1、NASA Techn ica I Paper 2047 1982 National Aeronautics and Space Administration Sclentlflc and Technical lnformatlon Branch Effect of Shot Peening on Surfiwe Fatigue Life of Carburized and Hardened AIS1 9310 Spur Gears Dennis P. Townsend and Erwin V. Zaretsky Lewis Research Center Cleveland, Ohio Pro
2、vided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Summary Gear surface fatigue endurance tests were condu :ted on two groups of 10 gears each of carburized and hardened AISI 9310 spur gears manufactured from the same heat of material. Both groups were man
3、ufactiired with standard ground tooth surfaces. The second group was subjected to an additional shot-peening proces!, on the gear tooth surfaces and root radius to produce a residual surface compressive stress. The gear pitch diameter was 8.89 cm (3.5 in.). Test conditions were a gear temperature of
4、 350 K (170“ F), a maximum Hertz stress of 1.71 x 109 N/m2 (248 OOO psi), and a speed of 10 OOOrpm. The shot-peened gears exhibited pitting fatigue 1 ives 1.6 times the life of the standard gears without shot peening. Residual stress measurements and ana ysis indicate that the longer fatigue life is
5、 the result of the higher compressive stress produced by the shot peening. The life for the shot-peened gear was calculated to be 1.5 times that for the plain gear by using the measured residual stress difference for the standard and shot- peened gears. The measured residual stress for the shot- pee
6、ned gears was much higher than that for the standard gears. Introduction Shot peening has long been used as a method for improving the bending strength of gear teeth (refs. 1 to 3). However, shot peening has not been considered as a means of extending the surface fatigue life of gears. In essence, s
7、hot peening induces a residual comprea sive stress below the surface of the gear tooth. Studie,; of residual stresses in rolling-element bearings have shown that increased residual compressive stress will incr :ase rolling-element (surface) fatigue life (refs. 4 and 5). There is always a need to imp
8、rove the surface fatigue lift: of aircraft gears, especially in helicopter and V/STOL aircraft. The objectives of the research reported herein werc: (1) to investigate the effects of shot peening of gear teeth on the surface fatigue life of standard ground, case- carburized, and hardened AISI 9310 s
9、pur gears, (2) to compare the life of shot-peened gears to that of non-siot- peened gears manufactured with the same material and specifications, and (3) to determine the residual stress produced by shot peening and its effect on the surface fatigue life. To accomplish these objectives, 20 spur gear
10、s were manufactured from a consumable-electrode-vacuum- melted single heat of AISI 9310 material. Ten of t lese gears were shot peened after finish grinding. The ,gear pitch diameter was 8.89 cm (3.5 in.). Both the shot- peened and non-shot-peened gears were then tested to fatigue by surface pitting
11、 under identical test conditions. These test conditions included a gear temperature of 350 K (170“ F), a maximum Hertz stress of 1.71 x 109 N/m2 (248 OOO psi), and a speed of 10 OOO rpm. Apparatus, Specimens, and Procedure Gear Test Apparatus The gear fatigue tests were performed in the NASA Lewis R
12、esearch Centers gear test apparatus (fig. 1). This test rig uses the four-square principle of applying the test gear load so that the input drive only needs to overcome the frictional losses in the system. A schematic of the test rig is shown in figure 10). Oil pressure and leakage flow are supplied
13、 to the load vanes through a shaft seal. As the oil pressure is increased on the load vanes inside the slave gear, torque is applied to the shaft. This torque is transmitted through the test gears back to the slave gear, where an equal but opposite torque is maintained by the oil pressure. This torq
14、ue on the test gears, which depends on the hydraulic pressure I temperature measure- ment location la) Cutaway view. .-Slave qear Ted gears r toque pressure2 View A-A lbl Schematic diagram. CD-11124-1 Figure 1. - NASA Lewis Research Centers gear fatigue test apparatus. Provided by IHSNot for ResaleN
15、o reproduction or networking permitted without license from IHS-,-,-applied to the load vanes, loads the gear teeth to the desired stress level. The two identical test gears can be started under no load, and the load can be applied gradually, without changing the running track on the gear teeth. Sep
16、arate lubrication systems are provided for the test gears and the main gearbox. The two lubrication systems are separated at the gearbox shafts by pressurized labyrinth seals. Nitrogen is the seal gas. The test gear lubricant is filtered through a 5-pm-nominal fiberglass filter. The test lubricant c
