Stribeck curve

The Stribeck curve is a fundamental concept in the field of tribology. It shows that friction in fluid-lubricated contacts is a non-linear function of the contact load, the lubricant viscosity and the lubricant entrainment speed. The discovery and underlying research is usually attributed to Richard Stribeck and Mayo D. Hersey, who studied friction in journal bearings for railway wagon applications during the first half of the 20th century; however, other researchers have arrived at similar conclusions before. The mechanisms along the Stribeck curve have been in parts also understood today on the atomistic level.

Concept

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. The Hersey number is defined as:

Hersey number

=

η

⋅

N

P

,

{\displaystyle {\begin{aligned}{\text{Hersey number}}={\frac {\eta \cdot N}{P}},\end{aligned}}}

where η is the dynamic viscosity of the fluid, N is the entrainment speed of the fluid and P is the normal load per length of the tribological contact.

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).

Thus, for a given viscosity and load, the Stribeck curve shows how friction changes with increasing velocity. Based on the typical progression of the Stribeck curve (see right), three lubrication regimes can be identified.