29 is an electromagnetic valve configured to be switchable between a closed position F and an open position G under the control of the controller 50. The closed position F is a spool position that closes the first makeup line L10. The open position G is a spool position that opens the first makeup line L10.

[0045] The third check valve 33 is disposed downstream in the flow of hydraulic oil from the first on-off valve 29 in the first makeup line L10. The third check valve 33 allows the flow of hydraulic oil from the first on-off valve 29 to the supply line L3 and blocks the flow of hydraulic oil from the supply line L3 to the first on-off valve 29.

[0046] The pressure reducing valve 34 is disposed in the first makeup line L10 upstream of the first on-off valve 29 in the flow of hydraulic oil. The pressure reducing valve 34 reduces the pressure of the hydraulic oil stored in the accumulator 24 to a third pressure (e.g., 0.2 MPa) that is higher than the minimum pressure and lower than the second pressure.

[0047] When the first on-off valve 29 is switched to the open position G, if the hydraulic pressure in the supply line L3 falls below the third pressure, the hydraulic oil reduced to the third pressure by the pressure reducing valve 34 passes through the first on-off valve 29 in the open position G and flows into the supply line L3. On the other hand, even if the hydraulic pressure in the supply line L3 exceeds the third pressure, the hydraulic oil in the supply line L3 does not flow into the first on-off valve 29. Furthermore, when the first on-off valve 29 is switched to the closed position F, the hydraulic oil reduced to the third pressure by the pressure reducing valve 34 does not pass through the first on-off valve 29.

[0048] The relief valve 35 is a safety valve that discharges the hydraulic oil in the second pump line L2 to the hydraulic oil tank 21 when the pressure of the hydraulic oil in the second pump line L2 exceeds a threshold value. The relief valve 36 is a safety valve that discharges the hydraulic oil in the discharge line L4 to the hydraulic oil tank 21 when the pressure of the hydraulic oil in the discharge line L4 exceeds a threshold value. The pressure sensor 37 detects the hydraulic pressure in the accumulator line L9 and outputs a pressure signal indicative of the detected hydraulic pressure to the controller 50.

[0049] Fig. 3 is a layout diagram of hydraulic components constituting the control valve 25. The control valve 25 is composed of a plurality of valves. As shown in Fig. 3, the control valve 25 is composed of a plurality of directional control valves 41, 42, 43 and relief valves 44, 45. However, the specific configuration of the control valve 25 is not limited to the example in Fig. 3.

[0050] The directional control valve 41 is an electromagnetic valve that can be switched between an accumulation position H, a return position I, and a bypass position J under the control of the controller 50. The accumulation position H is a spool position that supplies the hydraulic oil supplied through the first pump line L1 to the accumulator 24 and the hydraulic auxiliary device 15 through the accumulation line L9. The return position I is a spool position that bypasses the hydraulic oil supplied through the first pump line L1 to the directional control valves 42, 43 and returns the hydraulic oil discharged from the accumulator 24 to the hydraulic oil tank 21 through the return line L11. The bypass position J is a spool position that bypasses the hydraulic oil supplied through the first pump line L1 to the directional control valves 42, 43.

[0051] The directional control valve 42 is an electromagnetic valve that can be switched to a hoist raising position K, a hoist floating position L, and a hoist holding position M under the control of the controller 50. The hoist raising position K is a spool position that communicates the first pump line L1 with the bottom line L7 and communicates the rod line L8 with the return line L11 through the directional control valve 41 in the bypass position J. The hoist floating position L is a spool position that communicates the bottom line L7 with the return line L11. The hoist holding position M is a spool position that blocks the supply and discharge of hydraulic oil to and from the rod chamber and bottom chamber of the hoist cylinder 7 and bypasses the first pump line L1 to the return line L11.

[0052] The directional control valve 43 is an electromagnetic valve that can be switched to a hoist raising position N, a hoist lowering position O, and a hoist holding position P under the control of the controller 50. The hoist raising position N is a spool position that communicates the first pump line L1 with the bottom line L7 and communicates the rod line L8 with the return line L11 through the directional control valve 41 in the bypass position J. The hoist lowering position O is a spool position that communicates the first pump line L1 with the rod line L8 and communicates the bottom line L7 with the return line L11 through the directional control valve 41 in the bypass position J. The hoist holding position P is a spool position that blocks the supply and discharge of hydraulic oil to and from the rod chamber and the bottom chamber of the hoist cylinder 7 and bypasses the first pump line L1 to the return line L11.

