Generally, the optimal bearing must be selected to

suit the nature of the machine, the area within the

machine, the spindle specification, bearing type,

lubrication system and drive system of the intended

machine through considerations of the design life,  precision, rigidity and critical speed of the bearing.

Accuracies of rolling bearings, that is, dimensional accuracy and running accuracy of rolling bearings are defined by applicable ISO standards and JIS B 1514 standard. Usually, a high-precision bearing is JIS Class 5 or higher. In particular, the radial run-out, axial run-out and non- repetitive run-out of a main spindle bearing greatly affect the running accuracy of the main spindle and therefore have to be strictly controlled. With the recent super high-precision machine tools, the control of N.R.R.O. (Non-Repetitive Run-Out) has increasing importance, and the main spindle on a turning machine or machining center incorporates an N.R.R.O. accuracy controlled bearing.