Dump truck

Abstract

To inhibit a dump truck from traveling with a rear opening of a cargo box left open.SOLUTION: A dump truck 1 comprises: a cargo box 3 provided to be tiltable with respect to a vehicle body 2; a tail gate 4 that is rotatably provided at a rear end of the cargo box 3 to open and close a rear opening 3b of the cargo box 3; a dump cylinder 23 that tilts the cargo box 3 by extension operation and lays down the cargo box 3 by contraction operation; tail gate cylinders 6 that rotate the tail gate 4 by extension operation so as to open the rear opening 3b and rotate the tail gate 4 by contraction operation so as to close the rear opening 3b; a hydraulic pump 32 for supplying hydraulic oil to the dump cylinder 23 and the tail gate cylinders 6; and communication oil passages 39 for providing communication between first oil chambers 6d and second oil chambers 6e of the tail gate cylinders 6.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to a dump truck. [Background technology]

[0002] For example, the dump truck described in Patent Document 1 includes a cargo box tiltably mounted on the vehicle body, a dump cylinder for tilting the cargo box, a tailgate rotatably mounted at the rear end of the cargo box, and a tailgate cylinder for rotating the tailgate. In such a dump truck, when the dump cylinder is extended to tilt the cargo box, the tailgate cylinder is extended to rotate the tailgate away from the cargo box and open the rear opening of the cargo box. This allows the load, such as soil and sand, in the cargo box to be discharged to the outside through the rear opening of the cargo box. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 54-11163 Summary of the Invention [Problem to be solved by the invention]

[0004] In the dump truck described in Patent Document 1, after discharging the cargo inside the cargo box to the outside, when the worker starts to operate a specified switch, the dump cylinder contracts and the tailgate cylinder also contracts. As a result, the cargo box falls over, and the tailgate rotates to approach the cargo box, closing the rear opening of the cargo box. At that time, hydraulic oil is supplied to the tailgate cylinder from the hydraulic pump, so that a relatively high hydraulic pressure is generated in the tailgate cylinder, allowing the tailgate cylinder to contract quickly.

[0005] However, in the case of a dump truck in which the tailgate cylinder is retracted by the weight of the tailgate, the hydraulic pressure generated in the tailgate cylinder is relatively low, so the tailgate cylinder retracts slowly. This makes it easy for the tailgate cylinder to not finish retracting when the dump cylinder has finished retracting and the cargo box has fallen onto the vehicle body, and the tailgate does not close the rear opening of the cargo box. When this happens, the operator needs to keep operating the switch until the tailgate closes the rear opening of the cargo box.

[0006] However, some workers may mistakenly assume that the tailgate has also closed its rear opening when the cargo box is collapsed onto the vehicle body, and may end the switch operation. In particular, when a worker operates the switch from the driver's seat or nearby in order to enable the dump truck to move quickly after the switch operation, it is difficult to visually confirm the rotation state of the tailgate, and this mistake is likely to occur. If a worker makes such a mistake and ends the switch operation, there is a risk that the dump truck will move with the rear opening of the cargo box open.

[0007] The present invention has been made in consideration of the above circumstances, and has an object to prevent a dump truck from traveling with the rear opening of a cargo box left open. [Means for solving the problem]

[0008] (1) A dump truck of the present invention is a dump truck including a cargo box tiltably provided with respect to a vehicle body, a tailgate rotatably provided at a rear end of the cargo box for opening and closing a rear opening of the cargo box, a dump cylinder for tilting the cargo box by an extension operation and for laying down the cargo box by a contraction operation, a tailgate cylinder for rotating the tailgate to open the rear opening by an extension operation and for rotating the tailgate to close the rear opening by a contraction operation, and a hydraulic pump for supplying hydraulic oil from an oil tank to the dump cylinder and the tailgate cylinder, wherein the tailgate cylinder has a first oil chamber to which hydraulic oil from the hydraulic pump is supplied during an extension operation of the tailgate cylinder and from which hydraulic oil is discharged during a contraction operation of the tailgate cylinder, and a second oil chamber to which hydraulic oil is supplied during a contraction operation of the tailgate cylinder, and is provided with a communicating oil passage for communicating the first oil chamber and the second oil chamber of the tailgate cylinder.

[0009] According to the present invention, when the tailgate is rotated to close the rear opening of the cargo box by contracting the tailgate cylinder, the hydraulic oil discharged from the first oil chamber of the tailgate cylinder can be supplied to the second oil chamber of the tailgate cylinder via the communicating oil passage. This makes it possible to prevent the speed of the tailgate cylinder from slowing down even when the tailgate cylinder is contracted by its own weight. As a result, while the dump cylinder is contracting to lay down the cargo box, the tailgate is likely to finish rotating and close the rear opening of the cargo box, so it is possible to prevent the dump truck from traveling with the rear opening of the cargo box open.

