After looking into how Danfoss non-reaction orbital steering valves work in detail and seeing that it's not that complicated, I then thought to myself:
"Wouldn't it be a great thought experiment to take what I learned and imagine a design for a reaction steering valve?" By "reaction" I mean a steering valve that allows the steering cylinder to push the steering wheel back even when there's no steering input, which results in a vehicle that can, for example, straighten the wheels after a turn or follow a rut "by itself".
So, I knew that in order to achieve the reaction, I needed to come up with a spool valve that connects the work ports to the orbital motor when the spool is centered, and then does all the rest it needs to do when the spool is steered. And I confess that I did give it a couple of honest tries, but then said "na-a-a... I'm too lazy for this", and shamelessly succumbed to the temptation of peeking inside a "factory-made" reaction steering valve. Shame on me, I guess, but I will share what I learned anyways.
But before we go into the exploration of the reaction steering design, let me show you something cool real quick. I just got this Danfoss steering valve for an overhaul. Long story short - it's busted beyond repair, but check out its sleeve valve:
See something interesting? No tank slots in the inner sleeve! (Like the ones we saw when we were back-engineering an open-center Danfoss steering).
This can only mean one thing - this is a closed-center steering valve! See how nice it is to know the design details of stuff you work with? If this steering showed up without a nameplate, first - I would be able to tell it's closed-center just by looking at the parts, and second - I would also be able to take the orbital gear, measure it with a caliper, and calculate its displacement (something that we figured out how to do by ourselves when we studied the orbital motor simulation last week). Pretty neat, don't you agree?
Now, let us go back to exploring steering valves. Here you have a reaction steering spool valve next to a non-reaction one:
Aha! Things are getting clearer. The secret is in the double slots! If you peek inside the holes of the L and R ports and the orbital section, you can tell that when the spool valve is centered, the L and R ports are connected directly to the orbital via the two parallel slots:
Very clever. I would have never thought of something like this. Once again, this shows how ingenious the mechanical design engineers who work with hydraulic components need to be! And we are barely scratching the surface here. There are a ton more subtle details in making all this steering thing work in a smooth and stable fashion. For example - why the P to T by-pass is so different on this model in comparison to the one we saw in our back-engineering session? They obviously perform the same function, but why go through the hassle of machining these rectangular slots? Why not just drill the sleeve though like we saw in the other model and be done with it? And why the tank port connection is now a slot "shooting" the oil forward and not a simple pair of holes? I am sure there's a technical reason for it, and I am also sure that I am not smart enough to tell why! Well, at least knowing that the world is full of people that are so much smarter than me is comforting...
Let us finish with representing the feedback principle in a diagram - something that always helps me make things clearer. Black represents the inner sleeve and blue is the outer sleeve of the spool valve):
Great! Now we can look at a spool valve and tell if a steering valve has reaction or not. And I want to tell you where I want to go from here - load sensing! Both dynamic and static. That's my next stop. Unfortunately, I have only been receiving static LS units lately, but as soon as I get one with dynamic LS, I am back-engineering the heck out of it, so stay tuned!