Counter balance system

[jnmaney]

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. 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 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.

 

[maytag]

Can you post a schematic?? This would help. Maytag

 

[jnmaney]

I have the schematic on pdf, how do I attach it ?

 

[jnmaney]

I have posted here are a couple schematics of the circuit

 

[maytag]

What brand and model is the Pres. reducing/relieving valve??Its likely you are hearing the fluid do exactly as you say if the P.O. check is remaining open. When system pressure drops to what your N precharge is you will experience the "rushing of fluid"-this is a cheap, noninvasive way to check the approx. N precharge. How are the the cylinders oriented is relation to load and any electro-mechanical load?? Maytag

 

[jnmaney]

see valve pictue

. Cylinders are oriented as shown in schematic, rod end pointing down. System is designed so cylinders hold the load so motor can work up / down without lot of effort. Check valve will hold system pressure when hyd off for saftey to hold load in event motor brake not able to hold the load alone. This is 5th machine i've installed and is first to exhibit this holding of pressure after turned off and back rushing of fluid / check valve dropping then coming back up. see cyl orientation.

 

[hydromech]

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

 

[jnmaney]

Here is a picture of the check valve mounting surface

 

[jnmaney]

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.
Hydromech replied as follows :

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

 

[jnmaney]

Thanks Hydromech, I don't think the drain is tied into the pressure / return lines going out to the Y cable carrier. attached pic

which shows the check valve mounting surface.

 

[hydromech]

The two smallest holes that you can see in the block are the X and Y ports. X is the pilot pressure to open the valve, Y is the drain port.

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