Can two gearbox service factors be compared without their definitions?

The short answer

No. Service factor has meaning only within its manufacturer’s rating and selection method. Different factors can include different duty assumptions or corrections. A wind-yaw gearbox comparison should reconstruct the assessed demand and rating basis rather than rank offers solely by the larger factor.

Identify what the factor multiplies

Ask whether the factor applies to nominal torque, equivalent duty, application severity or another defined quantity. Confirm which starts, shocks, reversals and daily operating duration are represented. A value without its base equation cannot demonstrate mechanical reserve. Applying one supplier’s factor to another’s catalogue torque can combine incompatible methods and produce a misleading comparison even when the arithmetic is correct.

Keep separate checks separate

A favorable torque service factor does not necessarily cover output overhung load, thermal capacity, mounting position or a stationary holding duty. Those can govern independently. SEW project-planning material treats several such checks as distinct subjects. A higher factor also need not mean better performance or longer actual life if the other assumptions differ. Compare the actual operating cases before interpreting the apparent margin.

Request a common-duty selection

Give both suppliers the same torque history, motion/reversal pattern, external loads and mounting condition. Ask each to show its own selection method and identify the governing limitation. No MARSTON gearbox factor or life is asserted here. Preserve the original manufacturer definitions in the comparison rather than normalize factors by guesswork. The decision should state which offers cover the same documented duty and which require clarification, not assign universal meaning to an isolated service-factor number. A common comparison can retain manufacturer-specific calculations side by side. It is better to compare their coverage of the same duty than to convert unlike service factors into a fabricated universal reserve percentage.