Wind turbine gearboxes - KISSsoft AG

Wind turbine gearboxes 


KISSsoft AG 

Frauwis 1 

CH-8634 Hombrechtikon 

www.KISSsoft.ch 

Slide 1, August 05, H. Dinner, hanspeter.dinner@KISSsoft.ch

Gear analysis for the gear buyer 

Qualified interaction between 

gear manufacturer and gear buyer 

Quick yet 

detailled 

concept 

analysis 

Second 

opinion 

Buil up of 

know-how, 

„Local 

content“ 

Gear, shaft, bearing, shaft-hub analyis with KISSsoft & KISSsys 



KISSsoft / KISSsys in wind turbine gear analysis 

-Calculate the Kinematics (power split in compound gearboxes) 

-Do a quick and fairly accurate strength / life rating 

-Size a set of gears (helical or planetary) quickly for a given load and 

required safety / life rating 

-Consider shafts, gears shaft-hub connections and bearings 

simultaneously 

-Compare different gearbox concepts, good preliminary design 

-80% of the answers at 20% of the costs 

-No dynamic analysis 

-No load pattern analysis (LVR) 

-Support, training, consultancy services available 



Development of wind turbine power 

Today: 

Multibrid M5000: 5MW by 

PROKON Nord using Renk 

Multibrid gearbox 

REpower 5M: 5MW by 

REpower using Winergy 

(two planetary + one helical 

stage) or Renk Aerogear 

gearbox, tip height 183m 

On-shore -> Off-shore 



Gearboxes in a wind turbine 

Focus in this presentation: main drive between 

rotor and generator 



Design driver 

1. Power generation is a function of third power of rotor 

diameter 

2. Optimum blade tip speed however is a linear function of 

wind speed, maximum blade tip speed is given 

3. Increasing power with maximum speed -> increasing the 

torque 

4. Gondola mass should be minimised 

-> Gearboxes with high power density and high input torques 



Gondola masses 

•Tower gondola mass directly affects costs of foundation, tower, 

transport, assembly (availability of cranes) 

•Gearbox mass (current maximum): 60’000kg 

Turbine 

Nominal Power 

Rotor Diameter 

Gondola mass 

Specific mass 

Vestas V90 

3MW 

90m 

108to 

17kg/m 2 

GE-3.6, offshore 

3.6MW 

104m 

272to 

32kg/m 2 

NEG Micon NM110 

4.2MW 

110m 

219to 

23kg/m 2 

Enercon E-112 

4.5MW 

114m 

500to 

49kg/m 2 

Multibrid 

5MW 

116m 

285to 

27kg/m 2 

Repower 5M 

5MW 

126.5m 

352to 

28kg/m 2 




Loads on main drive 

•Production loads 

•Speed / torque for different wind speeds 

•Wind speed profile (depending on site, simulated by special 

software) results in time dependent torque / speed profiles 

•Torque / speed is classfied into a load spectra, used for design 

•Load assumptions are verified once wind turbine is operational 

•Braking loads 

•Peak loads 

•Negative torque 

•Exceptional loads 

•Start up (cold start with cold lubricant) 

•Operation at small speed / torque (idling) 

•-40C…+50C, salty environment 


Gearbox principles 

•Low noise, low vibration over a wide torque / speed range 

•Low mass, low stiffness (of gearbox and of the support) 

•Differenet loads: operating, braking, emergencies 

•Load peaks, unknown loads 

•Environment 

6 

•Or compound / 

differential gearboxes 

•Not used anymore: 

average power rating of 

new turbine today > 

1.5MW 

5 

4 

3 

2 

1 

0 

Tw o / three stage helical 

Nominal Pow er, [MW] 

Planetary stage / tw o 

helical stages 

Tw o planetary stages / 

helical stage 

Others 



One planetary, two helical stages 

•Helical gears for low 

noise 

•Input on carrier 

•Output on sun 

•Offset required 




•100% of power flows through 

first planetary stage 

•Ring gear is part of casing, 

structure borne sound 

•Shrink disk to attach to propeller 

shaft 

•Usually three planets 

•Sun free to move, not directly 

connected to next gear 




•CAD model 

•KISSsys model of 

power flow 

•KISSsys tree 

•Arrangement of gears, 

couplings, shafts 

(bearings not included) 

•Three kinematic 

constraints (input speed, 

input torque, ring speed 



Two planetary, one helical stage 

•Driving the ring planet carrier with a 

fixed ring gear allows for high ratios 

•Winergy 2.5MW 

•KISSsys schematic 

•KISSsys tree 

•KISSsys 3D 

•Load spectra 



Compound planetary gearboxes 

•Internal load splitting 

•Ring gear may not be part of casing 

•Ring gear may be driven 

•Load is shared between planetary stages 

•Fixed planet carrier simplifies lubrication 

•Axial loads on carrier from ring gear 




•Solution proposed by MAAG, 

eta=97.8% 

•Sound at max power = 99dB(A) 

