Crane and crane derailment prevention method

Abstract

To provide a crane and a crane derailment prevention method capable of further reliably preventing the derailment of a wheel rolling on a rail during an earthquake.SOLUTION: In a crane derailment prevention method, a wheel 11 rolling on a rail R installed in a running gear 4 is connected to a leg structure 2 in a relatively movable state in a vertical direction Z by a connection mechanism 4 installed in a crane 1, and the state where an energizing force is always given to the wheel 11 is set in the direction of pushing the wheel 11 toward the rail R by energization means 16 installed in the crane 1.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a crane that travels on rails and a method for preventing the crane from coming off, and more particularly to a crane that can more reliably prevent wheels rolling on rails from coming off during an earthquake and a method for preventing the crane from coming off. [Background technology]

[0002] In the case of cranes such as container cranes and unloaders that run on rails laid on the ground, there is a risk that the cranes will sway and tilt significantly in the event of a large earthquake, causing some of the wheels rolling on the rails to lift off the rails and derail. When a wheel derails from the rail, it takes a lot of time and effort to return the wheel back to the rail. Therefore, various inventions have been proposed to prevent crane wheels from derailing (see, for example, Patent Document 1).

[0003] The invention described in Patent Document 1 is configured such that, under normal circumstances when no earthquake is occurring, a hydraulic mechanism (hydraulic cylinder) restrains the upper equalizer beam of the bogie in a predetermined position, preventing the seismic isolation device from activating. When an earthquake occurs and the hydraulic pressure in the cylinder chamber of the hydraulic cylinder changes due to seismic load, and the detected value exceeds a set value, the hydraulic mechanism releases the restraint between the bogie and the upper equalizer beam, activating the seismic isolation device. In this invention, the seismic isolation device does not activate under normal circumstances, but after an earthquake occurs, the hydraulic mechanism releases the restraint, and the seismic isolation device activates. Therefore, there is a time lag between the occurrence of an earthquake and the activation of the seismic isolation device. Therefore, during this time lag, there is a risk that the wheels will lift off the rails and derail. Therefore, there is room for improvement in order to more reliably prevent wheel derailment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-284230 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a crane and a method for preventing wheels from coming off of a crane that can more reliably prevent wheels rolling on rails from coming off during an earthquake. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, the crane of the present invention comprises a traveling device provided at the bottom of a leg structure constituting the crane and allowing the crane to travel on rails, and wheels provided on the traveling device and rolling on the rails, characterized in that the crane also comprises a connecting mechanism that connects the wheels to the leg structure in a state in which they can move relative to each other in the vertical direction, and a biasing means that constantly applies a biasing force to the wheels in a direction pressing the wheels against the rails.

[0007] The method for preventing wheels from coming off of a crane of the present invention is a method for preventing wheels from coming off of a crane that has a running device that is provided at the bottom of a leg structure that constitutes the crane and allows the crane to run on rails, and wheels that are provided on the running device and roll on the rails, characterized in that a connecting mechanism provided on the crane connects the wheels to the leg structure in a state that allows them to move relatively in the vertical direction, and a biasing means provided on the crane constantly applies a biasing force to the wheels in a direction that presses the wheels against the rails. [Effects of the Invention]

[0008] According to the present invention, the wheels are connected to the leg structure of the crane by the connecting mechanism in a state in which they can move up and down relative to the leg structure, and the biasing means constantly applies a biasing force that presses the wheels against the rail, so that the wheels can be kept pressed against the rail at all times, both under normal conditions and during an earthquake. This more reliably prevents the wheels from lifting off the rail immediately after an earthquake occurs, and more reliably prevents the wheels from coming off the rail. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an explanatory diagram illustrating a normal state of a crane according to an embodiment of the present invention as viewed from the front. [Figure 2] FIG. 2 is a view taken along the arrow A in FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along the arrows BB in FIG. 2. [Figure 4] 3 is an explanatory diagram illustrating a state in which an earthquake occurs from the state of FIG. 2 and the crane tilts; FIG. [Figure 5] 5 is a cross-sectional view taken along the arrow CC in FIG. 4. DETAILED DESCRIPTION OF THE INVENTION

[0010] The crane and crane wheel derailment prevention method of the present invention will be described below based on the embodiment shown in the drawings. In the drawings, the X direction indicates the lateral movement direction X of the crane, the Y direction indicates the travel direction Y of the crane, and the Z direction indicates the vertical direction Z. The lateral movement direction X is perpendicular to the travel direction Y and the vertical direction Z.

