Master of Science in Mechanical Engineering (MSME), Wright State University, 2021, Mechanical Engineering

This study describes contact fatigue behavior of spur gear contacts operating under mixed elastohydrodynamic condition. The focus is placed on the starvation effect on fatigue crack initiation. With the model, parametric simulations are carried out with different contact parameters. In the process, the lubricant supply is varied to alter the lubrication condition from fully flooded to severely starved circumstance. Multi-axial stress fields induced by surface normal and tangential tractions are evaluated, whose amplitudes and means are used in a multi-axial fatigue criterion to determine the crack initiation life. It is found a lower lubricant viscosity elongates fatigue life when severe starvation occurs, which is opposite to the EHL rule under fully flooded lubrication condition. However, it's in line with the experimental observation [1], where film thickness was shown to increase when moving from high viscosity base oil to a lower one under starvation condition.

Committee: Sheng Li Ph.D. (Advisor); Ahsan Mian Ph.D. (Committee Member); Joy Gockel Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2021, Engineering Education

Contact fatigue behavior of a gear pair is typically gauged by size of macro-pits formed during the operation of the gear pair. It is common that very small pits form in certain tooth contact locations first. Macro-pits typically initiate from the boundaries of these micro-pitted zones. As most macro-pitting evaluations were performed at high- stress, medium-life conditions, micro-pit formation has often been overlooked. In this study, a set of spur gear experiments are performed under typical automotive speed, temperature and operating conditions with a modest torque transmitted, to investigate micro-pitting performance during ultra-long cycle operation up to 400 million cycles. Evolution of contact surfaces throughout each test are documented through interim inspections, including surface geometries, roughnesses and micro-pitted area. Digital imaging techniques are employed to automate the quantification of micro-pitted regions for each tooth image. The micro-pitting experiments are simulated by using an existing spur gear micro-pit life prediction methodology. Based on comparisons between predictions and measurements, an assessment on the accuracy and effectiveness of this methodology is made along with the potential areas of improvement.

Committee: Ahmet Kahraman Dr. (Advisor); David Talbot Dr. (Committee Member) Subjects: Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2019, Mechanical Engineering

Pitting is a rolling contact fatigue phenomenon commonly observed in mechanical rolling elements, such as gears and bearings. In case of gear contacts, pitting usually takes place in the dedendum region, where both sliding and contact load are high. In this study, a model is developed to predict surface breaking crack formation fatigue lives, including both nucleation and propagation stages, for spur gear contacts operating under mixed elastohydrodynamic lubrication (EHL) condition. The model utilizes a gear load distribution model for tooth contact Analysis. A mixed EHL formulation is implemented to evaluate the surface normal pressure and tangential shear, incorporating the lubricant non-Newtonian behavior, which is influential on lubrication film thickness and surface tractions under high sliding condition. According to the surface tractions, a boundary element formulation is utilized to determine the stress fields, whose contribution to fatigue damage accumulation is assessed using a multi-axial fatigue criterion, predicting the crack nucleation life. As for the crack propagation life evaluation, the Paris and Erdogan's formula is adopted. With the developed contact fatigue model, a parametric investigation is performed considering a spur gear pair, operating under different loads and different surface roughness conditions.

Committee: Sheng Li Ph.D. (Advisor); Ahsan Mian Ph.D. (Committee Member); Joy Elizabeth Gockel Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2019, Mechanical Engineering

In this study, a generalized methodology is designed and implemented to characterize dynamic behavior of a certain class of gear test machines, namely back-to-back setups. This methodology relies on two independent measurement systems. One system focuses on measurement of tooth root strains to be directly correlated to dynamic gear mesh forces transmitted by the gear mesh. A compliantly connected slip ring system is incorporated with this measurement system for noise-free transmission of strain signals to a stationary DAQ. A data analysis procedure is developed to process the strain data to determine dynamic stress factors within a range of speed and torque under both steady-state and transient operating conditions with appropriate filtering and statistical analysis procedures applied. The other measurement system uses an array of accelerometers mounted at stationary locations along the test gear pair bearing caps. The methodology is adapted to two separate back-to-back gear test machines, one machine being an ISO standard FZG machine, and the other being a newly developed GL-100 machine. The measurements from both machines are compared at the end to determine that the difference in their dynamic behavior is minimal.

