Reprint from Fastener World Magazine, Vol. 174

To understand why fasteners are manufactured to different strength levels, one must first understand how a fastener is intended to work. To work properly, a fastener must possesses some elasticity and stretch as it is tightened. For many this is counterintuitive because, for example, a large 1” diameter steel bolt hardly seems like an item that would stretch. However, stretch it must if it is going to function the way it was intended.

Figure 1 illustrates a demonstration that one can use to understand the behavior of a bolted joint. To conduct this demonstration, one places their thumb, index, and middle fingers together, wraps a wide rubber band twice around, and then opens their fingers against the resistance of the rubber band. In the second part of the demonstration a third wrap of the rubber band goes around the fingers and reopening the fingers is, once again, attempted. Individuals attempting this demonstration should notice that with the third wrap in-place the rubber band is much more highly stretched and it becomes significantly more difficult to open the fingers.