L10 Life Research Articles

The influence of external dynamic loads on the lifetime of rolling element bearings: Experimental analysis of the lubricant film and surface wear

Precise prediction of the lifetime of rolling element bearings is a crucial step towards a reliable design of many rotating machines. For bearings subjected to highly varying loads, recent research emphasises a strong reduction of the actual bearing lifetime w.r.t. the classically calculated bearing lifetime. This paper experimentally analyses the influence of external dynamic loads on the lifetime of rolling element bearings. A novel bearing test rig is introduced. The test rig is able to apply a fully controlled multi-axial static and dynamic load on a single test bearing. Also, different types and sizes of bearings can be tested. Two separate investigations are conducted. First, the behaviour of the lubricant film between the rolling elements and raceways is analysed. Increased metallic contact or breakdown of the film during dynamic excitation is investigated based on the measured electrical resistance through the bearing. The study shows that the lubricant film thickness follows the imposed variations of the load. Variations of the lubricant film thickness are similar to the variations when the magnitude of the static bearing load is changed. Second, wear of the raceway surfaces is analysed. Surface wear is investigated after a series of accelerated lifetime tests under high dynamic load. Due to sliding motion between asperities of the contacting surfaces in the bearing, polishing of the raceway honing structure occurs. This polishing is clearly observed on SEM images of the inner raceway after a test duration of only 0.5% of the calculated L10 life. Polishing wear of the surfaces, such as surface induced cracks and material delamination, is expected when the bearing is further exposed to the high dynamic load.

A finite element model for rolling contact fatigue of refurbished bearings

Bearing refurbishing has become a popular method of extending the life of rolling element bearings. In the refurbishing process the raceways of the bearing may be ground to remove any surface damage prior to repolishing and reassembly with larger sized rolling elements. In the current study a continuum damage mechanics finite element model was developed to quantify the damage in original and refurbished bearings. After calculating the damage accumulation for a set number of contact cycles with the original bearing geometry, refurbishing is simulated by removing a layer of the original surface. The refurbished microstructural model is then subjected to additional computational contact cycles until a fatigue crack reaches the surface, signifying failure. This model preserves the fatigue damage accumulated prior to refurbishing and evaluates its influence on the refurbished bearing fatigue life. All refurbished bearing surfaces showed a significant amount of life after refurbishing with L10 lives from the point of refurbishment, varying from 20% to 94% of the original L10 life. The results indicate that the remaining life of the refurbished bearing population is inversely related to the time before refurbishing and is proportional to the depth of the regrinding. Results obtained from this investigation are in good agreement when compared to the Lundberg-Palmgren bearing life equation modified for analyzing the life of a refurbished bearing.

Improved rolling contact fatigue life for an ultrahigh-carbon steel with nanobainitic microstructure

In the present work, the influence of nanobainite on the rolling contact fatigue (RCF) performance of an ultrahigh-carbon steel has been studied. The results show that nanobainite can effectively improve the RCF life of the steel. In particular, the L10 life of specimens with 21 ± 1 vol.% nanobainite is approximately 3.3 times longer than that of specimens without nanobainite. The improvement in the RCF life of the steel is attributed to the nanobainite, which can delay the crack initiation and propagation.

Improvement in Efficiency of Ultrasonic Tests for the Macroscopic Inclusions Evaluation

Abstract Rolling-contact-fatigue life is a key characteristic of bearings. Reliable bearings are expected to have long L10 life as a bearing performance index, as well as long commercial life without unexpected premature failure on any pieces of bearings in use. It has long been known that cleanliness is an important material factor to determine the fatigue life. The aforementioned early failure could be caused by macroscopic inclusions, which have small density of presence in recent clean steel and, thus, detection for those inclusions requires evaluation on a large volume of steel. Although higher-frequency ultrasonic tests have advantages for non-metallic inclusion detections, relatively low frequency of 10- to 20-MHz ultrasonic tests are useful for both evaluation efficiency of large volume and for detection performance of macroscopic inclusions. The authors apply a focused-beam-type 15-MHz ultrasonic test to detect 100-μm or larger macroscopic inclusions, where the efficient evaluation is needed because of the assessment for a large volume of steel. In this paper, the basic characteristics of several procedures of ultrasonic tests will be discussed from the viewpoint of the evaluation efficiency for macroscopic inclusions.

