US20120210990A1

US20120210990A1 - Arrow shooting device

Info

Publication number: US20120210990A1
Application number: US13/502,391
Authority: US
Inventors: Kyung Sin Park
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-12-03
Filing date: 2010-11-23
Publication date: 2012-08-23
Also published as: KR20100133346A; WO2011068319A3; CN102597688A; WO2011068319A2

Abstract

The present invention relates to an arrow shooting device, and more specifically to, a new arrow shooting device and a draw weight adding method, wherein an arrow is shot not by a compound arrow or a compound crossbow, which have been conventionally used, but by a rubber band and a differential motion cam. The arrow shooting device according to the present invention is formed in the straight simple shape, wherein, as shown in FIG. 6, the rubber band having superior elasticity is used rather than a limb in the energy storage method thereof and the retraction speed of the rubber band is converted using a differential motion cam or a differential motion cam so as to be suitable for shooting the arrow. In addition, it is possible to additionally hang the rubber band in the state when drawing is finished, or pull the rubber band by means of a winch, so that even people in general may shoot an arrow with a strong draw weight. Therefore, the arrow shooting device is light enough to easily be carried, is easy to manufacture and repair, and has much storage power so that an arrow may be shot far and shot straight. According to the present invention, it is possible to resolve the conventional problems of the complicated structure, heavy weight, noise and shock in the compound arrow and the compound crossbow and reduce the weight for convenience in movement.

Description

TECHNICAL FIELD

The present invention relates to an arrow shooting device, and in particular to a new arrow shooting device which can shoot arrows using a rubber band and a differential motion cam in such a way to add a draw weight after drawing.

BACKGROUND ART

FIG. 1 shows an energy input of energy which is to be used for shooting arrows.
The accumulations of energy input in a traditional bow are shown in Graph TB of FIG. 1. When a person is of a large build, a power stroke length increases, and a draw weight might increase depending on the intensity of is exercise.
The compound bow has a large power stroke length as compared with the motion of a limb by using an eccentric cam. A fast arrow speed can be made by changing a recovery speed of a limb, and a lot of energy can be accumulated when drawing a string, and an aiming is easy with an adaptation of a let-off technology. The energy input in terms of a compound bow is shown in Graph CB of FIG. 1, and the compound bow can reach the maximum draw weight earlier than a traditional bow, thus implementing an efficient energy accumulation, and the aiming is easy now that the let-off technology is adapted, in which technology the draw weight fast decreases as the drawing is getting finished. The power stroke length of the compound bow changes depending on the strong build and the kinds of the releases; however it is similar with a traditional bow.
The compound bow is adapted to a traditional type of a crossbow, thus having developed a compound crossbow. It is known that the compound to crossbow can accumulate the energy most as a device which can load and shoot arrows with only the manpower without using charge or compressed air among various devices so far known to shoot arrows. The accumulation of energy in terms of the compound crossbow is shown in Graph CC of FIG. 1. The compound bow can be drawn with much larger draw weight and can be loaded and hooked at a hooking member with the aid of both arms and a drawing assistant tool as compared with a traditional bow or a compound bow which is drawn with one arm, provided that magnificent weight and durability are needed to have the power stroke length similar with that of the compound bow, and the power stroke length is much shorter than the compound bow.
The energy input and output procedures accumulated in the compound bow are shown in CB of FIG. 12.
In terms of the energy output procedure, it has a mirror image of the energy input procedure; however the total energy output amount has 15% less than the energy input amount due to friction resistance, noise, vibration, etc.
A rubber band is widely used in the industry thanks to its good elasticity, and a rubber-mounted spear gun can shoot with a lot of energy of 300 Joule.
A differential motion pulley is a pulley assembly in which pulleys with different diameters are coupled and is generally used for a speed change or a tensile force change.
In the present invention, a thing like a cam is also used in addition to the type of a pulley, so the thing is called a differential motion cam, and the differential motion pulley belongs to a differential motion cam.
As shown in FIG. 2, a winch is directed to winding or unwinding a winch cable WC the function of which is combined with a ratchet function which allows a winch gear to rotate in one direction using a winch pawl WP. The winch is a kind of a device tugging a heavy thing by winding a rope on a cylinder, so it is possible to lift or tug a heavy thing using a smaller power with the aid of the principles of a pulley and a lever.

