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:

C/L=C1/L1+C2/L2+C3/L3+...+Cn/Ln

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.

 

Oxidation life of mineral oils under ideal conditions

Oil typeklMax temperature for 1,000-hr life, °C

Uninhibited (used in once-through systems)-10.6475
Extreme-pressure gear lubricant-10.3184
Hydraulic-8.7699
Turbine-8.45106
Heavily refined, hydrocracked-8.05121
Greases typically fail after losing about half their initial oil content. At this point bearing friction and noise generally rise along with grease iron content. Further operation can cause severe wear and early bearing failure.

 

Speed reduction factor kf
for reducing grease life

Bearing typekf

Deep-groove, single-row ball bearing 1.0
Angular contact, singe-row ball bearing 1.6
Self-aligning ball bearing 1.3 to 1.6
Thrust ball bearing 5-6
Cylindrical, single-row roller bearing 1.8 to 2.3
Needle roller bearing 3.5
Tapered roller bearing 4
Spherical roller bearing 7-12
Halved initial oil content corresponds to the percentage of soap (thickener), Sf, contained in a grease at failure:

 

Sf = 100 32S0/(100 + S0)

where S0 = the percent thickener in fresh grease. A fresh grease containing 10% soap, for instance, would be expected to fail at Sf = 18%.

As a rule, larger bearings and those that run at high speeds shorten grease life. Grease time-to-failure typically halves when bearing rotational speeds reach DN limits (bore diameter D, mm 3 speed N, rpm). Operation at even higher speeds can trigger early bearing failure, partly because centrifugal force throws grease from cage and raceway surfaces.

Speed reduction factor kf is yet another indicator of grease life. It is directly proportional to how far grease must travel to feed the width of ball or roller tracks. Higher values (for a given bearing type) apply to those with larger cross sections or higher load capacity and vice versa. Larger speed reduction factors shorten grease life. For reference, conventional single-row, deep-groove ball bearings have a kf =1.0 and a DN limit of about 300,000.

Speed effects vary among greases as well. For example, so-called channeling-type greases often used in double-shielded and double-sealed ball bearings probably won't last as long as some other grease types when operating near limiting kfDN values. And silicone greases with their low surface tension oils and poorer lubricating properties for steel-on-steel surfaces dictate 35% lower DN speed limits. An additional 25 to 50% DN reduction is called for when operating bearings on vertical shafts.

Elevated operating temperature is also an enemy of greases. In fact, bearings run at temperatures above 70°C cut grease life by a factor of 1.5 for each 10°C rise. And above 150°C, rapid oxidation boosts that factor to 2.0 for each 10°C rise. High temperatures promote oxidation and raise oil evaporation rates and oil loss by creep, all of which accelerate grease drying and shorten life.

Grease life LG, can be estimated for operating temperatures above 70°C with moderate loads and no contamination by:

log LG = -2.60+2,450/(T+273)-1 x 10-6kfDN

Here, LG is time-to-grease-failure for 10% of applications using fresh industrial greases of Grade 2 consistency with thickeners such as lithium, complex metal soaps, and polyureas. n