If you have performed the experiment properly, you will have experienced a profound change in the resistance from two wraps of the rubber band to three wraps. With the greater spring compression obtained by stretching the rubber band into three wraps, the resulting resistance to our attempts to separate our fingers was noticeably increased. In the exact same manner, when the preload is increased in a bolt, it will be able to withstand greater service loads that are attempting to separate the joint.

The behavior described here is expected for a joint loaded in tension. However, the same theory applies to joints loaded in shear. In these cases the designer wishes to prevent the clamped material from slipping sideways (perpendicular to the fastener axis) and creating a shearing load on the fastener. By inducing a higher preload the joint is more tightly compressed resulting in greater friction between the items clamped and greater resistance to slipping in service.

Naturally, this behavior is not infinite. There is a point at which every fastener is no longer capable of stretching elastically and begins to permanently (or plastically) stretch, ultimately leading to breaking or failure. The point where this transition occurs is referred to as the Yield Point. Although it is not uncommon to tighten a bolt up to the yield point, it is an operation that must be done with care and control.

This also explains why designers might choose a high strength bolt over a lower strength one. As bolt strength increases the amount the bolt can elastically stretch also increases. However, there are tradeoffs including the fact that as bolt strength increases the materials they are made from exhibit less ductility and so control of the tightening process, especially at values near the yield point become ever more important.

In summary there are several guidelines that should be applied to designing a robust joint:

1. To maintain a robust joint, it is important to maintain a clamp load at values above the service loads experienced by the joint.

2. The bolt should not be tightened beyond a predictable and controllable limit. In most cases one would never exceed the yield stress and, more typically, designs use some fraction of the yield stress.

3. When designing the joint, one must consider the ability to control tightening at both assembly and during maintenance operations.  Circling back to the study of truck wheels separating, the vast majority, if not all of the failures attributed to improper tightening, are probably the result of poor maintenance practices that have occurred during servicing of these vehicles.

4. Although preload is the top priority in the bolted joint, there are many factors that influence the ability to achieve or retain the desired preload, such as operating temperatures, harsh and corrosive environments, and whether the joint is a “hard” or “soft” one.  When designing the joint all of these factors must be considered and properly factored.

Engineering the proper joint can be a complicated task. In this regard, the fastener is more often than not, misunderstood and not given the respect that it is due. However, it remains true that when designed and installed with intentionality and care, a properly tightened joint will generate the requisite preload and will seldom come loose in the field.