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  1. #1
    Veteran Member dstig1's Avatar
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    Default Diverter valve - spool type question

    I am upgrading my tractor and will need to do a 3rd function again. I'm trying to source the components at lower prices now that I know what I'm looking for (mostly), which leads me to my question. The Bosch valve on my old tractor uses the "6B" spool type from the drawing below where the work ports are open to the input ports during the transition from position 1 to 2 (brief as it is). It looks like they would have to special order that, but can get the "6A" style from stock. 6A is closed at the transition point. I doubt it will matter either way, but I figured I could ask here and get confirmation or pointed another direction. Thoughts/concerns/ideas?

    Diverter valve - spool type question-bosch_valve_types.jpg
    -Dave

    "Being a pessimist is great. You can't lose. Either you end up being right...or you are pleasantly surprised."

    L5240HST, QA, 824 Loader, 48" Forks, 48" Grapple, Ancient Farmi Skidding winch
    Trailer - 10k/16' twin axle w/elec brakes
    2005 F250 5.4V8(3V) 3.73/4wd tow vehicle

  2. #2
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    Default Re: Diverter valve - spool type question

    Dave:

    The "6B" selector valves have an overlap so that if you shift while pressurized fluid is flowing into the SV and out a work port (you are extending a cylinder) there is at every moment during the transition a place for the pressurised fluid to go (i.e., to extend the cylinder in the other circuit). This avoids a sudden stoppage of the pressurized flow and the resultant pressure spike. It is just like the water hammer in home plumbing when the faucet is suddenly closed at the end of long piping; the kinetic energy of the moving fluid creates a pressure spike much higher than the normal system pressure. This pressure spike may ultimately trigger the system relief valve, but only after it reverberates off the closed selector valve and travels back to the relief; in the meantime the pressure spike can damage hoses, etc.

    The disadvantage to selector valves with overlap is that if you are using the valve to actually hold the cylinder extended after the work ports are shifted to the other cylinder, there will be pressure from the extended cylinder that may overcome the pressure (if any) in the other work port while they are connected, which would allow the extended cylinder to fall a bit and force fluid out the other work port. As a practical matter, selector valves usually have too much leakage even when closed to hold a load in position for very long.

    So the "6A" selector valve with no overlap should work fine if you remember to shift it only when no fluid is flowing from the pump port through a work port. If the SV is in its typical location downstream from a directional control valve, then the DCV should be in neutral, so that no fluid is flowing to or from the SV, when the SV is shifted.

  3. #3
    Veteran Member dstig1's Avatar
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    Default Re: Diverter valve - spool type question

    Thanks for a very thorough explanation. Based on this, I think I should stick with the 6B style. Since this is a button on my loader joystick, it is very easy to hit it a bit early or late when trying to clamp/open the grapple. If the risk of pressure spikes is that significant, I should aim to avoid that problem by going to the B valve.

    Thanks!
    -Dave

    "Being a pessimist is great. You can't lose. Either you end up being right...or you are pleasantly surprised."

    L5240HST, QA, 824 Loader, 48" Forks, 48" Grapple, Ancient Farmi Skidding winch
    Trailer - 10k/16' twin axle w/elec brakes
    2005 F250 5.4V8(3V) 3.73/4wd tow vehicle

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