Tossing around the idea of building a tractor again. I drew up this feedback linkage for controlling a 3 point setup with a position controlled lever using a standard valve. Nothing to scale, just a diagram to get the idea across. Red circles are hard pivot points, yellow is a "walking beam" type lever, and blue are the pushrods. front of tractor to the left. As lever is pulled back it pushes the top of the walking beam back, angling it and pushing in the valve, which pushes out the cylinder, moving the bottom of the walking beam out and releasing the valve to stop cylinder motion. Won't be able to slam it up and down obviously, but with a fairly quick cylinder on the rock rod it should work fine. Rock rod would be connected to lift arms with chain to facilitate floating, or a single acting cylinder could be used. I like the double acting because I could replace the chain with solid bars to give downforce if wanted. Let me know what you guys think, and sorry about the .jpg, it's hard to see, I'll try to upload the .dxf too.

teoc, I couldn't get much from the drawing but My old Allis Chalmers tractors employ "rock rods" if I understand you and I could post some pics of my set-ups and mods.

This design uses rock rods as well. diagram is more to show the feedback linkage to the hydraulic control valve. Pretty sure commercial tractors use a special valve, but if you have pictures of a linkage that allows a standard valve I would appreciate it!

Okay, here's a couple of pics of my AC-B set-up.
Sorry, I have either lost the ability or forgotten the procedure for posting pics.
If you want to PM me your e-mail address I can send you the pics.

I posted pictures today.

Tom...is that your backyard??

that picture ls from the mainland. here are some from around the island and a deer that came to visit.

Nice pics tom, even if they are a bit off topic.

OK: I forget everytime that on the new site that pictures will not show unless logged in. My machinebuilders.net is always logged in except when it's not. After trying to get a picture to show (the old site did not require you to be logged in) for several sec. I just logged in...much better now.

I could see the diagram above before logging in however. Logging in did not improve picture. What red circles?

Nice pictures.

that is why I put up with long winters and short summers.