Understanding the dark art of double nuts

Part 1

A commonly used method to prevent the self-loosening of bolted connections is the so-called ‘double-nut’ method, where two nuts are tightened onto a single threaded fastener. The jamming action associated with this practice, creates high frictional loads that resist relative movement of the nuts and provides a degree of resistance to self-loosening to the bolted connection – but only when correctly applied.

Typically, the use of double nuts involves preloading of the nut closest to the members being joined, which is often a ‘thin-nut’, to between 25% and 50% of the final required tightening torque and then tightening the second nut to the full torque required to achieve preload. It is obviously important to ensure that both the first nut and the fastener are held stationary and do not turn while the second nut is being tightened.

In order to gain a full understanding as to why both nuts need to be tensioned in a controlled manner (torque tensioning) and why the ‘thin-nut’ is used adjacent to the members (rather than on top of the full nut), it is important to understand what happens as the nuts are tightened. When the first nut is tightened the members are compressed, the bolt is stretched elastically and the load is carried by the upper flanks of the thread of the nut and lower flanks of the fastener, Figure 1. As the second nut is tightened onto the first-nut, and the preload (torque) is increased, the load on the upper/pressure flanks of the first-nut is reduced to zero (at which time the first nut acts as a simple washer). This occurs when the load (torque) applied by the second nut reaches that induced during the initial preloading (torque) of the first-nut, Figure 2 and Figure 3. As preloading continues, the bolt stretches and the members compress until the upper flanks of the bolt thread make contact with the lower flanks of the first-nut, Figure 4. Continuing to tighten the top nut results in the ends of the threads of the upper and lower nut wedging against i) each other, and ii) the faces of the bolt thread at the nut interface. It is this jamming action that creates high frictional loads that resist relative movement of the nuts (and gives the joint its self-loosening resistance).

Understanding the interaction between the two nuts highlights that care must be taken when tightening the respective nuts. This is particularly important in friction grip joints where a specific preload is required after the bolts have been tightened. As opposed to the case of a single nut, where the preload induced in the bolt is dependent on the tightening torque and the friction, the preload induced in the case of a double nut is dependent on i) the preload applied to the first and second nut, ii) the tolerance in the threads, and iii) the stiffness (and hence grip length) of both the bolt and member.

As such, although some resistance to self-loosening is achieved, it is very difficult to ensure what preload is actually achieved and it is recommended that in critical situations the joint be modelled carefully and the tolerances of the actual nut and bolt are specified/measured. Alternatively, trials using representative bolts and nuts (i.e. of similar size and supply) should be undertaken in a Skidmore machine (or other recognised tension measuring device). This is especially important when the bolts are galvanised, which affects the tolerances and deformation of the thread surfaces under load. In these cases alternative techniques to prevent self-loosening including inter alia Nordlock washers, chemical thread locking compounds or Nylock nuts should be considered.

It is also important to note that depending on the geometry/tolerances, self-locking may not be achievable. This will be addressed in our next tech tip.