IEC/DTS 61400-4-1

FINAL DRAFT
Technical
Specification
ISO/DTS 61400-4-1
ISO/TC 60
Wind energy generation systems —
Secretariat: ANSI
Part 4-1:
Voting begins on:
2025-10-13
Reliability assessment of drivetrain
components in wind turbines
Voting terminates on:
2025-12-08
Systèmes de génération d'énergie éolienne —
Partie 4-1: Évaluation de la fiabilité des composants de la chaîne
cinématique des éoliennes
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This draft is submitted to a parallel vote in ISO and in IEC.
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
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Reference number
ISO/DTS 61400-4-1:2025(en) © ISO 2025

FINAL DRAFT
ISO/DTS 61400-4-1:2025(en)
Technical
Specification
ISO/TC 60
Wind energy generation systems —
Secretariat: ANSI
Part 4-1:
Voting begins on:
Reliability assessment of drivetrain 2025-10-13
components in wind turbines
Voting terminates on:
2025-12-08
Systèmes de génération d'énergie éolienne —
Partie 4-1: Évaluation de la fiabilité des composants de la chaîne
cinématique des éoliennes
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPORTING D OCUMENTATION.
© ISO 2025
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
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This draft is submitted to a parallel vote in ISO and in IEC.
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TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
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Published in Switzerland Reference number
ii
IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

1 CONTENTS
3 FOREWORD . 3
4 INTRODUCTION . 5
5 1 Scope . 6
6 2 Normative references . 6
7 3 Terms, definitions, abbreviated terms, units and conventions . 6
8 3.1 Terms and definitions . 6
9 3.2 Abbreviated terms and units. 8
10 4 System reliability analysis model . 10
11 4.1 General . 10
12 4.2 Failure mode identification, classification, and system element assignment . 10
13 5 Component reliability analysis models . 12
14 5.1 Individual failure modes . 12
15 5.2 Gear reliability calculation . 12
16 5.2.1 General . 12
17 5.2.2 Estimation of service life for defined failure probabilities . 12
18 5.2.3 Extrapolation of failure probabilities to the design life . 16
19 5.3 Rolling bearing reliability calculation . 16
20 5.4 Shaft reliability calculation . 17
21 Annex A (informative) Example reliability calculations . 20
22 A.1 General . 20
23 A.2 Example system reliability model . 20
24 A.2.1 Identification of components . 20
25 A.2.2 Identification of system elements . 20
26 A.2.3 Classification and selection of system elements . 21
27 A.2.4 Arrangement of reliability model and calculation of system reliability . 21
28 A.2.5 Example system assumptions . 21
29 A.3 Example gear tooth surface durability (pitting) reliability calculation . 22
30 A.3.1 Example gear assumptions . 22
31 A.3.2 Estimation of service life for defined failure probabilities . 22
32 A.3.3 Extrapolation of failure probabilities to the design life . 24
33 A.4 Example gear tooth bending strength reliability calculation . 25
34 A.4.1 Example gear assumptions . 25
35 A.4.2 Estimation of service life for defined failure probabilities . 26
36 A.4.3 Extrapolation of failure probabilities to the design life . 27
37 A.5 Example rolling bearing contact fatigue reliability calculation . 28
38 A.6 Example shaft fatigue fracture reliability calculation . 29
39 A.6.1 Example shaft assumptions . 29
40 A.6.2 Estimation of life for the nominal failure probability . 29
41 A.6.3 Extrapolation of service life for 10% failure probability . 30
42 A.7 Example system reliability calculation . 30
43 Annex B (informative) Application of the Weibull distribution to rolling bearing fatigue
44 life . 31
45 Bibliography . 34
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

