Bearing life in a gearbox is not a guess and it is not a warranty claim waiting to happen — it is a number you can calculate before the unit is ever ordered. The L10h figure tells you the number of operating hours that 90% of an identical bearing population will survive under a defined load and speed, and for an industrial gear unit it is the single most useful predictor of when a planned shutdown should be scheduled. This guide gives you the formula, the terms that actually move the answer, a worked example, and the practical limits of the method.

What L10h Actually Means

L10h is a statistical life, not a promise about one specific bearing. It says that in a large population of bearings running under exactly the same conditions, 90% will still be running at the end of that many hours and 10% will have failed. The "h" simply marks the result in hours rather than millions of revolutions.

That statistical framing matters when reading supplier data. A unit quoted at 20,000 hours L10h is not guaranteed to fail at 20,000 hours — it is the point at which the risk of fatigue failure reaches one in ten. For a plant that replaces a gearbox on a planned outage, a one-in-ten risk at the planned interval is usually acceptable, because the replacement is scheduled anyway. For a critical drive that cannot be stopped, the same number means the bearing should be replaced earlier.

The Basic Bearing Life Formula

For a single bearing under a constant load and speed, the life in hours is:

L10h = (10^6 / (60 × n)) × (C / P)^p

Where:

• n = rotational speed in r/min
• C = basic dynamic load rating of the bearing in newtons, taken from the bearing catalogue
• P = equivalent dynamic bearing load in newtons
• p = life exponent, 3 for ball bearings and 10/3 for roller bearings

The term (10^6 / (60 × n)) converts from millions of revolutions into hours. The ratio C / P is where the leverage sits: because it is raised to the third power for ball bearings and to roughly the 3.33 power for rollers, a doubling of the load factor cuts life by a factor of about eight to ten. That is why an apparently small overload — a slightly undersized gear unit running a duty it was not selected for — destroys bearing life far faster than intuition suggests.

Which Load Goes into P

The hardest part of the calculation is not the arithmetic, it is deciding what P should be. In a real gearbox drive the load is never perfectly constant: a conveyor surges when it starts loaded, a crusher sees tramp metal, a mixer sees viscosity changes as the batch heats. The standard approach is to reduce the duty cycle to a single equivalent dynamic load using the load spectrum:

P = [ Σ (P_i^p × q_i) ]^(1/p)

Where each P_i is a load level and q_i is the fraction of the total running time spent at that level. In practice most selections compress the spectrum to three or four representative blocks — nominal running, full-load running, start-up, and occasional peak — which is accurate enough for a comparison of candidate frame sizes.

Treat the table as orientation for comparing candidate sizes. The load factor that matters is the one derived from your own duty cycle, not a generic figure copied from a table.

Worked Example: A 15 kW Conveyor Drive

Take a helical gear unit on a belt conveyor. The motor is 15 kW at 1,470 r/min, the required output speed is 60 r/min, so the total ratio is about 24.5:1 and the output torque at nominal load is roughly 2,390 Nm. The output shaft is carried by a roller bearing with a dynamic load rating C of 180,000 N, and the bearing sees an equivalent radial load P of 32,000 N at nominal running.

Substituting for a roller bearing, p = 10/3:

• C / P = 180,000 / 32,000 = 5.625
• 5.625^3.333 ≈ 341
• 10^6 / (60 × 60) = 278 hours per million revolutions
• L10h ≈ 278 × 341 ≈ 94,800 hours

Now apply the realistic duty. The conveyor starts loaded twice an hour, and during those starts the load factor jumps to 2.0 for about 1% of running time. Blending the spectrum with the formula above brings the equivalent load to about 33,500 N, and the life recomputes to roughly 82,000 hours — still comfortably beyond a 50,000-hour design target, so the frame size is sound.

The instructive part is what happens if the same unit is specified at 40 r/min output instead. Speed enters the first bracket linearly, so dropping from 60 to 40 r/min raises the hours per million revolutions by a factor of 1.5, and life improves proportionally — slow drives are kind to bearings for the same torque. Conversely, running the drive 20% faster than intended while keeping the same output torque cuts bearing life to roughly 70% of the design value.