TECHNICAL PROBLEM

Since the invention of a compound bow was made in 1969, a compound bow and a compound crossbow have advanced a lot; however the compound bow and the compound crossbow have some problems as follows.
The shooting is made after it is drawn only one time, so a lot of energy cannot be stored.
A principle of a lever works at a portion from a pivot point of a grip in a compound bow using a limb to the end of the limb in such a manner that the pivot point becomes a supporting point during the drawing, and the end of the limb becomes a force-applied point, and besides another principle of a lever works, in which principle a portion where a riser and a limb are connected becomes a supporting point, and the end of the limb becomes a force-applied point. Since more power is applied to the riser, so it is needed to manufacture a stronger and durable compound bow, which brings in an increased weight.
The compound bow or a crossbow with a limb is capable of storing is energy above or below about 100 Joule, which energy is to be used for shooting arrows. The rubber band used in a rubber-mounted spear gun is capable of storing about 300 Joule, which energy is used to shoot arrows, provided that the above rubber band is limited in terms of the speed of contraction, so it is not applicable to arrow shootings now that arrow should be shot at a high speed.

TECHNICAL SOLUTION

Accordingly, it is an object of the present invention to provide an arrow shooting device which adapts a method of shooting arrows following storing a lot of energy and a method of converting the accumulated energy into a fast speed using a differential motion cam.
The method of storing a lot of energy is formed of a method in which a plurality of rubber bands extendable with a users power are prepared, and the rubber bands are pulled and hooked one by one, thus accumulating energy, and a method in which more energy is accumulated using a winch, and then arrows are shot, and the contracting speeds of the rubber bands are converted using a differential motion cam, thus shooting arrows for the purposes of hunting or sports.
The present invention is directed to using a rubber band as an energy storing means without using a limb, the rubber band being extended and is contracted in the same direction as the body of the arrow shooting device, so the principle of a lever in which a limb works as a lever is not adapted. Since the force the body of the arrow shooting device receives is smaller than a bow, the arrow shooting device can be made lighter.

ADVANTAGEOUS EFFECTS

Since an ordinary person can shoot arrows by using a lot of stored energy, a fast arrow speed can be obtained. Thanks to the fast arrow speed, an accuracy rate and a hunting success rate can be enhanced, and a straight trajectory can be obtained.
Since a rubber band is used as an energy storing medium, the force is applied in the same direction as the body of the arrow shooting device, so the twists of the body and the bending force decrease a lot as compared with when using a limb, thus making the body much lighter, which results in an easy-to-carry performance and lowered manufacture costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy accumulation graph of a traditional bow (TB), a compound bow (CB) and a compound crossbow (CC).

FIG. 2 is a view illustrating a latchet function that a winch (WP) is works on a winch gear (WG).

FIG. 3 is an upper, back and side view of a bow string roller stopper (RS).

FIG. 4 is a side, back and bottom view of a differential motion cam 1 (DC1) of an arrow shooting device according to the present invention.

FIG. 5 is a side, back and bottom view of a differential motion cam (DC2) of an arrow shooting device according to the present invention.

FIG. 6 is a side and top view of an arrow shooting device according to the present invention.

FIG. 7 is a view illustrating a drawing of an arrow shooting device and an energy accumulation procedure following the drawing according to the present invention.

FIG. 8 is a view illustrating an arrow shooting procedure of an arrow shooting device according to the present invention.

FIG. 9 is a side and top view of an arrow shooting device with a winch according to the present invention.

FIG. 10 is a view illustrating an energy accumulation procedure of an arrow shooting device with a winch according to the present invention.

FIG. 11 is a view of an arrow shooting procedure of an arrow shooting device with a winch according to the present invention.

FIG. 12 is an energy accumulation and discharge graph of an arrow shooting device with a compound bow (CB), a differential motion cam (DC1), and a differential motion cam 2 (DC2).

FIG. 13 is an energy accumulation and an energy discharge graph when a winch is provided at an arrow shooting device with a compound bow (CB), a differential motion cam 1 (DC1), and a differential motion cam 2 (DC2).

