1、08FTM17AGMA Technical PaperInnovative Concepts forGrinding Wind PowerEnergy GearsBy A. Trich, C. Kobialka, andD. Vucetic, Gleason-PfauterMaschinenfabrik GmbHInnovative Concepts for Grinding Wind Power Energy GearsDr.-Ing. Antoine Trich, Dr.-Ing. Claus Kobialka and Dipl.-Ing. Dragan Vucetic,Gleason-P
2、fauter Maschinenfabrik GmbHThe 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.AbstractOver the past years wind power energy is gaining higher importance to reduce CO2emissions
3、 and thuscounteract global warming. The development of wind power engines is driven by increased performancewhichrequireslargerwindturbinesandgearboxes.Thequalitydemandsofthosegearsareincreasingwhiletheproductioncostmustdecrease.Thisrequiresnewproductionmethodstogrindthegearsatlowcostandahigh qualit
4、y level. Profile grinding is known as a pro-cess to achieve the highest possible quality even forcomplex flank modifications while threaded wheel grinding is known for high productivity. New machineconcepts make it now possible to use both advantages at the same time. The reduction of non productive
5、auxiliary time is a very important key aspect to becoming more productive.The article will show the newest developments to reduce the overall cycle time. This includes aspects toreduce setup time, idle time, productive time as well as dressing time.Copyright 2008American Gear Manufacturers Associati
6、on500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2008ISBN: 978-1-55589-947-93Innovative Concepts for Grinding Wind Power Energy GearsDr.-Ing. Antoine Trich, Dr.-Ing. Claus Kobialka and Dipl.-Ing. Dragan Vucetic,Gleason-Pfauter Maschinenfabrik GmbHProductivity in profile grinding
7、The gears used in wind turbine gear boxes have totransferhighloadswhichrequireshardenedmateri-al on one hand and an exact geometry on theotherhand. Thus, those gears have to be hard finished.Discontinuous profile grinding with dressablewheels is actually the most appropriate process tohardfinishgear
8、soflargemodules(m8mm:DP3).Duetotheongoingboominthewindenergymarket,the gear box manufacturers are focusing on in-creasing their capacity and productivity of existingmachine tools.Inprofilegrinding,thetotalcycletimetogrindagearconsists of idle time and main production time.Many approaches to optimize
9、 cycle time just con-centrates on improving the production time itselfwithout considering the idle time. The idle time,which can cover up to 50% of the total cycle time,consists of setup time, centering time, dressingtime, time for overtravel and pitch movements dur-ing grinding as wellas onmachine
10、measuringtime.Figure1showstimes foratypicalexampleinprofilegrinding of large gears. The effective grinding timeinroughgrinding(41min)is only34%orjust 17%ofthetotal cycletime (240min). This exampleshowsa dramatic inefficiency of the process.Thegrindingtimecanbecalculatedwiththespecificmaterialremoval
11、rateQwwhichrepresentsthepro-ductivity of agrinding process. Thehigher theQw,the shorter the grinding time. Figure 2 shows thedefinition of Qw for discontinuous profile grinding.Qwis theproduct of radialinfeedx and axialfeedspeedfa. Toreducethegrindingtime,Qwhastobeincreased either by larger radial i
12、nfeed or fasteraxialfeedspeedorevenboth. Thelimitingfactorforsuch an increase is usually the appearance ofgrinding burn.effective grinding time inrough grinding is only 34%Figure 1. Typical cycle time in profile grinding of large gears4Figure 2. Definition of Qw in discontinuous profile grindingFigu
13、re 3 shows the principle relation between theradial infeed x and the axial feed speed fa.Asanexample, a specific material removal rate ofQw = 10mm#/mmscanbeachievedbyusingara-dialinfeedofx =0,15mmandanaxialfeedspeedof fa= 4000mm/minas wellas usinga radialinfeedof x = 0,05 mm and an axial feed speed
14、offa= 12000mm/min. Inprincipletherearetwostrat-egies to reduce the total amount of stock xxtotal.One is using high feed speeds and the other one isusing a high radial infeed. It is obvious that using ahigher axial feed speed for example by a factor ofthree results in anincrease of strokes by a facto
15、r ofthree as well.Figure 3. Relation between radial infeed and axial feed speed at constant Qw5Running more strokes effects a longer cycle timebecause each stroke needs an approach travel toacceleratetheaxistotheaxialfeedspeedaswellasan overtravel to decelerate as shown in figure 4.Table1shows acycl
16、e timecomparison fordifferentgrindingstrategies. Thefirst strategy is runningthecycle with a high radial infeed. For a given specificmaterial removal rate of 10 mm3/mms and an axialfeed speed of 4.000 mm/min the radial infeed perstroke results in 0,15 mm. To remove the totalamount of radialinfeedxto
17、talof 3,0mm 20strokesare necessary. The second strategy is running thecycle with an axial feed speed of 12.000 mm/minwhich effects 60 necessary strokes to remove thetotal amount of stock. The idle time per strokedepends on the acceleration and deceleration timeof the axial axis. Figure 5 explains th
