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The mysterious shrinking micro-V belt.

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  • The mysterious shrinking micro-V belt.

    The brake on my Briggs winch stopped working today. I was able to make some adjustments to get it working again, but I couldn't understand why it quit.

    Here's a picture of the winch when I first started to use it:



    Here's a picture I took after I put everything back like it was before I made adjustments at the field today:



    It's the same belt, and the ends are in the same position they were in a month ago. Notice the difference in the angle of the arm.

    That's a LOT of difference, and I was really stumped to explain it until I compared the belt on the winch to an unused section of the same belt in the shop:



    The belt off the winch is on the left. The unused belt is on the right.

    The belt doesn't get hot from the braking action, but it is exposed to the sun while it's going to/from the field and while we're using it. The belt is labelled "Goodyear NACIII". It shrank 1/4" in a 9" section.

    Is it common for automotive belts to shrink from sun exposure? I've never seen it in the Gates micro-V belts I've used. Is there any other explanation?

    Roger
    Last edited by winchman; 08-09-2008, 02:58 PM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

  • #2
    Wow. If that was a toothed belt running on cogs, the mechanism would be totally unworkable in short order. It would not seem to me that there would be that much difference in construction or materials between the types of belts, but obviously there is. It seems that the cord material would have had to shrink along with the rubber, unless the method of laying it has left the belt free to shrink or expand. If heat was a problem, the belt would also probably be useless as it could easily get heated quite much in use. Exposure to sunlight I can see changing the belt, but I wouldn't have expected that to still be an issue with a quality product. Best guess is it's a crappy belt, cheaply made in relatively uncontrolled conditions with cheap labor and with no regard to its longevity in use. If I was Goodyear, I'd be embarassed to have my name on 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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    • #3
      My boss at my first job used to say "Everything expands when hot and shrinks when cold except rubber which expands with cold and shrinks when hot." I never could figure out what he was talking about. Maybe now I know.

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      • #4
        Yeah, I'm really surprised because both pieces were cut from the same belt, which I bought about a year ago.

        I just hope it doesn't continue to shrink.

        Roger
        Any products mentioned in my posts have been endorsed by their manufacturer.

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        • #5
          Is the over-all length the same, or shorter?

          Perhaps there was a production change in the belt and the cogs were redesigned
          "the ocean is the ultimate solution"

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          • #6
            Possibility exists of a change in humidity in belt in the sun. Soaking with some water should prove insightful. Have 13 yrs. experience with monofilament type of drive belts, can take the camber out of a belt just by setting it in some water for about 1/2 hour.
            mark costello-Low speed steel

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            • #7
              Yeah, I'm really surprised because both pieces were cut from the same belt, which I bought about a year ago.
              Aha. You are using the belt as a friction brake, right? Any idea how hot it is getting in use? Many of the cord materials used in belts are made using a process that pre-stretches the fibers while cold. This both reduces the possibility of further stretching and aligns the long chain molecules in the polymers the cords are made from which increases strength. As long as the fibers remain below their design working temperature all is well. However, if the working temperature is exceeded the fibers act just like heat shrink tubing and resume the state they were in before being pre stretched. Presto-Changeo, the incredible shrinking belt...
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              • #8
                The overall length of the section of belt I used for the brake is about 16 inches. The shrinkage over that length was about 3/8". That's why the brake arm angle changed so much.

                The belt is used as a friction brake, but it's only operating for about two seconds per launch cycle. We've only made about sixty launches with the winch.

                It gets MUCH more heat input from just sitting in the sun for several hours, but I doubt it's getting hotter than the typical underhood temperature you'd find on any modern car, though.

                This is the first Goodyear belt I've used. All the others have been Gates. But in today's world you can never tell where anything really comes from or who really makes it. All the others had continuous ribs instead of the little blocks.

                I'm going to carefully measure the piece of unused belt which hasn't been exposed to the sun for more than a few minutes. I'll leave it out in the sun for several days, check the length periodically, and see if it changes over time. Might as well pre-age it, I guess.

                Roger
                Any products mentioned in my posts have been endorsed by their manufacturer.

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                • #9
                  Heat and temperature aren't the same thing. Even a few seconds of high friction can produce a high temperature pulse. Also, in your application there is nothing to prevent the belt from shortening as it isn't under constant tension.
                  Free software for calculating bolt circles and similar: Click Here

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