Threads & Stress Estimation

[SMF1964]

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?

 

[rb1957]

i think you mean that the tip of the threads has been worn off. i don't think this would double the stress in the thread, but i think it would reduce the tension load that the thread can manage ... because of the reduced engagement of the male and female threads.

 

[unclesyd]

Are you talking about wearing of ACME threads like on a valve stem?

 

[SMF1964]

rb1957 - The threads faces are worn away, not the tips.
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.

 

[unclesyd]

You need to replace the worn nut as soon as possible as the threads will probably deform prior to stripping and jam, rendering the valve inoperable will the gear head.

 

[SMF1964]

Yeah, that was already a known factor/recommendation. The threads broke off in bits and pieces, with fragments getting further crunched and chomped into littler bits and pieces. Valve stem travel was somewhat slowed. It was a mess.

 

[GregLocock]

If the face has worn by 50% then the stresses at the root of the tooth may have increased by a factor of 4, crudely. In practice it is not quite that bad, on the other hand you may well have some new stress raisers.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Cockroach]

Obviously the thread experience load on her flanks, as it should. The load on the stem, already correctly stated, comes from valve operation. But the bronze nut is suppose to wear, that is the purpose of using the soft metal. As UncleSyd suggested, you need replace the nut.

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

 

[handleman]

Just for the sake of curiosity, why were you wanting to state how much the stress increased? Are you having to convince some really tight person to drop the cash for a new brass nut?

 

[SMF1964]

handleman - One might be tempted to say that, but the more significant issue was a question one person asked about whether they should change their operation of the valves to reduce the stress on the threads. I was under the opinion that they would probably not get a signficant improvement since even if you limit your closing force, that is not when the majority of the wear is occurring. Rather, the wear is happening when the valve is moving and the estimated stress is at about a tenth of the closure force (according to the plant engineer who is paid to know these things - I'm just a humble metallurgist).