AN EXPERIMENTAL INVESTIGATION OF MATERIALS AND SURFACE TREATMENTS ON GEAR CONTACT FATIGUE LIFE. A Thesis

Transcription

1 AN EXPERIMENTAL INVESTIGATION OF MATERIALS AND SURFACE TREATMENTS ON GEAR CONTACT FATIGUE LIFE A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Mark Andrew Klein, B.S. * * * * * The Ohio State University 9 Master s Examination Committee Approved by Dr. Ahmet Kahraman, Advisor Dr. Donald Houser Advisor Graduate Program in Mechanical Engineering

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3 ABSTRACT This study consists of experimental studies involving two modes of gear contact fatigue failure: gear pitting (spalling) and gear scuffing. For pitting studies, several materials and surface treatments were investigated at various stress levels. These surface treatments included (i) hobbed and shaved (baseline), (ii) chemically polished, (iii) shot peened and plastic honed, and (iv) ground gears. Pitting fatigue lives of chemically polished gears were greater than those of baseline specimens. Both shot peened and plastic honed gears and ground gears were shown to have greater pitting fatigue lives than baseline gears. The improved pitting fatigue life of ground gears over baseline gears appears related to the improved involute profile shapes of the specimens. For gear scuffing experiments, the standard ISO 465- FZG Scuffing Test was performed on AISI 86 type A spur gears. These experiments included four uncoated gear pairs and one gear pair coated with an experimental PVD coating. Uncoated gears encountered scuffing during Stages and. A high correlation between temperature and scuffing results was detected for both coated and uncoated specimens. ii

4 Dedicated to my mother, who encouraged me to apply to Graduate School iii

5 ACKNOWLEDGMENTS I would like to thank Neil Anderson, Don Maddock, and Avinash Singh of GM Powertrain for their guidance with this project. I would like to express my gratitude to Jeremy Bluestein for his contribution in teaching me the measurement and testing procedures for the pitting portion of this study. I would also like to give my sincerest gratitude to Dr. Ahmet Kahraman for his assistance and advice throughout this endeavor. I would like to thank Dr. Donald Houser for the insight and knowledge of gears he has provided me, and for the thorough review of this thesis study. Lastly, I would like to thank Sam Shon for assisting with the occasional unwilling dispositions of the FZG and measurement systems used for these experiments. iv

6 VITA October 4, Born - Youngstown, Ohio... B.S. Mechanical Engineering, Boston University 8 - Present... Graduate Research Associate, The Ohio State University Major Field: Mechanical Engineering FIELDS OF STUDY v

7 TABLE OF CONTENTS Page Abstract... ii Dedication... iii Acknowledgements. iv Vita... v List of Figures...viii List of Tables... xi Chapter Page. Introduction Motivation... Thesis Objectives 4. Thesis Outline. 6. Gear Pitting Experiments Experiment Test Set-up Test Procedures. Test Specimens..4 Test Matrix....5 Gear Pitting Test Results...6 Conclusions...5. Gear Scuffing Experiments 55. Scuffing Test Procedure Scuffing Gear Test Specimens. 6. Scuffing Test Results 66.4 Conclusions.. 75 vi

8 Chapter Page 4. Conclusions and Recommendations for Future Work Thesis Summary Conclusions Recommendations for Future Work. 8 Bibliography... 8 APPENDICES Appendix A Images of Pinions from Pitting Tests. 84 Appendix B Appendix C Surface Roughness Traces of Pinions from Pitting Tests...9 Surface Roughness Traces of Gears from Pitting Tests vii

9 LIST OF FIGURES Figure Page. A detailed FZG schematic from Bluestein [] Photographs of 86 and 46M ground gear specimens specific to this study Normalized pinion torque versus pinion cycles for HS gears Normalized pinion pitch line stress versus pinion cycles for HS gears. 5.5 HS pitted and suspended tests with 9% Confidence Intervals. 6.6 Weibull distributions of HS pitted tests at different load levels 7.7 Normalized pinion torque versus pinion cycles for CP gears 9.8 Normalized pinion pitch line stress versus pinion cycles for CP gears..9 CP pitted and suspended tests with 9% Confidence Intervals.. Weibull distributions of CP pitted tests at different load levels. Weibull distributions of HS and CP pitted tests at load level L6... Normalized pinion torque versus pinion cycles for SP+PH gears. 5. Normalized pinion pitch line stress versus pinion cycles for SP+PH gears Pitted and suspended SP+PH tests versus 9% Confidence Intervals for HS and CP pinions. 7.5 Weibull distributions of HS, SP+PH, and CP pitted tests at load level L 9.6 Weibull distribution comparison of CP tests at load L and SP+PH tests at load level L... 4 viii

10 Figure Page.7 Involute pinion wear during a load level L6 test on (a) HS specimen and (b) SP+PH specimen Figure.8: Involute pinion tooth traces ( per tooth) during initial measurements for (a) HS specimen and (b) GD8 specimen Normalized pinion torque versus pinion cycles for GD8 gears. 44. Normalized pinion pitch line stress versus pinion cycles for GD8 gears Pictures of GD8 pinion tooth and involute traces of a L test at (a) cycles, (b) 6M cycles, and (c) 8.6M cycles 46. Normalized pinion torque versus pinion cycles for GD4 gears. 47. Normalized pinion pitch line stress versus pinion cycles for GD4 gears Pictures of GD4 pinion tooth and involute traces of a L test at (a) cycles, (b) 9.M cycles, (c) 8.M cycles, and (d) 6.7M cycles Pitted GD8 tests versus 9% Confidence Intervals for HS and CP pinions Pitted GD4 tests versus 9% Confidence Intervals for HS and CP pinions.. 5. Schematic diagram of FZG test gearbox with approximate thermocouple location Thermal camera shown in front of gearbox during a scuffing experiment 58. Digital images of various amounts of scuffing on pinion tooth surfaces Digital images of pinion surface after their final test stage Display of recorded temperatures at Stage of each scuffing test performed Display of recorded temperatures at Stage of each scuffing test performed ix

11 Figure Page.7 Thermal camera image shown of gearbox front during a scuffing experiment Recorded temperatures of thermocouple (TC) versus maximum temperatures of thermal imaging camera (TI) during Stage of 4978R test Maximum temperatures recorded by thermal imaging camera during Stage for each scuffing test performed.. 74 A. Pictures of a HS pinion tooth of a L4 test at (a) cycles, (b) 8.4M cycles, (c) 6.7M cycles, (d) 55.M cycles, (e) 64.M cycles. Picture (f) shows the pitted tooth at 64.M cycles. 85 A. Pictures of a HS pinion tooth of a L6 test at (a) cycles, (b).m cycles, (c) 6.7M cycles, and (d) M cycles.. 86 A. Pictures of a CP pinion tooth of a L6 test at (a) cycles, (b) 8.4M cycles, (c) 6.7M cycles, (d) 55.M cycles, (e) 64.M cycles, and (f) 8.6M cycles 87 A.4 Pictures of a SP+PH pinion tooth of a L test at (a) cycles, (b).6m cycles, (c) 6.M cycles, (d) 8.6M cycles, and (e) 97.9M cycles 88 A.5 Pictures of a GD8 pinion tooth of a L test at (a) cycles, (b) 4.6M cycles, (c) 9.M cycles, and (d).7m cycles A.6 Pictures of a GD4 pinion tooth of a L test at (a) cycles, (b) 6.M cycles, and (c) 7.M cycles 9 A.7 Pictures of a GD4 pinion tooth of a L6 test at (a) cycles, (b) 4.5M cycles, (c) 48.9M cycles, (d) 79.5M cycles, (e) M cycles, and (f) 9.8M cycles... 9 B. Surface roughness traces of a HS pinion tooth during a L4 test... 9 B. Surface roughness traces of a HS pinion tooth during a L6 test B. Surface roughness traces of a CP pinion tooth during a L6 test. 95 B.4 Surface roughness traces of a SP+PH pinion tooth during a L test x

