ISO 281 guide

Equivalent Dynamic Bearing Load Calculation

Combine radial and axial loads into the equivalent dynamic load used by the ISO 281 rating-life equation.

Prepared by BearingSolve7 min readUpdated 13 July 2026

Why an equivalent load is needed

The ISO 281 life equation uses one load value, P. A bearing may instead carry simultaneous radial and axial loads. The equivalent dynamic bearing load is a constant hypothetical load that would produce the same rating life as the actual combined loading under the conditions covered by the method.

Combined radial and axial load

Select the catalogue condition before calculating P. A common form for radial bearings is shown below, but it is not universal: bearing type, contact angle, internal design, and which ring rotates relative to the load can change the applicable equation and factors.

P = {\begin{cases} F_{r} & when \frac{F_{a}}{F_{r}} \leq e \\ X F_{r} + Y F_{a} & when \frac{F_{a}}{F_{r}} > e \end{cases}

common catalogue form for a radial bearing—not a universal equation

Equivalent dynamic load variables
Symbol	Meaning
`F_r`	Applied radial bearing load
`F_a`	Applied axial bearing load
`X`	Radial load factor from the bearing data
`Y`	Axial load factor from the bearing data

The values of X, Y, and e depend on bearing type, geometry, contact angle, and sometimes load level. Always use the complete equation, conditions, and factors from the manufacturer data for the selected bearing.

Worked example

Suppose the applicable catalogue row gives X = 0.56 and Y = 1.63 for a bearing carrying F_r = 8 kN and F_a = 3 kN.

Radial contribution: 0.56 × 8 = 4.48 kN
Axial contribution: 1.63 × 3 = 4.89 kN
Equivalent load: P = 4.48 + 4.89 = 9.37 kN

This example demonstrates the arithmetic only. Before using these factors, confirm that the catalogue's load-ratio condition, bearing arrangement, and direction of axial load all apply.

Variable load and speed

If operating conditions change over time, first determine the equivalent load for each duty segment. A weighted equivalent load can then be established using the life exponent and the number of revolutions accumulated in each segment. A simple arithmetic average of the loads is generally incorrect because bearing life changes nonlinearly with load.

Common mistake

Do not add radial and axial load directly. Their contributions depend on the bearing-specific X and Y factors.

Standards and further reading

This educational guide does not reproduce the standards. Its equations and examples have been checked against the cited public references, but the guide has not been independently certified or reviewed for your application. Use the current standard, manufacturer data, and an appropriate engineering review for final bearing selection.

Apply the method

Run a complete bearing calculation

Model bearing geometry, loading, lubrication, life, load distribution, and contact stresses in one workflow.

Open BearingSolve