Interference fit (also called a press fit or shrink fit) is an assembly arrangement where the shaft diameter is larger than the bore diameter of the bearing inner ring — or the bearing outer diameter is larger than the housing bore — creating a contact pressure between the two surfaces after assembly.
This pressure holds the bearing fixed on the shaft or in the housing without bolts or pins, and prevents slipping under load. Per ISO 286-1, the degree of interference is specified through ISO tolerances: k5, k6, m5, m6 for shafts; K5, K6, M6 for housings. See the practical fit selection guide at how to choose a bearing and browse deep groove ball bearings by bore tolerance class. Selecting the wrong interference is the direct cause of two opposing problems: insufficient interference causes creep (shaft fretting), while excessive interference cracks the inner ring or eliminates radial clearance.
ISO Tolerances and Interference Levels
The fundamental rule for bearing fitting: the rotating ring must be interference-fitted; the stationary ring uses a loose or transition fit.
In the most common configuration (inner ring rotating with the shaft, radial load), the inner ring is interference-fitted to the shaft. Recommended shaft tolerances by load:
| Load Level P/C | Recommended Shaft Tolerance | Typical Interference |
|---|---|---|
| Light (P < 0.05C) | j5, j6 | 0–5 µm |
| Normal (0.05C–0.15C) | k5, k6 | 5–15 µm |
| Heavy (0.15C–0.30C) | m5, m6 | 15–25 µm |
| Very heavy (P > 0.30C) | n5, n6 | 20–35 µm |
These interference values apply to shaft diameters 40–80 mm. Larger shafts require greater absolute interference to achieve the same contact pressure.
Effect on Radial Clearance
This is the most important connection between interference fit and bearing behavior. Each 1 µm of interference on a d = 40 mm shaft reduces the inner ring radial clearance by approximately 0.7–0.8 µm (the ratio depends on material and geometry). With a k6 fit on d = 40 mm shaft (approximately 15 µm interference), clearance is reduced by roughly 12 µm.
Therefore, a CN-clearance 6308 bearing (clearance 11–25 µm) after mounting with a k6 fit may have operating clearance from −1 to +13 µm. Negative clearance is a problem — a C3 bearing is required to retain sufficient post-fit clearance.
Practical Example: 30 kW Pump Motor
Bearing 6308 C3 (d = 40, D = 90, B = 23 mm, C = 32.5 kN) on a 30 kW pump motor shaft. Radial load P = 6 kN (P/C = 0.18 → heavy load category). Shaft tolerance m6 (interference 15–28 µm). Housing tolerance H7 (clearance fit — outer ring does not rotate).
Installation procedure: heat the bearing in oil to 80°C (expansion of approximately 9 µm over 40 mm reduces the interference enough for assembly). Do not use a hammer — force must be applied through the inner ring, never through the rolling elements. After cooling, verify the bearing rotates smoothly by hand — if noticeably stiffer than normal, the interference is excessive or clearance has gone negative.
At a water pumping station in Can Tho, switching from k6 to m6 on a 40 mm shaft and from CN to C3 clearance eliminated the creep problem — previously the bearing required replacement every 3–4 months due to shaft fretting wear.