Section 8.4 - Friction in Lubricated Contacts

Abstract

This chapter deals with explanation of friction in lubricated contacts. Friction is the resistance to motion that occurs when one body slides over another. The resistive force acting in the opposite direction of the relative motion is known as the friction force. The friction force has two forms: the force required to initiate sliding, known as the static friction force, and the force necessary to maintain the sliding, called the kinetic friction force. This chapter discusses four regimes of Stribeck's curve that shows different regimes of lubrication can occur in a lubricated sliding contact, depending on the relative speed. These regimes also depend on the pressures occurring in the lubricating film. Regime I: it is called boundary lubrication for conformal contacts, whereas for nonconformal contacts, this regime is known as extreme pressure (EP) lubrication. The main feature of boundary lubrication in comparison with solid-to-solid contact is that although the reduction in the friction coefficient is less important, the reduction in wear is impressive. Regime II: corresponds to mixed lubrication in which several mechanisms act simultaneously. This regime is a transition regime between boundary lubrication and hydrodynamic lubrication for low-pressure films or between extreme pressure lubrication and elastohydrodynamic lubrication for highpressure films. Regime III: corresponds to hydrodynamic (HD) lubrication for low-pressure films, or to elastohydrodynamic (EHD) lubrication for high-pressure films. And lastly, regime IV: it is characterized by superlaminar flow in hydrodynamic lubrication. This regime occurs for high-relative-speeds, low-viscosity lubricants or very large dimensions.