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Thread: air over hydraulic system

  1. #1
    Associate Engineer
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    air over hydraulic system

    We are designing a machine that uses a saw that is controlled by a cylinder. My boss is worried about chatter as the saw cuts due to the air compressing in the cylinder. he would like me to design an air-over-oil system to make the cut smoother. I've never done this but I envision a tank with pressured air in the top, oil in the bottom; pushing oil through a flow control valve and actuating the cylinder. the cylinder we are using has a 2" bore and 18" stroke. Any one ever designed a system like this? How would I size the holding tank?

  2. #2
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    Unless I am misunderstanding the air-over-hydraulic concept you have in mind, the air will still act as a spring in the set up you have described. It would achieve nothing towards what you are asking in the smoother cut. The oil will simply act a medium to transfer the movement back to the air.

    If you must push the saw hard into the material (not a good thing) rather than allowing it's own weight to cut, then you would be better off with a full hydraulic system or better yet a lead screw drive and stepper motor. There is little sense in pushing a blade hard into the material being cut. It will do more damage to the blade than it will aid the speed of cutting. Metal can only be removed at the rate the teeth can scrape up a mouthful.

    Generally metal-cutting saws are actually limited in the reverse in that they are damped from lowering too fast. They use an hydraulic cylinder and an adjustable bleed valve to control the rate of downward pressure on the blade. Usually around 10 to 20 lbs load on the cutting teeth is about as hard as you can push it and still get good blade-life.

    Dave

  3. #3
    Administrator Kelly Bramble's Avatar
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    I think Nate400 is refereing to a "OLEO Actuator". The advantage is that it take way more volume of air to displace the hydraulic side of the actuator resulting is smoother and slower extension or retraction.

    There might be another technical name for these types of air driving fluids actuators.

  4. #4
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    Kelly,

    I don't think I have seen anything like that with air and oil in the same space, certainly not for saw-feed. I have seen air/oil systems where a larger diameter air cylinder was used to power a smaller diameter hydraulic to increase output pressure when the max air pressure was not high enough. But, even then, any vibrating or reciprocating load is transferred back to the air cylinder which acts like a spring at max air pressure.

    It is not pressure that makes a good saw cut. It is the load per tooth. If a smooth steady cut is important then a mechanical system (lead screw) is the only way. However, if they are cutting various types of material and various cross sections, then they are unlikely to need more than about 20-lbs contact pressure at the teeth and then adjust the down-speed for a constant tooth load. I think the "Boss" is looking at this from the wrong end. Let Engineers do the Engineering.

    Dave
    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  5. #5
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    Perhaps, the problem is that the air cylinder has no flow control valve which would control the speed of the cylinder and possibly avoid the chatter.

  6. #6
    Technical Fellow jboggs's Avatar
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    That's what I've been thinking all along. Even if the motive power is pneumatic, as long as the fluid in the cylinder is incompressible (hydraulic) and the ports have properly adjusted flow controls, that should control any chatter originating in the cylinder.

  7. #7
    Lead Engineer RWOLFEJR's Avatar
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    Folks have been making cylinders that are piggy backed like this for years. There's a hydraulic cylinder on the back of an air. They have flow controls for adjusting your speed of exit oil. We've been using a pair of them on a dual head abbrasive cut-off saw for years. The particular model we run if you want to do a search and see what they look like etc. is an Allenair CHFLH... Others make these as well. You will just need to start off choked back and slowly tweek it up until you're getting a good feed rate for your material and cutting tool.

    If you want steadier yet... the way to provide a rock solid steady feed... with hydraulics... in a critical application is to apply pressure to both ends of the cylinder. Give the cylinder far more work than required for the motion it needs to perform on the workpiece. By fighting a check it'll move as steady as all get out. Draw back is the extra heat generated but if you need dead solid with oil then you do what you have to do and install a bigger cooler.

    Good luck!!

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