Speed of a hydraulic cylinder.

[willpower100]

Y  I have figured out who the main pain in the rear is there.  guy has something to prove.

[dale1627]

Man, it never ceases to amaze me how much there is to learn. In our plant we have very little hydraulic, actually I can only think of three machines. And I can never remember having to do anything hydraulic on any. Here is my question though, would the same basic principles discussed here also pertain to pneumatic? Common sense would tell me yes, but it has failed me before......

[Peter Nachtwey]

Here is my question though, would the same basic principles discussed here also pertain to pneumatic? Common sense would tell me yes, but it has failed me before......

Pneumatic systems are very similar to hydraulic systems, in fact pneumatics is just a subset of hydraulics.   The main difference between the two is the compressibilty of air verses oil.   The difference is quantified in a term called the Bulk Modulus.   The higher the bulk modulus the less compressible the fluid is.

I have figured out who the main pain in the rear is there. guy has something to prove.

The troll is in Portland, Or.  I am in Vancouver, Wa. just across the Columbia river, however I have never met him.  I think he is just a bitter old man that hasn't kept up with changes.   He does have a lot of experience but his style of hydraulic and mine are totally different.  
There some very progressive hydraulic people in your neck of the woods at PQ Systems.

Back to the question.

Did I answer the question well enough?   I have a mass of equations that I derived to prove what I had been seeing for years.   I don't think anyone would care to see them.   Jack Johnson has a book on electro -hydraulic control.   He has an equation ( VCCM ) that can be used to calculate the cylinder gains in each direction.   I used that equation in making the cylcalc.xls spread sheet.

More data

The ratio of the extend to retact gains ( speeds ) is:

sqrt(area cap side/area rod side)

This assume there is no load and the cylinder is mounted horizontally.  This matches my observations on my system with the 2 inch diameter cylinder and 1.375 inch diameter rod.

sqrt((2^2)/(2^2-1.375^2)) = 1.37

Notice I factored out the PI/4 for each area calculation.
1.37 is the ration of my systems extend to retract gain.

[gbradley]

Here are couple of differences between pneumatic and hydraulic.

Air is compressable.
Oil is not compressable.
Air can be vented to the atmosphere.
Oil must be returned to the tank. (or you need a lot of Kitty litter)

That's just two I can think of.
There are probably ten or more though.


BTW
I'm still not convinced Peter.
Your Excel file may show the answer clearly, but to tell you the truth, it's a little over my head.
Maybe a 2X4 up side the head???
Is there another way you can convince me?

[Peter Nachtwey]

Here are couple of differences between pneumatic and hydraulic.

Air is compressable.
Oil is not compressable.
Air can be vented to the atmosphere.
Oil must be returned to the tank. (or you need a lot of Kitty litter)

That's just two I can think of.
There are probably ten or more though.

The compressible part I covered with the Bulk Modulus of oil.   Yes, there is the enviromental difference.

BTW
I'm still not convinced Peter.
Your Excel file may show the answer clearly, but to tell you the truth, it's a little over my head.
Maybe a 2X4 up side the head???
Is there another way you can convince me?

Just remember that Newton left flow out of the equation F=MA.   Flow by itself just compresses the fluid.  There must be a net force to actually move an object.

Acceleration = Net Force / mass  or
Acceleration = ( P cap * Area cap - P rod * Area rod ) / mass.

When extending the Pcap is Ps - pressure drop across the valve.   P rod is thre pressure drop across the valve.   What should be noted is that because the flow out the rod end of the cylinder is half the flow into the cap end the pressure drop across the valve to the cap end is 4 time greater than the pressure drop valve from the rod end.

[gbradley]

Ok let me try it this way.

Lets say you have this really big cylinder....
It has a volume on the Cap side of 55 gallons.
It has a volume on the Rod side of 35 gallons.
Now you have a pump that is pumping oil at 55 gallons per minute.

You start pumping oil into the cap side. (Open Rod side port to tank)
One minute later, the Ram is fully extended.

Now you start pumping oil into the Rod side.  (Open Cap side port to tank)
In less than a minute, the Ram has retracted completely.

