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Get the Air Out!

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  • Get the Air Out!

    I put together a small hydraulic dampening system consisting of a small double acting cylinder, elbow, tubing and a valve. The valve is used to control the rate of flow between the two sides of the cylinder. (Closed system)

    The problem I'm having is getting the last bit of air out of the system. I can hear it "squirt" when it gets to the ends of the movement. I've tried turning it every which way, moving the rod then opening a connection and refilling but can't get the last bubble out.

    Any suggestions?

  • #2
    Can you get them in a pot of boiling water somehow and use the resulting expansion of the hydraulic fluid to get that last bit out? Seal while hot?

    SP

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    • #3
      I have one of those little "Mity Vac" pumps that seems to be of use in these type of situations, as well as brake bleeding. Might not work too well though if the air is hung up or dead-headed somewhere it cannot rise from. Sidegrinder.

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      • #4
        Ken, is there any way to slip a straw in through an open hole before you close it up? Used to have to slip a WD-40 straw into an angled wheel clyinder on the the brakes of some old Chevy trucks to get that last little bit of air out. James

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        • #5
          Can you give it a high spot? Add a short lengh of tube to the circuit get the air out of the way? Sort of the opposite of a drop tube in a air compressor circuit to get rid of water.
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          • #6
            You will never get all the "air" out of a system like that if it has no provision for expansion and contraction of the fluid with temperature change. Fluids expand and contract much more than the metal and will either pull air in through seals or will pull a vacuum that will be filled with vaporized fluid when the fluid contracts on cooling.

            A small accumulator added to the system with an adjustable needle valve will allow for expansion and contraction of the oil. The needle valve is adjusted to provide what amounts to very tiny leak into the accumulator and allows the oil to change volume without changing the damping action.

            The accumulator is placed at the highest point of the circuit and will also allow for any remaining air to escape as the system is used.
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            • #7
              I agree with Evan, you in effect can get all the "air" out and you in effect can have positive locking seals that keep any air from coming into play --- but what you cant do is stop the "cavitation" effect,,, this is the same thing boat propellers go through, you essentually create a vacuum inside the chamber, its not air --- but it is empty space... Evan has givin you one example but there are different ways of trying to remedy this situation, for example; if the temperature variation is not extreme there are fluids that are designed to not have as detrimental expansion/contraction characteristics and can actually be all you need to take care of certain situations...

              Really without the heating and cooling factor and atmospheric pressure changes most of this would be unessessary, (until one of your seals started leaking)

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              • #8
                What about a small expansion chamber?
                Fork lifts use a steel chamber with a N2 filled rubber bladder that would work. For this application, I would look making at a small cyinder and piston connected to the system. Overfill the unit till the piston is moved half way up the cylinder. A light weight spring could be added once filled to keep a slight positive pressure and to over come the drag of the oring on the piston.
                Last edited by JPR; 11-18-2006, 01:12 PM.
                John

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                • #9
                  I think something like your talking about is fine for the aplication that your speaking of, you have an unlimited supply of "pump under pressure" fluid and can take up the slack in one direction or the other and then get things moving just as if the unit didnt exist, but on a push pull shock type system that is being used for dampening you would work the daylights out of your piston and also create the same effect as having air in the system for the first so much of piston travel, this would not be welcomed, its why Evan gave the example of the slow bleed valve,,, to small of a port to make any kind of difference in normal operations but large enough to allow compensation with expansion/contraction rates while running or at idle...
                  Last edited by A.K. Boomer; 11-18-2006, 01:14 PM.

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                  • #10
                    A needle valve to the accumulator isn't required either. Just a very small orifice will do the trick.
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                    • #11
                      This all really depends on what CCW is using this for and how many reps per second or minute its going through, so until I find out more info I wont speculate because your fork lift example could work for him if this is a very slow rep device that doesnt need imediate dampening responce...

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                      • #12
                        Thanks guys but this sounds like it's getting way too complicated.

                        I'm rebuilding my old generator test stand. The motor went out a few weeks ago so while replacing that, I fell into the trap of "might as well fix this too" quagmire. (Painting, replacing old wire, cleaning, etc.) It's always had a little shake so I thought I'd add a dampener to the motor. It's free floating (weight of the motor) to keep tension on the pulleys--One on the motor, different sets/sizes for various generators.

                        I tried my cylinder and it didn't help much. I suppose it's because of the small motor movement and the play in the connection/linkage. I'd have to rig up some contraption or arm to multiply the movement.

                        In fiddling with it today, I think I found the cause of the shakes.
                        Yep, the dang motor pulley isn't very true. This is causing the belt to ride up the side a little then drop back down. I could have saved a lot of time if I had checked that first.

                        I did get all the air out of cylinder and line. I ended up sinking the whole works into a bucket of hydraulic fluid and working it until there were no more bubbles. I just hope my arms don't rot and fall off. What a mess that was.

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                        • #13
                          I had a 'bouncing' motor problem on my bandsaw. Cured that by changing to a link belt. Every machine that I've changed over to the link belt has improved.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            I'm curious why you went to so much trouble to build a shock absorber. You know, like the ones on cheap bicycle front forks, cars, screen door closers etc.
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                            • #15
                              [[Just a very small orifice will do the trick]] And, just what are the dimensions of this orifice?

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