A A10VSO Variable displacement pump controls 2

[aijazblr]

Seeking guidance on controlling a Rexroth A A10VSO variable displacement pump.

I have this A10VSO pump that is used to power a closed centre hydraulic machine with multiple actuators (pistons and motors) operating at 2500 psi. The pump was recently replaced and I believe has not been set up correctly.
Although the pump nameplate says it is a DFR1 (pressure/flow control), there is no external line between the pump discharge and the pilot valves mounted on the pump.
The pump puts out a constant 2500 psi and maintains the pressure whether the machine is in use or in neutral, consequently the system overheats as there are long periods of machine inactivity with the pump running at 2500 psi.

With the DFR1 control, is the pump pressure supposed to drop to a lower idle pressure when the machine is in neutral? If so, how is this achieved?
How can I set up the pump to sit at an idle pressure with zero output and switch to 2500 psi on demand when the machine is moved out of the closed centre position?

Thanks in advance for your guidance.

 

[HPost]

Have a look at thread number 1083-399236, there is plenty in there to look at.

On a closed centre system, with no pressure at the 'X' port, the pump will go to standby pressure. This pressure is set by the margin compensator on the pump. Of the two compensators on the pump, the margin compensator is the smaller of the 2.

If your standby pressure is 2500 PSI, then it's because someone has wound the margin compensator all the way in. That should be set to 300 PSI.

You should have a load sensing line coming back from the valve or valves. The pump uses that line to respond to the demand of flow and pressure in the system. There is a small port on the top of the compensator, it will have the letter 'X' cast on the outside. That's where LS supply is connected to pump.

Your system is most likely very hot as the pump will never go to low pressure.

You need to back the standby pressure off to 300 PSI, then add an LS line at the 'X' port to bring the pump off standby when pressure and or flow are required.

Cheers

 

[aijazblr]

Thanks Adrian, for your response.

For the LS line, do I need to introduce an additional orifice or will the orifice built into the X port of the 'margin compensator' suffice?
If there is to be an additional orifice, can it be on a branch line Tee'd off from the main pump discharge?

 

[HPost]

You just need to connect the load from any and all actuators back to the 'X' port. It's a pressure signal, no orifice is required.

 

[aijazblr]

Hello Adrian and others,

Please give me some Engineering tips on this issue again.

I have this A10VSO pump that is used to power a closed centre hydraulic machine with multiple actuators (pistons and motors) operating at 2500 psi. The pump was recently replaced and I believe has not been set up correctly. Although the pump nameplate says it is a DFR1 (pressure/flow control), there is no external line between the pump discharge and the pilot valves mounted on the pump.
The pump puts out a constant 2500 psi and maintains the pressure whether the machine is in use or in neutral, consequently the system overheats as there are long periods of machine inactivity with the pump running at 2500 psi.

With the DFR1 control, is the pump pressure supposed to drop to a lower idle pressure when the machine is in neutral? If so, how is this achieved?
How can I set up the pump to sit at an idle pressure with zero output and switch to 2500 psi on demand when the machine is moved out of the closed centre position?

As suggested by HPost, I connected a load sensing line from the pump discharge to the X port on the margin compensator. See picture of the arrangement attached.
With the pump discharge valve closed (the tool in neutral closed centre position) and both compensators on the pump backed out, I started the pump. Adjusted the pressure compensator first to 2500 psi. And then tried adjusting the margin compensator, but it does not have any effect on the pump discharge pressure which remains at the setting of 2500 psi. I am expecting this pressure to drop to 300 psi on adjusting the margin compensator but it does not seem to have any effect.

What am I doing wrong ?

Thanks for your tips.

Aijaz

 

[HPost]

Aijaz...

Does the pump have a minimum displacement setting on it?

You have effectively dead headed the pump. The compensators are working, the pump has gone to minimum displacement, but is pumping against a closed valve.

Stick your hand on the casing drain, is it very hot?

It's either the above or the following.

The pump is responding to its own outlet pressure. If that hose to 'X' port is from upstream of the valve, there will be outlet pressure in the LS line.

The LS signal must come from the other side of the blocking valve.

It's hard to see on the photo where the LS pressure is coming from, but I suspect it's the pump side.

Connect the hose to an actuator or motor.

Try the above and let us know how it goes.

Cheers

Adrian

 

[HPost]

Aijaz...

With a closer look, I think you have the following.

That pressure you are sending to the 'X' port is trying to keep the pump at max displacement.

The pressure cut off compensator, the one without the hex wrench, should limit the pump pressure by pulling the servo piston back to put the pump at minimum displacement.

However, if there is a minimum displacement limiter...the pump cannot reduce the flow enough, so the pump output will be against a blocked valve. The pressure you are seeing is what the pump is producing at minimum flow, in a closed system.

As I said before (last post), the pump is responding to it's own pressure. You need to move the hose to the cylinder or motor, then the pump will respond to the load at the actuator. This is the way load sensing works.

Does your system need a minimum flow from the pump? If not, back the min displacement off to allow the pump to go to zero flow.

If you do need some constant flow from the pump, then leave the min displacement alone...or set it where you want it.

The main thing is to re-route the LS line. Where there is no load, the pump will sit at stand-by. You are sending 2500 PSI back to the pump, so the pump is trying to give necessary flow to maintain that pressure.

Cheers

Adrian

 

[aijazblr]

Greetings Adrian and all,

There is no displacement limiter on the pump.

You have rightly pointed out that the pump is responding to its own pressure as it is being dead headed. By closing the discharge valve and deadheading the pump, I was simulating the neutral position of closed centre hydraulic machine that the pump feeds.

With the machine in neutral and the pump running, I want the pump to drop down to a lower idle (standby) pressure and then as soon as any function on the machine is actuated, the pump should ramp up to the 2500 psi machine operating pressure as demanded by the machine.

The machine is a fair distance from the pump and has multiple different actuators each controlled by separate closed centre directional valves. One or more actuators are used at any time depending on requirement. This would mean the LS line would have to come off from the actuator side of the directional valve(s) and it would not work as one or more functions could be in neutral while the others are in use.

Is there any another way around this problem?

Thanks for your guidance

Regards

Aijaz

 

[HPost]

Aijaz...

You need a resolving network, This is a series of shuttle valve arranged in a way that allows the highest pressure in the system to control the pump.

While you are sending pressure back to the pump, the pump will respond. If you remove the hose from the 'X' port, the pump will go to stand-by. The pump needs to be against a closed centre valve, with no pressure from the actuators. With the LS hose being down stream of the blocked valve and having no pressure, the pump will be on stand by.

Most load sensing valves have the resolving network inside the valve and 1 hose coming from the valve to the pump. If you have distributed system, then you need to come up with a way of doing the same thing on a larger scale.

Cheers

Adrian

 

[Bumblyari]

It could be that a bleed-down orifice in the LS line would do the trick but we really need a circuit diagram to see what's going on.