47 Figure 1 – Calculated design versus apparent failure probability . 5
48 Figure 2 – Example of a system elements tree . 11
49 Figure 3 – Service-life calculation for a 50 % failure probability (top) from a scaled LRD
50 (bottom) . 13
51 Figure 4 – S-N curve for different failure probabilities (Hein et al. 2018) . 15
52 Figure A.1 – Functional elements . 20
53 Figure A.2 – System elements . 21
54 Figure A.3 – Service-life calculation with 1 % failure probability S-N curve (bottom) and
55 failure probability (top) . 24
56 Figure A.4 – Lognormal fit for gear pitting failure probability . 25
57 Figure A.5 – Lognormal fit for gear bending failure probability . 28
58 Figure B.1 – Evaluation of identical data by 2- and 3-parameter Weibull distributions . 31
59 Figure B.2 – Estimation of Weibull parameter γ by 3-parameter Weibull evaluation . 32
60 Figure B.3 – Evaluation of identical data by five test scenarios . 33
62 Table 1 – Failure probability conversion factors for permissible gear stresses . 15
63 Table A.1 – Simplified LRD . 22
64 Table A.2 – Contact stress for each load bin . 22
65 Table A.3 – Number of cycles for each load bin . 23
66 Table A.4 – Number of cycles for each load bin . 23
67 Table A.5 – Damage for each load bin . 23
68 Table A.6 – Damage sum, lifetime margin factor, and lifetime . 24
69 Table A.7 – Bending stress for each load bin . 26
70 Table A.8 – Number of cycles for each load bin . 26
71 Table A.9 – Number of cycles for each load bin . 27
72 Table A.10 – Damage for each load bin . 27
73 Table A.11 – Damage sum, lifetime margin factor, and lifetime . 27
74 Table A.12 – Torsional stress for each load bin. 29
75 Table A.13 – Number of cycles for each load bin . 29
76 Table B.1 – Effect of test scenario on L and Weibull parameters β and η . 33
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

80 INTERNATIONAL ELECTROTECHNICAL COMMISSION
81 ____________
83 Wind energy generation systems -
85 Part 4-1: Reliability assessment of drivetrain components in wind turbines
88 FOREWORD
89 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
90 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
91 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
92 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
93 Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
94 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
95 may participate in this preparatory work. International, governmental and non-governmental organizations liaising
96 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
97 Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
98 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
99 consensus of opinion on the relevant subjects since each technical committee has representation from all
100 interested IEC National Committees.
101 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
102 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
103 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
104 misinterpretation by any end user.
105 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
106 transparently to the maximum extent possible in their national and regional publications. Any divergence between
107 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
108 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
109 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
110 services carried out by independent certification bodies.
111 6) All users should ensure that they have the latest edition of this publication.
112 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
113 members of its technical committees and IEC National Committees for any personal injury, property damage or
114 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
115 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
116 Publications.
117 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
118 indispensable for the correct application of this publication.
119 9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
120 patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
121 respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
122 may be required to implement this document. However, implementers are cautioned that this may not represent
123 the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
124 shall not be held responsible for identifying any or all such patent rights.
125 IEC TS 61400-4-1 has been prepared by IEC technical committee 88: Wind energy generation
126 systems. It is a Technical Specification.
127 The text of this Technical Specification is based on the following documents:
Draft Report on voting
88/XX/DTS 88/XX/RVDTS
129 Full information on the voting for its approval can be found in the report on voting indicated in
130 the above table.
131 The language used for the development of this Technical Specification is English.
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132 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
133 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
134 at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
135 described in greater detail at www.iec.ch/publications.
136 A list of all parts of the IEC 61400 series, published under the general title Wind energy
137 generation systems, can be found on the IEC website.
138 The committee has decided that the contents of this document will remain unchanged until the
139 stability date indicated on the IEC website under webstore.iec.ch in the data related to the
140 specific document. At this date, the document will be
141 • reconfirmed,
142 • withdrawn, or
143 • revised.
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

145 INTRODUCTION
146 Gearboxes historically have been and still are a large contributor to wind turbine operating
147 expenses and downtime. IEC 61400-4 describes requirements for the specification, design, and
148 verification of gearboxes in wind turbines. This Technical Specification (TS) accompanies
149 IEC 61400-4 and describes a method for the calculation of the design reliability of gearboxes
150 for wind turbines. Neither IEC 61400-4 nor this document specify a minimum value of design
151 reliability.
152 The method enables comparison of the calculated reliability of gearbox designs as a function
153 of time. It allows gearbox suppliers, wind turbine manufacturers, wind plant owners, and others
154 to compare different gearbox designs on equal terms. For example, the design reliability can
155 be compared between different gearbox designs for the same load conditions or for the same
156 gearbox in different load conditions. Wind turbine manufacturers and operators can also use
157 the information for defining field service and repair strategies.
158 Currently, not all failure mechanisms that occur in the field have accepted theoretical models.
159 Therefore, the method only provides a quantitative assessment method of the failure
160 mechanisms that can be described with accepted mathematical models for the complete
161 gearbox, stages (functional units), field replaceable units, and individual components. For the
162 calculable failure mechanisms, it is possible to compare the reliability between different gearbox
163 designs within the limitations of the theoretical models. The use of field-based statistical
164 parameters can improve the accuracy of the calculated reliability.
165 As illustrated in Figure 1, there is a difference between the calculated reference and apparent
166 failure probability observed by the users because not all failure modes experienced in the field
167 have a standardized or generally accepted calculation method (Hovgaard 2015).
168 The method described in this document can accommodate additional failure modes in the future,
169 as long these modes are calculable according to a standardized method and are time related.
170 Figure 1 indicates how future inclusion of additional failure modes potentially reduces the gap
171 between calculated and apparent failure probability. Further information can be found in
172 Strasser et al. (2015).
174 Figure 1 – Calculated design versus apparent failure probability
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