12 Figure Page B.5 Surface roughness traces of a GD8 pinion tooth during a L test B.6 Surface roughness traces of a GD4 pinion tooth during a L test.. 99 B.7 Surface roughness traces of a GD4 pinion tooth during a L6 test... C. Surface roughness traces of a HS gear tooth during a L4 test... C. Surface roughness traces of a HS gear tooth during a L6 test.. 4 C. Surface roughness traces of a CP gear tooth during a L6 test.. 5 C.4 Surface roughness traces of a SP+PH gear tooth during a L test... 7 C.5 Surface roughness traces of a GD8 gear tooth during a L test... 8 C.6 Surface roughness traces of a GD4 gear tooth during a L test... 9 C.7 Surface roughness traces of a GD4 gear tooth during a L6 test... xi

13 LIST OF TABLES Table Page. Basic design parameters of the spur gear pair..... Elemental Composition by Weight % of test specimens Contact fatigue test matrix with related pinion and gear surface treatments. 5.4 Average initial Ra roughness values for various surface treatments Test Load Levels and their related normalized torques and contact stresses.. Applied pinion torques and related contact stresses for the ISO 465- FZG Scuffing Test Basic design parameters of type A spur gears Initial Ra and Rq type A specimen values from the Talysurf- surface profiler Scuffing test summaries with related scuffing stages and total scuffed areas xii

14 CHAPTER INTRODUCTION. Motivation Within geared transmissions, gear failures occur in many ways and often without advanced notice. While engineers have developed, over the years, a greater understanding of these failures, there is still a need for a thorough understanding of how involute gears fail and how they can be made to last longer. There are many types of gear failures, each influenced by a variety of gear, surface, lubricant and contact parameters. This thesis focuses on two of these failure modes, namely gear pitting (spalling) and gear scuffing. Pitting develops over time from recurring contact stresses between the teeth of two gears during rotation. Pitting can be described as visual surface fractures on the gear teeth usually preceded with hairline cracks that develop on or below the tooth surface. Once gear pitting has initiated, gear noise may become more prevalent and the gear surface continues to degrade until complete gear failure has occurred.

15 Kaneko [] did an extensive study on gear pitting with various carbon and alloy steels. In the study, quenched and annealed gears made of CrMO steel alloy and a carbon steel were subjected to various loads in a power circulation loop configuration similar to the FZG machines described later in this thesis. Flame hardened and induction hardened test specimens, which were ground afterward to remove heat treatment deformations, were also made of the same two materials. The hardened specimens were able to endure higher loads and the CrMo steel alloy had greater pitting fatigue life than samples made of carbon steel. Also in the study, NiCrMo and NiCr carburized steel gears were subjected to pitting tests and endured the highest loads. The NiCrMo carburized steel specimens had greater contact fatigue lives than those made of NiCr carburized steel. In a study by Townsend et al [], the pitting fatigue lives of spur gears was studied using two different materials while keeping other variables, such as speed (, rpm), hardness (HRC 6), and inlet lubricant temperature (7ºF ± 5ºF), constant. Specimens manufactured from the AISI M-5 steel had 5% longer pitting contact fatigue lives than those manufactured from Super Nitralloy (5Ni-Al). In addition to having longer contact fatigue lives, the AISI M-5 specimens had more wear than the Super Nitralloy gears. In the study, these differences in contact fatigue lives, as well as wear, were noted as not being statistically significant. In a study by Bluestein [], the influence of contact stress variations on gear pitting fatigue life was studied extensively. Bluestein, along with the sponsor of his study, produced a variety of successful gear contact fatigue experiments. In those experiments, various torque loads were applied to the gear specimens in order to

16 determine the influence of contact stress amplitudes on the pitting contact fatigue life of the gears. In addition to contact stress variations, the influence of various surface treatments on the contact fatigue life of the specimens was also studied. All of these surface treatments were also tested at different torque settings. In Bluestein s experiments [], tests at each test condition were repeated several times in order to provide statistically meaningful sets of data. With this data, the probabilities of pitting failure under each load for each surface treatment were calculated. Additionally, Weibull analysis was applied for various sets of data in order to provide a failure analysis comparison between identically loaded gears with different surface treatments. Gear scuffing on tooth surfaces is another mode of contact failure. Unlike pitting, the scuffing process initiates suddenly and does not require a high number of gear cycles to occur. Gear scuffing usually occurs in the addendum area of the gear tooth surface and involves the material transfer between two tooth surfaces in contact. This transfer of material occurs between the gear teeth as a combined effect of high loads (i.e. high friction forces), and high sliding velocities. Such contact conditions create high lubricant and tooth surface temperatures, which decrease the viscous properties of many lubricants utilized in transmissions. Upon scuffing initiation, the change in gear tooth surfaces from the material transfer between the teeth increases frictional forces during gear rotation. In a study by Castro and Seabra [4], scuffing tests were performed with ISO VG 68, VG, and VG 68 lubricants. The VG 68 lubricant had the lowest viscosity while the VG 68 had the highest viscosity of the three lubricants. Among these scuffing tests,

17 the FZG-A/8.6/9 standard test was performed for all three lubricants. The VG 68 lubricant faired the poorest during testing while the VG 68 lubricant prevented scuffing at much higher loads than the other lubricants. Other scuffing tests in the study included tests held at constant pitch line velocities. Results from these tests showed the temperatures for higher viscosity lubricants were able to increase to higher levels than those with lower viscosities. This study concluded that lubricant viscosity is highly influential on lubricant film thickness, and as the lubricant temperature increases, the lubricant film thickness decreases accordingly. In a study by Snidle et al [5], scuffing experiments with steel disks of varying surface conditions were performed at several sliding speeds. During testing, the applied force between the steel disks was increased stepwise until scuffing was initiated. Both the friction force between the disks and the disk bulk temperatures were recorded during testing. In the findings, a large increase in friction occurred once scuffing was initiated in the scuffing disk experiments. Also in the study, the friction force between the disks decreased before scuffing initiation.. Thesis Objectives In order to better appreciate pitting and scuffing failure modes of gears, many of the variables that influence these failures can be held constant in order to develop understandings of the dependencies upon a few allowable variables. 4

18 This thesis study continues with the experimental studies and methods developed by the sponsor and Bluestein []. These studies include some of the same gear surface treatments, as well as the addition of new surface treatments and materials. In addition, this study investigates gear scuffing of gears made of the same material as many of the specimens studied in the gear pitting experiments. This study can be viewed as the second phase of the pitting investigation initiated by Bluestein []. The specific objectives of this study include the following: ) Complete the experimental studies initiated by Bluestein [] of various surface treatments and the influence of those variations on gear contact fatigue life. ) Expand the existing pitting database from the previous study in order to develop the confidence intervals needed to make comparisons between different surface treatments, which did not have a statistically significant number of data points. ) Expand the fatigue pitting experimental studies to include additional surface treatments, as well as additional gear materials, and find their influence on gear pitting contact fatigue life at various contact stresses. 4) Develop and investigate experimental studies on gear scuffing failures for gears with different surface treatments. 5) Record and compare lubricant temperatures of each scuffing test and determine any relationships between temperatures and scuffing failures. 5

19 6) Record and investigate increases in test machine drive motor power input scuffing tests that have resulted in scuffing failures. The experimental data obtained from this study is expected to provide quantitative information on the impact of these variants on the resultant contact fatigue lives of gears. In addition, it will provide much needed experimental data for companion pitting and scuffing studies by Li [6] and Liou [7], respectively. The study by Li [6] includes the development of a model to predict gear scuffing failures. Liou s study [7] involves the modeling of gear scuffing based on various surface conditions and sliding velocities.. Thesis Outline Chapter discusses the experimental pitting tests performed in this study. Test procedures as well as of the test specimen details are described in depth. The test matrix, which includes various degrees of contact stresses, and gear pitting failure criteria are also included. Confidence interval and Weibull distribution analyses of the pitted data points and comparisons of these analyses between various surface treatments and gear materials are also included in this chapter. Chapter describes the scuffing tests performed on five different spur gear pairs. The test procedures and the geometric details of these gears are included in this chapter. The results of these tests and observations from the recorded thermal and amperage data during these tests are discussed in detail. Comparisons between lubricant temperatures 6

20 and scuffing results are made. Conclusions based on these comparisons are also discussed. Chapter 4 further discusses the conclusions from the pitting and scuffing tests performed in this study. Additionally, recommendations for future work in these two areas of gear failure are also provided. 7