Committee: Ahmet Kahraman Dr. (Advisor); David Talbot Dr. (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2019, Mechanical Engineering

In this study, a new state-of-the-art gear contact durability test machine is developed to eliminate some key limitations, issues and inconveniences associated with conventional FZG machines. Based on lessons learned from prior in-house studies using conventional FZG machines, a number of requirements are specified. A new four-square concept is designed and fabricated to gain various advantages with respect to foot-print, temperature regulation, and long-lasting auxiliary components. The mechanical layout and out-of-the-loop torque application methodology are described along with the heat management and lubrication systems. Various new safety provisions are highlighted, in addition to a streamlined interim gear inspection procedure during long-cycle contact fatigue tests performed on these new machines. For demonstration purposes, the proposed machines are used to conduct an example contact fatigue test program to evaluate a stress-life curve of ground spur gears made of a typical automotive gear steel. The test procedures, test conditions and the test specimens are described. Various results containing digital images of the gear teeth, surface wear and roughness progression over the fatigue life of the gears are detailed. A detailed statistical analysis is presented to define a stress-life curve and confidence intervals. This example fatigue study confirms the suitability of the new machines to perform high-fidelity gear contact fatigue experiments.

Committee: Ahmet Kahraman (Advisor); David Talbot (Committee Member) Subjects: Mechanical Engineering

Master of Science in Engineering, Youngstown State University, 2017, Department of Mechanical, Industrial and Manufacturing Engineering

Powder bed-based EBM technology has been utilized for biomedical implant manufacturing. Anisotropic microstructure of the EBM built implant surface is closely related to wear and corrosion behavior during active biomechanical loadings. A series of experimental studies are performed in ambient and in simulated body fluids. Before the wear tests, local properties are characterized by depth-sensing nanoindentation technique to obtain mechanical properties due to surface anisotropy. Microscale (fretting) contact experiments are conducted using the pure titanium spherical head of instrumented nanoindenter as a well-characterized single asperity to apply controlled sliding contact motions on the EBM-built Ti6Al4V surfaces. Removed volume of the surface is measured to determine the influence of surface anisotropy, contact stress, and synovial environment effect on surface fatigue response. Experimental results indicate that the layer-by-layer fashion of the EBM-built part develops anisotropic microstructure and the microstructure leads to variable tribological properties. In ambient environment with controlled humidity (35%), the wear behavior of the EBM built Ti6Al4V displays a significant dependence on both build orientation and sliding motions. In phosphate buffer saline (PBS) solution, wear rate for most of the EBM built parts increase under the cyclic sliding contacts, while the surface anisotropy effect becomes less significant. The result implies electrochemical attack largely affects the wear of transversely developed parts. However, wear rate of millannealed Ti6Al4V decreases in PBS compared to the wear in ambient. Material removal rates in protein-hyaluronic acid solution are significantly reduced in all specimens and sliding directions. Presence of major components in synovial fluids improves lubricative effect under the same mechanical stimuli. In conclusion, the grain morphology and iv orientation significantly changes the tribological and electrochemical pe (open full item for complete abstract)

Committee: Jae Ryu PhD (Advisor); Guha Manogharan PhD (Committee Member); Virgil Solomon PhD (Committee Member); Brett Conner PhD (Committee Member); Hazel Marie PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2017, Engineering PhD

Gears are vital power transmitting mechanical components, in both automotive and aerospace applications, and commonly operate within relatively high rotational speed ranges. Therefore, the dynamic behavior of gears is inevitable and can be quite significant under certain circumstances. The gear dynamics introduces not only noises and vibrations, but also large tooth force amplitudes, and consequently large amplitudes of bending stresses and contact stresses, and high surface temperatures, promoting the failures of tooth bending fatigue, contact fatigue, and scuffing. This study focuses on the mechanism by which the gear dynamic responses affect the flash temperature rise and contact fatigue life using a gear tribo-dynamic formulation. The significance of this work is that it connects the gear dynamics and gear tribology disciplines and shows the importance of dynamic response on the two critical failure modes; scuffing and pitting. A six degree-of-freedom transverse-torsional discrete gear dynamics equation set is coupled with a thermal mixed elastohydrodynamic lubrication formulation to include the interactions between the gear dynamics and the gear tribological behavior. The flash temperature rises are quantified within a wide speed range under the different operating and surface conditions. The results indicate evident deviations of flash temperature rise between quasi-static condition and tribo-dynamic condition especially in the vicinities of the resonances. The interactive model of gear dynamics and gear tribological behavior is bridged through an iterative numerical scheme to determine the surface normal pressure and tangential shear under the tribo-dynamic condition. The resultant multi-axial stress fields (from these surface tractions) on and below the surface are then used to assess the fatigue damage. A comparison between the tribo-dynamic and quasi-static life predictions is performed to demonstrate the important role of the gear tribo-dynamics in (open full item for complete abstract)

