Air cylinders?

[hackberry jake]

My place of employment is throwing away 5 good air cylinders. They are roughly 5" diameter by about 3' of throw. I have been thinking about adding a log turner to my mill, and I'm not sure if air would have enough power to turn bigger logs. Has anyone had experience with large air powered cylinders?

[mad murdock]

you just have to do a little cypherin' with the numbers.  air pressurexthe area of the piston (3.14159xradius squared) should be about 19.6 square inches.  if you applied 100 psix 19.6 you would be able to push 1,960 lbs of force at the cylinder, going out.  going back in you would have a little less, since the area of the ram would be subtracted from the square inch area of the piston on the return stroke.  Add mechanical advantage with the length of your bellcrank, and I think you could do some decent work, but it would take a decent air pump to make it work.  The cylinders you are talking about are pretty good size..  You could possibly convert them to hydraulic, if the seals were the right type.  You would need to know some more information to do that safely.  Probably would have to be a low pressure hydraulic system, say no more than 300-400 psi.  Need to cross check compatibility with seal material/oil, seal design type etc.  definately worth grabbing ahold of those pieces, especially for the price 8) 8)

[hackberry jake]

Wouldn't they be more "springy" since air compresses and hydrolic fluid doesn't?

[eastberkshirecustoms]

Jake, out of curiosity, are they festoon cylinders? Hydraulic fluid does compress at a rate of 0.4% - 0.5% by volume per 1,000 PSI. It is water that 'doesn't' compress.

[bandmiller2]

Jake,I'd run them with hyd.oil after looking them over to be sure their strong enough.Many times they just use hyd. cylinders for air as there more commonly available.Air is not good for lifting loads as all of a sudden it will move the load with little control. Frank C.

[tommone]

Ebw, I'm sure you're wrong there ,water compresses a lot more than hydraulic fluid for the same pressure.

[CalebL]

Water and hydraulic oil will compress given enough pressure.  They don't use water in hydraulics because it is corrosive, doesn't lubricate very well and it cavitates.   

[wannasaw]

 Jake, Iv'e wanted to ask the same question? Ihave acess to lots of (unusuable) air cyl.Would this be non mobil or thinking would the cyl. hold up under hyd pressure?

[Jeff]

The first mill I ran was a Renco. It had air dogs. After what I had to endure with those, I'd never have air anything on a mill.

[eastberkshirecustoms]

Ebw, I'm sure you're wrong there ,water compresses a lot more than hydraulic fluid for the same pressure.

Theoretically, any gas, solid, or liquid will compress with enough pressure. Water is considered incompressible because it takes somewhere near 290,000 PSI to compress it 1%, hydraulic fluid only takes 2000 PSI for the same percentage. That is why when you get a significant water leak into a engine's combustion chamber, you can say goodbye to a rod. 

[eamassey]

Air cylinders are good in the right application.  An air cylinder does a good job at holding tension, non-critical movement like a valve actuator. An air cylinder would do a good job of holding tension on a bandsaw blade--- though a more practical setup might be air cylinder powering a small hydraulic system.  Air cylinders are terrible- and can hurt you, can kill you--for delivering power and movement when smoothness and control are needed.  An air cylinder will pressure up against a load, and then jump the load uncontrollably.  I would not use an old air cylinder as a hydraulic cylinder -- they are significantly thinner and lighter.  It is not worth taking much risk on hydraulics-- a single blown seal--or hose, etc.-- can quickly put $100 of fluid on the ground.  I have a lot of empty 5-gallon buckets to prove that point.

[Jeff]

I had several holes in the back wall of that mill to show how unreliable air dogs were where logs sailed off the carriage when the air failed while in saw was in the cut of the vertical edger. Cold weather is a killer for them. Even with an air dry system, something will happen.

As a post script, I can't tell you how many farmers I had to chase off when I'd find outside standing there watching me saw through those gaping holes in the pole barn wall...

[hackberry jake]

Nope not to a festoon. They did go to an old unwind though. The mecinism that raises the roll into place.

[Brucer]

The problem with air cylinders is that they don't give you a lot of fine control. Two laws of physics conspire against it. First, there's more friction between two surfaces when they aren't moving than when they are, and second, air is very compressible.

