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107 Combined reduction and cogging wheel gear <strong>of</strong> a billet millOperating data<strong>The</strong> billet mill is designed for a monthly output <strong>of</strong>55,000 tons. <strong>The</strong> mill comprises a roughing and a finishingsection, each with two vertical and two horizontalstands in alternate arrangement. <strong>The</strong> drive <strong>of</strong> thevertical stands is on top; with this arrangement thefoundations are not as deep as for a bottom drive; onthe other hand, the top drive involves a greater overallheight.Rated horsepower 1,100/2,200 kW;motor speed 350/750 min –1 .Compared to two angular contact ball bearings, a fourpoint bearing <strong>of</strong>fers the advantage <strong>of</strong> smaller widthand, compared to a deep groove ball bearing, the advantage<strong>of</strong> smaller axial clearance and higher thrust carryingcapacity. <strong>The</strong> use <strong>of</strong> four point bearings is, however,limited to applications where the thrust load isnot constantly reversing. <strong>The</strong> bevel gear shafts featurethe smallest possible axial clearance to ensure perfectmeshing <strong>of</strong> the spiral-toothed gears. This is achievedby one duplex pair <strong>of</strong> angular contact ball bearingseach on the pinion shaft and on the bevel shaft. <strong>The</strong>yalso accommodate the thrust load whereas the radialload is taken up by cylindrical roller bearings.Machining tolerances<strong>Bearing</strong> selection, dimensioningRadial loads and thrust loads are accommodated separately:the radial loads by cylindrical roller bearings, thethrust loads by angular contact ball bearings and fourpoint bearings. Cylindrical roller bearings <strong>of</strong>fer thebest radial load carrying capacity in a limited mountingspace, thus keeping the distance between the gearshafts to a minimum. One decisive factor in the selection<strong>of</strong> the bearing size is the diameter <strong>of</strong> the individualgear shafts determined in the strength calculation.<strong>The</strong> two largest cylindrical roller bearings <strong>of</strong> the gearare situated on the cogging wheel side and have thefollowing dimensions: 750 x 1,000 x 250 mm. Axiallocation <strong>of</strong> the four gear shafts is provided by one fourpoint bearing each which are double direction angularcontact ball bearings.Cylindrical roller bearings: Shaft to p6; housing toH6/H7.Four point bearings and angular contact ball bearings:Shaft to f6; housing to D10.<strong>The</strong> outer rings <strong>of</strong> the four point bearings and angularcontact ball bearings are fitted into the housing withclearance to relieve them <strong>of</strong> radial loads; thus, they accommodateonly thrust loads.LubricationCirculating oil lubrication. <strong>The</strong> bearings and gearsshare the same lubrication system. <strong>The</strong> oil is directlysupplied to the bearings via an oil filter which preventscontamination <strong>of</strong> the bearings by particles abradedfrom the gears.FAG 150