Self-aligning ball bearings are constructed with the inner ring and ball assembly contained within an outer ring that has a spherical race way. This construction allows the bearing to tolerate a small angular misalignment resulting from shaft or housing deflections or improper mounting. 
The bearing was used mainly in bearing arrangements with very long shafts, such as transmission shafts in textile factories. 
One drawback of the self-aligning ball bearings is a limited load rating, as the outer raceway has very low osculation (radius is much larger than ball radius).