176 1 Scope
177 This part of IEC 61400 specifies a method to calculate the design reliability of wind turbines
178 gearboxes covered by IEC 61400-4 based upon failure modes where standardized calculation
179 methods are publicly available. A minimum value of design reliability is not specified. This
180 document does not consider repairable system analysis.
181 The calculated design reliability can provide information for the lifecycle management strategy.
182 However, this document does not provide trade-off decisions between higher design reliability
183 and maintenance strategies (e.g. preventive or predictive maintenance).
184 2 Normative references
185 The following documents are referred to in the text in such a way that some or all of their content
186 constitutes requirements of this document. For dated references, only the edition cited applies.
187 For undated references, the latest edition of the referenced document (including any
188 amendments) applies.
189 IEC 61400-4, Wind energy generation systems - Part 4: Design requirements for wind turbine
190 gearboxes
191 IEC 61400-8, Wind energy generation systems - Part 8: Design of wind turbine structural
192 components
193 ISO 6336-2:2019, Calculation of load capacity of spur and helical gears - Part 2: Calculation of
194 surface durability (pitting)
195 ISO 6336-3:2019, Calculation of load capacity of spur and helical gears - Part 3: Calculation of
196 tooth bending strength
197 ISO 6336-5:2016, Calculation of load capacity of spur and helical gears - Part 5: Strength and
198 quality of materials
199 ISO 6336-6, Calculation of load capacity of spur and helical gears - Part 6: Calculation of
200 service life under variable load
201 ISO 16281, Rolling bearings - Methods for calculating the modified reference rating life for
202 universally loaded bearings
203 DIN 743 (all parts), Shafts and axles, calculations of load capacity
204 3 Terms, definitions, abbreviated terms, units and conventions
205 3.1 Terms and definitions
206 For the purposes of this document, the following terms and definitions apply.
207 ISO and IEC maintain terminological databases for use in standardization at the following
208 addresses:
209 • IEC Electropedia: available at http://www.electropedia.org/
210 • ISO Online browsing platform: available at http://www.iso.org/obp
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

211 3.1.1
212 apparent failure probability
213 failure probability observed in the field, including all failure modes, whether they are considered
214 in the calculated system reliability as described in this document or not
215 3.1.2
216 component
217 part in the gearbox system comprising one or more functional elements
218 EXAMPLE Gear wheel.
219 3.1.3
220 failure mode
221 manner in which a failure occurs
222 EXAMPLE Gear tooth root bending fatigue fracture.
223 3.1.4
224 failure probability
225 F(t)
226 unreliability function
227 probability that the time to failure is lower or equal to the time, t
228 3.1.5
229 failure probability density function
230 derivative of the distribution function, which describes the amount of failures as a function of
231 time
232 3.1.6
233 field replaceable unit
234 individual component or gearbox sub assembly that can be replaced with relatively low cost and
235 effort, considerably less so than a complete gearbox replacement
236 EXAMPLE In typical wind turbine gearboxes, the high-speed shaft and bearings are considered as field
237 replaceable units.
238 3.1.7
239 functional element
240 element of a component providing a specific function
241 EXAMPLE Gear tooth.
242 Note 1 to entry: A component can have several functional elements.
243 3.1.8
244 reliability
245 probability that a product does not fail during a defined period of time under given functional
246 and environmental conditions
247 Note 1 to entry: The term probability takes into consideration that various failure events can be caused by
248 coincidental, stochastic distributed causes and that the probability can only be described quantitively. See
249 Bertsche (2008).
250 3.1.9
251 reliability function
252 R(t)
253 survival probability
254 probability of survival until time, t
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