21 CHAPTER GEAR PITTING EXPERIMENTS. Experiment Test Set-up Experimental contact fatigue tests utilizing spur gears were performed using three similar FZG gear test machines. These FZG machines create a four-square power circulation loop, within which specified test torque loads are applied. A schematic of the FZG machines used for testing in this thesis is shown in Figure.. The FZG machines have two gearboxes each. The reaction gearbox on the motor side includes a reaction gear pair which has a greater face and hence very low contact stresses and greater life expectancy than the test gears. The reaction gearbox used a more viscous gear lubricant to offer greater wear protection to the reaction gears. The test gear pair is installed in the test gearbox farthest from the motor. This allows greater accessibility to the test gears for easier visual and measured inspections. 8

22 Figure.: A detailed FZG schematic from Bluestein []. 9

23 Initial tests performed by a sponsor used Dexron III automatic transmission fluid as the lubricant. Another variation, Dexron VI, was used in pitting tests performed by Bluestein []. Dexron VI was also used for all testing performed for this thesis study. Although there were concerns for potential disparities in the results from the use of these two lubricants, these differences were found to be insignificant when fatigue data for these two lubricant variants were compared. For this reason, the test data sets for each lubricant have been combined into one data set. After the spur gear pairs were installed within the FZG machines, the lubricant reservoir was filled with Dexron VI until the spur gears were halfway immersed within the fluid. Fresh lubricant was used at the start of each test, and the same fluid was used throughout the test until the test was completed. During the progression of a test, any metallic debris that might be produced due to wear or small pits on the pinion surface were separated from the oil bath through a magnet placed at the bottom of the gearbox.. Test Procedures Before the start of each fatigue test, pinions and gears were thoroughly cleaned with isopropyl alcohol and then put on a Taylor-Hobson Form Talysurf- surface profiler to measure their surface roughnesses in the direction of sliding (profile direction). These measured roughness profiles were quantified by using R a (centerline average roughness) and R q (root-mean-square roughness) values as the roughness parameters. Pinions and gears that exhibited any unusual surface roughnesses, such as those having higher R a and R q values than the typical values associated with the finishing process,

24 were discarded. In addition, a Gleason M&M 55 gear coordinate measurement machine (CMM) was used to measure four different teeth on each pinion and gear. Three involute and three lead profile traces were obtained on each tooth surface. Similar to the procedure for surface roughness measurements, any pinion and gear that exhibited excessive deviations from the intended leads and involute profiles, were not used for testing. Lastly, an optical microscope with a /X objective lens and 5X magnification was utilized for inspecting and taking digital pictures of the pinion tooth surfaces before testing. Once the pinions and gears were measured, a fatigue test was initiated with a minute run-in test period at a low pinion torque level of 4 Nm and speed of 44 rpm. After this run-in period, the pinion and gear were wiped clean and visually inspected for any abnormalities. Following visual inspection, a fatigue test was started under the decided upon torque value. The FZG machines were run at a constant gear speed of 44 rpm ± 5 rpm, and the temperature of the Dexron VI was kept at 9 o C ± o C throughout both the run-in period and the remainder of each test. During testing, the pinion and gear specimens were inspected every % of their predicted life with each inspection alternating between visual inspections and those involving physical inspections. During visual inspections, the lubricant was drained from the test gear box, and the test gear box lid and front cover were then removed. Both the pinions and gears were wiped with a clean rag and visual changes to the pinion and gear tooth surfaces were noted. If any fatigue pits or crack initiations were observed during a visual inspection, a physical inspection was obtained before any additional testing cycles were performed. If none of the failure criteria was met, additional testing resumed.

25 Physical inspections of test specimens involved removing the pinion and gear from the FZG machine and thoroughly cleaning the pinion and gear surfaces with isopropyl alcohol. Upon cleaning the specimens, measurements were obtained from both the Talysurf roughness machine and the gear CMM. An optical microscope was also utilized to obtain digital pictures of the pinion tooth surfaces. All measurements obtained with the Talysurf and the Gleason M&M were always made on the same gear teeth as those measured during the initial measurements. If fatigue pitting prevented an accurate surface roughness from being obtained, the nearest tooth surface without any signs of pitting was utilized for the interim roughness measurement.. Test Specimens The spur gear specimens utilized for experimental testing in this thesis work had identical gear geometries to those utilized by the sponsor during the phase-one study and those tested in the study by Bluestein []. Design parameters of the test gear pair is shown in Table.. All tested gear specimens were heat treated to a surface hardness value of 6 HRC with a case depth of approximately. mm. The test gears were made of AISI 86 and AISI 46M steel alloys. Differences in elemental composition of the AISI 86 and AISI 46M alloys are shown in Table.. In order to determine the significance of various surface treatments on gear pitting fatigue life, several types of treatments were used for the AISI 86 gears. In addition to studying contact fatigue dependence on surface treatments, ground gears manufactured from AISI 46M steel were also tested in order to investigate impact of changing the material type. A summary

26 Parameter Pinion Gear Module (mm) 4. Center Distance (mm) 9.5 Number of Teeth 7 6 Pressure Angle (deg).5 Face Width (mm) 4..9 Pitch Diameter (mm) Base Diameter (mm) Outside Diameter (mm) Root Diameter (mm) Start of Active Profile (mm) Circular Tooth Thickness (mm) Table.: Basic design parameters of the spur gear pair.

27 Element AISI 86 AISI 46M Cr Max Ni Si P.5 Max.5 Max Mn Cu..5 Max Mo S.4 Max.5 Max C Table.: Elemental Composition by Weight % of Test Specimens 4

28 Test Description Abbreviation Material Pinion Gear Hobbed/Shaved HS 86 Hobbed/Shaved Hobbed/Shaved Chemically Polished CP 86 Chem. Polished Chem. Polished Shot Peened + SP+PH 86 Shot Peened + Shot Peened + Plastic Honed Plastic Honed Plastic Honed Ground AISI 86 GD8 86 Ground Ground Ground AISI 46M GD4 46M Ground Ground Table.: Contact fatigue test matrix with related pinion and gear surface treatments. 5

29 of the specimens in this study, their manufacturing process, and their surface treatments are listed in Table. and are described below: (i) Hobbed and Shaved (HS) These AISI 86 gears were hobbed and shaved before heat treatment. Irregularities in the involute and lead profiles from the shaving process and the heat treatment process can occur on the tooth surfaces. For this reason, all specimens were physically measured before all testing. (ii) Chemically Polished (CP) These AISI 86 gears are identical to the HS gears described above with the addition of a chemical polishing process. This process is also known as super finishing or isotropic finishing. This process reduced the R a and measurements. R q roughness values significantly, as verified by surface roughness (iii) Shot Peened and Plastic Honed (SP+PH) These AISI 86 gear specimens were first hobbed and shaved. The tooth surfaces were then shot peened to create compressive residual stresses along the entire tooth surface. These residual stresses are believed to improve the contact fatigue life of gears. The tooth surfaces were then plastic honed to reduce any surface irregularities induced by the shot peen process. Surface roughnesses of these gears were similar to the HS gear specimens. (iv) Ground AISI 86 (GD8) These AISI 86 gear specimens were cut and then heat treated. After heat treatment, the gears are ground to create a consistent involute gear profile on the gear teeth. Because the grinding process removes any irregularities that may have occurred from heat treatment, ground gears are 6

30 believed to improve contact fatigue life of gears through accuracy improvements. (v) Ground AISI 46M (GD4) Similar to the GD8 specimens, these gears are also hard ground, utilizing the same manufacturing process but are manufactured from AISI 46M gear steel. As can be expected from the wide variety of gear surface treatments tested in this study, the average initial roughness amplitudes varied with each surface treatment. In Table.4, the average R a values obtained with the Talysurf surface profiler before testing are listed for each of these surface treatments. As mentioned previously and shown in the table, the surface roughnesses for the CP specimens were significantly less than those for the HS specimens, as well as all other specimens in this study. Surface roughness traces made during tests for both pinions and gears are shown in Appendix B and Appendix C, respectively. It should be noted that the tests performed in this study with the HS, CP, and SP+PH specimens were performed with the same batch of gears produced by the sponsor during the phase-one study. The use of these gears for experimental testing ensures that the resulting data points will be consistent with the test results from the sponsor during the phase-one study, as well as those results from Bluestein []. The ground gear specimens were added to this study at a later date, and therefore, were not part of the original batch of gears produced by the sponsor. Photographs of the 86 and 46M ground gear specimens are shown in Figure.. 7

31 Surface Treatment Average Initial Roughness - R a (µm) HS.5 CP. SP+PH. GD8.8 GD4.9 Table.4: Average initial treatments. R a roughness values for various surface 8

32 Ground 86 Ground 46M Figure.: Photographs of 86 and 46M ground gear specimens specific to this study. 9

33 Load Level Normalized Torque Normalized Pitch Line Normalized Maximum Contact Stress Contact Stress L L L L L L5...9 Table.5: Test Load Levels and their related normalized torques and contact stresses.

