TWM572837U - Multi-point damping omni-directional wheel - Google Patents

Abstract

A multi-point shock absorbing omnidirectional wheel includes a hub, a plurality of dampers and a plurality of roller sets. The plurality of array slots are circumferentially disposed along the outer peripheral surface of the hub. The damping devices are respectively disposed in the grouping grooves. Each of the roller sets includes a wheel base and two rollers, each wheel seat has an upper portion and a lower portion, and a lower portion of each wheel seat protrudes from a bottom end of an upper portion of each wheel seat, and lower portions of the wheel bases are respectively disposed in the groups The groove is received and the bottom end thereof contacts the damping device, and the two rollers of each roller group are respectively disposed on two sides of the upper portion of the wheel base of each roller group and rotatable about an axis. Thereby, each damper device absorbs the vibration force generated by each roller group to achieve the multi-point shock absorbing buffer effect, and improves the stability of the vehicle when moving and the comfort of riding.

Description

Multi-point shock absorption omnidirectional wheel

The present invention relates to an omnidirectional wheel, and more particularly to a multi-point shock absorbing omnidirectional wheel for damping the shock generated when traveling on the ground.

The wheels of a typical vehicle are simply axially moved and can only move and rotate in a straight line and cannot move laterally in any direction. The advent of the omnidirectional wheel, in addition to providing linear movement and rotation of the carrier, can also provide lateral movement of the vehicle in any direction. The omni-directional wheel carrier has the advantage of flexible moving steering, which can move in multiple directions, and can travel anywhere without any obstacles even in a small space. Therefore, vehicles such as robots and wheelchairs are ideal for assembling omnidirectional wheels.

However, when the conventional omnidirectional wheel travels on an uneven road surface or a high or low drop road section or a debris on a road surface, vibration is easily generated, and the stability of the vehicle movement and the comfort of riding are reduced.

The main purpose of this creation is to provide a multi-point shock absorbing omnidirectional wheel. Each damper absorbs the vibration force generated by each roller set to achieve the multi-point shock absorbing buffer effect, and avoids the vibration force to further penetrate the hub and the axle. Pass to the vehicle to improve the smoothness of the vehicle and the comfort of the ride.

In order to achieve the foregoing objectives, the present invention provides a multi-point shock absorbing omnidirectional wheel including a hub, a plurality of dampers, and a plurality of roller sets. The hub defines a shaft hole and a plurality of array slots, wherein the shaft holes are for a plurality of axles to be disposed therein, and the assembly slots are circumferentially disposed along an outer circumferential surface of the hub. The damping devices are respectively disposed in the grouping grooves. Each of the roller sets includes a wheel base and two rollers, each wheel seat has an upper portion and a lower portion, and a lower portion of each wheel seat protrudes from a bottom end of an upper portion of each wheel seat, and lower portions of the wheel bases are respectively disposed in the groups The groove is received and the bottom end thereof contacts the damping device, and the two rollers of each roller group are respectively disposed on two sides of the upper portion of the wheel base of each roller group and rotatable about an axis.

Preferably, each set of slots includes a groove and a slot, the grooves are circumferentially disposed along the outer peripheral surface of the hub and extend axially parallel to the axis of the shaft hole, and the slots are respectively from the grooves The bottom wall of the groove extends radially in the direction of the shaft hole, and the damping devices are respectively disposed in the slots, and the bottom ends of the upper portions of the wheel seats are respectively disposed in the grooves, and the wheel seats are respectively The lower portions are respectively inserted into the slots and the bottom ends thereof are in contact with the damping devices.

Preferably, the length of each groove is larger than the diameter of each slot, the width of each groove is smaller than the diameter of each slot, the upper part of each wheel seat is plate-shaped and its thickness is equal to the width of each groove, and the width of each wheel seat The lower portion has a rod shape and has a diameter equal to the diameter of each of the slots, and the thickness of the upper portion of each of the wheel seats is smaller than the diameter of the lower portion of each of the wheel seats, and the length of the upper portion of each of the wheel seats is larger than the diameter of the lower portion of each of the wheel seats.