[0010] (2) The dump cylinder has a hydraulic oil chamber to which hydraulic oil from the hydraulic pump is supplied when the dump cylinder is extended and from which hydraulic oil is discharged when the dump cylinder is contracted, and the dump truck includes a main oil line, a discharge oil line connected to the hydraulic pump and through which hydraulic oil discharged from the hydraulic pump flows, a return oil line connected to the oil tank, a change-over valve that is switched between a state in which the main oil line and the discharge oil line communicate with each other and a state in which the main oil line and the return oil line communicate with each other, a first branch oil line branching from the main oil line and connected to the hydraulic oil chamber of the dump cylinder, and a second branch oil line branching from the main oil line and connected to the first oil chamber of the tailgate cylinder. a relief valve provided in the first branch oil passage for allowing hydraulic oil to flow from the main oil passage side of the first branch oil passage to the hydraulic oil chamber side when the hydraulic pressure of the hydraulic oil flowing from the main oil passage to the first branch oil passage reaches a set pressure equal to or higher than the hydraulic pressure required for the extension operation of the tailgate cylinder; a bypass oil passage branching off from the main oil passage side of the first branch oil passage relative to the relief valve and connected to the hydraulic oil chamber side of the first branch oil passage relative to the relief valve; and a check valve provided in the bypass oil passage for allowing hydraulic oil to flow from the hydraulic oil chamber side of the bypass oil passage to the main oil passage side and for regulating the flow of hydraulic oil from the main oil passage side of the bypass oil passage to the hydraulic oil chamber side.

[0011] In this case, when the hydraulic pump is driven with the switching valve switched so that the main oil passage communicates with the discharge oil passage, the hydraulic oil discharged from the hydraulic pump passes through the discharge oil passage, the switching valve, and the main oil passage, and flows into the first branch oil passage and the second branch oil passage. The hydraulic oil that flows into the first branch oil passage is not supplied to the hydraulic oil chamber of the dump cylinder until its hydraulic pressure reaches the set pressure of the relief valve. In addition, the hydraulic oil that flows from the first branch oil passage to the bypass oil passage is also not supplied to the hydraulic oil chamber of the dump cylinder by the check valve. On the other hand, the hydraulic oil that flows into the second branch oil passage is supplied to the first oil chamber of the tailgate cylinder, so that the tailgate cylinder extends before the dump cylinder. After the tailgate cylinder extends, when the hydraulic oil in the first branch oil passage reaches the set pressure of the relief valve, the hydraulic oil is supplied from the first branch oil passage to the hydraulic oil chamber of the dump cylinder, and the dump cylinder begins to extend. Therefore, since the dump cylinder can be extended after the tailgate cylinder has been extended, the rear opening of the cargo box can be opened and then the cargo box can be tilted.

[0012] After tilting the cargo box, when the change-over valve is switched so that the main oil passage communicates with the return oil passage, the hydraulic oil in the hydraulic oil chamber of the dump cylinder flows through the first branch oil passage via the bypass oil passage and into the main oil passage, and the hydraulic oil in the first oil chamber of the tailgate cylinder flows through the second branch oil passage and into the main oil passage. The hydraulic oil that flows from both cylinders into the main oil passage is returned to the oil tank via the change-over valve and the return oil passage. This allows the tailgate cylinder to be contracted while the dump cylinder is contracted, so that the cargo box can be tilted while the tailgate is rotated to close the rear opening of the cargo box.

[0013] As described above, a single switching valve can be used to switch between a state in which the tailgate cylinder and the dump cylinder are extended in that order, and a state in which both the tailgate cylinder and the dump cylinder are retracted. This eliminates the need to provide separate switching valves for supplying and discharging hydraulic oil to the tailgate cylinder and to the dump cylinder, allowing the hydraulic circuit to be constructed at low cost.

[0014] (3) The dump truck is provided with a main oil line through which hydraulic oil discharged from the hydraulic pump flows, a branch oil line branching from the main oil line and connected to the first oil chamber of the tailgate cylinder, and a pilot check valve provided in the branch oil line for allowing the flow of hydraulic oil from the first oil chamber side of the branch oil line to the main oil line side, and it is preferable that the pilot check valve restricts the flow of hydraulic oil from the main oil line side of the branch oil line to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil line side of the branch oil line relative to the pilot check valve is less than a predetermined pressure, and allows the flow of hydraulic oil from the main oil line side of the branch oil line to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil line side of the branch oil line relative to the pilot check valve is equal to or greater than the predetermined pressure.

[0015] In this case, since the hydraulic pump is generally not driven while the dump truck is traveling, the hydraulic pressure of the hydraulic oil on the main oil line side of the branch oil line relative to the pilot check valve becomes lower than a predetermined pressure, and the flow of hydraulic oil from the main oil line side of the branch oil line to the first oil chamber side is restricted. This suppresses the extension operation of the tailgate cylinder, thereby suppressing the tailgate from rotating in the direction that opens the rear opening while the dump truck is traveling. When the dump truck is stopped and the rear opening of the cargo box is opened, the hydraulic pump is driven to flow hydraulic oil from the main oil line to the branch oil line, and the hydraulic pressure of the hydraulic oil on the main oil line side of the branch oil line relative to the pilot check valve becomes equal to or higher than a predetermined pressure, allowing the hydraulic oil to flow from the main oil line side of the branch oil line to the first oil chamber side. This causes the tailgate to extend, opening the rear opening of the cargo box.

