Single-row angular contact ball bearings with a contact angle between 12° and 25° are termed spindle bearings. They comprise solid inner and outer rings as well as solid window cages and cannot be dismantled. In comparison to other bearings, they can be better lubricated thanks to their smaller contact surfaces. For this reason, spindle bearings are used for mounting high-speed main spindles in machine tools, and are distinguished from conventional angular contact bearings by their accuracy and the smaller contact surfaces for the balls. In order to guarantee a defined rolling behaviour of the balls, spindle bearings always require pre-tensioning in axial direction.
As a result of the increasing requirements, particularly for the maximum rotational speed, high-speed bearings (HS bearings) and hybrid bearings (HC bearings) have been developed. Hybrid bearings use ceramic balls, which have lower density and a higher modulus of elasticity than conventional roller bearing steel. This results in greater stiffness with the same pre-tensioning. However, this also causes higher Hertz pressings and lower load ratings because a smaller pressure ellipse arises in the contact area due to the lower elastic deformation of the rolling elements and running track, and this can be reduced through a lower pre-tensioning force. The ceramic material in conjunction with steel has very good tribological properties resulting in improved friction characteristics and also improved wear characteristics in comparison to HS bearings. The ball diameter also offers further potential to increase the rotational speed. Due to the lower circumferential speed, smaller contact surfaces and lower masses, roller bearings are therefore equipped with a large number of smaller balls for the highest rotational speeds. The lubrication of the spindle bearings can be implemented on the one hand as lifetime lubrication and sealed grease lubrication, or as oil / oil-air lubrication. In doing so, the oil-air lubrication can be applied directly through the outer rings of the bearing or through an additional lubrication channel in the housing.
The selection of the bearing as well as the lubrication is heavily influenced by the speed range and the required stiffness in the processing. The selection of the mounting should therefore always be defined by the application.
[WECK2], [BRAENDLEIN95]
