What is common between ball bearings and solid bearings?

• A. Both need lubricant
• B. Both require cooling
• C. Both are magnetic
• D. Both are gear-driven

Answer: (A)

Lubrication is the shared requirement. Bearings are meant to let rotating parts move with minimal resistance and wear, and the way they achieve that depends on the type. In ball bearings, the balls roll between inner and outer races, which greatly reduces friction, but a lubricant still forms a protective film that carries heat away, prevents corrosion, and keeps metal surfaces from welding together under load. In solid (plain) bearings, surfaces slide against each other, so lubrication is even more crucial to create a separating layer that minimizes wear and heating; many systems use oil or grease, though some high-temperature or dry-operation designs employ solid lubricants. The other statements aren’t universal traits: cooling isn’t required for every bearing type, magnetic bearings are a specialized category, and bearings themselves aren’t inherently gear-driven.