Air Step Board For Health

Abstract

An air step board for health is disclosed. The step board has an upper plate on the upper surface of a board frame, and a shock absorbing leg on a corner of an underside of the board frame. The board frame is formed using a synthetic resin material through injection molding such that a seating part is provided on the upper surface of the board frame, and a reinforcing rib is provided in the seating part to reduce a weight of the board frame and impart sufficient strength. A handle is provided on at least one side of the board frame. In this invention, the board frame having the reinforcing rib is manufactured through injection molding, thus enhancing the durability of the board frame, permitting mass production, and ensuring easy workability, and a reinforcing plate is provided between the reinforcing ribs and the upper plate, thus reliably withstanding shocks.

Description

TECHNICAL FIELD

The present invention relates, in general, to an air step board for health, which is used for conducting repeated skipping and landing, and absorbs shocks generated during exercise, using air pressure, thus mitigating the shocks. More particularly, the present invention relates to an air step board for health, in which a board frame is manufactured through injection molding using synthetic resin, and reinforcing ribs are provided in the board frame in such a way as to cross each other, thus increasing strength and realizing lightness, and in which an upper plate made of a shock absorbing material is welded to the reinforcing ribs, thus ensuring easy workability and preventing the upper plate from being removed from the board frame, and in which an air circulation unit having a perforated member surrounded with a casing is fitted into a ventilation opening formed in the upper end of a shock absorbing leg, thus preventing the removal of the air circulation unit and preventing noise from being generated by the sudden compression of the shock absorbing leg, when the air is discharged from the shock absorbing leg.

BACKGROUND ART

Generally, an air step board for health is sports equipment that is placed on a living room or a room floor and is used to repeatedly perform stationary jumping, skipping, and landing. The air step board includes a board frame which comprises a flat plate, a sheet member which surrounds the upper surface or the entire outer surface of the board frame, and shock absorbing legs which are attached to corners of the underside of the board frame, thus spacing the board frame apart from the floor and reducing the noise that is generated when landing.

Various kinds of sports equipment for repeatedly performing skipping and landing, like the above-mentioned air step board for health, have been proposed.

According to Korean U.M. Registration No. 20-298847, an air step board includes a board frame having the shape of a plate. Shock absorbing legs are provided on corners of the underside of the board frame, and function to absorb shocks applied to the board frame, in addition to supporting the board frame. Each shock absorbing leg includes a body which is open at the top thereof and has air circulation holes in the upper surface thereof. An air control member having flanges at regular intervals is fitted into the upper portion of the body. A disc-shaped cover is attached to the air control member.

An air step board of Korean U.M. Registration No. 20-385431 includes a rectangular board frame. Shock absorbing legs are mounted to corners of the underside of the board frame. Each shock absorbing leg includes an integral body, with a coil spring installed in the body. Alternatively, the shock absorbing leg may include a fixed body and a movable body which are separately manufactured and are subsequently coupled to each other, with a coil spring elastically installed in the space between the fixed and movable bodies. When a user jumps on the board frame, the coil spring is compressed and extended along with the body of the leg, thus absorbing shocks. Alternatively, the movable body moves relative to the fixed body to compress or extend the coil spring, thus absorbing shocks.

A shock absorbing leg for an air step board according to Korean U.M. Registration No. 20-421118 is mounted to the underside of a board frame, and functions to absorb and withstand a load applied to the board frame. The shock absorbing leg is open at the top thereof, and is constructed so that a cushioning member having in the center of the bottom thereof an air discharge hole is integrally provided on the bottom of a leg body. A vertical passage is formed through the cushioning member in such a way as to communicate with the air discharge hole, which is formed in the center of the cushioning member, and a plurality of horizontal passages is formed in the bottom of the cushioning member in such a way as to communicate with the vertical passage. Thus, when shocks are applied to the board frame, the leg body is compressed, so that the air is discharged from the body through the air discharge hole, the vertical passage, and the horizontal passages to the outside. In this way, shocks are absorbed.

