••••••••••••• _TECHNICAL FOCUS••••••••••••• _II

New Guidelines ForWind Turbine Gearboxes

IntroductionThe wind turbine industry has been

plagued with gearbox failures, whichcause repair costs, legal expenses, lostenergy production and environmentalpollution.

A wind turbine gearbox has failed if it:• Jams and stops rotating;• Breaks and allows the rotor to turn

without turning the generator;• Exceeds allowable sound level or

vibration limits;• Has excessive oil leakage;• Requires excessive maintenance.Failed gears, shafts, keys, bearings

and housings have caused jamming orbreaking. Bending fatigue is usually theultimate failure mode when gearboxesjam or break. However, bending fatigueis often a secondary failure mode initiat-ed by other failure modes such as macro-pitting or scuffing. Some wind turbineshave been destroyed when gearboxesbroke and allowed rotors to overs peed.

Strict sound regulations in theUnited States and Europe require shut-down of noisy turbines because failedgears or bearings cause excessive soundor vibration.

Failed shafts, seals and housingscause excessive oil leakage and, in somecases, loss of lubricant and gearbox fail-ure. Less severe leakage increases main-tenance and costs. Soil contaminationrequires costly cleanup.

Three years ago, a group of individu-als dedicated to wind power recognizedthat many gearbox failures were due to alack of understanding of the severity ofthe wind turbine operating environment.They sought to define this environment

and improve communication betweengear and wind turbine manufacturers soreliable gearboxes could be obtained.

Therefore, the American Gear Manu-facturers Association (AGMA) and theAmerican Wind Energy Association(AWEA) formed a committee of windturbine manufacturers and operators,researchers, consultants and gear andlubricant manufacturers.

The committee developed AGMAIAWEA 921-A97, "Recommended Prac-tices for Design and Specification ofGearboxes for Wind Turbine GeneratorSystems." The document reflects the lat-est knowledge about wind loads and fail-ures, plus the insights of gear and windturbine manufacturers and operators. Itdescribes wind turbine configurations,operating conditions and environmentalfactors that affect gearbox life. It offersguidelines for defining wind loads andspecifying gears, bearings, operation andmaintenance. All of these guidelinesfocus on one goal: ensuring reliable windturbine gearboxes.

Wind Thrbine Configurationand Operation

Two wind turbines with the same gen-erator can experience extremely differentgearbox loads. A good control systemmaximizes energy capture and minimizesstructural loads. AGMA/AWEA 921describes common wind turbines and thesignificance of their architecture andoperation on gearbox loads.

Rotors can be horizontal, as in aHAWT (horizontal-axis wind turbine); orvertical, as in a vertical-axis wind turbine(VAWT). Some HAWTs are integratedsystems where the gearbox supports the

Turbine at the National Wind Technology Center,Boulder. CO.

rotor and, in some cases, the generator andother components such as yaw drives.With this configuration, the gearbox hous-ing must transmit rotor loads to the sup-porting tower without incurring excessivestresses or deflections. Integrated systemsrequire detailed communication betweenthe wind turbine designer and gear manu-facturer to properly design interfaces.

Robert Errichellois the principal of GEARTECH, agear consulting firm in Townsend,MT He is a member of AGMA,ASME and a licensed professionalengineer in the State of California.He is also one of GearTechnology's technical editors.

Brian McNiffis president of McNiff LightIndustry of Blue Hill, ME, a com-pany providing wind turbine testingand analysis services.

_____--_---11TECHNICAL FOCUS .-.- _

By contrast, a modular system eon-sists ofa rotor, rotor support. shaft: andbearings, gearbox. and generator, eachmounted to a common support plate. Inthis case, the gearbox doesn't need 110

support external loads.Rotor torque is co.ntrol1ed by adjusting

blade pitch (active control) or by .aerody-namic stall (passive control). Active con-

trol increases gearbox complexity because,in many cases, the mechanical, electrical. or

hydraulic centrol passes through a bollowgearbox shaft to blade acmaters, Pas ivecontrol is simpler, but creates higher peaktorques than active oontrol.

