Introduction of the new gearbox design standard: a gap analysis between IEC and ISO

Transcription

1 Introduction of the new gearbox design standard: a gap analysis between IEC and ISO Giovanni Nappi, M.Sc. DNV GL Renewables Certification Giovanni.Nappi@dnvgl.com Niels Lerke, B.Sc. DNV GL Renewables Certification Niels.Lerke@dnvgl.com Summary The IEC standard was published on December 2012, thereby becoming a mandatory precondition for Type Certification of Wind Turbine Generators (WTGs) according to the IEC scheme. This long-awaited standard cancels and replaces the controversial ISO , published in 2005 and applicable only to WTGs with rated power under 2 MW. Even though several gearbox suppliers for long time have delivered products already meeting or in some cases even surpassing the new requirements of IEC , some of these are expected to raise the bar with regards to minimum certification deliverables for design, testing, monitoring and maintenance of gearboxes. The present paper intends to analyze the major differences between the design and testing requirements now enforced and compare them to the ones previously enforced. At the same time, this paper provides an overview on the certification requirements that need to be fulfilled by WTG and gearbox suppliers in order to certify gearboxes according to IEC scheme. 1 Historical background The very first attempt to build a common understanding on design and specification of Gearboxes can be dated back to 1993, when the American Gear Manufacturers Association (AGMA) published Information Sheet AGMA/AWEA 921-A97 Recommended Practices for Design and Specification of Gearboxes for Wind Turbine Generator Systems. During years serial gearbox failures started to hit wind turbines. AGMA therefore decided to take the lead on the topic, replacing the Information Sheet with a proper standard in October 2003, when the ANSI/AGMA/AWEA 6006-A03 was published. However, standardization efforts in the field of Wind Turbine Generator Systems had historically been a prerogative of the IEC body, which had also kick-started a preliminary work on design and specification of Gearboxes for Wind Turbine Generator Systems in May 2003 (New Work Item Proposal stage). In order to simplify the normative framework and avoid a conflict of attribution between the ISO and the IEC bodies [1], a Joint Working Group (JWG) between ISO and IEC was started in May Each country was represented by its National Body (be it ISO, IEC or both) and a maximum of two votes per country were assigned to resolve issues. Alongside with the ISO/IEC National Bodies, other interested parties such as certification companies and industry players were invited to take part in the works. The intention was to have ISO quickly releasing the first edition of the WTG Gearbox standard and afterwards have IEC to publish the following editions under the umbrella of the IEC series. In fact, the degree of maturity of the ANSI standard was such that the standard could soon be submitted for approval as a draft by the ISO/TC 60, Gears, Technical Committee, following the ISO fast-track procedure. ISO , ed.1:2005. Wind turbines Part 4: Design and specification of gearboxes [4] was therefore published in October 2005, with the approval of both ISO and IEC national bodies. However, contrarily to the intentions, the replacement and the update of [4] with the IEC document took much longer than anticipated, fundamentally due to a lack of consensus between the parts involved [1]. The first Committee Draft (CD) of [3] was released for ballot only in October 2009 and during 2010 two meetings of the JWG were necessary to adjust it following the JWG members commentaries [2]. The result of this was the release of a Committee Draft for Vote (CDV) in December 2010, which again underwent a rather complicate ballot process, finalized only during 2012.

