Knowledge

Welcome to our knowledge centre. Here you can find a selection of resources and articles on our products and industries we are involved with.

Paper

The Effect of Lubricant Composition on White Etching Crack Failures

White etching cracks (WECs) are the dominant mode of failure for wind turbine gearbox bearings. These failures are characterized by …

White etching cracks (WECs) are the dominant mode of failure for wind turbine gearbox bearings. These failures are characterized by subsurface initiation and local region of microstructural alterations adjacent to the crack faces. The definitive cause of WECs within the field is unknown, because of this laboratory replication has proved difficult. At a benchtop scale, specific lubricant formulations referred to as “bad reference oils” (BROs) are often employed to aid in the formation of WECs; however, exactly how these lubricants induce WECs is unknown. The present work intends to elucidate how these lubricants facilitate the formation of WECs by systematically varying the additives which are found in BROs and studying the effect that these additive combinations have on time until failure, as well as tribofilm development. It was found that the lubricant containing Zinc dialkyldithiophosphate alone led to the formation of WECs sooner than any lubricant studied. It was also documented that a lubricants frictional characteristics play a more dominant role than the tribofilm characteristics.

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Paper

Effect of Retained Austenite on Micropitting Behavior of Carburized AISI 8620 Steel under Boundary Lubrication

The objective of this study was to investigate the effect of retained austenite (RA) on the micropitting behavior of carburized …

The objective of this study was to investigate the effect of retained austenite (RA) on the micropitting behavior of carburized AISI 8620 steel under boundary lubrication condition. Samples with RA ranging from approximately 0 to 70% were prepared using specific carburizing schemes. Rolling contact fatigue tests were carried out at maximum contact pressure 1.5 GPa using a benchtop test rig. Samples were subsequently analyzed using noncontact white light profilometry and micro X-Ray Diffraction to observe the evolution of micropitting and RA respectively. An increase in RA% resulted in increased micropitting life. The failure mechanism for the lowest RA samples was dominated by early crack initiation and rapid crack propagation, whereas samples with medium and high RA showed initiation and propagation of micropitting with clear evidence of RA transforming to martensite. Higher levels of RA% to ensure a stable amount remains after this transformation is desirable for extended micropitting life. An extended finite element method (XFEM) which included a Voronoi tessellation to randomly generate the steel microstructure was used to simulate the experiments, and showed that consistent with experimental findings, crack propagation was increasingly hindered in the case of microstructure with increasing RA%.

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Paper

A Review on Micropitting Studies of Steel Gears

With the mounting application of carburized or case-hardening gears and higher requirements of heavy-load, high-speed in mechanical systems such as …

With the mounting application of carburized or case-hardening gears and higher requirements of heavy-load, high-speed in mechanical systems such as wind turbines, helicopters, ships, etc., contact fatigue issues of gears are becoming more preponderant. Recently, significant improvements have been made on the gear manufacturing process to control subsurface-initiated failures, hence, gear surface-initiated damages, such as micropitting, should be given more attention. The diversity of the influence factors, including gear materials, surface topographies, lubrication properties, working conditions, etc., are necessary to be taken into account when analyzing gear micropitting behaviors. Although remarkable developments in micropitting studies have been achieved recently by many researchers and engineers on both theoretical and experimental fields, large amounts of investigations are yet to be further launched to thoroughly understand the micropitting mechanism. This work reviews recent relevant studies on the micropitting of steel gears, especially the competitive phenomenon that occurs among several contact fatigue failure modes when considering gear tooth surface wear evolution. Meanwhile, the corresponding recent research results about gear micropitting issues obtained by the authors are also displayed for more detailed explanations.

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Paper

The Impact of Steel Microstructure and Heat Treatment on the Formation of White Etching Cracks

White etching crack (WECs) associated macro-pitting is a dominant mode of damage within wind turbine gearbox bearings. This damage is …

White etching crack (WECs) associated macro-pitting is a dominant mode of damage within wind turbine gearbox bearings. This damage is characterized by broadly branching crack networks surrounded by local regions of microstructural alterations. The definitive cause of WECs in the field is unknown; therefore, the implementation of effective mitigation techniques has proven difficult. The current work utilized an accelerated benchtop testing technique in order to quantify differences in performance between through-hardened AISI 52100 steel and carburized AISI 3310 steel. It was found that the carburized AISI 3310 steel demonstrated a more than twofold increase in the time elapsed until WEC macro-pitting damage occurred compared to the through-hardened AISI 52100 steel samples.

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Paper

The Influence of Steel Microstructure and Inclusion Characteristics on the Formation of Premature Bearing Failures with Microstructural Alterations

The formation of premature cracking damage associated with local microstructural alterations, commonly referred to as “white etching cracks” (WECs) has …

The formation of premature cracking damage associated with local microstructural alterations, commonly referred to as “white etching cracks” (WECs) has led to unpredictable bearing failures in numerous industrial applications. WECs are known to occur in the field independent of material characteristics such as heat treatment, predominant microstructure and surface coatings, as well as lubricant characteristics such as additive packages and viscosity. However, WECs have proven difficult to recreate at a benchtop scale; to date, no benchtop test has been reported to recreate WECs using commercially available field lubricants. The present work intends to identify key differences between the microstructure of field bearings and the microstructure of benchtop samples, focusing on the characteristics of the inclusions contained in both steels. Synchrotron scale X-ray tomography revealed that AISI 52100 steel used in industrial scale field bearings contained a drastically different inclusion microstructure then the AISI 52100 steels used in standard benchtop test specimens. Additionally, the authors were able to form WECs in test samples manufactured out of the steel of a field bearing using fully formulated field lubricants; a finding which has not yet been reported in open literature.

