Dump truck

Abstract

To provide a dump truck which enables shortening of lodging time at a load-carrying platform while preventing cavitation from occurring at a fan motor.SOLUTION: This dump truck comprises: a control valve switchable among a load-carrying platform lifting position for supplying a working fluid directed to erect a load-carrying platform at a hoist cylinder, a load-carrying platform suspension position for supplying a working fluid directed to lodge the load-carrying platform at the hoist cylinder, and a floating position for returning the working fluid discharged from the hoist cylinder when the load-carrying platform lodges by self-weight through a return line to a working fluid tank; a first opening / closing valve capable of switching a bypass line for connecting the upstream side and a downstream side of the working fluid of a back pressure holding valve of the return line between an opening position for opening and a closing position for closing; and a controller for switching the first opening / closing valve from the closing position to the opening position and then switching the control valve to the floating position when a float-down instruction for causing the load-carrying platform to lodge by self-weight is received.SELECTED DRAWING: Figure 5

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, dump trucks have been known that include 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 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 is equipped with a switching valve that can switch the hydraulic oil discharged from the hydraulic pump between a state in which it 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 or lower the loading platform, it is necessary to prevent negative pressure at the fan motor inlet, which can cause cavitation. Therefore, the pump discharge flow rate is slowly reduced and the fan motor rotation speed is sufficiently reduced before the switching valve is switched. As a result, it takes time to raise or lower the loading platform, which could worsen the cycle time at the customer's site. Therefore, the challenge is to prevent cavitation in the fan motor and shorten the loading platform raising or lowering time (especially the time it takes to lower the loading platform).

[0006] The present invention has been made in consideration of the above-described circumstances, and its object is to provide a dump truck that can reduce the time it takes for the loading platform to collapse while preventing cavitation from occurring in the fan motor. [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, a hydraulic circuit that supplies hydraulic oil to the hoist cylinder and the fan motor, and a controller that controls the hydraulic circuit, The hydraulic circuit includes a hydraulic oil tank that stores hydraulic oil, a hydraulic pump that discharges the hydraulic oil stored in the hydraulic oil tank, a control valve that can be switched to a platform-raising position that supplies hydraulic oil to the hoist cylinder in a direction that raises the platform, a platform-lowering position that supplies hydraulic oil to the hoist cylinder in a direction that lowers the platform, and a float position that returns hydraulic oil discharged from the hoist cylinder to the hydraulic oil tank through a return line when the platform collapses under its own weight, and a switching valve that can be switched to a fan position that supplies hydraulic oil discharged from the hydraulic pump to the fan motor through the supply line, and a hoist position that supplies hydraulic oil to the hoist cylinder. a back pressure maintaining valve disposed in the return line to maintain the hydraulic pressure of the hydraulic oil discharged from the hoist cylinder at a pressure higher than the minimum pressure required for the fan motor to function as a hydraulic motor; a check valve disposed in a first makeup line that branches off from the return line upstream of the back pressure maintenance valve in the flow of hydraulic oil and leads to the supply line, the check valve allowing the flow of hydraulic oil from the return line to the supply line and blocking the flow of hydraulic oil from the supply line to the return line; The bypass line connecting the upstream and downstream sides of the flow of hydraulic oil from the back pressure maintaining valve of the return line is provided with a first opening / closing valve that can be switched between an open position to open it and a closed position to close it, and when the controller receives a float down command to cause the loading platform to tip over under its own weight, it switches the first opening / closing valve from the closed position to the open position and then switches the control valve to the float position. [Effects of the Invention]

[0008] According to the present invention, it is possible to reduce the time it takes to collapse a loading platform while preventing cavitation from occurring in the fan motor. Note that other problems, configurations, and effects will become clear from the following description of the embodiments. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a side view of a dump truck according to an embodiment of the present invention. [Figure 2] FIG. 2 is a circuit diagram of a drive circuit mounted on a dump truck. [Figure 3] FIG. 2 is a layout diagram of hydraulic components that constitute the control valve. [Figure 4] FIG. 2 is a hardware configuration diagram of the dump truck. [Figure 5] 10 is a flowchart of an operation control process. [Figure 6] 10 is a flowchart of a loading platform lowering start process. [Figure 7] 10 is a flowchart of a loading platform lowering completion process. [Figure 8] 1 is a timing chart of float down in a conventional hydraulic circuit. [Figure 9] 4 is a timing chart of float down in the hydraulic circuit of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[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. Note that, unless otherwise specified, front, back, left, and right in this specification are based on the viewpoint of an operator who is on board and operating the dump truck 1.

