Simple in concept, correctly tightening the ubiquitous threaded fastener turns out to be a complex process.
Assembly with threaded fasteners is an essential element of modern industrial production, and bolts are among the most common and widely used types of threaded fasteners. Bolts, like all threaded fasteners, are ideal for recycling and conform to standard safety practices. Because bolted joints are so widely used, their economic importance cannot be overstated.
The principles of bolted joints, and the commonly used methods of controlling the tightening process, are deceptively simple. A bolt is just a threaded fastener, with a thread that does not extend all the way to the head, designed for use with a nut. But tightening the bolt correctly is a complex undertaking that requires understanding the characteristics of the bolted joint and the various tightening methods.
Without a working load, the bolt is merely a clamping device. In a bolted joint, the parts are clamped in place between the bolt and nut. The bolt preload is equal but opposite to the
preload of the clamped parts. In the assembled state without an external load, the assembly preload is equal to the bolt force. The assembly preload in the clamped parts is equal to the clamp force.