but instead by being screwed into the sheath 4 with the abutment pieces and the sheath being provided with corresponding respective engaging external and internal threads in the fourth embodiment of fig4 the glass fibre rod 1 is provided with a chamfer at its bottom end the rod 1 is pushed into the sheath 4 until the cup spring 13 on the bottom of the sheath is deformed on removal of the load on the rod 1 self locking of the rod occurs and the rod 1 can no longer be moved back or up in fig4 for this purpose in fig4 the shoulder pieces 5 which are movable with respect to the sheath are in the form of unidirectionally self locking internally feathered toothed discs 15 and the abutment pieces 7 which are fixed with respect to the sheath but are movable with respect to the glass fibre rod 1 are in the form of externally feathered self locking toothed discs 16 moreover exact matching of the toothed discs to one another is important full details of the discs 15 16 are apparent from fig5 a 5b 6a and 6b the externally toothed discs 16 brace themselves with respect to the internal bore of the sheath 4 either through friction against the plain internal bore 17 fig6 a or positively by means of a rigid one way motion toothing 18 on the inner face of the sheath 4 fig6 b the toothing only permits descent of discs 16 the solution according to fig6 a has the advantage in comparison with that of fig6 b that it can be optimized an equalization of load being obtained by slipping of some of the toothed discs 15 in the event of local overloading.