[0016] (4) The dump truck is provided with a pilot check valve provided in the second branch oil line and allowing the flow of hydraulic oil from the first oil chamber side of the second branch oil line to the main oil line side, and the pilot check valve regulates the flow of hydraulic oil from the main oil line side of the second branch oil line to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil line side of the second branch oil line relative to the pilot check valve in the second branch oil line is less than a predetermined pressure, and allows the flow of hydraulic oil from the main oil line side of the second branch oil line to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil line side of the second branch oil line relative to the pilot check valve in the second branch oil line is equal to or greater than the predetermined pressure, and it is preferable that the predetermined pressure of the pilot check valve is smaller than the set pressure of the relief valve.

[0017] In this case, since the hydraulic pump is generally not driven while the dump truck is traveling, the hydraulic pressure of the hydraulic oil on the main oil line side of the pilot check valve in the second branch oil line is lower than a predetermined pressure, and the flow of hydraulic oil from the main oil line side of the second branch oil line to the first oil chamber side is restricted. This suppresses the extension operation of the tailgate cylinder, so that it is possible to suppress the tailgate from rotating in the direction that opens the rear opening while the dump truck is traveling. When the dump truck is stopped and the rear opening of the cargo box is opened, the hydraulic pump is driven to flow hydraulic oil from the main oil line to the second branch oil line, and the hydraulic pressure of the hydraulic oil on the main oil line side of the second branch oil line relative to the pilot check valve reaches a predetermined pressure or higher, allowing the hydraulic oil to flow from the main oil line side of the second branch oil line to the first oil chamber side. This causes the tailgate to extend, opening the rear opening of the cargo box. Furthermore, since the predetermined pressure of the pilot check valve is smaller than the set pressure of the relief valve, the dump cylinder can be extended after the tailgate cylinder is extended. As a result, even if the pilot check valve is used as described above, the rear opening of the cargo box can be opened before the cargo box is tilted. Effect of the Invention

[0018] According to the present invention, it is possible to prevent a dump truck from traveling with the rear opening of a cargo box open. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a side view showing the rear side of the dump truck according to the embodiment of the present invention. [Diagram 2] FIG. 2 is a side view for explaining a function of the dump truck. [Diagram 3] FIG. 4 is an enlarged side view showing the rear of the packing box. [Figure 4] FIG. 4 is an enlarged side view illustrating the operation of the tailgate. [Diagram 5] FIG. 4 is an enlarged side view illustrating the operation of the tailgate. [Figure 6] FIG. 2 is a hydraulic circuit diagram of the dump truck. [Figure 7] FIG. 4 is a hydraulic circuit diagram illustrating the flow of hydraulic oil when the packing box is tilted. [Figure 8] FIG. 4 is a hydraulic circuit diagram illustrating the flow of hydraulic oil when a packing box is toppled. [Figure 9] FIG. 11 is a hydraulic circuit diagram showing a modified example of a communication oil passage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. [Overall configuration] Fig. 1 is a side view showing the rear side of a dump truck according to an embodiment of the present invention. Fig. 2 is a side view for explaining the function of the dump truck 1. Hereinafter, in this specification, the front-rear direction, the up-down direction, and the left-right direction refer to the front-rear direction, the up-down direction, and the left-right direction of the dump truck, respectively. In Fig. 1 and Fig. 2, the dump truck 1 includes a vehicle body 2, a cargo box 3, a tailgate 4, a dump device 5, and a tailgate cylinder 6.

[0021] The vehicle body 2 has a chassis frame 2a and a subframe 2b fixed onto the chassis frame 2a. The cargo box 3 is mounted on the vehicle body 2 and is tiltable relative to the vehicle body 2. Specifically, the cargo box 3 is tiltable via a hinge shaft 7 at the rear end of the subframe 2b. The cargo box 3 has a pair of left and right side walls 3a and a rear opening 3b.

[0022] FIG. 3 is an enlarged side view showing the rear part of the packing box 3. FIG. 4 and FIG. 5 are enlarged side views explaining the operation of the tailgate 4. In FIG. 3 to FIG. 5, the tailgate 4 opens and closes the rear opening 3b of the packing box 3. The tailgate 4 of this embodiment has a pair of left and right arm parts 4a arranged on the upper side of each side wall 3a of the packing box 3, and a gate main body part 4b connected to these arm parts 4a. The front end part of each arm part 4a is rotatably attached to the upper side of the rear part of each side wall 3a via a first hinge pin 8. The upper end part of the gate main body part 4b is rotatably attached to the upper rear end part of each arm part 4a via a second hinge pin 9.

[0023] The gate body 4b extends in the left-right direction (vehicle width direction) and closes the rear opening 3b of the cargo box 3 in the state shown in Fig. 3. A lock pin 10 is fixed to each of the left and right side surfaces of the gate body 4b. A hook 11 that opens upward is fixed to each of the left and right side walls 3a of the cargo box 3. In the state shown in Fig. 3, each lock pin 10 is engaged with the corresponding hook 11. This makes it possible to prevent the tailgate 4 from flapping against the cargo box 3 while the dump truck 1 is traveling.

