Hydraulic device and dump vehicle

Abstract

To inhibit temperature rise of a hydraulic fluid effectively to prevent seizure of a hydraulic pump.SOLUTION: A hydraulic device 4 includes: a hydraulic cylinder 10; and a hydraulic pressure supply / discharge device 5 which expands or contracts the hydraulic cylinder 10. The hydraulic pressure supply / discharge device has: a bypass oil passage B which is branched from a first oil passage 41 extending from the discharge side of a hydraulic pump 15 to a spool of a switching valve 16 and disposed so that its downstream end returns a hydraulic fluid to an oil tank 17; a bypass valve 19 which may open or close the bypass oil passage; and linkage means which links the spool and the bypass valve to each other. At least a part of the bypass oil passage B is formed in a bypass pipe 18 provided outside of casings 24, 31. The linkage means links the spool and the bypass valve to each other so that the hydraulic fluid discharged from the hydraulic pump flows through the bypass pipe when a switching position of the spool is in a neutral position and the hydraulic fluid discharged from the hydraulic pump does not flow through the bypass pipe when the switching position of the spool is in a lifted position.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a dump vehicle and a hydraulic device for tilting a cargo box of the dump vehicle. [Background technology]

[0002] In general, in a dump vehicle, a dump cylinder is connected between a chassis frame and a cargo box tiltably mounted thereon. Hydraulic oil is supplied to and discharged from the dump cylinder via a selector valve or the like to tilt the cargo box. The selector valve spool is linked to an operating lever located in the driver's seat. When the operator pulls up the operating lever while the hydraulic pump is operating, the selector valve spool is switched to the "up" position, and the dump cylinder extends to its maximum stroke, tilting the cargo box upward. When the operator lowers the operating lever, the selector valve spool is switched to the "down" position, and the dump cylinder retracts to its minimum stroke, tilting the cargo box downward. Furthermore, when the operator operates the operating lever to an intermediate position while the hydraulic pump is operating, the selector valve spool is switched to the "neutral" position, and the extending / retracting dump cylinder stops extending and maintains the inclined position of the cargo box. A dump vehicle equipped with this type of hydraulic device is disclosed in Patent Document 1.

[0003] To stop and hold a cargo box at a desired tilt angle between 0° and the maximum tilt angle (hereinafter referred to as an "intermediate tilt angle"), the control lever is moved to an intermediate position to switch the switching position of the spool of the switching valve to the "neutral" position, or the PTO button is turned "off" while the cargo box is tilting. However, because the control lever is raised when tilting the cargo box upward and lowered when tilting the cargo box downward, operating the control lever to the intermediate position is more intuitive than turning the PTO button "off" when stopping and holding a cargo box at an intermediate tilt angle. For this reason, the operation of the control lever to the intermediate position tends to be more frequently used than the operation of turning the PTO button "off."

[0004] In the hydraulic device for tilting the cargo box of a dump vehicle disclosed in Patent Document 1, the casing of the hydraulic pump and the casing of the switching valve are integrated, and a circulation passage is provided within the integrated casing for circulating hydraulic oil from the discharge side to the suction side of the hydraulic pump when the switching valve is switched to the "neutral" position. Because the circulation passage is provided within the casing, it is difficult to dissipate heat from the hydraulic oil flowing through the circulation passage. Unless measures are taken (measures described below in Patent Document 1), if the switching valve is held in the "neutral" position for a long period of time, the temperature of the hydraulic oil will rise above an allowable value, potentially causing the hydraulic pump to seize. For example, when spreading gravel loaded into the cargo box of a dump vehicle, the cargo box must be kept tilted at a predetermined angle, and during such work, the switching valve is held in the "neutral" position for a long period of time. In addition, in dump vehicles with manual transmissions, the hydraulic oil temperature can be prevented from rising by stepping on the clutch pedal to stop the hydraulic pump, but in dump vehicles with automatic transmissions that do not have a clutch pedal, such a method cannot be used to prevent the hydraulic oil temperature from rising.

