Roll Threading Existing Cut Threads for Fatique Improvement 1

[ERAU82]

Not sure if this is the right Forum. I have an commercial aircraft MLG axle that due to corrosion was machined 1/16" undersized. The OEM approved the repair but reduced the life limit due to fatigue issues with the newly cut undersized threads. The OEM did state that the original life limit could be restored if a thread rolling die was used to added compression to the root radius. I understand the concept but since the threads were already cut, will the die add compression to the root radius? We were looking at the torque load to roll the threads on a test section that did not have threads cut, but I do not know how to translate this to a torque required for the die to roll over the cut threads. Any help would be appreciated.

Regards,

ERAU82

 

[CoryPad]

To get the residual compressive stresses, you need to plastically deform the thread roots. Usually, this is done by deforming the entire thread onto a cylindrical blank. However, there are special tools to form the root of a cut thread. Check into the Cut&Form products from Emuge:

http://www.emuge.com

Also, you could use deep rolling, take a look at Ecoroll:

http://www.ecoroll.com

 

[rb1957]

there would be some benefit to rolling the thread (after machining it), sort of like shot peening, but i'd've thought that to get much benefit you'd have to really reform the thread (so the the original nut wouldn't fit).

if you've got some documentation from the OEM, why hot approach them for clarification ?

 

[ornerynorsk]

I'm no metallurgist, but I believe that you would have cut through a good share of the material that received the benefit of the original rolled threads, and introduced significant stress riser. I don't know that re-rolling is going to do you much good, unless you can go another step undersized.

 

[tbuelna]

ERAU82,

Thread rolling produces a more fatigue resistant grain flow in the metal thread structure than a cut thread. The thread rolling process is like forging, in that it plastically displaces material from the root area up into the crests. So the resulting grain flow follows the profile of the thread. Cutting, milling or green grinding a thread interrupts this desirable grain flow.

If the reworked (undersized) thread has the same pitch and essentially "chases" the original thread, then a rolling operation on the new thread may actually be of some benefit since the original rolled grain structure may still partially exist. If your thread roller is very careful, and very clever, he might be able to roll the cut threads with a very slight pitch offset. This would produce a cold working effect that may restore the local grain flow. (Of course, getting your DER to buy into this theory is a whole separate matter )

But if the original rolled grain structure has been machined away, then rolling an already cut thread will not be of much benefit. It will only produce some surface compression, and the same result could more easily be achieved thru a process like shot peening.

Good luck.
Terry

 

[RPstress]

"...the original life limit could be restored if a thread rolling die was used to added compression to the root radius..."

I do not think that you will be able to quantify the effects of rolling an already-cut thread. Yes, you can say that 'rolling will likely add usefully to the life', but that is probably as far as engineering will take you.