An air step board for health and a shock absorbing leg thereof according to Korean Patent No. 10-446588 are constructed as follows. That is, a bolt is provided on each corner of the underside of the air step board, and a screw hole is formed in the upper portion of the shock absorbing leg, so that the bolt is screwed into the screw hole. The shock absorbing leg includes a body which is open at the bottom thereof. Shock absorbing means, each having an air control part and a closing part, are provided in the body in multiple layers. A plurality of air discharge holes is formed through the closing part. Thus, when shocks are applied to the air step board, the body is compressed, and the air is discharged from the body through the air discharge holes in the closing part to the outside.

A memory foam shock absorbing step board for health according to Korean Patent No. 10-574199 includes a rectangular board frame. A shock absorbing leg is attached to each corner on the bottom of the board frame. The shock absorbing leg includes a body which is open at the bottom thereof, with a coil spring installed in the wall of the body. A groove and an air hole are formed, respectively, in the outer circumference and the lower end of the body. The body is filled with sponge. When the body is compressed by shocks applied to the board frame, both the coil spring and the sponge are compressed, so that the shocks are absorbed.

DISCLOSURE OF INVENTION

Technical Problem

The conventional air step boards for health, constructed as described above, are problematic in that the board frame of each air step board is made of wood, such as plywood, so that the board frame made of the wood is easily deformed or damaged due to moisture, and in addition, the board frame does not withstand shocks which are repeatedly applied to the board frame during the use of the air step board, and thereby the board frame may break.

Further, in order to protect the board frame from moisture, the outer surface of the board frame may be covered with the sheet member. The sheet member is made of synthetic resin or rubber. Thus, an adhesive is applied to the surface of the board frame, and the sheet member must be pressed against the board frame for a predetermined period of time so as to be reliably attached to the board frame. However, it is very difficult to attach the sheet member to the board frame. Moreover, even though the sheet member is attached to the board frame using the adhesive, the sheet member may easily become detached from the board frame during use.

Further, the air discharge hole and the vertical passage are formed by simply boring holes through predetermined portions in the shock absorbing leg. Thus, the conventional air step boards are problematic in that noise is generated when a user repeatedly skips and lands on the board frame, so that the user cannot continue exercise, and thus avoids exercise.

Particularly, according to Korean U.M. Registration No. 20-298847, the air control member is fitted into the entire opening of the top of the leg body. The entire top of the leg body, including the air control member is covered with the cover, so that the construction is complex, and many parts are required, so that the cost of materials is high.

Technical Solution

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an air step board for health, in which a board frame having on an upper surface thereof reinforcing ribs is manufactured through injection molding, thus enhancing the durability of the board frame, permitting mass production, and ensuring easy workability, and in which a reinforcing plate is provided between the reinforcing ribs and an upper plate, thus reliably withstanding shocks that are repeatedly applied to the board frame due to repeated skipping and landing.

Another object of the present invention is to provide an air step board for health, in which an upper plate is simply fitted into the upper surface of a board frame in a one-touch manner, and is welded or attached at predetermined portions to the board frame, thus preventing the upper plate from being removed from the board frame or breaking during exercise, and in which a handle is provided on each side of the board frame, thus allowing a user to more conveniently hold the air step board, therefore making it easy to carry.

A further object of the present invention is to provide an air step board for health, in which an air circulation unit, having a perforated member surrounded by a casing, is fitted into a ventilation opening formed in the upper end of a shock absorbing leg, thus preventing the removal of the air circulation unit and preventing noise from being generated, due to the sudden compression of the shock absorbing leg when the air is discharged from the shock absorbing leg.

Yet another object of the present invention is to provide an air step board for health, in which a locking projection and a locking hole are provided in a leg fastening groove of a board frame and a leg fastening protrusion of a shock absorbing leg, respectively, such that the locking projection is locked into the locking hole, thus preventing the shock absorbing leg from being undesirably released during use, by fitting the leg fastening protrusion of the shock absorbing leg into the leg fastening groove of the board frame, therefore allowing the shock absorbing leg to be stably used for a lengthy period of time without the separation of the shock absorbing leg.

Advantageous Effects

According to the present invention, an air step board for health is constructed so that a board frame having on an upper surface thereof reinforcing ribs is manufactured through injection molding, thus enhancing the durability of the board frame, permitting mass production, and ensuring easy workability, and a reinforcing plate is provided between the reinforcing ribs and an upper plate, thus reliably withstanding shocks that are repeatedly applied to the board frame due to repeated skipping and landing.

