Rotary Seal Design Guide - Seals Unlimited

Catalog EPS 5350/USAEngineeringShaft Misalignment and RunoutApplications with rotating shafts come withtheir own set of common problems. Among theseare those associated with the shaft not beingaligned properly with the surrounding hardware.Misalignment most commonly manifests itself asEccentricity and Runout. Every shaft has somedegree of both as described in Figure 2-22.Eccentricity of a rotating shaft creates twoproblems. One is that it forces the seal lip to followa shaft that is not centered in the bore, wearing thelip more on one side. Because they are lesselastic, PTFE seals are more susceptible to failure,misalignment and runout conditions thanelastomeric lip seals. The second potentialproblem is that it enlarges the extrusion gap onone side, which could be detrimental if highpressure is involved. Extended heel designs willreduce seal extrusion.Shaft Runout is when the shaft is spinning onan axis of rotation that is offset from the geometriccenter of the shaft at the point of seal lip contact.Runout can be caused by a bent shaft or bywhirling deflection while spinning. The seal mustbe sufficiently compliant to maintain contact withthe shaft despite being compressed and extendedeach revolution. It follows that shaft runoutbecomes more of a problem at high speeds.inch0.0110.0100.0090.0080.0070.0060.0050.0040.0030.0020.0011222Bore CenterlineECCENTRICITYWith eccentricity, onlystatic radial deflection isimposed on the seal.Bore CenterlineShaft CenterlineSeal HousingCavity Diameter(Bore)MISALIGNMENTShaft RunoutEnvelopeShaft Centerline(Orbits aroundaxis of rotation)inch1/16 3/32 1/8 3/16 1/4FlexiSeal Cross-Section1 Maximum Eccentricity with No Runout2 Maximum Shaft Runout (less than 500 RPM) with No EccentricityFigure 2-23. FlexiSeal Eccentricity and RunoutLimitsAll rotating shafts have eccentricity and runoutto some degree. The risk of failure increasessignificantly if a system has a considerableamount of both. Figure 2-23 shows the acceptablemaximum for these parameters for all rotaryFlexiSeal profiles except the FFN-H. Figure 2-24shows the limits for FlexiLip and FlexiCase profiles.Shaft GeometricCenterlineECCENTRICITYIn this case, cyclical radial deflection dueto runout is superimposed on static radialdeflection due to eccentricity.Shaft Axisof RotationSHAFT RUNOUTRunout (inches)0.0040.0030.0020.001High Leakage andWear RangeAcceptable RangeAssuming Shaft Speedof 1750 RPMAXIS OF ROTATIONFigure 2-22. Eccentricity and Shaft Runout0.001 0.002 0.003 0.004 0.005 0.006Eccentricity (inches)Figure 2-24. FlexiLip and FlexiCase Eccentricityand Runout Limits03/28/062-25 Parker Hannifin CorporationEPS DivisionToll Free: (800) 233-3900www.parkerseals.com