Bearing Fatigue Life Tests of Two Advanced Base Oils for Space Applications Under Vacuum and Atmospheric Environments

Four series of rolling-element bearing fatigue tests were conducted with 51104 size thrust ball bearings with three balls made from SUJ2 (AISI 52100) steel lubricated with two advanced synthetic base oils used for space applications. The test lubricants were perfluoropolyether (PFPE) and multiply alkyated cyclopentane (MAC). Each oil was tested with bearings under vacuum and atmospheric environments. The bearings were tested at a maximum Hertzian stress of 4 GPa on the inner and outer races. The outer race was rotated at a speed of 250 rpm. A pool lubrication system was used. Fresh lubricant was used for each test bearing. Testing in vacuum conditions was at 5 × 10−2 Pa. The test oils were analyzed to determine whether changes occurred as a result of operating in air and in a vacuum. In a vacuum environment, the PFPE 815Z oil exhibited a longer fatigue life than the MAC 2001A oil. However, in an air environment, the MAC 2001A oil had a longer L10 fatigue life than the PFPE 815Z oil. The fatigue life tests of PFPE 815Z oil in vacuum resulted in a longer L10 life than when tested in an air environment. In an air environment, hydrogen fluoride was generated in the bearing tests with the PFPE 815Z oils. Under vacuum conditions, hydrogen fluoride was not generated with the PFPE 815Z oil, resulting in longer bearing fatigue lives. The fatigue life tests of MAC 2001A oil in a vacuum resulted in shorter L10 fatigue life than in an air environment. The shorter life was attributed to the lower elastohydrodynamic oil film formation with the MAC 2001A oil because of a higher operating temperature and decomposition of the oil in vacuum.

Wear behaviour and rolling contact fatigue life of Ti/TiN/DLC multilayer films fabricated on bearing steel by PIIID

In order to improve the friction and wear behaviours and rolling contact fatigue (RCF) life of bearing steel materials, Ti/TiN/DLC (diamond-like carbon) multilayer hard films were fabricated onto AISI52100 bearing steel surface by plasma immersion ion implantation and deposition (PIIID) technique. The micro-Raman spectroscopy analysis confirms that the surface film layer possess the characteristic of diamond-like carbon, and it is composed of a mixture of amorphous and crystalline phases, with a variable ratio of sp2/sp3 carbon bonds. Atomic force microscope (AFM) reveals that the multilayer films have extremely smooth area, excellent adhesion, high uniformity and efficiency of space filling over large areas. The nanohardness (H) and elastic modulus (E) measurement indicates that the H and E of DLC multilayer films is about 32 GPa and 410 GPa, increases by 190.9% and 86.4%. The friction and wear behaviours and RCF life of DLC multilayer films specimen have also been investigated by ball-on-disc and three-ball-rod fatigue testers. Results show that the friction coefficient against AISI52100 steel ball decreases from 0.92 to 0.25, the longest wear life increases nearly by 22 times. In addition, wear tracks of the PIIID samples as well as wear tracks of the sliding steel ball were analyzed with the help of optical microscopy and scanning electron microscopy (SEM). The L10, L50, La and mean RCF life L of treated bearing samples, in 90% confidence level, increases by 10.1, 4.2, 3.5 and 3.4 times, respectively. Compared with the bearing steel substrate, the RCF life scatter extent of Ti/TiN/DLC multilayer films sample is improved obviously.

