P. L. Wong

Tribological performance of a magneto-rheological suspension

Abstract The boundary lubrication performance of typical electro and magneto-rheological fluids are compared for friction and wear. Given that the MR species seems to have advantages over the ER variety, the tribological mechanism of the former is investigated from the point of interest of providing a basis from which to reach the optimisation potential of the fluid. The investigation involves changing the iron particle loading of the suspensions, measuring the roughness of the surfaces and includes supporting SEM photographs, all from tests using a block on ring apparatus. From these data a model for the tribological scenario is elucidated, albeit for low MR particle concentrations.

A new optical technique for roughness measurement on moving surface

A new optical technique which allows the roughness of moving surfaces to be determined was developed. The new technique which is called the dark/bright ratio (DBR) method utilizes the combined effects of speckle and scattering phenomena. The roughness of surfaces is inferred from the dimensions of the recorded dark or bright area in the speckle pattern. Although it is a relative method, it has great potential to be used for in-process measurement and automation owing to the simplicity of both its principle and required optical set-up. The new technique has also been proved to have large measuring range and with high precision. The principle of this technique and the set-up of the measuring system are described. Experimental results for both static and dynamic conditions, which were compared to those obtained using the traditional stylus technique, were found to be in good agreement. The reliability of the new technique in obtaining roughness data of surfaces under various speed conditions (from 0 to 0.017 m/s) was validated.

The Effect of the Electric Double Layer on a Very Thin Water Lubricating Film

This paper discusses the effect of the electric double layer on a very thin water lubricating film with and without consideration of the elastic deformation of the opposing surfaces. A modified Reynolds equation that considers the electric double layer is used in a numerical analysis. The effect of zeta potential on the film thickness and pressure is numerically calculated. For both hydrodynamic and elastohydrodynamic cases, the electric double layer significantly increases the lubricating film thickness. The pressure is also marginally increased, as illustrated in the hydrodynamic analysis. However, the effect on pressure is almost unnoticeable in the elastohydrodynamic analysis. Overall, the electric-double-layer effect is only significant for a water-film thickness of less than approximately 100 nanometers.

An anomalous elastohydrodynamic lubrication film: Inlet dimple

This paper presents a deliberately designed elastohydrodynamical lubrication (EHL) experiment for the study of the individual effect of the limiting shear stress and wall slippage. Very slow entrainment speeds were employed to avoid influential shear heating and oils of high viscosities were chosen to ensure that the conjunction was under typical EHL. An anomalous EHL film, characterized by a dimple at the inlet region, was obtained. Literature revealed that this inlet dimple was reported in some numerical studies taking into consideration the limiting-shear-stress characteristics of the lubricant and wall slippage. It was found that even under the same kinematic conditions, different types of film shape would be generated by simple disc sliding and simple ball sliding. Simple disc sliding produces an inlet dimple with a comparatively thick inlet film thickness, which droops rapidly toward the outlet region. For simple ball sliding, there is also an inlet dimple but the central film thickness is rather uniform. However, by prerunning the conjunction at a zero entrainment velocity (at the same linear speeds but in opposite directions) before the sliding experiment, the slope of the central film of simple disc sliding becomes smaller. It is probably due to the modification of solid-liquid interface, i.e., the slippage level, by the highly pressurized and stressed prerunning conditions. With a prescribed prerunning, which can produce very similar films at simple disc sliding and simple ball sliding, variation of film thickness was studied and it was found that the inlet dimple film has obvious dependence on entrainment speeds, but was not sensitive to loads. The present experimental results can be considered as direct evidence for those numerical findings of the inlet dimple. Tentatively, an effective viscosity wedge is proposed to account for the formation of the inlet dimple. @DOI: 10.1115/1.1866165#

Experimental study of the real time change in surface roughness during running-in for PEHL contacts

An optical system for the implementation of a new real time roughness measuring technique was incorporated to a two-disk machine. The change in surface roughness during the running-in stage of partial elastohydrodynamic lubricated wear tests was measured in a real time mode. The results were compared with the experimental data, which were measured, in a conventional manner, at discrete intervals of time by stopping the wear test. The apparent discrepancies reveal that the wear is enhanced by the stop/start actions. The effects of different initial roughness, sliding/rolling ratios and loading on the change in surface roughness during lubricated running-in tests were also studied with the current set up.

