Someone just had to ask.

Peter Nachtwey · Oct 19, 2008

The argument is like the one about the solar system long ago

The sun rises in the east and sets in the west so the sun most rotate around the earth. We all observe that. However, that theory it didn't work mathematically and didn't explain the motion of Mars and Venus so the whole idea of the sun rotating around the earth was questioned.

You have provide no formula for flow makes it go or pressure is resistance to flow. How can one do fluid power engineering or design without formulas for these basics?

Newton's laws of motion apply in all cases, not just some. Newton didn't mention flow.

I am sitting in the Denver airport now running on batteries.
Since my run for the Presidency got deleted I will make up a Friday night quiz similar to the problem on the patchn forum only it will deal with flow rather than compressing oil by moving a piston. Thats a heads up. This to will require some calcululs but a clever person may be able to find a solution with an excel spread sheet.

fluidpower1 · Oct 19, 2008

Peter asked:

"How can one do fluid power engineering or design without formulas for these basics?"

In my opinion it's due to the lack of TRAINING offered for persons that somehow got into the Fluid Power regimen. You will notice the one and two week offerings is about the main way most get even a meager amount of Fluid Power TRAINING. There are a few schools with several semesters of Fluid Power classes and most Mechanical Engineers may get one or two semesters in the basics.

Why do we not have Fluid Power Engineers like the Mechanical and Electrical fiels Have? They would have books with all the formulas and make Mathematics be a mainstay of the training program. Then there would be knowledgeable persons designing circuits and putting them in operation.

Unfortuanately the lack of TRAINING has Semi-Worked and is still the norm for almost everyonne in the Fluid Power business.

As I have written before, I started out as a Salesmen for a Fluid Power manuacturer and almost immediately was sizing components and within a short time I was also designing circuits and making schematics. Only very few could read the schematics but they were required for the documentation for a machine.

That is not the way any equipment should be handled but has been the way Fluid Power, in general, has been implemented forever.

However, only a meager minority see the lack of Trained/Dedicated persons in the Fluid Power arena as a problem and from the bulk of feedback on the Fluid Power Forums it is not going to change in the near or even distant future.

Oh well, Someday Maybe?????????????????????????

TimWilborne · Oct 20, 2008

Peter Nachtwey said:
TW, pressure is not force. Again, look at Alaric's problem. He had enough pressure but not enough force because the pressure was only acting on a 1 inch diameter surface.

Yes, I'm aware I left out the area, but at least in laymens terms, is my correlation of why it is pressure and not flow that makes it go correct?

So, just throwing some numbers out there, if we have a 25 gpm pump that has a pressure of 1000psi with the cylinder stationary and we actuate the cylinder the pressure may drop to 700 psi while in motion. If we increase our pump capacity to 50 gpm and stick with the 1000psi, when we actuate the cylinder we may only see the pressure drop to 800psi while in motion. We have "increased the flow" which in fact has increase the pressure while in motion

fluidpower1 · Oct 20, 2008

BTW Peter, the way I figure the amount of flow a Cylinder requires for a certain stroke in a specified amount of time is:

GPM=GPI*IPM (Gallon Per Minute)=(Gallons Per Inch of the Piston or Rod Annulus Area)*(Inches Per Minute Travel Speed)

This comes up with a GPM rate that would move the Cylinder in the allotted time if it instantaneously attained full speed and instantaneously stopped at the end of stroke. It seldom is a flow rate that is matched by a Pump Catalog Flow Rate so it is always necessary to go to the next larger pump and use that flow rate. If that rate is close to the flow figure needed or there was any doubt about the cycle time not meeting the requested cycle time, then I go to the next higher flow rate pump. NOT REAL SCIENTIFIC BUT ALWAYS WORKED FOR THIS MATH CHALLENGED INDIVDUAL with a 2 year Industrial Design degree and a mechancal aptitude. No one ever complained about a cycle time 10% faster that requested but it was a hard sell if it was 1% slower than required.

On top of all that, any pump I'm familiar with, continuously decreases in flow as it runs due to wear and if the original pump is sized to close to the actual flow required the machine cycle soon falls below the requested cycle time.

