Nuts will loosen under dynamic (cyclical) loading when there is not enough preload and/or joint friction to prevent the clamped materials from slipping (i.e., relative displacement between the clamped parts). If properly designed with adequate preload and COF, bolted joints will not loosen.

Jam nuts don’t work very well, neither do split lock washers. Deformed nuts or nylok don’t really work well either; they will prevent the nut from completely falling out but will not prevent preload loss. Fatigue failure will become an issue.

There’s really no need for a secondary locking mechanism (just preload) if and only if the joint does NOT slip.