Michael N. Kotzalas

Attaining High Levels of Bearing Performance with a Nanocomposite Diamond-Like Carbon Coating

Mechanical systems that operate in agricultural, construction, and mining and mineral processing applications typically operate in debris-containing and thin-film lubrication environments. Consequently, the lives of rolling element bearings in these applications are often limited by surface fatigue arising from thin-film lubrication, damage from the debris, false brinelling, and scuffing. Traditional approaches utilizing specialized heat treatments and lubricant additives can sometimes delay the onset of early raceway fatigue, but even in these cases the actual bearing life is still usually less than that for which the bearing was designed. Bearings with a highly durable W-aC:H coating applied to the rolling elements are shown to have significantly longer fatigue lives in debris-containing and thin-film lubrication environments and are highly resistant to wear from false brinelling and scuffing.

A Theoretical Study of Residual Stress Effects on Fatigue Life Prediction

Recently, the trend has been toward the use of the full subsurface stress field in rolling element bearing fatigue life prediction (stress field-based life models). By using the stress field-based bearing life models, more accurate assessments of such things as fitting practice and thermal treatments on the bearing performance are achieved. However, one aspect missing in most models has been the consideration of the changing residual stress during operation of the bearing. This study was conducted to investigate the time dependent residual stress on contact fatigue life predictions. This study concluded that the changes in residual stress during operation were most likely a fatigue reaction of the material to the pre-fatigue residual stress and cyclic contact stress fields. The materials fatigue response changes the instantaneous values of the material constants in most stress field-based life equations, thus making them in-calculable. As such, the pre-fatigue residual stress field should be used in the stress field-based models. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in San Francisco, CA October 21–24, 2001

A Method to Calculate Frictional Effects in Oil-Lubricated Ball Bearings

Friction and heat generation in oil-lubricated ball bearings is mainly the result of sliding in the ball-raceway contacts and agitation of the lubricant in the free space between the balls, cage. and bearing rings. Endurance of a ball bearing is highly dependent on the thicknesses of the oil films which serve to separate the balls from the raceways in a well-lubricated bearing. The film thicknesses, in turn, are dependent upon the lubricants viscous properties. These are functions of bearing temperatures and are determined by the balance between the rates of frictional heat generation and heat dissipation. Therefore, in the design of a ball bearing application such as the high speed rotor and low speed rotor support bearings in an aircraft gas turbine, it is important to be able to predict the bearing frictional heat generation rates with reasonable accuracy. This paper presents a method to perform the required calculations considering hearing loading and speeds, realistic lubricant rheological properties, and a relatively simple heat transfer system between the hearing rings, halls. And lubricant. The results of the analysis are shown to compare favorably with experimental data. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in San Francisco, CA October 21–24, 2001

Predicting Micro-Pitting Occurrence in Wind Turbine Gearbox Roller Bearings

The standard rolling contact fatigue life calculations currently in use by the rolling bearing industry is based on the first occurrence of subsurface-initiated spalling of a raceway or roller surface. However, wind turbine gearbox roller bearings have been suffering from another damage mode, which manifests itself as micro-pitting. The micro-pitting, which is spalling on a micro scale, by itself can be tolerated in its early stages; i.e. the roller bearing will still function properly. As the damaged bearing continues to operate, the micro-pitting propagates and at the later stages, often termed peeling, the pitting becomes deep enough to reach the appearance of traditional subsurface-initiated spalling. To better understand the phenomenon micro-pitting and its causes, this study was conducted to review published literature on the topic as it relates to bearing operation. The key findings were the need for a low specific lubricant film thickness parameter, and some component of sliding velocity in the contacting surface. With this knowledge, a proposed test scheme including these variables could be created from which a method to predict the risk of micro-pitting may be determined.Copyright

Composite Skewness for Two Rough Surfaces in Contact

Typically, the contact of two rough bodies is transformed into an equivalent rough surface contacting a flat. Due to the mathematics involved, this was usually conducted for surfaces assumed to be Gaussian, where distribution of peaks and valleys are equal. These assumptions were acceptable when dealing with ground surfaces, which are typically Gaussian; however, other finishing techniques often result in skewed surface distributions. As such, an equation was developed numerically within, based on the Weibull distribution, to estimate the composite skewness of contacting rough bodies. The assumption of using the Weibull distribution to describe the surface variation was validated with measured data from different finishing techniques, as was the equation for composite skewness developed within. Presented at the STLE Annual Meeting in Las Vegas, Nevada, May 15-19, 2005 Review led by Alan Lebeck

Statistical Distribution of Tapered Roller Bearing Fatigue Lives at High Levels of Reliability

The original two-parameter Weibull distribution used for rolling element bearing fatigue tends to greatly underestimate life at high levels of reliability. This fact has been proven for through hardened ball, cylindrical and spherical roller bearings, as well as linear ball bearings, by other researchers. However, to date this has not been done with tapered roller bearings (TRB) or case carburized materials, and as such this study was conducted. First, the three-parameter Weibull distribution was utilized to create a mathematical model, and statistical data analysis methods were put into place. This algorithm was then investigated as to its ability to discern the shape of the reliability distribution using known, numerically generated, data sets for two and three-parameter Weibull distributions. After validation, an experimental data set of 9702 TRB’s, 98% of which were case carburized, was collected. Using the developed algorithm on this data set, the overall RMS error was reduced from 26.0% for the standard, two-parameter to 12.2% for the three-parameter Wiebull distribution. Also, the error at 99.9% reliability was reduced from 95.8% to 37%. However, as the results within varied from previously published values at high reliabilities, there is likely a difference in the underlying population and/or dependency on the statistical and mathematical methods utilized. Therefore, more investigation should be conducted in this area to identify the underlying variables and their effects on the results.© 2005 ASME

A Simple Calculation of EHL Traction at Small Slide/Roll Ratio Including Roller Elasticity

Elastohydrodynamic point contact traction may be predicted accurately for the small slide-to-roll ratio, Σ, isothermal case by first calculating the shear response of the liquid in a parallel gap with a hemispherical pressure distribution and then correcting Σ for the elastic roller compliance.Copyright

Statistical Analysis of Traction Measurements with Rolling, Sliding, and Spin

Traction measurements, like any other physical testing, contain variability associated with the test rig, test samples, and data acquisition systems. These natural sources of variation increased when simulating contacts with spin velocity in the most common rolling and sliding test conditions. Averaging the results within one test sequence with spinning, rolling, and sliding velocities did not always reflect the expected results from the entire population of possible measured values. To be confident in the conclusions that were drawn from the traction testing, it was important to repeat the test sequence several times and apply statistics for defining error bands around the expected traction values. To allow for easy application of error bands, simplified equations were curve fit to the data from the numerical routines. By making use of error bands, an easy determination of the test repeatability can be discerned, and quantified comparisons between data sets can be made with a higher degree of confidence.