•Five / seven planets 

•Study by Winergy 




•KISSsys allows for 

quick modelling of 

compound planetary 

gearboxes of any type 




•Only one planet 

shown 

•Automatic collision 

check is performed 

•Input: on first ring and second planet carrier (red) 

•First sun connected to second ring (green) 

•Followed by helical (white) and high speed 

planetary stage (pink) 



Differential gears 




•1a: Fixed ring gear, carrier 

is driving (epicyclic set) 

•1b: Fixed planets, ring is 

driving the sun through the 

planets (stationary set) 

•1c: Ring and carrier are 

driven by previous stages, 

sun speed is difference of 

ring and carrier speed 

•1d: Helical offset 

•Not only power is 

summarised but also speeds 




•Rexroth (Bosch Group) 

drive 

•Target power range: 5MW 

•High power density due to 

power split 

•Requires same space as 

typical 2MW gearbox (!?) 

•Three planets per stage, 

floating sun 

•i=25…60 

•Power on first stage: 65% 

•m=18.7to (Pnom=3.1MW) 

•Mass saving 10%-22% 



Integrated design: Gearbox, 

generator and azimuth drive in 

one part 

Planet carrier is fix 

Climate control 

Gondola mass: 300to 

Multibrid M5000, Renk 



Multibrid M5000, Renk 

Four planets 

Hydrodynamic bearings 



Aerogear, Renk 

•Compound planetary 

gearbox, high ratio using few 

elements (reduces losses) 

•Driven on the ring gear 

•Helical stage for offset 

•Planetary bearings are on a 

fixed location, simplifying 

lubrication 

•Double row bearings are 

lubricated from centre 

outwards 

•Specific tooth loads are 

reduced to acceptable levels 



Aerogear, Renk 

•Total efficiency of gearbox >95%, Lhmin=20 years 

•d~2m 

•Four planets 

•Problem of hydrodynamic bearings: start / braking 

•Low noise since ring gear is seperated from casing 

•Principle 

arrangement 

•KISSsys 

tree 

•KISSsys 

schematic 



Torque limiting gearboxes 

•Early wind turbines were operated at one or two fixed rotor speeds 

•Leads to high torques or low efficiency 

•Large turbines are operated with variable rotor speed to generate 

energy also at low or high wind speeds 

•Rotor of a wind turbine takes maximum power from wind flow only 

at a certain ratio of blade tip speed / wind speed 

•Demands with respect to grid compatibility has increased, therefore, 

load peaks should be eliminated 

•Load peaks are driving the gearbox mass 



Henderson Gearbox 

Torque on ring gear of last planetary stage 

is controlled by a hydraulic unit 

However, power is lost 



Henderson Gearbox 

Uses a planetary set where the torque on the ring gear is controlled by 

means of a hydraulic pump with adjustable torque 


KISSsys model of 

power flow 


Voith WinDrive 

Generator 

•Basic problem: Speed at 

generator should remain 

constant but input speed 

(wind speed) is fluctuating 

with low (e.g. morning 

winds) and high frequency 

(gusts) 

•Rotor drives single planetary stage (or double planetary stage), 

combined with single helical stage for axis offset 

•Output speed of single helical stage is then controlled by a 

variable speed gearbox (CVT gearbox) 




•Red: epicyclic gear 

•Blue: stationary gear 

•Yellow: converter fluid 

•Green: converter control blades 


•WinDrive by Voith 

(Germany) 

•Output speed may be 

independent from input 

speed over considerable 

speed range 

•Hydrodynamic torque 

converter reduces load 

peaks and torsional 

vibrations (flexible 

coupling) 

