Tension makes the bolt self-locking

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The rise of the cams between the Nord-Lock wedge lock washers is greater than the pitch of the bolt. In addition, there are radial teeth on the opposite side. The wedge lock washers are installed in pairs, cam face to cam face. So to put it simply - it takes more force to loosen the nut then to tighten it - so it doesn't loosen!

Source:  https://www.nord-lock.com/nord-lock/products/washers/