1、91FTM17The Influence of Lubrication on the Onset ofSurface Pitting in MachinableHardness Gear Teethby: C.E. Massey and C. R. Reeves, NAVSEASYSCOM;, E.E. Shipley, MTIAmerican Gear Manufacturers AssociationTECHNICAL PAPERThe Influence of Lubrication on the Onset of Surface Pitting inMachinable Hardnes
2、s Gear TeethC. E. Massey and C. R. Reeves, NAVSEASYSCOM;E. E. Shipley, MTITheStatementsandopinionscontainedhereinarethose of theauthorandshouldnotbeconstruedasanofficial action oropinion of the AmericanGear ManufacturersAssociation.ABSTRACT:Tests have been runon machinablehardnesshelical gearsto stu
3、dy the influenceof the changesin calculatedoil filmthicknessin the operatinggearteeth on theonset of surfacepitting. Conlroltests were runat constantload to developtypical pitting patterns on the gear teeth within a reasonable test timeperiod. Subsequenttests were carried out toevaluatethe changesin
4、 resistancetopittingthatoccmred whentheoil film thicknesswas varied. All tests were operatedwith an ample controlledsupply of a petroleum-based lubricant,symbol 2190 TEP, that met the specifications ofMff.,-L-17331.Copyright 1991American Gear Manufacttm_ Association1500 King Street, Suite 201Alexand
5、ria, Virginia, 22314October, 1991ISBN: 1-55589-605-7THE INFLUENCE OF LUBRICATION ON THE ONSET OFSURFACE PITTING IN MACHINABLEHARDNESS GEAR TEETHAUTHORS: C.E. Massey, Naval Sea Systems Command, Code 56X44C.R. Reeves, Naval Sea Systems Command, Code 56X4E.E. Shipley, Mechanical Technology Incorporated
6、INTRODUCTIONThe objective of the work cov- are higher than the compressive fa-ered in this paper was to obtain tigue life of the gear materials.insight into the benefits gained interms of K-factor (surface stress) by Gear tooth pitting is a fatiguethe influence of Lambda ratio on the phenomenon that
7、 is best prevented byonset of surface pitting of machin- proper design and selection of mate-able hardness test gears using 2190 rials and hardnesses that are ade-TEP oil. quate for the specified operatingconditions and life. However, theTypical pitting failure patterns choice of lubricant also affe
8、ctswere developed on gear surfaces under resistance to pitting which may occurmixed lubrication conditions (Lambda prematurely when the film thicknessration of 0.7) at 350K-factor tooth (strongly affected by lubricant vis-load level. These results were used cosity) is small compared to theas a base
9、line failure point. Once surface finish of the mating gearpitting conditions were established, parative tests were run with high-er viscosity fluids to investigate For the most part, in thin filmthe benefits of improved lubrication operation, pits originate at theconditions (Lambda ratio of 1o4). su
10、rface. Surface imperfections andAdditional tests were run to evaluate asperity contact smears act as crackthe ability of the higher viscosity nucleation sites. The cracks propa-fluids to retard the growth of sur- gate inward from the surface and thenface pitting once pitting had been branch out. Whe
11、n some branches turnestablished on a given set of test back toward the surface, a smallgears, piece of metal is removed from thesurface. The result is a void area,BACKGROUND or pit, with rough edges and withsmall cracks extending out from somePits on gear teeth are caused by of the sharp corners.the
12、 Hertzian stresses on the gearsurfaces that exceed the capacity of The influence of the lubricantthe gear material. The onset of on the onset of surface pitting haspitting is a function of the contact been recognized for a long time.stress, the hardness of the gearmaterial and the number of applied
13、Earl Ryder (1)in 1959 presentedstress cycles, information pertaining to the pittinglife of standard Ryder test gearsClassical pitting is subsurface that varied over a 10:1 range whenin origin; i.e., the fatigue cracks tests were run with a constant gearinitiate below the surface at the load of 3300
14、ibs. per inch of face.depth where the subsurface shear The fluids tested were both petroleumstresses, developed by the applied based and synthetic based lubricants.surface loading, are highest. Mostoften, subsurface flaws or inclusions A. Ishibashi, et al _ in refer-serve as nucleation sites and the
15、 ence to pitting tests conducted onfatigue ruptures travel to the sur- rolls, suggested in 1982 that therefacet forming the pits. Classical are two kinds of S-N curves. A high-pitting will occur in gear elements er pitting limit and a lower pittingwhen the surface loading and cycles limit. The highe
16、r pitting limit is1obtained when loaded rolls are oper- quate oil film thickness can promoteated with full elastro-hydrodynamic gear tooth pitting and how sufficientlubrication (the classical type of oil film thickness can prevent orsub-surface pitting is formed). The control surface pitting on mach
17、inablelower fatigue limit is obtained hardness gearing.under marginal lubricant film condi-tions relative to the surface finish TEST FACILITYof the rolls. Their experimentalresults showed a significant differ- An existing, four-square testence in life. loop was adapted and utilized forthese tests. F
