A warehouse forklift

Abstract

The present invention discloses a warehouse forklift. It includes a mobile chassis and a fork assembly. The mobile chassis is U-shaped, including a walking mechanism symmetrically arranged on the left and right and a connecting plate connecting the rear ends of two walking mechanisms. The fork assembly is arranged between the two walking mechanisms. The mobile chassis is provided with a translation mechanism for driving the fork assembly to translate forward and backward. The fork assembly includes a frame, a fork and a lifting mechanism. The lifting mechanism is arranged on the frame, and the fork is connected to the lifting end of the lifting mechanism. The forklift aisle occupied by the present invention is narrow, and does not require a turning radius, so that the shelf density in the storage area can be greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of warehousing technology, and in particular to a warehousing forklift. Background Art

[0002] Forklifts are industrial handling vehicles, which refer to various wheeled handling vehicles for loading and unloading, stacking and short-distance transportation of palletized goods. With the development of the social logistics industry, the corresponding logistics forklifts are more and more widely used. In many storage warehouses of logistics companies, it is often seen that staff drive animal logistics forklifts to carry out cargo handling.

[0003] The forklift aisle that traditional forklifts need to occupy is relatively wide. Since traditional forklifts require a turning radius, a larger space must be reserved at the entrance and exit of the forklift aisle for traditional forklifts to turn. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a warehouse forklift, which needs to occupy a narrow forklift aisle and does not require a turning radius, so that the shelf density in the storage area can be greatly improved.

[0005] In order to solve the above problems, the present invention adopts the following technical solutions:

[0006] A warehouse forklift of the present invention comprises a mobile chassis and a fork assembly, wherein the mobile chassis is U-shaped and comprises a walking mechanism symmetrically arranged on the left and right and a connecting plate connecting the rear ends of the two walking mechanisms, the fork assembly is arranged between the two walking mechanisms, and a translation mechanism for driving the fork assembly to translate forward and backward is provided on the mobile chassis, the fork assembly comprises a frame, a fork and a lifting mechanism, the lifting mechanism is arranged on the frame, and the fork is connected to the lifting end of the lifting mechanism.

[0007] In this solution, the mobile chassis is used to drive the warehouse forklift to move. When putting goods, the mobile chassis drives the warehouse forklift to move to the corresponding shelf's loading position, the lifting mechanism drives the fork to rise to the height of the corresponding loading position, the translation mechanism drives the fork assembly to move forward, so that the goods on the fork are moved to the top of the corresponding loading position, then the lifting mechanism drives the fork to descend a certain distance, so that the goods are placed on the corresponding loading position, and finally, the translation mechanism drives the fork assembly to move backward, and the fork moves out of the loading position to complete the loading. When picking up goods, the mobile chassis drives the warehouse forklift to move to the corresponding shelf's loading position, the lifting mechanism drives the fork to rise to the height of the corresponding loading position, the translation mechanism drives the fork assembly to move forward, so that the fork extends under the goods on the corresponding loading position, then the lifting mechanism drives the fork to rise a certain distance, lifts the goods up and does not touch the loading position, and finally, the translation mechanism drives the fork assembly to move backward, and the fork carries the goods out of the loading position to complete the picking.

[0008] The warehouse forklift of this solution sets the fork assembly in a U-shaped mobile chassis. When picking up and placing goods, the fork assembly can be moved forward and backward independently to pick up and place goods, and only a narrow forklift aisle is needed to work normally. Unlike traditional warehouse forklifts, the forks are set on the front side of the vehicle body, and the entire vehicle body needs to be moved forward and backward to pick up and place goods, which requires a wider forklift aisle.

[0009] Preferably, the walking mechanism comprises a base, a walking assembly is provided at the bottom of the base, a rotating motor for driving the walking assembly to rotate is provided on the base, and the walking assembly comprises a driving wheel and a walking motor for driving the driving wheel to rotate.

[0010] The rotating motor can adjust the direction of the travel assembly so that the travel assembly can move in any direction. The warehouse forklift moves horizontally left and right in the forklift aisle. After the warehouse forklift leaves the forklift aisle, the rotating motor adjusts the travel assembly to rotate 90 degrees, so that the warehouse forklift can move forward and backward. Therefore, the warehouse forklift of this solution does not need a turning radius, does not need to occupy a large space at the entrance and exit of the forklift aisle, and the width of the forklift aisle does not need to be very wide.

