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marksl

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Reply with quote  #1 
Hi Gents,

We are monitoring a fan which runs an SNR cartridge housing containing two deep groove ball bearings which has a high blade pass frequency and may have since it was commissioned. The bearings in the cartridge are only lasting around 7 months. Could the blade pass frequency be the cause of the short bearing life and if so, what damage would you expect to see in the bearings when they are removed from the fan and inspected?

Thanks
David Eason

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Reply with quote  #2 
My first question would be to ask why is the vane pass high? Where is it on the fan curve? Is Production suffering any ill effects of the fan not doing what it is supposed to do, or maybe the fan is being asked to run outside of design specs to achieve what production wants? In other words, the bearing failures may be collateral damage from the fan being operated outside of design and the vane pass you're seeing may be the tattle-tale. 

David
Walt Strong

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Reply with quote  #3 
I would consider it highly unlikely that high blade pass frequency pulsations would be the direct cause of a bearing failure, but rare situations do occur. High pulsations could be a factor/contributor to the dynamic force that directly or indirectly causes a bearing to fail. It would be easy to get side-tracked on discussing your OP. As you know, there would be a lot of issues more likely to cause a bearing failure (correct bearing, correct installation, lubrication, rotor balance, shaft alignment, etc.), so have you exhausted investigating those possibilities?

Can you provide basic details about the machine (arrangement, speed, power, etc.), vibration spectra after new bearing, and vibration spectra just prior to bearing failure?

"what damage would you expect to see in the bearings when they are removed from the fan and inspected?" I assume the bearings would not be removed for inspection, if they are in known good condition. Since, they were removed for previous replacement, then visual inspection should be easy and obvious. If the bearings are believed to be bad, then what does the vibration data show?

Walt
Shurafa

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Reply with quote  #4 
I second what Walt mentioned. With the limited info you gave, my reply would be generic and it is unlikely to have a special affect to damage the bearings in months.

Maybe you can calculate the required forces (static and dynamic) on the bearings (for your machine) to fail in seven months.

Please give us more details like spectra and machine drawings. How high is high vibration in your perspective or in this case? I hope it not more than 1 ips rms.

Regards- Ali M. Al-Shurafa
OLi

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Reply with quote  #5 
I have seen exactly that happening in a glass, window related industry where you instantly required a flow of cold air on instant process demand so the setup was to run the fans full flow against a closed valve for 90-95% of the time and opening the valves for a very short time so it mostly gave the result as OP describe. Bearings were upgraded and rebuilt with circulating oil lube and cooling and I think speed was reduced for some time of the cycle and it improved the situation. I suggested a bypass of some kind but it would have been complicated so it was not installed.
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arie mol

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Reply with quote  #6 
Vibration amplitude at blade pass frequency can be high because frequency coincides with a local structural resonant condition. Speed variation will confirm this.
I should not worry too much about the detrimental effect of vibration energy at BPF and centrifugal fans. However considering centrifugal pumps I have seen premature bearing failures with BPF running at or near resonant condition. Water is 700 times more powerfull than air!

ruben

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Reply with quote  #7 
[QUOTE = marksl] Hi Gents,

estamos monitoreando un ventilador que tiene una carcasa de cartucho SNR que contiene dos rodamientos rígidos de bolas que tiene una frecuencia de paso de cuchilla alta y puede tener desde que se puso en marcha. Los rodamientos en el cartucho solo duran alrededor de 7 meses. ¿Podría la frecuencia de paso de la cuchilla ser la causa de la corta vida del cojinete y, si es así, qué daño esperaría ver en los cojinetes cuando se retiran del ventilador y se inspeccionan?

Gracias [/ QUOTE]

How many blades has the fan?
It´s a cantilivered or supported between two bearings rotor?
how is the configuration, moved by belts or by direct coupling?
Has you checked balance and alignment?
What´s the temperature on the bearing after more than 120 hours working after overhaul?
What´s the process liquid temperature? The Grease is correct for that equipment (rpm, load and temperature?
Do you know the critical speeds?

RBT

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Reply with quote  #8 
Several years ago we had an issue on a large centrifugal pump similar to this, it was a pump not a fan. The manufacturer had changed the bearing arrangement, opposite of what they had used in years past. The change, unknown to us had caused our pump to have a very high 4x and chewing up bearings every few months. The 4x{VPF} had the famous hump at 4x, of course you know the rest of the story, the pump shaft was in resonance enough to damage bearings. The bearings were changed back to the original configuration and the problem went away.It took awhile to figure the issue out, or what was causing the 4x to rise.

Something you might want to do, if you can get to the shaft {not running}, do a bump test on it and the entire fan, also do a coast down.

Good Day Gentleman,
Curran919

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Reply with quote  #9 
Quote:
Originally Posted by RBT
Several years ago we had an issue on a large centrifugal pump similar to this, it was a pump not a fan. The manufacturer had changed the bearing arrangement, opposite of what they had used in years past. The change, unknown to us had caused our pump to have a very high 4x and chewing up bearings every few months. The 4x{VPF} had the famous hump at 4x, of course you know the rest of the story, the pump shaft was in resonance enough to damage bearings. The bearings were changed back to the original configuration and the problem went away.It took awhile to figure the issue out, or what was causing the 4x to rise.

Something you might want to do, if you can get to the shaft {not running}, do a bump test on it and the entire fan, also do a coast down.

Good Day Gentleman,


This all sounds strange. These were rolling element bearings? And you confirmed that there was a lateral rotor dynamic critical speed that was leading to high casing vibration at VPF?
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