Which statement correctly describes the Stribeck curve, which plots the friction coefficient of a plain bearing against the bearing characteristic number ηN/P formed from the lubricant viscosity η, the rotational speed N and the bearing pressure P?

Answer: A. In the boundary-to-mixed region, raising ηN/P lowers the friction coefficient; past the minimum, in hydrodynamic lubrication, viscous drag makes it rise gently

When ηN/P is small, that is, at low speed, high pressure and low viscosity, the oil film is thin and asperities touch, giving boundary lubrication with a large friction coefficient of about 0.1. As ηN/P increases, the bearing passes through mixed lubrication, in which pressure in the oil film generated by the wedge effect carries part of the load, and reaches hydrodynamic lubrication, in which the oil film fully separates the surfaces, and the friction coefficient falls to about 0.001–0.01. Raising the speed further in the hydrodynamic region increases the viscous resistance from shearing the oil, so the friction coefficient rises gently. The statement that friction is smallest in boundary lubrication has it backward, and the lubrication regime depends not only on pressure but also on viscosity and speed. A bearing runs stably when operated slightly on the hydrodynamic side of the minimum.