[0024] [Tailgate cylinder] The tailgate cylinders 6 are arranged outside the left and right side walls 3a of the cargo box 3 so as to be capable of expanding and contracting in the vertical direction. Each tailgate cylinder 6 is a double-acting hydraulic cylinder. Each tailgate cylinder 6 has a cylindrical cylinder body 6a, a piston 6b that is provided so as to be able to reciprocate in the vertical direction within the cylinder body 6a, and a rod 6c whose lower end is fixed to the piston 6b. The upper end side of the rod 6c passes through the end wall of the cylinder body 6a.

[0025] The lower end of the cylinder body 6a is connected to the side wall 3a. The upper end of the rod 6c is connected to the lower rear end of the arm portion 4a of the tailgate 4. Within the cylinder body 6a, a first oil chamber 6d is formed below the piston 6b, and a second oil chamber 6e is formed above the piston 6b (see FIG. 6).

[0026] When hydraulic oil is supplied to the first oil chamber 6d and the second oil chamber 6e of the cylinder body 6a from the state shown in Fig. 3, the tailgate cylinder 6 is extended due to the difference in pressure-receiving area of ​​the piston 6b. In the initial stage of extension of the tailgate cylinder 6, as shown in Fig. 4, the arm portion 4a rotates upward about the first hinge pin 8, and the gate body portion 4b moves upward while rotating little by little in the clockwise direction in Fig. 3 about the second hinge pin 9. This releases the engagement between the lock pin 10 and the hook 11. In addition, since the upper end portion of the gate body portion 4b abuts against the rear end surface of the arm portion 4a, the clockwise rotation of the gate body portion 4b is restricted.

[0027] When the tailgate cylinder 6 is further extended from the state shown in Fig. 4, the arm portion 4a and the gate main body portion 4b rotate further upward integrally about the first hinge pin 8 as shown in Fig. 5. This causes the tailgate 4 to rotate upward and open the rear opening 3b of the cargo box 3.

[0028] In the state shown in Fig. 5, when hydraulic oil is discharged from the first oil chamber 6d of the tailgate cylinder 6 and hydraulic oil is supplied to the second oil chamber 6e, the tailgate cylinder 6 contracts. This contraction causes the arm portion 4a and the gate main body portion 4b to rotate downward integrally about the first hinge pin 8. Then, when the lock pin 10 abuts against the upper portion of the hook 11 at the end of the contraction of the tailgate cylinder 6 shown in Fig. 4, the arm portion 4a rotates downward about the first hinge pin 8, and the gate main body portion 4b moves downward while rotating little by little in the counterclockwise direction in Fig. 4 about the second hinge pin 9. As a result, as shown in Fig. 5, the rear opening 3b of the cargo box 3 is closed by the tailgate 4, and the lock pin 10 is engaged with the hook 11.

[0029] [Dump device] 1 and 2, the dump device 5 is disposed between the subframe 2b and the cargo box 3. The dump device 5 includes a triangular lift arm 21, a rod-shaped tension link 22, and a dump cylinder 23. One top of the lift arm 21 is rotatably attached to the subframe 2b via a connecting shaft 24. One end of the tension link 22 is rotatably attached to the other top of the lift arm 21 via a connecting shaft 25. The other end of the tension link 22 is rotatably attached to the bottom of the cargo box 3 via a connecting shaft 26.

[0030] One end of a dump cylinder 23 is rotatably attached to the remaining top of the lift arm 21 via a connecting shaft 27. The other end of the dump cylinder 23 is rotatably attached to the bottom of the cargo box 3 via a connecting shaft 28. The dump cylinder 23 is a single-acting hydraulic cylinder, and has a hydraulic oil chamber 23a into which hydraulic oil is supplied and discharged when the dump cylinder 23 extends and retracts (see FIG. 6).

[0031] When hydraulic oil is supplied to the hydraulic oil chamber 23a of the dump cylinder 23 from the state shown in Fig. 1, the dump cylinder 23 is extended. When the dump cylinder 23 is extended, the lift arm 21 rotates upward about the connecting shaft 24 as shown in Fig. 2, and the cargo box 3 is in a dumped state tilted with the front side raised relative to the vehicle body 2. When hydraulic oil is discharged from the hydraulic oil chamber 23a of the dump cylinder 23 from the dumped state shown in Fig. 2 to contract the dump cylinder 23, the lift arm 21 rotates downward about the connecting shaft 24, and the cargo box 3 is in a landing state in which it is laid down on the vehicle body 2 as shown in Fig. 1.

[0032] [Hydraulic circuit configuration] 6 is a hydraulic circuit diagram of the dump truck 1. This hydraulic circuit mainly includes an oil tank 31, a hydraulic pump 32, a discharge oil passage 33, a return oil passage 34, a switching valve 35, a main oil passage 36, a first branch oil passage 37, and a second branch oil passage (branch oil passage) 38. In the following description, the side closer to the hydraulic pump 32 is referred to as the "upstream side", and the side farther from the hydraulic pump 32 is referred to as the "downstream side". In addition, in each of the oil passages 33, 34, 36 to 38, the road end closer to the hydraulic pump 32 is referred to as the "upstream end", and the road end farther from the hydraulic pump 32 is referred to as the "downstream end" (the same applies to a communication oil passage 39, a first bypass oil passage 44, and a second bypass oil passage 48 described later).

