Liu et al

Liu et al. (2016) identified four different types of loading bolts experience under vibratory conditions: axial tensile load, transverse (shear) load, torsional load and prying load. The load applied in the transverse and rotational directions are considered the primary loading modes causing self-loosening (Yokoyama et al., 2012) and was demonstrated by Junker (1969) who also showed that loosening ultimately results from gross slip at the head and thread interfaces. Pai and Hess (2002) conducted research in support of Junker (1969) and concluded that loosening can occur due to localised slip at contact regions and that the minimum shear load needed to start the loosening process is significantly lower than the shear load required to cause head slip. These mechanisms occur as soon as the applied shear overcomes friction in the transverse direction. The joint becomes free of circumferential friction, and the loosening moment developed due to the component of the preload around the thread helix causes the fastener to loosen (Pai and Hess, 2002). Furthermore, Liu et al. (2016) determined the preload of identical bolted joints are not constant values despite being generated under the same conditions.