Further, a handle is provided on each side of a board frame, thus making it more convenient for a user to hold an air step board, therefore making it easy to carry. An air circulation unit, having a perforated member surrounded by a casing, is fitted into a ventilation opening formed in an upper end of a shock absorbing leg, thus preventing the removal of the air circulation unit and preventing noise from being generated due to the sudden compression of the shock absorbing leg when the air is discharged from the shock absorbing leg.

Furthermore, a locking projection and a locking hole are provided in a leg fastening groove of a board frame and a leg fastening protrusion of a shock absorbing leg, respectively, such that the locking projection is locked into the locking hole, thus preventing the shock absorbing leg from being undesirably released during use, by fitting the leg fastening protrusion of the shock absorbing leg into the leg fastening groove in the board frame, therefore allowing the shock absorbing leg to be stably used for a lengthy period of time without the separation of the shock absorbing leg.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an air step board for health, according to the present invention;

FIG. 2 is a perspective view showing the air step board of FIG. 1, in which one shock absorbing leg is separated from the bottom of a board frame;

FIG. 3 is a plan view showing the board frame of the air step board for health, according to the present invention, in which an upper plate is removed from the board frame;

FIG. 4 is an exploded perspective view showing the shock absorbing leg of FIG. 2;

FIG. 5 is a vertical sectional view showing the state in which the shock absorbing leg is fastened to the board frame of the air step board for health, according to the present invention;

FIGS. 6 and 7 are sectional views showing air step boards for health, according to other embodiments of the present invention; and

FIG. 8 is a view showing an air circulation unit of an air step board for health, according to a further embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In order to accomplish the above objects, the present invention provides an air step board for health, having an upper plate on an upper surface of a board frame, and a shock absorbing leg provided on a corner of an underside of the board frame and stably supporting the board frame, wherein the board frame is formed using a synthetic resin material through injection molding such that a seating part is provided on the upper surface of the board frame to hold the upper plate seated on the seating part, and a reinforcing rib is provided in the seating part to reduce a weight of the board frame and impart sufficient strength.

In the air step board of the invention, a handle is provided on at least one side of the board frame to allow the air step board to be easily held and carried.

Further, a reinforcing plate is provided between the reinforcing rib of the board frame and the upper plate so as to impart sufficient strength to the board frame.

A fitting projection may protrude upwards from the reinforcing ribs, and a fitting hole may be formed in the underside of the upper plate or the reinforcing plate, so that the fitting projection is fitted into the fitting hole.

Further, the present invention provides an air step board for health, having an upper plate on an upper surface of a board frame and a shock absorbing leg provided on a corner on an underside of the board frame and stably supporting the board frame, wherein the shock absorbing leg includes at least one ventilation opening which is formed in an upper end of the shock absorbing leg and allows air to circulate in and out of the shock absorbing leg as it is compressed or extended, and an air circulation unit installed in the ventilation opening and including a perforated member which functions to prevent a noise from being generated when air circulates in and out of the shock absorbing leg, and a casing which has a hole to allow air to flow from an interior or exterior of the shock absorbing leg through the ventilation opening and the perforated member and surrounds the perforated member so as to prevent the perforated member from being removed from the ventilation opening by the air flowing through the ventilation opening.

The casing of the air circulation unit is made of metal or synthetic resin, and has a “U” shape to surround the perforated member. Further, the casing is made of synthetic resin, the casing comprises an open sheet having a bottom surface and first and second side surfaces which are to be folded along folding lines, a coupling pin protruding from the first side surface and a pin hole formed in the second side surface so that the coupling pin is inserted into the pin hole, whereby the casing is assembled into a “U” shape by folding the side surfaces along the folding lines.

Further, in order to prevent the air circulation unit from being undesirably removed from the ventilation opening of the shock absorbing leg, a removal prevention step is provided on an upper end in the ventilation opening of the shock absorbing leg, or a locking step is provided on the casing of the air circulation unit.

The air step board for health according to the present invention includes a locking means for preventing the undesirable separation of the shock absorbing leg, when the leg fastening protrusion of the shock absorbing leg is fitted into the leg fastening groove of the board frame in a one-touch manner and then a user continuously and repeatedly performs skipping or stepping. The locking means includes a locking projection which is provided in the leg fastening groove of the board frame contacting the lower surface of the leg fastening protrusion of the shock absorbing leg, and a locking hole which is formed in the lower surface of the leg fastening protrusion of the shock absorbing leg to correspond to the locking projection of the leg fastening groove.