[0053] The relief valve 44 is a safety valve that discharges the hydraulic oil in the first pump line L1 to the hydraulic oil tank 21 when the pressure of the hydraulic oil in the first pump line L1 exceeds a threshold value. The relief valve 45 is a safety valve that discharges the hydraulic oil in the accumulator line L9 to the hydraulic oil tank 21 when the pressure of the hydraulic oil in the accumulator line L9 exceeds a threshold value.

[0054] By switching the directional control valve 41 to the accumulation position H, the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is accumulated in the accumulator 24 through the accumulation line L9. This is an example of the accumulation position of the control valve 25. Furthermore, by switching the directional control valve 41 to the return position I, the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is bypassed to the directional control valves 42, 43, and the hydraulic oil discharged from the accumulator 24 is returned to the hydraulic oil tank 21 through the return line L11. Furthermore, by switching the directional control valve 41 to the bypass position J, the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is bypassed to the directional control valves 42, 43.

[0055] By switching the directional control valve 41 to the bypass position J and switching the directional control valves 42, 43 to the hoist raising positions K, N, the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is supplied to the bottom chamber of the hoist cylinder 7 and the hydraulic oil is discharged from the rod chamber of the hoist cylinder 7. This causes the hoist cylinder 7 to extend and the platform 5 to change position from the seated position to the release position (i.e., stand up). This is an example of the hoist position of the control valve 25.

[0056] By switching the directional control valve 41 to the bypass position J, the directional control valve 42 to the hoist holding position M, and the directional control valve 43 to the hoist lowering position O, the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is supplied to the rod chamber of the hoist cylinder 7, and the hydraulic oil is discharged from the bottom chamber of the hoist cylinder 7. This causes the hoist cylinder 7 to contract, and the platform 5 changes position from the release position to the seated position (i.e., falls down). This is another example of the hoist position of the control valve 25.

[0057] The directional control valve 41 is switched to the bypass position J, the directional control valve 42 is switched to the hoist floating position L, and the directional control valve 43 is switched to the hoist holding position P, thereby allowing the hydraulic oil to be discharged from the bottom chamber of the hoist cylinder 7. As a result, the hoist cylinder 7 contracts due to the weight of the platform 5, and the platform 5 changes its position from the release position to the seated position (i.e., falls over).

[0058] By switching the directional control valve 41 to the bypass position J and switching the directional control valves 42, 43 to the hoist holding positions M, P, the supply and discharge of hydraulic oil to the hoist cylinder 7 is blocked, and the hydraulic oil supplied from the first hydraulic pump 22 through the first pump line L1 is returned to the hydraulic oil tank 21. This is an example of the center bypass position of the control valve 25.

[0059] In addition, the CV line L6 is connected to the control valve 25 downstream of the directional control valve 41 in the flow of hydraulic oil. That is, the hydraulic oil supplied from the second hydraulic pump 23 through the switching valve 26 at the hoist position B is supplied to the directional control valves 42, 43 without passing through the directional control valve 41. That is, the control valve 25 can extend and retract the hoist cylinder 7 with the hydraulic oil supplied from the second hydraulic pump 23 while the hydraulic oil supplied from the first hydraulic pump 22 is being accumulated in the accumulator 24.

[0060] By switching the switching valve 26 to the hoist position B, switching the directional control valve 41 to the pressure accumulation position H, and switching the directional control valves 42, 43 to the hoist raising positions K, N, the hydraulic oil supplied from the first hydraulic pump 22 is accumulated in the accumulator 24, and the hydraulic oil supplied from the second hydraulic pump 23 through the switching valve 26 at the hoist position B is supplied to the bottom chamber of the hoist cylinder 7, and the hydraulic oil is discharged from the rod chamber of the hoist cylinder 7. This is an example of the pressure accumulation & hoist position of the control valve 25.

[0061] The switching valve 26 is switched to the hoist position B, the directional control valve 41 is switched to the pressure accumulation position H, the directional control valve 42 is switched to the hoist holding position M, and the directional control valve 43 is switched to the hoist lowering position O, whereby the hydraulic oil supplied from the first hydraulic pump 22 is accumulated in the accumulator 24 and supplied to the rod chamber of the hoist cylinder 7 supplied from the second hydraulic pump 23 through the switching valve 26 in the hoist position B, and the hydraulic oil is discharged from the bottom chamber of the hoist cylinder 7. This is another example of the pressure accumulation & hoist position of the control valve 25.