DESCRIPTIONS OF REFERENCE NUMERALS OF KEY ELEMENTS OF THE DRAWINGS

TB: traditional bow
CB: compound bow
CC: Compound Crossbow
IC: Inner Cam
IP: Inner Pulley
OP: Outer Pulley
DC1: Differential motion Cam 1
DC2: Differential motion Cam 2
RB: Rubber Band
S: String
SA: String Anchor
BA: Band Anchor
Ar: Arrow
T: Trigger
RS: Roller Stopper
SC: Stopper Cable
IS: Inner String
WC: Winch Cable
WG: Winch Gear
WP: Winch Pawl
FP: Fixing Point
Sh: Shield

BEST MODES FOR CARRYING OUT THE INVENTION

The preferred construction and operation according to the present invention for achieving the above objects will be described with reference to the accompanying drawings.
A bow string roller stopper (RS) is manufactured in the way shown in
FIG. 2.
It is engaged in the way shown in FIG. 6 or 9, and a central shaft is durably manufactured to stand a large power, and a groove through which a string passes can move a bit in a horizontal direction; however it should move in is a vertical direction to allow an arrow to shoot in stable.
The differential motion cam 1 (DC1) is manufactured in the ways of FIGS. 3 and 4.
The portions are prepared for fixing a bow string (S) and a rubber band inner string (IS), and the fixing method generally adapted in a compound can be used, and the fixing point (FP) means a theoretical fixing point, not an actual fixing point. The both sides of the inner pulley (IP) are made to form a cylindrical shape, with their central portions being equipped with a groove to allow a rubber inner string (IS) to be hooked in a full draw state. The let-off is determined based on a radius from the groove to the axis. If the groove is formed to be 30% of the radius of the inner pulley (IP), the let-off becomes 70%.
The ratio of the radius of the outer pulley (OP) and the radius of the outer pulley (OP) serves to determine the flying speed of the arrow as compared with the rubber band contraction speed. When the radius of the outer pulley (OP) is three times the radius of the inner pulley (IP), the arrow can fly three times the to rubber band contraction distance.
The rubber band (RB) has various thicknesses and lengths and can be selected for a desired draw weight. Two rubber bands are cut, and the rubber bands are connected with a bow string or something, thus producing multiple rubber bands in loop shapes.
FIG. 6 is a side and top view illustrating a state that the differential motion cam 1 (DC1) and the bow string roller stopper (RS) are installed, and the rubber band (RB), the bow string (S), the string anchor (SA), the rubber band anchor (BA), the trigger (T), the string roller stopper (RS), the stopper cable (SC), and the rubber band inner string (IS) are set.
The string S has one end connected with the fixing point (FP) of the outer pulley (OP), and the other end connected to the fixing point (FP) of the outer pulley (OP). The length of the string (S) is determined so that the string (S) keeps tensioned around the string anchor (SA) with a desired draw length when the central groove of the inner pulley (IP) is positioned at the lowest position.
The rubber band inner string (IS) is divided into two parts the one part of which is fixed to the rubber band (RB), and the other part of which is fixed to two fixing points (FP). The rubber band inner string (IS) is installed in such a manner that the rubber band (RB) can be fixed at the fixing point (FP) of the inner pulley (IP) depending on the length for the rubber band (RB) to have a proper initial tensile force from the moment the string (S) is wound on the outer pulley (OP) and is hooked at the string roller stopper (RS).
One side of the rubber band (RB) is fixed at the rubber band anchor (BA), and the other side of the same is connected with the rubber band inner string (IS).
As shown in FIG. 6, two more rubber bands (RB) might be connected to the portion connected with the rubber band inner string (IS). The rubber band to be pulled later is pulled without using the differential motion cam, so it should be preferably weaker than the engaged rubber band for easier pulling.
One side of the stopper cable (SC) is connected with the fixing point (FP) of the body, and the other side of the stopper cable (SC) is connected with the portion to which the rubber band (RB) and the rubber band inner string (IS) are connected. The length of the stopper cable (SC) and the installation of the fixing point (FP) are determined so that the rubber band (RB) can be loosened when the string (S) is fully drawn as shown in FIG. 8 and the rubber band (RB) can have a proper initial tensile force when the string (S) is wound on the outer pulley (OP) and is hooked at the string roller stopper (RS).
FIG. 7 is a view illustrating a procedure of a drawing at the arrow shooting device which has finished a basic setting and a procedure that energy is further accumulated after drawing.
The string (S) hooked at the string roller stopper (RS) is drawn and is hooked at the string anchor (SA) as shown in the second drawing, and when the string (S) is pulled, the string (S) wound on the outer pulley (OP) is unwound, and the differential motion cam 1 (DC1) rotates in the clockwise direction, and the rubber band inner string (IS) is wound on the inner pulley (IP), and the rubber band (RB) extends by as much length as the wound length. Since the rubber band inner string (IS) is fixed at the outer side of the inner pulley (IP) at the initial stage of drawing, it is wound on the cylindrical portion of the inner pulley (IP); however the rubber band inner string (IS) gathers at the central portion when the drawing is almost finished, so it becomes naturally supported at the groove of the central portion of the inner pulley (IP). When the rubber band inner string (IS) becomes supported at the groove of the central portion of the inner pulley (IP) just before the drawing is finished, the draw weight being applied to the string (S) decreases to 30% as compared with just before. In other words, 70% of the let-off effects can be obtained. In this state, since the string S is supported by the string anchor (SA), the force being applied to the trigger largely decreases, so the durability of the trigger might be enhanced, along with the increased accuracy rate.
The energy accumulation procedure following the drawing will be described. One of the relaxed rubber bands (RB) is pulled and hooked at the rubber band anchor (BA) as shown in the third drawing, and the remaining rubber bands (RB) are pulled and hooked at the rubber band anchor (BA) as shown in the fourth drawing.
FIG. 8 is a view illustrating a procedure that an arrow is loaded by hooking at the string (S) after the energy accumulation procedure is finished following the drawing, and the arrow is shot by trigging the trigger after aiming.
When the string anchor (SA) falls as the trigger is triggered, the rubber band contracts, and the differential motion cam rotates in the counterclockwise direction, and the rubber band inner string (IS), which has wound on the inner pulley (IP) during the drawing is unwound, and the string (S) is wound on the outer pulley (OP). When the radius of the outer pulley (OP) becomes three times the radius of the inner pulley (IP), and the same rotations are made, the moving distance of the string (S) becomes three times the moving distance of the rubber band inner string (IS), so the flying speed of the arrow becomes three times the contraction speed of the rubber band based on the above principle.