Also, all the circuits I designed used Valves and Plumbing that were sized for a 25-50 PSI Pressure Drop for their run length. Hardly ever did I win a sale on price but could easily point out the energ waste from using undersized components plus the need for a Heat Exchanger and its energ consumption. At least 90% of the customers had enough experience to see that "Cheaper is Not Always Less Expensive" and paid the higher bid price.

I know you are not convinced, that anyone with minimal Math experience and who cannot quote a Formula for how they come up with workable answers, can make a living in the Fluid Power field. Or that type individual could not write training books or articles for Hydraulics & Pneumatics Magazine about Fluid Power implementation that make any sense or have any value. But, that is the REAL WORLD of FLUID POWER where training and training material is mostly left to Fluid Power Manufacturers and distributors.

Should notbe that way, But I don't see a Rosy future for change, especially not to something as ridiculous as Fluid Power Engineers an Fluid Power Maintenance Technicians.

Oh well, Someday Maybe??????

fluidpower1 · Oct 21, 2008

OT An Informative Hydraulic Newsletter

For anyone interested in Hydrauilc systems.

Take a look at this Newsletter from Down Under that has excellent information for anyone interested in Hydraulic equipment implementation and operation basics to advanced.

http://www.hydraulicsupermarket.com/oct.html

Clay B. · Oct 21, 2008

Think I have heard this one before

I have heard the whole argument before though I never put the math to it Peter did. To prove my point I made a simple test. I put an air cylinder in a vice and had the perticular hardhead in question move the rod in and out of the cylinder. I asked if he noticed that he could pull the rod out alot faster than he could push it back. If the cylinder is long enough you can actually feel the air compressing.

You using the same engery ( or very close to the same to move the rod in either direction) so why the difference.... my understadning (and I may be explaining this wrong) is that when you retracting the cyclinder you are moving more volume so therfore more mass so the amount of energy required goes up. Since for the same speed you would need more energy then you can "feel" more resistance. I do not see why this would be different in a hydraulic system since all your changing is your engergy transfer for air to a fluid.

Well anyway thats how I look at it and so far nobody has proved me wrong.

Peter Nachtwey · Oct 21, 2008

Clay B. said:
I have heard the whole argument before though I never put the math to it Peter did. To prove my point I made a simple test. I put an air cylinder in a vice and had the perticular hardhead in question move the rod in and out of the cylinder. I asked if he noticed that he could pull the rod out alot faster than he could push it back. If the cylinder is long enough you can actually feel the air compressing.

That isn't the same thing as which direction the actuator will move faster. BTW, what direction did the hard head say he could push it the easiest? You will be surprised at the answer. Pnuematica calculations are much more difficult than hydraulioc ones. Are you ready for some math?

You using the same engery ( or very close to the same to move the rod in either direction) so why the difference.... my understadning (and I may be explaining this wrong) is that when you retracting the cyclinder you are moving more volume so therfore more mass so the amount of energy required goes up.

Moving more mass or compressing more gas? What about the other side? The gas expands and the pressure drops

Since for the same speed you would need more energy then you can "feel" more resistance. I do not see why this would be different in a hydraulic system since all your changing is your engergy transfer for air to a fluid.

What if I told you the force required to externally move the cylinder changed as a function of position AND direction?

I can see the Friday night math quiz will be good one. It will have many phases.

Well anyway thats how I look at it and so far nobody has proved me wrong.

Wait. This is similar to Bud's observations. Just because you see it once doesn't mean it is always true.

fluidpower1 · Oct 25, 2008

Peter wrote:
"Wait. This is similar to Bud's observations. Just because you see it once doesn't mean it is always true."

How about if you just have a Cylinder Tube with a Piston and Rod with no Head or Cap with their RESTRICTED FLOW PORT FLOW PATHS?

I believe the force required to move the Piston in either direction would be so close to the same that it would take some sophsticated instruments to detect the difference.

Any FLOW RESTRICTED DEVICE will require more force to move in the direction of the highest FLOW when using the same SIZE FLOW PATH no matter the FLUID used.

Seems like common sense to me and has allowed a couple of 50" Bore Cylinders, in my area, with 48 7/8" Rods to retract 50" in 8 seconds with 370 GPM FLOW since 1952.

Peter Nachtwey · Oct 25, 2008

fluidpower1 said:
Peter wrote:
"Wait. This is similar to Bud's observations. Just because you see it once doesn't mean it is always true."

