SIST EN IEC 61400-4:2025

Wind energy generation systems - Part 4: Design requirements for wind turbine gearboxes (IEC 61400-4:2025)

IEC 61400-4:2025 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. This document is supported by two Technical Reports: IEC TR 61400‑4‑2 provides additional information on lubrication of wind turbine drivetrains and IEC TR 61400‑4‑3 contains explanatory notes and supportive information to the requirements specified in this document.
It is published as a double logo standard.
This second edition cancels and replaces the first edition published in 2012. This edition includes the following significant technical changes with respect to the previous edition:
- extension of the scope to wind turbines above 2 MW reference power;
- considerations for converging differing approaches to reliability in gear, bearing and wind turbine standards;
- new clause on wind turbine loads specific to drivetrains;
- revised clause on verification and validation;
- new clause on design requirements for plain bearings;
- revised and expanded design considerations for rolling bearings;
- revised clause on considerations and requirements in the design and analysis of gearbox structural elements;
- updated considerations and requirements on lubricants and lubrication systems;
- removal of requirements for documenting the compliance of a design with the requirements of the document in favour of reference to IECRE OD‑501‑2.

Windenergieanlagen - Teil 4: Auslegungsanforderungen für Getriebe von Windenergieanlagen (IEC 61400-4:2025)

Systèmes de génération d’énergie éolienne - Partie 4: Exigences de conception des boîtes de vitesses des éoliennes (IEC 61400-4:2025)

L’IEC 61400-4:2025 s’applique aux boîtes de vitesses sous carter pour trains d’entraînement d’éolienne à axe horizontal dont la puissance nominale dépasse 500 kW. Le présent document s’applique aux boîtes de vitesses de conception nouvelle pour les éoliennes installées sur terre et en mer. Les exigences techniques indiquées dans le présent document ne sont pas destinées aux boîtes de vitesses réparées ou remises à neuf ni à la prolongation de la durée de vie en service au-delà de la durée de vie théorique.
Le présent document fournit des exigences et des recommandations sur l’analyse des charges d’éolienne en tenant compte de la conception des éléments d’engrenage et de boîte de vitesses. Les éléments d’engrènement couverts par le présent document comprennent les engrenages à dentures droite, hélicoïdale simple ou double et leurs combinaisons dans des configurations parallèles et épicycloïdales dans le trajet de puissance principal. Le présent document ne s’applique pas aux engrenages de prise de force (PTO - power take-off).
Le présent document comprend les exigences, les recommandations de conception et l’étalonnage des boîtes de vitesses équipées de paliers à roulement, de paliers lisses ou d’une combinaison de ces deux types de paliers. Le présent document est étayé par deux rapports techniques: L’IEC TR 61400‑4‑2 fournit des informations supplémentaires sur la lubrification des trains d’entraînement des éoliennes et l’IEC TR 61400‑4‑3 contient des notes explicatives, ainsi que des informations complémentaires aux exigences spécifiées dans le présent document.
Elle est publiée comme norme double logo.
Cette deuxième édition annule et remplace la première édition parue en 2012. Cette édition inclut les modifications techniques majeures suivantes par rapport à l’édition précédente:
a) l’extension du domaine d’application aux éoliennes dont la puissance de référence est supérieure à 2 MW;
b) des considérations pour la convergence de différentes approches en matière de fiabilité dans les normes relatives aux engrenages, paliers et éoliennes;
c) un nouvel article concernant les charges des éoliennes, spécifiques aux trains d’entraînement;
d) un article révisé concernant la vérification et la validation;
e) un nouvel article concernant les exigences de conception des paliers lisses;
f) des considérations de conception révisées et étendues pour les paliers à roulement;
g) un article révisé concernant les considérations et les exigences en matière de conception et d’analyse des éléments structurels d’une boîte de vitesses;
h) des considérations et exigences mises à jour concernant les lubrifiants et les systèmes de lubrification;
i) la suppression des exigences relatives à la documentation de la conformité d’une conception aux exigences du document au profit d’une référence à la norme IECRE OD‑501‑2.

