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gguevara

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Posts: 6
Reply with quote  #1 
Dear all,
I have a very interesting case here that I would like to share. Any opinions, or suggestions are welcome.

A couple of years ago we replaced 3 big roller bearings (oil lub., 1500 mm shaft, 36 rollers 70x75 mm each, 1820 mm outside diameter) in a 500 rpm tubular stranding machine:
Machine.png 

Two of them, the most loaded ones, begun showing very high vibration and BPFO harmonics after 100 hours working at 450 rpm. They had to be replaced inmediatly, we installed the old bearings that where "in a better shape"...
The third one, about half loaded, worked for 6 month at 300 rpm before showing the same symptoms. 

There is a 4th bearing, smaller one, coupled int the same shaft, that had no trouble at all... neither the other bearings of the machine (drive, and so on)

According to the bearing supplier, the most possible cause of the bearing failure is current passage - we are still trying to know what really happened... If you see the outer race photos it really makes you think of a current prolem: 

Bearing damage.png 

20160106_125216.jpg 

DSC05243.JPG 

However, we suspect there may be another cause, because:
1- No galvanic coupling exist between the drive and machine (belt coupled 300 kw DC motor) - there is no other electrical power source in the near-hood of the bearings - just 24v swithces/sensors.
2- Never in the last 20 years (as long as i have been working here) had a bearing problem in the motor, always replaced during PM after 3 years of working and no strange marks seen on the races.
3- The tube of the machine (i.e. ineer race) is grounded through slip-rings, the outer race is also grounded, no considerable voltages were detected
4- Previus bearing were working for a couple of years. We decided to replace them after a huge flood that left corrosion marks on the races, noise and BPFI in considerable magnitud, but contiue working for a couple of years before replacing them...
5- No sign of damage on the NU small bearing (grease lub., )
6- The damage could not be confirmed through 1000 X electronic microscope (according to the lab they can´t confirm it but  neither they can discard it)

Here are the velocity and enveloped spectrums:
Bearing N° 3 300 rpm BPFO veloc.png  Bearing N° 3 300 rpm BPFO.png 




Now, recently we replaced the bearings - but from a different manufacturer. They are similar, but not the same: 31 rollers instead of 36, different roller dimensions... but the bearings are equivalent. They have been working for 500 hours at 450 rpm. Some small amplitud BPFO harmonics appear in one of the bearings (the most loeaded one):

small BPFO 2017.pngSome other data: the oil circuit is quite safe, has many swithces and flow transducers that we monitor online, and check periodically. The temperature of the bearings is also monitored, no more than 80 ° C during summer (as much as 45 ° C room temp.)... during winter no more than 65 ° C (5 - 15 ° C room temperature). 


What do you think may happened with the bearings that made them work only for 100 hours?
What do you think is happening now withe these new bearings that show small BPFO components in the enveloped spectrum?

Thanks!



John from PA

Sr. Member
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Posts: 963
Reply with quote  #2 
Even though this says "Welcome- Start Here" you might do better if you move it to the forum area titled Vibration Analysis - Balancing - Alignment.   You can find it at http://www.machineryanalysis.org/?forum=473361.  Some other categories that might be of interest are at http://www.machineryanalysis.org/categories

As to your problem...any chance you use induction heaters for mounting bearings?
gguevara

Member
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Posts: 6
Reply with quote  #3 
Thanks for your advice, i will move the post as suggested.
No induction heaters used to install them, we used an oil bath with torches... 100 C
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