What is the best thread profile for fatigue?
What is the best thread profile for fatigue?
(OP)
Hi,
I have an application where a threaded element is subject to fatigue loading in tension. There have been some failures in the past using UNC threads and I am wondering if another thread profile such as acme could help improve the fatigue resistance. I have the impression that acme is used primarily for moving parts, is this correct? Would it be wise to use this thread profile for a static application?
Thanks for your response.





RE: What is the best thread profile for fatigue?
Whitworth thread profile was very good for fatigue because of radiused thread root. Hard to find these days. Most UN fasteners are actually UNR, R for rounded. Are yours?
There are some propriatery threads that are similar to Whitworth, but I am at a loss at the moment to name them.
How about just a larger UNC fastner, or more of the same size?
UNF has sightly larger minor diameter for same nominal size, which might also help you.
RE: What is the best thread profile for fatigue?
RE: What is the best thread profile for fatigue?
A short answer is, everything else being equal, the change form UNC to UNF will give you the best return for the money.
If you need a more detailed answer come back with more specific details, application, size and type fastener, material, etc.
RE: What is the best thread profile for fatigue?
Which ever thread form you choose from above posts the method of manufacture is important and for fatigue resistance threads that have been formed by rolling are the best.
regards desertfox
RE: What is the best thread profile for fatigue?
increase number of fasteners
increase major diameter
increase material fatigue strength
use rounded root thread form
increase root diameter (UNJ or MJ thread forms)
pitch change (complex interaction, see Proc. IMechE 211 (1997), 591-600)
roll threads after heat treatment
Using a combination of these methods can have enormous influences on fatigue behavior.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: What is the best thread profile for fatigue?
RE: What is the best thread profile for fatigue?
RE: What is the best thread profile for fatigue?
jetmaker
RE: What is the best thread profile for fatigue?
A further improvement may be made by having a properly designed, polished thread run out groove with adequate radius and diameter slightly less than the thread root radius.
RE: What is the best thread profile for fatigue?
Some bolted connections were failing. It turns out that some bolt/nut joints were not mating well and the load was only on a few threads.
They went to a system where each bolt was tightened, and then using dial indicaters, over torqued by a fixed amount. They were then loosened and re-torqued. The slight overload seated everything and brought the threads into full contact.
= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: What is the best thread profile for fatigue?
RE: What is the best thread profile for fatigue?
1. Carry out thread rolling after heat treatment. It will introduce compressive stresses at the root of the threads. This has a fairly significant effect on fatigue strength. However the die life will be shorter.
2. Increase the fillet radius under the head. This may lead to a slightly larger head size.
3. Reduce the shank diameter below the thread root.
Gurmeet