34 .4 Test Matrix Each of the surface treatments described in Section. were tested under six different torque levels. These torque levels and their corresponding maximum pitch line contact stresses were normalized by dividing these values by a reference torque T R and reference contact stress σ R, respectively. These reference values were identical to those used by Bluestein []. The normalized values for input torque and pitch line contact stress for each of the load levels tested during these experiments are shown in Table.5. A test was concluded when the specimens failed under one of the three criteria defined prior to the test program. These failure criteria are as follows:. Total area of pits on a single tooth is greater than.8 mm (.589 in ). Total area of pits on all teeth greater than 6 mm (.5 in ). Maximum surface wear on any of the measured teeth exceeds 5 m (. in) for SP+PH tests and m (.5 in) for all other tests. The first two failure criteria were included in the pitting database as tests with successful pitting failures. The tests with the third failure criterion were included in the database as tests with excessive wear. Any tests interrupted due to unforeseen reasons such as test machine shaft failures or gear tooth breakage were discarded. The majority of successful tests resulted in a single tooth pitting failure (the first criterion). The area of the surface pits was obtained utilizing calibrated software, which measured the dimensions of tooth pits at a 5X magnification.

35 In addition, any tests that reached a life of million pinion cycles were typically suspended without failure and these points were distinguished in the presentation of the data. In a few instances, when a test was believed to be close to pitting failure due to indications of failure from previous tests, the tests were extended beyond million cycles. Confidence intervals, similar to those in Bluestein [], were also calculated when at least five data points were available at each test load level. The equation used for calculating the confidence interval is where is the confidence interval, is the mean (i.e. 5% confidence interval), the value is.645 for a 9% confidence interval, is the standard deviation, and is the number of data points. Tests suspended for wear were not considered in these confidence intervals. In the study by Bluestein [], data points, which were obtained from suspended tests of million cycles or more, were not included in the calculation of the confidence intervals for HS and CP specimens. However, in this thesis, those data points were also included. During previous HS and CP testing by the sponsor and by Bluestein [], as well as testing performed in this study, very few tests resulted in wear failure. Of those tests resulting in wear failure, wear occurred after million cycles or less. Weibull distributions were also calculated in this study when five or more data points at a particular load occurred. Unlike the confidence interval calculations, all Weibull distribution calculations in this thesis study did not include suspended tests at

36 M cycles or more. This was done because the Weibull distribution comparisons were used for pitting failure analysis only..5 Gear Pitting Test Results In previous testing preformed by Bluestein [], the majority of experimental tests were performed at higher torque values. These high torque values produced a large number of successful experiments in a relatively short period. In order to obtain confidence data for lower torques and contact stresses, the majority of testing included in this thesis involved lower loads. All tests performed in this study had procedures and failure criteria that were consistent with the original study. For the tests using Hobbed and Shaved (HS) gears, there were a total of 5 successful tests (46 pit failures and 4 suspended tests). Of these successful tests, 8 tests (5 pit failures and suspended tests) resulted from this study. One suspended test occurred at load L6, and the rest of the tests occurred at load L4. Stress-life (S-N) charts for both normalized pinion torque and normalized contact stress of the HS tests are shown in Figures. and.4, respectively. The large number of successful tests at load L4 allowed the confidence intervals to be calculated and plotted on the S-N curves along with the test data points as shown in Fig.5. As mentioned previously, tests that were suspended without failure were included when calculating the confidence intervals. The inclusion of suspended tests within the confidence data was done in order to more accurately capture the contact fatigue life of the specimens. The effect of including suspended HS tests within

37 Normalized Torque points points 8 points 7 points 9 points point Pitted Wear Suspended without failure Millions of Pinion Cycles Figure.: Normalized pinion torque versus pinion cycles for HS gears. 4

38 Normalized Stress points points 8 points 7 points 9 points point Pitted Wear Suspended without failure Millions of Pinion Cycles Figure.4: Normalized pinion pitch line stress versus pinion cycles for HS gears. 5

39 Normalized Stress Pitted Suspended without failure Mean 9% Confidence Interval Millions of Pinion Cycles Figure.5: HS pitted and suspended tests with 9% Confidence Intervals. 6

40 Percent Failed Probability Plot of HS - L, HS - L, HS - L, HS - L4, HS - L6 Weibull HS - L HS - L HS - L HS - L4 HS - L6 Millions of Pinion Cycles Figure.6: Weibull distributions of HS pitted tests at different load levels. 7

41 confidence interval calculations can be observed in Figure.5. Although the number of cycles for the pitted specimens does not exceed 6 million pinion cycles, the 9% Confidence Interval occurs at approximately 7 million cycles. This result occurs from the inclusion of the two tests suspended at million cycles with no pits. In Figure.6, Weibull distributions were calculated for each of the load levels of the HS tests. Unlike the confidence intervals mentioned above, these Weibull distributions did not include suspended tests and, therefore, have less data points. Compared to the large number of HS tests, the total number of successful Chemically Polished (CP) tests was only ( pit failures and suspended tests). The S-N charts for these data points are shown in Figure.7 and Figure.8. Six of these tests (5 pit failures and suspended test) at load L6 resulted from experiments in this study. Although there were a smaller number of CP tests, the number of tests was great enough to calculate the confidence intervals at L6. Previously, in Bluestein [], the confidence intervals were calculated for loads L, L, and L. In Figure.9, the data points and confidence intervals in this study along with those determined previously are shown. As performed for the HS test results, Weibull distributions were also calculated for CP tests at various loads. If there were five or more successful pit failures at a particular load, Weibull distributions were calculated. In Figure., those Weibull distributions for various loads are shown. When comparing the CP 5% Confidence Interval test results with the HS 5% Confidence Interval, the contact fatigue life of CP gears is substantially longer than the life of HS gears at all load levels. With the addition of the HS and CP test points 8

42 Normalized Torque points 5 points 9 points 6 points point Pitted Suspended without failure Millions of Pinion Cycles Figure.7: Normalized pinion torque versus pinion cycles for CP gears. 9

43 Normalized Stress points 5 points 9 points 6 points point Pitted Suspended without failure Millions of Pinion Cycles Figure.8: Normalized pinion pitch line stress versus pinion cycles for CP gears.

44 Normalized Stress Pitted Suspended without failure Mean 9% Confidence Interval Millions of Pinion Cycles Figure.9: CP pitted and suspended tests with 9% Confidence Intervals.

45 Percent Failed Probability Plot of CP - L, CP - L, CP - L, CP - L6 Weibull CP - L CP - L CP - L CP - L6 Millions of Pinion Cycles Figure.: Weibull distributions of CP pitted tests at different load levels.

46 Percent Failed Probability Plot of HS - L6, CP - L6 Weibull Millions of Pinion Cycles HS - L6 CP - L6 Figure.: Weibull distributions of HS and CP pitted tests at load level L6.

