Someone just had to ask.

[mordred]

I would say any trades peron reading this if ticketed must have a certain amount skills in algebra so with comments they should be able to follow it

 

[TimWilborne]

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.

I believe must people could follow it. Many technicians use some type of algebra while troubleshooting though they may not even realize it. No hurry, but I look forward to reading it along with any other content you could contribute

My person feelings are if someone doesnt understand this they should leave the design of servo hydraulics to someone else that does.

I completely agree. I was just in with one of the local hydraulic guys specing some pumps today. No matter how confident you are in your abilities, you should always seek advice when you are out of your specialty. At the same time it is very good to know the fundamentals so you can provide your specialist with good information. Their recommendation will only be as good as the information you give them

 

[TimWilborne]

Peter, I have added a note for now to warn viewers of the error until the equation is corrected
http://www.patchn.com/index.php?option=com_content&task=view&id=31&Itemid=74

 

[fluidpower1]

Peter said way back:

Quotte:
Twice the pressure yields 1.414 times the speed. That isn't insignificant.


Peter, How much does flow, to and from the cylinder change, when it goes 1.414 times faster?

If you say Flow Does Not Increase, I will have to go along with Force Makes it Go and take Flow Makes it Go out of my books.

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

I believe Peter's statement: "servo hydraulic actuators extend faster than they retract" is the key to his saying what he does about the speed of a cylinder is dependent on how much force it has not how much flow it receieves.

Servo and Proportional valve controlled cylinder that use a Symetrical Spool (EQUAL FLOW PATHS BETWEEN PUMP AND CYLINDERS PORTS) are Flow Restricted. That means the greater the Rod Diameter the greater the flow difference and also Back Pressure between Extending and Retracting motion. Therefore, raising pressure can attain greater speed since it overcomes the Greater Back Pressure while retracting and force more oil through the Servo Valves Restricted Orifice.

However, a cylinder circuit using valves that do not restrict flow often use oversize Rods to return a cylinder to its start position fast while using a low volume pump. In my area there are some Trim Presses in a Diecast plant that have 8" Bore Cylinders with 7" Dia. Rods for fast return. They also use a Free Fall Extend to the work and a Pre-Fill valve to fill the Cap End of the cylinder as it fast advances. The presses use a small pump and motor and have very quick cycle times.

There are a couple of 50" Bore 2,500 Ton presses, 2.5 miles from where I live, that have a 48.875 Rod with a similar setup to the Trim Presses and also use the large Rod for fast return and reduced cycle time at decreased flow. On the Retract portion of the press the Rod End is receiving 370 GPM while the Blind End is sending almost 9,000 GPM back to tank through (2) 12" and (1) 8" Pre-Fll valves. It returns 70" in about 8 Seconds.

Any Cylinder that is not Flow Restricted will always Retract faster than Extend when receiving the same flow. They also have more force on the Extend Stroke since the Area on the Blind End is greater.

At least that has been my experience in the field of Fluid Power.

 

[Peter Nachtwey]

Peter said way back:

Quotte:
Twice the pressure yields 1.414 times the speed. That isn't insignificant.


Peter, How much does flow, to and from the cylinder change, when it goes 1.414 times faster?

Steady state that would be 1.414 time more flow. But flow doesn't make it go. flow only equalizes pressure.

If you say Flow Does Not Increase, I will have to go along with Force Makes it Go and take Flow Makes it Go out of my books.

I don't understand what you are trying to say.

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

I believe Peter's statement: "servo hydraulic actuators extend faster than they retract" is the key to his saying what he does about the speed of a cylinder is dependent on how much force it has not how much flow it receieves.

Force=Mass*acceleration
When the valve is opened the actuator will accelerate if there is a net force. It will accelerate until the net force is 0. This applies to servo systems and your press systems.

Servo and Proportional valve controlled cylinder that use a Symetrical Spool (EQUAL FLOW PATHS BETWEEN PUMP AND CYLINDERS PORTS) are Flow Restricted. That means the greater the Rod Diameter the greater the flow difference and also Back Pressure between Extending and Retracting motion. Therefore, raising pressure can attain greater speed since it overcomes the Greater Back Pressure while retracting and force more oil through the Servo Valves Restricted Orifice.

