Small Hydraulic Cylinder Diagram?

There is often a "captive gas" space which will not cause rebound in the working (down; not extended or retracted, but "down") direction, except upon the very first stroke in that orientation. Thus, total system pressure may change as rod-displaced oil volume either takes up gas space or gives it back, but the pressure across the piston will remain equal, minus the bias pressure of the check valve.

If such a cylinder is inverted, the first stroke toward the now-down gas bubble will cause it to be expressed through the check valve to the high end of the cylinder. After that, all down strokes are against 'hard' oil. The gas only acts to take up rod volume. Think about it, and you'll see it works in either orientation after one stroke. You'll hear such a cylinder grumble on the first stroke.

(ever think about auto shock absorbers? )

LLoyd

LLoyd

Does it have the control valve off to the side to adjust the feed rate? I think that the valve assembly also provides some air chamber space to accommodate the difference in bore area between the sides of the cylinder. Certainly the valve assy. appears to be considerably larger than would be needed for a simple metering valve on the units I've seen.

Pete C.

Still can't wrap my brain around it, but I did think that there had to be air or something in there that did compress to allow it to work. Still can't get a hold of the concept that it doesn't cause rebound or resistance from the compressed gas, but that's step #2 in my understanding this stuff...

Only when they go bad - and then I ignore them as long as possible anyhow.

Regards, Joe Agro, Jr. (800) 871-5022

01.908.542.0244 Automatic / Pneumatic Drills: Spindle Drills: V8013-R

It's actually easier than you're making it. Regardless of which direction the thing is working, the "bottom" oil is either under compression (which it cannot do) or rarifaction (which it cannot do without boiling). The gas space is under the same conditions, but it cannot be compressed or rarified unless the volume of oil in the entire cylinder changes. When the rod is moving, the volume changes, but at a tiny fraction of the total displacement per unit of stroke. So... no (well, virtually no) bounce, even though there is gas trapped in the system.

LLoyd

On my saw, it works like this (yes, I had to open the #@*##* thing up and do a bit of overhaul...).

Mine sits vertically and has a tube connecting the top and bottom with a valve assembly where the tube connects to the top. Other designs may have some of this stuff internally to the cylinder assy.

The valve mechanism consists of a check valve that allows oil to flow freely from top to bottom and a needle valve that controls the flow from bottom to top.

Anyhow, when the cylinder is fully extended, i.e., the saw is up, the piston is near the top and the oil has been forced through the check valve into the bottom of the cylinder. The travel of the piston is such that it only moves through the top half of the cylinder's length. The bottom of the cylinder then contains most of the oil with some air-space above it.

When you lower the saw, it comes to rest on the oil in the bottom, with a cushion of compressed air on top of it (the saw can bounce a little on it...). Then, when you open the valve, the oil is allowed to flow back into the top and the saw comes down the rest of the way at a rate controlled by the valve.

The system is self-adjusting. If you get too much oil into it, it will eventually leak out the top of the cylinder (actually it does it fairly expeditionsly...). If you have too little oil in it, the point at which the saw stops when you bring it down with the valve closed will be lower. Anywhere in between, the air/oil balance will maintain itself such that the system works properly.

Jerry

Hi Joe,

Just think of it as two reservoirs connected by a needle valve and check-valve in parallel. The whole system is closed loop.

In one direction fluid flows freely past the check valve. In the reverse direction the check valve closes and the flow is restricted by the needle valve. Don't worry about the gas/liquid stuff.

You can build the same thing using a 2 way air cylinder and a needle/check valve. The only difference is you get more bounce with air as the medium. Try it out.

Mike

CLIP

I understand how that would work, but what happens when the cylinder moves

1" on side "A" and displaces an oz of oil where as it vacates enough space on side "B" to allow for 1.2 ox of oil due to the lack of a rod in the center of the plunger? There has to be something to make up the difference in volume...

Regards, Joe Agro, Jr. (800) 871-5022

01.908.542.0244 Automatic / Pneumatic Drills: Spindle Drills: V8013-R

I just answered this for someone, don't know where my answer went. ??

The bandsaw cylinder is actually open at the top. Just enough for a little air to account for the volume of the rod.

The piston has a round rubber flapper valve on it, that allows the bottom of the space to fill. (Basically the one way valve.) The needle valve is used to allow the oil from the bottom to "go around" to the top, and thus it controls the downstroke of the blade. The upper "seal" is not air tight.

How Do I know? The flapper valve in mine rotted, and I cut the cylender apart to see what was going on, it was a sealed unit, so there was nothing to lose. It turns out the end can be carefully turned off, to free the end from the tubing. I cu it loose on the lathe at the Uper end, since that end has the "ventable" seal in it. The rubber flapper valve was cut out of a piece of neoprene. Ready to go again. BTDT. Pete

(snip)

(snip)

You have the basic idea. The key here is the rate at which it can equalize in each direction. On the upstroke flow is impeded little by the checkvalve that opens when moving in that direction. On the downstroke (with checkvalve closed) the oil must go thru a much smaller passage -- the needle valve. For given pressure (weight) it can only flow so fast thru that small opening so it falls slowly. Rate of descent can be controlled with the needle valve.

In a perfect world there isn't a net difference . You might have some air in the lines that makes the system a little "spongy" due to compressibility of gas. Some gas is entrained in the liquid.

Try raising the saw too fast - notice how it wants to drop back down like its attached to a spring? When you do this any dissolved gas is removed from the liquid. If it's truly a sealed system if you displace 1oz on one side 1oz is displaced on the other. If you have a little air in the system then it might seem like you are getting more.

Mike