Speed of a hydraulic cylinder.

[Peter Nachtwey]

http://www.patchn.com/tutr-hyd.htm

Will a hydraulic cylinder with a piston rod extend faster or retract faster if the controlling servo valve is given the same amplitude control signal for each direction?

Why?

[Eric Nelson]

OK, so what prompted this quiz, Peter? Great question for a quiz regardless! I won't give away the answer (so others will investigate), but it's in the Area of a Cylinder section...

I don't deal with hydraulics, but I'm thinking you have an easier time than I do with the pneumatic equivalent (unless you have air in the system). I get the same dumb stares every time when asked why an air cylinder can't be stopped (and held indefinitely) at mid-stroke... :roll:

:prost

-Eric

[willpower100]

I have a better question, why did you post that question on this board?  And why post the question at all, I didn't see anyone asking you to post random quiz questions for electricians to answer.  Are you bored Peter?

[rsdoran]

I will answer that, I did. This site is for ANYONE in the industrial engineering and/or maintenance field not just electricians.

I am glad Peter takes the time to participate on this forum with any and all information. Questions like this get people to think and look for the answers (if they dont know them). Technically I dont know the answer but I would think it would depend on load and position, I dont have time to look it up now.

[dale1627]

This is one of my weakest areas. Not often that I have to get into hydraulics but it is something that I need to learn. Glad that Eric gave a hint because I actually would have thought the opposite.

[Steve Bailey]

Before answering the question, I need to ask one. When you talk about the "Controlling servo valve", does that include the controller so that the "same amplitude control signal" is applied to the same device that the feedback signal is connected to?

[Peter Nachtwey]

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

OK, to be more specific.   If I have a 2" diameter cylinder with a 1 3/8 inch rod and I apply 1 volt to extend the cylinder and then -1 volt to retract the cylinder, which way will the piston go the fastest.

Willpower100, you better know the answer to this question as you work in hydraulics.   Please leave the answering to another.  BTW,  I can't even convince the people on the www.ifps.org board the real answer is correct.  Finally  Willpower100,  this site is much less hostile if you know what I mean.

There will be a follow up question.

I do training on hydraulic motion control.  I like to ask this question every class.   Rarely does 25% of the class get the answer right.   Sometimes no one gets the answer right.

[chriswarrn]

Think volumns.....

[gbradley]

You have more force Extending the cylinder...... at the same pressure.

That doesn't make it faster..... in fact it's downright s l o w e r.

Wait.. what was the question??
OK.
I'm gonna go with Retract.

[Wino]

I don't know squat about the inside of a hydraulic cylinder, but I'd think that you are exerting a hydraulic force on each side of the cylinder rod.

On the retracting end, this force is applied to the end of the cylinder rod itself, while on the extending end, the same force is applied to the box, giving less force per square unit on the face of the rod.

The long and the short, I think it will retract faster.

Hmmm, doesnt the rod itself take up some of the space in the cylinder, therefore requiring less volume, when retracting? Is this where were going with this or am I thinking too hard. The term amplitude always throws me for a loop.
I would say retracting is faster.

[Eric Nelson]

Ut oh, I'm beginning to think this might be a trick question, with all the different 'Whys' we're getting...

My 'guess' is that it will retract faster because there is less overall volume to fill on the side with the piston. To put it another way, if it takes 'x' amount of fluid fluid to fill the non-piston side, the piston side will take "x - piston volume" amount of fluid, and therefore less time to move the same overall distance, given equal flow rate.

If I'm wrong, then you'd better provide a convincing arguement as to 'Why'...  

I'm thinking retract MIGHT be slower based on the fact that since you have less piston area, you have less force (THIS PART better be right!). Assuming the load, friction, etc., etc. is equal for both directions, retract might have a harder time moving the same distance? Again, you'd have to 'splain the heck out of that theory to convince me...  :?

EDIT: I see Eddie snuck in while I was typing. At least we agree!...

:prost

-Eric

[chriswarrn]

Eric,

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.

Ah Ha!
 

 It will retract faster but without as much force when extending
  [End quote]
 I would have never thought about that.
 Thats good stuff to know.
  Thats why I like this sight...Thanx RSDORAN :!:

[Peter Nachtwey]

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?

I am surprised so many got involved :patchn .