Bearing Selection Basics: L10 Fatigue Life Explained
Selecting the correct bearing is vital to ensure structural reliability in machinery. Bearings must support combined radial and axial loads while keeping friction low.
Dynamic Load Rating and Equivalent Radial Load
Every bearing has a Dynamic Load Rating (C), which is the constant load under which the bearing can survive 1 million revolutions. When a bearing undergoes combined loading, we calculate the Equivalent Dynamic Load (P) using P = X·Fr + Y·Fa.
ISO 281 Fatigue Life Exponent
The fatigue life in millions of revolutions is L10 = (C / P)^p. For ball bearings (point contact), p = 3. For roller bearings (line contact), p = 10/3.
Formulas
Basic L10 Life
L10 = (C / P)^p — Calculates the dynamic rating life in millions of revolutions at 90% reliability.
L10h Life in Operating Hours
L10h = (10^6 × L10) / (60 × n) — Converts millions of revolutions into operating lifetime hours.
Worked Example
A deep groove ball bearing (C = 14.0 kN) operates at 1500 rpm under a pure radial load of 2.5 kN.
L10 = (14.0 / 2.5)³ = 175.6 Million Revolutions. L10h = 175,616,000 / 90,000 ≈ 1,951 hours.
Design Tips
- Ensure proper lubrication viscosity for full L10 life
- Check shaft-housing fits — too tight eliminates clearance
- Heavy thrust loads need angular contact or tapered roller bearings
Common Mistakes
- Wrong exponent — p=3.33 for rollers, not 3
- Ignoring axial thrust from helical gears or thermal growth
- L10 is statistical — 10% of bearings fail before reaching it
FAQs
What does L10 life mean?
90% of bearings survive that duration without fatigue damage. 10% may fail before.
How do contaminants impact bearing life?
Contaminants create stress concentrations that reduce actual life below L10.