Preload is an effective way to ensure the bearing having the high working stiffness and accuracy. The analysis and calculation of preload and precompact of a kind of bearing is given in this paper. This bearing is a combo of double row angular contact ball bearing and double row self-aligning ball bearing. The main feature of double row contact angular bearing is that it can be preloaded during the integration. The direct result is that the working stiffness of the rotator

supported by the double row contact angular bearings

can be improved greatly. For the double row self-aligning ball bearing

just as its name states that its main feature is that it can compensate the misalignment of the shaft supporting bases. So

the main features of the combo are: preloaded to enhance stiffness and self-aligning to compensate the misalignment. We call this bearing two-piece outer ring double row self-aligning ball bearing. In order to be preloaded properly

the outer ring is consisted of two spherical inner face raceways

which form a spherical surface whose center is common to the bearing center

and a in-between spacer. Controlling the thickness of spacer is a way to control the bearing preload. The inner ring has two raceways. The steel balls

cage and inner ring can rotate freely at a certain angle. The allowable misalignment angle of this bearing is about 3 °. The relationship between preload and bearing supporting load is analysed and formulated with the help of mechanics theory. Meanwhile

based on the Hertz contact theory

the formular to determine the spacer thickness is given in this paper.This kind of bearing was adopted in China national key scientific project LAMOST telescope focal rotating axis and got perfect performance: the radial runout is less than 0.02mm and axial runout is less than 0.03mm of a φ1100mm bearing at arbitrary 50°angle（working range）. The tracking accuracy of focal rotation axis is better than 0.3″supportted by this kind of bearings.