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

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

(OP)
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

LunchBoxSession

Insane Hydraulics

### RE: Fire torch warning... Pressure created by resistance to flow??

"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! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
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" ??

I = V/R

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

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/

### RE: Fire torch warning... Pressure created by resistance to flow??

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."

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
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.

### RE: Fire torch warning... Pressure created by resistance to flow??

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)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Fire torch warning... Pressure created by resistance to flow??

2
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.

### RE: Fire torch warning... Pressure created by resistance to flow??

2
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.

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
Thanks, Bumblyari!

#### Quote (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"

#### Quote (Bumblyari)

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.

#### Quote (Bumblyari)

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.

### RE: Fire torch warning... Pressure created by resistance to flow??

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! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)

#### Quote (IRstuff)

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.
• Constant flow pumps with fixed displacement in a system where we vary the pump pressure to control various speeds and forces of the actuator.
• Constant pressure pump with variable displacement in a system where we vary the pump flow to control various speeds and forces of the actuator.
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

### RE: Fire torch warning... Pressure created by resistance to flow??

This thread is going in circles.

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

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.

#### Quote:

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

#### Quote:

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.

#### Quote:

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/

### RE: Fire torch warning... Pressure created by resistance to flow??

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

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

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 !

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

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'

#### Quote:

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).

#### Quote:

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/

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

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.

#### Quote (PNachtwey)

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.

### RE: Fire torch warning... Pressure created by resistance to flow??

"resistor doesn't create the voltage"

And how would it? No resistor on Earth will generate a voltage without a voltage field, ever.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote (IRStuff)

No resistor on Earth will generate a voltage without a voltage field, ever.

Try this:
No woman on Earth will generate a baby without a man, ever.

Nobody was arguing that.

So this brings us to the crux of the discussion. There is no such thing as a voltage field without a resistance to current flow, though I’ll listen if someone wants to explain how you could have both voltage and no resistance to current flow.

The implication in the original post refers to the resistor in a circuit.

So, I presume that you would not disagree that the resistor creates the voltage in a circuit, recognizing that a voltage field is solely created and solely exists by the prevention of (resistance to) current flow.

Therefore, I will not entertain the notion that a battery creates a voltage or that a pump create a pressure or that an accumulator creates it, either, and that neither voltage nor pressure exists until something is introduced (resistor, air gap, switch, valve, plug) to prevent or resist the flow.

Pressure is created by resistance to flow as voltage is created by the resistor in a circuit . You do not get pressure or voltage, either one, unless and until you are preventing or restricting flow.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote (akkamaan)

If we have a "12V" battery resting and no circuits that draw currency there is no voltage differential?

Sure there is — solely because between the positive terminal and the negative terminal there is an enormous resistance to flow - a nearly infinite resistor commonly referred to as “air.” You do understand that an infinite or nearly infinite resistor is still a resistor, right? That resistor is what is allowing for a voltage to exist between the battery terminals.

You can replace that resistor with any of a great number of other resistors and your voltage will drop accordingly. Lower the resistance to flow and you get lower voltage measured on your battery. Put a small LED lamp on it and you’ll see nearly 12 volts across your terminals, due to the extremely high resistance. Put a lower resistance resistor, such as a lawnmower blade, across the terminals and then see what your voltmeter reads. Since you are lowering the resistance, you get a lower voltage. Try it sometime.

#### Quote (akkamaan)

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?

Um... yes — there is a pressure differential between the contained fluid and the place that you’re standing, presumably outside of the accumulator or hydraulic circuit. This is because the flow of the fluid is resisted by the shell of the accumulator and the body of the hydraulic cylinder and the pipes, hoses, etc. The pressure is created by resisting the natural tendency of the contained fluid to find its way to an area of lower pressure; the differential area is created by the boundary of the containment system. If you crack a valve, the pressure as measured at the outlet port of the accumulator or pump will be reduced. Open it up wide and it will be significantly reduced, because you have reduced the resistance to flow.

#### Quote (akkamaan)

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?

It is all of that plus the closed hydraulic system itself containing the fluid, resisting it from flowing anywhere, i.e., back to the tank or onto the shop floor.

#### Quote:

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.

