Infoscience

Thesis

Lubrification thermoélastohydrodynamique: une approche globale

The present work is part of a general research program conducted by the Machine-Tools and Automata Laboratory of the Federal Institute of Technology in Lausanne on "Synthetic Intelligence". The final goal is to automatize design using computers in mechanical engineering. More specifically, this thesis deals with one of the many steps in the design of high performance spindles and bearings for machine-tools, namely their thermomechanical behaviour. In our opinion, this subject has not yet been fully understood, and has led to unreliable products. This work mostly consists of a two-dimensional lubricated thermo-elasto-hydrodynamic contact study (that can be extended to three-dimensional cases) using the TACT FEM program which allows for solid/lubricant coupling. The objective is to take into account the power consumption mechanism in this type of contact. Analytical, numerical and experimental investigations are conducted in parallel. The analytical part deals with the isoviscous rigid regime. We prove that the dissipated power is exactly given, in pure rolling conditions, by the resistant torque due to the unsymmetric pressure distribution, and that the amount of sliding occurring at the two contacts of the rolling element of a bearing, is the one which leads to a minimum of dissipation. Otherwise, we can show that the power losses, in adiabatic conditions, strongly decrease with starvation, and that the film temperature is minimum when the regime is so-called "without reverse flow". The numerical part covers the discrete lubrication operator with its distinctive characteristics. New penalty techniques have been used in order to solve the free boundary situations at the inlet of the contact zone – variable oil quantity, and at the outlet – cavitation. Implementation of an upwind SU/PG scheme was necessary in ensuring solution stability for the heat equation. Various numerical cases show that, in an adiabatic hypothesis, the starvation conditions in the inlet zone, leading to a decreasing temperature of the film, is responsible for an increase of the TEHD power losses, even when the EHD power losses become smaller. This result raises the question of thermal modelisation and boundary conditions of the contacting bodies. In the experimental part, the approach consists of measurements of the power consumption of EPFL-developed bearings and spindles, and comparisons with calculations. Experimental and numerical values are in good agreement. An apparatus for measuring viscosity of usual lubricants at high pressures and different temperatures is presented. The results differ from that of the manufacturer by about 15% on viscosity values. The thermo- and piezo- viscous coefficients are determined with good accuracy but are respectively 10% and 43% less than what we initially estimated. The measurements show that the change-of-phase of oil occurs at very low pressures (about 1'700 bar) and at room temperature (21 °C).

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