Environmental ConsiderationsWind turbines must withstand aggres-

sive environments ranging from hot, dustydeserts tocold, wet marine environments.

Temperatures can vary widely" whichaffects lubrication. The oil SUID-p tempera-ture should be at least .soC above the lubri-

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cant pour point during start-up and lesthan 9.5"C during, operation. Otherwise,heaters or coolers may be necessary.

All sites require rugh-capacity, filteredbreathers and positive shaft seals to controlcontamination. Desiccant Drealhers mini-mize water contamination, Protective coat-ings help' prevent corm ion.

Defining Gearbox LoadsMany early wind turbine gearboxe

failed because designers were uncertainof loads such as:

• Long periods of small oscillationwhen 'the brake stop_llIe high-speedslilaft, and the wind buffets the roter:

• Long periods of low speed and light[cads during light winds;

• Long periods of high speed and lightloads when winds are below cut-in speed;

• High tran ient loads when thegener-ator connects to Ibepowergrid;

• Rapid load fluctuations during nor-mal operation;.

• High transient and impact loads dur-ingbraking ..

Defining load spectra for these eondi-tions is difficult due to the u:llcertalnty ofpredicting I.cads. However, experiencebas made predictions more reliable.AGMAJAWEA 921l:elli how to assem-ble load' pectra that. include both. windloads and tim ient loads, such as: tan-upsynchronization, rapid blade p.itell. a nia-tion of bladecontrol surfacesaad bralting.

Until recently, wind turbines were notclassified accor1iing to their capabilitie .However; '!:he International Eleceoteeb-meal Commission's IEC 1400 initiativebas established four classes based onwind severity. A Class 1 wind turbine, forexample, can. operate in the most severeconditions. with an average wind speedof 10 mis, an extreme wind speed of 50m/s and .high turbulence intensity ..

Specifying Gearbox. ComponentsExperience bas shown which gearbox

components. perform best in wind turbines.The t:oUowing material . ummarizes rec-ommendations for gears, bearings andlubrication systems.

Gears. Wind turbine gears are sub-jectedto wind roads that vary from lightto very heavy (wind gusts). Carburized,

hardened and ground gears have provenretiableunder widely varying loa ' .

_------------TECHNICA1.FOCUS-------------Induction-hardened gears were tried.

but lacked toad capacity because of inade-quate accuracy. Carburizedand groundgears are me best choice because they haveme necessary load capacity and accuracy,

Nitricling is not appropnate for exter-nal gears for wind turbines because it hapoor resistance to overloads, It. has beensucce sful for annulus gears.

Through-hardened gears do not havesufficient load capacity because of theirrelatively low hardness. hl addition.when run with carburizedpinions, theypromote polishiog wear.

Metallurgical quality should meetrequirements for Grade 2 material in.accordance with ANSIIAGMA 2001-C9S.

Gear teeth should be designed to max-imize load capacity as explained inAGMA 901-A92, For example •

• Use at least 20 teeth on pinions toachieve good balance between pilti~gresistance. bending strength and scuffingresistance;

• Because wind tUIbine gearboxes arespeed increa ers, design profile shrift forbalanced specific sliding;

··1b achieve uniform gear tooth loaddistribution. lise a low aspect ratio (pin-ion face wi.dth to diameter ratio) andmodify profiles and helices to compen-sate for deflections and manufacturiagvariations;

• To obtain quiet operation. use gearswith trig!! contact ratios and higb accura-cy (AGMA 'Quality No. 11 or higher),

Preferred gear types tor w:ind turbinesare spur, helical and double helical,

Helical gears are Quieter than spurgears and have more load capacity.Double helical gears may balance thrustloads; however. they require high accura-cy to control dynamic loads, and shaftcouplings must be designedaad appliedto limit external thrust loads.