Sistemi za proizvodnjo energije na veter - 4. del: Konstrukcijske zahteve za menjalnike vetrnih turbin (IEC 61400-4:2025)

Ta del standarda IEC 61400 se uporablja za zaprte menjalnike, ki povečujejo hitrost, za prenosnike moči na vodoravni osi za vetrne turbine z nazivno močjo, ki je večja od 500 kW. Ta dokument se uporablja za nove menjalnike vetrnih turbin, ki so nameščene na kopnem ali na morju. Tehnične zahteve, navedene v tem dokumentu, niso namenjene za popravljene ali obnovljene menjalnike ali za podaljšanje delovanja po predvideni življenjski dobi.
Ta dokument podaja zahteve in smernice za analizo obremenitev vetrnih turbin glede na konstrukcijo elementov gonil in menjalnikov. Elementi gonil v tem dokumentu vključujejo ravne, vijačne ali dvojne vijačne zobnike in njihove kombinacije v vzporednih in epicikličnih razporeditvah v glavni poti napajanja. Ta dokument se ne uporablja za gonila za prenos moči (PTO).
Ta dokument vključuje zahteve, priporočila za konstruiranje in ocene menjalnikov s kotalnimi ležaji, drsnimi ležaji ali kombinacijami obeh vrst ležajev.
Vključene so tudi zahteve in smernice za projektiranje jaškov, vmesnikov pesta jaška, mazanja in nadzornega vmesnika vetrne turbine ter konstrukcijo škatle za sklopke, s čimer se doseže konstrukcija, ki lahko prenese okoljske pogoje in pogoje delovanja vetrne turbine.
Zahteve za dinamično analizo menjalnika znotraj sistema vetrne turbine so določene z namenom ugotavljanja ravni obremenitve, ki presegajo napovedi globalne aeroelastične simulacije. Analiza prenosa in emisij hrupa (npr. tonalne emisije pri frekvencah zobniškega prenosa) ne spada na področje uporabe tega dokumenta.
Poleg tega ta dokument podaja zahteve in smernice za preverjanje konstrukcije menjalnika, preskušanje prototipov in proizvodno preskušanje ter upoštevanje konstrukcije pri servisiranju in vzdrževanju.
Metoda za sistematično ocenjevanje konstrukcijske zanesljivosti menjalnika v referenčnih pogojih delovanja je določena v standardu IEC TS 61400-4-11.
Ta dokument je podprt z dvema tehničnima poročiloma: standard IEC TR 61400-4-22 vsebuje dodatne informacije o mazanju prenosnikov moči vetrnih turbin, standard IEC TR 61400-4-33 pa vsebuje pojasnjevalne opombe in podporne informacije k zahtevam, določenim v tem dokumentu.

General Information

• Status: Published
• Publication Date: 11-Jun-2025
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: IEHT - Electrotechnics - Hydraulic turbins
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 04-Jun-2025
• Due Date: 09-Aug-2025
• Completion Date: 12-Jun-2025
• Ref Project: EN IEC 61400-4:2025 - Wind energy generation systems - Part 4: Design requirements for wind turbine gearboxes

Relations

• Replaces: SIST EN 61400-4:2013 - Wind turbines - Part 4: Design requirements for wind turbine gearboxes
• Effective Date: 01-Jul-2025

Overview

SIST EN IEC 61400-4:2025 (IEC 61400-4:2025) defines design requirements for wind turbine gearboxes used in horizontal-axis drivetrains. The standard applies to enclosed, speed-increasing gearboxes for turbines with a rated power above 500 kW and - with this second edition - extends considerations to turbines above 2 MW reference power. It covers new gearbox designs for both onshore and offshore installations and excludes repaired/refurbished gearboxes, life‑extension beyond design life, and power-take-off (PTO) gears. Published as a double‑logo standard, this edition supersedes the 2012 version and adds significant technical updates.