[0011] Preferably, the translation mechanism includes a translation component symmetrically arranged on two walking mechanisms, the translation component is arranged along the front and rear direction of the walking mechanism, the translation component includes a transmission component, a support, a guide rail arranged on the support, a slider arranged on the guide rail and slidable along the guide rail, the slider is fixedly connected to the frame, the transmission component includes a sprocket arranged at the front and rear ends of the walking mechanism, a chain mounted on the two sprockets, and a chain connecting block arranged on the chain, the direction of the chain is consistent with the direction of the guide rail, the chain connecting block is fixedly connected to the frame, the sprocket is rotatably connected to the walking mechanism, and the translation mechanism also includes a driving module for driving the sprocket to rotate.

[0012] The support is arranged along the front and rear direction of the walking mechanism, and the direction of the guide rail is consistent with the support. The driving module drives the sprocket to rotate, the sprocket rotation drives the chain to rotate, and the chain rotation drives the frame to move forward and backward through the chain connecting block, thereby realizing the forward and backward translation function of the translation mechanism.

[0013] Preferably, the support includes a pin hole plate and a plurality of pin shafts arranged on the upper surface of the walking mechanism, the pin hole plate is provided with pin holes at positions corresponding to the pin shafts, the pin shafts are inserted into the corresponding pin holes, the diameter of the pin holes is larger than the diameter of the pin shafts, and the guide rail is arranged on the pin hole plate.

[0014] The pin hole diameter is slightly larger than the pin shaft diameter, so that when the warehouse forklift moves on uneven ground, there is flexibility between the traveling mechanism and the fork assembly, which plays a shock-absorbing role.

[0015] Preferably, the driving module comprises a driving motor and a transmission shaft, both ends of the transmission shaft are coaxially connected to sprockets at the rear ends of the two walking mechanisms, and the driving motor is used to drive the transmission shaft to rotate.

[0016] Preferably, the cargo fork comprises a vertical plate and two first horizontal plates arranged at the bottom of the vertical plate, the vertical plate is connected to the lifting end of the lifting mechanism, the first horizontal plate is arranged horizontally along the front-back direction, and a first camera is arranged on the front side of the vertical plate. The first camera is used to locate the height and position of the cargo, and the first horizontal plate is used to carry the cargo.

[0017] Preferably, the bottom of the frame is provided with two second horizontal plates arranged horizontally along the front-back direction, the two second horizontal plates are respectively located directly below the two first horizontal plates, and the bottom of the second horizontal plates is provided with a first universal wheel. The provision of the second horizontal plates makes the frame more stable, so that the forklift can carry heavier goods, and avoids the forklift from tipping over due to the unstable center of gravity when carrying heavier goods.

[0018] Preferably, a first guide wheel is provided inside the walking mechanism, and the first guide wheel abuts against the adjacent second transverse plate. The first guide wheel abuts against the second transverse plate on the corresponding side, guiding the forward and backward translation of the second transverse plate, and reducing the interaction force between the guide rail and the slider.

[0019] Preferably, when the first cross board is lowered to the lowest position, the distance between the upper surface of the first cross board and the ground is ≤ the height of the bottom groove of the pallet for placing goods; the width of the first cross board and the width of the second cross board are both ≤ the width of the bottom groove of the pallet for placing goods.

[0020] The goods are placed on the pallet. When the pallet on the ground and the goods on it need to be moved, the fork is lowered to the lowest position, and then the first cross plate and the second cross plate below it are inserted into the groove at the bottom of the pallet on the ground, and then the pallet is lifted for transportation.

[0021] Warehouse forklifts can carry pallets placed on the ground and the goods on them, improving the utilization rate of the storage area. If the goods cannot be stacked, the pallets and the goods on them can be placed on the shelves; if the goods can be stacked (such as the goods are in the shape of a rectangular block), the pallets used to place the upper goods can be directly stacked on the lower goods without the need for shelves, further improving the cargo density of the storage area.

