1、11FTM18AGMA Technical PaperLongitudinal ToothContact Pattern ShiftBy J.B. Amendola,J.B. Amendola, III, andD. Yatzook, Artec MachineSystemsLongitudinal Tooth Contact Pattern ShiftJohn B. Amendola, John B. Amendola III, and Dereck Yatzook, Artec MachineSystemsThe statements and opinions contained here
2、in are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractAfter a period of operation high speed turbo gears may exhibit a change in the longitudinal tooth contactpatternreducingfullfacewidthcontactandtherebyincreasi
3、ngtheriskoftoothdistressduetothedecreasedloaded area of the teeth.The phenomena may or may not occur. In some units the shift is more severe than others and has beenobservedincaseswherethereisaslittleas16,000hoursofoperation. Inothercasesthereisnoevidenceofanychangeforunitsinoperationformorethan170,
4、000hours. Thisconditionhasbeenobservedprimarilyinhelical gears. All recorded observations have been with case carburized hardened and ground gear sets.This document describes the phenomena observed among some of many installed high speed gear units infield operation which have been inspected. The au
5、thors have not found any written material describing thisbehavior and upon further investigation suggest a possible cause. Left unchecked and without correctiveaction, this occurrence may result in tooth breakage.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAl
6、exandria, Virginia 22314October 2011ISBN: 978-1-61481-017-93 11FTM18Longitudinal Tooth Contact Pattern ShiftJohn B. Amendola, John B. Amendola III, and Dereck Yatzook, Artec Machine SystemsIntroductionField inspection of high speed turbo gears after an undefined period of operation may exhibit a cha
7、nge in themating contact pattern of the gear mesh.This has been observed on gear rotors with the following characteristics:S Gear elements which are case carburized and groundS Relatively large face widths (greater than 300 mm)S Pitch line velocities greater than 100 m/sS Modified leads accounting f
8、or both mechanical deflections and thermal deformationS Operating hours with as little as 16,000 hrs and as much as 170,000 hrs.Lead corrections for high speed gearsetsEvery gearset is subject to torque, resulting in elastic deflection of the gear tooth parts as well as the entirerotor body. Individ
9、ual teeth bend, thepinion andwheel bodiestwist, bend,and expandunder theeffect ofthetorque, load, and centrifugal forces; see Figure 1.Forhighspeedgearsetsthemechanicaldeflectionsdescribedhereinarecompoundedwithadditionalfactorswhich result in further deformation of the gear teeth:- Churning of the
10、lube oil and frictional losses in the bearings cause the rotor bodies to heat up andconsequently expand.- Pumping action of the oil and air entering the gear mesh produces an increased asymmetricaltemperature gradient of the gear tooth along the length of the tooth flank resulting in added deformati
11、on.Special consideration must be given to compensatefor thesemechanical deflectionsand thermaldistortionsso that the load across the gear face is uniformly distributed under normal operating conditions.Toothingmodificationforgearsetswithpitchlinevelocitiesgreaterthan100m/srequirecarefulconsideration
12、of thermal distortion. The resulting thermal deformation, in addition to the mechanicaldeflections, requiresacompositeprofileandlongitudinalmodificationtoachieveproperloaddistributionunderoperatingconditions;see Figure 2 and Figure 3.Figure 1. Tooth contact patterns of a spur toothed pinion without
13、longitudinal correction at rest(top) and with load with load (bottom) applied4 11FTM18Figure 2. Schematic of single helical toothing deflection with added parameter for thermaldeformation and the associated lead correctionFigure 3. Schematic of double helical toothing deflection with added parameter
14、 for thermaldeformation and the associated lead correction5 11FTM18InvestigationsbyMartinagliawereperformedtoaccuratelydeterminetemperatureconditionsinahighspeedpinion, see Figure 4.The dimensional parameters of the rotor configuration define how the lead is modified. Its derivation isspecific for a
15、 continuous single or double helix. Therefore, this dimension is largely dependent on theexperience gained by years of observation of the tooth bearing patterns at nominal loads leading to accumu-lated empirical values.Early observationsEvidence of a longitudinal tooth contact shift was first observ
16、ed by Artec in the early 1990s. When firstobserved,itwasinitiallythoughttobetheresultofchangesinthefoundationorotherexternalinfluencesofthegear unit inducing a small misalignment in the mesh. However subsequent inspection(s) of thesame and/orotherinstallation(s)revealedacurious shiftof thecontact pa
17、ttern. Sincethis observationdid notconsistentlymanifest itself when inspecting other installations, the cause of the phenomena remained questionable for anumberofyears. Whenacontactshiftwasobserved,oftentimesheavyvarnishingofthelubricantonthegearteethinthevicinityofthecontactfacewasnotedintheareaoft
