Question:
This is the case of a overhung fan ,located on 2 spherical roller bearings and bearings located in Plummer blocks . The vibrations measured on bearings showed high 3x component on both the bearings . Recently some where at this forum only I had red that 3x means triangular displacement of the shaft . In this case also I felt the same. What I did is to reduce the effect of this triangular displacement , I slightly started loosening the Plummer Block top cover bolts while doing the vibration measurements simultaneously . At one time , 3X component dropped substantionally and vibrations reduced to normal . I did the same trick on the other bearing as well and the same result of reduced vibrations with 3x component almost disappearing . Great , we could just bring the machine to normal in running condition.

Just to share with u all .

Good Day ,

Anurag

Choices:
shaft displacement was restricted too much.

 

Are you possibly suggesting that this is a corrective measure that should be adopted as a maintenance practice? Do sights like big yellow birds and the smell of petroleum enter into the picture?

This sort of sounds like the test many do on a motor to confirm softfoot problems that cause a high 2X LF through distorting the motor casing. These motor bolts are not left loose, the softfoot is repaired the next time the motor is down.

But, I have seen people loosen bolts on motors or pumps to bring the alignment into tolerance rather than adjusting the shims to bring it into tolerance, and they do leave the bolts loose. Bad practice I think.

Anurag,
I hope you did not leave the bearing cap bolts loose, but merely used this as a test on what to repair next time the fan is down.

Curious and interesting your sharing Anurag. I would like to know about your two last posted, about the late solution. They are :
Higher vibration problem and High vibration in 1X horizontal. I wish to know what was the end of history.
Regards

It doesn't sound like Anurag left them loose enough to pull them out--just looser than he found them. As if they were overtight to start with. Maybe they were a little egg-shaped to begin with--housings or bearings--and loosening them a bit relieved some stress. Would I advocate this? Probably not without doing a lot of homework.

I do agree with u lawrancep.we should not advocate this . But sometimes such tricks do count. my fan would nt have run for long had i not done this .

Anurag

both the problems are still existing bcos of nonabailability of the machine for production reasons . I wish to do balancing of the rotor.
Regards ,
Anurag

believe me Dear Ralph , I didnt leave the cap bolts loose .
Regards,
Anurag

Anurag,

I tend to agree with Ralph. It sounds to me more like it was an effect of misalignment that you diminished by loosening up the bolts. I came to realize that misalignment will manisfest itself in vibration response in many different ways (btw what kind of coupling was being used?) sometimes even without affecting vibration at all.

Of course, in your case this is just my hypothesis, but it could be easyly verified. I'm almost sure that while loosening up the bolts, the rest of the harmonic amplitude content has also changed and likely not favorably.
It would be nice if you could post the before and after data.

I'd like to refer back to this problem. I also have an overhung fan belt driven which has 3X & 4X dominant peaks throughout both ACC & VEL. It's not an imbalance problem obviously. Both bearings are in alarm range.

These bearings are Dodge Imperial with split housings. Are these peaks perhaps caused by misalignment, or looseness? We had a millwright install these about 2 years ago and they have never ran good and smooth. He even put a machinist level on the mounting surface and used all sorts of shims under the housings...

Never have like this fan, and it's been hammering away out there for months! Don't know what to do with it other than go back to the SKF spherical roller bearings which it had previously. At least they ran smoother and in the green, other than an occasional imbalance, 1X

We have a turnaround coming up soon on this unit and I'm considering changing these bearings back out to the SKF...

Thanks,

In todays industrial maintenance world, that would be an activity I could and "would" be fired over.

Safe work permits,Lock out Tag Out Procedures, etc are thrown out the roof when you put wrenches on rotating equipment.

Consider the "IF's" - If your shirt sleeve got caught in the shaft, what if the wrench slipped and your hand went under the shaft... etc etc...

There is no machine operating condition that warrants loosening bearing cap bolts during operation!!!

The guy busting his tail, taking chances, getting the job done anyway possible is likely the next guy that will get fired because of safety.

Just my 2 cents... Be Safe !!

Mike

Rockin' Rod,
I have had experiences with the Dodge bearings. Because of the feature that makes setting them so easy, they are very critical on the sizing. One of my customers had been through two sets before calling me and we found that the bore was slightly on the tight side.
You should check the sizes of the shaft and the bearing ID.
For what it's worth, I did have a fan with SKF double row spherical bearings and it drove us crazy for two reasons;
The SKF rep would not budge on his tolerance specs (C3), and the fan was pushing hot air (400 degrees F), and the bearing clearances were closed before the rest of the bearing could come up to temperature.

Ron,

Thats the main reason we are trying these Dodge bearings on three of our fans. Their ease of installation, and time saving features.

The fan in question (3&4X peaks) had a new shaft and wheel installed in it when the Dodge bearings were used, so the tolerances should be okay.

This fan is also pushing hot air about 350-400 degrees. We use the SKF double row spherical bearings C3 in most all of our machines as well. Including our other hot fans.

Let me ask you while we're at it. When you set the clearances on the SKF bearings, do you put some sort of load on the bearing to push the clearance to the top, so you can check it with the feeler guage?

Unfortunately all of the SKF training films, and techs will tell you how to set the bearings to the shaft without the housings. We cannot do that here. We have to install the bearings in the housings then slide them onto the shaft, locate them precisely, move the shaft & wheel in the housing correctly, for clearance, then set the bearings...The question here is how to set the mounted clearance. Load the bearing into the housing all the way, then check clearance, or put just enough pressure on the bearing outer race and still be able to turn it (sort of hard) then set clearance using half of the mounted clearance. Not really a good way I guess. If too tight, then you throw in the heat factor from the hot fan probably making nit tighter, then the bearing overheats...Of course there is no load on the bearing when doing the actual tightening, just when checking with the feeler guage.