2 The last round of technical commentaries were reconciled and led to the Final Draft International Standard (FDIS) being circulated for formal approval in September The FDIS was finally approved in November 2012 and in December 2012 [3] was published. The standard therefore cancelled and replaced [4] and thereby became a mandatory precondition for Type Certification of Wind Turbine Generators (WTGs) according to scheme [6]. 2 Range of applicability IEC , ed.1:2012. Wind turbines Part 4: Design requirements for wind turbine gearboxes [3] is applicable to enclosed speed increasing gearboxes for horizontal axis WTG drivetrains, with power rating equal or higher 500 kw, regardless onshore or offshore installation. No upper limit on the power rating is enforced. This is the first relevant difference compared to [4], whose applicability was strictly limited to WTG power ratings in the range of kw. Applicability to larger WTG power ratings was conditioned to appropriate adjustments in the specifications. It should be noted that Amendment 1:2010 to IEC , ed.3:2005 [5] already requires WTG gearboxes to be designed according to [4] and to [3], whenever the latter would have been published. This formulation in the past led to frequent misunderstandings between WTG manufacturers, gearbox designers and certification bodies. As a result, there should be no further doubt about the applicability of this standard to all the HAWT generators. Concerning design concepts covered, [3] covers spur, helical and double helical gears in either parallel or epicyclical arrangement, with rolling bearings. Use of plain bearings is permitted, but rating is not in the scope of the standard. Previously, [4] was applicable to WTG gearboxes in parallel axis arrangement, as well as combined epicyclical and parallel axis arrangements. No significant changes with regards to the gearbox design concepts are identified. In conclusion, [3] is applicable for design, testing and manufacturing of most gearbox models installed on medium/large size WTGs and it erases one of the formal weaknesses of its predecessor [4]. 3 Gearbox design: Gear rating 3.1 Gear rating standards IEC enforces the use of widely recognized gearing standards for calculation against bending, pitting and scuffing failure modes. Moreover, new failure modes to be taken into consideration by the designers have been introduced, namely micropitting (recommended) and subsurface initiated fatigue. The table below summarizes the normative framework for gear rating according to [3] and [4]: Pitting resistance Bending strength Scuffing Micropitting (recommended) ISO [4] IEC [3] ISO 6336:1996 (all parts) ANSI/AGMA 2101-D04 ISO 6336 (all parts) ISO 6336:1996 (all parts) ANSI/AGMA 2101-D04 ISO 6336 (all parts) ISO/TR ISO/TR AGMA 925-A03 DIN DNV Classification Note ANSI/AGMA 925-A02 + ISO/TR (Worst case between total contact temperature and (total contact temperature method integral temperature method) recommended) N/A ISO/TR Subsurface initiated N/A DNV Classification Note 41.2 fatigue Table 1: Reference design gear rating standards

3 The following conclusions can be drawn. The use of ISO is enforced in a definite fashion This requirement was not explicit in [5] which only referred to use of the Palmgren Miner s rule. For scuffing calculations, use of the integral temperature method according ISO/TR is now required, whereas in [4] it was recommended to use only the total contact temperature method. Concerning selection of the valid result, it is specified that the worst case between total contact temperature and integral temperature method shall be considered. Calculation of gears against micropitting is recommended. This is an item that previously was not part of [4]. Hence, for certification purposes it is suggested that either calculations against micropitting are provided or gear designers could justify why this failure mode is not likely to occur with the gearbox under certification. Calculations of the gears against subsurface initiated fatigue (case/core crushing) should be provided or designers should justify why this failure mode is not likely to occur with the gearbox under certification. This item as well was previously not part of [4]. 3.2 Gear rating calculation factors Concerning application-specific calculation factors, [3] bring few substantial changes in comparison with what was already required by [4]. The table below summarizes the main differences identified between [3] and [4] in terms of gear rating calculation factors: Dynamic factor K V Mesh load factor K Face load factor K H Gear accuracy (ISO ) Y Sg ISO ISO , ed.1:2005 [4] IEC , edition 1:2012 [3] 1,05 minimum 1,05 minimum Method B, ISO Results from accurate and proven Multi- Body Dynamic analysis allowed 1:2006 Results from measurements allowed N/A 1,10 (3 planets, for reference) 1,25 (4 planets, for reference) 1,15 minimum Method B, ISO :2006 Numerical analysis of the gear face in helix and profile directions Worst case value to be used for the whole load spectrum 6 maximum (external gears, carburized) 7 maximum (internal gears, carburized) 7 maximum (internal gears, nitrided) 8 maximum (internal gears, through hardened) 1,15 minimum Method B, ISO :2006 Numerical analysis of the gear face in helix and profile directions Results of the calculation to be verified by testing 6 maximum (external gears) 7 maximum (internal