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Thread: Dynamic Sealing - what am I doing wrong?

  1. #1
    Adobe Evangelist emisnug's Avatar
    Join Date
    Dec 2013
    Birmingham, UK (originally a Seattle native)

    Dynamic Sealing - what am I doing wrong?

    I've been trying to design to the Parker O Ring guide, but I'm having real difficulty making sense of it.

    What I'm trying to design is a pneumatic cylinder for my little combat robot, and as far as I can tell, there's no commercially available cylinders that are both lightweight and can deal with 68bar pressures. To this end, I thought I'd try my hand at designing the following

    80mm Bore
    200mm Stroke

    What I've been using is this: - more specifically, designing around the following O ring:

    -337 75.57 5.33

    I've done my gland/groove depths with the design guidelines that that document gives.

    I've also attached a drawing - basically, I'm confused as hell because I'm following the design guidelines and yet the O rings look like they're far too large to actually work. Any pointers on what I'm doing wrong would really help.

    Thanks!Piston with O rings.jpg
    No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.

  2. #2
    Just at a quick glance, it looks like you missed that the width is added up on both sides of the inner diameter to get to the total outer diameter. You are trying to get an 86.23mm OD o-ring into an 80mm bore. That's damn near a quarter inch too big.

  3. #3
    Insider AndrewTheWookie's Avatar
    Join Date
    Feb 2013
    Central Coast, CA
    Looks like you did the same thing with your o-ring groove, double how much you cut on one side is subtracted from the diameter. Currently the groove OD is 70.7 compared to the o-ring's ID of 75.57.
    I don't know, fly casual

  4. #4
    That groove is probably not too far off (at least by the Parker guide standards) if it was the correct size o-ring. There just isn't a chance in hell of that o-ring fitting properly into an 80mm bore.

  5. #5
    Join Date
    Apr 2012
    Manchester, England
    Supports Inception Designs
    Well that chart is confused to start with!

    It says Metric o-ring chart on the front cover, but those are imperial size o-rings inside.

    For an 80mm bore I would go with 4072 (72mm id and 4mm c/s - total 80mm outer diameter) and then a groove of 72.2-72.5mm diameter and 4.5mm wide

    That would be my starting point.

    Dear boy, I work at Planet Eclipse, don't you know..

  6. #6
    Join Date
    Apr 2012
    Knoxville, TN
    Wait.... little combat robot?

    68 bar? 1000psi? A 1000psi unit isn't light weight for a reason though, and you have seen the 3000psi hydraulic units I imagine.

    80mm bore is 3.2", so about 8 square inches of surface area, so 8000lbs of force? Over nearly 8 inches of travel? That means you are looking at about 20" long just for the ram? Little robot?

    Ah... you might want to put a lever in there somewhere if you are trying to get 8000 lbs of force. You can lift a Suburban with that.
    Josh Coray
    J4 Paintball
    Lead Design

  7. #7
    Insider PBSteve's Avatar
    Join Date
    May 2012
    Los Angeles
    Probably also want to look into backup rings and other high-pressure considerations as well, at that pressure you'll see extrusion of a duro 70A ring into the typical 0.010" clearance used in many paintball parts. See section 3.1.4 of the Parker handbook for more info.

    At 1 kpsi you really need to thoroughly look at at high-pressure considerations.

    Here's the actual Parker handbook:
    Ever so many citizens of this republic think they ought to believe that the Universe is a monarchy, and therefore they are always at odds with the republic. -Alan Watts

    I work for the company building the Paragon

  8. #8
    Join Date
    May 2012
    I don't know if you're doing an inertia labs thing but unregged CO2 would have cryogenic considerations as well.

    I always thought using the cylinder as a heatsink and siphon CO2 would be the play for maximum effective flowrates in those systems.

    I think they actually let people build their own valves now, check out the Chomp build thread (same dudes as The Judge).
    "So you've done this before?"
    "Oh, hell no. But I think it's gonna work."

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