Hsing-Sen S. Hsiao

A Complete Solution for Thermal-Elastohydrodynamic Lubrication of Line Contacts Using Circular Non-Newtonian Fluid Model

A complete solution is obtained for elastohydrodynamically lubricated conjunctions in line contacts considering the effects of temperature and the non-Newtonian characteristics of lubricants with limiting shear strength. The complete fast approach is used to solve the thermal Reynolds equation by using the complete circular non-Newtonian fluid model and considering both velocity and stress boundary conditions. The reason and the occasion to incorporate stress boundary conditions for the circular model are discussed

Non-newtonian and thermal effects on film generation and traction reduction in EHL line contact conjunctions

The thermal circular non-Newtonian model accompanied with three specialized models was used to study the mechanisms of film generation and traction reduction in EHL line contact conjunctions. Results revealed that the film generation capability is mainly controlled by the inlet zone pressure buildup and the inlet zone piezothickening. The diffusion time effect enhances the thermal thinning that reduces this capability. On the other hand, the piezoviscosity and the shear rate in the central contact zone are the main traction generation factors whereas shear skidding and thermal skidding are the traction reduction mechanisms. Results also showed that neglecting either viscous heating or the combination of shear thinning and shear stress reduction in formulating an EHL simulator is inaccurate for many cases

Ultrathin Liquid Lubrication of Magnetic Head-Rigid Disk Interface for Near-Contact Recording: Part I—A Closed-Form Solution to the Reynolds Equation

A closed-form approximate solution to ultrathin and near-zero-pitch liquid lubrication of magnetic head-rigid disk interfaces for near-contact recording is derived. This study focuses on the steady state at the fully flooded rail-disk contact condition. It is assumed that the lubricant in the interface is isoviscous and the slider is in the noroll and no-yaw situation. Simple dimensionless groups of operating factors are identified to facilitate the parametric study of performance. Results of interface geometry effects on load, location of pressure center, friction force, and friction coefficient are presented. Typical distributions of pressure and shear stress at rail surface are also presented. The important effects such as shear thinning and thermal thinning are incorporated by introducing shear thinning and thermal thinning factors. Implementation of these correction factors will be presented in Part II.

Finite Element System Approach to EHL of Elliptical Contacts: Part I—Isothermal Circular Non-Newtonian Formulation

The column continuity equation is used in formulating a modified Reynolds equation for elastohydrodynamic lubrication of elliptical contacts. A finite element method (FEM), here the Galerkin weighting method with isoparametric Q9 elements, is used to discretize the weak form of the Reynolds equation. In addition to the nodal pressures and the offset film thickness, the locations of the two-dimensional irregular free boundary are explicitly solved for by simultaneously forcing the essential and the natural Reynolds boundary conditions. Newton-Raphsons iterations with a user-friendly yet efficient meshless scheme (i.e., automatic meshing-remeshing, are finally applied to solve these equations. A decoupled circular non-Newtonian fluid model is adapted in a way to illustrate the implementation of this new solution method. Extensive results will be given in Part II.

Ultrathin Liquid Lubrication of Magnetic Head–Rigid Disk Interface for Near-Contact Recording: Part II—Shear Thinning and Thermal Thinning

The Carreau shear thinning equation and the modified Roelands thermal thinning equation are combined by using the generalized master curve approach to construct a new thermal non-Newtonian rheology model. The Newton-Raphson method is then used to curve-fit existing experimental data for two perfluoropolyethers to this new model. The thermophysical properties thus found are then used to calculate thermal and shear thinning correction factors for the ultrathin liquid lubrication of magnetic head-rigid disk interfaces. The results of rheology modeling show excellent fits of the widely ranging raw data with the new thermal non-Newtonian model. The results of hydrodynamic analysis indicate that the friction force-isothermal Newtonian shear stress curve for these interfaces is simply a lubricant characteristic curve of thermal-shear thinning.

Finite Element System Approach to EHL of Elliptical Contacts: Part II—Isothermal Results and Performance Formulas

Extensive isothermal solutions for elastohydrodynamic lubrication (EHL) of elliptical contacts under pure rolling are obtained by using the newly developed finite element system approach. The Hamrock-Dowson type of performance formula is revisited with these close-to-Newtonian results. Additional solutions are also compared with those obtained by a multi grid technique. The current solutions show good agreement with their existing counterparts.

Stream Functions and Streamlines for Visualizing and Quantifying Side Flows in EHL of Elliptical Contacts

Three-dimensional stream functions for flows in elastohydrodynamically lubricated elliptical conjunctions are formulated. Closed-form and numerical solutions for the stream functions on special planes are obtained. Streamlines on these special planes are plotted to reveal the trajectories of the lubricant particles that pass by, pass through, or flow back from the Hertzian contact zone. Furthermore, a conceptual column stream function and column streamlines are introduced to present the three-dimensional flow in a two-dimensional manner. Thereby, the column streamlines can be plotted to visualize and quantify the flow rates of the lubricant that passes by or passes through the Hertzian zone.

Transition from Elastohydrodynamic to Partial Lubrication

The recognition and understanding of elastohydrodynamic lubrication (EHL) represents one of the major developments in the field of tribology in the last half of the twentieth century. The revelation of a previously unsuspected lubrication film is clearly an event of some importance in tribology. In this case it not only explained the remarkable physical action responsible for the effective lubrication of many nonconformal machine elements such as gears, rolling-element bearings, cams, and continuously variable traction drives, but also brought order to the understanding of the complete spectrum of lubrication regimes, ranging from boundary to hydrodynamic.