Short answer
Grease is mostly oil held in a thickener, and that base oil does the actual lubricating in the bearing or joint. Its viscosity, reported in cSt at 40 °C and 100 °C, is a separate property from the NLGI grade, which only describes firmness. A soft NLGI 1 grease can have a heavy base oil, and a firm NLGI 2 grease can have a light one.
Two numbers that answer different questions
The NLGI grade describes how stiff the grease is and therefore how it stays in place, pumps and seals. Base oil viscosity describes how thick the oil film will be at the contact surfaces. Bearing selection depends heavily on the second number, yet it is easy to overlook because it is printed further down the data sheet.
Base oil viscosity is measured by the same kinematic viscosity methods used for liquid oils, such as ASTM D445, and is sometimes stated as an ISO VG equivalent. General-purpose greases sit in the middle of the range, while greases for high-speed spindles may be much lighter and greases for slow, heavily loaded bearings much heavier.
Speed, load and temperature
Slow-turning, heavily loaded or shock-loaded bearings, such as those on crushers, kiln rollers or large pivot pins, need a thicker base oil to keep surfaces apart. Fast-turning bearings, such as electric motors and spindles, need a lighter base oil to limit heat from churning. Bearing manufacturers use speed factors, often the bearing speed multiplied by its mean diameter, to guide that choice.
Temperature works in the same direction as with oils: base oil thins as it heats, so a hot application may need a heavier base oil to keep adequate film, while a cold application may need a lighter or high-viscosity-index base oil so the grease can still be pumped and turned.
Base oil type
The base oil may be mineral or synthetic, such as polyalphaolefin or ester. Synthetic base oils generally offer a wider temperature range and better low-temperature flow for a given viscosity, but the formulation as a whole determines performance. Food-grade greases use base oils and additives acceptable for incidental food contact.
Putting it together
When comparing greases, line up the thickener type, NLGI grade, base oil type and viscosity at 40 °C, and any listed specifications. If the bearing maker or equipment manual gives a required base oil viscosity, that requirement matters as much as the NLGI grade.
Frequently asked questions
Does NLGI 2 grease always have the same oil inside?
No. NLGI grade is only consistency. Two NLGI 2 greases can have very different base oil viscosities and types.
Where do I find a grease’s base oil viscosity?
It is listed on the product data sheet, usually as kinematic viscosity in cSt at 40 °C and 100 °C.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].