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Fastening molded nylon gear to brass

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  • Fastening molded nylon gear to brass

    I need bevel gear sets 48 Diametral Pitch x .375" & .5" Pitch diam. Works out to 1/1.33 ratio and cannot be changed. I need these sizes to fit into a "spot".

    I can get a .375PD gear in metal but the .5PD only come in Molded nylon (Boston Gear). The .5 gear needs to be modified so that the existing hub is removed and a new hub is on the opposite side compared to a normal gear. How can I fasten the nylon gear to a brass hub. There is no room for a screw etc.

    Will any type of glue work?
    If the nylon gear is bored out can a brass hub be pressed into the bore and stay tight?

    Any other suggestions. This is becoming a "pulling hair out" situation.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

    Southwestern Ontario. Canada

  • #2
    Knurl it, heat it, press it in.
    https://www.youtube.com/channel/UCIF...7S66kX1s8rd0qA

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    • #3
      Probably depends on the unspecified load...

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      • #4
        Not too much in the way of glues and adhesives make a reliable joint to nylon.

        J&B Plastic Weld is reported to get the job done:

        http://info.craftechind.com/blog/bid...ue-for-Plastic


        I like the knurled/heated/pressed idea.

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        • #5
          If the gear is to turn in only one direction perhaps you could use an appropriate thread, right or left hand depending upon direction of rotation, on the brass shaft and inside the nylon gear, to a shoulder on the brass shaft. This would require a shaft larger than the threaded hole in the gear, of course, at least att he point of the shoulder after which the shaft could be necked down if necessary.

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          • #6
            No it turns in goth directions. Not much of a load, connects to a tiny hand wheel, part of the elevating gear on a artillery model but it dose need to work.

            How much heat, to soften it I'm assuming and how much of an interference should I use.
            The shortest distance between two points is a circle of infinite diameter.

            Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

            Southwestern Ontario. Canada

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            • #7
              A small pin pressed into a through hole in the shaft and sticking out on each side that fits in a slot you would cut in the hub?

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              • #8
                The hub is going to be removed so no pinning.
                The shortest distance between two points is a circle of infinite diameter.

                Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                Southwestern Ontario. Canada

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                • #9
                  When you say no room for a set screw, I am guessing that you mean from the side to lock the two together. Can you press them together and then drill and tap at the seam for a set screw to lock them together that way?

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                  • #10
                    It would seem like the method of choice is to use splines. Just pressing a plastic gear onto a hub is likely to put too much stress on it if it's to not slip- unless the application requires very low torque. You could raise splines on a hub by knurling with the right wheels- I think a pattern like typical knurling would not be the best, but straight lines would be good. If a plastic gear is a light press fit onto a hub before the splining takes place, that should be an appropriate fit for the gear over the raised splines. JMO
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      Nylon is slightly hygroscopic (I read somewhere that it can absorb up to 6% of its own mass in water), and swells a little when immersed in water. I've never tried this, but I wonder if you could effect a kind-of shrink fit by making the brass a thou or two over size, then soaking the nylon for a week, putting the two together and letting it dry out.
                      (This swelling, incidentally, is the reason some boats have stiff steering. If the Ertalon rudder bushings are bored by an engineer unaware of this phenomenon to a minimal clearance on the rudder stock, then when held around the outside by the rudder tube they can't swell outward in the usual fashion, so they swell inward when immersed, and grip the stock.)

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                      • #12
                        Put a pin straight between the teeth. But make the pin short enough that is won't stick out past the root dia .
                        Last edited by ahidley; 08-02-2015, 08:18 PM.

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                        • #13
                          I have also seen gears with set screws run in through the teeth. Drill at the root. You did say a light load.



                          Originally posted by oxford View Post
                          When you say no room for a set screw, I am guessing that you mean from the side to lock the two together. Can you press them together and then drill and tap at the seam for a set screw to lock them together that way?
                          Paul A.
                          SE Texas

                          Make it fit.
                          You can't win and there IS a penalty for trying!

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                          • #14
                            Where on a bevel gear would you put the pin?

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                            • #15
                              The gears I've seen that aren't press fits use a straight knurl. With something as small as you're talking about, it may not be necessary to heat the part before you install the knurled axle. I'd probably try a press fit first.

                              Threading may work as well, if you either apply thread locker or use a jam nut. For what it sounds like you're doing, red loctite will probably be fine.

                              FWIW, I've glued Delrin gears to steel shafts with CA glue. It seems to hold up for model railroad use. Nylon would probably do the same.

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