255 3.1.10
256 system element
257 unique combination of a failure mode with a functional element of a component
258 EXAMPLE Gear tooth root bending fatigue fracture of the sun pinion.
259 3.1.11
260 system element reliability function
261 R (t)
SE
262 failure behaviour of a system element
263 3.1.12
264 system reliability function
265 R (t)
S
266 failure behaviour of the complete system, calculated from the system element failure behaviours
267 and Boolean system theory
268 3.2 Abbreviated terms and units
269 This document uses equations and relationships from several engineering specialties. As a
270 result, there are, in some cases, conflicting definitions for the same symbol. All the symbols
271 used in the document are nevertheless listed, but, if there is ambiguity, the specific definition
272 is presented in the clause where they are used in equations, graphs, or text.
F(t) failure probability %
f conversion factor for tooth root bending strength for failure –
F1
probability, F
f conversion factor for pitting strength for failure probability, F –
H1
f failure free time factor –
tB
i load bin index –
j system element index –
L lifetime where failure of F % of the elements is expected h
F
L bearing rating life for 10 % failure probability (i.e. 90 % reliability) h
L bearing combined modified reference rating life for 10 % failure h
10mr
probability (i.e. 90 % reliability)
N number of shaft load cycles for endurance strength according to –
D
DIN 743 series
N number of gear load cycles for endurance limit according to –
L ref
ISO 6336-6
N allowable number of load cycles for tooth root bending strength for –
FF,i
failure probability, F, and load bin, i
N allowable number of load cycles for pitting strength for failure –
HF,i
probability, F, and load bin, i
N allowable number of load cycles for shaft fatigue fracture for failure –
SF,i
probability, F, and load bin, i
n number of cycles for load bin, i –
i
n shaft rotational speed r/min
R
n relative shaft rotational speed for load bin, i –
rel,i
p material S-N curve slope exponent –
R(t) reliability function –
R (t) system element reliability function for element, j –
SE,j
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R (t) system reliability function –
S
S lifetime margin factor for bending strength for failure probability, F –
LFF
S lifetime margin factor for pitting strength for failure probability, F –
LHF
s relative standard distribution for bending strength for failure %
F50%
probability, F
s relative standard distribution for pitting strength for failure %
H50%
probability, F
T relative torque for load bin, i –
rel,i
t time h
t design life, typically 175 000 h for a 20-year design life h
d
t tooth root bending strength service life h
sF
t pitting strength service life h
sH
t relative time for load bin, i %
rel,i
U critical damage sum –
crit
U damage sum for gear tooth root bending strength for failure –
FF
probability, F
U damage for gear tooth root bending strength for failure probability, –
FF,i
F, and for load bin, i
U damage sum for gear pitting strength for failure probability, F
HF
U damage for gear pitting strength for failure probability, F, and for –
HF,i
load bin, i
U damage sum for shaft fatigue fracture for failure probability, F
SF
Z quantile of the standard normal distribution for failure probability, –
F
F
Weibull shape parameter –
β
γ Weibull location parameter h
η Weibull characteristic life or scale parameter h
σ lognormal distribution standard deviation h
σ shaft endurance strength according to DIN 743 series MPa
ADK
σ shaft fatigue strength for each load bin MPa
ANK,i
σ gear tooth root stress for load bin, i MPa
F,i
σ gear tooth root bending stress limit for failure probability, F MPa
FPF
σ gear tooth contact stress for load bin, i MPa
H,i
σ gear tooth pitting stress limit for failure probability, F MPa
HPF
274 DIN Deutsches Institut für Normung
275 FVA Forschungsvereinigung Antriebstechnik
276 IEC International Electrotechnical Commission
277 ISO International Organization for Standardization
278 LRD load revolution distribution
279 VDMA Verband Deutscher Maschinen- und Anlagenbau
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

