Thick-film lubrication, also known as hydrodynamic or full-film lubrication, can be described using several equations that model the conditions and parameters involved.

Below are the key aspects along with the relevant indicative equations:

1. Complete Separation: The lubricant film thickness is sufficient to fully separate the contacting surfaces. The minimum film thickness can be estimated by the equation: where is the dynamic viscosity of the lubricant, is the relative surface speed, and is the load per unit width on the bearing.
2. Pressure Generation: The pressure generated within the lubricant film due to hydrodynamic action is given by Reynolds’ equation: where and are the coordinates along the length and width of the bearing, respectively.
3. Viscous Forces: The force balance is maintained by the viscous forces in the lubricant, which can be represented by: where is the force due to viscous shear, is the contact area, and is the velocity gradient perpendicular to the flow direction.
4. Applications: Common applications include journal bearings and gear systems where the surfaces are smooth, and the operating conditions are such that a full lubricant film can be maintained.
5. Dependence on Speed and Viscosity: The effectiveness of thick-film lubrication is influenced by the Sommerfeld number, which characterises the lubrication regime: Higher values of indicate a thicker lubricant film, ensuring better separation of surfaces.
6. Advantages: This lubrication regime provides low friction and wear, characterised by the coefficient of friction , which can be approximated by: where is the normal load. In thick-film lubrication, is typically low, reflecting the effectiveness of the lubrication film.

These equations provide a theoretical basis for understanding and analysing thick-film lubrication, helping to predict and optimise the performance of mechanical systems under various operating conditions.