•Increases grid 

compatibility and reduces 

load on machine elements 






•Only small amount of power flows through hydrodynamic path, 

therefore, overall efficiency is maximised 

•Maximum of 10% of the power is taken from the main shaft 

•Output speed is constant 

•Main gear: i=20…30 

•Hydrodynamic “gear” with variable ratio 

•Speed control through hydrodynamic converter 

•Vibration analysis has been performed using multi body 

analysis (3D model) including grid simulation 

•Gondola mass can be reduced since load peaks are reduced 

•Requires control electronics 



Basic formulas for root and flank stress 

•Root stress acc. ISO6336 

•Flank stress acc. ISO6336 

σ 

F 

= σ 

FO 

⋅ K 

A 

⋅ K 

V 

⋅ K 

Fβ 

⋅ K 

Ft 

σ 

FO−B 

= ⋅YF 

⋅YS 

⋅Y β 

bm 

σ 

FO−C 

n 

Ft 

= 

bm 

n 

⋅Y 

Fa 

⋅Y 

Sa 

⋅Y 

ε 

Fα 

⋅Y 

≤ σ 

β 

FP 

σ 

σ 

H 

HO 

= Z 

B 

= Z 

⋅σ 

H 

HO 

⋅ Z 

E 

⋅ 

⋅ Z 

K 

ε 

A 

⋅ Z 

⋅ K 

β 

⋅ 

v 

⋅ K 

Hβ 

⋅ K 

Hα 

Ft 

u + 1 

⋅ 

d ⋅b 

u 

1 

≤ σ 

HP 

Index “0”: Nominal stress level 

K factors: Load increasers 

These stresses are compared with permissible stress levels 

For gear pair. For planetary set, Kγ is considered too 



K factors, KA 

•Application factor 

•Can be used during sizing of gear sets or simplified rating 

•Use either nominal torque with KA or equivalent torque 

•Equivalent torque is calculated according to DIN3990, Part 6, 

Method III or AGMA6006 

•The result (equivalent torque) is a torque with equivalent 

damage as the torque spectrum 

•Limitation: other K factors (Kv, KH/Fbeta, KH/Falpha) are 

assumed to be constant 

•Teq=(∑n*T^p/Neq)^1/p 

Input of KA in KISSsoft 



K factors, KV 

•Dynamic factor 

•Considers dynamic tooth load based on gear speed and tooth 

stiffness 

n E 1 

30⋅10 

= 

π ⋅ z 

1 

3 

⋅ 

cγ 

mred 

Input of KV in KISSsoft 



K factors, KHβ, KFβ 

•Face load factor for flank and root 

•Considers uneven load distribution over face width 

•Due to deformation of gear, shafts, ring gears 

•Due to misalignement of shafts, gears 

•Deformation of tooth flank is corrected by form grinding 

•Such that under load, even load distribution is achieved 

Input of KHβ, KFβ in KISSsoft 



K factors, Kγ 

•Load sharing factor Kγ considers that the load is not equally shared 

by the n planets 

•Regulations by DNV, GL or standards like ISO6336, AGMA6006 

give information on Kγ to be used 

•Kγ=1+0.25*√(n-3) according to GL, DNV 

•Kγ=1.15…1.23, for four planets, according ISO 

•AGMA6006? 

•AGMA6123? 

•Low Kγ values require accurate machining / assembly or a flexible 

planet carrier pin (see below) 



K factors, Kγ 

•Torsional wind-up of double sided planet carrier 

•Position accuracy of the pins on the carrier 

•Misalignement between carrier and ring gear 

•Statically overdertimed support of floating sun with number of 

planets higher than three 

•Bearing clearances (of the planet bearing) 

•Floating sun gear is required 

Input of Kγ in KISSsoft 



K factors, Kγ, the FlexPin 

•Introduced by Ray Hicks in the sixties 

•Widely used e.g. by MAAG 

•Allows simple planet carrier (disk) which 

allows for tighter positioning of planets 

•Also for misalignment between ring gear 

and planetary carrier 

•Sun gear need not be floating anymore 

F 








•Pretensioned tapered roller bearings (no 

axial play, beneficial at low loads) 

•Bearing cages are directly integrated into 

flex pin and gear 

•Pretension reduces axial play wich reduces 

tilting of helical planets, reducing Khβ 

•Single sided planet carrier will not tilt 

planet carrier shaft (compared to double 

sided carrier) 

•No wandering of races possible 

•Flexibility may reduce load peaks (?) 

•However, spur gears are recommended 

•Gear wall height should be at least 3*mn 


Damages on gearboxes 

•20-25% of all wind turbine damages are on the gearbox 

•Toothing and bearings are critical 

•Gearbox is located between the two large rotating masses, that is the 

propeller and the generator -> dynamic torque peaks (up to 3.5) 

•Wind turbine gearboxes therefore should use higher safety factors 

compared to gearboxes used in stationary power generation 

•Failure are typcially premature 

Probability of failure 


Typical product 

Wind turbine gearbox 

Time 



•Load assumptions 

•Knowledge of load spectra is required, requires dynamic analysis 

of whole drivetrain, wind-propeller-gearbox-generator-grid 

•Not only torsional vibration, also translational vibration 

•Torque load spectra will not be sufficient any more 

•Multi body analysis is time intensive / expensive 

•Lubrication 

•Filtering of oil 

•Heating / cooling of oil 

•Pre-heating at start of wind turbine 

•Oil injection 




•Markings due to load during stand still of gearbox 

•Poor load sharing patterns 

•Load on coasting flank (torques can be negativ in braking operations) 

