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Leque, NicholasDevelopment of an Experimental Methodology for Evaluation of Gear Contact Fatigue under High-Power and High-Temperature Conditions
Master of Science, The Ohio State University, 2011, Mechanical Engineering
Contact fatigue failures in the form of pitting or micro-pitting have been a perennial problem in power transmission applications. These failures are dictated by a large number of parameters including loading conditions, gear geometry and tooth modifications, kinematics (rolling and sliding velocities), lubricant parameters (viscosity, pressure-viscosity behavior), and material parameters (material type, hardness, case depth, residual stresses). As such, theoretical treatment of contact fatigue failures has been rather challenging, directing the focus to the experimental investigation of the problem. Most of the experimental gear pitting studies to date were limited to low-speed and low-temperature operating conditions. This study aims at developing a methodology for evaluating the contact fatigue lives of gears under high-speed (pitch-line velocities up to 50 m/s), high-stress (contact stresses up to 2 GPa) and high-temperature (oil inlet temperatures up to 150C). Specifications of a test machine concept that meets these requirements are defined and two test machines are designed and procured for this purpose. Gear test specimens that result in pits consistently are developed with the other competing failures (wear, scuffing, tooth breakage), as well as the high vibration conditions, avoided. Preliminary high-speed tests are presented at the end, representing both automotive and aerospace conditions to show that pitting and micro-pitting failures can be produced with the proposed methodology.

Committee:

Ahmet Kahraman, PhD (Advisor); Carlos Castro, PhD (Committee Member)

Subjects:

Engineering; Mechanical Engineering

Keywords:

Gears; fatigue life; pitting; micro-pitting; high-speed; high-temperature; aerospace; automotive

Tilson, Nial RobertAn Experimental Evaluation of Micro-pitting Performance of Two Bearing Steels
Master of Science, The Ohio State University, 2013, Mechanical Engineering
This experimental study focuses on the impact of operating conditions including entraining speed, sliding ratio, and load on micro-pitting performance of two typical bearing steels. Case hardened roller specimens made of alloy 9310 and through-hardened roller specimens made of alloy 485-2 are procured. Test matrices are defined according to the Latin Hypercube Space Filling Design of Experiment approach and executed to evaluate the micro-pitting performance of both materials by manipulating the speed, sliding ratio, load, and material. A two-disk test set-up is used for this purpose with each test documented by initial, interim, and final roughness, wear, and micro-pitting measurements. Before each test, a run-in process is implemented for the purpose of surface roughness break-in. By counting the distributed micro-pitted areas on the surface, a Micro-pitting Severity Index (MSI), defined as the ratio of the total micro-pitted area to the entire inspection area, is quantified for every test condition. Results indicate that the 9310 specimens showed significantly better resistance to micro-pitting compared to the 485-2 specimens, with nearly ten times lower MSI values. The data collected in this study establishes a database for micro-pitting failure of these two steel alloys that is required for validation of point contact lubrication models with circumferentially oriented surface roughnesses.

Committee:

Ahmet Kahraman, Dr. (Advisor); Robert Siston, Dr. (Committee Member)

Subjects:

Engineering; Materials Science; Mechanical Engineering

Keywords:

Micro-pitting; micro-pit; elastohydrodynamic lubrication; ground specimens; surface failure