[0011] As shown in Fig. 1, a crane 1 according to an embodiment of the present invention is a crane that travels on rails R laid on the ground, and examples thereof include a quay crane or unloader that loads and unloads containers onto and from a ship docked at a quay, and a gantry crane that handles containers at a container yard. In this embodiment, a quay crane is used as the crane 1. The crane 1 travels along two rails R laid on the ground. The two rails R each extend in a traveling direction Y and are spaced apart from each other in a lateral direction X.

[0012] The crane 1 includes a leg structure 2 and a traveling device 4 provided at the bottom of the leg structure 2 to allow the crane 1 to travel on rails R. The crane 1 further includes a girder and a boom provided at the top of the leg structure 2 and extending in the lateral direction X, a trolley that moves in the lateral direction X along the girder and the boom, and a loading device disposed below the trolley.

[0013] The leg structure 2 has two legs 3 extending in the vertical direction Z on each side in the lateral direction X, and a traveling device 4 is provided at the bottom of each of the four legs 3. Each traveling device 4 has a plurality of wheels 11 that roll in contact with the rail R.

[0014] 2, the traveling device 4 of this embodiment is configured to have a main equalizer 5, an intermediate equalizer 7, a bogie 9, and wheels 11. The main equalizer 5 is connected to the lower part of the leg structure 2 via an equalizer pin (main equalizer pin) extending in the lateral direction X so as to be tiltable relative to the leg structure 2. The intermediate equalizer 7 is connected to the lower part of the main equalizer 5 via another equalizer pin 6 (intermediate equalizer pin) extending in the lateral direction X so as to be tiltable relative to the main equalizer 5.

[0015] A bogie 9 is tiltably connected to the lower part of the intermediate equalizer 7 via a bogie pin 8 extending in the lateral direction X. Each bogie 9 is provided with a plurality of wheels 11. On each intermediate equalizer 7, bogies 9 having wheels 11 (drive wheels) equipped with a drive mechanism such as a motor or a reducer, and bogies 9 having wheels 11 (driven wheels) not equipped with a drive mechanism are installed at intervals in the traveling direction Y.

[0016] The shape and structure of the leg structure 2 that constitutes the crane 1, and the structure, number, arrangement, etc. of the traveling gear 4 are not limited to this embodiment and may be configured differently. Furthermore, the number and arrangement, etc. of the intermediate equalizers 7, bogies 9, and wheels 11 that constitute the traveling gear 4 are not limited to this embodiment and, for example, a configuration in which the bogie 9 is connected to the lower part of the main equalizer 5 without the intermediate equalizer 7, or a configuration in which the intermediate equalizer 7 and bogie 9 are connected to the lower part of the main equalizer 5 in parallel in the traveling direction Y, may be used.

[0017] A major feature of the crane 1 according to the embodiment of the present invention is that it includes a connecting mechanism 14 provided between the leg structure 2 and the wheels 11, and a biasing means 16. As illustrated in Figures 2 and 3, the connecting mechanism 14 is a mechanism that connects the wheels 11 to the leg structure 2 so that they can move relatively in the vertical direction Z. The biasing means 16 is a means that constantly applies a biasing force to the wheels 11 in a direction that presses the wheels 11 against the rail R.

[0018] 2, in this embodiment, a connecting mechanism 14 and a biasing means 16 are provided at the connecting portion between the bogie 9 and the wheels 11 (driven wheels) and at the connecting portion between the main equalizer 5 and the intermediate equalizer 7. The connecting mechanism 14 and the biasing means 16 can also be provided, for example, at the connecting portion between the lower part of the landing gear structure 2 and the main equalizer 5, or at the connecting portion between the intermediate equalizer 7 and the bogie 9.

[0019] As illustrated in Figures 2 and 3, a connecting mechanism 14 is provided at the connecting portion between the bogie 9 and the wheel 11, so that the wheel 11 is connected to the bogie 9 in a state where it can move relative to the bogie 9 in the vertical direction Z, and a biasing means 16 is provided between the bogie 9 and the wheel 11 so that the wheel 11 is always biased downward relative to the bogie 9.

