Understanding Bearing Life: L10, L10h, and Service Life Calculations

Bearing Life Review Before RFQ

Bearing life is one of the most misunderstood concepts in mechanical engineering. The L10 rating life is a statistical prediction, not a guarantee. It tells you the number of revolutions (or hours) that 90% of a group of identical bearings will complete or exceed under defined operating conditions before the first signs of fatigue spalling appear.

This means:

• 90% of bearings will reach or exceed L10 life
• 10% will fail before reaching L10 life
• The L10 value is a reliability-based prediction, not a warranty period
• Actual bearing life in a specific application depends on many factors beyond the basic load rating

The Basic Rating Life (L10)

The fundamental equation from ISO 281:

L₁₀ = (C/P)^p million revolutions

Where:

• C = Basic dynamic load rating (from bearing catalog, in N or kN)
• P = Equivalent dynamic bearing load (calculated from actual loads, in N or kN)
• p = Exponent: p = 3 for ball bearings, p = 10/3 for roller bearings

For spherical roller bearings (roller bearings), the exponent of 10/3 means that load has a particularly strong effect on life:

• Double the load = life reduced to 1/10 of original
• Halve the load = life increased by approximately 10×

This non-linear relationship is why even small reductions in operating load can produce significant extensions in bearing life.

Converting to Hours (L10h)

For rotating machinery, hours are more useful than revolutions:

L₁₀h = (10⁶ / 60n) × (C/P)^p hours

Where n = speed in RPM.

Example calculation for a spherical roller bearing:

• Bearing: 22320 CCK/W33, C = 670 kN
• Radial load: Fr = 45 kN
• Axial load: Fa = 8 kN
• Speed: n = 1500 RPM

Step 1: Calculate equivalent load P: P = Fr + Y₁ × Fa (simplified; actual calculation uses X and Y factors from the catalog) P = 45 + 1.8 × 8 = 59.4 kN

Step 2: Calculate L10: L₁₀ = (670/59.4)^(10/3) = 11.28^(3.333) ≈ 3,175 million revolutions

Step 3: Convert to hours: L₁₀h = 3,175 × 10⁶ / (60 × 1500) = 35,278 hours

This means 90% of these bearings will operate for at least 35,000 hours before fatigue spalling appears — approximately 4 years of continuous operation.

Adjusted Rating Life (Lnaa)

ISO 281 provides adjustment factors for more accurate life prediction:

Lnaa = a₁ × aISO × L₁₀

Reliability Factor (a₁)

The basic L10 assumes 90% reliability. For applications requiring higher reliability:

For a critical steel mill application where a bearing failure would stop production, 97-99% reliability may be required — effectively reducing the calculated life to 25-47% of the basic L10 value.

Life Modification Factor (aISO)

The aISO factor accounts for:

• Lubrication condition (viscosity ratio κ)
• Contamination level
• Fatigue load limit of the bearing

For well-lubricated, clean applications, aISO can exceed 1.0 (extending predicted life). For poorly lubricated or contaminated applications, aISO can be as low as 0.1 (reducing life to 10% of the calculated value).

Practical Implications for Bearing Selection

For New Equipment Design

1. Calculate the required bearing life for the application
2. Select a bearing with L10h ≥ required life at the desired reliability level
3. Use the adjusted life formula (aISO) to account for actual operating conditions
4. Validate with field experience or testing

For Replacement Bearings

1. The replacement bearing should have C ≥ original bearing’s C
2. If upgrading to E1 series, the higher C value can extend life or allow downsizing
3. Always verify that the replacement clearance and cage type match the application

Typical Required L10h Values

For spherical roller bearings in continuous-process industries (steel, paper, cement), target L10h values of 60,000–100,000 hours are typical.