# Does 6/2 valve make any sense

As for the 6/2 valve commonly used, there’s typically no technical need for it, except as a safety cutoff. If you omit the valve, only technical issue is that to use pressure disengage on autosteer, you need to steer in the opposite direction to the autosteer steering to generate the LS pressure from orbitrol. I was mapping the alternatives, and I think with the 6/2 the safety is not really increased, see attached pdf for the different options.

steering_safety.pdf (37.5 KB)
EDIT 01/12/21 Reupload: steering_safety1.pdf (118.5 KB)

If you have a 6/2 valve in between the autosteer and orbitrol, the safety relies heavily on the 6/2 valve, should the valve remain in autosteer for any reason (spool stuck, electrical fault) there’s no manual steering available. It doesn’t really matter what happens on the autosteer valve side.

If you just omit the 6/2 valve completely and tee directly into steering lines, the failure mode causing no steering is that your autosteer valve spool doesn’t return to the middle when powered off and steering pressure bleeds through the open A/B channels.

So given the above scenarios, the 6/2 valve doesn’t really decrease the failure probability of the system, instead it’s just defined by the failure rate of the 6/2 valve instead of the autosteer valve. With a high quality autosteer valve, you might have actually a lower rate of failure that with some off the shelf 6/2.

Now, if you install a 4/2 cutoff valve to the steering lines, the failure of probability drops, as in order to lose manual steering both the 4/2 cutoff must be stuck open and the autosteer spool not be in the middle at the same time.

So if you have a failure probability of 0.1% on all of the individual valves, both 6/2 and direct tee connection have the same failure probability of 0.1%, but with the 4/2 cutoff, you have a failure probability of 0.0001 %.

Furthermore, bit depending on the autosteer valve type, you will have different amounts of safety built in. With a direct PWM drive, anything that gives voltage to the spools will move the valve. If you on the other hand have a valve with some built-in error control, it will go to a safe mode if something weird happens on the command signals or can communication.

Thoughts?

If you look at Danfoss SIL certified OSPE system, for example, they are using a cut-off spool that cuts off the autosteer valve and the pilot pressure to the autosteer valve, not a 6/2 selector. They also use a steering wheel angle sensor for disengage. When in electrohydraulic mode, the steering lines are just tee’d to the orbitrol.

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I think you need a 6/2 valve, if the orbitrol is reaction-type. The steering wheel will turn every time the autosteering is active. For a non-reaction orbitrol, i agree with you.

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I agree with Martin. The only reason I got a 6/2 valve was to stop steering wheel rotating.
But there is a guy in DK who use this double overcentre valve instead, adjusting is as low as possible, and that way have almost no feeling of resistance when autosteer is not in use. (a normal double piloted cut off valve, would shift at around 3 to 6 bar)
It could be one like this: VBCD DE A CC - Double Overcentre Valves For Closed Centre - Type A - Oleodinamica Marchesini

True, the OSPE internally cuts off the A/B reaction lines to not rotate the wheel when steering with EH valve.

Steer hydraulics is not something I’ve looked at much apart from studying how Fendt systems operate.

Looking at your PDF the one scenario I would definitely not want is the first, for the obvious failure reasons. Even with scenario 3 you’d have a fighting chance of combating AS valve flow with the orbitol.

Are people actually using this modification (1) on road going machines?

I just used me 6/2 valve as a 2 channel on / off valve… Stock steering pipes are just T’d…then the 6/2 valve has black plugs in 2 ports, and the other two go to the proportional valve… So the 6/2 valve is essentially just turning flow on / off to the proportional valve… That way, worst case if the steering valve did go crazy AND the 6/2 valve failed in the on position, the steering wheel is still connected, so with force / speed you would have some control. Chances of all this failing at once obviously extremely low, but do also have lever valves directly after the T’s so can be mechanically isolated also when on the road as double failsafe…

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That sounds a lot better than scenario (1) in the PDF above!!

Well looking at that PDF, my setup is essentially scenario 2…but with a 6/2 valve with plugs in 2 ports instead of a 4/2 valve…

Yes, that is what you get, when you buy an aftermarket valve unit.
That is how the 10 year old SBC system on our NH 135 works.
And as Flyidsystem block is made as well, the 6/2 is spring loaded to return to orbitrol setting.

BUT as you say?? it can get stuck or 12 V hangs or!

I’d be very concerned about sticking in the orbitol blocking position. I’ve had valves stick on more than one occasion and the problem with this design is you’ve got no chance to fight it.

I have a hole in the roof of one of my sheds where the slew spool of my Hitachi EX135 jammed and put the bucket straight through it. Turned out to be a pump issue that put debris into the line, jamming the valve open.

I am a little worried as well, but as the first screen on startup of SBC system " Must not be operated on road" and also the pictures we saw recently of tractor in ditch, human fault is more likely.
You cant leave the field if valve is stuck, so to happen on road either 12 V should happen by shortcut (because autosteer switch with 12 v to system is off), or the inside spring should suddenly break.

Most of the retail steering kit on the north American market tee directly into the existing steering lines. I don’t know about the newer factory stuff. I believe most manuals tell you to shut off power to the system when you are roading. .

“Tee-ing in to the steer lines seems a lot more sensible than adding a blocking valve to me.

Having a reactive orbital is a separate issue that perhaps needs addressing separately?

I am firmly in the scenario 2 camp, and my Pcb and ino have extra control logic to control the 4/2 valve separately from the proportional, so hopefully it is robust against program errors. Mosfet driver power is derived from the e-stop circuit, which must be hit for road transport.
Seeing as a hydraforce 4/2 valve only costs about £80, in a reactive orbitrol situation I think it’s best to put a second 4/2 valve, oppositely ported like the SV08-42, in the orbitrol lines, sharing power with the safety valve.

If you still have this pdf can you reupload it?

As a help for @nut I did the necessary modification to the file steering_safety, so you now can download it. It is just below the original one in post 1.
Since we moved to our own discourse the files (from old discourse.group ) must be saved as another pdf to get around the UPS wrong homepage problem. (rename pdf is not enough, but I suppose making a new pdf from original word/writer file would work)

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