Selecting lubricating oils
Selecting oil is primarily based on the viscosity required to form a sufficiently thick hydrodynamic film at normal operating temperature. The viscosity of oil is temperature dependent, becoming lower as the temperature rises. The viscosity-temperature relationship of an oil is characterized by the viscosity index (VI). For rolling bearings, oils with a viscosity index of at least 95 (little change with temperature) are recommended.
To form a sufficiently thick oil film in the contact area between the rolling elements and raceways, the oil must retain a minimum viscosity at normal operating temperature. The rated viscosity ν1 required at normal operating temperature to provide adequate lubrication can be determined from diagram 1, provided a mineral oil is used. 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 (105 °F), i.e. the oil ISO VG viscosity class, can be obtained from diagram 2, which is compiled for a viscosity index of 95.
Certain bearing types, such as spherical roller bearings, toroidal roller bearings, tapered roller bearings, and spherical roller thrust bearings, typically have a higher operating temperature than other bearing types such as ball bearings and cylindrical roller bearings, under comparable operating conditions.
When selecting an oil, consider the following:
- Bearing life may be extended by selecting an oil where the viscosity n at normal operating temperature is higher than the obtained viscosity ν1 (diagram 1). The condition ν > ν1 can be obtained by choosing a mineral oil with a higher ISO VG viscosity class or by selecting an oil with a higher viscosity index, provided the oil has the same pressure-viscosity coefficient. Since higher viscosity increases operating temperature, there is frequently a practical limit to the lubrication improvement that can be obtained using this method.
- If the viscosity ratio κ < 1 (→ Viscosity ratio κ), SKF recommends using an oil containing EP additives. If κ < 0,4 an oil with EP additives must be used. Oils with EP additives may also enhance operational reliability in cases where κ > 1 and medium and large-size roller bearings are used. It should be remembered that some EP additives may cause adverse effects.
- For exceptionally low or high speeds, for critical loading conditions, or for unusual lubricating conditions, contact the SKF application engineering service.
A bearing with a bore diameter d = 340 mm and an outside diameter D = 420 mm is required to rotate at a speed ν = 500 r/min. What is the required viscosity ν at the reference temperature of 40 °C (105 °F)?
From diagram 1 with dm = 0,5 (340 + 420) = 380 mm and n = 500 r/min, the rated viscosity ν1 required for adequate lubrication at normal operating temperature is approximately 11 mm2/s. From diagram 2, assuming that the normal operating temperature is 70 °C (160 °F), a lubricating oil in the ISO VG 32 viscosity class, with an actual viscosity ν ≥ 32 mm2/s at the reference temperature of 40 °C (105 °F), is required.