A curve plotting the friction coefficient of a plain bearing or similar contact against the bearing characteristic number, viscosity × speed ÷ pressure. It shows the transitions among boundary, mixed and hydrodynamic lubrication.
Japanese name: ストライベック曲線
Stribeck curve appears in 1 practice question across 1 topic, including Surface treatment and tribology.
The Stribeck curve plots the bearing characteristic number ηN/P (η is the lubricant viscosity, N the rotational speed and P the bearing pressure) on the horizontal axis against the friction coefficient on the vertical axis. The region of small ηN/P is boundary lubrication, where asperities on the surfaces touch and the friction coefficient is about 0.1. As ηN/P increases, the bearing enters mixed lubrication, in which pressure in the oil film carries part of the load, and the friction coefficient falls sharply. Increasing it further gives hydrodynamic lubrication, in which the oil film fully separates the surfaces; the friction coefficient passes its minimum and then rises gently because of the viscous resistance of the oil.
In hydrodynamic lubrication, oil is drawn toward the direction in which the gap between the sliding surfaces narrows like a wedge, and pressure develops there to support the load. This is called the wedge effect, and the higher the speed and viscosity, the thicker the oil film. Where contact pressures are very high, as in rolling bearings and gears, elastic deformation of the contact and the rise in oil viscosity with pressure come into play, giving elastohydrodynamic lubrication in which even an oil film thinner than 1 µm can separate the surfaces. The film thickness ratio, the ratio of film thickness to surface roughness, serves as a guide to the lubrication state.
A bearing operated slightly on the hydrodynamic side of the friction minimum keeps a stable friction coefficient even when speed or load changes. On the boundary side of the minimum, a temperature rise that lowers viscosity moves the bearing still closer to boundary lubrication, increasing friction and possibly leading to seizure. Starting and stopping always pass through boundary lubrication, so materials for rubbing surfaces must resist adhesion even when the oil film breaks. Oil additives, surface treatments and solid lubricants are used mainly to reduce friction and wear in this boundary lubrication region.
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.
It is a state in which the oil film has become very thin, asperities touch, and friction is governed by molecules adsorbed on the surfaces and thin films formed by reaction with additives. Oil viscosity has little effect, and the friction coefficient is about 0.1. It occurs at starting and stopping, and at low speed under heavy load.
Not necessarily. In the boundary and mixed lubrication regions the oil film thickens and friction falls, but once the bearing is already in the hydrodynamic region, viscous resistance increases and friction loss actually grows. This is why low-viscosity oils are used to save energy.
Pressure in a hydrodynamic oil film grows in proportion to viscosity and speed and opposes the bearing pressure. The combination viscosity × speed ÷ pressure therefore expresses whether the oil film can carry the load. Experiments under different conditions can be collapsed onto a single curve using this number.
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