[0011] As shown in Figure 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 body axles with respect to suspension arms supported swingably on the body frame 2 and the left and right body axles, a pair of rear tires 4L, 4R rotatably supported via rear axles 60 on both the left and right ends of the rear of the body frame 2, a loading platform 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 in response to steering operation by the operator. Meanwhile, the pair of rear tires 4L, 4R are drive wheels that rotate when a driving force from a travel motor (not shown) is transmitted to them. The dump truck 1 is equipped with a pair of left and right travel motors in order to transmit 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 (for example, 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 body frame 2 as the hoist cylinder 7 extends and retracts. One end of the hoist cylinder 7 is connected to the body frame 2, and the other end is connected to the loading platform 5, and the hoist cylinder 7 extends and retracts by receiving hydraulic oil from the first hydraulic pump 22 or the second hydraulic pump 23 (see Figure 2). When the hoist cylinder 7 extends, the loading platform 5 stands up and assumes a release posture (discharge posture), and when the hoist cylinder 7 retracts, 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 cargo is released to the rear of the dump truck 1. The seating position is the position of the loading platform 5 when it is seated on the vehicle body frame 2 and a cargo is loaded. Note that the cargo loaded on the loading platform 5 is not limited to earth and sand, but since earth and sand is a typical example of a cargo, the following description will be given assuming that the loading platform 5 is loaded with earth 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 in response 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 traveling motor to function as a regenerative brake, and a hydraulic brake pedal that drives a hydraulic brake. The steering 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.

[0018] The hoist lever is an operating device that is operated to command the elevation (seating position, earth-discharging position) of the platform 5. The hoist lever receives, for example, a platform-raising command, a platform-lowering command, and a float-down command from the operator. The platform-raising command is a command to supply hydraulic oil to the bottom chamber of the hoist cylinder 7 and discharge hydraulic oil from the rod chamber of the hoist cylinder 7 to raise the platform 5. The platform-lowering command is a command to supply hydraulic oil to the rod chamber of the hoist cylinder 7 and discharge hydraulic oil from the bottom chamber of the hoist cylinder 7 to actively lower the platform 5. The float-down command is a command to connect both the bottom chamber and the rod chamber of the hoist cylinder 7 to the hydraulic oil tank, allowing the hoist cylinder 7 to extend and retract. The hoist cylinder 7 then contracts due to the weight of the platform 5 (approximately 20 tons) and the hoist cylinder 7 (approximately 500 kg), resulting in the platform 5 lowering.

[0019] 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.

[0020] The engine 10 generates driving force for operating the dump truck 1. The driving force of the engine 10 generates electric power in a generator (not shown), which is then supplied to a travel motor, causing the dump truck 1 to travel. In addition, the driving force of the engine 10 rotates a first hydraulic pump 22 and a second hydraulic pump 23, which will be described later.

[0021] 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 through 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 rotating when the driving force of the fan motor 14 is transmitted to it. The fan motor 14 is supplied with hydraulic oil discharged from the second hydraulic pump 23 and generates driving force to rotate the cooling fan 13. The hydraulic accessories 15 are a general term for hydraulic equipment that operates by being supplied with hydraulic oil.

[0022] 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 maintenance valve 27, a second back pressure maintenance 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.

[0023] The hydraulic circuit 20 also includes, as main oil passages through which 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 return line L10, a bypass line L11, a first makeup line L12, and a second makeup line L13. The lines L1 to L13 are formed, for example, by combining metal piping and flexible hoses.

[0024] The first pump line L1 is an oil passage that runs 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 discharged from the first hydraulic pump 22 to the control valve 25. The second pump line L2 is an oil passage that runs 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 discharged from the second hydraulic pump 23 to the switching valve 26.

[0025] The supply line L3 is an oil passage that runs 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 discharged from the second hydraulic pump 23 to the fan motor 14. The discharge line L4 is an oil passage that runs 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 upstream of the second back pressure maintenance valve 28 in the flow of hydraulic oil, passes through the second check valve 32, and runs to the supply line L3.

[0026] 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 discharged from the second hydraulic pump 23 to the control valve 25.

[0027] The bottom line L7 is an oil passage that leads 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 that supplies and discharges hydraulic oil between the control valve 25 and the bottom chamber of the hoist cylinder 7. The rod line L8 is an oil passage that leads 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 that supplies and discharges hydraulic oil between the control valve 25 and the rod chamber of the hoist cylinder 7.

[0028] The pressure accumulation line L9 is an oil passage that runs from the control valve 25 to the accumulator 24. In other words, the pressure accumulation line L9 is an oil passage that accumulates the hydraulic oil supplied from the control valve 25 in the accumulator. The second makeup line L13 is an oil passage that runs from the accumulator 24 to the supply line L3 via the pressure reducing valve 34, the second on-off valve 30, and the third check valve 33. In other words, the second makeup line L13 is an oil passage that supplies the hydraulic oil accumulated in the accumulator 24 to the supply line L3.

[0029] The return line L10 is an oil passage that runs from the control valve 25 to the hydraulic oil tank 21 via the first back pressure maintenance valve 27. In other words, the return line L10 is an oil passage that returns the hydraulic oil discharged from the control valve 25 to the hydraulic oil tank 21. The bypass line L11 is an oil passage that connects the upstream and downstream sides of the return line L10 from the first back pressure maintenance valve 27 in the hydraulic oil flow direction. In other words, the bypass line L11 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 maintenance valve 27. The first makeup line L12 is an oil passage that branches off from the return line L10 from the upstream side of the first back pressure maintenance valve 27 in the hydraulic oil flow direction, passes through the first check valve 31, and leads to the supply line L3. In other words, the first makeup line L12 is an oil passage that supplies the hydraulic oil passing through the return line L10 to the supply line L3.

