The invention relates to a nut for a moment screw, comprising an upper torsion member, a lower clamping member of the same material with the tor- sion member, and a weakening groove between the torsion member and the clamping member, the weakening part being dimensioned so as to establish a fraction point between the torsion member and the clamping member as a given torque occurs between the torsion member and the clamping member. This type of nut for a moment screw is known from the Applicant's patent publication FI-102120. This prior known and other similar moment screw nuts involve drawbacks as follows. The nut breaks up sharply without advance warning when subjected to a maximum torque. Consequently, the tense arm of a fitter makes an abrupt movement, whereby the hand or tor- sion wrench may hit the head of a fitter or a hazardous situation may develop at high level installation site in case the fitter's balance becomes unsteady. Another drawback is waste resulting from a detached torsion member (environmental burden). Publication JP 2005163299 A discloses a moment screw nut, wherein the screw thread continues substantially through the entire nut for retaining the torsion member as a locknut. This is difficult to dimension for a given torque and the fraction point is difficult to detect, because the fraction point material must also become upset for a fracture to occur.

An object of the invention is to provide an improved nut for a moment screw, whereby these drawbacks can be alleviated or even eliminated.

This object is achieved on the basis of characterizing features presented in the appended claim 1. Preferred embodiments of the invention are presented in the dependent claims. The invention will now be described more closely by way of exemplary embodiments with reference to the accompanying drawing, in which

Fig. 1 shows a nut according to a first embodiment of the invention in an axial section and in a plan view;

Fig. 2 shows a nut according to a second embodiment of the invention in an axial section and in a plan view; and Fig. 3 shows a nut according to a third embodiment in an axial section and in a plan view.

The proposed moment screw nut comprises an upper torsion member 1 and a lower clamping member 2 of the same material with the torsion member 1. Between the torsion member 1 and the clamping member 2 is a weakening groove 3, which is dimensioned to establish a fraction point between the torsion member and the clamping member as a given torque occurs between the torsion member and the clamping member. The lower clamping member 2 has its thread 4 continue, according to the invention, across the fraction point 3 of a nut 6 to a bottom edge or bottom part of the torsion member 1. A portion of the thread 4 lying above the fraction point 3 is preferably sufficiently long to resist continued twisting of the torsion member 1 after the nut 6 has fractured, yet with a substantially less- er force (counter torque) than prior to fracturing of the nut 6. This portion of the thread 4 present above the fracture point 3 is preferably long enough to retain the torsion member 1 in its place in engagement with the moment screw's thread even after the fracture of the nut 6. The embodiments of figs. 1, 2 and 3 only differ from each other in terms of where a deepest point 3a of the weakening groove 3 is located. In fig. 1 the deepest point 3a lies at a bottom edge of the weakening groove, in fig. 2 the deepest point 3a is located in the middle of the groove 3, and in fig. 3 the deepest point 3a lies at a top edge of the groove 3. In all these cases, the invention achieves an additional benefit of being able to make the nut 6 shorter in its entirety as the thread 4 becomes longer. In a preferred em- bodiment of the invention, the clamping member 2 has its thread 4 continuing at least one full circle to a position above the deepest point 3a of the weakening groove 3. Modern manufacturing tolerances are so precise that a fracture of the nut is always enabled at the same point, i.e. at the deepest point 3a of the weakening groove 3.