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Effectiveness of Diameter Reduction in Hydraulic Cylinder Blind End

Hydraulics is not my specialty, so please excuse me if I mis-use terminology, etc.

We have a design with a hydraulic cylinder that is required only for emergencies; it is very important for it to remain retracted during normal operation. The extension pressure line has a closed ball valve to prevent accidental operation. The retraction line remains pressurized (3000 psi) to keep the cylinder retracted, and a pilot operated check valve prevents rod end fluid from returning to tank unless there is extension pressure.

I am wondering about the reliability of this design, because it seems to me that fluid could leak past the piston seal from the rod end to the blind end. The difference in surface area (due to the presence of the rod) might result in the piston creeping, even though fluid cannot return to tank. There is no load on the rod in either direction.

However, here is the counter-argument. The piston was designed to hit a mechanical stop of machined steel in the blind end of the cylinder when fully retracted. I do not know the finish specification for the contacting areas, but am guessing it is pretty smooth but not mirror finish. This mechanical contact area is supposed to reduce the effective surface area of the piston face. Indeed, if it did so effectively, the rod side would have a greater surface area and the cylinder would remain solidly retracted regardless of leakage.

I am questioning this on a couple of points: a) a single piece of grit could completely eliminate the surface area reduction; and b) it seems implausible to me that the pressurized fluid could be excluded completely from the contact area.

Let's forget the grit issue for a moment. The main question is then, will the fluid that penetrates into the crack between piston and hard stop exert a force? I assume the scale of surface features even on a relatively smooth surface is going to be bigger than the hydraulic oil molecules, but perhaps surface tension is important in such small spaces.

If anyone can share their thoughts on this, I would be grateful.

Robert

Reply to
Robert Harris-Stoertz
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Stop the person who designed this working for you on hydraulics.

With the return line shut of, any leakage past the piston seal will equalise the pressure on both sides of the ram, the ram will be forced out as the piston rod reduces the piston area on the front side of the piston.

There will be no effective reduction of area over the stop unless a seal is used and the area it seals is provided with a port relieving the pressure.

You must use a valve which relieves the pressure by opening a return to the reservoir of the system as well as shutting of the pressurised supply.

Reply to
Jonathan Barnes

snipped-for-privacy@odim-spectrum.com (Robert Harris-Stoertz) wrote in news: snipped-for-privacy@posting.google.com:

My thoughts are, that in order for a cylinder to actuate, you must have a pressure 'differential' from the front to the back of the piston. If the pressure is equal on both sides of the piston, irregardless of the surface area, no movement occur as long as no other forces are enacted upon the piston (gravity, for example). With that said, in order to be more safe, the return to tank should be opened, so that a pressure differential is maintained in the retracted position.

Reply to
Anthony

In most hydraulic systems, the "extend" line would be vented to the tank while the cylinder is in the retracted position. So any fluid that leaks around the piston-seal simply drains into the reservoir. I think that adding the "safety" ball valve alone was a mistake. It should have included a check valve in shunt with the ball valve so that it prevents extending the cylinder if the line is pressurized, yet allows the oil to bypass in the other direction to drain the cylinder. Otherwise I think you are right: it will eventually creep.

And I hope the ball valve is simply to prevent inadvertent operation and it's not under pressure all the time. Then you're really f*cked.

Don

Reply to
Don A. Gilmore

Replace the 2-Way Ball Valve with a 3-Way Ball valve that blocks pressure to the extend side of the cylinder and drains it freely to tank. This way it will not matter if the ball valve or the cylinder seals leak since it will be impossible to build pressure on the extend side until shifting the 3-Way ball valve.

I fully agree with the other post that the idea of trying to reduce the extend side area with a machined diameter without some sort of seal and a path to tank will not work Also the cylinder will try to extend if there is seal bypass like any single rod cylinder with flow to both sides simultaneously. The motion would be slow unless there was a catostrophic seal failure but the rod would extend.

Bud Trinkel fluidpower 1 @hotmail.com

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Reply to
Bud Trinkel

Thanks to all of you who replied.

This hydraulic circuit is an example of what can happen when a lot of people have input into a design without sufficient review by a specialist. The original design had only a directional control valve without the 2-way ball valve, but the customer did not like being able to activate the circuit so easily. The ball valve has a padlock and a position sensor. The directional control valve is still there.

I agree with what a couple of you suggested--that the ball valve should be a 3-way valve draining the extend side of the cylinder to tank.

I'm having difficulty visualizing a check valve shunt past the ball valve. As long as the cylinder is pressurized in the retract position, a simple check valve back to tank could prevent build-up of pressure on the wrong side of the piston. But then, when you actually wanted to extend the cylinder...well, that's something for the designers to mull over!

Anthony, you had me wavering for a moment there, with the point about equal pressure. If a cylinder had identical rods sticking out both ends, I would think equal pressure on both sides would result in no movement. But suppose you have a cylinder cross-section of 10 square inches, and a rod of 1 square inch attached to one piston face only, and you pressurize both sides to 100 psi through a tee fitting. One side of the piston would see 1000 pounds of force while the other side only sees 900 pounds. Even without a return to tank, the piston should extend, assuming there is no significant external force resisting movement of the rod. The volume available for fluid inside the cylinder will increase the same amount as the volume of the extended rod, making a return to tank unnecessary.

Again, thanks to all!

Robert Harris-Stoertz

Reply to
Robert Harris-Stoertz

Dear Robert Harris-Stoertz:

The intention was to only bypass the ball valve, to allow flow *out* of the piston-end and to flow its intended path through the directional-control valve (then to tank). Now that you cite that this is a directional control valve, that could be unintenionally actuated, then this solution will not work either. Ball valves have been found to leak, and occasionally check valves can have leaks (depending on their history). The 3-way is pure genius. I'd also consider putting a mechanical lockout on shifting the position of the directional control valve.

You wouldn't be able to constrain such a system to have any particular piston position, with the circuit originally described. Over time, any position would be possible. Leakage past the cylinder would allow "drift". The 3-way solution solves all.

David A. Smith

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