Committee: Sheng Li Ph.D. (Advisor); Joseph Slater Ph.D. (Committee Member); Ahmet Kahraman Ph.D. (Committee Member); Ha-Rok Bae Ph.D. (Committee Member); Nikolai Priezjev Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2015, Mechanical Engineering

A global study performed in 1966 revealed that nearly 30% of the energy produced is spent to overcome friction and associated losses. A new interdisciplinary domain "Tribology" was defined in the "Jost Report"' as the branch of science that essentially deals with friction, wear and lubrication. Increasing demand to improve efficiency of mechanical systems has stimulated the research in tribology over the last few decades. Surface engineering methods are state of art techniques that are being adapted by industries including bearing manufacturers to address friction and wear issues. Many new and novel coatings have been developed for specific applications, but few if any, have improved the tribological performance of the most widely used components: ball bearings. Thus, there is a need for new tribological research designed to understand the influence of the coatings deposited onto spherical rolling elements in tribological contacts, and minimize losses due to friction and wear. In this work, thin films were deposited onto spherical rolling elements and the performance of the coated balls was evaluated under different conditions. The study revealed that ball coatings improves the performance of bearings, but coatings need to be selected based on application requirements to avail the benefits of coated balls.

Committee: Gary Doll (Advisor) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2015, Civil Engineering

Spherical roller bearings (SRBs) utilized in the gearboxes of wind turbine generators are known to be especially susceptible to premature failure due to low cycle micropitting of the raceways. Micropitting in rolling element bearings is believed to arise from significant roller/raceway sliding in thin film lubrication conditions. Roller/raceway sliding occurs in SRBs as a consequence of their geometry, and almost all the bearings in wind turbine gearboxes operate in thin film (or low lambda) lubrication conditions. There is currently no accepted solution to mitigate micropitting in wind turbine gearboxes that are equipped with SRBs. Since WC/a-C:H coatings on rolling elements have been effectively used to solve wear issues encountered by SRBs in other industrial applications, these coatings have been offered as a solution to low cycle micropitting in wind turbine gearbox SRBs. This research plan has been developed to test the hypothesis that a WC/a-C:H coating will mitigate or eliminate micropitting such as that experienced by SRBs in wind turbine gearboxes. The laboratory tool that is used to create micropitting on test specimens is the PCS Instruments Micropitting Rig (PCS MPR). The MPR is a three-contact disc machine in which there are three rings of equal diameter positioned at 120 degrees apart with a smaller diameter roller located in the middle and in contact with all the rings. This arrangement allows the test roller to be subjected to a large number of rolling contact cycles in a short period of time and hence significantly reduces testing time. At a typical entrainment speed of 3.5m/s, the central test roller will experience approximately one million contact cycles per hour. Since the controls of the PCS MPR allow the speed, slide-roll ratio, temperature, and load to be automatically and independently controlled, the thin film lubrication and slide/roll ratio conditions that generate micropitting on SRBs can be reproduced in the laboratory. Mo (open full item for complete abstract)

Committee: Gary Doll Professor (Advisor); Evans Ryan Doctor (Committee Member); Binienda Wieslaw Doctor (Committee Member); Dong Yalin Doctor (Committee Member); Menzemer Craig Doctor (Committee Member); Sancaktar Erol Doctor (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Materials Science; Mechanical Engineering