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Paper

Effect of Retained Austenite on White Etching Crack Behavior of Carburized AISI 8620 Steel under Boundary Lubrication

The formation of white etching cracks (WECs) is a dominant failure mode in wind turbine gearbox bearings that can significantly …

The formation of white etching cracks (WECs) is a dominant failure mode in wind turbine gearbox bearings that can significantly shorten their operating life. Although the phenomenon of WECs has been communicated in the field for more than a decade, the driving mechanisms are still debated, and the impact of proposed mitigation techniques is not quantified. Leading hypotheses to inhibit the formation of WECs center on material solutions, including the use of steel with high levels of retained austenite (RA). The present work aims to explore the impact of RA on the formation of WECs within AISI 8620 steel under boundary lubrication. A three ring-on-roller benchtop test rig was used to replicate WECs in samples with different levels of RA. While varying levels of RA had a minimal effect on time until failure, a significant effect on crack morphology was observed. Additionally, potential underlying mechanisms of White Etching Area formation were elucidated. Under the current test conditions, the microstructural alterations adjacent to the cracks in the lower RA samples were more developed compared to those of the higher RA samples. Additionally, the WEC networks in the high RA samples contained significantly more crack branches than those of the low RA samples.

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Paper

Investigating the Micropitting and Wear Performance of Copper Oxide and Tungsten Carbide Nanofluids under Boundary Lubrication

The potential of copper oxide (CuO) and tungsten carbide (WC) nanofluids in enhancing micropitting and wear behavior of AISI 8620 …

The potential of copper oxide (CuO) and tungsten carbide (WC) nanofluids in enhancing micropitting and wear behavior of AISI 8620 steel under boundary lubrication conditions was investigated. The nanofluids consisted of 1% nanoparticles by weight and 1% by weight of oleic acid surfactant in Polyalphaolefin (PAO). Rolling contact fatigue tests were conducted using a micropitting test rig (MPR). Both the nanofluids exhibited increased micropitting life compared to the base oil. Tungsten carbide nanofluids showed significantly higher micropitting and wear resistance behavior than the CuO nanofluids under the boundary lubrication regime. Analysis of the surfaces showed different mechanisms to inhibit micropitting and wear for the two nanofluids. The WC nanofluid formed a tribofilm whereas the CuO nanofluids tended to fill surface cracks with the nanoparticles.

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Paper

The Influence of Slide–Roll Ratio on the Extent of Micropitting Damage in Rolling–Sliding Contacts Pertinent to Gear Applications

Micropitting is a type of surface damage that occurs in rolling–sliding contacts operating under thin oil film, mixed lubrication conditions, …

Micropitting is a type of surface damage that occurs in rolling–sliding contacts operating under thin oil film, mixed lubrication conditions, such as those formed between meshing gear teeth. Like the more widely studied pitting damage, micropitting is caused by the general mechanism of rolling contact fatigue but, in contrast to pitting, it manifests itself through the formation of micropits on the local, roughness asperity level. Despite the fact that micropitting is increasingly becoming a major mode of gear failure, the relevant mechanisms are poorly understood and there are currently no established design criteria to assess the risk of micropitting occurrence in gears or other applications. This paper provides new understanding of the tribological mechanisms that drive the occurrence of micropitting damage and serves to inform the ongoing discussions on suitable design criteria in relation to the influence of contact slide–roll ratio (SRR) on micropitting. A triple-disc rolling contact fatigue rig is used to experimentally study the influence of the magnitude and direction of SRR on the progression of micropitting damage in samples made of case-carburised gear steel. The test conditions are closely controlled to isolate the influence of the variable of interest. In particular, any variation in bulk heating at different SRRs is eliminated so that tests are conducted at the same film thickness for all SRRs. The results show that increasing the magnitude of SRR increases the level of micropitting damage and that negative SRRs (i.e. the component where damage is being accumulated is slower) produce more micropitting than the equivalent positive SRRs. Measurements of elastohydrodynamic film thickness show that in the absence of bulk heating, increasing SRR does not cause a reduction in EHL film thickness and therefore this cannot be the reason for the increased micropitting at higher SRRs. Instead, we show that the main mechanism by which increase in SRR promotes micropitting is by increasing the number of micro-contact stress cycles experienced by roughness asperities during their passage through the rolling–sliding contact. Therefore, the asperity stress history should form the basis of any potential design criterion against micropitting.

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Paper

Film Formation and Friction in Grease Lubricated Rolling-Sliding Non-conformal Contacts

This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact …

This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact temperatures. The effects of grease composition are assessed by employing custom made, additive free, lithium and diurea thickened greases, whose composition is systematically varied so that the effects of the thickener and the base oil can be isolated. All film thickness and friction measurements were conducted under fully-flooded conditions. It was found that at low speeds all tested greases are able to form thicker films than the corresponding base oils. The thickness and behaviour of these films is determined by the thickener type and is independent of the base oil viscosity and the test temperature. At higher speeds, the film thickness is governed by the base oil properties alone and can be predicted by the EHD theory. At low speeds, films with diurea greases grow with time under constant speed and residual films persist under load after contact motion ceases. The real lambda ratio, based on measured grease film thickness, was shown to correlate well with contact friction. The transition from the thickener dominated behaviour to that dominated by the base oil occurs at a relatively constant film thickness, regardless of the base oil viscosity and test temperatures, rather than at a given entrainment speed. Based on the presented evidence, it is here proposed that the mechanism of formation of grease films at low speeds, is analogous to that reported to operate in EHL contacts lubricated with colloidal dispersions, namely the mechanical entrapment and deposition of thickener fibres, and that, rather than the widely quoted ‘transition speed’, it is the ratio of the thickener fibre size to prevailing film thickness that determines the range of conditions under which the film enhancement due to the action of thickener is present.

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