MODES FOR CARRYING OUT THE INVENTION

1. The differential motion cam 1 (DC1) and rubber band (RB) are additionally pulled and hooked, which has been described in the best mode for carrying out the invention.
2. The differential motion cam 2 (DC2) and rubber band (RB) are additionally pulled and hooked.
It is possible to obtain a linear increase effect in the draw weight after the completion of the drawing with the aids of the let-off effects and the hooking of additional rubber bands like the differential motion cam 1 (DC1) of FIG. 12 in terms of the energy accumulation and the energy output aspects when the differential motion cam 1 (DC1) is used; however the energy accumulation and energy output graphs similar with the traditional bow are obtained, so the efficiency decreases as compared with the compound bow. It is needed to change the design of the inner cam so as to accumulate and output the energy in an efficient way like the compound bow.
FIG. 5 is a side, back and bottom view illustrating the differential motion cam 2 (DC2). The outer side of the inner cam has a large radius, and the inner side has a small radius, and a straight groove is formed at the central portion, by the construction of which the radius can sharply decrease. Like the differential motion cam 2 (DC2) of FIG. 12, it is possible to obtain the effects same as the compound bow in terms of the energy accumulation and let-off. to Since additional rubber bands are engaged, thus increasing a linear increase effect in terms of the draw weight in the let-off state. When an arrow is shot, it is shot with a trajectory similar with the compound bow. The basic setting method in case of the use of the differential motion cam 2 (DC2) is the same as when the differential motion cam 1 (DC1) is used.
3. The number of the rubber bands (RB): Even when the rubber band (RB), which is basically engaged, has a tensile force stronger than the rubber band (RB) which is additionally pulled, it can be easily pulled with the aid of the differential motion cam 1 (DC1) or the differential motion cam 2 (DC2). The rubber band (RB), which is additionally pulled, is pulled without using the differential motion cam 1 (DC1) or the differential motion cam 2 (DC2), it should have a tensile force weaker than the basically engaged rubber band. It is preferred that the number of the rubber band (RB), which is to be additionally pulled, is not limited, but 2˜4.
4. The use of winch
FIG. 9 is a side and top view illustrating an arrow shooting device which is drawn using a winch. The method for setting the differential motion cam 1 (DC1), the string (S), the string roller stopper (RS), the stopper cable (SC) and the rubber band inner string (IS) is the same as the earlier setting method, provided that as different matters, the rubber band with a strong tensile force is basically used, and there is provided a shield Sh for covering the rubber band, and one of the rubber band (RB) is connected with the winch cable (WC). Since there is not a function for additionally pulling the rubber band and hooking the same, it is needed to select and engage a stronger rubber band which is basically engaged. When there is a shield Sh for covering the rubber band, it is helpful to support with one hand during the aiming. One side of the rubber band (RB) is connected with the winch cable (WC), and when the winch cable (WC) is wound by rotating the winch handle (WH), it is easy to pull the rubber band even though it has a stronger tensile force.
As shown in FIG. 2, a winch pawl (WP) is installed at the winch, which leads to obtaining a ratchet function. The clockwise rotation or the counterclockwise rotation can be implemented with one winch pawl (WP).
