1、12FTM13AGMA Technical PaperGear Material Selectionand Construction forLarge GearsBy F.C. Uherek RexnordIndustriesGear Material Selection and Construction for Large GearsFrank C. Uherek, Rexnord IndustriesThe statements and opinions contained herein are those of the author and should not be construed
2、 as anofficial action or opinion of the American Gear Manufacturers Association.AbstractFor gears larger than 3 m (10 feet) in diameter, construction of gear blanks tend to be divided into cast steel,ductileiron,andforgedrimweldedwebstructuresforuseincylindricalgrindingmillsandkilns. Thispaperwillre
3、viewtheapplication,variousoptionsformaterialselection,andtheimpactofselectionontoothgeometry. Agroup of sample gears are developed to compare each of the materials and methods of blank construction.Each sample is discussed in light of structural stress, deflection, expected life, handling weight, ma
4、terialorigin, fabrication method, inspection requirements during construction, and impact of selection on fieldperformance. Basedontheabove,aroadmapisdevelopedlistingcriticalconsiderationsandoptimaluseofeach material and method of construction in this application.Copyright 2012American Gear Manufact
5、urers Association1001 N. Fairfax Street, Suite 500Alexandria, Virginia 22314October 2012ISBN: 978-1-61481-044-53 12FTM13Gear Material Selection and Construction for Large GearsFrank C. Uherek Rexnord IndustriesIntroductionThe purpose of any gear mesh is to transmit rotary motion and torque from one
6、location to another at aconsistent rate. Various rating practices from AGMA, ISO, and others go into great detail about the toothproportions, accuracy requirements, materialselection,andcuttingmethods toproduceatooththatsatisfiesthe requirements of the application. However, standards do not provide
7、all information necessary to makesurethetorqueatthegeartoothisactuallymovedtothepieceofdrivenequipment,i.e.,gearblankdesign. Inmostencloseddriveapplications,adiskofthesamefacewithaboreandkeywayissufficient. However,whenin the realm of large gears, defined as 3 m (10 feet) in diameter and above, a so
8、lid blank fulfils the designengineers maxim of makingthepart difficult tomanufactureandimpossibletoinstall. Blankdesignneedstobe driven by the application and the range of materials available to ensure sufficient stress capacity is avail-ableattheteeth,aswellastheabilitytoconnectwiththedrivenequipme
9、nt. Thispapercoverstheseissuesina specific area of use: gearing for cylindrical grinding mills and kilns.BackgroundGrindingmillandkilnserviceareunusualinstallationsforgearingwhencomparedtotraditionalenclosedgeardriveinstallations, but theseapplications havebeenutilizedfor over eighty fiveyears. The
10、grindingprocess,moreaccurately termedatumblingprocess, uses horizontalrotatingcylinders that contain thematerial tobebroken potentially augmented by grinding media. The material moves up the wall of the drum until gravityovercomes centrifugal forces, and it drops to the bottom of the drum to collide
11、 with the remaining material.This breaks up the particles and reduces their size. Kilns rotate at far slower speeds to enableeven firingoftheircontents. Powerrequiredforthisprocessrangesfrom75to18000kW(100to24000HP),ineithersingleor dual motor configurations.In this type of application, the pinion i
12、s mountedon pillowblocks drivenby alow speedmotor or a motor andenclosed gear drive. For mill applications, the gear is mounted on the mill using a flange bolted connection(seeFigure 1for onetypeof flangeinstallation). For akiln, various types of spring plates are used. Boththecenter distanceandalig
13、nment areadjustableeither by shimmingthepillowblocks ormovingthemill. Lubric-antistypicallyeitherhighviscosityoil(1260cSt100C)sprayedonthegearin15minuteintervalsoralowerviscosityoilorgreaseproductsprayedonthepinioneveryfewminutes. Alternately,lubricationcanbeappliedby continuous spray or dip immersi
14、on methods.Figure 1. Grinding mill installation4 12FTM13Gear sizes can range up to 14 meters (46 feet) in diameter with face widths approaching 1.2 meters (50inches). Typical toothsizes rangefrom 20to 40module (1.25DP to0.64 DP). Singlestage reductiongearsrangefrom8:1toasmuchas20:1. Gearmaterialsare
15、typicallythroughhardenedcaststeel,fabricatedrolledsteel, or spheroidal graphitic iron. Pinions are carburized, induction hardened, or through hardened steels.Forsmallinstallations,eitheraoneortwopiecedesignisusedwiththesplitjointslocatedintherootofatooth.Four and six piece designs are also utilized
