Load and speed cases
| Case | Equivalent load P lbf | Speed n (rpm) | Share of time (%) | Static load P0 lbf |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
For one steady condition fill case 1 only and set its time share to 100. P is the equivalent dynamic load P = X·Fr + Y·Fa from the catalog. Leave unused cases at 0. The static load P0 is optional and only used for the static safety factor.
Rating life L10h
--Rating life in revolutions
-million revolutions (adjusted)Required C for target life
--Results
| Life exponent p | - |
| Equivalent load P (all cases) | - |
| Mean speed | - |
| Basic life L10 (90 %) | - |
| Basic life L10h (90 %) | - |
| Reliability factor a1 | - |
| Adjusted life Lnm | - |
| Adjusted life Lnmh | - |
| Load ratio C/P | - |
| Static safety factor C0/P0 | - |
Checks
Life against load (this bearing and speed)
How the calculator works
The calculator uses the basic rating life of ISO 281, which comes from Lundberg and Palmgren's fatigue theory of rolling contact. Life is the number of revolutions that 90 % of a large group of identical bearings complete before the first fatigue spall appears. It assumes the load is steady and constant on the bearing, the bearing is properly mounted, lubricated and kept clean, and the only failure mode is rolling-contact fatigue.
- Basic rating life. L10 = (C/P)p in million revolutions, with p = 3 for ball bearings and p = 10/3 for roller bearings. In hours, L10h = 106/(60·n) · L10, with n in rpm.
- Varying load and speed. For up to three operating cases with equivalent load Pi, speed ni and time share qi, the calculator applies the Palmgren-Miner linear damage rule. The mean speed is nm = Σ(qi·ni)/Σqi and the equivalent load is P = [Σ(qi·ni·Pip)/Σ(qi·ni)]1/p. The life then follows from P and nm. A case at zero speed contributes no revolutions and no damage here.
- Reliability and modification. The adjusted life is Lnm = a1·aISO·L10. The reliability factor a1 is 1 for 90 %, 0.64 for 95 %, 0.55 for 96 %, 0.47 for 97 %, 0.37 for 98 % and 0.25 for 99 %, the values given in ISO 281. aISO depends on the lubricant viscosity ratio, contamination and the fatigue load limit; read it from the bearing manufacturer's calculator and enter it, or leave it at 1 for the basic rating life.
- Required C. With a target life Ltarget in hours the dynamic rating you need is C = P·(60·nm·Ltarget/(106·a1·aISO))1/p.
- Static check. The static safety factor is s0 = C0/P0. Typical minimum values depend on the application, noise requirement and rotating or non-rotating bearing; the calculator warns below 1 and flags values below 2 for the largest static load.
The equivalent load P = X·Fr + Y·Fa uses factors X and Y that depend on the bearing type and the ratio Fa/Fr; they are bearing-specific and published in each manufacturer's catalog, so you enter the resulting P. For a radial bearing with no axial load, P = Fr. Ratings C and C0 are listed in the catalog for each part number (for example 14.8 kN and 7.8 kN for a common 6205 deep groove ball bearing).
Worked example: 6205 ball bearing on a gearbox shaft
A deep groove ball bearing with C = 14.8 kN carries a steady equivalent load P = 3.0 kN at 1500 rpm. C/P = 4.933, so L10 = 4.9333 = 120.1 million revolutions. In hours, L10h = 106/(60·1500) · 120.1 = 1,334 h. At 99 % reliability (a1 = 0.25) the life falls to 334 h. To reach 20,000 h at 90 % reliability the bearing needs C = 3.0 · (60·1500·20,000/106)1/3 = 36.5 kN, which points to a larger bearing, for example a 6308 or 6309 size part, or a lower load.
Duty cycle example: the same bearing runs 60 % of the time at 2 kN and 1500 rpm and 40 % at 5 kN and 600 rpm. Mean speed is 0.6·1500 + 0.4·600 = 1,140 rpm. The equivalent load is [(0.6·1500·23 + 0.4·600·53)/(0.6·1500 + 0.4·600)]1/3 = [37,200/1,140]1/3 = 3.196 kN, so L10h = 106/(60·1140) · (14.8/3.196)3 = 1,452 h.
Questions
How do you calculate bearing life?
The ISO 281 basic rating life is L10 = (C/P)^p million revolutions, where C is the basic dynamic load rating from the bearing catalog, P the equivalent dynamic bearing load and p the life exponent: 3 for ball bearings and 10/3 for roller bearings. In hours, L10h = 1,000,000 / (60·n) · (C/P)^p with n the speed in rpm. Example: a 6205 deep groove ball bearing (C = 14.8 kN) at P = 3 kN and 1500 rpm has L10 = (14.8/3)³ = 120 million revolutions, or 1,334 hours.
What does L10 life mean?
L10 is the life that 90 % of a large group of identical bearings, run under the same conditions, reach or exceed before the first sign of rolling-contact fatigue. It is a statistical life, not a guarantee for a single bearing: 10 % of the bearings fail earlier, and the median life is roughly four to five times the L10 life. For a higher reliability, such as 99 %, multiply L10 by the reliability factor a1 (0.25 at 99 %).
What is equivalent dynamic bearing load P?
P is the single constant radial load (for radial bearings) that would give the same life as the real combination of radial load Fr and axial load Fa. It is P = X·Fr + Y·Fa, where the factors X and Y depend on the bearing type and on the ratio Fa/Fr. Take X and Y from the bearing catalog. With purely radial load on a radial bearing, P = Fr. When the load or speed varies, use the duty cycle table: the calculator combines the load cases into one equivalent load.
How do I find the required dynamic load rating C for a target life?
Rearrange the life equation: C = P · (60·n·L10h / 1,000,000)^(1/p). Enter the target life in hours and the calculator returns the C you need, so you can pick a catalog bearing with a rating at or above it. Example: P = 3 kN at 1500 rpm for 20,000 hours with a ball bearing needs 60·1500·20,000/106 = 1,800 million revolutions, so C = 3 · 1800^(1/3) = 3 · 12.16 = 36.5 kN.
What is a good design life for a bearing?
It depends on the machine. Common planning figures are about 8,000 hours for intermittent-duty machines, 20,000 hours for machines running eight hours a day and 40,000 to 60,000 hours or more for continuous-duty industrial machines such as pumps and fans. Use the figure your customer or your own duty specification gives, and check with the bearing manufacturer for critical applications.
What does this calculator leave out?
It uses the basic rating life: the life modification factor a_ISO of ISO 281, which accounts for lubrication, contamination and the fatigue load limit, is entered by you (default 1). It does not choose X and Y factors, does not model preload, misalignment, high temperature, very low speed, or failure modes other than rolling-contact fatigue such as wear, corrosion or smearing. For those, use the bearing manufacturer's calculation.
Disclaimer
This calculator is an engineering aid based on the basic rating life method of ISO 281. It does not replace the bearing manufacturer's selection software or a design check to the relevant standard; you remain responsible for the design, and critical or safety-related designs must be verified independently. Nothing you enter leaves your browser except through a share link you copy yourself.