How to Calculate L10 Bearing Life Step by Step: What You Can Compute and What You Must Get from the Supplier

Decide whether radial load alone is enough for P

P is the load term in the life equation, so if the bearing sees axial load and you enter Fr alone, the calculated life comes out too high.

The conversion is P = X·Fr + Y·Fa, where X and Y depend on the bearing type and its geometry and are published by standards or by the manufacturer^9,18.

You cannot take those factors from this article; ask the supplier for the X and Y values that apply to your candidate bearing and your Fa/Fr ratio before you trust any life figure. Whether you need them at all depends on the load case.

A Deep Groove Ball Bearings can carry axial load in both directions, but one supplier states its axial capacity typically does not exceed about 20–30% of the static radial rating, and higher axial capacity calls for an angular contact design⁶. Treat that percentage as one supplier's observation, not a rule you can design to.

If your axial component is a small fraction of the radial load, the radial-only calculation may be adequate for a first pass; if it is not, the X and Y route is mandatory.

Fix the exponent and the base formula

The basic rating life is L10 = (C/P)^p × 10^6 revolutions, where C is the basic dynamic load rating, P is the equivalent dynamic load, and p is 3 for ball bearings or 10/3 for roller bearings^8–10,21.

Get the exponent wrong and every number downstream is wrong, so confirm the bearing family before you start: a roller bearing uses 10/3, not 3. The exponent is also the reason load changes matter so much.

Because life varies as the load ratio raised to that power, a small increase in applied load produces a large reduction in calculated life — doubling the load on a ball bearing cuts life to roughly one-eighth⁸. Roller bearings, with the higher exponent, are more sensitive still^1,9,21.

That is why a modest upsizing of the bearing, which raises C, can be cheaper than accepting a marginal life figure. One caution on C itself: catalogue dynamic ratings for tapered roller bearings are not always directly comparable between manufacturers because they may be calculated to different standards²⁹.

If you are comparing candidates from more than one maker, ask each to state the standard behind the rating.

Convert revolutions to hours at the application's speed

L10 comes out in revolutions; hours only exist once you fix a speed. Divide the revolution figure by the revolutions per hour, which is RPM × 60¹⁰.

The arithmetic shape is simple: with C = 10,000 N, P = 2,000 N and p = 3, C/P = 5 and (C/P)^3 = 125, giving L10 = 125 × 10^6 revolutions; at 1,000 RPM that is 125,000,000 ÷ 60,000 = about 2,083 hours^9,10.

That example is a generic illustration, not a motor, pump or conveyor case — no source in this set works one of those through with real loads. What you compare the result against depends on the duty.

One source puts typical target L10 life at 20,000–50,000 hours for most industrial equipment, and 80,000–100,000+ hours for critical continuously running pumps, fans and compressors²¹.

Treat those bands as a starting reference from a single source, not a specification, and set your own target from the maintenance interval and the cost of an unplanned stop.

A conveyor that runs one shift a day and a pump that runs continuously do not deserve the same hour target even if the bearing is identical.

Apply the reliability and operating-condition adjustments

The basic L10 figure is a 90% reliability number. If your application needs better than that, the modified rating life is Lna = a1 × a2 × a3 × L10, where a1 adjusts for reliability, a2 for material and a3 for operating conditions including lubrication and cleanliness¹¹.

The reliability factor is the one with published values: a1 = 1.0 at 90% reliability, 0.62 at 95% and 0.21 at 99%²⁰.

Moving a pump from 90% to 99% reliability therefore cuts the calculated life to about a fifth, which is the trade you are making when you write a reliability target into the specification.

The lubrication and contamination side enters through a3, or through the combined ISO 281 modified-life approach that accounts for lubrication condition, contamination and reliability targets together^11,21. No source in this set gives a_ISO values or a worked modified-life calculation with numbers, so you cannot complete this step from published data alone.

Ask the supplier to run the modified calculation for your lubrication and cleanliness conditions and to state which factor structure they used.

Set internal clearance and fit for the duty

Clearance is not part of the L10 arithmetic, but it decides whether the life you calculated is the life you get.

Internal clearance affects noise, vibration, heat stress, deflection, load distribution and fatigue life, and higher radial play is desirable when the inner ring or shaft runs hotter than the outer ring, because the play closes as the shaft expands⁷.

In a motor, pump or conveyor the inner ring normally runs hotter, which is why C3 clearance is commonly recommended for all three duties on the grounds of high speed, heat generation and thermal expansion²². Fit works the same way from the other direction.

A high-speed, high-load duty needs a more secure interference fit, while thermal expansion may push you toward a clearance or transition fit²³. The two choices interact: a tight fit on a hot shaft can consume the internal clearance you selected, and a loose fit lets the ring creep.

Before you release the enquiry, confirm the clearance class and the shaft and housing tolerances together against the actual temperature difference between inner and outer ring, rather than treating them as separate decisions.

Check whether C0 governs instead of dynamic life

The dynamic life formula assumes the bearing rotates under load.

For a bearing that is stationary or moving very slowly under load — a shaft that indexes and dwells, or a holding position — the static rating C0 and the static safety check take over, and shock loads, slow oscillation or long dwell times make that check as important as the dynamic one¹².

A conveyor drive with frequent starts and shock loading is the case to watch here. What the sources do not give you is the static safety factor formula or an acceptable minimum for any application, so you cannot complete this check from this article.

Ask the supplier for the static equivalent load method and the minimum safety factor they apply to your duty.

Data to demand from the supplier before you trust the figure

The calculation chain above is complete only if you have inputs the sources do not publish. Ask for these before you release the enquiry, and treat any life figure quoted without them as provisional.

• ✓X and Y factors for your candidate bearing at your Fa/Fr ratio, with the standard they are taken from
• ✓The standard behind the catalogue C rating, especially for tapered roller bearings where ratings may not be comparable between makers
• ✓C0 and the static safety factor method and minimum for your duty
• ✓a_ISO or equivalent lubrication and contamination factors for your actual lubrication and cleanliness conditions
• ✓Confirmation that the life figure assumes clean lubrication, correct mounting and normal operating conditions, and what changes if it does not
• ✓Whether the supplier's own life calculator can be run on your duty once you have a short list of candidates
• ✓DN value for the candidate bearing, since single-row bearings typically permit DN values 20–40% higher than double-row equivalents of the same bore

Where the sources disagree on the adjustment structure

The two adjustment routes give different arithmetic, so ask the supplier which one their quoted life uses before you compare figures.

What the sources do not establish

• No source provides the actual X and Y factor values needed to calculate P for specific bearing types.
• No source provides a fully worked L10 calculation for motor, pump or conveyor applications with specific loads, speed and resulting L10h.
• No source provides the s0 = C0/P0 formula or acceptable minimum values for any application.
• No source provides a_ISO values or a fully worked modified rating life calculation with numbers.
• No source provides actual catalogue data (C, C0, dimensions, speed limits) for specific candidate bearings.
• No source provides a step-by-step worked example that includes calculating P from radial and axial loads using X and Y factors.
• No source provides the conversion between L10 in revolutions and L10h in hours for variable-speed applications.