M.M. Khonsari
Mechanical Engineering Dept.
Louisiana State Univ.
Baton Rouge, La.

E.R. Booser
Consulting Engineer
Niskayuna, NY

Unless bearings come prelubricated and sealed from the factory, they will periodically need a change of lubricant. How often depends on operating conditions and lubricant type.

There are three basic kinds of bearing lubricants: synthetic oils, mineral oils, and greases. Synthetic lubricants generally last longer at elevated temperatures than their mineral-oil counterparts. And some types have special low-temperature and low-flammability properties. However, hydrolysis or the tendency to absorb water -- even from exposure to atmospheric humidity -- tends to shorten the life of some phosphate, silicate, and ester synthetic oils. Avoiding hydrolysis may require special additives, desiccated air, and filtration with activated alumina or clay-based Fullers earth.

Mineral lubricating oils deteriorate when they oxidize or react chemically with dissolved atmospheric oxygen. This raises oil acidity and encourages varnishlike surface deposits, both of which can shorten bearing life. Lubricant makers add oxidation inhibitors to help break down hydroperoxides that form during what's called the initial oxidation step.

Additives extend oil life by interrupting oxidation chain reactions and by deactivating any catalytic metal surfaces touching the oil. Oxidation-inhibiting additives are slowly consumed during the initial oxidation period. Adding more inhibitor within this time frame lengthens the induction period and delays acceleration of oxidation reactions.

Elevated temperature is probably the biggest contributor to oil oxidation. Oil life L, drops by a factor of two for each 10°C temperature rise between 100 to 150°C. Knowing oil operating temperature T (°C), lets you estimate oil life in hours:

logL= kl + 4,750/(T+273)

where kl depends on oil type. For example, the equation predicts oil in a turbine bearing at a temperature of 138°C degrades about 180 times faster than the same oil in a turbine oil reservoir at 71°C. It's not uncommon for lubrication systems to have several temperature zones. Each of the n zones with oil volume Cn has a deterioration rate of 1/Ln. Summing the individual contributions:


gives the overall deterioration rate for all oil in a system.

However, the above approach assumes no water or other contamination, no adverse catalytic effects from copper and iron surfaces, and no oxidation-inhibitor evaporation, all of which can cut expected oil life. Adjusting the above oil-life calculations with an equipment-dependent factor accounts for these items. Electric motors and hydraulic systems, for example, use a factor of three. In other words, oil in these systems lasts about 66% less than what oil-life equations predict. A factor of two to five works for steam turbines and compressors while a factor of 10 is good for heavy-duty gas turbines.

When to change the oil
In all cases, improved bearing life calls for periodic (typically monthly) laboratory checks of oil samples for oxidation, viscosity change, or contaminant accumulation. Oil should be changed when its acidity rises by 0.2 to 0.3-mg KOH/gm above that for new oil, or when viscosity changes more than 5%.

Fortunately, several methods are available to rapidly evaluate oil condition. These include electrochemical, microscale oxidation tests, differential thermal analysis, and high-pressure differential scanning calorimetry.

One test, called Fourier TranSform Infrared Spectroscopy, estimates remaining oxidation inhibitor in an oil sample by the amount of light it absorbs in the 2 to 50-micron wavelength range. An oil change or replenishing of oxidation inhibitor are signaled when oxidation-inhibitor concentration drops by half or more from original values. Alternatively, turbine and other circulating-system oils may use the ASTM D2272 rotating bomb oxidation test (RBOT). Here, a value below 50 min indicates marginal remaining life.

Grease metrics
As with oil, acidity and antioxidant content are important indicators of the life remaining in grease. But greases contain oil and thickeners, both of which also influence lifetime. Oil content is a measure of remaining life and can be quantified in a grease sample by atomic absorption spectroscopy or by solvent separation and weighing of the remaining oil.


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