Experimental observation of a dimple-wedge elastohydrodynamic lubricating film

Abstract One of the main features of typical elastohydrodynamic lubricating (EHL) contacts is the unique horseshoe film shape, which can be readily observed by using interferometry and quite accurately modelled by the well-established EHL theory. However, an anomalous EHL film, characterized by a wedge shape together with a tiny dimple at the inlet region, is observed under pure sliding conditions with ultra slow speeds of 3–800 μm/s in an optical EHL test rig. The variations of the wedge and the inlet dimple with different sliding speeds and loads are investigated using a series of polybutene oils of high viscosities. It is found that the inclination of the wedge is dependent on sliding speeds, loads and oil viscosities. The dimple always occurs at the inlet. The appearance of an inlet dimple together with a wedge film shape is reported for the first time. The phenomenon can be attributed to a non-Newtonian characteristic of the lubricant: the limiting shear strength. Additionally, the influence of starvation on the film shape is also examined.

A Wide Range Measuring System for Thin Lubricating Film: From Nano to Micro Thickness

This paper introduces a newly developed lubricating-film-thickness measuring system, which implements the multi-beam intensity-based (MBI) scheme proposed recently by the authors. Some details about the software of the testing system and instrumentation of the MBI approach are discussed. For efficient determination of the fringe order range of measured points, a simple counting strategy was suggested. It is shown that this measuring system can provide a measurement range from nano to micrometers and a high resolution on the nanometer level. Besides being able to detect ultra-thin lubricating film thickness, the system can also measure tiny local variations in film thickness on a nano-scale in the conventional EHL regime. The capability of the system is demonstrated by the determination of film thickness in the range of 1 nm to 2.542 μm with a standard deviation of 0.89 nm. In addition, some analyses are given for further understanding the optical EHL and the testing system in this paper.

Pressure and temperature dependence of the density of liquid lubricants

Abstract A theoretical model describing the influence of temperature T and pressure p on the density of liquid lubricants is described. It is based on the molecular interaction theory expressed in the van der Waals equation of state. Theoretical results for two different kinds of liquid lubricants were obtained. They correlate well with data measured by different experimental techniques. Characteristics of the p  T density curves including glass transition and flash point are correctly predicted by the model. The model contains three coefficients which can be deduced from low pressure and low temperature data that can easily be measured by conventional techniques. The model is not only valid under low pressure and temperature conditions but it also corresponds accurately with high pressure and high temperature results. The paper presents two cases for pressures up to 1.2 GPa and for temperatures up to and higher than the flash points of the lubricants. Further, it is shown that the model can be used for both mineral oils and synthetic oils.

Feasibility Study on the Storage of Magnetorheological Fluid Using Metal Foams

This article presents an original idea: the use of metal foams to store unexcited magnetorheological fluid (MRF) in such a way such that the fluid mixture can be propelled when excited. This work is motivated by the desire to overcome the need for costly dynamic seals for conventional MRF dampers. The results of metal foam characteristics, possible changes in the content of propelled MRF, and the responses of soaked MRF when excited are presented. A plate-on-plate test rig was developed to investigate the performance of the MRF-soaked metal foam set-up. The shearing performance, response time, gap effects and the influence of different metal foams were investigated experimentally. The results show that the MRF can be magnetically propelled into the gap between the plate and the metal foam surface, and shearing resistance is produced on the rotary plate. Shearing performance is sensitive to the volume of the propelled MRF, gap clearance, and the strength of the external magnetic field. The response of the proposed MRF-soaked metal foam set-up is slower than that of the conventional MRF-filled two-plate configuration simply because it takes time to propel the MRF. This primary investigation shows that MRF-soaked metal foams can produce MR effects.

Wear volume determination during running-in for PEHL contacts

Abstract The relation of wear volume and the change of average surface roughness under the “zero-wear” condition was derived, with the assumption that the original profiles of the surface below the wear plane remain exactly the same as before, i.e. no plastic deformation. The flattening of asperities on an engineering rough surface was simulated with numerical techniques. The variation in wear volume and average surface roughness with the depth of wear was studied. The pattern and the correlation length of rough surface were checked and found to have no effect on the relation of wear volume and change of average roughness. The simulated results show that the variation of wear volume and the change of average roughness can be described by a second order polynomial. The model was also validated with experimental results obtained by using a two-disc wear machine.