Pressure viscosity refers to the property of a fluid where its viscosity changes in response to applied pressure. While temperature is commonly known to affect viscosity, for many fluids, particularly lubricants under high pressure, viscosity also increases significantly as pressure rises.
This phenomenon is crucial in various engineering applications, especially where fluids are subjected to intense pressures in small contact areas. Understanding how viscosity behaves under pressure is vital for predicting fluid behavior and performance.
The Pressure-Viscosity Coefficient
To quantify this effect, especially in lubrication science, a specific parameter is used. As stated in the classical approach to elastohydrodynamic lubrication (EHL):
- In the classical approach to elastohydrodynamic lubrication (EHL) a single parameter, the pressure-viscosity coefficient, quantifies the isothermal pressure dependence of the viscosity for use in prediction of film thickness. Many definitions are in current use.
This coefficient provides a numerical value that describes how much a fluid's viscosity increases for a given increase in pressure, assuming temperature remains constant (isothermal).
Why is Pressure Viscosity Important?
The pressure-viscosity effect, quantified by the pressure-viscosity coefficient, plays a critical role in:
- Elastohydrodynamic Lubrication (EHL): This is a lubrication regime where lubricant viscosity and the elastic deformation of the contacting surfaces are significant. The pressure-induced increase in viscosity is essential for forming a load-carrying film that prevents surface contact, particularly in components like gears, rolling bearings, and cams.
- Film Thickness Prediction: As highlighted in the reference, the pressure-viscosity coefficient is directly used in formulas to predict the thickness of the lubricant film separating surfaces under high pressure. A higher pressure-viscosity coefficient generally leads to a thicker predicted film, which is beneficial for reducing wear and extending component life.
- Fluid Performance: It influences the flow characteristics and load-carrying capacity of fluids in high-pressure hydraulic systems and other components where fluids are compressed.
Pressure Viscosity vs. Pressure-Viscosity Coefficient
Here’s a simple comparison:
| Term | Description | Role |
|---|---|---|
| Pressure Viscosity | The physical phenomenon of viscosity changing with pressure. | A fundamental fluid property under pressure. |
| Pressure-Viscosity Coefficient | A parameter that measures or quantifies this phenomenon. | Used in calculations (e.g., EHL film thickness prediction). |
In summary, pressure viscosity is the characteristic of a fluid where its resistance to flow (viscosity) changes under pressure, and this effect is precisely measured by the pressure-viscosity coefficient, a key parameter in predicting lubrication performance in demanding conditions like EHL.
