Spindle bearings are single-row angular contact ball bearings that consist of one-piece outer and inner rings and ball-cage assemblies having one-piece window cages. They are not demountable. The bearings are available in open and sealed versions. Spindle bearings have restricted tolerances for abutment dimensions and radial runout accuracy. They are particularly suitable for bearings with the highest requirements regarding guide accuracy and suitability for high speeds. They have proven their worth when it comes to carrying spindles in machine tools.