A model can often be of help in understanding why the bolt does not sustain the full effect of the applied load. Figure 2 is an attempt to illustrate the load transfer mechanism involved in a bolted joint by the use of a special fastener. In the case of this fastener no significant load increase would be sustained by the fastener until the applied load exceeded the fastener's preload. (Preload is the term used for a bolt's clamp force.)

With the special fastener shown, the bolt is free to move within its casing, a compression spring is included within the casing so that if the bolt is pulled down the spring will compress. A scale on the side of the casing indicates the force present in the spring and hence the force present in the shank of the bolt. Figure 2A illustrates this special fastener in its untightened condition.

The bolt is now inserted through a hole in a support plate and a bracket attached to the special fastener by securing a nut to the threaded shank. If the nut is now rotated so that the head of the bolt is pulled down, the spring will be compressed. If the nut is rotated so that 2 force units are indicated on the casing, the compressive force acting on the spring will be 2 and the tensile force in the bolt shank will also be 2. This is illustrated in figure 2b; this is like a tightened bolt without any working load applied.

If a weight is now added to the bracket (figure 2c) of value 1, then the initial reaction is to think that the load in the bolt must increase, otherwise what happens to the additional force? Surprisingly it will keep at its existing value of 2 - it will not 'feel' any of the additional force. To visualise why this is so - imagine what would happen if the load in the bolt did increase. To do this it would compress the spring more and a gap would be made between the bracket and the plate. If such a gap was to form then it would mean that there would be 2 units of force acting upwards - due to the spring, and 1 unit of force acting downwards from the applied weight. Clearly this force imbalance would not occur. What does happen is that the effect of the applied load is to decrease the clamp force that exists between the plate and the bracket. With no load applied the clamp force is 2 units, with the load applied this decreases to 1 unit of force. The bolt would not actually 'feel' any of the applied force until it exceeded the bolts clamp force.

Older design procedures proposed calculation methods based upon the idea that the bolt will not 'feel' any of the applied load until it exceeds the bolts clamp force. That is, the bolt should be sized so that its clamp force is equal to the external load after a factor of safety has been included. With the special fastener used in this example the stiffness of the fastener is far smaller than the stiffness of the plate and bracket it clamps. Practical fasteners differ from that shown in figure 2 in that elongation of the fastener and compression of the clamped parts occurs upon tightening. This compression results in the bolt sustaining a proportion of the applied load. As the applied force reduces the clamp force existing within the joint an additional strain is felt by the bolt which increases the force it sustains. The amount of the additional force the bolt sustains is smaller than the applied force to the joint. The actual amount of force the bolt sustains depends upon the ratio of stiffnesses of the bolt to the joint material.

The best way to understand and visualise how the force sustained by the bolt depends upon the joint stiffness is by the use of joint diagrams. These are the subject of the next page in this basics of bolted joints tutorial.

You may also be interested in an article published on this topic:

 

The Basics of Bolted Joints Published in the Fastener and Fixing magazine in November 2011, this article presents some fundamental ideas about bolted joints. Over the last sixty years great improvements have been made by the fastener industry in improving the design and reliability of their products. However, no matter how well designed and made the fastener itself is, it cannot alone make the joint more reliable. Fastener selection, based upon an understanding of the mechanics of how a threaded fastener sustains loading and the influence that tightening procedures can play, is also needed. This article provides an introduction to the basics of bolted joints and the major factors involved in the design of such joints. It is not widely understood how a bolted joint carries a direct load. A fully tightened bolt can survive in an application that an untightened, or loose bolt, would fail in a matter of seconds. When a load is applied to a joint containing a tightened bolt it does not sustain the full effect of the load but usually only a small part of it. This seems, at first sight, to be somewhat contrary to common sense but hopefully this article will explain why this is the case.