How about if you just have a Cylinder Tube with a Piston and Rod with no Head or Cap with their RESTRICTED FLOW PORT FLOW PATHS?

I believe the force required to move the Piston in either direction would be so close to the same that it would take some sophsticated instruments to detect the difference.

Yes the force would be roughly equal IF the load is equal.

Any FLOW RESTRICTED DEVICE will require more force to move in the direction of the highest FLOW when using the same SIZE FLOW PATH no matter the FLUID used.

This needs clarification.
The flow through the valve is the same whether extending or retracing. The oil must flow in one side and out the other side of the cylinder. It makes no difference which way the cylinder moves. It makes no difference whether this is a bang-bang valve or servo valve. Regeneration is a different matter because the flow on the rod side does not go through the valve.

Seems like common sense to me and has allowed a couple of 50" Bore Cylinders, in my area, with 48 7/8" Rods to retract 50" in 8 seconds with 370 GPM FLOW since 1952.

Yes, but that argument has been applied by others here that say they have tuned 20 -30 PID system and think they know a lot about tuning a PID when in reality they know a lot about only one system.

fluidpower1 · Oct 27, 2008

Peter answered to my statement:
"

Any FLOW RESTRICTED DEVICE will require more force to move in the direction of the highest FLOW when using the same SIZE FLOW PATH no matter the FLUID used.

This needs clarification.
The flow through the valve is the same whether extending or retracing. The oil must flow in one side and out the other side of the cylinder. It makes no difference which way the cylinder moves. It makes no difference whether this is a bang-bang valve or servo valve. Regeneration is a different matter because the flow on the rod side does not go through the valve."

Do I hear you saying that no matter what the Pump Flow is the a Directional Control Valve that will only allow 50% of the pump flow to pass its designed in, internal Flow Path Restrictions is the reason you state that a "Single Rod Cylinder will Extend Faster than it Will Retract?"

I fully agree with that and have seen it happen on at least one occasion that I sized components for. However, adding valves to reduce the Flow Restricton made the cylinder Retract almost Twice as Fast as it Extended due to the Annulus Area of the Rod End being approxiamtely 1/2 of the Area of the Cap End. After that fiasco in the College of hard Knocks I paid more attention to Flow Path size and never had that problem again.

I have found over the years if Flow is Restricted a Cylinder will move at the Rate Determined by the Flow Restrction no matter if the Restriction is from an undersize directional control valve or a Fixed or Variable Orifice.

Seems so simple to me?????

Tom Jenkins · Oct 27, 2008

I think there is insufficient data in the original question for determining a correct answer.

The formula you gave, Peter, is for pressure drop across a fixed orifice or restriction. If the pump is a fixed displacement pump then indeed flow makes it go, and the speed of a cylinder will be unchanged up to the point the relief valve opens and some of the flow dumps back to the reservoir. The pump discharge pressure will simply rise to make the fixed flow rate go someplace.

If the pump is a pressure compensated variable displacement one then doubling the pressure setting may increase velocity as Peter indicates up to the max displacement of the pump.

If the pump is centrifugal then you need the pump curve as well as the system curve to find the intersection.

The load makes a difference too. If it is accelerating a mass throughout the stroke one kind of behavior occurs. If it is a long stroke at constant load like a hydraulic elevator then another behavior occurs.

And finally, don't confuse the behavior of a pneumatic (compressible fluid) system with a hydraulic (incompressible) one. They aren't the same.

This is a trick question, Peter.

Peter Nachtwey · Oct 27, 2008

Tom Jenkins said:
I think there is insufficient data in the original question for determining a correct answer.

Nathan got it.

The formula you gave, Peter, is for pressure drop across a fixed orifice or restriction. If the pump is a fixed displacement pump then indeed flow makes it go, and the speed of a cylinder will be unchanged up to the point the relief valve opens and some of the flow dumps back to the reservoir.

There is a valve between the pump and the cylinder. That is what the spool constant is for. The supply pressure is constant like an ideal voltage/pressure source.

The pump discharge pressure will simply rise to make the fixed flow rate go someplace.

I didn't say anything moved. The speed is 0. We are just adding oil until the pressure reaches in the cylinder reaches the supply pressure. On on the www.patchn.com forum there is a thread about compressing the oil volume. The quiz was to calculate the pressure change when the piston is pushed back 0.001. I didn't see a need to cover that again since the links have be published on the 'someone just had to ask' thread.