47 obtained in this study, a better comparison between the two surface treatments at lower loads was possible. In order to illustrate these differences at lower loads, Weibull distributions for both HS and CP treatments are shown at load L6 in Figure.. A large portion of surface treated specimens included in this study were Shot Peened and Plastic Honed (SP+PH). Combining the tests in this thesis with those previously conducted by Bluestein [], the total number of successful tests for SP+PH specimens was 5 with tests occurring from pit failures and tests suspended. In addition to these successful tests, 5 tests reached the 5 m wear failure criterion. Of the successful SP+PH tests, 8 pit failures, suspended test, and 4 wear failures occurred during this study. The S-N curves for normalized pinion torque and normalized pinion contact stress of SP+PH specimens are shown in Figure. and Figure., respectively. For the SP+PH surface treatment, the limited number of tests did not allow for the calculation of confidence intervals at most load levels. However, Figure.4 shows a comparison between the SP+PH data points along with the confidence intervals for both HS and CP gears. Most of the successful SP+PH tests had a greater contact fatigue life than the 5% confidence interval for HS gears. From this figure, it is observed that of the 5 successful tests had a greater number of cycles than the corresponding 5% HS confidence interval at the same load level. When trying to make a comparison of SP+PH specimen results with CP confidence intervals, the task is difficult due to the small number of successful data points. Although load level L has 8 data points, the data points range from the lower 9% HS confidence interval to above the upper 9% CP confidence interval. Although the contact pitting fatigue lives at load L vary a great 4

48 Normalized Torque points point 8 points points Pitted Wear Suspended without failure Millions of Pinion Cycles Figure.: Normalized pinion torque versus pinion cycles for SP+PH gears. 5

49 Normalized Stress points point 8 points points Pitted Wear Suspended without failure Millions of Pinion Cycles Figure.: Normalized pinion pitch line stress versus pinion cycles for SP+PH gears. 6

50 Normalized Stress points point 8 points points Pitted Suspended without failure HS Mean HS 9% Confidence Interval CP Mean CP 9% Confidence Interval Millions of Pinion Cycles Figure.4: Pitted and suspended SP+PH tests versus 9% Confidence Intervals for HS and CP pinions. 7

51 deal, the number of data points was sufficient for the calculation of confidence intervals. At the 5% confidence interval, the number of pinion cycles was 4.5 million. For the CP specimens at the same load, the number of cycles was 47.5 million. From this single data set, it appears the SP+PH specimens have lives somewhere between HS and CP specimens at load L. Another method of comparing SP+PH specimens with those from HS and CP tests is with the creation of a Weibull distribution for each of the specimens at the L load. In Figure.5, Weibull distributions for each of the three surface treatments are shown at load level L. From this figure, it appears the SP+PH specimens have a greater contact fatigue life at the same load as the HS gears; however, these SP+PH specimens do not have contact fatigue lives as great as those from the CP tests. A better comparison between SP+PH gears and CP specimens can be made when comparing Weibull distributions of the two surface treatments at different load levels. In Figure.6, a comparison of these Weibull distributions is made between SP+PH tests at load L and CP test results at load L. Although the variation of pitting failures is greater for the SP+PH specimens, these two scenarios are relatively comparable to each other. The tests performed on SP+PH specimens were more time consuming than those performed on HS and CP specimens due to the number of tests that were suspended do to wear. Although the amount of allowable wear depth for these specimens was double that of HS and CP gears, 5% of the tests were suspended for wear. The change in the wear criteria occurred because many of the successful tests would have otherwise been suspended for wear failures early in each test. This significant wear was characteristic for the majority of the SP+PH tests. Observations of these wear patterns with the use of 8

52 Percent Failed Probability Plot of HS - L, SPH - L, CP - L Weibull HS - L SP+PH - L CP - L Millions of Pinion Cycles Figure.5: Weibull distributions of HS, SP+PH, and CP pitted tests at load level L 9

53 Percent Failed Probability Plot of CP - L, SPH - L Weibull Millions of Pinion Cycles CP - L SPH - L Figure.6: Weibull distribution comparison of CP tests at load L and SP+PH tests at load level L 4

54 Involute Deviation (um) Involute Deviation (um) Involute Traces for BL L (a) M cycles M cycles M cycles Pinion Roll Angle (Deg.) Involute Traces for SP+PH L (b) M cycles M cycles M cycles Pinion Roll Angle (Deg.) Figure.7: Involute pinion wear during a load level L6 test on (a) HS specimen and (b) SP+PH specimen. 4

55 the involute tooth profile measurements appear to indicate that most of the wear occurred during the initial cycles of each test. Some comparisons of typical wear for SP+PH specimens versus with those of HS gears are shown in Figure.7. In this figure, the HS specimen has an even amount of wear that occurs throughout the entire test. This type of wear behavior was inherent of many of the successful HS tests performed during this study. SP+PH specimens, on the other hand, encounter a significant amount of wear during the initial cycles of the test. In the figure, most of the wear for the SP+PH specimen occurs during the initial 4.5M cycles. After this initial wear, very little additional pinion wear occurs in the subsequent 75M cycles. This type of wear behavior was evidenced by most of the SP+PH tests in this study. Ground gear experimental testing was specific to this study only. One specific difference between HS and ground gears was observed during physical measurements. The ground gears had a more accurate involute profile shape when performing initial measurements for the ground gears on the Gleason M&M machine. A comparison between HS gears and ground specimens is shown in Figure.8. In the figure, three involute traces are shown for four different teeth on the specimens. The HS specimen has a fairly large involute deviation while the pinion roll angle increases from to degrees. The GD8 ground gear traces shown in the figure have very little deviation between those angles. This was typical of all the GD8 specimens, as well as the GD4 gears, tested in this study. Experimental tests on ground gears for both 46M and 86 gears were performed at three loads: L, L, and L6. The S-N curves for normalized pinion torque and normalized pinion contact stress of GD8 tests are shown in Figure.9 and Figure 4

56 Involute Deviation (um Per Division) Involute Deviation (um Per Division) Tooth Involute Traces for HS Pinions 4-C 4-B 4-A -C -B -A 5-C 5-B 5-A -C -B -A (a) Pinion Roll Angle (Deg.) Tooth Involute Traces for 86M Ground Pinions 4-C 4-B 4-A -C -B -A 5-C 5-B 5-A -C -B -A (b) Pinion Roll Angle (Deg.) Figure.8: Involute pinion tooth traces ( per tooth) during initial measurements for (a) HS specimen and (b) GD8 specimen. 4

57 Normalized Torque points points Pitted, Millions of Pinion Cycles Figure.9: Normalized pinion torque versus pinion cycles for GD8 gears. 44

58 Normalized Stress points points Pitted, Millions of Pinion Cycles Figure.: Normalized pinion pitch line stress versus pinion cycles for GD8 gears. 45

59 Involute Deviation (um) (a) cycles (b) 6M cycles (c) 8.6M cycles Involute Traces for BL L M cycles -5 6.M cycles 8.6M cycles Pinion Roll Angle (Deg.) Figure.: Pictures of a GD8 pinion tooth and involute traces of a L test at (a) cycles, (b) 6M cycles, and (c) 8.6M cycles. 46

60 Normalized Torque points points point Pitted Suspended Millions of Pinion Cycles Figure.: Normalized pinion torque versus pinion cycles for GD4 gears. 47

61 Normalized Stress points points point Pitted Suspended Millions of Pinion Cycles Figure.: Normalized pinion pitch line stress versus pinion cycles for GD4 gears. 48

62 Involute Deviation (um) (a) cycles (b) 9.M cycles (c) 8.M cycles (d) 6.7M cycles Involute Traces for BL L M 9.M cycles -5 8.M cycles 6.7M cycles Pinion Roll Angle (Deg.) Figure.4: Pictures of a GD4 pinion tooth and involute traces of a L test at (a) cycles, (b) 9.M cycles, (c) 8.M cycles, and (d) 6.7M cycles 49

63 Normalized Stress points points Pitted HS Mean HS 9% Confidence Interval CP Mean CP 9% Confidence Interval Millions of Pinion Cycles Figure.5: Pitted GD8 tests versus 9% Confidence Intervals for HS and CP pinions. 5

64 Normalized Stress points points point Pitted Suspended HS Mean HS 9% Confidence Interval CP Mean CP 9% Confidence Interval Millions of Pinion Cycles Figure.6: Pitted GD4 tests versus 9% Confidence Intervals for HS and CP pinions. 5

