What is bearing preload and why is it applied?

• A. Preload is an imposed axial load to remove clearance, improve rigidity and concentricity, and extend life.
• B. Preload is an axial misalignment adjustment.
• C. Preload is an adhesive film applied to bearings.
• D. Preload is a radial clearance adjustment.

Answer: (A)

Bearing preload means applying an intentional axial load to a bearing to remove internal clearance between the raceways and rolling elements. This eliminates play, which makes the bearing stiffer and helps keep the shaft and components aligned more consistently.

Why it’s applied: the axial preload increases rigidity and concentricity, so loads are transmitted more evenly across the rolling elements. That reduces micro-motions, improves positioning accuracy, and can extend bearing life by minimizing wear and slip, especially in high-speed or high-load situations like machine tool spindles or precision assemblies.

How it’s done: preload is introduced as an axial force, typically by tightening the bearing or housing so the bearing elements are pressed together, or by using spacers, springs, or specially preloaded bearing designs. It is not an adjustment for misalignment, not an adhesive film, and not a radial clearance adjustment.

Note: too much preload raises friction and temperature and can shorten life, so the level is chosen within the bearing manufacturer's recommended range.