open vs closed center hydraulics

[jimg]

I understand what open and closed center hydraulics do but not why you would want one or the other. I'm sure one isn't 'better' but is used for differing applications. From the tractor literature I've read it seems
most tractors come w/ one or the other and load sensing is usually mentioned along w/ closed center. As these features that go together?
So, could someone enlighten me pls?

 

[BB_TX]

Here is a <font color="blue"> site </font> with a discussion of both types directly related to backhoe operation. Seems it is pretty much a matter of personal preference.

 

[grandad]

I seem to recall an earlier thread where one of the wizards said open-center systems were self-bleeding while closed systems had to be purged of air in the circuits. So, open-center is preferred in attach-detach situations like a removable FEL on our CUT's. With a full-size TLB, the FEL and BH are integral to the unit, so self-bleeding would be of less concern. Also, the open center system allows for the addition of other hydraulic circuits in series through the power-beyond of the control valve. If I got any of this wrong, I'm sure someone will correct me.

 

[pajoube]

You might check the Power Trac thread, this has been an on going discussion for some time.
PJ

 

[nailerguy]

Well, I guess I just can't resist taking a shot at this and let me apologize beforehand if I confuse you more, but here goes nothing, /forums/images/graemlins/tongue.gif

Closed center directional valves are used in conjunction with a variable volume pump. This means that the pump will go to a position where it maintains full pressure at 0 flow. There are several different types of this type of pump. The piston pump is probably the most widely used, but there are also variable volume vane pumps as well.

I have been told that the piston pump is also the most robust of this design which would seem to be the case if there are more of them used than the vane type. Neither of these pumps like any kind of contaminates and a good filtration system is a must.

Open center valves are used with fixed displacement pumps, meaning the pump has to move a certain amount of oil per each rotation of the shaft, which is why the valve is open center. When the oil from the pump is not used for work, the valve centers and redirects the pumped oil back into the reservoir or tractor gearcase.

Both of these systems require pressure relief valves which also redirect the oil back into the reservoir, but where it is a must have for a fixed displacement pump, it is more of a safety valve for a variable volume pump. Since the variable volume pump will go to 0 flow at full pressure, as long as the pump is working properly no oil will cross the relief and the pump will not stall. A fixed displacement pump on the other hand has to have this since it displaces oil every time it rotates. Let's say you are curling your bucket under a stump in the ground as you are trying to dig it up with your FEL and the bucket stalls because the stump is still stuck, in this position, a variable volume pump will go to 0 flow, but maintain pressure in the lines to the bucket cylinders as long as you hold back on the joystick. A fixed displacement pump in this situation will continue to pump oil at full pressure and flow, at this point the pressure relief in the system pops off it's seat and directs the oil back into the reservoir of the tractor. If the relief did not do this, /forums/images/graemlins/shocked.gif then the tractor engine would stall or the hoses or a cylinder would explode or the shaft of the pump or the pump case could possibly break.

There are probably more types of fixed displacement pumps than any other design and they include, but are not limited to,
Gear, Vane, Geroler, Gerotor, and even Piston pumps.

Fixed displacement Gear and Vane pumps are probably most resistant to contaminates.

Either of these types of pumps in a properly designed system will offer very little trouble. I personally favor the piston variety for larger volumes of oil and Gear, Vane, etc. for smaller volumes of oil. Piston pumps are also a bit quieter /forums/images/graemlins/smile.gif than Gear or vane pumps.

Hope this long post (sorry) makes some of the differences between these systems a bit clearer for you. /forums/images/graemlins/cool.gif

Nailerguy

 

[JerryG]

Willard,
You did a good job of explaining it.

 

[pajoube]

Great post, keep them coming.
PJ