18、hehighesttemperaturegradient;seeFigure 5.This is also the area where the deepest correction is required; see Figure 6. The phenomena had beenobserved in both gear sets where the thermal corrections were nearly negligible as well as units withsignificant thermal corrections.Figure 4. Effect of pitch
19、line velocity on tooth flank temperature, establishment of anasymmetrical speed/temperature gradient6 11FTM18Turbine side Compressor sideFigure 5. Varnish deposits on gear teethLoaded flanks - designFigure 6. Original toothing modification plan(Ordinate values intentionally omitted for proprietary r
20、easons)7 11FTM18PhenomenaIn all cases upon inspection, the contact pattern on both the loaded and non-loaded flank exhibited a contactshift deviating from the originally designed tooth contact pattern. In some units the contact shift is moreseverethanothers. Ifleftunattended,thiscontactshiftoftheint
21、endeddesignedcontactpatternmayprogressresultingineverincreasinglocallyhighloadedsegmentsoftherotorflankswhichmayeventuallyresultingearfailure.Case study - example installation verifying longitudinal contact shiftApplication: Driving: gas turbine (4,670 rpm)Driven: centrifugal compressor (2,926.5 rpm
22、)c-c center distance = 580 mm (22.853 in)B face width = 500 mm (19.764 in) helix angle = 10 (single helical) pressure angle = 20- rotor material = carburizing alloy steelP power = 37,285 kW (50,000 hp)PLV pitch line velocity = 109 m/sm module = 9.25Observations in the field: Inspection after 12 year
23、s and 105,000 hours of continuousoperation:Dynamic:After 105,000 hours the gearset showed no signs of tooth surface distress; i.e., scuffing, micropitting andvirtually no signs of pitting (a few pits were found on the dedendum of the pinion). Varnishing (on both pinionand wheel) was noted to be heav
24、iest in the flank section with the deepest lead correction. However underobservation it was readily noted that the “dynamic” load clearly favored the turbine end, little or no loadappeared to have been transmitted over the first150 mm of tooth face nearest the compressor end.Static:Thetoothcontactpa
25、tternoftheoldrotorspriortoremovalwasfoundtobeinverypoorconditionandwasnotinaccordance with the original manufacturers protocol. The tooth contact was found tobe heavyin thecentralleft portion of the tooth flank in a centralized area 100 175 mm from the pinion drive end of the flank. In thisregion co
26、ntact was found to be symmetrical to the pitch line (that is, distributed evenly along the profile) withequal load sharing along the addendum and dedendum.Attempt to correct in the fieldIn this case the unloaded static tooth contact pattern of the drive flank was originally designed to be at the(NDE
27、) compressor side of the mesh. After 12years ofcontinuous operationthis staticpattern shiftedtowardthe (DE) gas turbine side. Subsequently, the non-loaded tooth flank (originally designed with a unmodifiedstraight contact pattern) shifted toward the (NDE) compressor side.Anattemptwasmadetocorrectthe
28、noloadstatictoothcontactinthefieldbyshiftingthestaticcontacttowardthe NDE of the pinion. Only a fractional improvement was realized even with a relatively large adjustment.Improved contact is only possible if the longitudinal correction was no longer in accordance with the originalspecification. Thi
29、s had confirmed that the tooth form had become distorted with time.Onthisbasisadecision wasmade toreplace therotor setwith anunused spareset ofrotors. The newrotorswere installed and the no load tooth contact was recorded, and confirmed to be according to the originalconfigured protocol. An operatio
30、nal check of new spare rotors (dynamic tooth contact) via the inspection8 11FTM18cover revealed full load tooth contact after the operation reached steady state conditions. The observedcontact was 100% consistent across the entire tooth flank.Confirmation of hypothesisTheoldpinionandgearwhichweretak
31、enoutofservicewereplacedonaleadandprofilecheckingmachinetodetermine the as found lead and profile measurements. Figure 6 describes the original corrective values asmadeonthegearsetloadedtoothflankspriortooperation. Figure 7describestheasfoundloadedtoothflankmeasuredvalues. Comparisonofthesefiguresde
32、monstratestoothflankexpansionwhichshiftedtheoriginalno load static contact pattern and provided obvious evidence of permanent deformation of the gear teeth.Permanent deformation was also evident on the non-loaded tooth flanks (back side). Figure 8 and Figure 9provideagraphicalcomparisonofeachrotorno
33、n-loadedtoothflanksshowingtheamountofchangethattookplace for both the pinion and wheel.Figure 7. Loaded flank - design and as found(Ordinate values intentionally omitted for proprietary reasons)Figure 8. Pinion unloaded flank - design versus as found9 11FTM18Figure 9. Wheel unloaded flank - design v
