Everybody uses steering valves, but unfortunately, all of our clients, and probably most of the techs I know, approach orbital steering valves as "users" and never go into much detail about how exactly these beautiful things operate. If you do so too, you are likely to learn something new from the following series of articles. I won't be able to fit everything in a single post, so it's going to be steering valves for another couple of weeks, at least.
But let me begin with a story, that hopefully will "hook you up" on the steering valve "mysteries".
A client brought over a Danfoss OSPC ON (open-center, non-reaction) steering unit the other day, saying that "something strange" was happening with it. The tractor steering wheel would jerk back and forth while driving over rough terrain, and he was wondering if it could be the steering valve (as his mechanic suggested) or if it was the faulty pump. So, we tested the steering unit and it passed with flying colors. It had to be the pump then, and indeed it was the pump. But this seemingly simple case rose a very interesting question of how exactly a non-reaction steering unit could be casing steering wheel jerks. You know, with it being non-reaction and all...
Naturally, when you see something contradictory in hydraulics (and if you work in this industry you will be finding contradictory stuff all the time) what you do is run tests. So that is exactly what I did.
I hooked our bench up to the steering unit so that it was injecting oil into the work ports, and left the P port disconnected, to simulate a situation when the pump has failed and the steering cylinder is "pushing the oil back" into the valve. And, indeed, I saw something seemingly strange. When there was no input to the steering wheel, pressurizing the ports didn't result in any movement of the steering wheel - which was absolutely normal for a non-reaction steering valve, but when I would turn the steering wheel in the direction of the port that was currently being pressurized by the bench (our imaginary steering cylinder), I would immediately feel the strong reaction of the steering wheel pushing back with much force. This could still be explained by the fact that to reach the relatively high pressure in the work-port a more "muscular" input would be required for this pump-less system, but what I could not explain was why, if I didn't provide enough force at the steering wheel to overcome the port pressure, but simply kept only minor input in the "pressurized" direction - the steering wheel would rotate back with significant speed and force as long as I kept the light input. The only way to stop this was to release the steering input completely.
From the "tractor driver's" standpoint it would feel like this:
You are driving the tractor with your hand on the wheel, and if you chanced to slightly push or pull on the steering wheel towards a direction that was being currently "pushed back" by the steering cylinder due to the "tractor-ly terrain conditions" - the steering wheel would be immediately pushed back, and if, instead of grabbing it and applying enough force to stop it, you would only keep your hand sliding on the wheel, providing merely some steering wheel "braking", if you will - it would keep rotating until either you would grab it, or you would remove your hand from the steering wheel altogether, allowing for the non-reaction steering valve to close off the work ports and lock the steering cylinder. This looked strange to me because the "official" schematics seemed to suggest that when such a steering valve is in a controlled position (with a steering input applied), any pressure in a work-port should not be able to push the orbital motor section back because of the check valve in the P port:
I am having a really hard time explaining this with words, so let me show you exactly what I mean with a video:
OK, let us resume the facts:
1) When a work-port of this steering valve is pressurized, and there's no P supply, the steering wheel will rotate indefinitely as long as there's some "steering wheel braking" present.
2) The only three ways to stop it from rotating in this situation would be:
Now, look at the hydraulic diagram of the steering valve, and see if you can think of a valid explanation for this phenomenon. And I will be revealing why this happens in my next week's post. I see you all then!
I suppose steering valves aren't that simple after all, are they? This is not a broken valve, I promise. A brand new OSPC ON would do the same!