six turns of the grooves 29a 29b at a constant radius a region 31a of minimum radius wherein there are about three turns of the grooves 29a 29b at a minimum radius and an intermediate region 32a wherein the radius of the grooves 29a 29b increase smoothly and continuously from the region 31a to the region 30a as before the drum assembly 22a is releasably fixed to the shaft by a clamping screw 26 carried in a collar 25 the collar including sockets 33 for engagement by a torquing tool such as a rod to effect rotation of the drum assembly 22a relative to the shaft when the clamping screw 26 is disengaged at the opposite end of the drum there is a flange 24 provided with a clamping element 36a the ends of a pair of cables 35a 35b are releasably fastened to the drum assembly 22a by means of the clamping element 36a these cables being laid side by side in their respective grooves 29a 29b in essentially the same manner as described in relation to the cable 35 in fig3 and 4 from the drum assembly 22a the cables 35a 35b hang in loops 37a 37b the opposite ends of the cables being secured at 38a on a bracket 39a fixed to a.