342 coupled to which a washer 341 and the rear of which is coupled to a spring 343 having a hook protrusion 343 protruding outward the cover 310 has an insertion hole and a hook protrusion in which coupling protrusions are formed symmetrical to each other on the inner circumferential surface of the cover protruding outward also a plurality of protrusions 314 coupled to a winding drum around which the double fabric is wound are formed on the outer circumferential surface of the cover 310 any desired number of protrusions 314 can be provided on cover 310 the coupling grooves 321 which are symmetrical to each other and can be in the form of semicircular grooves are formed in the outer circumferential surface of the rotation component 320 the coupling hole 323 having the insertion grooves 322 where a plurality of balls 350 are inserted can be formed in the inner circumferential surface of the rotation component the insertion groove 323 can have a semicircular shape and extend in a longitudinal direction within the rotation component 320 which is illustrated in fig6 also the plurality of balls 350 can be inserted into the insertion groove 323 and used as discussed herein in particular four balls 350 can be inserted into the insertion groove or eight balls 350 may alternatively be inserted for use in other embodiments more than eight balls 350 may be used if desired the.