Large Diameter Threads subject to Bending Moment

[SteveyB]

Hi,

I was wondering if more of the technically adept people on here could offer some advice. I am basically after some means of calculating the bending moment capacity of a large diameter relatively short thread. So as an example a ring which screws onto a shaft which has a 500mm OD - 400mm internal thread and 80mm thick. I have had a search through various texts but seem to be struggling to find anything with this type of scenario.
I know its probably not the conventional and most appropriate way to load a thread but I would find this extremely useful if someone could point me in the right direction. Ideally I don't want to use FEA for this but on the other hand I realise it is extremely complicated with the ring being subject to a multitude of differing stresses i.e. hoop, shear and bending etc.

If anyone could offer any help I would be most grateful.

Thanks

Steve

 

[israelkk]

A picture or drawing can help.

 

[racookpe1978]

Are you trying to dynamically "bend" the large diameter threaded shaft, or expect a bending force large enough to force the threaded shaft out of straight - which will distort the threads and lock up the nut you are trying to turn around the now-bent threaded rod?

Or are you trying to find the force sufficient to bend a threaded rod that large?

 

[MikeHalloran]

It would be conservative to analyze the joint as a single turn of an Acme thread with zero pitch, i.e. an annular rib/groove joint (that is almost impossible to assemble, granted). Perhaps that small simplification will help.

It will not help if you are trying to make a joint that develops the full strength of an unbroken tube, of course.



Mike Halloran
Pembroke Pines, FL, USA

 

[SteveyB]

Hi,

Sorry for the late reply. I have now attached a image that will hopefully explain what I am trying to achieve. The threaded ring will basically be permanently attached to the shaft via a thread. The shaft is fixed at the opposite end to the flange and the flange end is free (i.e. a cantilever). The bending moment load will be applied directly to the threaded ring. I am therefore after determining the rings/threads resistance to the bending moment. I hope this explains things a bit better thanks for your help in advance.

 

[zcp]

Determine the needed contact area for the force couple to not fail the thread in bearing as the moment is transferred from the ring to the shaft. See if you have a whole lot more than that.

You can then use that same force to see how much thread length is needed to resist that force in shear. Hopefully you have a lot of area compared to force.....

ZCP

http://www.phoenix-engineer.com

 

[Tmoose]

How often is the bending moment applied?
Is it always applied in one direction?
What is the maximum acceptable motion when the moment is applied?

Is the moment really the result of the fixed end being "guided" and the ring being anchored, and a transverse force being applied to the fixed end?

 

[chicopee]

From the diagram and description, I am still trying to figure out where the ring is positioned. Now I just see an unthreaded solid shaft and a flange.

 

[chicopee]

Also, how is the moment created at the flange location. Is it a pure moment. or one caused by two equal opposite forces, or one that is caused by an end load?

 

[SteveyB]

Hi,

Thanks for the replies. Tmoose to answer you're questions, the bending moment is a constantly varying moment and is applied in multiple directions and may do 5,000,0000 cycles in a year. There are also no motion constraints. The moment is a result of the a transverse load being applied to another component which is attached to the flange via bolts.

Chicopee, The flange is the component at the bottom of the shaft which is secured to the shaft via a thread. The remaining portion of the shaft is un-threaded. For simplicity I would consider the moment is a pure moment applied to the flange.

 

[Tmoose]

"the bending moment is a constantly varying moment and is applied in multiple directions and may do 5,000,0000 cycles in a year."

Have any of these been built yet?
I'm expecting the mating threads to be severely worn by Christmas.

 

[MikeHalloran]

I'm expecting the threads to disengage completely in a week.

Seriously, if it exists already, there have to be better images.





Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

Suggest a Ringfeder type of shaft locking device along with your own flange plate - clamping ring, shrink disk, etc.

http://www.ringfeder.com

 

[SteveyB]

Hi Guys,

Thanks again for the replies. As I initially said this was somewhat unconventional and my immediate gut instinct looking at it was that it is less than ideal. Fortunately this thing hasn't been manufactured and I have been handed it as a concept. I was just more interested in the means of analysing such an I interface. Thanks dvd for the info on the ringfeder connections they look pretty useful.