Even if a CNC machining center is equipped with spindle bearings that are optimized for a duty cycle at the time of purchase, there is no guarantee the bearings will be optimized for future duty ...

Heavy press fits should be avoided. Accuracy of mounting surfaces should be equal to accuracy of mating bearing surface. Misalignment for low torque and running accuracy should not exceed 1/4°.

Rolling-element bearings in electric motors support and locate the rotor, maintain a small and consistent air gap between the rotor and stator, and transfer loads from the shaft to the motor frame.