The Design of Rolling Bearing Mountings

95 Ball tube millTube mills are mostly used in the metallurgical, miningand cement industries. The tube mill described isused in an Australian gold mine for grinding auriferousminerals (grain sizes 4...30 mm) into grit by meansof grinding bodies (balls). The grain size of the materialdepends on the number of balls and the quantity ofadded water. The grinding drum, which revolvesaround its horizontal axis, is lined with chilled-castiron plates. Charged with the grinding stock, it is veryheavy.Operating dataDrum: diameter 5,490 mm, length 8,700 mm; inputpower 3,850 kW; speed 13.56 min –1 ; drum mass whenloaded 400 t; maximum radial load per bearing F r =1,962 kN; maximum thrust load F a = 100 kN; bearingdistance 11,680 mm, throughput 250 t/h.The equivalent dynamic loadP = f z · F r + Y · F a = 2 · 1.5 · 1,962 + 4.5 · 100 =3,393 kNWith a dynamic load rating C = 12,900 kN the index ofdynamic stressing:f L = C/P · f n =12,900/3,393 · 1.31 = 4.98 (L h > 100,000 h).The bearings are very safely dimensioned with regardto nominal rating life.The bearings are mounted in split FAG plummerblock housings SZA48/1500HF (locating bearing) andSZA48/1500HL (floating bearing). The outer rings aretightly fitted into shell sleeves (e.g. made of grey-castiron) in the lower housing half. They facilitate compensationof axial length variations. The sliding effectis enhanced by grease injected into the shellsleeve/housing joint.Bearing selectionTrunnion bearingsAs the drum rotates, the bearings have to accommodate,in addition to the heavy weight, constant shocktypeloads caused by the grinding bodies. Both drumtrunnions are supported on spherical roller bearings ofseries 239, 248 or 249. The bearings compensate forstatic and dynamic misalignments that can be causedby misalignments of the bearing seats (large bearingdistance) or drum deflections. In this example, sphericalroller bearings with a tapered bore (K 1:30), FAG248/1500BK30MB are mounted both as the locatingbearing at the drive end and as the floating bearing atthe feed end. The bearings are mounted on the trunnionwith a wedge sleeve.Drive pinion bearingsThe drive pinion is supported on two spherical rollerbearings FAG 23276BK.MB with adapter sleeveFAG H3276HG, in plummer block housings withTaconite-seals FAG SD3276TST.Bearing dimensioningThe dimensioning of the drum bearings is based onhalf the weight of the loaded drum(400/2 · 9.81 = 1,962 kN).The shock loads are taken into account by a shock factorf z = 1.5. The required nominal rating life is100,000 h; this corresponds to an index of dynamicstressing f L = 4.9.Machining tolerancesThe circumferentially loaded inner rings are press-fittedon the trunnion. This is easily achieved by mountingthem hydraulically on wedge sleeves. The radial clearancereduction and the radial clearance of the mountedbearing have to be observed (see table in FAG catalogueWL 41 520, chapter on spherical roller bearings).The trunnions are machined to h9, with a cylindricitytolerance IT5/2 (DIN ISO 1101); the housing boresto H7.Lubrication, sealingGrease lubrication with a lithium soap base grease ofpenetration class 2 with EP additives, e. g. FAG rollingbearing grease Arcanol L186V. Continuous replenishment(approx. 5 g/h per bearing) ensures adequate lubrication.The bearings are sealed by multiple labyrinths. Due tothe extreme ambient conditions, the labyrinths arepreceded by dirt baffle plates and rubbing seals (Vrings).This combination is also referred to as Taconitesealing. The labyrinths are also continuously replenishedwith approx. 5 g/h per labyrinth.FAG 134