A ground shaft can normally meet a straightness requirement of less than 5 microns. Bearings have a positive effect on runout however, greatly reducing runout.

Mount both the front and rear bearings in a stator bore that has a single diameter. The single bore lines up the bearings precisely with each other. It is possible on certain lathes to hold that bore to within 1 to 2 microns of diameter and straightness variance.

Use a threaded bearing preload adjuster and wave spring to precisely set the preload. Until now, most designs use spacers and wave springs. There may be several thicknesses of spacers and wave springs stacked. An adjustable threaded spacer, combined with a single wave spring, is screwed into position while monitoring the effect on motor starting torque to reach a target close-loop preload.