[0020] The coupling mechanism 14 is configured to have a long hole 15 provided in the bogie 9 and extending in the vertical direction Z. The bogie 9 is a box-shaped casing that is open at the bottom, and wheels 11 are arranged in the hollow space inside the bogie 9. The wheels 11 are provided with bearings 12 through which axles 13 are inserted, and are configured so that the wheels 11 can rotate about the axles 13 that extend in the lateral direction X.

[0021] A long hole 15 is formed in the bogie 9, penetrating in the lateral direction X, and an axle 13 is inserted into the long hole 15, extending in the lateral direction X. The axle 13 is configured to be able to move in the vertical direction Z along the long hole 15 while inserted in the long hole 15. In normal times when no earthquake has occurred, the axle 13 abuts against the upper end of the long hole 15, and the load of the structure of the crane 1 is supported by the axle 13.

[0022] That is, the height position of the upper end of the elongated hole 15 is set to a height position where the axle 13 is in contact with the upper end of the elongated hole 15 when the crane 1 is in a normal state with almost no tilt. The height position of the lower end of the elongated hole 15 is set to the upper limit of the estimated distance (height) that the bogie 9 moves upward compared to normal times when the crane 1 tilts during an earthquake. The width dimension (dimension in the traveling direction Y) of the elongated hole 15 provided in the bogie 9 is set to be approximately the same as the diameter of the axle 14.

[0023] The biasing means 16 includes a compression coil spring 17 that is connected to the axle 13 and can expand and contract in the vertical direction Z. The expansion and contraction of the compression coil spring 17 biases the axle 13 downward relative to the bogie 9, so that a downward biasing force is constantly applied to the wheel 11.

[0024] In this embodiment, biasing means 16 (compression coil springs 17) are provided on both sides of the bogie 9 in the lateral direction X. Lower fixing portions 18 for fixing the lower ends of the compression coil springs 17 are provided at the upper portions of both ends of the axles 13 that protrude outward from the side surfaces of the bogie 9. Upper fixing portions 19 for fixing the upper ends of the compression coil springs 17 are provided at positions above the lower fixing portions 18 on the side surfaces of the bogie 9. For example, it is also possible to provide the biasing means 16 (compression coil springs 17) on only one side of the bogie 9 in the lateral direction X, or to directly connect the lower ends of the compression coil springs 17 to the axles 13 without providing the lower fixing portions 18 on the axles 13.

[0025] 2, a connecting mechanism 14 is provided at the connecting portion between the main equalizer 5 and the intermediate equalizer 7, and the intermediate equalizer 7 is connected to the main equalizer 5 in a state where the intermediate equalizer 7 can move relative to the main equalizer 5 in the vertical direction Z. Further, a biasing means 16 is provided between the main equalizer 5 and the intermediate equalizer 7, and the intermediate equalizer 7 is constantly biased downward with respect to the main equalizer 5.

[0026] The connecting mechanism 14 is configured to have an elongated hole 15 that is provided in the main equalizer 5 and extends in the vertical direction Z. An equalizer pin 6 that connects the main equalizer 5 and the intermediate equalizer 7 extends in the horizontal direction X and is inserted into the elongated hole 15 that is formed in the main equalizer 5 and penetrates in the horizontal direction X. The equalizer pin 6 is configured to be movable in the vertical direction Z relative to the main equalizer 5 along the elongated hole 15 while inserted in the elongated hole 15. During normal times when no earthquake has occurred, the equalizer pin 6 abuts against the upper end of the elongated hole 15, and the equalizer pin 6 supports the load of the structure of the crane 1 above the intermediate equalizer 7.

[0027] The biasing means 16 includes a compression coil spring 17 connected to the equalizer pin 6 and expandable in the vertical direction Z. The expansion and contraction of the compression coil spring 17 biases the equalizer pin 6 downward relative to the main equalizer 5, thereby constantly applying a downward biasing force to the wheel 11. In this embodiment, the compression coil springs 17 are provided on both sides of the main equalizer 5 in the lateral direction X. Lower fixing portions 18 for fixing the lower ends of the compression coil springs 17 are provided at the tops of both ends of the equalizer pin 6 that protrude outward beyond the side surface of the main equalizer 5, and upper fixing portions 19 for fixing the upper ends of the compression coil springs 17 are provided at positions above the lower fixing portions 18 on the side surface of the main equalizer 5. For example, it is possible to provide a biasing means 16 (compression coil spring 17) on only one side of the main equalizer 5 in the lateral direction X, or to provide no lower fixing portion 18 on the equalizer pin 6 and connect the lower end of the compression coil spring 17 directly to the equalizer pin 6.

