A10VSO Variable displacement pump power control

[simran1983]

I am maintaining a hydraulic press which is having a piston of 600 mm. It works up to 280 bars and creates a maximum force of 800 Tons. It is having a Rexroth A10VSO 71 DRS/32 pump which delivers 100 lpm of oil at 1440 rpm. It is coupled with a 35 HP electric motor. Please find attached images of the pump and also the schematic of the hydraulic circuit.

Pump

Pump's name plate

Hydraulic Schematic

Pump's Control Circuit Zoomed

**********************************************************************************
(These are the links to the images in case they don't open up in the post directly)

**********************************************************************************

Now my problem:

By looking at the schematic, it suggests the pump to be a power control pump (i.e DFLR in terms of Rexroth’s terminology). But physically (see the image attached) it is NOT a power control pump. Practically, power control is taking place in the circuit - otherwise a pump delivering 100 lpm of oil cannot go up to a pressure of 280 bars with a 35 HP Motor. My question is how power control is taking place in the circuit?

The manufacturer has installed a proportional relief valve (Rexroth DBETE) in the pilot line of the controller. In the machine’s manual it says that by varying the pressure setting of this valve, the flow of the pump can be changed. Again, my question is how is it happening?

As far as I know (and as Rexroth’s product manual says) the flow of A10VSO 71 DRS/32 can be dynamically changed by creating a pressure drop across a variable orifice in the delivery line of the pump. But I don’t see any variable orifice in the circuit. If the flow is dynamically controlled, then power control can be achieved. But, in this circuit, how is flow being dynamically reduced with the rise is pressure so as to achieve power control.

Please help me out. Thank you.

 

[HPost]

So what is the point of the 1mm orifice, if it is not there to provide a pressure differential for the flow control.

Yes, it's not a load sensing system as such, but the principle of controlling the flow is to use the proportional valve to change the pressure drop over the orifice.

The power control does what its name suggests.

The margin compensator controls the pump angle to set flow.

The cut off compensator is a spool that sees pump outlet pressure and works against a spring to directly control the servo piston to de-stroke the pump to hold 280 BAR.

I'm done arguing about it, it's pointless. Nothing to be gained...let's move on!

 

[Jacc]

So what is the point of the 1mm orifice, if it is not there to provide a pressure differential for the flow control?

It can serve two purposes as far as I can see:
- a dampening orifice to calm down the controller
- limiting the flow through the proportional pressure relief valve

 

[HPost]

Nope...

The valve will have its own orifice to dampen its action.

Limiting the flow will drive the upstream pressure up and that is pointless.

I wouldn't normally be so insistent, but they way I have explained it really is the way it works.

The orifice is there to provive a pressure differential to control the pump awash plate angle and with it the flow rate.

People come here for advice and if we gave the wrong advice then that would be no good at all.

On the machines I am responsible for, we use the main spool as the variable restriction and we have similar pump that responds to the changing pressure differential. So as the pump speed changes or the spool flow area changes, the pump will respond to hold the flow rate. With this subtle difference as shown above, the speed is fixed and so the flow can be changed by changing the pressure drop across the orifce.

In all honesty, the way pressure and flow control systems work are very similar. It is only the fact that we make so many machines with this sort of control that means I can spot the difference.

It makes no difference to me at all, I just think that it's important to get the facts right.

 

[Bumblyari]

Here we go again.

1. Of course the orifice is there to limit the flow as Jacc rightly says. If it wasn’t there you’d be trying to push 100 lpm through a valve rated for probably 5 lpm.

2. The orifice also creates a pressure differential to enable the pump to de-stroke when the pressure differential reaches the setting of the bias spring. However allowing the pump to de-stroke is not the same as controlling the pump flow (try controlling the pump flow with its outlet connected back to tank and see what happens).

