The unresolved definition of the pressure-viscosity coefficient

Area of Science:

• Tribology
• Fluid Mechanics
• Materials Science

Background:

• Classical elastohydrodynamic lubrication (EHL) relies on the pressure-viscosity coefficient to model fluid behavior under pressure.
• Current definitions of the pressure-viscosity coefficient are inadequate for predicting EHL film thickness, especially at elevated temperatures.
• Accurate piezoviscous effect measurements, existing for a century, are often overlooked in EHL research.

Purpose of the Study:

• To highlight the limitations of classical EHL models regarding the pressure-viscosity coefficient.
• To emphasize the need for improved definitions that account for piezoviscous effects at higher temperatures.
• To address the long-standing issue of inadequate EHL film thickness prediction for non-Newtonian fluids.

Main Methods:

• Review of existing definitions of the pressure-viscosity coefficient in EHL.
• Analysis of the limitations of classical EHL formulas, including the Hamrock and Dowson model.
• Comparison of theoretical EHL models with experimental piezoviscous data.

Main Results:

• Classical EHL formulas, including Hamrock and Dowson, are limited to high Newtonian limit liquids and low temperatures.
• The pressure-viscosity coefficient definition and assumed equation of state restrict the applicability of current models.
• Accurate piezoviscous measurements are not adequately incorporated into classical EHL film thickness predictions at temperatures above ambient.

Conclusions:

• No current definition of the pressure-viscosity coefficient can accurately quantify piezoviscous response for analytical EHL film thickness calculations at temperatures above ambient.
• The classical approach to EHL lubrication requires significant revision to incorporate real-world fluid behavior at higher temperatures.
• Resolving the limitations of pressure-viscosity coefficients is crucial for advancing EHL research and applications.