Key topics and technical requirements

• Load analysis and drivetrain dynamics: Requirements and guidance for analysing wind turbine loads in relation to gear and gearbox design, including modelling, excitation evaluation and verification in the system environment.
• Gearing types covered: Spur, helical, double-helical and combinations in parallel and epicyclic arrangements in the main power path.
• Gears and gear rating: Calculation of gear load capacity, materials, accuracy and manufacturing considerations; reliability convergence with other gear and bearing standards.
• Bearings: Requirements for rolling bearings and plain bearings, including selection, materials, interfaces, lubrication and rating calculations.
• Structural elements: Design and analysis requirements for gearbox housings and supporting structures.
• Lubrication and lubrication systems: Updated considerations and requirements; supported by IEC TR 61400‑4‑2 for lubrication guidance.
• Verification & validation: Revised clauses on methods to verify dynamic behaviour and validate designs; documentation requirements aligned with IECRE OD‑501‑2.
• Supporting guidance: IEC TR 61400‑4‑3 provides explanatory notes and supportive information to the normative requirements.

Practical applications

• Designing and rating wind turbine gearboxes for new turbine projects (onshore/offshore)
• Selecting bearings and lubricants and specifying lubrication systems for drivetrain reliability
• Performing gearbox structural and dynamic analyses, including excitability and verification in system models
• Preparing submissions for conformity assessment and certification under IEC/EN processes
• Informing OEMs, drivetrain suppliers and asset owners about design best practices and reliability expectations

Who should use this standard

• Wind turbine gearbox designers and drivetrain engineers
• Turbine OEMs and component suppliers (gears, bearings, lubrication systems)
• Certification bodies, test laboratories and consultants
• Turbine asset owners and technical due‑diligence teams

Related standards (select)

• IEC 61400‑1 (Design requirements)
• IEC 61400‑3‑1 (Offshore design)
• IEC TR 61400‑4‑2 (Lubrication guidance)
• IEC TR 61400‑4‑3 (Explanatory notes)
• ISO 6336 series (Gear load capacity), ISO 281 (Rolling bearings)

Keywords: IEC 61400-4:2025, wind turbine gearbox design, wind energy generation systems, gearbox bearings, lubrication, drivetrain loads, EN IEC 61400-4:2025.

Standards Content (Sample)

SLOVENSKI STANDARD
01-julij-2025
Nadomešča:
SIST EN 61400-4:2013
Sistemi za proizvodnjo energije na veter - 4. del: Konstrukcijske zahteve za
menjalnike vetrnih turbin (IEC 61400-4:2025)
Wind energy generation systems - Part 4: Design requirements for wind turbine
gearboxes (IEC 61400-4:2025)
Windenergieanlagen - Teil 4: Auslegungsanforderungen für Getriebe von
Windenergieanlagen (IEC 61400-4:2025)
Systèmes de génération d’énergie éolienne - Partie 4: Exigences de conception des
boîtes de vitesses des éoliennes (IEC 61400-4:2025)
Ta slovenski standard je istoveten z: EN IEC 61400-4:2025
ICS:
27.180 Vetrne elektrarne Wind turbine energy systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61400-4

NORME EUROPÉENNE
EUROPÄISCHE NORM May 2025
ICS 27.180 Supersedes EN 61400-4:2013
English Version
Wind energy generation systems - Part 4: Design requirements
for wind turbine gearboxes
(IEC 61400-4:2025)
Systèmes de génération d'énergie éolienne - Partie 4: Windenergieanlagen - Teil 4: Auslegungsanforderungen für
Exigences de conception des boîtes de vitesses des Getriebe von Windenergieanlagen
éoliennes (IEC 61400-4:2025)
(IEC 61400-4:2025)
This European Standard was approved by CENELEC on 2025-05-20. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61400-4:2025 E

