Bearing Fit Chart

A shaft fit refers to the tolerance applied to the shaft where the bearing inner ring sits. A housing fit refers to the tolerance applied to the bore in the housing where the bearing outer ring is mounted. The shaft fit typically uses lowercase ISO tolerance codes (j5, k5, m5, etc.) while housing fits use uppercase codes (H7, J7, M7, etc.).

Use an interference (press) fit on the ring that rotates relative to the load direction. For most applications with a rotating shaft, use an interference fit on the shaft (k5 or m5) and a clearance fit on the housing (H7 or J7). If the housing rotates instead, reverse the approach: clearance on the shaft and interference on the housing.

ISO tolerance codes consist of a letter and number. The letter indicates the position of the tolerance zone relative to nominal size (e.g., "h" is at nominal, "k" is above nominal for interference). The number indicates the grade or width of the tolerance band (5, 6, 7, etc., with lower numbers being tighter). For example, k5 means a tight interference fit with IT5 grade tolerance.

For normal loads, a j5 or k5 shaft fit is common. For heavy or impact loads, use m5 or n6 for a tighter interference to prevent the inner ring from creeping on the shaft. For light loads or where easy assembly is needed, g6 or h6 provides a looser fit. The housing follows a similar logic: H7 for light loads, M7 or N7 for heavy loads.

Bearing seats typically require a surface finish of Ra 0.8 micrometers (32 microinches) or better for the shaft, and Ra 1.6 micrometers (63 microinches) or better for the housing bore. The roundness should be within IT5 or IT6 tolerance grade. Poor surface finish can cause uneven load distribution and premature bearing failure.