A commercial form of Dardelet external screw thread, such as that of a bolt, has a root locking surface 10 which, in cross section, extends the full width of the bottom of the thread groove. This surface slopes slightly with reference to the thread axis, the rise of its slope being in the direction of the tip of the bolt, that is, away from the bolt head. An internal thread, such as that of a nut, complementary to said external thread, has along its crest a locking surface II with a slope similar to that of the root surface 10. The degree of slope is within the angle of friction or the angle of repose of the two locking surfaces. In standard Dardelet threads the slope is six degrees.
The groove of each thread is materially wider than its rib to afford clearance for substantial crosswise axial displacement of one thread on the other.
When the nut is screwed home and its axial advance is arrested by contact with an abutment, further rotation of the nut displaces its thread to force its locking surface 1 into frictional locking engagement with the sloping locking surface 10 of the bolt thread...