Counter Balance Circuit Problem
Counter Balance Circuit Problem
(OP)
I have a system i want to get some advice on. Large milling machine, hyd system = counterbalance circuit : 2 counterbalance cylinders, pilot operated check valve, pressure reducing valve and a nitrogen charged accumulator. 1 additional pressure line going out to 1 other area of machine. My experience with this system is when hydraulics are shut off pump pressure immediatly drops to zero and counterbalance gage does not drop, holding system pressure for that circuit at the check valve. What I have on this machine is pressure is maintained at the pump for a couple seconds after pump is shut off, then a noise like fluid rushing back reversing the pump. Hyd gage drops to zero. C/bal gage also drops to zero, then quickly return's to system pressure. Is a brand new machine, have replaced the pilot operated check valve and the pressure reducing valve. Initially with the original components the c/bal pressure was slow to creep back up to system pressure after shut down, now with new valves = quicker response to system pressure.
http://i13.tinypic.com/6gbx8aq.jpg
http://i8.tinypic.com/689jpmw.jpg
http://i12.tinypic.com/6809rwg.jpg
http://i7.tinypic.com/67850d1.jpg
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http://i13.tinypic.com/6gbx8aq.jpg
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http://i7.tinypic.com/67850d1.jpg
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RE: Counter Balance Circuit Problem
Sound very much like the PO check valve is slow to close...
While the check valve is open, gravity induced load on the cylinders is pushing oil back through the system, motoring the pump. When the PO check valve eventually closes, the pressure rises because of the load on the cylinders.
The noise you can hear is high velocity fluid flowing over the PO check.
The schematic drawing shows the PO check with an external drain. If this drain is too small or blocked the PO check may be slow to operate.
First job would be check the drain line from the PO check.
The drawing shows that the PO check drain is connected to the Y-axis cable conveyor. High flows = high pressure. If the drian form the conveyor is full, it will stop the PO check resetting.
This post would be better in the Fluid Power Engineering forum.
'hope this helps...
Adrian
see view of the check valve mtg surface.
[IMG]http://i18.tinypic.com/54lwmzp.jpg[/IMG]
RE: Counter Balance Circuit Problem
You are correct...sorry I did not read the drawing correctly. There is a seperate drain line for the y port. It goes back to DR2 on the power unit. That would be the place I would look.
I am sure the problem is with the PO check not closing.
Is the schematic correct because it also shows the counterbalance pressure at 5-5.5 MPA which is 50-55 BAR. The pressure gauge in the picture only goes upto 16 BAR?
Adrian
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
The GA of the power unit and the schematic only show DR2, which is the Y port drain coming from one side of the manifold. It then connects to the power reservoir via a hose.
You say you replaced the valves and the system worked better. That would rule out the drain line as being the problem.
I think the original PO check had a fault...simple as that.
Adrian
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
On the bottom of the valve, the X and Y port should be open.
If the valves have been setup for internal drain there will be a plug in the Y port. If there is no plug, it is setup for external drain. A plug in the Y port will stop it closing. Sometime valve suppliers forget to remove the plug...?
Also check the pre-charge pressure in the accumulator. If it is too low it will fill with too much oil and when the PO check is open the oil will rush out of the acc' very very quickly. The flow forces are another thing that could stop PO check from closing. The acc will empty in a couple of seconds and then thePO chack will close.
Just some things to check...I will keep giving it some thought...
Adrian
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Counter Balance Circuit Problem
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Counter Balance Circuit Problem
The pressure line through the PO check is tied to the pilot line. You can see on the schematic that just after the pump flow enters the manifold, it goes up to the pressure reducer, left to the Y axis conveyor and right to the PO check as the pilot to open the valve.
With the PO check being open and having equal pressure both sides and with a heavy bias toward being opening, what is there to stop the valve staying open?
With the PO check open and gravity acing on the cylinders and the accumulator emptying, the pump is being motored. If there is enough pressure to turn the pump and the electric motor, there is surely enough to open the PO check.
I have never seen a system where a setup like this operates safely.
I think the pilot line needs to be supplied from somewhere else...not from the main pressure line.
Adrian
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
RE: Counter Balance Circuit Problem
What is the point of the web site?
Call me cynical, but nobody gives away information for free.
There are no advertisements on the site and there is no clue as to who is behind it.
Strange things are afoot at about-hydraulic-rams.co.uk..?
RE: Counter Balance Circuit Problem
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