maybe I can shed some light on this.
How we define the pre-stress in the bolt similar to what is done in an interference fit: we intentionally design the geometry of the bolt a little shorter and the repositioning of the contacting nodes on the surface of the contacted body will result in a strain and thus a stress in the bolts. In real life interference fits are created with expansion/shrinkage during heat treatment (and normally not on bolted connections), but for our purposes this should work fine with the shortening of the bolt.
The question is now, why do we define this mysterious remote displacement?
The short answer: to make our life easier.
The longer one: If we use a frictionless physical contact, the bolts movements are not statically defined, they are free to rotate around their middle axes and also free to slide in X- and Z-direction (imagine the contact zones being made of ice). To prevent this we use remote displacements, put each somewhere at the center axes of a bolt and restrict rotation around the Y-axis and the two rotations. So we create a new node, restrict its rotation and displacement accordingly and connect via RBE3 connections to the bolts.
Now this should in theory should be enough to get a statically defined system, but still the z-movement may cause problems. The bolts are only restricted from moving along the z-axis by the two opposite contact zones, so finally the contact forces. If those two opposite forces are not exactly equal (which is very likely in a numerical simulation), we get a resulting force which will accelerate the bolts along z. In a static analysis this will lead to an unsolvable system or totally unphysical results (almost infinite displacement). So we use a trick: We pose the remote displacements at the two opposing ends of each bolt and restrict the z-displacement. BUT since we selected a deformable behavior, the two ends are still able to deform in z-direction, only the total “average” of the displacements has to be zero. And we actually would expect this to happen.
I hope this is clearing up some of your question marks regarding the remote displacement boundary condition and why it is used here. You can also find some additional information about this boundary condition in our documentation.
We could actually check if our approximations are correct if we would set up a frictional contact (and possibly use a dynamic analysis for the z-Force problem) and compare the results with our approach. If you want to go this extra mile I would be happy to assist you if problems appear.
Concerning the total displacement at the end of the arm, I’ll have to take a closer look at the model to see what actually makes sense. Theoretically both positive and negative values could be correct because of the pre-load, only a positive trend has to be visible when applying a positive Y-force. I’ll check it and come back later with my conclusion.