What causes loss of preload?

The biggest challenges for a bolted joint, causing it to lose preload and fail, is spontaneous loosening. This graph clearly shows Nord-Lock wedge-locking washer have consistent preload thanks to consistent friction​. Low and constant friction values allows a more efficient use of fasteners. The preload is needed to prevent slippage and opening of construction parts.

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Spontaneous loosening: shock, vibration, dynamic loads
Spontaneous loosening occurs when a bolt rotates loose due to external factors such as vibration, shock and dynamic loads (flexing). The most demanding applications are continually subjected to dynamic loads and even a slight rotation of the fastener can be enough for the joint to lose much of its preload.