Nut design for increased fatigue resistance - 2
In the previous article, the subject matter was the work done in the former USSR on fastener fatigue and particularly the positive effect that nut design and material selection can have on the fatigue life of studs and other male fasteners. In this article we will look at attempts by fastener manufacturing companies to achieve the same effects using relatively subtle changes in the fastener geometry.
There have been several attempts to manufacture nuts with a thread form that distributes the load evenly between threads. You will recall from the previous article that use of standard nuts places the vast majority of the load on the first few engaged threads.
The Spiralock thread form has been shown to improve load distribution, and nuts with this thread are commercially available in a number of forms, all of which are compatible with standard male fasteners. It is also possible to buy taps in various configurations, including cold-forming taps (which form threads by material displacement rather than removal). It is therefore possible to design and manufacture small batches of bespoke fasteners that use this thread form.
The action of the thread relies on there being line contact between the outside diameter of the male thread and a specially designed portion of the female thread. Contact pressures are therefore high (standard threads assume face contact) and some permanent deformation of the female thread is expected. There is also a change in the torque-tension relationship owing to two factors - effective contact angle and contact diameter. The effective contact diameter moves from the pitch diameter to the outside diameter of the male thread. A Spiralock thread is said typically to require 10-20% extra torque in comparison with a standard nut to attain a given level of tension in the fastener.
Equa-Stress threads were a patent of ESNA, now part of SPS. The modern equivalent is called Double Durability, and a number of different pattern nuts are available, although little literature exists to explain where the improvement in fatigue life comes from, and whether there is any effect on the torque-tension relationship.
Bolted joints using differing flank angles on the male and female parts have proved effective in increasing the fatigue life of bolted joints. A study in the former USSR some years ago showed a 26% increased in endurance limit for standard bolts engaged with nuts having a smaller included thread angle.
Asymmetric threads have also proven positive in this regard, with both nuts and bolts having asymmetric threads showing improved fatigue limit when engaged with standard mating fasteners. However, care needs to be taken in designing and using such components, as changing the angle in the wrong direction will lower the endurance limit.
Over-pitching of nuts - that is, increasing the pitch slightly compared to the mating component - has also proven successful in improving fatigue properties by distributing the load more evenly over the engaged threads. The same effect can be achieved by de-pitching of bolts. The philosophy behind this is simple. For standard fasteners, as the joint is loaded, the bolt is stretched and the nut is compressed, giving a pitch error that causes a poor distribution of load. In over-pitching the nut thread, the thread pitches can be engineered to match in the loaded condition. The optimal amount of over-pitching (or de-pitching if applied to a bolt) depends on the material properties, bolt dimensions and thread form.
Fig. 1 - The internal thread form of a nut, in conjunction with a mating part having a conventional thread, can improve endurance limit markedly
Written by Wayne Ward