a shoulder surface 20 extends radially outwardly with a cylindrical surface 21 extending axially from the shoulder 20 an annular bushing 19 is threadedly supported on the lowermost end of the drive nut 3 and the downward axial end face of the bushing 19 is fitted with a friction ring 7 formed of a material having a high coefficient of friction with respect to the bobbin 8 the gripping cage 4 includes a ring like base mounted annularly about the bushing 19 in axial facing relation to the shoulder 20 of the drive nut 3 with a plurality of resilient spring like gripping fingers 13 extending axially downwardly from the cage base annularly about the bushing 19 the thrust ring 6 is of an inner diameter corresponding to the outer diameter of the cylindrical surface 21 of the drive nut 3 the upper end of the thrust ring 6 being formed with a radially outwardly projecting shoulder 15 which axially faces a radially inwardly projecting shoulder 16 formed at the lower end of the housing 1 a helical coil type spring 5 is disposed annularly about the cylindrical surface 21 of the drive nut 3 to extend axially between the drive nut 2 and the upper end of the thrust ring 6 to bias the drive nut 3 and thrust ring 6 away from one another into a normal relative axially spaced relationship the downwardly projecting ends of the gripping fingers 13 are radially outwardly tapered with the lower.