•Micro pitting (Graufleckigkeit) 

•Pitting 

•Flank failure 

•Root failure (less common) 

•Load distribution on ring gear is not even (relative position of planet 

carrrier to ring varies 

•Bearings are moving in their seat, resulting in play, resulting in tilting 

of planet resulting in poor load bearing over tooth thickness 

•Load distribution on three planets may be poor (high Kγ) 



Damages on gearboxes: bearings 

•Bearing failure may not be due to fatigue but due to a change in the 

environment 

•If e.g. a modified lifetime of 300’000h is calculated, the calculation 

method is valid for maybe 150’000h only, then it is more important to 

control lubrication 

•Operating conditions are relevant for bearing life, not the fatigue 

•For high loads, shocks, misalignement, oil contamination: 

hydrodynamic bearings 

•ISO281, AMD 4, extended lifetime analysis, based on load bearing 

coefficient C 

•AGMA6006: uses local contact pressures 

•Specialised software by FAG/INA, Timken, SKF 

•Movement of outer ring leads to axial load on bearings 



Damages on gearboxes: bearings 

During idling or 

braking, thrust (due to 

helical gears and 

torque reverse) reverse 

may occur resulting in 

high axial loads at low 

speeds -> static 

overload of bearings 

Pre-tensioned tapered 

bearings with rib ring 

proposed by Timken 



Hydrodynamic vs. roller bearings 



Number of planets used in planetary gears 

Zähnezahlverhältnis 

Z 3 /Z 1 

12.0 

5.2 

3.4 

2.7 

2.2 

2.0 

Mögliche Anzahl Planeten 

3 

4 

5 

6 

7 

8 




380 

160 

415 

517 

1540 

130 

1522 

Lower load per planet allows 

for thinner planets 

Thinner planets will result in 

lower Khβ values and lower 

overall length of gearbox 

However, Kγ increases 


535 

1522 



PU - 3 PU - 8 PU - 10 

z1/z2/z3 [-] 21/37/96 53/23/99 63/22/107 

i Stufe [-] 5.57 2.87 2.70 

a mm 415 517 535 

b mm 380 160 130 

m mm 14 13.5 12.5 

Ft kN 442 133 103 

kMAAG % 100 63 63 

SigmaB MAAG % 100 73 75 

Lh10 26‘000 71‘000 148'000 



Analysis of gears using load spectrum 

•Step 1: Definition of load spectrum 

•Step 2: Sizing of a suitable gear set (helical stage or 

planetary set) based on nominal load or equivalent load 

•Step 3: Lifetime calculation of given gear set based on 

nominal load and load spectrum 

•Step 4: Test 



Step 1: Definition of load spectrum 

•Knowledge of load spectra is required, requires dynamic analysis of 

whole drivetrain, wind-propeller-gearbox-generator-grid 

•Not only torsional vibration, also translational vibration 

•Torque load spectra will not be sufficient any more 

•Multi body analysis is time intensive / expensive 

•For load spectra with torque reverse 

•Method FA: S-N curve is used where torque reverse is 

considered. Requires extensive measurements or calculation of 

synthetic S-N curve. 

•Method FB: S-N curve is modified 

•Method FC: Steps in load spectrum with reversed torque are 

multiplied by 1/0.7 

•Method FD: Complete load spectrum is multiplied by 1/0.7 



Step 2: Sizing of gear set 

Sizing using KISSsoft sizing functions 

•Input, load: KA, speed, nominal torque (equivalent 

torque=KA*nominal torque) 

•Input, gear data: ratio, module/helix angle/pressure 

angle/… range 

•Output: Possible solutions for gear pair or planetary 

sets 




•Rough sizing and 

fine sizing 

•For helical gear pair 

and planetary set 

•Variation of 

reference profile, 

pressure angle, helix 

angle, module, … 






Step 3: Rating of gear set 

•According DIN3990, ISO3660, AGMA2001, (VDI2737), GL 

and DNV regulations 

•For ring gear: YS, YF: do not use 30deg rule (see planned 

imrovements in ISO6336 or VDI2737) 

•K factors (Kv, KH/Fbeta, KH/Falpha) are calculated for each 

steps individually 

•Calculate lifetime with required safety factors 

•Calculate safety factors for required lifetime 

•Calculate maximum permissible nominal torque based on 

required lifetime and safety factors 

•Use different modifications of S-N curve 







•AGMA 6006 in KISSsoft/KISSsys: 