34、ersus actualThe expansion is remarkable insofar as it has a magnitude nearly 75% of the amount of the design leadmodification.Theory as to causeDuring gear unit operation thereis ahigh velocityaxial flowof lube,oil andair acrossthe helix(s)of themesh.Through mesh engagement, oil and air are mixed an
35、d compressed resulting in an asymmetrical (uneven)tooth flank temperature gradient; see Figure 4.Inthecasestudyhighlylocalizedtemperatureshadreachedlevelsthatcausedametallurgicaltransformationwherein entrained austenite was transformed to martensite. This transformation resulted in an expansion ofth
36、ecasematerialtherebyaffectingthe tooththickness whichresulted ina contactpattern changeon boththeloaded and non-loaded flanks. The mechanism of expansion is due to the density difference betweenausteniteandmartensite. AsnotedfromFigure 9thisexpansiondoesnotoccurinauniformmannersincethetemperature ri
37、se from the meshing action is asymmetrical over the tooth length.Dudleys Handbook of Gear Design states: “After quenching, steels are usually tempered or stress-relieved.The tempering operation may be used to reduce the hardness of a part and increase the toughness. Byproperly adjusting the temperin
38、g temperature hardness values can be obtained over a wide range. Evenwhennoreductioninhardnessisdesiredalow-temperature(250to350F)temperingoperationisdesirabletoreduce stresses in the steel and produce a kind of martensite that is tougher than the kind producedimmediately upon quenching.” This tempe
39、rature range may be achievable in the operational environment ofsome running gear units.Load conditions, partial loads vs. full load, tooth module, level of operating stress numbers relative to geartoothdesignmayalsohavecontributinginfluences. Animproperdesignleadcorrectionand/ormanufacturingerrorso
40、ftheunit,oranimproperlysetupgeartoothcontactpatternduringtheoriginalinstallation,orchangesdue to uneven bearing wear or soft foot may also contribute to the described phenomena.Considering the successfullength ofservice inthe notedexample itcan bedetermined withcertainty thatthegearset was furnished
41、 with an optimized lead and at the outset of service the unit operated with uniform con-tact across the entire full face width of the toothing. As service time was accumulated a segmentof thetoothflankincreasedintemperatureandreachedthetemperingtemperaturerange. Thismostlikelywastheresultof accumula
42、tedvarnish stainingof thegear teeth. Transformationoccurred slowlyat firstand thenincreasedat a faster rate reducing flank contact over time. The appearance of significant varnishing of the gear teethwas the cause of this rise in tooth temperature.10 11FTM18Varnishing does two bad things:S Varnish i
43、s an insulator on the gear teeth thereby reducing the efficiency of the lube oil to cool the gearteeth. As the varnish builds up a worsening condition develops. As a result, the gear teeth will gethotterthereby encouraging tempering of the gear teeth in that regime of the flank.S Varnishing increase
44、s the frictional affects of the compressed lube oil and air as it travels longitudinallyacross the flank thereby adding additional heat to the tooth flanks.Regardlessofthemeansofentrainment,theactionthatleadstovarnishisinplace. Fromhere,thefailurecanproceed with adiabatic compression in the load zon
45、e of a lubricationsystem. Adiabatic compressionoccurswhen air bubbles travel from low pressure to high pressure. The air bubble compresses rapidly (implosion)resulting in intense entrapment of the heat and extreme rise in temperature.In this example during the earlier years of service prior to the s
46、low development tooth varnishing, themaximum temperature along the tooth length was most likely below the tempering temperature range. Asvarnish began to deposit on the gear teeth, the temperature gradient increased and gradually entered thetempering temperature range beginning the process of transf
47、orming retained austenite to martensite andthereby began its expansion. What was discovered finally was the result of what may have occurred over aperiod of operation not consistent in a linear sense with the running life of the gearset. In fact, the majortransformation period likely started quite s
48、ometime after the start of service and then changed rapidly, mostlikely in the last few years of service. So while there has not been a failure, evidence of surface distress wasbeginning to develop with small pitting located on the dedendum of the pinion, a failure may have beenimminent and was caug
49、ht in time. Inspection of the rotors revealed an increase in surface hardness of34Rc(original58-59Rcnow61-62Rc)asmeasuredatvariouspointsonseveralgear teeth. This is theresult of untempered martensite transformed from retained austenite. While the transformed surface isharderthantheearlierstartingconditionitcouldnotpossiblysustaintheloadoversuchanarrowportionoftheface for very much longer. Considering the locally high loads no additional surface distress was observedsuch as more pitting or scuffing. We attributed this to the improved surface condition over running t
