A locking device (1) is fixed to a nut (4) and includes a coupling unit (42) insertable into a hexagonal configuration (11) of a spigot end (2) at a reduced axial distance being coupled by mutually interpenetrable gears (49, 51) with a stop unit (38) which axially slides in a body (18) opposite to a compression spring (36). The gears (49, 51) are symmetric and are insertable into each other with a predetermined elastic resistance by the action of compression or loosing torque. The device is embodied in one piece which is mounted by assembling with the nut without affecting the spigot ends (2, 3).