Spur gears are often used in epicyclicgearboxes because tbey avoid the tenden-cy 'of some helical gears tornisalign plan-et gears and interfere with floating sungears. A combination of spur gears in thelow-speed epicyclic stages and helicalgears in the bigh-speed stage provide acompact. quiet gearbox.

As power increases, there are incen-tives to use epicyclic, split-power-path or

bybrid gear arrangements to meet.capacity while limiting size and weight.

Bearings, Severe vibration in wind tur-bines precludes standard industrial prac-tice for fitting bearing outer races andhousing bore . Vibration may cause outerrace to spin within their bore , can ingwear that misaligns gears, generates weardebris, damages gears and bearings and

steel sliding between aliter races and thehousing cause scuffing and severe wear.Damage may be less in cast iron hou ingsbecause dissimilar materials are less likelyto scuff. Nevertheless, for reliable windturbine service, bearing fits must be tight.or races must. be mectumicaJJy re trainedfrom spinning.

Preferred bearingtypes for wind tur-bine gearboxes are spherical .roller.tapered roller. double-row, cylindrical

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Full-complement, cylindrical roller

bearings offer high load capacity.However, rollers have counterformal con-tact and opposed sliding that disrupt thelubricant film, allow steel-on-steel contact,create friction and make rollers prone toscuffing. Therefore, use these bearingsonly 011 low-speed shafts where slidingspeeds are low enough to avoid scuffing,

Wind turbines frequently rotate at fullspeed under light loads. Under these con-ditions. bearing loads may be insufficient

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to maintain rolling contact betweenrolling elements and raceway . A aresult, skidding. overheating and cuffingmay cause bearings to fail. Tile highestrisk occurs 011 high-speed sll3flS wi thfour-point or angular contact ball bear-ings. Therefore. specify preload if neces-sary to prevent skidding.

Wind turbines with high-speed shaftbrakes subject a gearbox to reversingloads during a brake stop. Therefore,minimize bearing end play to reduceimpact loads on bearings caused byreversing thrust loads from helical gears.

Other tips:• Equip bearings with bronze retain-

ers. Plastic retainers are susceppble tocontamination by hard particles thatabrade rolling elements.

• For shaft-mounted gearboxes. mini-mize radial clearance of low-speed bear-ings to avoid misaligning the low-speedgear mesh. Modify the helix of the low-speed pinion or gear to compen ale formisalignment.

• Except for epicyclic gearboxes,where the sun pinion has no bearings.mount all pinions between bearings.Don't use overhung pinions; they maycause gear misalignment and high bear-ing loads.

LubricationMicropitting, rnacropitring, adhesion

scuffing. abrasion and other lubrication-related failures are widespread in windturbine gearboxes. To avoid uch fail-ures, the lubrication system should pro-vide an adequate amount of cool, cleanand dry lubricant to gear and bearings.

Pressure-fed system should have afilrer to clean tile oil and can have a healexchanger to cool the oil,

Maintenance and condition mo.nitoringshould be considered at the design stage.

Many factors need to be consideredwhen selecting a lubricant, including vis-cosity. viscosity index, pour point. addi-tives. rnicropiuing resistance, scuffingresistance and.cost. There are debates onthe relative advantages of synthetic vs,mineral oils, especially regarding micro-p:itting resistance, Operators have learnedthat experiments are required to findacceptable lubricants,

Wind turbine gears have relatively

_ •••••••••••• -TECHNIICAl'fO'CUS--------------To compare competing gearbox

designs. AGMN AWEA 921. recommends

these standards for rati~g the components:

• Gearbox: ANSIJA:GMA 600 1, ANSI!AGMA 6010 or ANSIIAGMA 6023.

