Hi Guys,

Hoping you can help me with a overhung fan which has an increasing 3x fan frequency which is dominant in the axial direction.
Motor 45Kw
V-Belt Drive
Overhung Fan which cartridge containing 2 x 6310 bearings.

Fan was rebuilt around 4 months ago where motor was replaced (old motor was 37Kw), New pulleys, new belts, new fan shaft, rebuilt impeller and new bearings in existing cartridge.
Recently vibration is showing an increasing frequency at what could be either 3xF or BPO for the 6310 bearing which is highest at the fan side bearing. The fault does not appear to be a bearing as there is no harmonic series nor are there any indications of a BPO in the demodulation. The amplitude of this frequency is currently 10.98mm/sec RMS. Thermal imaging of the cartridge shows an even temperature of 32 degrees C with a 19 degree ambient. This frequency appears in all locations but increases the closer you get to the fan impeller. (i.e. you can see it in the belt side fan bearing @ 3.68mm/sec RMS.
During our inspections we also noted that the motor DE bearing appears to be on the hot side at 73 degrees C with 19 degree ambient but no evidence of any issues in the vibration.
We reported the fan fault last survey and told the customer to inspect the belt drive pulleys for runout, belts and fan shaft for wear etc. Customer reports that no faults were found during their inspections.
I have read some case studies of air flow issues in overhung fans causing high 3xF in axial direction but I don't see any raised floor noise in sub run speed frequencies.

Any ideas you may have would be greatly appreciated

Mark

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Original Post
quote:
Originally posted by Oil Can Boyd:
I guess the first thing I would want to know is; bearing defect or 3x. Take a time synchronous averaging measurement and see if the peak goes away. At least then you will know if its synchronous or not and can start ruling out some possibilities.

Hi Boyd,

Thanks for the suggestion I will give that a try on Monday and then at least I will know whether it's a bearing defect or not.
Is this a monoblock housing? This is a Cylindrical housing with a bearing at each end and 4 mounting feet. The shaft is locked in the housing and a bearing change requires removal of the fan wheel to change the bearing. If so check to make sure you have about 0.003 inch shaft float in the housing. If there is no float you can have a stiff axial spring. Do a bump check with the accel on the sheave end of the shaft to see if shaft axial natural frequency is near 3x shaft speed. You can increase the bearing end cover gasket thickness to pick up some float.Too much float is also bad.
quote:
Originally posted by Oil Can Boyd:
I guess the first thing I would want to know is; bearing defect or 3x. Take a time synchronous averaging measurement and see if the peak goes away. At least then you will know if its synchronous or not and can start ruling out some possibilities.


I was finally able to perform the time synchronous averaging test this morning, along with a standard survey so I could confirm the suspect frequency was still present which is was. The results of the TSA showed that it is a non-synchronous frequency and therefore as it lines up to a BPO defect the frequency much be related to the bearing. There still is no harmonic series or evidence of the BPO in the demodulation and therefore must be in it's early stages.
If we split the housing do you think we would be able to see the wear if we measured the clearance between the ball and outer race given the bearings is 6310 and compare the RIC to the OEM specifications.
quote:
Originally posted by spherical:
10.98 mm/s RMS amplitude? Just curious, can a bearing fault frequency amplitude this much run without obvious bearing defect noise or hgih temperature?


This is what has been bothering me, it just doesn't follow the "normal" (if there is such a thing) appearance of a bearing defect
In a typical outer race defect, the ball rides directly over a defect and causes a sharp impact, with high harmonics.

As Gary mentioned, this pattern is more typical pattern associated with an irregularity in the housing supporting the outer race. Possibly consider it as oblong. Or else consider it as an irregularity at one point (the splitline, or some debris between housing and outer ring). Then as each ball goes over that area, there is flexing of the outer race, but not an impact. So it results in a force periodic at BPFO, but not the harmonics. We've talked about it before and it makes good sense although I've never followed a case like this all the way through to see anything conclusive found during repairs.