European foreword
The text of document 88/971/CDV, future edition 2 of IEC 61400-4, prepared by TC 88 "Wind energy
generation systems" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-05-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-05-31
document have to be withdrawn
This document supersedes EN 61400-4:2013 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61400-4:2025 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 61400-24 NOTE Approved as EN IEC 61400-24
ISO 9000 NOTE Approved as EN ISO 9000
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 61400-1 2019 Wind energy generation systems - Part 1: EN IEC 61400-1 2019
Design requirements
IEC 61400-3-1 - Wind energy generation systems - Part 3- EN IEC 61400-3-1 -
1: Design requirements for fixed offshore
wind turbines
IEC/TS 61400-3-2 - Wind energy generation systems - Part 3- - -
2: Design requirements for floating offshore
wind turbines
IEC 61400-8 - Wind energy generation systems - Part 8: EN IEC 61400-8 -
Design of wind turbine structural
components
IEC/TS 61400-30 - Wind energy generation systems - Part 30: - -
Safety of wind turbine generators - General
principles for design
ISO 281 - Rolling bearings - Dynamic load ratings - -
and rating life
ISO 683 series Heat-treatable steels, alloy steels and free- EN ISO 683 series
cutting steels
ISO 683-17 - Heat-treatable steels, alloy steels and free- EN ISO 683-17 -
cutting steels - Part 17: Ball and roller
bearing steels
ISO 898-1 - Mechanical properties of fasteners made of EN ISO 898-1 -
carbon steel and alloy steel - Part 1: Bolts,
screws and studs with specified property
classes - Coarse thread and fine pitch
thread
ISO 898-2 - Fasteners - Mechanical properties of EN ISO 898-2 -
fasteners made of carbon steel and alloy
steel - Part 2: Nuts with specified property
classes
Publication Year Title EN/HD Year
ISO 898-3 - Mechanical properties of fasteners made of EN ISO 898-3 -
carbon steel and alloy steel - Part 3: Flat
washers with specified property classes
ISO 1328-1 - Cylindrical gears - ISO system of accuracy - -
- Part 1: Definitions and allowable values of
deviations relevant to corresponding flanks
of gear teeth
ISO 3104 - Petroleum products - Transparent and EN ISO 3104 -
opaque liquids - Determination of kinematic
viscosity and calculation of dynamic
viscosity
ISO 4042 - Fasteners - Electroplated coating systems EN ISO 4042 -
ISO 4406 - Hydraulic fluid power - Fluids - Method for - -
coding the level of contamination by solid
particles
ISO 6336 series Calculation of load capacity of spur and - -
helical gears
ISO 6336-1 2019 Calculation of load capacity of spur and - -
helical gears - Part 1: Basic principles,
introduction and general influence factors
ISO 6336-2 2019 Calculation of load capacity of spur and - -
helical gears - Part 2: Calculation of
surface durability (pitting)
ISO 6336-3 2019 Calculation of load capacity of spur and - -
helical gears - Part 3: Calculation of tooth
bending strength
ISO/TS 6336-4 - Calculation of load capacity of spur and - -
helical gears - Part 4: Calculation of tooth
flank fracture load capacity
ISO 6336-5 2016 Calculation of load capacity of spur and - -
helical gears - Part 5: Strength and quality
of materials
ISO 6336-6 2019 Calculation of load capacity of spur and - -
helical gears - Part 6: Calculation of
service life under variable load
ISO/TS 6336-20 - Calculation of load capacity of spur and - -
helical gears - Part 20: Calculation of
scuffing load capacity - Flash temperature
method
ISO/TS 6336-21 - Calculation of load capacity of spur and - -
helical gears - Part 21: Calculation of
scuffing load capacity - Integral
temperature method
ISO/TS 6336-22 - Calculation of load capacity of spur and - -
helical gears - Part 22: Calculation of
micropitting load capacity
ISO 6618 - Petroleum products and lubricants - - -
Determination of acid or base number -
Coulour-indicator titration method
Publication Year Title EN/HD Year
ISO 6619 - Petroleum products and lubricants; - -
neutralization number; potentiometric
titration method
ISO 7146-1 - Plain bearings - Appearance and - -
characterization of damage to metallic
hydrodynamic bearings - Part 1: General
ISO 8579-1 - Acceptance code for gear units - Part 1: - -
Test code for airborne sound
