A standard bearing of identical type and size will have the L10 fatigue life of around double its equivalent manufactured 50 years ago, and this is reflected in the increased dynamic load ratings published by bearing makers. This is because contemporary rolling bearings produced by reputable manufacturers are made from clean alloy steels with applied processing technologies which result in a much lower incidence of premature surface contact fatigue.
However, in certain operating situations, industry is still experiencing premature fatigue failures of bearings well before their design life is achieved, according to Ross Lee, technical manager of Strategic Partnerships —Bearings for Motion Australia.
Overwhelmingly, these bearing failures are due to what is known as “surface-initiated fatigue,” he says. “As this naming suggests, the fatigue and subsequent cracking or flaking of bearing rings and rollers in contact originates at the surface, and results from one or a combination of operating conditions.”
Firstly, the presence of hard particle debris circulating through the bearing. “This type of contamination causes indentions with crater like edges, or high points, that form rolling contact stress raisers, and so the nucleus of fatigue flaking,” says Ross.
Another common cause of surface fatigue, he explains, is improper or diminished lubrication. “In this instance, the applied oil viscosity at the bearing operating temperature has insufficient surface separating film strength to form effective lubrication condition,” says Ross. “The result is metal to metal contact at the high points of rollers and raceway surfaces, and fine surface layer fatigue develops in the form of peeling damage.”
“Furthermore, if there is a dispersal of fine contaminants in the lubricant, that will actually wear the rolling contact surfaces,” he elaborates. “This will act much the same as a lapping agent, leading to loss of the bearing surface geometry, increased clearance, and ultimately failure.”
Lastly— heavy or excessive loading conditions will not only increase the cyclic stresses in bearings and shorten fatigue life, but can also exceed the capability of the lubricant, explains Ross. In the instance of this combination of factors, premature surface-initiated fatigue failure can sometimes be observed.
NTN Corporation has been observing these application issues and how they have been affecting their customer base in major heavy industrial segments. From this, they put together an R&D team including Design Engineers, Metallurgist and Tribologists, who set about addressing these life impacting factors one by one.
The summit of their findings on surface technology is evident in the resulting EA Long-Life bearings, which were developed with a two-stage heat treatment process that is comprised of conventional case carburising and compound carbon nitriding.
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