hamster wrote:Rolled threads are stronger as the grain is not interrupted across the thread edge, also a hardened surface is preserved.
Also, the bottom of the thread has a radius so hass less stress concentration for cracking of the thread off the bolt.
So there are very good reasons for a rolled thread on a spoke which is thin and has cyclic stresses.
Correct, and the surface finish is better than tapped or turned. The grains and forging flow lines are changed so that the orientation of potential defects is more favourable for the forces applied, and this reduces the potential for fatigue failure. In this case, the forces in the actual thread are shear forces.
Rolled on the left, tapped on the right.
Also, from a cleaning and removal point of view, which can be difficult, no excess swarf or chips are created. These can also act as stress concentrators and crack initiators if present.
The surface is work-hardened, so strength is not just preserved, it is enhanced, and typically has a compressive stress which resists the cyclic forces which cause fatigue. This is similar to the surface hardening, say, of compressor blades in aircraft engines, which can be peened to induce compressive surface stresses and improve fatigue properties.
The reduced shank diameter is not an issue, as the weakest part of a threaded fastener is the minor diameter within the threads, where the work-hardening has occurred - it is also the same diameter!
The cited example of Grand Prix is probably because of more traditional "prejudices" amongst designers, and that it is easier to turn or tap threads in a jobshop environment, where production quantities are low, than to also have a complete range of rolling dies/tools. The associated time-consuming set-up of a rolling die-set for only a few parts versus going with a manual die could well make it wasteful of resources.
