Someone just had to ask.

Peter Nachtwey said:

You are not clear
Qe and Qr being the flow from the pump or the flow from the extend and retract side of the piston?

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Peter Nachtwey said:

I read what Tom has to say. I think there is hope for Tom unlike others posting here. Tom is still and engineer, I hope.

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Tom Jenkins said:

You are correct - I wasn't sufficiently clear. The e and r refer to the extend and retract side of the piston, or as us old timers called them the rod end and cap end of the cylinder. The flow referenced is the flow in and out of the cylinder. V is the velocity of the piston. V = Qe/Ae = Qr/Ar.
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Yes, that is part of the proof of the VCCM equation. Keep going, you will soon come to the same conclusion as to why the speed doesn't double when the supply pressure doubles. In stead it is a square root function.

Well, Peter, that's what it says on my business card and my PE license anyway!

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I am more impressed if the PE believes in Newton's law of motion rather than "flow makes it go". Did you see the part in one of my PDFs about

ΔPressure=β*ΔVolume/Volume

If you let ΔVolume=Flow-Velocoty*Area then you can see that If Velocity=Flow/Area that ΔPressure must be 0. You can say that Velocity=Flow/Area only when ΔPressure needs to be 0 but that doesn't make the system go. There needs to be a net force to make it go. It may keep the system going under ideal conditions. It depends on other factors.

The VCCM equation provides the maximum steady state velocity IF the system has the ability to accelerate to that velocity. The differential equations explain how the system will accelerate or decelerate. The available flow makes no differences as long as there is enough flow. The speed at which the sum of forces is 0 is the steady state speed. The flow will then fill the volume using Velocity=Flow/Area. "Flow happens". Those people that see the Velocity=Flow/Area are only looking at the steady state condition. Those that work on big presses normally have more than enough force to make the actuator accelerate so they doesn't make much difference to them. In ALL cases Newton's laws applies Velocity=Flow/area applies only when the pressure doesn't need to change and steady state velocity is achieve but this is a special case of keeping the system going. It doesn't make the system go in the first place.

Tom, look at the PDFs again and try to follow them.

I am getting tired of those that keep on insisting the sun rise in the east and set in the west. That is just how it appears. The sun doesn't rise or set at all, the earth rotates. One can always find a special case that appears to justify their position. Newton's laws always apply. There is no other way to skin tha cat.

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