[0033] The hydraulic pump 32 sucks and discharges hydraulic oil in the oil tank 31. The discharge oil passage 33 is connected to the hydraulic pump 32, and the hydraulic oil discharged from the hydraulic pump 32 flows into the discharge oil passage 33. The discharge oil passage 33 is provided with a check valve 40 that regulates the hydraulic oil from flowing upstream. The return oil passage 34 is connected to the oil tank 31, and is an oil passage that returns the hydraulic oil to the oil tank 31. A filter 41 and a check valve 42 are connected to the return oil passage 34 as shown in the figure.

[0034] The switching valve 35 is switched between a state in which the discharge oil passage 33 communicates with the main oil passage 36 and a state in which the return oil passage 34 communicates with the main oil passage 36. The switching valve 35 in this embodiment is, for example, a three-port, three-position electromagnetic switching valve, and each port is connected to the downstream end of the discharge oil passage 33, the upstream end of the return oil passage 34, and the upstream end of the main oil passage 36. The switching valve 35 has a pair of solenoids 35a, 35b. Each solenoid 35a, 35b is excited when an operator operates the switch.

[0035] The switching valve 35 is in a neutral position (the center position in FIG. 6) when both solenoids 35a, 35b are in a non-excited state. In this neutral position, the main oil passage 36 does not communicate with either the discharge oil passage 33 or the return oil passage 34, but the discharge oil passage 33 and the return oil passage 34 communicate with each other.

[0036] When the solenoid 35a is excited, the switching valve 35 switches from the neutral position to the first communication position (the position on the right side in FIG. 6). In this first communication position, the discharge oil passage 33 communicates with the main oil passage 36, and the return oil passage 34 does not communicate with any oil passages. This causes the hydraulic oil to flow from the discharge oil passage 33 to the main oil passage 36.

[0037] When the solenoid 35b is excited, the switching valve 35 switches from the neutral position to the second communication position (the position on the left side in FIG. 6). In this second communication position, the return oil passage 34 communicates with the main oil passage 36. This allows the hydraulic oil to flow from the main oil passage 36 to the return oil passage 34.

[0038] The first branch oil passage 37 branches off from the downstream end 36a of the main oil passage 36, and is connected to the hydraulic oil chamber 23a of the dump cylinder 23. A relief valve 43 is provided in the first branch oil passage 37. The relief valve 43 opens when the hydraulic pressure of the hydraulic oil flowing from the main oil passage 36 to the first branch oil passage 37 reaches a set pressure, and allows the hydraulic oil to flow from the upstream side to the downstream side of the first branch oil passage 37. The set pressure of the relief valve 43 is set to be equal to or higher than the hydraulic pressure required for the extension operation of the tailgate cylinder 6.

[0039] The first branch oil passage 37 is provided with a first bypass oil passage (bypass oil passage) 44 that bypasses the relief valve 43. Specifically, the first bypass oil passage 44 branches off from the first branch oil passage 37 upstream of the relief valve 43 and is connected to the first branch oil passage 37 downstream of the relief valve 43. The first bypass oil passage 44 is provided with a first check valve (check valve) 45. The first check valve 45 allows the hydraulic oil to flow from the downstream side to the upstream side of the first bypass oil passage 44 and regulates the flow of the hydraulic oil from the upstream side to the downstream side of the first bypass oil passage 44.

[0040] The second branch oil passage 38 branches off from the downstream end 36a of the main oil passage 36 and is connected to the first oil chamber 6d of the tailgate cylinder 6. In this embodiment, the second branch oil passage 38 has a first path portion 38a and a pair of second path portions 38b. The upstream end of the first path portion 38a is connected to the downstream end 36a of the main oil passage 36. The pair of second path portions 38b branch off from the downstream end 38a1 of the first path portion 38a and are connected to the first oil chamber 6d of each tailgate cylinder 6, respectively.

[0041] A pair of communication oil passages 39 for communicating the first oil chamber 6d and the second oil chamber 6e of each tailgate cylinder 6 are connected to the second branch oil passage 38. Each communication oil passage 39 branches off from a midway portion 38a2 slightly upstream of a downstream end 38a1 of the first path portion 38a of the second branch oil passage 38, and is connected to the second oil chamber 6e of each tailgate cylinder 6. As a result, the first oil chamber 6d and the second oil chamber 6e of the tailgate cylinder 6 are communicated via the second path portion 38b, the portion from the downstream end 38a1 to the midway portion 38a2 of the first path portion 38a, and the communication oil passage 39.

[0042] A pilot check valve 46 is provided in the first path portion 38a of the second branch oil path 38 upstream of the intermediate portion 38a2. The pilot check valve 46 allows the flow of hydraulic oil from the downstream side to the upstream side of the first path portion 38a. When the hydraulic pressure of the hydraulic oil upstream of the pilot check valve 46 in the first path portion 38a is lower than a predetermined pressure, the pilot check valve 46 restricts the flow of hydraulic oil from the upstream side to the downstream side of the first path portion 38a.

[0043] The pilot check valve 46 allows the flow of hydraulic oil from the upstream side to the downstream side of the first path portion 38a when the hydraulic oil pressure on the upstream side of the pilot check valve 46 in the first path portion 38a reaches or exceeds a predetermined pressure. The predetermined pressure is set to a value that is greater than the hydraulic oil pressure acting on the upstream side of the pilot check valve 46 in the first path portion 38a when the switching valve 35 is in the neutral position, and is smaller than the set pressure of the relief valve 43.

