Speed of a hydraulic cylinder.

[Eric Nelson]

Both points you make are correct. The cylinder will retract faster but with less force then it will when extending, given egual pressure and flow in both directions.

Let's hope Peter agrees with you...

I want Ron to update the page I refered to in the first post.

This leads me to believe that 'retract' is the WRONG answer :?:

:prost

-Eric

[gbradley]

The flow through the valve is the same, but you asked about the speed of the cylinder.
It's gonna retract faster.

[Eric Nelson]

It doesn't make any difference which way the piston is moving.  The flow through the valve WILL BE THE SAME when extending and retracting at the same speed!    Now does the flow argument make sense?

See? I didn't think he would... :lol:

First off, will you agree that the piston side and non-piston sides have differing volumes? For arguements sake, lets say the non-piston side has a volume of 1 gallon, and the non-piston side has a volume of 1/2 gallon. If you pump is flowing at 1 gallon per hour, retract will take 1/2 hour, while extend will take 1 hour (hurry up already!).

Please elaborate... banghead

:prost

-Eric

[Peter Nachtwey]

FORCE = MASS X ACCLERATION

Notice that Newton neglected to mention flow.

BTW,  it is actually the sum of forces = mass x acceleration.

What is the sum of forces when the piston and rod are moving at the highest speed at constant velocity and acceleration is 0?
By definition the maximum velocity is reach when the accleration is 0.

I will answer this evening when I have time to explain.  Meanwhile the torture continues. I'm bad :twisted:

BTW, how many of you guys do hydraulics?  I know willpower100 does.
How many do hydraulic servo systems?  Just curious.

[Steve Bailey]

The answer to the question I posted earlier is "It doesn't matter".

Now, consider the fact that the flow from the high pressure side to the cylinder and the flow from the cylinder to the low pressure side both go through the valve.  Does this affect your reasoning?

To answer Peter's question about working with hydraulics, I've had to troubleshoot hydraulic systems, including a few with servos, but I've never had to design them.

[gbradley]

In response to Steve.
I guess if both sides of the cylinder goes through the same valve then that would affect it.
Is that the gotcha peter?

I am pretty sure you can plumb it so that if your are extending the cylinder, the Rod side port dumps to tank, without metering through the valve.
I know that you can T it, and use a bang bang valve to dump it to tank.
BTDT

[willpower100]

Peter, I cant believe how many people got involved with this question, its great.  And you are right, the FPS board has a lot of people with chips on thier shoulders.  Not very many people actually work with servoes up here, there are a lot of them in the mills but not many people actually get to test or repair them.  I was lucky I had the opportunity to do servo rebuilds and repairs.  I couldnt drink too much coffee, or my hands would shake too much to put them together..

[croakus]

Peter,
I agree with the "volume group".  Less volume to fill, assuming same gpm, the cylinder will retract faster.

To answer your other question, the last four years I've worked on large injection molding machines (1500 - 3000 ton).  A lot of time spent troubleshooting an add-on AB Proset 600 control system - is it electrical or hydraulic?  Where can I put my hydraulic meter leads?
No formal training on servo valves, just OJT.

Keep the questions coming and Thanks Ron for this site.

Mike

[Icky812]

I think I see where Peter is going with this.   I don't have the math skills to explain it, so here goes in laymans terms.

If I am right, then the answer has to do with the forces acting on the cylinder with regard to the amount of mass that has to be moved.  Flow certainly does enter into it.  Fluid has mass, mass has to be moved and you can only push so much, so fast through a knothole.  So assuming that the valve is a normal hydraulic valve, you can only flow 'X' amount through the valve given 'Y' amount of pressure.  Since you have put 'X' amount to fluid into the extended cylinder, you have to remove that same 'X' amount of fluid upon retraction.  Again assuming that the pressure is equal for extend and retract, the cylinder will move at the same pace extending and retracting since flow through the valve should be the same when extending and retracting.

This would be much easier to explain with  drawings.  Unfortunately I don't have time to do them right now.

Am I close Peter?

[Peter Nachtwey]

The piston and rod will move faster in the extend direction than in the retract direction.  This is due to that fact that the piston and rod will accelerate until the net force on it is zero.  At that point it is at maximum speed.  When extending the piston has a large area to apply pressure AND the rod side has a small area to apply the back pressure.  It takes pressure to push the oil out of the cylinder.    The piston and rod will accelerate while the pushing force is greater than the back pressure force.  Eventually the pressure drop across the valve is so high that the pushing force drops to equal the back pressure force and the piston and rod stop accelerating and have reached the maximum speed

See the cylinder calculations spread sheet.
http://www.patchn.com/cylcalc.xls

Make the cylinder diameter 2 and the rod diameter 1.375

When retracting, the speed at which the net force is zero is reached much more quickly because it takes A LOT of pressure to push out all the oil on the cap end of the cylinder.   This means that the force due to back pressure will be much higher than when extending.   In addition the system pressure has much less surface area to push on when retracting.   The result is th speed at which the net force is zero is much lower while retracting.    However, this maximum retract speed can go up significantly on a regen system where the rod end is piped directly to system pressure.

Play with the spread sheet.  Got to go, more later.

Good lord Peter, that board is horrible!!!

What the hell is wrong with those guys?

[Peter Nachtwey]

Good lord Peter, that board is horrible!!!

Are you refering to the FPS board?

Icky, you must have posted while I was positng.  You are very close.

[Eric Nelson]

As George mentioned, are you porting the return oil through the same valve? I don't know hydraulic valves, but I'm picturing something similar to how a pneumatic 4-way/5-port valve operates (in a way like electrical flow, what goes out must return). If so, then I'd agree with your 'extend' answer.

Much like you can dump to tank, if I stick a quick exhaust valve on the exhaust port of a cylinder, I can get it to FLY since I'm not restricting the air back through the valve porting.

My only experience with servo hydraulics was with the Moog valves on the Netstal injection molding machines at my last job. They would get 'stupid' on occasion, and command the press to open at full speed into the hard stops. You could hear the press slam from across town!... :shock:

Since neither Nestal nor Moog was able to resolve the issue, I ended up adding a relay to electrically disable the valve if the press ever opened farther than normal. The sensor used to detect overtravel was nicknamed the 'slam imminent' sensor. Killing the control signal was enough to stop the press before it hit the hard stops. This was rough on the clamp, but better than having the entire press rip loose from its moorings!... :?

:prost

-Eric

Are you refering to the FPS board?

Yes.

These message boards are here to help each other, not cut each other down. I would never post a question there, as my ignorance would not be tolerated.

[Peter Nachtwey]

Are you refering to the FPS board?

Yes.

These message boards are here to help each other, not cut each other down. I would never post a question there, as my ignorance would not be tolerated.

I know what you mean.  The forum troll there thinks I am a punk kid that wants to electrify his hydraulics.   You can see he rarely passes a chance to give me grief.

The topic we are dicussing now came up about two years ago.   The webmaster deleted the thread because of the troll giving everyone and espicially me sh!t.  I am a big boy.  I can take it.

Willpower100, have you figured out who the troll is?

BTW, I am just a month younger than RSDoran.