Hmmmm.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
Your argument fails because there is always an active force, pressure or voltage behind any motion. And there is no active energy component in a dead resistor or load. You can't increase the voltage or pressure by increasing the resistance. Pressure or voltage has to be increased on the prime mover side of the equation.

Usually, your arguments come in combination with the "flow makes it go theory". There is no energy in "flow", Flow is just a way to describe the velocity of gases an fluids. Only torque (Nm), force (N), pressure(N/m2 has the necessary "Newton"-component. That's why pressure/voltage can come from the resistance.
But resistance is a conditional factor for how high pressure or voltage we need to express from the source of energy to be able to move the load or overcome the resistance.

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

Your argument fails because there is always an active force, pressure or voltage behind any motion.

Let's see how that works. Try this real-world experiment. Put your hand straight out, facing up and place a postage stamp on your palm. Now apply 10 pounds of force straight upwards to the postage stamp. Can you do it? Why not? You can raise your hand as fast and as hard as you like, but you are still never going to apply 10 pounds to a postage stamp in free air. Why can you not do it?

Now take a 9.9 pound package of printer paper. Place it in your outstretched palm and apply 10 pounds of force upwards. Hmmm... where you could not create any appreciable force on the postage stamp, now suddenly, you can apply a force to this. In other words, your attempts at applying a force failed until you have a resistance. Note that this will work whether you try to do it up, down or sideways.

Your statement that an active force, pressure or voltage being behind a motion does nothing to refute the statement that pressure is created by resistance to flow.

#### Quote:

You can't increase the voltage or pressure by increasing the resistance.

Yes. Yes, you can and that is exactly what happens when you close a valve or turn up the knob on a potentiometer. I have explained that twice in what happens with your car battery.

#### Quote:

Pressure or voltage has to be increased on the prime mover side of the equation.

That is wrong because it is not possible to do anything with your battery, generator or pump to increase pressure.

The electrical analogy:

Yank the rope on your generator and where does the voltage come from? Where is the voltage once it starts reading 120V? If you tied the two terminals together, where would the voltage go?

If you replaced the windings with solid copper plate, what would happen to the plate as the magnetic field passed by it 60x/second? What is eddy current heating? If you took the copper plate and cut a big hole in the middle and then saw cut one side so that you have an incomplete hoop, what would you see between the two faces of the saw cut? Would it be some measurable voltage? If you pushed the two faces together, where would the voltage go? Could you measure any voltage anywhere on the entire loop with the two faces pushed together? If you introduced a resistance to current flow by opening up the saw-cut gap again, would you measure a voltage between the two faces? If there is no voltage when the air gap is gone and you get a voltage reading when you re-introduce the air gap, then what creates the voltage? How is that similar or dissimilar to a single turn of bare wire? How is that similar or dissimilar to a single turn of insulated wire? In what way is that similar or dissimilar to a hydraulic pump?

The point is that neither a generator, nor a power plant, nor a battery "create voltage".

The mechanical analogy:

Take a gear pump and disassemble it. Surprisingly, or perhaps not, the driving pinion is often an involute spur gear profile that will mate quite nicely on a rack. Now mate this exact gear pump pinion to a rack and turn the pinion. Voila! The rack moves back and forth, just as it will on the steering in your car. Now, allow unrestricted movement of the rack and tell me what kind of stresses (coincidentally, measured in psi) are created in the cross-section of the rack. Save for the inertia of the rack itself, you get pretty much zero stress (psi) in a cross-section of the rack when there is no resistance to movement of the rack.

Now, attach the far end of the rack to a plow or shovel or some form of resistance and observe the stresses in the rack. More resistance from the load and you get higher stresses in the cross-section of the rack. Now attach the end of the rack to something that will not move. Attempt to turn the pinion. Extreme stresses (psi) in the rack and yet... no movement. That is to say, there is no flow of teeth passing a fixed observer. Apply a load cell into the system while the far end is fixed. Try to turn the gear back and forth. What does the load cell read? Is that similar to a pressure gauge in a hydraulic system? Does the load cell show fluctuations in force as you attempt to turn the pinion back and forth? If you know the cross-sectional area of the rack, can you use the information from the load cell to determine the stress in the working medium of the power transmission element (i.e., the rack in this case or the fluid in a hydraulic system)?

