Fire torch warning... Pressure created by resistance to flow?? 11

[akkamaan]

For a hydraulic guy to say that pressure is created by resistance to flow is like an electrician would say that the resistor creates the voltage...Right!?

Listen to
Jim Pytel, Columbia Gorge Community College
in his Youtube lesson

LunchBoxSession

Insane Hydraulics

 

[IRstuff]

"For a hydraulic guy to say that pressure is created by resistance to flow is like an electrician would say that the resistor creates the voltage...Right!?"

No, the corresponding analogy would be "voltage is created by the resistance to current flow" More properly, though, it's voltage DROP that is created by resistance to current flow.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[akkamaan]

So how much "current flow" is needed to make a "voltage DROP"...
ie
How much "fluid flow" is needed to make a "pressure drop"?

Btw, shouldn't we call it "voltage differential", "delta U" or "pressure differential", "delta p" ??

 

[hydtools]

I = V/R

Ted

 

[PNachtwey]

Its pop corn time! I might get a gas can.

20 years ago we argued two things for about 10 years, maybe more.
1 "flow makes it go".
2. "does a pump create pressure or flow"
It is amazing that people still haven't got it right.




Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

A child had hold of a cat's tail.
The cat howled at the insult.
The mother demanded the child stop pulling the cat's tail.
The child responded "It's the cat that is doing the pulling."

 

[IRstuff]

A voltage drop is a differential.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[akkamaan]

Since you guys, associated voltage drop with current and also that voltage drop is a voltage differential
the pressure drop must be pressure differential and associated with the fluid flow?
If we have a "12V" battery resting and no circuits that draw currency there is no voltage differential?
Or a source of hydraulic pressure but no flow, an accumulator, a cylinder at rest on confined pressurized fluid, or a constant pressure pump, there is no pressure differential?
And what is inducing the pressure in the pressurized hydraulic system with 0 GPM flow? Is it the "bottom of the pressurized cylinder at rest" or the force from the mass (source of energy, prime mover) resting on the cylinder piston?

I can't understand how people high up in the recognized fluid power hierocracy (see the 3 examples in my opening post) can say that pressure comes from the resistance to flow.

 

[IRstuff]

Voltage and pressure are ALWAYS measured relative to SOMETHING; in the case of batteries, it's understood that the voltage of a battery is measured across its two terminals, so it is technically a differential measurement, but it is not a voltage "drop", as a battery is standalone source, while a resistor is not. An isolated resistor will have no voltage drop, EVER; likewise, a standalone pipe will have no pressure drop.

I was originally, and purposefully, addressing only the semantics of your OP, not its physical validity. In school, we learned that the term voltage refers to an "electro-motive force," i.e., a potential that can cause current to flow. Since one can measure a voltage very little, or zero, current flow, I would argue that voltage drives current and not the other way around. Aside from superconductors, there is no way to get current to flow without something that has an electro-motive force, or voltage, present at its terminals.

TTFN (ta ta for now)
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[Compositepro]

This is a chicken or egg argument. Which came first? There is no one correct answer, and it does not matter. Look at it in the most useful way for solving your problem. Light can be thought of as a wave or as a particle at the same time, and you can choose the best model to solve your current problem. The danger you have to avoid is setting up a circular argument that results in confusion or just plain wrong conclusions.

In fluid power the pumps are almost always positive displacement so it is common to start the thinking process with the pump causing fluid motion (flow) and continue from there.

 

[Bumblyari]

It's not quite a chicken and egg situation since, (assuming the egg has been fertilised), both chicken and egg are active components.

It's true that a pump on its own won't create pressure. It's also true that there won't be pressure in the system if the pump is removed. However, in this case one component is active (the pump) and one is passive (the system).

Hence, the system provides the resistance which enables the pump to create pressure.

 

[akkamaan]

Thanks, Bumblyari!

It's true that a pump on its own won't create pressure.

That's true, the pump needs both energy input from a "prime mover" and some sort of fluid or gas which basically is a "mass". Newton's Second Law F=m×a, It takes a force to accelerate a mass. And there is also friction to overcome even if not "measurable"

However, in this case one component is active (the pump) and one is passive (the system).

That's the key in the discussion, in what end of the system, the pump end or the actuator end, is there an "intent" of motion.

Hence, the system provides the resistance which enables the pump to create pressure.

Here is where people get it wrong. Pressure is just another way to express "force". If the fluid instead was a not confined solid we would use the term force, and no one would even think about that the big rock on the ground would "create the force" to move itself when a dozer is pushing. The force (or pressure) is coming from the dozer or the prime mover via a pump.

If we have a system with a simple piston (pump) in one end and a cylinder piston in the other end attached to a load (resistance) and we hit with a sledgehammer on to the pump piston, a pressure wave will transfer from the pump to the load. The pump creates pressure.

 

[IRstuff]

But, pumps DO create pressure without flow; just like batteries have voltage without current. If you take any primed fluid or air pump and block its outlet, there will be a pressure; that's why a pump curve shows maximum pressure at zero flow.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[akkamaan]

But, pumps DO create pressure without flow; just like batteries have voltage without current.

That was my point above.
What differs an electric system from a hydraulic system is that there are two different types of (hydrostatic) pumps.
[ul]
[li]Constant flow pumps with fixed displacement in a system where we vary the pump pressure to control various speeds and forces of the actuator.[/li]
[/ul]
[ul]
[li]Constant pressure pump with variable displacement in a system where we vary the pump flow to control various speeds and forces of the actuator.[/li]
[/ul]

An electric system "always" or usually work with constant voltage (pressure) and variable current or amperage.