65 .9, respectively. The GD8 gears had wear characteristics similar to those of HS specimens. Both had a large amount of micro-pitting in the dedendum area of the gear teeth, and both had moderate amounts of wear accumulated during the duration of each tests. In Figure., pictures of GD8 specimens as well as CMM measured tooth involute traces are shown for a L test. For the GD4 specimens, the S-N curves for normalized pinion torque and normalized pinion contact stress of GD8 tests are shown in Figure. and Figure., respectively. Similar to HS and GD8 tests, GD4 gears had a large amount of micropitting in the dedendum area of the gear teeth; however, the wear on these specimens was greater than that of HS and GD8 specimens. Pictures of wear patterns on a L loaded GD4 specimen and the CMM involute measurements are shown in Figure.. As observed in the figure, the involute wear on the GD4 specimen is almost twice that of the similarly loaded GD8 pinion. Because of the limited number of experimental tests performed on GD8 and GD4, confidence intervals were not calculated. To help compare the ground gear specimen results with those of HS tests, S-N plots of the GD8 data points and GD4 data points along with the confidence intervals for HS gears are shown in Figure.4 and Figure.5, respectively. The ground gears for both materials had contact fatigue lives greater than the 9% Confidence Interval for HS gears for all performed tests under all three loads. Although the Ra roughness values for the HS and ground specimens were very similar to each other, it was also worthwhile to make a comparison of R q roughness 5

66 values between the specimens. Higher values in these Rq values would suggest a greater variation in asperity contact magnitudes along the tooth surface and may influence the contact fatigue life of each specimen. There were moderate differences in R q values between the HS and GD8 specimens. For the HS gears the average R q value was.5 µm while the average for GD8 gears was.4779 µm. The significance of this difference on pitting fatigue life is difficult to determine because of the higher involute deviation on the HS specimens. The average R q value for the GD4 gears was.4984 µm. Although this value is similar to the HS surface treatment, a direct comparison between two different steel alloys cannot be made..6 Conclusions Based on the tests results presented in the previous section, the following conclusions can be listed in regards to the effects of surface treatments and material variations on fatigue pitting of gears: At lower load levels, the expected fatigue life for CP gears was significantly longer than that for HS gears at the same load. However, the difference in measured fatigue life was not as great at higher contact stress conditions. SP+PH gear specimens showed a greater tendency to wear, much more than HS and CP gears at the same loads. Although the amount of acceptable wear was doubled for SP+PH tests, 5% of the tests still exceeded that increased wear limit. 5

67 Available tests suggest SP+PH specimens have greater fatigue lives than HS gears at the same loads. Further testing is needed to better determine the significance of these differences between SP+PH gears and tests for HS and CP specimens at loads other than L. Available tests indicate ground gears for both AISI 86 and AISI 46M steel alloys may have longer contact fatigue lives than HS 86 specimens at the same load levels. Additional testing at different load levels and an increased number of data points at previously tested load levels are needed to confirm this suggestion. Limited tests appear to indicate ground AISI 46M specimens tend to wear more than ground 86 gears. This wear may require the acceptable wear depth for these gears to increase in similar fashion to the SP+PH experiments. Additional testing is needed to determine if this is the case. 54

68 CHAPTER GEAR SCUFFING EXPERIMENTS. Scuffing Test Procedure In this study, the ISO standard regarding scuffing evaluation of gears (ISO 465- FZG Scuffing Test) was applied to determine the test stages at which AISI 86 FZG type A gears encountered scuffing failures within Dexron VI automatic transmission fluid. Similar to pitting fatigue testing in Chapter, a standard FZG machine was utilized for these scuffing experiments. The lubricant reservoir was also filled until the gears were halfway immersed in oil bath. The FZG machine used for these scuffing tests was slightly modified from the machines utilized for fatigue pitting experiments. This modification involved the removal of the copper cooling tubes that were designed to maintain the lubricant temperature at 9ºC during fatigue testing. This change allowed the lid of the test gearbox reservoir to be quickly removed during scuffing tests for visual inspections. A Type J Thermocouple was placed within the test gearbox of the FZG machine. The thermocouple was placed approximately cm below the static lubricant fluid level. The location of the thermocouple is shown in Figure.. During testing, the lubricant 55

69 temperature was recorded once per second using DASYLab data acquisition software. The thermocouple was also utilized for verifying the correct starting temperature at various stages of testing. An inductive current clamp was also utilized during these experiments. The inductive current clamp was attached to the FZG motor to record the amount of amperage sent to the motor by the FZG controller in order to maintain a constant machine speed during testing. During the scuffing process, a material transfer between the gear and pinion teeth takes place. This material transfer increases the friction between the tooth surfaces, causing the FZG controller to increase the amount of amperage delivered to the motor. The motor current readings were captured utilizing DASYLab in order to determine the specific point at which the scuffing process initiates. Lastly, a Fluke Ti45FT Thermal Imager was utilized to obtain the temperature distribution of the outside of the test gearbox. A photo of the thermal camera placed in front of the FZG machine during testing is shown in Figure.. Thermal images were taken at least once every minute during experimental testing. Capturing thermal images at regular frequency throughout testing provided additional information about the thermal state of the gearbox during the scuffing test, complementing the thermocouple readings. The ISO 465 FZG Scuffing Test consists of possible stages with each stage increasing the amount of applied torque to the test pinion. A list of the applied torque levels for each stage, and their related pitch line contact stresses are given in Table.. Each stage of testing consists of,7 gear revolutions, corresponding to approximately 5 minutes of testing at an input speed of 455 rpm. 56

70 Lubricant Fluid Level Figure.: Schematic diagram of FZG test gearbox with approximate thermocouple location. Thermocouple Location 57

71 58 Figure.: Thermal camera shown in front of gearbox during a scuffing experiment.

72 Stage Pinion Torque Normal Tooth Load Pitch Line Contact Stress (Nm) (N) (N/mm) Table.: Applied pinion torques and related contact stresses for the ISO 465- FZG Scuffing Test. 59

73 (a) No Scuffing Marks (b) No Measurable Scuffing Marks (c) Approx. mm Scuffing Marks (d) Approx. 4 mm Scuffing Marks (e) Approx. 6 mm Scuffing Marks (f) Approx. mm Scuffing Marks Figure.: Digital Images of various amounts of scuffing on pinion tooth surfaces. 6

74 levels for each stage, and their related pitch line contact stresses are given in Table.. Each stage of testing consists of,7 gear revolutions, corresponding to approximately 5 minutes of testing at an input speed of 455 rpm. At the start of Stage, the temperature of the lubricant for all tests was ambient, approximately ºC. Once the FZG machine completed the stage after,7 revolutions, the torque for Stage was applied and the machine was brought up to speed to complete another,7 revolutions. This process was repeated through Stage 4. During these first 4 stages, the temperature of the lubricant was allowed to rise freely. The test gearbox lid was left in place and no visual inspections were made during these first 4 stages. After Stage 4, the lid to the lubricant reservoir was quickly removed, the gears were visually inspected, and digital pictures of various teeth on the pinion were taken. These inspections were made to check for scuffing initiation, as well as any uneven wear due to shaft, gear, or other misalignments. Upon inspection, the test gearbox lid was secured into place, the Stage 5 torque was applied to the gear pair, and the Dexron VI was heated to 9ºC ± ºC as specified by the ISO scuffing test. These inspections occurred after each additional stage until a scuffing failure occurred. According to the ISO scuffing test, a scuffing failure is defined as a pinion with mm of total scuffed area on the sum of the pinion tooth widths. An example of various degrees of scuffing is shown in Figure.. 6

75 . Scuffing Gear Test Specimens As mentioned previously, the gears utilized for testing were type A spur gears and pinions. The design parameters of these gears are specified by ISO and are shown in Table.. The pinions for this type A design have a very long addendum in order to create high sliding velocities toward the tip of the pinion tooth. These high velocities help induce scuffing patterns on the pinion teeth due to the decreased viscous properties of the Dexron VI at high temperatures. In addition to the uncoated 86 type A gears, an additional test was performed utilizing the same gears coated with an experimental PVD coating provided by the sponsor. This PVD coating was applied to both the gear and pinion surfaces. The sponsor believes this hard coating may provide better scuffing protection when applied to gears with high contact stress applications. A scuffing test using gears with this PVD coating was performed to determine the usefulness of this coating under such applications. Before all testing, the gear surface roughnesses were measured on the pinions and gears utilizing the Taylor-Hobson Form Talysurf- surface profiler. roughness was measured in the profile direction on one tooth, and both the The surface R a and R q values for the pinions and gears were recorded. These roughness values are listed in Table.. CMM inspections, with the use of a Gleason M&M 55, were also performed on the test gears to measure three lead and three involute traces on four teeth of each specimen. This was done to check for any unknown tooth irregularities. Lastly, an 6