281 4 System reliability analysis model
282 4.1 General
283 This document specifies a Boolean approximation for calculating the system reliability function,
284 R (t), of the gearbox. The gearbox elements are represented as a series of blocks, each of
S
285 them representing the probability of a gearbox element failing in one calculable failure mode.
286 The approach makes the following assumptions:
287 • The gear elements currently considered in the model do not have any redundancies.
288 Therefore, the reliability elements are arranged in sequential blocks. This indicates that the
289 model considers the gearbox as failed if any of the individual elements fail. In particular, the
290 individual elements of a planetary gear system are not considered redundant or have
291 parallel paths.
292 • All individual elements within the gearbox are assumed to be independent.
293 NOTE Several other approaches for reliability analysis exist. Standardizing one method for wind turbine drivetrains
294 improves the opportunities to share data between different stakeholders (e.g., operators, manufacturers of wind
295 turbines or gearboxes) and thereby accelerates the validation of the analysis against field data and enriches the
296 model with empiric data.
297 Within these assumptions, the system reliability function, R (t), is calculated as the product of
S
298 the system element reliability functions, R , (t)
SE j
R (t) = R (t)
S ∏ SE,j
(1)
∀j
299 If a system is comprised of multiple identical components, the reliability of each component
300 shall be considered in Formula (1).
301 An example of a system reliability calculation is described in Annex A.
302 4.2 Failure mode identification, classification, and system element assignment
303 For analysing system reliability, the gearbox system is structured according to its sub-systems,
304 components, functional elements, and failure modes. A system element is a unique combination
305 of a failure mode of a functional element of a component. The structured system can be
306 illustrated as a system element tree as outlined with dashed lines in Figure 2, with an example
307 system element "pitting on sun pinion gear teeth".
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

309 Figure 2 – Example of a system elements tree
310 All relevant failure modes are categorized as A1, A2, B or C as described in IEC 61400-4.
311 IEC 61400-4 recommends that all failure modes be identified by the critical system analysis
312 (e.g. failure mode and effects analysis) and failure risks mitigated by analysis or the verification
313 plan. The present reliability calculation method includes only the A1 failure modes described
314 in 5.1.
315 The failure modes should be described in accordance with standardized definitions. ISO 15243
316 provides failure mode definitions for rolling bearings and ISO 10825-1 and ISO/TR 10825-2
317 provide failure mode definitions for gears.
318 NOTE Describing the failure modes in accordance with standards supports the comparison of calculated reliability
319 with field data if the field data follow the same definitions. A calibration of the calculation model by means of field
320 data can be considered to improve calculation model accuracy.
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

321 5 Component reliability analysis models
322 5.1 Individual failure modes
323 The design reliability calculation shall include all category A1 failure modes described in
324 IEC 61400-4 for which a life can be estimated by means of a standardized calculation method,
325 currently including:
326 • gear tooth surface durability (pitting) according to ISO 6336-2:2019;
327 • gear tooth bending strength according to ISO 6336-3:2019;
328 • bearing rolling contact fatigue according to ISO 16281;
329 • shaft fracture according to DIN 743 series or IEC 61400-8.
330 The calculation methodology and assumptions for each failure mode shall be documented.
331 The methodology described herein can be expanded to include additional unique failure modes
332 that demonstrate a similarly standardized life calculation or are supported by empirical data.
333 Plain bearings are designed such that no fatigue failure is expected during the design life.
334 Hence, they are currently not considered in the reliability model.
335 5.2 Gear reliability calculation
336 5.2.1 General
337 The failure probability for gears is calculated for surface durability (pitting) and tooth root fatigue
338 fracture failure modes using the same methodology. The calculation procedure can also be
339 applied to any failure mode for which an S-N curve and corresponding statistical distribution
340 function are available. The basic principle is the damage accumulation according to Miner.
341 The gear reliability calculation is conducted in two parts: estimation of the service life for defined
342 failure probabilities as described in 5.2.2, followed by extrapolation of those service life
343 estimates to very low failure probabilities typical for wind turbine gearboxes as described
344 in 5.2.3. This extrapolated failure probability is then used to estimate the gear system element
345 reliability, R (t), at or even below the design life, t .
SE d
346 5.2.2 Estimation of service life for defined failure probabilities
347 The first part of the calculation estimates the service life of a gear wheel for a defined set of
348 failure probabilities, F, from 0,1 % to 99,9 % in accordance with the service-life calculations
349 described in ISO 6336-6. The load spectrum is determined as described in ISO 6336-6:2019,
350 4.1 based on the specified gearbox loads. The result is represented by an LRD. The stress
351 spectrum and the service life are calculated according to ISO 6336-6:2019, 4.2. The S-N curves
352 for different failure probabilities are determined based on FVA project 304 (Stahl et al. 1999).
353 These S-N curves include the translation of the failure probability from a single tooth to the
354 entire gear wheel.
355 Figure 3 illustrates an example of the service life estimation procedure. The example shows a
356 design LRD for 20 years, which has a failure probability below 1 %. The design LRD leads to a
357 damage sum less than 1, so it can be scaled in time until the damage sum equals 1. The scaled
358 LRD results in a failure probability of 50 %. The resulting scaling factor in time is designated as
359 the lifetime margin factor for failure probability, F. With this calculation, one point in the failure
360 probability progression over time, F(t), is determined.
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362 Figure 3 – Service-life calculation for a 50 % failure probability (top) from a scaled LRD
363 (bottom)
364 The calculation consists of the following steps. The damage sums, U and U , for an LRD
HF FF
365 representing the design life for the S-N curve for a given failure probability, F, are
UU=
∑
HF H,Fi
(2)
∀i
UU=
∑
FF F,Fi
(3)
∀i
366 The damages for each load bin, U and U , are determined from the ratio of the number of
HF,i FF,i
367 cycles in each load bin, n , to the number of allowable cycles in each load bin, N and N
i HF,i FF,i
n
i
U =
H,Fi (4)
N
H,Fi
n
i
U =
F,Fi (5)
N
F,Fi
368 where
p

