In full-film lubrication, the moving surfaces are separated by what?

• A. Air film
• B. Hydraulic pressure
• C. Magnetic field
• D. Gas cushion

Answer: (B)

In full-film lubrication, the moving surfaces are kept apart by the pressure generated inside the lubricant film. As parts slide past each other, the oil is drawn into the gap and, due to the viscosity and the geometry of the clearance, a load-bearing pressure builds up in the film. This hydraulic (or hydrodynamic) pressure supports the external load and creates a continuous film that keeps the surfaces from touching.

This is different from an air film or gas cushion, where separation would be provided by a layer of gas rather than oil. The magnetic field option isn’t relevant to how a lubricating film works in this context. The key idea is that the oil film itself develops pressure that maintains separation between the surfaces.