Rotary Seal Design Guide - Seals Unlimited

Catalog EPS 5350/USAEngineeringLubricant ConsiderationsShaft Considerations22HousingLubricantShaftSpeedA proper shaft finish provides small pockets tohold the needed oil film between the lip and shaft,preventing direct contact that would otherwisecause friction and wear as the shaft rotates. Theshaft surface must also be smooth enough toavoid peaks that are large enough to breakthrough the lubrication film.PressureFigure 2-5. Sealing SystemThe contact lip is designed to run on a thin filmof oil. Without the oil film, the seal lip will rundirectly on the rotating shaft and generateexcessive friction and fail within hours. Thelubricant selected needs to remain viable over theexpected service life. If the underlip temperatureexceeds the lubricant rating, carbonization of theoil will occur.Abrasive carbonized oil particles will build upat the seal lip and accelerate lip and shaft wear. Asthe oil film becomes less than optimal, the lipfriction increases, as does lip wear.When selecting a lubricant keep the followingin mind:1. Do temperature limits of the lubricant matchthe underlip operating temperature of the seal?2. Are the base oil and additives compatiblewith the lip material?3. Does the oil level provide adequatelubrication and cooling at the seal lip?The optimal surface for elastomeric shaft sealsis a plunge ground finish of 8 to 17 µin Ra (0.20 to0.43 µm Ra) (0.010" [0.25 mm] cutoff) with a leadangle below 0.05 degrees. (See Table 2-5 onPage 2-21 for shaft finish requirements for PTFEseals.)Recent studies show that the Ra measurementalone is insufficient to quantify a proper surface.The surfaces below have the same Ra finish, butthe impact on seal performance will vary.Surface Finishes18 Ra18 Ra18 RaTwo additional requirements are needed: Rz(the average peak to valley height) of 65 to 115µin (1.65 to 2.90 µm), and RPM of 20 to 50 µin(0.5 to 1.25 µm), the average peak to meanheight. For additional information, refer to RubberManufacturers Association Technical BulletinOS-1-1.03/28/062-6 Parker Hannifin CorporationEPS DivisionToll Free: (800) 233-3900www.parkerseals.com