In a paper converting mill, one of our draw roller shaft failed at the weakest point (near the bearing and chamfer with the radius of 0.5). Rachet marks (but shiny) indicates that fatigue cracks were initated at a few locations along the groove. The eccentric pattern tells me that the load on the shaft was imbalanced BUT it's only pressing on tissue papers, and to our knowledge, there was no excessive loads. Two shafts failed within a month... it's really frustrating.

Do you have a picture for us?

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams

Thanks Scott, yes I have picture for you to look at. In fact, I was told that it was fretting corrosion that caused it but we are iffy about this because it's not subject to high loads with other parts and it was in the mill for two weeks.... again, I still haven't figured out what to do next to resolve this issue.

• http://files.engineering.com/getfile.aspx?folder=b5c029e8-fa2a-40b3-8a04-97

Roller could be resonating at its natural frequency. Was much vibration noticed before breakdown? Have conditions changed like speed or loading. Higher loading may even dampen out vibration.

Thanks asimpson....speed was up 30% but not loading. Just a note too although speed was up 30%, it's still way below the designed speed, so it's just mind boggling and I don't see where the fretting corrosion came from? Between the shaft and the bushing????
Thanks.

Just remember, one of the criteria for the 'design speed' may have been to find a point at which there were no harmonic issues. However, reducing the speed may actually have dropped it to where a harmonic could have become a problem. If there's a potential for a natural harmonic frequency situation, lowering the speed is not always the right answer.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

sokenyou,

It was a bit difficult to see details of the fracture zone due to the picture's low resolution. After looking at the picture, can you confirm a couple things for me?

First, it looks like the bearing used is a ball bearing unit with a self-aligning flange mount and a set screw on the inner race. Is that correct?

Second, what was the fit between the shaft and bearing bore? If there was clearance and relative movement, another possible cause of the fracture (besides contact fretting) may have been shaft surface gouging damage produced by the set screw tip.

Regards,
Terry

Hi Terry,

Yes, the bearing used is a ball bearing unit with a self-aligning flange mount and a set screw on the inner race. The fit between the bearing zone and the shaft is a couple of thou according to what the Mechanic was saying.

However, I don't understand why this (clearance and relative movement) is an issue, it's been like this for a good number of years according to what I heard (I just started this job a month ago). Well, we have seen signs of the shaft surface gouging damage produced by the set screw tip though...

I am puzzled about this issue as well...not understanding why it wasn't an issue before and now it's giving us problems.

Thanks everyone if you have an idea to determining the root cause of this.

Thanks.



Regards,
Kenneth


 

One of the things you will find out is that all problems are "new".

"They've been doing it like this for years and never had a problem"

"We've just started seeing this; it's never happened before"

For nearly all of these types of problems, if you dig deep enough, you will find that either
1) They haven't always been doing it like this. They've either used a higher (or lower) rated motor/shaft/some other component or have increased the loading/production/or some other usage variable.
OR
2) They've always had problems with it.
Usually, both.

The reality is that nearly all of the problems are "old". They rarely ever get solved, someone just puts a band-aid on it and they go on. Sometimes the band-aid will help, sometimes not, but since the broken component has been replaced with a new one, it works for a while. People forget/get moved to another job/some new guy comes along.

Easy problems get fixed and since they no longer exist, nobody remembers them.
Tough ones get recycled.

rp

Even if the clearance was only a "couple a thou" it could still result in some degree of eccentricity which would exacerbate the potential for harmonics. Anything 'high-speed' should really use press-fit bearings.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

sokenyou,

Clearance between the shaft and bearing bore is a big issue, since it's this clearance that sets up the conditions for fretting to occur. Rather than installing the shaft into the bearing bore with clearance and relying on a set screw to hold things secure, I would suggest using a bearing that has a locking tapered sleeve on the inner race. This would eliminate the possibility of fretting on the shaft surface. These bearing are offered by SKF (and others) in a flanged self-aligning pillow block design like the one you are currently using.

As for this situation never occurring in the past, try to be optimistic about it. You had a great run of good luck!

Hope that helps.
Terry

• http://files.engineering.com/getfile.aspx?folder=ee0fd4c9-7748-48b3-afc8-52

Sokenyou

I wanted to add to the very good advice giving all above.
To my experience a bearing that spins in or outside of it's mounted surface,
is a very bad to occur. Like instant failure.
I like both recommendations above, but feel a pressed bearing(with Loctite) or thermal assembled, is the the better method.
This is just my opinion & experience.

Mfgenggear

Hi Kenneth,

Can you provide pictures of OD the hunk of broken shaft that was in the bearing (the bearing "seat"), and the OD of the broken stub on the roll, and the bearing ID?

In the picture it looks to me like the shaft extends out a ways, for a coupling or gear or sheave or something.
Do you have details of that area too, like sheave width and number of belts, belt tensioning method, type of cou0pling, etc.

Dan T

Hello Dan T,

Please see attached - dimensions of the shaft and the roller.
Thanks.