Now, imagine that you place a clamp on the rack that is attached to the source of resistance to movement of the rack, such that extreme forces in the rack will cause the clamp to slide along the joint. What will be observed when the final load is fixed and the pinion/rack is forced to move? Will there be significant heating caused by the clamp dragging? Will the work/energy of the rack moving back and forth be dumped into friction between the clamp and the rack? Can it be noted that on the downstream side of the clamp, there will be no movement of the load? Could you call something like that clamp a stress relieving mechanism? Is that similar to a pressure relieving valve in a hydraulic system?

Now, put the pinion back into a housing and fill it with a fluid. The exact same pinion is now moving fluid instead of a rack. Whether it is moving a rack or a volume of oil, the movement of the medium (rack or oil) must take place for the shovel or plow or whatever implement to move or "go." In other words, the flow of either fluid or solid must occur for something to go, or more succinctly, the flow makes it go.

Finally:

I have seen the idea posted multiple times on this board that somehow "flow makes it go" is wrong. Each time I have seen it, there has been a significant amount of detail that has a "then a miracle occurs" step between the theory and the conclusion that somehow flow doesn't make it go. So here we are. I have put my cards on the table and will eat crow if someone will correct my thinking and analogies, but the correction has to be correct both theoretically and in the real world. I'm sure that it's been explained elsewhere, so I'll read an article or three that is on a non-ET website if it will explain just how "flow makes it go" is wrong. So let's see it.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

I suppose the is a corollary, if it does not go there is no flow.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

I see the hydraulic system as an energy transfer device or mechanism or system. Both pressure and flow or force and velocity must be present. Otherwise you have only the capacity to do anything.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

Pressure and flow are interrelated. So it can be somewhat confusing as to which causes what. As hydtools points out, though, the flow of energy through the process is pretty straightforward. Remember one of the most fundamental principles of science, energy cannot be created or destroyed, only converted from one form to another.

### RE: Fire torch warning... Pressure created by resistance to flow??

Do the pneumatic people also say that 'flow makes it go' ?

If so I'd better make sure my bike is chained down the next time I pump up its tyres (sorry, tires).

### RE: Fire torch warning... Pressure created by resistance to flow??

Solving any engineering problem involves simplifying assumptions. "Flow makes it go" is not an expression that I would use, but inherent in it is the assumption that liquids are incompressible and and that expansion of hoses are not an issue. These assumptions are very common in many engineering problems, but not always accurate.

### RE: Fire torch warning... Pressure created by resistance to flow??

Can anybody think of cases where there is pressure without flow being involved? If I put a book on the table the pressure on the table is mass*g/area. Does the table create the pressure or the book?

#### Quote:

Do the pneumatic people also say that 'flow makes it go' ?
I have never heard that from someone doing pneumatic controls but then I haven't seen many people do real pneumatic controls. Obviously it is not true. Both air and oil are compressible it is just a matter of magnitude. So why do hydraulic people say flow makes it go?

#### Quote:

Solving any engineering problem involves simplifying assumptions. "Flow makes it go" is not an expression that I would use, but inherent in it is the assumption that liquids are incompressible and and that expansion of hoses are not an issue. These assumptions are very common in many engineering problems. but not always accurate.
Our tech support guys hate hydraulic designers that make the assumption stated.
Oil is definitely compressible. It is more compressible if there is entrained air. Hydraulic designers often do not know the formula for natural frequency. The bulk modulus of oil, compressibility, is part of that formula. The natural frequency limits how fast a hydraulic cylinder can accelerated or decelerate. I wrote an H&P article about this. Hoses expand a lot. They are the main culprit in lowering the natural frequency. What makes matters even worse is that the hose, or solid piping, add to the compressed volume of oil and even worse is the fact that the speed of sound or pressure waves are not infinitely fast in oil. It is roughly about 4ft/millisecond. The length of hose on this project
was 40 ft which created a 10 millisecond dead time. There is no point in having fast motion controller if the hydraulic 'designer' is going to handicap the controller with hose.
One of our engineers was operating the hydraulic controls. The normal TV guys may have had PhDs but they knew nothing about hydraulic controls. Most so called 'hydraulic designers' no little about designing a hydraulic servo system.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

### RE: Fire torch warning... Pressure created by resistance to flow??

Peter, you are completely correct, but most hydraulic power systems are are not servo-systems. Hydraulic systems have been around for hundreds of years, long before electricity was commercialized. It was electronic controls that allowed servo-systems to be developed, first with electric motors and that slowly trickled into hydraulics. You do not need Laplace transforms to design a wood splitter. Just keep that in mind. You need to know your audience.