[0005] In order to suppress the temperature rise of the hydraulic oil described above, the invention disclosed in Patent Document 1 employs a passage configuration in which, when the switching valve is in the "neutral" or "lowering" position, a portion of the hydraulic oil circulating through the circulation flow path is returned to the oil tank via the oil return flow path, and low-temperature oil is sucked from the oil tank. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-160574 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, when an oil return passage is provided in the circulation passage connecting the discharge side and suction side of a hydraulic pump, as in the invention disclosed in Patent Document 1, the hydraulic oil discharged by the hydraulic pump flows into the oil return passage after passing through a switching valve. However, the hydraulic oil circulating in the circulation passage tends to be hottest immediately after the hydraulic pump's discharge port, so the switching valve spool and its surrounding area are likely to become hot due to the high temperature of the hydraulic oil. Furthermore, because the switching valve casing and the hydraulic pump casing are integrated, heat is trapped inside the casing. As the temperature of the switching valve rises, the temperature of the hydraulic pump itself also rises. As the switching valve is maintained in the "neutral" position for a long time, the hydraulic pump is increasingly likely to seize.

[0008] The present invention was devised in light of such problems, and aims to provide a hydraulic system and a dump vehicle that can suppress temperature rise of hydraulic oil more effectively than conventional systems, thereby preventing seizure of the hydraulic pump. [Means for solving the problem]

[0009] A hydraulic system according to a first aspect of the present invention includes a hydraulic cylinder and a hydraulic supply / discharge device that extends and retracts the hydraulic cylinder. The hydraulic supply / discharge device has a hydraulic pump and a switching valve. The hydraulic pump includes a rotating part that rotates due to an external force and a pump casing that rotatably houses the rotating part. The switching valve includes a spool that is displaceable by user operation and a valve casing that slidably houses the spool. The hydraulic supply / discharge device is configured such that when the spool is in the "up" position, the hydraulic cylinder extends due to pressurized oil discharged by the hydraulic pump. When the spool is in the "down" position and an external force acts on the hydraulic cylinder in a contracting direction, the hydraulic cylinder contracts. When the spool is in the "neutral" position, the hydraulic cylinder does not extend or retract even when the hydraulic pump is operating and an external force acts on the hydraulic cylinder in a contracting direction. The pump casing and the valve casing are integrally formed. The hydraulic supply and discharge device further includes a bypass oil passage branching from a first oil passage extending from the discharge side of the hydraulic pump to the spool of the switching valve and arranged so that its downstream end returns hydraulic oil to an oil tank, a bypass valve capable of opening and closing the bypass oil passage, and interlocking means for interlocking the spool and the bypass valve. At least a portion of the bypass oil passage is formed in a bypass pipe provided outside the valve casing and the pump casing. The interlocking means interlocks the spool and the bypass valve so that hydraulic oil discharged from the hydraulic pump flows through the bypass pipe when the switching position of the spool is in the "neutral" position, and so that hydraulic oil discharged from the hydraulic pump does not flow through the bypass pipe when the switching position of the spool is in the "raising" position.

[0010] In a hydraulic device having this configuration, when the spool is in the "neutral" position, the hydraulic oil flowing through the bypass oil passage passes through a bypass pipe provided outside the valve casing and pump casing and is returned to the oil tank. Heat from the hydraulic oil is dissipated more efficiently when the hydraulic oil passes through an oil passage in the bypass pipe provided outside the valve casing and pump casing than when it passes through an oil passage inside the valve casing and pump casing. In addition, because the high-temperature hydraulic oil discharged from the hydraulic pump is returned to the oil tank before flowing into the spool of the switching valve, the hydraulic pump draws low-temperature hydraulic oil from the oil tank, effectively suppressing a rise in the temperature of the hydraulic oil.

[0011] A hydraulic system according to a second aspect of the present invention is the hydraulic system according to the first aspect, wherein the bypass valve is provided in the bypass pipe.

[0012] According to the hydraulic device of the second aspect, the bypass valve is provided outside the casing, which is prone to trapping heat, together with the bypass piping, so that the temperature rise of the bypass valve can be suppressed, and accordingly the temperature rise of the hydraulic oil passing through the bypass valve is also suppressed.

