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Thread: pneumatics questions - diagram attached

  1. #1
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    pneumatics questions - diagram attached

    My brother mentioned that he had read of using mineral oil instead of air in a pneumatics circuit. But I cannot find any examples or information on it.

    His reasoning was that my application would be a light duty hydraulic system. The mineral oil certainly has some advantages.

    Since air is easily compressed, his thinking was that if the cylinder rod encountered more resistance for a bit (like a knot in the wood), the rate of travel could slow and then jump forward when the resistance was passed. At the least the wood would burn. At the worse, the entire set-up could let loose with dangerous results.

    Oil does not compress so this scenario doesn't play out. He claims that the pneumatic cylinders and solenoids would not be harmed using oil.

    Other reasons included built-in lubrication, less likelihood of leaks as oil molecules are much bigger than air molecules, no rust, the relative inexpensiveness of the mineral oil, and fewer components in the system. Added to this is the fact of Arizona's dryness. Seals in an oil environment are less likely to dry out.

    An inexpensive RV water pump (fixed 43 PSI) could provide the pressure to the system, as opposed to an air compressor. The supply reservoir would be a small "tank" made of 4" PVC capped at both ends, but vented to the atmosphere. (ala Joe Woodworker's vacuum pump systems.)

    So there is compelling reason to explore this.

    "Real" hydraulics are ruled out of my project due to overkill with regard to power and the much, much higher cost of components.

    Also, 'real' hydraulics systems operate at a much, much higher PSI than pneumatics. Those high pressures are way unnecessary to my project.

    Thus far Mr. Google has failed me in finding if this has been done before. I probably don't know exactly what to ask.

    What does the collective trust know about this?
    Last edited by Carol Reed; 10-30-2010 at 04:42 AM.

  2. #2
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    Just wondering if an RV pump could actually move mineral oil. Interesting idea though...

  3. #3
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    I deal with industrial hydraulics all the time and a little pneumatics.

    The leaking issues are not a reason to use the oil, you're likely to have more problems with the oil than the air. However you should have some oil drops inside your air system if it's going to be in continuous use. It will make your pneumatic valves last longer. You can buy oilers to do this, or like an oiled air nailer just drop a few drops into the air line now and then for something used less often.

    All the pneumatic solenoids I'm familiar with are very different than hydrualic valves and I'm fairly certain there will be issues with the oil. They'll likely be sluggish as the passages are made for air to travel. They're also very lightly constructed, but you're looking at low pressure anyway, not so big of deal. The general function also I'm skeptical of them working with only 43 psi oil and all the backpressure and oriface size issues, but they might.

    Your cylinder seals might not cut it for continued oil use either. Small air cylinders typically have really light seals that barely rub the shaft and often leak a small amount of air past. It's to get lower seal friction since they don't have a lot of force to work with. It should work for a while but I think you'll get some leakage from the oil. It's really hard for me to give you any idea how long the'll last without seeing the cylidner though. I'm just speaking in general terms from the small air cylinders I'm familiar with. Larger cylinders 2"+ bore often are built just like hydraulic cylinders though.


    I've never seen an RV water pump so not 100% sure what you're referring to there. Your pump is probably going to be fine, for the most part anything that can pump water pumps mineral oil fine. There are some problems switching from water service to oil with seal softening for a lot of materials, but at low pressures it doesnt' matter. Also it's probably a centrifugal pump so the seal will be a hard material and not matter.

    Now for air systems there's a couple ways to help get around the problem you stated, jumping from changes in force. The first way is you use meter out flow controls. A needle valve on the side of the cylinder going out restricts the flow. The compressed air flowing across the restrictions slows the cylinder down. If you get to an area of increased force, the bleeding air keeps goign out lowering it's force restricting the cylinder from moving, applying more force on the object that's stopping the cylinder. If it suddenly moves freely again, the outgoing air from the cylinder quickly compresses and starts slowing the cylinder again.

    The other way is to put a secondary dampner on the cylinder, works generally on the same principle as the meter out flow control but you can use oil filled shock absorber with piston holes to do the job.

    Also you generally just need to make sure the cylinder has ample force to move whatever resists it. You're speaking of putting 43 psi on a cylinder with hydraulics. That's going to have even less force than the same cylinder with say 100 psi of air and be more likely to stall.

    If you need some more help with pneumatics I can ask a co-worker that deals with the pneumatic equipment more. I mostly work with hydraulic and lubrication systems.
    Last edited by Jeb Taylor; 10-30-2010 at 02:01 AM.

