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Short throw, leakproof hydraulic actuator

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  • Short throw, leakproof hydraulic actuator

    I have a need for a pair of clamping devices. When energized they will prevent a mechanism from sliding. When de-energized, the mechanism would move easily. I have a space about 1-1/4 square and up to about 1/2 inch high for these 'actuators' to fit within. The 'stroke' would theoretically be zero, as the parts could remain in contact, yet free to slide with low friction. Put a clamp over the parts and you haven't brought them any closer together, but now they can't slide.

    Zero stroke isn't quite accurate, but 10 thou of motion would be more than enough. So-

    The device I have in mind could be made to expand within a gap, and it would do the job. Essentially, an extremely short hydraulic actuator would be ideal. What I'm thinking is to make a steel ring to bolt down over a piece of sheet rubber onto the base. A hole in the base would connect through tubing to a hand powered pump. Squeeze the handle and the rubber would be expanded upwards, bringing the parts into tight contact. The hand pump could be a similar diaphram type pump, and in this case it would feed two actuators. The handle would latch mechanically and would thus maintain pressure in the system- provided there are no leaks whatsoever.

    A further refinement of the system would have the rubber being the contact surface, such that when it was inflated it would become one of the friction surfaces. The base that the rubber piece is mounted on is thus prevented from sliding relative to the guide piece.

    The rubber is going to be forced up against the edges of the ring and the guide bar. I would probably put a bevel on the bottom inner of the ring so the rubber has a smoother edge to be forced up against. Other than that, the rubber has nowhere to go when inflated except to be forced to fill the small gap, so there should be no blowout problem.

    The system would only work for me if there's no leakage of the actuating fluid. I'd be relying on the ring to hold the diaphragm tightly enough to the base to prevent leakage. I don't think I'll be needing more than about 300 lbs of pressure to make this work.

    Comments and ideas welcome.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Guy Lautard described a "rubberdraulic" clamping device in The Machinist's Second Bedside Reader for locking a graduated hand wheel dial. The oil was replaced with silicone rubber cast in place. Rubber displaces rather than compressing. For very short strokes as you describe, this might be an option. Fill the rubber diaphragm and passages with the RTV silicone, put a piston in the bore of the "pump" area and move it with a lever, or for more force drive the piston with a screw. Using the rubber, no o-rings needed.
    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
    ~Practitioner of the Electromechanical Arts~

    Comment


    • #3
      You could also butcher a hydraulic brake for a mountain bike. Might need some rework to fit within 0.5" though, but maybe you can steal the piston with its seal.

      Igor

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      • #4
        For such a short stroke, how about using metal shim in place of rubber?

        Ian
        All of the gear, no idea...

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        • #5
          Hi,

          How many cycles do you expect it to last? At some point all hydraulic cylinders will leak. Perhaps going pneumatic might be a better choice if oil leakage is a problem.

          dalee
          If you think you understand what is going on, you haven't been paying attention.

          Comment


          • #6
            Part of the idea is to eliminate the need for sliding seals and pistons, which will eventually wear out.

            I like the idea of a metal shim in place of the rubber, but are you suggesting that the shim is flexible, or is it just placed on top the rubber as a wear piece? I like that idea-

            One of my concerns with the rubber is that it will eventually need replacing due to age, and it will also have a flexural lifetime. I'm ok with the need to replace it from time to time- I would suggest a 5 year or more lifetime would be alright. My bigger concern is leakage- I want to be able to apply the pressure and find that it's still holding a few days or a week later. If there's no leakage, there's nowhere for the pressure to dissipate to. This is one of the things that made this idea attractive to me in the first place.

            I also though of another way to make this. I would make the disc from steel, and recess the edge to some degree- enough to accommodate an O-ring. When pressure is applied, the lip of the disc presses upwards on the O-ring, compressing it slightly. With the disc being thicker in the center, it's high enough to contact the guide piece without interference from the clamping ring that holds it together. This idea however means that the O-ring has to seal perfectly under both conditions- whether just touching or when compressed. Even though there is no sliding action going on, as would be the case with a normal piston seal, I still don't think I'd get a reliable seal under all conditions.

            The job here is probably best done mechanically with linkages, so chances are that's how I will do it. The hydraulic method with the rubber membrane does seem workable, so I'll still think about it.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #7
              Bourdon tube.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                Turn it sideways.....