gears) Grinding notches to be analyzed with FEA or alternatively ISO Y NT, Z NT 0.85 at 1x10 10 cycles 0.85 at 1x10 10 cycles Gear surface roughness R a 0,8 m (with exceptions) R z 5,0 m (with exceptions) Flank shot peening not allowed Material strength ISO (MQ quality level as a minimum, with exceptions) Table 2: Gear rating calculations calculation factors R a 0,8 m (external gears, flank) R z 4,8 m (external gears, ground regions) R a 1,6 m (internal gears flank) R z 9,6 m (internal gears ground regions) Flank shot peening not allowed ISO The following observations can be drawn from the table below. First, [3] introduces standard values for the mesh load factor K, a factor which accounts for uneven distribution of the torque between gear stages (torque split designs) or between planet gears in case of epicyclical gear stages. The mesh load factor is to be seen as a multiplicative factor of the application factor K A. Default values for K are now given in [3] for planetary gear stages. For reference, only the values to be used in case of three and four planet gears are reported above. It shall be added

4 that the more the planet gears per stage, the higher the mesh load factor will be according to Table 3 in [3]. The second important change with the introduction of [3] is that the values of the face load factors K H and K F are to be calculated using numerical contact analysis according to Method B from ISO and ISO respectively. The method for determining the extreme value of the mesh misalignment is not as per ISO :2006, but instead follows an ad-hoc formula provided in [3]: For fatigue calculations For extreme calculations The mesh misalignment factor takes into account the helix slope deviation of the pinion, of the gear and the resulting mesh misalignment caused by deviations of the shaft alignment due to manufacturing variations that might affect relative shaft parallelism. Examples of influences that have to be taken into account and documented individually are: In- and Out-of-Plane deviations of the housing or planet carrier, as per ISO/TR Variation of bearing clearance and deflection under load Variation of deflection of housing and planet carriers under load Influence of planet carried alignment tolerance The numerical contact analysis shall be carried out to derive the load distribution along the profile direction and the helix directions, if any. As a minimum, it should account for: Influence of adjacent meshing teeth Influence of bearing operating clearances Influence of deflections of shafts, housings, carriers and bearings Influence of local discontinuities in the stiffness at the extremities of the contact area Apart from the requirements concerning the execution of the numerical analysis, it is worth mentioning that a minimum value of 1,15 for K H and K F shall be used, even if the results of the numerical analysis should show a lower one. Furthermore, [3] requires that final validation of the face load factors takes place when the results of the gearbox prototype testing (contact load pattern) are available. Concerning the load enhancing gear velocity factor, K v, [3] opens the possibility of using not only results from numerical analysis allowed (this was already allowed by [4]), but also results from specific testing, in order to derive a more accurate value. Nevertheless, the minimum required value shall be 1,05. This requirement is in line with the provisions of [4]. With regards to strength of materials, tooth shape factor and operating condition factors, there are no changes compared to [4]. Concerning tooth stress correction factor Y SG, [3] prescribes that unintended notch factors at the root of the tooth caused by grinding should be analyzed using the Finite Element Method in order to derive a more accurate value. This is a slightly more demanding requirement compared to [4], which referred only to calculation carried out using ISO factors. However, this factor Y SG has an influence only for gear teeth with small modules, thus for a standard, well designed gearbox, its effect should be negligible. Last but not least, it shall be mentioned that [3] extends the requirement of following up the inspection of the gear surface temper after grinding to 100% of the manufactured gears, compared to [4] which only required it for a sample of the manufactured gear population. This requirement surely mitigates the risk of manufacturing and shipping gears with an undesirable tooth surface quality. On the other hand, though, the cost of this quality assurance measure is going to be sensibly higher, as a larger number of inspections and/or inspectors will be needed.

