The Design of Rolling Bearing Mountings

73 Run wheel of a material ropewayOperating dataSpeed n = 270 min –1 ; radial load F r = 8 kN. Thrustloads as guidance loads only, considered by 20 % ofthe radial load: K a = 1.6 kN.Bearing selectionEach run wheel is supported by two tapered rollerbearings FAG 30306A. The bearings are assembled inO arrangement which provides for a wider bearingspread than an X arrangement. The wider the spread,the lower the additional bearing load from thrust loadK a .Bearing dimensioningAs thrust load K a acts at the wheel circumference, itgenerates radial reaction forces at the bearing locations.Thus,F rA /Y = 6.1/1.9 = 3.2; F rB /Y = 1.9/1.9 = 1 andconsequently F rA /Y > F rB /YThe second condition proven isK a > 0.5 · (F rA /Y – F rB /Y) = 0.5 (3.2 – 1) = 1.1For calculation of bearing A the following thrust loadF aA must, therefore, be taken into account:F aA = K a + 0.5 · F rA /Y = 1.6 + 0.5 · 1.9/1.9 = 2.1 kNConsequently, the equivalent dynamic load P A of bearingA is:P A = 0.4 · F rA + Y F a = 0.4 · 6.1 + 1.9 · 2.1 = 6.45 kNWith this load, the indicated dynamic load rating andthe speed factor f n = 0.534 (n = 270 min –1 ) the index ofdynamic stressing.f L = C/P A · f n = 60/6.45 · 0.534 = 4.97This value corresponds to a nominal rating life of morethan 100,000 hours. Since this calculation is based onthe most unfavourable load conditions, the thrust loadacting constantly at its maximum and only in one direction,the bearing is adequately dimensioned with regardto fatigue life. The service life will probably be terminatedby wear, especially under adverse operatingconditions (high humidity, heavy contamination).The load carrying capacity of bearing B does not needto be checked since its loading is much less than thatof bearing A.Bearing A:F rA = F r /2 + K a · (D/2)/l = 4 + 1.6 · 125/95 = 6.1 kNThe thrust load K a = 1.6 kN acts toward bearing A.Bearing B:F rB = F r /2 – K a · (D/2)/l = 4 – 1.6 · 125/95 = 1.9 kNRadial loads acting on a shaft supported on two taperedroller bearings generate axial reaction loadswhich have to be considered in the calculation of theequivalent dynamic load. These internal loads togetherwith the external thrust loads should, therefore, be takeninto account for life calculation (see FAG catalogueWL 41 520, chapter "Tapered roller bearings").Data for tapered roller bearings FAG 30306A (designationto DIN ISO 355: T2FB030):dynamic load rating C = 60 kN,Thrust factor Y = Y A = Y B = 1.9.Machining tolerancesThe run wheel mounting is a so-called hub mounting,i.e. the run wheel, with the two cups, rotates about astationary shaft. The cups carry circumferential loadand are thus tight-fitted. The shaft is machined to h6,the hub bore to M6.Lubrication, sealingThe bearings and the free spaces have to be filled duringmounting with grease, e. g. FAG rolling bearinggrease Arcanol L186V. The grease filling will last forapproximately one year.In the example shown, the bearings are sealed byspring steel seals (Nilos rings).FAG 102