Preferably, the hub includes a first plate body and a second plate body. The first plate body defines a plate body shaft hole, a plurality of notches, and a plurality of recesses. The notches of the first plate body are along the first plate body. The outer peripheral surface is disposed circumferentially and extends axially parallel to the axis of the plate body shaft hole of the first plate body, and the concave portions of the first plate body are respectively close to the first plate body from the bottom walls of the notches of the first plate body One side of the inner side surface extends radially toward the shaft hole of the first plate body, and the second plate body defines a plate body shaft hole, a plurality of notches and a plurality of recesses, and the notches of the second plate body are along the second The outer peripheral surface of the plate body is circumferentially disposed and extends axially parallel to the axis of the plate body shaft hole of the second plate body, and the concave portions of the second plate body are respectively close to the second bottom wall of the notches of the second plate body One side of the inner side surface of the plate body extends radially toward the shaft hole of the second plate body, and the inner side surface of the first plate body is coupled to the inner side surface of the second plate body, and the plate body shaft hole of the first plate body Cooperating with the shaft hole of the plate body of the second plate body to form a shaft hole of the hub, the gaps of the first plate body are respectively opposite Such gap and the second plate member together form the trenches, such recess portion of the first plate respectively correspond to those of the recess of the second plate member and configured such socket together.

Preferably, the inner side surface of the first plate body is recessed with a ring groove and has a convex portion surrounding the outer side of the convex portion, and the plate body shaft hole of the first plate body penetrates the axis of the convex portion, and the second plate body a flange is protruded from the inner side surface, and the shaft hole of the second plate body penetrates the axial center of the flange, and after the inner side surface of the first plate body is coupled to the inner side surface of the second plate body, the convexity of the first plate body The portion is embedded in the shaft hole of the plate body of the second plate body, and the flange of the second plate body is embedded in the ring groove of the first plate body.

Preferably, the axes of the rollers are all disposed in a plane orthogonal to the axle, the axes of the rollers intersecting the radial direction of the hub such that the outer contours of the rollers are disposed on the same circumference centered on the axle.

Preferably, the upper part of each wheel base is provided with a perforation, and each roller has a roller shaft hole. Each roller set includes a rotating shaft and a plurality of bearings. The rotating shaft passes through the perforation of the upper part of each wheel base and the two ends thereof are disposed on the respective rollers. The roller shaft holes of the two rollers of the group are respectively disposed in the roller shaft holes of the two rollers of each roller group and are sleeved on the rotating shaft.

Preferably, the curvature of the outer contour of the rollers coincides with the curvature of the circle centered on the axle.

Preferably, the axes of the rollers intersect the radial direction of the hub and are perpendicular to each other, and each roller has a large diameter end and a small diameter end, and the diameter of each roller gradually decreases along the large diameter end toward the small diameter end. The large diameter end of each roller is close to the upper part of each wheel seat, the small diameter end of each roller is away from the upper part of each wheel seat, and the small diameter end of each roller is recessed with a notch; wherein, in the adjacent two roller group, The small diameter end of one of the rollers extends into the recess of the small diameter end of the other roller.

The effect of this creation is that when the vehicle equipped with the multi-point shock absorbing omnidirectional wheel of the present invention travels on uneven road surface or high and low drop road sections or debris on the road surface, the roller group will be generated. Violent vibration. At this time, each wheel seat can be slightly moved in each of the joint grooves and compressed to each damping device, and each damping device can absorb the vibration force generated by each roller group to achieve multi-point shock absorption. The effect of the cushioning is to prevent the vibration force from being transmitted to the carrier through the hub and the axle, thereby improving the smoothness of the carrier and the comfort of the ride.

The implementation of the present invention will be described in more detail below with reference to the drawings and component symbols, so that those skilled in the art can implement the present specification after studying the present specification.

Please refer to FIG. 1 to FIG. 7. FIG. 1 is a perspective view of the multi-point shock absorbing omnidirectional wheel 1 of the present invention, and FIG. 2 is a perspective view of another angle of the multi-point damped omnidirectional wheel 1 of the present invention. The exploded view of the multi-point shock absorbing omnidirectional wheel 1 is created. FIG. 4 is a perspective view of the first plate body 13 of the hub 10 of the multi-point damped omnidirectional wheel 1 of the present invention, and FIG. 5 is a multi-point shock absorption of the present invention. A perspective view of the second plate body 14 of the hub 10 of the omnidirectional wheel 1, FIG. 6 is a front perspective view of the multi-point shock absorbing omnidirectional wheel 1 of the present invention, and FIG. 7 is a multi-point shock absorbing omnidirectional wheel 1 of the present invention. Sectional view. The present invention provides a multi-point shock absorbing omnidirectional wheel 1, comprising a hub 10, a plurality of damper devices 20, and a plurality of roller sets 30. The hub 10 defines a shaft hole 11 and a plurality of array slots 12. The shaft hole 11 is for a shaft (not shown) to be inserted therein. The set of grooves 12 are circumferentially disposed along the outer peripheral surface of the hub 10. The damping devices 20 are respectively disposed in the assembly slots 12. Each of the roller sets 30 includes a wheel base 31 and two rollers 32, 33. Each of the wheel seats 31 has an upper portion 311 and a lower portion 312. The two rollers 32, 33 of each roller group 30 are respectively disposed on two sides of the upper portion 311 of the wheel base 31 of each roller group 30 and are rotatable about an axis.

