Lubrication conditions – the viscosity ratio K

The effectiveness of the lubricant is primarily determined by the degree of surface separation between the rolling contact surfaces. If an adequate lubricant film is to be formed, the lubricant must have a given minimum viscosity when the application has reached its normal operating temperature. The condition of the lubricant is described by the viscosity ratio K as the ratio of the actual viscosity V to the rated viscosity V1 for adequate lubrication, both values being considered when the lubricant is at normal operating temperature, see the section Selection of lubricating oil.

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where

K = viscosity ratio

V = operating viscosity of the lubricant [mm2/s]

V1 = rated viscosity depending on the bearing mean diameter and rotational speed [mm2/s]

In order to form an adequate lubricant film between the rolling contact surfaces, the lubricant must retain a minimum viscosity when the lubricant is at operating temperature. The rated viscosity V1, required for adequate lubrication, can be determined from tha diagram 1, using the bearing mean diameter dm = (D + d)/2, mm, and the rotational speed of the bearing n, r/min. This diagram has been revised taking the latest findings of tribology in rolling bearings into account.

Diagram 1. Estimation of the minimum kinematic viscosity at operating temperature

When the operating temperature is known from experience or can otherwise be determined, the corresponding viscosity at the internationally standardized reference temperature of 40�C can be obtained from diagram 2, or can be calculated in the Viscosity calculator. The diagram is compiled for a viscosity index of 95. Table lists the viscosity grades according to ISO 3448:1992 showing the range of viscosity for each class at 40�C. Certain bearing types, e.g. spherical roller bearings, tapered roller bearings, and spherical roller thrust bearings, normally have a higher operating temperature than other bearing types, e.g. deep groove ball bearings and cylindrical roller bearings, under comparable operating conditions.

Diagram 2. Conversion to kinematic viscosity at reference temperature (ISO VG classification)

Calculation example

A bearing having a bore diameter d = 340 mm and outside diameter D = 420 mm is required to operate at a speed n = 500 r/min. Since dm = 0,5 (d + D), dm = 380 mm, from diagram 1, the minimum rated viscosity V1 required to give adequate lubrication at the operating temperature is approximately 11 mm2/s. From diagram 2, assuming that the operating temperature of the bearing is 70�C, it is found that a lubricant to an ISO VG 32 viscosity class, with an actual viscosity ? of at least 32 mm2/s at the reference temperature of 40�C will be required.