splitter control valve...

[AKKAMAAN]

AKKAMAAN, when will you learn that knowledge and facts mean nothing.

I hear ya....LOL....I think and hope I'll never learn that...

Ultimately, it's not the valve opening that determines flow, up until full valve opening. The load downstream of the valve determines flow. Up until full valve opening, a certain amount of fluid is ported back to tank.

I would like to say it is the pump pressure and the load pressure, that determines the flow thru the work port. How much flow depends on how much bigger pump pressure is than load pressure. That is what we call DELTA P, pressure differens.

Basically, whatever the load couldn't handle.

I do not like that way to express it....This is the tricky part of the thing...

Let us say we have a 10GPM CONSTANT FLOW from pump.
The load on the actuator creates a pressure. Let us say load pressure is 1000PSI!
If we move the lever (spool) we will restrict the Open Center, so pressure increases from pump. Now work port is open a bit! as long as pump pressure is 1000PSI or less, ther will be no flow through work port. All 10GPM will go thru center to tank!! We will need a pressure higher than 1000PSI, how much higher depends on the friction in the cylinder (seals).
Let us say cylinder starts to move just a little (0.1 GPM) at 1050PSI. Then 9.9 GPM will be directed to tank. To ACCELERATE the speed, we will need to increase pump pressure by restricting center more.

This is the part most have a hard time with, most think that it is enough to open work port, and forget that we need to increase pressure to accelerate,. It is basically pure mechanics and Newtons laws that rules. Both the "port valve" and the "center valve" is synchronized because the are sitting on the same spool.

A valve rated for 10 gpm or a million gpm won't matter. The valve is still just an opening that fluid flows through. Whatever can't be used by the load is ported back to tank, until full valve stroke, no matter the size of the spool.

When we use a manual control valve we usually want to be able regulate cylinder speed. We want as many "speeds" as possible. We call this PROPRTIONALITY. Let us say, that we want "20" different speeds or maybe more. Then the "ball head" on the valve lever, need 20 different positions within its movement. If it moves 2", then each "speed" is within 1/10 of an inch.
In our case here, we were discussing how an over sized valve operates on a small flow.

REMEMBER we are talking about a CONSTANT FLOW system. We need to introduce the term PROPORTIONALTY! PROPORTIONALTY means that a certain spool movement restricts center to a proprotional pump pressure.

PROPORTIONALTY in a CONSTANT PRESSURE system means that a certain spool movement allows a proportional flow thru the work port.

What happens when we use an oversized valve is, that, maybe the first 1" of lever movement, never restricts the center enough, to increase pump pressure. NOW we only have 10 "speeds" left. WE HAVE LOST 50% OF THE VALVE PROPORTIONALTY!!
If flow gets lower, or if we use an even larger (way oversized valve), there will be NO PROPORTIONALTY left! We just got an ON-OFF valve at the very end of the lever (spool) stroke! NOT AT THE BEGINNING!!!

This is the way I always understood it. AKKAMAAN, is my logic correct?

I think you have a GOOD idea about it!

 

[Leejohn]

Very understading. I miss something before, was easy to get side tracked.

 

[Wayne County Hose]

Interesting reading AKKAMAAN, thank-you for taking the time to go over not only this but the many other subjects you have covered in numerous posts. I know it takes a lot of time and is mostly a thankless effort, but I know I speak for many of us here when I tell you that we do appreciate your efforts.

Andy

 

[spruce Deere]

Is there an issue if using a larger flow control valve but actually having lesser flow? That is, would it be okay to flow less than 20 gpm through a 20 gpm valve?

NO PROBLEM. If that is what you have on hand, use the D*MN thing. It IS like going 35mph in a vette. Akkamaan, nice schematics Proof TBN is a wealth of knowledge!

 

[Redneck in training]

I have only basic knowledge of hydraulics but I agree with AKKAMAAN. The spool is a flow divider until the center is fully closed. That means that the "orifice" that lets the oil drain back to the tank is also large when the valve is half way so it is hard to built pressure. You have to move past the halfway point to restrict return (create pressure) and direct the flow to the cylinder. Restricting the return matters.

 

[AKKAMAAN]

Interesting reading AKKAMAAN, thank-you for taking the time to go over not only this but the many other subjects you have covered in numerous posts. I know it takes a lot of time and is mostly a thankless effort, but I know I speak for many of us here when I tell you that we do appreciate your efforts.

Andy

Thanks Andy!!

I have made my living, for 25+ years, on teaching basic hydraulics, and when students starts to ask questions....You just need, starighten it all out and tell the truth, or say, "I DO NOT KNOW, BUT I WILL BRING AN ANSWER TOMORROW!!!"....I my world it has been impossible to make up stories, because the students will come back next day and say...."THAT WAS NOT RIGHT!!!!".....and then you are the only one that got fooled.....

I love to learn more about this, and I love to share my part of it too!!

All You guys have taught me a lot about tractor hydraulic now....and I love to read all your posts!!

 

[mrcaptainbob]

WOW! I sure am the better for having posed THAT question!! Thank you ALL so much. Great dialogue from all.

 

[AKKAMAAN]

I have only basic knowledge of hydraulics but I agree with AKKAMAAN. The spool is a flow divider until the center is fully closed. That means that the "orifice" that lets the oil drain back to the tank is also large when the valve is half way so it is hard to built pressure. You have to move past the halfway point to restrict return (create pressure) and direct the flow to the cylinder. Restricting the return matters.

 

[dcyrilc]

JJ,
Maybe I can explain it in a simpler way. Take a 3/4" garden hose connected to a faucet and nothing on the other end (simulates return to tank). Use a nail to put a hole in the hose (simulates work port). Turn on the faucet at a trickle (simulates large valve with small flow). Now begin pinching the hose near the end (simulates actuating the OC valve. How much will you have to pinch off the hose before you begin spraying water from the nail hole in the hose? Try this with the faucet all the way on (simulates mached flow and valve). The water will begin spraying through the nail hole much sooner (less valve actuation to generate power to the work port).

Clear as mud?

 

[AKKAMAAN]

JJ,
Maybe I can explain it in a simpler way. Take a 3/4" garden hose connected to a faucet and nothing on the other end (simulates return to tank). Use a nail to put a hole in the hose (simulates work port). Turn on the faucet at a trickle (simulates large valve with small flow). Now begin pinching the hose near the end (simulates actuating the OC valve. How much will you have to pinch off the hose before you begin spraying water from the nail hole in the hose? Try this with the faucet all the way on (simulates mached flow and valve). The water will begin spraying through the nail hole much sooner (less valve actuation to generate power to the work port).

Clear as mud?

Excellent!!