As shown in FIG. 8, the multi-point shock absorbing omnidirectional wheel 1 of the present invention can be used as the front wheel of a wheelchair 2. However, not limited to this, the multi-point shock absorbing omnidirectional wheel 11 of the present invention can be installed on any vehicle (for example, a robot), which will be described first.

When the carrier of the multi-point damper omnidirectional wheel 1 (such as the wheelchair 2 shown in FIG. 8) equipped with the present invention travels on an uneven road surface or a high-low drop road section or a debris on a road surface, The roller set 30 will generate severe vibrations. At this time, each wheel base 31 can be slightly moved in each of the joint grooves 12 to be compressed to each of the damper devices 20, and each damper device 20 can absorb the vibration force generated by each of the roller groups 30, The multi-point shock absorption buffer is achieved, and the vibration force is further transmitted to the carrier through the hub 10 and the axle, thereby improving the stability of the carrier and the comfort of the ride.

Specifically, as shown in FIG. 3 , FIG. 6 and FIG. 7 , each of the set of slots 12 includes a groove 121 and a slot 122 . The grooves 121 are circumferentially disposed along the outer circumferential surface of the hub 10 and extend axially in parallel with the axis of the shaft hole 11. The slots 122 extend radially from the bottom wall of the grooves 121 toward the direction of the shaft hole 11, respectively. The damping devices 20 are respectively disposed in the slots 122. The bottom ends of the upper portions 311 of the wheel seats 31 are respectively disposed in the grooves 121. The lower portions 312 of the wheel bases 31 are respectively inserted into the slots 122 and the bottom ends thereof contact the damping devices 20. Thereby, each of the wheel seats 31 can be slightly moved radially in each of the joint grooves 12, and the bottom ends of the upper portions 311 of the wheel seats 31 are still held in the grooves 121, and the wheel seats 31 are held. The lower portion 312 remains in the slots 122 so that the wheel seats 31 do not disengage from the hub 10.

Preferably, as shown in FIG. 7, the length L1 of each of the grooves 121 is larger than the diameter D1 of each of the slots 122; as shown in FIG. 6, the width W1 of each of the grooves 121 is smaller than the diameter of each of the slots 122. D1; as shown in FIG. 3, FIG. 6, and FIG. 7, the upper portion 311 of each of the wheel bases 31 has a plate shape and has a thickness equal to the width W1 of each of the grooves 121, and the lower portion 312 of each of the wheel bases 31 has a rod shape and The diameter of the upper portion 311 of each of the wheel bases 31 is smaller than the diameter of the lower portion 312 of each of the wheel bases 31. The length of the upper portion 311 of each of the wheel bases 31 is greater than each of the wheel bases 31. The diameter of the lower portion 312. In other words, the length L1 of each of the grooves 121 is larger than the diameter of the lower portion 312 of each of the wheel bases 31, as shown in FIG. 7; the width W1 of each of the grooves 121 is smaller than the diameter of the lower portion 312 of each of the wheel seats 31, as shown in the figure. 6 is shown. When the wheel seats 31 are moved in the radial direction away from the shaft hole 11 of the hub 10, the top ends of the lower portions 312 of the wheel seats 31 contact the top walls of the slots 122. Therefore, the bottom ends of the upper portions 311 of the wheel seats 31 can be surely held in the grooves 121, and the lower portions 312 of the wheel seats 31 can be surely held in the slots 122, and thus the wheel seats 31 are not Will break away from the hub 10.