[0030] The hydraulic oil tank 21 stores hydraulic oil to be supplied to and discharged from hydraulic equipment (for example, the hoist cylinder 7, the fan motor 14, the hydraulic accessories 15, the accumulator 24, etc.). The hydraulic oil tank 21 is preferably positioned 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.

[0031] 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 discharging 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 discharging 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 whose pump displacement per rotation can be changed under the control of the controller 50.

[0032] The accumulator 24 is connected to the pressure accumulation line L9. The accumulator 24 compresses and accumulates (i.e., accumulates pressure) the hydraulic oil discharged 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. Furthermore, the accumulator 24 supplies the accumulated hydraulic oil to the supply line L3 through the second make-up line L13.

[0033] 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 accumulator line L9, and the return line L10. The control valve 25 supplies hydraulic oil discharged from the first hydraulic pump 22 to the hoist cylinder 7, the accumulator 24, and the hydraulic oil tank 21. The control valve 25 is an electromagnetic valve configured to be switchable between a platform raising position, a platform lowering position, a float position, an accumulator position, and a center bypass position under the control of the controller 50, for example. Note that the first pump line L1 and the CV line L6 may be joined and then connected to the control valve 25, and hydraulic oil discharged 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.

[0034] The platform raising position is a spool position that supplies hydraulic oil discharged from one or both of the first hydraulic pump 22 and the second hydraulic pump 23 to the hoist cylinder 7 in a direction that raises the platform 5. More specifically, the platform raising position is a spool position that raises the platform 5 by supplying hydraulic oil to the bottom chamber of the hoist cylinder 7 through the bottom line L7 and returning hydraulic oil discharged from the rod chamber of the hoist cylinder 7 to the hydraulic oil tank 21 through the rod line L8 and the return line L10.

[0035] The platform lowering position is a spool position that supplies hydraulic oil discharged from one or both of the first hydraulic pump 22 and the second hydraulic pump 23 to the hoist cylinder 7 in a direction that tilts the platform 5. More specifically, the platform lowering position is a spool position that actively tilts the platform 5 by supplying hydraulic oil to the rod chamber of the hoist cylinder 7 through the rod line L8 and returning hydraulic oil discharged from the bottom chamber of the hoist cylinder 7 to the hydraulic oil tank 21 through the bottom line L7 and the return line L10.

[0036] The float position is a spool position that returns hydraulic oil discharged from the hoist cylinder 7 when the platform 5 collapses under its own weight to the hydraulic oil tank 21 through the return line L10. More specifically, the float position is a spool position that allows the hydraulic oil to be freely supplied and discharged to the hoist cylinder 7, thereby returning hydraulic oil discharged from the bottom chamber under its own weight to the hydraulic oil tank 21 through the bottom line L7 and the return line L10, and sucks up hydraulic oil from the hydraulic oil tank 21 into the rod chamber, which has become negative pressure, thereby collapsing the platform 5. In addition, when in the float position, hydraulic oil discharged from the first hydraulic pump 22 and the second hydraulic pump 23 is not supplied to the hoist cylinder 7.

[0037] 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 discharged from one or both of the first hydraulic pump 22 and the second hydraulic pump 23 to the hydraulic oil tank 21 through the return line L10 without supplying it to the hoist cylinder 7 and the accumulator 24. More specifically, the center bypass position is a spool position that returns hydraulic oil discharged from one or both of the first hydraulic pump 22 and the second hydraulic pump 23 to the hydraulic oil tank 21 while preventing the supply and discharge of hydraulic oil to the hoist cylinder 7 (i.e., extension and contraction of the hoist cylinder 7).

[0038] 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 discharged from the second hydraulic pump 23 to one or both of the fan motor 14 and the control valve 25. In other words, the switching valve 26 switches the supply destination of the hydraulic oil discharged from the second hydraulic pump 23. 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.

[0039] The fan position A is a spool position that supplies the hydraulic oil discharged from the second hydraulic pump 23 only to the hoist cylinder 7 and the fan motor 14 out of the fan motor 14. The hoist position B is a spool position that supplies the hydraulic oil discharged from the second hydraulic pump 23 only to the hoist cylinder 7 out of the hoist cylinder 7 and the fan motor 14. The distribution position C is a spool position that supplies the hydraulic oil discharged from the second hydraulic pump 23 to both the fan motor 14 and the hoist cylinder 7.

[0040] The first back pressure maintenance valve 27 is disposed in the return line L10. The first back pressure maintenance valve 27 returns the hydraulic oil in the return line L10 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 return line L10 in the flow of hydraulic oil from the first back pressure maintenance valve 27 and the first makeup line L12) 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.

[0041] The first on-off valve 29 is disposed in the bypass line L11. The first on-off valve 29 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 L11. The open position E is a spool position that opens the bypass line L11.

[0042] The first check valve 31 is disposed in the first makeup line L12. The first check valve 31 allows the flow of hydraulic oil from the return line L10 to the supply line L3 and blocks the flow of hydraulic oil from the supply line L3 to the return line L10.

