Ring Magnet on a Ferromagnetic Shaft

What happens when i use a Ring Magnet e.g. MAG10-2R on a on a Ferromagnetic Shaft?

This depends on two factors: 1) The permeability of the shaft and 2) the shape of the shaft.

Let’s address (1) first. Though motor shafts are often steel, my experience is that they are commonly some type of stainless steel which has pretty negligible permeability. If we are dealing with a relatively high permeability, then we need to consider point (2): Is the shaft axially symmetric? If so, the field will not be deformed, so we don’t need to worry about any effect on non-linearity. The B field will, however, be weakened as the shaft will tend to “suck” some of the field into itself. In that case we may go with a slightly larger magnet or a higher grade of magnet which has a higher remnant field to compensate for that.
Our worst case scenario is if the shaft is high permeability AND non-axially symmetric. In that case, we have to look at either a different mechanical approach or try to compensate in some creative way.
I’m happy to say that in my experience, I cannot recall a customer application where we have encountered this scenario; we’ve always been able to solve with a simple approach.