OT, Hydraulic accumulators

You were too simplistic though. All pumps create flow, not pressue. The system creates the pressure. I dunno about you but a good system will maintain pressure i.e. there is nothing to leak. USUALLY a cylinder has nothing to do with the pressuriztion. If a system has a measurable leak... IT HAS A PROBLEM.

I have not figured out the manometer thing, that is usually used around atmospheric pressure.

rsdoran, it must be my post you refer to.

All pumps create flow, not pressue.

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So what happens if you block the outlet or the inlet of the pump, then there will be no flow, but the pump will generate a pressure difference. So from that I conclude that it is the other way around.

If a system has a measurable leak... IT HAS A PROBLEM.

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Most systems are not ideal. With "leak" I do not mean a leak to the outside the system but between the inlet and outlet sides of the pump.

Anyway, I am obviously out of my league here. I just wanted to comment that there is an alternative to using an accumulator for saving power.

edit:
And "manometer", sorry sorry maybe "pressure gauge" is the correct term.

Ron is absolutly correct.

Fluid power 101 "Fluid pressure cannot be created without resistance to flow".
A good example is a hydraulic bottle jack. It does not build pressure until it contacts the load.

gas, I disagree 100% with what you wrote, but I dont want to continue this tangent.

OK, lets continue then.

A pump plus load is a coupled system.
Does the pressure generate the flow, or does the flow generate the pressure ? The point is that you cannot ask it like that.
The pump has a operating curve pressure=f(flow), you can invert the curve so that flow=g(pressure) if you like.
The load also has a similar curve.
Pressure and flow settles where the two curves intersect.

A positive displacement pump will have a curve that is very "steep" when compared against a centrifugal pump for example.

Some loads are just resistive. If you add a pressure regulator that opens at a certain pressure, then it changes the load curve.

If you block the outlet of the pump, then you have exchanged the load curve for another curve which is actually a straight line where flow=0 at any pressure. This "curve" will then intersect with the pump curve probably at a higher pressure than at the normal load curves intersection point.

Now go the other way. Completely open the outlet of the pump. How much pressure does it generate?

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That would be a curve that is almost flat, but not completely flat. There will be a minute pressure.

All of the above assumes a static system. A dynamic system is more complicated.

When I said that a pump generates a pressure and not a flow, is because at a blocked outlet the flow will be zero. The pump will not generate an unlimited pressure to overcome the 'resistance'. This is a simplification of course. To explain in detail why this simplification is not correct is a tangent if you ask me.

To gas' hydraulic jack: It is a special application. Here the load is changing dynamically (as long as the piston is free to move, then the volume in the system expands when the pressure increases).

To the original topic:

If you stop the pump, and there is no accumulator, then the pressure will drop to zero "instantly" in any practical meaning of the word.

One way to save energy is to use an adjustable pump. That means that the pumps curve is variable in stead of fixed.

I did not think it was a tangent either, I can not get into the detailed math Peter can but I have worked with hydraulics a little.

So what happens if you block the outlet or the inlet of the pump, then there will be no flow, but the pump will generate a pressure difference. So from that I conclude that it is the other way around.

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If you block the outlet you would get pressure, probably enought to stall or tear up the pump. Blocking the outlet does not stop it from TRYING to create flow.

If you block the inlet then it cavitates and in most cases gets ruined.

I have not seen Bud (fluidpower) in quite a while, his biggest beef is that not enough people actually learn about fluidpower.

You have to be aware that Peter works with motion control; which is slightly different in some aspects to bang-bang systems. To get a better understanding take a look at some of the posts here:
http://www.patchn.com/SMF/index.php?board=7.0
Most of the ones posting there are primarily fluidpower people.

Want to look at even more: http://www.nfpa.com/
They had a forum but I have not visited it for awhile.

Not sure where Peter wants to go with this accumulator thing but they are energy storing devices. The most common usage is to supplement pump flow and shock absorbing. I am not sure about the energy saving aspect.

If you would like to see some circuits that Accumulators go to this site: www.hydraulicspneumatics.com and look at the second ebook link.

It is a book I use to teach an advanced class and the first chapter is on Accumulators.

The other ebook there is the one I use to teach Basic Fluid Power.

rsdoran said:

If you block the outlet you would get pressure, probably enought to stall or tear up the pump. Blocking the outlet does not stop it from TRYING to create flow.

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Blocking the inlet or outlet, and opening the outlet to allow free flow are just imaginary examples to illustrate what happens at these extremes.

rsdoran said:

If you block the inlet then it cavitates and in most cases gets ruined.

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This illustrates that there will be no flow, but the pump will generate a negative pressure (relative to atmospheric pressure) and that again causes the "boiling" of the liquid inside (= cavitation).

Ron and Fluidpower 1

Go easy on this guy, I've taught this stuff for years and that is often the one thing that students get hung up on. Now I just tell them to memorize it and the concept will sink in later.
He may just be right though, I haven't been involved for a few years and since we haven't heard from Pascal in a while perhaps he has revised his work.

Don't quote the mindless hydraulic sayings.

It is all about energy

A hydraulic pump just converts eletric or diesel energy to hydraulic energy. No pressure, no energy. Oil flows from high to low energy. Therefore the oil must be losing head or pressure if there is flow. If there is no flow, doesn't the pump increase the pressure? When the pressure increases it is increasing the energy of the oil as the energy from the pump is coverted into energy in the oil. If so, then is the pump producing flow or energy. Hydraulics is just another way to get energy from one place to another like a crankshaft, cables, or a bunch of gears.

Fluid power 101 "Fluid pressure cannot be created without resistance to flow".

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What if I pump oil up hill into a tank? The oil can flow freely but the pressure still rises at the pump. In this case one is increasing head.

There is pressure at the bottom of the ocean.

I like to shake up the hydraulic forums with my engineering/physics approach to hydraulics. Hydraulic people think in terms of flow. I think in terms of energy.

It is all about energy.

gas said:

Go easy on this guy, I've taught this stuff for years and that is often the one thing that students get hung up on.

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I say bring it on.
So you teach your students that pumps generate flow, eh ? Sounds interesting.

JesperMP said:

You posted about the accumulator being used to save energy, and my point was that this kind of pump system works in alternative way to also save energy. Rather than using an intermediary buffer, this system only uses just the energy needed to create the required flow and pressure at any given time.

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You are right in a steady state sense but motion control is not about steady state. In motion control applications one can't wait 100+ milliseconds ( an eternity in motion control ) for the pump to respond. Some day I hope to fix that by tying the motion control of the actuators and pumps together.

Our system is like "slow" motion control (pun intended).