Rotary Seal Design Guide - Seals Unlimited

Catalog EPS 5350/USAEngineeringShaft ConsiderationsThe shaft finish required for PTFE seals is justas critical as that for elastomeric lip seals (seePage 2-6).Proper surface finish is critical to ensure positivesealing, and achieve the longest seal life possiblein rotating applications. Rotating surfaces that aretoo rough can create leak paths and can be veryabrasive to the seal. Unlike elastomer contactseals, PTFE-based Flexi designs can run on verysmooth surfaces with or without lubrication. Due tothe toughness and low coefficient of friction ofPTFE, Flexi designs, unlike seals made of othermaterials, slip over the high points of the matingsurface and resist abrasion. To maximize sealperformance, the recommendations for surfaceroughness in Table 2-5 should be followed.Dynamic surfaces with relatively rough finisheswill result in higher wear rates, which decrease theseal life and may compromise performance.Additionally, dynamic surfaces which have a finishsmoother than recommended may also decreasethe seal’s effectiveness. The optimum surfaceroughness allows a film of the fluid being sealed toflow between the seal and the mating surface, whicheffectively lubricates and extends the life of the seal.Table 2-5. Surface Roughness, R aMedia BeingSealedCryogenicsHelium GasHydrogen GasFreonAirNitrogen GasArgonNatural GasFuel (Aircraft andAutomotive)WaterHydraulic OilCrude OilSealantsDynamicSurfacesStaticSurfacesµ inch µ m µ inch µ m04 max. 0.1 max.0 08 max. 0.2 max.06 max. 0.15 max. 12 max. 0.3 max.08 max. 0.2 max. 16 max. 0.4 max.12 max. 0.3 max. 32 max. 0.8 max.PTFE rotary seal applications require a hardrunning surface on the dynamic portion of thehardware. The harder surface allows the use ofhigher reinforced seal materials that will increasethe seal and hardware life. Softer running surfacesmust use lower wear resistant materials that willnot damage the hardware and normally yieldshorter seal life. A balance between seal materialand dynamic surface hardness must be met toensure that the seal remains the sacrificialcomponent. Table 9-3 includes minimumrecommended surface hardness for Parkermaterials in dynamic applications, based ontemperature, motion and speed.When the dynamic surface hardness is below45 Rc, most seal materials will polish the runningsurface of the hardware and the seal. This initialbreak-in period will cause seal wear to taper offover a period of time, depending on the sealmaterial, surface finish and PV of the application.When hardness exceeds 45 Rc, the initial surfacefinish is very important since the surface is muchharder to polish and the time to achieve break-in ismuch longer. Surface hardness above 65 Rc willgenerally not polish and therefore the initialsurface finish is even more critical to seal life. Thehardness of the dynamic hardware surface affectsthe wear rate of the seal. Additionally, some seallip materials are abrasive and will wear softermetal shafts or dynamic components. In general,higher surface finish results in better overall sealand hardware performance. The ideal hardness ofthe dynamic surfaces of the hardware is 50 to 60Rockwell C. The actual hardness used is normallya balance between the additional cost associatedwith finishing harder materials versus themaximum seal life that will be achievable.30°22wAt 30°See Table 2-2a on Page 2-8for chamfer lengthFigure 2-18. Shaft Profile03/28/062-21 Parker Hannifin CorporationEPS DivisionToll Free: (800) 233-3900www.parkerseals.com