The Design of Rolling Bearing Mountings

93 Hammer millHammer mills are mainly used for crushing ores, coal,and stone.Operating dataHourly throughput 90...120 t of iron ore; input power280 kW; rotor speed 1,480 min –1 , rotor weight includinghammers approximately 40 kN; bearing centre distance2,000 mm.Bearing selectionDue to the high loads and rugged operation, hammermill rotors are mounted on spherical roller bearings.This self-aligning bearing type can compensate for misalignmentsof the two plummer block housings, andpossible rotor deflections. Two spherical roller bearingsFAG 23228EASK.M.C3 are mounted, one acting asthe locating bearing, the other one as floating bearing.The increased radial clearance C3 was selected becauseof the high speed. The bearing inner rings heat upmore than the outer rings, causing the bearing clearanceto be reduced during operation.Bearing dimensioningThe rotor weight imposes a radial load on the bearings.Added to this are unbalanced loads and shock loadswhose magnitude can only be estimated. These loadsare introduced in the nominal rating life calculation bymultiplying the rotor weight G R with a supplementaryfactor f z of 2.5...3, depending on the operating conditions.The thrust loads acting on the bearings are sosmall they need not be taken into account in the lifecalculation.With the dynamic load rating C = 915 kN, the speedfactor f n = 0.32 (n = 1,480 min –1 ) and the rotor weightG R = 40 kN, the index of dynamic stressing f L for onebearing:f L = C · f n / (0.5 · G R · f z ) = 915 · 0.32 / (20 · 3) = 4.88An f L value of 3.5...4.5 is usually applied to hammermills. Thus the bearings are adequately dimensionedwith regard to nominal rating life (L h approximately100,000 h).Bearing mountingThe bearings are mounted on the rotor shaft withwithdrawal sleeves FAG AHX3228. They are fittedinto plummer block housings MGO3228K. Bothhousings (open design) are available for locating bearings(design BF) and for floating bearings (design BL).The split housings of series MGO were especially developedfor mill applications. They are designed for oillubrication and feature particularly effective seals.Machining tolerancesFor mounting with sleeves, the shaft seats are machinedto h7, with a cylindricity tolerance IT5/2 (DIN ISO1101). The housing bores are machined to G6. Thusthe requirement that the outer ring of the floating bearingmust be displaceable within the housing is met.Lubrication, sealingFor reliable operation at high speeds, the bearings areoil bath lubricated. Grease-packed labyrinths preventthe ingress of foreign matter. To increase the sealingefficiency, grease is replenished frequently. Flingergrooves on the shaft, and oil collecting grooves in thehousing covers retain the oil within the housing.LocatingbearingFloatingbearing93: Hammer mill mounting131 FAG