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

Peter, you are completely correct, but most hydraulic power systems are are not servo-systems.
Ask yourself "where is the money?". Do you want to be in the wood splitter business or the high tech business?
Why don't we weight what is important by how profitable the business is? Think value added.
Back in the mid 1990s I was sitting in front of the VP of a major hydraulic company. He dismiss us by saying that hydraulic servo control is only about 5% of the market and high risk. At the time my thoughts were "there is an opportunity to grow"

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

### RE: Fire torch warning... Pressure created by resistance to flow??

I retired from what I thought was a very satisfying career. The design, development, and production of pneumatic and hydraulic powered tools for the mining and construction markets. Servo controls were not involved. There is a bigger world in which to work.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)

#### Quote (EngineerTex)

Let's see how that works. Try this real-world experiment. Put your hand straight out, facing up and place a postage stamp on your palm. Now apply 10 pounds of force straight upwards to the postage stamp. Can you do it? Why not? You can raise your hand as fast and as hard as you like, but you are still never going to apply 10 pounds to a postage stamp in free air. Why can you not do it?
Why would I want to push "a stamp out to the stratosphere in one second"?
Pressure is not the purpose with hydraulics, it is the motion of an object we want to accomplish. If I want to move that stamp 1 foot upwards in 1 second with my arm I do it by lifting my 10000 times heavier arm. Not a lot of extra force needed
Let's say that stamp has a mass of 0.0001kg I apply 50N (approx 10 lbs)it will accelerate with Newtons Second law of motion, F=m×a, or a=F/m, a=50/0.0001=500000m/s2.
But where is the engineer that tries to design an application that induces a certain pressure or force rather than a certain motion with a specific velocity? Whatever application there is, work has to be done. Work takes energy. Work is the product of force and distance. Even if it is a "static" press, there has to be some compression or motion (work/energy/pressure×volume) to accomplish the applied force. And the object in the press is not moving so the compressing (force/pressure) motion has to come from the prime mover.

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

Why would I want to push "a stamp out to the stratosphere in one second"?

Who knows? Even if desirable, such a thing is impossible with the prime mover (your hand.) It illustrates that even if your arm is capable of applying a force of 10 pounds, it cannot do so without something to resist that 10 pounds, similar to how a pump that can push fluid against all sorts of pressures, can't do anything besides spill fluid all over the floor if there is nothing to resist it.

#### Quote (akkamaan)

Pressure is not the purpose with hydraulics, it is the motion of an object we want to accomplish.

#### Quote (akkamaan)

And the object in the press is not moving

Um. Ok.

Anyway. For everyone else reading this, the fluid doesn't really care about what we want to accomplish. The object in the press is moving. It is being compressed. And the press itself and the object in the press together resist the movement of the fluid in all three dimension. This is where the pressure comes from. The pump provides fluid flow. The resistance to that flow is what creates the pressure, whether the calculations describing the system include high frequency effects like fluid inertia and elasticity of a hose, or low frequencies, like the movement of a cylinder rod.

Resistance to twisting is what causes torque in a driveshaft.
Resistance to movement of the crane hook is what causes tension in a cable.
Resistance to movement of the bucket is what causes bending in a backhoe boom.
Resistance to fluid flow is what causes pressure in a hydraulic system.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

Tex your statements are useless for any type of problem solving. So pipes create pressure. That's useful knowledge.

### RE: Fire torch warning... Pressure created by resistance to flow??

If resistance to flow does not result in pressure, why are we concerned with pressure at the outlet of actuators in motion? That pressure we call backpressure. There is only resistance to flow in a return line as fluid flows from the actuator to the reservoir which is usually vented to atmosphere.
In the force balance equation for extending a cylinder there is a force term resisting piston motion and is described as that force necessary to push fluid out. Force to push fluid seems to imply a resistance to flow.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote (Compositepro)

Tex your statements are useless for any type of problem solving.

Respectfully, I disagree.

Identical engines are used in a wide variety of vehicles. Knowing that vehicle weight, tire size, rated approach angle, etc. are what dictate drive shaft size is an important thing to know.
Knowing that the load on the end of the cable is what controls the cable tension is an important thing to know if you need to keep a crane mast from being torn down.

