Manual vs automated fault detection by vibration
Manual vs automated fault detection by vibration
(OP)
Hi everyone!
As a newly graduated control system and automation engineer I got a bit of a shock when I discovered that the automatic fault detection at the company I work at is non existing. Sure, they measure vibrations once in a while manually to detect bearing fault and imbalances but considering how cheap and powerful microchips and PLCs are today I can't see why they don't do it automatically all the time. And since I've not gotten any good answers I'm curious about how it looks at the industry today. Is automated fault detection just something they teach in school but that's to expensive to implement in real life today or is just my company 10 years behind? It's a large truck manufacturer by the way, so there are a lot of turning, milling and drilling going on and it's not a small 10 machine business.
Some of your inputs and thoughts about this subject would be deeply appreciated. Thanks in advance!
Axel
As a newly graduated control system and automation engineer I got a bit of a shock when I discovered that the automatic fault detection at the company I work at is non existing. Sure, they measure vibrations once in a while manually to detect bearing fault and imbalances but considering how cheap and powerful microchips and PLCs are today I can't see why they don't do it automatically all the time. And since I've not gotten any good answers I'm curious about how it looks at the industry today. Is automated fault detection just something they teach in school but that's to expensive to implement in real life today or is just my company 10 years behind? It's a large truck manufacturer by the way, so there are a lot of turning, milling and drilling going on and it's not a small 10 machine business.
Some of your inputs and thoughts about this subject would be deeply appreciated. Thanks in advance!
Axel





RE: Manual vs automated fault detection by vibration
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RE: Manual vs automated fault detection by vibration
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Greg Locock
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RE: Manual vs automated fault detection by vibration
RE: Manual vs automated fault detection by vibration
Further to that, when attended machines, or machines that are near attendants, begin to fail, their sound signature changes. That's basically what you can detect with accelerometers and such, but it happens that people can detect sounds too, and will report a change in funkiness of any given machine. ... if they are allowed to, by local custom and contract.
Remote/unattended monitoring might make sense for unattended machinery, or for attended machinery in a politically hostile environment. Some union shops are like that, with a constant subliminal war being conducted by people who are ill-equipped for head games.
Unattended factories have other means of detecting mechanical failure early, by means of examining the data already being collected for quality assurance purposes.
Which is not to say that you'll never make a dime installing machinery monitoring equipment, just that it's a harder sell than educators might suggest.
Mike Halloran
Pembroke Pines, FL, USA
RE: Manual vs automated fault detection by vibration
You have a good point about the machine operator being able to detect many faults just like you do on your car. But at least according to my experiences so far that is more detect when things already have gone wrong for real. And the whole point is to detect the small changes in vibrations that indicates a later failure before it happen, so that the bearing fault of one bearing doesn't take others with it because of more vibrations and so on. Sure, replacing the bearings with short intervals will decrease the risk of a large failure but my idea is to cut the maintenance costs and more importantly unexpected downtime by as much as possible.
The picture you're giving me is about the same I've gotten from the more experienced coworker at my job so I'm starting to guess that still is valid. I did check with a manufacturer of automatic fault detection equipment today though and the price per unit he gave me right away for a simple system that you connect to the PLC of the machine was a bit short of $1000 (per accelerometer channel) and that seems like no money at all to me. But the question remains, haven't the price of this technology dropped so much in the last few years (to where it is now) so that's a reasonable investment or is it still only something to consider for very few branches like the paper industry that have used it for a couple of years already?
RE: Manual vs automated fault detection by vibration
From what I've read in the prognostics literature, controlled experiments often result in a 50% spread, which is too large to be actionable.
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RE: Manual vs automated fault detection by vibration
I'm easy to find. Send me $1000.
Thanks.
Mike Halloran
Pembroke Pines, FL, USA
RE: Manual vs automated fault detection by vibration
Or did I get you wrong?
Well played Mike, well played
RE: Manual vs automated fault detection by vibration
For example, our company did a life test on a bunch of pumps and used a 10% increase in power draw as an indicator of a possible fail. One pump did indeed fail soon after that, but another one ran for an additional 10,000 hrs and never actually failed its other performance requirements in the duration of the test.
