The Design of Rolling Bearing Mountings

100 Vibrator motorThe vibrations of vibrating equipment are generatedby one or several activators. An electric motor with animbalance rotor is an example of such an activator. It isreferred to as a "vibrator motor". Vibrator motors areprimarily mounted in machinery for making prefabricatedconcrete parts, in vibrating screens and vibratingchutes.Operating dataInput power N = 0.7 kW, speed n = 3,000 min –1 .The bearings are loaded by the rotor weight and thecentrifugal forces resulting from the imbalances: maximumradial load on one bearing F r = 6.5 kN.Bearing selection, dimensioningDue to the high centrifugal forces, the load carryingcapacity of the deep groove ball bearings usually usedfor medium-sized electric motors is not sufficient forthis application. Vibrator motors are, therefore, supportedon cylindrical roller bearings. The arrangementshown incorporates two cylindrical roller bearingsFAG NJ2306E.TVP2.C4; the dynamic load rating ofthe bearings is 73.5 kN.The adverse dynamic bearing stressing by the centrifugalforces is taken into account by a supplementaryfactor f z = 1.2. Considering this supplementary factor,the equivalent dynamic loadP = 1.2 · F r = 7.8 kN.With the speed factor f n = 0.26 (n = 3,000 min –1 ), theindex of dynamic stressingf L = C/P · f n = 73.5/7.8 · 0.26 = 2.45This f L value corresponds to a nominal rating life of10,000 h. Thus the bearings are correctly dimensioned.Machining tolerancesShaft to k5; housing to N6.The bearing outer rings carry circumferential load andare, therefore, tight fits. Since the inner rings are subjectedto oscillating loads, it is advisable to fit themtightly onto the shaft as well. With non-separable bearingsthis requirement would make bearing mountingand dismounting extremely complicated. Therefore,separable cylindrical roller bearings of design NJ areused.Bearing clearanceThe initial radial clearance of the bearings is reducedby tight fits. Further radial clearance reduction resultsfrom the different thermal expansion of inner andouter rings in operation. Therefore, bearings of radialclearance group C4 (i. e. radial clearance larger thanC3) are mounted.To prevent detrimental axial preloading, the innerrings are assembled so that an axial clearance of0.2...0.3 mm exists between the roller sets of the twobearings and the lips (floating bearing arrangement ).Lubrication, sealingBoth bearings are lubricated with grease. Lithium soapbase greases of penetration class 2 with EP additiveshave proved successful. Relubrication after approximately500 hours.Since the vibrator motor is closed at both ends, gaptypeseals with grooves are satisfactory.100: Imbalance rotor bearings of a vibrator motor143 FAG