1、09FTM15AGMA Technical PaperHigh PerformanceIndustrial GearLubricants for OptimalReliabilityby K.G. McKenna, J. Carey,N.Y. Leon, andA.S. Galiano-Roth, ExxonMobilResearch and EngineeringHigh Performance Industrial Gear Lubricants for OptimalReliabilityK.G. McKenna, J. Carey, N.Y. Leon, and A.S. Galian
2、o-Roth, ExxonMobil Researchand EngineeringThe 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.AbstractIn recent years gearbox technology has advanced and Original Equipment Man
3、ufacturers (OEM) haverequired gear oils to meet the lubrication requirements of these new designs. Modern gearboxes operateundersevereconditionsandmaintaintheirreliabilitytoensureend-userproductivity. Thelatestgenerationofindustrial gear lubricants can provide enhanced performance even under extreme
4、 operating conditions foroptimal reliability and reduced cost of operation.Thispaperdescribeshowgearlubricantsfunctioningearboxesanddiscussesthefactsvs.mythsofindustrialgear lubricants. The paper will show how advanced gear lubricant technology can optimize the life of thegears, bearings and seals,
5、resulting in reduced cost of operation. Opportunities to use advanced syntheticgear lubricants to achieve operational benefits in the areas of improved energy efficiency, wider operatingtemperature ranges, extended oil drain intervals and equipment life will be discussed.Copyright 2009American Gear
6、Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314September 2009ISBN: 978-1-55589-968-43High Performance Industrial Gear Lubricants for Optimal ReliabilityK. G. McKenna, J. Carey, N.Y. Leon, and A.S. Galiano-Roth,ExxonMobil Research and EngineeringTypes of lubricati
7、ng film classificationsKnowledge of the types of lubricating film will assistin understanding the formulation and application ofgear lubricants.The two types of lubricating film relevant to gearlubrication are boundary and elastohydrodynamic(EHL). Understandingthecharacteristicsofeachofthese is impo
8、rtant in order to understanding thelubricant performance requirements.Figure 1 shows that boundary lubrication existsduring sliding motion where metal to metal contactoccursbetweenthetwosurfaces. Thecoefficientoffriction ranges from 0.1 to 0.15 between the metalsurfacesinthislubricationregion. Inthe
9、absenceofspecialized antiwear additive technology increasedwear rates will occur during boundary lubrication.The amount of wear will depend on temperature,speed,surfacefinishes,material,lubricantviscosityand effectiveness of the additives. Gears operatewith combined sliding and rolling motion above
10、andbelow the pitchline. Underlow speedand hightem-perature conditions the EHL film will be relativelythin and boundary conditions will dominate (seefigure 2).Elastohydrodynamiclubrication(EHL)occurswhenthe lubricant film thickness reduces metal to metalcontactandlocalcontactpressurebetweenthesur-fac
11、es is high enough to cause elastic deformation.This creates a small but finite area of contact oftenreferred to as the Hertzian contact zone. The highcontact pressure also acts to increase the lubricantviscosity as it is drawn into the contact zone. Thisincrease in viscosity helps generate the lubri
12、cantfilm that maintains the separation of the twosurfaces. Withthishighviscosityandtheshorttimeinthecontactarea,thelubricantcannotescapeandseparation of the surfaces is achieved. The filmthickness generated in EHL contacts of this type isvery thin and is typically between 0.1 to 0.5 micrometers. Fil
13、mthicknessisafunctionoftemperature,speed, load, geometric conformity of the surfaces,initial lubricant viscosity and the rate at whichviscosityincreases withpressure. This lastcharac-teristic is often quantified by the pressure-viscositycoefficient of the lubricant and varies with itscomposition.Sur
14、facefinishalsoinfluencesthestateoflubricationbetween two surfaces. The more polished thesurface,thelowerthelubricant filmthickness thatisrequired to achieve separation between thecontacting surfaces. This is often quantified in theLambda value or specific film thickness. This ismerely the ratio of t
15、he EHL film thickness to a mea-sure of the combined surface roughness. Thus ahigh specific film thickness indicates that thesurfaces are well separated. Conversely a lowspecific film thickness indicates poorer surfaceseparation which may result in higher friction andpotentially increased rates of we
16、ar.Two examples of EHL lubrication classification arewhen gear teeth mesh at the pitch line and in theload zone of anti-friction bearings (see Figures 3and 4).Figure 1. Boundary lubrication4Figure 2. Gear rolling and sliding lubricationGear lubricant requirementsThe lubricant formulator must conside
17、r manyfactors and components in developing a properlubricantforanenclosedgearbox. The mostimpor-tant components are the gears (i.e. gear teeth),bearings and seals. The factors influencing thelubricant and the reliability of the gears, bearingsand seals are;S Gear typeS Gear speedS Reduction ratiosS