17、an be heated electrically with an immersion heater. The temperature of the heater skin is controlled to prevent overheating the test lubricant. to automatically shut off the test rig when a gear surface fatigue occurs. The gearbox is also automatically shut off if there is a loss of oil flow to eith
18、er the main gearbox or loss of seal gas pressurization. speeds by changing pulleys. The operating speed for the tests reported herein was 10 000 rpm. I I A vibration transducer mounted on the gearbox is used I the test gears, if the test gear oil overheats, or if there is a The belt-driven test rig
19、can be operated at several fixed , I E 1 ement C Mn Si Ni Cr Mo cu P S I Test Materials Composition wt% 0.10 .63 .27 3.22 1.21 .12 .13 .005 .005 The test gears were manufactured from consumable- electrode-vacuum-melted (CVM) AISI 93 10 steel from the same heat of material. Both sets of gears were ca
20、se hardened to a case hardness of Rockwell C 58 and a case depth of 0.97 mm (0.038 in.). The nominal core hardness was Rockwell C 40. One set of the gears was shot peened, after finish grinding, on the tooth root and the tooth profile according to the specifications given in table I. The chemical co
21、mposition of the material is given in table 11. Both sets of gears were case carburized and heat Process Temperature - K OF Preheat in air _- - 1172 1650 Carburize Air cool to room _- - Copper plate all over - - 922 1200 Reheat Air cool to room _- - Austeni t ize 1117 1550 Oil quench Double temper 4
22、50 350 Finish grind _-_- - Stress relieve 450 350 temper at ure temperature _ - Subzero cool 180 -120 TABLE I. - SHOT-PEENING SPECIFICATION Time, hr - 8 - - 2.5 - 2.5 - 3.5 2 each - 2 Specification . MIL-S-131658 Shot size . 070 Shot type Cast steel Intensity (height of Almen 0.18 to 0.23 (0.007 to
23、0.009) Coverage (sides and root 200 BPS FW 4409 strip, type A), mm (in.) only) percent - Step _ 1 2 3 4 5 6 7 8 9 10 11 12 TABLE 11. - NOMINAL CHEMICAL COWOSITION OF CVM AISI 9310 GEAR MATERIAL TABLE 111. - HEAT TREATMENT FOR AISI 9310 treated in accordance with the heat treatment schedule of table
24、111. Figure 2 is a photomicrograph of an etched and polished gear tooth surface showing the case micro- structure of the AIS1 9310 material. Test Gears Dimensions of the test gears are given in table IV. All gears have a nominal surface finish on the tooth face of 0.406 pm (16 pin.) rms and a standa
25、rd 20“ involute profile with tip relief. Tip relief was 0.0013 cm (0.0005 in.), starting at the highest point of single-tooth contact. Surface traces of the standard gear and the shot-peened gear are shown in figure 3. Provided by IHSNot for ResaleNo reproduction or networking permitted without lice
26、nse from IHS-,-,-la) Standard gear case. (cl Shot-peened gear case, Ibl Standard gear core. (dl Shot-peened qear core. Figure 2. - Photomicrographs of case and core for standard and shot-peened spur gears. 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IH
27、S-,-,-TABLE IV. - SPUR GEAR DATA Test Lubricant Gear tolerance per ASMA class 12.1 Number of teeth . 28 Diametral pitch . 8 Circular pitch, cm (in.) . 0.9975 (0.3927) Whole depth, cm (in.) 0.762 (0.300) Addendum, cm (in.) . 0.318 (0.125) Pressure angle, deg . 20 Pitch diameter, cm (in.) 8.890 (3.500
28、) Outside diameter, cm (in.) . 9.525 (3.750) Root fillet, cm (in.) 0.102 to 0.152 (0.04 to 0.06) Measurement over pins, . 9.603 to 9.630 cm (in.) (3.7807 to 3.7915) Pin diameter, cm (in.) . 0.549 (0.216) Backlash reference, cm (in.) . . 0.0254 (0.010) Tip relief, cm (in.) . 0.001 to 0.0015 (0.0004 t
29、o 0.0006) Tooth width, cm (in.) . 0.635 (0.25) Chordal tooth thickness . 0.485 (0.191) (reference), cm (in.) la) Standard gear. (b) Shot-peened gear. Figure 3, - Surface finish of standard ground and shot-peened gears. All the gears were lubricated with a single batch of synthetic paraffinic oil. Th
30、e physical properties of this lubricant are summarized in table V. Five percent of an extreme-pressure additive, designated Lubrizol 5002 (partial chemical analysis given in table V), was added to the lubricant. Test Procedure After the test gears were cleaned to remove the preservative, they were a
31、ssembled on the test rig. The 0.635-cm (0.25-in.) wide test gears were run in an offset condition with a 0.30-cm (0.12-in.) tooth-surface overlap to give a load surface on the gear face of 0.28 cm (0.1 1 in.), thereby allowing for the edge radius of the gear teeth. If both faces of the gears were te
32、sted, four fatigue tests could be run for each set of gears. All tests were run in at a pitch-line load of 1225 N/cm (700 lb/in) for 1 hour, which gave a maximum Hertz stress of 0.756 x 109 N/m2 (1 11 OOO psi). The load was then increased to 5784 N/cm (3305 lb/in), which gave a pitch-line maximum He
33、rtz stress of 1.71 x IO9 N/m2 (248 OOO psi). At this pitch-line load the tooth root bending stress would be 0.21 x 109 N/m2 (30 OOO psi) if plain bending were assumed. However, because there was an offset load, an additional stress was imposed on the tooth bending stress. Combining the bending and t
