The Design of Rolling Bearing Mountings

13 High-speed motor milling spindleOperating dataInput power 11 kW;maximum spindle speed 28,000 min –1 .Bearing selectionThe bearings must be suitable for very high speeds andfor the specific thermal operating conditions in a motorspindle. Hybrid spindle bearings with ceramic ballsare particularly suitable for this application.Milling spindles must be guided extremely accuratelyboth in the axial and in the radial direction.Work end:1 spindle bearing set FAG HC7008E.T.P4S.DTL intandem arrangement.Drive end:1 spindle bearing set FAG HC71908E.T.P4S.DTL intandem arrangement.The bearing pairs at drive end and work end aremounted in O arrangement and elastically adjusted bymeans of springs (spring load 300 N), correspondingto a medium preload. The bearing pair at the drive endis mounted on a sleeve which is supported on a linearball bearing with zero clearance so that axial lengthvariations of the shaft can be freely compensated for.Bearing dimensioningBearing size and bearing arrangement are selected onthe basis of the specified speed and on the spindlediameter.Two other factors that have to be taken into accountare the heat generated by the motor, which causes amajor temperature difference between the inner ringand the outer ring of the bearing, and the ring expansionwhich makes itself felt by the centrifugal force resultingfrom the high speed. In a rigid bearing arrangement,this would considerably increase the preload.Due to the spring preload, both these influences areeasily compensated for. As a result, the contact pressurein the rolling contact area of the bearing is relativelylow (p 0 ≤ 2,000 N/mm 2 ), and the bearings arefailsafe. Consequently, the service life of the bearings isdictated by the grease service life.Lubrication, sealingThe bearings are lubricated with rolling bearing greaseArcanol L207V which is particularly suitable for thegreater thermal stressing and for high speeds.To protect the grease from contamination, and consequentlyto increase the grease service life, the bearingsare sealed by labyrinths consisting of a gap-type sealwith flinger grooves and a collecting groove.Machining tolerancesBearing Seat Diameter Form tolerance Axial runout tolerancetolerance (DIN ISO 1101) of abutment shoulderSpindle bearing Shaft +6/+10 µm 1 µm 1.5 µm(drive/work end) Housing –3/+5 µm 2 µm 3 µm13: Bearing arrangement of a high-speed motor milling spindle25 FAG