Master of Science, The Ohio State University, 2013, Mechanical Engineering

In this study, a number of spur gear tests were performed under high-power and high-temperature conditions representative of certain aerospace gearing applications. As the first type of tests, long cycle tests of 100 million cycles were performed at set operating speed, load, and temperature conditions. The second type of tests, load-staged scuffing tests, implemented an incrementally increased torque schedule under constant speed and oil temperature conditions. Two different gear tooth surfaces were considered in these tests: hard ground surfaces representative of rough, as machined gear surfaces and chemically polished gear surfaces that were an order of magnitude smoother than the ground surfaces. The primary failure mode of concern was scuffing of the contact surfaces due to temperature build up. The impact of surface roughness amplitudes, contact stress, and oil inlet temperature on scuffing failures were investigated. Effects of ramp up procedures for the speed and torque, as well as the introduction of a break-in test stage were also investigated to show that they are critical to the scuffing performance of gears.

Committee: Ahmet Kahraman (Advisor); Brian Harper (Committee Member) Subjects: Mechanical Engineering

PhD, University of Cincinnati, 2004, Engineering : Mechanical Engineering

The dissertation is to address the need, in contact mechanics, of efficient and effective solutions to certain 3-D contact problems. The solutions developed here are based on underlying analytical solutions to pyramidal loading elements. This feature, along with other characteristics, distinguishes this method from other numerical solutions. The research work is logically divided into three subsequent parts, each of which addresses a particular aspect of the project: (1) Developed analytical solution sets in closed form to pyramidal loading profiles. First, a set of Boussinesq-Curruti equations to linear/bilinear distribution of normal and tangential loading over a triangular area are derived and evaluated. Second, solution sets to normal and tangential surface loading pyramids are constructed. The work provides a solution set to a basic loading element, which is the foundation of the development of effective and efficient semi-analytical solutions to 3-D contact problems with general geometry and loading profile. (2) Developed a semi-analytical approach (non-incremental algorithm) to 3-D normal contact problems with friction. This approach treats normal contact (indentation) phenomenon as a static problem. Based on fully coupled governing equations, the algorithm of contact detecting and stick/slip partitioning is designed as nested iterations, to fulfill contact boundary conditions. The computation shows that it is an efficient algorithm. Numerical examples are presented to show the accuracy and efficiency of the method.(3) Developed a semi-analytical approach (incremental algorithm) to 3-D contact problems with friction. This approach treats contact as a dynamic problem. The general dynamic models are simplified into quasi-static models in many practical cases that inertial force can be ignored. The incremental algorithm is designed to solve the quasi-static problems. The computation shows that the algorithm works very well for cases featuring both similar and di (open full item for complete abstract)

Committee: Dr. EDWARD BERGER (Advisor) Subjects: Engineering, Mechanical

Master of Science, The Ohio State University, 2012, Mechanical Engineering

In this study, gear contact tests were performed using a recently developed test methodology capable of both high-power (pitch-line velocities up to 50 m/s and pinion torques up to 450 N-m) and high-temperature (oil inlet temperatures up to 150C) operating conditions. Test specimens and operating conditions were chosen in order to simulate high-power automotive and aerospace applications. Automotive test specimens were made from a typical automotive transmission gear steel, SAE 4118M, at surface roughnesses typical of hard ground gears. Aerospace test specimens were made out of a high performance (high-temperature) proprietary gear steel. These aerospace specimens were either chemically polished or super-finished following grinding to achieve roughness amplitudes more than 10 times smoother than typical ground surfaces. Throughout each test interim inspections were used to identify and monitor failure modes. Experimental testing for automotive applications is shown to consistently produce contact fatigue failures in the form of micro-pitting and macro-pitting. Tests were suspended when macro-pits exceeded the test methodologies pre-determined failure criteria. Experimental testing for aerospace applications is shown to be absent of any contact fatigue failures due to the extremely smooth contact surfaces. The primary mode of contact failure in aerospace tests is observed to be scuffing.

Committee: Ahmet Kahraman (Advisor); Gary Kinzel (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Automotive Engineering; Automotive Materials; Engineering; Mechanical Engineering

Master of Science, The Ohio State University, 2009, Mechanical Engineering

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 14635-1 FZG Scuffing Test was performed on AISI 8620 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 11 and 12. A high correlation between temperature and scuffing results was detected for both coated and uncoated specimens.

Committee: Ahmet Kahraman PhD (Advisor); Donald Houser PhD (Committee Member) Subjects: Mechanical Engineering