[0028] The biasing means 16 is configured so that even when the wheel 11 moves in the up-down direction Z relative to the leg structure 2, the magnitude of the biasing force applied to the wheel 11 by the biasing means 16 is large enough to enable the wheel 11 to roll on the rail R and prevent the wheel 11 from lifting off the rail R. That is, each compression coil spring 17 is in its most compressed state when the wheel 11 is at the highest position in the range of movement allowed by the connecting mechanism 14, and the biasing force applied to the wheel 11 from the compression coil spring 17 is at its largest, but even at this time, the compression coil spring 17 applies to the wheel 11 a biasing force large enough to enable the wheel 11 to roll on the rail R. Furthermore, even when the wheel 11 is at the lowest position in the range of movement allowed by the connecting mechanism 14, each compression coil spring 17 is in its compressed state, and the compression coil spring 17 constantly applies a biasing force to the wheel 11 in a direction pressing the wheel 13 against the rail R.

[0029] In this way, in this crane 1, the wheels 11 are connected to the leg structure 2 by the connecting mechanism 14 in a state in which they can move relative to each other in the vertical direction Z, and the biasing means 16 constantly applies a biasing force that presses the wheels 11 toward the rail R. Therefore, the wheels 11 can be maintained in a state in which they are constantly pressed against the rail R, both under normal conditions and during an earthquake. In other words, as illustrated in FIGS. 4 and 5 , even if the crane 1 tilts in the traveling direction Y when an earthquake occurs, the biasing means 16 constantly applies a biasing force that presses the wheels 11 toward the rail R, so that the wheels 11 naturally move relative to the leg structure 2 of the crane 1 in the vertical direction Z, and the wheels 11 are maintained in contact with the rail R. Therefore, the wheels 11 can be more reliably prevented from lifting off the rail R immediately after an earthquake occurs, and the wheels 11 can be more reliably prevented from running off the rail R.

[0030] If the configuration is such that the connecting mechanism 14 and the biasing means 16 are provided at the connecting portion between the bogie 9 and the wheels 11 as in this embodiment, even if the bogie 9 tilts during an earthquake, for example, each wheel 11 moves individually in the vertical direction Z relative to the bogie 9, making it more difficult for each wheel 11 to lift off the rail R. Furthermore, the connecting mechanism 14 and the biasing means 16 provided at the connecting portion between the bogie 9 and the wheels 11 can be configured to be relatively compact compared to the connecting mechanism 14 and the biasing means 16 provided at the connecting portion between the lower part of the leg structure 2 and the main equalizer 5 or between the main equalizer 5 and the intermediate equalizer 7.

[0031] If the connecting mechanism 14 has elongated holes 15 extending in the vertical direction Z provided in the bogie 8, and the axles 13 are inserted into the elongated holes 15 so that the axles 13 can move in the vertical direction Z along the elongated holes 15, the axles 13 can be moved up and down stably on a constant trajectory relative to the bogie 8, despite the simple configuration. This reduces wobbling and vibration of the wheels 11 when the axles 13 move up and down relative to the bogie 8. Furthermore, if the axles 13 are configured to abut the upper ends of the elongated holes 15 during normal times when no earthquake has occurred, the load of the crane 1 structure is stably supported by the axles 13 during normal times, thereby enabling the crane 1 to travel stably during normal times despite the simple configuration. The vertical length of the elongated holes 15 can be set appropriately depending on the expected range of vertical movement of each axle 13 during an earthquake. For example, the vertical length of all the elongated holes 15 may be set to the same dimension, or the dimension may be different for each axle 13.

[0032] When there are multiple bogies 9 that make up the traveling unit 4, there may be a difference in weight between the bogies 9 that make up the same traveling unit 4. In the event of an earthquake, a bogie 9 that is relatively light in weight is more likely to be lifted up than a bogie 9 that is relatively heavy. For this reason, it is advisable to provide a coupling mechanism 14 and a biasing means 16 between a bogie 9 that is particularly light in weight and each of the wheels 11 provided on that bogie 9. Generally, a bogie 9 that has wheels 11 (driven wheels) without a drive mechanism is relatively light in weight, and a bogie 9 that has wheels 11 (driving wheels) with a drive mechanism is relatively heavy in weight. For this reason, it is advisable to provide a coupling mechanism 14 and a biasing means 16 especially for a bogie 9 that has wheels 11 (driven wheels) without a drive mechanism.

