When we tighten a fastener, the aim is to impose an axial load. The method of doing this is normally either to turn a nut on a stationary fastener, or to turn the fastener within a fixed female thread. This is something with which we are all quite familiar. What is less obvious though is what is happening to the female threaded component, and how this might affect the strength of the joint and the relationship between torque and tension in the joint.
There is a phenomenon called nut dilation at play to some extent in all female threaded components. As the name suggests, nut dilation is concerned with an increase in size of the nut brought about through the act of tightening the fastener, although the effect is not confined to nuts alone but to all female threaded parts.
A normal thread has a definite flank angle and, as we tighten the fastener, the natural behaviour of the parts in contact acts to move the nut material along the 'ramp' of the thread flank. The female part is generally the less stiff part in the radial sense, hence the effect of dilation.
The strength of the female thread is affected by nut dilation in terms of its resistance to thread stripping under load. As thread dilation moves the female thread radially outwards (and to a much lower extent the male thread is compressed radially inwards), the 'overlap' of the inner and outer threads is reduced. As the female thread dilates, the shear area of the female thread at the maximum contact diameter is reduced. Where the stress exceeds the shear strength of the nut material in shear, the female thread will strip, assuming that the male thread is strong enough to make the female thread strip first. The same applies to the male thread in the case that it is likely to strip first, except that the shear area is calculated at the inner diameter of the female thread. Whether it is the male or female thread which strips first, nut dilation means this undesirable result will happen at a lower than expected load.
Nut dilation also affects the torque-tension relationship in two ways. As the nut dilates, the effective thread contact diameter also increases, leading to an increase in the thread friction term of the equation. Additionally, the nut underhead face is also increased in terms of mean diameter, and this means that underhead friction is correspondingly increased too.
There are a number of ways to counter the effects of nut dilation, but all generally mean an increase in component mass. The aim is to increase the radial stiffness of the component. One might decide to increase the height of the nut or the length of the engaged thread. However, as the load is taken preferentially by the first few threads of the joint, this has little effect. Increasing the across-flats dimension of the nut - or the amount of material surrounding a female thread in a machined part - is effective, as is choosing a material of increased modulus.
Written by Wayne Ward