The Design of Rolling Bearing Mountings

GlossaryPreconditions for normal cleanliness (V = 1):– good sealing adapted to the environment– cleanliness during mounting– oil cleanliness according to V = 1– observing the recommended oil change intervalsPossible causes of heavy lubricant contamination(V = 3):– the cast housing was inadequatly cleaned– abraded particles from components which are subjectto wear enter the circulating oil system of themachine– foreign matter penetrates into the bearing due tounsatisfactory sealing– water which entered the bearing, also condensationwater, caused standstill corrosion or deterioration ofthe lubricant propertiesThe necessary oil cleanliness class according to ISO4406 is an objectively measurable level of the contaminationof a lubricant.In accordance with the particle-counting mehod, thenumber of all particles > 5 µm and all particles > 15 µmare allocated to a certain ISO oil cleanliness classs.For example, an oil cleanliness class 15/12 accordingto ISO 4406 means that between 16,000 and 32,000particles > 5 µm and between 2,000 and 4,000 particles> 15 µm are present per 100 ml of a fluid.A defined filtration ratio x should exist in order toreach the oil cleanliness required.The filtration ratio is the ratio of all particles > x µmbefore passing the filter to the particles > x µm whichhave passed the filter. For example, a filtration ratio 3 ≥ 200 means that in the so-called multi-pass test(ISO 4572) only one of 200 particles ≥ 3 µm passesthe filter.Counter guidanceAngular contact bearings and single-direction thrustbearings accommodate axial forces only in one direction.A second, symmetrically arranged bearing mustbe used for "counter guidance", i.e. to accommodatethe axial forces in the other direction.Curvature ratioIn all bearing types with a curved raceway profile theradius of the raceway is slightly larger than that of therolling elements. This curvature difference in the axialplane is defined by the curvature ratio . The curvatureratio is the curvature difference between the rollingelement radius and the slightly larger grooveradius.curvature ratio =groove radius – rolling element radiusrolling element radiusDynamic load rating CThe dynamic load rating C (see FAG catalogues) is afactor for the load carrying capacity of a rolling bearingunder dynamic load. It is defined, in accordancewith DIN ISO 281, as the load a rolling bearing cantheoretically accommodate for a nominal life L of 10 6revolutions (fatigue life).Dynamic stressing/dynamic loadRolling bearings are dynamically stressed when onering rotates relative to the other under load. The term"dynamic" does not refer, therefore, to the effect of theload but rather to the operating condition of the bearing.The magnitude and direction of the load can remainconstant.When calculating the bearings, a dynamic stress is assumedwhen the speed n amounts to at least 10 min –1(see Static stressing ).Endurance strengthTests by FAG and field experience have proved that,under the following conditions, rolling bearings can befail-safe:– utmost cleanliness in the lubricating gap(contamination factor V = 0.3)– complete separation of the components in rollingcontact by the lubricating film (viscosity ratio ≥ 4)– load according to stress index f s* ≥ 8183 FAG