Sleeve bearing lube oil turned black 2

[Hamza1988]

Hi guys, in our company we have a FD fan which is supported by two pillow blocks (sleeve bearings). One is 'fixed' while the other is 'free'. I would like to know what is the difference between the two.
Also, quite recently we noticed that the color of bearing lube oil turned black. We tried to flush the oil online several times (meaning draining and filling the oil simultaneously as we cannot stop the FD fan due to operational reasons). But the oil color did not change and is still dark greyish or blackish in color. Later I found that the oil which we have been using was Regal R&O 100 but the recommended one was Regal R&O 68. So, I would also like to know what can cause the color of oil to turn black. There was no apparent metallic debris in the drained oil as well. Also, would this wrong oil or higher viscosity oil that we have been using might have caused bearing damage of some kind and turn the oil black.

Thanks for the help...

 

[JJPellin]

The fixed bearing is designed to carry a thrust load. In the case of a pillow block sleeve bearing, it probably has a thrust collar in a groove in the center of the bearing or two collars on either end of the bearing. The free bearing does not have the thrust bearing feature and can only take radial load. For a forced draft fan, the thermal growth of the shaft should be small, but you cannot fix both bearings or you will have a problem with axial thermal growth. The bearings probably have brass slinger rings hanging in grooves in the bearing which provide oil to the bearings. They probably have constant level oilers to maintain the proper oil level.

The oil can turn dark from high temperature. For a forced draft fan, this is unlikely. For a very large FD fan, you might have cooling water circulating in a cast-in cooling jacket in the bearing liner. But, none of our FD fans have cooling water in this arrangement. Oil will turn dark if it is exposed to brass particles. Even if you don’t see metallic particles in the oil, if the brass oil rings are rubbing and wearing, the oil will turn dark quickly. You should have view ports to look at the tops of the oil rings. Check to see if they are turning smoothly and delivering a good stream of oil. If the oil ring is jumping around or running hard to one side of the groove, this could be the problem. The oil ring could become unstable if the oil level is too low or too high. Verify that the oil level is at the manufacturer’s recommended level.

I would not expect the one grade heavier oil to cause rapid darkening of the oil. But, you can eliminate this variable by changing the oil to the lighter grade on the run.

The bearing liner may be set in a spherical fit within the housing to allow the liner to align to the shaft. The liner may be locked down in this fit with a jam bolt coming down on the top of the liner. This jam bolt is supposed to be torqued to a certain torque and then locked with a jam nut. You can release this jam bolt and retorque it on the run to be sure that the bearing is properly aligned to the shaft.


Johnny Pellin

 

[Tmoose]

Fixed bearings mechanically prevent the shaft from moving axially. "Free" bearings can slide axially to accommodate thermal growth etc. The installation manual for your bearings should make it clear how that is done.

Time for oil analysis.

I would not expect the slightly higher viscosity oil ( 100 ) to cause any problems.

What are the bearing temperatures? How are they being measured?

 

[Hamza1988]

Thank you guys,

Spot on Johnny Pellin, the arrangement we have perfectly matches the description you have provided except that we do have a cooling water jacket. I forgot to mention about the oil level which was quite high. After draining about 4 liters of oil, the level could be seen through the sight glass. I suppose the level should be maintained at 75% of sight glass.

Tmoose, The bearing temperature was 40 degrees C and it is measured using a thermocouple probe and is read on a temperature gauge.

 

[JJPellin]

For most of our bearings, the oil level is supposed to be at the center (50%) of the round, bulls-eye sight glass. This may not be true for all bearings.

Johnny Pellin

 

[Tmoose]

"The bearing temperature was 40 degrees C and it is measured using a thermocouple probe and is read on a temperature gauge......."

Is the temp probe spring loaded to touch the back side of the bearing insert? Or in some less useful location?
Also is the thrust bearing a separate insert, and its temp measured separately and directly?

 

[electricpete]

Aside from investigating temperature, I would revisit your assumption of no particles. Brass oil rings are common and associated wear products can easily turn the oil black and be difficult to detect visually or by putting a magnet near the bottle. Best way to investigate is oil sample elemental analysis using ICP method. Copper is most distinctive since it is not normally in additives (zinc may be present as additive). Additional investigation would be close inspection of oil rings looking for wear when machine shutdown for maintenance. Another data point is to look for abnormal oil ring movement as possible cause of oil ring wear.

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