[0022] Preferably, the frame includes uprights symmetrically arranged on the left and right sides, and vertical guide grooves are symmetrically arranged on the inner sides of the two uprights. The left and right sides of the rear surface of the vertical plate are symmetrically provided with guide mechanisms, and the guide mechanisms include a second guide wheel and a third guide wheel, the second guide wheel is embedded in the guide groove and contacts the front and rear side walls of the guide groove, the third guide wheel is perpendicular to the second guide wheel, and the third guide wheel contacts the inner wall of the guide groove. The second guide wheel and the third guide wheel play a guiding role in the lifting of the fork.

[0023] Preferably, the bottom of the base is further provided with two second universal wheels, which are respectively located at the front and rear sides of the walking assembly.

[0024] Preferably, a second camera is provided at the bottom of the base of one of the walking mechanisms. The second camera is used to identify the guide line or navigation QR code on the ground of the storage area, to help the storage forklift to locate, and to cooperate with the mobile chassis to realize free movement in the storage area.

[0025] The beneficial effects of the present invention are as follows: the forklift aisle to be occupied is narrow, no turning radius is required, and the shelf density in the storage area can be greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of an embodiment;

[0027] Figure 2 is a schematic diagram of a fork of an embodiment extending forward;

[0028] Figure 3 It is a schematic diagram of the connection state between the mobile chassis and the fork assembly;

[0029] Figure 4 yes Figure 3 Rear view of

[0030] Figure 5 yes Figure 3 A top view of

[0031] Figure 6 It is a structural schematic diagram of a mobile chassis;

[0032] Figure 7 It is a bottom view of the mobile chassis;

[0033] Figure 8 It is a bottom view of the fork assembly;

[0034] Fig. 9 It is a schematic diagram of the structure of the fork;

[0035] Fig.10 yes Fig. 9 Rear view of

[0036] Fig.11It is a schematic diagram of the structure of the pallet;

[0037] Fig.12 This is a schematic diagram of a warehouse forklift working in a storage area with shelves;

[0038] Fig.13 It is a schematic diagram of a warehouse forklift working in a storage area without shelves.

[0039] In the figure: 1, mobile chassis, 2, fork assembly, 3, walking mechanism, 4, connecting plate, 5, frame, 6, fork, 7, lifting mechanism, 8, base, 9, rotating motor, 10, driving wheel, 11, walking motor, 12, guide rail, 13, slider, 14, sprocket, 15, chain, 16, chain connecting block, 17, pin hole plate, 18, pin shaft, 19, pin hole, 20, driving motor, 21, transmission shaft, 22, vertical plate, 23, first horizontal plate, 24, first camera, 25, second horizontal plate, 26, first universal wheel, 27, first guide wheel, 28, pallet, 29, groove, 30, column, 31, guide groove, 32, second guide wheel, 33, third guide wheel, 34, second universal wheel, 35, second camera, 36, warehouse forklift, 37, shelf, 38, cargo. DETAILED DESCRIPTION

[0040] The technical solution of the present invention is further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0041] A storage forklift of this embodiment, such as Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 As shown, it includes a mobile chassis 1 and a fork assembly 2. The mobile chassis 1 is U-shaped, including a walking mechanism 3 symmetrically arranged on the left and right and a connecting plate 4 connecting the rear ends of the two walking mechanisms 3. The fork assembly 2 is arranged between the two walking mechanisms 3. The mobile chassis 1 is provided with a translation mechanism for driving the fork assembly 2 to translate forward and backward. The fork assembly 2 includes a frame 5, a fork 6 and a lifting mechanism 7. The lifting mechanism 7 is arranged on the frame 5, and the fork 6 is connected to the lifting end of the lifting mechanism 7.

[0042] like Figure 6 , Figure 7 As shown, the walking mechanism 3 includes a base 8, a walking component is provided at the bottom of the base 8, a rotating motor 9 for driving the walking component to rotate is provided on the base 8, the walking component includes a driving wheel 10 and a walking motor 11 for driving the driving wheel 10 to rotate, and two second universal wheels 34 are also provided at the bottom of the base 8, and the two second universal wheels 34 are respectively located at the front and rear sides of the walking component, and a second camera 35 is provided at the bottom of the base 8 of one of the walking mechanisms 3.