18、igure 1 is a schematicIn 1984 S. Tanaka, A. Ishibashi of this test facility, and Figures 2aand S. Ezoe published their test work and 2b show photographs of the facil-on super-finished gears. Their ity.results indicated that gears withvastly reduced surface roughness, Briefly, referring to Figure i,w
19、hich almost assures completes elimi- the test rig is driven by a 300 HP,nation of metal surface contact dur- 2-speed motor, through a variableing operation, show an appreciable speed clutch. The speed rangegain in surface durability (resis- achievable is from 500 rpm to 3500tance to pitting), rpm on
20、 the gear shaft (1077 rpm to7539 rpm on the pinion shaft) withThese tests point out again that the present 2.154 pinion-to-gearimproved Lambda ratios permit the speed ratio.gear elements to operate at the high-est fatigue limit possible consistent Torque is applied and lockedwith the material hardne
21、ss values and into the loop through the torquecleanliness characteristics of the applier which moves the helical pin-steel, ion in the slave box axially towardsthe right in Figure i. The axialIn 1987, Dr. H. Winter and Dr. motion of the slave box pinion isPo Oster of the Technical University accommo
22、dated in the flexible splineof Munich, reported the results of coupling.some of their work _ pertaining tothe influence of the lubricants on The test pinion is spline mount-pitting and micro-pitting of case ed on its shaft. The test gear iscarburized gears using the FZG back- bolted to its shaft and
23、 has a rabbetto-back test rig. fit. The slave gear box and the testgear box have fully separate andW. R. Alexander, et al _8 dis- independent lube oil systems.cussed the development of a laborato-ry gear oil spalling (pitting) test The test gear box has a “lexan“machine and test procedure developed
24、dome cover which can be lifted toby Maok Truck to evaluate the pitting provide access to the gear meshcapabilities of various transmission within seconds after shutdown (seeoils used in their truck axles. The Figure 2).test specimens were hardened spiralbevel gears and the tests recognized The pinio
25、n support bearings inthat various oils and additives can the test gear box are mounted inprolong the bevel gear life in terms calibrated, eccentric bushings, whichof time to formation of spalling can be rotated as necessary during(pitting). alignment of each gear set to achievefull face contact.Many
26、 gear tooth pitting problemshave been reduced or solved by simply When contact checks indicate thatchanging the lubricant although many realignment is desired0 the clampingtimes the reasons for the improvement bolts of either bushing are relaxedwere not fully understood. The bushing is then rotated
27、to raiseor lower the shaft end by the prede-The test results covered in this termined amount_ following the call-report will show some interesting bration markings on the bushing. Theaspects of how the lack of an ade- clamping bolts are then retightenedand operation resumedThe gears were designed to
28、 beThe primary documentary data fitted on the shaft both on the in-from the tests were: (a) the visual ternal diameter and by dowels.inspections, (b) the tapes of thetooth surfaces that were made at The series of tests reportedperiodic intervals during each test here were all run at 1750 RPM gearto
29、document the pitting and its prog- shaft speed, which corresponds to aress and, (c) the photographs of the pitch line velocity of 5300 ft/min.pitted surfaces.All but one of the tests wereTEST SPECIMENS AND TEST CONDITIONS conducted using symbol 2190 TEP oilper MIL-L-17331 as the lubricant.Figure 3 i
30、s a photograph of a One test was conducted with a syn-typical set of new test specimens thetic oil, Mobil 529 SHC. Figure 9(pinion and gear), and Figure 4 il- lists the viscosities and other rele-lustrates the pitting obtained in the vant properties of these two oils.gear during the tests. Figure 5s
31、hows the operating parameters during The elastohydrodynamic (EHD)the baseline tests and Figure 6 shows film thickness and the correspondingthe calculated surface and bending Lambda factor (ratio of calculatedstresses at the 350 K-Factor, base- EHD film thickness to combine surfaceline load condition
32、, as well as the finish of the mating parts) werematerial allowable stresses, calculated for the test elements.The calculations were made using theThe test elements used in most EHD film thickness equation for hell-of the tests were finish ground with ca1 gears _ which essentially followsa _paraboli
33、c profile“, that is the the Grubin analysis. The calculatedstart of modification was just about film thicknesses and correspondingat the pitch line and followed a Lambda factors are tabulated in Fig-smooth, parabolic curve to the tips ure i0. These Lambda factors areof the teeth. The profile modific