Effect of raceway geometry parameters on the carrying capability and the service life of a four-point-contact slewing bearing

The carrying capability and the service life of a four-point-contact slewing bearing are influenced significantly by its raceway geometry parameters. In this paper, the carrying capability is characterized by the maximal contact stress in the paper. The usual industry practice is to specify the service life as the L10 life. The geometry parameters, cross section clearance, ratio of curvatures, rolling ball diameter, and initial contact angle, are also considered. The sensitivity of each parameter influencing the carrying capability and the service life is discussed as well. The results show an interesting phenomenon, indicating that the maximal contact load and the maximal stress do not always have the same varying trend when discussed parameters vary. The developed design can be utilized as a reference for an optimal slewing bearing design. If a slewing bearing of high static carrying capability is required, it can be designed with large ratio of curvatures and rolling ball diameter. In terms of service life, the cross section clearance and the initial contact load should also be considered.

Rolling Contact Fatigue Life Test Design and Result Interpretation Methods Maintaining Compatibility of Efficiency and Reliability

Abstract In this report, several methods for both rolling contact fatigue (RCF) life test design and result interpretation are introduced. These methods generate results using random numbers followed by Weibull distribution (i.e., Weibull random number). The first method illustrates a relationship between the minimum number of test specimens and the suspension time in a fixed time test required at L10 and L50 lives with an arbitrary reliability. This relationship is useful to maintain the qualitative reliability and avoid excessive quantitative testing. The second method can clarify a relationship between the given number of test specimens and resultant significant differences at L10 and L50 lives in an accelerated test with an arbitrary reliability. This relationship is also useful to estimate the appropriate number of test specimens based on statistical logic. Of note, calculations employing the Weibull random number can apply to not only RCF life test design but also estimations of the test results. The third method enables the determination of a range of L10 and L50 lives with an arbitrary reliability even if the number of test specimens is too small to estimate L10 or L50 lives from the Weibull plots. The fourth method can determine significant differences of L10 and L50 lives between any two given lots and allow a quantitative estimation of the minimum difference between their lives from data obtained by experiments. These methods provide techniques that are easier to understand as compared to the recent mathematical model, and they show enough flexibility to apply to almost all types of testing. These systems will therefore eliminate the need for qualified experiences related to the statistical design and result interpretation for RCF life testing.

A Numerical Model for Life Scatter in Rolling Element Bearings

Fatigue lives of rolling element bearings exhibit a wide scatter due to the statistical nature of the mechanisms responsible for the bearing failure process. Life models that account for this dispersion are empirical in nature and do not provide insights into the physical mechanisms that lead to this scatter. One of the primary reasons for dispersion in lives is the inhomogeneous nature of the bearing material. Here, a new approach based on a discrete material representation is presented that simulates this inherent material randomness. In this investigation, two levels of randomness are considered: (1) the topological randomness due to geometric variability in the material microstructure and (2) the material property randomness due to nonuniform distribution of properties throughout the material. The effect of these variations on the subsurface stress field in Hertzian line contacts is studied. Fatigue life is formulated as a function of a critical stress quantity and its corresponding depth, following a similar approach to the Lundberg–Palmgren theory. However, instead of explicitly assuming a Weibull distribution of fatigue lives, the life distribution is obtained as an outcome of numerical simulations. A new critical stress quantity is introduced that considers shear stress acting along internal material planes of weakness. It is found that there is a scatter in the magnitude as well as depth of occurrence of this critical stress quantity, which leads to a scatter in computed fatigue lives. Further, the range of depths within which the critical stress quantity occurs is found to be consistent with experimental observations of fatigue cracks. The life distributions obtained from the numerical simulations are found to follow a two-parameter Weibull distribution closely. The L10 life and the Weibull slope decrease when a nonuniform distribution of elastic modulus is assumed throughout the material. The introduction of internal flaws in the material significantly reduces the L10 life and the Weibull slope. However, it is found that the Weibull slope reaches a limiting value beyond a certain concentration of flaws. This limiting value is close to that predicted by the Lundberg–Palmgren theory. Weibull slopes obtained through the numerical simulations range from 1.29 to 3.36 and are within experimentally observed values for bearing steels.

