Roller shaft failed
Roller shaft failed
(OP)
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.





RE: Roller shaft failed
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams
RE: Roller shaft failed
RE: Roller shaft failed
RE: Roller shaft failed
Thanks.
RE: Roller shaft failed
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.
RE: Roller shaft failed
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
RE: Roller shaft failed
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
RE: Roller shaft failed
"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
RE: Roller shaft failed
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.
RE: Roller shaft failed
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
RE: Roller shaft failed
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
RE: Roller shaft failed
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
RE: Roller shaft failed
Please see attached - dimensions of the shaft and the roller.
Thanks.
Regards,
Kenneth
RE: Roller shaft failed
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
RE: Roller shaft failed
RE: Roller shaft failed
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
RE: Roller shaft failed
RE: Roller shaft failed
@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
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Nvidia Quadro 2000
RE: Roller shaft failed
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
RE: Roller shaft failed
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
RE: Roller shaft failed
RE: Roller shaft failed
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...
RE: Roller shaft failed
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!
RE: Roller shaft failed
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
RE: Roller shaft failed
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.
RE: Roller shaft failed
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
RE: Roller shaft failed
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
RE: Roller shaft failed
"On the human scale, the laws of Newtonian Physics are non-negotiable"
RE: Roller shaft failed
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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RE: Roller shaft failed
see the above article it gives clear concise failure analysis examples.
note page 14 shafts
Mfgenggear
RE: Roller shaft failed
Bearing Failures,Causes & counter measures
Mfgenggear
RE: Roller shaft failed
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
RE: Roller shaft failed
Pay attention to page P5-22 & on
Mfgenggear
RE: Roller shaft failed
like this
http://www.me.mtu.edu/~mavable/MEEM2150/Chap5.pdf
Mfgenggear
RE: Roller shaft failed
http://www.nd.edu/~manufact/FME%20pdf_files/FME_Ch...
Mfgenggear
RE: Roller shaft failed
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.
RE: Roller shaft failed
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
RE: Roller shaft failed
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