Harvey P. Nixon

On Mechanisms of Fatigue Life Enhancement by Surface Dents in Heavily Loaded Rolling Line Contacts

This paper studies mechanisms of surface dents in enhancing the fatigue life of rolling bearings previously reported in Akamatsu et al. (1). First, transient micro-EHL analyses of heavily loaded contacts between rough surfaces with multiple dents are conducted under near rolling conditions. Contacts with various dent dimensions, dent arrangements under different loading and kinematic conditions are investigated. Results show that surface dents generate no favorable micro-EHL effects to enhance the contact fatigue life. Subsequent analyses, in conjunction with other published studies, suggest that the fatigue life enhancement likely comes from the reduced local traction at asperity contacts through the oil pots effects of the dents. The effects of the surface dents on contact fatigue life may depend on the lubrication regime in which the contact is operating, being favorable in poor lubrication conditions but adverse in well-lubricated contacts. Since rolling bearings are usually designed to operate in a healthy regime of lubrication, fatigue life enhancement by artificially introducing dents on bearing surfaces may not extend to field applications.

Fatigue Life Reduction of Roller Bearings Due to Debris Denting: Part I — Theoretical Modeling

Stress concentration due to debris denting in EHL contacts has been analyzed. Quantitative relationships between the geometry of surface indentation, contact load and the maximum stress and its location have been established. Based on these relationships and the Weibull weakest-link theory, a fatigue life reduction model is proposed. Parametric study shows that life reduction is primarily determined by dent slope and life reduction factor decreases as dent slope increases. Indentation area-density is another major factor that affects contact fatigue life. Life reduction factor decreases as dent area-density increases. Life reduction is also affected by contact load. While fatigue life decreases with increasing load, life reduction factor increases as load increases. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999

Transmission Electron Microscopy of Boundary-Lubricated Bearing Surfaces. Part II: Mineral Oil Lubricant with Sulfur-and Phosphorus-Containing Gear Oil Additives

Transmission electron microscopy (TEM) was performed on cross-sectional samples of tapered roller bearing cone surfaces that were tested at two levels of local boundary lubrication severity, Λ ∼ 1.1 and 0.3. Unlike our previously reported work in which a base mineral oil was used, the bearing tests were conducted in mineral oil with sulfur- and phosphorus-containing gear oil additives. Structural and compositional characterization of undetached antiwear surface layers on the base steel (cone raceway) revealed that the films contained crystalline and amorphous regions. A sharp interface (<∼10 nm) that separated the surface layer and base steel was imaged. The surface layer for the cone tested at Λ ∼ 1.1 consisted of Fe, O, and P, whereas that for the cone tested at Λ ∼ 0.3 consisted of Fe, O, P, C, Ca, and S. Various TEM analytical techniques were used to study the segregation of these elements throughout the antiwear surface layer volume.

Fatigue Life Reduction of Roller Bearings Due to Debris Denting: Part II - Experimental Validation

Life tests have been conducted for tapered roller bearings. Bearing race ways are pre-dented with particles of various types and sizes. Life reductions due to surface indentation are calculated by a stress-based life model using the measured raceway topography. The predictions are compared and validated with testing results. The study concludes that bearing life reduction is primarily determined by the slopes and area densities of surface indentations. Life reduction factor decreases as dent slope and area-density increase. The hardness and fracture toughness of the denting particles have a noticeable effect on bearing life reduction. Ductile particles cause more severe life reduction than the brittle particles. Surface failures associated with small dents are superficial and often appear in forms of “micro-pitting” and “micro-peeling.” Large dents generated by ductile particles cause severe damage to bearing raceways and are more likely to induce surface pits and spalls. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999

Methods for Assessing the Bearing Surface Durability Performance of Lubricant Formulations

Lubricant formulations and lubricant additives have been demonstrated to have a major impact on the surface durability of rolling element bearings. However, there are very few standard tests used to assess the performance aspects of lubricants as it relates to bearing surface performance. Lubricant formulations have been slanted heavily toward protecting gear concentrated contacts from galling and wear. In addition, much of the performance differentiation of lubricants has been dependent on highly accelerated standardized laboratory tests related to gears. Methods have been developed for properly evaluating a lubricant’s performance characteristics as it relates to bearings. These methods are explained and the corresponding test results are reviewed to show their effectiveness as a lubricant performance evaluation tool. The implications of these findings provide direction and suggestions for ways to minimize or avoid potential detrimental performance effects of lubricant formulations on rolling element bearings. INTRODUCTION It has been demonstrated that lubricant additive formulations have an impact on surface durability of rolling contact bearings (1)(2), see Figure 1. The surface durability in this context is pitting fatigue performance, as opposed to wear of the surfaces due to surface adhesive, the latter is seen in sliding surfaces. While sliding protection is important, the damage mode of surface fatigue should also be addressed when evaluating lubricants for bearing performance characteristics. A standard test protocol would be of valuable assistance in determining performance. In the following discussion an approach is outlined which can be used to evaluate surface durability performance of lubricants on a relative basis. It can be used to bring important lubricant performance parameters into consideration for such evaluations. IMPORTANCE OF TEST PARAMETERS Many of the lubricant evaluation tests being used by the industry involve fixing various operational parameters and then making a comparison of one lubricant formulation relative to another. In actual applications the lubricant itself can have an influence on some of the very parameters being fixed in the evaluation test. A prime example of this issue is the operating temperature of any equipment for which the lubricant is intended. One lubricant may allow the system to operate at a lower temperature while another may drive the operating higher. Therefore, it is important to include such operating variables into evaluation tests and thus to have appropriate comparisons in performance. In our example here, forcing a lubricant to operate at a fixed operating temperature would not be a fair comparison relative to any lubricant, which on its own, will operate at a much higher temperature. Figure 1 – Performance Variation Due to Chemistry of Additive Packages 1 Copyright

Experimental and Analytical Methods for Assessing Bearing Performance Under Debris Contaminated Lubrication Conditions

Debris particle contamination in lubricants is a major cause of premature bearing and gear failure, responsible for causing accompanying costs in equipment downtime, warranty claims, and lost productivity. Various experimental and predictive methods have been developed to assist design engineers in analysis and development of equipment that is less sensitive to such contamination. This paper provides an overview of debris particle contamination in lubricants. It also presents new data that compares bearing life test results and predictive analysis methods for various tapered roller bearings operating under debris-contaminated conditions. As a baseline, some previous work in these areas is briefly summarized and referenced. More recent work has refined one analytical method (using a surface characterization technique), correlated this method with bearing test lives in debris conditions, and pointed to design and manufacturing modifications in the bearings themselves. This has resulted in designs that improve bearing life in debris-contaminated environments.