[0043] When the first on-off valve 29 is switched to the closed position D, the hydraulic pressure in the first makeup line L12 and upstream of the first back pressure maintenance valve 27 in the return line L10 becomes a first pressure. When the hydraulic pressure in the supply line L3 falls below the first pressure, the hydraulic oil in the return line L10, which is maintained at the first pressure, flows into the supply line L3 through the first makeup line L12. On the other hand, even if the hydraulic pressure in the supply line L3 exceeds the first pressure, the first check valve 31 is shut off, so the hydraulic oil in the supply line L3 does not flow into the return line L10 through the first makeup line L12.

[0044] Furthermore, when the first on-off valve 29 is switched to the open position E, the hydraulic oil in the return line L10 returns to the hydraulic oil tank 21 without passing through the first back pressure maintenance valve 27. In other words, no hydraulic pressure is generated upstream of the first back pressure maintenance valve 27 in the flow of hydraulic oil in the return line L10, and the hydraulic oil discharged from the control valve 25 returns to the hydraulic oil tank 21 without resistance.

[0045] The second back pressure maintenance valve 28 is disposed in the discharge line L4. The second back pressure maintenance valve 28 maintains the hydraulic pressure of the hydraulic oil discharged from the fan motor 14 (i.e., the upstream side of the second back pressure maintenance valve 28 in the discharge line L4 in the hydraulic oil flow and in the loop line L5) at a second 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, while returning the hydraulic oil in the discharge line L4 to the hydraulic oil tank 21. 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.

[0046] 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.

[0047] 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, which is 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.

[0048] The second on-off valve 30 is disposed in the second makeup line L13. The second on-off valve 30 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 second makeup line L13. The open position G is a spool position that opens the second makeup line L13.

[0049] The third check valve 33 is disposed in the second makeup line L13 downstream of the second on-off valve 30 in the flow of hydraulic oil. The third check valve 33 allows the flow of hydraulic oil from the second on-off valve 30 to the supply line L3 and blocks the flow of hydraulic oil from the supply line L3 to the second on-off valve 30.

[0050] The pressure reducing valve 34 is disposed in the second makeup line L13 upstream of the second on-off valve 30 in the flow of hydraulic oil. The pressure reducing valve 34 reduces the pressure of the hydraulic oil accumulated 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.

[0051] When the second on-off valve 30 is switched to the open position G, if the hydraulic pressure in the supply line L3 falls below the third pressure, the hydraulic oil whose pressure has been reduced to the third pressure by the pressure reducing valve 34 passes through the second on-off valve 30 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 second on-off valve 30. Furthermore, when the second on-off valve 30 is switched to the closed position F, the hydraulic oil whose pressure has been reduced to the third pressure by the pressure reducing valve 34 does not pass through the second on-off valve 30.

[0052] 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 indicating the detected hydraulic pressure to the controller 50.

[0053] Fig. 3 is a layout diagram of hydraulic components that make up the control valve 25. The control valve 25 is made up of a plurality of valves. As shown in Fig. 3, the control valve 25 is made up of a plurality of directional control valves 41, 42, and 43, and relief valves 44 and 45. However, the specific configuration of the control valve 25 is not limited to the example shown in Fig. 3.

[0054] The directional control valve 41 is a solenoid 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 hydraulic oil supplied through the first pump line L1 to the accumulator 24 and the hydraulic accessories 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 L10. 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.

[0055] The directional control valve 42 is a solenoid valve configured to be switchable among 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 connects the first pump line L1 to the bottom line L7 and connects the rod line L8 to the return line L10 through the directional control valve 41 in the bypass position J. The hoist floating position L is a spool position that connects the bottom line L7 to the return line L10. The hoist holding position M is a spool position that blocks the supply and discharge of hydraulic oil to the rod chamber and bottom chamber of the hoist cylinder 7 and bypasses the first pump line L1 to the return line L10.

[0056] The directional control valve 43 is a solenoid valve configured to be switchable among 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 L10 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 L10 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 bottom chamber of the hoist cylinder 7 and bypasses the first pump line L1 to the return line L10.

[0057] 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. 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.

[0058] By switching the direction switching 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 direction switching 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 direction switching valves 42, 43, and the hydraulic oil discharged from the accumulator 24 is returned to the hydraulic oil tank 21 through the return line L10. Furthermore, by switching the direction switching 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 direction switching valves 42, 43.

[0059] When the directional control valve 41 is switched to the bypass position J and the directional control valves 42, 43 are switched 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 position of the platform 5 changes from the seated position to the release position (i.e., stands up). This is an example of the platform raising position of the control valve 25.

[0060] When the directional control valve 41 is switched to the bypass position J, 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, 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 position of the platform 5 changes from the release position to the seated position (i.e., it falls over). This is an example of the platform lowering position of the control valve 25.

[0061] The directional control valve 41 is switched to the pressure accumulation position H or 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 connecting both the bottom chamber and the rod chamber of the hoist cylinder 7 to the hydraulic oil tank. As a result, the hoist cylinder 7 contracts due to the weight of the platform 5, and the platform 5 changes position from the release position to the seated position (i.e., falls over). This is an example of the float position of the control valve 25.

[0062] By switching the direction switching valve 41 to the bypass position J and the direction switching 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.

[0063] In addition, the CV line L6 is connected to the control valve 25 downstream of the direction switching 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 direction switching valves 42, 43 without passing through the direction switching 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.

[0064] By switching the switching valve 26 to the hoist position B, switching the direction switching valve 41 to the pressure accumulation position H, and switching the direction switching 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 & platform raising position of the control valve 25.

