Static load rating C₀ (basic static load rating) is the equivalent static load at which the total permanent contact deformation between rolling elements and raceways reaches approximately 0.0001 times the rolling element diameter.
This is the deformation threshold that does not affect normal bearing operation, as defined by ISO 76. C₀ applies to bearings that are stationary or rotating very slowly (below 10 rpm) — where there is no hydrodynamic effect from the lubricant. Loads exceeding C₀ do not cause immediate failure, but they create indentations on the raceway (brinelling) that generate vibration and noise when the bearing resumes operation. See the full C and C₀ calculation methodology in bearing life calculation guide and browse spherical roller bearings for high-C₀ applications.
The Difference Between C and C₀
Many engineers confuse these two parameters. C (dynamic load rating) determines fatigue life when the bearing rotates — it relates to cumulative revolutions. C₀ (static load rating) determines resistance to plastic deformation when the bearing is stationary or subject to shock loads — it is independent of revolution count.
The C₀/C ratio depends on bearing type. Deep groove ball bearings: C₀ ≈ 0.6–0.8 × C (6308: C = 32.5 kN, C₀ = 21.2 kN → ratio 0.65). Tapered roller bearings: C₀ ≈ 1.0–1.2 × C (32220: C = 290 kN, C₀ = 400 kN → ratio 1.38). Spherical roller bearings: similar C₀/C ratio to tapered rollers. A higher ratio means better resistance to static deformation relative to fatigue capacity.
Static Safety Factor s₀
Engineers use C₀ through the static safety factor s₀ = C₀/P₀, where P₀ is the equivalent static load. Minimum s₀ values by operating condition:
| Condition | Min s₀ (ball bearings) | Min s₀ (roller bearings) |
|---|---|---|
| Smooth load, no shock | 0.5 | 1.0 |
| Normal load | 1.0 | 1.5 |
| Light shock | 1.5 | 2.5 |
| Heavy shock (crushers, hammers) | 2.0–3.0 | 3.0–5.0 |
Worked example: a 5-tonne crane, bearing housing under static load P₀ = 50 kN with light shock. Required s₀ = 1.5 → need C₀ ≥ 75 kN. Bearing 6208 has C₀ = 25.5 kN — insufficient. Select 22209 EK (C₀ = 96 kN) — achieves s₀ = 1.92.
Practical Example: Bearing 32220
The tapered roller bearing 32220 (d = 100, D = 180, B = 49 mm, C = 290 kN, C₀ = 400 kN) is used on the axle of a 12-tonne truck. Maximum static load when the vehicle is parked on a 15° slope and jacked: estimated P₀ = 180 kN. Static safety factor s₀ = 400/180 = 2.22 — adequate for shock loading during jack release. The dynamic load during normal driving, P = 80 kN, gives a fatigue life calculation consistent with the design intent using C = 290 kN.