Why does torque vary when tightening and untightening?

Why is the untightening torque lower than the tightening torque when tightening a bolt or nut?

When tightening a bolt or nut, the applied torque must overcome the thread friction, the friction under the bolt head or nut and the inclined plane of the thread, to obtain a bolt preload (see Fig. 1A).

While untightening, provided that the assembly remains in the same condition, the applied torque only has to overcome frictional forces. As a result, untightening a fastener requires a lower torque (see Fig. 1B).

There are some exceptions to this rule. Since friction conditions vary, higher torque can be required to untighten a fastener. Corrosion, seizing, or surface roughness can considerably increase friction and subsequently the untightening torque.

With Nord-Lock washers, the difference between the ­tightening and untightening torque is even more significant. During ­tightening, sliding occurs between the bolt head or nut and the serrated surface of the upper Nord-Lock washer (see Fig 2A).

However, during untightening, sliding occurs between the cam faces of the washers, where ­friction is ­significantly lower (see Fig 2B).

This is a valuable feature of Nord-Lock washers, as a low untightening torque facilitates maintenance and thereby reduces downtime.