[0062] FIG. 4 is a hardware configuration diagram of the dump truck 1. The dump truck 1 includes a controller 50. The controller 50 includes a central processing unit (CPU) 51 and a memory 52. ​​The memory 52 is configured, for example, with a read only memory (ROM), a random access memory (RAM), a hard disk drive (HDD), or a combination of these. The controller 50 realizes processing described below by the CPU 51 reading and executing program code stored in the ROM or HDD. The RAM is used as a work area when the CPU 51 executes a program.

[0063] However, the specific configuration of the controller 50 is not limited to this, and may be realized by hardware such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0064] The controller 50 controls the overall operation of the dump truck 1. More specifically, the controller 50 controls the engine 10, the first hydraulic pump 22, the second hydraulic pump 23, the control valve 25, the switching valve 26, the first on-off valve 29, and the second on-off valve 30 in accordance with an operation signal output from the operating device 9 and a pressure signal output from the pressure sensor 37.

[0065] The controller 50 controls the torque of the travel motor according to the depression amount of the accelerator pedal, and controls the rotation direction of the travel motor according to the position of the travel lever. The controller 50 also brakes the dump truck 1 according to the depression amount of the hydraulic brake pedal. The controller 50 also changes the position of the loading platform 5 by extending and retracting the hoist cylinder 7 according to the operation of the hoist lever. The processing when the hoist lever is operated will be described later with reference to FIG. 5.

[0066] Furthermore, in response to the hydraulic pressure in the accumulator line L9 detected by the pressure sensor 37 becoming lower than the threshold value, the controller 50 switches the control valve 25 to the accumulator position (more specifically, the directional control valve 41 to the accumulator position H) to accumulate the hydraulic oil pumped from the first hydraulic pump 22 in the accumulator 24. This process is repeatedly executed irregularly (for example, several times per minute) while the engine 10 is driven (more typically, while the dump truck 1 is traveling).

[0067] Fig. 5 is a flowchart of the bed hoisting process. Fig. 6 is a diagram showing a combination of positions of the control valve 25 during the bed hoisting process. The controller 50 executes the bed hoisting process shown in Fig. 5 in response to the operation of the hoist lever. At the start of the bed hoisting process (typically, when the dump truck 1 is traveling), the engine 10 is running, the first hydraulic pump 22 and the second hydraulic pump 23 are rotating at a predetermined tilt, the control valve 25 is in the accumulation position or the center bypass position, the switching valve 26 is in the fan position A, the first opening / closing valve 29 is in the closed position F, and the second opening / closing valve 30 is in the closed position D.

[0068] When the dump truck 1 is traveling, the hydraulic circuit 20 is maintained in the above-mentioned state. As a result, the fan motor 14 is rotated by the hydraulic oil supplied from the second hydraulic pump 23, and cooling air is supplied from the cooling fan 13 to the radiator 11 and the oil cooler 12. The tilt of the second hydraulic pump 23 is appropriately adjusted according to the temperature of the coolant passing through the radiator 11 or the temperature of the hydraulic oil passing through the oil cooler 12.

[0069] Furthermore, the controller 50 accumulates the hydraulic oil pumped from the first hydraulic pump 22 in the accumulator 24 at the timing when the hydraulic pressure in the accumulator line L9 detected by the pressure sensor 37 falls below the threshold value. Furthermore, the operator stops the dump truck 1 before raising or lowering the bed 5. In other words, the operator operates the hoist lever with the dump truck 1 stopped. Therefore, the control valve 25 may be in the accumulation position or the center bypass position at the timing when the hoist lever is operated.

[0070] First, the controller 50 determines the operation direction of the hoist lever (S11 & S12). More specifically, the controller 50 determines whether the hoist lever is operated in a direction to raise the platform 5 (i.e., platform raising), whether the hoist lever is operated in a direction to lower the platform 5 (i.e., platform lowering), or in any other direction.

