Stribeckkurver

Stribeckkurver, or Stribeck curves, are graphical representations used in tribology to describe how the coefficient of friction between sliding surfaces varies with a dimensionless speed parameter under lubricated conditions. The most common form plots the friction coefficient μ against a Sommerfeld number S, defined as S = ηu/p (or equivalently ηv/p in some texts), where η is the lubricant viscosity, u or v is the sliding speed, and p is the contact pressure. The x-axis can also be labeled by speed or by related lubrication-performance measures, but the underlying idea is to capture how lubrication regimes change with operating conditions.

Typical shape and regimes: At very low speeds, asperity contact dominates, producing high friction in boundary lubrication. As speed increases, the lubricant film thickens and enters mixed lubrication, where asperity contact and hydrodynamics coexist, and friction decreases. At higher speeds, a full fluid film develops, entering hydrodynamic or elastohydrodynamic lubrication (EHL) regimes, where friction reaches a minimum or stabilizes. In some cases, friction may begin to rise again at very high speeds due to increased viscous shear within the film or temperature effects.

Dependencies and use: The exact position and depth of the minimum μ_min depend on factors such as load, lubricant viscosity, temperature, material properties, and surface roughness. Stribeck curves are used to compare lubricants and surface treatments, to aid bearing and gear design, and to predict frictional losses and efficiency across operating conditions.

Limitations: The curve is an idealized, empirical tool. Real systems may not follow a single universal curve, as friction is influenced by transient conditions, contamination, and non-Newtonian lubricant behavior.