18、e relationbetweentheaccelerationanddecelerationtimeperstrokeindependency of theaccelerationrateof theaxis. The accelerating and decelerating time perstrokeatatypicalaxisaccelerationrateof1m/s2for4.000 mm/min axial feed speed takes about 0,35 swhile this timeincreases to0,6 s at aspeed levelof12.000
19、mm/min. The effect of this increase can beseen in Table 1. The pure grinding time for bothstrategiesisstillthesamebuttheidletimeisgettingmuchlonger. Thisisthereasonforatotalcycletimewhich is 34% longer compared to the strategy ofhigh infeed. Even when running the machine at anacceleration rate of 2
20、m/s (strategy no. 3 “highspeed 2”) the total cycle time is still increased by23%.Figure 4. Approach and overtravel in profile grindingTable 1. Cycle time comparison for different grinding strategieshigh infeed high speed 1 high speed 2Total radial infeed xtotalmm 3,00 3,00 3,00Axial feed speed famm/
21、min 4000 12000 12000Spec. material removal rate Qw mm3/mms 10 10 10Radial infeed per stroke Xm 0,15 0,05 0,05Face width beffmm 278 278 278Grinding time per stroke thstrokes471,39 1,39Axis acceleration rate am/s21 1 2Acceleration and deceleration timeper stroketn1s0,350,6 0,4Idle time for radial infe
22、ed and pitchmovementtn210,1 0,1Number of strokes n - - 20 60 60Total grinding time ths 83,4 83,4 83,4Total idle time tns942 30Total time per tooth slot ttotals 92,4 125,4 113,4Time ratio (idle time/grinding time) tn/th- - 11%+36%50%+23%36%6Figure 5. Acceleration and deceleration time per strokeFurth
23、ermore an increase of axis acceleration haslimitationsduetohigherloadofallmechanicalcom-ponents such as bearings, spindles and guideways.Thestrategy of grindingat higher axial feedspeedsfinally results in longer idle times although thespecific material removal rate stays constant andthus is not appr
24、opriate. Experimental trials doneatGleason Pfauter have shown that grinding typicalwind turbine gears at 12.000 mm/min axial feedspeedhave36%longeridletimes(seetable1)thangrinding at 4.000 mm/min axial feed speed andhigherradialinfeed. Tocompensatethistimedelay,an increase of the specific material r
25、emoval ratefrom Qw = 10 mm#/mms to Qw = 14 mm#/mmswould be necessary. But this would tremendouslyincreasetheriskofgrindingburn. Inotherwordsitisnot possible to achieve a higher productivity byhigher feed speeds without an increased risk ofburn. Furthermore such an increased materialremoval rate woul
26、d just affect the 41 min effectivegrindingtimeasshowninfigure1whichagainisjust17% of the total cycle time. Increased mechanicalload on axis components and increased electricalpower consumption are disadvantages at thiscomparison.Thus Gleason Pfauter is focusing on otherstrategiestoincreasetheproduct
27、ivityonbothsides:- the main grinding time as well as- the non productive times.Multiple wheel profile grindingTheuseofmultiplewheelprofilegrindingofferssev-eralpossibilities toincrease theperformance or thework piece quality. In case that the teeth have nospecial profile modifications, four instead
28、of twoflanks can be ground simultaneously in roughingandfinishingoperationswhichwillreducethegrind-ingtimebyafactoroftwo. Justaswell,itispossibleto reduce the risk of burn significantly without im-pacting the productivity compared to conventionalprofile grinding. Therefore, just the two externalwhee
29、lsaregrindingandthemiddleoneisnottouch-ing the teeth. This effects a smaller contact anglebetween the grinding wheels and the tooth flanks.Schlattmeierdescribesin3thattheriskofgrindingburn is getting lower the smaller the contact anglegets. Thereverseconclusionallowstoincreasethespecific material re
30、moval rate at the same burningriskwhengrindingwithjusttheexternalwheelsthusthe cycle time will be reduced.7Another possibility is to grind four instead of twoflanks at a time with a lower Qw thus theproductivityisthesamebutatamuchlowerburningrisk.Another important point for ground gears is thesur-fa
31、ce finish. Investigations 6 have shown that theloadcapacityofagroundgearcanbetremendouslyincreased by a very good surface finish of Ra 0,2mm. In conventional profile grinding using just onegrinding wheel such a good surface finish is notachievable because the grinding wheel is designedas a compromis
32、e for rough and finish cutting.Therefore, for high quality gears an additionalprocess called barrel finishing is used to achievethis surface finish. The use of multiple wheels asshowninfigure6allowstheuseof differentgrindingwheel specifications for rough and finish operationthus beingcapabletoachiev
33、e agood surfacefinishofRa0,2mmwithouttheadditionalbarrelfinishingprocess. To realize such good surface finish, thetwo external wheels are just used for the roughingoperationandthemiddleone, withafinegrit size,isused for the finish operation. During roughing themiddle wheel is dressed to a smaller di