Mode for the Invention

Hereinafter, an air step board according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a perspective view showing an air step board for health, according to the present invention, FIG. 2 is a perspective view showing the air step board of FIG. 1, in which one shock absorbing leg is separated from the bottom of a board frame, FIG. 3 is a plan view showing the board frame of the air step board for health, according to the present invention, in which an upper plate is removed the board frame, FIG. 4 is an exploded perspective view showing the shock absorbing leg of FIG. 2, and FIG. 5 is a vertical sectional view showing the state in which the shock absorbing leg is fastened to the board frame of the air step board for health, according to the present invention.

As shown in the drawings, the air step board for health according to the present invention includes a board frame 10, an upper plate 20, and shock absorbing legs 30. The board frame has the shape of a rectangle having rounded corners. The upper plate is attached or mounted to the upper surface of the board frame 10. The shock absorbing legs are mounted to four corners of the bottom of the board frame 10, and function to absorb and mitigate shocks.

A seating part 12 is provided on the upper surface of the board frame 10, so that the upper plate 20 is seated and fitted into the seating part. Further, reinforcing ribs 11 are provided in the seating part 12 so as to conserve the material of the board frame 10, that is, synthetic resin, reduce the weight of the board frame, and impart sufficient strength. Leg fastening grooves 13 are formed in four corners of the board frame, so that the shock absorbing legs 30 are fastened to the board frame.

The reinforcing ribs 11 may have various shapes, for example, circular, radial, or rectangular shapes. Further, various shapes of reinforcing ribs may be used together. As shown in the drawing, the reinforcing ribs 11 are provided on the upper surface of the board frame 10 in such a way as to cross each other. However, the reinforcing ribs 11 may be provided on the lower surface of the board frame 10 in such a way as to cross each other.

Further, a handle 14 is provided on at least one side of the board frame 10 to allow a user to easily hold and carry the air step board. The board frame 10 is formed through injection molding using a synthetic resin material. Preferably, when the reinforcing ribs 11, the leg fastening grooves 13, and the handle 14 of the board frame 10 are formed, they are formed integrally with the board frame 10 using one mold.

As long as the shock absorbing legs 30 are fastened to the corresponding leg fastening grooves 13 of the board frame 10 in a one-touch manner, any shape of fastening groove is possible. When shown from the bottom, L-type grooves each having the shape of an arc which is wide at one end thereof are formed along the same circumference in such a way as to be opposite each other.

Each shock absorbing leg 30 is a hollow bellows tube, and is open at the top thereof. Two arc-shaped leg fastening protrusions 31 are provided on the upper end of each shock absorbing leg, and correspond to the leg fastening grooves 13, which are provided in the lower surface of the board frame 10. Each leg fastening protrusion 31 has an angled cross-section which has a neck portion. Thus, when the leg fastening protrusion enters the wide end of each leg fastening groove 13 and thereafter is rotated, the neck portion of the leg fastening protrusion is locked to the leg fastening groove.

When the leg fastening protrusions 31 of each shock absorbing leg 30 are fitted into the corresponding leg fastening grooves 13 of the board frame 10 in a one-touch manner, and then a user continuously and repeatedly performs skipping or stepping, a locking means for preventing the undesirable separation of the shock absorbing leg 30 is required. The locking means for preventing the undesirable separation of the shock absorbing leg 30 includes a locking projection 13a, which is provided in a portion of the leg fastening groove 13 contacting the lower surface of each leg fastening protrusion 31, and a locking hole 32, which is formed in the lower surface of each leg fastening protrusion 31 of each shock absorbing leg 30 and is locked to the corresponding locking projection 13a of the leg fastening groove 13.

Further, one or more ventilation openings 33 are provided in the upper end of each shock absorbing leg 30, and cause the air to circulate in and out the shock absorbing leg 39 as it is compressed or extended. An air circulation unit 35 is installed in each ventilation opening 33 in such a way that it is not undesirably removed and prevents noise from being generated when the air is circulated.

