Counter Balance Circuit Problem 1

[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. 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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[jnmaney]

hydromech replied :

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.

 

[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

 

[jnmaney]

Regarding the gage, it reads in Mpa's, Help me with the x y are you looking at the 689 jpeg which shows dr 2 coming out on both sides of the manifold? Really appreciate the help.

 

[hydromech]

The pressure reducer/relief is also pilot operated and the drain from the valve is also connected to DR2. Its always good to run pilot drains seperate from the main return line.

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

 

[jnmaney]

I think the only thing that improved was the check valve is closing everytime but there is still issues. it drops to zero first which is unusual. the inrush sound is even of concern to the customer as it is so loud. I see where the check / pressure reducing valve drain seem to be tied into each other. i want to take the components off and take a look at the manifold and see if anything looks suspicious.

 

[hydromech]

Something else to check...

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

 

[jnmaney]

I will check the pressure. Last time a tank was low the up stroke pressure was really fluctuating ( not able to assist heavy demand ). One procedure I got on this job was how to set up the pressure reducing valve. Turn in the valve to almost closed, then open till you see pressure drop, then turn in a 1/4 turn then its set. Do this also if you make a change to pump pressure.

 

[EdDanzer]

We have documented that check valves can float open during a pressure balance and a slow change of pressure across the valve. If this is a concern you would need to look at the valve construction and compare it to a HydraForce valve

http://www.hydraforce.com/Directnl/Dvck.htm.

With this information I can determine if this is a possible problem.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[budt]

Put a Check Valve in the pump outlet line that is free flow to the circuit to eliminate back flowing the pump.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydromech]

Having read Bud's comments and looking at the schematic again, I now see a problem with the system.

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

 

[jnmaney]

Thanks for your replies. I am not at a point yet to do any digging on the system. The response I am getting from engineering is that this is a issue they don't see very often. They do say reversing the pump is not good. This is a system that is on a number of our machines and works fine. I have a drawing of the manifold and hope to get a look at this sometime in next couple weeks. To see if there is any obstruction in the drain line. I't does not happen all the time, just 80% of the time when shutting system down at end of day.

 

[jnmaney]

Just making an update to this thread. I did remove the manifold the checkvalve is mounted to and all piping connects to. The checkvalve drain line is correct to drawing and seemed good. Shutting off system now works fine most of the time. Have no idea how condition improved but is not fixed for sure. Thanks for all your help.

 

[juzboarder]

http://www.about-hydraulic-rams.co.uk

 

[hydromech]

What is the point of that post...?

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..?

 

[ScottyUK]

Well, the only link in the main text goes to Apex Hydraulics with there (sic) advice, pictures and unlimited knowledge of hydraulic rams. Seems the 'unlimited knowledge' does not extend to English grammar...


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