σ
HPF
(6)
N = N

HFi, L ref

σ
H,i

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p

σ
FPF
(7)
NN= 
FFi, L ref

σ
i
F,

369 and N and p are material parameters. The lifetime margin factors, S and S , are
L ref LHF LFF
U
crit
S =
(8)
LHF
U
HF
U
crit
S =
(9)
LFF
U
FF
370 The critical damage sum, U , is assumed to be 1. Multiplying the lifetime margin factors with
crit
the design lifetime, t , yields the service lives, t and t
d sH sF
t = tS
(10)
sH d LHF
t = tS
(11)
sF d LFF
372 The S-N curves for tooth surface durability and tooth bending strength can be derived for
373 different failure probabilities using appropriate statistical methods applied to suitable fatigue
374 test data. Comprehensive fatigue test data for tooth surface durability and tooth bending
375 strength derived from test gears in FVA project 304 (Stahl et al. 1999) can be described by a
376 standard normal distribution in the endurance range. Figure 4 shows S-N curves for different
377 failure probabilities, F, and their failure probability density functions.
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IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

379 NOTE 1 For purely illustrative purposes, the strength values are shown as normal distributions even though the S-N
380 curves are plotted on a logarithmically scaled diagram.
381 NOTE 2 For purely illustrative purposes, the endurance limit is shown as a horizontal line, whereas ISO 6336-2 and
382 ISO 6336-3 also define a slope in the endurance region.
383 Figure 4 – S-N curve for different failure probabilities (Hein et al. 2018)
384 According to FVA project 304 (Stahl et al. 1999), the relative standard deviation (standard
385 deviation as a percent of mean) for 50 % failure probability is s = 3,4 % for tooth bending
F50%
386 endurance strength of case carburized, shot blasted or shot peened gears and s = 3,5 %
H50%
387 for surface durability endurance strength of case carburized gears. Based on these standard
388 deviations, FVA project 304 yields factors to convert the stress limit from 50 % to 1 % failure
389 probability for case carburized gears. In a similar way, FVA projects 615 yields conversion
390 factors for nitrided gears. Table 1 lists these conversion factors.
391 Table 1 – Failure probability conversion factors for permissible gear stresses
Material processing Surface Root bending Source
durability strength
(endurance) (endurance)
f f
H1% F1%
Case carburized 0,92 0,86 FVA project 304
(Stahl et al. 1999)
Case carburized with controlled shot 0,92 0,92 FVA project 304
blasting or shot peening of root fillet (Stahl et al. 1999)
Nitrided 0,88 0,86 FVA project 615/III
(Hoja et al. 2024)
393 ISO 6336-5:2016 provides stress limits for 1 % failure probability. These stress limits can be
394 converted to 50 % failure probability by
© IEC 2025 – All rights reserved

IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

σ
HP1%
σ =
(12)
HP50%
f
H1%
σ
FP1%
σ =
(13)
FP50%
f
F1%
395 Stress limits for different failure probabilities can be calculated by means of the median, the
396 relative standard deviation, and the quantile of the standard normal distribution for either pitting
397 or tooth root strength
σ σ 1+sZ
( ) (14)
HPFFHP50% H50%
σ σ 1+sZ
(15)
( )
FPF FP50% F50% F
398 where
−1
Z 2 erf 2Ft−1 (16)
( ( ) )
F
-1
399 NOTE erf is the inverse Gaussian error function.
400 Repeating the calculation procedure for several S-N curves with different failure probabilities,
401 F, ranging from 0,1 % to 99,9 % gives the service life estimation shown in Figure 3. The service
402 life should be estimated with at least 12 steps of 0,1 %, 1 %, 2 %, 5 %, 10 %, 30 %, 50 %, 70 %,
403 90 %, 95 %, 98 %, and 99,9 % to achieve sufficient convergence for the lognormal curve fitting
404 and extrapolation of failure probabilities to the design life described in 5.2.3.
405 5.2.3 Extrapolation of failure probabilities to the design life
406 A wind turbine gear component with safety factors according to IEC 61400-4 typically has a
407 failure probability, F, much lower than 0,1% at typical design lifetimes (e.g. 20 years). The
408 second part of the gear reliability calculation extrapolates the failure probabilities, F(t),
409 from 5.2.2 to or even below the design life, t , to determine the gear system element reliability,
d
410 R (t).
SE
411 A lognormal function should be fit to extrapolate the service life estimates from the higher failure
412 probabilities of 0,1 % to 99,9 % (see Bertsche (2008) and VDMA 23904) to the failure
413 probability at the design life, F(t ). The mean, 𝜇𝜇, and the standard deviation, σ, of the lognormal
d
414 function should be obtained using a numerical regression method such as maximum likelihood
415 estimation or rank regression (least squares). The system element reliability based on the
416 lognormal function is
lnτμ−
( )
t
−
1 2
2σ (17)
R t 1− e dτ
( )
SE, j ∫
τσ 2π
417 Other distribution functions such as Weibull can be used if proven by test data.
418 5.3 Rolling bearing reliability calculation
419 The relevant failure modes for rolling bearings in wind turbine gearboxes are described in
420 IEC 61400-4. Of these modes, only rolling bearing contact fatigue is considered in the reliability
421 calculation because it is the only one that has a standardized calculation model. However, it is
© IEC 2025 – All rights reserved
=
=
=
=
IEC DTS 61400-4-1 © IEC 2025 88/1124/DTS

422 not advisable to select or optimize a rolling bearing arrangement based on a calculated
423 theoretical reliability alone, because the actual bearing reliability in service is also dependent
424 on minimum load, rolling element slip, lubricant, lubricant supply, and heat dissipation.
425 For values of t below L , the rolling bearing fatigue life is usually described by a 3-parameter
426 Weibull distribution (see also Annex B). The theoretical reliability value for a 3-parameter
427 Weibull distribution is
β
t−γ
−

(18)
η

R te=
( )
SE, j
428 ISO/TR 1281-2 provides values for the Weibull shape parameter of β = 1,5 and a lower limit for
429 the Weibull location parameter of γ = 0,05 × L , which are also used for the calculation of the
430 life modification factor for reliability specified in ISO 281. This estimation of the Weibull location
431 parameter, γ , was chosen to be deliberately conservative, so the actual reliability of a rolling
432 bearing will be generally higher than calculated.
433 Using the specified values of β and γ from ISO/TR 1281-2 and the bearing rating life
434 requirements from IEC 61400-4, η can be calculated as
1− 0,05
ηL ≈×4,259 L
10mr 10mr
1 (19)
1,5
−−ln 1 0,1
( )
435 which yields
1 if t ≤×0,05 L

10mr

1,5


1,5
R t = −ln(0,90) 
( ) t (20)
 
SE, j
− −0,05


0,95 L
10mr


e if tL>×0,05
10mr

436 for the theoretical reliability value of a bearing where L is the modified reference rating life
10mr
437 calculated for a failure probability of 10 % in accordance with ISO 16281 and IEC 61400-4.
438 IEC 61400-4 limits the ratio of modified reference rating life, L , to the nominal reference
10mr
439 rating life, L . This limitation should not be applied for the reliability analysis described in this
10r
440 document unless experience from operation of similar gearboxes indicates otherwise. The
441 operating conditions of the bearing as specified in IEC 61400-4 can have a significant influence
442 on the calculated bearing life and failure probability.
443 5.4 Shaft reliability calculation
444 Shafts can include features where local
...