The air circulation unit 35 includes a perforated member 36 and a casing 37. The perforated member prevents noise from being generated when the air is circulated in and out each shock absorbing leg 30 through the air circulation unit. Holes 38 are formed in the casing 37 such that the air flows in and out of each shock absorbing leg 30 through the ventilation opening 33 and the perforated member 36. The casing surrounds the perforated member 36 so as to prevent the perforated member from being removed from the ventilation opening 33 by the flow of the air passing through the ventilation opening 33.

Further, a removal prevention step 34 is provided on the upper end in an associated ventilation opening 33 of each shock absorbing leg 30. When the air circulation unit 35 is inserted into the corresponding ventilation opening, the air circulation unit is stopped by the removal prevention step, and thus the removal of the air circulation unit is prevented. As such, the removal prevention step 34 is provided on the upper end in an associated ventilation opening 33 to restrain both ends of the casing 37. Further, locking steps 39 having shapes corresponding to the removal prevention step may be provided on the casing 37 of the air circulation unit 35.

In the air step board for health according to the present invention, which is constructed as described above, the reinforcing ribs 11, which are provided on the upper surface of the board frame 10 and the bottom of the upper plate 20, which is to be seated on the reinforcing ribs 11 are attached at predetermined positions to each other using an adhesive, or are welded to each other, and the edges of the upper plate 20 are fitted into the seating part 12 of the board frame 10. In comparison with the prior art, in which the entire upper plate is attached to the board frame using an adhesive, the present invention considerably enhances productivity, and prevents the upper plate from being detached from the board frame during use.

Further, since the reinforcing ribs 11 are provided on the upper surface of the board frame 10 in such a way as to cross each other, the board frame 10 has sufficient strength, in addition to saving a material and achieving lightness. Hence, the upper plate 20 is firmly supported, so that the ability to withstand the load that is applied when skipping and landing are repeatedly performed is remarkably increased. The upper plate 20 may be made of the same material as the board frame 10, or may be made of a soft material so as to perform a shock absorbing operation to some extent.

FIG. 6 is a view showing the construction of an air step board for health, according to another embodiment of the present invention. This embodiment is equal to the first embodiment except that a reinforcing plate 15 is provided between the reinforcing ribs 11 provided on the upper surface of the board frame 10 and the upper plate 20 seated on the seating part 12 of the board frame 10. The reinforcing plate 15 may be secured to the reinforcing ribs 11 provided on the upper surface of the board frame 10, using a screw or an adhesive. However, ultrasonic welding, which melts and attaches surfaces of the reinforcing plate 15 and the reinforcing ribs 11, is preferred to ensure more reliable and firmer coupling.

In the air step board for health according to this embodiment, the reinforcing plate 15 is provided between the board frame 10 and the upper plate 20. Thus, the impact load generated when a user jumps or lands is primarily absorbed by the upper plate 20, and is indirectly transmitted to the reinforcing ribs 11. As such, since shocks and repeated load applied to the upper plate 20 are indirectly transmitted, the board frame 10 is more reliably reinforced.

FIG. 7 is a view showing the construction of an air step board for health, according to a further embodiment of the present invention. According to this embodiment, a reinforcement protruding part 16 protrudes from the center of the bottom of the board frame 10, and contacts the ground when each shock absorbing leg 30 is contracted by the load generated when a user jumps and lands on the board frame 10. The reinforcement protruding part 16 is formed integrally with the board frame 10 through injection molding using the same material as the board frame 10 or a material that is different from that of the board frame.

Therefore, the board frame 10 is reinforced by the reinforcement protruding part 16, which is provided on the lower surface of the board frame. Moreover, the reinforcement protruding part increases the ability of the board frame 10 to withstand the repeated load generated when a user repeatedly jumps and lands on the upper plate 20, which is provided on the upper surface of the board frame 10.

In a detailed description, as jumping action is repeatedly performed, the user's impact load is applied to the board frame 10. Due to the impact load, each shock absorbing leg 30 is contracted, and simultaneously, the board frame 10 is elastically deformed. At this time, the reinforcement protruding part 16, provided on the lower surface of the board frame 10, is supported by the ground on which the air step board is placed. As such, when the central portion of the board frame 10 is supported by the reinforcement protruding part 16, the impact load is reduced. Consequently, the effect of increasing the strength of the board frame is obtained. Therefore, the air step board according to this embodiment actively copes with the repeated load applied due to the jumping action, so that the durability and stability of the board frame 10 are further increased.