Yes, but the ratio of the extend to retract speeds is sqrt((CapArea*SupplyPressureWhileExtening)/(RodArea*SupplyPressure WileRetracting));
This formula assume the load is zero. If the pressure is pushing down and the load must be taken into account then the ratio formula is
sqrt((CapArea*SupplyPressureWhileExtening+Load)/(RodArea*SupplyPressure WileRetracting-Load));

On a servo system the supply pressure should not change much whether the system is extending or retract. To make it fit you applications I only need to have a Ps_ext and a Ps_ret or a more general Ps(t) which is the supply pressure as a function of time. That way one can see how the gain changes as a function of distance moved and load changes.

However, a cylinder circuit using valves that do not restrict flow often use oversize Rods to return a cylinder to its start position fast while using a low volume pump.

So how does the oil escape on the cap side? There must be big valves to let the oil escape easily so the back pressure does't build up.

In my area there are some Trim Presses in a Diecast plant that have 8" Bore Cylinders with 7" Dia. Rods for fast return. They also use a Free Fall Extend to the work and a Pre-Fill valve to fill the Cap End of the cylinder as it fast advances. The presses use a small pump and motor and have very quick cycle times.

That is a smart way to do it because otherwise the cap side would cavitate, this trick is used with servo systems too.

There are a couple of 50" Bore 2,500 Ton presses, 2.5 miles from where I live, that have a 48.875 Rod with a similar setup to the Trim Presses and also use the large Rod for fast return and reduced cycle time at decreased flow. On the Retract portion of the press the Rod End is receiving 370 GPM while the Blind End is sending almost 9,000 GPM back to tank through (2) 12" and (1) 8" Pre-Fll valves. It returns 70" in about 8 Seconds.

You have different valves on the top and bottom so you effectively have different valve constants. This means the Kvpl is different when extending and retracting too.

Any Cylinder that is not Flow Restricted will always Retract faster than Extend when receiving the same flow.

NO!!!! Look at the equation in the PDF. You are looking at a specific case. Just because a few presses you have seen work that way because you designed them that way doesn't mean all presses are the same. I bet you can't tell me what the flow constants are for those valves or what the system pressure is when extending versus retracting.

If there isn't enough force the press will not move. This gets back to Alaric's problem of last week. His press didn't have enough pressure to move the press even though it had the flow capability to move it. THIS is why I got the answer quickly and ruined Alaric's fun ( My hat is off to Mildrone too he typed faster whereas I was writing a length post until I saw his answer ).

In your case you have different valve flow constants while extending and retracting AND you have different supply pressure when extending and retracting.

Force makes it go. Flow just equalizes the pressure difference across the valve. You have admitted as much in your example. The force of gravity is making the system go down and the oil is sucked from the tank to equalize the pressure so there is no cavitation.

They also have more force on the Extend Stroke since the Area on the Blind End is greater.

That depends in your systems the pumps don't maintain the same system pressure while extending and retracting. That is why I was careful to use SupplyPressureWhileExtending and Supply PressureWileRetracting. In your systems the pumps can't maintain a constant pressure while extending. Therefore the SupplyPressureWhileExtending is often much less that the SupplyPressureWhileRetracting. This is why you see the system retract faster. Your system doesn't violate the laws of motion. My equation in the PDF file assumes that an accumulator will keep the servo systems supply pressure relatively constant. In your systems you don't keep the supply pressure constant.

I think your design and technique is good. It is obviously efficient. I just object to the way you assume that because your system doesn't behave the way a servo system that Newton's laws of motion doesn't apply.

 

[fluidpower1]

OK, This is what I think I've gleaned from all the conversations:

Force, F=PA, moves a Cylinder, Flow only determines how fast the Steady State Speed of that Cylinder will be as long as there is enough Force available to overcome Opposing Forces.

Maximum Force is determined by the System Relief Valve or Pump Compensator Set Pressure, X the Area of the Cylinder and determines how soon Steady State Speed is reached when Opposing Forces are less than Available Force.

If Opposing Forces require a pressure less than Maximum Set Pressure the Load will accelerate to Steady State Speed as fast as the available Force, at or below Maximum Set Pessure. After reaching Steady State Speed, Pressure will only be as high as it takes to overcome the Opposing Forces when no Flow Restrictors are set to require less than Maximum Pump Flow.

????????

 

[Peter Nachtwey]

OK, This is what I think I've gleaned from all the conversations:

Force, F=PA, moves a Cylinder, Flow only determines how fast the Steady State Speed of that Cylinder will be as long as there is enough Force available to overcome Opposing Forces.

