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Thread: Hydraulic cylinder piston o-ring ?

  1. #1
    craigS is offline Aluminum
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    Default Hydraulic cylinder piston o-ring ?

    I need a suggestion for the O-Ring thickness to use on a hydraulic cylinder I am making for a project. Choices are: .070, .103 or .139 thick.

    The cylinder has a 1.125" bore, 2.25" stroke is single acting with a miximum force of about 800 lbs (800 - 1200)PSI. Operation is via a hand pump so its slow and the number of operations would be less than 20 cycles per year.

    I found pistons in Bailey's but they don't detail out the O-ring groove

    Thanks,
    Craig

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    HuFlungDung is offline Diamond
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    The rule of thumb that I use is that the minor diameter of the piston groove should be exactly twice the O-ring's nominal cross section smaller. And you determine the OD of the O-ring's major diameter from the exact honed bore of the cylinder. So you're commonly working with exact fractional amounts in the inch system: .125 difference for an .07 O-ring.

    The length of the groove should, as a rule of thumb, be about 25 to 30% longer than the O-ring's nominal cross section, plus additional amounts for any backup rings. The dynamic action of a piston O-ring requires that it roll slightly in the groove so it can bring a lubricated edge into contact with the cylinder wall. Otherwise it will wear flat. Probably that is not a concern to you, but you might as well make it right.

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    craigS is offline Aluminum
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    Not sure I follow your logic.

    I found a lot of references on how to design the O-ring groove but I am looking for some rule of thumb on what thickness of O-Ring to use in the first place on my application. Once I have choses the correct thickness of O-Ring I can go on to calculating the proper groove geometry, and cut the grooves in my piston.

    Craig

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    autofrite is offline Hot Rolled
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    this makes sense,but for all the o-ring fitted applications i have seen-the grooves are usually tight.maybe slightly wider to allow room for the ring to compress into.i guess i would have to find an o-ring catalog with technical data -as sizes change the tolerances need to be adjusted,i would think.800 psi is not that great a pressure,but the harder o- rings will give you better service (90 durometer)
    also,the theory would be different with square rings.but,there isnt any place where a square ring is used that an o-ring wont work.
    thicker ring-more piston to wall tolerance is acceptable
    thinner ring- less drag.
    it would be easier to make the groove wider if the narrow groove isnt working.

  5. #5
    HuFlungDung is offline Diamond
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    Quote Originally Posted by craigS View Post
    Not sure I follow your logic.

    I found a lot of references on how to design the O-ring groove but I am looking for some rule of thumb on what thickness of O-Ring to use in the first place on my application. Once I have choses the correct thickness of O-Ring I can go on to calculating the proper groove geometry, and cut the grooves in my piston.

    Craig
    I'd use 3/32 or 1/8, whatever is commonly available in the size range your working in. I don't know of much advantage to going heavier except that perhaps more wear surface is available. The clearance between the piston and the cylinder bore defines the extrusion gap, and it may be that more clearance requires a heavier O-ring to resist extruding at extreme pressures? However, a backup ring is typically provided whenever that possibility exists.

    I'd like to read any explanations for cross sectional area selection that are offered, too.

  6. #6
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    Usually, O-rings don't make a good seal for a hydraulic cylinder unless it's low pressure. The pressure of the fluid will just pass around an O-ring. Lip seals and U-cups are much more common for this. With both of these, the hydraulic pressure actually provides a tighter seal by forcing the seal area out.

    Here's some basic engineering:
    Hydraulic Seals, Mechanical Seals, O-rings
    A lip seal will provide about 4 times the sealing area under load vs an O-ring. The higher the pressure, the tighter the seal. For a single acting cylinder, you need one seal on the pressure side. For a double acting, you need 2 opposing seals.
    JR

  7. #7
    CalG is offline Titanium
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    Default O-ring seals are hydro-dynamic just as lip seals

    When the mounting configuration is correct, the fluid pressure behind the o-ring presses the ring into the corner that requires sealing. As do internal combustion piston rings.

    This hydraulic pressure is what extrudes the o-rings into the gap and causes failure. ( the action of the moving parts acts on this aspect too) This pressure also causes friction to build with load. That is where proper hydraulic seals shine.

    Heck, an o-ring becomes flattened in short order when in service on a moving rod. (wear and deformation) A hybrid o-ring, quad ring, squar packing.