[0043] like Figure 3 , Figure 8 , Fig. 9 , Fig.10 As shown, the fork 6 includes a vertical plate 22 and two first horizontal plates 23 arranged at the bottom of the vertical plate 22, the vertical plate 22 is connected to the lifting end of the lifting mechanism 7, the first horizontal plate 23 is horizontally arranged along the front-to-back direction, a first camera 24 is arranged on the front side of the vertical plate 22, and two second horizontal plates 25 are horizontally arranged along the front-to-back direction are arranged at the bottom of the frame body 5, the two second horizontal plates 25 are respectively located directly below the two first horizontal plates 23, and two first universal wheels 26 are arranged at the bottom of the second horizontal plate 25.

[0044] like Fig.11 , 12 As shown, the goods 38 are placed on the pallet 28, which is in a rectangular shape. A groove 29 for inserting the first transverse board 23 is provided at the bottom of the pallet 28 corresponding to the first transverse board 23. When the first transverse board 23 is lowered to the lowest position, the distance between the upper surface of the first transverse board 23 and the ground is ≤ the height of the bottom groove 29 of the pallet 28 for placing the goods 38; the width of the first transverse board 23 and the width of the second transverse board 25 are both ≤ the width of the bottom groove 29 of the pallet 28 for placing the goods 38.

[0045] In this solution, the mobile chassis is used to drive the warehouse forklift to move. The rotating motor can adjust the direction of the travel assembly so that the travel assembly can move in any direction. The warehouse forklift moves horizontally left and right in the forklift aisle. After the warehouse forklift leaves the forklift aisle, the rotating motor adjusts the travel assembly to rotate 90 degrees, so that the warehouse forklift can move forward and backward. Therefore, the warehouse forklift of this solution does not need a turning radius, does not need to occupy a large space at the entrance and exit of the forklift aisle, and the width of the forklift aisle does not need to be very wide.

[0046] The second camera is used to identify guide lines or navigation QR codes on the ground in the storage area to help locate the storage forklift and cooperate with the mobile chassis to achieve free movement in the storage area.

[0047] The goods are placed on the pallet, and the first camera is used to locate the height and position of the pallet to facilitate adjustment of the warehouse forklift so that the fork can smoothly move the pallet and the goods on it.

[0048] When putting goods, the mobile chassis drives the storage forklift to move to the corresponding shelf's storage position, the lifting mechanism drives the fork to rise to the height of the corresponding storage position, and the translation mechanism drives the fork assembly to move forward, so that the pallet on the fork and the goods on it move to the top of the corresponding storage position, then the lifting mechanism drives the fork to descend a certain distance, so that the pallet and the goods on it are placed on the corresponding storage position, and finally, the translation mechanism drives the fork assembly to move backward, and the fork moves out of the storage position to complete the putting of goods. When picking up goods, the mobile chassis drives the storage forklift to move to the corresponding shelf's storage position, the lifting mechanism drives the fork to rise to the height of the corresponding storage position, and the translation mechanism drives the fork assembly to move forward, so that the fork extends into the bottom groove of the pallet on the corresponding storage position, then the lifting mechanism drives the fork to rise a certain distance, lift the pallet and the goods on it upwards without contacting the storage position, and finally, the translation mechanism drives the fork assembly to move backward, and the fork holds the pallet and the goods on it and moves out of the storage position to complete the picking up.

[0049] When the pallet on the ground and the goods on it need to be moved, the fork is lowered to the lowest position, and then the first cross plate and the second cross plate thereunder are inserted into the groove at the bottom of the pallet on the ground, and then the pallet is lifted up for moving.

[0050] The warehouse forklift of this solution sets the fork assembly in a U-shaped mobile chassis. When picking up and placing goods, the fork assembly can be moved forward and backward independently to pick up and place goods, and only a narrow forklift aisle is needed to work normally. Unlike traditional warehouse forklifts, the forks are set on the front side of the vehicle body, and the entire vehicle body needs to be moved forward and backward to pick up and place goods, which requires a wider forklift aisle.

[0051] The warehouse forklift can also carry the pallet 28 and the goods on it placed on the ground, thereby improving the utilization rate of the storage area. If the goods 38 cannot be stacked, the warehouse forklift 36 places the pallet 28 and the goods 38 on it on the shelf 37, such as Fig.12 If the goods 38 can be stacked (such as the goods are in the shape of a cuboid), the warehouse forklift 36 can directly stack the pallet 28 for placing the upper goods 38 on the lower goods 38 without the need for shelves, further increasing the density of goods in the storage area, such as Fig.13 shown.

