Threads & Stress Estimation
Threads & Stress Estimation
(OP)
I have a threaded stem nut that drives a valve shaft up and down. The threads on the stem nut have been worn to 50% width (estimated). Would I be able to state that the stress in the threads is therefore doubled?





RE: Threads & Stress Estimation
RE: Threads & Stress Estimation
RE: Threads & Stress Estimation
unclesyd - I'm not sure what ACME threads are. The steel valve stem is threaded, the bronze stem nut threads over the threads on the valve stem and has longitudinal gear teath on the outer surface that engage with the operator. Turning the stem nut (which is held in place) causes the valve stem to go up or down. The threads on the steel valve stem are intact, the threads on the stem nut are worn away on both faces, indicating that the wear condition exists when the valve opens and closes, not just one or the other.
RE: Threads & Stress Estimation
RE: Threads & Stress Estimation
RE: Threads & Stress Estimation
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Threads & Stress Estimation
But to answer your question, no you cannot make a generalized statement that thread stress is doubled because of the wear. First of all, wear is secular and cannot to be constant over an epoch. Second, you need to classify the thread and obtain the geometry of the profile. You can then apply the equations from ASME/ANSI B1.5 (Acme Threads), ASME/ANSI B1.8 (Stub Acme), ASME/ANSI B1.1 (UN)or ASME B1.2M (Metric) to obtain the thread area per unit length engagement. This is simply multiplied by the nut height for a numher giving you thread shear area. Knowing the material stress, you can obtain the maximum allowable stem force or figuring out the physical load experienced by the stem, then obtain the actual stress on the threads.
I think that UncleSyd is correct in assuming the thread profile to be Acme. This is fairly common in valve stem design.
Good luck with it.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Threads & Stress Estimation
RE: Threads & Stress Estimation