Take a look at this site for an interestion discussion on what makes a hydraulic cylinder move.
http://www.plctalk.net/qanda/showthread.php?t=42533
Seewhat you think on the subject.

I agree, Bud; I think this is an interesting discussion.
I understand Peter's point, but his statements that most hydraulics guys think "flow makes it go" is inaccurate. Maybe the majority of hydraulics guys he has talked with think that, but the people I talk with know that a piston moves because force on one side exceeds that on the other.
Peter also stated that, "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."
It's not my place to do so, but if he decides to post his comments here, he might be surprised at the outcome.

Ooops!

Alan, I'm one of those that has always thought and taught that "Flow Makes it Go" and even wrote it in my Basic book. Even Peter has not given a reason I understand or believe that backs up his "Force Makes it Go" statement. Maybe you can give me an explanation of why it is "Force That Makes it Go" that I can follow.

My thought is, I can have Force that is ample to overcome the load but without flow entering the actuator it is not moving.

When i'm trouble shooting a non-working hydraulic circuit that is operating at a reduced cycle time but has full system design pressure showing on a known good gauge, I don't raise pressure to get it back to design speed. In that case I check Flow and if the pump is putting out rated Flow I start looking for Restrictions to Flow, Kinked Hoses Mashed Plumbing or other Flow Reducers or places where Flow is Bypassing such as blown Cylinder Seals or partially open Bleed Valves.

Always seemed so logical to me and no one ever refuted the statement, BUT, Now I'm really beginning to wonder. I don't mind being wrong since I have heard so many wrong ideas aboout how Fluid Power functions over the years that I don't want to add another wrong idea.

Even Peter post to prove "Force Makes it Go" by using the example of oil going through a Servo Valve set at 10 GPM Flow at 1,000 PSi will pass 1.414 times more flow if the input pressure to the Servo valve is increased to 2,000 PSI. At the same time he answered my question about whether the Flow changed when speed increased due to the higher Force he repilied that Flow did increase when the speeed increased from theincreased pressure supply. To me the increased speed ame from the increased Pressure Drop across the Orifice.

I never got an answer I understood when I asked about a cylinder moving along 500 PSI contacted another load that raised the pressure to 1,500 PSI, How much faster would it go if flow remained the same?

A better one yet is Peter's statement that a Single Rod Cylinder wi;; Exrend faster that Retract when a valve is given the same signal input. Certainly is true of Proportional or Servo valve circuits since the Spool moves the same amount with the same signal input for either direction of cylinder travel so the restricted flow from the full area is trying to be overcome by reduced force of the Rod End area.

Peter thinks all hydraulic circuits should be controlled by Proportional or Servo valves and all his assumptions are based on that input.

I believe Peter's assertions are all based on Proportional or Servo valve circuits that have Symetrical Spools and have the same Flow Areas in both directions of travel.

At least tha is the way I am making my deductions. If I am wrong I will readily admit it, In Writing, as soon as I hear a vable reason that this math challenged person can follow.

quote:

Originally posted by Peter Nachtwey:
[QUOTE]
I never got an answer I understood when I asked about a cylinder moving along 500 PSI contacted another load that raised the pressure to 1,500 PSI, How much faster would it go if flow remained the same?

How many times must I refer you to the VCCM equation?

The flow wouldn't stay the same. The flow would increase. Speed would increase based on the supply pressure, the piston area and the load. The velocity would increase by roughly the sqrt(3) or 1.732 if the load force is 0. 3 is the ratio of 1500/500.

/QUOTE]
Gentlemen, I know I'm stepping into a very deep discussion about something I thought was very elementary and basic, and easy to understand. I have read this thread over and over, even on that plctalk-forum. It has been hard to organize everything that have been said from you guys..I can see a few contradictions in arguments. I really want to understand the logic in this discussion, and how flow, pressure, resistance (load) and force, interacts with each other. Now I'm ready to step in with some statements/questions. Please reply to them you disagree with.....To simplify, let’s not bring “internal leaks" into the flow discussion, they wouldn’t increase the speed anyway, or..

...If a cylinder moves at a "certain" CONSTANT speed, a proportional amount of oil is moving at a "certain" flow...OK!?

...This oil(fluid) could basically be replaced with another media, like air, or even a piece of rebar grinded into steel powder, or even a solid rebar to push the cylinder, the speed of the rebar would the same as the cylinder...OK!?

...In a friction free, non gravity environment, we don't need to apply a force to maintain speed. Only to change speed. There will be no pressure to maintain speed...OK!?

...Pressure is just force spread out on a certain area...OK!?

...With some friction and a load we need to apply force to maintain speed, and we will get pressure...OK!?

...We have 500psi making a force to maintain a cylinder speed...OK!?

...If load increases, force (pressure) have to increase to maintain speed...OK!?

...Now we have 1500psi, AND THE SAME MAINTAINED SPEED...OK!?

...Did we get an increased speed from increasing the force (pressure)? NO, we just maintained THE SAME speed, NO INCREASE OF SPEED...OK!?

...PRESSURE and FLOW is just another way to describe FORCE and SPEED, we can say that PRESSURE is FORCE and FLOW is SPEED...OK!?

...Break now…BUD asked: “How much faster would it go if flow remained the same?”...This is the same mechanical parallel as, when we drive our car on the highway, we set the Cruise Control to maintain speed. When we hit an upgrade, increased load, the CC make the engine apply more force to maintain speed. No increase in rpm (flow), just more fuel to increase torque or force...OK!?