[0013] A dump vehicle according to a third aspect of the present invention includes a cargo box tiltably mounted on a vehicle frame, and the hydraulic device according to the first or second aspect. The hydraulic cylinder is connected between the vehicle frame and the cargo box as a dump cylinder that tilts the cargo box. When the spool switching position is in the "raised" position, the cargo box tilts upward due to pressurized oil discharged by the hydraulic pump. When the spool switching position is in the "lowered" position, the cargo box tilts downward due to its own weight. When the spool switching position is in the "neutral" position, the cargo box does not tilt even when the hydraulic pump is driven. [Effects of the Invention]

[0014] According to the hydraulic system and dump vehicle of the present invention, the temperature rise of the hydraulic oil can be suppressed more effectively than in the past, and seizure of the hydraulic pump can be prevented. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a left side view of a dump vehicle according to an embodiment of the present invention. [Figure 2] 1 is a plan view of a dump vehicle according to an embodiment of the present invention. [Figure 3] FIG. 2 is a rear view of the dump vehicle according to the embodiment of the present invention. [Figure 4] FIG. 2 is a schematic diagram showing a hydraulic pump, a switching valve, and the like, with an integrated casing. [Figure 5] 1 is a hydraulic circuit diagram of a hydraulic device according to an embodiment of the present invention. [Figure 6] 10 is a graph showing the relationship between the flow rate of hydraulic oil flowing through a bypass pipe and the position of a spool. DETAILED DESCRIPTION OF THE INVENTION

[0016] A dump vehicle according to an embodiment of the present invention will be described below with reference to the drawings. As shown in Figs. 1 to 5, a dump vehicle 1 according to this embodiment includes a cargo box 3 tiltably mounted on a vehicle frame 2, and a hydraulic device 4. The vehicle frame 2 includes a chassis frame 2a extending in the front-to-rear direction of the vehicle, and a sub-frame 2b fixed on the chassis frame 2a. The hydraulic device 4 has a dump cylinder 10 consisting of a hydraulic cylinder connected between the vehicle frame 2 and the cargo box 3, and a hydraulic supply / discharge device 5 (see Fig. 5) for extending and contracting the dump cylinder 10.

[0017] A cargo box 3 having an openable and closable rear gate 7 is mounted on a vehicle frame 2 of the dump vehicle 1. Hinge brackets 8, integrally provided on the left and right rear portions of the cargo box 3, are rotatably connected to the rear portion of the subframe 2b via hinge shafts 9. The base end of a cylinder barrel 10b of a dump cylinder 10 is rotatably supported on the vehicle frame 2, and the intermediate portion of a lift arm 11 is pin-connected to the tip end of a cylinder rod 10r of the dump cylinder 10. One end of the lift arm 11 is pin-connected to the bottom beam of the cargo box 3, and the other end of the lift arm 11 is pin-connected to one end of a tension link 12. The other end of the tension link 12 is rotatably supported on the subframe 2b. Therefore, the cargo box 3 tilts upward as the dump cylinder 10 extends, and tilts downward as the dump cylinder 10 retracts.

[0018] A driving engine E for the dump vehicle is provided at the front of the chassis frame 2a. A hydraulic pump 15 included in the hydraulic pressure supply / discharge device 5 is connected to the driving engine E via a power take-off device PTO and a drive shaft 21 (see FIG. 4).

[0019] As shown in FIG. 5, the hydraulic supply / discharge device 5 includes a hydraulic pump 15, a switching valve 16, an oil tank 17, a bypass pipe 18, a bypass valve 19, and an interlocking means 20 (see FIG. 4).

[0020] In this embodiment, the hydraulic pump 15 is a gear pump. As shown in Fig. 4, the hydraulic pump 15 has a pair of meshed pump gears 26, 27 (corresponding to rotating parts that rotate due to an external force) rotatably housed in a pump casing 24. One pump gear 26 is fixed to a drive shaft 22, and the other pump gear 27 is fixed to a driven shaft 23. The drive shaft 22 and the driven shaft 23 are rotatably supported within the pump casing 24 via bearings. The drive shaft 22 is connected to a drive shaft 21, and the rotational force of the drive shaft 21 rotates the pump gears 26, 27 together with the drive shaft 22.

