What is the best thread profile for fatigue? 5

[Rpierre]

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.

 

[MintJulep]

Acme thread is designed for power transmission, not clamping.

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.

 

[israelkk]

UNJ (UNJF, UNJEF) threads has larger radius

 

[unclesyd]

There quite few variables that enter the equation for increased fatigue life of fastener.

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.

 

[desertfox]

Hi Rpierre

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

 

[CoryPad]

Variables that will improve fatigue performance are:

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.

 

[BillPSU]

In hydraulic cylinders buttress threads are used for this application as it reduces radial forces generated by the angle of the thread. I don't know if your application has limited ring strength or your mode of failure. Buttress threads are expensive to make as all cutting tools and gaging is special compared to V threads.

 

[Screwman]

For high fatigue applications the use of the UNJ thread profile with enlarged root radius is recommended. This root profile follows closely with Whitworth forms (withstanding the flank angle differences). Since this is a standard thread profile it will be relatively easy to find both male and female fasteners. This is the thread of choice for critical aerospace applications.

 

[jetmaker]

Keep in mind that selection of the nut can also have an effect on the fatigue life of the bolt. There are some special nuts out there for fatigue applications.

jetmaker

 

[MOB1]

Threaded element sounds like a one off type unit which happens to ahve a thread. This makes some adjustments difficult (eg changing to a rolled thread).
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.

 

[EdStainless]

I ran into a case recently that has some bearing on this. The issue was thermal fatigue, but the same principles.
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

 

[arto]

How about Turbine bolts using tapered threads for improved load distribution?

 

[gurmeet2003]

Some other important things to improve fatigue strength of fasteners:

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