Why is oil cleanliness critical for generator bearings?

• A. Clean oil raises viscosity and increases friction.
• B. Oil cleanliness has no effect on bearings.
• C. Clean oil improves electrical insulation between windings.
• D. Clean oil reduces wear, carries away heat and contaminants, and supports bearing life.

Answer: (D)

Oil cleanliness is critical for generator bearings because bearings rely on a thin lubricating film to keep metal surfaces apart. When oil is dirty, particles and sludge act as abrasives, causing scoring, pitting, and accelerated wear of the raceways and rollers. Clean oil also carries away heat from the bearing area; if contaminants clog passages or hinder circulation, the bearing can overheat and fatigue sooner. Additionally, clean oil helps remove corrosive by-products and prevents deposits that degrade lubrication over time. All of this directly affects bearing life and reliability.

The other ideas don’t fit as well because clean oil isn’t about raising viscosity or increasing friction; it’s about maintaining proper lubrication and heat removal. And while oil can provide some electrical insulation, that’s not the primary benefit for bearings—the main impact of oil cleanliness in this context is reducing wear and aiding cooling to extend bearing life.