[0071] Then, when it is determined that the loading platform is to be raised (S11: Yes), the controller 50 switches the directional control valve 41 to the bypass position J, and switches the directional control valves 42, 43 to the hoist raising positions K, N (S13), as in the first pattern of Fig. 6. That is, if hydraulic oil is being stored in the accumulator 24 at the timing when the hoist lever is operated (that is, if the directional control valve 41 is in the storage position H), the storage in the accumulator 24 is interrupted, and the loading platform is given priority. On the other hand, if the directional control valve 41 is already in the bypass position J, the switching of the directional control valve 41 is omitted. Then, the controller 50 executes the processes from step S16 onwards.

[0072] Furthermore, when the controller 50 determines that the bed is being lowered (S11: No & S12: Yes), it switches the directional control valve 42 to the hoist holding position M and switches the directional control valve 43 to the hoist lowering position O (S14) while maintaining the directional control valve 41 in the current position, as in the second pattern of Fig. 6. As a result, if the directional control valve 41 is in the pressure accumulation position H, pressure accumulation in the accumulator 24 continues. Then, the controller 50 executes the processes from step S16 onwards.

[0073] Furthermore, if the controller 50 determines that the lever operation is other than "bed raising" or "bed lowering" (S11: No & S12: No), it carries out control processing of the control valve 25 etc. according to the lever operation position (S15), and terminates the bed raising and lowering processing without executing the processing from step S16 onwards.

[0074] More specifically, when the controller 50 determines that the lever operation is "platform float", it switches the directional control valve 42 to hoist float position L and switches the directional control valve 43 to hoist hold position P while maintaining the directional control valve 41 in its current position, as in the third pattern of Fig. 6. On the other hand, when the controller 50 determines that the lever operation is "platform hold", it switches the directional control valve 42 to hoist hold position M and switches the directional control valve 43 to hoist hold position P while maintaining the directional control valve 41 in its current position, as in the fourth pattern of Fig. 6.

[0075] Next, the controller 50 switches the first on-off valve 29 to the open position G (S16). As a result, the hydraulic oil stored in the accumulator 24 is reduced to the second pressure by the pressure reducing valve 34 and passes through the first on-off valve 29 in the open position G. Next, the controller 50 switches the second on-off valve 30 to the open position E (S17). As a result, the hydraulic oil discharged from the control valve 25 bypasses the first back pressure maintenance valve 27 and flows back to the hydraulic oil tank 21. Note that the processing of steps S16 to S17 is executed in an extremely short time, although the order in FIG. 5 is important.

[0076] Next, the controller 50 minimizes the tilt of the second hydraulic pump 23 (S18). This minimizes the amount of hydraulic oil supplied from the second hydraulic pump 23 to the fan motor 14 through the switching valve 26 in the fan position A, but because the hydraulic oil that has passed through the first on-off valve 29 in the open position G is supplied to the supply line L3 through the third check valve 33, the hydraulic pressure at the inlet of the fan motor 14 is maintained at the second pressure, which is higher than the minimum pressure. As a result, cavitation due to a drop in the hydraulic pressure at the inlet of the fan motor 14 is prevented.

[0077] Next, the controller 50 switches the switching valve 26 to the hoist position B (S19). Next, the controller 50 increases the tilt of the second hydraulic pump 23 to a predetermined tilt (S20). As a result, the hydraulic oil pumped from the second hydraulic pump 23 is supplied to the hoist cylinder 7 through the switching valve 26 at the hoist position B and the control valve 25 at the pressure accumulation & hoist position. In addition, the hydraulic oil discharged from the hoist cylinder 7 flows back to the hydraulic oil tank 21 through the control valve 25 and the second opening / closing valve 30 at the open position E. As a result, the hydraulic oil discharged from the hoist cylinder 7 flows back to the hydraulic oil tank 21 without being resisted by the first back pressure retention valve 27, so that the loading platform 5 is smoothly raised and lowered.

[0078] In addition, in step S20 when the hoist lever is operated in a direction instructing the bed 5 to be raised, the controller 50 maximizes the tilt of the second hydraulic pump 23. On the other hand, in step S20 when the hoist lever is operated in a direction instructing the bed 5 to be lowered, the controller 50 maximizes the tilt of the second hydraulic pump 23 to a predetermined value smaller than the maximum.

[0079] The controller 50 maintains the states of steps S16 to S17 and S19 to S20 until the operation of the hoist lever is completed (S21: No). Then, when the operation of the hoist lever is completed (S21: Yes), the tilt of the second hydraulic pump 23 is minimized (S22). The same applies when the platform 5 is raised or lowered to the end of the movable range. The completion of the platform raising / lowering operation in step S21 may be determined based on the platform angle, etc., other than the lever operation.