If the pump is a pressure compensated variable displacement one then doubling the pressure setting may increase velocity as Peter indicates up to the max displacement of the pump.

Again, volume is fixed. There is no velocity, but I can add one for next Friday.

If the pump is centrifugal then you need the pump curve as well as the system curve to find the intersection.

Don't assume too much. The supply pressure is fixed at 2000 psi in the quiz. In reality it will drop at bit as flow increases but an accumulator can be added on the output of the pump. This is like putting a huge capacitor on a power supply. It helps to keep the pressure ( voltage ) constant. However, if you want to simulate pumps and accumulators that can be done too. It is too much for a Friday night quiz though. I am still working on the java program but it you want to see what I am doing.
http://forums.hydraulicspneumatics.com/eve/forums/a/tpc/f/8621030121/m/3541030003
I need to improve the interface a bit but I didn't even know how to program in java when I started in August.

And finally, don't confuse the behavior of a pneumatic (compressible fluid) system with a hydraulic (incompressible) one. They aren't the same.

Oil and air are both fluids and they both compress. Obviously oil compresses much less than air but it still compresses.
ftp://ftp.deltacompsys.com/public/PDF/SpringEffectEffBulkModl.pdf

This is a trick question, Peter.

No it isn't. Nathan nailed it. Josh at least figure out many cubic inches of oil that is required using the rule that oil compers 0.5%/1000 psid

My goals was to three fold.
1. To introduce people to the idea of solving interative problems on a spread sheet or a programming language like what Nathan used. I know my eyes were opened when I started to use computers to do the number crunching.
2. To introduce people to one of the most basic formulas which is

Code:

Δp=β*Δv/v

3. To show that there would be an answer here first and it has nothing to do with the amount of training specific to hydraulics.

I am at the Fluid Power Conference and Expo in Dallas ( Frisco ) Tx right now. On Wednesday I give a presentation on this topic and a lot more. My fear is that my topic will be too much for the audience to understand even if only a few people understand that will be enough. Hopefully it will get a few people thinking about hydraulics in a whole new way or at least know what they don't know.

fluidpower1 · Oct 29, 2008

Tom Jnkins wrote:
"I think there is insufficient data in the original question for determining a correct answer."
Tom;
Peter forgot to tell us that he was referring to a Proportional or Servo Valve that had the spool shifted the same amount in both directiions. That means the FLOW is ORIFICED to the same amount in both directions of actuator travel.

So, Inreasing Pressure Drop across an ORIFCE will INCREASE FLOW and INCREASED FLOW will make aCylinder move faster.

However, It was not the INCREASED PRESSURE that made the cylinder go faster it was the fact that the INCREASED PRESSURE forced more FLUID through the RESTRICTION and that INCREASED AMOUNT OF FLUID FLOW made the cylinder go faster.

Seems so simple to this MATH CHALLENGED individual. I still strongly believe "Flow Makes it GO" is not just a Ditty to repeat over and over but a proven fact.

Tom Jenkins · Oct 29, 2008

Math challenged or not, I agree with you Bud.

Pumps create flow, system resistance to flow produces pressure.

Flow dictates how fast, pressure dictates how hard. Obvioulsy there are relationships between the two in orifices etc. but fundamentally flow does make it go, as long as there is enough pressure available to make the flow go.

TConnolly · Oct 29, 2008

Tom Jenkins said:
Math challenged or not, I agree with you Bud.

Pumps create flow, system resistance to flow produces pressure.

Flow dictates how fast, pressure dictates how hard. Obvioulsy there are relationships between the two in orifices etc. but fundamentally flow does make it go, as long as there is enough pressure available to make the flow go.

Tom, I almost never disagree with you, but this time I respectfully disagree. Pumps transfer energy. They don't create anything. Resistance doesn't do anything except consume energy, it produces nothing - although resistance at the end actuator comes from doing useful work.

The energy transfer is in proportion to flow * pressure, the ratio of the two being dictated by system resistance.

If the pump made only flow then the pressure it produced would be zero, making the energy transfer zero, and hydraulics would be useless.

The old "pumps make flow not pressure" phrase was probably repeated today in 50 different hydraulics training seminars across the nation. But that doesn't make it accurate.

Someone just had to ask.

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