ISO/TR 10064-3 - Code of inspection practice - Part 3: - -
Recommendations relative to gear blanks,
shaft centre distance and parallelism of
axes
ISO 10683 - Fasteners - Non-electrolytically applied EN ISO 10683 -
zinc flake coating systems
ISO 10825-1 - Gears - Wear and damage to gear teeth - - -
Part 1: Nomenclature and characteristics
ISO 12925-1 - Lubricants, industrial oils and related - -
products (class L) - Family C (Gears) - Part
1: Specifications for lubricants for enclosed
gear systems
ISO 14104 - Gears - Surface temper etch inspection - -
after grinding
ISO 14635-1 2023 Gears - FZG test procedures - Part 1: FZG - -
test method A/8,3/90 for relative scuffing
load-carrying capacity of oils
ISO 15243 2017 Rolling bearings - Damage and failures - - -
Terms, characteristics and causes
ISO 16281 2025 Rolling bearings - Methods for calculating - -
the modified reference rating life for
universally loaded rolling bearings
ISO 17956 2025 Rolling bearings - Method for calculating - -
the effective static safety factor for
universally loaded rolling bearings
ISO 21920-1 - Geometrical product specifications (GPS) - EN ISO 21920-1 -
Surface texture: Profile - Part 1: Indication
of surface texture
ISO 21920-2 - Geometrical product specifications (GPS) - EN ISO 21920-2 -
Surface texture: Profile - Part 2: Terms,
definitions and surface texture parameters
ISO 21920-3 - Geometrical product specifications (GPS) - EN ISO 21920-3 -
Surface texture: Profile - Part 3:
Specification operators
ANSI/AGMA 6001 - Design and selection of components for - -
enclosed gear drives
ANSI/AGMA 6123 - Design manual for enclosed epicyclic gear - -
drives
ASTM D445 - Standard test method for kinematic - -
viscosity of transparent and opaque liquids
(and calculation of dynamic viscosity)
Publication Year Title EN/HD Year
ASTM D664 - Standard test method for acid number of - -
petroleum products by potentiometric
titration
ASTM D974 - Standard test method for acid and base - -
number by color-indicator titration
ASTM D5185 - Standard test method for multielement - -
determination of used and unused
lubricating oils and base oils by inductively
coupled plasma atomic emission
spectrometry (ICPAES)
ASTM D6304 - Standard test method for determination of - -
water in petroleum products, lubricating
oils, and additives by coulometric Karl
Fischer titration
DIN 471 - Retaining rings for shafts - Normal type - -
and heavy type
DIN 472 - Circlips (retaining rings) for bores: Normal - -
type and heavy type
DIN 743 series Calculation of load capacity of shafts and - -
axles
DIN 3990-16 - Calculation of load capacity of cylindrical - -
gears - Part 16: Determination of the
micro-pitting load-carrying capacity of
lubricants using FZG test method GT-
C/8.3/90
DIN 6885-1 - Drive type fastenings without taper action; - -
Parallel keys, keyways, deep pattern
DIN 6892 - Mitnehmerverbindungen ohne Anzug - - -
Passfedern - Berechnung und Gestaltung
DIN 7190 - Interference fits - Calculation and design - -
rules
DIN 51399-1 - Testing of lubricants - Determination of - -
elements content in additives, wear and
other contaminations - Part 1: Direct
determination by optical emission spectral
analysis with inductively coupled plasma
(ICP OES)
DIN 51777 - Petroleum products - Determination of - -
water content using titration according to
Karl Fischer
DIN 51819-3 - Testing of lubricants - Mechanical-dynamic - -
testing in the roller bearing test apparatus
FE8 - Part 3: Test method for lubricating
oils - Applied test bearing: axial cylindrical
roller bearing
FVA 563 I - FVA Guideline: Recommendations for the - -
standardization of load carrying capacity
test on hardened and tempered cylindrical
gears, Research Association for Drive
Technology (FVA)
Publication Year Title EN/HD Year
VDI 2230-1 2015 Systematic calculation of highly stressed - -
bolted joints - Joints with one cylindrical
bolt
VDI 2230-2 2014 Systematic calculation of highly stressed - -
bolted joints - Multi bolted joints
VDI/VDE 2862-2 - Minimum requirements for application of - -
fastening systems and tools - Applications
in plant construction, mechanical
engineering, equipment manufacturing and
for flange connections in components
under pressure boundary
IEC 61400-4
Edition 2.0 2025-04
INTERNATIONAL
STANDARD
Wind energy generation systems –

