1、- 99FTMllPerformance of Nitrided Gears in HighSpeed Epicyclic Gearbox Used in GasTurbogenerators - A Case Studyby“ A.K. Rakhit, Gear ConsultantAmerican Gear Manufacturers AssociationTECHNICAL PAPERPerformance of Nitrided Gears in High Speed EpicyclicGearbox Used in Gas Turbogenerators - A Case Study
2、Dr. Ajit K. Rakhit, Gear ConsultantThe statements and opinions contained herein are those of the author and should not be construed as an official action oropinion of the American Gear Manufacturers Association.AbstractHigh speed epicyclic gearboxes used in gas turbogenerators are typically designed
3、 with either carburized and hardened ornitrided gears. Each of these types of gear has its own merits and limitations. While carburized and hardened gears offeroptimum design, nitrided gears are sometimes preferred because of their low heat-treat distortion. In terms of loadcapacity, carburized and
4、hardened gears perform satisfactorily not only under steady load but also heavy shock load.Nitrided gears, on the other hand, are known to operate successfully only under steady state load. The fatigue life of thesegears reduces significantly under heavy shock load. That there is also aconsiderable
5、reduction of nitrided gears life underrated load with wide fluctuations is unknown. A recent failure of an epicyclic gearbox with nitrided gears that wasoperating under such loading confirms this fact. In this case, the gearbox was used to reduce the speed of a gas turbinerated 10.4 roW. During the
6、engine wash cycle, the load at the generator end varied from 0 to 10 mW at 1.5 cycles persecond for about 25 seconds duration, with the peak occasionally reaching to 12 mW. No such failure is reported insimilar gearboxes that are operating at various installations that did not experience this type o
7、f load fluctuations.Copyright 1999American Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 1999ISBN: 1-55589-749-5,AVPERFORMANCE OF NITRIDED GEARS IN HIGH SPEED EPICYCLIC GEARBOXUSED IN GAS TURBOGENERATORS m A CASE STUDYA. K. RakhitGear ConsultantLaguna N
8、iguel, CaliforniaIntroduction FailureIncidentCase carburized and hardened gears are known to operate According to a field representative, gearbox failure occurredsatisfactorily both under steady and overload conditions, after three hours of operation following a scheduled on-lineNitrided gears, on t
9、he other hand, seem to perform well only turbine water wash cycle. In general, all turbines go through aunder steady load 1. Shock loads detrimentally affect nitrided water wash cycle after a few thousand hours of operation.gears life 2. Thus, nitrided gears are rarely used in applications During su
10、ch a cycle for this turbine, the generator load is broughtthat experience occasional shock loads. However, that the life of down to about the 80 percent level and then water is injectednitrided gears may also deteriorate when subjected to a rated load through the compressor section of the turbine. F
11、ollowing wash,that varies widely in a very short time period is relatively the load is raised to the maximum continuous level. After theunknown. Although, such load spectrmns are not very common, last wash cycle, high vibration levels of the gearbox weresometimes these are observed in gas turbogener
12、ator applications, reported just before the gearbox failed. Unforamately, no dataAn analysis of a gas turbogenerator load during start-up shows were taken. The gearbox had a total of about 16,000 hours ofload fluctuations to occur whenever the fuel control system service before the failure.malfuncti
13、ons. In this paper, a case-study on the failure of Following tiffs incident, to ensure the failure was not causednitrided gears used in a gearbox subjected to a load with wide bymisalignmentofequipment or any malftmction ofcomponentsfluctuationsis presented, such as bearings, couplings, shaft, a tho
14、rough inspection wasconducted. The equipment alignment of the package was foundFailed Gearbox to be within the allowable tolerance. Also, no visible damage ofany component was noticed. This led the inspection toward theThe gearbox is of star epicyclic configuration and is used as gearbox internal co
