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

Rheological and Wetting Properties of Environmentally Acceptable Lubricants (EALs) for Application in Stern Tube Seals

The use of Environmentally Acceptable Lubricants (EALs) for stern tube lubrication is increasing. Although the machine components of a sailing …

The use of Environmentally Acceptable Lubricants (EALs) for stern tube lubrication is increasing. Although the machine components of a sailing vessel are designed to operate together with mineral oil-based lubricants, these are being replaced by the less environmentally harmful EALs. Little is known about the rheological performance of EALs in particular at the high shear rates that occur in stern tube seals. In this study, the viscosity and wetting properties of a set of different EALs is analysed and compared to traditional mineral oil-based lubricants using a set of experimental techniques. Some of the EALs present Newtonian behavior whereas other show shear thinning. No significant difference in surface tension was observed between the different lubricants.

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Paper

Polymer-Thickened Oil Rheology When There is No Second Newtonian

The recent development of quantitative elastohydrodynamics makes the accurate description of the temperature, pressure, and shear dependence of viscosity extremely …

The recent development of quantitative elastohydrodynamics makes the accurate description of the temperature, pressure, and shear dependence of viscosity extremely important. It has been customary for tribologists to expect a second Newtonian plateau to appear in any flow curve for a polymer-blended lubricant and, since viscometers at ambient pressure cannot reach such a plateau, procedures have been suggested to extrapolate to a second Newtonian from commercial high-shear viscometer data. Two examples of oils, characterized in pressurized thin-film Couette viscometers, are presented for which there is no second Newtonian. Extrapolation from ambient-pressure high-shear viscometer data, by fixing the second Newtonian viscosity at the viscosity of the base oil, is not useful. Apparently, the second Newtonian will not appear when the base oil begins to shear thin at the shear stress for which the second Newtonian inflection might appear.

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Paper

Capillary Rheometer for Magnetic Fluids

Magnetic fluids have been around since the 1940’s. They come in different forms: Magnetorheological fluids (MR fluids) and Ferrofluids. MR …

Magnetic fluids have been around since the 1940’s. They come in different forms: Magnetorheological fluids (MR fluids) and Ferrofluids. MR fluids characterise themselves by having a large change in viscosity under the influence of a magnetic field. Ferrofluids have a significantly smaller change in viscosity however ferrofluids are colloidal suspensions. After their discovery many applications followed, such as the MR clutch, magnetic damper and bearing applications, in which the fluids are subjected to ultra high shear rates. Little knowledge is available on what happens to the rheological properties under these conditions. In general, the characteristics determined at lower shear rates are extrapolated and used to design new devices. Magnetic fluids have potential in the high tech and high precision applications and their properties need to be known in particular at shear rates around 106 s −1 . Commercially available magnetorheometers are not able to measure these fluids at ultra high shear rates and are limited to 105 s −1 . Therefore a new magnetorheometer is required to measure ultra high shear rates. In this paper the physical limitations of current measuring principles are analysed and a concept is designed for ultra high shear rate rheometry in combination with a magnetic field. A prototype is fabricated and the techniques used are described. The prototype is tested and compared to a state of the art commercial rheometer. The test results of the prototype rheometer for magnetic fluids show its capability to measure fluids to a range of 104 s −1 to 1.16 × 106 s −1 and the capability to measure the magnetorheological effect of magnetic fluids.

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Paper

In-situ Observations of the Effect of the ZDDP Tribofilm Growth on Micropitting

The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical …

The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical systems, means that machine components are required to operate for longer periods under thin film, mixed lubrication conditions where the risk of surface damage is increased. For this reason, the role of zinc dialkyldithiophosphate (ZDDP) antiwear lubricant additive has become increasingly important in order to provide adequate surface protection. It is known that due to its exceptional effectiveness in reducing surface wear, ZDDP may promote micropitting by preventing adequate running-in of the contacting surfaces. However, the relationship between ZDDP tribofilm growth rate and the evolution of micropitting has not been directly demonstrated. To address this, we report the development of a novel technique using MTM-SLIM to obtain micropitting and observe ZDDP tribofilm growth in parallel throughout a test. This is then applied to investigate the effect of ZDDP concentration and type on micropitting. It is found that oils with higher ZDDP concentrations produce more micropitting but less surface wear and that, at a given concentration, a mixed primary-secondary ZDDP results in more severe micropitting than a primary ZDDP. Too rapid formation of a thick antiwear tribofilm early in the test serves to prevent adequate running-in of sliding parts, which subsequently leads to higher asperity stresses and more asperity stress cycles and consequently more micropitting. Therefore, any adverse effects of ZDDP on micropitting and surface fatigue in general are mechanical in nature and can be accounted for through ZDDP's influence on running-in and resulting asperity stress history. The observed correlation between antiwear film formation rate and micropitting should help in the design of oil formulations that extend component lifetime by controlling both wear and micropitting damage.

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Paper

Prediction of Micropitting Damage in Gear Teeth Contacts Considering the Concurrent Effects of Surface Fatigue and Mild Wear

The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts …

The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing subsequent micropitting, followed by zero or negligible wear for the rest of the gear pair life. The complexities of the interaction between the contact conditions, wear and surface fatigue, as evident in the present results, mean that a full treatment of gear micropitting requires a numerical model along the lines of that applied here, and that use of overly simplified criteria may lead to misleading predictions.

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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

Method For Forming Pitting Resistant Carbon Coating

A hydrogenated diamond-like coating (“H-DLC”) for metallic substrates provides improved reliability. The H-DLC is relatively soft and elastic. Unlike hard …

A hydrogenated diamond-like coating (“H-DLC”) for metallic substrates provides improved reliability. The H-DLC is relatively soft and elastic. Unlike hard and/or inelastic coatings in the prior art, the present coatings do not exhibit a loss of adhesion (delamination). A bonding layer may be used between the metallic substrate and the H-DLC. H-DLC coatings can, for example, be used in bearings and gears to reduce the occurrence of micropits and, ultimately, product failure.

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Paper

Effect of Retained Austenite on Spalling Behavior of Carburized AISI 8620 Steel Under Boundary Lubrication

The objective of this study is to investigate the effect of retained austenite (RA) on the macropitting or spalling behavior …

The objective of this study is to investigate the effect of retained austenite (RA) on the macropitting or spalling 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. Macropitting tests were carried out at maximum contact pressure 1.9 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 contact surface and RA respectively. Results indicated that increasing RA and minimizing pre-existing sub-surface cracks in the material help to enhance macropitting life. The failure mechanism for all samples was from spall formation due to sub-surface crack initiation and propagation with clear evidence of RA transforming to martensite. Pre- and post-experiment residual stress showed the impact of phase transformation on enhancement of residual stress during RCF experiments, however residual stress was not a significant factor in impacting the life.

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