[0052] The provision of the second cross plate makes the frame more stable, enables the forklift to carry heavier goods, and prevents the forklift from tipping over due to unstable center of gravity when carrying heavier goods.

[0053] The mobile chassis is also provided with a controller and a wireless communication module, and the controller is electrically connected to the mobile chassis, the fork assembly, the translation mechanism, and the wireless communication module. The controller controls the operation of the entire warehouse forklift, and the wireless communication module is used to receive wireless instructions sent by the management center.

[0054] like Figure 3 , Figure 4 , Figure 5 , Figure 6 As shown, the translation mechanism includes translation components symmetrically arranged on two walking mechanisms 3, the translation components are arranged along the front and rear directions of the walking mechanisms 3, the translation components include a transmission component, a support, a guide rail 12 arranged on the support, a slider 13 arranged on the guide rail 12 and capable of sliding along the guide rail 12, the slider 13 is fixedly connected to the frame 5, the transmission component includes a sprocket 14 arranged at the front and rear ends of the walking mechanism 3, a chain 15 sleeved on the two sprockets 14, and a chain connecting block 16 arranged on the chain 15, the direction of the chain 15 is consistent with the direction of the guide rail 12, the chain connecting block 16 is fixedly connected to the frame 5, the sprocket 14 is rotationally connected to the walking mechanism 3, the translation mechanism also includes a driving module for driving the sprocket 14 to rotate, the support includes a pin hole plate 17 and a plurality of pin shafts 18 arranged on the upper surface of the walking mechanism 3, a pin hole 19 is provided at a position corresponding to the pin shaft 18 on the pin hole plate 17, the pin shaft 18 is inserted into the corresponding pin hole 19, the diameter of the pin hole 19 is larger than the diameter of the pin shaft 18, the guide rail 12 is arranged on the pin hole plate 17, the driving module includes a driving motor 20 and a transmission shaft 21, both ends of the transmission shaft 21 are coaxially connected to the sprockets 14 at the rear ends of the two walking mechanisms 3, and the driving motor 20 is used to drive the transmission shaft 21 to rotate.

[0055] The support is arranged along the front and rear direction of the walking mechanism, and the direction of the guide rail is consistent with the support. The driving module drives the sprocket to rotate, the sprocket rotation drives the chain to rotate, and the chain rotation drives the frame to move forward and backward through the chain connecting block, thereby realizing the forward and backward translation function of the translation mechanism.

[0056] The pin hole diameter is slightly larger than the pin shaft diameter, so that when the warehouse forklift moves on uneven ground, there is flexibility between the traveling mechanism and the fork assembly, which plays a shock-absorbing role.

[0057] A first guide wheel 27 is provided inside the walking mechanism 3, and the first guide wheel 27 abuts against the adjacent second transverse plate 25. The first guide wheel abuts against the second transverse plate on the corresponding side, guides the forward and backward translation of the second transverse plate, and reduces the interaction force between the guide rail and the slider.

[0058] like Figure 3 , Fig. 9 , Fig.10As shown, the frame 5 includes upright posts 30 symmetrically arranged on the left and right sides, and vertical guide grooves 31 are symmetrically arranged on the inner sides of the two upright posts 30. A guide mechanism is symmetrically arranged on the left and right sides of the rear surface of the vertical plate 22. The guide mechanism includes two second guide wheels 32 and two third guide wheels 33. The second guide wheel 32 is embedded in the guide groove 31 and contacts the front and rear side walls of the guide groove 31. The third guide wheel 33 is perpendicular to the second guide wheel 32. The third guide wheel 33 contacts the inner wall of the guide groove 31. The two second guide wheels 32 and the two third guide wheels 33 are arranged in a straight line from top to bottom, and the two third guide wheels 33 are located between the two second guide wheels 32. The second guide wheel and the third guide wheel guide the lifting of the fork.