15、mponents.a speed reducer in between a 10.5 mW gas turbine and an electric Visual inspection of the disassembled gearbox revealedgenerator. The general characteristics of the gearbox are: severe mechanical damage to all gears. The failure was socatastrophic that one complete tooth fell off from one o
16、f theHorsepower rating (hp) “ 16,500 planet gears and was found at the bottom of the housing. FigureInput speed (rpm) 8625 1 shows the damaged planet gear and Figure 2 the severed tooth.Output speed (rpm) 1,500 Tooth debris was noticed in various gear meshes. No abnormaldistress was found on planet
17、sleeve bearings, although the thrustThe sun pinion and planet gears are nitrided whereas the faces of output shaft sleeve bearings were completely wiped out.ring gear is just through-hardened. The design life of the gears Also, sealing surfaces of output shaft and labyrinth seal wereis 100,000hoursm
18、inimum, foundto beburnt.Ito.Figure 1 - Overall View of One Damaged Planet Gear Figure 3 - Detail View of the Received FragmentFrom the Failed tooth. Arrows indicatethe crack propagation direction Mag: 4XFigure 2 - View of Dislocated ToothTo determine if there were any discrepancies with the qualityo
19、f gear material and heat treatment, detailed metallurgicalevaluation was carried out with a section from the failed tooth.Metallurgical EvaluationFigure 4 - SEM View of the Fracture Surfaces of theExamination of the fragmented tooth revealed some well- Gear Fragment. Mag: 100Xdefined beach marks (Fi
20、gure 3) suggesting a typical fatiguefailure. The tooth segment was also analyzed by EnergyDispersive X-ray Spectroscope (EDS) and examined with aScanning Electron Microscope (SEM). The primary fracturesurface was relatively flat and showed well-defined crack growthmarks as illustrated in Figure 4. F
21、eatures of transgranularfatigue propagation were identified on the fracture surfaces(Figure 5), confirming cracking of the tooth due to bendingfatigue mechanism. Material certification shows gears weremade from European nitriding grade steel (EN-4B)and thechemical composition seems to meet the requi
22、rements as shownbelow:0.2/ 0.10/ 0.45 3.00! 0.45/ 0.025 0.0250.28 0.35 max. 3.50 0.65 max. max.2gThe case hardness profile and the effective case depth of Figure 5 - SEM View of the Fracture Surfaces of theteeth were determined by a microhardness tester and are shown Gear Fragment. Mag: 500Xin Figur
23、e 6. The effective case depth was found to be 0.015 inch. Hardness and case depth profile, and the case microstructureTiffs satisfies the design requirements for these gears. The core indicate there was not much discrepancy during heat treatment.hardness of tooth varied from 26 HRC to 28 HRC. The de
24、sired Lower core hardness of the planet gears has some influence oni,qm-i minimmn core hardness is 28 HRC. the reduction of bending fatigue life but not to the extent as inthis case. Since there were no records of any sudden overload, it7 is believed the major cause of failure was load fluctuations.
25、! .Analysis of the generator load spectrum (Figure 7)a _ _ _ subsequentto an on-lineenginewashcycleshowsquitea bitof- - _ loadfluctuation- from4 mWto 12mWin a shortintervalof_ f _ time (1.5cyclesper second for about25 seconds).Field reportsw _ indicatethis typeof loadingto occur manytimes sincethe,
26、_ generator set was commissioned. Nobody paid any attention tot9re u L this loading pattern because it did not represent a real overload conditionfor the gearboxthat is designedwith an overloadt_ capacity of four times the nominal rating. That a high,- _ fluctuatingnominalloadcouldbeas damagingasove
27、rloadtothe_ nitrided gears was simply unknown. Although reduced allowable-I- 46 _x bendingfatiguestrengthwasconsideredin designingthe planetgears for reverse bending 3, it did not take into account of any._ possible change in the slope of S-N curve (damage line) due to4o high magnitude of a fluctuat
