Measuring circularity of installed seal
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    Default Measuring circularity of installed seal

    We have a medium hard (say 90A) seal that is pressed into a block on the OD. So OD is static and ID is dynamic.
    We are having issues with leakage around the ID, likely due to the circularity of ID edge.

    I can't figure out a good way to measure this circularity. I think Go/No go gages would deform the seal enough to throw off measurements. We don't have a CMM and it would be a challenge to fixture the assembly in a way to use a dial indicator reliable, especially if we need to measure every part.

    Any ideas?

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    Quote Originally Posted by Shapeofwater View Post
    We have a medium hard (say 90A) seal that is pressed into a block on the OD. So OD is static and ID is dynamic.
    We are having issues with leakage around the ID, likely due to the circularity of ID edge.

    I can't figure out a good way to measure this circularity. I think Go/No go gages would deform the seal enough to throw off measurements. We don't have a CMM and it would be a challenge to fixture the assembly in a way to use a dial indicator reliable, especially if we need to measure every part.

    Any ideas?
    Can you use an optical comparator on it ?

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    What diameter is the shaft? What's the RPM? What material? Is this some sort of special seal or just an
    ordinary lip seal like you would buy from National or CR? In my experience ordinary seals will acomodate
    a reasonable amount of irregularity on the surface of the shaft so I'm not sure if the circularity of the seal
    would be an issue. Is it possible that the seal was damaged during installation? Or could it have been a
    dud right from the factory?

    There are a myriad of different seal types out there. Are you sure the seal you've chosen is correct for
    the application?

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    You could visualize the circularity of the seal using a transparent glass or acrylic shaft with polished ends. By Looking at the width of the band where the rubber seal makes contact with the clear shaft you can determine how circular the seal is. A thin band indicates poor contact and a wide band indicates a tighter fit in that area.

    If you etch lines on the shaft you can make a easy to read Go/No gauge showing the upper and lower range for required contact area.

    A clear syringe demonstrates where the rubber seal is making contact.

    download.jpg

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    Quote Originally Posted by digger doug View Post
    Can you use an optical comparator on it ?
    That's a great suggestion thank you. An optical comparator works well in the quality lab. It'll be a great way for me to pull some data sets and test limits, but is there a more on the fly method assemblers could use for pass/fail criteria?

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    Quote Originally Posted by LKeithR View Post
    What diameter is the shaft? What's the RPM? What material? Is this some sort of special seal or just an
    ordinary lip seal like you would buy from National or CR? In my experience ordinary seals will acomodate
    a reasonable amount of irregularity on the surface of the shaft so I'm not sure if the circularity of the seal
    would be an issue. Is it possible that the seal was damaged during installation? Or could it have been a
    dud right from the factory?

    There are a myriad of different seal types out there. Are you sure the seal you've chosen is correct for
    the application?

    All good questions. It's a 7/8 shaft at about 60 spm. Very specific geometry and material for our application, but similar to ordinary lip seal. We have very high requirements for acceptable leakage. It's ongoing research to find better seals, but regardless these seals have worked, we are having issues within the same batch. Likely install procedure affects sealing, but my question is still around validating the installed seal is circular and not damaged.

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    Quote Originally Posted by Shapeofwater View Post
    ...we are having issues within the same batch. Likely install procedure affects sealing, but my question is still around validating the installed seal is circular and not damaged...
    If you're having varying issues within one batch you're probably pushing up against the limits of the
    manufacturers tolerances. And I'm thinking that, however you manage to measure the seal, it will have
    to be a non-contact method--anything you do that touches the surface is going to distort it.

    I'm not familiar with air gauges but is there a possibility that one might work in this situation? Also
    wondering if you've contacted any of the seal manufacturers about your problem?

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    As has been stated already, the best thing you can do is use a non-contact method to measure the thickness/Cross-section of your seal all the way around for consistency. Ovality of the seal itself in the free state is mostly irrelevant at the 90A hardness level. Deviations from circularity in your bore and/or shaft will define the seal's installed shape and squeeze.

