Analysis of the Thermo-Viscous Effect on Friction and Energy Dissipation in Oil Lubricated Interfaces

The last decade an increasing amount of work are performed with the aim of enhancing efficiency and reliability of fluid power components. Consequently, the loss mechanisms of lubricated tribological interfaces are of particular interest. This has led to development of sophisticated three-dimensional thermo-elastohydrodynamic lubrication models of fluid power components. The computational efforts involved in simulation with such models entail that design optimization are to some extend impractical. However, such models are also pursued in theoretical tribology with the aim to study loss and wear mechanisms, which is very difficult to study experimentally. In consequence, advanced numerical models are the state of the art approach in theoretical fluid power tribology research. However, a downside of modern numerical models is the inability to provide a practical tool for wide-scale parameter sweep investigations, due to computational effort, whereby analytical research in loss mechanisms still have certain advantages. In this paper, the thermo-viscous effect of a lubricant is included in an analytical study of the friction and energy dissipation of oil hydraulic thin-films. This analytical study is based on an asymptotic approximation of the laminar lubrication thermal field at low reduced Peclet and Brinkman number, where viscosity is included as a function of temperature. The asymptotic series is truncated at first order and used to derive an expression of the viscous friction on a sliding surface. This reveal an influence from the surface temperature gradient on the viscous friction, which id not revealed when applying classical isothermal analysis. The significance of the thermo-viscous effect on friction and energy dissipation is analyzed analytically in order to provide a qualitative insight to the relation between thermo-dynamic properties, film thickness, sliding velocity, and viscous friction.