FIG. 10 is a view illustrating a drawing procedure of the arrow shooting device with a winch installed therein. The winch gear (WG) can be rotated in the clockwise direction by pressing the winch pawl (WP), and as the string (S) is pulled, the winch cable (WC) can be wound without having the rubber band (RB) extended, so a drawing can be made possible when a draw weight is very low. The tensile force being applied to the string (S) can decrease to 30% as compared to just before in such a manner that the rubber band inner string (IS) is supported at the grove of the central portion of the inner pulley (IP) just before the completion of the drawing. In other words, it means that 70% of the let-off effect can be obtained. In this state, since the string (S) remains supported by the string anchor (SA), the force being applied to the trigger significantly decreases, which leads to enhancing the durability of the trigger, along with enhanced accuracy rate. The winch gear (WG) can be made to rotate only in the counterclockwise direction by pressing the winch pawl (WP) in the opposite direction after the string (S) is hooked at the string anchor (SA) until the let-off state is obtained, and then the winch gear (WG) is rotated in the counterclockwise direction using the winch handle (WH). As the winch cylinder rotates, the winch cable (WC) is wound on the winch cylinder, and the rubber band is extended, and the draw weight of the string (S) being in the let-off state increases.
FIG. 11 is a view illustrating an arrow shooting procedure of an arrow shooting device with a winch installed therein. The arrow (Ar) is loaded at the string (S) of the arrow shooting device which has completed the drawing, and the cover is supported using one hand, and the handle is held by the other hand, and then the aiming is done, so the arrow is shot by trigging the trigger (T). When the string anchor (SA) fells, the rubber band contracts, thus pulling the rubber band inner string (IS). The rubber band inner string (IS) wound on the inner pulley (IP) during the drawing is unwound, and the differential motion to cam 1 (DC1) rotates counterclockwise. The string (S) is wound on the outer pulley (OP) as many turns as the rotation, so the arrow is shot. When the radius of the outer pulley (OP) becomes three times the radius of the inner pulley (IP), and the same rotations are made, the moving distance of the string (S) becomes three times the moving distance of the rubber band inner string (IS), so the flying speed of the arrow becomes three times the contraction speed of the rubber band based on the above principle.
Here the differential motion cam 2 (DC2) might be used instead of using the differential motion cam 1 (DC1). It is possible to have the energy accumulation and output aspects like the compound bow in the same manner as in the differential motion cam 2 (DC2) of FIG. 12 by variously changing the radius of the groove of the first inner cam (IC) of the differential motion cam 2 (DC2).
FIG. 13 is a view illustrating an energy accumulation and output when a winch is used.
The draw weight is very weak now that the rubber band is not extended during the drawing.
The draw weight being in the let-off state vertically rises as the rubber band is extended by turning the winch in the let-off state following the drawing.
When the arrow is shot by trigging the trigger, the arrow shooting device with the differential motion cam 1 (DC1) emits energy with a trajectory similar with the traditional bow except for the let-off, and the arrow shooting device with the differential motion cam 2 (DC2) emits energy with a trajectory similar with the compound bow.
Since the strong rubber band is pulled using the winch, an ordinary person can store a lot of energy and easily shoot arrows with the aid of the stored energy.
5. The present invention serves to manufacture an arrow shooting device which can shoot an arrow by using a release without adapting a trigger device with the aid of the differential motion cams such as a differential motion cam 1 (DC1) and a differential motion cam 2 (DC2).
In the body with a simple-looking grip are disposed the differential motion cam (DC1) or the differential motion cam 2 (DC2), the string roller stopper (RS), the rubber band (RB), the string (S), the rubber band anchor (BA).
The release is used without the trigger being fixed at the body, and if the differential motion cam 2 (DC2) is adapted, the energy accumulation and output graphs are similar, so the drawing feeling and let-off effects might be similar with the compound bow; how performances might be a bit low now what the position of the pivot point differs.
6. The arrow shooting device with a smaller body might be manufactured using a metallic spring instead of a rubber band.

