Case hardened 8620 will work good if you want a hardened clamping surface. For a softer clamping surface try 5160, 4140, or 4340 heat treated to 36-40 RC. For cheep and dirty use C1040.
Yeah, I agree with EdDanzer, AISI 4140/4130 are typically used in the oilfield, but the condition is HRc 18-22.
I've designed and manufactured many of these for various downhole equipment usage in the past fifteen (15) plus years, no problems.
A word to the wise, pay attention to bending stress at the root of the fingers around the collet base. Ensure a very healthy fillet to keep stress concentrations down, the predominant mode and location of failure. You can very easily compute stresses using Circular Beam Theory or get an estimate from cantilever analysis and using end beam or tip deflection as your variable input. I have found that my mathematical analysis, particularly pull-out force, stab-in load, etc mimick reality in lab experiments.
Good luck with the design!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
Thanks a bunch for your imput. I myself have designed them
and have used 4140 hardened to Rc 38-42. I have always
seemed to have good luck, but the last ones I designed
are failing. I am researching how our shop processed
them, I am suspicious that they have done something to
cause this. Once again, thanks for letting me pick your
brains!