[0044] A flow rate adjustment valve 47 is provided in the first path portion 38a of the second branch oil path 38 upstream of the pilot check valve 46. The flow rate adjustment valve 47 adjusts the flow rate of the hydraulic oil flowing through the first path portion 38a. In addition, a second bypass oil path 48 that bypasses the flow rate adjustment valve 47 is provided in the first path portion 38a.

[0045] The second bypass oil passage 48 branches off from the first passage portion 38a at a position upstream of the flow rate adjustment valve 47, and is connected to the first passage portion 38a at a position downstream of the flow rate adjustment valve 47 and upstream of the pilot check valve 46. A second check valve 49 is provided in the second bypass oil passage 48. The second check valve 49 allows the hydraulic oil to flow from the upstream side to the downstream side of the second bypass oil passage 48, and regulates the flow of the hydraulic oil from the downstream side to the upstream side of the second bypass oil passage 48.

[0046] As a result, when the tailgate cylinder 6 contracts, the hydraulic oil discharged from the first oil chamber 6d and flowing from the downstream side to the upstream side of the first path portion 38a always passes through the flow control valve 47, and by adjusting the flow rate of that hydraulic oil with the flow control valve 47, the contraction speed of the tailgate cylinder 6 can be adjusted.

[0047] [Hydraulic circuit when driving] When the dump truck 1 is traveling, as shown in Fig. 6, the switching valve 35 is held in the neutral position, so that the main oil passage 36 does not communicate with either the discharge oil passage 33 or the return oil passage 34. The dump cylinder 23 and the tailgate cylinder 6 are both in a contracted state. That is, the cargo box 3 is in a landing state, and the tailgate 4 closes the rear opening 3b of the cargo box 3.

[0048] When vibration occurs during the travel of the dump truck 1, the tailgate cylinder 6 in the contracted state may attempt to extend. In that case, the hydraulic oil remaining in the second branch oil passage 38 is sucked into the first oil chamber 6d and the second oil chamber 6e of the tailgate cylinder 6. However, in this embodiment, since the switching valve 35 is in the neutral position during the travel of the dump truck 1, the hydraulic pressure of the hydraulic oil upstream of the pilot check valve 46 of the second branch oil passage 38 is less than a predetermined pressure. As a result, the flow of the hydraulic oil from the upstream side to the downstream side of the second branch oil passage 38 is regulated by the pilot check valve 46, so that the extension operation of the tailgate cylinder 6 is suppressed. As a result, it is possible to suppress the tailgate 4 from rotating in a direction to open the rear opening 3b of the cargo box 3 during the travel of the dump truck 1.

[0049] [Hydraulic circuit when tilting the cargo box] Fig. 7 is a hydraulic circuit diagram explaining the flow of hydraulic oil when the cargo box 3 is tilted. When the worker operates a switch to tilt the cargo box 3 while the dump truck 1 is stopped (see Fig. 1), the solenoid 35a of the switching valve 35 is excited. As a result, as shown in Fig. 7, the switching valve 35 switches from the neutral position to the first communication position, and the discharge oil passage 33 and the main oil passage 36 are communicated with each other.

[0050] Next, when the hydraulic pump 32 is driven, the hydraulic oil discharged from the hydraulic pump 32 branches and flows into a first branched oil passage 37 and a second branched oil passage 38 (first passage portion 38a) via the discharge oil passage 33, the switching valve 35, and the main oil passage 36. The hydraulic oil that flows into the first branched oil passage 37 is not supplied to the hydraulic oil chamber 23a of the dump cylinder 23 until the hydraulic pressure reaches the set pressure of the relief valve 43. In addition, the hydraulic oil that flows from the first branched oil passage 37 to the first bypass oil passage 44 is also not supplied to the hydraulic oil chamber 23a of the dump cylinder 23 because the flow of the hydraulic oil to the downstream side is restricted by the first check valve 45.

[0051] On the other hand, the hydraulic oil that has flowed into the first path portion 38a of the second branch oil path 38 passes through the flow rate regulating valve 47 and the second check valve 49 of the second bypass oil path 48, and flows to the upstream side of the pilot check valve 46. Then, when the hydraulic pressure of the hydraulic oil on the upstream side of the pilot check valve 46 reaches or exceeds a predetermined pressure, the flow of hydraulic oil from the upstream side to the downstream side of the pilot check valve 46 is permitted.

[0052] Then, the hydraulic oil flowing through the first path portion 38a of the second branch oil path 38 is supplied to the first oil chamber 6d and the second oil chamber 6e of each tailgate cylinder 6 via the second path portion 38b and the communicating oil path 39. When the hydraulic oil is supplied to the first and second oil chambers 6d, 6e, the tailgate cylinder 6 is extended. As a result, the tailgate 4 starts to extend, and the rear opening 3b of the cargo box 3 can be opened.

