Originally Posted by
Onceapilot
I think if you look very carefully, you will see in that diagram that the outer races are made with a deeper track on their inside faces so that they can carry a greater axial load in one direction. The diagram does show that it is designed to carry equal load in each direction but, offset. Obviously, there are a huge variety of options that suit different applications. I believe that the "preload" term may be something of a misnomer. The manufacturing tolerances are designed so that as the inner races are clamped, and move together, the clearances of the balls in their races are positioned where they are designed to run so, this part of the process is one of correctly positioning the bearing components. Also worth noting that, unless the parts are Murphy proof, the component bearings might be assembled in wrong order(s), with their characteristics corrupted. Depending upon the method of clamping and holding the inner races, the required torque load on the retaining nut will also be defined and this will be part of the overall design.
OAP
I had already looked carefully and saw that the "load lines" on the diagram show equal angles once the pre-load is applied. I'm familiar with Duplex bearings to some extent (I needed to research the subject in the past before buying some for my various vehicle projects).