Bearing Variable Duty Equivalent Dynamic Load Calculator

Variable-load and variable-speed formula

For stepped duty segments, combine already-derived segment loads using their revolutions:

Calculate the time-weighted mean speed over the same cycle:

Because every duration is converted to canonical hours, all segments use the selected common time unit. Multiplying every duration by the same factor leaves P_eq and mean speed unchanged, although the displayed cycle duration and cycle revolutions scale with that factor.

Worked three-segment example

The default ball-bearing cycle contains:

The cycle totals 8 hours and 576,000 revolutions. For p = 3:

1. Compute each revolution weight nᵢtᵢ: 4,800, 1,800, and 3,000 rpm·h.
2. Compute Σ(Pᵢ³nᵢtᵢ) = 1,602,600 kN³·rpm·h.
3. Divide by Σ(nᵢtᵢ) = 9,600 rpm·h.
4. Take the cube root: P_eq = 5.506191 kN.
5. Mean speed: 9,600 / 8 = 1,200 rpm.

The 8 kN segment governs the calculated fatigue contribution even though it has fewer revolutions than segments 1 and 3 combined. This is why a simple arithmetic-average load is not equivalent.

Derive P_i before combining the duty cycle

Each Pᵢ must be the applicable equivalent dynamic load for one constant segment. If a segment contains radial and axial load, determine its Pᵢ from the exact bearing table and decision branch first. The Bearing Equivalent Dynamic Load Calculator can evaluate P = XFr + YFa when verified X and Y factors are known.

Do not combine raw Fr and Fa values across different segments, and do not reuse one segment’s X/Y branch when the load ratio, direction, clearance, or bearing arrangement changes.

Time weighting is only valid at constant speed

When every segment has the same rotational speed, n cancels from the equivalent-load expression and duration weighting alone gives the same answer. When speed changes, using only time shares incorrectly gives equal fatigue exposure to segments with different revolution counts.

This calculator reports both P_eq and time-weighted mean speed so the pair can reproduce the same basic rolling-fatigue arithmetic for the entered repeating cycle.

Stationary, idle, and zero-load segments

A positive-duration segment at zero rpm contributes to total cycle time and reduces mean speed, but contributes no rolling-fatigue revolutions. If it carries load, the calculator raises a warning because static equivalent load P₀ and static safety factor s₀ may govern.

A rotating segment with Pᵢ = 0 contributes revolutions but no fatigue term in this simplified equation. The calculator preserves the arithmetic and warns that real rolling bearings commonly require a minimum load to avoid skidding or damaging operating conditions.

Set duration to zero to disable a segment. Nonzero load or speed in a zero-duration segment is ignored and reported as a warning so stale values are visible.

Continue to L10 or required C

Two related links transfer P_eq and mean speed:

• the Bearing L10 Basic Rating Life Calculator receives C = P_eq as a deliberately visible placeholder; replace C with the exact catalog rating;
• the Bearing Required Dynamic Load Rating Calculator receives a placeholder 20,000-hour target; replace it with the actual basic-life requirement.

These handoffs verify arithmetic continuity. They do not independently validate the duty cycle, segment loads, bearing data, or application.

Engineering scope and limitations

This calculator covers up to four stepped, repeating, constant-within-segment operating conditions. It excludes:

• automatic derivation of segment Pᵢ, X/Y factor selection, bearing reactions, preload, load sharing, or moment loads;
• continuously varying load integration, measured time-series import, random spectra, transient peaks, shock, impact, reversal, acceleration, or start-stop dynamics;
• oscillating-motion equivalence, partial rotations, raceway load-zone changes, or direction-dependent load distribution;
• modified rating life, reliability other than 90%, fatigue load limit, lubrication, contamination, viscosity, or surface-condition factors;
• static equivalent load, static safety, permanent deformation, minimum load, skidding, speed limits, thermal balance, clearance, fits, mounting, or misalignment;
• wear, corrosion, electrical erosion, false brinelling, sealing, maintenance, or actual service-life prediction;
• bearing catalog selection, product availability, dimensional compatibility, cost, warranty, certification, or final engineering approval.

Use current manufacturer documentation and qualified application review when the duty cycle affects bearing selection or machine safety.