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Hex lock nuts, also called as stiff nut, locking nuts, and self-locking nuts, are nuts that have a locking feature to resist loosening due to vibrations and torque. They replace standard nuts in machinery and other applications exposed to vibrations. Locking nuts usually have a deformable section in the thread that generates friction to prevent further movement. Jam nuts however use two shorter nuts that are driven against each other to prevent loosening.