I think most people get stuck in the "constant flow" system and from there argue the "pressure is created by resistance to flow"-principle.

If we have a force resultant that makes a load move it must be the force behind the motion that creates the pressure.
But the size of the pump pressure (force) depends on the product of load resistance and what load acceleration we are trying to accomplish. But that doesn't mean that the resistance creates the pressure

 

[Compositepro]

This thread is going in circles.

 

[PNachtwey]

In fluid power the pumps are almost always positive displacement so it is common to start the thinking process with the pump causing fluid motion (flow) and continue from there.

No! Not anymore. That is old technology and wasteful because excess oil goes through a relief valve. Now fixed displacement pumps should be used with a variable speed motor.
Most pumps we see are variable displacement pumps with a pressure regulator that adjusts the swash plate as needed.

It's true that a pump on its own won't create pressure

Newton's Second Law F=m×a, It takes a force to accelerate a mass. And there is also friction to overcome even if not "measurable"

Akkamaan remembers my argument well. Yes, it takes just force to accelerate the oil out of the pump. The force divided by the surface area is the pressure that the pump generates. Once the oil is accelerate is has just kinetic energy in free space.

IRStuff is ruining our fun.

This thread is going in circles.

No!

More later. I need to make more pop corn.





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[Compositepro]

It is not an important point but variable displacement and variable speed pumps are still positive displacement pumps.

 

[Bumblyari]

20 years ago we argued two things for about 10 years, maybe more.
1 "flow makes it go".
2. "does a pump create pressure or flow"
It is amazing that people still haven't got it right.

I wasn't around here then but yes, it is amazing. Not to mention all the other myths, fallacies and inexactitudes put about by our industry's educators. It's not only Sir Isaac who must be turning in his grave !

 

[PNachtwey]

I wasn't around here then but yes, it is amazing.

I don't think this forum was around then. I was referring to the NFPS.org and ifps.org then the Hydraulics and pneumatics forums. LinkIn also had a hydraulic forum where people that were supposedly looking for jobs or trying to impress people, would embarrass themselves. These forums are gone now along with the all the good, the bad and the ugly. I hated that so much effort and knowledge was gone but it was difficult for the new guys to tell which is which because of all the 'noise'

Not to mention all the other myths, fallacies and inexactitudes put about by our industry's educators.

This is a shame. What hydraulics is often taught at a 'dumbed down' level that perpetuates these myths. When I see the equation V=Q/A my integrator winds up into saturation. (IRStuff will get it).

It's not only Sir Isaac who must be turning in his grave !

I am doing my best to calm him down but along the way I have made lots of enemies.

Now I have a pulpit from which to write. Maybe I can do more. I could make this a topic in H&P but it would be short.

People only look at the output but not what is actually happening in the pump. I would explain that a hydraulic pump simply converts energy from electric to hydraulic energy. The energy can be in the form of kinetic energy ( moving ) and potential energy ( pressure). The pump must to do work on the oil. Work is the integral of force over distance. The distance is the length of stroke of a pump cylinder. The force can be thought of as F=P*A. Inside the pump the equation Power = mass*velocity*acceleration assuming the oil is exhausting into free space In free air the pump no longer exerts a force on the oil after the pump piston has reached the end of stroke. Therefore it is no longer accelerating except from gravity. When flow is restricted a there is still the pressure caused by accelerating the oil but also pressure to overcome the any opposing pressure. The pump is still accelerating a mass to add to the kinetic energy put the force available for accelerating the mass is reduced by the opposing force or pressure at the outlet of the pump. When the forces are equal there is no flow but pressure like IRStuff mentioned. This is very inefficient. The energy must go somewhere if the pump doesn't stall, there must be some place for the oil to leak within the pump or the pump has a swash plate in the near neutral position providing just enough flow to to cover the leakage.

In short, the pump needs to exert a force on the oil to move it out of the pump. Any force that exceeds the opposing force will be used to accelerate the oil so it an move out of the pump. If the oil doesn't move fast enough the oil pressure will increase which is a form of potential energy.

To really understand you need to look at this as if you are modeling a system and must know all the equations.

A good example is the 'flow makes it go' equation V=Q/A. This equation is wrong because it would compute that a rodded cylinder will retract faster than extent. This is not so. The V=Q/A doesn't explain how the cylinder accelerates. If you look at a simulation you will find that force makes the piston go. Flow only equalized energy by flowing from a higher energy ( pressure ) to a lower energy. TIn the case of a hydraulic cylinder, the flow occurs only because there is motion. I think Akkamaan put this in his signature on the H&P forum.

BTW, I still have some of the old Excel spread sheets from those discussions. Not everything was lost.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[EngineerTex]

For a hydraulic guy to say that pressure is created by resistance to flow is like an electrician would say that the resistor creates the voltage

Where is the problem here? Who would say that the resistor doesn't create the voltage?

Get your video camera and prove me wrong. Take the battery out of your car and remove the giant resistor on it, i.e., take away the 9" air gap by using a 2" diameter copper bar. Tell me where the voltage is when there is no resistor. Link me the video, I enjoy a good show. I'll make some popcorn, myself.

A good example is the 'flow makes it go' equation V=Q/A. This equation is wrong because it would compute that a rodded cylinder will retract faster than extent. This is not so.

lolwut?

Engineering is not the science behind building. It is the science behind not building.