Part 4: Design requirements for wind turbine gearboxes

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180 ISBN 978-2-8327-0225-3

– 2 – IEC 61400-4:2025 © IEC 2025
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 10
3 Terms, definitions, abbreviated terms, units and conventions . 13
3.1 Terms and definitions . 13
3.2 Abbreviated terms and units. 15
3.3 Conventions . 18
3.3.1 Shaft designations – examples for typical wind turbine gearbox
architecture . 18
4 Design process . 20
4.1 General . 20
4.2 Gearbox type . 21
4.3 Design life, service life, and reliability . 22
4.4 Component class and consequence of failure . 22
4.5 Design process . 22
5 Gearbox interfaces and loads . 25
5.1 General . 25
5.2 Interfaces . 25
5.3 Loads. 25
5.4 Drivetrain dynamics . 26
5.4.1 General . 26
5.4.2 Model requirements . 26
5.4.3 Required analyses . 27
5.4.4 Evaluation of excitability . 28
5.4.5 Verification of dynamic behaviour in system environment . 28
6 Design and rating requirements . 29
6.1 Gears. 29
6.1.1 Reliability considerations . 29
6.1.2 Calculation of gear load capacity . 29
6.1.3 Load factors . 33
6.1.4 Materials . 35
6.1.5 Accuracy . 35
6.1.6 Manufacturing . 35
6.2 Rolling bearings . 36
6.2.1 Reliability considerations . 36
6.2.2 Bearing selection . 39
6.2.3 Materials . 40
6.2.4 Interface requirements . 40
6.2.5 Design considerations . 41
6.2.6 Bearing lubrication . 43
6.2.7 Rating calculations . 44
6.3 Plain bearings . 47
6.3.1 Reliability considerations . 47
6.3.2 Design load cases and associated risks . 47
6.3.3 Interface requirements . 49

IEC 61400-4:2025 © IEC 2025 – 3 –
6.3.4 Shaft and housing fits . 50
6.3.5 Hydrodynamic lubrication regime . 50
6.3.6 Lubricant regime analysis . 51
6.3.7 Bearing requirements . 51
6.4 Shafts, keys, housing joints, splines, and fasteners . 52
6.4.1 Shafts . 52
6.4.2 Shaft-hub connections . 53
6.4.3 Shaft seals . 53
6.4.4 Fasteners . 53
6.4.5 Bolted joints. 54
6.4.6 Circlips . 55
6.5 Structural elements . 55
6.5.1 General . 55
6.5.2 Interfaces, boundary conditions, and loads . 56
6.6 Lubrication . 56
6.6.1 General . 56
6.6.2 Lubricant performance characteristics . 56
6.6.3 Lubricant viscosity . 56
6.6.4 Method of lubrication and cooling . 57
6.6.5 Quantity of lubricant in the lubrication system . 57
6.6.6 Operating temperatures . 58
6.6.7 Temperature control . 58
6.6.8 Lubricant condition monitoring . 59
6.6.9 Lubricant filtration . 59
6.6.10 Ports. 60
6.6.11 Lubricant level indicator . 60
6.6.12 Magnetic plugs . 60
6.6.13 Breather port . 60
6.6.14 Flow sensor . 61
7 Design verification and design validation . 61
7.1 General . 61
7.2 Design verification and validation plan . 61
7.3 Failure mode categorization . 61
7.4 Verification methods . 62
7.4.1 General . 62
7.4.2 Testing . 62
7.4.3 Similarity . 64
7.4.4 Simulation . 65
7.5 Verification and validation matrix . 65
8 Manufacturing and quality assurance . 69
8.1 General . 69
8.2 Quality plan . 69
8.2.1 General . 69
8.2.2 Surface temper inspection of gears after grinding . 69
8.2.3 Surface roughness inspection . 69
8.3 Critical processes . 69
8.4 Statistical process control . 69
8.5 Factory acceptance testing . 70
8.5.1 Test objectives . 70