Regards,
Kenneth

• http://files.engineering.com/getfile.aspx?folder=5e4dcc5f-b935-4978-bac7-42

Hello All,

We found some cracks on the shaft very close to the bearing sleeve. We are now leaning towards fretting corrosion, BUT here is the thing, we are pointing our fingers at everything that could possibly leading to microscopic motions of tightly fitting parts, however, why didn't it happen for the last so many years before we turn up the speed of the mill and this shaft started failing?

The thing is, the design for the machine is to run it at 2 times of what we are running at right now!!!

It's just mind boggling....


Thanks.

Regards,
Kenneth

http://www.mt-online.com/july2012/failure-analysis...

Hi Sokenyou,

I,m still interested in pictures of the damaged shaft and bearing race.

If the bearing ID is a slip fit on the shaft (diametral clearance greater than 0.0002 inch) and the bearing uses any commercial combination of setscrews, eccentric collars, or even slit race extension and clamping collars then radial forces from a belt drive or even just roll weight will be enough to create micromotions that will first slowly, then more quickly eat the shaft, setscrew and bearing race.
The greater the clearance, the more rapidly wear will progress.
The greater the radial load the more rapidly wear will progress. Excessive belt tension can be a killer two ways - hastening the micromotion, and greater bending stress on the shaft.

regards,

Dan T

I tend to agree that this is probably a resonance problem, but want to point out that "j6" is not a slip fit. It falls under the category of interference fits and is a shaft fit recommended for normal to heavy loads.

If they are running at (exactly) half the design speed, wouldn't the resonance created there also be there at the normal speed? (Been a while since I did any vibration work but I seem to remember something like this)

@Sokenyou:
Look up old orders on that machine and try to find out how frequently the shaft broke before you arrived there. If it indeed wasn't a problem before, find out what they changed and, if possible, change it back.

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM
Nvidia Quadro 2000

The shaft broke within two weeks last time and the shortest time it lived there was 5 days. The one we have in now has been there for 10 days and we are praying hard that it will run for a long time.

Is there a possibility that the root cause of the problem is heat treatment of the steel or the heat treatment wasn't done properly?

Also, what are the consequences if I make the shaft with a more ductile material??? what would be the trade-off? I am not solving the problem but if the shaft distorts then I know something is wrong and can plan ahead to change it, to buy some time.

Please advise if you can.
Thank you

sokenyou

Do a full failure analysis.
don't jump to conclusions.

what is different now than what was before.
verify verify verify, or measure three times then cut.
was it the correct bearing clearance/press fit , contact the bearing manufacture make sure what is recommended by the mfg.
Is there history that can be looked at?
what is it the correct material & heat treat.do a sample spectra analysis, run met labs of the failed shafts. does it meet print?
is the correct hardness?
was is it processed correctly during manufacturing?
was any short cuts taken during manufacturing cycle.
are the certifications required for a paper trail? verifies the correct material, heat treat, any process, NDT.
have the bearings mfg changed, is it the correct class of precision bearings, who installed them?
was there any critical operations where operators changed.
was the out side diameters ground or machined correctly?
was there non destructive test completed, on the shafts,IE mag
and so on.

Mfgenggear

If this equipment is covered by your insurance company, have its representative do a vibration analysis.

Very generally, probably too generally, a material with greater ductility will also have lower yield, ultimate and endurance limits, so fail sooner if subjected to the same (alternating bending loads). It won;t bend unless the stresses exceed the yield strength.

if you really want some advance warning I'd use Zyglo or spot check or some similar dye penetrant product to look for cracks initiating. Might have to shift the bearing over to get a look at the critical area.

The new condition causing failure >>could<< be as simple as a new belt drive assembled with greater overhang distance and tightened by a muscular young millwright.
Or, maybe your roll supplier already (and unbeknownst to you) switched to some low grade badly processed steel.
or is adding a stub or creating a radius by adding material using questionable and uncontrolled weld techniques.


I'd definitely be doing a lot more inspections of broken and new parts, and the installation looking for clues, and some basic design checking of what's there.

Look at the effect corrosion has on steel's endurance limit. page 82 here -
http://www.timken.com/en-us/Knowledge/engineers/ha...

Another shaft went this morning! Same failure - rotation bending.
I am out of idea except spending $30,000 for a brand new roller from the vendor

This is a bitch!

I have gone through a few directions
1. Alignment - we used self-aligned bearings
2. Heat Treatment - hot rolled steel and don't know how to proof
3. Fretting corrosion - why is it only happening to this guy and not the upper roller

But none of them seems to be the root cause.
DAMN!

Obviously you have a loading issue. It cannot be this " fretting corrosion"' whatever that is, or heat treating. This would happen to so many shafts in such a small period of time?

Look at the bending moment diagram and you will probably see a maximum stress just outside that bearing position. How do you know you don't have a rotating deflection? That would mean you have some sort of bend that when rotating, it goes from a compressive state to tensile condition twice per cycle. So you are old working the shaft at that location.