[0033] If a configuration is provided in which a connecting mechanism 14 and a biasing means 16 are provided in at least one of the connecting portion between the lower part of the leg structure 2 and the equalizer (main equalizer 5), the connecting portion between the equalizer (main equalizer 5) and another equalizer (intermediate equalizer 7) connected to its lower part, or the connecting portion between the equalizer (main equalizer 5 or intermediate equalizer 7) and the bogie 9, a small number of connecting mechanisms 14 and biasing means 16 can effectively prevent a large number of wheels 11 from coming off.

[0034] The connecting mechanism 14 has an elongated hole 15 extending in the vertical direction Z provided in the lower part of the leg structure 2 or in the equalizer (main equalizer 5 or intermediate equalizer 7). A connecting pin, either the equalizer pin 6 or the bogie pin 8, is inserted into the elongated hole 15, and the connecting pin is configured to be movable in the vertical direction Z along the elongated hole 15. This allows the connecting pin to move stably up and down relative to the leg structure 2 or the equalizer 6 on a constant trajectory, despite the simple configuration. This reduces rattle and vibration during the vertical movement of the equalizer or bogie 9. Furthermore, if the connecting pin is configured to abut the upper end of the elongated hole 15 during normal times when no earthquake has occurred, the load of the crane 1 structure is stably supported by the connecting pin during normal times. This allows the crane 1 to travel stably during normal times, despite the simple configuration.

[0035] In this embodiment, an example is given in which a connecting mechanism 14 and a biasing means 16 are provided at the connecting portion between the main equalizer 5 and the intermediate equalizer 7, but the same effect can also be obtained when a connecting mechanism 14 and a biasing means 16 are provided at the connecting portion between the lower part of the landing gear structure 2 and the main equalizer 5, or at the connecting portion between the intermediate equalizer 7 and the bogie 9.

[0036] The locations where the coupling mechanism 14 and the biasing means 16 are provided are not limited to this embodiment, and they can be appropriately selected from the coupling portion between the bogie 9 and the wheels 11, the coupling portion between the intermediate equalizer 7 and the bogie 9, the coupling portion between the main equalizer 5 and the intermediate equalizer 7, or the coupling portion between the lower part of the landing gear structure 2 and the main equalizer 5. Preferably, the coupling mechanism 14 and the biasing means 16 are provided at a plurality of coupling portions in the vertical direction Z. More preferably, the coupling mechanism 14 and the biasing means 16 are provided at a plurality of coupling portions in the vertical direction Z, including the coupling portion between the bogie 9 and the wheels 11.

[0037] By providing the coupling mechanisms 14 and the biasing means 16 at multiple connection points in the vertical direction Z, it becomes easier to ensure a long range of movement of the wheels 11 in the vertical direction Z. Furthermore, by providing the coupling mechanisms 14 and the biasing means 16 at the connection points between the bogie 9 and the wheels 11, each wheel 11 moves up and down individually, making it more difficult for the wheels 11 to lift off the rail R, which is more advantageous in preventing the wheels 11 from coming off.

[0038] As in this embodiment, a configuration in which coupling mechanisms 14 and biasing means 16 are provided for all of the wheels 11 of the crane 1 is even more advantageous in reducing the risk of wheels 11 coming off. In this embodiment, the coupling mechanisms 14 and biasing means 16 provided at the connecting portions between each main equalizer 5 and intermediate equalizer 7 are configured to prevent all of the wheels 11 of the crane 1 from coming off, but for example, coupling mechanisms 14 and biasing means 16 can also be provided at the connecting portions between all of the wheels 11 of the crane 1 and the bogie 9.

[0039] When the biasing means 16 is configured with a compression coil spring 17, the biasing means 16 can be configured simply and inexpensively, and the risk of malfunctioning of the biasing means 16 can be reduced. The installation position and fixing method of the compression coil spring 17 are not limited to this embodiment. For example, the compression coil spring 18 can be disposed inside the bogie 9 by providing a lower support portion 18 and an upper fixing portion 19 inside the bogie 9 (inner hollow portion).

