This part of IEC 61400 is applicable to enclosed speed increasing gearboxes for horizontal axis wind turbine drivetrains with a power rating in excess of 500 kW. This document applies to newly designed gearboxes for wind turbines installed onshore or offshore. The technical requirements given in this document are not intended for repaired or refurbished gearboxes, or for the extension of the service life beyond the design life.

This document provides requirements and guidance on the analysis of the wind turbine loads in relation to the design of the gear and gearbox elements. The gearing elements covered by this document include such gears as spur, helical or double helical and their combinations in parallel and epicyclic arrangements in the main power path. This document does not apply to power take off (PTO) gears.

This document includes requirements, design recommendations, and rating of gearboxes with rolling bearings, plain bearings, or combinations of both bearing types.

Also included are requirements and guidance on the engineering of shafts, shaft hub interfaces, lubrication, wind turbine controller interface, and the gear case structure to achieve a design that is capable of withstanding the environment and operating conditions of a wind turbine. Requirements for dynamic analysis of the gearbox within the wind turbine system are specified for the purpose of identifying load levels exceeding the predictions of the global aeroelastic simulation. The analysis of noise transmission and emission (e.g. tonal emission at gear mesh frequencies) is not within the scope of this edition of the document.

Further, this document provides requirements and guidance on gearbox design verification, prototype testing and production testing, as well as consideration of design for service and maintenance.

A method for a systematic assessment of the design reliability of a gearbox design under reference operating conditions is specified in IEC TS 61400‑4‑11.

This document is supported by two Technical Reports: IEC TR 61400‑4‑22 provides additional information on lubrication of wind turbine drivetrains and IEC TR 61400‑4‑33 contains explanatory notes and supportive information to the requirements specified in this document.