Bearing life according ISO 281
The bearing life according ISO 281 is calculated using dynamic load capacity C and dynamic equivalent load P = X*Fr + Y*Fa. An influence on load distribution by bearing clearance or tilting moments is not considered. The basic rating life is defined as L10 =(C/P)^p. Lubrication and reliability have no influence on this value. The L10 life is associated with 90% reliability.
The modified rating life is defined as Lnm = a1*aISO*L10. The modification factor a1 is considering reliability and the factor aISO is considering lubrication and contamination. The modification factor aISO is dependent of viscosity ratio κ, fatigue load limit Cu, equivalent load P and the contamination factor eC. There are two sources for the contamination factor eC: Either table 13 in ISO 281 or appendix A. According appendix A the factor eC is dependent on lubricant cleanliness, bearing size and viscosity ratio κ. According appendix A usually smaller values for eC result than according table 13, especially for small bearings.
The modified life Lnm can be larger or smaller than L10. At small speed the lubricant film thickness will be small which leads to small values of aISO and therefore to a reduction of bearing life. If load spectra should be considered an equivalent load can be calculated from the spectrum for the calculation of L10, but for modified life, the modification factor aISO will be different for each load case. Therefore, modified life has to be calculated for each load spectrum element first and can then be combined to a resulting life.
Bearing life according ISO/TS 16281
The calculation of bearing life according ISO/TS 16281 is based on the calculation of load distribution within the bearing. By using the load distribution the influence of bearing clearance, tilting angle or centrifugal forces can be considered. For ball bearings an equivalent load is calculated for inner and outer race. Roller bearings are divides in minimum 30 sections (standard value of 41 in the software), for each section then an equivalent load is calculated for inner and outer race contact.
The basic reference rating life L10r according ISO/TS 16281 is calculated using dynamic load capacity C and these equivalent loads. Lubrication and reliability have no influence. The L10 life is associated with 90% reliability. Bearing clearance or tilting moments are considered by using the load distribution.
The modified reference rating life Lnmr is considering the modification factors a1 for reliability and aISO for lubrication and cleanliness. aISO is calculated for each section in roller bearings. Like above the modification factor aISO is dependent of loading, contamination, fatigue load limit and viscosity ratio.
The bearing life according ISO/TS 16281 can be larger than life according ISO 281 for good conditions. For large tilting angle or large bearing clearance, a reduction is seen. As additional influence factors are considered the reference rating life according ISO/TS 16281 should be more accurate than bearing lice according ISO 281. Bearing life according ISO 281 had been mainly added to the software for comparison to reference life, so it can be quickly assessed which method shows larger or smaller values.
If the calculation is done using load spectra, the load distribution has to be calculated for each load case. Therefore using an equivalent load for the whole spectrum is not appropriate.