What is Rolling Contact Fatigue in Bearings?April 30, 2019
Rolling point fatigue, as the term implies, occurs as a consequence of point contact stress. With these focal points rubbing together, material wear is inevitable. The hardness-to-malleability coefficient of the bearing alloy counteracts the effect, at least as much as is possible, but there comes a moment when those stress-focusing contact regions can no longer soak up this aggressive rubbing. What comes next will be unpleasant, to say the least.
Point-Contact Subtractive Fatigue
At low velocities, and lower load factors, the rolling elements shift finitely against one another. There’s very little fatigue to worry about as yet, for the strengthened alloy uses its polished finish as a staunch surface shield. That microcrystalline finish, milled to perfection, stays shiny and smooth. For the next phase of operation, picture a heavier application force. The load is heavier, thrust forces are in play, and higher velocities are throwing the bearing parts outwards. As the stronger forces hit hard, the contact stress points rub against one another. Every ball bearing, rubbing against its neighbour or pressing down against a bearing raceway, experiences the effect. Materials are removed from the fine-grained alloy surfaces now, and it’s only a matter of time before cracks propagate between the fatigue zones.
Assessing the Cyclical Stress Factors
Progressive by nature, Rolling Contact Fatigue (RCF) will continue as long as a cyclical fault mechanism exists. A heavy load, one that’s not consistently applied, will worsen the problem. Other causative factors include faulty or substandard rolling element cage designs, the presence of brinelling contaminants, or the selection of a substandard device alloy. For that latter issue, RCF analysis techniques are yielding superior products, thanks to a finer focus on heat treatment procedures. With stronger martensitic heat treatment approaches applied, the material surface resists the micro-pitting effect that propagates when the stress becomes entirely unmanageable. As for the contaminants, mechanically sealed bearings don’t allow such foreign particles entry.
There are a number of corrective moves that can be actioned if Rolling Contact Fatigue becomes a major challenge. First off, it’s better to stop the surface pitting before it exacts its device debilitating cost. Seals and heat treatment technologies are options, as are the newer, slipperier lubricating agents that forestall this abrasive consequence. Better yet, however, an installed bearing should be designed to better distribute those point stresses. Sometimes, just by replacing smaller ball bearings with larger balls or cylinders, the fatigue is neutralized. Otherwise, it’s back to the drawing board, where the stress factors can be dampened and managed, the bearing surface areas broadened, and the fatigue-induced pitting counteracted.
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