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RE: Manual vs automated fault detection by vibration
If you _could_ get the bearings off and disassemble them without damage, which in an industrial environment is not likely, then it would make sense, upon finding no significant damage, to reassemble and reinstall the bearings and run them to some further indication without damaging them during installation, which again in an industrial environment is somewhat less than completely likely.
You need to find a different windmill to tilt at.
Mike Halloran
Pembroke Pines, FL, USA
RE: Manual vs automated fault detection by vibration
Could performing a shock pulse test as confirmation of the vibration analysis give more accurate data? I've seen some automatic fault detection systems that perform this test and that seems more reliable to me.
RE: Manual vs automated fault detection by vibration
Also in many power plants on mills, pumps and fans.
Back in the 1990s GM published a comprehensive VIBRATION STANDARD FOR THE PURCHASE OF NEW and REBUILT MACHINERY AND EQUIPMENT.
http://maintenanceforums.com/groupee_files/attachm...
It was adopted almost verbatim by NASA for "Reliability based Maintenance."
Pages 42 and 43 have moderately detailed vibration limits for new or rebuilt machine tool spindles. In some instances new spindles are tested on a dedicated test stand before shipping. I would expect the vibration to be "different" when installed.
They would be a decent starting point for continuous monitoring.
RE: Manual vs automated fault detection by vibration
RE: Manual vs automated fault detection by vibration
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RE: Manual vs automated fault detection by vibration
But this might be after a long "startup period" where you have to do many unnecessary replacements before you have found limits that'll work?
RE: Manual vs automated fault detection by vibration
You need two sets of things, the instrumentation for monitoring the equipment condition and process measurements where you can quantitatively identify when the process becomes unacceptable. The latter is a matter of trending data and trying to find when the process goes our of its control range. You correlate that with the instrumentation measurements to determine which of the measurements or combinations thereof result in a tolerably acceptable criteria for stopping the machine to do maintenance. Once you have these two sets of things, how you go about it is dependent on your cost constraints.
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RE: Manual vs automated fault detection by vibration
Some processes, and even power requirements could not care less about a ball/roller bearing that is softly crying itself to sleep due to lubricant degradation. But, if I don't sample vibration often enough, or ignore the signs that are pretty well detected by a little more sophisticated vibration analysis I will miss the golden opportunity to re-lubricate and in a few weeks or minutes (dn related, among other factors) the bearing will un-necessarily be turned into smoking junk requiring extensive rework to the shaft and housing AND replacement bearings with a 12 week delivery.
Temperature measurements made on the outer race can be pretty good for detecting bearing issues too, as can oil analysis looking for both wear debris and dissolved metals, especially for gears and plane/plain/journal bearings.
The equipment sellers and service providers that refer to their wares as "Predictive Maintenance" "Reliability based maintenance" or "condition monitoring" are telling the truth.
RE: Manual vs automated fault detection by vibration
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Greg Locock
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RE: Manual vs automated fault detection by vibration
Don't forget sleeve bearing.
RE: Manual vs automated fault detection by vibration
You said - "In 35 years of using lathes and milling machines I've never had one of those faults come up. "
I was thinking you were discounting the "bearing faults" AX3L more than hinted at, of which there are several ( discrete spalling on inner race, discrete spalling on outer race, discrete spalling on balls, general roughness, plus a few more.) Granted they don't happen every day, and if lubrication is reasonably good then catastrophic failure is generally not imminent, so a periodic walk around program would catch many if not most, but lube related problems can develop thermally reduced preload and then progress quickly (just seconds until the smoke gets out at 15,000 rpm).
IRstuff said - ' My point is that you need to have a rather extensive database of failures and symptoms to even begin to decide whether to perform a repair. This is further compounded by the fact that many symptoms don't necessarily lead to immediate failure."
...." For example, our company did a life test on a bunch of pumps and used a 10% increase in power draw as an indicator of a possible fail. One pump did indeed fail soon after that, but another one ran for an additional 10,000 hrs and never actually failed its other performance requirements in the duration of the test."
My 27 June reply was intended to suggest that -
1 - some fairly detailed generic vibration limits exist that could be used right out of the box as a starting point for effective condition monitoring. Yes, calling for overhaul at the first indication of BPFO may be a little premature.
2 - I'd expect looking for increased power as an indicator of a developing problem would likely miss a LOT of problems.
