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John stated: "I was specifically after evidence of wild excursions in the supply temperature that could be correlated to the erratic behavior in the embedded temperature at the affected bearing.  The beginnings of varnish in a journal can be caused by an unusual temperature swing in supply temperature that causes a subsequent increase in the bearing temperature sufficiently high enough to initiate varnish formation.  Then, even though the supply temperature is reduced, the varnish has decreased the bearing clearance, causing an increased temperature and further formation of varnish."

I was asking the same question as it might apply to the discharge temperature. Thanks for your comments about bulk oil discharge temperature, especially the 20-40% attributed to gear mesh.

The last temperature plots are not very readable, especially the time scale. The temperature spikes appear to ramp up in about 3-steps, and then abruptly drop. There is more going on in the bearing than varnish causing the temperature increase. Not much of a temperature rise at about 10-C from inlet to outlet. I would expect that it would take a lot of varnish to reduce the oil film gap, but I will go along with your statement. Is the varnish getting wiped-off during the temperature spikes?

Here are some articles describing causes of varnish formation:

The Root Cause of Varnish Formation

 10 Things You Should Know About Varnish

From <https://www.machinerylubricati...6/know-about-varnish>

Discovering the Root Cause of Varnish Formation

From <https://www.machinerylubricati...98/varnish-formation>

Continuous electrostatic oil cleaner

Varnish Potential Analysis

See Related Articles about Varnish

From <https://www.machinerylubricati...h-potential-analysis>

2 Ways to Manage and Control Varnish

From <https://www.machinerylubricati...nage-control-varnish>  

The Varnish Issue: Strategies for Successful Monitoring and Acceptable Levels

From <

Journal Bearings and Their Lubrication

From <https://www.machinerylubricati...-bearing-lubrication>


@Walt Strong posted:

Walt said: "I would expect that it would take a lot of varnish to reduce the oil film gap, but I will go along with your statement. Is the varnish getting wiped-off during the temperature spikes?

What varnish can do and how it behaves is often misunderstood.  See Influence of Varnish on Bearing Performance and Vibration of Rotating Machinery for some insight as to the phenomenon.  You don't have to go beyond the abstract to understand the impact of varnish on a bearing.  In high speed gearboxes where the oil film is likely 2 mils or less in a bearing with 10 mils clearance, varnish can be quite disastrous.  Something key to remember here is that gravity loaded rotors (turbines, generators, compressors, etc.) often operate with relatively low bearing unit loading (<200 psi) and subsequent high film thickness.  Gearboxes, due to the forces generated in mesh often have bearing unit loading well above 200 psi and operate with very small oil film thickness.

Hi Walt,

"Is the varnish getting wiped-off during the temperature spikes?"

I think the mechanism of temperature is that high temperature --> oil locally reduce viscosity --> reduce load capacity of oil wedge --> oil wedge can not balance with load force --> journal act with load --> shaft position change --> oil come more --> temp reduce. In this process, the shaft journal could be touch with bearing wall and vanish could be come out by friction (if fortunately).


Those orifice sizes could be normal.  The orifice that supplies the "Low speed non coupling side" might control the flow for both the radial and thrust bearing. You would have to check some drawings or examine the lubrication method to see if this is the case.  Are the orifices externally accessible?  If so make a test run with it opened up a bit. 

Last edited by John from PA
@Ngo Dinh posted:

It is not externally accessible. But you are right, John, I forget the thrust bearing there.

I never liked the designs where the flow control method wasn’t accessible.  With my former β€œgear” employer we typically did one of two things.  Quite common was external piping with a plate between two flanges.  That plate had the orifice.

In designs where The oil passages were internal, there was a plug that was bolted to the main casing.  From the outside it would like a bolted on circular plate.  But when removed you would find an orifice that would sit in the internal passage.  You might want to verify on the FG design that you don’t have something similar.

I'm not even an "INteRrneT eXpert" on turbines or gearboxes, but my experiences with Mobil's Synthetic lubricants has been quite positive.

In particular the generic high temperature behavior and "varnish cleaning" of Mobil's synthetic automotive grade oils ( not PAO or whatever the heck turbine oil is made of) is remarkable.


It looks like Mobil SHC 800 oils meet some of the same specs as the BP mineral product you are using. 

I'm guessing Maybe BP also has some "synthetic" version of Turbinol X/46 too.


Last edited by Dan Timberlake

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