34、orsional moments gave a maximum stress of 0.26 x 109 N/m2 (37 O00 psi). This bending stress does not include the effects of tip relief, which would also increase the bending stress. TABLE V. - PROPERTIES OF SYNTHETIC PARAFFINIC OIL Additive aLubrizol 5002 Kinematic viscosity, cm2/sec (cs) at- 244 K
35、(-20 F) . 311 K (100 F) . 372 K (210“ F) 477 K (400“ F) 250010- (2500) 31.610- (31.6) 5.7x10-* (5.7) 2.010- (2.0) Flashpoint, K (OF) 508 (455) Fire point, K (OF) 533 (500) Pour point, K (OF) 219 (-65) Vapor pressure at 311 K (100 F), . 0.1 Specific heat at 311 K (100 F), . . 676 (0.523) Specific gra
36、vity 0.8285 mm tig (or torr) J/kg K (Btu/lb OF) aAdditive, Lubrizol 5002 (5 01%); content of additive: phosphorus, 0.6 wt%; sulfur, 18.5 wt%. 4 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Operating the test gears at 10 OOO rpm gave a pitch-line v
37、elocity of 46.55 m/sec (9163 ftlmin). Lubricant was supplied to the inlet mesh at 800 cmVmin at 319*6 K (1 16“ * 10“ F). The lubricant outlet temperature was nearly constant at 350*3 K (170“ *5“ F). The tests ran continuously (24 hr/day) until they were automatically shut down by the vibration detec
38、tion transducer, located on the gearbox adjacent to the test gears. The lubrkant circulated through a 5-pm fiberglass filter to remove vear particles. After each test the lubricant and the filter element were discarded. Inlet and outlet oil temperatures were continuously recorded on a strip-chart re
39、corder The pitch-line elastohydrodynamic (EHD) film thickness was calculated by the method of reference f. It was assumed, for this film thickness calculation, that the gear temperature at the pitch line was equal to the outlet oil temperature and that the inlet oil temperature to the contact zone w
40、as equal to the gear temperature, even though the inlet oil temperature was considerably lovier. It is possible that the gear surface temperature was egen higher than the outlet oil temperature, especially at the end points of sliding contact. The EHD film thickness for these conditions was computed
41、 to be 0.33 pm (13 pili.), which gave an initial ratio of film thickness to compo site surface roughness h/a of 0.55 at the 1.71x109-N/mz (248 OOO-psi) pitch-line maximum Hertz stress. Results and Discussion Gears manufactured from CVM AISI 9310 mate-ial were tested in pairs until failure or for 500
42、 hours. One- half of the gears were shot peened on the tooth root and profile. Nineteen tests were run with standard-finish ground test gears, and 24 tests were run with standard- finish ground gears that had been shot peened. ?est results were analyzed by considering the life of each air of gears a
43、s a system. Surface (pitting) fatigue results for the standard-finish AISI 9310 gears are shown in figure 4(a). These data wxe analyzed by the method of reference 7. The 10- and 50-percent fatigue lives were 18.8 106 and 46.1 x 106 stress cycles (31.3 and 76.8 hr), respectively. These resL Its are s
44、ummarized in table VI. The failure index (Le., ihe number of fatigue failures out of the number of sets tested) was 18 out of 18. A typical fatigue spall is shown in figure 5(b). A cross section of a typical fatigue spa1 is shown in figure 5(a). The surface pitting failure occiirs slightly below the
45、 pitch line in the area of highest Hertz stress and is of subsurface origin. Pitting fatigue life results for the gears that were sl.ot peened are shown in figure 4(b). The failure index was 24 out of 24. A typical fatigue spall for the shot-peened gears is shown in figure 6(a). A cross section of a
46、 typical fatigue spall for the shot-peened gears is shown in figure 4!f 9 , I , y /i p, , O 20 40 6080 100 2wx106 10 20 40 6080100 200x106 Number of cycles (a) Standardgears. (b) Shot-peened gears Figure 4. - Comparison of surface (pitting) fatigue lives of standard ground and shot-peened carburized
47、 and hardened CVM AIS1 9310 steel spur gears. Speed, loo00 rpm; lubricant, synthetic paraffinic oil; gear temp- erature, 350 K (1700 F); maximum Herh stress, 1.7x109NlmZ1Z48000 psi). TABLE VI. - FATIGUE RESULTS WITH AISI 9310 STANDARD AND SHOT-PEENED TEST GEARS life, life, percent S t andard Shot pe
48、ened aIndicates numbers of failures out of total number of tests. brobabi lity. expressed as a percentage. that the 10-percent life with the baseline AIS1 9310 gears is either less than. or greater than, that of the particular lot of gears being considered. 6). The 10- and 50-percent surface pitting
49、 fatigue lives were 30.1 x 106 and 67.5 x 106 stress cycles (50.3 and 112.6 hr), respectively. These results are summarized in table VI. The shot-peened gears exhibited a 10-percent fatigue life 1.6 times that of the standard ground AISI 9310 gears. The confidence number for the difference in life was 83 percent. The mean life ratio for the shot- peened over the sta
