The term ‘fatigue resistant’ might be a little misleading, as it is not a quality that is in black and white but shades of grey, and any component can be made to fail through fatigue by subjecting it to a high enough level of cyclic stress. However, when it comes to fasteners, there are some design features, material choices and manufacturing methods that we can use to improve fatigue strength. Many of these have been covered in previous RET-Monitor articles in this channel.
One fundamental choice that we need to make is that of threadform. We need to carefully select the correct size of fastener so that it can provide sufficient pre-load and withstand the service loads we expect to subject it to. There is a wide variety of threadforms to choose from; not all threads are designed for use as fasteners, so those such as acme and trapezoidal threads can be discounted instantly. We will generally choose between metric (M) and unified (UN) threadforms, both of which have flank angles of 60°. Unified threads are imperial (inch) sizes and are most widely used in the US, which continues to use the inch as its preferred unit of length.
There are fatigue-resistant versions of both these types of threads, known as J-form threads. For any given thread pitch, these have a more generous root radius on the male thread than the ‘non-J’ equivalent. The increase in root radius increases the fatigue strength of the fastener for two reasons. The first is that the stress concentration is reduced slightly owing to the radius increase, and the second is that the minor thread diameter is also increased. J-form threads are denoted by MJ for metric and UNJ for imperial sizes, and there are only certain combinations of nominal diameter and pitch for which J-form threads are available.
Whitworth threads incorporate a controlled radius on the major diameter of the tap, so that the female threaded component is rendered more resistant to fatigue by having a reduced stress concentration at its major diameter.
The aero thread is a very clever but complex thread system that has never gained widespread acceptance. It incorporates a number of features that make it resistant to fatigue but it is rarely (if ever) used for new designs, and I have never seen an example of this in anything other than a textbook. The male thread is semicircular and relatively shallow. It therefore resists fatigue for the same reasons as the J-form threads but to an even greater extent.
The female thread is cut into a nut or casting, for example, and is of a completely different form to the male thread, being similar in terms of geometry to a common 60° flank angle thread. There is an intermediate member which is a thread insert to be installed into the female component. The thread insert has a 60° flank angle thread on the outside and the semicircular thread on the inside. Thread inserts de-stiffen the female thread, and this is known to improve the distribution of load along the thread. In a conventional metric or unified thread, the first loaded thread takes very much more of the load than any of the other threads, and the use of female components of lower stiffness than the male thread makes the load distribution much more even.
Aero threads are expensive to produce, and the lack of availability of suitable inserts makes them impractical. However, the concept of using thread inserts to improve load distribution (and hence reduce stress concentration factor or improve fatigue strength) is very valid, and suitable thread inserts are available in most metric and imperial thread sizes.
Written by Wayne Ward