This paper describes a study of EHD film thickness in non-steady state contact conditions. A modification of ultrathin film interferometry is employed which is able to measure both central film thickness and film thickness profiles 50 times a second. Film thickness with two perfluoropolyethers and two mineral base oils are investigated in a number of different types of non-steady state motion, including acceleration/deceleration, stop/start and reciprocation. The results demonstrate a range of transient behaviors of EHD film whose thicknesses deviate from those in steady state conditions.

EHD Film Thickness in Non-Steady State Contacts

This paper presents an automatic system for the real-time evaluation of elastohydrodynamic (EHD) film thickness and shape that incorporates an advanced experimental apparatus controlled by a computer with an extensive interferogram processing software. This software coordinates the data acquisition and instrument control and also provides real-time data processing and displaying. Film thickness evaluation technique based on the colorimetric interferometry successfully overcomes limitations of conventional optical interferometry. Colorimetric interferometry combining conventional chromatic interferometry with image processing and differential colorimetry gives instantaneous detailed information on EHD film thickness and shape for various regimes. This approach makes it possible to determine film thickness in the range of 1 to 800 nm with a resolution of about 1 nm. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, Octob...

An Automatic System for Real-Time Evaluation of EHD Film Thickness and Shape Based on the Colorimetric Interferometry

This paper describes an experimental study of elastohydrodynamic (EHD) lubricating film formation during the start-up of motion of a point contact from rest. EHD film thickness was measured using ultra-thin optical interferometry. It was found that film thickness behaviour depends strongly upon acceleration. When motion starts, most lubricants form a front that travels across the contact with its initial thickness unchanged. At some point, depending upon the acceleration, a second front often forms, so that the lubricant film has a characteristic stepped profile. This behaviour occurs in both pure sliding and pure rolling conditions. The development of film profile over time can be used to chart the speed of motion of the fluid film in the contact. In pure rolling conditions, the first film front travels through the conjunction at a speed lower than the entrainment speed in the first half of the contact and higher in the second half. In pure sliding it travels at a velocity higher than the entrain...

Elastohydrodynamic film formation at the start-up of the motion

This paper describes a study of the behavior of elastohydrodynamic lubricated contacts subjected to rapid halting. Experiments have been carried out using ultrathin interferometry coupled to a high-speed camera to measure the change in lubricant film thickness and shape during fast, controlled deceleration, both in pure sliding and pure rolling conditions. Film collapse is seen to occur in two stages. The first persists throughout the deceleration period and, during this stage the film geometry remains almost constant across the contact. In this stage of film collapse, the film thickness lags behind the value predicted from steady-state theory, which means that when motion ceases, a thicker than expected film is present. The second stage of film collapse ensues when the entrainment speed falls below a critical value of approximately 0.002 m/s and is characterized by the formation of a central entrapment and classical, normal approach, squeeze behavior.

Elastohydrodynamic Film Collapse During Rapid Deceleration. Part I—Experimental Results

The mechanism of fluid film lubrication in ultra-thin conjunctions under counterformal concentrated circular point contacts is discussed in this paper. The significance of squeeze film action in increased load carrying capacity is highlighted. The numerical predictions made in acceleration-deceleration motion of a ball against a flat race is found to conform with the experimental findings of other research workers for polar branched lubricants down to a film thickness in the region of 20 nm. This conformance with the experimental findings has enabled a fundamental understanding of the multi-physics of film formation when conditions promoting the formation of ultra-thin films of the order of few nanometres have been employed, with the use of a non-polar lubricant. This paper provides the first ever solution of the combined effect of viscous, surface and molecular forces under transient conditions.

https://iopscience.iop.org/article/10.1088/0022-3727/34/17/307/pdf

D. Zhu

Papers

Experimental method for quantitative analysis of transient EHL