Spindle bearings are single-row angular contact ball bearings with a contact angle between 12° and 25°. They consist of solid inner and outer rings as well as solid window cages and cannot be dismantled. Compared to other bearings, they can be lubricated better by smaller contact surfaces. For this reason, spindle bearings have become established for the bearing of fast rotating main spindles of machine tools which are distinguished from conventional angular contact bearings in the accuracy and small contact surfaces of the balls. To ensure a defined rolling of the balls, spindle bearings always require a preload in axial direction. High-speed bearings (HS bearings) and hybrid bearings (HC bearings) were developed in response to increased requirements, particularly for maximum speeds. Hybrid bearings contain ceramic balls which have a lower density and a higher modulus of elasticity than conventional rolling bearing steel. This results in increased stiffness with the same preload. However, this also results in higher Hertzian pressures and lower load ratings since the lower elastic deformation of the rolling elements and raceway results in a smaller pressure ellipse in the contact surface which can be reduced by a lower prestressing force. In combination with steel, the ceramic material has very good tribological properties resulting in improved friction behavior and also improved wear behavior compared with HS bearings. The ball diameter offers further potential for increasing the speed. Due to the lower peripheral speed, smaller contact surfaces and lower mass, rolling bearings are therefore equipped with a large number of small balls for maximum speeds. The spindle bearings can be lubricated either as lubricated for life and sealed grease lubrication or as oil/air-oil lubrication.
The oil-air lubrication can take place directly via the outer rings of the bearing or via an additional lubrication channel in the housing. The choice of bearings and lubrication strongly depends on the speed range and the required rigidity during machining. The choice of bearing should therefore always be defined by the intended use.
Sources: Wake-up, M.+ Breaker, C.: Werkzeugmaschinen, volume 2, Konstruktion und Berechnung VDI-Verlag, 8th completely revised edition, 2006 Bründlein, Eschmann, Hasbargen, Weigand: Die Wälzlagerpraxis, Wiesbaden, 1995