18、Operating temperaturesS FilterabilityS Input powerS Load characteristics- Shock in a steel mill- Steady in a power plant cooling towerFigure 3. EHL in bearingFigure 4. EHL in gears5S Drive typeS Application methodS Water contaminationS Ambient conditions- Arctic temperatures below -20F- Tropical - h
19、ighhumiditytemperaturesabove100FS Maintenance access- Easy access - walk up to the gearbox- Located under an evaporative type coolingtower- Located above the ground in a wind turbineor overhead craneS Industrial specifications- AGMA (American Gear ManufacturingAssociation)- DIN (Deutsches Institut f
20、r Normung)S Original equipment manufacturers (OEM)specificationsThe goal is to develop a lubricant that uses highperformance base stocks balanced with the properadditive technology to achieve the optimumperformance and reliability of the gearbox.When gearbox operating conditions are severe,such as e
21、xtreme temperatures, loads, and speeds,synthetic lubricants may be necessary for reliableoperations. A synthetic lubricant that offers ex-tended drain interval may also be desirable whereequipment is not readily accessible. An example ofan application that meets the preceding criteriawould be gearbo
22、xes in wind turbines.Gear lubricant characteristicsThenecessarycharacteristicsforgearlubricantcanbe stated as:S Correct viscosity at operating temperatures toassure distribution of the lubricant to all contactsurfaces and formation of an EHL film over therange of operating speeds and loads.S Adequat
23、e low temperature fluidity to permitcirculation at the lowest expected start-up tem-perature.S Chemical stability to minimize oxidation underelevatedtemperaturesand agitationin thepres-ence of air and to provide the desired lubricantlife for the maintenance service intervals.S Good demulsibility to
24、permit water separationfor removal.S Good antiwear performance to protect againstwear under boundary lubricationS Extreme pressure additivesto minimizeweldingof metals under excessive loads.S Low traction to control operating temperaturesunder severe service.S Anti-rust properties to protect gears a
25、ndbearing surfaces from rusting.S Non-corrosive so that gears and bearings willnot be subjected to chemical attack by thelubricant.S Foamresistanttoallowentrainedairtoseparatefrom the lubricant.S Compatible with commonly used seals.A properly enclosed gear lubricant is a balancedformulationthatwillp
26、rovidegearprotection,bearingprotection, corrosion/rust resistance, sealcompatibility, filterability, oxidation resistance andanti-foam/air release (see figure 5).Gear protectionThe gear lubricant functions are to cool, to reducewear, and to assist in sealing for optimal protectionof the gearbox comp
27、onents. An areaof concernforlubricantgearprotectionisexcessivewear. Severaltypes of wear might take place including pitting,micropitting, and scuffing.Pitting can be in the form of micropitting or macro-pitting. Micropitting is surface metal fatigue thatcausesthetoothprofileshapedeviations whichcanr
28、educe gearbox efficiency, increase noise, andvibrations. Two commonly used terms to describemicropitting are ”grey staining” or ”frosting” of thegeartoothface. Contactstresseslocatedbelowthepitch line (dedendum) of the driving gear tooth arehigher because of the shorter radii of the toothcurvature (
29、see figure 6).Gears that are overloaded for any reason willdevelop fatigue failure and pitting of surface metalwilloccurinthededendumareaafterlongperiodsoftime. Asthepittingincreaseitcanbecalledmacro-pitting. If an overload is great enough this type offatiguefailurecouldoccurinarelativelyshortperiod
30、of time (see figure 7).6Figure 5. Balanced gear oil formulationFigure 6. Micropitting exampleFigure 7. Macropitting example7Micropitting istalked aboutmore incurrent gearde-signsthandesignsofgears30yearsago. Therearemany operational and design factors that increasethe tendencies for micro-pitting. L
31、isted below arepotential solutions to reduce micro-pitting in gears.Solutions for reducing macro/ micro-pittingmechanicallyS Use quality steel, properly heat-treat to desiredhardness.S Reduce contact stresses by reducing load.S Optimize gear geometry.S Polish to smoother surface finishes.S Assure un
32、iform load distribution.Solutions for reducing pitting throughlubricationS Check to ensure the use of the proper viscosity.Higher viscosity lubricant directionally may be asolution however, beware that the higherviscosity may cause issues with the bearings orthe other gears in the gearboxS Use a lub
33、ricant containing micropitting resistantadditivesS Reduce lubricant operating temperatureS Use synthetic lubricant to provide higher filmthickness at operating temperatures and toreduce shear forces in the sliding contact areathroughtheirinherentlylowertractioncoefficientversus mineral oil.Scuffing1
