Flow makes it go,,,,, NOT!!! Newton did not include flow in his 3 laws of motion.

[PNachtwey]

Unless there is cavitation in the oil, the equation, V=Q/A, is always true.

No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No! No!
It is people like you that screw up hydraulic servo designs that others need to fix or live with.
Flow doesn't cause motion. Flow is the result of motion.
Flow equalizes pressure or energy. It is force that makes things go.
Also, oil compresses or expands so velocity cannot be proportional to flow during these times.
Newton did not include flow in his 3 laws of motion.
You can not calculate how a mass will accelerate using V=Q/A
In the recent thread it was pointed out that the press would fall down if oil was not metered out on the rod side and that there would be cavitation. No oil would be necessary, only gravity.

I have been arguing this point on different hydraulic forums for 20 years now. I don't know who teaches V=Q*A. They harm the hydraulic industry by their failures and make people think motors are the only way to go.

There are two equations that are very important to hydraulic servo control
one is how pressure increases and decreases

There is an important equation missing.
dP/dt=E*(Q(t)-Velocity(t)*Area)/(DeadVolume+Position(t)*Area)
P(t)=P0+integral of E*(Q(t)-velocity*Area)/(DeadVolume+Position*Area) dt

E is the bulk Modulus of oil.
Velocity is positive when extending
Position increases when extending

Notice there is a term Q(t)-Velocity(t)*Area. When this term is 0 the pressure does not change so the force does not change.
This equation must also be applied to the opposing side of the piston.
Then it is possible to calculate the net force using Pa*Aa-Pb*Ab-friction
Then it is possible to calculate the acceleration and integrate acceleration to get velocity and integrate the velocity to get position.
That is a lot of calculus or differential equations that must be solved simultaneously for a good simulation.

The other must know equation is the VCCM equation. The VCCM calculates the peak steady state velocity

https://forum.deltamotion.com/viewtopic.php?f=18&t=497

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[HPost]

Peter

There is no doubt that you are correct in your statements on cylinder velocity calculations. There are no arguments.

As I see it, this forum is about giving free advice and “tips”. The folks that ask questions about fluid power don’t really have much of a clue about what they are doing and based on vague details, we try to help people and often get no thanks or no reply.

I like to start with the basics that people can get their heads around. If they come back asking for more details, we can elaborate slightly on the basics and discuss KV values, VCCM equations and the finer points of motion control theory. Unloading control theory on someone that wants to know about basic hydraulic theory will just scare them off.

V = Q/A is an appropriate approximation for cylinder velocity. Should the OP need to know more, we would rightly expect them to pay for professional advice rather than get it for free on this site.

I am as vehemently opposed to the statement “flow makes it go” as you are. However, the fact is that professional training organisations use that phrase and it’s plain wrong. They won’t stop using it though...

When people start to pay for the advice I give here, I will give more precise and qualified data. Until then, I will be helpful, but I won’t give away what I can charge for. That means they get basic help, not much more.

 

[PNachtwey]

The forum's name if Fluid Power Engineer, not hacking or kludging.
What you and many others don't understand is that we get support calls from customers with bad designs. It happens all the time. The bad designs are not our fault.

As I see it, this forum is about giving free advice and “tips”

V=Q/A is not a tip. It is not free when it costs money later.

Unloading control theory on someone that wants to know about basic hydraulic theory will just scare them off.

Possibly or maybe they will hire someone that knows what they are doing and save money in the long run.

V = Q/A is an appropriate approximation for cylinder velocity

only when not accelerating or decelerating. You CANNOT use V=Q/A to calculate speed when you do not know flow.

They won’t stop using it though...

The people that use "flow makes it go" should NOT be teaching. They create problems down the road when there are screwed up designs. This makes hydraulic motion look bad.





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[HPost]

Those of us that don’t add our company name and web link are giving advice for altruistic reasons. You seem to want to fight and argue with everyone and advertise Delta Motion and Control at the same time...good luck.

 

[PNachtwey]

Those of us that don’t add our company name and web link are giving advice for altruistic reasons.

You think so but is it good advice?

You seem to want to fight and argue

You have not disputed what I have said. I am not arguing. I am telling the truth. There are those that have gone before like Taco Viersma, George Keller and Jack Johnson. I have the advantage of knowledge of math, physics and control theory plus many system.

advertise Delta Motion and Control at the same time...good luck.

I have our links in my signature but I haven't said anything about using our controller specifically.
Leonardo could buy a system from MTS. They have PhDs that how to design test systems. I know how to design them too. The difference is they will sell a complete system and I only sell the motion controller and tell the customer how to design the system. MTS will sell a complete system that works but it will cost.

I would prefer not to have to argue with the "flow makes it go" people but it must be done. The "flow makes it go" people are not helping the hydraulic industry. I am not the enemy. I/we provide options to use hydraulic fluid power instead of servo motors.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

Finally found it, VCCM = valve control of cylinder motion. Just had to go through about 50 articles, some basically titled VCCM; only one bothered to define it, almost accidentally.

Also came across IDAS Engineering.

http://www.idaseng.com

which has what look to be very useful and relatively inexpensive training/teaching materials for electrohydraulic motion control systems. Books and DVDs sold through Hydraulics & Pneumatics publications.

