Ph.D. Thesis Defense by Fanghui Shi
Tuesday, November 23, 1999

"A Deterministic Mixed-Elastohydrodynamic Lubrication Model and Analysis of Lip-Seal Performance"

Abstract

Mixed-lubrication refers to the cases where the intervening fluid film between two solid surfaces of a tribological element is so thin that the asperities on the relatively moving surfaces interact. This happens when either the fluid film is not fully established or when it breaks down.  It happens inevitably during the transients of startup and shutdown.  As a consequence, the accumulated asperity contacts can change the geometry of the asperities, and damage to the rough surfaces may occur.  Therefore, an in-depth study of mixed-lubrication problems would help to improve design and prevent failure.

In the thesis, a deterministic model for mixed-lubrication problems is developed. Several coupled processes are included in the model: transient hydrodynamic lubrication, cavitation, and rough surface contact.  A simplified approach to determine the film rupture condition, when asperity contact occurs, is adopted to establish the link between micro-elastohydrodynamic lubrication and mixed-elastohydrodynamic lubrication. This model allows the identification of almost all the important parameters in a given mixed-lubrication problem, such as the load sharing between hydrodynamic and contact pressures, cavitation and contact areas, minimum and average film thickness, and the alteration of the rough surface pattern. Based on a mathematical analogy between transient lubrication and contact problems, a generalized formulation is derived and a unique solution scheme is constructed to solve these two seemingly different types of problems.  The efficiency is compared between this new scheme and some advanced iterative schemes, such as the SLOR and the ADI accelerated by the multi-grid method.

The above mixed-lubrication model is applied to the analysis of the rotary lip seal. Such seals are widely used in machines containing rotating shafts.  They are usually protected from thermal and mechanical damage by a thin film of lubricant that separates the lip from the rotating shaft under steady-state conditions.  However, under some transient conditions, the seals work in the mixed-lubrication regime. The implementation of the deterministic mixed-lubrication model in the performance analyses for rotary lip seals helps to better understand the sealing mechanism and extend the seal life.  A new pumping mechanism, based on surface tilting, is introduced.  A parametric study has been conducted for design improvement.