• Gears: ANSliAGMA 2001,• Bearings: ANSIJAFBMA 11.•.Shafts and keys: ANSI! AGMA 6001,Plans beuld clearly define all accep-

tance criteria before manufacturing starts.After awarding the contract. the purchas-

low pitch line velocity (typically W-IOmis) and high loads (contact stress over

1400 MPa). These conditions requiresynthetic or mineral. gear oil. with antis-cuff additives and the highest practicalviscosity (at least [SO VG 320). Too rowa viscesity causes micropirting, macro-

pitting. adhesive wear and scutfiag.Micropitting is prevalent in wind tUI-

bine gearboxes. Though it does notalways lead to catastrophic failure, itgenerally reduces gear tooth accuracy.

increases noise and can progre s to full-

scale macropitting and gear failure.To prevent micropitting. maximize

specific film thicknes by using gearswith smooth tooth surfaces and an. ade-quate amount of cool. clean and drylu.brica:l1t of the highest viscosity pennis-sible, You may need to experiment w.ith

different lubricants to find one with ade-quate rnicropiuing resistance. FVA Pro-

ject Number 54 0) is a standardized lestfor rnicropitting resistance of lubricants.

A rea-stage pass is minimum acceptableperformance,

Running-in gears under controlled

loads reduces tooth surface roughness andrisks of micropitting and scuffing, How-

ever, manufacturers and purchasers dis-agree on who should perform this task,Gear manufacturers are reluctant 10 run-in

gears because their test facilities are limit-ed; and purchasers are reluctant becauseLhey can', always control loads. Therefore.

run-in must be negotiated between the gearmanufacturer and the purchaser,

QuaUty AssuranceThe purchaser should write .<"1 procure-

ment specificarion for wind turbine gear-boxes that fuUy describe.slhe application.load spectrum and minimum requirementsfor design. manuJactwlng. qualiry assur-ance, tesling and gearbox perfonnance.

The gear manufacturer should prepareplans for quality assurance. inspectionsand tests.

The purchaser hould ccnfirm thai thegear manufacturer understands all require-ments of the procurement pecificatinnbefore awardjng the contract. Criteria for

judging gear manufacturer competence

are design review meetings. evaluations ofmanufacturing and QA plans and visits tomanufacturing facilities.

er should audit manufacturing, inspectionand testing for conformance to the pro-curement specification.

Prototype gearboxes should be quali-fied before full-scale production begins.Prototype gearboxes should not be manu-factured until the purchaser approves all.elements of first-article jnspcctions and

tests. After prototypes have met therequirements. of the procurement spccifi-

cation, and manufacturing plans are

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sound, the transition to production houldbe straightforward.

Operation and MaintenanceThe gear manufacturer, lubricant up-

plier and purchaser should establishplansfor start-up, operation and maintenancebefore gearboxes are placed in service.Gearboxes should be run-in under con-trolled loads to reduce the ri k of scuffingand mlcropitting and prolong gear andbearing lives.

Regulargearbox inspections should bedone at run-in and at specified intervals.AGMN AWEA 9021 gives gu:idelines foron-site monitoring and laboratory analyses

of lubrica_nts for viscosity, water content,add number, solid contaminants and addi-live depletion. It also describes methodsfor monitoring vibration and temperature.

Safety is a major concern for wind tur-bine operators because of the ri k offalling, being trapped in rotating compo-nents orbeing injured while maneuveringin restricted spaces. Gearboxes shouldhave all adequate number of steps, hand-holds and lanyard hook points. Housingsshould have rounded comers, and aUexposed rotating components shouldhave safety guards. Because access is dif-ficult and work: space is limited, designthe gearbox toea e maintenance ..0

References:L FVA·Lnformation Sheet, "Micropining,"

o. 54n, July, 1993. PorschungsvereinigungAatricbstechnik e.V., Lyoner Strasse 18, D·60428. Frankfurt/Main. Germany,

Acknewledgement: This article was presented atrile .1997 AGMA Fall Technical Meeting in SQ/I

Diego, CA. CJ 997, AGMA. Used with permission.

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