More specifically, the hub 10 includes a first plate body 13 and a second plate body 14. As shown in FIG. 3, FIG. 4, FIG. 6, and FIG. 7, the first plate body 13 defines a plate body shaft hole 131, a plurality of notches 132, and a plurality of recesses 133. The notches 132 of the first plate body 13 are along the first The outer peripheral surface of the plate body 13 is circumferentially disposed and extends axially parallel to the axis of the plate body shaft hole 131 of the first plate body 13, and the recesses 133 of the first plate body 13 are respectively offset from the first plate body 13 The side wall of the bottom wall of 132 is radially adjacent to the side of the inner side surface of the first plate body 13 in the direction of the plate body shaft hole 131 of the first plate body 13. As shown in FIG. 3 , FIG. 5 and FIG. 7 , the second plate body 14 defines a plate body shaft hole 141 , a plurality of notches 142 , and a plurality of recesses 143 . The notches 142 of the second plate body 14 are along the second plate body 14 . The outer peripheral surface is disposed around and extends axially parallel to the axis of the plate body shaft hole 141 of the second plate body 14, and the concave portions 143 of the second plate body 14 are respectively from the bottom of the notch 142 of the second plate body 14. The side of the wall adjacent to the inner side surface of the second plate body 14 extends radially toward the plate body shaft hole 141 of the second plate body 14. As shown in FIG. 1 , FIG. 2 , FIG. 3 and FIG. 7 , the inner side surface of the first plate body 13 is coupled to the inner side surface of the second plate body 14 , and the plate body axial hole 131 and the second plate body of the first plate body 13 . The plate body shaft holes 141 of the first plate body 13 and the notches 132 of the second plate body 14 respectively constitute the groove 121, and the plate holes 141 of the second plate body 14 respectively constitute the groove 121. The recesses 133 of a plate 13 respectively correspond to the recesses 143 of the second plate body 14 to form the slots 122.

In this embodiment, as shown in FIG. 4, the inner side surface of the first plate body 13 is recessed with a ring groove 134 and has a convex portion 135. The annular groove 134 surrounds the outer side of the convex portion 135, and the plate of the first plate body 13 The body shaft hole 131 penetrates the axis of the convex portion 135. As shown in FIG. 5, a flange 144 protrudes from the inner side surface of the second plate body 14, and the plate body shaft hole 141 of the second plate body 14 penetrates the axial center of the flange 144. As shown in FIG. 7 , after the inner side surface of the first plate body 13 is coupled to the inner side surface of the second plate body 14 , the convex portion 135 of the first plate body 13 is embedded in the plate body shaft hole 141 of the second plate body 14 . The flange 144 of the second plate body 14 is embedded in the ring groove 134 of the first plate body 13.

The damper devices 20 can be respectively disposed in the recesses 133 of the first plate body 13 or the recessed portions 143 of the second plate body 14 during assembly. The bottom ends of the upper portions 311 of the wheel seats 31 can be first The notches 132 of the first plate body 13 or the notches 142 of the second plate body 14 are respectively disposed. Next, the inner side surface of the first plate body 13 is bonded to the inner side surface of the second plate body 14, and assembly is completed. Therefore, the multi-point shock absorbing omnidirectional wheel 1 of the present invention is easy to assemble.

At the time of disassembly, the damper device 20 and the roller groups 30 can be separated from the hub 10 by separating the first plate body 13 and the second plate body 14. Therefore, the multi-point shock omnidirectional wheel 1 of the present creation is easy to disassemble.

