Hydraulic flow

[3Ts]

Here is my understanding of how this works. The system starts with a pump @ 11 gpm and 2800 psi and is open center. This means that the pump is running when the engine is running and pumping fluid thru the valves and into the tank so nothing is happening which also means the pressure in the system is much less than rated. It follows the red lines to the backhoe valve, then since there are only 2 lines in the backhoe valve (one for pressure) the other has to be the dotted blue line to the tank.

- Why can't the blue dotted line be replaced by the red dotted line as long as the 3PH valve is always centered and not moved? It produces
the same result, the fluid flowing thru the valve to tank.

Now, to actuate a cylinder a valve is moved to block the flow and the valve diverts the flow into the cylinder at one end and provides a "drain" for the other end of the cylinder (we'll leave regen out for now). So, now that the valve has blocked the fluid flow, the downstream pressure falls to zero and the draining fluid continues on down the line thru the centered valves to the tank. Now, if it could be guaranteed that only one valve could ever be activated at a time, only the red lines would be needed. Right? The problem comes when a loader valve and a backhoe valve are activated at the same time. If this happens the fluid from the pump is directed into a cylinder, the fluid from the opposite end of that cylinder instead of meeting zero pressure is then directed into the next cylinder and builds up to system pressure and the fluid from the opposite end of the 2nd cylinder is drained to the tank. If the valves are held in position the overpressure valves will release the pressure downstream, but since the 1st valve directs the flow into a pressurized line then what? Wouldn't the pressure valve on the 2nd bank then release the pressure downstream? Or does the 2nd overpressure valve not protect the input port and only the actuated valves? I don't think I understand why a 3rd line is needed except as a case drain for valve leakage.

My backhoe is a subframe mount and I plan to pull it off and replace it with a set of stack valves to control a hydraulic blade with 4 cylinders. One of the cylinders I will put on this is 2x8 so with 11 gpm the extension time will be less than 2 seconds. I obviously need some sort of metering valve on this. I know about the disk with the 0.03 hole in the fitting and a needle valve to control the flow rate. I've also come across a metered valve as one option in a stack valve that Surplus Center has which appears to do the same thing but at a variable rate. Is there any reason not to use a metered stack valve for all cylinders? Why aren't they always used, their cost is only a few dollars more.

Any thoughts or comments are welcome.

 

[MossRoad]

Could it be that the 3pt hitch valve is only a one-way valve? It's power up, but it's gravity down. So it needs a separate return to the tank.

 

[MossRoad]

Here's a pretty good animation video that shows the operation of an open center valve. It has to have a return to tank. And, it appears that loader valve has power beyond port to feed the next valve down the line. And so on.

 

[MossRoad]

Here's a link to Army Field Manual FM 5-499 hydraulics.

http://www.derijcke.com/dl/manual.pdf

Go to section 2 and it explains open center pretty well. There's a good diagram on page 2-4 after you read the explanation at the start of section 2-2.

 

[MossRoad]

Here's a screenshot of that diagram. It pretty much shows your exact circuit, doesn't it? Each valve (or valve bank) feeds the next valve power, except for the last valve in the line, and each valve (or valve bank) has a return to tank.

View attachment 587357

 

[Tractor Seabee]

The key to multiple functions and more than one valve in an open center system is the power beyond (PB) line from one valve to another. Each valve has its own return to tank (T) line. Open circuit valves require a plug added to activate the PB.

Ron

 

[texasranger556]

- Why can't the blue dotted line be replaced by the red dotted line as long as the 3PH valve is always centered and not moved? It produces
the same result, the fluid flowing thru the valve to tank.

View attachment 587347

You're correct that you can replace the blue dotted line with the red dotted line if you can guarantee that the 3 point valve would not add resistance (pressure) to the backhoe's tank return port. Manufacturers don't do it that way because it exposes them to warranty claims if the lowest common idiot that purchases their tractor dead ends the 3 point and damages the backhoe valve by over pressuring the tank return port. Look at the specs of spool valves, they will specify a maximum back pressure on the tank port, often a couple hundred PSI versus a power beyond port rated at full system pressure.

Edit: My reply is assuming your backhoe valve only has 2 ports (IN and OUT) without provision for power beyond. My backhoe is the same way.

 

[texasranger556]

My backhoe is a subframe mount and I plan to pull it off and replace it with a set of stack valves to control a hydraulic blade with 4 cylinders. One of the cylinders I will put on this is 2x8 so with 11 gpm the extension time will be less than 2 seconds. I obviously need some sort of metering valve on this. I know about the disk with the 0.03 hole in the fitting and a needle valve to control the flow rate. I've also come across a metered valve as one option in a stack valve that Surplus Center has which appears to do the same thing but at a variable rate. Is there any reason not to use a metered stack valve for all cylinders? Why aren't they always used, their cost is only a few dollars more.

View attachment 587347

My guess is a metering valve costs more, is one more place to leak, and in most cases unnecessary. If it's too difficult to simply modulate flow with the spool valve, OEMs install a small orifice inside the line on the implement side to limit flow. Then it isn't up to the customer to remember to adjust the metering valve depending on which attachment is installed. Again, lowest common idiot. Can you tell I work for an OEM? Different industry, but same concepts.

 

[jaydee325]

Here's a link to Army Field Manual FM 5-499 hydraulics.

http://www.derijcke.com/dl/manual.pdf

Go to section 2 and it explains open center pretty well. There's a good diagram on page 2-4 after you read the explanation at the start of section 2-2.

I would suggest starting with section 1. Many don't understand the basics of hydraulics and this leads to confusion from the start.

Just remember, a fluid under pressure will take the path of least resistance.:thumbsup:

 

[3Ts]

When I mentioned a metered valve, what I had in mind was something like this that has the notches in the spool where a small lever movement would start allowing fluid to flow via the notch. And the farther it was pulled, the more fluid would flow.


Instead of a separate valve in-line that would have to be adjusted each time. The cost seems to be only a few dollars per spool. I've seen the fittings with the .03" hole, but don't know where to find them. Not even sure what they're called so don't know what to search for.