[0065] By switching the switching valve 26 to the hoist position B, switching the direction switching valve 41 to the pressure accumulation position H, switching the direction switching valve 42 to the hoist holding position M, and switching the direction switching valve 43 to the hoist lowering position O, 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 at the hoist position B, and the hydraulic oil is discharged from the bottom chamber of the hoist cylinder 7. This is an example of the pressure accumulation & platform lowering position of the control valve 25.

[0066] 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 CPU (Central Processing Unit) 51 and a memory 52. ​​The memory 52 is configured, for example, with a ROM (Read Only Memory), a RAM (Random Access Memory), an HDD (Hard Disk Drive), or a combination of these. The controller 50 realizes the processing described below by having the CPU 51 read and execute program code stored in the ROM or HDD. The RAM is used as a work area when the CPU 51 executes the program.

[0067] However, the specific configuration of the controller 50 is not limited to this, and may be realized by hardware such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array).

[0068] 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 operation device 9, a pressure signal output from the pressure sensor 37, a weight signal output from the weight sensor 38, and a seating signal output from the seating sensor 39.

[0069] The controller 50 controls the torque of the travel motor according to the depression amount of the accelerator pedal, and also 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 extends and retracts the hoist cylinder 7 in response to operation of the hoist lever, thereby changing the posture of the bed 5. The processing when the hoist lever is operated will be described later with reference to FIG. 5.

[0070] 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), thereby accumulating the hydraulic oil discharged from the first hydraulic pump 22 in the accumulator 24. This process is executed repeatedly at irregular intervals (for example, several times per minute) while the engine 10 is being driven (more typically, while the dump truck 1 is traveling).

[0071] The weight sensor 38 detects the weight of the cargo loaded on the loading platform 5 (hereinafter referred to as "load weight") and outputs a weight signal indicating the detected weight to the controller 50. The method for identifying the load weight is not particularly limited, but may be determined based on the hydraulic pressure in the rod chamber of the hoist cylinder 7, for example, or may be determined by any other well-known method.

[0072] The seating sensor 39 detects that the loading platform 5 has assumed a seating position and outputs a seating signal to the controller 50. The method for determining that the loading platform 5 has assumed a seating position is not particularly limited, but may be determined, for example, by a touch sensor that comes into contact when the loading platform 5 assumes a seating position, or may be determined based on the angle of the loading platform 5, or may be determined based on the elapsed time since the loading platform 5 began to tip over, or may be determined by any other well-known method.

[0073] Fig. 5 is a flowchart of the operation control process. The operation control process is a process for controlling the operation of the dump truck 1 (more specifically, the hydraulic circuit 20) in response to an operation of the operating device 9 by the operator. The controller 50 repeatedly executes the operation control process shown in Fig. 5 at predetermined time intervals, for example, while the dump truck 1 is being driven (while the engine 10 is being driven). It is assumed that, at the start of the drive control process, the engine 10 is being driven, the first hydraulic pump 22 and the second hydraulic pump 23 are rotating at a predetermined tilt, the control valve 25 is in the float position, the accumulation position, or the center bypass position, the switching valve 26 is in the fan position A, the first on-off valve 29 is in the closed position D, and the second on-off valve 30 is in the closed position F.

[0074] First, the controller 50 controls the rotation of the cooling fan 13 in accordance with a predetermined control logic (S11). For example, the controller 50 adjusts the tilt of the second hydraulic pump 23 so that the volume of cooling air generated by the cooling fan 13 increases (i.e., the rotation speed of the cooling fan 13 increases) as the temperature of the coolant passing through the radiator 11 and the temperature of the hydraulic oil passing through the oil cooler 12 increase. As a result, the hydraulic oil discharged by the second hydraulic pump 23 is supplied to the fan motor 14 through the switching valve 26 in fan position A, causing the cooling fan 13 to rotate.

[0075] Next, the controller 50 waits to execute the processing from step S14 onwards until the operation device 9 is operated by the operator in the cab 6 (S12: No & S13: No). Then, when the controller 50 receives an instruction other than a float-down instruction through the operation device 9 (S12: No & S13: Yes), the controller 50 controls the operation of the dump truck 1 (for example, traveling of the dump truck 1, turning of the dump truck 1, raising of the loading platform 5, etc.) in accordance with the instruction of the operator (S14).

[0076] On the other hand, when the controller 50 receives a float down command through the operation device 9 while the platform 5 is upright (S12: Yes), it executes the processing from step S15 onwards. Note that the position of the platform 5 at the time of receiving the float down command is not limited to the earth-releasing position, but may be any position between the earth-releasing position and the seated position. Also, the control valve 25 may be in the pressure accumulation position, the center bypass position or the float position at the time of receiving the float down command.

[0077] The controller 50 compares the load weight detected by the weight sensor 38 with a predetermined threshold value (S15). The threshold value is set, for example, to a load weight at which the platform 5 should tip over under its own weight while braking with the first back pressure maintenance valve 27. If the controller 50 determines that the load weight is less than the threshold value (S15: Yes), it switches the first on-off valve 29 from the closed position D to the open position E (S16). As a result, the hydraulic oil discharged from the control valve 25 bypasses the first back pressure maintenance valve 27 and returns to the hydraulic oil tank 21. On the other hand, if the controller 50 determines that the load weight is equal to or greater than the threshold value (S15: No), it skips the processing of step S16. As a result, the hydraulic oil discharged from the control valve 25 returns to the hydraulic oil tank 21 through the first back pressure maintenance valve 27.

