Heat and contaminants are the biggest enemies of bearing grease and oil.
What is in this article?:
- Predicting lube life
- Oxidation life of mineral oils under ideal conditions
- Speed reduction factor kffor reducing grease life
Speed reduction factor kf
for reducing grease life
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:
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