FIG. 8 is a view showing a casing of an air circulation unit of an air step board for health, according to another embodiment of the present invention. According to this embodiment, when a casing 35a is made of synthetic resin, it has the shape of an open sheet, which has a bottom surface 37a and both side surfaces 37b and 37c folded along folding lines 37d. Coupling pins 37e protrude from one side surface 37b, while pin holes 37f are formed in the other side surface 37c so that the coupling pins 37e are inserted into the corresponding pin holes. Thus, when the side surfaces are folded along the folding lines, the casing has a “U”-shaped profile. As in the embodiment of FIG. 4, a plurality of holes 38 must be formed in both side surfaces 37b and 37c of the casing 35a.

Therefore, when the air step board for health according to the present invention is manufactured, the casing 37 or 370 for the air circulation unit 35 may be manufactured using metal or synthetic resin to thus have a predetermined shape. Further, the casing may be manufactured by preparing an open sheet and then folding the sheet according to the predetermined shape. As such, the casing may be manufactured in various methods.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides an air step board for health, which allows a user to repeatedly perform skipping, landing, and stepping indoors without generating vibrations or noise.

Claims

1. An air step board for health, having an upper plate on an upper surface of a board frame, and a shock absorbing leg provided on a corner of an underside of the board frame and stably supporting the board frame, wherein

the board frame is formed using a synthetic resin material through injection molding such that a seating part is provided on the upper surface of the board frame to hold the upper plate seated on the seating part, and a reinforcing rib is provided in the seating part to reduce a weight of the board frame and impart sufficient strength.

2. The air step board according to claim 1, wherein a handle is provided on at least one side of the board frame to allow the air step board to be easily held and carried.

3. The air step board according to claim 1, wherein a reinforcing plate is provided between the reinforcing rib of the board frame and the upper plate so as to impart sufficient strength to the board frame.

4. The air step board according to claim 1, wherein a reinforcement protruding part protrudes from a center of a lower surface of the board frame, and increases ability to withstand a load which is repeatedly applied by a user.

5. An air step board for health, having an upper plate on an upper surface of a board frame and a shock absorbing leg provided on a corner on an underside of the board frame and stably supporting the board frame, wherein the shock absorbing leg comprises:

at least one ventilation opening formed in an upper end of the shock absorbing leg, and allowing air to circulate in and out of the shock absorbing leg as it is compressed or extended; and

an air circulation unit installed in the ventilation opening, and comprising:

a perforated member functioning to prevent a noise from being generated when air circulates in and out of the shock absorbing leg; and

a casing having a hole to allow air to flow from an interior or exterior of the shock absorbing leg through the ventilation opening and the perforated member, and surrounding the perforated member so as to prevent the perforated member from being removed from the ventilation opening by the air flowing through the ventilation opening.

6. The air step board according to claim 5, wherein the casing of the air circulation unit is made of metal or synthetic resin, and has a “U” shape to surround the perforated member.

7. The air step board according to claim 6, wherein, when the casing is made of synthetic resin, the casing comprises an open sheet having a bottom surface and first and second side surfaces which are to be folded along folding lines, a coupling pin protruding from the first side surface and a pin hole formed in the second side surface so that the coupling pin is inserted into the pin hole, whereby the casing is assembled into a “U” shape by folding the side surfaces along the folding lines.

8. The air step board according to claim 5, further comprising:

a removal prevention step provided on an upper end in the ventilation opening of the shock absorbing leg, thus preventing the inserted air circulation unit from being removed from the ventilation opening.

9. The air step board according to claim 8, further comprising:

a locking step provided on the casing of the air circulation unit, and locked by the removal prevention step provided on the upper end in the ventilation opening.

10. The air step board according to claim 5, wherein a locking projection is provided on a predetermined portion of a leg fastening groove which contacts a lower surface of a leg fastening protrusion of the shock absorbing leg, and a locking hole is formed in a lower surface of the leg fastening protrusion of the shock absorbing leg to be locked to the corresponding locking projection of the leg fastening groove.