Selection Factors That Sit Beside the Calculation

The L10h figure covers rolling-contact fatigue only. A real gearbox selection also has to clear the factors that the formula does not model:

• Service factor. The application service factor multiplies the transmitted power before the gear unit is chosen, covering shock, starting frequency and running hours per day. A unit selected on nominal power with no service factor is selected for a duty that does not exist.
• Thermal rating. A gear unit can be mechanically adequate but thermally limited — the oil runs too hot at continuous full load, and viscosity drops until the gears and bearings stop being lubricated as designed. Thermal rating is quoted separately from mechanical rating and must be checked against ambient temperature.
• Gear tooth life. Bearing life and tooth flank life are separate calculations. A unit can have ample bearing life and still pit its gear flanks if the surface durability limit is exceeded.
• Lubrication and contamination. L10h assumes clean oil at the correct viscosity. Water ingress, wear debris and extended oil change intervals shorten real life well below the calculated figure, which is why an oil analysis programme pays for itself on any drive that matters.

You can run the torque and speed side of a selection directly on the online selection system and compare the resulting frame against the bearing data sheet.

Translating L10h into a Maintenance Interval

A calculated L10h of 80,000 hours does not mean 80,000 hours of neglect. The standard practice is to derive an inspection interval from the figure and then shorten it for operating environment. For a typical industrial drive:

• Inspect and sample oil at roughly L10h ÷ 40 intervals, so about 2,000 hours for the example above — which usually lands on a quarterly or half-yearly schedule.
• Vibration trend continuously or monthly, because bearing defect frequencies appear in the spectrum before the surface degrades enough to change performance.
• Plan bearing replacement at around L10h ÷ 3 for critical drives that cannot tolerate an unplanned stop, and later for drives with redundancy.

The L10h number is the input to those intervals, not a replacement for them. Treat it as the outer envelope of the maintenance plan.

FAQ: Gearbox Service Life and Bearing Life

What is a good L10h value for an industrial gearbox?
For general industrial duty, a design target of 20,000 to 50,000 hours is common, and heavy continuous drives such as mill or kiln gear units are often specified above 50,000 hours. The right target depends on running hours per day, whether the drive is critical, and whether a planned outage is available for replacement. There is no universal pass mark, but a figure below roughly 10,000 hours on a continuously loaded drive is worth challenging.

Does a higher service factor automatically give longer bearing life?
Indirectly. The service factor drives you to select a larger frame, and a larger frame brings bigger bearings and therefore a higher dynamic load rating C. The life improvement comes from the larger bearings rather than from the service factor number itself, and it only materialises if the duty cycle used in the calculation is honest.

Why did the calculated life come out lower than the catalogue figure?
Catalogue life figures are usually quoted at a nominal load and a specific speed. Once your actual output speed and your actual equivalent load — including start-up and shock blocks — are substituted, the result normally drops. A gap of two to four times between catalogue and calculated life is common and is not a sign of an error.

Can I extend bearing life by changing the oil?
You can protect the calculated life, but you cannot exceed it by lubrication alone. Synthetic oil with a higher viscosity index and better oxidation stability holds its film at higher temperatures, which keeps the drive at its design life rather than letting contamination and thermal degradation push it below. This is why oil selection and oil analysis are part of the same decision as the frame size, and the M-series industrial gear reducers page lists the frame sizes and their catalogue data for comparison.

How do I confirm the life calculation before ordering?
Send the driven machine, the motor power and speed, the required output speed, the daily running hours and the shock characteristics of the duty. The selection is confirmed against the equivalent load spectrum and returned with the rated torque, the thermal rating and the expected bearing life, so the number is on the record before the unit ships.

Next Step

Bearing life is a selection input, not an afterthought. Run your duty point through the online selection system to get a candidate frame, then ask for the bearing life calculation against your actual load spectrum — including the start-up blocks that most specifications quietly omit. Get the number before the order, and the maintenance plan writes itself.