Flow just equalizes pressure. Flow doesn't determine anything except how fast the pressure equalizes.

Maximum Force is determined by the System Relief Valve or Pump Compensator Set Pressure, X the Area of the Cylinder and determines how soon Steady State Speed is reached when Opposing Forces are less than Available Force.

The VCCM equation determines what the maximum speed will be but not how soon. To determine how soon the maximum velocity is reached requires solving a system of linear and non-linear differential equations.

If Opposing Forces require a pressure less than Maximum Set Pressure the Load will accelerate to Steady State Speed as fast as the available Force, at or below Maximum Set Pessure. After reaching Steady State Speed, Pressure will only be as high as it takes to overcome the Opposing Forces when no Flow Restrictors are set to require less than Maximum Pump Flow.
????????

Yes

 

[fluidpower1]

Peter wrote:

"Flow just equalizes pressure. Flow doesn't determine anything except how fast the pressure equalizes."

Do I hear you saying Pump GPM has nothing to do with how fast the cylinder moves?

No matter what size Pump I specify the Cylinder will go the distance in the specified time?

 

[TimWilborne]

Do I hear you saying Pump GPM has nothing to do with how fast the cylinder moves?

No matter what size Pump I specify the Cylinder will go the distance in the specified time?

I'm still trying to follow this too...I think I'm beginning to understand. Flow is necessary to maintain pressure on a moving cylinder.

"Flow just equalizes pressure. Flow doesn't determine anything except how fast the pressure equalizes. "

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

Going back to my high school physics class, I think I understand. The law of inertia states that an object in motion stays in motion and an object at rest stays at rest unless acted upon by an external force. That external force would be pressure?

Am I in left field?

 

[Tharon]

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

That's how I'm understanding it. We experience the bad side of this all the time at work. We have a small pump running two machines, and it only designed to run one. According to management they were never supposed to produce at the same time. Of course 2 or 3 years down the road suddenly they wish to have both running at the same time.

If the operators cycle their machines at the same time, the pressure just isn't there, and one sits and waits for the other to finish before its pressure switches are activated and the cycle continues. The operators usually stagger their cycles now since the powers that be don't want to pay to have the pump upgraded.

 

[Peter Nachtwey]

Look at the pdf file I posted a link to.

Do I hear you saying Pump GPM has nothing to do with how fast the cylinder moves?

Not directly.

The pump GPM and pressure determine how fast energy is converted from the electrical form to the hydraulic form and therefore the systems ability to do work. That is it. All the pump is a energy conversion device. Get over Bud. That is the way it is. There is no such thing has hydraulic power but there is hydraulic power transmission. Hydraulics does not create the energy.

No matter what size Pump I specify the Cylinder will go the distance in the specified time?

There are two limits.
1. The ability of the pump to convert energy. If you get a small pump you will be energy limited. You would call this flow limited.
2. The point where the sum of forces are equal determines the velocity when you aren't flow limited.

In both cases the VCCM equation applies but in the energy or flow limited case the supply pressure will not be constant or the same in each direction.

In all cases I don't care how big the pump is if there isn't enough force to over come the opposing force. The actuator won't move just as in Alaric's case.

Now if the pump can't maintain pressure then one must recalculate using the VCCM formula with the new pressure. In this case the pressure will build up just enough to move the actuator. This is what happens with flow limited pumps. This is the mode in which Bud operates but the velocity then is indeterminate without complex math.

That external force would be pressure?

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.

I will be giving a presentation in Dallas on this topic
www.fluidpowerexpo.com/

Tharon, there may not be enough energy to run both machines at the same time but what if you got a big accumulator. The accumulator stores energy. The pump would then run flat out even when the machines are at a dwell period but then the energy will go to the accumulator. Should both machines run at the same time the accumulator will release some of its energy. I would need to know more but think of accumulators as capacitors or batteries. Accumulators store energy.

What this means is that my 5 GPM pump set at 1500 psi can move my actuator 24" long 2 inch diam with a 1.375 in diameter rod at 46 inches per second. I also have a 5 gal accumulator. I can turn the pump off and still move at 46 inches per second. Do the math Bud? How do I do it? What difference does the size of the pump make when it is off?

 

[fluidpower1]

Anyone who does'nt mind goimg through 26 pages of a question posed by Peter in October 2004 on "why a Single Rod Cylinder extends faster than it retracts" can see this scenarion has been hashed out before by many posters.