XRD Residual Stress Analysis for the Clarification of Failure Modes of Rolling Bearings*

Abstract Material conditions of rolling bearings, which are assessed by X-ray diffraction (XRD) measurements, point to a variety of load possibilities especially at raceway surfaces. Due to clearly distinguishable damage symptoms, it is differentiated between the (near-) surface and sub-surface failure mode in the literature. Surface distress of different intensity can be generated by particle-contaminated lubricants that result in raceway indentations. These micro-Hertzian contacts may lead to changes in residual stress and line broadening and in the microstructure. Another cause of surface distress is boundary lubrication. Relevant position and nature of the failure mechanisms are characterized. In the case of initial material stabilization, the time alterations of the XRD parameters correlate with the statistical parameter of the 10% bearing life. Contrary to the L10 equivalent value for sub-surface failures, i.e. classical rolling contact fatigue, which lead to spalling only a long time after incipient material softening, in the surface damage mode the L10 life roughly coincides with the beginning of the instability phase. Surface pitting or gray staining occrs frequently without distinct XRD indication of material aging. Here, scanning electron microscopy and electron microprobe analysis point to corrosion rolling contact fatigue as crack propagation mechanism. The interaction between material and lubricant opens up new research areas in the field of tribology.

X-Ray Diffraction in Failure Analysis of Rolling Bearings

By X-ray diffraction (XRD) measurements, material conditions of rolling bearings are red that point to a variety of load possibilities especially at raceway surfaces. Due to unambiguously distinguishable damage symptoms, according to H. Muro, it is differentiated between the surface and sub-surface failure mode in the literature. Surface distress of different intensity can be generated by particle-contaminated lubricants that result in raceway indentations. These micro-Hertzian contacts may lead to changes in residual stress and line broadening and in the microstructure. Another cause of surface distress is boundary lubrication. Relevant position and nature of the failure mechanisms are characterized. In case of initial material stabilization, the time alterations of the XRD parameters correlate with the statistical parameter of the 10 % bearing life. Contrary to the L10 value for sub-surface fatigue, which leads to spalling only a long time after incipient material softening, in the surface damage mode the L10 life roughly coincides with the beginning of the instability phase. Surface pitting or gray staining turns up frequently with low XRD indication of material aging. Here, scanning electron microscopy and electron microprobe analysis point to corrosion fatigue. The interaction between material and lubricant opens research in the field of tribology.

Evaluation Procedures of Nonmetallic Inclusions in Steel for Highly Reliable Bearings

Abstract The improvement in rolling contact fatigue life is a key subject to increase the reliability of bearings. It has been well known that the fatigue properties are significantly affected by nonmetallic inclusions in steel. The rolling contact fatigue life may be divided into two types, according to the type of reliability requirements. One is “L10 life,” which represents general bearing performance. The other is “accidental short life,” where a bearing prematurely fails in its service period and its calculated life is hardly met. Since the probabilities of these failures are significantly different, the distribution densities of the corresponding life-limiting inclusions are also expected to be widely different. When nonmetallic inclusions are evaluated as an index of fatigue life, therefore, testing volume should be appropriately determined according to the concerned type of life. From this point of view, it is effective for the reliability assessment of bearing steel to combine the evaluations of large microscopic inclusions by statistics of extreme value and macroscopic inclusions in larger volume by a high-frequency ultrasonic test. It was found that the L10 life determined by thrust-type rolling-contact fatigue tests was well correlated with maximum micro-inclusion diameter predicted by the statistics of extreme value, where both oxide and sulfide played as life-limiting inclusions. Fifteen MHz ultrasonic testing was employed to evaluate the macroscopic inclusions. The combination of these two procedures was able to demonstrate the difference in reliability clearly, even when other cleanliness indices could not characterize the concerned materials.

