Hydrodynamic lubrication occurs when a thick film of lubricant is interposed between surfaces in relative motion. Pressure builds up in the fluid film due to the motion, reducing friction. It is used in journal and thrust bearings. The fluid forms an oil wedge that supports the load and separates the surfaces. As load increases, pressure rises to counteract the load and maintain film thickness. Viscosity, velocity, and load determine the characteristics. A bearing number relates these factors and indicates if hydrodynamic lubrication can be achieved.