The Goldilocks Zone and the Coefficient of Friction of Threaded Fasteners

In July 2023, Bill Eccles of Bolt Science presented a paper titled: ‘The Goldilocks Zone and the Coefficient of Friction of Threaded Fasteners’. This summarises the paper and adds some background.

When a nut is tightened, the bolt is stretched and is resisted by a tensile force within the bolt, the preload. The preload is reacted by a clamp force that pushes the joint plates together.

In most applications, it is the clamp force acting on the joint interface that is the critical parameter. There is no low cost means of measuring or controlling the clamp force so some other, easily measurable quantity is needed. Controlling the torque value applied to the nut being the most popular approach.

Most of the torque that is applied when tightening a nut is used in overcoming friction. The pie chart shows the case for an M12 property class 8.8 bolt being fully tightened with the friction coefficient being 0.12. The torque value is very specific to the coefficient of friction. On the positive front, friction is needed since otherwise the nut would spin off when the socket was removed following tightening.

When a nut is tightened, besides the tensile stress, the bolt is also subjected to torsion. This arises from the thread friction torque and the torque that is needed to stretch the bolt. Accordingly, higher the thread friction, higher will be the torsion in the bolt.