76 Parameter Pinion Gear Module (mm) 4.5 Center Distance (mm) 9.5 Number of Teeth 6 4 Pressure Angle (deg). Working Pressure Angle (deg).5 Face Width (mm).. Pitch Diameter (mm) Outside Diameter (mm) Profile-shift Coefficient Addendum Engagement (mm) 4.7. Table.: Basic design parameters of type A spur gears. 6

77 Test Ra Pinion Rq Pinion Ra Gear Rq Gear Ra Ave Rq Ave 4977R L R L L Table.: Initial profiler. Ra and Rq type A specimen values from the Talysurf- surface 64

78 Test Material Scuffing Stage Total Scuffed Area 4977R AISI 86 Stage Approx 6 mm 4978L AISI 86 Stage Approx.5 mm 4978R AISI 86 Stage Approx 6 mm 4979L AISI 86 Stage Approx 8 mm 496L AISI 86 w/pvd Did Not Scuff None Table.4: Scuffing test summaries with related scuffing Stages and total scuffing areas. 65

79 optical microscope with a /X objective lens and.5x magnification was utilized to obtain digital pictures of the pinion surfaces.. Scuffing Test Results In this study, there were a total of five scuffing tests performed. Four tests involved 86 type A gears and pinions while the other test had an experimental PVD coating applied to the 86 type A gear and pinion specimen. The four pairs of uncoated specimens successfully scuffed. The pair of PVD coated specimens did not scuff. A summary of these tests is shown in Table.4. As can be seen by the amount of scuffed area listed in the table, the amount of scuffing at the end of each test varied a great deal. From the limited amount of roughness data taken with the Talysurf- shown in Table., scuffing appears unrelated to initial surface roughness. However, only one tooth was measured along the involute direction in one location. To find a more thorough and better understanding of the relationship between surface roughness and scuffing, each of the teeth would need surface roughness measurements taken in various locations along the tooth surface. The resulting scuffing patterns from the successful scuffing failures were generally evenly distributed amongst all the pinion teeth. In a few instances, some pinions had teeth that were unmarked while other teeth had -4 mm of scuffed area along the face width. Digital photos of test specimens after their final stage are shown in Figure.4. 66

80 (a) 4977R Pinion after Stage (b) 4978L Pinion after Stage (c) 4978R Pinion after Stage (d) 4979L Pinion after Stage (e) 496L Pinion with PVD coating after Stage Figure.4: Digital Images of pinion surfaces after their final test Stage. 67

81 Temperature [ºC] R 4978L 4978R 4979L 496L Time [Seconds] Figure.5: Display of recorded temperatures at Stage of each scuffing test performed. 68

82 Temperature [ºC] L 4978R 4979L 496L Time [Seconds] Figure.6: Display of recorded temperatures at Stage of each scuffing test performed. 69

83 7 Figure.7: Thermal camera image shown of gearbox front during a scuffing experiment

84 In Figure.4, the temperature recordings made with the submerged thermocouple are shown for all tests at Stage of testing. The test with the lowest temperature, called 496L, was the test with the experimental PVD coating provided by the sponsor. The coating appears to keep the specimens at a lower temperature throughout testing. These lower temperatures may help keep the viscosity lower and therefore help reduce scuffing. In Figure.5, the 4977R test, which resulted in a successful scuffing at Stage (6 mm), had one of the higher temperature recordings throughout Stage testing. Similar to the figure discussed above, Figure.6 also shows the temperature recordings made obtained with the thermocouple. However, these recordings were made at Stage. As with Stage, the specimens with the PVD coating had a smaller temperature rise then those without the coating. The 4979L specimen had the highest temperature readings throughout Stage and also finished with the highest amount of scuffed area (8 mm). The temperatures captured utilizing the Fluke Ti45FT Thermal Imager were higher than those recorded with the thermocouple. As an example, Figure.7 shows a photograph of the temperature distribution during testing. A mark at the approximate thermocouple location and a mark at the maximum temperature location are shown within the figure. The temperature difference between the two locations is approximately 7ºC in this example. Although the thermal camera was used to capture thermal images once per minute during the majority of testing, the camera settings were changed for one of the experiments to capture images every seconds. This was done to obtain a better trend 7

85 of temperature data throughout the testing stage. Figure.8 shows the results of this comparison. Throughout testing, the maximum temperature of the gearbox cover is higher than the temperature obtained with the thermocouple. At the start of testing, the maximum temperature from the thermal image was above the 9ºC thermocouple temperature because of the previous testing in Stage. When the FZG machine stopped after the completion of Stage, the top of the test gearbox cooled at a slower rate than the bottom because the FZG steel table surface acts as large heat sink. In Figure.8, the temperature difference between the thermocouple and thermal images is roughly 5ºC after seconds and increases to approximately ºC toward the end of the Stage. If the maximum temperatures captured by the thermal camera are consistent, the temperatures from the tests at Stage should have similar trends to the thermocouple temperatures discussed previously and shown in Figure.6. As seen in Figure.9, the PVD coated specimens had the lowest thermal imaging temperatures throughout Stage while specimen 4979L, which had the most scuffed area, had the highest temperatures throughout the stage. These thermal imaging results are in agreement with those from the thermocouple. The motor current data obtained during testing may have shown some useful trends during testing. During testing, the FZG motor current increased with each subsequent stage of testing. This was expected because the amount of current needed to keep the machine running at a constant speed would need to increase due to the larger magnitudes of applied torques and increased frictional forces for each stage. In particular interest was the FZG motor current recorded data from the final stage of testing. 7

86 Temperature [ºC] R_TC 4978R_TI Time [Seconds] Figure.8: Recorded temperatures of Thermocouple (TC) versus maximum temperatures of Thermal Imaging (TI) Camera during Stage of 49789R test. 7

87 Temperature [ºC] L_TI 4978R_TI 4979L_TI 496L_TI Time [Seconds] Figure.9: Maximum Temperatures Recorded by Thermal Imaging Camera during Stage for each scuffing test performed. 74

88 The recorded motor currents and a 6 second moving average for each of the tests at Stage were recorded. The moving average was applied in order to better determine if and when substantial changes to the FZG motor current occurred. For the 4978L test during Stage, the FZG machine had a constant current during the initial minutes of testing and showed a decrease and later an increase in motor amperage. If the friction between the tooth surfaces did decrease, the amount of motor amperage needed to keep the FZG machine rotating at the same speed would also decrease. This result for 4978L implies that the amount of friction decreased in advance of scuffing initiation. For the 4979L test during Stage, the FZG machine had a constant current during the initial 9 minutes of testing and later showed an increase in motor amperage. These increases in current for these tests may indicate that the FZG controller needed to provide the motor more current in order to overcome increased frictional forces created by scuffing initiation..4 Conclusions From the experiments performed in this thesis study with AISI 86 type A gears, the effects of surface treatments on gear scuffing have produced the following conclusions: The resulting scuffing stage of specimens in Dexron VI under dip lubrication conditions occurs at Stages and based on limited testing. 75

89 The experimental PVD coated type A gears provided by the sponsor improved the scuffing resistance of the gears and prevented scuffing initiation through Stage. The test specimen that successfully scuffed at Stage also resulted in having the highest recorded temperatures during Stage. The experimental PVD coated specimen, which did not scuff, had the lowest recorded temperatures throughout Stage when compared with other specimens. A change in FZG motor amperage was detected when scuffing initiation occurred during testing. 76

90 CHAPTER 4 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK 4. Thesis Summary In this thesis study, the influences of gear pitting (spalling) and gear scuffing were investigated with experimental testing on spur gears. With all tests performed under dip lubrication conditions with Dexron VI automatic transmission fluid as the lubricant, the impact of various parameters on gear life were investigated. For gear pitting experiments, specimens with four different surface treatments were studied: Hobbed and Shaved, Chemically Polished, Shot Peened and Plastic Honed, and Ground. While the majority of the specimens were made of AISI 86 steel, this study also included Ground specimens made of AISI 46M steel. Each of the surface treatments were tested using six different load levels. Gear scuffing tests were performed using the standard ISO scuffing test. The standard ISO test utilizes type A gears, which were made of AISI 86 steel for these experiments. There were a total of 5 tests performed, one of which included specimens coated with an experimental PVD coating. During each test, a Type J thermocouple with 77

