I have a 1200 rpm motor at a paper mill that I monitor and for the past 2 months it has had this peculiar frequency at around 14,353 cpm or 12.04 orders. It has sidebands of 1X rotational speed and bearing defects have been ruled out. But what is really weird is that is comes and goes. It isn't always there. When it is you definitely know. It's very audible.

This motor is your run of the mill induction, AC motor - 71 rotor bars, 90 slots. There is nothing special about it. It's vertically mounted and it belt drives an ultra screen. The screen runs around 600 rpm

I have attached some spectral data. I am at a loss here. I have no idea where this frequency is coming from and why it comes and goes. It doesn't seem to have anything to do with process or any apparent event.

I would appreciate any advice.

Filler_loop_ultra_screen.doc (341 Kb, 42 downloads) Filler Loop Ultra Screen

Smitty,

Divide 14,353 cpm by 12 and get 1196 cpm that is probably the actual motor speed. I believe that it is 12xSS. Does motor have a 12-blade fan, either internal or external? A blade-rate pulsation at 239.2 Hz would be very audible if produced by a fan. If this hypothesis makes sense, then perhaps the fan is loose on shaft, housing is loose, or there is material on blades or on shroud that reduces blade clearance and increases pulsation. The 1xSS sidebands (indicating modulation) fits this scenario.

Walt

Walt,

Thanks for the reply. Yes I do realize that the actual motor speed is 1196 or so. SS? I assume you are refering to synchronous speed?? (1200rpm) I'm not too sure how many blades are on the cooling fan but I can check tomorrow. But if you are right, then why would it come and go? Blade pass frequency usually stays when it starts doesn't it? Plus I would think that if the fan is loose on the shaft there would be more harmonics of rotational speed. I could be wrong. I appreciate it.

Smitty,

SS = Service Speed

The crud on the motor in the photo could be on the fan as well.

Another possibility is a belt resonant frequency (close to 12xSS) that varies with belt tension and that varies with motor load. Take a look at belt tension and sheave alignment.

Walt

Walt,
Ah...service speed! Sorry about that one. The pic is old. This motor has only been on line less than 6 months. There's not really any pulp on the motor at all. The load is pretty consistent. As far as belt resonance goes I'd have to check so see how many belts there are. I believe there are 4-5 V belts. Most resonance cases I have seen there is usually just one lone peak and a somewhat sinusoidal waveform. Plus with really no load change the resonance would stay pretty consistant. But I'm always learning something new. I mean when this frequency goes away it just stops and the same for when it starts. Thanks again!

Do I understand; the motor has a noise also that comes and goes? It's there at start-up and goes away after a run-in or heat-up period? And then may return? Check axial for physical movement. Looseness in a part heats and locks until cools then starts again. To me that accounts for 12X or could. I had one in my lifetime that did that way; a Lincoln 50 HP motor. But Walt could be on track; however, the noise seems odd and hard to explain outside the motor if I understand everything alright.

Sam,
The noise and the frequency are simultaneous. No, it has nothing to do with start up. This week when I first heard it the unit had been running for weeks. The motor constantly runs. The noise just comes and goes. The motor does have some actual displaced movement but it is related to screen speed. Plus it has always has a little movement to it. The unit probably could use some re alignment, but I'm not convinced that is the root cause of the mysterious frequency/noise.

Smitty,

What version of RBMware do you have? If it is not the latest, 4.9x or whatever the latest is (I have 4.8) and you would extract out the machine into a small database and send to me, I would be glad to look at it. Not to say I can see something you might have missed but sometimes a different view helps.

address

rottenrocky at comcast dot net

Smitty,

This will be just a guess, but modulated 12x with harmonics could be originating from the driven equipment. It is worth to see what is inside. I'd take readings on it.It even could come from a neighboring equpment, so you may want to find the outside source using coherence function.

As far as noise goes, firstly, the frequency is in audible range and of decent amplitude, secondly, it is modulated, which makes it easy to distinquish on the background.

David

Ralph,

I am running v4.9, so I won't be able to send you any data. I wish I could. I wouldn't mind getting a second pair of eyes on the data.

David,

The motor drives a Black Clawson Ultra Screen. If you're not familiar with this type of machine, it has 4 wipers inside. Each wiper is sort of shaped like an airplane wing and as they rotate they create a suction that draws moisture out of the stock and into the center. Now if what your saying about the modulation originating from the driven piece then wouldn't I have sidebands of the driven piece of equipment(612cpm), or no? As far as searching for an outside source I won't be able to do that now. This is an account I only go to once a month and I am back home now.

I took one of Walt's suggestions and inspected the motor with a strobe and I was not really able to count the cooling fan blades convincingly (hard to see) although I believe there are 12. But I think more importantly, the fan was not loose on the shaft and it had a good 2 inches or so of clearance all the way around.

When I took the strobe down to the unit to look at it it was making that noise. So I inspected the fan. Then I inspected the driven end and looked at the sheave closely. Nothing looked abnormal. The shaft wasn't jerking, or thrusting or anything odd. I tried to look at the screen the best I could but it is very difficult to see and other than confirming screen speed I couldn't determine anything there either. (Note that there is not access to the screen bearings at all so all the data I have is just for the motor). So, after looking at the screen I went back to the motor and just watch the motor shaft. The noise was still going at this point. As I am sitting there looking at the stopped motor shaft, I could tell that the noise was starting to fade a little in and out. Then all of a sudden it just stopped and nothing changed. The shaft didn't jerk, didn't speed up, and didn’t slow down....nothing.

I then went up to the control room and spoke with one of the operators. He pulled up the process signal/trend screen and we looked at it going all the way back 30 minutes and nothing changed there either.

I really hope I am missing something here and there is a very simple explanation. But I have not found that yet.

Can you provide horizontal, vertical, axial vibration data in a waterfall plot? Does this show up in all axis? Does this same data show up on driven component? 14330/600 is 24 x TS. It's a matter of identifying the 12 x or 24 x vibration source. If you have modulation (audible noise coming and going) you will have sidebands. You are correct the sideband spacing is coming from the machine at fault. Do you have some information on the interals of the driven piece? web addrees to OEM?