5 In conclusion, the major changes introduced by [3] with regards to gear rating calculations are mostly connected to a more accurate validation of many design hypothesis that are made when calculating using ISO 6336 series. Most of these validation steps require a certain engineering manpower as well as adequate testing resources. 3.3 Gearbox design: gear rating safety factors Regarding minimum safety factors allowed for gears, based on 99% quantile material properties, the major change introduced by [3] is the definition of safety factors for static root bending and tooth surface failure under extreme loads, which were not explicitly stated in ISO It shall be added though, that these requirements had always formed an integral part of [5]. In general the updated safety factors for gears are a synthesis of the values previously reported in [4] and [5]. The static strength safety factors have however been lowered compared to the requirements in [5], as the table below reports: ISO [4] IEC [5] IEC [3] Pitting resistance, S H 1,25 1,20 1,25 Surface static resistance, S H N/A 1,20 1,00 Bending fatigue strength, S F 1,56 1,45 1,56 Bending static strength, S F N/A 1,45 1,40 Max scuffing risk allowable (ANSI/AGMA 925-A02) 5% - 5% Scuffing S S worst case between ISO/TR and 1,25 1,30 1,30-2 to be considered Table 3: Minimum safety factors for gearings (based on 99% quantile material properties) For gear manufacturers that have historically followed provisions of [5] and [6], the safety factor table above will not imply any radical change. Adding on to that, the new table puts an end to all the inconsistencies between standards with regards to safety factors for gears, and does not introduce radical changes compared to what required in standard IEC certifications. At the same time, though, neither requirements nor recommended safety factors are given for micropitting and subsurface failure calculations. In principle this could lead to misunderstandings and conflicts between gear manufacturers, WTG manufacturers, certification bodies and wind farm owners. A possible way forward could consist in leaving the choice of the desired safety against this failure mode directly to the WTG manufacturers and their technical product specifications to the gearbox suppliers. 3.4 Bearing rating With regards to bearing rating calculations, several new requirements are introduced with [3]: Bearing design considerations First of all, a large part of the section on Gearbox design is dedicated to general considerations on bearing design and reliability. Compared to the advices on choice of bearings that were included with [4], IEC specifically mentions that the bearing damages historically seen in wind turbine gearboxes could depend on many different failure modes, as defined in ISO 15243:2004: Subsurface initiated fatigue Surface initiated fatigue Adhesive wear Moisture corrosion Frictional corrosion Excessive Electrical Voltage (lightning strikes) Electrical current leakages

6 Overload Indentation from debris Indentation from handling For many of these failure modes, no calculation methods are available, but only general design measures that might be adopted. Nevertheless, mentioning these possible failure modes in [3] creates more awareness in bearing suppliers, gearbox manufacturers and WTG manufacturers, which are somehow compelled to adopt an integrated design, manufacturing and assembly approach in order to mitigate the risk of these failures occurring serially. Second, it is explicitly stated that the steel quality of bearings to be used in Wind Turbine Gearboxes shall meet ISO 683 requirements with regards to chemical composition, steel cleanliness, steelmaking process, heat treatment and micro-structure. Referring openly to ISO 683 series for bearing steel quality is an important step forward: ISO 281:2007, the calculation standard most often used for bearings, requires only steel bearings of a good quality to be used for the standard to be applicable. No reference to what good quality might mean has ever been explicitly given and no direct link between ISO 281 and ISO 683 has ever been mentioned. As [3] asserts this link so directly, it could be expected that this ambiguity of ISO 281 is now removed. This is only partially true: in fact, limits for steel cleanliness as given in Table B.1 of ISO :1999 are meant to be only informative, not normative. However, next edition of ISO (already at FDIS stage) will define normative limits and evaluation methods with regards to bearing steel cleanliness. It is thus expected that also this bearing design issue can be finally clarified Bearing static rating [3] requires bearings to be rated using the internal load distribution from detailed models such as the ones described in ISO/TS Instead, [4] did not mention explicitly how to account for the internal load distribution, and referred to ISO 76 in case methods to account for this were not available. The requirements for the static safety factors are unchanged compared to [4]: at least 2,0 against extreme loads and 3,0 against the maximum operating load as described in the load spectrum Bearing lifetime rating With regards to lifetime rating, again it is expected that detailed models as per ISO/TS are used to account for the internal load distribution. This is the first large change compared to [4] which was instead referring to ISO 281 BBl.4, until this became finally superseded by ISO/TS in 2008.As a minimum, 90% survival probability should be used, but wind turbine manufacturers are free to specify higher survival probability for the bearings. Proprietary calculation methods as the ones developed by many bearing suppliers are also allowed, but the results obtained should still be compared with the ones obtained using ISO/TS This corresponds to the provisions of [4], and discrepancies between the two methods are still expected to be justified by the parties involved in the design. In terms of lifetime requirements, the modified reference rating life must meet or exceed the design lifetime for the gearbox, which is supposed to be equivalent to the design lifetime of the WTG. An upper limit to the modified reference rating life is imposed: in case L 10mr is ten times greater than the basic reference rating life L 10r, then L 10mr shall be set to 10 times L 10r. This is to avoid too optimistic considerations concerning lubrication conditions and material fatigue limits. It is mandatory that the rating life calculations are performed bin-by-bin using the load spectrum specified by the Wind Turbine Manufacturer. The fractions of the bearing life consumed in each bin shall then be added linearly. An exhaustive guidance on how to reduce the number of bins in the spectrum is given, in order to facilitate execution of calculations with flexible surrounding environment. L 10mr shall thus include the effects of:

