What is bearing modified rating life Lnm?

ISO 281 expresses modified rating life as Lnm = a1 × aISO × L10. The subscript n is failure probability in percent, a1 corresponds to the selected reliability, and aISO must be determined from the ISO method or verified bearing-manufacturer data for the actual operating conditions.

What is the difference between L10 and Lnm?

L10 is basic rating life at 90% reliability under the conventional conditions stated by ISO 281. Lnm is modified rating life after applying reliability factor a1 and the condition-dependent aISO factor. Neither is a guarantee of actual service life.

How do I choose the reliability factor a1?

Use the ISO 281:2007 factor for the required reliability: 1.00 at 90%, 0.64 at 95%, 0.55 at 96%, 0.47 at 97%, 0.37 at 98%, and 0.25 at 99%. Higher required reliability gives a lower rating life.

What does the life exponent p mean?

The life exponent p is 3 for ball bearings and 10/3 for roller bearings. It appears in the basic rating life formula L10 = (C/P)^p and reflects the different contact geometry (point vs line contact).

How do I obtain aISO?

Do not choose aISO from a generic clean or dirty label. Determine it with the ISO 281 method or verified bearing-manufacturer software or diagrams using the actual bearing type, viscosity ratio, contamination factor, fatigue load limit and equivalent load. This calculator only applies the value supplied.

Free Engineering Tool #044

Bearing Modified Rating Life L_nm Calculator

Calculate modified rating life L_nm using ISO 281:2007 reliability factors and an a_ISO value determined for the actual bearing and operating conditions.

ISO 281:2007 Lnm Rating Life Ball & Roller

Dynamic Load Rating C (kN)

Equivalent Dynamic Load P (kN)

Speed (RPM)

Bearing Type

Reliability Factor a₁

Verified Life Modification Factor a_ISO (from ISO/manufacturer method)

Quick presets

a_ISO is not estimated here. Obtain it for the actual bearing and duty from the ISO 281 method or verified manufacturer software/diagrams using viscosity ratio, contamination, fatigue load limit and load. Generic labels such as “clean oil” are insufficient.

Results

Modified Rating Life L10m

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Basic Life L10

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L10 (million revolutions)

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L10m (million revolutions)

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C/P Ratio

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Basic Rating Life L₁₀

Per ISO 281, the basic rating life in millions of revolutions is:

C — basic dynamic load rating (kN)
P — equivalent dynamic bearing load (kN)
p — life exponent: 3 for ball bearings, 10/3 for roller bearings

Modified Rating Life L_nm

The ISO modified rating life applies the reliability factor a₁ and the condition-dependent factor a_ISO:

a₁ — life adjustment factor for reliability (1.00 for 90%, 0.25 for 99% in ISO 281:2007)
a_ISO — life modification factor for lubrication, contamination, and fatigue load limit

Conversion to Hours

Reliability Factor a₁

Reliability (%)	a₁	L_n notation
90	1.00	L_10m
95	0.64	L_5m
96	0.55	L_4m
97	0.47	L_3m
98	0.37	L_2m
99	0.25	L_1m

Practical Example

Worked example — stated rating and verified factors

Given: C = 52.7 kN, P = 10 kN, n = 1500 RPM, a₁ = 1.0, a_ISO = 1.0

L₁₀ = (52.7 / 10)³ = 146.36 million rev

L_10h = 146.36 × 10⁶ / (60 × 1500) = 1,626 hours

L_10m = 1.0 × 1.0 × 1,626 = 1,626 hours

⚠️ Scope: Modified rating life is a statistical fatigue rating, not guaranteed service life. ISO 281 does not cover wear, corrosion or electrical erosion. Values of a_ISO above 50 are not accepted here because public manufacturer guidance states that such values are not meaningful for this simplified calculation.

Bearing Modified Rating Life Calculator | ISO 281

Results

Basic Rating Life L10

Modified Rating Life Lnm

Conversion to Hours

Reliability Factor a1

Practical Example

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Support

Contact

Basic Rating Life L₁₀

Modified Rating Life L_nm

Reliability Factor a₁