1、10FTM03AGMA Technical PaperA Novel Approach tothe Refurbishment ofWind Turbine GearsBy M. Michaud, REM SurfaceEngineering, G.J. Sroka andR.E. Benson, REM ResearchGroupA Novel Approach to the Refurbishment of Wind TurbineGearsMark Michaud, REM Surface Engineering, Gary J. Sroka and Ronald E. Benson,
2、REMResearch GroupThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractMulti-megawatt wind turbine gearboxes operate under demanding environmental conditions includingcon
3、siderable variation in temperature, wind speed, and air quality. It is not uncommon for gearboxes rated fora maintenance free 20-year lifespan to fail after only a few years. These gearboxes experience several typesof repairable damage including micropitting or “gray staining”, abrasive wear, foreig
4、n object debris (FOD)damage, surface corrosion and fretting corrosion. Wear is greatest on the input stage, especially on the sunpinion gear. Historically, grinding is utilized to refurbish these damaged gears. However, there are numerousdrawbacks including but not limited to high capital investment
5、 and the extraordinary amount of time and skillinvolved in the grinding process. Moreover, nitrided gears cannot be ground and must be scrapped. However,chemically accelerated vibratory finishing, or isotropic superfinishing (ISF), represents a value adding,low-cost option for refurbishing both case
6、 carburized and nitrided gears. Isotropic superfinishing removeslight to moderate gear flank surface damage. The result is a surface with a non-directional pattern with aroughness of approximately 0.08 mm or less. Moreover, evidence suggests that isotropic superfinishingimparts a finish that increas
7、es gear durability and service life in the field. A case study on a sun pinion gear ispresented.Copyright 2010American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October 2010ISBN: 978-1-55589-978-33A Novel Approach to the Refurbishment of Wind Turbine Ge
8、arsMark Michaud, REM Surface Engineering, Gary J. Sroka and Ronald E. Benson, REMResearch GroupIntroductionTypical multiple megawatt (MW) gearboxes, 1.5MW and higher, are designed to operate for 20years without requiring major maintenance to thedrive train. However, many owners areexperiencing gearb
9、ox failures after only a few yearsof service. Gearbox repairs may cost $360,000(USD) or more for a complete replacement by 2009estimates 1. Gears fail for several reasons. Windturbine gears operate under extreme environmentalconditions including highly variable temperature,wind speeds and air qualit
10、y. These conditionscause variable high loading and torque. During peri-ods of low or no wind, the loading on slowly movingor stationary gears is exacerbated. Moreover,moisture can contaminate the lubricant andcondense on the gear surfaces forming sludge,corrosion and micropitting. Finally, dust and
11、otherforeign debris in the air can contaminate the lubric-ant during maintenance leading to abrasive wear.Fortunately condition monitoring systems allowgearbox problems to be discovered before seriousgear damage occurs 2, 3.BackgroundWind turbine gears experience several types of re-pairable damage
12、including micropitting or “graystaining”, abrasive wear, foreign object damage(FOD), surface corrosion and fretting corrosion. Anexample of each is depicted in Figure 1 4, 5. Thereare three main approaches to repairing gear dam-age: refurbishment by regrinding, refurbishment bysurface finishing or r
13、eplacement with a new gear.The refurbishment process is a combination of re-clamation and reconditioning of a used gear or bear-ing 6. New gearing is expensive which makes sur-face finishing and regrinding the preferred low costalternatives. Regrinding is necessary when leadand profile corrections a
14、re required on the workingsurfaces of the gear teeth 7.Figure 1a. Micropitting or gray staining ongear flanksFigure 1b. Heavy abrasive wear on gear flankand a SEM image of trail left by an abradingparticle 4Figure 1c. Foreign object debris (FOD)damage4Figure 1d. Corrosion looking down on topland 4Fi
15、gure 1e. Fretting corrosion 4The Isotropic Superfinish or ISFProcess, hence-forward referred to as superfinishing, is analternative time and cost efficient method of gear re-furbishment. The process utilizes conventionalvibratory finishing equipment and high density,nonabrasive finishing media to pr
16、oduce isotropicsurface finishes with a final surface roughness (Ra)below 0.10 mm.The superfinishing process is easily understood byreferring to Figure 2. At the start of the superfinish-ing process shown in Step 1 of Figure 2, the originalmetal surface reacts a first time with the activechemistry, f
17、orming the first conversion coating(Step 2) 8. The vibratory machine andnonabrasive media produce an effective rubbingmotion on the surface of the gear (Step 3). Thisexposes the peak asperities of the metal surfaces toa second reaction (Step 4), re-forming the completeconversion coating. The process
