Broken Pulley Shaft Fracture Surface 1

[Vibroman]

Hi everyone,
What you think about this fracture surface.
On a newly installed conveyor this pulley shaft is broken after 1,5 month of operation time. Slow speed application app. 120 rpm.
I don't have much experience on failure analysis of machine shafts. Son can anyone please help me out on this issue.

 

[Vibroman]

Another picture from the moment shaft broken.

 

[israelkk]

Looks like fatigue.

 

[metengr]

Agreed. Looks like rotating bending fatigue. Beyond this diagnosis what are you looking to do? Have you had repeated failures at the same location? A proper metallurgical analysis will help you to solve this problem. Be prepared to spend some money for this analysis.

 

[winstonsk]

Looks like the fatigue crack grew a really long way, too

 

[Vibroman]

I was looking the type of the fatigue(i guess it is rotating bending fatigue), which point it started and how it progressed, approximately how many cycles it has run after crack started type of information.
I know these are not easy questions to answer without analysis. I was looking just for ideas.
This is the 1st time this shaft failed, because it is a pretty new conveyor like i mentioned. Unfortunately I cant efford to spend money for this failure now.
Thanks,

 

[metengr]

In situations like this if you can't afford a proper failure analysis replace it and monitor. Fatigue failures normally ocurr in these types of applications at sharp transitions. Make sure the replacement pulley has no sharp corners or transitions. Check the replacement pulley material to see if it is a low alloy steel and heat treated.

 

[mfgenggear]

I suggest to mag the new parts & verify there are no indications. what is the hardness requirements of the parts. check the hardness, ask the suppliers who made the parts to show certifications for the material, and the heat treatment. eliminate these possibilities. make sure the parts were heat treated properly a cut piece then mounted and etch with show the correct transformation. these are simple test and verification.

 

[arunmrao]

"Unfortunately I cant efford to spend money for this failure now"
Your comment, suggests, that you are not willing to spend for testing and verification,but you shall have to source a replacement shaft, to keep your machine running!!.
Perform some of the basic tests to understand failure.





"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[mrfailure]

Fatigue is the least interesting aspect to this failure - one would typically expect fatigue as the likely failure mode if the shaft fractures. You need to ask yourself: is it worth the cost of a proper metallurgical failure analysis from a failure specialist to prevent another failure and the costs associated with downtime? You should at least get a quote from a local lab specializing in industrial failures to know the answer to that question.

 

[mfgenggear]

while I totally agree with every one here about the failure analysis. but before he does that the points I made earlier needs to be completed as part of the failure analysis. is it the correct material , was it heat treated correctly, were there or are there any defects in the existing new parts.

 

[Vibroman]

"Unfortunately I cant efford to spend money for this failure now"
I guess you guys tangled up with this sentence. Sorry about that, maybe i misexplained my self.I don't afraid to spend money for analysis, just i think i don't need to for this case.
We got more then 100 pullies in this plant with similar size, similar material, similar applications. I don't think it is related with shaft material. This recent upgrade project conveyor pulley shaft is failed. There may be 2 reasons for that.
Either the take up unit attached to this pulley was tightened too much or tightened unevenly on both sides or because of the slightly bended installation of the pulley housing base frame every thing is flexing in and out at every turn of pulley shaft.I realized that couple of days ago top of the bearing housings flexing (moving) in and out every turn.So my original question was is it a sudden failure(brittle kind of failure) or happened slowly with time (because of over tightening)
I'm not sure about the condition of the new pulley now. I'm a vib. guy so my way to check this pulley is to follow the phase of 1X vibration with time. Didn't seem getting worse last 3 days.
I'm working both on frame and tightening procedure now anyway.
Thanks,

 

[mfgenggear]

that was the next phase I was going to suggest is the assembly procedure.

 

[mrfailure]

You are making a fundamental mistake in assuming that there are, maybe, 2 causes for failure and identification of failure mode will thus tell you which one. That mode of thinking very often leads to mistaken diagnoses and continued failures. There may well be other causes you did not think of. Appropriate testing does not get performed to determine whether there was a material issue (for example, was the shaft properly heat treated), a design issue (fundamental design will often induce this type of failure), or, as you suggest, an installation issue.

It is in your interest to have a specialist in metallurgical failure analysis work with you to identify the root cause, even if that person ends up confirming your diagnosis. You are a vib guy, and you need someone with appropriate experience to help you.

 

[mfgenggear]

I concur with Mrfailure

 

[saplanti]

Could the reason be an installation mistake such as an unparalleled driver and follower shafts? This can explain the fatigue and resultant failure.

 

[RolMec]

Hello OP,

pls. have a look at my last post in the parallel thread,

http://www.eng-tips.com/viewthread.cfm?qid=407934

KR
R.

RSVP

 

[Maui]

If you have an additional 100 other components operating in your plant that are subject to the same operating conditions as this one then you should inspect them [italic]now[/italic] for this same type of defect. Don't wait until they fail.

Maui