Bearing arrangements for greater rigidity and higher speeds
When higher speeds are required, as is the case for high-speed machining centres (A > 1 200 000 mm/min), there is typically a compromise between rigidity and load carrying capacity. In these applications, the spindle is usually driven directly by a motor (motorized spindles or electro-spindles), or through a coupling. Therefore, there are no radial loads on the non-tool end as is the case with a belt driven spindle. Consequently, single row angular contact ball bearings mounted in sets and single row cylindrical roller bearings are frequently used (fig. 1). In this bearing arrangement, the tool end bearing set is axially located, while the cylindrical roller bearing on the non-tool end accommodates thermal expansion of the spindle shaft relative to the housing within the bearing.
Other arrangement examples for spindles in high-speed machining centres, and milling machines are shown in fig. 2 and fig. 3.
If enhanced performance is required, SKF recommends using hybrid bearings equipped with rolling elements made of bearing grade silicon nitride (Si3N4).
A particular example is that of high speed air-cooled electro-spindles (A up to 1 600 000 mm/min) typically used in routers for machining wood, stone and glass (fig. 4) using two S70 W series sealed angular contact ball bearings mounted in tandem at both tool and non-tool ends in a back-to-back arrangement (spring preloaded).