INDUSTRIAL APPLICABILITY

The present invention is directed to obtaining a wanted arrow speed by adjusting the ratio of an outer pulley radius and an inner cam radius of the differential motion cam 1 (DC1) or the differential motion cam 2 (DC2) which is adapted to increase the contraction speed of the rubber band, and the draw weight can be increased by the method of adding the draw weights. So, an ordinary person can easily shoot arrows with the draw weight of hundreds of pounds.
The present invention is directed to providing a simple-looking straight structure by adapting a rubber band or a strong spring instead of using a limb to store energy.
Much less air resistance is obtained as compared with a conventional compound bow and a compound crossbow, so noises and vibrations are low during the shooting.
With the help of the above-mentioned advantages, the arrow shooting device of the present invention might substitute a compound bow and a compound crossbow which are generally used for the purposes of sports and hunting. When the present invention is used for the purpose of a rubber spear gun, it is possible to manufacture a rubber spear gun which is smaller than the conventional rubber spear gun but has a lot of output energy.

Claims

1. An arrow shooting device using a rubber band, comprising:

a differential motion cam including:

a pair of outer pulleys each having a groove for receiving a string; and

an inner cam in which a rubber inner string connected with the rubber band is fixed at a fixing point of the outer side, and a spiral groove is symmetrically formed, the radius of which decreasing as the rubber inner string is inwardly wound, the inner cam having a straight groove the radius of which is smallest at the central portion as it meets the spiral groove.

2. An arrow shooting device using a rubber band, comprising:

a differential motion cam including:

a pair of outer pulleys each having a groove for receiving a strip; and

an inner pulley in which a rubber inner string connected with the rubber band is fixed at a fixing point of the outer side, the rubber inner string being inwardly wound after passing through an outer cylindrical portion, and passing through a diamond-shaped groove at the central portion.

3. An arrow shooting device according to claim 1, wherein there is provided a trigger, and there is provided a way of increasing a draw weight in such a way that a string is drawn, and a rubber band is added and pulled toward the rubber band anchor and is hooked.

4. An arrow shooting device according to claim 1, wherein there is provided a trigger, and there is provided a winch helping stop the rotation with a winch pawl in such way that a string is drawn, and a winch cable connected with the rubber band is pulled by turning a winch handle.

5. An arrow shooting device according to claim 1, wherein there is provided a pair of string roller stopper which permit the movement of the string in a horizontal direction when the string starts moving after shooting, and limit the movement of the string in a vertical direction and hooks and stops the string.

6. An arrow shooting device according to claim 1, wherein there is provided a stopper cable one end of which is connected to a portion where the rubber band inner string and the rubber band are connected, and the other end of which is connected to a fixing point of the body, thus stopping the contraction of the rubber band.

7. An arrow shooting device according to claim 2, wherein there is provided a trigger, and there is provided a way of increasing a draw weight in such a way that a string is drawn, and a rubber band is added and pulled toward the rubber band anchor and is hooked.

8. An arrow shooting device according to claim 2, wherein there is provided a trigger, and there is provided a winch helping stop the rotation with a winch pawl in such way that a string is drawn, and a winch cable connected with the rubber band is pulled by turning a winch handle.

9. An arrow shooting device according to claim 2, wherein there is provided a pair of string roller stopper which permit the movement of the string in a horizontal direction when the string starts moving after shooting, and limit the movement of the string in a vertical direction and hooks and stops the string.

10. An arrow shooting device according to claim 2, wherein there is provided a stopper cable one end of which is connected to a portion where the rubber band inner string and the rubber band are connected, and the other end of which is connected to a fixing point of the body, thus stopping the contraction of the rubber band.