18、 of conversioncoating re-formation and removal (Step 5) iscontinued through many successive cycles thusplanarizing the original rough machined ordamaged surface. The final required surface finishgoverns the total number of cycles. This planarizingprocess is continued until the gears are smoothed tot
19、he required surface finish quality. Once therequired surface finish quality is achieved, theactive chemistry from the smoothing stage of thesuperfinishing process is drained away, and a neut-ral, burnishing soap is introduced into the vibratorymachine. The burnish removes all remaining con-version c
20、oating (Step 6) from the surface of thegear, producing a mirror-like appearance, whileimparting a mild rust preventive to the surface. Thegear is ready for unloading and the superfinishingprocess is complete. Figure 3 shows a comparisonof a ground gear surface versus a superfinishedgear surface.Figu
21、re 2. Superfinishing processThe initial selection of the proper media shape, size,and mixture is a significant part of the art to success-fully superfinishing gears. Media is specificallychosen based on the following criteria:S Root diameterS Diametral pitchS Gear sizeS Gear massS Alloy5Figure 3. Gr
22、ound surface (top) versus anIsotropic Superfinish surface (bottom)Once the media is chosen, the superfinishingprocess repeatedly finishes gears identically. Themedia is nonabrasive, and therefore has a very lowattrition rate. The size, shape, and density of themedia remain stable over thousands of h
23、ours ofvibratory machine operation.The superfinishing process possesses several idealfeatures.1. Superfinishing removes metal uniformly fromevery tooth of the gear with the ability to controltotal stock removal down to below 2.50 mm.2. There is no discoloration or temper burn, a riskthat is associat
24、ed with grinding.3. Superfinishing does not destroy residualcompressive stress surface layers 9. Residualcompressive stress is imparted on the surfaceduring case hardening. Compressive stressslows the rate of surface wear and inhibitscorrosion 10. Hence, the superfinished gearexhibits a superior sur
25、face versus used gearsthat are refurbished by grinding.4. The process works on nitrided as well as casecarburized surfaces.5. The superfinishing process is an ideal techniquefor improving the inspection of refurbishedgears. There is sufficient material removal to re-veal subsurface damage from micro
26、pitting,scuffing and corrosion that may be masked bythe texture of the used surface. Grinding andin-service wear may cause a “smearing” of themetal on the gear surface resulting insubsurface damage not only being invisible tothe naked eye, but also potentially going un-detected by other inspection m
27、ethods 11. Forexample, Figure 4 depicts a cracked gear tooththat was delivered by a customer and deemedsuitable for refurbishment. Only aftersuperfinishing did the crack become visible.Consequently, this gear was scrapped and thepotential catastrophic failure of the gearbox wasavoided. In general, s
28、uperfinishing can.Figure 4. Crack on a used gear near theaddendum of the tooth was revealed onlyafter superfinishing6. Superfinishing can also remove light (25 mmdepth) to moderate (130 mm depth) damagefrom the gear tooth surface while maintaininggeometric tolerances. Figure 5 shows imagesof the gra
29、phite tape lift method that is used tomeasure and record the amount of micropittingon a gear flank 12. The deepest micropittingdamage is approximately 150 mm (see arrows).In this instance, it is apparent that somemicropitting remains after superfinishing. Theremoval of all damage may have compromise
30、dthe geometry of the gear. However, the peakasperities or “stress raisers” were removed andthe existing micropitting will, in all likelihood, notprogress to pitting and eventual spalling 13.Recently, gearbox manufacturers indicatedbetween 150-250 mm can be safely removedwithout compromising the gear
31、 geometry.11Maximum removal amount is per customer specifications.6Figure 5. Graphite tape lifts from damaged gear surface before (top) and after (bottom)superfinishing refurbishmentGear refurbishment via superfinishing wasevaluated for bending fatigue, contact fatigue, andscoring resistance tests o
32、n military helicopter gearsby the Gear Research Institute (GRI) 14, 15.Importantly, GRI results suggest that “in all threetests, the repaired gears met or exceeded theperformance of the new gears”. Figure 6 depictsprofilometer traces of (a) new ground gear, (b) usedgear prior to refurbishment and (c
33、) used gear afterrefurbishment via superfinishing.Figure 6a. Surface trace of a new ground low-speed stage pinion before superfinishing (2282)Figure 6b. Surface trace of a used low-speed stage pinion before superfinishing (2308)7Figure 6c. Surface trace of low-speed stage pinion after superfinishing