Thats an ongoing problem we've had here, how to set the clearance properly. Seems like different technicians use different methods. There must be a better way to do this with bearing using the tapered sleeve. Any ideas?

Regards,

quote:

Originally posted by Ron Brook:
Rockin' Rod,
I have had experiences with the Dodge bearings. Because of the feature that makes setting them so easy, they are very critical on the sizing. One of my customers had been through two sets before calling me and we found that the bore was slightly on the tight side.
You should check the sizes of the shaft and the bearing ID.
For what it's worth, I did have a fan with SKF double row spherical bearings and it drove us crazy for two reasons;
The SKF rep would not budge on his tolerance specs (C3), and the fan was pushing hot air (400 degrees F), and the bearing clearances were closed before the rest of the bearing could come up to temperature.

Hey Rockin'
Sounds like we are on to something. What you are experiencing is exactly what we found. The C3 fit was too tight for the application. If you have other 'hot' fans that have no problem with a C3 fit, I would ask you if maybe they give the bearings a chance to heat up before the hot air is introduced? The fan we had issues with went from colt to hot almost instantaneously.
As far as taking out the clearance, I agree with you that taking it all out and then backing off is probably not good. Ask SKF if it were possible to put a downward load on the fan shaft, say with a come along, take my feeler guage readings and then close the clearance.
As far as 3x and 4x predominant vibration, that sounds like a loose bearing, probably exciting a natural frequency (near the 3x and/or 4x).
Change the y axis to log scale and see if those two peaks have wide bases.

The fan with the problem is the one with the Dodge bearings. I was suspecting either misaligned bearing and or looseness on the Dodge Imperials.

We devised a fixture about 4 years ago which will allow us to push down on the shaft and rotate at the same time, thus controlling the amount of tension on the bearing. I normally push it down snug while still turning the outer race, then measure the clearance using half the mounted clearance. Seems to be working good.

We used a come along like you said, before that. But you always had to fight it too much.

quote:

Originally posted by Ron Brook:
Hey Rockin'
Sounds like we are on to something. What you are experiencing is exactly what we found. The C3 fit was too tight for the application. If you have other 'hot' fans that have no problem with a C3 fit, I would ask you if maybe they give the bearings a chance to heat up before the hot air is introduced? The fan we had issues with went from colt to hot almost instantaneously.
As far as taking out the clearance, I agree with you that taking it all out and then backing off is probably not good. Ask SKF if it were possible to put a downward load on the fan shaft, say with a come along, take my feeler guage readings and then close the clearance.
As far as 3x and 4x predominant vibration, that sounds like a loose bearing, probably exciting a natural frequency (near the 3x and/or 4x).
Change the y axis to log scale and see if those two peaks have wide bases.

Rod,

I am glad to hear that others are having problems with these Dodge Imperials. A few years back the maintenance superintendant at a plant that I do work at made the decision to switch to these Imperial bearings on all the fans in the plant. His reasoning was that "they are easier to install" than the old SKF spherical rollers they used for years. However I believe he either got a good lunch out of the salesman or he owned some stock in the company. Anyway... they've been a problem ever since. High 1X with harmonics and yes often times 3X & 4X dominate. Classical looseness... Fortunately a new regime is now in control of maintenance and they seem more willing listen and do away with Dodge altogether.

I'd love to hear from someone who's actually had success and likes these bearings. Personally, I wouldn't wish them on anyone... especially a vibration guy.

quote:

Originally posted by Michel:
Anyway... they've been a problem ever since. High 1X with harmonics and yes often times 3X & 4X dominate. Classical looseness... .

Frankly, I am not familiar with Dodge Imperial bearing but always interested in losseness symptoms. The symptoms above, I think, are not always manifest looseness. In particular, 3x and 4x are most likely not. As the author of the thread pointed out ,just the opposite has happened - loosening the bolts resulted in 3x, 4x reduction. Also, multiple harmonics although are indicators of some looseness is not always an indication of a detremental condition.

Personally, I'd get real nervous only when fractional harmonics show up.

In this case 3x and 4x phenomenon present interest IMO and probably are caused by misaligned shafts ( possibly coupling dependant) or internal bearing misalignment. I wonder if both of those were ruled out?

Dave

I was actually thinking it was more of a cocked bearing or misaligned bearings on the shaft. Perhaps loosening the cap bolts relieved it somewhat, in the case of Anurag.... When this fan in question was installed, our millwright put a machinist level on top of the bearing pedestal, and he had all sorts of crazy readings. There are shims all under those bearings, different sizes. I'm thinking this was the problem all along since it has never run that good.

The ideal thing would have been to machine the surface, but we did not have that luxury, nor time.

I went back to the spectrums on this fan looking at some of the smaller peaks. I see a lot of 225cpm sidebands around the 3X & 4X running speeds too averaging about .215 ips...Fan speed is 2160cpm

Anurag,
Have you checked for a resonant condition near 3*RPM? Perhaps you have tuned the resonance frequency by manipulating the coupling between bearings and supporting frame! Came across such a case before where everybody focussed on misalignment and bearing design flaw however at the end of the day resonance was the root-cause.
Arie Mol, NL