I'll take a shot at it /forums/images/graemlins/smile.gif
I've never done this, but have wanted to for some time and I don't see why it wouldn't work (add homemade power steering to an old tractor that is)...
On most older manual steering tractors you should have easy access to the rod that connects the steering box arm to the arm that actually turns the wheels (sorry, not sure of all the proper terminology). Imagine that you remove the front (wheel) end of this rod and attach it to the spool of a hydraulic valve instead. Now when you turn the wheel just a little to each side the valve spool will be pushed in or out just like when you push or pull on the handle that might normally have been connected to this valve.
Now imagine that there is a hydraulic cylinder with the ROD end attached to a bracket on the tractor frame and this cylinder is plumbed into the above valve. When you turn the wheel a bit each way, the cylinder will extend or retract and since the rod end is fixed, the outer cylinder moves back and forth.
Next we will fasten the valve body to the outer part of the hydraulic cylinder so that when the cylinder moves, so does the valve. Now move the wheel a little bit to one side, the steering rod pushes on the valve spool, the cylinder starts to move, and the valve body "catches up" with the spool, thereby shutting the valve off again and everything stops moving. Move the wheel a bit more, everything catches up again.
So now we have a hydraulic cylinder that moves in proportion to steering wheel movements but with the power of, well, a hydraulic cylinder. The cylinder body will basically just follow the position of the steering rod. We have essentially built a hydraulic servo. Now all we have to do is also connect the cylinder body to the wheel side of the original steering arrangement and you should have a reasonably decent power steering arrangement.
You will have to make sure the valve to cylinder plumbing is the correct way around of course, and without a source of hydraulic pressure you will probably not be able to steer the tractor at all. The orbital steering valves on modern hydrostatic steering systems act like a pump when you turn the wheel with the motor off and so you can still steer with a bit more effort. The above system will not be able to do that, so consider the possible safety implications.
I hope that long winded description is clear enough to understand. If not, let me know and I'll try to draw a picture or two of what I have in mind. Be warned, however, I can't draw worth a cra.... um, darn. /forums/images/graemlins/tongue.gif
Brad.