    "Chicago Rawhide is my favorite reference

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    RDCutter is offline Aluminum
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    Do a search for "quad ring+quad seal+x ring" Be sure to finish the bore in a cross hatch pattern, to reduce the wear on the seals.
    Regards,
    Bob

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    I would go with the .139" thick and machine the groove .139" wide and .134" deep or a pinch less. thats .005" radial crush and should seal up good.

    For the head go with a poly pack and an outboard wiper seal, they can be a bitch to install but hold back the oil real well.

  10. #10
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    From post No 1.
    Quote
    "I need a suggestion for the O-Ring thickness to use on a hydraulic cylinder I am making for a project."

    Personally, I would just buy a suitable crocodile style car Jack and use the cylinder and actuating pump as the start point for such a project .

    Why re-invent the wheel ?.
    I suppose it depends whether you want the project finished to use it, or whether you want to make the hydraulic cylinder just for the hell of it.

    Davycrocket

  11. #11
    wlbrown is offline Hot Rolled
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    Default o ring groove?

    HELLO,
    suggest you do a google search. this
    should tell you what you want to know.
    just type in o ring groove.
    good luck.
    wlbrown

  12. #12
    craigS is offline Aluminum
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    Everyone,
    Thanks for the replys, I think I will start out with the .139 thick O-Ring. If that gives me any trouble I will look into lip seals. I did a job in the past with a lip seal and it worked out just fine. Only problem with lip seals is finding the one you need and paying more than $.05 for an O-Ring. The usage is low and slow and I can always make new aluminum pistons with lip seals if necessary.

    Learning a lot about hydraulics.

    Thanks,
    Craig

  13. #13
    HuFlungDung is offline Diamond
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    They most likely measure .135" and the correct groove minor diameter is .250 under the bore diameter of the cylinder.

    While you can make the O-ring fit in a groove .135" wide (actually called 'the length'), that is a little too tight. Better be about .150" wide.

  14. #14
    little.one.der is offline Aluminum
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    Hey CraigS

    For the best outcome, head over to Parker's website. They have an o-ring book you can download for free. Inside they have toleranced dimensions for every o-ring size! No guess work involved.

    Parker O-ring Catalogs
    jackal and autofrite like this.

  15. #15
    thruthefence's Avatar
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    If I'm not mistaken, o-rings only became successful after someone discovered they need to "roll" in the groove a little in service. Apparently, the clearance in the groove is a subtle thing, to much, or too little, it won't work.

    Regarding "High Pressure" & o-rings; what are you defining as "high"?

  16. #16
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    Quote Originally Posted by RDCutter View Post
    Do a search for "quad ring+quad seal+x ring" Be sure to finish the bore in a cross hatch pattern, to reduce the wear on the seals.
    Regards,
    Bob
    I know of using cross-hatch patterns to INCREASE wear in order to break in cast iron piston rings, but never heard of using it to decrease wear. All the hydraulic cylinders I've seen have mirror-finish chrome bores. Am I missing something in my knowledge base?

  17. #17
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    75% of my work is for a hydraulics repair shop.

    They use Hercules parts.

    Here is a catalog reference with all of the specs. Download the [Engineering Catalog].

    Hercules Sealing Products-Hydraulic cylinder repair seals and seal kits, replacement cylinders and cylinder components for construction and many other heavy equipment industries.


    Sometimes they will need a modification. They give me the old rod , the tube diameter and the seal numbers.

    There is enough info to tell you piston dimensions, clearance, groove width, type of finish on grooves, tolerances, etc.

    Chesterton is another good company brand, but I haven't worked with their parts in over 8 years.

    JAckal

  18. #18
    wesg is offline Stainless
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    Too smooth and the surface gets wiped clean, burning the surface of the seal away. You need some roughness to the surface to hold oil. Same reason there's a cross hatch honed into a cylinder, to hold oil for break-in.

    Much better than an o-ring is a T-seal. Avoids the issue of an o-ring possibly twisting in the groove and cutting itself to ribbons. And a whole lot easier to install than one of the Polypack style lip seals. Man are those a bear to get in for a shaft seal. We finally gave up and used a screw on cap.

    The hydraulics guys we consulted made the point that the Polypacks were best to be avoided unless we were running high pressures, like over 2500psi IIRC. Maybe 'avoided' is a bad word to use. How about 'not needed'?

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