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Old October 14th, 2008, 03:23 PM   #1
Peter Nachtwey
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Someone just had to ask.

Does doubling the pressure double the speed of a hydraulic cylinder provided all other things stay the same? It is safe to say the answer is no because Q=Kv*sqrt(ΔP) where Q is the flow and Kv is the valve flow constant. It is easy to see the flow will be proportional to the square root of the pressure drop but there is more because flow doesn't move things. Force does. Newtwon didn't include flow in his laws of motion.
I love saying that because it ruffles the feathers of the hydraulic guys.

So the velocity will not double when the pressure is doubled. For extra credit and a lot of brownie points, what will the velocity be in the extend direction and in the retract direction. This is something you must solve symbolically.

Steve Bailey figured out the formula years ago and he doesn't even do hydraulic systems. I just saw that Steve was on-line and I know he could spoil the fun like Mildrone and I spoiled Alaric's fun.

Anyway, turn about is fair play.

I would post this at the hydraulic forum but they would never figure this out as they still think flow makes it go. When in reality flow just equalizes pressure.

I heard the info on patchn was gone. That is probably just as well as there was too much noise to filter through to get to the data.

This should keep a few people busy for a few PLC scans.
 
Old October 14th, 2008, 03:34 PM   #2
Steve Bailey
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It's still on the server Peter, just not accessible through the front door.

http://www.patchn.com/SMF/index.php?topic=353.0
 
Old October 14th, 2008, 07:05 PM   #3
Peter Nachtwey
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Steve, you realized you ruined the fun.

No brownie points for you this time. I do wish that thread could be cleaned up.

Since Steve gave the answer away
ftp://ftp.deltacompsys.com/public/NG...CM%20Proof.pdf

One can see steady state velocity does increase as a function of the square root of the pressure increase IF there is no load. It also assumes the system is perfect as in no plumbing losses, filters, and the pressure is constant. In reality you should subtract about 20% off the ideal speed to account for these losses.

The VCCM equation should be used to calculate peaks steady state speeds in the extend and retract direction ASSUMING that the steady state speeds can be obtained. Often our customers need to go point to point as quickly as possible so the actuator is always accelerating and decelerating. In this case the calculations get to be very difficult. It requires solving a system of linear and non-linear differential equations.

Many hydraulic designers screw up because they think that flow makes it go and they divide the flow by the area of the piston to calculate the velocity. Then they or their customers call us when the actuators don't move as fast as promised.
 
Old October 14th, 2008, 07:12 PM   #4
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Quote:
Originally Posted by Peter Nachtwey
Many hydraulic designers screw up because they think that flow makes it go and they divide the flow by the area of the piston to calculate the velocity. Then they or their customers call us when the actuators don't move as fast as promised.
We use a position/velocity transducer to get feedback from our system and use those Velocity and Pressure feedbacks to adjust our pump RPMs. There are no hydraulic flow transducers on it at all.
 
Old October 14th, 2008, 08:22 PM   #5
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I had that conversation to which Peter refers here, no matter how hard I tried I never convinced the other guy that doubling the cylinder retract pressure would not significantly speed up the cylinder retraction. I couldn't get an agreement that the cylinder would accelerate only until the net forces on the rod and cap end were equal and that would be max velocity. What really made me shake my head was when I tried to argue that increasing the rod pressure would cause a proportional increase in cap pressure, -- if one thinks that it wont, then what pray tell would make them believe that increasing retract pressure would do any good at all then because without it there would be no gain in fluid outflow rate?




Quote:
Originally Posted by Peter Nachtwey
I love saying that because it ruffles the feathers of the hydraulic guys.
Apparently if ruffles the feathers of some control guys as well.
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Last edited by TConnolly; October 14th, 2008 at 08:24 PM.
 
Old October 14th, 2008, 09:29 PM   #6
Peter Nachtwey
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Quote:
Originally Posted by Alaric
I had that conversation to which Peter refers here, no matter how hard I tried I never convinced the other guy that doubling the cylinder retract pressure would not significantly speed up the cylinder retraction.
Twice the pressure yields 1.414 times the speed. That isn't insignificant.