7 Radial, axial and moment loads Internal arrangement of bearing Operating internal clearance Elasticity of surrounding structures (carriers, rings, shafts, etc.) Load sharing between rolling elements Load distribution along rolling element length, taking into account the profile and the truncation of the contact area Lubricant viscosity and cleanliness at operating temperature (assumptions to be verified during Type Testing module, within the gearbox field test campaign) Gear load offset (deviation of the axial mid position of the gear to resulting load impact. The assumed constant gear load offset for the whole load spectrum shall be verified by calculation) Mesh load factor K as discussed in section 3.2. Finally, contact stresses calculated using the dynamic equivalent bearing load shall also be documented and not exceed reference values, as given in Table 7 of [3]. In this case, compared to [4], the only relevant change is that for planet bearings the maximum contact stress allowable, p max, grows from 1450 MPa to 1500 MPa. In conclusion, compared to [4] IEC introduces a set of relatively extensive and welldefined design requirements for WTG gearbox bearings which will definitely help mitigating the risk of a serial failure in this delicate component. 3.5 Design of shafts Regarding design of shafts, there are no significant changes in [3] compared to [4]. The design standards accepted are DIN 743:2000 (now replaced by DIN 743: ) and ANSI/AGMA The element of novelty consists in the indication of the minimum required safety factors, which are now found to match the indications given in IEC , ed.3:2005, as it can be seen in the table below: ISO [4] IEC [3] Design standard DIN 743 or ANSI/AGMA 6001 DIN 743 or ANSI/AGMA 6001 Minimum SF yielding N/A (1,2 recommended by DIN 743) 1,30 DIN 743 (includes the SF of 1,2 recommended by DIN 743) 1,30 ANSI/AGMA 6001 Minimum SF 1,75 DIN 743 fatigue N/A (1,2 recommended by DIN 743) (includes the SF of 1,2 recommended by DIN 743) 1,75 ANSI/AGMA 6001 Table 4: Comparison of shafts design requirements The major change for gearbox designers is that the safety factors recommended in DIN 743 (1,2 for both fatigue and ultimate limit states) are no longer accepted and are overruled by the ones given in [3]. It is a precondition that design loads including dynamic effects are used (calculation based on nominal torque only are not allowed). 3.6 Housing joints With regards to housing joints, it is required that fasteners are made of at least of ISO 898 grade 8.8, selected following the requirements of ANSI/AGMA 6001 or by material tests, accompanied by corresponding certificate. However, compared to [4] provisions, Grade 10.9 and 12.9 are allowed but only with the following restrictions: Sourcing of high strength steel shall be subject to strict control Phenomena of hydrogen embrittlement have to be prevented Each batch of hardware shall be sampled and subject to tensile stress In case of epicyclic gears, where the ring gear is in-built with the flanged part, [3] requires that the joint shall transmit the maximum design load by means of friction. Here the change

8 compared to [4] is substantial, since [4] had only formulated a generic recommendation that the joint should transmit the maximum design load by means of friction. According to [3], use of solid pins is mandatory in case the friction capacity of the bolted connection is not sufficient. However, the contribution of the bolted joint friction capacity shall not be accounted for in the pin calculation. This apparently minor change between [3] and [4] leads to a very practical consequence, where the number and sometimes the size of the bolts needed in the housing joints is considerably increased with regards to [4]. On the other hand, this higher requirement has the effect of mitigating the risk of several issues connected with relative movement of the flanges, such as fretting, flanged parts wearing, corrosion and spills of lubricant. 