Hi guys,

In one of our overhung fan, we got some interesting sidebands at 75.6CPM around 2X, 3X, 4X... Can anybody give some clue about this?

Itz an overhung fan.(Hankook Blower)
Motor RPM :720
No. of blades :12
DE&NDE bearing : 6320

Have a nice day!!

Fan_Problem.pps (60 Kb, 131 downloads)

Looks like around 0.1 orders?

What's between the motor and the fan? Belt, gears, coupling?

Assuming no belt, gears, or vfd, some guesses:
1 - process oscillation. Check for indication of pressure, flow, or valves might be oscillating at around 1.2 hz ~70cpm.
2 - Machine sits on shock isolators and resonant frequency is around 70cpm (I have seen
it once... resonant frequency of skid on springs was ~ 200cpm and showed up as sidebands around running speed harmonics... don't have any explanation).
3 - Loss of preload can cause non-sync frequencies in rolling bearings.
4 - Inner ring spinning on shaft or less likely outer ring spinning in housing.
5 - Pole pass sidebands - if you give us exact motor speed we can cacluate pole pass frequenc, although usually it is not as high as 0.1 orders.

At first glance, I didn't catch on to the magnitudes (I'm used to looking at ips). You of course have some pretty high amplitude harmonics. 12 mm/sec peak/0 magnitude at a frequency of 2x on fan NDE... that's about 0.5 ips !?!

Not directional -> might not amplified by resonance.

I think there must be some serious looseness problems going on. Most likely in or around the bearings. Or else if you have those spring isolators under the skid, they might be bottoming out.

Also, I would check for low-frequency non-sync patterns other than these sidebands... check for bearing defect patterns.

This message has been edited. Last edited by: electricpete,

Hi Pete,

Thanks for your valuable comments!!

The motor and the fan is coupled through Grid Coupling!! Actually we aligned this equipment last downday since there was slight angular mis-alignment!! It is not a variable speed machine..

And about your guesses:

1. Process oscillation:-That is one of the thing we were also doubting. But where to start is a problem!! It is a Reverse Fan which is blowing air to the baghouse(for cleaning)..There are two dampers operating in the process-Absorb damper and Exhaust damper. Absorb damper will always remain open and this is which we can adjust manually. Exhaust damper will close and open in every "X" minutes(cycling time is not available.we will try to get it later) automatically since we have 14 baghouses..When the Exhaust damper is closed, we experienced very high vibration and sound. The vibration signature above is taken during the Exhaust damper in open position.
2.Shock Isolator:- The machine is not sitting on a shock isolator.
3.Preload:- But the acceleration and enveloping spectrums are not showing any distinct peaks.
4.Inner Ring Spinning :- Both the fan bearings showing the same pattern and having the same amplitude in both horizontal and vertical directions. We will check this if possible!!
5.Pole Pass Frequency :- Will pole pass frequency sidebands appear on the Fan bearings also? I think Itz close to our pole pass frequency.But anyway i will check the Motor RPM tomorrow.

By the way Pete, the acceleration signature is not showing any distinct peaks for matching with a defect pattern!!

Have a nice day!!

I believe the SBs are at PPF. If LF=50 HZ, then it appears to be a 8 pole motor with synchronous speed of 750 RPM. If actual speed is ~ 740 RPM then PPF=10x8=80 CPM
I suggest verifying RPM and taking a better resolution spectrum.

David

What kind of base is the fan sitting on? I have seen thin flimsy bases "resonate" causing subsynchronous sidebands.

David,

LF is 60Hz here!! Synchronous speed 720.. I will try to get the actual speed today..

Ralph,

I will try to get a picture.. But itz not that much flimsy..

Have a nice day!!

Here it is...

Have you checked the amplitudes at different spots around the base of the bearings and between them? Are the ends of the pedestal open, with no supports under the bearings?

Looks like the coupling is strange. Is it broken apart in this picture?

hello

PPF sidebands around harmonics of foundamintal speed may due to an elelctric fault in the induction motors


thank you

quote:

Originally posted by David_G:
For an AC induction motor to run at ~720 RPM there must be a VFD.

Also there are some other problems with this power train.

A 10 pole motor will have a nominal speed of 720 rpm. Our seal oil pumps run around 714 rpm.

Steve, you are right... sorry, having math problems. 10 poles - this is a rarity though.

Then again, with typical slippage of about 1% the run speed is 712 CPM
PPF= 8 x 10 = 80 CPM

But having high 2x, 3x, and multiples suggests misalignment/looseness and as a result motor rotor dynamic eccentricity causing ultimately the PPF SBs.

Hi guys,
Lot of good comments.. Thanks!!

Ralph,

I will answer ur first questions later after i check the fan!! About the coupling, the picture was taken with the coupling bolts removed. By the way itz a grid coupling!!

Steve and David,
I think itz a 10 pole motor with no vfd.. Alignment we already checked and itz within the tolerance for this speed..

Have a nice day!!

Hi guys,

Got some information about this fan!!

Ralph,

The end of the pedestal is not open.. I'm attaching some more spectrums.. Itz strange to have 7275CPM harmonics in the motor. Also the sidebands around the 2X,3X.. peaks were at 69.3CPM and is present both in the Motor and Fan.

