Because the steel grain structure is cold forged, rather than cut, rolled threads are up to 30% stronger…
While the strength of a thread is a function of section thickness, a thread rolled rather than cut thread is usually superior in mechanical properties, all other things being equal- here are 6 reasons why:
- The flow of the material by cold work during rolling reinforces the shape.
- The cold working strain increases mechanical properties, Surface Hardness,Tensile Strength, Yield Strength, and the Yield Strength / Tensile Strength ratio.
- The surface finish of the thread flanks is usually smoother due to the burnishing action of the rolls.Smoother finish means better fatigue life and fewer opportunities for stress risers.
- There is more material (section thickness). This results in material savings*, since the diameter of the blank will be between the major and minor diameter of the thread, rather than greater than the major diameter for a cut thread.
- The compressive stresses on the threads resulting from rolling improves the fatigue life.
- The root of the thread has a smoother radius, improving fatigue life.
Pictures courtesy PMPA member Ray Industries
*I got my first “learnin” on rolled threads at my customer Keystone Threaded Products back when my hair was not silver and B.K. (Before Kids): “Why do you always buy funny sizes Jim?” I asked. That was a great first lesson on how engineering can add savings: