super precision bearings

LIFE CALCULATION FOR SUPER PRECISION BEARINGSLife Calculation for SuperPrecision BearingsSuper precision bearings must locatemachinery components withhigh accuracy and support loads atup to very high speeds. They arepredominantly selected for their• accuracy• rigidity• running behaviour.These demands can be met overan expected life span only if nobearing wear occurs. This is dependentupon the generation of asupportive hydrodynamic lubricantfilm in the rolling contact area. Underthese circumstances rollingbearings achieve ultimate life in avariety of applications. From theload point of view, the stress occurringin the contact points aswell as the bearing kinematics areof decisive influence on bearingservice life. Therefore the traditionaldesign in keeping withDIN ISO 281 has proved inexpedientwhile the modified life calculationcomes closer to field experience.Yet especially for high-performanceunits it is better to determineindividual bearing arrangementswith the help of special calculationprograms.Bearing LoadDynamic EquivalentLoad PFor dynamically loaded bearings,the loads are combined into a dynamicequivalent load P. This is theconstant load derived from• combined load (radial and axial)• temporarily alternating loadsto give the same calculated life asthe actually acting combined load.For bearings that can accommodateradial and axial load components,the equivalent load is calculatedusing the equationP = X · F r + Y · F aThe factors X and Y are derivedfrom the ratio of F a /F r compared tothe bearing specific factor e.Spindle BearingsContact Angle α = 15°F a /F r ≤ eX = 1, Y = 0.F a /F r > e (Tables 14 and 15)P = 0.44 · F r + Y · F aContact Angle α = 25°With bearings of α = 25°, the contactangle changes very little evenunder axial load and therefore theaxial factor Y is taken as a constant.F a /F r ≤ 0.68P = F rf 0 · F a Spindle Bearingsi · C 0α = 15°e X YF a /F r > 0.68P = 0.41 · F r + 0.87 · F a0.3 0.4 0.44 1.40.5 0.43 0.44 1.310.9 0.45 0.44 1.231.6 0.48 0.44 1.163 0.52 0.44 1.086 0.56 0.44 1i = number of bearings that accommodate the axial load14: Radial and axial factorsFAG · 134