Bearing L10 rating life (ISO 281 / ABMA 9) and combined system life, checked against the API 610 §6.10.1.11 25,000 / 16,000 h requirement
Understand what L10 really means, the (C/P)p law, system-life combination, and the API 610 25,000 / 16,000 h requirement
L10 is the life that 90% of a population of identical bearings will reach or exceed before fatigue spalling — i.e. only 10% are expected to fail earlier. It is not an average or guaranteed life.
C = basic dynamic load rating (bearing maker); P = dynamic equivalent load (radial + axial analysis); N = shaft rpm. Source: ISO 281 / ABMA 9.
API 610 (12th ed.) §6.10.1.11 sets the normative minimum bearing system rating life:
Each bearing's L10h is computed per ABMA 9 (§6.10.1.10), then combined into system life. Because the combination is always shorter than the weakest bearing, two passing bearings can still fail the system requirement.
L10 is the basic rating life — the life that 90% of a population of identical bearings will reach or exceed under a given load before the first signs of fatigue. Per ISO 281 / ABMA 9 it equals (C/P)p million revolutions, where C is the basic dynamic load rating, P is the dynamic equivalent load, and p is 3 for ball bearings or 10/3 for roller bearings. Dividing by 60·N (rpm) and multiplying by one million converts it to hours, L10h.
API 610 §6.10.1.11 requires a minimum bearing system rating life of 25,000 hours of continuous operation at rated conditions, and at least 16,000 hours at maximum radial and axial loads with rated speed. Each bearing's L10h is computed per ABMA 9, then combined into a system life that must meet these thresholds.
A pump rotor runs on two or more bearings, and failure of any one takes the machine down. API 610 §6.10.1.11 combines the individual L10h values as L10h,system = [Σ(1/L10hi)1.5]−2/3, which is always shorter than the weakest single bearing. Two identical bearings each rated 37,037 h, for example, give a system life of about 23,300 h — below the 25,000 h requirement even though each bearing alone passes.