[0080] Next, the controller 50 switches the control valve 25 to the bypass position (i.e., the directional control valve 41 to the bypass position J, the directional control valve 42 to the hoist hold position M, and the directional control valve 43 to the hoist hold position P) as in the fifth pattern of FIG. 6 (S23). However, if the pressure accumulation in the accumulator 24 has not ended (i.e., the oil pressure detected by the pressure sensor 37 is still below the threshold), the controller 50 switches the control valve 25 to the pressure accumulation position (i.e., the directional control valve 41 to the pressure accumulation position H, the directional control valve 42 to the hoist hold position M, and the directional control valve 43 to the hoist hold position P) in step S23. This stops the supply and discharge of hydraulic oil to the hoist cylinder 7.

[0081] Next, the controller 50 switches the second on-off valve 30 to the closed position D (S24). As a result, the upstream side of the flow of hydraulic oil from the second on-off valve 30 in the return line L11 and the second make-up line L13 are maintained at the first pressure.

[0082] Next, the controller 50 switches the first on-off valve 29 to the closed position F (S25). This stops the supply of hydraulic oil from the accumulator 24 to the supply line L3 through the first makeup line L10. Meanwhile, hydraulic oil is supplied from the return line L11 to the supply line L3 through the second makeup line L13, so that the hydraulic pressure at the inlet of the fan motor 14 is maintained at the first pressure, which is higher than the minimum pressure.

[0083] Next, the controller 50 switches the switching valve 26 to the fan position A (S26) and ends the bed hoisting process. Then, the controller 50 supplies the required amount of hydraulic oil to the fan motor 14 through the switching valve 26 in the fan position A by adjusting the tilt of the second hydraulic pump 23 according to need (for example, the temperature of the coolant passing through the radiator 11 or the temperature of the hydraulic oil passing through the oil cooler 12).

[0084] Fig. 7 is a time chart of the bed raising and lowering process in a conventional hydraulic circuit. The conventional hydraulic circuit corresponds to, for example, the hydraulic circuit 20 shown in Fig. 2 from which the first on-off valve 29, the second on-off valve 30, the third check valve 33, the pressure reducing valve 34, the first make-up line L10, and the bypass line L12 have been removed. Although Fig. 7 describes the process of raising the bed 5, the same process can be applied to the case of lowering the bed 5.

[0085] In the conventional hydraulic system, steps S16 to S17 in FIG. 5 are omitted, and step S18 is executed after steps S13 / S14. In this case, in order to prevent cavitation at the inlet of the fan motor 14, it is necessary to slowly decelerate the cooling fan 13. Also, in order to enable the supply of hydraulic oil from the return line L11 to the supply line L3 through the second makeup line L13, it is necessary to wait until the directional control valve 41 switches from the pressure accumulation position H to the bypass position J and the tilt of the first hydraulic pump 22 becomes sufficiently large (period A+B). Furthermore, after slowly decreasing the tilt of the second hydraulic pump 23 to the minimum, it takes time for the switching valve 26 to switch from the fan position A to the hoist position B and for the tilt of the second hydraulic pump 23 to become maximum (period H+I+J).

[0086] Fig. 8 is a time chart of the bed raising / lowering process in the hydraulic circuit 20 according to this embodiment. Although the process of raising the bed 5 will be described in Fig. 8, the process of lowering the bed 5 can be similarly considered.

[0087] In the hydraulic circuit 20 according to this embodiment, the first on-off valve 29 is switched to the open position G (S16) prior to step S18, so that the tilt of the second hydraulic pump 23 can be instantly minimized (S18) and the switching valve 26 can be instantly switched from the fan position A to the hoist position B. As a result, the period H+I+J can be shortened compared to the case of FIG. 7. Furthermore, by switching the second on-off valve 30 to the open position E (S17), the back pressure in the return line L11 is minimized, so the operating speed of the hoist cylinder 7 is improved. As a result, the time required to raise and lower the platform 5 is shortened.

[0088] The controller 50 may switch the spool positions of the solenoid valves (26, 29, 30, 41-43) instantly or slowly. For example, the controller 50 may switch the spool positions instantly when responsiveness to an operator's operation is required, and may switch slowly when responsiveness has a low priority.

[0089] According to the above embodiment, for example, the following advantageous effects are achieved.

