We have had a couple of split bearing failures recently. We checked the date of installation and consider the failures premature. The past couple of days I have been investigating bearing lubrication and the application of these bearings. Found that the bearing manufacturer (tech support) has a specification for ISO-VG (ISO-Viscosity Grade). Spoke with our lubrication manufacturer (tech support) and they were more familiar with grease specification in NLGI 1, 2 or 3 and not heard of ISO-VG. After some discussion with technical support from both manufacturers, I found that the ISO VG was a designation of the viscosity of the oil that is blended in the grease. This is the first time I have heard of this specification. I am familiar with the standard NLGI 1, 2 or 3 for grease but never heard of ISO-VG? Do you think this should be included in the NLGI specifications?

Yes, viscosity of the base oil is an important property and is not addressed in the NLGI grade.

The viscosity of the base oil is not a secret, it is available from the manufacturer and should be considered along with other paramters in grease selection.

For the Mobilith SHC series discussed in the other thread, the viscosity of the base oil is part of the grease identification. SHC100 has viscosity grade 100 for base oil, SHC46 has VG46 for base oil, etc. All in the SHC series are NLGI 2 which is mostly a function of the thickener, not the base oil.

According to most sources, the minimum base oil viscosity at operating temperature in grease for greased bearing is the same as minimum oil viscosity at operating temperature for oil in oil-lub'd bearing.... a function of D*N which allows lower minimum viscosity for higher speed, higher diameter. See minimum viscosity chart on 4th page of this document:
http://www.mrcbearingservices.com/docs/lubrication.pdf

Not meeting minimum viscosity at operating temp can cause acclerated wear. Excessivly high viscosity far above the minimum causes unnecessary heating.

Also a lot of people say pressure viscosity coefficient of the base oil is important. For non-synthetic oils there is not much variation in this parameter, but for synth oils there is wide variation. But this parameter you will usually not find much information published for the oil in your grease.

The end result of this and a lot of other stuff is that selecting grease is not a simple task which can be narrowed down to 1 parameter (NLGI), 2 parameters (add viscosity), 3 parameters (add pv coefficient). We have to look carefully at the grease literature to see what is the range of intended application for the product, or ask lube supplier or bearing supplier for help.

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