I'm trying to rig a Walschaerts valve gear, which looks like this:
I've got some of it working, but I can't figure out how to get the combination lever (the mostly vertical bar between the wheels and pistons) to be influenced from both sides while attached to the valve stem (the upper piston) which it rotates about. The point where the valve stem attaches to the combination lever must not move vertically on the z-axis, but does need to move horizontally along the y-axis, which is what controls the valve stem.
I've got one IK constraint on the large yellow bone + the two bones guiding the lower piston. I have another IK constraint on the small yellow bone all the way through the two bones controlling the upper piston. I just don't know how to influence that from the top as well.
I've tried adding another IK constraint to the bone beneath the highlighted radius rod, but I can't get it to work. I basically have one rod, and each end needs to be acted on by a different IK constraint.
In case it is of use, here is my blend: