Selecting Shaft and Bearing Fits for Reliable Performance

Choosing the right fit between a shaft and a bearing is critical for system reliability. The fit determines whether the inner ring rotates with the shaft or slips, affecting load distribution and wear. Manufacturers specify recommended fits based on bearing type and application conditions, typically using ISO tolerance grades like j6, k6, or m6 for the shaft.

For a rotating shaft, the inner ring must have an interference fit to prevent fretting and rotation relative to the shaft. Common choices are k6 for normal loads and m6 for heavier loads. For a stationary housing, a slight clearance fit (e.g., H7) allows for thermal expansion and easier assembly. Always consult the bearing catalog for specific recommendations.

Load magnitude and direction also influence fit selection. Under heavy radial loads, tighter interference is needed to prevent ring creep. Additionally, consider surface finish—rougher surfaces reduce effective interference. Roundness and taper of both shaft and housing must be within specified limits to ensure uniform contact.

Practical tip: Before assembly, measure the shaft diameter and housing bore at several points. Use a micrometer and bore gauge, and compare readings to the tolerance limits. If the fit is too loose, the bearing may spin on the shaft; too tight can cause excessive preload and premature failure. When in doubt, start with the manufacturer’s standard recommendation.

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