16、when weight or pouring capacity becomes an issue.Structure requirementsBased on the application, these gears need to have large bores to accommodate the mill or kiln shell. Thisenables useof reductionratios not normally thought of as reasonable(i.e.,8:1to20:1) inasinglestage. Thegears are bolted to
17、the mill through a flange connection or mounted on tangential spring plates to allow forthermal growth. See Figure 2.Thenextstepistoconnecttheboreofthegeartotheteeth. ThisisdonebyeitherusingaBox,alsoknownasaDelta or Y shape, or tee shape structure. See Figure 3.A typical ring gear has a series of wi
18、ndows cut into the material for handling and weight considerations, asshown in Figure 4.Over time design rules have been developed to address the material shape distribution of the variouselements of the ring gear structure. Rexnord has over 5000 gears in service with design lives exceeding 25yearst
19、hatconfirmstheserulesandcalculationsarereflectfieldrequirements. Thepurposeofthestructureistoprovidestability at thetoothlocation toensure theassumptions madeat therating phaseof gear develop-ment are supported by the actual blank design. Annex C of ANSI/AGMA 6014-A06 discusses the followingconsider
20、ations for blank designS reduction of strength rating by moving the location of bending fatigue failure into the gear rim from thetooth root (KBmfactor);S effect of rim deflection on the load distribution factor, Km;S influence of the mating element on load distribution factor, Km;S definition of dy
21、namic alignment techniques to achieve correct mesh patterns.Figure 2. Flange mounting and spring mounting options5 12FTM13Figure 3. Box/Y/delta and tee shape cross sectionFigure 4. Side view of ring gearRimthicknessisasignificantparameterinthedesign. Thereisaminimumvalueofthethicknessspecifiedbyther
22、atingstandards toensureanybendingstrengthfailureofteethwouldtravelthroughthebaseof thetoothand not through the rim of the blank. Based on field experience, AGMA 6014 suggests designs having abackup ratio mB1.0mB=tRht(1)wheremBis back-up ratio;tRis gear rim thickness below the tooth root, in;htis gea
23、r tooth whole depth, in.This avoids the need to derate the gear to move the failure mode to a more conventional area. Otherstandards,suchasANSI/AGMA 2001-D04,feelavalueof1.2ismoreappropriate. Apoint ofdebateiswhat6 12FTM13isconsideredtheinsiderimofagear. Conservativethinkingwouldrequirethatanymissin
24、gmaterialbelowthetooth root is the start of the inside rim diameter. Many designs feature a groove in the side of the gear formounting of a dust shield. This groove is located to generate a backup ratio of0.65 to 0.80. The loss ofsupport,typically13mm(0.5inch),isnotconsideredsignificantwhenworkingwi
25、thfacewidthsof380to1015mm(15to40inches). Thenextlossofmaterialunderneaththetoothrootisanindicatingbandtypicallyturnedontheinsideoftherimdiametertofacilitateinstallation,as showninFigure 5. Thisloss ofsupport istypically25mm(1inch)wide. Finallythereisthetrueinsiderim diameterthat istypically 13mm (0.
26、5inch) beyondthisvalue. Reasonable design practice tends to use the machined indicating band as the location of the insidediameter for the purposes of determining a value for the rim thickness factor, KBm.Since 60% of the weight, and therefore cost, is tied to rim size and thickness, optimization pa
27、ys largedividends. A start point for rim thickness values are abackup ratioof 1.10for box section gears and 1.25forteesectiongears. As gears movetowardfiner pitches (i.e., 1.0DP) what tends todrive rim thick-ness is the tapped hole beneath the guard groove for support of the external dust guard. At
28、larger modules,deflection tends to be the controlling factor.Achieving calculated values of load distribution, Km, is a function of tooth generation accuracy and rimsupport. Basedontheratingpractice, thesetypes of gearsaretypicallyA9toA7(Q8toQ10)for helixaccur-acy. Typicalverificationmethods areahel
29、ix check of the pinionand acontact check with thegear toconfirmmeshcompatibility. Onetypicaldeflectionsourceiscomefromfacemovementawayfromthepinioneitherinthe center portion of box Y rims or the end portions of tee rims.Two other deflection modes are rim deflection and face deflection illustrated in
30、 Figure 6. Rim deflectionoccurs when the rim sags between the arms of the gear. Face deflection arises when the entire gear bendsfrom the mounting flange due to thrust force of the teeth. A good design practice is to limit maximumdeflections of these three modes to be less than 25 mm (0.001 inches).