34、a- based on a surface finish of 32tion of the teeth is illustrated in microinches in both the pinion andFigures 7a and 7b for the pinion and gear. That is, a combined surfacegear. In some of the later tests a finish of (322 + 322)_ = 45“modified/true involute“ profile was microinches.used in which t
35、he start of modifica-tion was approximately at mid-length With 2190 TEP oil at 130 sup-of the addendum region, as illustrat- ply, the calculated Lambda ratio ised in Figures 8a and 8b. In all 0.7. It is substantially improved bycases, the test elements were fin- reducing the oil supply temperatureis
36、hed by grinding to ensure uniformi- or changing to a higher viscosityty between all the sets of test spec- lubricant. It should be noted, how-imens, ever, that the Lambda ratios in Fig-ure i0 are based on the average sur-The test gears and pinions were face finish of 32 microinches of thedesigned fo
37、r testing on both flanks, test pinions and gears, prior toand therefore needed to be readily installation in the test rig andreversible in the test stand. The operation through the “break-in“pinions were attached to the shaft cycle.through a flat root, major diameterfit, 30 pressure angle, straight
38、TESTS CONDUCTEDspline. The internal spline teeth inthe pinion were broached and mated A total of 15 tests were con-with hobbed and nitrided, external ducted, as listed in Figure 11, andspline teeth in the shaft which were discussed below.ground both on the sides and on theOoD. to maintain a consiste
39、nt fit. Tests 1 and 2 were the baselineThe combination of broached internal tests conducted to determine whetherteeth and nitrided and ground exter- the selected test conditions (350 K-nal teeth maintained mounting consis_ Factor load and 2190 TEP oil suppliedtency and accuracy from test to test. at
40、 125/130F) would develop the type3and extent of pitting of interest in range.this study, in an acceptable testperiod. In summary, the test resultswere:Tests 3 and 4 were run to eval-uate the increased pitting resistance Very fine pitting was ob_achieved by higher capacity lubricant served on a few g
41、ear teeth after 12films, hours of operation+ The pits werequite small, less than 1/64 in. inTests 5, 6, 7 and 8 were run diameter, and shallow.with previously pitted elements toevaluate whether the high capacity Pitting continued at a verylubricant films would arrest the slow pace andw after 24 hour
42、s ofpitting. In the case of gear and operation, the most pitted tooth hadpinion set No. 2 (Test Series 6, 7 developed a very thin line of theand 8), the total number of cycles fine diameter pits.run at this condition was carriedthrough to slightly more than 108 After 36 hours of operation,cycles, th
43、e pitting had become more prominentand the fine pits had started to joinTest 9 was conducted to check up in places Other teeth showedwhether pitting resumed when opera- similar random pitting, and stilltion was returned to the baseline other teeth had no pitting at all.condition. At the end of 61 ho
44、urs, theTest I0 consisted of operating test was shut down_ since virtuallythe parabolic profile test gears with all of the gear teeth showed evidence2190 TEP oil at the 90/95F tempera- of fine pitting. Some teeth had ature range. Previous tests indicated minimal amount of fine pitting;that this comb
45、ination of test condi- other teeth showed fine pitting moretions at the 350 K-factor level pro- or less all along the face.duced excellent results. The pitting originated at andTests 10 and ii were run to try just below the pitch line.to quantify, in terms of K-Factorloading, the pitting resistance
46、of There was no evidence ofthe test elements in 2190 TEP oil at pitting on the pinion teeth.the two values of oil supply tempera-ture used, 90/95F and 125/130F.These test runs completed the testing Figure 4 in this report shows awith the parabolic profile pinions photograph of a pitted tooth fromand
47、 gears (Figure 7A and 7B)o this test.Tests 12 through 15 were run TEST #2with new test pinions and gears,finish ground with the modified/true This was the second “baseline“involute profile (Figures 8a and 8b) test, run with a new set of testthat maintained the true involute specimens_ utilizing 2190
48、 TEP oil+atcurve over one tooth interval along 125/130F supply. Again, all oper-the line of action. The purpose of ating conditions were held constantthese tests was to evaluate the im- during the test.provements in pitting resistance thatare gained by the profile change. In summary, the test result
49、sshowed:TEST RESULTS Very fine pitting was oh-TEST #I served on a few random gear teethafter 17 hours of operation+ Again,This was the first of two “base- the pits were very fine, 1/64 in.line“ tests run with the test condi- diameter or less, and most teeth hadtions were held constant and the 2190 only one or two pits.TEP oil was kept in the 125/130F4 After 37 hours of operation, At the end of 54.5 hours, theboth the number of pitted teeth and condition of the teeth was about thethe average number of pits per tooth same. One tooth had two very smallhad i
