Dr Bill Eccles
A frequent question that is asked relating to bolting is: 'We've just checked a nut that we tightened and it's well below what we tightened it to just minutes earlier ‑ what's wrong?'.
It's not widely known that the torque for a newly tightened nut or threaded fastener, in general, is different depending upon whether you attempt to untighten it or tighten it a bit more. Typically the torque needed to untighten a newly tightened fastener is around 10% to 30% less than the torque to tighten it further. When you are tightening a threaded fastener a significant amount of torque is needed to overcome friction in the threads and under the nut face (or the bolt head, if the bolt is rotated). The proportion of the torque that is used to overcome friction depends upon the friction value but is typically in the 85% to 90% region. This is illustrated in Figure one, which shows that when tightening a nut/bolt with a coefficient of friction of 0.12, only about 14% of the torque is used to stretch the fastener producing the clamp load with 86% of the torque being lost overcoming friction. The torque needed to stretch the fastener always acts in the untightening direction and itís for this reason that the untightening torque is less than the tightening torque.
Figure two shows what happens when you tighten, and then untighten, a threaded fastener. The tightening torque TOn produces a certain clamp force (F), it then takes a torque TOff to untighten it. For a newly tightened fastener, TOn is greater than TOff. If the fastener is left for a prolonged period, changes in friction can result in the untightening torque TOff becoming greater than the original tightening torque TOn.
In many applications the clamping force provided by tightening fasteners is of critical importance in determining the success, or otherwise, of the structural integrity of an assembly. A great deal of attention is often placed on ensuring that bolted connections are installed in a controlled manner so that a predictable clamping force is achieved. The most popular controlled method of tightening a threaded fastener is by applying a specific tightening torque. Below the yield point of the fastener, the relationship between the applied torque and the clamp force provided by the fastener, is linear. That is, double the torque and you double the clamp force. Once tightened, the clamp force provided by a bolted connection can decrease. The decrease can occur without any rotation of the thread, as in the case of stress relaxation, embedding, creep and similar effects, or, the bolt or nut may rotate decreasing the clamp force as in the case of self-loosening. Subsequently, concern over the loosening of bolts, in many applications, necessitates quality control measures to check their tightness. Tightness is usually assumed to be the measured torque value but in the majority of applications, it is the clamp force rather than the torque that is the critical factor. The problem is that there is no low cost method of assessing the clamp force provided by a previously tightened fastener. Currently, the tightness of a bolt/nut assembly is usually assessed by a torque based method, the approach is referred to as torque auditing.
From an article featured in Fastener & Fixing Magazine Issue 90 November 2014.
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