In reply to climbingjoca:
> you think? I would limit the movement i.e put stops so there is only a limited travel for the spring on the shaft this in a way wouldn't be to much different to the springs on standard cams only used to hold the cam in place
If your expansion framework only has a spring to keep it in place, it will:
a) fight with the cam springs
b) compress when the cams try to engage and apply an inward load.
b) is how the cams work; you need a sideways force between cam and rock to create a frictional force to prevent the cam slipping. And, given the shallow angle, that inward force is significantly larger than the vertical force (imagine rigging a sling at that angle; you're into Death Triangle territory). The ratio of these two forces gives you the theoretically available limiting coefficient of friction.
You'd need some locking mechanism to hold the framework fixed once it's been placed. Taking design_crisis' comment "If you get a standard wine bottle opener", you might envisage a vertical shaft that had annular notches, like the wine bottle opener, and some mating structure that would swing onto the shaft (and lock in place...) to prevent it moving.