Calculating KT for a hole

[itadekimasu]

I am new to this website so forgive me. My question is I am in need of information that describes how to calculate stress concentration in a localized area due tapping an existing hole that has a cyclic life. To expand my question there is a rotating disk that has a certain amount of holes in it. Each one of these will be tapped and a threaded plug will go inside. I am trying to look for information that would tell me how to calculate what the consequences are from tapping the hole. By consequences I mean will I have a localized stress concentration, will the cyclic life of the disk be reduced, may I induce a localized crack that could propagate and reduce my life cycle, calculating Kt factor?

Thanks ahead of time for any information Matthew.

 

[desertfox]

hi itadekimasu

We need more information what is this disc, whats its function, what load is it under? can you upload a sketch.

desertfox

 

[itadekimasu]

It is a turbine disk. The function is to let cooling air pass through the passage way. The load that the disk sees come from the transfer of power on the curvic clutch gear system to a torque tube. I am currently reading parts of "HANDBOOK OF BOLTS AND BOLTED JOINTS By JOHN H. BICKFORD, SAYED NASSAR" since I do not own the book I am trying to piece things together since the whole book is not available online.

 

[desertfox]

Hi itadekiasu

What load if any is put on the threaded plugs?
Also what stress levels do you have on the disc during service.

desertfox

 

[itadekimasu]

There should not be any real loads put on each one of the holes. The curvic clutch connection is only located on the rear of the disc on a meaty section. There are no clutch interfaces in front of the disc. I currently know the cyclic life of the disc with the current holes but I cannot seem to find any information about how to calculate what possible problems could occur due to tapping the holes and threading a plug in there. Welding is not an option due to the fact that, the disc cannot be removed. In addition, the holes do not play a large part in the life of the disc. There are many other reasons that the disc life is short. The current hole is not intended to have a plug in the area. Really what I am looking at is how to calculate what the stress due to tapping the area and if that will have an effect on my know cyclic life of the disc. If I can calculate the stress and compare it to the Kt values I will be golden.

 

[desertfox]

Hi itadekimasu

If the holes are not in a stressed area then I don't think you have a problem and the only stresses you will generate is from screwing the plugs in.
Stresses can be estimated for threaded joints at this site below and I also give you the link for stress concentrations for holes for reference:-

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

http://www.roymech.co.uk/Useful_Tables/Fatigue/Stress_concentration.html

desertfox

 

[itadekimasu]

Thanks for the information it was very informative and I believe you are right about the stress coming from the plug being screwed in.

I do have another question though, what should I read to find out information about the way to predict the residual stresses caused at the root of the tapped hole? I am looking to find a difference between fine, coarse, or rolled threads and there impact on tapping the hole?

 

[desertfox]

Hi itadkimasu

Look at this site and scroll down to class numbers 28 to 32

http://www.utm.edu/departments/engin/lemaster/machine_design.htm

desertfox

 

[itadekimasu]

Desertfox,


Thank you very much for your thoughts and the information you have shared with me.

 

[unclesyd]

Evidently there is some problems associated with threaded holes in turbine disks as there are tons of information on eddy current probes to check same. If I recall all the problems are where the threads are loaded in tension with things like tie rods or in other words an external force is acting on the threads unlike your case where there is no axial load on the threaded hole. I also know that fretting and wear can be the initiator of cracking in bolt holes, untreated, tight clearances are recommended for pass through fasteners.

I've been looking for a paper I had that had considerable information about holes in disks and fatigue cracking from knife edge holes and rounded edge holes. I haven't found it as yet. Here is one article that may be of assistance.

http://www.sciencedirect.com/scienc...serid=10&md5=2425877f66eb9f9049d1ff3a06db3c3d

I would look to form the threads if possible instead of cutting same.

http://www.precisiontwistdrill.com/techhelp/help_pages/thread_forming_prod_taps.asp

NASGRO:

You might be able to find someone who has access to NASGRO to checkout your exact conditions.

http://www.swri.org/4org/d18/mateng/matint/nasgro/Demo/default.html

 

[itadekimasu]

@ unclesyd


Sir did you happen to come across the paper you discussed in your post??

 

[unclesyd]

I haven't found it yet, probably one of the things I filed for safekeeping. I haven't even come to close as this paper had hand drawn images.

A. F. Grandt, Jr., "Stress Intensity FacTors for Some Thru-Cracked Fastener Holes," International Journal of Fracture, vol. 11, No.2, 1975.

You might do a search using Dr Grandt as keyword as for some reason his name popped up in the windmills of my mind.

http://cobweb.ecn.purdue.edu/~grandt/

I would also do a search on NASA. For some reason my system is getting error messages saying to allow Java Scripts when I Try to look at papers on their website.