[0078] That is, when the first on-off valve 29 is set to the open position E, the return resistance of the hydraulic oil returning from the hoist cylinder 7 to the hydraulic oil tank 21 through the control valve 25 and the return line L10 decreases compared to when the first on-off valve 29 is set to the closed position D, so the speed at which the platform 5 is lowered increases. On the other hand, when the first on-off valve 29 is set to the closed position D, the return resistance of the hydraulic oil returning from the hoist cylinder 7 to the hydraulic oil tank 21 increases compared to when the first on-off valve 29 is set to the open position E, so the platform 5 can be lowered while being braked.

[0079] Next, the controller 50 switches the control valve 25 to the float position (i.e., while maintaining the directional control valve 41 at its current position, the directional control valve 42 to the hoist float position L and the directional control valve 43 to the hoist hold position P) (S17). This allows the hydraulic oil to be discharged from the bottom chamber of the hoist cylinder 7. As a result, the platform 5 slowly falls over under its own weight, and the hydraulic oil discharged from the bottom chamber of the hoist cylinder 7 is returned to the hydraulic oil tank 21 through the bottom line L7 and the return line L10. However, if the control valve 25 is already in the float position, the processing of step S17 is skipped.

[0080] Next, the controller 50 waits for the processing from step S19 onwards until a bed lowering command is received through the operation device 9 or until the seating sensor 39 detects that someone is seated on the bed 5 (S18: No & S22: No). Then, if the controller 50 receives a bed lowering command through the operation device 9 before the bed 5 is seated (S22: No & S18: Yes), it executes the bed lowering start processing shown in Fig. 6 (S19). Fig. 6 is a flowchart of the bed lowering start processing.

[0081] First, when the controller 50 determines that the first on-off valve 29 is in the open position E (S31: Yes), it switches the second on-off valve 30 from the closed position F to the open position G (S32). 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 second on-off valve 30, which is in the open position G. On the other hand, when the controller 50 determines that the first on-off valve 29 is in the closed position D (S31: No), hydraulic oil has already been supplied from the return line L10 to the supply line L3 via the first makeup line L12. Therefore, even if the second on-off valve 30 remains in the closed position F, the hydraulic pressure at the inlet of the fan motor 14 is maintained at the first pressure, which is higher than the minimum pressure.

[0082] Next, if pressure is not being accumulated in the accumulator 24 (i.e., the directional control valve 41 is not in the accumulation position H) (S33: No), the controller 50 switches the control valve 25 to the platform lowering position (i.e., 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) (S34). On the other hand, if pressure is being accumulated in the accumulator 24 (i.e., the directional control valve 41 is in the accumulation position H) (S33: Yes), the controller 50 switches the control valve 25 to the accumulation & platform lowering position (i.e., the directional control valve 41 to the accumulation position H, the directional control valve 42 to the hoist holding position M, and the directional control valve 43 to the hoist lowering position O) (S35).

[0083] Next, the controller 50 minimizes the displacement of the second hydraulic pump 23 (S36). 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 fan position A, but because hydraulic oil is supplied to the supply line L3 via the first makeup line L12 or the second makeup line L13, the hydraulic pressure at the inlet of the fan motor 14 is maintained at the first pressure or the second pressure, which is higher than the minimum pressure. As a result, cavitation caused by a drop in the hydraulic pressure at the inlet of the fan motor 14 is prevented.

[0084] Next, the controller 50 switches the switching valve 26 from the fan position A to the hoist position B (S37). Next, the controller 50 increases the tilting of the second hydraulic pump 23 to a predetermined value (S38). As a result, the hydraulic oil discharged 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. In addition, the hydraulic oil discharged from the hoist cylinder 7 is returned to the hydraulic oil tank 21 through the control valve 25 and the return line L10. As a result, the platform 5 is actively tilted down by the force of the hydraulic oil.

[0085] Returning to Fig. 5, the controller 50 waits to execute the processing of step S21 until the instruction to lower the loading platform to the operation device 9 is completed (S20: No). It is assumed that the instruction to lower the loading platform is completed before the loading platform 5 assumes a seating position. Then, when the instruction to lower the loading platform is completed (S20: Yes), the controller 50 executes the loading platform lowering completion processing shown in Fig. 7 (S21). Fig. 7 is a flowchart of the loading platform lowering completion processing.

[0086] First, the controller 50 minimizes the tilt of the second hydraulic pump 23 (S41). Next, the controller 50 switches the control valve 25 to the float position (i.e., while maintaining the directional control valve 41 at its current position, the directional control valve 42 to the hoist float position L and the directional control valve 43 to the hoist hold position P) (S42). Next, the controller 50 switches the switching valve 26 from the hoist position B to the fan position A (S43). As a result, the hydraulic oil discharged from the second hydraulic pump 23 is supplied to the fan motor 14 through the switching valve 26 in the fan position A.