                Can you use a 2-piece wedge locking action, and put the actuator to the side? If you can get the hose to it, presumably you can get an actuator rod to it as well.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  I know that this will sound "stone age," but how about a very short screw jack with a large fine thread. Whatever will give you a .010" throw in a quarter turn. Link the two together with a single actuator. If the base was 0.125" thick with a delrin "nut" of about 0.300" with a 20 TPI thread it should just about work and not jam at the wrong time.
                  Duffy, Gatineau, Quebec

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                  • #10
                    How about an electric trailer brake?

                    if 5 years replacement is 'ok', just about anything will do really as long as it does not need to actualy slow down heavy loads often (wearing out the components) and is not in a harsh envorment, and is used often enough to prevent siezing.
                    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                    • #11
                      Originally posted by darryl
                      Part of the idea is to eliminate the need for sliding seals and pistons, which will eventually wear out.

                      I like the idea of a metal shim in place of the rubber, but are you suggesting that the shim is flexible, or is it just placed on top the rubber as a wear piece? I like that idea-

                      One of my concerns with the rubber is that it will eventually need replacing due to age, and it will also have a flexural lifetime. I'm ok with the need to replace it from time to time- I would suggest a 5 year or more lifetime would be alright. My bigger concern is leakage- I want to be able to apply the pressure and find that it's still holding a few days or a week later. If there's no leakage, there's nowhere for the pressure to dissipate to. This is one of the things that made this idea attractive to me in the first place.

                      I also though of another way to make this. I would make the disc from steel, and recess the edge to some degree- enough to accommodate an O-ring. When pressure is applied, the lip of the disc presses upwards on the O-ring, compressing it slightly. With the disc being thicker in the center, it's high enough to contact the guide piece without interference from the clamping ring that holds it together. This idea however means that the O-ring has to seal perfectly under both conditions- whether just touching or when compressed. Even though there is no sliding action going on, as would be the case with a normal piston seal, I still don't think I'd get a reliable seal under all conditions.

                      The job here is probably best done mechanically with linkages, so chances are that's how I will do it. The hydraulic method with the rubber membrane does seem workable, so I'll still think about it.

                      If you need it to hold for that long then why not spring load it and use air or hydrolics to release it.
                      Craftsman 101.07403
                      Grizzly G0704
                      4x6 Bandsaw

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                      • #12
                        Some interesting ideas, thanks. This has become of academic interest only now, since I decided to go strictly mechanical. I thought about the screw jack idea, which is what I have on my table saw fence at home. It works, but connecting the front and rear to operate from one handle turned out to be finicky. I didn't want to have this on the saw at work, so I was looking for other ideas. After a lot of thinking about how to do this, it came together as a fairly simple mechanism.

                        What I have now is a pair of L-shaped pivoting levers, each one with a contact pad which comes up to pinch against the guide bar. The bottom of the L rises up when the upright part of the L is moved left. The bottom of the L carries the contact pad.

                        So now there will be two push-rods, each connected to one lever. The operating handle connects to the inner ends of the push-rods. When you lay the handle flat it pushes the rods apart, activating the clamping mode. When you raise the handle, the rods come together, releasing the pressure. One of the push rods, the longer one, runs in a guide very near the handle (and another one about halfway along) and it carries the handle. The second rod just swings up or down to suit. Because the handle is not fixed to anything but the rods, the clamping force is automatically equalized. This is something else that I was having a hard time figuring out how to do simply, but now it is simple and automatic.

                        The pivot points on the handle are drilled in such a way that when the handle is laid flat against the fence, the pivot points are slightly 'over center'. This gives a locking action so it doesn't come loose from vibration, etc. There will be an adjustment point somewhere so you can set the amount of pressure.

                        I have one question- the contact pads are steel, and the guide rails are steel. There seems to be lots of grip with only moderate pressure, so I'm happy with how it is. Some time ago someone, Evan I think, posted something about the relationship between pressure, friction, and contact area. Am I correct in my recollection that if pressure remains constant, so will the friction, regardless of the area of contact? My contact pads turned out to be 1/2 x 5/8, which doesn't seem like much, but it grips well. I could make them larger by making some minor modifications to the levers, but I don't want to do that unless I'm getting some advantage. Here's a drawing of what I came up with-



                        By the way, you're looking at an end-on view of the guide rails. They are over 8 ft long, and the fence itself is 6 ft long. The push-rods will be square tubing- the long one may need a support at its center. I don't think it will bend, but I'm prepared to add the support anyway.
                        Last edited by darryl; 03-19-2012, 04:13 AM.
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                        • #13
                          Add a slip joint and a die spring to one of the rods so you maintain your clamping force regardless of pivot and pad wear.

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                          • #14
                            There will be a point somewhere that has an adjustment. I'm not sure if there will be enough flex in everything so I don't have to use a spring- I guess I'll be finding out in a day or so.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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