Viscosity ratio κ
The importance of the oil viscosity to form a hydrodynamic oil film that separates the bearing contact surfaces is already mentioned under Lubrication conditions – the viscosity ratio κ. That information applies equally to the base oil viscosity of lubricating greases and oils.
The condition of the lubricant is described by the viscosity ratio κ as the ratio of the actual viscosity n to the rated viscosity ν1 for adequate lubrication, when the lubricant is at normal operating temperature.
|ν||=||actual operating viscosity of the lubricant [mm2/s]|
|ν1||=||rated viscosity of the lubricant depending on the bearing mean diameter and rotational speed [mm2/s]|
To separate the bearing contact surfaces, a minimum viscosity ratio κ = 1 is required. Full-film conditions exist when κ ≥ 4, i.e. a sufficient hydrodynamic film is formed for adequate lubrication. However, SKF recommends limiting κ to 4, otherwise the frictional heat decreases the operating viscosity.
In applications where κ < 1, a sufficient hydrodynamic film cannot be formed and metal-to-metal contact may occur. The use of lubricants containing extreme pressure or anti-wear additives (→ Load carrying ability) might extend bearing service life.
For oil lubricated applications where κ < 0,4, an oil with EP additives must be used.
In applications where κ < 1, hybrid bearings (→ Hybrid bearings) can be considered. Even under inadequate lubrication conditions, there is reduced risk of smearing between silicon nitride and steel surfaces.