[0087] Next, the controller 50 switches the second on-off valve 30 from the open position G to the closed position F (S44). This stops the supply of hydraulic oil from the accumulator 24 to the supply line L3 through the second makeup line L13. However, because a minimum amount of hydraulic oil is supplied to the fan motor 14 from the second hydraulic pump 23 at the minimum displacement, cavitation of the fan motor 14 is prevented. Furthermore, if the second on-off valve 30 is already in the closed position F, the processing of step S44 is skipped. Furthermore, the controller 50 controls the rotation of the cooling fan 13 in accordance with predetermined control logic (S45).

[0088] 5, the controller 50 waits to execute the processing from step S23 onwards (S22) until the seating sensor 39 detects the seating posture of the bed 5 (S22: No). That is, if a bed lowering command is not input to the operation device 9 (S18: No), the bed 5 changes its posture to the seating posture due to its own weight. On the other hand, if a bed lowering command is input before the bed 5 reaches the seating posture (S18: Yes), the bed 5 is actively lowered by the force of the hydraulic oil, thereby shortening the time it takes to reach the seating posture.

[0089] Then, when the seating sensor 39 detects the seating posture of the loading platform 5 (S22: Yes), the controller 50 switches the control valve 25 to the center bypass position (i.e., the directional control valve 41 to the bypass position J, and the directional control valves 42, 43 to the hoist holding positions M, P) (S23). The controller 50 also switches the first on-off valve 29 from the open position E to the closed position D (S24). However, step S23 may be omitted and the control valve 25 may be maintained in the float position. Also, if the first on-off valve 29 is already in the closed position D, the processing of step S24 is skipped.

[0090] FIG. 8 is a timing chart of float-down in a conventional hydraulic circuit (without the first on-off valve 29 and the second on-off valve 30). Because the conventional hydraulic circuit does not have the first on-off valve 29, after step S12, the process skips step S16 and proceeds to step S17. If a lowering command is received in step S18, the process proceeds to lowering start processing (S19). Also, because the conventional hydraulic circuit does not have the second on-off valve 30, the process skips step S32 and proceeds to step S34. Therefore, in order to prevent cavitation at the inlet of the fan motor 14, it is necessary to slowly decelerate the cooling fan 13 (A). Therefore, after the tilting of the second hydraulic pump 23 is slowly reduced to the minimum, it takes time to switch the switching valve 26 from fan position A to hoist position B (A+B+C).

[0091] FIG. 9 is a timing chart of float-down in the hydraulic circuit 20 of this embodiment. In the hydraulic circuit 20 of this embodiment, after receiving a float-down command (S12: Yes), the first on-off valve 29 is set to open position E in step S16, and then the process proceeds to step S17. Furthermore, in the loading platform start process, the second on-off valve 30 is switched to open position G in step S32, eliminating the risk of cavitation in the fan motor 14. This allows the tilt of the second hydraulic pump 23 to be instantly minimized in step S36, and the switching valve 26 to hoist position B in step S37 can also be instantly switched (D). Furthermore, by increasing the tilt of the second hydraulic pump 23 to a predetermined value in step S38, the time required for the hydraulic oil discharged from the second hydraulic pump 23 to be supplied to the hoist cylinder 7 can be shortened (A+B+C). Furthermore, by setting the first on-off valve 29 to open position E, the return back pressure of the control valve 25 is minimized, thereby improving the operating speed of the hoist cylinder 7. This reduces the time required to tip over the loading platform 5. During period E in Fig. 9, the order in which the second on-off valve 30, the first on-off valve 29, and the switching valve 26 are opened and closed is important, but since this order is executed in an extremely short time, there is not much difference between Figs. 8 and 9.

[0092] According to the above embodiment, for example, the following advantageous effects are achieved.

[0093] According to the above embodiment, by providing the first back pressure retention valve 27, the first on-off valve 29, and the first check valve 31, the return back pressure generated in the reflux line L10 can be supplied to the supply line L3 (the inlet of the fan motor 14) through the first makeup line L12. This makes it possible to prevent the inlet of the fan motor 14 from becoming negative pressure and causing cavitation, even if the switching valve 26 is switched instantaneously from the fan position A to the hoist position B.

[0094] On the other hand, if the first on-off valve 29 is set to the closed position D to generate return back pressure in the return line L10, the operating speed of the hoist cylinder 7 will decrease, particularly when the platform 5 is toppled under its own weight. Therefore, when floating down the platform 5, the first on-off valve 29 is switched to the open position E to remove the return back pressure, thereby increasing the speed at which the platform 5 is toppled.

[0095] Furthermore, according to the above embodiment, during the unloading start process, by setting the second opening / closing valve 30 to the open position G and supplying hydraulic oil from the accumulator 24 to the inlet of the fan motor 14 through the second makeup line L13, even if the back pressure of the first makeup line L12 is removed, cavitation of the fan motor 14 can be prevented when the switching valve 26 is switched from the fan position A to the hoist position B.

[0096] Furthermore, according to the above embodiment, in the bed lowering completion process, after the switching valve 26 is switched from the hoist position B to the fan position A, the second opening / closing valve 30 is switched to the blocking position F, so that even if the supply of hydraulic oil through the second makeup line L13 is stopped, cavitation of the fan motor 14 can be prevented.