It is worth mentioning that the multi-point damped omnidirectional wheel 1 shown in the figure is a single-row omnidirectional wheel, and the axes of the rollers 32, 33 are all arranged in a plane orthogonal to the axle (not shown). The axes of the rollers 32, 33 intersect the radial direction of the hub 10 such that the outer contours of the rollers 32, 33 are disposed on the same circumference centered on the axle. More specifically, as shown in FIG. 3 and FIG. 7, the upper portion 311 of each wheel base 31 defines a through hole 3111; each of the rollers 32, 33 has a roller shaft hole 321 and 331; and each roller group 30 includes a rotating shaft. And the plurality of bearings 351-354, the rotating shaft 34 passes through the through hole 3111 of the upper portion 311 of each wheel base 31 and the two ends thereof pass through the roller shaft holes 321 and 331 of the two rollers 32 and 33 of each roller group 30, The bearings 351-354 are respectively disposed in the roller shaft holes 321 and 331 of the two rollers 32 and 33 of each roller group 30 and are sleeved on the rotating shaft 34. The curvature of the outer contour of the rollers 32, 33 coincides with the curvature of the circle centered on the axle. The axes of the rollers 32, 33 intersect the radial direction of the hub 10 and are perpendicular to each other. As shown in FIG. 1, FIG. 2, FIG. 3 and FIG. 6, each of the rollers 32, 33 has a large diameter end 322, 332 and a small diameter end 323, 333, and the diameter of each of the rollers 32, 33 is along a large diameter. The ends 322, 332 gradually become smaller toward the small diameter ends 323, 333. The large diameter ends 322 and 332 of the rollers 32 and 33 are adjacent to the upper portion 311 of each of the wheel bases 31, and the small diameter ends 323 and 333 of the rollers 32 and 33 are away from the upper portion 311 of each of the wheel bases 31. A notch 3231, 3331 is recessed in the small diameter ends 323, 333 of the rollers 32, 33. In the adjacent two roller sets 30, the small diameter ends 323, 333 of one of the rollers 32, 33 extend into the recesses 3231, 3331 of the small diameter ends 323, 333 of the other rollers 32, 33.

The above description is only a preferred embodiment for explaining the present creation, and is not intended to impose any form of restriction on the creation, so that any modification or change related to the creation under the same creative spirit is provided. , should still be included in the scope of this creative intent.

1‧‧‧Multi-point shock absorbing wheel

10‧‧·wheels

11‧‧‧Axis hole

12‧‧‧ group slot

121‧‧‧ trench

122‧‧‧Slots

13‧‧‧First board

131‧‧‧Board shaft hole

132‧‧‧ gap

133‧‧‧ recess

134‧‧‧ Ring groove

135‧‧‧ convex

14‧‧‧Second plate

141‧‧‧ plate body shaft hole

142‧‧ ‧ gap

143‧‧‧ recess

144‧‧‧Flange

20‧‧‧ damping device

30‧‧‧Roller set

31‧‧‧ wheel seat

311‧‧‧ upper

3111‧‧‧Perforation

312‧‧‧ lower

32, 33‧‧‧ Wheels

321,331‧‧‧Roller shaft hole

322, 332‧‧‧ large diameter end

323, 333‧‧‧ small diameter end

3231, 3331‧‧ ‧ notches

34‧‧‧ shaft

351~354‧‧‧ bearing

2‧‧‧ Wheelchair

D1‧‧‧ diameter

L1‧‧‧ length

W1‧‧‧Width

Figure 1 is a perspective view of the multi-point shock absorbing omnidirectional wheel of the present invention. 2 is a perspective view of another angle of the multi-point shock absorbing omnidirectional wheel of the present invention. Figure 3 is an exploded view of the multi-point shock absorbing omnidirectional wheel of the present invention. 4 is a perspective view of the first plate body of the hub of the multi-point shock absorbing omnidirectional wheel of the present invention. Fig. 5 is a perspective view of the second plate body of the hub of the multi-point shock absorbing omnidirectional wheel of the present invention. Fig. 6 is a front perspective view of the multi-point shock absorbing omnidirectional wheel of the present invention. Figure 7 is a cross-sectional view of the multi-point shock absorbing omnidirectional wheel of the present invention. FIG. 8 is a schematic view of the multi-point shock absorbing omnidirectional wheel of the present invention as the front wheel of the wheelchair.

Claims (9)