34、ameter inorder not to touch the flanks while during finishingthe two external wheels are dressed to a smallerdiameter.Last but not least it is even possible to increase theperformance when using Gleasons patented antitwist grinding method. The unique point in thismethod is to achieve the twist modif
35、ication in dualflankgrindingwhilethecompetitionhastodoitflankby flank which doubles the grinding time. With theuse of multiple grinding wheels it is now evenpossible to further increase theperformance by us-ing four flanks for roughing and two flanks for finishgrinding including the anti twist modif
36、ication.Threadedwheelandprofilegrindingincombination:Threadedwheelgrindingisknownasamuchfastergrindingprocessbecausetherearenoidletimesforpitch movements between the teeth and thusappropriate for grinding gears with large number ofteeth. Threaded wheel grinding of large modulegears has limitations r
37、egarding the achievablequality.Figure 6. Multiple wheel profile grinding8The highest quality levelcan stillbeing achievedbyprofile grinding but a lot of investigations are beingmade to use threadedwheel grindingfor roughandfinish grinding of large module gears 5. GleasonPfauter is following a new st
38、rategy to use threadedwheel grinding as a fast roughing cycle and profilegrindingtoachieveahighqualitylevelwhichforhighmodule gears is typically in the range of DIN 1-2.Butthistechnologyrequiresnewmachineconceptscapabletorunboth cycles. Gleason-Pfauter is de-veloping a new machine series capable of
39、combin-ing the advantages of both cycles. The grindinghead of that machine is designed to use threadedgrindingwheelsaswellasprofilegrindingwheelsasshowninFigure7. Inaddition,themachineiscapa-ble of changing those wheels fully automaticallywithin a grinding cycle by a special tool changer.Figure 8 is
40、 showing an example for cycle time re-duction when using threaded wheel grinding forroughingandprofilegrindingfor finishing. Thetotalcycle time can be reduced from 127 min to 77 minwhich is a reduction of 40%!Figure 7. Grind head for profile and threaded wheel grindingFigure 8. Comparison between co
41、nventional and new technology9Adaptive grinding technologyAnother important aspect to reducethe maingrind-ingtimeistoavoidthesocalled“airgrinding”. Ifusing the conventional technology as describedabove the maximum amount of stock is subdividedinto a certain number of strokes which will bepassed thro
42、ugh with the programmed axial feedspeed. Butduetohardeningdistortionstheamountof stock is not constant over the tooth flanks andaround the gear as shown in figures 9 and 10 withthe effect that a considerable amount of strokelength is not grinding the gear.Figure 9. Adaptive technology to reduce cycl
43、e time and dressing timeFigure 10. Typical runout of hardened gears10To avoid this unproductive air grinding, Gleason isusinganacoustic emissionsensor technique tode-tect whether the grinding wheel has contact to theworkpieceor not. Incasethewheelis runningwith-out contact, the axial feed speed is i
44、ncreased to amaximumspeedeffectingtimesavings. Additional-ly the dressing intervals can be increased as well.Insteadofdressingthewheelafteracertainnumberof strokes, the wheel is dressed after a certainamount of effectively ground stroke length. This isreducingdressingtimeas wellas toolcost, suchasgr
45、indingwheelanddresser. Theadaptivetechnolo-gyallowstimesavingsupto33%. Sothemorecriti-cal the hardening distortions the more effective isthe adaptive technology.Power dressingThe reduction of dressing timeis another importantaspect forproductivity improvement. Whenmount-ing a new grinding wheel the
46、shape does not fit tothe required profile thus has to be dressed.Inconventionaldressing,thetargetprofile(redline)is dressed as shown in Figure 11. The dresser isstarting to follow path no. 1 then 2 and so on until itreachesthefinalprofile. Thiseffectsthat thedress-eriscoveringavolume(blue)whichissho
47、wnontheright hand side of Figure 11. This volume is muchhigher that the real dressed volume showing thatthis dressing method is not very efficient.To avoid this ineffective dressing volume Gleasonhas developed the so called “Power Dressing” me-thod. Its principle is shown inFigure 12. Insteadofdress
48、ing pass by pass parallel to the target profilethe dresser works in radial direction. Thereforetheraw profile of the wheel is programmed to the ma-chine which allows starting the dressing cycle justoutside the wheel and infeeding in radial directionuntil the target profile is reached. The effect is
49、thatthe processed volume (blue) is almost the sameasthe real dressed volume representing the highefficiency of this method.Dependingontherawprofileof the grindingwheelsaswellasthegeardatathedressingtimecomparedto conventional dressing can be 9 times faster asshown in Table 2.grinding wheel blankFigure 11. Conventional dressingTable 2. Performance comparison of power dressing versus conventional dressingModule 14 10 8 5No.ofteeth 15 10 34 18Wheel width 50 40 30 20Wheel characteristicConvention (mm:ss) 20:27 55:37 k.A. k.A. 9:47 31:30 4:48 13