[0053] When the extension operation of each tailgate cylinder 6 is completed and the rear opening 3b of the cargo box 3 is opened (see FIG. 5), the hydraulic pressure of the hydraulic oil in the first branch oil passage 37 reaches the set pressure of the relief valve 43, and the relief valve 43 opens. Then, the hydraulic oil in the first branch oil passage 37 passes through the relief valve 43 and is supplied to the hydraulic oil chamber 23a of the dump cylinder 23. As a result, the dump cylinder 23 starts to extend, and the cargo box 3 tilts upward with respect to the vehicle body 2 (see FIG. 2). Therefore, after the tailgate cylinder 6 is extended, the dump cylinder 23 can be extended, so that the rear opening 3b of the cargo box 3 is opened and then the cargo box 3 can be tilted. This allows the load such as soil and sand in the cargo box 3 to be reliably discharged to the outside from the rear opening 3b.

[0054] [Hydraulic circuit when the box is toppled] Fig. 8 is a hydraulic circuit diagram explaining the flow of hydraulic oil when the container 3 is tilted (see Fig. 2). When the worker operates a switch to tilt the container 3 while the container 3 is tilted (see Fig. 2), the solenoid 35b of the switching valve 35 is excited. As a result, as shown in Fig. 8, the switching valve 35 switches from the neutral position to the second communication position, and the return oil passage 34 and the main oil passage 36 are connected to each other.

[0055] When the return oil passage 34 and the main oil passage 36 communicate with each other, the hydraulic pressure of the working oil in each of the oil passages 36-39, 44, 48 downstream of the switching valve 35 is released. As a result, the dump cylinder 23 in the extended state starts to contract due to the weight of the cargo box 3, and the tailgate cylinder 6 in the extended state starts to contract due to the weight of the tailgate 4.

[0056] When the dump cylinder 23 is contracted, the hydraulic oil in the hydraulic oil chamber 23a passes through the first bypass oil passage 44 and the first check valve 45, passes through the first branch oil passage 37, and flows into the main oil passage 36. When the tailgate cylinder 6 is contracted, the hydraulic oil in the first oil chamber 6d passes through the second branch oil passage 38 and flows into the main oil passage 36. The hydraulic oil that has flowed from each of the first branch oil passage 37 and the second branch oil passage 38 to the main oil passage 36 passes through the switching valve 35 and the return oil passage 34 and returns to the oil tank 31. As a result, the tailgate cylinder 6 can be contracted while the dump cylinder 23 is contracted, so that the cargo box 3 can be collapsed onto the vehicle body 2 while the tailgate 4 is rotated downward to close the rear opening 3b of the cargo box 3.

[0057] When the tailgate cylinder 6 is contracted as described above, a part of the hydraulic oil flowing from the first oil chamber 6d to the downstream end 38a1 of the first path portion 38a through the second path portion 38b is supplied to the second oil chamber 6e through the intermediate portion 38a2 of the first path portion 38a and the communicating oil path 39. This makes it possible to increase the contraction speed of the tailgate cylinder 6 even when the tailgate cylinder 6 is contracted by the weight of the tailgate 4. As a result, while the dump cylinder 23 is contracted to lay the cargo box 3 down, the tailgate 4 is likely to finish rotating downward and close the rear opening 3b of the cargo box 3, so that the dump truck 1 can be prevented from traveling with the rear opening 3b of the cargo box 3 open.

[0058] Also, as mentioned above, the lock pin 10 on the tailgate 4 side engages with the hook 11 on the cargo box 3 side at the end of the contraction of the tailgate cylinder 6. As a result, the speed of the contraction operation of the tailgate cylinder 6 increases, and the lock pin 10 can be reliably engaged with the hook 11 while the dump cylinder 23 is contracting to tip the cargo box 3.

[0059] As described above, in the hydraulic circuit of this embodiment, the single selector valve 35 can be used to switch between a state in which the tailgate cylinder 6 and the dump cylinder 23 are extended in that order, and a state in which both the tailgate cylinder 6 and the dump cylinder 23 are retracted. This eliminates the need to separately provide a selector valve for supplying and discharging hydraulic oil to the tailgate cylinder 6 and a selector valve for supplying and discharging hydraulic oil to the dump cylinder 23, allowing the hydraulic circuit to be configured at low cost.

[0060] In addition, since the predetermined pressure of the pilot check valve 46 is smaller than the set pressure of the relief valve 43, the dump cylinder 23 can be extended after the tailgate cylinder 6 is extended. As a result, even when the pilot check valve 46 is used as described above, the rear opening 3b of the cargo box 3 can be opened before the cargo box 3 is tilted.

[0061] [others] It should be noted that the embodiments disclosed herein are illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not by the above meaning, and is intended to include the equivalent meaning of the claims and all modifications within the scope.

[0062] For example, the communication oil passage 39 in this embodiment communicates the second branch oil passage 38 (first passage portion 38a) with the second oil chamber 6e of the tailgate cylinder 6, but may directly communicate the first oil chamber 6d with the second oil chamber 6e of the tailgate cylinder 6 as shown in Fig. 9. In addition, the dump device 5 only needs to include at least the dump cylinder 23.

[0063] The tailgate 4 of this embodiment is composed of the arm portion 4a and the gate body portion 4b, but may be composed of only the gate body portion 4b. In that case, the end of the tailgate cylinder 6 on the tailgate 4 side may be connected to the gate body portion 4b.