[0021] The switching valve 16 has a spool 30 that is displaced by a user's operation, and a valve casing 31. The switching valve 16 is a three-port, three-position switching valve, and the spool 30 is slidably housed in a spool chamber formed in the valve casing 31. An operating rod 32 is connected to one end of the spool 30. The operating rod 32 extends outside the valve casing 31 and is connected via a wire (not shown) to an operating lever 33 provided in the driver's seat of the dump vehicle 1 so as to operate in conjunction with the operating lever 33. Therefore, by operating the operating lever 33, the user can change the switching position of the spool 30, and selectively connect or disconnect two ports p and t and one port c of the switching valve 16 shown in FIG. 5. Specifically, when the operating lever 33 is raised to the uppermost position, the switching position of the spool 30 becomes the "up" position 16a, when the operating lever 33 is lowered to the lowermost position, the switching position of the spool 30 becomes the "down" position 16c, and when the operating lever 33 is moved to a position intermediate between the uppermost and lowermost positions, the switching position of the spool 30 becomes the "neutral" position 16b. Note that the "intermediate position" referred to here does not necessarily mean a position halfway between the uppermost and lowermost positions, but rather means a position between the uppermost and lowermost positions.

[0022] As shown in FIG. 5, a first oil passage 41 to a fifth oil passage 45 and a bypass oil passage B are formed inside the pump casing 24 and the valve casing 31 (hereinafter, these casings will also be collectively referred to as "casings 24, 31"). The first oil passage 41 communicates between the discharge port 35 of the hydraulic pump 15 and the port p of the switching valve 16. The second oil passage 42 communicates between an oil tank port 37 facing the outside of the casings 24, 31 and the suction port 40 of the hydraulic pump 15. The third oil passage 43 communicates between the port t of the switching valve 16 and the second oil passage 42 formed inside the casings 24, 31. The fourth oil passage 44 communicates between the port c of the switching valve 16 and a cylinder port 39 facing the outside of the casings 24, 31. A variable throttle valve 50 is provided in the fourth oil passage 44. The fifth oil passage 45 branches off from an intermediate portion of the first oil passage 41 and communicates with the fourth oil passage 44 formed in the casings 24, 31. The fifth oil passage 45 is provided with a check valve 38 that allows flow from the bypass oil passage B toward the fourth oil passage 44 and blocks flow in the reverse direction. The bypass oil passage B branches off from an intermediate portion of the first oil passage 41 and communicates with a bypass port 36 that faces the outside of the casings 24, 31. In this embodiment, the first oil passage 41 side of the fifth oil passage 45 and the first oil passage 41 side of the bypass oil passage B share a single oil passage.

[0023] Outside the casings 24 and 31, an oil tank pipe 51, a first cylinder pipe 52, a second cylinder pipe 53, and a bypass pipe 18 are provided.

[0024] The oil tank piping 51 is disposed inside the oil tank 17, and the other end is connected to the oil tank port 37. The second oil passage 42 described above is formed not only inside the casings 24 and 31, but also continues into the oil tank piping 51.

[0025] One end of the first cylinder piping 52 is connected to a cylinder port 39 provided in the casings 24, 31, and the other end is connected to the high-pressure chamber of the dump cylinder 10. The above-mentioned fourth oil passage 44 is formed not only inside the casings 24, 31, but also continues into the first cylinder piping 52.

[0026] One end of the second cylinder piping 53 is connected to the low-pressure chamber of the dump cylinder 10 , and the other end is disposed inside the oil tank 17 .

[0027] One end of the bypass piping 18 is connected to a bypass port 36 provided in the casings 24, 31, and its downstream end is located inside the oil tank 17 so as to return hydraulic oil to the oil tank 17. The bypass oil passage B described above is formed not only inside the casings 24, 31, but also continues into the bypass piping 18. The bypass piping 18, oil tank piping 51, and second cylinder piping 53 are combined into a single pipe near the oil tank 17.