[0097] Furthermore, according to the above embodiment, the first on-off valve 29 is switched to the closed position after the loading platform 5 is seated, so the bypass line L11 functions effectively until the loading platform 5 is seated. As a result, the return resistance is maintained low until the loading platform 5 is seated, further shortening the time it takes for the loading platform 5 to collapse.

[0098] Furthermore, if the load weight of the bed 5 is equal to or greater than the threshold, and the first on-off valve 29 is set to the open position E to float down the bed 5, the bed 5 may tip over too quickly. Therefore, in such a case, the bed 5 can be floated down with the first on-off valve 29 set to the closed position D, allowing the bed 5 to float down slowly while being braked by the return back pressure of the return line L10. This reduces the impact when the bed 5 lands. Furthermore, since the second on-off valve 30 is maintained in the closed position F while the bed 5 floats down, hydraulic oil in the accumulator 24 can be saved. However, from the viewpoint of increasing the tipping speed of the bed 5 due to its own weight, the weight sensor 38 and the processing of steps S15 and S31 can be omitted.

[0099] The above-described embodiments are merely 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 spirit of the present invention. [Explanation of symbols]

[0100] 1: Dump truck 2: Body frame 3L, 3R: Front tires 4L, 4R: Rear tires 5: Cargo bed 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 shut-off valve 30: Second shut-off 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: Reflux line L11: Bypass line L12: 1st Makeup Line L13: Second Makeup Line

Claims

1. The car body and a loading platform supported on the vehicle body so as to be able to rise and fall; 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 the cooling fan; a hydraulic circuit for supplying hydraulic oil to the hoist cylinder and the fan motor; In a dump truck including a controller for controlling the hydraulic circuit, The hydraulic circuit includes: a hydraulic oil tank for storing hydraulic oil; a hydraulic pump that discharges hydraulic oil stored in the hydraulic oil tank; a control valve that can be switched to a platform-raising position in which hydraulic oil is supplied to the hoist cylinder in a direction that raises the platform, a platform-lowering position in which hydraulic oil is supplied to the hoist cylinder in a direction that lowers the platform, and a float position in which hydraulic oil discharged from the hoist cylinder when the platform lowers under its own weight is returned to the hydraulic oil tank through a return line; a switching valve that can switch between a fan position in which the hydraulic oil discharged from the 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 back pressure maintaining valve disposed in the return line to maintain the hydraulic pressure of the hydraulic oil discharged from the hoist cylinder at a pressure higher than the minimum pressure required for the fan motor to function as a hydraulic motor; a check valve disposed in a first makeup line that branches off from the return line upstream of the back pressure maintenance valve in the flow of hydraulic oil and leads to the supply line, the check valve allowing the flow of hydraulic oil from the return line to the supply line and blocking the flow of hydraulic oil from the supply line to the return line; a first on-off valve that is switchable between an open position that opens a bypass line that connects the upstream side and downstream side of the back pressure maintenance valve in the flow of hydraulic oil of the return line and a closed position that closes the bypass line, When a float down command to cause the loading platform to collapse under its own weight is received, the controller switches the first opening / closing valve from the closed position to the open position, and then switches the control valve to the float position.

2. The dump truck according to claim 1, The hydraulic pump A first hydraulic pump that discharges hydraulic oil to be supplied to the hoist cylinder; a second hydraulic pump that discharges hydraulic oil to be supplied to the hoist cylinder or the fan motor, The switching valve switches a supply destination of the hydraulic oil discharged from the second hydraulic pump, The hydraulic circuit includes: an accumulator that accumulates pressure in the hydraulic oil discharged from the first hydraulic pump; a second on-off valve that is switchable between an open position where a second makeup line extending from the accumulator to the supply line is opened to supply hydraulic oil from the accumulator to the fan motor, and a closed position where the second makeup line is closed; When the controller receives a platform lowering instruction to supply hydraulic oil to the hoist cylinder to lower the platform while the platform is lowered by its own weight, switching the second on-off valve from the closed position to the open position; Minimizing the tilt of the second hydraulic pump; switching the switching valve from the fan position to the hoist position; Switching the control valve from the float position to the bed lowering position; The dump truck is characterized in that the tilting amount of the second hydraulic pump is increased to a predetermined value.

3. The dump truck according to claim 2, When the instruction to lower the loading platform is completed, the controller Minimizing the tilt of the second hydraulic pump; switching the control valve from the bed lowering position to the float position; switching the switching valve from the hoist position to the fan position; switching the second on-off valve from the open position to the closed position; The dump truck is characterized in that the tilting amount of the second hydraulic pump is increased to a predetermined value.

4. The dump truck according to claim 1, The dump truck is characterized in that the controller switches the first on-off valve from the open position to the closed position when the loading platform falls over under its own weight and sits on the vehicle body.

5. The dump truck according to claim 1, When the controller receives the float-down command, When the weight of the cargo loaded on the loading platform is less than a threshold value, the first on-off valve is switched from the closed position to the open position, and then the control valve is switched to the float position; When the load weight is equal to or greater than the threshold value, the dump truck switches the control valve to the float position while keeping the first on-off valve in the closed position.

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