ESDU 87023 gives guidance on fastener devices commonly available to prevent joint loosening under severe vibration in service. The bolted assembly will usually be designed with the bolt(s) pretensioned to a prescribed load (see ESDU 86014)@ and it is methods of maintaining that pretension that are considered. In particular@ the double lock nut is examined and tightening procedures recommended. The influence of tolerances on the preload is considered@ with a worked example@ and the effect of the vibration cycles on loss of preload illustrated in comparison with the use of a single nut. Fatigue strength is also improved@ and the reasons for that are explained and the extent of the improvement shown on a stress-endurance (S-N) curve using experimental data extracted from the literature.