Hydraulic cylinder, single-acting, but not vented - how does that work??

# Thread: Hydraulic cylinder, single-acting, but not vented - how does that work??

1. ## Hydraulic cylinder, single-acting, but not vented - how does that work??

A friend of mine has been asked to do some work on a hydraulic cylinder from the mast of a forklift. It is single acting, and there is no vent - but instead, there is some sort of bypass hole in the piston that apparently allows oil to pass from the bottom up to the top, and vice-versa.

We're both puzzled on how this works. When pressure is applied to the bottom of the cylinder, what keeps it from feeding oil into the top through the bypass hole, or what allows the piston to reach the top without getting stopped by the oil in the top? And likewise, when the valve is released to allow the piston to fall, oil is draining back into the reservoir ... but since there is no vent, oil must also be sucked into the top of the cylinder, right?

As my questions indicate, I am a total noob when it comes to hydraulics. My friend has a good bit of experience using cylinders, and is a very experienced machinist, but now in his retirement he is taking on repair work here and there, mostly from farmers and such, and repairing hydraulic cylinders is outside his previous experience.

If anyone can help us understand, or point me to some resources, I will be most grateful, so grateful that I will reward you with - wait for it - a LIKE on your post!!

2. Stainless
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That is interesting. I presume the rationale would be to minimize contamination.

I'm no expert either, but in thinking about it this is how I'd analyze it. The hole means there's equal pressure (psi) on both sides of the piston. The actual force exerted is psi x piston surface. The two sides of the piston are not equal since one side's area is reduced by the area occupied by the rod. So, given equal pressure, the piston will move in the direction with the largest piston area and with a force equal to the difference in projected area. Hmm.

3. Stainless
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Could it be a plunger type cylinder? That would have the gland at the top and the rod large enough to nearly fill the barrel. The oil would act against the cross section of the plunger. There would probably be a bearing at the bottom of the rod but no piston seal.

4. Diamond
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It's a displacement cylinder. It does not have seals on the piston, only the rod.

5. Aluminum
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What you describe sounds a lot like the end of stroke cushioning mechanism used in some single acting mast cylinders. The annular side does not fill up with oil. There is more than likely a one way valve installed in the piston head that prevents oil flowing from the full bore to the annular side. Typically there is also a large cavity drilled in the rod. The cavity is somewhat proportional to the volume of the annular side(When we make longer cylinders we drill deeper holes).

When assembling these cylinders you typically have to put in a certain volume of oil(in our cylinders somewhere in the range of 20CC) in the annular side after you have pushed the piston head into the tube but before screwing/pushing in the gland. The combination of the oil on this side and the volume of air compressed in the cavity somehow magically combine and you get cushioning when the cylinder reaches the end of it's outward stroke. I think the valve allows a small volume of oil to flow from the annular side to the full bore side if the pressure gets too high.

All being said I don't design the cylinders i just make them.

6. Aluminum
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#4 post is right.

7. Stainless
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+1 on number 4. My Yale lift has this type of cylinder. The rod seal takes the pressure.

8. Diamond
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That gives us a name for it: "Displacement Cylinder". But it in no way does it explain how it works.

If the piston has no seals, then I would expect that leakage would have the pressure equal on both sides of the piston. And then, the rod would, in effect, become the piston. So, why even have a piston? Guidance perhaps? And external force would be required to return the piston/rod back to the start position.

Is that it? Or is there some other kind of magic here?

A web search seems to confirm my guess:

How does a displacement cylinder work? - Cylinder Services Inc

Originally Posted by tdmidget
It's a displacement cylinder. It does not have seals on the piston, only the rod.

9. Hot Rolled
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I know what a displacement cylinder is,but I have never seen one as lift ram for a forklift....forklifts are usually piston seals,with a drain on the top end of the cylinder.Lift cylinders are often a bit mangled on top,and hard to see details.........one biggie I worked on,after rainstorm,the ram would shoot out oil and water if run up to the stop.....As mentioned ,any hole in the piston is a stroke limiter that unseats before dead end...often before cylinder stop,the lift chains come up tight,sometimes hoses for a sideshift too.....EDIT.....farm tractor front loader rams are often displacement,cause you only need a few hoses.

10. Diamond
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The advatage is chromed rod is cheap and readily available, the cylinder can be more or less any bit of seamless tube going and the seals are easily changed on the machine, just jack mast up a foot or so or lower it on block then let the cylinder down, undo gland, some have a bolt holding end of rod to the mast, some just locate in a pocket. Its just dirt simple design. Piston is more just a guide element.

Other advantage is the rod can be longer than a typical cylinder of that bore as its wider hence less inclined to buckle, it goes up faster and down faster as you have to pump less oil, hence smaller pump and smaller line sets. A typical circa 2 ton forklift mast cylinder is a pretty low effort thing for hydraulics. But you have to make it a certain minimum dia to get the buckle loading resistance do to cylinder length, hence you might as well use it displacement style rather than suffer the speed penalty of a large volume of pumped oil.

Glands are also less circumference so its easier to get a good seal than on a piston and that limits your creep down rates. Also the fully sealed nature helps eliminate contamination issues, lots of forklifts go a long time between full boom ups and condensation and such can destroy the cylinder and then the seals when they run over that bit of cylinder wall.

Hence this is not a cost cutting measure, it has some real long term benefits to the user and the OEM.