A multi-point shock absorbing omnidirectional wheel comprises: a hub, a shaft hole and a plurality of array slots, wherein the shaft hole is for a wheel axle to be inserted therein, the group groove is arranged around the outer circumference of the hub a plurality of damper devices respectively disposed in the set of slots; and a plurality of roller sets each including a wheel base and two rollers, each of the wheel seats having an upper portion and a lower portion, each of the lower portions of the wheel base being convexly disposed a lower end of the upper portion of the wheel base, the lower portions of the wheel bases are respectively disposed in the assembly grooves, and the bottom ends thereof are in contact with the damping devices, and the two rollers of each roller group are respectively disposed on the roller groups The two sides of the upper portion of the wheel base are rotatable about an axis. The multi-point shock absorbing omnidirectional wheel according to claim 1, wherein each of the set of slots includes a groove and a slot, the grooves are circumferentially disposed along the outer circumferential surface of the hub and axially Extending parallel to the axis of the shaft hole, the slots respectively extending from the bottom wall of the groove to the direction of the shaft hole, and the damping devices are respectively disposed in the slots, the wheels The bottom ends of the upper portions of the seats are respectively disposed in the grooves, and the lower portions of the wheel seats are respectively inserted in the slots and the bottom ends thereof are in contact with the damping devices. The multi-point shock absorbing omnidirectional wheel according to claim 2, wherein each of the grooves has a length greater than a diameter of each of the slots, and each of the grooves has a width smaller than a diameter of each of the slots, and each of the slots The upper part of the wheel base is plate-shaped and has a thickness equal to the width of each of the grooves, and the lower part of each of the wheel bases has a rod shape and a diameter equal to the diameter of each of the slots, and the thickness of the upper portion of each of the wheel bases is smaller than each of the wheels The diameter of the lower portion of the seat, the length of the upper portion of each of the wheel seats is greater than the diameter of the lower portion of each of the wheel seats. The multi-point shock absorbing omnidirectional wheel according to claim 3, wherein the hub includes a first plate body and a second plate body, the first plate body defines a plate body shaft hole, a plurality of notches, and a plurality of recesses, the notches of the first plate body are circumferentially disposed along an outer circumferential surface of the first plate body and extend axially parallel to an axis of the plate body shaft hole of the first plate body, the first plate body The recesses extend from a side of the bottom wall of the notches of the first plate adjacent to an inner side surface of the first plate body to a direction of a plate body shaft hole of the first plate body, the second plate The body defines a plate body shaft hole, a plurality of notches, and a plurality of recesses. The notches of the second plate body are circumferentially disposed along the outer circumferential surface of the second plate body and extend axially with the plate body axis of the second plate body. The axes of the holes are parallel, and the recesses of the second plate body respectively from the bottom wall of the notches of the second plate body to the side of the inner side surface of the second plate body to the plate body axis of the second plate body The direction of the hole extends radially, and the inner side surface of the first plate body is coupled to the inner side surface of the second plate body, the first plate body The shaft hole of the plate body communicates with the shaft hole of the plate body of the second plate body to jointly form a shaft hole of the hub, and the notches of the first plate body respectively meet the notches of the second plate body to form the same The grooves, the recesses of the first plate body respectively corresponding to the recesses of the second plate body, together constitute the slots. The multi-point shock absorbing omnidirectional wheel of claim 3, wherein the inner side of the first plate body is recessed with a ring groove and has a convex portion surrounding the outer side of the convex portion, a shaft hole of the first plate body penetrates the axis of the convex portion, a flange is protruded from an inner side surface of the second plate body, and a shaft hole of the plate body of the second plate body penetrates the axis of the flange After the inner side surface of the first plate body is coupled to the inner side surface of the second plate body, the convex portion of the first plate body is embedded in the shaft hole of the plate body of the second plate body, and the convexity of the second plate body The edge is embedded in the annular groove of the first plate body. The multi-point shock absorbing omnidirectional wheel according to any one of claims 1 to 5, wherein the axes of the rollers are disposed in a plane orthogonal to the axle, and the axes of the rollers are The radial intersection of the hubs is such that the outer contour of each of the rollers is disposed on the same circumference centered on the axle. The multi-point shock absorbing omnidirectional wheel according to claim 6, wherein each of the wheel seats has a perforation, and each of the rollers has a roller shaft hole, and each roller group includes a rotating shaft and a plurality of bearings. The rotating shaft passes through the perforations of the upper portions of the wheel bases and the two ends thereof are disposed in the roller shaft holes of the two rollers of the roller groups. The bearings are respectively disposed on the roller shaft holes of the two rollers of the roller groups. Medium and sleeved on the shaft. The multi-point shock absorbing omnidirectional wheel of claim 6, wherein the curvature of the outer contour of the rollers coincides with the curvature of a circle centered on the axle. The multi-point shock absorbing omnidirectional wheel according to claim 6, wherein the axes of the rollers intersect the radial direction of the hub and are perpendicular to each other, and each of the rollers has a large diameter end and a small diameter end. The diameter of each roller gradually decreases along the large diameter end toward the small diameter end, and the large diameter end of each roller is close to the upper part of each wheel seat, and the small diameter end of each roller is away from each wheel seat. In the upper part, a recess is recessed in the small diameter end of each roller; wherein, in the adjacent two roller sets, the small diameter end of one of the rollers extends into the recess of the small diameter end of the other roller.