Bolted joints are used everywhere, but how much do you really understand about how they work? How are these joints able to transfer large forces and resist cyclic loads? It’s not quite as straightforward as it might seem.
The strength and effectiveness of a bolted joint all comes down to how it is assembled. When the nut is tightened the threads on the bolt and the nut engage, and rotation of the nut pulls the bolt threads down, which causes the bolt to stretch. This stretching creates a tensile force in the bolt, pulling the two joined members into contact with one another and compressing them.
This tensile force in the fastener is called preload. It is intentionally applied to the bolt or screw before any external loads that act on the joint. Preload is surprisingly powerful. It makes the joint stronger, more resistant to fatigue damage, and much less likely to fail.
The video below covers this in detail, and explains why preload is a key consideration when working with bolted joints.