31、The other two parameters affecting Kmare influence of the mating element and dynamic alignmenttechniques to achieve correct mesh patterns. These are beyond the scope of this paper.When designing large gear blanks, the major factors to be considered are:S loadS face widthS rim thicknessS stiffener sp
32、acing and number of windowsS window sizeS support web thicknessS material.Figure 5. Locations of dust guard groove and indicating band7 12FTM13Figure 6. Overhang deflection modesLoading on these blanks come from three sources: the amount of power being transmitted though mesh,handling as horizontal
33、rings during manufacturing, and handling as vertical segments or semi rings duringinstallation.Typically the requirement for maintainingtooth alignment is thechief driver for dimensionalselection. Widerfacewidthstendtorequireadditionalrimthicknesstomanageoverhangdeflection. Forcaststeeldesignsthecro
34、ssover point between tee and box Y section designs is 760 mm (30 inches) of face width. In a specificexample, a 6250 kW Ball mill gear at 16.76 rpm output speed has a required rim thickness value of 210 mm(8.26in) inateeconfigurationwhereastheboxY gearhas 165mm (6.51in). This reducestheoverallweight
35、of thegear to61600kg(135700lbs) inabox Y configurationbut 67500 kg(148 700lbs) as a teeconfigura-tion. Fabricated steel and ductile iron designs cannot take advantage of a box Y design due to cost of con-structionandmaterialflowduringtheproductionprocessandasaresultwillhavethicker rimswiththesefacew
36、idths.As noted above, rim thickness is driven mainly by requirements for failure though the tooth root and not theblank. Locations of customer supplied guarding and deflection also drive this parameter.The number of stiffeners is a function of the web height of the gear (i.e., distance between the b
37、ore and theinsiderimdiameter),thenumberofwindows,andtheamountofhelixangleofthegeartopreventfacedeflec-tion. Forgearsoftightcrosssection(3350kW4500HP). Whenusingductileirongears intheseapplications,thereductioninbendingstrengthrequireseither widerfacewidthsor largermod-ules (coarser pitches).Another
38、consideration is the yield strength of the material. Figure 9 illustrates that ductile iron has60 to70% of the yield strength of its steel counterpart for the same material hardness.This becomes an issue when reviewing the performance of mill gears in low cycle inching or maintenancedrive usage when
39、 the number of load cycles is expected to be less than 10 000. For cycles greater than10 000, there is no fatigue life performance difference between steel and ductile iron per AGMA 6014.9 12FTM13Figure 7. Comparison of material related factors for pitting resistanceFigure 8. Comparison of material
40、related factors for bending strength10 12FTM13Figure 9. Comparison of material related factors for yield strengthConstruction considerationsEach of the three methods of ring gear fabrication offer significant benefits as well as noteworthy disadvant-ages that can be used as a guideline for the selec
41、tion process.Theinitialconsiderationis theclient interfacedimensions. Gear designers havelittlecontrolover theboreofthegearandtheconnectioninterfacetothestructure. For applicationsthat featureagearreducer inadditiontothegearset,thedistributionofratiobetweenthegeardriveandthefinalstagereductionwillha
42、veasignific-ant impact on cost. An initial conjecture is to wrap the gear as closely as possible around the mill or kilnandplace the remaining ratio in the gear drive based on the assumption that a carburized hardened and groundenclosed drive is more cost efficient in torque transmittal capabilities
43、 than the open set. This needs to bebalancedbythelossinefficiencyifamultiplestagereductiondriveisnecessaryfortheratiorequired. Ifoneisusing a line of catalog gear drives, the steps in torque transmittal capacity as a function of unit size will alsodrivethe selection. Thefinal considerationis theover
44、all cost of providingtorque tothe millor kilnin terms ofselecting a low speed (200 rpm) motor and directly connecting it to the mill pinion in place of a higher speedmotor(1170- 740rpm)andincludingageardriveinthetrain. Itisbesttoadvisethegearsupplierofeitherthedirect driven or reducer driven option
45、and let them work out the most cost efficient solution to size the gear/gear drive combination. Forcing a mill pinion speed in a reducer drive train or selecting too fast of a motorspeedcanleadtolowcostitemssuchasinputshaftbearingsinthegeardriveconstrainingtheentiredesignofthedrivetrain. Anexampleof
46、 this is thecombinationof highpower (over 5000kW 6700HP) high speedmo-tors with L10 bearing requirements greater than the design amount based on the service factor of the drive.Requesting100000hoursofL10lifewitha2.0servicefactorthatimplies50000hoursofliferequiresthedrivedesigner to increase the size
47、 of the input shaft bearings to achieve the liferequirement. This may lead toanincrease in drive size to achieve the L10 life requested. Not allowing the ratio in the drive to increase to usemore of the excess torque capacity of the gear drive by slowing down the pinion speed causes an unevendistrib
48、ution of torque between the drive and the gear set thus increasing costs.11 12FTM13Cast Ductile IronThe next consideration is to select the material for construction. When designing with ductile iron, the firstconsiderationasnotedaboveisthereducedbendingstrengthrating. Thiswilltendtodrivethedesignto
49、largermodules that require greater rim thickness due tothe requirements of therim thickness factor, KB.Havingathicker rim will also help in controlling overhang deflection of the rim. Having a modulus of elasticity11%less than steel, will result in ductile iron moving more under the sameload. To controlthis, rim and websec-tions tend to be larger than on a comparable sized steel gear. For successful casting, abrupt section sizechanges should be avoided with this material due to solidification dynamics. In addition, to achieve uniformcooling, mold chills are required in the ri
