How do we know for sure which bolt is the right size for each unique joint interface? Could we get away with a smaller bolt without risking the integrity of the bolt/joint? If so how?

Joint diagrams can display a significant amount of information about the joint but in our experience many people find them difficult to interpret and understand. Preload Requirement Charts are a way to graphically display the results of a joint analysis in a clear and understandable manner.

By way of example, consider the joint shown below that is subject to combined axial and shear loading. For information, the bolt is M12 property class 10.9, the joint thickness is 20 mm with an axial load of 15 kN and a shear force of 4 kN being applied. (If the joint consists of several bolts, it is first necessary to determine the loading on an individual bolt.)

One key aspect to appreciate is that the root cause of the majority of bolt/joint failures is due to insufficient preload. It is unusual for the bolt to be overloaded. If the preload provided by the bolt is insufficient, joint separation and movement can occur resulting in possible bolt fatigue and self-loosening issues. In order that such problems do not occur it is vital that there is sufficient residual clamp force acting on the joint interface after accounting for the effects of the applied forces and embedding losses. A Preload Requirement Chart graphically illustrates this point as it looks at the forces acting on the joint interface. Such a chart is shown below for the above joint.
The above chart was produced by the BOLTCALC program, but such charts can be produced manually. Explaining each of the parts of the chart in turn:

The above chart was produced by the BOLTCALC program, but such charts can be produced manually. Explaining each of the parts of the chart in turn:

Embedding Loss: Embedding is localised plastic deformation that occurs under the nut face, in the joint faces and in the threads as a result of flattening of the surface roughness. Embedding results in a loss of clamp force acting on the joint. If the joint and bolt stiffness can be established, the amount of this force loss can be quantified if the surface roughness of the contact surfaces is defined. In the above chart, a loss of 10 kN is anticipated. Large amounts of embedding loss can occur in joints with a short grip length consisting of many interfaces.

Axial Force Requirement: In a preloaded joint, the majority of the applied axial load reduces the clamp force on the joint interface rather than increasing the load in the bolt (see an earlier tutorial for an explanation). The amount of the axial load that unloads the joint interface can be determined from the joint/bolt stiffness calculations. In this example, of the 15 kN applied force, 13.8 kN reduces the clamp force on the interface (the remaining 1.2 kN increases the load in the bolt). To simplify, when hand calculations are being completed, the conservative assumption is often applied that all the applied axial load reduces the clamp force on the joint interface.

  Anti-friction coatings
AF coatings' are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder. Such coatings can be applied to fastener threads to replace metallic coatings such as zinc and cadmium and offer maintenance free permanent lubrication. By careful selection of the lubricants, AF coatings can be designed to meet specific applications. The coatings are permanently bonded to the metal surface and provide a lubricating film preventing direct metal to metal contact.

 
About the Author
Bill Eccles formed Bolt Science in 1992.  The company is a provider of independent technical expertise in bolted joint technology.  Bill has extensive experience in the design, analysis and installation of bolted joints and has published several technical papers on the subject.  He has a Doctorate in Engineering on The Self-Loosening of Threaded Fasteners and has delivered training courses around the world on the analysis of bolted joints and bolting technology.