Ball Bearing Equivalent Radial Load Factors

Select the correct bearing and condition

Equivalent dynamic load converts a combined radial and axial bearing load into a hypothetical constant radial load for a bearing-life calculation. SKF gives two branches for single-row deep-groove ball bearings: P = Fr when Fa/Fr is at or below e; otherwise P = XFr + YFa. The table reports the limiting value e and factors X and Y by Fa/C0 ratio and radial-clearance group.

Fr is the radial load, Fa is the axial load, and C0 is the basic static load rating. Use loads in the same force units. The ratio Fa/C0 selects the factor row, while Fa/Fr is compared with the row’s e value to choose the equation. These are different ratios and must not be confused. For the listed bearing type and arrangement, the factors X and Y are dimensionless.

The SKF catalog explains that clearance affects the factors and that the usual mounting fits are part of the stated conditions. It also notes that higher internal clearance can enable the bearing to carry heavier axial loads. A clearance selected for a thermal operating condition should be evaluated as an installed operating clearance, not copied from a nominal label without context.

Follow the factor lookup in order

First confirm that the bearing is a single-row deep-groove ball bearing and that the load arrangement is covered. Then calculate Fa/C0 and choose a corresponding catalog row. Select Normal, C3 or C4 factors that match the source conditions. Compare Fa/Fr with e. If the ratio is no greater than e, P is Fr; if it exceeds e, calculate P = XFr + YFa. Do not combine e from one clearance group with X and Y from another.

As a worked example, take the Normal-clearance row at Fa/C0 = 0.13, which lists e = 0.31, X = 0.56 and Y = 1.4. For Fr = 2 kN and Fa = 1 kN, Fa/C0 must be 0.13 for that row to apply, and Fa/Fr is 0.5, which exceeds e. Then P = 0.56(2) + 1.4(1) = 2.52 kN. This arithmetic demonstrates the lookup; it does not verify the bearing’s suitability or life.

If the actual Fa/C0 ratio falls between catalog rows, use the interpolation method permitted by the applicable manufacturer documentation or engineering tool. Do not automatically round to a favorable row. The table has a finite set of ratios and factor values, so a bearing series or load condition that is not covered needs a current catalog lookup or manufacturer calculation.

Know which arrangements are excluded

The table’s scope is single bearings and bearing pairs arranged in tandem for the stated single-row deep-groove design. Back-to-back and face-to-face pairs use different equations in SKF’s current product guidance. Bearings with filling slots also have a separate restriction: SKF states a condition on Fa/Fr and directs users to an un-slotted design when axial load is above its stated limit. Do not apply this factor table to other bearing types or arrangements.

The calculated equivalent load is not the same as rating life. A life calculation also requires the basic dynamic load rating, speed, reliability target and any applicable modification factors. Lubrication, contamination, temperature, internal load distribution, fits and operating clearance can influence the application. Do not interpret the factor lookup as an approval of a load or an estimate of service life by itself.

This article transcribes Table 2 from SKF’s 2018 Bearings and Mounted Products catalog. It is intentionally labeled by edition because manufacturer catalogs can change. Before a release drawing, safety-critical application or bearing substitution, check the current SKF catalog for the exact product and operating condition. If the bearing is not an SKF design, use that manufacturer’s own X, Y and e values.

Check the calculation before design use

A good calculation record includes the bearing series, arrangement, clearance, Fr, Fa, C0, both load ratios, selected e/X/Y and resulting P. Retain the source edition and the reason the table conditions match the application. Keep units consistent, and carry unrounded values through the factor calculation before formatting the answer. A second reviewer can then reproduce the lookup and identify an incorrect row quickly.

Use the manufacturer’s engineering tool or consult bearing support if operating clearance is uncertain, the loading fluctuates, the bearing pair is not tandem, or the ratio lies outside the listed table. This reference helps calculate the equivalent dynamic radial load within its explicit scope; it does not certify a bearing arrangement or guarantee reliability.

Source and scope

Factor source: SKF Bearings and Mounted Products catalog (2018), Table 2.