In light of the description of that lubricant, I'm a bit surprised on using it on this type machine. Are you certain it is used based on the gearbox OEM recommendations?

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All RENOLIN DTA products are DIN 51524-1 (HL) hydraulic oils and DIN 51517-2 (CL) general lubricating oils, based on mineral oils, demul-sifying (water-repellent) and free of zinc.

ADDED INFORMATION

One of the best references on high speed gearing lubrication, titled Cooling and Lubrication of High-Speed Helical Gears can be found at http://turbolab.tamu.edu/proc/...oproc/T33/t33-06.pdf. If you seek out "axial meshing velocity" on page 2 by Acrobat reader (42 as the article is numbered) you will find the reason for the burnt coloring all the way toward one end of the single helical gearing.

Beyond what you learn there I would do an internal inspection of the gearing through an inspection cover and make sure that a spray nozzle hasn't become plugged. Check with the OEM or your lubrication vendor and see what methods you might be able to use to clean off the varnish through the inspection opening. Then verify you are using a recommended lubricant.

You might ask yourself as well about the possibility of varnish on bearing surfaces. Do you have a history or sudden onset of elevated bearing temperatures?

Thanks John,

It is recommended by OEM (Siemens) to use ISO VG 46 class viscosity lubricant.
In Jan-13 we changed all gears set because of this problem and afterword OEM recommended to apply the ESP cleaning system to reduce the built-up of varnish in the system. Additionally the oil should be topped up with 20% new oil to increase the level of antioxidants again.
We are following the recommendation but still problem is not resolved.
Now we are looking to change the oil type. Do you have any suggestion.

the varnish and the brown color are a indication that the operating temperature is way too high and/or the oil has been used quite a bit longer then it is capable to withstand oxidation.

have you made any temperature measurements? the oil used matches the required viscosity grade but given the fact that it only meets the DIN HL and CL specifications will not be that good in respect to oxidation resistance. if there is no actual requirement of the oil being zincfree (zincfree oils are used in certain applications with silver coated parts and applications with very specific filterability requirements where filters may block when moisture and zinc comes together), you probably would be better of using a DIN HLP type hydraulic oil which will contain both a certain amount of antioxidant and antiwear additive.

You really should go back to Siemens and pressure them for a very specific list of lubricants. The selection of a lubricant for an integrally geared compressor, your mention of five stages makes me think that is what you have, is far more complex than just stating ISO VG46 is acceptable. I believe that somewhere around 2005 Siemens acquired Flender-Graffenstaden (FG), a major reputable gear manufacturer. If that is the actual supplier of your gearbox attempt to go directly to FG for a recommendation. You may wish to ask about the use of a synthetic hydrocarbon (SHC) type of lubricant recommendation. That family of lubricants is considerably more expensive but may offer significant benefits.

Just as a reference API 671 (2002) states the following "general" comment about lubrication for an integrally geared compressor drive.

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2.10.2 Unless otherwise specified, the lubricant shall be hydrocarbon oil of viscosity Grade 32 with an FZG load stage of 5, in accordance with ISO 8068. When required for gearing, viscosity Grade 46 with an FZG load stage of 5 may be used, with purchaser’s approval. Oils with extreme pressure (EP) additives shall not be used.

We have a number of these types of machines installed on our site and found similar problems, although not as severe as this and normally only after 4 years operation. API672 refers to a FZG load stage of 5, but we have found that a FZG load stage greater than 8 is much better. There are tremendous forces generated between the bull gear and the pinion teeth which compress the oil film, this pressure converts to heat energy and if the oil film is not strong enough to withstand these forces the oil simply oxidise. In high speed turbo machine less additives are better than more as it is the additives which tend to oxidise faster than the base oil. It is however the base oil that creates the pressure force stability and is recommended to use a group base 3 oils in cases like this, in standard high speed applications group base 2 oils are normally sufficient. The 20% yearly fresh oil top up is also a good practise. It is recommended to check for contaminants in the oil, water is considered one of the biggest culprits and will have a significant impact on varnishing. The ISO VG32 is recommended for cooler regions like Europe, although ISO VG46 is commonly used in hotter regions. Hydraulic oil is NOT recommended for these applications. Also verify your oil feed temperature, this should be in the region of 100-115 degree Fahrenheit (38-45°C)