Stribeck Curve

The Stribeck Curve, named after German engineer Richard Stribeck, illustrates the relationship between friction and lubrication regimes, through which we can determine how friction varies based on lubricant viscosity, velocity and load in fluid-lubricated contacts. Prof. Richard Stribeck conducted extensive experiments while working on railway axles, journal and ball bearings to arrive at the relation between coefficient of friction and speed.

Categorized into Three Lubrication Regimes:
1. Boundary Lubrication – High friction occurs due to direct surface contact with minimal lubrication, which is common in low-speed and high-load applications.
• A grease with high viscosity base oil and solid additives is preferred to maintain a protective film
2. Mixed Lubrication – A transition phase where friction decreases as a thin lubricant film begins to separate the surfaces, but asperities still make contact. Partial fluid film forms, reducing friction.
• Grease with moderate-viscosity base oil is required, balancing load-carrying capacity and fluid film formation.
3. Hydrodynamic Lubrication – The condition where a full lubricant film completely separates the surfaces, minimizing friction and wear. Occurs at high speeds with sufficient lubricant film.
• Grease with low viscosity base oil is ideal to reduce internal resistance and overheating.

Choosing the right lubricant based on the Stribeck Curve ensures smoother operations.
• Low-speed, high-load applications Grease with high base oil viscosity for better film strength
• High-speed, low-load applications Grease with low base oil viscosity to minimize drag
How Engineers use the Stribeck Curve for grease selection?
The Stribeck Curve guides engineers in selecting the right lubricant for specific applications by analyzing key parameters that influence the lubrication regime.
• Viscosity of the lubricant – Higher viscosity fluids form thicker films. Ideal for boundary and mixed lubrication operating conditions.
• Operating speed – At low speeds, boundary lubrication is prevalent, requiring greases with extreme pressure (EP) or anti-wear (AW) additives.
• Load conditions – High loads can cause film breakdown, necessitating advanced lubricants that can maintain film integrity.

Graphical Representation of the Stribeck Curve illustrating the relationship between friction coefficient (µ) and the lubrication parameter (ηN/P), or the Hersy number, where:
η = lubricant viscosity
N = speed of rotation
P = applied load
Hersey Number is a non-dimensional number that combines fluid viscosity (η) and velocity (N) of the relative motion. While velocity is given in revolutions per second, P is the average pressure (load), which means that you either increase the velocity or reduce the average pressure along the horizontal axis.