[0090] According to the above embodiment, the first makeup line L10 is newly provided from the accumulator 24 to the supply line L3, and the first on-off valve 29 is provided to open and close the first makeup line L10. As a result, if the first on-off valve 29 is switched to the open position G in advance, it is possible to prevent the inlet of the fan motor 14 from falling below the minimum pressure and causing cavitation when the switching valve 26 is switched from the fan position A to the hoist position B. Also, as explained by comparing Figures 7 and 8, the time required to raise and lower the loading platform 5 can be shortened.

[0091] Furthermore, according to the above embodiment, by providing the first back pressure retention valve 27, the first check valve 31, and the second makeup line L13, when the bed 5 is not being raised or lowered (i.e., when the switching valve 26 is in the fan position A), the first opening / closing valve 29 can be switched to the closed position F. This makes it possible to suppress the outflow of hydraulic oil from the accumulator 24. However, from the viewpoint of suppressing cavitation when the switching valve 26 is switched from the fan position A to the hoist position B, the first back pressure retention valve 27, the first check valve 31, and the second makeup line L13 can be omitted.

[0092] Furthermore, according to the above embodiment, by providing the second on-off valve 30 and the bypass line L12, the back pressure in the return line L11 can be reduced when the loading platform 5 is raised or lowered. This makes the loading platform 5 rise or lower more smoothly. However, from the viewpoint of suppressing cavitation when the switching valve 26 is switched from the fan position A to the hoist position B, the second on-off valve 30 and the bypass line L12 can be omitted.

[0093] In the above embodiment, by providing the second back pressure maintenance valve 28, the second check valve 32, and the loop line L5, it is possible to utilize the hydraulic oil returning to the hydraulic oil tank 21 to suppress a drop in the hydraulic pressure at the inlet of the fan motor 14. From the viewpoint of suppressing cavitation when the switching valve 26 is switched from the fan position A to the hoist position B, the second back pressure maintenance valve 28, the second check valve 32, and the loop line L5 can be omitted.

[0094] Furthermore, in the above embodiment, the third check valve 33 and the pressure reducing valve 34 are provided in the first makeup line L10, and the inlet pressure of the fan motor 14 is set to be less than the adjustment pressure of the pressure reducing valve 34 and the relief pressure of the second back pressure maintenance valve 28, so that the outflow of hydraulic oil from the accumulator 24 to the supply line L3 can be suppressed. More specifically, if the inlet pressure of the fan motor 14 becomes smaller than the adjustment pressure of the pressure reducing valve 34, the third check valve 33 opens, and makeup is possible at the inlet of the fan motor 14. On the other hand, the second back pressure maintenance valve 28 does not open, so that it is possible to prevent the hydraulic oil stored in the accumulator 24 from flowing out more than necessary due to excessive makeup. However, from the viewpoint of suppressing cavitation when the switching valve 26 is switched from the fan position A to the hoist position B, the third check valve 33 and the pressure reducing valve 34 can be omitted.

[0095] The above-described embodiments are illustrative examples of the present invention, and are not intended to limit the scope of the present invention to these embodiments. Those skilled in the art can implement the present invention in various other forms without departing from the gist of the present invention. [Explanation of symbols]

[0096] 1: Dump truck 2: Body frame 3L, 3R: Front tires 4L, 4R: Rear tires 5: Cargo platform 6: Cab 7: Hoist cylinder 8: Hinge pin 9: Operating device 10: Engine 11: Radiator 12: Oil cooler 13: Cooling fan 14: Fan motor 15: Hydraulic auxiliary equipment 20: Hydraulic circuit 21: Hydraulic oil tank 22: First hydraulic pump 23: Second hydraulic pump 24: Accumulator 25: Control valve 26: Switching valve 27: First back pressure maintaining valve 28: Second back pressure maintaining valve 29: First opening and closing valve 30: Second opening and closing valve 31: First check valve 32: Second check valve 33: Third check valve 34: Pressure reducing valve 35, 36, 44, 45: Relief valve 37: Pressure sensor 41, 42, 43: Directional valve 50: Controller 51: CPU 52: Memory 60: Rear axle L1: First pump line L2: Second pump line L3: Supply line L4: Discharge line L5: Loop line L6: CV line L7: Bottom Line L8: Rod line L9: Accumulator line L10: 1st Makeup Line L11: Reflux line L12: Bypass line L13: 2nd Makeup Line

