A double lock nut assembly for preventing loosening comprises a main nut (5) and an auxiliary nut (6) tightened on the main nut. The main nut (5) comprises a body portion (7) including female threads (5A) adapted to bear the axial bolting force, a locking portion (8) including female threads (58) adapted to bear the locking force, and at least one joint portion (9) by way of which the body portion and locking portion are integrally connected to one another with a predetermined clearance (L) therebetween. The joint portion (9) includes a rigid support (10) and a resilient joint (11) integral with the support. The joint portion (9) is located at the corner of the hexagonal configuration of the main nut or disposed along a radius of the main nut. The auxiliary nut (6) serves merely for resiliently deforming the locking portion (8) toward the body portion until a required locking force is generated due to pressed contact between the surfaces, directed to the body portion, of the female threads (5B) of the locking portion (8) and the flanks of the male threads of a bolt (2), directed toward the free end of the latter. The support may be formed by welding.
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