Show the complete shaft with all loads and positions of these bearings. Clearly you are missing a major issue in the dynamics, I can't see a word solution such as the discussions to date really helping you much.

Regards,
Cockroach

I'm sorry you're having trouble.

Are the broken shaft from one supplier, and was that "the vendor."

Depending on your location there may be shaft repair geniuses around who can securely attach new stubs of good quality and accurate manufacture.

What is the heat treat you were expecting?
If this is a previously successful design then the manufacturing is highly suspect. There may be some clues in the broken pieces, but you seem to have given up on the investigation.

Do a full failure analysis.
don't jump to conclusions

you are not giving enough information to the senior design engineers here to help you.
I see repeated questions that has been given above by all who have posted.
take a check list & verify what each of the questions that have been asked.

if this beyond your experience, " not your capability" then seek help from an outside source.
discuss the issues with your supplier, do the vibration analysis as suggested.
eliminate what it is not.

mfgenggear

Thanks mfgenggear,

I want to do a full analysis but budget is an issue here. I can't just do a vibration analysis and it turns out it's a waste of money.

You said I am not giving out enough information, but what information you think it's missing that I haven't provided?. Please kindly advise. I have some experience doing failure analysis but I do admit this one is a challenge for me.

We discussed the issue with the supplier but we have tried what they suggested but the root cause is still not determined and nothing much too helpful from the local shop that refurbished our roller shafts either.

We plan to install another refurbished one and do an alignment check or if budget permits, vibration analysis.

If there is non-proprietary information that you think it's useful for the group to help me resolve the failure, I am glad to provide.

Thank you very much

How much does it cost each time you have to shut down your operation and replace the part? I can't imagine it would take many iterations of that to pay for a proper analysis of the problem.

"On the human scale, the laws of Newtonian Physics are non-negotiable"

Sokenyou, after reading all the above posts, I don't think anyone is going to be able to solve your problem for you at this point. However, just replacing parts is obviously not a reasonable solution. At some point, it is likely your management is going to decide to replace the person (like they did with your predecessor) as the issue isn't getting fixed.

You earlier stated that a new roller will cost you $30,000. I suggest going to management with an engineering analysis that is something along the lines of "We've broken x number of shafts in the past y months. Each time one breaks, it costs us this much in downtime $$$ as well as this much in costs $$. Here's what I've tried so far, for a cost of this $$$. I could recommend ordering a new roller for a price of $30,000 and this much in time and labor $$$ to install, not to mention this much lost production time $$$. I don't want to keep throwing money on this. I think it would be wise to do a complete analysis, including some vibration analysis, to get an idea of where the problem is really occurring. Here's my list of what I think needs to be done and what it will cost. Yes, in the end we'll probably have to make some changes, but at least then we'll know what we're doing so it will be one-time changes not this constant drain. I think this will be money well spent in keeping us up and running."

Saying you have the money to keep replacing parts but not the money to find out what the problem is poor engineering. Your management might not want to hear that, so you need to make your case. But sometimes there's a limit to what you can do over an Internet forum.

Patricia Lougheed

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see the above article it gives clear concise failure analysis examples.
note page 14 shafts

Mfgenggear

http://www.koyousa.com/brochures/pdfs/catb3001e.pd...

Bearing Failures,Causes & counter measures

Mfgenggear

I'm done with it.

The magic words, "budget is an issue here" told me everything I needed to hear. That coupled with wordy problem solving, no free body diagrams, no loading, lack of clear, concise engineering practice. I've asked for some numbers, I don't think you got a handle on what the problem really is. So we could be talking about fairies dancing on the end of a pin. You just can't do engineering like that, which makes me wonder about your practice.

Good luck with the wonder through the wilderness.

Regards,
Cockroach

http://www.engineersaustralia.org.au/sites/default...

Pay attention to page P5-22 & on

Mfgenggear

Torsion of Shafts
like this

http://www.me.mtu.edu/~mavable/MEEM2150/Chap5.pdf
Mfgenggear

like this too
http://www.nd.edu/~manufact/FME%20pdf_files/FME_Ch...
Mfgenggear

Thanks mfgenggear for all the useful links....greatly appreciate it.
After reading all your comments, especially vpl and cockroach's, I feel bad.... I agree I should've done the engineering analysis properly and get to the bottom of it to provide good recommendations.

Thank you very much.

If the broken pieces are around it's not too late to learn something.
A first cut at stress in the shaft can be done just knowing the geometry.
I'd spend $30 at the welding supply store and get a few cans of SpotChek dye penetrant chemicals to inspect the "refurbished" shaft before installing it.
Also the areas on either side of the busted shaft(s).

They don't happen to be welding up the bearing journals to refurbish the shafts, to they?

Dan T

I realy agree with Tmoose,
or send it out to a third party NDT lab.
use MAG & DEMAG per ASTME1444/E1444M & MIL-STD-1907 Grade A
it would be a minimum charge.

Mfgenggear