Claims

1. The car body and A loading platform supported on the vehicle body in a foldable manner; A hoist cylinder to which hydraulic oil is supplied to raise and lower the platform; a fan motor that receives hydraulic oil through a supply line and generates a driving force for rotating a cooling fan; A dump truck including a hydraulic circuit for supplying hydraulic oil to the hoist cylinder and the fan motor, The hydraulic circuit includes: A hydraulic oil tank for storing hydraulic oil; A first hydraulic pump and a second hydraulic pump that pump out hydraulic oil stored in the hydraulic oil tank; an accumulator that accumulates hydraulic oil; a control valve that can be switched between a hoist position in which the hydraulic oil pumped from one or both of the first hydraulic pump and the second hydraulic pump is supplied to the hoist cylinder, a pressure accumulation position in which the hydraulic oil is accumulated in the accumulator, and a center bypass position in which the hydraulic oil is returned to the hydraulic oil tank; A switching valve that can be switched between a fan position in which the hydraulic oil pumped from the second hydraulic pump is supplied to the fan motor through the supply line and a hoist position in which the hydraulic oil is supplied to the hoist cylinder; a first make-up line extending from the accumulator to the supply line; a first on-off valve switchable between an open position for opening the first makeup line to supply hydraulic oil from the accumulator to the fan motor, and a closed position for closing the first makeup line.

2. The dump truck according to claim 1, The hydraulic circuit includes: a first back pressure maintaining valve arranged in a return line extending from the control valve to the hydraulic oil tank, for maintaining the hydraulic pressure of the hydraulic oil discharged from the control valve at a first pressure higher than a minimum pressure required for the fan motor to function as a hydraulic motor; a first check valve disposed in a second makeup line that branches off from the upstream side of the hydraulic oil flow of the first back pressure maintenance valve of the return line to the supply line, and allows the hydraulic oil to flow from the return line to the supply line and blocks the flow of hydraulic oil from the supply line to the return line.

3. The dump truck according to claim 2, The hydraulic circuit is characterized in that it includes a second opening / closing valve that can be switched between an open position that opens a bypass line that connects the upstream and downstream sides of the flow of hydraulic oil from the first back pressure maintenance valve of the return line and a closed position that closes the bypass line.

4. The dump truck according to claim 1, The hydraulic circuit includes: a second back pressure maintaining valve arranged in a discharge line that returns the hydraulic oil discharged from the fan motor to the hydraulic oil tank, and that maintains the hydraulic pressure of the hydraulic oil discharged from the fan motor at a second pressure higher than a minimum pressure required for the fan motor to function as a hydraulic motor; a second check valve arranged in a loop line that branches off from the upstream side of the flow of hydraulic oil from the second back pressure retention valve of the discharge line to the supply line, and allows the flow of hydraulic oil from the discharge line to the supply line and blocks the flow of hydraulic oil from the supply line to the discharge line.

5. The dump truck according to claim 4, The hydraulic circuit includes: a pressure reducing valve disposed upstream of the first on-off valve in the first makeup line in the flow of hydraulic oil, the pressure reducing valve reducing the hydraulic oil stored in the accumulator to a third pressure that is higher than the minimum pressure and lower than the second pressure; a third check valve disposed downstream of the first on-off valve in the first makeup line in the flow of hydraulic oil to allow a flow of hydraulic oil from the first on-off valve to the supply line and to block the flow of hydraulic oil from the supply line to the first on-off valve.

6. The dump truck according to claim 1, the control valve is capable of storing hydraulic oil pressurized from the first hydraulic pump in the accumulator, and switching to a storage & hoist position in which the control valve in a hoist position supplies hydraulic oil pressurized from the second hydraulic pump to the hoist cylinder.

7. The dump truck according to claim 3, An operating device that is operated to instruct the raising and lowering of the loading platform; a controller for controlling the hydraulic circuit in accordance with an operation of the operating device, The controller: When the operating device is operated in a direction to raise the loading platform or in a direction to lower the loading platform, the first opening / closing valve and the second opening / closing valve are switched to an open position and the switching valve is switched to a hoist position; A dump truck characterized in that, when operation of the operating device is completed or when the loading platform is raised or lowered to the end of its movable range, the first opening / closing valve and the second opening / closing valve are switched to a closed position and the switching valve is switched to the fan position.

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