11. Originally Posted by awake
A friend of mine has been asked to do some work on a hydraulic cylinder from the mast of a forklift. It is single acting, and there is no vent - but instead, there is some sort of bypass hole in the piston that apparently allows oil to pass from the bottom up to the top, and vice-versa.
See post #4

That's not a piston on the bottom of the rods (or tubes), it’s a bearing. If it were a piston (for pressure) it would have V seals on it.

Good luck,
Matt

12. Diamond
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Originally Posted by tdmidget
It's a displacement cylinder. It does not have seals on the piston, only the rod.
Sounds allot like the old single post, in ground automotive lift.

13. Thanks, all. I have sent these replies to my friend - I'll be curious to hear from him more details on the piston/bearing. Displacement makes perfect sense - now I don't know why it was so hard to figure that out!

Sent from my Lenovo TB-8504F using Tapatalk

14. Diamond
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The design is also a safety feature. This is why you need to fix your forklift mast IMMEDIATELY if it is leaking at the rod. That's the only thing between the load and gravity.

15. Hot Rolled
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Lift cylinder on my Daewoo forklift is like that, rebuilt it last summer, super easy! Remove hose/chain guide from top of cylinder, remove gland nut, replace seals, and back together! Easiest cylinder I ever rebuilt, no need to remove cylinder, no need to pull rod.

16. Again, many thanks for the helpful replies. I copied them and sent to my friend; he responded that the cylinder he is working on doesn't quite match the displacement design you all have described, because there are seals on the piston ... as well as a hole in the piston. I've asked him to take pictures so we can get a better idea.

Meanwhile, though, here's my theory - likely to be shot down, since I'm guessing without experience!! My theory is that, with a hole in the piston, it must effectively act as a displacement cylinder as far as the power stroke is concerned - the hole effectively negates the effect of the pressurized oil on the piston. BUT on the release stroke, I theorize that the piston with hole acts as a damper, slowing the descent of the cylinder to the rate that oil can pass through the hole.

Thoughts? Reasonable theory, or totally out to lunch?

17. Originally Posted by awake
Again, many thanks for the helpful replies. I copied them and sent to my friend; he responded that the cylinder he is working on doesn't quite match the displacement design you all have described, because there are seals on the piston ... as well as a hole in the piston. I've asked him to take pictures so we can get a better idea.

Meanwhile, though, here's my theory - likely to be shot down, since I'm guessing without experience!! My theory is that, with a hole in the piston, it must effectively act as a displacement cylinder as far as the power stroke is concerned - the hole effectively negates the effect of the pressurized oil on the piston. BUT on the release stroke, I theorize that the piston with hole acts as a damper, slowing the descent of the cylinder to the rate that oil can pass through the hole.

Thoughts? Reasonable theory, or totally out to lunch?
Kinda, maybe? The pressure release is usually handled by the lift circuit. But they may want the cylinder wet with oil above the piston (& bearing now) and all air bleed (up stroke) goes down thru the piston into the mast.

I’d be nice to see a picture of it. All single acting cylinders just need a seal on the pressure side. BUT it’s a very good idea to NOT have steel on steel sliding against each other & seals aren’t made for that. You have brass pistons, cast iron (not that good) or split rings of some other suitable that go in grooves of the piston.

Good luck,
Matt

18. Diamond
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Originally Posted by awake
slowing the descent of the cylinder to the rate that oil can pass through the hole.

Thoughts? Reasonable theory, or totally out to lunch?
IMHO irtrelevent, piston can just pull a vacume and vacume only gives you just shy of 15 psi, hence irrelivent. More likely the seals not a seal but a polymer bearing ring, you have to have some kinda sliding element on the piston for this to work. A ptfe - teflon sintered polymer ring works great, just unlike a seal it won't have the typical rubber seal arrangement under it that expands it onto the bore. Or if it does, theres just a real good chance its OEM reusing existing parts in there parts libary.

19. Plastic
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IMHO irtrelevent, piston can just pull a vacume and vacume only gives you just shy of 15 psi, hence irrelivent. More likely the seals not a seal but a polymer bearing ring, you have to have some kinda sliding element on the piston for this to work. A ptfe - teflon sintered polymer ring works great, just unlike a seal it won't have the typical rubber seal arrangement under it that expands it onto the bore. Or if it does, theres just a real good chance its OEM reusing existing parts in there parts libary.
If I'm understanding it correctly, the size of the hole would control the rate at which the load on the cylinder could pump oil through. So the variables would be force on the shaft, hole area, oil viscosity, and fluid flow rate (the final determiner of descent speed).

The bearing rings are also commonly PTFE or Nylon filled with bits of bronze to give it more guts. If it's got an angle/skive or straight cut in it, that's a sure way to tell, since those bearing surfaces are mostly contained in a groove with metal on both sides. A seal will have a V or Y shape to it and often will be snapped in to a partially open groove on one end.

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As stated, it is a displacement style cylinder. The rod diameter area is the acting force. The larger the rod, the more work can be done. I designed many displacement type cylinders during my career, including several mast cylinders for forklifts as described. If there is even a piston it will be to act as a bearing. If it has a beefy enough gland it might even just have some sort of stop ring on the rod to keep it retained in the cylinder at the end of the stroke.

P.S.I. x area of the rod diameter is what it will lift. Overall bore size means nothing in this application. It's just a container to hold and guide the rod.