When mounting a pair of angular contact bearings, the best technique is:

The "O" arrangement, or face-to-face mounting of angular contact bearings, is a highly effective technique when it comes to managing axial load and ensuring rigidity in the bearing setup. This configuration allows both bearings to support axial loads in both directions, which enhances the overall stability and performance of the assembly.

In the "O" arrangement, the contact angles of the bearings are oriented towards one another. This configuration leads to a more uniform load distribution and minimizes the potential for misalignment or differential thermal expansion between the two bearings. Additionally, this setup helps to limit the risk of point loading, which can lead to premature wear or failure of the bearings.

Using this arrangement is particularly beneficial in applications requiring high precision and tight tolerances. It provides a robust solution for dealing with radial loads while also effectively managing axial forces, making it a preferred choice for assembly in various mechanical systems, such as spindle applications in machine tools.

Other arrangements, such as the "X" (back-to-back) or tandem configurations, while useful in different contexts, do not offer the same balance of load distribution and axial support as the "O" arrangement does, particularly in applications where axial forces in both directions need to be managed effectively.