Take a look here: http://www.patchn.com/SMF/index.php?topic=353.0

At least you have to give Peter points on his tenacity concerning the subject.

So far I've not read a viable reason to not use the so called Ditties, Flow Makes it Go and Pressure is Resistance to Flow in training books and Fluid Power training sessions. After 40 years of using these quotes Peter is the first one to question them. That is not a real good reason for doubting his position since Fluid Power training is less than adequate in general.

As soon as I hear a viable reason to change the Ditties I will readily admit the error of my ways and openly state that I have been giving out wrong information. That would be the right thing to do after all my writings about the sad state of the training offered for the Fluid Power field.

 

[Peter Nachtwey]

Anyone who does'nt mind goimg through 26 pages of a question posed by Peter in October 2004 on "why a Single Rod Cylinder extends faster than it retracts" can see this scenarion has been hashed out before by many posters.

Take a look here: http://www.patchn.com/SMF/index.php?topic=353.0

At least you have to give Peter points on his tenacity concerning the subject.

That is because we sell thousands of axes of hydraulic control a year and the 1% of the people that really screw things up take a lot of tech support and they don't like being told they made a very expensive mistake.

So far I've not read a viable reason to not use the so called Ditties, Flow Makes it Go

How can you ignore Alaric's problem?

and Pressure is Resistance to Flow in training books and Fluid Power training sessions.

Show me a formula for calculating pressure. Pressure is just one of the engergy attributes of a fluid. Heat and elevation are others. There is pressure at the bottom of the ocean. There is pressure in the center of a gases planet. Where is the resistance to flow? You think in very specific terms.

Show me the equation for pressure is resistance to flow!

After 40 years of using these quotes Peter is the first one to question them. That is not a real good reason for doubting his position since Fluid Power training is less than adequate in general.

Maybe someone else on this forum will fall into the trap and try to find a "pressure is resistance to flow" formula for you.

The electicians here would equate this to voltage is resistance to current. The electrician knows that if he puts a resistor in series with a current source that will create a voltage DROP but the voltage is unknow until the once side of the resistor is referenced to some other voltage or ground.

If you reword your statement to be the pressure drop is proportional to flow squared I would buy that but it does not say what the actual pressure is.

 

[fluidpower1]

Peter asked:

"How can you ignore Alaric's problem?"

I believe Alaric's problem was due to flow restriction. The pump was adequate but Resistance to Flow raised the pressure high enough that excess pump flow went to tank through a relief valve or a pressure compensated pump reduced flow after reaching its set pressure.

Had that same proble many yeasr back. Added a Tee in the line to the Cap End of the cylinder and put a Normally Non-Passing 2-Way Pilot Operated valve with its Inlet piped to the Tee Branch and its Otlet piped to Tank.

Then I piped the Pilot Port that shifts the 2-Way valve to open it to a Tee in the Cylinder Rod End line. Anytime the Cylinder was retracting it had an added path to Tank that reduced Back Pressure on the Cod Side so all Pump Flow could enter the cylidner. Problem Solved, Return speed elevated as first design was planned.

"Maybe someone else on this forum will fall into the trap and try to find a "pressure is resistance to flow" formula for you."

I don't have a Formula Peter, However, I can observe the phenomenom on any hydraulic cylinder circuit in the world.

Pump Flow Free to return to Tank through a 4-Way Directonal Control.
Little or no discernible Pressure when using a DCV with a rated flow at or above Pump Flow.

Pump Flow "Entering a Cylinder Port that is doing work,
Pressure is equal to Cylinder Force divided by Cylinder Area plus the Little or no discernible pressure above and the flow resistance of the oil from the opposite end of the cylinder that is returning to tank.

At least that always satisfied this inquiring mind.

 

[fluidpower1]

Peter also asked:

"There is pressure at the bottom of the ocean. There is pressure in the center of a gases planet. Where is the resistance to flow? You think in very specific terms."

Anytime a fluid is stopped from spreading out and therefore gains Depth, by any type of container, there is Resistance to Flow. Hence, the Ocean is a container and Gravity gives weight to the water and that weight makes each foot of depth of the water equal 0.433 PSI.

Note the Water Pressure from a City Water System is there even when there are no pumps running. That is due to the weight of the water and the height of the Water Towers strategically located at elevated sites. A very large container but a container none the less.