National/ Timken Seals Catalog - Norfolkbearings.com

SECTION 1-C TECHNICALSealing System Preparation And Seal InstallationSHAFT REQUIREMENTSProper engineering of the components of the assembly is necessary for seal reliability. Shaft requirements have beendetermined by the Rubber Manufacturer’s Association, through thousands of hours of testing. The following items shouldbe considered at the original equipment design stage or whenever anexisting application is to be upgraded.Shaft Finish — Shaft finish, a prime factor in proper functioning of a lip seal, should be specified as 10-25microinches Ra (0.20-0.60 micrometers Ra) with no machine lead.Shaft Lead — Maximum permissible lead angle is 0 ± 0.05°. The most acceptable method for obtaining thissurface characteristic is plunge grinding.Shaft Diameter — Shaft diameter tolerances are shown below. Tolerances greater than those shown should be usedonly if agreed upon between user and supplier.Shaft Diameter (inches) Tolerances Shaft Diameter (mm) TolerancesTo and including 4.000 ± .003 To and including 100 ± 0.084.001 through 6.000 ± .004 100.01 through 150 ±0.106.001 through 10.000 ± .005 150.01 through 250 ± 0.1310.001 and larger ± .006 250.01 and larger ± 0.15Shaft Hardness — Radial lip type seals will function satisfactorily on mild steel, cast iron or malleable iron shafts.Under normal conditions, however, the section of the shaft contacted by the sealing lip should behardened to Rockwell C30 minimum. If, however, the shaft may be nicked or damaged duringhandling or assembly, a Rockwell C45 minimum is recommended.Shaft Chamfer — To aid installation and help prevent damage to the seal lip, the leading edge of the shaft should havea chamfer or radius which must be smooth and free of nicks or rough spots.Shaft-to-BoreMisalignment(STBM) — The distance the center of rotation is displaced from the center of the bore. It usually exists to somedegree due to normal machining and assembly tolerances. Misalignment results in uneven wear and,if excessive, will shorten seal life. To measure, attach a dial indicator to the shaft and indicate off theseal bore while rotating the shaft. For specific limitations see the National Oil Seal Design Type andDescription charts on page 6 & 7.Dynamic Runout — Twice the distance the center of the shaft is displaced from the center of rotation. It can result from abent shaft, lack of shaft balance or other manufacturing variables. To measure, find the total movementof an indicator held against the seal area of the slowly rotating shaft. For specific limitations see theNational Oil Seal Design Type and Description charts on page 6 & 7.Shaft Material— Steel, stainless steel and certain cast irons all provide good sealing surfaces if finished properly.Brass or bronze shafts are not recommended nor are alloys of aluminum, zinc, magnesium andother similar elements. If plating is considered, it should be hard nonporous chrome plate. Flaking orplating exposes razor edges which may cut the seal lip.XVI