– 4 – IEC 61400-4:2025 © IEC 2025
8.5.2 Acceptance test plan . 70
8.5.3 Factory test sequences . 70
8.5.4 Acceptance measurements . 71
8.6 Non-conforming components. 71
8.6.1 General . 71
8.6.2 Grinding notches . 72
9 Design for service and operation . 72
9.1 General . 72
9.2 Service and operation design requirements. 72
9.3 Service and operation documentation requirements . 72
9.4 Safety . 73
Annex A (informative) Examples of drivetrain interfaces and loads specifications . 74
A.1 General . 74
A.2 Common wind turbine drivetrain architectures . 74
A.2.1 Non-integrated drivetrain with 4-point suspension . 74
A.2.2 Non-integrated drivetrain with 3-point suspension . 75
A.2.3 Integrated drivetrain . 75
A.2.4 Interfaces . 78
A.2.5 Coordinate system . 78
A.2.6 Interface descriptions . 78
A.2.7 Engineering data at the interface . 80
A.3 Wind turbine load descriptions . 80
A.3.1 Load description formats . 80
A.3.2 Rainflow matrices . 81
A.3.3 Load revolution distribution . 82
A.3.4 Extreme load descriptions . 83
A.4 Wind turbine reference power and speed . 84
A.4.1 General . 84
A.4.2 Wind turbine power control theory . 84
A.4.3 Practicalities of power control . 85
Annex B (informative) Dynamic gearbox model verification and validation. 87
B.1 General . 87
B.2 Verification of drivetrain dynamics models . 87
B.3 Validation of drivetrain dynamics models . 87
Bibliography . 90

Figure 1 – Shaft designation in 3-stage parallel shaft gearboxes . 19
Figure 2 – Shaft designation in 3-stage gearboxes with one planet stage . 19
Figure 3 – Shaft designation in 3-stage gearboxes with two planet stages . 20
Figure 4 – Shaft designation in 4-stage gearboxes with three planet stages . 20
Figure 5 – Design process flow chart . 23
Figure 6 – Definition of planet gear rim thickness . 31
Figure 7 – Examples of bearing selection criteria . 39
Figure 8 – Notional operational conditions and plain bearing risk regions. 49
Figure A.1 – Non-integrated drivetrain with 4-point suspension . 74
Figure A.2 – Non-integrated drivetrain with 3-point suspension . 75
Figure A.3 – Rotor-side integration with rigid main shaft connection . 76

IEC 61400-4:2025 © IEC 2025 – 5 –
Figure A.4 – Rotor-side integration with flexible main shaft connection . 76
Figure A.5 – Generator-side integration with rotor support in generator . 77
Figure A.6 – Generator-side integration with rotor support in gearbox and generator . 77
Figure A.7 – Generator-side integration with rotor support in gearbox . 77
Figure A.8 – Example of rainflow cycles per design load case . 82
Figure A.9 – Example of a load revolution distribution . 83
Figure A.10 – Wind turbine power control regions . 84
Figure A.11 – Ideal power and speed control strategy . 85
Figure A.12 – Control strategy compared to actual response . 85

Table 1 – Definition of a gearbox type . 21
Table 2 – Minimum safety factors for pitting resistance and bending strength . 30
Table 3 – Mesh load factor for planetary stages . 33
Table 4 – Required gear accuracy . 35
Table 5 – Typical temperature differences for calculation of operating clearance . 42
Table 6 – Bearing lubricant temperature for calculation of viscosity ratio . 44
Table 7 – Guide values for maximum contact stress. 46
Table 8 – Plain bearing risks coupled to operating conditions . 48
Table 9 – Minimum safety factors. 52
Table 10 – Failure mode categorization . 62
Table 11 – Verification and validation matrix . 66
Table A.1 – Analysis information at interfaces for non-integrated drivetrains . 79
Table A.2 – Analysis information at interfaces for integrated drivetrain . 80
Table A.3 – Engineering data and design load descriptions. 80
Table A.4 – Rainflow matrix example . 81
Table A.5 – Extreme load matrix example . 83

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 4: Design requirements for wind turbine gearboxes