[0028] The bypass pipe 18 is provided with a bypass valve 19 that can open and close the bypass oil passage B. In this embodiment, a two-port two-position solenoid directional control valve is used as the bypass valve 19. When the bypass valve 19 is energized and its switching position is a right position 19a, the bypass oil passage B is closed, and when the bypass valve 19 is deenergized and its switching position is a left position 19b, the bypass oil passage B is opened. Note that the bypass valve 19 is not limited to the above-mentioned directional control valve as long as it is a valve that can open and close the bypass oil passage B and can be linked to the spool 30 by a linking means 20, which will be described later.

[0029] The interlocking means 20 interlocks the spool 30 and the bypass valve 19. When the switching position of the spool 30 is in the "neutral" position 16b, the interlocking means 20 interlocks the spool 30 and the bypass valve 19 so that, with the bypass valve 19 in an open state, the hydraulic oil discharged from the hydraulic pump 15 flows through the bypass oil passage B in the bypass piping 18, and when the switching position of the spool 30 is in the "raising" position 16a, the interlocking means 20 interlocks the spool 30 and the bypass valve 19 so that, with the bypass valve 19 in a closed state, the hydraulic oil discharged from the hydraulic pump 15 does not flow through the bypass oil passage B in the bypass piping 18.

[0030] As shown in FIG. 4, the interlocking means 20 includes a position detection sensor 29 that detects the position of the spool 30 and a control unit 28 that opens and closes the bypass valve 19 based on an output signal from the position detection sensor 29. A proximity sensor or an optical sensor can be used as the position detection sensor 29. A programmable logic controller (PLC) can be used as the control unit 28. In this embodiment, when the switching position of the spool 30 is in the "down" position 16c or the "neutral" position 16b, the control unit 28 opens the bypass valve 19 to allow hydraulic oil to flow through the bypass pipe 18, as shown in FIG. 6, and when the switching position of the spool 30 is in the "up" position 16a, the control unit 28 closes the bypass valve 19 to prevent hydraulic oil from flowing through the bypass pipe 18. Therefore, the control unit 28 opens the bypass valve 19 when the switching position of the spool 30 is from the "down" position 16c to a position between the "neutral" position 16b and the "up" position 16a, and closes the bypass valve 19 when the switching position of the spool 30 is from the intermediate position to the "up" position 16a.

[0031] In the dump vehicle 1 having the configuration described above, when the loading box 3 is tilted upward from a horizontal state as shown by the solid line in Figure 1 to a state where the loading box 3 is tilted upward as shown by the two-dot chain line in the same figure, the power take-off unit PTO is connected and the hydraulic pump 15 is driven by the power of the traveling engine E. Then, when the operating lever 33 arranged in the lowest position is pulled up to the highest position by the operator, the spool 30 of the switching valve 16 moves from the "down" position 16c, passes through the "neutral" position 16b, and to the "up" position 16a, and the ports p, t, and c of the switching valve 16 are all closed. Furthermore, when the spool 30 of the switching valve 16 is in the "raising" position 16a, the bypass valve 19 is in a closed state, and therefore the hydraulic oil discharged from the hydraulic pump 15 is supplied to the high-pressure chamber of the dump cylinder 10 via the first oil passage 41, the fifth oil passage 45, and the fourth oil passage 44 without flowing into the bypass piping 18, and the hydraulic oil is discharged from the low-pressure chamber of the dump cylinder 10. As a result, the dump cylinder 10 extends and the cargo box 3 tilts upward. The hydraulic oil discharged from the low-pressure chamber of the dump cylinder 10 is returned to the oil tank 17 via the second cylinder piping 53.

