Shaft failed due to reversed torsional fatigue?

[sokenyou]

Hi Everyone,

I got this failed shaft from work (please see the attached). It's the main drive shaft for our Bath Tissue Converting machine - we called it the knock off paddle. Based on what I saw and what I read from the book "How Components Fail", it looks like a Reversed torsional fatigue, but there is a plstic deformation in the centre, so I hesitated a bit to come to conclusion. Unless it goes again, I probably won't spend the time per my Manager to do a full scale stress analysis, bending moment diagrams and stuff (not to say we don't have the resources but the lack of that kind of information provided by OEM). What I wish to see is based on your experience, at a glance, do you think it is a reversed torsional fatigue?


Thanks a lot!


K

 

[CoryPad]

Yes, reversed torsion.

 

[3DDave]

The plastic part is all that was still holding together up to the last moments of use. It looks like there was a groove, based on the step just outside the fracture. If that is, then it's probably a stress concentration that started the fatigue fracture.

 

[desertfox]

Hi

Yes I agree with the other posters, have a look at this link just for further information

http://www.scarborough.gov.uk/pdf/0...e-of-GearBox-From-The Whitby-Swing Bridge.pdf

 

[Dougt115]

Changes in geometry act as a focus for stresses. Even if all parts of the assembly are strong enough the change in geometry will focus the stress and may result in failure. Even a collar around a part can result in a stress concentration.

Check out any Mechanical Engineering Design books that deal with stresses and you will find a section on stress concentration factors. This and maintaining a Factor of Safety of >1.5 as served me well. (Personally I try to stay above a Factor of Safety of 2 if at all possible.)

 

[LiftDivergence]

That is a very interesting surface. It definitely looks like classic torsional fatigue. The cracks growing in from the surface probably reduced the net section capability too far to resist yielding. Would be fun to try to figure out the critical point where plastic deformations start. Would have to know the loads, though. I could be wrong, but I know those color bands are typically indicative of variances in the crack growth rate. The density of striations can make some sections look darker than others. Any thoughts on if that is occurring here?

 

[Tmoose]

What attaches to the missing shaft?
How long was it in service?

 

[sokenyou]

Thanks everyone for your input

The shaft was in a pulley/sprocket

No record of replacement of this shaft since the machine was commissioned but our work order system is only good from year 2010.


Thanks!