  4. #4
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    I must have missed the memo

    What are you planing on actuating with the pneumatic or hydraulic system?
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
    William Arthur Ward

  5. #5
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    Stu, I am modifying a radial arm saw into a dedicated dado machine. The cylinder advances the saw by simply pushing a button. Total control over the saw cut.

    Jeb, here is a diagram that I think will work. Maybe your friend could take a look?

    The pneumatic part I picked off a YouTube video of a guy explaining how a pneumatic circuit works. The electrical our very own Vaughn helped with.

    I can't show the mechanical until I get the parts. But the cylinder will attach to the motor, pushing it along the arm. A YouTube of this was put up here a couple of weeks ago and it stirred the creative juices.

    Here is how it works: (starting at the bottom)

    The momentary NO switch activates the solenoid.

    Air flows to the A port of the cylinder, extending the piston.

    When the NC switch is opened, the solenoid is de-energized

    Air then flows to the B port of the cylinder and the piston retracts.

    The flow control valve in the EA part of the circuit controls the speed of the extension.

    The flow control valve in the EB part of the circuit controls the speed of the retraction.

    At least that is what I intend.

    The NC switch will be movable along the path of the saw motor, allowing the length of the cut to be selective.

    What else do I need? All your input is appreciated, especially before I start ordering parts.
    Attached Thumbnails Attached Thumbnails diagram.jpg  

  6. #6
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    It's a simple control circuit and with flow controls on the exhaust it should work fine. I'm not sure that you need a check valve on the supply, but it won't hurt.

    Keeping your oiler close to the valve and cylinder helps. You don't want a long run of pipe after it or the oil can collect in the pipe. It's best to stay within 20' and not have a large elevation change. A few feet is ok. I'd check harbor freight for FRL prices.

    I'll see if anyone at work has other ideas on stabilizing the cylinder against cutting resistance in the wood on monday though.

  7. #7
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    Thanks, Jeb. I'll keep the oiler location in mind when I get to the mechanical side of things. The check valve was to keep oil out of the supply. So I see that I need to reverse their locations. Thanks!

    What's FRP?

    Also, for the electrical guru's, there will be a fuse in the electrical circuit to protect the solenoid. I have more work to do on the electrical side. I may have different voltages to deal with, depending on the solenoid I can get.

    Stay tuned.

  8. #8
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    FRL = filter regulator lubricator. Cleans the air, regulates pressure, and has oil tank with a screw adjustment that puts a drop of oil into the line over time. Something like this

    http://www.harborfreight.com/125-psi...tor-92261.html

    but that one's got bad reviews, so probalby should choose another brand. This is the typical way of donig the oiling/filtering of air to make your eqiupment last longer. You just fill the oiler up with some generic air tool oil every few weeks. It will also keep oil from going back upstream as long as it's in a horizontal run. The filter can trap water out the line too and you can drain it as needed, though that might not be a problem in arizona.

    The check valve isn't going to do much for keeping oil out of your supply. If you're really worried, make a "trap" by running the pipe down and back up, and put a tee and small ball valve at the bottom. You can open it up periodically and drain anything sitting at the bottom out. In my opinion though, as long as your supply line runs slightly downhill to the oiler and you keep a watch on the filter bowl and drain it if it fills, you should have zero problems backfeeding any liquids into the air supply.

    A word of caution on oil mist though. Especially at startup the exhaust off the valve, a visible oil mist is very hazardous to breath. Sometimes on startup of something all the excess oil inside starts spraying out the exhaust. Just make sure it's ventilated when you start it up.

  9. #9
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    The only thing that jumps out at me. Is when you use a RAS you hold it back almost as much as you pull it through the cut. I would be concerned that the piston would not be able to hold the saw back when it hits the wood.
    If it was me doing this I might be looking at a screw drive rather than an air piston.http://www.youtube.com/watch?v=aEdvuc2Qxyw
    That way you could control speed and the saw could not take off on it's own.
    It could be worse You could be on fire.
    Stupid hurts.

  10. #10
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    On advancing the cut, the piston is pushing. The amount of 'push' is controlled by the flow control valve which is on the exhaust side of the cylinder.

    Don't think that is an issue, Chuck, but perhaps Jeb can explain it better.

    The screw drive is mucho money!! Muy mucho dinero! Out of the budget.

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