Claims

1. A warehouse forklift, characterized in that: The invention comprises a mobile chassis (1) and a fork assembly (2), wherein the mobile chassis (1) is in a U shape and comprises a walking mechanism (3) symmetrically arranged on the left and right and a connecting plate (4) connecting the rear ends of the two walking mechanisms (3), wherein the fork assembly (2) is arranged between the two walking mechanisms (3), and the mobile chassis (1) is provided with a translation mechanism for driving the fork assembly (2) to translate forward and backward, wherein the fork assembly (2) comprises a frame (5), a fork (6) and a lifting mechanism (7), wherein the lifting mechanism (7) is arranged on the frame (5), and the fork (6) is connected to the lifting end of the lifting mechanism (7); wherein the translation mechanism comprises a translation assembly symmetrically arranged on the two walking mechanisms (3), wherein the translation assembly is moved forward and backward along the walking mechanisms (3), and wherein the fork (6) is moved forward and backward along the walking mechanisms (3). The direction setting, the translation assembly includes a transmission assembly, a support, a guide rail (12) arranged on the support, a slider (13) arranged on the guide rail (12) and capable of sliding along the guide rail (12), the slider (13) being fixedly connected to the frame (5), the transmission assembly includes a sprocket (14) arranged at the front and rear ends of the walking mechanism (3), a chain (15) sleeved on the two sprockets (14), and a chain connecting block (16) arranged on the chain (15), the direction of the chain (15) is consistent with the direction of the guide rail (12), the chain connecting block (16) is fixedly connected to the frame (5), the sprocket (14) is rotationally connected to the walking mechanism (3), and the translation mechanism also includes a driving module for driving the sprocket (14) to rotate; The support comprises a pin hole plate (17) and a plurality of pin shafts (18) arranged on the upper surface of the walking mechanism (3); a pin hole (19) is arranged at a position corresponding to the pin shaft (18) on the pin hole plate (17); the pin shaft (18) is inserted into the corresponding pin hole (19); the diameter of the pin hole (19) is larger than the diameter of the pin shaft (18); and the guide rail (12) is arranged on the pin hole plate (17); The driving module comprises a driving motor (20) and a transmission shaft (21), the two ends of the transmission shaft (21) are respectively coaxially connected to the sprockets (14) at the rear ends of the two walking mechanisms (3), and the driving motor (20) is used to drive the transmission shaft (21) to rotate; The cargo fork (6) comprises a vertically arranged vertical plate (22) and two first horizontal plates (23) arranged at the bottom of the vertical plate (22); the vertical plate (22) is connected to the lifting end of the lifting mechanism (7); the first horizontal plate (23) is horizontally arranged along the front-back direction; the bottom of the frame body (5) is provided with two second horizontal plates (25) horizontally arranged along the front-back direction; the two second horizontal plates (25) are respectively located directly below the two first horizontal plates (23); and the bottom of the second horizontal plate (25) is provided with a first universal wheel (26).

2. A warehouse forklift according to claim 1, characterized in that: The walking mechanism (3) comprises a base (8), a walking assembly is provided at the bottom of the base (8), a rotating motor (9) for driving the walking assembly to rotate is provided on the base (8), and the walking assembly comprises a driving wheel (10) and a walking motor (11) for driving the driving wheel (10) to rotate.

3. A warehouse forklift according to claim 1, characterized in that: A first guide wheel (27) is provided inside the walking mechanism (3), and the first guide wheel (27) abuts against an adjacent second transverse plate (25).

4. A warehouse forklift according to claim 1, characterized in that: When the first transverse plate (23) is lowered to the lowest position, the distance between the upper surface of the first transverse plate (23) and the ground is ≤ the height of the bottom groove (29) of the tray (28) for placing the goods (38); the width of the first transverse plate (23) and the width of the second transverse plate (25) are both ≤ the width of the bottom groove (29) of the tray (28) for placing the goods (38).

5. A warehouse forklift according to claim 1, characterized in that: The frame (5) comprises upright posts (30) symmetrically arranged on the left and right sides, and vertical guide grooves (31) are symmetrically arranged on the inner sides of the two upright posts (30). Guide mechanisms are symmetrically arranged on the left and right sides of the rear surface of the vertical plate (22), and the guide mechanisms comprise a second guide wheel (32) and a third guide wheel (33). The second guide wheel (32) is embedded in the guide groove (31) and contacts the front and rear side walls of the guide groove (31). The third guide wheel (33) is perpendicular to the second guide wheel (32), and the third guide wheel (33) contacts the inner wall of the guide groove (31).

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