Is this fan running in a piece of equipment that is also vibrating or is there another fan nearby? If so, it is possible that you are hearing a beat note as the two sources of vibration drift in and out of synchronization about once a second.

There is other equipment running nearby, but we didn't observe a beat.

Drew K

What type of sensors?

Do you have a frequency analysis?

Sub-1rpm suggests two pieces of equipment running at slightly different speeds.

Yes I agree.. I have also seen a beat frequency develop in CSI equipment with a very low battery and yes it tells you the battery needs charging.. Maybe he is in an earth quake zone.

Any additional details available? Size of fan, speed, mounted to equipment or mounted in a room, etc.

Maybe I'm just dumb, but does 1 RPM=1 Hz?

Dunno...checked magic box and didn't have a converter for rpm (angular velocity) and frequency.

Actually I feel pretty dumb about this whole thread. First I seem to have got most of the details wrong.

It is a pump not a fan, it is in a coal power plant. There are nearby pulverizers that are running at the spike in the spectrum. There are other pumps running nearby but we don't suspect a beat.

I have only a fingertip grasp of plant vibration analysis but I recall (in the book I can't find) that there are only a few things that can cause sub 1 times rpm spikes...I just can't remember them.

Drew K

My problem is that a 1 RPM event will take a minute to occur.

1 Hz, well, you get it.

What is the period?

Period? Like in the period of a pendulum?

I won't get a copy of the spectrum analysis until Wednesday.

Drew K

1rpm=360º/60seg=6º/seg=2•Π/60seg ; 1hz=1/seg ; anyway don't worry: Drew k don't even know what he is talking about...

Wow, that is harsh. And obvious. If I knew what I was talking about I wouldn't be here asking.

Drew K

Hi Sir: I am afraid you are in a mistake. Although rpm is used commonly as angular velocity, in fact is frequency, because is an event (the revolution) by a time (a minute). You divide rpm by 60 and obtain Hz (cicles by second).

Best Regards.

Gabriel

I think he's got it. I was confused not seeing the x, which would have made it clear.

Isolation could be a problem here, too with all the other stuff going on.

No.

RPM = revolutions per minute, Hz = cycles per second.

Therefore 1 RPM = 60 Hz

The "M" pire rules

↑12 (You need to divide rather than multiply.)

How stable is the power source voltage AND frequency? Relevant source fluctuations can give that kind of vibration (on low inertia-stabilized systems) S.M.

This is where I would get a short piece of pipe and use it like a stethascope at varipous points to find the location of the maximum amplitude. I've seen badly anchored piping move on a split of the pump frequency but this sounds ....unusual.

If we are really talking about i Hz, you can't hear that anyway.

I don't know how to relate 1 rpm to any sound.

I think OP has not expressed it clearly. It must be 1xrpm, and not 1 rpm. In vibration spectrum if peak appears at machine speed it is called 1xrpm. Sub 1xrpm means he is getting vibration peak at less than machine speed, but OP has not specified, how much less? Normally 0.5xrpm is due to oil film in bearings or oil whirl.

Yup...that is what I was trying to say (vibration analysis is new to me).

We don't think it is oil whirl either.

I will try to get the spectrum up tomorrow.

Drew K

1 rpm ≡ 1/60 Hz.

The frequency is an order of magnitude lower than that experienced by skyscraper buildings wobbling, for example.

What is the consequence of these very low frequency vibrations? What sort of problems do they cause? Is it the sort of thing that can be corrected by stiffening things or adding mass, so as to lower the resonant frequency of the objects and thereby eliminate the wobble?

One cause is cage damage in roller bearings. The cage that keeps the bearings separate can be damaged and because it is moving slower than the 1 times rpm it will show up as a spike in the spectrum.

We know this isn't the problem because this machine doesn't have roller bearings.

Drew K

Why is every one skirting round the issue here:-

RPM normally means Revs Per Minute.

Is that what RPM in the OP means? Or is it something different?

I know several others have alluded to this but no one seems to have said it.

In the vibrations class the instructor called it 1 times rpm. In the spectrum he showed us there was usually lots of noise and spikes at the 1 times rpm then resonance at higher frequencies that could indicate many other problems.

As I recall, there were only a few significant instances of sub 1 times rpm problems...i just can't recall them.

Drew K

Yes,1 times RPM is turning speed and if it 1750 RPM/60=29.16 revs.per second and that is where VIB. shows all "Loosness or Out of Balance" the Turning speed times 1.

Well, so far we have identified that I don't know what I am talking about (but I am trying to learn).

I have learned the correct terminology, and explained that it isn't oil whirl or cage failure...any other ideas?

Just so you know, I did google this before asking here..._{just sayin}

Drew K

Are you referring to Sub-harmonics of shaft rpm?

Beside the oil film whirl or whip in journal bearings, Journal bearings loose in housing also could exhibits sub-harmonics. Normally they use to be 1/2 or 1/3 the rpm. Say, if the running speed is 3000rpm, if journal bearing looseness exists, the dominant vibration frequency could be around 1500 or 1000 rpm.

I think so, I should find out tomorrow when I get the spectrum analysis.

Drew K

We are waiting for the spectrum.

Yup...me to. Had another problem pop up that took priority. Will upload it as soon as I get it.

Drew K

OK I think we're all getting the idea now.

What is the frequency of rotation (RPM)?

And when you say sub 1xRPM do you mean a fraction like ½ or ¼ or ¹/₃ or do you mean just below like say 99% ?

Ok, got some pictures of the spectrum.

It looks like it is at about.45 X RPM.

This is right where we would expect to see oil whirl but this is a vertical mounted boiler circ pump that has water cooled bearings. Can you get oil whirl with a vertical water cooled bearing?

This is a close up of the .454 X RPM

This is a unzoomed shot showing higher frequency spikes.

This shows the 1 X RPM .

Drew K