Have a nice day!!

Reverse_Fan.pps (1,334 Kb, 37 downloads)

Jenish-

You've done a very good job here. Nice internet troubleshooting. The amplitudes really are not jumping around as you move down through the bearing and the base until you jump down to the bottom which is expected. Are you able to check the phase of these as you move through that? I am somewhat concerned with the alignment issues. What temperature are the bearing operating at. Do you have access to an infrared camera to look at them and see what is going on. You grease issue there is of concern too. Strongly suggest taking off the bearing caps and cleaning that out during another down day and starting fresh. Might want to work with your greaser to go a little lighter on the grease and not run with the if a little is good a whole lot is better idea. Has the opposite affect where now all that is left is hardened grease soap. Attached are the harmonics for you 6230 bearing using an NTN bearing. The Enveloping looks very characteristic to me bearing although the bearing faults don't line up. Can you do a waveform check. What are the impacting levels at. As far as the Motor Slip Pole Issue- to really tell the levels of concern here you need to change it into dBs and see if the sidebanding levels are 75% of the turning speed but seeing how this is a synchronous unit it is a little different ball game.

This is my suggestion on this. Take a full set of readings all 12. H-V-A on each bearing. The pull a 1200CPM/6400LOR reading on the motor in the h-v on DE and ODE. Also pull a Envelop reading on each bearing in the vertical.

This ball game is running around in 20 different directions here. I will be glad to analyze this and help you out. From the looks of things you are using SKF. What version of software do you have? If you can extract that data I can possibly pull it into mine. Would really like to look at the waveforms as well as the spectrum. Looking at only half means you only get half the picture.

Take Care and Keep up the Good Work!

Good comments.

Some more thoughts. 6320 I think is the fan bearing? What about the motor bearings? I'll bet maybe that 7275cpm is a fault frequency for a motor bearing. But tough to get a handle on the magnitude from "gse" spectrum. My own personal preference (I guess everyone's got one) would be to see veloctiy spectrum out to 300kcpm, and acceleraiton TWF to help guess the severity.

Looks like an outdoor area exposed to some junk falling from above (birds? product?).

Hi OJU and Pete,

Thanks for the great comments and support..

By the way, i got some more information and i'm attaching the file.

1. The amplitudes in vertical direction were remaining the same till we reach the foundation. No phase readings available.
2. Alignment was already checked and even though itz not a perfect alignment, i think it will not cause that much problem in this speed. Itz only my opinion and please correct me if i'm wrong.
3. The IR images were provided in the file attached. Not much problem with temperature.
4. Checking the grease in the fan bearings and applying fresh grease, we will plan to do in next down day.
5. Sorry for the confusions about the bearing details. Fan bearings are 6320 and the motor bearings - DE is 6315 and NDE is 6319. Pete, I don't know whether you will win the bet or not , but the peaks are not matching the bearing fault frequencies in the Motor.
6. This equipment is sitting in an outdoor area with lots of dust around.

Let me summarize the problem:

1. Overhung fan with high looseness signature in the Fan bearings. Levels were almost same in both vertical and horizontal directions with lots of harmonics.
2. High 2x found in all the bearings.
3. Sidebands around 2X, 3X... were changing frequency on different set of readings in the Fan bearings. The 1X stayed same with no sidebands.
4. Enveloping spectra of the Motor showing distinct peaks harmonics which are not matching any fault frequencies.

Have a nice day!!

Reverse_Fan_IR.pps (216 Kb, 33 downloads)

Hi OJU,

Thanks for your interest in looking at our data!!

We are using SKF Machine Analyst V 3.2. Any way, i'm attaching the exported file from the database.

Thanks in advance!!

Have a nice day!!

reverse_fan_export.mab (571 Kb, 26 downloads)

Jenish-

Sorry for my absence. Just from looking at the infrared images quickly you can note their is a difference from one side of that drive end bearing to the other. My initial thought is there could have a cocked DE bearing there.
Also many times heating at the drive end bearings on motors is due to 2 things- bad air circulation or misalignment. I suggest doing a thermal profile and being sure to perform a hot alignment check if possible. With the infrared imaging, I would suggest changing to rainbow and squeezing in the thermal focus to see what is happening better.
I will look through things and see if I can import that data into my version of SKF, can't remember which it is.
Will look more and post back.

Hi guys,

More headaches!!

I got some additional information from electrical section. This fan is getting supply from our new Power Plant and according to them sometimes the supply is not synchronous!! That confirms the slight change in 2*LF peaks!!

Hi OJU,
Thanks for you comment!!
I didn't understand which bearing you are talking about!! Actually in the right end of the image is the NDE bearing of the Fan with the temperature of 72.2ºC. I found more than 80ºC in the impeller side.. I think the temperture is conducted from the hot gas to the bearing on that side(May be i'm completly wrong!!).There was a difference in temperature from DE of the Motor(60.8ºC) to the DE of the Fan(62.5ºC). Is that a big difference, if we consider the NDE Fan bearing temperature?


Have a nice day!!