3.7 Gearbox structural parts [3] defines gearbox structural parts as non-axisymmetric components capable of transferring variable as well as non-variable loads of the wind turbine. In a typical gearbox the following components are considered as structural parts: Torque arms or torque reaction flanges Planet carriers Gearbox housings or any other component which transfers major loads Typical assessments to be carried out for these parts are deflection analyses and strength verifications. Analysis of the deflection of the structural parts is important in order to verify that a too flexible structural part causes excessive misalignments in gears and bearings, improper function and/or interference with other components. The strength verification is relevant because the gearbox by all means is the component that transfers the rotor torque reaction to the rest of the WTG structure. [3] allows for calculation of stress and strains for the structural parts by means of either analytic or numerical models, such as FEA. Recommendations on the use of FEA are given, such as focus on mesh sensitivity and inclusion of neighbouring elements to avoid stress fields disturbed by the application of the boundary conditions. Not only external forces, moments and displacements should be considered as loads, but also shrink fits and bolt preloading among others. Concerning material behaviour, in general linear elastic behaviour is accepted as long as the components do not show significant yielding. In case of a brittle material however, the limit state is component rupture, described by the maximum principal stress hypothesis. For ductile components, the Von Mises or Tresca criteria and the yield limit state, with limited capability of stress redistribution, are better suited. With regards to material properties, reference is made to either statistically assured material test, including determination of S/N curves, or to literature data and/or methods, such as [7] for synthetic S/N curves. Partial material safety factors for structural parts are clearly indicated in the standard. These detailed requirements clarify the requirements and the rules for certification of the gearbox structural parts. Several new aspects are considered compared to [4], which took explicitly into account only the following ones: Housing distortion from thermo-mechanical deformations Accuracy of housing machining and insurance of shaft alignment tolerances Allowed materials are ductile cast iron, cast steel or fabricated steel Alongside, general requirements from [5] could be applied to the gearbox housing. The benefit of having explicated this in [3] is obviously large and contributes to further clarity in the certification requirements and in the assurance of the gearbox integrity. 3.8 DriveTrain Dynamic Analysis The greatest element of novelty introduced by [3] with regards to WTG gearbox design is perhaps the DTD analysis, which was previously not part of [4]. Three goals are to be attained by means of the Drivetrain Dynamic Analysis:

9 1. Verify and confirm the modeling of the gearbox in the WTG aero-elastic model (i.e. gearbox stiffness) 2. Verify the occurrence of gearbox-specific loads due to dynamic amplification 3. Assess influence of boundary conditions on the internal gearbox loading The DTD analysis is normally carried out by setting up a detailed model of the gearbox and its subcomponents in an appropriate simulation environment. The model should also include components of the drivetrain whose dynamic properties are expected to contribute to the gearbox dynamic response (e.g. low speed rotor including coupling, shaft, hub and eventually blades, high speed rotor including coupling, shaft and generator). The modal response of the gearbox and the gearbox behavior is simulated in the time and frequency domains, in order to verify the occurrence of dynamic amplification phenomena caused by fundamental forcing frequencies of the WTG and gearbox specific frequencies, such as gear mesh frequencies, shaft rotational frequencies and their harmonics. Minimum requirements for documentation of this exercise are included in the standard. This requirement is considered to have the following effects: 1) Once again, ascertain that the design of the gearbox cannot be seen as independent from the design of the WTG and in particular its aeroelastic response 2) In order to carry out such studies, specialized software (Multi-Body Dynamics software) and the engineering manpower needed to manage it is needed, with the consequent increase of the complexity level and costs 3) A close cooperation is needed between gearbox designers and WTG aeroelastic designers, in order to exchange data that are mutually needed for the analysis 4) Results from the DTD analysis are then validated using experience from the field or from the workshop test benches. 4 Gearbox Testing Requirements Requirements for testing of gearboxes are much stronger with the introduction of [3] compared to [4]. First of all, the scope of the test campaign is to be agreed not only between the gearbox manufacturer and the wind turbine manufacturer, but also with the bearing supplier and even the certification body. The gearbox test plan can be seen as a combination of two test campaigns, each consisting of different tests: 4.1 Gearbox Prototype Testing 4.2 Gearbox Serial Testing 4.1 Gearbox Prototype Testing The gearbox prototype Test Campaign consists of the following tests: Workshop Test Concerning Workshop Testing, there are few differences compared to the requirements described in ISO Notably, mandatory quantities to be measured include: Gear contact pattern at each load step (same as ISO ) Lubricant cleanliness Temperatures at each load step Vibrations at each load step The gearbox shall be completely disassembled after the test and inspected. Compared to [4], where duration of the prototype testing was not specified, [3] brings clarity with regards to this point, requiring that the test is run for each defined load step at least until the temperature is stabilized, and minimum six hours.

