NKE Hauptkatalog / General Catalogue 2011

Designof Bearing LocationSealing of Bearing ArrangementsGeneralRolling bearings are high precision machineelements that are produced with tolerances ofclose microns [μm].For an optimum function they have super finishedrunning surfaces featuring surface roughnessof some 10th microns (0,1 μm).This is why rolling bearings are very sensitiveto damage caused by solid contaminations andimpurities.The efficient sealing of a bearing arrangementis thus one of the major pre-conditions for thesuccessful performance of a rolling bearingarrangement.Seal TypesIn the field of bearing sealing there are manyproven designs and design variations. To providethe optimum solution for each application andspecifi c problems this practical experience shouldalways be considered.For rotary movement, dynamic seals are usuallyused for sealing bearing arrangements.To satisfy the specific problems for each applicationthere are in principle two main differences,namely.- non-contacting seals- contacting (rubbing) sealsFor some applications it may become necessaryto combine both types.Non-Contacting SealsThe principle function of non-contacting sealsis based on the sealing effect of narrow gapsbetween stationary and rotating machinecomponents.In their simplest form, non-contacting sealsare simple, straight gaps as shown in fig. 7.9a.Their effectiveness may be increased by designimprovements up to complex shaped labyrinthseals.Gap seals do not have any contacting parts, theygenerate practically no friction and thus no wearwhich make this type suitable for high speedoperations.The width of sealing gap should be approximately0,1 ÷ 0,3 mm, according to the accuracyof shaft guidance and dependant upon thebearing size.Some compensation of alignment errors betweenshaft and housing may be possible based on theseal arrangement to be used, particularly for selfaligning bearing types (e.g. ball bearings and/orspherical roller bearing).A signifi cant improvement in the effectiveness ofa gap seal may be achieved by grease filling ofthe sealing gaps. By this measure the penetrationof fi ne dust particles may be avoided.A higher efficiency of sealing may also beachieved by a combination of non-contactingseals with sealed or shielded bearings, (suffi xes-Z, -2Z, -RS, -2RS, -RS2, -2RS2, - 2LFS).For variations of non-contacting seals, (see fig 7.8).Fig. 7.8a)A straight gap between shaft and housing coverbuilds the simplest form of a gap seal. Suitablefor grease lubricated bearing applicationsrunning under dry surroundings where less dustmay occur.www.nke.at 313