[0040] The biasing means 16 is not limited to the compression coil spring 17, and various other configurations may be used as long as it is configured to constantly apply a biasing force to the wheel 11 in a direction pressing the wheel 11 against the rail R. For example, the biasing means 16 may be configured as a pressure mechanism that applies a biasing force using hydraulic pressure, air pressure, or the like.

[0041] Furthermore, in the embodiment illustrated above, the connecting mechanism 14 allows the wheel 11 to move relatively to the leg structure 2 in a direction substantially parallel to the vertical direction Z, but the connecting mechanism 14 may be configured to connect the wheel 11 to the leg structure 2 in a state where the wheel 11 is movable relatively to the leg structure 2 in the vertical direction Z. For example, the connecting mechanism 14 may be configured to allow the wheel 11 to move relatively to the leg structure 2 in an oblique direction (on the XZ plane) that includes a component in the vertical direction Z. Similarly, the direction of the biasing force applied to the wheel 11 by the biasing means 16 may also be set to an oblique direction (on the XZ plane) that includes a component in the vertical direction Z. That is, for example, the elongated hole 15 constituting the connecting mechanism 14 may be configured to extend in an oblique direction (on the vertical direction Z and the running direction Y) that includes a component in the vertical direction Z, or the compression coil spring 17 may be configured to expand and contract in an oblique direction. [Explanation of symbols]

[0042] 1 crane 2 leg structure 3 legs 4 Running gear 5 Main Equalizer 6 Equalizer Pin 7. Mid-Equalizer 8 Bogey Pin 9 bogeys 10 Bearing section 11 wheels 12 bearings 13 axles 14 Connection mechanism 15 long hole 16. Actuation means 17 Compression coil spring 18 Lower fixing part 19 Upper fixing part R rail

Claims

1. A crane comprising a traveling device provided at the bottom of a leg structure constituting the crane and allowing the crane to travel on rails, and wheels provided on the traveling device and rolling on the rails, a connecting mechanism that connects the wheels to the leg structure in a state in which the wheels can move relatively in the vertical direction, and a biasing means that constantly applies a biasing force to the wheels in a direction pressing the wheels against the rail.

2. 2. The crane according to claim 1, wherein the connecting mechanism and the biasing means are provided at a connecting portion between the bogie and the wheels that constitute the traveling device.

3. 3. The crane according to claim 2, wherein the connecting mechanism has a long hole extending in the vertical direction provided in the bogie, the axle of the wheel is inserted into the long hole so that the axle can move in the vertical direction along the long hole, and during normal times when no earthquake has occurred, the axle is in contact with the upper end of the long hole.

4. The crane according to any one of claims 1 to 3, wherein the connecting mechanism and the biasing means are provided in at least one of a connecting portion between the lower part of the leg structure and an equalizer constituting the traveling device, a connecting portion between the equalizer and another equalizer connected to the lower part thereof, or a connecting portion between the equalizer and a bogie constituting the traveling device.

5. 5. The crane according to claim 4, wherein the connecting mechanism has a vertically extending elongated hole provided in the lower part of the leg structure or in the equalizer, a connecting pin, either an equalizer pin or a bogie pin, is inserted into the elongated hole, and the connecting pin is configured to be movable vertically along the elongated hole, and during normal times when no earthquake has occurred, the connecting pin is in contact with the upper end of the elongated hole.

6. The crane according to any one of claims 1 to 5, wherein the coupling mechanism and the biasing means are provided for all of the wheels of the crane.

7. The crane according to any one of claims 1 to 6, wherein the biasing means has a compression coil spring, and the biasing force is constantly applied to the wheel by expansion and contraction of the compression coil spring.

8. A method for preventing wheels from coming off a crane, the method comprising: a traveling device provided at the bottom of a leg structure constituting the crane and allowing the crane to travel on rails; and wheels provided on the traveling device and rolling on the rails, A method for preventing a crane from coming off, characterized in that a connecting mechanism provided on the crane connects the wheel to the leg structure in a state in which the wheel can move relatively in the vertical direction, and a biasing means provided on the crane constantly applies a biasing force to the wheel in a direction pressing the wheel against the rail.

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