(Edit because there's a crumb under the 'd' key that I can't get to come loose. If I don't smack it hard enough it doesn't register the hit.)

 

[PNachtwey]

Finally found it

Very Good!
If you just search for VCCM there are too many unrelated uses of the letters VCCM so you must narrow down the search by searching for VCCM hydraulic. Then you will see a link to one of Jack Johnson's articles and the next link is to the one I provided on our forum.

I have Jack Johnson's "Design of Electrohydraulic Systems For Industrial Motion Control.
Chapter 9 is loaded with good stuff.
If you know how fast the load must move and how quickly it must accelerate then the formulas in chapter 9 help you calculate the cylinder area and the valve size that will achieve that goal.
Jack Johnson only used algebra to derive the equations so a high school student should be able to follow how the VCCM equation was derived.
I have only a few problems with the book.
George Keller's
Hydraulic System Analysis also solved the problem but he used differential equations. George Keller's formulas calculate the acceleration rate too. Basically the equation calculates the velocity as a function of time. The problem is that this book is 50 years old. The examples are dated.
Both books can be bought at the H&P book store.

So why bother?

The VCCM equation calculates the maximum steady state speed when the valve is fully open. This provides a good estimate of the open loop gain of the hydraulic actuator in both directions. If you do the math you will find that hydraulic actuators extend faster than they retract when the pressure supply is equal both ways. The flow makes it go equation predicts the opposite.

I/we see way too many poor designs. We get phone calls from people that wonder why the motion controller isn't fast enough when the motion controller is sending 10 volts to their valve?
Everyone wants to move faster and cheaper. Most people try to use cylinders that are too small in diameter. These cylinders may not have enough area to accelerate to the desired speed in the desired distance. Another problem is cavitation. When the cylinder is extending rapidly with an over hauling load, there are limits as to how fast the load can be decelerated without causing cavitation. This is especially a problem with presses where the rod is pointing down with a have slider or platten on it. Top chipper heads in saw mills also have this problem.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[3DDave]

I searched for "VCCM equation," so no false leads there. I came across bunches of articles and comments about hydraulics. None of them explained the abbreviation. I lucked into an abstract that followed publishing conventions for such things, and it spelled out the source.

The main failures I've come across are from people who don't look at the basics** of pressure loss due to flow in hoses. One product I was peripherally involved with kept blowing motor seals. The motors all tested great on the bench, but the undersized case drains built up too much back pressure. Another, similar problem, had designers not understand the area ratio between the sides of the piston. Instead of increasing the diameter of one of the supply lines to reduce the flow-related back pressure, they added a second line back to tank and a solenoid to close it off during extension and cantilevered the hose, the solenoid, and a stack of adapters from the cylinder port on a mobile application. Basically a static load of 5 pounds with a 2 foot moment arm on a 3/8" OD tube. Since this was on a trailer, there was no on-board power source for the solenoid, so they tapped off the power for the anti-lock brake system. For some reason no one put together why the stubby 3/8" tube in the middle of the adapter would fracture. On another project, electrical with the same lack of attention, a failure to understand that 24V on a 300 foot extension cord might not work out well. No one looked at resistance to flow.

**I could really stop the sentence there as it was pretty universally the case.

 

[TimSchrader2]

Quote "It is force that makes things go". True. But how far and fast can it go without flow. Not too far, until a vacuum stops it. Although with enough force it could keep going, even without flow.

How often is a servo control required where V = Q/A (an appropriate approximation for cylinder velocity) is not accurate enough? I would guess 20-25% of the applications???? As mentioned it is accurate in steady state conditions. No accel or deceleration.

Anytime "accelerating or decelerating" are important or must be accurately controlled would probaly be the most common reason for servo control.

Accurate wording can be important. I see that. There have been many many times where workers have mixed up flows relation ship to speed with pressure. For example, "The gage says 2000 psi so the cylinder should be moving". I've Seen and heard countless variations of this error.

 

[PNachtwey]

Quote "It is force that makes things go". True. But how far and fast can it go without flow. Not too far, until a vacuum stops it.

A load can fall a long way due to gravity alone. A perfect vacuum can only hold up about 15 lbs / sqin so a load can drop a long way until mechanically stopped.

How often is a servo control required where V = Q/A (an appropriate approximation for cylinder velocity) is not accurate enough?

All the time!
Too many design disaster occur every week.
"Flow makes it go" or V =/Q/A cannot be justified.
V=Q/A does not explain how the load will accelerate.
V=Q/A predict that a cylinder will always retract faster than it extends. This is the opposite of what happens in a servo system.

Anytime "accelerating or decelerating" are important or must be accurately controlled would probaly be the most common reason for servo control.

If you control acceleration and deceleration you also control velocity and position.

Flow only equalizes energy. Flow goes from higher elevation to lower. Flow goes from higher pressure to lower pressure. Flow is the result of motion. If a cylinder is dead headed there is no flow. What does V=Q/A say about that?

I thought I was done arguing this a decade ago.

I wish there was a trade show where experts could be on a panel an try to one up each other with stupid hydraulic people stories. I would love to have this recorded and put on YouTube for all to see. You can't believe the bad designs, pissing contests, the bad excuses, the wasted time, the extra costs and finally the guilty blaming the innocent.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/