...But of course, if we drive on that same highway with constant force and speed, and suddenly start giving engine more fuel, torque (force) is going to increase and……what happens…we accelerate, till we get to a speed that matches the applied force>...OK!?

...Why does the acceleration stop and we get settled at a new higher speed…..because friction increases when speed increases. If it wasn’t for the friction and gravity speed would keep increasing for ever when force is applied...OK!?

...“faster” is a speed increase...OK!?

...we had to increase force (pressure from 500psi to 1500psi) to maintain speed due to increased load...OK!?

...We just agreed that FLOW is SPEED...OK!?

How can you Peter N. then say we get a speed increase if we have the SAME flow?????


ps.
...This is the same mechanical parallel as, when we drive our car on the highway, we set the Cruise Control to maintain speed. When we hit an upgrade, increased load, the CC make the engine apply more force to maintain speed. No increase in rpm (flow), just more fuel to increase torque or force...OK!?

...But of course, if we drive on that same highway with constant force and speed (rpm), and suddenly start giving engine more fuel, torque (force) and rpm's is going to increase and……what happens…we accelerate, till we get to a speed (rpm) that matches the applied force...OK!?

...Why does the acceleration stop and we get settled at a new higher speed…..because friction increases when speed increases. If it wasn’t for the friction and gravity speed would keep increasing for ever when force is applied...OK!?

quote:

Originally posted by Peter Nachtwey:

quote:

Originally posted by AKKAMAAN:
...In a friction free, non gravity environment, we don't need to apply a force to maintain speed. Only to change speed. There will be no pressure to maintain speed...OK!?

What force are you talking about then? The force on the cap side, rod side or the net force. Your statement is true for the net force but realize that it takes force to push the oil out the other side of the cylinder.

Of course i'm talking net force, actuallly I said that there was no friction or gravity....like out in the space...no resistance from oil we push out on cap side....

quote:

Originally posted by Peter Nachtwey:
So where does the energy go?

Do you need energy, any more than the kinetic that you already have, to maintain speed when there is no friction and no gravity to fight?

quote:

Originally posted by Peter Nachtwey:

quote:

Originally posted by AKKAMAAN:

quote:

Originally posted by Peter Nachtwey:
So where does the energy go?

Do you need energy, any more than the kinetic that you already have, to maintain speed when there is no friction and no gravity to fight?

No but the problem stated the pressure increased from 500 to 1500 psi and the flow is the same. Where does this extra energy go?

Ok thanks for your patience Peter...I think I get your point now....man, its hard to think about it this way, would be very valuble if we all had the same imaginary hydraulic system to relate to in front of us, when we talk pressure 500 to 1500 I see you are talking pressure in front of a valve, pump pressure, and others are talking load pressure on the cylinder side, and it's DELTA p over the valve that is the key, and it all gets confusing when I see two of you discuss and not agree...I need more time on tghis one.....this energy deal is the key point...It basicly doesnt matter if it is a hydraulic or mechanic system....the energy have to go some where....either heat (friction), increased potential energy(gravity) or increased kinetic energy(mass)...
I'll sleep on this one now....

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This is Newton's first Law usually called Law of Motion.

As long as there is a balance among the Force of the Load, force on the cap of the piston, and the force on the rod side of the piston the cylinder will travel at a constant velocity. The velocity can be anywhere between 0 and 100%. Any unbalance of forces will make the cylinder accelerate or decelerate to a new velocity until the forces are once again in balance. Force on both sides of the piston are directly proportional to pressure on both sides. Pressure can only be changed by adding or subtracting oil (FLOW).

quote:

Originally posted by Jim K:
Pressure can only be changed by adding or subtracting oil (FLOW).

I like that statement too, even the last sentence make sense to me...Good job Jim K!!

Your absolutely correct. That's why I started my explanation with balance of forces takes into account Fc,Fa,Fload

I want to change my last sentence to address the initial question - What makes a cylinder move.

Newton's first Law

The Inbalance of Forces as Newton refers to causes acceleration or deceleration. The inbalance of forces can be caused by a change in flow or a change in load. Once the forces come back into balance the cylinder will continue on in constant velocity from 0-100%

Does flow make a cylinder move? YES
Does a change in load make a cylinder move? YES
What causes a cylinder to undergo a velocity change (0-100%)- See Newton's Law of Motion.

He was a smart Dude!

I forgot.
Yes, I am familiar with the VCCM and have all of Jack's books.

He's a smart Dude too!

One final comment:

In the real world, a change in load is the net change(friction, weight, wind, etc)

Peter wrote:

quote:

Pressure can be changed by compressing the oil. In vertical systems it can be load induced. In dynamic systems the energy of a moving mass can compress the oil.

Let's say the Cylinder was a Single Acting Ram like you might find in a Service Station Automobile Lift. No Piston, just a large Rod in a Bore with a seal around the Ram/Rod.

How far would the Load Induced Ram move when the Pressure Increased due to the added Load???

What allowed that Movement????? I believe you will say the Compression of the Liquid. But, isn't that Flow of molecules being foceed into closer contact with each other?? Still sounds like it could be caused by Flow?????

Also, If I want to raise that Ram 12" do I add more Pressure or more Flow?????

Jim K;
You might be interested in this same discussion on an Electrical (PLC) forum at this address:

http://www.plctalk.net/qanda/subscription.php?

Go to the "Someone just had to ask" discussion and enjoy.