The point of bringing up all of these analogies is that just as in hydraulic systems, in order to determine certain aspects of the system, one must look at the load, rather than the energy source.

Most importantly here, the audience of the videos originally referenced is filled with introductory-level fluid power designers, mechanics and technicians. Understanding why a pump being turned at a constant speed will show widely changing pressures, due solely to a changing load, is a very important thing to know. Resistance to flow creates the pressure in a hydraulic system in the same way that resistance to current flow is what creates the voltage in an electrical system and if a person does not have his mind wrapped around that concept, he will have zero understanding of the why and how a fluid power system works. This concept is equally important to know for advanced fluid power system designers.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

EngineerTex, you are missing the point. Resistance to flow does not create energy. This is similar to IRStuff's comment about resistors do not create voltage. You also seem to forget that it takes pressure/force to accelerate a mass of oil out into a void.
Pressure is just a form of energy a fluid might have. There is heat, pressure, kinetic, potential etc.
So how does one use an equation to calculate pressure?
Look at this very long and pretty detailed simulation of a hydraulic cylinder.
Where do you find an equation that indicates pressure is resistance to flow?
Your argument is similar to the "flow makes it go" guys that say V=Q/A but they assume the know what the flow is when in truth the flow happens because of motion, flow doesn't create it.
The closest equation I can think of is Q=K*sqrt(ΔP)or (Q/K)^2=ΔP but that is just a change in pressure, not the absolute pressure.
Look at the simulation, basically it is integrating changes in pressure with respect to time. The pressure changes add or subtract from the absolute value.
In short, P = P0 + (Q/K)^2
The best you can state is that pressure is the resistance to flow plus the pressure of some other pressure state. P0 could be a 0 or it could be 1 atmosphere.

Technically HydTool's post above should be I = ΔV/R

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

### RE: Fire torch warning... Pressure created by resistance to flow??

Does the Darcy-Weisbach equation for pressure loss due to flow friction mean anything here? It was derived based on momentum principle. Newton may be okay with that.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

Does the Darcy-Weisbach equation for pressure loss due to flow friction mean anything here? It was derived based on momentum principle. Newton may be okay with that.
Yes, it is useful to computer pressure drops but it does compute pressure. This is the distinction that most people miss. Also, there is no pressure drop if there isn't a difference in energy potential. The pipe by it self does not create energy.

BTW, I wish they had specifications for pressure drop for pipes and hoses. Also capacitance. There should be a specification for "resistance to flow"/meter and capacitance/meter. Both values I could use in a simulator. However, I have yet to see an equation that calculates and absolute pressure except by integration.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote (PNachtwey)

The pipe by it self does not create energy.

Neither does the pump. That is the whole point here. All of the parts of the system convert energy.

I will not be condescending and act like you don’t know that energy is neither created nor destroyed. I will, however, point out that you conveniently and, I can only assume, purposely ignore it in the attempt to make your point.

Whether we talk about a simple energy conversion device like a resistor or an antenna, or we talk about more complex energy conversion devices, such as steam engines or desk fans, the energy is only being converted from one form to another. You are correct to point out that pressure is an indirect measure of a part of total internal system energy.

With that in mind, what does a bridge abutment do to a speeding truck? Its kinetic energy is converted into noise, heat and metal deformation by the abutment. The truck did not make this energy conversion. The wall did.

Now consider a nozzle, yet another energy conversion device. This particular one converts potential energy into kinetic energy. A nozzle fitted to the plumbing at the bottom of a water tower will accelerate water based on the shape of the nozzle. When comparing dozens of shapes of nozzles, you will find the flow characteristics to be determined by the shape of the nozzle and the plumbing, as opposed to the water tower. The energy conversion device is the nozzle, not the water tower.

What about an injection pump, which, with no moving parts, uses the steam from a boiler to pull water from an outside tank into the boiler that is producing the steam to begin with? It seems impossible, but by carefully noting where the energy is and how it is converted back and forth from potential to kinetic and then back again, it is seen that pressure and flow are changing based on the confinement and release of potential and kinetic energies.