34、)is severe adhesion and metal transferbetween teeth due to welding. Under conditions ofheavy loads, extreme temperatures, rough andirregular surfaces, loss of or inadequate oil supply,or the use of a lubricant with too low in viscosity willresult in only a partial lubricant film present in theloaded
35、 contact area. This partial lubricant filmconditioncausesa degreeof metalto metalcontactbetween the surfaces that will tear and weld thegear material (see figure 8). Listed below arepotential solutions to reduce scuffing in gears.Solutions for reducing scuffing mechanicallyS Use proper initial start
36、ing run in procedures.S Optimize gear geometry and use precision gearteeth design and maintain good helix alignment.S Use smoother surface finishes.S Useproperlyengineeredmaterialsformaximumscuffing resistance.(Photo courtesy of GEARTECH)Figure 8. ScuffingSolutions for reducing scuffing throughlubri
37、cationS Use the proper viscosity lubricant. Higher vis-cosity lubricant directionally may be a solution,however, beware that the higher viscosity maycause issues with the bearings or the othergears in the gearbox.S Use a lubricant containing anti-scuffingadditives - sulfur, phosphorous or borate.S R
38、educe lubricant operating temperature.S Use synthetic lubricant to provide higher filmthickness at operating temperatures andreduced contact area temperatures through itsinherently lower traction coefficients.Shock loading is a sudden application of excessiveloads on the gear teeth, which can result
39、 in plasticdeformation of the gear teeth. What is plastic de-formation of a metal? When a metal is loaded orstressed it causes strain (stretches - -similar to arubber band when pulling on the ends but not asnearlyasmuchmovement)tothematerial. Whenaload (stress) is maintained in the elastic region of
40、the material and then, when the load (stress) isremoved the metal will return to its original size.However, if the load (stress) exceeds the elasticregion of the metal, it goes into the plastic region.When this occurs, the metal does not return to itsoriginal size after the load is removed. When the
41、load (stress) exceeds the yield point of the metal itwill fracture (see figure 9)._1)Scuffing sometimes is referred as scoring by users of industrial gear oils.8Figure 9. Stress/strain curveShock loading reduces the life of the gears. It iscausedbytheoperationalconditionsintheprocess,which is being
42、driven by the gearbox. Until theshock loads are reduced in frequency and/oramplitude the gears will not achieve their optimumlife.Solutions for reducing shock loadmechanicallyS If the loads are resulting in gear fracture andunscheduled downtime. Change operationalconditions to reduce the shock loads
43、. Becausethere is a balance between optimum gear lifeand maximum production, overall knowledge ofthe plant operational goals are required.S Use higher horsepower rated gearboxes.(Typically, the user will push the limits of thedesign to achieve maximum production.)Solutions for reducing wear rates ca
44、usedfrom shock load affects on gears throughlubricationS Loads typically exceed the elastic region of themetal and a higher viscosity can not ”cushion”the force. Therefore, continue use of theproperorOEMrecommendedviscositylubricanttopre-vent other issues that can occur with a heavierviscosity lubri
45、cant.S Use anti-scuffing additives - sulfur,phosphorous or borate to reduce the welding ofthe metals during the shock load.S You can never reduce a mechanically inducedshock load through lubrication.Bearing and seal lifeWhen a gear lubricant is formulated, considerationfor the bearings and seals is
46、also important. Ifpremature bearing failure occurred then damage ofthegearsmayoccur. Ifthesealsarenotfunctioningasdesignedorprematurelyfail,otherconcernsmayarise. These concerns areincreased lubricantcon-sumption and increased level of detrimentalcontamination in the gearbox. The contaminationresult
47、s in decreased reliability of the gearbox.Reportsvary,but40% - 60%ofgearboxfailuresareinitially bearing failures2). The bearing failuremodes are micropitting, macro-pitting, spallingcaused by high surface stresses, abrasive wear,etching/plastic deformation caused by hardparticles. Hard particles com
48、e from externalcontaminants, corrosion particles (rust), and wearparticles from components in the gearbox. Bear-ings also fail because of insufficient lubricant orimproper lubricant viscosity and/or additives.A standard test is the FAG3)FE-8 roller bearingwear test. This multi-purpose laboratory rig
49、 testcan evaluate friction, bearing wear, anddeposit-forming tendency of the lubricant.As shown in figure 10, the lubricant using highquality base stocks and the advanced balancedlubricant technology achieves improved resultsover the standard gear lubricant technology.SKF, an international roller bearing manufacturerhas done extensive work to develop detailed bear-inglifeequation. Theequationconsidersloads,reli-ability, and life adjustment factors. The life adjust-ment factors include the effects of lubrication andexternal contamina
