Stribeck curve

[6] For a contact of two fluid-lubricated surfaces, the Stribeck curve shows the relationship between the so-called Hersey number, a dimensionless lubrication parameter, and the friction coefficient.

Hersey's original formula uses the rotational speed (revolutions per unit time) for N and the load per projected area (i.e. the product of a journal bearing's length and diameter) for P. Alternatively, the Hersey number is the dimensionless number obtained from the velocity (m/s) times the dynamic viscosity (Pa∙s = N∙s/m2), divided by the load per unit length of bearing (N/m).

Richard Stribeck's research was performed in Berlin at the Royal Prussian Technical Testing Institute (MPA, now BAM), and his results were presented on 5 December 1901 during a public session of the railway society and published on 6 September 1902.

The reason why the form of the friction curve for liquid lubricated surfaces was later attributed to Stribeck – although both Thurston and Martens achieved their results considerably earlier – may be because Stribeck published his findings in the most important technical journal in Germany at that time, Zeitschrift des Vereins Deutscher Ingenieure (VDI, Journal of German Mechanical Engineers).

The VDI journal provided wide access to Stribeck's data and later colleagues rationalized the results into the three classical friction regimes.

This may be the reason why the minimum in the coefficient of friction for a liquid-lubricated journal bearing was not discovered by him, but was demonstrated by the graphs of Martens and Stribeck.

The graphs plotted by Martens show the coefficient of friction either as a function of pressure, speed or temperature (i.e. viscosity), but not of their combination to the Hersey number.