Prevention of fastener loosening, part 3

In the first two instalments in this series on the prevention of fastener loosening, we talked about methods of chemical thread-locking before starting to look at some of the mechanical methods in common used, of which there are many.

As mentioned before, axial thread interference is occasionally used, mainly as a way to improve the distribution of load between threads, but it has the happy coincidence of introducing some extra friction into the relationship, which is useful when trying to avoid bolts coming undone. This practice is nearly always used for very highly loaded fasteners.

Also mentioned was that radial interference on threads is not common; that is to say, ‘bulk’ interference over the entire length of the thread is not often used. It gives an inconsistent level of additional friction during tightening, which is very unhelpful when trying to achieve a known level of load in a fastener. However, interference fits on a single or small number of threads is used, as this approach gives a constant and repeatable amount of extra friction when tightening. This extra friction can be measured using a torque wrench or electronic device, and can be accounted for in the torque-tension relationship.

Thin-walled metallic nuts are commonly supplied with specially deformed threads that act to give a controlled and constant amount of friction. Originally designed for use in aerospace applications, these are widely used in motorsport. Most people tend to call them K-nuts, and they come in various types. The locking action is achieved by the elliptical deformation, and the most commonly used are the hexagonal K-nuts with a flange, which also exist in a high-temperature silver-plated version. Other types that are riveted to sheet metalwork or composite panels provide thread-locking in applications such as engine intakes.

All-metal locking nuts are a real favourite in aerospace applications, so we also find them used in motorsport. Another popular type of nut, often known as aero nuts or aerotight nuts, have slots cut towards the top of the nut, a couple of threads down from the top, de-stiffening the top part of the nut which is then deformed slightly in the axial direction.

Various suppliers of wire thread inserts market inserts with a single deformed thread to provide a constant amount of friction when installing a fastener. Other than chemical thread-locking methods, deformed wire inserts are the main method of positively locking a standard fastener into a threaded hole, as they have the advantage of providing a constant amount of additional friction to the system. Chemical thread-locking methods are prone to inconsistencies between operators and differences in application, speed of tightening and so on, and here the mechanical wire insert is much more predictable, albeit at a much higher cost.

Locking thread inserts are sometimes installed in nuts that are made of soft materials or those prone to surface damage during tightening: titanium and aluminium nuts are available in this locking configuration. The wire insert in the nut also provides an improved load distribution between threads, leading to a lower stress concentration factor on the male fastener.

Written by Wayne Ward