Does Open Center = Draft, Sag?

[Colgate2004]

Hi all,

I am a complete newbie when it comes to hydraulics, so please pardon the newbie question.

In doing some reading on open-center vs. closed-center valves, I understand that the kind you need to buy is based on the type of pump in your tractor (whether it's constantly pumping or variable-flow). The pump in my soon-to-be tractor is open-center (it's a JD 3025E), so I understand I need to buy open-center valves if I add spools (which I plan on doing).

I also read that open-center valves allow fluid to flow from the extend-side of the cylinder to the retract-side while the valve is in neutral (center) position.

It occurs to me that if that's the case, wouldn't that mean that the cylinder would be prone to drifting, i.e., moving slowly on its own, without you telling it to, based on the forces on it? For example, if a cylinder was holding something up (say, a heavy implement), the force of gravity would push on the cylinder, and since fluid could freely flow from one side of the cylinder to the other, the implement would drift lower over time?

This seems like it would be completely unacceptable so I have to imagine it isn't the case, but if an open-center valve allows fluid to flow from one side of the cylinder to the other, I don't understand how it wouldn't be.

Any wisdom/knowledge/explanation is much appreciated!

 

[bigdeano]

Hi all,

I also read that open-center valves allow fluid to flow from the extend-side of the cylinder to the retract-side while the valve is in neutral (center) position.

This isn't correct unless it's a a valve intended to run a hydraulic motor. A valve for a cylinder blocks the work ports in neutral. Open center means the oil from the pump can flow to the tank when the valve is in neutral.

 

[oldnslo]

Hi all,

I am a complete newbie when it comes to hydraulics, so please pardon the newbie question.

In doing some reading on open-center vs. closed-center valves, I understand that the kind you need to buy is based on the type of pump in your tractor (whether it's constantly pumping or variable-flow). The pump in my soon-to-be tractor is open-center (it's a JD 3025E), so I understand I need to buy open-center valves if I add spools (which I plan on doing).

I also read that open-center valves allow fluid to flow from the extend-side of the cylinder to the retract-side while the valve is in neutral (center) position.

It occurs to me that if that's the case, wouldn't that mean that the cylinder would be prone to drifting, i.e., moving slowly on its own, without you telling it to, based on the forces on it? For example, if a cylinder was holding something up (say, a heavy implement), the force of gravity would push on the cylinder, and since fluid could freely flow from one side of the cylinder to the other, the implement would drift lower over time?

This seems like it would be completely unacceptable so I have to imagine it isn't the case, but if an open-center valve allows fluid to flow from one side of the cylinder to the other, I don't understand how it wouldn't be.

Any wisdom/knowledge/explanation is much appreciated!

The discussion of valve and spool styles can be very confusing.

Open center "systems" which use constant displacement pumps require open center style valves to operate correctly on tractors

Closed center "systems " which have variable displacement pumps require closed center valves to operate correctly on tractors.

Valves that have all the ports common in the spring center condition can also be called open center valves.

Valves that have all the ports blocked in the spring center condition can also be called closed center valves.

Hope this doesn't confuse you even more.

 

[Texasmark]

I also read that open-center valves allow fluid to flow from the extend-side of the cylinder to the retract-side while the valve is in neutral (center) position.


Any wisdom/knowledge/explanation is much appreciated!

The cylinder, if double acting has a moving piston connected to a rod just like your truck or tractor's engine. Oil is not "designed" to be transferred internally between ports....if it does your "packing" seals on the piston, is/are leaking.

The open center valve has a clearly marked in the casting, in port and out port. Fluid from your pump follows that logic. It continuously flows out the out port as long as a diversion valve isn't moved to the diversion position....aka you have a cylinder you want to run and you push/pull the lever to make it happen.

Fluid goes out of the diversion spiggot to a cylinder port and starts filling that side with fluid forcing the ram to the far end if carried all the way. The fluid that was on the other side of the piston is forced out of the second port on the cylinder and back into the controller via the return hose to be sent out the out port of the controller and back to the fluid tank.

It's common practice to have the engine running any time pressure is required on a cylinder. Reason is to keep fresh fluid available if you have "blow by" aka leaking seals somewhere in the system. It is common practice also to ensure that any load on a cylinder is removed when the tractor is turned off....aka 3 pt or FEL bucket would be lowered to it's lowest position.

One thing to consider however is temperature. If you are using a cylinder for depth control you can have some movement due to temperature of the fluid. Since you have a closed system when you have applied fluid to a cylinder to establish it's position and released the control handle, there is no place for the fluid to go. If the fluid gets hot, like when you are plowing all day, start in the morning when it's cool and no sun and plow all day with everything in the system heating up and all, you can have fluid expansion which can change the position of your implement.....not all that much, but (as happened to me) may be enough to mess up your plowing depth. Just something to be aware of.

 

[Colgate2004]

This isn't correct unless it's a a valve intended to run a hydraulic motor. A valve for a cylinder blocks the work ports in neutral. Open center means the oil from the pump can flow to the tank when the valve is in neutral.

Okay then it sounds like what I read was incorrect.

I've now read a few other places that say the same as what you described, i.e. when in the neutral position the extend/retract ends are closed off and the open center just allows the inlet to flow through to the outlet/back to the reservoir.

Seems that one source was just wrong! Thank you for clearing it up!