[0032] After the cargo box 3 has been tilted to its maximum inclination angle, the operator lowers the operating lever 33 from the uppermost position to the lowermost position. This moves the spool 30 of the switching valve 16 from the "up" position 16a, through the "neutral" position 16b, to the "down" position 16c, and ports p, t, and c of the switching valve 16 become interconnected. As a result, hydraulic oil is discharged from the high-pressure chamber of the dump cylinder 10 and returned to the oil tank 17 via the fourth oil passage 44, the third oil passage 43, and the second oil passage 42. At the same time, hydraulic oil flows into the low-pressure chamber of the dump cylinder 10 from the oil tank 17 via the second cylinder piping 53. This causes the dump cylinder 10 to contract due to the weight of the cargo box 3, causing the cargo box 3 to tilt downward.

[0033] If the operator moves the operating lever 33 to the intermediate position while the cargo box 3 is tilting upward or downward, the spool 30 of the selector valve 16 moves to the "neutral" position 16b, and as shown in Figure 5, ports p and t of the selector valve 16 communicate with each other, while port c does not communicate with either port p or t. At this time, a force in the contracting direction acts on the dump cylinder 10 due to the weight of the cargo box 3, but the check valve 38 prevents flow in the fifth oil passage 45, and at the same time, port c of the selector valve 16 is closed, so the dump cylinder 10 does not contract, the cargo box 3 stops tilting, and that state is maintained.

[0034] As described above, when the spool 30 of the switching valve 16 is in the "neutral" position 16b, the hydraulic oil discharged from the hydraulic pump 15 forms a circulation flow that returns to the suction port 40 via the first oil passage 41, port p, port t, the third oil passage 43, and the second oil passage 42. However, because the bypass valve 19 is opened by the interlocking means 20, a portion of the hydraulic oil discharged from the hydraulic pump 15 is returned to the oil tank 17 via the bypass oil passage B. The hydraulic oil flowing through the bypass oil passage B efficiently dissipates heat as it passes through the bypass piping 18 provided outside the casings 24 and 31, thereby effectively suppressing a temperature rise of the hydraulic oil. In addition, a portion of the high-temperature hydraulic oil discharged from the hydraulic pump 15 is returned to the oil tank 17 via the bypass oil passage B before flowing into the spool 30 of the switching valve 16. Therefore, the hydraulic pump 15 draws low-temperature hydraulic oil from the oil tank 17, thereby further effectively suppressing a temperature rise of the hydraulic oil. Furthermore, according to the dump vehicle 1 equipped with the hydraulic device 4, the temperature rise of the hydraulic oil can be suppressed, and the cargo box 3 can be stopped at a desired inclination angle between 0 degrees and the maximum inclination angle by intuitive operation using the operating lever 33.

[0035] <Other embodiments> As described above, the tilting operation of the cargo box 3 can be stopped midway and the inclination angle of the cargo box 3 can be maintained by switching the switching position of the spool 30 of the switching valve 16 to the "neutral" position 16b, but the tilting operation of the cargo box 3 can also be stopped and the inclination angle of the cargo box 3 can be maintained even if the spool 30 of the switching valve 16 is located at a position between the "neutral" position 16b and the "raised" position 16a rather than at the "neutral" position 16b. Such a stop of the tilting operation (hereinafter also referred to as "apparent stop") occurs when the thrust of the cylinder rod 10r, which is generated by hydraulic oil being supplied to the high-pressure chamber of the dump cylinder 10 through the narrow flow path of the spool 30, balances with the weight of the cargo box 3, and the position of the spool 30 when the "apparent stop" occurs varies depending on the load weight of the cargo on the cargo box 3. In other words, when an "apparent stop" occurs, the position of the spool 30 moves toward the "raised" position 16a as the load weight of the cargo in the cargo box 3 increases, and moves toward the "neutral" position 16b as the load weight of the cargo in the cargo box 3 decreases. When an "apparent stop" occurs, as when the switching position of the spool 30 is in the "neutral" position 16b, the hydraulic oil discharged from the hydraulic pump 15 forms a circulating flow that returns to the suction port 40 via the first oil passage 41, port p, port t, the third oil passage 43, and the second oil passage 42. However, even when the spool 30 is disposed in a position where an "apparent stop" occurs, the bypass valve 19 is opened so that a portion of the hydraulic oil discharged from the hydraulic pump 15 is returned to the oil tank 17 via the bypass oil passage B. This suppresses a temperature rise of the hydraulic oil and prevents the hydraulic pump 15 from seizing.