3. The proportional relief valve cannot control the pressure drop across the orifice as you say. It can only control the pressure on one side of it. The pressure on the other side will be determined by the load resistance. The point at which the pump flow rate will start to reduce is therefore determined by the setting of the proportional relief valve provided it is set lower than the load pressure. Now, if the magnitude of the load is speed dependent (such as on a fan drive for example) then and only then will controlling the compensator pressure determine the actuator speed.

4. It’s a similar situation to controlling the speed of a cylinder with a simple needle valve. For a given load if you increase the pump relief valve setting upstream of the needle valve the cylinder will move faster but what is the relief valve controlling – flow or pressure ?

5. The way pump pressure and flow control systems work are not similar, they are fundamentally different. To achieve true flow control from the pump alone (ie. not via an external flow control valve) you require a stroke controlled pump with mechanical or electronic feedback from the swashplate angle.

6. So in order to fully explain this circuit we need to know what the press is actually pressing. The proportional relief valve is controlling the pump cut-off pressure so whether that determines the press speed or not depends upon the characteristics of the load. However, in my experience, you are more likely to want to control the pressing force of a press electronically than you are its speed.

 

[HPost]

You keep saying that the proportional relief valve is to limit the pressure.

Why have a pressure cut off compensator on the pump and a relief valve on the press as well then?

You will never get full pump flow across the orifice, the delta P over the orifce is sensed by the pump and the pump will back to hold the margin set on the compensator.

Yes, the valve can and does control the flow across the orifice. However, it's all relative to what the press is doing.

Pressure is a direct result of resistance to flow, so the control of pressure and flow is similar, not the same, just similar.

 

[Jacc]

Sorry Adrian, I agree with 73lafuite and Bumblyari and I don't follow your logic.
An orifice to provide a pressure differential is indeed the core of a Load Sensing principle. However, such an orifice is always in the line of the output flow from the pump. The flow in the orifice on the T'ed of branch to the regulator does not represent the flow from the pump.

As an example, simply connecting the LS port to the P port of an LS pump will force the pump up to maximum flow. Basically it is the same as having an infinitely large orifice in the line of flow, the pump will never be able to deliver enough flow to reach the pressure differential that is set in the pump controller (margin pressure). The pump will swivel to max displacement.
This is how this system is connected as well with the addition of a proportional pressure relief valve on the LS signal line. A similar setup can be seen in the attached pdf, page 25, fourth figure from top. You just have to imagine that the variable orifice in the line of flow is not there (or just fully open all the time).

On mobile machines with LS system the pressure out to cylinder from a certain directional control valve is sometimes limited to a lower setting (on some Caterpillar machines also maybe?). This is achieved by having a small pressure relief valve on the LS signal line on that DCV. The same principle is used on this system, there is a small proportional pressure relief valve on the LS line. This valve limits the maximum pressure of the pump to setting of the prop pressure relief + the margin pressure. Since it is an electrical proportional relief valve the pressure can be controlled at any time.

A small orifice is needed upstreams to limit the flow to the small prop relief valve. Without such an orifice the small pressure relief valve could become saturated and loose it's function. It is common to use a small relief valve in combination with an orifice to pilot control another valve (very common on larger logic element type of valves).

Like you say Adrian, there is most likely an orifice in the regulator itself but for this application it may have been removed. This is also mentioned on page 25 in the pdf.


The application also says a lot about the hydraulic system here. LS systems are indeed brilliant for use in many mobile machine applications where the operator constantly regulates the speed and position of multiple cylinder powered functions. In this case it is a hydraulic press, accurate control of speed and position is not so important, the important thing is to control is pressure. From what I can see on the pictures that prop. relief valve looks like a high end Rexroth unit.

HPost 7 Jan 16 17:06
When valve EV3 opens to move the press, the pump outlet pressure would drop and oil would start to flow, but while the delta pressure over the 1mm orifice is higher than the margin compensator setting, the pump flow will remain low. This is where the flow control works.

