Variable displacement axial piston pumps 1

[tactical14]

Just have a quick questions. In variable displacement pumps, does the swash plate continuously move in a reciprocating motion? Or once you set the swash angle, does it stay stationary at this angle?

 

[Oldhydroman]

The angle of the swash plate determines the displacement of the pump (think of "cubic centimetres per revolution" [cc/rev] or "cubic inches per revolution" [in³/rev] whichever is most comfortable for you depending on which part of the world you're in). The swash plate pivots on an axis which is at right angles to the shaft of the pump. The swash plate is pushed to greater or lesser angles by the action of springs and small hydraulic actuators: either cylinders or vane type rotary actuators depending on the make and size of the pump. (There are a few versions where the swash plate angle is adjusted by pure mechanical means or by a small electric motor and a leadscrew.)

When the swash plate angle is at maximum the pump displacement is at a maximum. A lot of pumps have stop screws that allow you to limit this maximum angle and so deliberately reduce the maximum displacement. When the swash plate is precisely at right angles to the shaft then the displacement will be zero and the pump will deliver no flow no matter how fast you turn the shaft.

The pump displacement controller (usually a type of hydraulic valve) will pressurise or de-pressurise the small hydraulic actuator(s) in order to achieve the desired effect - and the type of controller installed will determine what this effect is. The pump might have a controller which attempts to adjust the angle in such a way as to keep the pump outlet pressure constant, or the controller might be one that sets the pump displacement to a value in proportion to the pilot pressure signal you give it.

So, in answer to your question, if you deliberately drive the swash plate to reciprocate then it will. The swash plate is not connected to the pump shaft in any way: the rotation of the pump shaft does not cause the swash plate to reciprocate.

Are you thinking of a wobble plate pump by any chance?

DOL

 

[hydtools]

The swash plate does not continuously move in an oscillating fashion. Its controls hold the swash plate an angle depending on the demands of the system. The barrel containing the pistons spins on the shaft axis and the piston shoes follow angle of the swash plate moving the pistons in an oscillating motion in the barrel to pump fluid. As the angle of the swash plate is changed in response to system demand, the distance the pistons move, hench the displacement, changes.

Ted

 

[Rhymond]

Gents.
We have got system with axial piston pump pressure compensator set 800 psi and relief valve set 2200 psi (plus unloading valve set 2000 PSI) and actual system pressure is 2000 psi.
If compensator valve set 800 psi why we still can raise system pressure after it? Does that mean the swash plate never goes to 0 angle and we still build up pressure to RV settinng just with minimum flow? I tried to find answer but everywhere said that compensator pressure setting is maximum system pressure.

 

[Oldhydroman]

Hi Rhymond
You might be right. If the mechanism is working correctly then the swash plate should get back to zero (theoretically) when the outlet pressure rises above the setting of the compensator.

In practice, the pump manufacturers set the pumps up so the swash plate movement stops just a fraction on the positive side. The displacement doesn't quite reach zero and the minimum flow you get is enough to [partially] compensate for any internal leakage in the pump. A typical setting procedure would be to set the compensator to zero and have the pump outlet dead-headed (but protected by a low leakage relief valve). Such a low setting on the compensator causes the swash plate to [try to] return to "zero" regardless of the actual outlet pressure. Then the swash plate minimum-angle stop screw is adjusted until the outlet pressure (deadheaded) rises to 50 bar (Rexroth A4VSO) or 80 bar (Kawasaki K3VL). [I think these are the numbers but they're of that order anyway]. If it is ever your intention to run one of these pumps at a very low compensator setting then you have to tell the manufacturer so the minimum swash plate angle can be set a little lower.

In your case the minimum angle is too much and the pump is still delivering when it is not supposed to. Unless the whole displacement changing mechanism is non-functional and the pump is running like a fixed displacement pump all the time. Is there an indicator somewhere on the pump so you can see the angle changing?

DOL

 

[Rhymond]

Oldhydroman, thanks for quick answer. The pump is Rexroth AV10 52 Series, there is no indicator on it but if you have a look on data sheet

http://www.boschrexroth.com/modules...20&sid=29226A0BD74142174CAC4302FA88DABC&sch=M

Page 10, DR type. They say
The pressure control limits the maximum pressure at the pump
output within the pump control range. The variable pump only
supplies as much hydraulic fluid as is required by the consumers.
If the operating pressure exceeds the target pressure set
at the pressure valve, the pump will regulate towards a smaller
displacement. The pressure can be set steplessly at the control
valve.

Does that mean this particular pump always build up pressure up to 2000 PSI but with small displacement when consuming circuits require it. Actually there is no some operational problem, system pressure is 2000 PSI, many of the circuits working with this pressure as per schematic, I just want to understand phisically how its possible if we got compensator set to much lower setting 800 PSI.

 

[Oldhydroman]

Hi Rhymond

How do you know the compensator is actually set to 800 psi? In the absence of an indicator on the pump can you actually measure the flow (with a flow meter installed on the pump outlet) or can you calculate the flow approximately (from motor current and outlet pressure)?

