Tolerances control the dimensional accuracy of a bearing. We use ISO bearing tolerances which start at P0 and then move upwards in precision grade to P6, P5 and then P4. ISO tolerances are measured in thousandths of a millimetre (or microns). AFBMA (ABEC) bearing tolerances are also commonly used and these are measured in ten-
Tolerances have no effect on radial play although it is sometimes mistakenly thought that improving the tolerances will produce a bearing with less play. Assuming that the shaft and housing are manufactured to the same tolerances as the bearing, higher bearing tolerances will produce better mating between shaft/housing and bearing, lower noise and vibration due to improved roundness and lower starting and running torque (also subject to radial play and lubricant).