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GlossaryViscosityViscosity is the most important physical property <strong>of</strong> alubricating oil. It determines the load carrying capacity<strong>of</strong> the oil film under elastohydrodynamic lubricatingconditions. Viscosity decreases with rising temperatureand vice-versa (see V-T behaviour). <strong>The</strong>refore it is necessaryto specify the temperature to which any givenviscosity value applies. <strong>The</strong> nominal viscosity 40 <strong>of</strong> anoil is its kinematic viscosity at 40 °C.SI units for the kinematic viscosity are m 2 /s andmm 2 /s. <strong>The</strong> formerly used unit Centistoke (cSt) correspondsto the SI unit mm 2 /s. <strong>The</strong> dynamic viscosity isthe product <strong>of</strong> the kinematic viscosity and the density<strong>of</strong> a fluid (density <strong>of</strong> 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). <strong>The</strong>refore, 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. <strong>The</strong> effective bearing spread in anX arrangement is less than in an O arrangement.<strong>The</strong> viscosity ratio, being the quotient <strong>of</strong> the operatingviscosity and the rated viscosity 1 , is a measure <strong>of</strong> thelubricating film development in a bearing, cp. factora 23 .Viscosity-temperature behaviour (V-T behaviour)<strong>The</strong> term V-T behaviour refers to the viscosity variationsin lubricating oils with temperature. <strong>The</strong> V-T behaviouris good if the viscosity varies little with changingtemperatures.Wear<strong>The</strong> life <strong>of</strong> rolling bearings can be terminated, apartfrom fatigue, as a result <strong>of</strong> wear. <strong>The</strong> clearance <strong>of</strong> aworn bearing gets too large.One frequent cause <strong>of</strong> 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.<strong>The</strong>refore, wear can be considerably reduced by providinggood lubrication conditions (viscosity ratio > 2 if possible) and a good degree <strong>of</strong> 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 <strong>of</strong> the left-hand bearing points to the rightand that <strong>of</strong> the right-hand bearing points to the left.FAG 194