If the compensator is actually set to 800 psi and is working properly but your minimum displacement is set to a significantly non-zero value then, as the outlet pressure increases, you will see this: full flow from the pump when the pressure is less than ~715 psi (slightly lower than this if your pump is larger than 45 cc/rev), then the flow will gradually decrease to a minimum value as the pressure rises to 800 psi, and then the flow will stay at this small value (ignoring losses in the pump) until the pressure reaches an upper limit - that being the setting of your unloading valve. If your pump IS changing displacement as the pressure increases then you will hear a difference in the noise level as you approach 800 psi and there will be a change in the relationship between motor current and pressure. [I'm assuming here that the pump isn't driven by anything other than an electric motor.]

If the compensator is broken (and jammed in the "not reducing the pump displacement" position), or if the pump displacement changing mechanism is jammed at full displacement, or if the compensator is set higher than the unloading valve, then, as the pressure increases, the flow will stay [nominally] the same - just as if you had a fixed displacement pump. There will be no noticeable change in noise (it will just get louder) and the motor current will go up steadily as the pressure goes up.

The link you sent doesn’t work where I live but I know which pump you mean. There is no adjustment of maximum or minimum displacement as standard, but it is possible you have a special version which has the adjustments available or has been set-up in a special way (with a spacer built in to limit the minimum displacement to some particular value).

Check the model number of the pump against the coding key in the datasheet. If there is a number at the end of your pump’s code like “SO xyz” (xyz standing for any number) then you have a “Special Order” or “Spezial Ordnung” version. With these SO numbers you have to contact Rexroth to find out what is special about the component: it could be something small (the label being stainless steel rather than aluminum) or it could be a huge list of major changes to the way the thing works. Rexroth might not even be able to tell you what your SO means - sometimes they work closely with an OEM to create a configuration that suits a particular machine and Rexroth are contractually constrained to only sell that version to the OEM and to keep the special formulation confidential.

I did once encounter a system where the variable displacement pump had a deliberate and significant positive setting for minimum displacement and had a pressure compensator setting well below the relief valve setting. The control philosophy was that at low pressures the pump worked as a variable displacement unit but at high pressure it became a [small] fixed displacement unit. It was a kind of “expensive HI-LO, but cheap horse-power limiter” system. I suppose it saved a few valves and gave an easily adjustable power limiter but I couldn’t help thinking it was odd. Anyway, is it possible that’s what you have - the significant non-zero minimum displacement might be deliberate. I’m also persuaded that this might be the case because you have an unloading valve in your circuit; a simple pressure compensated pump circuit wouldn’t have one of these.

Hope this helps.

DOL

 

[RobertHasty]

I find this presentation from Parker to be very good and brief.

 

[Rhymond]

RobertHasty, thanks for good presentation.

Oldhydroman, correct, our pump number has SO letters, also note that min and max volume stops are not adjustable. I attached schematic piece showing the pump, unloading and relief valves.

http://files.engineering.com/getfil...83b-4328-a9f1-d30fd0c35e0e&file=schematic.pdf

We have got procedure to set the compensator valve, measuring motor current with UV1 set to maximum, adjusting RV1 from 0 to max, looking on current, and it should drop down on 800 PSI.

 

[Cbrunner]

If you take the hose diameter and flow rate into account while designing the system, you can minimize the pressure drop to the point that it becomes negligible. Steel tubing is a great idea where feasible.

When working with rotary flow dividers, it is advisable to select a divider slightly undersized, as their accuracy is when running at full speed (and they become less accurate the slower they run). Getting rotary flow dividers with built in pressure relief valves prevents problems like pressure intensification.

Accuracy is directly proportional to cost; piston/spool type dividers work fairly well for short stroke applications.

Chris Brunner
Brunner Equipment LLC

 

[Cbrunner]

Please ignore my last post - I hit the wrong thread in a hurry.

Chris Brunner
Brunner Equipment LLC

 

[kcj]

Oldhydroman, correct, our pump number has SO letters, also note that min and max volume stops are not adjustable


If the pump has a minimum stop (I have only seen this on motors, are you sure it is a pump?) then regardless of controls, the pump will push out a certain flow at minimum, destroked condition. If the circuit leakage or demand is less than that flow at 800 psi drop, then the pressure will rise until the load absorbs that flow through higher leakage at higher pressure. Either the pressure rises until leakage balances, or in your case until a relief valve or unloading setting is reached and the flow passes at this limiting pressure.


As a test, try capping all lines but leave the relief valve in place. See what the minimum pressure is. Add a small needle valve or leakage to tank and see how much flow is needed before the pump regains control at 800 psi. That would tell you if the pump does not truly reach 0 flow.

 

[Oldhydroman]

Hi kcj

There's no need to do a test to see if the minimum displacement is greater than a nominal zero - the circuit diagram actually has the pump minimum displacement written on it (0.5 cu in/rev). It is a deliberate setting by the manufacturer but I don't think Rhymond previously appreciated the consequences of this non-zero minimum displacement, hence his query to this forum.

I do, however, agree with you. It is rare to see a minimum displacement adjustment on a pump; so much so that Rhymond's original search for information completey bypassed this peculiarity. There are one or two pumps around which have this facility even if the adjustment is not documented in the operation and maintenance manuals. This would be because it's not usually a good idea to adjust the pump manufacturer's original minimum displacement setting.

DOL