10 4.1.2 Robustness test The robustness test is a new addition to the WTG gearbox certification requirements compared to the provisions of both [4] and [6]. It consists in an overload test whose aim is to identify weak links in the design of the gearbox. However torque level and duration of the test are not specified in [3], leaving it to the WTG and gearbox manufacturers to define its scope. From the certification point of view, it is important that the test is carried out relatively early in the process, so that the identified weak link and the corrective measures can be part of the certification scope, increasing the value of the certification Field test Field test was not originally part of [4]. This activity was instead introduced only with [6] as a mandatory part of the Type Certification activities. With [3] the minimum requirements for Gearbox Field Testing are now fixed. The scope of the field test is to measure: Gearbox design loads as calculated by the WTG manufacturer aeroelastic model and DTD Analysis Torsional vibrations at selected events, such us run up through several speed ranges, cut-in at transition winds and high winds, shutdown events, emergency stops, brake application, idling and backwind idling, low voltage ride-through Measured Campbell diagram HSS torque and rotational speed LSS torque, if applicable Drivetrain resonances are avoided and vibration levels correspond to workshop test locations Lubrication and cooling efficiency, by measuring oil inlet/outlet and oil sump temperature. Attention is put into avoiding aliasing phenomena in the data acquisition system: therefore the sampling frequency shall at least be 3-5 times the relevant vibration frequency which is to be measured. 4.2 Gearbox Serial Testing Requirements Serial testing was originally not included in the scope of [4], although some of these tests were executed only on the gearbox prototype. The scope of this mandatory testing activity includes: Factory Acceptance Testing Sound Emission Testing Vibration Testing Factory Acceptance Testing This is a bench test that shall include at least three load steps, including nominal torque loading conditions. The test specifications and the corresponding acceptance criteria are to be agreed between the gearbox manufacturer and the wind turbine manufacturer. Lubrication cleanliness and system temperatures shall also be measured at the same location where they will be monitored in the Wind Turbine Sound emission testing This activity was only carried out for the prototype gearbox in [4], while with [3] becomes an element of the serial testing. Reference standard for test execution is ISO , as in [4] Vibration Testing Also this activity was carried out only for the prototype gearbox in [4], while with [3] becomes an element of the serial testing. Reference standard for test execution is ISO , as in [4].

11 4.3 Conclusions on Gearbox Testing All in all, [3] puts a strong focus on testing activities compared to [4]. It defines more clearly the scope of each test, while maintaining sufficient flexibility for the wind turbine manufacturer and gearbox manufacturer to select the applicable test acceptance criteria. Many tests have been designed to reproduce the actual operating and extreme conditions that the gearbox will experience during its lifetime, which creates added value for all the WTG industry players and boosts the confidence in the product delivered. On the other hand, though, both prototype gearbox tests and factory acceptance serial tests compel gearbox manufacturer to sensible investments in terms of test benches and manpower to manage this. 5 Integration of gearbox Component Certificate into WTG Type Certificates A consistent integration of an gearbox Component Certificate according to [3] into a WTG, whether already covered or not by a type certificate according to [6], requires some further steps, which are summed up below: 1) Comparison between gearbox design loads and WTG certified loads, including justification or update of load reserve factors. 2) Execution or update of the DTD analysis, including physical properties of the WTG where the gearbox is meant to be installed on. 3) Integration of the mechanical, electrical and control interfaces of the gearbox in the WTG design. 4) Integration of the Installation, Operation and Maintenance procedures of the gearbox in the WTG manual system. 5) Execution of the gearbox field test campaign: obviously this requires a prototype of the WTG under certification where the gearbox can be installed for the duration necessary for the test campaign. The applicability of these steps is subject to a joint evaluation by the certification bodies involved, the WTG manufacturer and the gearbox manufacturer. 6 Conclusions The recently published IEC brings a number of novelty elements into design and validation of WTG gearboxes. The present paper has shown how some of the new requirements impact stakeholders, not only in terms of certification efforts, but also in terms of knowledge and test equipment. However, a better definition of the design and certification requirements has also a positive impact. In particular it allows a more efficient process for design and validation of gearboxes, by better defining the rules that need to be followed. Not only that, defining strong yet flexible requirements increases also the added value that all industry stakeholders are able to deliver thanks to a solid, trusted and internationally recognized product. 7 References [1] Bradley B. Gear Technology; Randall Publications, July 2009; [2] McGuinn J. GearTechnology; Randall Publications, March/April 2011; [3] IEC , ed.1:2012. Wind turbines Part 4: Design requirements for wind turbine gearboxes [4] ISO , ed.1:2005. Wind turbines Part 4: Design and specification of gearboxes [5] IEC , ed.3:2005 including A1:2010. Wind turbines Part 1: Design requirements [6] IEC , ed.1:2010. Wind turbines Part 22: Conformity testing and certification [7] Gudehus H, Zenner H. Leitfaden für eine Betriebsfestigkeitsrechnung; Stahleisen-Verlag, January 1999