The Stribeck Curve and Lubrication Regimes

Understanding the Stribeck Curve in Tribology

Introduction to the Stribeck Curve

• The video introduces the concept of the Stribeck curve, a foundational element in tribology, which studies friction, lubrication, and wear.

• The origins of the curve trace back to late 1800s investigations by Thurston, who identified a minimum point in the relationship between friction and speed in journal bearings.

Development of the Curve

• Stribeck expanded on Thurston's findings by examining the coefficient of friction versus speed rather than just friction itself.

• In 1914, Goombal and Herze independently explored this relationship further using a term called z/p , where z represents viscosity, n is rotational speed, and p denotes load.

Sections of the Stribeck Curve

• The classical representation of the Stribeck curve is divided into three sections: boundary lubrication, mixed lubrication, and hydrodynamic lubrication.

Boundary Lubrication

• In boundary lubrication, surfaces are imperfectly lubricated or not lubricated at all; metal surfaces come into direct contact affecting friction based on surface finish and material properties.

Mixed Lubrication

• As load increases with lubricant support, mixed lubrication occurs where both lubricant film and solid surface interactions are present.

Hydrodynamic Lubrication

• Hydrodynamic lubrication features a full lubricant film that supports loads with almost complete separation between surfaces.

Film Thickness Relationship

• The log of film thickness shows a linear relationship with z/p , while the coefficient of friction does not follow a linear pattern across different lubrication regimes.

Conforming vs. Non-Conforming Contacts

• Conforming contacts (like journal bearings) have parallel surfaces; non-conforming contacts (like gear teeth) do not maintain parallelism throughout their interaction.

Impact on Film Thickness and Friction

• In non-conforming contacts during hydrodynamic conditions, film thickness becomes determined by oil viscosity at contact points rather than solely by surface interactions.

Effects of Lubricant Properties on the Stribeck Curve

• Understanding how lubricant properties influence the shape of the Stribeck curve helps assess their impact on contact surfaces.

Reducing Coefficient of Friction

• Lowering EHL (Elastohydrodynamic Lubrication) friction through better lubricants can reduce overall coefficient of friction significantly.

Increasing Viscosity

• Higher viscosity lubricants can initiate boundary lubrication earlier due to their ability to support more load effectively.

Surface Modifications