You have to compress a lot of air to get the pressure up enough to make the piston move. Then when it starts to move the friction drops and you don't need as much pressure to move it. So the air keeps expanding (and the piston keeps moving) until the air pressure drops to the point where the piston stops.

Air cylinders are great for clamping something when you don't have to worry about applying excess pressure. Just open the valve wide and let the cylinder slam into whatever you're clamping. Keep the clamping pressure high enough that the cylinder will always want to move.

They are also great when you want to maintain a certain pressure but may have some movement in whatever you are clamping, as in blade tensioning devices.

If you try to push something with them slowly, they'll hop and skip.

If you must use an air cylinder in a controlled application, best to place the control valves in the exhaust ports.

Air cylinders are also useful if you want to move something very quickly. You can fill a large air tank over a long period of time, and then discharge it all into a cylinder in a very short time. With a typical hydraulic system your cylinder speed is limited by the flow rate in the pump.

[sandhills]

I wouldn't go with air powered cylinders at all, if you can use them with hydraulics that would be my choice.  I pulled one of the stupidist moves I ever made a few years ago trying to dismantle a hydraulic cylinder with compressed air to rebuild it.  To this day I'm not sure why I still have my right thumb and index finger (thumb still won't work quite right but it's still there) like others have stated there is no fine "control" trust me compressed air is an explosion compared to hydraulic pressure. All I would've had to do was plug the cylinder back into the outlets on my tractor and push the ram out SLOWLY and safely :.  I have worked with air cylinders in the right applications but I don't think it would apply to yours, there is enough force to kill you, anyone here can correct me but if memory serves correct you have to take the psi that you are applying and X that by the square inches of the cylinder you're filling to get the true force, it adds up to a lot!

[hackberry jake]

I got a couple shorter ones. A couple 5" by 14" throw cylinders. Wonder how to tell if they're air or hydraulic? They sure look heavy built. I guess an air cylinder that big is going to be heavy built tho. I don't know what I'll use em for. Maybe dogs?

[york]

Air,no good for-but take them home,how fat is the rod?3' thro,that means the rod is at least 6' and they could be used in a mill build...You want fluid power not air....albert

[bandmiller2]

Jake,just compare them to a hydraulic cylinder.Do they have four tie rods or are they a crimped/welded cylinder. Is there a manufactures plate with model on them.?? Frank C.

[hackberry jake]

No nameplate that I see

[eastberkshirecustoms]

The fittings that I see look like lower pressure barbs and black iron 'street' elbows, definitely not meant for hydraulic pressure. So my vote would be air cylinders.

[steamsawyer]

No nameplate that I see

Positively air cylinders.... Without a doubt.
These are unacceptable for hydraulic system use.

[mad murdock]

from the pics you posted, I'd say they are aluminum air cylinders.  If you had a need, you could rig up a low pressure hydraulic system, just set the pressure relief low, say 200 psi, that would be a safe range for those cylinders, and you still got some mechanical advantage.  You definately don't want to put them in a hydraulic system with a lot of working pressure available. We use low pressure hydraulics on the flight controls of one of the helicopter types in our fleet, obviously not the same as on a mill, but for the price, I would not throw them out.  200PSI on a 4in piston is a lot of force, to put towards some useful work, IMO.

[hackberry jake]

Guess I'll just put em in storage til I find a use for em. They will be throwing away a hydraulic pump and two hydraulic motors shortly. I'll definately try to get them. Thanks for the insight fellas. It's crazy what manufactering plants waste. I've scavenged a few gearboxes, zero-max speed controllers, carts, casters, bearings, fittings, etc and a lot of them were new, never installed.

[eastberkshirecustoms]

Yeah Jake, the company I worked for would have an annual cleaning and throw out ton's of stuff. Huge IR compressors, air dryers, a complete oil reclaimer set up, industrial racks, complete roll stands, etc. I mean this stuff wasn't junk either. I managed to get some of the racks, miles of #40 and #60 chain, and full cabinets of NEW nuts & bolts simply because they changed vendors. I have enough hardware to build 20 mills now!!! ;D

[millwright]

I have tried to use air cyls. on a lower pressure hyd. system and it worked for a short time, but then they always started to leak. Then I had to refabricate everything a second time plus get the right cyls.

[bandmiller2]

The size of the rod give them away as low pressure cylinders,their just not designed for much force. Frank C.