Friction is the mechanical resistance to relative motion between two surfaces. In engineering, it is generally minimized because it causes energy losses, heat generation and wear.
The coefficient of friction can be determined using Coulomb's relationship. Types of friction include sliding, spinning, rolling and combined rolling-sliding friction.
An effective solution requires an understanding of all the factors in a tribological system and their complex interactions. Environmental conditions such as dust, temperature and humidity, structural factors such as material, surface finish and geometry, and operating loads such as speed, contact pressure and vibration all influence the choice of lubricant.
The Stribeck curve illustrates friction in oil- or grease-lubricated plain bearings under different lubrication conditions. During startup, static friction is followed by dry friction, with a high coefficient of friction and high wear. As speed increases, the surfaces become partially separated by a lubricant film in the mixed-friction regime. A protective film formed by solid lubricants can help protect the contact surfaces in this stage, as illustrated by the red curve. At higher speeds, a hydrodynamic film fully separates the surfaces. Friction and wear are lowest in this fluid-friction regime.
Lubricating Greasecan serve several functions, individually or in combination, depending on the application. In addition to transmitting power with minimal friction and wear, lubricants may need to provide water resistance, chemical resistance, compatibility with plastics and corrosion protection.
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