Sign up Calendar Latest Topics Donate
 
 
 


Reply
  Author   Comment   Page 1 of 2      1   2   Next
Noknroll

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 776
Reply with quote  #1 
Do you think it would be possible that motor DE bearing with BPFO at 3.094X would come in and out of phase with a 6 bolt shim pack coupling?

recent compressor survey revealed elevated 3 & 4 vane screw frequency along with motor outer race frequencies and elevated high frequency noise floor.

some greasing of the motor was done and the compressor technician also made an adjustment to the spiral loading/unloading valve together both of these actions bought vibration down to normal acceptable levels

Now I have a significant beat frequency measured in the axial direction at DE of motor.
the 3 vane screw is not really in the equation as there is a step up gear drive from the motor to the male/female screws (its a sullair TS32 if anyone is familiar with them) comp b m2a.png  compbm2aspec.png

dnk

Member
Registered:
Posts: 59
Reply with quote  #2 
2 questions. 
Are there other compressors running near this one?
Is your resolution high enough to separate 3X from 3.09?
Ralph Stewart

Avatar / Picture

Sr. Member
Registered:
Posts: 226
Reply with quote  #3 
Noknrol,

I see your "c" frequency marker is not aligning with neither of the 2 peaks we are looking at.

Wondering why this is?

Is the speed possibly wrong by 0.19 Hz, in order to make the 6320 bearing BPFO align with the 76.30 Hz position?

Is there a different bearing on the opposite end of the motor, like maybe a 6319 bearing?

Is it possible that there is a BPFO on both bearings (DE and NDE) and this is making the 2 frequencies perform a beat frequency at the fundamental (3.073x and 3.096x) of both bearing defects?


Just Wondering. 😉

Thanks and Have a Merry and Happy Holiday Season,
Ralph

__________________
Ralph Stewart
AlertAnalytical.com
ivibr8

Sr. Member
Registered:
Posts: 145
Reply with quote  #4 
Nok

I have never seen such a significant beat frequency as you show for bearing frequency defects.... 
It seems a bit strange it would be in the axial direction...do you also see this in radial?

If this was a bearing defect in the motor, would you not also expect to see it prior to the recent "adjustments"?

I am wondering if the unloader valve (my small experience is that it was a throttle valve...semantics?) is causing some issues here

Just my 2 cents.....

Have a Merry  !!!!

Jim P
fburgos

Sr. Member
Registered:
Posts: 538
Reply with quote  #5 
I don't believe it's from the bearing, quick search this is a two stage screw compressor, Do you know the ratios?
Noknroll

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 776
Reply with quote  #6 
Thanks all
Correction to opening post.
Its not a 6 bolt shim pack coupling. Its a jaw type coupling wth individual H shaped rubber blocks I think it has 8 pieces?
I'll answer what questions i can in no particular order.
DNK
There are other compressors running nearby
Resolution is at maximum as annotated on spectrum with clear differentiation from 3x
Ralph
Although speed detection was done at start of data collection there may have been a slight drift from the detected speed by the time i measured the DE, i didnt see a bpfo at NDE I'm pretty sure its a different brg but will have tocheck what it is when I'm back in the office
Jim
Not even a hint of a beat detected on any other readings, opening post 2nd paragraph mentions bpfo present before lubrication, throttle valve :-) adjustment reduced screw vane pass frequency, not sure if it influenced temp but it was also noted that temp was reduced from 96 to 90 deg C
Fburgos
Not sure of the exact ratio off the top of my head but its approx 1.1 :1 step up ratio. Screw vane pass frequency is in the vicinity of 108Hz
Danny Harvey

Sr. Member / Moderator / Supporter
Registered:
Posts: 1,302
Reply with quote  #7 
The only times I see a high component at 1 x bpfo is when a bearing is cocked on the shaft or pinched by a high spot in the outer race. Axial is probably the most common direction and I have always attributed it to a cocked bearing but never confirmed it.  Usually the motor is replaced, the 1 x bpfo goes away, and I never find out what was the cause.
ivibr8

Sr. Member
Registered:
Posts: 145
Reply with quote  #8 

Quote: The only times I see a high component at 1 x bpfo is when a bearing is cocked on the shaft or pinched by a high spot in the outer race.