Effect of Rolling Bearing Refurbishment and Restoration on Bearing Life and Reliability

For nearly four decades it has been a practice in commercial and military aircraft application that rolling-element bearings removed at maintenance or overhaul be reworked and returned to service. The work presented extends previously reported bearing life analysis to consider the depth (Z45) to maximum shear stress (τ45) on stressed volume removal and the effect of replacing the rolling elements with a new set. A simple algebraic relationship was established to determine the L10 life of bearing races subject to bearing rework. Depending on the extent of rework and based on theoretical analysis, representative life factors for bearings subject to rework ranged from 0.87 to 0.99% of the lives of new bearings. Based on bearing endurance data, 92% of the bearing sets that would be subject to rework would result in L10 lives equaling and/or exceeding that predicted for new bearings, with the remaining 8% having the potential to achieve the analytically predicted life of new bearings when one of the rings is replaced at rework. The potential savings from bearing rework varies from 53 to 82% that of new bearings depending on the cost, size, and complexity of the bearing.

Bearings for IEEE 841 motors

As the IEEE 841 specification becomes widely used outside of the petrochemical industry, more review is required to assure the intent is understood. A look at competitive literature in the context of IEEE 841, finds many different terms to describe bearing life. Terms like 'Bearing Life', B10, L10 life, and L10a. Belted life claims vary from 26280 hours to 70000 hours for the same bearing, in motors with very similar geometry. Clearly there are questions in the industry about the evaluation of IEEE 841 bearing systems. This paper reviews the application issues surrounding bearings in IEEE 841 motors.

An Engineering Approach to Confidence Intervals and Endurance Test Strategies

Confidence intervals defined by their lower and upper bounds have been define for Weibull slope and L10 life using Monte Carlo simulations. Miscellaneous testing Strategies have been explored, namely failing all N specimens or failing only the first specimen out of N (the sudden death approach), this being repeated NR times using NR groups of N specimens. Statistics about the median duration time of the endurance test are also reported, showing that the sudden death approach is particularly attractive. Confidence intervals for the L10 can be significantly improved (ratio of two or more) by making use of a 'trick' that assumes that the Weibull slope is known form many previous tests, and to use it. Recommendations are given for using a (N = 8, NR = 3) strategy instead of (N = 4, NR = 6), in order to reduce the testing time by two without affecting too much the accuracy on L10 (if use id made of the trick). If the slope needs to be measured, duplicating the (N = 8, NR = 3) tests allows analysis of the results as a (N = 8, NR = 6) case which provides the same testing time and same accuracy of the Weibull slope as in a (N = 4, NR = 6) case, but with a narrower confidence interval on L10. The zero failure strategy has also been shown to be the one that provides the shortest testing time for checking that the tested specimen life exceeds an expected life within a given probability. The inconvenience of such an approach is that the β and η values are not evaluated. Presented at the 57th Annual Meeting in Houston, Texas May 19–23, 2002

Determination of Rolling-Element Fatigue Life From Computer Generated Bearing Tests

Two types of rolling-element bearings representing radial loaded and thrust loaded bearings were used for this study. Three hundred and forty (340) virtual bearing sets totaling 31400 bearings were randomly assembled and tested by Monte Carlo (random) number generation. The Monte Carlo results were compared with endurance data from 51 bearing sets comprising 5321 bearings. A simple algebraic relation was established for the upper and lower L10 life limits as function of the number of bearings failed for any bearing geometry. There is a fifty percent (50%) probability that the resultant bearing life ill be less than the calculated. The maximum and minimum variation between the bearing results life and the calculated life correlate with the 90-percent confidence limits for a Weibull slope of 1. 5. The calculated lives for bearings using a load-life exponent p of 4 for ball bearings and 5 for roller bearings correlated with the Monte Carlo generated bearings lives and the bearing data. STLE life factors for bearing steel and processing provide a reasonable accounting for differences between bearing life data and calculated life. Variations in Weibull slope from the Monte Carlo testing and bearing data correlated. There was excellent agreement between percent of individual components failed from Monte Carlo simulation and that predicted. Presented at the 58th Annual Meeting in New York City April 28–May 1, 2003

