Double Nutting Part 2: Length Limitations

January 2023 | Technical Tips

Part 2

The first ‘Technical Tip on Double Nutting’ provided a description and methodology of the process and the mechanics of ‘double nutting’ – the process of adding a second (typically thicker) nut to a preloaded fastener to help prevent self-loosening. The process has been fully described in this article which is available on our website.

Unfortunately, there is still a lack of understanding of the correct practice and application, especially when the threaded fastener (or bolt) is of limited length and the elastic deformation during pre-loading is limited, (particularly with regard to matters of tolerances and fit.) Based on the theory is described in the former Technical Tip on Double Nutting, difficulties arise if the bolts are too short and the elastic deformation at full preload is less than the tolerance between the thread of the first nut and the bolt. In this case the upper surfaces of the threads of the bolt never make contact with the lower surface of the thread in the nut and as such jamming/self-locking action is not achieved i.e. the condition shown in Figure 4 is never achieved. Essentially the first nut simply acts as a thick washer.

Figure 1 Diagram depicting the first stage of the double nut assembly, (first/thin nut preloaded to 25-50% of required preload, second/top nut loose) highlighting contact (highlighted red) on the lower flanks of the bolt with the first nut.

Figure 2 Diagram depicting the second stage of the double nut assembly, (first/thin nut preloaded to 25-50% of required preload, second/top nut partially preloaded) highlighting both nuts are in contact with the lower threads of the bolt, when the second-nut preload ≤ first-nut preload.

Figure 3 Diagram depicting the third stage of the double nut assembly, (first/thin nut preloaded to 25-50% of required preload, second/top nut partially preloaded) highlighting the second nut is in contact with the lower threads of the bolt and the first nut is not in contact with the thread of the bolt (acting as a washer), when the second-nut preload > first-nut preload.

Figure 4 Diagram depicting the fourth stage of the double nut assembly (first/thin nut preloaded to 25-50% of required preload, second/top nut preloaded to exceed initial preload on bottom nut, bolt extension is greater than thread tolerance), highlighting the second nut is in contact with the lower threads of the bolt and the first nut is in contact with the upper thread of the bolt and the jamming action occurs.

An extensive experimental study undertaken by Origen revealed a wide variation in the effectiveness of double nutting in achieving the goal of achieving i) self-locking and ii) preload. Owing to the variance in thread tolerance and the friction at the thread interfaces, the self-locking action and preload is highly variable. Numerous experimental tests revealed that for shorter threaded fasteners that double nutting does not result in the desire locking action being achieved. In the case of longer bolts where the jamming action is achieved preload is highly variable and is dependent on thread tolerances.

The threaded fastener, depending on the tolerances and surface detail needs to be sufficiently long to develop pre-load as described particularly in Fig4 of Double Nutting, Part 1. In general, this is seldom achieved in practice and the value of “double nutting” on short bolts of various tolerances or reproducibility is dubious at best, and completely ineffective in short bolts, art worst. This is further exacerbated by imperfect surface finishes and or galvanising and its use in practice is of dubious value and needs to be accurately analysed and modelled (as well as measured) to have any confidence in its usage. Alternative systems such as Palnuts or Nordlockmethods may be feasible.

Because double nutting to maintain load and avoid loosening, is seemingly ineffective (despite its widespread use) is of questionable value, is not recommend, particularly of short bolt applications.