28、ing load.It has been shown 4 and, also in Germany (H. Wintersunpublishedwork)thattheslopeof S-Ncurvesforbothbendingo 2 4 6 lo 12 _4 1 is 2o and pitting shift downward when nitrided gears are subjected toDistance from the Surface (x 10a in.) any overload as illustrated in Figure 8. Conversely, slope
29、of S-Ncurves for carburized and hardened gear steels changes very littleFigure 6 - Gear Tooth Microhardness Profile as shown in Figure 9. So, it is certain that the allowable fatiguestrengths for both sun and planet gears that were nitrided hadbeen reduced due to high fluctuating load. With reducedF
30、ailure Analysis allowable fatigue strength, one of the planet gears wlfich mighthave been metallurgically weakest of all, failed first. It isIn a mature design of a gearbox as in tiffs case, the failure is believed the fraganents of the failed tooth then went into meshbelieved due to influence of ei
31、ther one or more of the following causing the premature failure of the gearbox.factors: Defective gear material or forging Improper heat treatment of gears 15000 Sudden overload beyond design consideration12000 High fluctuating load9000In this analysis, failure initiation due to inadequatelubricatio
32、n is ruled out because no evidence such as discoloration _“of tooth or any burn marks were observed on teeth surfaces, o 6000Also, vibration was not believed to be the cause because noabnorlnal vibration levels were reported while the generator set 3000was nmning following turbine wash cycle. Thus,
33、it is justified toassume that gearbox health was reasonably satisfactory for 0sometime before the failure occurred. 0 10 2o 30 40In regard to material, the chemical analysis shows an TIMEDURATION,sec ,99go2roanuacceptable alloy composition. Also, there were no adverse Figure 7- Fluctuation of Genera
34、tor Load Afterreports on forging quality such as banding, or grain flow. So, On-line Engine Washingmaterial quality did not seem to play any significant role in thefailure mechanism.= !_ ConclusionsKto The fatigue life of nitrided gears is reduced not only underLU_.“ overload but may also be reduced
35、 under high fluctuating nominalOriginalMaterial load. The failure due to overload or fluctuating nominal loadz .,.,. , definitely accelerates with the level of fatigue already induced by“-., previousstresscyclesin thenitrided In this it isgears. case,uJ believed 16,000 hours of turbine operation con
36、tributed to some-J Damaged _ _. _.“_ i “_- extent in the failure of gears. So, it is advisable to consider lowerI “.-. allowable fatigue strengths in designing nitrided gears that areI subjected to overload or occasional high fluctuating load. Also,10a a cleaner material that meets Aerospace Materia
37、l SpecificationLOADCYCLES.N (A/_S) 2300 or 2304 is recommended in such applications forreliable and high fatigue strength.Figure 8 - Bending Fatigue Life of Original andDamaged Nitrided Tooth References325 1. French, H., “Fatigue and the Hardening of Steels,“ Trans.260 - Am. Soc. Steel Treat., 212 (
38、1933) 10, S. 899.2t; 195 - 2. Albertin, L., Frolich, R. L., Winter, H., Hohn, B.R. andco_ -“_ J Michaelis, K., “Load Carrying Capacity of Nitrided Gears,“130 OriginalMaterial AGMA Fall Technical Meeting, St. Louis, Missouri, paperNo. 94FTM4.0 Damaged I_z G1 ! 3. AGMA Standard 2001-B95, 1995, “Fundam
39、ental Rating65 -I II Factors and Calculation Methods for Involute andSpurG252 I III II I HelicalGearTeeth“.I II I39 -.I gk I1_ 4. Rakhit, A., Heat Treatment of Gears for Optimum QualityI I /ll /t_ I I12 103 104 n:l 10NI-nl06N1 107 108 109 N_010 and Performance - A Practical Guide for MechanicalNUMBE
40、ROF LOADCYCLES,N Engineers, Book to be published.19990270-001MFigure 9 - Bending Fatigue Life of Original andDamaged Carburized and HardenedGear ToothDiscussionThe fluctuating load following a turbine water wash cyclewas due to some instability in the fuel control system. This sortof instability and hence the load fluctuation may be avoided byreplacing on-line wash of turbine with stationary wash procedure.Such an arrangement will definitely improve the life of suchgearboxes. In fact, in installations with stable fuel controlsystems alone, there was not a single instance of gearbox failure.