34、 (2282)Superfinishing has several distinct advantages overregrinding in terms of time and cost savings.1. The process does not require engineering draw-ings.2. All teeth are uniformly and simultaneouslyfinished. For example, the cost per tooth is thesame for a small gear with 59 teeth versus alarge
35、gear with 113 teeth.3. As described superfinishing requires less setuptime and potential complications versusgrinding. Consequently, the turnaround time israpid.4. Typically the ring, sun and pinion gears of thelow-speed stage and the output pinions of thehigh speed stage receive the most wear and a
36、rerefurbished. However, the gears of theintermediate stage especially an assembled in-termediate unit also can be superfinished withlittle additional expense since the entire gearboxis removed for maintenance. Figure 7 depicts arefurbished assembled intermediate stage gear.Case studyA case study was
37、 performed on the input stage of a1.5 MW wind turbine gearbox. The low-speed sunpinion gear usually shows the most damage and willbe used here to describe the superfinishingrefurbishment process. It should be noted that cus-tomers frequently request complete refurbishmentof the planetary gears and h
38、ollow wheel gear aswell. The sun pinion gear is shown in Figure 8a andFigure 8b.Figure 7a. Superfinished intermediate gearassembly(photo courtesy of Moventas)Figure 7b. Characteristic IsotropicSuperfinish on the flanks of the intermediategear assembly pictured above(photo courtesy of Moventas)8Figur
39、e 8a. Sun pinion gear with severalmodes of damage including hard linemicropittingFigure 8b. The same sun pinion gear afterrefurbishmentThe following protocol was used to refurbish thispinion gear:1. After the gear was received it was inspected fordamage. Micropitting and light abrasive wearwere note
40、d. Pre-finishing inspection is crucialas gears may be damaged duringdecommissioning and/or transport.2. The initial average surface roughness (Ra)wasmeasured to be 0.31 mm. RefertoTable1.3. The gear was placed in a vibratory finishingapparatus with an optimized media mixture toeffect uniform stock r
41、emoval on the flank.4. It was processed in active chemistry for a shortduration and then carefully inspected for hiddenserious damage such as cracks or deep pitting.No serious damage was detected.5. The gear was further processed in activechemistry and its surface roughness wasperiodically monitored
42、 with a skidded portableprofilometer2until the targeted Rawas achieved.6. The pinion was then burnished to remove alltraces of the conversion coating.7. The Raof the superfinished refurbished gearwas determined to be 0.07mm. Refer to Table 2.Table 1. Raand Rzmeasurements taken atdifferent locations
43、on the sun pinion prior torefurbishmentTraceRa(mm)InitialRz(mm)Initial1 0.34 1.952 0.29 2.123 0.32 2.194 0.29 1.55Average 0.31 1.95SD 0.02 0.29Table 2. Four Raand Rzmeasurements takenat different locations on the sun pinion afterrefurbishmentTraceRa(mm)FinalRz(mm)Final1 0.06 0.462 0.06 0.443 0.07 0.
44、504 0.07 0.50Average 0.07 0.48SD 0.01 0.03Results and discussionIn the Case Study discussed above, the planet andhollow wheel (ring) gears were also successfullyrefurbished. Customers utilize CMM to determine ifthe gears are still within tolerances after the partsare returned. All gears, more than 2
45、300 refurbishedto date, remain within the required tolerance. In themajority of cases, all traces of micropitting and other2A Hommel T1000 Basic with a skidded T1E probe and a 5 mm stylus was used for the case study. Theprofilometer is calibrated to ISO 5436 standard.9moderate tooth damage can be su
46、ccessfullyremoved with superfinishing. Even if the micropit-ting is too deep to remove completely, the peak as-perities or “stress raisers” are removed and micro-pitting will not progress to pitting and eventualspalling. Moreover, superfinishing will remove theraised lip around the lip of a FOD dent
47、 even if thevalley of the damaged area is not completelyremoved. Gears with severe subsurface micropit-ting or in need of tooth profile correction must bereground first. However, it is still advantageous touse superfinishing as the final step in order to imparta much longer service life. Ongoing fie
48、ld tests, nowmore than two years old, are being conducted tocorroborate that superfinished wind turbine gearshave equivalent or superior performance versusnew ground gears. Currently, refurbishing windturbine gears via superfinishing is in commercialoperation as a viable replacement to regrinding by
49、 amajor wind turbine gearbox manufacturer.ConclusionsIn terms of time, cost, and in-service performancesuperfinishing has several distinct advantages.S The majority of damage sustained on in-servicegears in wind turbine gearboxes can besuccessfully refurbished by superfinishing.S Superfinishing is time efficient and economical.- All teeth are finished simultaneously versusa few teeth at a time.- Large gears finish in the same amount oftime as small gears.- Proprietary drawings or expensive reverseengineering is not required.- Nital etch inspection i
