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Danny Harvey

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I have the TA section on analysis of these compressors and it says that 1/3 harmonics of shaft speed should never be in the spectrum and that it is a sign of shaft looseness at any level. It sets Alert at .017 ipsp, Alarm at .055 and shut down at .167.  

It also identifies things in this frequency range as undetectable by Shock Pulse and Spike Energy which I take to mean PeakVue also.

I'm not even at Alert level in the velocity spectrum but it's there and it appears quite clearly in PeakVue spectrum and at about 2 g's in the twf. Autocorrelating the twf shows that 1/3 has a high correlation factor.

I realize that this text was written long before we had a good understanding of demod readings. Does anyone have any experience with what's really a problem when it comes to 1/3 x running speed?
John from PA

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Reply with quote  #2 
Danny,

Do you know if there is any kind of a ratio between the male and female screws.  In a competitors drive, I have often see a 3:2 ratio and a 1/3 X is not uncommon.  You also should have a set of timing gears and it would be good to know the tooth counts there as well.  Attached picture are the timing gears from a Z compressor.

z timing gears.jpg

Danny Harvey

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Reply with quote  #3 
John,

There are 4 and 6 lobes so the ratio of the timing gears is 2:3 but I don't see how that would relate to 1/3 of shaft speed.

I don't have the specifics of this particular compressor but the TA publication shows enough examples to know what is typical.

It does leave me making the assumption that what I'm looking at is actually running speed, though.

I had all the info for the Sullair that this AC replaced but haven't gotten the model number and serial number from the client yet.


Danny Harvey

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...and thanks for the pic.  That's always handy to have.
MarkL

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Reply with quote  #5 
Christ, Thanks, John!! ( I mean that sincerely)  that pics just made me reconsider the way I've been mounting and setting out the collection points on some of these compressors.
I check a few of them, and going by those bearing locations I was...how should I say a bit off the mark location wise [confused][confused] 

Danny, I have been struggling with Atlas Copco here in Ireland trying to get the teeth count lobe counts etc, the service guys are decent enough but they are being supplied the bits in OEM part numbered kits nowadays and they are treated as 'plug and play' items almost so just junked after with no thought to check bearing numbers etc as most aren't doing any vibe monitoring(I know Ingersol rand use a badge-engineered Ancient SPM unit for their service guys)

I just go By My Envelope and trend it and watch the acceleration for any changes.

John from PA

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Reply with quote  #6 
Quote:
Originally Posted by Danny Harvey
John,

There are 4 and 6 lobes so the ratio of the timing gears is 2:3 but I don't see how that would relate to 1/3 of shaft speed.


Most of my experience with these screw compressors has been on a test stand and during a final acceptance test.  These machines were large enough that the rotors were supported by fluid film bearings and the OEM did not have the equipment to monitor/document the proximity probe data during test.  As you likely are aware, in units with timing gears, the rotors ideally do not contact each other.  In the instances where I've seen 1/3X there were issues with the timing gear positioning which allowed the rotors to touch.

Danny, you've seen the 1/3X before; see the thread at http://www.maintenance.org/topic/1-3-harmionics-in-compressor?reply=399590942962601442 and http://www.maintenance.org/topic/screw-compressor-4x6 (scroll down for your your comments 04/23/07 at 9:30 AM.

A 1/3X was also discussed in the thread at http://www.machineryanalysis.org/post/twin-screw-compressor-8118015.  See in particular the comments by Big Al mentioning a paper discussing the 1/3X.  I remember looking for the paper back then and not finding it.  Such was the case today as well.
Walt Strong

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So Danny, you have had this issue for 10 years?

Walt
Big Al

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Reply with quote  #8 
I haven't seen the paper to which John refers for years. It was a real paper document and not an electronic file. There is a strong chance that it may have left the office roughly around the same time as a  colleague left the company ;-)

Anyway I'm assuming that this is the updated version:

http://www.technicalassociates.net/online-store.html#!/Rotary-Screw-Air-Compressors-2nd-Edition/p/67464105/category=19517717
Danny Harvey

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Reply with quote  #9 
John,

Thanks for the reminder.  I didn't make the connection because while they are both AC twin screws, the old one is a GA series with a shaft speed of about 1900 and the new one is a turbo screw compressor and the shaft speed is about 25,000 or so.

I'm still hesitant to make any sort of a call based on the mere presence of 1/3 harmonics. I don't see any signs of significant impacting in the accel twf but it's there in PeakVue. 

I don't know if this client does regular oil analysis on this or not but I recommended that they start.

Does anyone have any insight as to the effectiveness of oil analysis to determine looseness in on an oil free screw compressor? It seems to me that the only wear particles would come from the bearings and gears and not from the screws themselves.
Danny Harvey

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Reply with quote  #10 
Walt,

Many of my "issues" are more than 10 years old but not this particular one. 
RustyCas

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Reply with quote  #11 
John, in that old post Danny was asking about "harmionics" which might not be the same thing... ;-)

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