RE: Manual vs automated fault detection by vibration
Tmoose: That's exactly what I'm talking about. Just to give an example a couple of days ago we had to order a ball screw with plane and pay a couple of thousand bucks just to get it here quickly, way more than what the screw itself was. And this is something we have to do on a more or less regular basis. Using automated fault detection, we probably could have detected the problem at least a day or two earlier and sent it by regular express delivery or even standard delivery instead (ball screw faults is something you can, at least in theory, detect using above mentioned analysis).
Greg: Speaking of the investment cost, it should be said that most (if not all) of our machines already have a PLC that have more than enough power to make some vibration analysis as well. So the only investment, besides all the man hour to fit such a system, is the sensors (that even sometimes are fitted to make the manual vibration analysis easier) and some chip that read the sensors and send them to the PLC.
spciesla: Well, the list of possible faults to detect could be made very very long. I'm not sure how many extra sensors would be needed to do this though, and have therefore concentrated on the bearings at first. Of course I'm open to detect all kinds of fault though, so if anyone have any additional information of those I would love to hear them as well.
Tmoose (last post): Thanks for clearing that out! The win in reduced predictive maintenance is just as important (if not more) to me as the decreased risk for immediate failure. Besides, knowing what parts to change in the predictive maintenance would be very useful indeed. The procedure now is usually "check ... for faults", and if any is detected we have to order that part and do a second stop of the machine to actually change it. But I don't know, this theory might fail because of the limits in reliability of this analysis and we therefore still would have to check it manually with given intervals?
Thanks again for all great answers, I've learned a lot and have been given many great aspects I hadn't thought of already!
Axel
RE: Manual vs automated fault detection by vibration
Let me go back to the comment above, and related it to an experience of mine. As a much younger engineer, I got a call from the factory floor one Friday (always Friday, isn't it?) and they reported a faulty piston. Somehow this one had slipped through all kinds of QC procedures and made it into production. How did they find it? The test cell operator thought the engine sounded funny, so he took it offline and they tore it down to investigate. The piston was out of tolerance - a lot - but I'll be damned if I could hear the difference. This guy heard it from in front of a bank of test cells which all had running engines, and was confident enough to pull it out of production.
...So, now we're worried that there might be more. We stop production. We get the piston supplier on the phone and "persuade" him to come help us figure out how to find engines with bad pistons in them - given that we know that they sound different. Unfortunately, with all manner of instrumentation on the engine we were not able to devise a repeatable method of detecting this defect - even knowing that we had it. "It sounds funny" was a perfect explanation for the guy who runs the test cells every day, but just try to convince a computer of it. In the end we were able to trace the defect back to a process exception at the piston plant and contain the problem within a group of about a hundred engines, which we then had to tear down and inspect (beginning Sunday evening, in another state).
Would automatic fault detection have helped? I doubt it...we certainly gave it a shot after we knew there was a problem. The manual version worked well enough though.
RE: Manual vs automated fault detection by vibration
About the computers ability to find such things I don't have the experience to tell (obviously, that's why I started this thread in the first place) but I think it would. With continuous monitoring one can detect even very small changes in the vibration spectra and relate them to the rotation speeds. You're probably right that just doing one analysis wouldn't tell if there was a problem, but if you can see the change over a period of time I think you at least can detect that there's something wrong.
By the way, does anyone know if using a regular 100mV/g accelerometer (measuring up to 20kHz), a inductive sensor to get the RPM and a relatively simple DAQ connected to a regular PC works "in the real world"? To me this seems like a great alternative, where you with cheap components can do advanced analysis using a MATLAB software. At least in theory, you could even communicate with the PLC of the machine with the DAQ and more importantly, since the computer have internet access it can send reports and data to the service engineers.
Thanks everyone who have posted so far and took time to read my long and probably bad written posts, I really really appreciate it!
Sincerely
Axel
RE: Manual vs automated fault detection by vibration
Cheers
Greg Locock
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RE: Manual vs automated fault detection by vibration
Here's a link to a sensor of this kind I got from Google, just to give you an idea of what I'm talking about: http://www.monarchinstrument.com/product.php?ID=41
RE: Manual vs automated fault detection by vibration
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Manual vs automated fault detection by vibration
RE: Manual vs automated fault detection by vibration
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Manual vs automated fault detection by vibration
In some cases I think even the machine itself measure its rpm, and communicate with the PlC of the machine using either a digital or analogue output can't be too difficult.