What defines plain bearings in contrast to rolling-element bearings?

• A. They use sliding contact between surfaces
• B. They use rolling elements
• C. They are always maintenance-free
• D. They require no lubrication

Answer: (A)

At the heart of what defines plain bearings is sliding contact between bearing surfaces. No separate rolling elements—such as balls or rollers—sit between the shaft and the bearing. The load is carried by the shear of the lubricant film or direct contact between a sleeve/bearing surface and the journal. This means performance depends a lot on proper lubrication, surface finish, and alignment. Plain bearings are typically sleeves or bushings made from materials like bronze, bronze-graphite, or polymers, and they’re valued for simplicity, compactness, and the ability to handle large loads at moderate speeds, but friction and wear can be higher without good lubrication.

Rolling-element bearings, by contrast, use balls or rollers to create rolling contact, which reduces friction and allows higher speeds with less wear under the same loads. So the defining feature here is sliding contact between surfaces, not rolling elements.

The other statements don’t fit: plain bearings aren’t inherently maintenance-free, and they generally do require lubrication (they may use self-lubricating materials in some cases, but that doesn’t make lubrication unnecessary). They also don’t use rolling elements.