SEED Guides Bolted Joints and Threaded Fasteners < >

General Principles
It is important to remember when designing a bolted joint that the fastener is essentially a device to sustain a tensile load only. When the applied load is in the plane of the joint, it should be resisted by friction at the joint faces provided by tensioning the bolt, and not by shear in the bolt itself - see Figure 2.1.

Figure 2.1 Loading in plane of joint faces

If the fastener is permitted to take a transverse load, it will in fact be subjected to a combination of shear and bending stresses, in addition to the stress resulting from any axial tension applied by the nut. Clearly, it would be possible to select a fastener to sustain this combined loading, but the result would be a design of excessive proportions compared with a correctly sized tensile fastener.

Figure 2.2 Showing a fastener under shear and bending stresses

The above example of a clevis joint illustrates the additional stresses to which the threaded fastener is subjected in this type of connection. The deflections are greatly exaggerated to illustrate the issues. This type of connection is often used, but its design is outside the scope of this guide.

If it proves impossible to sustain a transverse joint load by friction, the joint faces may be stepped or serrated to provide a more positive restraint, for example see Figure 2.3. However, in such cases it is often better to reconsider the whole situation, and rotate the plane of the joint face to a position in which the transverse loading is lower.

This guide assumes that the component parts being connected are sufficiently rigid so as not to subject the threaded fasteners to bending stresses when the joint is being tightened or when it is under load in service. Fig 2.4 illustrates this point by considering a cylindrical pressure vessel with a flanged end cap and illustrates in exaggerated form the tendency for the elements of the joint to rotate and distort. Pressure Seal in groove

Figure 2.4 Deflection of joint elements causing bending of fasteners

The analysis of the deflections of the elements being joined is outside the scope of this guide but should not be ignored in those cases where it may add significantly to the bolt stresses by causing additional bending stresses. In cases such as the above, consideration should be given to making the flat end plate into a Ścup' shape so moving the flanged joint away from the end of the pipe so that the bolt load becomes purely axial.