Danny, this is interesting to me.   In all the years I can recall only one instance of high 1 X bpfo (with only very slight bpfo harmonics out near 2500 hz). It was on a motor-generator set and was also in the axial direction. We replaced the bearing and the high 1Xbpfo disappeared.
The MDiv LPO indicated that there was a single large spall in the outer race.
I was much more interested in knowing why the high 1xbpfo instead of the typical dominant bpfo harmonics; however they had already thrown the bearing into scrap bin and I couldn't recover it to inspect it and satisfy my curiosity --->e.g. did the spall have "flattened" edges so that it reduced the ball-to-race impact sufficiently enough so as not to result in significantly higher harmonics???  

Interestingly, the MG set continued to show relatively high axial levels. I too thought it might be due to cocked bearing but a check by the M-div indicated it was in squarely.  <---I was not there to see the actual checks so I can only assume it was done properly.

This will be interesting to find out what Nok learns.  

Jim P
Nok - the fact that it is only showing on the DE bearing would further boost your suspicion of a bpfo related beat (and not an issue with unloader valve which, I think, would show up in all bearings).  Danny's comment about a "cocked bearing" is something to look at when you open it up

Danny Harvey

Sr. Member / Moderator / Supporter
Registered:
Posts: 1,302
Reply with quote  #9 
Jim,

I've got two cases at one site. This graph is of one of them over the last three years or so. Further back, I'd have to get from our old server and our CSI stuff.

They changed the motor when I found the first one some years ago but have let these two run for a long time.

I think the downward trend in this graph is likely due to wear relieving the clearance issue that is the root cause. 

I'm not sure of the mechanics of why 1 x bpfo becomes predominant but my experience leads me to believe that it is caused by a single point of closed clearance and that it is directionally sensitive. Something like a metal chip in the bearing bore or a dropped or mishandled bearing might cause it.

I can't prove it though, because they send the motors to the same shop that rebuilt them and we rarely get any word on what was found.






 
Attached Files
docx cocked bearing.docx (65.64 KB, 15 views)

Ralph Stewart

Avatar / Picture

Sr. Member
Registered:
Posts: 226
Reply with quote  #10 
Quote:
I'm not sure of the mechanics of why 1 x bpfo becomes predominant but my experience leads me to believe that it is caused by a single point of closed clearance and that it is directionally sensitive.


I am a little inclined to think the way Danny is talking, except that I think there could be another possible reason.

I basically have data that sort of confirmed my original theory, and after watching a slow paper machine roll (approx. 1.5 revs per minute) which had a sheet of plexiglass fitted into the cover over the front of the bearing as opposed to a metal one and also was running on oil.

I could actually see that the rollers were ceasing to "turn" when out of the load zone (possible due to the viscosity of the oil and clearance) and then starting to roll again when they came in contact with the outrace. This created a "defect" which I "labeled" as a "slide" because the rollers sort of slide as they entered the load zone of the roll before beginning to "roll" again.

What may be happening with Noknroll's motor bearing after "greasing the bearings", the balls stopped when out of the load zone (or contact with the outer race) due to the excessive or extra grease was added. And then beginning to turn/roll when entering the load.

A pinched area in the outer race, as Danny spoke of, could also be the problem. But base on Noknroll's quote, it may be as I mentioned, stiffer and new grease stopping the rollers from turning when out of the load zone.
Quote:
Now I have a significant beat frequency measured in the axial direction at DE of motor.


I have some data and pictures of this (from paper machine rolls), if I can locate them, and if anyone would like see them. I may even have some from a fan, but they may be a little more difficult to located these. Most are from the 1990's. [smile]

Thanks and Have a Happy and Prosperous New Year,
Ralph


__________________
Ralph Stewart
AlertAnalytical.com
Danny Harvey

Sr. Member / Moderator / Supporter
Registered:
Posts: 1,302
Reply with quote  #11 
Ralph,

Did the skidding bearing on the Yankee roll generate a high 1 x bpfo?
Ralph Stewart

Avatar / Picture

Sr. Member
Registered:
Posts: 226
Reply with quote  #12 
Quote:
Ralph,

Did the skidding bearing on the Yankee roll generate a high 1 x bpfo?


