of the inner sleeve is truncated into a cylindrical portion 8 and the bottom of the thread flanks of the outer sleeve is filled out into a corresponding cylindrical portion except in an area 9 at one end of the flanks thereby the right hand end of the thread of the outer sleeve is wedged against the area 9 in connection to the thread of the inner sleeve when the outer sleeve is screwed to the right over the inner sleeve fig3 shows an embodiment in which the thread flank of the inner sleeve is prolonged in an area 10 at one end of the thread when the outer sleeve is screwed to the right on the inner sleeve a portion of the thread flank of the outer sleeve is wedged against the prolonged thread flank in the embodiment according to fig4 the flank of the thread of the outer sleeve is prolonged in an area 11 at the left hand end the inner sleeve is depressed in the corresponding area in order to permit the outer sleeve to be displaced over the threads on which the flange 5 is mounted this embodiment operates in a manner similar to the operation of the embodiment of fig3 fig5 shows an embodiment in which a cylindrical portion 12 at one.