FOREWORD
1) ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission)
form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC
participate in the development of International Standards through technical committees established by the
respective organization to deal with particular fields of technical activity. ISO and IEC technical committees
collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental,
in liaison with ISO and IEC, also take part in the work.
2) The formal decisions or agreements of IEC and ISO on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested IEC and ISO National bodies.
3) IEC and ISO documents have the form of recommendations for international use and are accepted by IEC and
ISO National bodies in that sense. While all reasonable efforts are made to ensure that the technical content of
IEC and ISO documents is accurate, IEC and ISO cannot be held responsible for the way in which they are used
or for any misinterpretation by any end user.
4) In order to promote international uniformity, IEC and ISO National bodies undertake to apply IEC and
ISO documents transparently to the maximum extent possible in their national and regional publications. Any
divergence between any IEC and ISO document and the corresponding national or regional publication shall be
clearly indicated in the latter.
5) IEC and ISO do not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC and ISO marks of conformity. IEC and ISO are not
responsible for any services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this document.
7) No liability shall attach to IEC and ISO or their directors, employees, servants or agents including individual
experts and members of its technical committees and IEC and ISO National bodies for any personal injury,
property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including
legal fees) and expenses arising out of the publication, use of, or reliance upon, this ISO/IEC document or any
other IEC and ISO documents.
8) Attention is drawn to the Normative references cited in this document. Use of the referenced publications is
indispensable for the correct application of this document.
9) IEC and ISO draw attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC and ISO take no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, IEC and ISO had not received notice of
(a) patent(s), which may be required to implement this document. However, implementers are cautioned that this
may not represent the latest information, which may be obtained from the patent database available at
https://patents.iec.ch and www.iso.org/patents. IEC and ISO shall not be held responsible for identifying any or
all such patent rights.
IEC 61400-4 has been prepared by IEC technical committee 88: Wind energy generation
systems, in co-operation with ISO technical committee 60: Gears. It is an International
Standard.
It is published as a double logo standard.
This second edition cancels and replaces the first edition published in 2012. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) extension of the scope to wind turbines above 2 MW reference power;
b) considerations for converging differing approaches to reliability in gear, bearing and wind
turbine standards;
IEC 61400-4:2025 © IEC 2025 – 7 –
c) new clause on wind turbine loads specific to drivetrains;
d) revised clause on verification and validation;
e) new clause on design requirements for plain bearings;
f) revised and expanded design considerations for rolling bearings;
g) revised clause on considerations and requirements in the design and analysis of gearbox
structural elements;
h) updated considerations and requirements on lubricants and lubrication systems;
i) removal of requirements for documenting the compliance of a design with the requirements
of the document in favour of reference to IECRE OD-501-2.
The text of this International Standard is based on the following documents:
Draft Report on voting
88/971/CDV 88/1003A/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table. In ISO, the standard has been approved by 11 P-members out of 12 having
cast a vote.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of the IEC 61400 series, under the general title: Wind energy generation
systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
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INTRODUCTION
IEC 61400-4 outlines minimum requirements for specification, design, and verification of
gearboxes in wind turbines. It does not serve as a complete design specification or instruction
manual. It is intended for use by experienced designers of wind turbine drivetrains, gearboxes,
and gears or bearings who are capable of selecting reasonable values for the design factors,
based on knowledge of similar designs and the effects of such items as lubrication, deflection,
manufacturing tolerances, metallurgy, residual stress, and system dynamics.
Any of the requirements of this document may be altered:
• if more accurate data are available from full scale load tests, precise measurements,
comprehensive mathematical analysis, or any combination of these;
• or on the basis of proven operating experience;
• and if the evidence is accessible for independent assessment (e.g. by an accredited
renewable energy certification body);
• and if the accuracy and reliability of the alternative method is demonstrated with respect to
the safety and reliability of the complete wind turbine drive system.

IEC 61400-4:2025 © IEC 2025 – 9 –
WIND ENERGY GENERATION SYSTEMS –

Part 4: Design requirements for wind turbine gearboxes

1 Scope
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 61400-4-1 .
This document is supported by two Technical Reports: IEC TR 61400-4-2 provides additional
information on lubrication of wind turbine drivetrains and IEC TR 61400-4-3 contains
explanatory notes and supportive information to the requirements specified in this document.
___________
Under preparation. Stage at the time of publication: IEC/DTS 61400-4-1:2024.
Under preparation. Stage at the time of publication: IEC/DTR 61400-4-2:2024.
Under preparation. Stage at the time of publication: IEC/DTR 61400-4-3:2024.

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