In which conditions is a self-locking nut most likely to fail?

A self-locking nut is designed to resist loosening due to vibration and movement. However, if a self-locking nut is able to be loosened by hand, it indicates that the locking mechanism is not functioning effectively. This could be due to wear, improper installation, or fatigue in the locking feature. When a self-locking nut loses its gripping capability and can be loosened without tools, it fails to provide the necessary retention needed to secure the assembly.

In contrast, being pressed tightly typically ensures that the locking mechanism is engaged more firmly. High temperatures can affect materials, but many self-locking nuts have tolerance levels for various degrees of temperature. Keeping the nut under consistent tension generally enhances its ability to remain locked rather than leading to failure. Thus, when the self-locking feature is compromised to the point where it can be loosened by hand, it represents a clear failure of the nut's design and purpose.