The Design of Rolling Bearing Mountings

107 Combined reduction and cogging wheel gear of a billet millOperating dataThe billet mill is designed for a monthly output of55,000 tons. The mill comprises a roughing and a finishingsection, each with two vertical and two horizontalstands in alternate arrangement. The drive of 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 offers the advantage of smaller widthand, compared to a deep groove ball bearing, the advantageof smaller axial clearance and higher thrust carryingcapacity. The use of four point bearings is, however,limited to applications where the thrust load isnot constantly reversing. The bevel gear shafts featurethe smallest possible axial clearance to ensure perfectmeshing of the spiral-toothed gears. This is achievedby one duplex pair of angular contact ball bearingseach on the pinion shaft and on the bevel shaft. Theyalso accommodate the thrust load whereas the radialload is taken up by cylindrical roller bearings.Machining tolerancesBearing 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 offer thebest radial load carrying capacity in a limited mountingspace, thus keeping the distance between the gearshafts to a minimum. One decisive factor in the selectionof the bearing size is the diameter of the individualgear shafts determined in the strength calculation.The two largest cylindrical roller bearings of the gearare situated on the cogging wheel side and have thefollowing dimensions: 750 x 1,000 x 250 mm. Axiallocation of 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.The outer rings of the four point bearings and angularcontact ball bearings are fitted into the housing withclearance to relieve them of radial loads; thus, they accommodateonly thrust loads.LubricationCirculating oil lubrication. The bearings and gearsshare the same lubrication system. The oil is directlysupplied to the bearings via an oil filter which preventscontamination of the bearings by particles abradedfrom the gears.FAG 150