Quote:
I couldn't get an agreement that the cylinder would accelerate only until the net forces on the rod and cap end were equal and that would be max velocity.
What about Newton's first law of motion?

Now tell the other PLC person that servo hydraulic actuators extend faster than they retract given other things are equal. Bet him lunch.

Quote:
What really made me shake my head was when I tried to argue that increasing the rod pressure would cause a proportional increase in cap pressure, -- if one thinks that it wont, then what pray tell would make them believe that increasing retract pressure would do any good at all then because without it there would be no gain in fluid outflow rate?
At least you know. How does one expect to control the pressure on just the rod side? Two valves would be required to control the pressures on each side independently. That could be done but it would require two servo valves.

Quote:
Apparently if ruffles the feathers of some control guys as well.
Yes, I hope Bud see this. He is under the impression that PLC people are better trained than hydraulic people. That may be so but it isn't the rule.
 
Old October 14th, 2008, 10:11 PM   #7
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Quote:
Steve, you realized you ruined the fun.
I don't know about ruining any fun. There's a lot you have to wade through before you get to the answer. At any rate, I had fun rereading it.
 
Old October 15th, 2008, 01:19 AM   #8
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Guys,
I tried to follow this thread but failed. I'm a complete newbie when it comes to hydraulics and pneumatics. It would be good to see some kind of sketch or image to visualise this. Actuators are my weak point. I'll try to understand that hydraulics tutorial on patchn web site.

Hope I'll be better next time...
 
Old October 15th, 2008, 01:43 AM   #9
Peter Nachtwey
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There is too much 'noise' on that thread.

Hydraulic actuators and the servo valves that are used to control them are much different from the motorized valves that you use.

There is a picture of a hydraulic actuator in the pdf
http://www.deltamotion.com/pdf/hydr2.pdf
The pdf shows an actuator configured with position and force feedback. Most have only position feedback. Few use only pressure sensors or load cells.

You should be able to follow the VCCM Proof.
 
Old October 15th, 2008, 07:21 AM   #10
TimWilborne
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Quote:
Originally Posted by Peter Nachtwey
I heard the info on patchn was gone. That is probably just as well as there was too much noise to filter through to get to the data.
The info isn't gone. I moth balled everything including several unfinished articles Ron was working.

There was some problems with the old forums database so we had to start from scratch to preserve the information. The old forum threads will be imported into the new forum but it is a one shot deal as far as giving users credit for their post. Every time I think that all the users of the old forum have signed up on the new forum that want to participate, another one signs up. For example you, Peter, have 310 very valuable post there. If you aren't signed up then those post will be credited to an "Unknown User" when imported. I'm trying to give everyone a fair chance to sign up.

The new forum is
http://www.patchn.com/index.php?option=com_fireboard

The old forum is
http://www.patchn.com/SMF
 
Old October 15th, 2008, 09:17 AM   #11
Peter Nachtwey
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There, I signed up.

I will not post there often. I have been trying to cut back since I have a never ending hydraulic simulator project to work on now.

I like the new look. I like the fact the titles can be seen. Before you would barely notice them.

BTW, there were two spreed sheets that I had contributed to the site. One did the VCCM calculations, the other calculated the natural frequency of a hydraulic actuator. Do you have then or should I send them to you. In any case I would like to update the VCCM one to modify the graphs.

Finally, there reason I started that post is that there is a section at the bottom of the hydraulic section that calculates speed of a hydraulic actuator incorrectly. It basically is the 'flow makes it go' equation where the flow is divided by the area of the cylinder to calculate a velocity. The flow makes it go equation is wrong and gets many into trouble. Those that use the 'flow makes it go' ,V=Q/A, equation don't take into account the force required to get to the desired speed. If there isn't enough force then the desired speed can't be reached. I/we get tired of explaining to customers that they or their hydraulic designer screw up the design after the blame our controller for not make the system go fast enough.

That thread did not convince Ron to remove the "flow makes it go equation" even though Steve Bailey independently proved that it was the forces that made the actuators move and determined the steady state velocity.
 
