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  • Plastic Armature Gear

    I notice even though Aurora AFX, JL and AW went with plastic gears for the idler, driven, pinion and cluster gears, they all used a brass armature gear. I'm guessing this is because of the heat generated by the armature. However, with new plastics today is it possible to have a plastic armature gear? Has anyone tried it?

    Thanks...Joe

  • #2
    I think you are right about the heat issue.

    I haven't been active in designing plastic parts in high-end thermoplastics for many years, but back when I was "Ryton" was the ultimate resin for such.

    Ryton was used to make circuit board mounts for CPU's (Central Processing Units) back then -- back when CPU's were prodigious heat-producers. Fortunately Ryton could stand up to the heat, which was a good thing because producing those parts in any other material would have been really difficult. Not only were parts made of Ryton highly heat-resistant, it allowed you to maintain the really tight dimensional tolerances needed for the tiny, close-spaced leads of those CPU's

    Interestingly, those parts were so rigid they would 'ring' if you dropped one -- like ceramics.

    Copied from an online source:

    Ryton® polyphenylene sulfide (PPS) is a semi-crystalline, high-heat polymer that offers exceptional dimensional stability for precision-molded components needing to withstand prolonged, high-temperature service.

    'Course when we are talking about premium thermoplastics, they tend to be expensive. Also, producing parts in high-temperature thermoplastics can be tricky. Which might discourage folks designing slotcars from using it.

    The biggest issue might be maintaining a press-fit on an armature shaft. When the shaft heats up and transfers heat to a pressed-on gear differential rates of thermal expansion come into play. Any plastic part expands more when heated than steel will, and Ryton is no exception. Combined with any softening of that plastic due to heat there is a good chance the press-fit would fail under high torque.

    This is a case where the traditional solution is probably the best. Brass will stand up to the heat better than any plastic I know of.

    These days there may be better, cheaper and easier-to-process heat-resistant thermoplastics plastics that could stand up in this service. Like I said, I've not kept up with the technology.

    And any of the thermoset plastics (plastics that can't be injection molded, like phenolic) should do better than the best thermoplastics. But they are hard to process

    Sticking with brass probably makes the best sense.

    Ed Bianchi

    PS - I always wondered if the name Ryton was a due to a marketing Joe trying to be clever and trendy. Ryton/Right On! Get it? Yeah, corny.
    Last edited by HO RacePro; 10-22-2020, 08:28 AM.

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    • #3
      Kapton is another polymer that can take a lot of heat. https://en.wikipedia.org/wiki/Kapton I see that it can be used in 3D printing.
      I'm guessing that the very fact that these polymers have very high melting points make them unsuitable for injection molding. Possibly either polymer could decompose, or at least be degraded if it was heated enough to melt. Other factors like shrinkage upon cooling would have to be taken into account. Gears could also be CNC machined from round stock, possibly neither of these polymers is easy to machine.
      There is one more issue with pancake top gears, they are thin so they would be more likely to slip on a motor or cluster gear shaft. Johnny Lightning/Auto World Thunderjet cars have splined cluster gear shafts.
      Last edited by RichD; 10-22-2020, 11:16 AM.

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      • #4
        You can buy brass gear stock in rod form, and slice off gears like baloney.

        Ryton is a thermoplastic and can be injection molded. Those CPU circuit board mounts had incredibly thin, fine details, all molded in one shot. The fact Ryton could do it had a major impact on computer technology. Expensive and fussy? Almost certainly. But at least there was one thermoplastic that could do the job.

        'Course that doesn't mean Ryton would be a good gear material. Gears have to stand up to rubbing wear and impact stress -- very different requirements than for a CPU mount. Nylon, especially Nylatron -- a grade that includes a solid lubricant -- is a commonly used resin for injection molded gears.

        As for machining, paper-reinforced phenolic thermoset has been a common material for machined gears. Though gears as small as the T-Jet armature pinion gear would probably not be successful in phenolic. The paper reinforcement might not work well in parts that small.

