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Deciphering hydraulic spool valve diagrams

  1. #1
    Lifer ZX-12R's Avatar
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    Deciphering hydraulic spool valve diagrams


    So I'm in the market for a tractor and I haven't been impressed with the 3rd function options for the loader that the dealers are suggesting. For that reason, I'm investigating what it will take to build my own. I'm looking at both diverter valve options linked to one of the loader functions as well as a fully independent controlled implementation. In the latter case I would need a 4-way, 3-position valve with a tandem center for correct operation. In my quest to find these valves, I came across spool valve diagrams that have a total of 5-states when the unit is ultimately a 3-position device. See the attached picture as a reference and click it to make it bigger:

    Deciphering hydraulic spool valve diagrams-4way3pos5state-png

    My question is, what are the 2 extra states in-between the neutral and end states? I can't say I've encountered this nomenclature before and my best guess is this shows what the spool does while transitioning between states. If that's the case, since these are solenoid actuated valves the transition state wouldn't mean much but the 3C6 spool would be the safer implementation since the loader valve contains a relief valve.

    What says the horde?

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    Last edited by ZX-12R; 10-08-19 at 09:03 AM.
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    Lifer Kurlon's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    AvE has a vid on deciphering those IIRC.

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    Senior Member MHenry600's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    Yes, the middle ones are the transition state. On the top one, all ports are blocked. On the bottom one, all ports are open to each other. Although the valve moves quickly, this can be important. For example, if your pressure line has no other route to follow, and you block the port, you are deadheading the pump for a moment. Typically when using a switching valve, there will be another path (from P to T) with a fixed pressure valve to act as a bypass. Set this to you max system operating pressure so that you don't pop something.

    On the other hand, if all ports are open to each other when switch, you might loose a little pressure that you wanted held in there (for location or pressure control reasons), because your A and B lines are allowed to dump to T.

    Both valves are spring to center, and dual solenoid as well...

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    Lifer loudbeard's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    Hell yeah. I read this earlier and just thought "yeah, it's been about 12 years since my basic hyd. class where I 'learned' this stuff" and was hoping someone with actual knowledge on the subject would reply.

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  5. #5
    ^ It's my bike and my car tls25rs's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    Is there a hydraulics for dummies website you can point me to so I can try to figure out what it is you guys are discussing for myself?

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    Lifer ZX-12R's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    Quote Originally Posted by tls25rs View Post
    Is there a hydraulics for dummies website you can point me to so I can try to figure out what it is you guys are discussing for myself?
    I don't have a good reference for you to look at but the concepts aren't too complicated.

    Take a look at this valve diagram which is easier to see than what I posted earlier:


    This valve has 4 connection points (P, T, A, B) and 3 positions it can be in which are designated by the 3 large squares. This makes it a 4-way, 3-position valve. P is the high pressure input port for the valve which is fed from an upstream valve or directly from your pump, T is the low pressure return port to tank, and A & B are the work ports that connect to the hydraulic device you are trying to actuate. In my case, it would be a double acting cylinder that can be powered in both directions.

    On the left side of the diagram you see a handle symbol which means it's manually actuated by a handle. The symbol all the way on the right means it has detents and will hold whatever position you put the handle in.

    When the handle is in the center position, you see there is a line connecting P to T while both A and B are blocked. In this state, all the flow from the pump through the P port is returned to the T port which would return to your fluid reservoir if it's the last valve in the chain, or it could be connected to the P port of a downstream valve that controls another function. With A & B blocked, no fluid can enter or leave the cylinder so it's effectively locked in place.

    When the handle is moved to position A (leftmost square), picture the blocks sliding to the right so the vertical arrows are lined up under the P, T, A, B ports. In this state, high pressure fluid from P would be directed out work port A causing the cylinder to move in one direction. The fluid being forced out the other end of the cylinder at low pressure would flow back into work port B and back to the tank.

    When the handle is moved to position B (rightmost square), the crossed arrows are lined up under the P, T, A, B ports. In this state, high pressure fluid from P would be directed out work port B causing the cylinder to move in the other direction. The fluid being forced out the opposite end of the cylinder at low pressure would flow back into work port A and back to the tank.

    The valve diagram in my first post has a 4th and 5th box which shows what happens while the spool inside the valve is in transit. I hadn't seen a 3 position valve illustrated that way before which is what led to my question. It also has different nomenclature on the very ends. The squiggles on both sides means there are springs that put the valve in its center state when no input is provided to it. The rectangle with the slash through it means the side (both in this case) are controlled by an electric solenoid. The narrow rectangle above that (also on both sides) means the valve also has a manual control as well.

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    Last edited by ZX-12R; 10-09-19 at 07:58 AM.
    "...i would seriously bite somebody right in the balls..." -bump909

  7. #7
    Lifer ZX-12R's Avatar
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    Re: Deciphering hydraulic spool valve diagrams

    There are tons of valve options available and these are just some of the options for just directional control valves.

    Deciphering hydraulic spool valve diagrams-spool_options-jpg

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