Monday, February 23, 2009

Piston Seal Leakage - Rod Down

We have a cylinder mounted with the rod pointing down. It has a weight on the end of the rod.
There is a ball valve at each cylinder port.

Assumptions:
  1. Piston seals are leaking badly
  2. Rod seal does not leak
  3. Ball valves, when closed, do not leak

What happens when the cylinder is fully retracted and both ball valves are closed?

13 comments:

woodygb said...
This comment has been removed by the author.
woodygb said...

Piston seals are leaking badly.

OK ..Lets just say that they aren't there at all.
I'd then assume that the rod and piston will simply drop thru the fluid volume in the retract side....the extend side will then contain the fluid volume from the retract side and an air space / vacuum?

JDK said...

"OK ..Lets just say that they aren't there at all."

Fair enough. What happens to the rod and piston?

woodygb said...

Well....I think it would drop till it either reached a physical stop i.e. the rod end cap ..or the vacuum / atmospheric pressure force difference created by the rod exiting the cylinder balanced the load.

JDK said...

"Well....I think it would drop till it either reached a physical stop i.e. the rod end cap ..or the vacuum / atmospheric pressure force difference created by the rod exiting the cylinder balanced the load."

Thanks Woodygb; you're right on this one. "Spot on" as the Australian guy would say.

Just to add- if the piston and rod stop midstroke, it's because 14.7 PSI or less of vacuum times the rod area is enough to support the load. It's also important to realize that if they stop midstroke the pressure on the rod and cap side of the piston must be equal according to Pascal's Law.

woodygb said...
This comment has been removed by the author.
woodygb said...

"It's also important to realize that if they stop midstroke the pressure on the rod and cap side of the piston must be equal according to Pascal's Law."

But there isn't a "rod and cap side" ..the seals aren't seals and can be ignored...thus it's just one volume...but we do have an inside and outside.:¬)

Tsk...I wish that there was an edit facility...

JDK said...

If you want to call it brown and I want to call it black; I think that's OK. The main thing is to understand what happens. Hopefully your OK with that.

woodygb said...

Apologies JDK ..I was just being pedantic.

Alan Wood ( Woody )

Anonymous said...

If the piston seals were not leaking the cylinder should stay up.
Since all oil would be trapped by the closed ball valves.
Since the seals are leaking the cylinder will fall at that leakage rate.

JDK said...

How far do you think it will fall?

marc said...

if you completely remove the piston seals and plug both ports the piston would basically stay right where it is. it would not creep down to its full stroke. its hard for me to say in words why but it is an equalibrium thing.

JDK said...

Marc:

Thank you for your comment.
An Example: 4" bore cyl. with a 2 1/2" rod
cap area = 12.56 in sq
rod area = 4.91 in sq
net(rod side)area = 7.65 in sq
Since pressures on both sides are equal, area to resist extension is 7.65 in sq.
Apply a 105 lb weight
Generated PSI = 13.73 PSI; if atmospheric PSI in your area is greater than that, the piston will stop at some point. If not it will extend all the way.