REVIEW OF FAN LIFE EVALUATION PROCEDURES

Improving the reliability of air cooled electronic equipment must include focusing on the life expectancy of the fans or blowers. Evaluating fan failure behavior, however, is not a trivial problem, as vendors report very little information on this subject. Even when product literature provides such data, one vendor’s results are generally impossible to compare with another’s due to different test procedures, different assumptions, and different calculation methods, not all of which are explicitly defined. This paper is designed to help thermal and component evaluation engineers who have been assigned the task of sorting through these sometimes conflicting, often incompatible, claims regarding fan quality. We start with a definition of fan failure in terms of rotational speed, running current, and acoustic noise. Some basic statistical principles, such as Weibull hazard rate, Mean Time To Failure (MTTF) and L10 life, are presented in the reliability section, leading into methods for the estimation of fan life. Booser’s equation for grease life is included and its limitations for modern greases are noted; the standard equation for bearing rating life is also covered. To verify calculated fan life estimates, a designed experiment may be performed. A basic formula for the design of a fan life experiment is given, together with a brief analysis of published failure data from such an experiment. Most fan life tests are conducted under stress conditions, such as high temperature, that accelerate fan failures. The final section discusses accelerated life testing, including potential pitfalls. Simulating a mini-textbook, the paper contains valuable everyday reference material, including sample calculations to illustrate the concepts reviewed.

浸炭鋼の疲れ特性に及ぼすＳｉ，Ｖの影響

Influence of silicon and vanadium contents on fatigue properties of carburized steels have been investigated. The rolling con;tact fatigue strength and rotating bending fatigue strength increase with addition of silicon and vanadium. The L10 life of 1.5%Si-0.3%V steel is five times longer than that of SCr420 steel. The microstructural changes induced by rolling contact load have been observed below the raceway surface of the tested specimens by the optical microscopy. These microstructural changes are affected by silicon and vanadium contents and for that reason the rolling contact fatigue life increases with an increase of silicon and vanadium content.

Rolling Contact Fatigue and Sliding Wear Performance of Ferritic Nitrocarburized M-50 Steel

The rolling contact fatigue and sliding wear performance of two types of ferritic nitrocarburizing techniques (gaseous and fluidized bed) were evaluated on AISI M-50 steel. These processes produced a thin compound layer over a nitrogen-rich diffusion zone. It was observed that compound layer thicknesses less than 1 μm increased the rolling contact fatigue (RCF) L10 life up to a factor of six over the untreated M-50 steel. Compound layers greater than 2 μm had a tendency to flake off the substrate near the spalled region, producing lower L10 lives than the baseline M-50 steel. Sliding wear tests on ferritic nitrocarburized M-50 showed that the compound layer exhibited slightly larger friction coefficients than the nitrogen-rich diffusion zone, although this layer had slightly greater resistance to catastrophic wear at higher contact stresses. Presented at the 46th Annual Meeting in Montreal, Quebec, Canada April 29–May 2, 1991

Overview of Condition Monitoring Methods with Emphasis on Industrial Fans

In any industrial fan, the rotating elements are located within some form of bearing arrangement. Defects, such as unbalance, will lead to an increase in vibration which is transmitted via these elements to the fan supports. As the bearings wear or as impeller balance deteriorates due to the effects of erosion, corrosion or dust build-up, there will be a corresponding increase in vibration magnitude. The rate of change of amplitude is a useful indication of the fan's ‘health’. The paper considers the fundamental equation of vibration and the disturbing forces involved. It is shown that a baseline fan signature is necessary if maximum benefits are to be achieved by any fan health programme. Ultimately, well-designed and installed bearings will fail due to fatigue. The concept of L10 life is also introduced and some of the methods for predicting the onset of failure are described.