•Load spectra analysis: ok (KISSsoft) 

•Different configurations of gearboxes: ok (KISSsys) 

•5.2.2: Sizing of profile shift: ok (KISSsoft) 

•AGMA 6001: will be included in KISSsoft (shaft analysis) 

•Shaft rating acc. DIN 743: ok (KISSsoft) 

•Gear rating acc. AGMA2001, ISO6336: ok (KISSsoft) 

•Scuffing acc. AGMA925: ok (KISSsoft) 

•Bearing acc. ISO281 AMD 4: ok (KISSsoft) 

•Contact stress in bearings: to be included in KISSsoft 

•Equivalent torque calculation : to be included in KISSsoft 




Step 4: Testing of gears 

Back to back 

testing 

Bosch Rexroth 

Differential 

gearbox, 3.1MW 

Overload test: 

2xnominal max 

load for 20 

Minutes 

5MW test stand 


Step 4: Testing of gears 

Back to back 

testing 

Winergy currently 

uses a 7.5MW test 

rig 

12MW is planned 



References 

•U. Giger, G.P. Fox, Leistungsverzweigte Planetengetriebe in Windenergieanlagen mit flexibler Planetenlagerung, ATK03 

•F. D. Krull, T. Siegenbruck, Windenergieanlagen fordern hohe Leistungsdichten, Antriebstechnik 9/2004 

•M. Bodmer, F. Sxhwingshandl, Windenergie optimal umgewandelt, Antriebstechnik 9/2004 

•M. Bodmer, Planetenkoppelgetriebe mit Leistungsverzweigung, Erneuerbare Energien 2/2004 

•M. Stöckl, Initialschäden an Getrieben sind vermeidbar, Erneuerbare Energien 5/2004 

•N. Fecht, Die Antwort kennt nur der Wind, antriebspraxis, 01/2005 

•Windflow Technology, Towards a Sustainable Future, company brochure 

•Voith, WindDrive, product brochure 

•W. Kretschmer, Wohin der Wind weht, changeX Partnerforum 11/2003 

•REpower, REpower MD70, product brochure 

•MAN Group News Service, Renk Aerogear – new solutions for the wind power industry 

•Winergy, Die Winergy AG eröffnete am 8. Juni 2004 erneut Serienprüfstand mit Rekordleistung 

•E. Bauer, Windenergieanlagen Schadenbetrachtungen, AZ Expertentage Windenergieanlagen 

•R. Poore, T. Lettenmaier, Alternative Design Study Report: WindPACT Advanced Wind Turbine Drive Train Designs Study 

•R. Dinter, Wie geht es weiter bei der Getriebeentwicklung?, Erneuerbare Energien 8/2003 

•Renk, Gear units for wind power plants, technical information 

•G. Berger, Differentialgetriebe – eine neue Getriebegeneration für Multi-Megawatt-Windturbinen, Fachbeitrag Bosch Rexroth 

•N. Erdmann, Die Offshore-Windenergieanlage Multibrid M5000, Erneuerbare Energien 10/2004 

•E. de Vries, Global wind technology, overview of developments 2003-2004, www.earthscan.co.uk 

•B. Schlecht, T. Schulze, T. Hähnel, Modelle für die Eigenfrequenzanalyse, Erneuerbare Energien 5/2005 

•J. Hermsmeier, C. Eusterbarkey, P. Quell, REpower 5M – Erste Betriebserfahrungen mit der grössten Windenergieanlage der Welt, ATK05 

•M. Tilscher, A. Basteck, Drehzahlgeregelte Getriebe für den Einsatz in modernen Windenergieanlagen der Multimegawatt-Klasse, ATK05 

•E. Bauer, F. Wikidal, T. Gellermann, Überblick über Schäden am mechanischen Strang von Windenergieanlagen, ATK05 

•G.P. Fox, E. Jallat, Use of the integrated Flexpin bearing for improving the performance of eplicyclical gear systems, Timken Technical Paper 

•R. Grzybowski, B. Niederstucke, Betriebsfestigkeitsberechnung von Getrieben in Windenergieanlagen mit Verweildauerkollektiven, Allianz Report 2004 

•J.B. Franke, R. Grzybowski, Lifetime prediction of gear teeth regarding micropitting in consideration of WEC operation states 

•DIN3990, ISO6336, AGMA6006, AGMA6123, VDI2737, GL Richtlinie, DNV Richtlinie 

•Vestas, Renk, Hansen, Winergy, Eickhoff, Siemens, Bosch Rexroth, REpower, Nordex, GE : company homepages

Wind turbine gearboxes - KISSsoft AG

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