The best I can recall they do. I will check my data and see if I have one.

Most are Felt rolls, wire rolls, guide rolls, Press Rolls, Pressure Rolls, Reel rolls, etc.,  at some time or another, would and/or could display this 1x BPFO.

But I must admit, I never saw one generate a "beat pattern" as does Noknroll's is showing. I thing a good test on this motor might be to check it after the grease, that was applied, gets to the point where the balls will not stop turning, if caused by fresh grease and then grease them again to see what happens.

Also there may be problem with the DE and NDE bearings causing this "beat", since motors usually have a different size bearing number on the DE and NDE.

Just a guess.

Thanks,
Ralph


__________________
Ralph Stewart
AlertAnalytical.com
RRS_Dave

Sr. Member / Supporter
Registered:
Posts: 215
Reply with quote  #13 
Quote:
Originally Posted by Ralph Stewart


I basically have data that sort of confirmed my original theory, and after watching a slow paper machine roll (approx. 1.5 revs per minute) which had a sheet of plexiglass fitted into the cover over the front of the bearing as opposed to a metal one and also was running on oil.

I could actually see that the rollers were ceasing to "turn" when out of the load zone (possible due to the viscosity of the oil and clearance) and then starting to roll again when they came in contact with the outrace. This created a "defect" which I "labeled" as a "slide" because the rollers sort of slide as they entered the load zone of the roll before beginning to "roll" again.

I used to find this on coal conveyor take up pulleys. They most generally were fairly big diameter, and since the load was very much single plane, the rollers would stop rolling until they came to the load zone, then they would go from 0 to a zillion RPM all at once. It would dish out an area in the outer race load zone. Spike energy would pick it up, and although it started as a high !X BPFO, it would turn into many harmonics in just a couple months. We changed several before questioning just how much life they still had in them. We had the bearing rep out and after much prodding, poking and feeling, decided that they would probably have lasted another 6 months before changing. We did that, and after changing them at that stage, poked and prodded some more, and decided that we would change them at 9 months from when we first saw the 1X (just after velocity would begin to show).  We never had any failures on them


A pinched area in the outer race, as Danny spoke of, could also be the problem. But base on Noknroll's quote, it may be as I mentioned, stiffer and new grease stopping the rollers from turning when out of the load zone.

This is what we thought the problem with the take up rollers was. They were big, and the housings most generally were not the best of shape. If you've ever changed on of them, you'll know that it is not an easy or friendly thing to do. Pinching the cartridge, while trying to get the cap back on one from a crane basket or hanging on to the side of the  structure, is easy to do. The logistics of the changes was also the reason for determining a good MTBF, the belt crew did not like the vibes guy.




Ralph Stewart

Avatar / Picture

Sr. Member
Registered:
Posts: 226
Reply with quote  #14 
Quote:
Dave's Quote:
and decided that we would change them at 9 months from when we first saw the 1X.  We never had any failures on them


Ditto to "no failures", [smile]

Thanks and "Happy New Year in 2020", and the rest of this year also,
Ralph


__________________
Ralph Stewart
AlertAnalytical.com
Noknroll

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 776
Reply with quote  #15 
sorry about the inactivity and no response, been on leave for Christmas - New year break.
Ralph
there maybe a beat between motor NDE 6316 Bpfo @ 76.03Hz/3.086X and DE 6320 Bpfo @ 75.91 Hz/3.073X
although my 2 frequencies in OP spectrum are 76.72/3.114X & 76.3Hz/3.097X. 

There maybe some slight frequency variations in what motor bearings are actually installed and a slight speed drift from what was detected.

I'll be checking this compressor again tomorrow hopefully something will reveal itself
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.