91 the use of data acquisition software captured lubricant temperatures within the gearbox, while a thermal imaging camera recorded the temperature distribution of the FZG test gearbox front. An inductive current clamp was also utilized to measure the FZG motor current throughout testing in order to detect changes in motor current, which may indicate scuffing initiation during testing. 4. Conclusions Based on the tests results presented in the previous section, the following conclusions can be listed in regards to the effects of surface treatments and material variations on fatigue pitting of gears: The expected fatigue life for CP gears was longer than that of HS gears, considerably so at higher load levels. SP+PH gear specimens showed a greater tendency to wear, much more than HS and CP gears at the same loads. The majority of this wear tends to occur during the initial testing cycles while wear for HS specimens gradually occurs throughout the entire test. Tests suggest SP+PH specimens have greater fatigue lives than HS gears at the same loads. Limited testing indicates ground gears for both AISI 86 and AISI 46M steel alloys may have longer contact fatigue lives than HS 86 specimens at the same load levels. This result seems due to a more consistent involute gear profile. 78

92 Limited tests appear to indicate ground AISI 46M specimens tend to wear more than ground 86 gears. This wear may require the acceptable wear depth for these gears to increase in similar fashion to the SP+PH experiments. Additional testing is needed to determine if this is the case. From the scuffing experiments performed in this thesis study with AISI 86 type A gears, the effects of surface treatments on gear scuffing have produced the following conclusions: The resulting scuffing stage of specimens in Dexron VI under dip lubrication conditions occurs at Stages and based on limited testing. The experimental PVD coated type A gears provided by the sponsor improved the scuffing resistance of the gears and prevented scuffing initiation through Stage. The test specimen that successfully scuffed at Stage also resulted in having the highest recorded temperatures during Stage. The experimental PVD coated specimen did not scuff and had the lowest recorded temperatures throughout Stage when compared with other specimens. Increases in FZG motor amperage were detected during scuffing stages of specimens. This may indicate scuffing initiation during testing. 79

93 4. Recommendations for Future Work From the extensive experimental tests performed in this study, much as been learned about the performance of gears under pitting and scuffing conditions for specimens with a variety of surface finishes and steel alloys. To further examine some of the conclusions and suggestions made by the pitting tests performed in this study and in previous studies, additional testing in the following areas can be performed: Different testing speeds. Decreasing the rotational speeds will increase the number of asperity contacts due to a thicker lubricant film thickness. A better understanding of the dependency of gear pitting on lubricant film thickness with these varied gear surfaces would be beneficial in determining if the performance of CP gears approaches that of HS specimens tested in this study. Additional testing of Ground Gears. Although it appears that ground gears have better pitting resistance than HS specimens, additional testing for statistical purposes is needed to determine the extent of this better performance. Ground gears that are chemically polished may also be beneficial in testing; however, if the pitting fatigue lives increase drastically, it may be difficult to obtain pitting results below load L. Variation of tooth thicknesses. Smaller tooth thickness results in more bending. This additional cyclic bending may increase/decrease pitting characteristics of spur gears for a given surface finish. 8

94 Use of acoustic software and accelerometers might be helpful in determining pitting initiation. This has been applied to single tooth bending tests by Singh et al [8] but not in dynamic testing within an FZG machine. Testing at different temperatures. In all tests, the temperature was always held constant at 9ºC. Influences of temperature can be further studied and compared in order to determine the changes to pitting initiation due to changes in lubricant fluid properties. To further enhance scuffing tests for AISI 86 type A gears, the following suggestions can be made: In normal running conditions, transmissions are not allowed to rise in temperature freely. Keeping the temperature at lower temperatures may provide more realistic conditions. Creating additional stages running additional tests may be helpful with a Stage B and Stage B, where those stages have torques/contact stresses between Stages,, and. The addition of accelerometers attached to the FZG gearbox may be helpful in determining when scuffing initiates. Small changes in vibrations during scuffing initiation may be able to be detected. Once it can be better determined what contact stress results consistently in scuffing, run additional scuffing tests that only run at one load torque to determine if there are consistencies in how long it takes for scuffing initiation to occur. 8

95 Similar to pitting tests, different gear materials can be tested to determine scuffing dependency on material. Based on the limited pitting tests with 46M, it appears that wear may be more prevalent in that material versus 86. From this, the 46M gears may be more susceptible to scuffing; however, more understanding of the material dependencies on scuffing initiation can be developed through experimental testing. 8

96 BIBLIOGRAPHY [] Kaneko, K., 97, The Pitting Phenomena in the Power Transmission Gears, Mechanisms Conference & International Symposium on Gearing and Transmissions, San Francisco, US. [] Townsend, D. P., Chevalier, J. L., and Zaretsky, E. V., 97, Pitting Fatigue Characteristics of AISI M-5 and Super Nitralloy Spur Gears, NASA Technical Note D-76. [] Bluestein, J. M., 7, An Experimental Study of the Impact of Various Tooth Surface Treatments on Spur Gear Pitting Life, M.S. Thesis, The Ohio State University. [4] Castro, J. and Seabra, J., 998, Scuffing and Lubricant Film Breakdown in FZG gears: Part I. Analytical and Experimental Approach, Wear, 5, pp. 4-. [5] Snidle, R. W., Dulipalla, A. K., Evans, H.P., and Cooper, C. V., 8, Scuffing Performance of a Hard Coating Under EHL Conditions at Sliding Speeds up to 6 m/s and Contact Pressures up to. GPa, Transactions of the ASME Journal of Tribology,, pp [6] Li, S., 9, Lubrication and Contact Fatigue Models for Roller and Gear Contacts, Ph.D. Dissertation, The Ohio State University. [7] Liou, J. J., 9, A Theoretical and Experimental Investigation of Roller and Gear Scuffing, Ph.D. Dissertation, The Ohio State University. [8] Singh, A., Houser, D. R., and Vijayakar, S.,999, Detecting Gear Tooth Breakage Using Acoustic Emission: a Feasibility and Sensor Placement Study, Transactions of the ASME Journal of Mechanical Design,, pp

97 APPENDIX A IMAGES OF PINIONS FROM PITTING TESTS 84

98 (a) cycles (b) 8.4M cycles (c) 6.7M cycles (d) 55.M cycles (e) 64.M cycles (f) 64.M cycles (pitted tooth) Figure A.: Pictures of a HS pinion tooth of a L4 test at (a) cycles, (b) 8.4M cycles, (c) 6.7M cycles, (d) 55.M cycles, (e) 64.M cycles. Picture (f) shows the pitted tooth at 64.M cycles. 85

99 (a) cycles (b).m cycles (c) 6.7M cycles (d) M cycles Figure A.: Pictures of a HS pinion tooth of a L6 test at (a) cycles, (b).m cycles, (c) 6.7M cycles, and (d) M cycles. 86

100 (a) cycles (b) 8.4M cycles (c) 6.7M cycles (d) 55.M cycles (e) 64.M cycles (f) 8.6M cycles Figure A.: Pictures of a CP pinion tooth of a L6 test at (a) cycles, (b) 8.4M cycles, (c) 6.7M cycles, (d) 55.M cycles, (e) 64.M cycles, and (f) 8.6M cycles. 87

101 (a) cycles (b).6m cycles (c) 6.M cycles (d) 8.6M cycles (e) 97.9M cycles Figure A.4: Pictures of a SP+PH pinion tooth of a L test at (a) cycles, (b).6m cycles, (c) 6.M cycles, (d) 8.6M cycles, and (e) 97.9M cycles. 88

102 (a) cycles (b) 4.6M cycles (c) 9.M cycles (d).7m cycles Figure A.5: Pictures of a GD8 pinion tooth of a L test at (a) cycles, (b) 4.6M cycles, (c) 9.M cycles, and (d).7m cycles. 89

103 (a) cycles (b) 6.M cycles (c) 7.M cycles Figure A.6: Pictures of a GD4 pinion tooth of a L test at (a) cycles, (b) 6.M cycles, and (c) 7.M cycles. 9

104 (a) cycles (b) 4.5M cycles (c) 48.9M cycles (d) 79.5M cycles (e) M cycles (f) 9.8M cycles Figure A.7: Pictures of a GD4 pinion tooth of a L6 test at (a) cycles, (b) 4.5M cycles, (c) 48.9M cycles, (d) 79.5M cycles, (e) M cycles, and (f) 9.8M cycles. 9