Vibration loosening analysis and simulation of preloaded threaded fasteners

Loosening is widely regarded as one of the main failure modes of threaded fasteners, as confirmed by a survey of 100 automobile dealers in the Detroit Metropolitan area, which revealed how an average of 23% of the service issues were (still in 1988) related to loosening of fasteners. Indeed, some automakers reported peaks of service issues related to loose fasteners close to 40%, the greatest areas of concern being rocker arms, the oil pan, engine mounts and the body in general. Over the years, the literature consistently reported several different failure scenarios of threaded connections whose root cause can be traced to loosening. The present chapter aims at giving a basic understanding of the phenomenon of bolt self-loosening, starting with a definition of self-loosening and, more specifically, vibrational loosening.