How about a resistor? How about an LED? What about a throttle? Where is the energy going in each of those systems? At the same time that resistors create heat, motors make rotation and light bulbs make light, none of these things are creating energy, which brings us to the heart of the issue. Neither a throttle nor a nozzle nor a motor nor a pump creates or destroys fluid energy, whether that energy is potential or kinetic. Your entire argument rests on the notion that because motive energy is imparted by the pump, that the pump is the origination of the pressure. And this is wrong, wrong, wrong. The pressure arises due to the overall system — whether that system is a plug or an accumulator stuck into the pump’s outlet port or any configuration of hoses, pipes and valves — converting kinetic energy into potential energy, heat energy or work outside of the fluid system. Kinetic energy is converted to a static pressure by the containment of the fluid that would otherwise continue with its imparted motion from the pump. It is this resistance to flow that creates pressure. The system itself converts electrical energy to mechanical energy and from mechanical (rotating) energy to another form of mechanical energy (fluid) and without a containment system existing in some form or another, the kinetic energy is never converted to potential energy and pressure would never exist. The resistance to flow creates pressure.

To drive home the point, how is pressure created in systems that do not have pumps or motive force applied to the fluid, i.e., when the fluid itself is expanding or contracting? What occurs inside of a wax motor? Why is force only exerted at the end of a wax motor’s shaft when there is a rigid shell to resist flow in all other directions? What happens when water is contained in a rigid vessel and then frozen? Does the lack of a pump to apply motion mean that there is no pressure? Would there be any pressure at all if there were no resistance to movement? Finally, what happens to a pump when you run it without its case? You would get all kinds of fluid motion inside of your tank, but it does nothing useful because there is nothing to resist flow in any direction and no pressure is developed anywhere. This is the same as inducing eddy currents in a copper plate. It does nothing useful and no potential is created.

TL;DR: The containment of fluid, which converts kinetic energy to potential energy (pressure) is as much of an energy conversion device as a resistor, a motor or a pump. Ignoring that does not mean that a pump creates pressure.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

#### Quote:

Neither does the pump. That is the whole point here. All of the parts of the system convert energy.
Yes! but that doesn't meant "pressure is created by resistance to flow" or what I hear more often is "pressure is resistance to flow".
Now if someone were to say "a pressure drop is created by resistance to flow", that is good.
This does not imply that resistance to flow creates energy nor to is imply that one knows an absolute pressure, just the pressure drop.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

### RE: Fire torch warning... Pressure created by resistance to flow??

That is because the ideal voltage source has unlimited current capacity. It is not a real life source.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

Yes. Yes it is, within limits.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
This boils down to energy. Yes, energy can't be created, only transferred and converted to different other forms of energy. A certain form of energy has a local source. We can mechanically transfer energy from point A to point B. During the transfer of energy we have losses, pressure drop, voltage drop, volumetric leakage etc, usually into heat. We can transfer mechanical energy with a drive belt, bicycle chain, a piston rod in an engine etc. "Flow" can't transfer energy but "oil" can. Flow is just a measurement of how fast we transfer energy. Flow only tells us that energy transfer is happening. Point A is the hydraulic pump and point B is somewhere downstream of the pump ie the actuator. Resistance to motion of energy transfer, which the flow is an indicator of, causes the losses and part of transferred energy are converted into "heat" or eventually another energy form. The force or pressurizing the oil that is behind the energy transfer cannot come from an energyless resistance. That means the pump is the source of pressure behind the transfer of energy and the motion of an actuator. The actuator only causes the pressure drop which means the actuator is just converting incoming pressure energy into mechanical work which results in other energy forms like potential energy when lifting a mass, friction, and heat when moving frictious loads like when a dozer push dirt on the flat ground.

### RE: Fire torch warning... Pressure created by resistance to flow??

In a dusty old bookshop in a back street of Cambridge (UK), an old manuscript has just come to light which is believed to be a previously unknown work of Sir Isaac Newton and entitled Principia Mathematica Part 2.

In it appears a corollary to his first law of motion which states:

‘Non quiescit quiescat urgeatur objecti manet volumen ex humore quodam tempore introitum hidrauliskās actus i.e. facit fluxus est ire. (Ut puto habeo hoc primum nefas) – I. N. (Sir)’.

Latin scholars have translated this as:

‘An object at rest stays at rest unless acted upon by the result of a volume of hydraulic fluid in a certain period of time entering an actuator, i.e. flow makes it go. (I think I may have got it wrong the first time) – I. N. (Sir)’.

