Hi Frederick,
The NodeEditor is not designed to do kinematics, but you can track the arm's rotation points via the "Select Filtered" node inside the Select menu. (Select/Select Filtered)

Pass the "Arm v01 Part 2 Shaft" (after rotation) into this node and set the mode to "Select Circle Edges", then use a "Bbox" node to determine its center point. (Objects/Bounding Box)

Here is the latest NodeEditor Plus version:
https://moi3d.com/forum/messages.php?webtag=MOI&msg=11614.1