In the world of rotating machinery, bearing life is a critical factor in ensuring equipment reliability and minimizing downtime. One of the most widely used measures for predicting bearing life is L10 life, also called the basic rating life.

L10 life is defined as the number of revolutions or operating hours at a given constant speed that 90 percent of identical bearings will statistically complete before the first sign of fatigue occurs on the rolling surfaces. In other words, if you have 100 bearings operating under identical load and speed conditions, 90 of them are expected to reach or exceed the L10 life, while 10 may fail earlier due to fatigue.

Figure 1: An NU1036/C3 bearing catastrophic failure per lubrication system failure (Photo courtesy of Upper Occoquan Service Authority)

The L10 life formula, as given in ISO281, the rolling bearings standard established by the International Organization for Standardization, is primarily based on bearing load and bearing size, assuming ideal operating conditions. It does not explicitly account for real-world factors, such as operating temperature, contamination, lubrication quality, or mounting errors.

Why Temperature and Lubrication Quality Aren’t Considered in Basic L10 Life

The original L10 model was developed as a statistical, load-based prediction method. Its intent was to give a standard reference for comparing bearings rather than predicting exact, real-world performance. For simplicity, it assumes:

• Proper lubrication is always maintained;
• Operating temperature is within a range where material strength and lubricant properties are unaffected;
• Contamination is negligible.

However, in actual service conditions, temperature and lubrication quality can have a dramatic impact on bearing life, sometimes more than the load itself. By ignoring these factors, the L10 model can be overly optimistic when conditions are harsh or unduly conservative if conditions are well controlled.

The Role of Temperature in Bearing Life

Temperature influences bearing life in two main ways:

1. Lubricant Viscosity – As the temperature rises, oil viscosity drops, reducing the lubrication’s film thickness. This increases metal-to-metal contact, accelerating wear and fatigue.
2. Material Properties – Elevated temperatures can reduce the hardness of bearing steel, lowering its fatigue resistance. In extreme cases, heat can cause dimensional changes, internal clearances shifts, and lubricant oxidation.

Even moderate increases in temperature can shorten bearing life dramatically if they push the lubrication out of its optimal viscosity range.

The Role of Lubrication Quality in Bearing Life

Lubrication quality depends on:

• Viscosity ratio (κ) – The ratio of actual lubricant viscosity in service to the required viscosity for adequate film formation in a given temperature.
• Cleanliness – Contamination (e.g., dirt, wear particles, water) increases surface fatigue and abrasive wear.
• Lubricant degradation – Oxidation, additive depletion, and contamination over time reduce lubricant effectiveness.

Figure 2: NU1036/C3 bearing fragments / catastrophic failure per lubrication system failure (Photo courtesy of Upper Occoquan Service Authority)

Benefits of Including Temperature and Lubrication Quality

Modern bearing life models, such as the Generalized Bearing Life Model (GBLM) or ISO281 adjusted life rating method, incorporate factors for temperature and lubrication quality. The benefits of using these enhanced models include:

1. More Accurate Life Predictions – By including temperature and viscosity ratio, life calculations better reflect actual operating conditions, reducing the gap between predicted and observed life.
2. Reduced Failure Rates – Recognizing the impact of temperature and lubrication allows maintenance teams to take proactive measures, such as improving cooling, optimizing lubricant selection, or enhancing contamination control, before failures occur.
3. Increased Reliability – Improved prediction accuracy and corrective actions translate into higher mean time between failures (MTBF), reduced unplanned downtime, and greater confidence in equipment performance.
4. Optimized Maintenance Intervals – Accurate bearing life estimates help plan maintenance at the right time, neither too early (wasting resources) nor too late (risking catastrophic failure).

How to Include Temperature in L10 Bearing Life Calculations

You can factor in temperatures by using the adjusted bearing life approach from ISO281, where temperature enters through lubrication quality (i.e., viscosity at operating temperature) and, if applicable, a temperature derating of the bearing’s dynamic load rating.

Adjusted Bearing Life Formula

L₁₀ₘ = a₁ × a ISO (κ, ηc, bearing type) × (Cₜ / P) ᵖ

Where:

a₁ = Reliability factor (1.0 for 90% reliability; use a table for 95%, 99%, etc.)
a ISO = Life modification factor for lubrication / contamination (temperature affects this strongly)
κ = Viscosity ratio = ν(T) / ν₁

• oν(T) = Actual kinematic viscosity of the lubricant at operating temperature T (cSt), obtained from the oil’s viscosity–temperature curve (ISO VG grade + VI or data sheet)
• oν₁ = Required kinematic viscosity at operating speed (from ISO / original equipment manufacturer (OEM) charts based on dₘn, where dₘ = (d + b) / 2)

ηc = Contamination factor based on cleanliness class / filtration
Cₜ = Temperature-adjusted dynamic load rating (apply OEM temperature factor to C when T is high, otherwise, Cₜ = C)
P = Equivalent dynamic bearing load
p = 3 for ball bearings or 10/3 for roller bearings

Practical Steps

1. Compute speed factor dₘn using mean diameter dₘ and rpm n.
2. Get required viscosity (ν₁) from the ISO / OEM required viscosity vs. dₘn chart (bearing-type-specific).
3. Get oil viscosity at temperature (ν(T)) from the lubricant data sheet using the viscosity index (VI), the ISO viscosity grade (VG), or ASTM International’s D341 and D2270 standard practices.
4. Calculate viscosity ratio: κ = ν(T) / ν₁; ratio is typically between 0.1 and 4.
5. Pick a contamination factor (ηc) based on cleanliness and/or filtration.
6. Find a ISO from ISO / OEM curves using κ, ηc and bearing type.
7. Determine temperature derating of C, if needed; for elevated T, apply the manufacturer’s temperature factor to get Cₜ.
8. Calculate L₁₀ₘ.

Worked Example (Illustrative)

Bearing type: Deep groove ball bearing
C = 50 kN, P = 5 kN → Basic life: (C/P) ³ = 10³ = 1000 million rev
d = 90 mm, b = 110 mm → dₘ = 100 mm
Speed: n = 1500 rpm → dₘn = 150,000
From chart: Required viscosity ν₁ ≈ 10 cSt
Oil: ISO VG 46, VI ≈ 95

• At 80° C → ν(80° C) ≈ 9 cSt → κ = 0.9 → a ISO ≈ 0.9
• At 120° C → ν(120° C) ≈ 3.5 cSt → κ = 0.35 → a ISO ≈ 0.4

• 80° C: L₁₀ₘ ≈ 0.9 × 1000 = 900 million rev
• 120° C: L₁₀ₘ ≈ 0.4 × 1000 = 400 million rev

Conclusion: A higher operating temperature reduces oil viscosity, which can cut bearing life by more than half for the same load and speed.

Data Needed to Run This Calculation

• Bearing type and size (d, b)
• Dynamic load rating C
• Load P and speed n
• Lubricant grade (ν at 40° C and VI, or full ν–T table)
• Cleanliness / filtration level
• OEM temperature factor for C, if available

Conclusion

While the L10 life model remains a valuable standard for basic bearing life estimation, it is only part of the story. Real-world applications demand consideration of temperature and lubrication quality, as these factors directly influence fatigue, wear and reliability. By integrating these elements into bearing life calculations, engineers can extend service life, lower failure rates, and increase operational reliability, turning a theoretical prediction into a powerful tool for real-world performance improvement.