I have to build a device to apply a pure normal force over a force plate. The requirement is that it doesn't apply any shearing (by means of friction) on the surface.
The problem is: for any given device structure, there will always appear some deformation, specially under load, and so I don't want to depend on geometric alignment, I would like to have my indenter to have an appropriate joint with the proper types of degrees of freedom.
I though about a ball bearing roller, but that would only decouple the force along one direction, since it could still transmit force by friction along the other direction.
So my question is:
What is the recommended way to eliminate coplanar reaction forces, so that only normal force is transmitted and coplanar forces are neutralized?
I think lubricant-based solutions don't fit well, since they are always quantitative in nature. I think a qualitative solution involving mechanical decoupling by a proper joint is better, similar to the ball-bearing one, but with an additional degree of freedom.