    More things to check are the shaft-bore misalignment (axis-axis static) and shaft runout (rotational dynamic). If not concentric, the seal will be extra squeezed on one side and out of contact with the shaft on the other.

    Axial perpendicularity misalignment "Seal Cock" is another possibility - how well the sealing interface is perpendicular to the shaft axis. If this is off more than about .008" per inch, the seal is presenting an elliptical sealing lip to the shaft, and that will result in varying squeeze as well. This can be traced by an indicator on the assembly fairly easily with a rotating fixture.

    The tolerance for any of these might be less if other factors are in play of course.
    Last edited by BoxcarPete; 11-27-2019 at 04:50 PM. Reason: clarity

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    Since you are having variation in the seal batches, I would be looking at other things that affect the seal performance.

    Things to check are the bore ID. Is it concentric? Is the seal always being installed perpendicular to the bore?

    I would also be looking at the shaft. Are there details such as a snap ring groove, splines, etc. that have a sharp edge. Have you check the leaking seal under a microscope paying particular attention to the edge of the lip itself. It does not take a very large nick to cause issues.

    Lip seals are very forgiving of fit up issues. When you have a leak issues it is more often an installation issue then the seal being concentric.

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    Are these ''bought in'' seals or are you machining them in house?

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    Seals are purchased to eng specs; we work closely with this supplier.

    Thanks for the feedback everyone. Yes there's a snap ring groove that may affect install. We check perpendicularity face to axis for every part, but not perpendicularity of installed seal. "Seal cock" will be good to investigate. There is very rarely any noticeable failure mode when we look at failed seals under a microscope.

    I don't think an air gage will work here because of how narrow the sealing edge is.

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    When I worked for the manufacturer of very high quality gearboxes we checked every installed seal with a tool that slipped on to the shaft an was pushed through the seal. If the seal was not concentric then it showed as a raised lever/pin and investigations began. The seals we used were very special indeed and were about 200 times what a normal seal cost!
    A related problem was that when CNC machine tools started machining the seal housings the company found that the bores were not round and final bores had to be done using boring heads, also bores could only be machined on vertical spindles, the sag on horizontal machines meant that bores were not within specifications.

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    I understand the holes are through-holes.
    Get a stepped gage made in suitable optical-qty material, in 0.01 mm or 0.001 mm steps as needed.
    Pyrex, any of the typical astronomy materials.
    Wont cost a lot.

    Then video the seal performance, as an increasingly large gage is inserted.
    It won´t "prove" anything but it will likely show You where the problem is.

    I fixed several multi-million-$ sets with similar stuff.


    Quote Originally Posted by Shapeofwater View Post
    Seals are purchased to eng specs; we work closely with this supplier.

    Thanks for the feedback everyone. Yes there's a snap ring groove that may affect install. We check perpendicularity face to axis for every part, but not perpendicularity of installed seal. "Seal cock" will be good to investigate. There is very rarely any noticeable failure mode when we look at failed seals under a microscope.

    I don't think an air gage will work here because of how narrow the sealing edge is.

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    Silly thought time - are the seal bores and shafts concentric?

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    Quote Originally Posted by Shapeofwater View Post
    ... but is there a more on the fly method assemblers could use for pass/fail criteria?
    What-if : instead of a dimensional Go/No-go inspection, change to a functional pass/fail check?

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    Quote Originally Posted by Buchanman View Post
    ... also bores could only be machined on vertical spindles, the sag on horizontal machines meant that bores were not within specifications.
    This one is giving me frown lines ... Dixi jig borers make pretty round holes, and they are horizontal.

    Was this assumption ever verified ? Or are we talking about 15' diameter bores in a tunnelling machine ?

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    Sealing a 7/8 shaft is pretty easy but groove/gland forms are not universal for rotary and reciprocating. You mentioned a speed of 60 SFM. Is that rotating or reciprocating SFM? What is the medium you're trying to seal? What pressure? Spring-loaded lip seal? O-ring loaded lip seal? O-ring? Open vee?


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