nodrama,
"Duplex bearings are matched pairs of bearings with “built-in” preload. The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set"
I think that this quote from your tutorial is probably the important bit. IMO, the "preload" in this case is the achievement of the design running clearance and this is mainly done by the machining process. Where your tutorial refers to preload, I believe that it means precisely this, the amount of bearing clearance (or interference) when the assembly is clamped together, achieved through the machined dimensions of the components. Yes, the assembly will come with a design clamping load or torque setting to achieve and maintain the integrity of the assembly in use.
Where your tutorial goes on to say these comments:
"How much preload should be applied?
In general, provided the design requirements are met, the least amount of preload is desired
What are the benefits of preload?
Rotational accuracy and precise shaft positioning, elimination or reduction of ball skidding, control and reduction of axial and radial deflection under applied load, noise reduction, load sharing between bearings
Does preload have any effect on a bearings operational life?
Bearing life decreases as preload is increased
What other side-effects of preload should I know about?
With increased or excessive preload, stresses are higher and excess heat is generated "
Again, these refer to the machined-in preload, not the amount that the retaining nut is tightened. Obviously, the installation / assembly instructions for the components will be specific.
OAP