EngineerBenchCalculating basic bearing rating life
Basic rating life compares a bearing's dynamic load rating C with its equivalent dynamic load P. The ratio is raised to a power: three for ball bearings and ten-thirds for roller bearings. The result describes a statistical fatigue-life basis, not the predicted failure time of one particular bearing.
Equivalent load is a separate calculation
Use the manufacturer's method to combine radial and axial loads into the equivalent load required here. Do not simply add them. The selected dynamic rating must match the bearing and its catalog units. RPM converts a life in revolutions into operating hours.
Life depends on more than load rating
Lubrication, contamination, alignment, fits, temperature, and installation affect service life. A reliability adjustment is not a correction for all of those conditions. Use the inverse mode to estimate a required dynamic rating, then check candidate bearings against the full application requirements.
Converting revolutions into operating hours
For a ball bearing with a dynamic rating ten times its equivalent dynamic load, the basic relationship gives 1,000 million revolutions. At 1,000 RPM, that corresponds to about 16,667 operating hours before any separately entered modifier. Speed changes the conversion to hours, while the load ratio governs the basic life in revolutions. Neither number is a scheduled replacement date by itself.
Because the load ratio is raised to a power, using a peak load, average load, or equivalent duty-cycle load can produce very different estimates. The correct equivalent load must come from an appropriate method for the operating cycle and bearing type. Avoid combining a rating from one manufacturer's convention with an unexplained load value from another calculation. For the required-rating mode, review the resulting candidate's speed, static capacity, lubrication, and mounting conditions as well as its dynamic rating. A longer calculated fatigue life does not rule out earlier failure from contamination, inadequate lubrication, corrosion, misalignment, or installation damage.
Formula
L10 = (C/P)^p million revolutions; p = 3 for balls or 10/3 for rollers. Adjusted hours = L10 × 10⁶/(60 RPM) × a₁ × modifier.