Technical question about hyd cyl.

[J_J]

If you closed off the fluid port on a SA cyl, and the seals were good, the load should hold and not move.

Actually the SA cyl is designed to suck in air when the valve is activated for retract, whether through the breather port or a small hole in the gland nut.

In an SA cyl with leaking seals, the fluid will transfer to the air side of the cyl and be pushed out when full.

Consider the air side of a SA cyl and the rod is retracted by gravity/load, the cyl will draw in air as a normal routine.

 

[CalG]

JJ

You are making confusion.

Certainly there is air on the dry side of a SA cylinder.
It need not be "sucked in". that side is OPEN to the air.

I guess what is obvious to some can get twisted and turned around in discussion. My habit is to explore the obvious, but there are always expectations to be justified.

 

[glastron23]

In an SA cyl with leaking seals, the fluid will transfer to the air side of the cyl and be pushed out when full. Most definitely !!!!!

Consider the air side of a SA cyl and the rod is retracted by gravity/load, the cyl will draw in air as a normal routine.[/QUOTE] ????? don't think so... even with the valve full open on retract, there is still pressure inside the cylinder due to restrictions/ back pressure pushing on the seals, air can not be drawn in...

 

[J_J]

Why do you think there is an air breather in the rod side of the SA cyl.

It has to have air or it would pull a vacuum.

 

[Leejohn]

Sure alot of if's here. The cyl is 100% sealed there is no if's.

 

[J_J]

In a SA cyl, there is always air on the rod side, except when the seals are leaking.

 

[CalG]

Even when the seals leak, there is still going to be air on the open side.

Just the way it is....

but back to the earlier comment. Air pressure is only 14.7 psi at sea level and is less with elevation...

(this implys that a "Vacuum" can only change things by the same amount....760mm of Hg, 32 feet of water, whatever you want to call it...)

IT will NOT effect the displacement of a hydraulic cylinder at "typical" operation conditions.

 

[glastron23]

JJ yes there is always air on the ROD side of a SA cylinder.. yes that is the reason for the vent, to prevent the vacuum, most SA's just use a wiper seal to keep some of the dirt and water out. so what is the point of this discussion when you were talking about a DA mounted with the rod down and a load pulling down on the it?? yes if the piston seal is leaking the fluid will travel from the rod side to the base side, the difference in volume will cause a vacuum to develop and it MAY draw air into the rod side until the piston stops traveling... ???? But if the piston/rod seals are doing it's job then the rod will not travel ....

 

[arlen4720]

A cylinder can extend by pulling in air as you say..if is has faulty seals on the puck, but it can't retract without external leakage...even if the puck seals are gone, because the rod is taking up more space as it retracts, and the cylinder volume is decreasing. There is no where for the fluid to go. The rod seal takes all the force and will quickly blow out.
When I worked in a manufacturing plant, we would diagnose large press cylinders by retracting them, and capping the ports. If the cylinder drifted down, then we repacked them, if not, we would look at the many control valves as the culprit.
If the seals are shot, and the cylinder is extending with a weight hung on it, the pressure is falling on both sides of the puck because the rod is exiting, and the cylinder volume is increasing, but there is a fixed volume of fluid in it. It WILL suck air around the rod seal. They are not made to hold a vacuum.

 

[CalG]

Extrapolation on the "thought problem"

If a fully retracted single acting cylinder were used to "hang" a load", what would be the action of the "piston".

This arrangement would certainly "pull a vacuum", and the answer lies in the vapor pressure (volatility) of the working fluid!