What are the common failure modes in gears and how do you prevent them?

Bending fatigue at tooth root is prevented by adequate module and fillet radius. Pitting (surface fatigue) is prevented by proper hardness and lubrication. Scoring/scuffing occurs due to lubricant film breakdown at high speeds/loads - use EP additives. Wear is minimized by adequate hardness and filtration. Plastic flow occurs in soft gears under overload. Design involves Lewis equation for bending strength and Buckingham equation for surface durability, with appropriate factors of safety.