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| Lubrication Tips for Reliability Professionals |
May
22, 2001 Subscribers: 6,825 |
“We have an application for which the OEM recommends an NLGI 00 grease with a minimum base oil viscosity of 460 cSt at 40°C. Is it normal to combine such a high viscosity oil with an NLGI 00 grease?”
It is important not to confuse the NLGI number of a grease which is a measure of the worked penetration at 25°C with the viscosity of the base oil contained in grease. Worked penetration measures the hardness or consistency of the soap medium containing the lubricant, whereas viscosity measures the resistance to flow of the base oil contained in the grease thickener.
Depending on the application and operational requirements, it is possible to get an NLGI 00 grease with a kinematic viscosity of 460 cSt at 40°C as in your case, or an NLGI grade 3 grease with a base oil viscosity of 15 cSt at 40°C. Generally, the NLGI requirements are selected based on speed and low temperature pumpability requirements while the base oil viscosity is selected for the load that has to be carried.
Here's a handy reference for selecting greases for different applications. Go to:
http://www.lubetraining.com/links/grease1.html
Most greases contain 75% to 96% oil. By design, the oil separates from the thickener, which is why it is common to see oil separating if the grease is idle.
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Diesel engines in trucks generally operate at more nearly constant speed and higher load factors than gasoline engines. Thus, combustion conditions are more nearly optimum, and water condensation and fuel dilution are not as serious problems. Where excessive fuel dilution occurs, it is usually the result of a mechanical problem, such as a faulty injector. Diesel fuel is not as readily evaporated from the engine oil as is gasoline; thus, if a problem exists, the concentration of diesel fuel will tend to increase steadily. This can lead to deposits and reduction of the oil viscosity to a sufficient extent that mechanical wear occurs.
For more information about the book "Lubrication Fundamentals" go to:
http://www.noria.com/secure/product_detail.asp?catalogid=2
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