[0064] The switching valve 35 is not limited to the present embodiment as long as it can be switched between a state in which the main oil passage 36 communicates with the discharge oil passage 33 and a state in which the main oil passage 36 communicates with the return oil passage 34. For example, the switching valve 35 may be a two-position electromagnetic switching valve. [Explanation of symbols]

[0065] 1. Dump truck 2. Body 3 packing boxes 3b Rear opening 4. Tailgate 6 Tailgate cylinder 6d 1st oil chamber 6e 2nd oil chamber 23 Dump cylinder 23a Hydraulic oil chamber 31 Oil Tank 32 Hydraulic pump 33 Discharge oil path 34 Return oil passage 35 Switching valve 36 Main Oil Line 37 First Branch Oil Pipe 38 Second branch oil passage (branch oil passage) 39 Communication oil passage 43 Relief valve 44 First bypass oil passage (bypass oil passage) 45 First check valve (check valve) 46 Pilot Check Valve

Claims

1. A cargo box tiltably provided with respect to the vehicle body; a tailgate rotatably provided at a rear end of the cargo box for opening and closing a rear opening of the cargo box; a dump cylinder that tilts the container by extending and tumbles the container by contracting; a tailgate cylinder that rotates the tailgate to open the rear opening by an extension operation and rotates the tailgate to close the rear opening by a contraction operation; A hydraulic pump that supplies hydraulic oil from an oil tank to the dump cylinder and the tailgate cylinder. The tailgate cylinder has a first oil chamber to which hydraulic oil from the hydraulic pump is supplied during an extension operation of the tailgate cylinder and from which hydraulic oil is discharged during a contraction operation of the tailgate cylinder, and a second oil chamber to which hydraulic oil is supplied during a contraction operation of the tailgate cylinder, A dump truck is provided with a communicating oil passage that connects the first oil chamber and the second oil chamber of the tailgate cylinder, discharges hydraulic oil from the first oil chamber to the outside of the tailgate cylinder, and supplies the hydraulic oil discharged to the outside to the second oil chamber.

2. the dump cylinder has a hydraulic oil chamber to which hydraulic oil from the hydraulic pump is supplied during an extension operation of the dump cylinder and from which hydraulic oil is discharged during a contraction operation of the dump cylinder, The main oil passage, a discharge oil passage connected to the hydraulic pump and through which hydraulic oil discharged from the hydraulic pump flows; A return oil passage connected to the oil tank; a switching valve that is switched between a state in which the main oil passage communicates with the discharge oil passage and a state in which the main oil passage communicates with the return oil passage; A first branch oil passage branched from the main oil passage and connected to the hydraulic oil chamber of the dump cylinder; a second branch oil passage branched from the main oil passage and connected to the first oil chamber of the tailgate cylinder; a relief valve provided in the first branch oil passage, the relief valve allowing the flow of hydraulic oil from the main oil passage side of the first branch oil passage to the hydraulic oil chamber side when the hydraulic pressure of the hydraulic oil flowing from the main oil passage to the first branch oil passage reaches a set pressure equal to or higher than the hydraulic pressure required for the extension operation of the tailgate cylinder; a bypass oil passage that branches off from the first branch oil passage at a side closer to the main oil passage than the relief valve and is connected to the first branch oil passage at a side closer to the hydraulic oil chamber than the relief valve; 2. The dump truck according to claim 1, further comprising: a check valve provided in the bypass oil passage, which allows a flow of hydraulic oil from the hydraulic oil chamber side of the bypass oil passage to the main oil passage side, and regulates a flow of hydraulic oil from the main oil passage side of the bypass oil passage to the hydraulic oil chamber side.

3. a main oil passage through which hydraulic oil discharged from the hydraulic pump flows; a branch oil passage branching from the main oil passage and connected to the first oil chamber of the tailgate cylinder; a pilot check valve provided in the branch oil passage and allowing a flow of hydraulic oil from the first oil chamber side to the main oil passage side of the branch oil passage, 2. The dump truck according to claim 1, wherein the pilot check valve restricts a flow of hydraulic oil from the main oil passage side of the branch oil passage to the first oil chamber side when a hydraulic oil pressure of the hydraulic oil on the main oil passage side of the branch oil passage relative to the pilot check valve is less than a predetermined pressure, and allows a flow of hydraulic oil from the main oil passage side of the branch oil passage to the first oil chamber side when a hydraulic oil pressure of the hydraulic oil on the main oil passage side of the branch oil passage relative to the pilot check valve is equal to or greater than the predetermined pressure.

4. a pilot check valve provided in the second branch oil passage and configured to allow hydraulic oil to flow from the first oil chamber side of the second branch oil passage to the main oil passage side; the pilot check valve restricts the flow of hydraulic oil from the main oil passage side of the second branch oil passage to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil passage side of the second branch oil passage relative to the pilot check valve is less than a predetermined pressure, and allows the flow of hydraulic oil from the main oil passage side of the second branch oil passage to the first oil chamber side when the hydraulic oil pressure of the hydraulic oil on the main oil passage side of the second branch oil passage relative to the pilot check valve is equal to or greater than the predetermined pressure, The dump truck according to claim 2 , wherein the predetermined pressure of the pilot check valve is lower than the set pressure of the relief valve.

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