Your shopping cart is empty.
Product Qty Amount

[email protected]
/ Categories: Archive, fasteners

Nut materials and their effect on stud fatigue

Fastener design is a critical area of any highly stressed machine operating with cyclic loads. Reciprocating internal combustion engines are an extreme example of this; there are a lot of bolted joints which are very highly loaded. Engine design engineers therefore need to pay close attention to the design and engineering of their fasteners. The rate at which load cycles are accumulated means that design mistakes are generally swiftly punished.

The design engineer might be mistaken for thinking that, having designed a stud from the strongest available material, having incorporated the most fatigue-resistant thread forms, taken great care over other design features and used the best manufacturing methods, he or she has done everything in their power to make the stud as reliable as possible. It is true to say that they have gone a long way towards this goal, but the nut also has a very important part to play in the endurance of the stud.

Even for a completely fixed nut geometry, there is one important factor over which the design engineer has control and which can lead to a significant improvement in stud fatigue resistance. This ‘magic’ factor is the material from which the nut is made. The design of the nut can also have a strong effect on the stress concentration factor experienced by the bolt or stud, but for this article we will look at the effect of material.

In order to understand this, let us reflect on the lesson first published in 1902 by Russian scientist Zhukovski, who postulated that the loads borne by threads in a nut were not evenly distributed, as we might imagine, but the threads closest to the load application bear a disproportionate share of the load. The distribution was shown to be a hyperbolic relationship depending on several factors. However, for the example of a steel nut and bolt with ten engaged threads, Zhukovski showed that the vast majority of the load was taken by the first few threads, with 34% taken by the first complete thread.

Since Zhukovski’s time, calculation methods and the computing power needed to carry them out have advanced, and more recent studies have put the stress concentration due to the first thread higher than Zhukovski’s figure. Several of these studies have taken a standard example of an M10 fastener with five complete engaged threads; both simulation and measurement show that the first thread takes between 36% and 41% of the total load.

The key to improving the load distribution in the threads is to make the nut threads less stiff. By doing this, the load is more easily transferred between threads by both shear and tension effects. The practical way to do this, for a nut of fixed geometry, is to change the material of the nut to one with lower elastic modulus. For nuts with a lower elastic modulus than that of the stud or bolt, the stress distribution is made more even than Zhukovski’s prediction, giving a more favourable stress concentration factor, but where the nut material’s modulus is greater then the load distribution will be worse, leading to a more severe stress concentration.

Written by Wayne Ward