Dynamic load rating C (also written C_r for radial or C_a for axial) is the constant, radial load that a group of identical bearings can carry under standard conditions to achieve the nominal rating life of L₁₀ = 1,000,000 revolutions.
This is the foundational parameter for bearing life calculation under ISO 281 — see worked calculations in bearing life calculation guide and find spherical roller bearings with the C rating your application needs. The basic formula is: L₁₀ = (C/P)^p, where P is the equivalent load, p = 3 for ball bearings and p = 10/3 for roller bearings. The C rating is measured by the manufacturer — not an estimate — under standardized ISO test conditions. Two bearings of the same size from different manufacturers may have C values that differ by up to 5–10%, depending on production process.
What the C Value Actually Means
C = 32.5 kN for a 6308 means: if 100 identical 6308 bearings run at a constant load of 32.5 kN, 90 of them will reach at least 1,000,000 revolutions (approximately 56 hours at 300 rpm) before contact fatigue spalling appears. This is a statistical threshold, not an immediate destruction limit.
In typical operating conditions, the actual load is much less than C. Example: a 15 kW motor with 5 kN radial load using a 6308 (C = 32.5 kN) gives a C/P ratio of 6.5. L₁₀ = 6.5³ × (1,000,000 / n × 60) hours. At 1,450 rpm: L₁₀ ≈ 16,400 hours — nearly two years of continuous three-shift operation.
The C value increases with: larger outer diameter D, more rolling elements, and larger rolling element diameter. Cylindrical roller bearings have 50–100% higher C than ball bearings of the same dimensions because line contact distributes load more effectively.
Comparing C Across Common Bearing Codes
Understanding C through specific comparisons is faster than reading tables.
| Bearing Code | d (mm) | D (mm) | C (kN) | C₀ (kN) | Type |
|---|---|---|---|---|---|
| 6205 | 25 | 52 | 14.8 | 7.8 | Deep groove ball |
| 6308 | 40 | 90 | 32.5 | 21.2 | Deep groove ball |
| NU308 | 40 | 90 | 55.0 | 60.0 | Cylindrical roller |
| 30207 | 35 | 72 | 56.0 | 63.0 | Tapered roller |
| 22220 EK | 100 | 180 | 365.0 | 490.0 | Spherical roller |
NU308 and 30207 have double the C of 6308 despite similar or smaller D — a clear illustration of line contact advantage over point contact.
Worked Calculation Example
At a paper mill in Phu Tho, a roll shaft requires: radial load P = 40 kN, speed n = 960 rpm, minimum life L₁₀h ≥ 20,000 hours. Calculate required C:
L₁₀ (million revolutions) = L₁₀h × n × 60 / 10⁶ = 20,000 × 960 × 60 / 10⁶ = 1,152 million revolutions
Required C = P × L₁₀^(1/3) = 40 × 1,152^(1/3) = 40 × 10.46 = 418 kN (ball bearing, p = 3)
For rollers (p = 10/3): C = 40 × 1,152^(3/10) = 40 × 6.07 = 243 kN → select 22220 EK/C3 (C = 365 kN), which provides a safety margin of 1.5×.