The calculation of modified reference life requires a viscosity ratio κ. This ratio is calculated by the lubricant viscosity and a reference viscosity ν1 or it can be calculated from the lubricant film parameter Λ as according (ISO 281, 2007) and (ISO/TR 1281-2, 2008).
Since the viscosity ratio is calculated using standard settings for surface roughness, loading, pressure viscosity coefficient of the oil and geometric properties, the usage of lubricant film thickness takes into account more parameters of the actual bearing. For the definition of the reference viscosity ν1 see the derivation in (Baalmann, 1994) or (Heemskerk, 1980).
If this setting is activated the lubricant film thickness is calculated according (Harris, et al., 2007). For ball bearings the minimal film thickness is calculated using equation 4.60 (Harris, et al., 2007) by Hamrock and Dowson. For roller bearings equation 4.57 (Harris, et al., 2007) according Dowson and Higginson is used. The same formulas are used for the calculation of Λ in (ISO/TR 1281-2, 2008). The software uses the minimal film thickness for the calculation of Λ, in contrast to (Heemskerk, 1980) who is using the central film thickness.
In addition, a thermal correction is applied from version 08-2016 on. The thermal correction factor is calculated according to (Koch, 1996) using the formula 4.62 from Wilson and Murch. Different literature provides different correction factors, see (Baly, 2005) or (Gohar, 2001). As temperature the input for the lubricant temperature is used. For high speed the thermal correction factor reduces the film thickness.
This calculation requires the input of surface roughness, oil density and oil pressure viscosity coefficient. The minimal film thickness for all contacts is then used for ball bearings. For roller bearings the minimal film thickness for each section is calculated and used for the calculation of life modification factor for systems approach aISO for this section.