The Design of Rolling Bearing Mountings

GlossaryViscosityViscosity is the most important physical property of alubricating oil. It determines the load carrying capacityof the oil film under elastohydrodynamic lubricatingconditions. Viscosity decreases with rising temperatureand vice-versa (see V-T behaviour). Therefore it is necessaryto specify the temperature to which any givenviscosity value applies. The nominal viscosity 40 of anoil is its kinematic viscosity at 40 °C.SI units for the kinematic viscosity are m 2 /s andmm 2 /s. The formerly used unit Centistoke (cSt) correspondsto the SI unit mm 2 /s. The dynamic viscosity isthe product of the kinematic viscosity and the densityof a fluid (density of mineral oils: 0.9 g/cm 3 at 15 °C).Viscosity ratio With an X arrangement, the bearing clearance is obtainedby adjusting one outer ring. This ring should besubjected to point load because, being displaceable, itcannot be fitted tightly (Fits). Therefore, an X arrangementis provided where the outer ring is subjected topoint load or where it is easier to adjust the outer ringthan the inner ring. The effective bearing spread in anX arrangement is less than in an O arrangement.The viscosity ratio, being the quotient of the operatingviscosity and the rated viscosity 1 , is a measure of thelubricating film development in a bearing, cp. factora 23 .Viscosity-temperature behaviour (V-T behaviour)The term V-T behaviour refers to the viscosity variationsin lubricating oils with temperature. The V-T behaviouris good if the viscosity varies little with changingtemperatures.WearThe life of rolling bearings can be terminated, apartfrom fatigue, as a result of wear. The clearance of aworn bearing gets too large.One frequent cause of wear are foreign particles whichpenetrate into a bearing due to insufficient sealing andhave an abrasive effect. Wear is also caused by starvedlubrication and when the lubricant is used up.Therefore, wear can be considerably reduced by providinggood lubrication conditions (viscosity ratio > 2 if possible) and a good degree of cleanliness inthe rolling bearing. Where ≤ 0.4 wear will dominatein the bearing if it is not prevented by suitable additives(EP additives).X arrangementIn an X arrangement, two angular contact bearings aremounted symmetrically in such a way that the pressurecone apex of the left-hand bearing points to the rightand that of the right-hand bearing points to the left.FAG 194