It is likely that engineering textbooks around the world will now have to be recalled for corrections to be made to long-held beliefs.

For more fairy stories, check out the Brothers Grimm.

### RE: Fire torch warning... Pressure created by resistance to flow??

Ok. Now we have two different views stating why

#### Quote:

pressure is created by resistance to flow
is incorrect.

If I can boil down the first school of thought, which I believe is held by akkamaan, it would be the following: Because the energy imparted to the fluid originates inside of the pump, both flow and pressure are created by the pump. * If that summation is not correct, then please correct it and perhaps I will see something that I have not seen so far.

However, if that distillation is a mostly-accurate representation of the argument, then I remain unconvinced. For my final example for why I do not believe this to be the case, I will raise the phenomenon of the simple water hammer in household plumbing. When a faucet is suddenly closed, there is a large pressure spike throughout the plumbing inside the house. The origination of the pressure spike is at the valve and if you had a number of fast-acting pressure transducers located all through the plumbing, it would be seen that the pressure spike through the fluid moves backwards in relation to the flow, i.e., outward from the valve. That is to say that the pressure increase begins at the valve, not the source of flow.

Now, to the second school of thought on why the quote above is argued to be incorrect. I do not see enough difference between:

#### Quote (PNachtwey)

a pressure drop is created by resistance to flow

and

#### Quote:

pressure is created by resistance to flow

to see where the problem lies. In a static system under pressure, is there not a pressure drop across a pipe wall from its inside to its outside? I will need further explanation before I can get on board. Again, I will read an outside article explaining it, if you have one to link.

Also:

#### Quote (PNachtwey)

This does not...imply that one knows an absolute pressure, just the pressure drop.
I do not follow what you are saying here.

*In this summation, I am not accusing anyone of claiming that energy is created or destroyed.

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

### RE: Fire torch warning... Pressure created by resistance to flow??

Your water hammer example relates to an instance of energy conversion. The kinetic energy of the flowing water was converted to potential energy when the valve closed. The valve's sudden resistance to flow caused a sudden rise in pressure which was reflected back into the water system as following elements of flowing water were each brought to zero velocity and resulting rise in static pressure.

Ted

### RE: Fire torch warning... Pressure created by resistance to flow??

At risk of reopening a shouting match....

Whether or not the "Pressure is caused by the load, not the pump" simplification is always (ever?) completely true, there are some people - myself included - who find it useful. Here's why.

I was brought up a sparky. Like most of my mob, I mis-spent my youth designing, building and setting fire to circuits that worked off (roughly) constant-voltage power supplies, and got myself thoroughly into a mindset that analysed things by starting with supply volts, then seeing what that would drive through the various impedances I'd stuck across it.

For a horrible first term at university, they made us do stuff like Norton's theorem, where we had to adapt our heads to the idea of constant-current sources - but then we dropped swiftly back into that comfortable constant-voltage headspace.

As a sparky moving into hydraulics, it was really useful to have that simple warning early on that you shouldn't try to draw too many analogies between batteries and hydraulic pumps - at a first approximation, pumps behave more like a constant-flow device than a constant-pressure device (so dead-heading them can be exciting in a way that open-circuiting a battery won't be).

I know the constant-flow model isn't universally true either - and actually have a fair number of systems that have constant pressure hydraulic ring-mains supported by big accumulators.

The aim of education isn't to tell you how it is - it's to help you to think for yourself.

A.

### RE: Fire torch warning... Pressure created by resistance to flow??

(OP)
A somewhat fair analogy between an electric DC system and a hydraulic system only work with a hydraulic CP (Constant Pressure) system with an accumulator. Constant voltage alternator=constant pressure hydraulic pump and battery=accumulator. It has to be a constant pressure system and a DC constant voltage vehicle system (alternator and battery) if you want the make a fair analogy between the two.
This is one of my favorite electric vs hydraulic analogies...

### RE: Fire torch warning... Pressure created by resistance to flow??

This topic has given me ideas about an article for H&P magazine.
BTW, the next article will be about valve flow constants.
I have queue of articles.
I will address the pressure is resistance to flow sometime later this year.

If only people would say a pressure DROP is caused by resistance flow.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/

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