Aaron Greco

Investigating the Process of White Etching Crack Initiation in Bearing Steel

White etching cracks (WECs) have been identified as a dominant mode of premature failure within wind turbine gearbox bearings. Though WECs have been reported in the field for over a decade, the conditions leading to WECs and the process by which this failure culminates are both highly debated. In previously published work, the generation of WECs on a benchtop scale was linked to sliding at the surface of the test sample, and it was also postulated that the generation of WECs was dependent on the cumulative energy that had been applied to the sample over the entirety of the test. In this paper, a three-ring-on-roller benchtop test rig is used to systematically alter the cumulative energy that a sample experiences through changes in normal load, sliding, and run-time, in an attempt to correlate cumulative energy with the formation of WECs. It was determined that, in the current test setup, the presence of WECs can be predicted by this energy criterion. The authors then used this information to study the process by which WECs initiate. It was found that, under the current testing conditions, the formation of a dark etching microstructure precedes the formation of a crack, and a crack precedes the formation of white etching microstructure.

Correlation of Polysiloxane Molecular Structure to Shear-Thinning Power-Law Exponent Using Elastohydrodynamic Film Thickness Measurements

Siloxane-based polymers (polysiloxanes) are susceptible to temporary shear-thinning that manifests as a reduction of elastohydrodynamic film thickness with increasing entrainment speed or effective shear rate. The departure from Newtonian film thickness can be predicted with the power-law exponent ns, an indicator of the severity of shear-thinning in a polymeric fluid that is influenced by the macromolecular structure. In this paper, a combination of extant rheological and tribological models is applied to determine the power-law exponent of several polysiloxanes using film thickness measurements. Film thickness data at several temperatures and slide-to-roll ratios are used to validate the methodology for several siloxane-based polymers with alkyl and aryl branches.

High-Performance Heterocyclic Friction Modifiers for Boundary Lubrication

The demand for increased energy efficiency continuously drives the development of new lubricants. Here we report the design and synthesis of hexahydrotriazine, triazine, and cyclen derivatives as friction modifiers (FMs) for enhanced fuel economy. This series of sulfur- and phosphorus-free oil-soluble heterocyclic ring-based molecules exhibits differing thermal and chemical stability depending on the degree of aromatization and number of linking spacers within the central heterocyclic ring. Thermally stable triazine and cyclen FMs significantly increase friction performance in the boundary lubrication regime. Cyclens in particular reduce friction by up to 70% over a wide temperature range. Detailed experimental investigations of the newly synthesized FMs at elevated temperatures demonstrate their favorable tribological performance under four operating conditions: variable-temperature sliding, linear speed ramping, reciprocating sliding, and rolling–sliding contact. These latest experimental findings suggest the potential of the application of “designer” heterocyclic FMs for reducing frictional loss in motor vehicles.

Empirical Investigation of Electricity Self-Generation in a Lubricated Sliding–Rolling Contact

The paper reports the empirical observations of voltage generation in a lubricated tribocontact with different oils altering load, sliding and temperature. The investigation is done in the context of research of the root cause of white etching cracks (WEC) failure in bearings. Tested oils of different additive packages found completely different electrical behavior. The oil, which is known to produce WECs in laboratory tests, demonstrated non-zero voltage generation.

Micro-Scale Surface Texture Design for Improved Scuffing Resistance in Gear Applications

The reliability and durability of gear components are critical issues especially in advanced power transmissions subject to increased power density and extreme operating conditions. In general the three main tribological failure modes present in gear tooth contact are: wear, contact fatigue/pitting, and scuffing. The present work investigates the influence of surface texturing in the form of micro-scale dimples as a method to enhance scuff resistance. Flat H13 steel samples were micro-dimpled using a laser surface texturing process. For scuffing and wear evaluation the flats were tested in point contact under lubricated linear reciprocating sliding. A threefold increase in scuffing resistance was observed for textured surfaces over comparable non-textured flats, however, for prolonged operation at high loads an increase wear rate of the counterface is observed. For gear application the design and location of surface texture requires careful consideration to balance scuffing resistance with potential influence to increased wear and fatigue pitting.Copyright