Fabrice Ville

Prediction of power losses due to tooth friction in gears

Sliding friction between gear teeth is recognized as one of the main sources of power loss in geared transmissions as well as a potential source of vibration and noise. Its accurate modeling is therefore of primary importance in efficiency and vibration analyses of mechanical transmissions. For lubricated contacts, various empirical friction laws based on results from simulators can be found in the literature. One of their limitations comes from the specimen surface texture, which is often different to that of actual gears. Moreover, most of these models were established for high slide-to-roll ratios and cannot be used for low slide-to-roll ratios as encountered near the pitch point in gears. In this article, friction measurements were derived from an EHL simulator with contact conditions and surface finish close to those in gears and which covers a wide range of sliding/rolling conditions. A new traction law is proposed and integrated in a three-dimensional dynamic model of gears with consideration of tooth friction. The numerical results are then compared with the experimental evidence from a gear test rig. It is observed that tooth friction can strongly affect dynamic transmissibility through bearing mounts. Finally, the need of an accurate friction model for reliable power loss predictions is stressed. Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Reviewed led by Liming Chang

Early fatigue failure due to dents in EHL contacts

An experimental and numerical study of early fatigue failure due to dents in EHL contacts is presented. Tests are conducted on a two-disk machine with different operating conditions. Disks are made of AISI 52100 bearing steel. The surface of one disk is smooth. The other one is initially indented by M50 steel powder using the procedure presented in previous papers (1), (2). The oil is a synthetic one qualified under the MIL-L-23699 specification. The lubricant is cleaned during tests by using a β3μm = 200 cleaning filter. An optical microscope and a profilometer are used to describe the dent evolution, as the tests are stopped at regular intervals. The test bench is described and the experimental procedure is presented. The role of sliding direction is underlined. Experimental results are finally compared with numerical calculations. It is shown that the spall starts ahead of the dent in the direction in which the surface travels through the contact when the dent is on the follower surface, it starts behi...

An abacus for predicting the rolling contact fatigue life reduction due to debris dents

Nowadays, Rolling Contact Fatigue (RCF) is essentially caused by surface originated failures. This failure mode is generally linked to oil contamination. In fact, as the filters are no absolute barriers, particles can pass through the contact and create dents. In this work, previous studies on the pressure perturbation due to a dent are complemented with an analysis of the elastic stress field. The magnitude and location of the maximum shear stress is first analyzed. The simple relations obtained between the pressure peak and the maximum shear stress lead to an endurance limit criterion. Completed with a stressed volume analysis, a damage risk is defined. Finally, a damage risk abacus as a function of dent geometry is presented.

Numerical and experimental investigations on rolling contact fatigue for dented surfaces

This paper is divided into two parts. An experimental part dealing with the indentation of surfaces by ductile of dented surfaces. It is shown that during the first cycles, the dent geometry is strongly modified and that there is a preferential damage location. Next, a numerical part allows to explain qualitatively these observations. A 2D dry contact model is used to estimate the 2D pressure distribution using measured dent topographies. In parallel, the use of a 1D EHL code allows to illustrate hydrodynamics effects occuring in Rolling Contact Fatigue.

Experimental investigations on rolling contact fatigue of dented surfaces using artifical, defects: subsurface analyses

It is no longer necessary to demonstrate the influence of oil contamination on Rolling Contact Fatigue (RCF). Previous studies have shown the consequences induced by the travel of solid particles inside the contacts. In fact, when particles enter the contact, they indent the surfaces. The resulting surface defects, called dents, become stress raisers and increase the risk of spalling and consequently of RCF. In order to compare numerical and experimental results, some tests were performed on a two disk machine with artificially dented surfaces. Cross-sections were performed to complete the surface observations, and to analyse the crack initiation sites and the crack propagation directions. This paper proposes an analysis of these cross-sections to identify the cracks that lead to micro and macro spalling.

Rolling contact fatigue crack propagation in nitrided alloyed steels

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.