Ball bearings may be open, shielded, or sealed.

• A. Open
• B. Shielded
• C. Sealed
• D. Open / Shielded / Sealed

Answer: (D)

Ball bearings can be configured to protect the rolling elements and retain lubricant in different ways: open, shielded, or sealed. An open bearing has no shields or seals, so it relies on the surrounding lubricant and environment; it offers maximum speed and cooling but is most exposed to dirt and lubricant loss. Shielded bearings have shields that block larger particles while still allowing lubrication to circulate, providing a middle ground of protection with easier maintenance. Sealed bearings use seals to keep contaminants out and lubricant in, offering the strongest protection in dirty or wet conditions, though they may limit lubrication options during service. Because a bearing can legitimately be found in any of these three forms, the statement that they may be open, shielded, or sealed is the best way to describe the common variations.