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Danny Harvey

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Reply with quote  #1 
https://www.machineryanalysis.org/post/velocity-pickups-why-9814821?pid=1305077759&highlight=velocity

I don't want to create a big drift from the subject of Velocity Probes but in this thread at Post #12 I had asked Oli about the use of velocity probes on cooling tower drives and he provided very useful information.

I also got very useful stuff about the gearbox from John.  

Thanks to both.

This location has their entire cooling tower system hooked into the DCS through an SKF IMX online system. This gives an overall RMS velocity for each probe every hour to the DCS. There is also a stand-alone PC that runs SKF @ptitude and is supposed to be hooked into the IMX system to get the data. It is also the PC that we have to use when we collect and analyze their data.

We have to have it moved from the control room to an office to do our stuff, then it doesn't get moved back because there is no one on site that would ever use it. So no data gets analyzed.

I did find one set of data from September, though and I took a look at it.  I reported that one unit had seen a sharp uptick at GMF with sidebands of secondary shaft speed.

Everything else looked ok but with GMF at about 28,500 cpm and fmax at 30,000 cpm, and no twf's being stored (they would be velocity twf's) I wasn't really all that comfortable with my call.

I went to speak to the Maint. Supv. and found that the drive that I was calling as looking like it had wear or damage to the bevel gear was brand new.  The sudden increase occurred immediately upon installation of the new drive.

Then he told me that he knew two others had broken teeth. He didn't know where in the drive the broken teeth were. I went back and tried to find anything that would indicate broken teeth and it's just not there.

I would like to change the fmax to about 120,000 cpm and start saving twf's but it's now subject to federal regulations because it is hooked into the DCS so it's not going to be easy to get permission.

Aside from prevention, how is best to detect broken teeth with velocity probes?
John from PA

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Reply with quote  #2 
Quote:
Originally Posted by Danny Harvey
https://www.machineryanalysis.org/post/velocity-pickups-why-9814821?pid=1305077759&highlight=velocity

I did find one set of data from September, though and I took a look at it.  I reported that one unit had seen a sharp uptick at GMF with sidebands of secondary shaft speed.

Everything else looked ok but with GMF at about 28,500 cpm and fmax at 30,000 cpm, and no twf's being stored (they would be velocity twf's) I wasn't really all that comfortable with my call.

I went to speak to the Maint. Supv. and found that the drive that I was calling as looking like it had wear or damage to the bevel gear was brand new.  The sudden increase occurred immediately upon installation of the new drive.

Then he told me that he knew two others had broken teeth. He didn't know where in the drive the broken teeth were. I went back and tried to find anything that would indicate broken teeth and it's just not there.

I would like to change the fmax to about 120,000 cpm and start saving twf's but it's now subject to federal regulations because it is hooked into the DCS so it's not going to be easy to get permission.

Aside from prevention, how is best to detect broken teeth with velocity probes?


Who is providing the rebuild service on these gearboxes?  If the drive involves a bevel gearset they can be difficult to set properly and if not set properly they will generate a GMF right from the start.

Broken teeth are difficult to diagnose with common walk around programs and their tools.  There are often multiple pairs of teeth in mesh, not just two, and hence when a broken tooth passes through mesh the impulse one might expect doesn't occur because several teeth share the load.  If the situation involves only a portion of a tooth being broken off, then it is even more difficult to capture.  This will be especially true if the bevel set is a spiral bevel gearset.  At the other extreme, and just mentioned as an example, two spur gears with a totally broken off tooth would cause a major impulse when the area of the broken tooth passed through mesh.

You might investigate if the monitoring system has the ability to measure, and preferably trend, crest factor.  Changes in crest factor are a reliable method of detecting impulsive type changes in the signal.  To do this reliably, it is essential to have permanently mounted transducers.

Danny Harvey

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Reply with quote  #3 
John,

I'm pretty sure that this was brand new from Amarillo.

I went straight to the same possibility that the bevel gearset was not properly set-up but I wouldn't expect that with a drive from the factory-either brand new or newly rebuilt. I can't post any data but it really jumped off the page on their first set of data after replacement. They don't do oil analysis but I think they should start.

I think you're right on the mark when you talk about broken teeth not making much in the way of vibration when it comes to spiral bevel gearsets.  With the load spread so effectively, you are really trying to measure the absence of material that doesn't cause any change in pitch line velocities so there is no impact.

The SKF IMX is a permanently mounted wireless system that uses @ptitude (their PDM software) in addition to sending an overall to their DCS. They selected velocity probes for what I assume are the same reasons that Oli cited. If I were to want to find a broken geartooth with vibration analysis, I would normally look to the acceleration twf both unfiltered and demodulated. But, like you said, it might not make any noise. Plus, I can't get that here because of the probes.

I'm hoping that they will see the advantage of preventing broken teeth in the first place as opposed to trying to detect them better. Better detection was the whole reason for installing this system but it really hasn't improved things. With sensors and transmitters, etc. the material cost was over $100,000 and they spent weeks getting it on-line. And they haven't found anything with it.
OLi

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Reply with quote  #4 

I had only once or twice a gbx out of factory completely wrong setup and it was during a period papermachine suppliers had a boom and accepted more orders than they could produce and subcontracted assembly to anyone. In the case I remember a drive gbx in a papermachine the gearmesh stuck out like a sour thumb and gbx was swapped directly and found totally wrong in the setup. It as not a spiral fancy gbx.

I have seen 2-3 fancy turbine gbx's where teeth disappeared. One was found on inspection as it only had eddy probes that could not detect it. One was found on the exactly the same setup bu with a CSi route collector. 3rd was on larger turbine set btw. HP and generator and did trip the Bently monitoring, it may have had a accel on the gbx. Gears was swapped, monitoring increased for 10 years. We saw very short periods of transients where dominating 1xgearmesh suddenly changed to dominating 2xgearmesh and back. Once we were on site and could get a correlating transinet from the steam pressure monitoring and valve regulation. Report were written, OEM said "no regulation can not be a problem, we never heard of any valves sticking" after summer overhaul it was never seen again, gbx been operating for many years now, interest of the problem totally lost. By the way those teeth disappeared w/o a trace likely as dust in the oil filters?
2 of them only half or partial but the last that tripped the Bently system it was mostly gone. It had by the way a std Bently accel that is not that hi freq.

So, yes once in a while a gbx from factory is bad. 
When gbx loose teeth it can be seen in trend if you are lucky but if there are nothing trending, inspection is the only thing left I guess part from oil analysis.
Classic what the book says, if you loose teeth you may have overload that may apply for one reason or another.


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