The Design of Rolling Bearing Mountings

Glossary1. Safe retention and uniform support of the bearingrings2. Simplicity of mounting and dismounting3. Axial freedom of the floating bearingThe simplest and safest means of ring retention in thecircumferential direction is achieved by a tight fit.A tight fit will support the rings evenly, a factor whichis indispensable for the full utilization of the load carryingcapacity. Bearing rings accommodating a circumferentialload or an oscillating load are always fittedtightly. Bearing rings accommodating a point load maybe fitted loosely.The higher the load the tighter should be the interferencefit provided, particularly for shock loading. Thetemperature gradient between bearing ring and matingcomponent should also be taken into account. Bearingtype and size also play a role in the selection of the correctfit.Floating bearingIn a locating/floating bearing arrangement the floatingbearing compensates for axial thermal expansion.Cylindrical roller bearings of NU and N designs, aswell as needle roller bearings, are ideal floating bearings.Differences in length are compensated for in thefloating bearing itself. The bearing rings can be giventight fits.Non-separable bearings, such as deep groove ball bearingsand spherical roller bearings, can also be used asfloating bearings. In such a case one of the two bearingrings is given a loose fit, with no axial mating surfaceso that it can shift freely on its seat.Floating bearing arrangementA floating bearing arrangement is an economical solutionwhere no close axial shaft guidance is required.The design is similar to that of an adjusted bearingarrangement. In a floating bearing arrangement, however,the shaft can shift relative to the housing by theaxial clearance s. The value s is determined dependingon the required guiding accuracy in such a way thatdetrimental axial preloading of the bearings is preventedeven under unfavourable thermal conditions.In floating bearing arrangements with NJ cylindricalroller bearings, length variations are compensated forin the bearings. Inner and outer rings can be fittedtightly.Non-separable radial bearings such as deep groove ballbearings, self-aligning ball bearings and spherical rollerbearings can also be used. One ring of each bearing –generally the outer ring – is given a loose fit.Grease, grease lubricationcp. Lubricating greaseGrease service lifeThe grease service life is the period from start-up untilthe failure of a bearing as a result of lubrication breakdown.The grease service life is determined by the– amount of grease– grease type (thickener, base oil, additives)– bearing type and size– type and amount of loading– speed index– bearing temperatureIndex of dynamic stressing f LThe value recommended for dimensioning can be expressed,instead of in hours, as the index of dynamicstressing f L . It is calculated from the dynamic load ratingC, the equivalent dynamic load P and the speedfactor f n .f L = C · f nPsThe f L value to be obtained for a correctly dimensionedbearing arrangement is an empirical value obtainedfrom field-proven identical or similar bearingmountings.The values indicated in various FAG publications takeinto account not only an adequate fatigue life but alsoother requirements such as low weight for light-weightconstructions, adaptation to given mating parts,higher-than-usual peak loads, etc. The f L values conformwith the latest standards resulting from technicalprogress. For comparison with a field-proven bearingmounting the calculation of stressing must, of course,be based on the same former method.Based on the calculated f L value, the nominal rating lifeL h in hours can be determined.FAG 186