[0036] Therefore, the heavier the load weight of the cargo in the cargo box 3, the more the position of the spool 30, which determines the timing for switching the open / closed state of the bypass valve 19, is shifted toward the "up" position 16a, thereby making it possible to accurately suppress the temperature rise of the hydraulic oil and prevent seizure of the hydraulic pump 15. Note that the load weight of the cargo in the cargo box 3 can be determined by mounting a camera on the dump vehicle 1 and providing weight estimation means for estimating the load weight from the cargo volume based on camera images, and the control unit 28 can change the timing for opening and closing the bypass valve 19 based on the value estimated by the weight estimation means. [Industrial Applicability]

[0037] The present invention is applicable to dump vehicles and hydraulic devices for tilting cargo boxes of dump vehicles. [Explanation of symbols]

[0038] 1 dump truck 2 Vehicle frame 3 packing boxes 4 Hydraulic system 5 Hydraulic supply and discharge device 10 Dump cylinder (hydraulic cylinder) 15 Hydraulic pump 16. Switching valve 16a "Raised" position 16b “Neutral” position 16c "lower" position 17 Oil Tank 18 Bypass piping 19 Bypass valve 20 Linkage means 24 Pump casing 26 Pump gear (rotating part that rotates due to external force) 27 Pump gear (rotating part that rotates due to external force) 30 spools 31 Valve casing 33 Operating lever 41 No. 1 oil road B Bypass oil passage

Claims

1. A hydraulic cylinder and a hydraulic supply / discharge device that extends and retracts the hydraulic cylinder. The hydraulic pressure supply / discharge device is a hydraulic pump including a rotating part that rotates due to an external force and a pump casing that rotatably houses the rotating part; a switching valve including a spool that is displaced by a user's operation and a valve casing that slidably houses the spool; and When the switching position of the spool is at the "raising" position, the hydraulic cylinder is extended by the pressure oil discharged from the hydraulic pump, When the switching position of the spool is in the "down" position and an external force is acting on the hydraulic cylinder in a contracting direction, the hydraulic cylinder contracts, When the switching position of the spool is in the "neutral" position, the hydraulic pump is driven and even if an external force acts on the hydraulic cylinder in a contracting direction, the hydraulic cylinder does not expand or contract, The pump casing and the valve casing are integrally formed. In hydraulic systems, The hydraulic pressure supply / discharge device is a bypass oil passage branching from a first oil passage extending from the discharge side of the hydraulic pump to the spool of the switching valve, the downstream end of which is arranged to return hydraulic oil to an oil tank; a bypass valve capable of opening and closing the bypass oil passage; an interlocking means for interlocking the spool and the bypass valve; and At least a portion of the bypass oil passage is formed in a bypass pipe provided outside the valve casing and the pump casing, the interlocking means interlocks the spool and the bypass valve so that, when the switching position of the spool is in the "neutral" position, the hydraulic oil discharged from the hydraulic pump flows through the bypass piping, and, when the switching position of the spool is in the "raising" position, the hydraulic oil discharged from the hydraulic pump does not flow through the bypass piping. A hydraulic device characterized by:

2. 2. The hydraulic system according to claim 1, The hydraulic device, wherein the bypass valve is provided in the bypass pipe.

3. a cargo box tiltably mounted on the vehicle frame; The hydraulic device according to claim 1 or 2; A dump vehicle comprising: The hydraulic cylinder is connected between the vehicle frame and the cargo box as a dump cylinder for tilting the cargo box, When the switching position of the spool is in the "raising" position, the cargo box is tilted upward by the pressure oil discharged from the hydraulic pump, When the switching position of the spool is in the "down" position, the cargo box tilts downward due to its own weight, When the switching position of the spool is in the "neutral" position, the cargo box does not tilt even when the hydraulic pump is driven. A dump vehicle characterized by:

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