You are telling me that this can't happen, and that it will take longer than a minute to retract.

So what you are saying is that because it has to push the oil in the Cap side out and back to tank, it can't do it faster than 55 GPM?
Why not?

[Peter Nachtwey]

You are telling me that this can't happen, and that it will take longer than a minute to retract.

So what you are saying is that because it has to push the oil in the Cap side out and back to tank, it can't do it faster than 55 GPM?
Why not?

Yes,  you are assuming that a 55 GPM pump will in fact pump 55 GPM.  If the oil has no where to go, a pressure compensated pump swash plate will go to the neutral position as long as pressure is maintained.   A positive displacement pump will push oil through the relief valve.  Your assumption that a pump will supply 55 GPM is like connecting a 1000 ohm resister across a 24 volt, 10 amp power supply.  Just because the power supply can supply 10 amps doesn't mean it will.   It depends on the resistance.   In the case of the cylinder retracting, the back pressure due to pushing out the oil in the cap end of the cylinder is a high resistance because the rod side has a fraction of the cap end sides surface area.  This keeps the pump from supplying all the oil it could supply with a lower back pressure.

If necessary I will find a way to demonstrate with my hydraulic system.

[Eric Nelson]

Aw c'mon, just bore out the port on the rod side to balance the system...

Hydraulic question for Peter...

On those PET molding machines I'm installing new controls on, there's an adjustable pressure switch for the blow mold clamp. It's just an on-off switch, similar in function to the pressure switch on an air compressor. It prevents expanding the bottles if the blow mold isn't clamped at the correct minimum pressure.

The oil feeding the pressure switch passes through a tiny orifice, which tends to plug up with crap (these are O-L-D machines). When it gets plugged, the pressure switch no longer sees the oil pressure.

What is the purpose of this orifice, and why does it have to have such a tiny hole? Isn't oil is going to reach the same pressure at the switch whether it's flowing through this tiny orifice, or a friggin' 6" I.D. pipe???

:prost

-Eric

My quick guess Eric, would be it prevents a hydraulic hammering effect on the switch.

Just a guess though.

[Eric Nelson]

My quick guess Eric, would be it prevents a hydraulic hammering effect on the switch.

The problem is that when the orifice gets plugged up, it encourages the use of an ACTUAL hammer on the switch...

Your guess sounds plasuible. tiny orifice = tiny flow.

Thanks, John.

:prost

-Eric

[gbradley]

Aw c'mon, just bore out the port on the rod side to balance the system...

Thanks Eric. That's what I'm talking about.

BTW a pneumatic cylinder can be stopped (and held indefinitely) at mid-stroke.
You just need to have different pressures on each end.
Put the same pressure on both sides and it's gonna extend.

[Peter Nachtwey]

What is the purpose of this orifice, and why does it have to have such a tiny hole? Isn't oil is going to reach the same pressure at the switch whether it's flowing through this tiny orifice, or a friggin' 6" I.D. pipe???

:prost

-Eric

You are right.   The size of the hole only determines how fast the pressure equalizes on either side of the orifice.

[Peter Nachtwey]

BTW a pneumatic cylinder can be stopped (and held indefinitely) at mid-stroke.
You just need to have different pressures on each end.
Put the same pressure on both sides and it's gonna extend.

Yes,  we are working on a pneumatic position controller now that can do just that.   The new controller specifically for pneumatic  applications and can position within .001".  This link shows some of our early successes with our hydraulic motion controller which was modified to handle the active damping required for pneumatic control.  

ftp://ftp.deltacompsys.com/public/pneu/

BTW,  I took these pictures to show that the weight didn't oscillate.  I later increased the gains more and found I could get the load to move nicely into position without the hesitaion at the end of motion due to friction.

[gbradley]

Peter please review this table

Retraction speeds are always faster, and the bigger the Rod, the faster still.

http://www.powerdrives.com/fluidpower/page17-18.htm

I observed a large rolling shear retracting alot faster than extending last night. No servoes involved just a simple sol. valve.  watsupwitdat?!