Old October 15th, 2008, 10:42 AM   #12
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Well, FWIW, it convinced me - and it explained what I was observing first hand on a 20" cylinder.
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Old October 15th, 2008, 11:03 AM   #13
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Peter, Just want to make sure... on www.patchn.com, there is a hydraulics tutorial, and everything is good except the last section:

"Speed of a cylinder

The speed of a cylinder is dependent on the flow rate to the cylinder and the area of the cylinder. This formula assumes no loss of fluid over a relief valve.

Velocity (Ft./Sec.) = 231 * Flow Rate (GPM)..."

So instead of this we need to use your calculations, right?

P.S. What exactly VCCM means (velocity, cylinder...!?!?)?

Last edited by Pandiani; October 15th, 2008 at 11:09 AM.
 
Old October 15th, 2008, 01:45 PM   #14
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Quote:
Originally Posted by Peter Nachtwey
I will not post there often. I have been trying to cut back since I have a never ending hydraulic simulator project to work on now.

I like the new look. I like the fact the titles can be seen. Before you would barely notice them.
Thank you for the compliments
Quote:
BTW, there were two spreed sheets that I had contributed to the site. One did the VCCM calculations, the other calculated the natural frequency of a hydraulic actuator. Do you have then or should I send them to you. In any case I would like to update the VCCM one to modify the graphs.
The spreadsheets in question can be found at...
http://www.patchn.com/index.php?opti...d=29&Itemid=74

Send me the updated files and I will replace them
Quote:
Finally, there reason I started that post is that there is a section at the bottom of the hydraulic section that calculates speed of a hydraulic actuator incorrectly. It basically is the 'flow makes it go' equation where the flow is divided by the area of the cylinder to calculate a velocity. The flow makes it go equation is wrong and gets many into trouble. Those that use the 'flow makes it go' ,V=Q/A, equation don't take into account the force required to get to the desired speed. If there isn't enough force then the desired speed can't be reached. I/we get tired of explaining to customers that they or their hydraulic designer screw up the design after the blame our controller for not make the system go fast enough.

That thread did not convince Ron to remove the "flow makes it go equation" even though Steve Bailey independently proved that it was the forces that made the actuators move and determined the steady state velocity.
Pandiani, I'm not the best at the hydraulics and you seem to be following Peter, can you write a replacement section for the "Speed of a Cylinder?

Thanks
Tim
 
Old October 15th, 2008, 02:13 PM   #15
Peter Nachtwey
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Quote:
Originally Posted by Pandiani
P.S. What exactly VCCM means (velocity, cylinder...!?!?)?
Valve control of cylinder motion.
I would prefer to call it the VCAM and substitute actuator for cylinder because rarely does the cylinder move. Usually it is the piston and its rod that moves.

Newton's first law should be expanded to use the term NET FORCE. As Alaric pointed out if takes force to push the oil out the other side of the piston and that force opposes the motion. The opposing force also goes up with the square of the velocity.

I didn't develop those equations. George Keller developed a training course for Boeing engineers back in the 1960s. He had a form of the equation but it was complicated. Jack L Johnson simplified it. Jack L Johnson wrote many books on hydraulics and is still an adviser for Hydraulics and Pneumatics magazine.
Jack is still around. I see Jack at the Hydraulic trade shows.

Back in the late 90s I developed the equations on my own because was just getting curious about auto tuning and why the extend feed forwards are less than the retract feed forwards and not the other way around. My equations are similar to Jack's but I developed one set of equations for extending and another for retracting whereas Jack uses the ratio of powered to exhausting side of the piston and valve spool.

TW, I will see what I have for replacing the old speed calculations but the equation in the .pdf is right. It is just a little terse for those that are looking for an explanation instead of seeing how the formula is developed. If I add some comments do you think the average maintenance man would follow the math? It is just algebra and making substitutions. I can explain the substitutions but all is lost if the reader doesn't understand algebra. My person feelings are if someone doesnt understand this they should leave the design of servo hydraulics to someone else that does.
 
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