Edit - I guess with high axial, we should consider that one bearing housing might be cocked with respect to the shaft. Shaft is trying to push that bearing housing into position where shaft is straight. Stiffness of the bearing itself varies at bpfo... highest when a ball is in-line with the direction the shaft wants to push the housing and lowest when that direction is between rolling elements. I'd think the variation in stiffness in a given direction would be very small, especially given how close together the rolling elements are (housing problem as in previous paragraph more likely). But you never know. Anyone seen it?

I didn't quite understand what Rick was saying.
A monoblock bearing is a long housing with a bearing in both ends. The shaft is integral to the bearing. There end covers at both ends and one bearing is a floater and the other end fixed (usually the drive end. A common assembly error is not allowing enough axial clearance on the fixed end. The end cover gasket thickness controls the end float(0.003" is ideal). If no clearance the outer race is preloaded against the inner race and can act like an axial spring.

Is this a monoblock bearing?
I looked through our history and found one somewhat similar case. 100hp 3600rpm ODP motor was showing 0.06 ips at BPFO fundamental (3.06 orders) with no harmonics and some other very low level non-sync peaks below 0.015ips. The bpfo peak showed up only on the outboard beairng and primarily in the horizontal direciton.

The pattern went away when bearings were replaced. I reviewed the paperwork today from that bearing replacement job which was done in 2013. No housing work done. Nothing unusual recorded. No bearing fit measurements recorded. Not sure what to make of it. Wish I would have followed the job closer at the time to see what they saw when they replaced the bearings.

Edit - just noticed 3x was also high (compared to the other running speedharmonics), so maybe some kind of resonance near that frequency may have played a role in the pattern also.

Rick - so your saying if the bearing gets in a bind axially it might give this pattern?

Attachments

quote:
Originally posted by Rick Little:
A monoblock bearing is a long housing with a bearing in both ends. The shaft is integral to the bearing. There end covers at both ends and one bearing is a floater and the other end fixed (usually the drive end. A common assembly error is not allowing enough axial clearance on the fixed end. The end cover gasket thickness controls the end float(0.003" is ideal). If no clearance the outer race is preloaded against the inner race and can act like an axial spring.

Is this a monoblock bearing?


Hi Rick,

Yes it's a monoblock housing with 2 x 6310 ball bearings fitted, see attachment

Attachments

Photos (1)
The bearing housing end cover has a raised ring that presses against the outer race and the inner race shoulders against a step in the shaft. There may or may not be some wavey washers between the opposite side of the outer race and the housing like some motors have. If there is not any float when the shaft is barred back and forth there will be an axial preload between the inner and outer race and set up sort of a spring that can cause axial vibration. Depending upon the magnitude of the preload the axial natural frequency may be what you're seeing. If you have no float you may have used a thinner gasket than you had before the repair. If so install a thicker gasket and check end float. I like 0.003" end float.
We repair a good number of these bearings and on a lot of them find it necessary to bore and sleeve the housing to correct excessive clearance between the outer race and the bearing bore. If this is not corrected the bearing can chatter in the bore and sometimes cock.
Hi Rick, thankyou for you replies they are much appreciated. This defect has only just appeared and the bearings were replaced some time ago now. What you cause the axial end float to become an issue now.
Is there an easy method to measure this end float by just removing the housing cap?

Thanks
Mark
Epete
We once had a 60 hp, 4 pole motor that had axial vibration close to the bearing outer race flaw frequency. We bumped the motor axially with the pickup on the shaft end and got the same frequency. We loosened the bolts that pull the plate up behind the bearing to clamp it in the bore and then carefully tightened them and the axial vibration disappeared. I wish we had done the bump check again but we didn't. I would, in retrospect, like to see if there was a change.
Mark,
Easiest thing in the world to check. Put dial indicator on shaft end. Then push sheave toward inlet and then pull it away from inlet and the total indicator movement will be the float. It should pop to one side and then the other. If it springs back after pull or push then you are probably seeing deflection and not end float.

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