Seriously Adrian, when EV3 is closed the pump will be trying to swivel to max as hard as it can. This is not a mobile LS valve, it's not about regulating the flow across that 1mm orifice. On mobile LS valves the LS line is drained to tank when the DCV spool is in neutral position and connected to the load when the DCV opens but that is not the case here.

Yes one could view it as the regulator is trying to keep a constant deltaP across that 1mm orifice (it does for sure) but that view is not helpful in this application (since the orifice sits in a T'ed of branch of the main flow). When the prop press valve is closed the LS port is connected straight to P and will cause the pump to swivel up as much as it can. When the pressure of the prop pressure relief valve is reached, the pressure in on the X port will be limited (thanks to the orifice) and hence the pumps pressure will be limited.
Viewing it like a pressure relief on an LS line makes more sense, the prop. pressure valve controls the pressure of the pump, nothing more, nothing less. You probably know very well how a pressure relief on a LS line work.

 

[Bumblyari]

Jacc's got it just about right. Those may be the damping orifices just to the left of the left-hand compensator but the resolution of the diagram isn't really good enough to say.

But in reply to HPost's comments:

The main pressure relief valve is a belt & braces requirement since pump compensator spools are prone to jam and not always in a safe condition.

The manually adjusted cut-off compensator performs a similar back-up function for the proportionally adjusted one (ie. protects against an electrical issue).

The proportional valve allows you to vary the pressing force electronically of course probably via a PLC or other digital controller.

I didn't say the control of flow and pressure are not similar I said that pressure controlled pumps are different to flow controlled pumps.

 

[Jacc]

HPost
Why have a pressure cut off compensator on the pump and a relief valve on the press as well then?

That is a relevant question. Sure it acts as a backup like Bumblyari says but that may not be the only reason that it is there.
Part of the reason for this long discussion in the first place is that this design is a bit weird, I give you that Adrian. Why is there an LS-spool to start with, why not just a pressure compensator spool?

Looking at the Parker PV pump pdf I posted, page 23, second figure from top, (remote pressure compensator). That pump controller would be perfect for this application and it has one spool less. I can't find a similar option for the A10, the only way using an A10 pumps seems to be this setup with a DRS controller.

It could be also be as simple as Rexroth stocking the A10 with DRS regulator and that there was a long lead time for something more suitable or maybe it was cheaper.

 

[Jacc]

Back to the original questions:

By looking at the schematic, it suggests the pump to be a power control pump (i.e DFLR in terms of Rexroth’s terminology). But physically (see the image attached) it is NOT a power control pump. Practically, power control is taking place in the circuit - otherwise a pump delivering 100 lpm of oil cannot go up to a pressure of 280 bars with a 35 HP Motor. My question is how power control is taking place in the circuit?

Can you confirm that power control is taking place? You can use a Fluke 434 or similar to measure actual consumed power.
It could like 73lafuite suggested, it is simply running on high power for a short period of time.
My suggestion; I recently saw an old SauerDanfoss amplifier card that had an integrated power controller based on input from the electric motor current measurement. I guess it would be possible to use something similar in the PLC?




The manufacturer has installed a proportional relief valve (Rexroth DBETE) in the pilot line of the controller. In the machine’s manual it says that by varying the pressure setting of this valve, the flow of the pump can be changed. Again, my question is how is it happening?

It is not directly changing the flow, just the pressure. Usually (but not always) an increase in pressure will increase the flow. I think maybe the person who wrote the manual was in a hurry.

 

[hydroman247]

Can OP post some better pictures please. This is an interesting topic that I am struggling to fully understand.

 

[HPost]

It could be this way...it could be that way, who knows and more to the point, who is interested enough to argue about the finer points???

Unless we are going to make arrangements to visit the press and instrument it, we will just be making educated guesses at how it actually does function.

I personally have nothing more to add, it looks like it works in a particular way,,,but I have bee doing this long enough to know that I could be completely wrong.

Good luck to you all...