        I didn't think Kapton would be a candidate for this job. I've only ever seen it as a film used to insulate electrical wiring, or for flexible circuit 'boards'. I'll do some research, just out of curiosity. The internet is my friend.

        Ed Bianchi

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        • #5
          Well, apparently Kapton (polyimide) is a true thermoplastic, and can be injection molded and 3D printed. Who knew?

          Kapton film has a checkered history as an insulator for aircraft wiring. Harsh environments and rubbing/vibration can cause it to fail -- which it has done, causing crashes and fatalities. As a result it has been replaced in older airframes at considerable expense.

          It is commonly used as an insulator for magnet wire, where it has had much more success.

          It looks like it can be injection molded into gears. Scroll down to see the photo in the link below:

          https://www.satplating.com/plating-plastics/pi/

          Ed Bianchi

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          • #6
            Originally posted by HO RacePro View Post
            You can buy brass gear stock in rod form, and slice off gears like baloney
            Ed Bianchi
            I recently got a quote to produce the T-Jet brass gears here in the US. $4 to $5 per gear...not per set, per gear! And you had to get 10,000 to get down to about $4 a piece. That was shocking.

            Joe

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            • #7
              Ouch! You can buy CNC gears for not a lot more. Ordinary brass pancake gears are made by pulling rod stock through a die to form the teeth then taking off thin slices and drilling the holes. If it was me I would not trust a plastic armature pinion gear, but it might work.

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              • #8
                Maybe you could have a 14-tooth pinion 3D printed...
                then I would have it tapped with a fine thread size and have the armature threaded with the same fine thread size.
                (BTW - it’s not the heat it’s the humidity....)
                Last edited by keven; 10-24-2020, 01:26 PM.

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                • #9
                  Originally posted by RichD View Post
                  Ouch! You can buy CNC gears for not a lot more. Ordinary brass pancake gears are made by pulling rod stock through a die to form the teeth then taking off thin slices and drilling the holes. If it was me I would not trust a plastic armature pinion gear, but it might work.
                  So the CNC T-jet gears from RT-HO and FLR. Are the teeth on those gears fully CNC cut, or do they just use a CNC baloney slicer, to use Ed's term.

                  I had always assumed they were fully CNC cut teeth, but I suppose one could use a CNC machine to slice and drill a gear extrusion and still rightfully call them CNC cut. But they would not be as cool.

                  Now that I think about it, $6-$8 per gear, at T-jet cottage industry volumes, is pretty good if they are actually milling every tooth from bar stock.

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                  • #10
                    Originally posted by HO RacePro View Post
                    I think you are right about the heat issue.

                    The biggest issue might be maintaining a press-fit on an armature shaft. When the shaft heats up and transfers heat to a pressed-on gear differential rates of thermal expansion come into play. Any plastic part expands more when heated than steel will, and Ryton is no exception. Combined with any softening of that plastic due to heat there is a good chance the press-fit would fail under high torque.
                    Just a curious question - when a plastic gear gets heated and expands, does the center hole get larger or smaller? In other words, does the gear not only expand outward but inward as well? If it expands inward then it shouldn't lose it's grip on the armature unless the plastic also gets soft.

                    Joe

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                    • #11
                      GCS -- When gears, or any other item, are heated ALL dimensions get larger. That includes the hole size.

                      In industry this effect is often used to assemble close-fitting parts. If, for example, you want to install a ball bearing in an aluminum wheel, you machine the hole in the wheel to be a few thousandths smaller than the ball bearing. Then you heat the wheel and drop the ball bearing into the enlarged hole. When the assembly cools the ball bearing will be clenched tightly in the wheel.

                      You can use this in reverse -- chill the ball bearing in, say, liquid nitrogen (−196°C; −320 °F). Drop the shrunken bearing into the hole. When the bearing warms up it will be tight in the wheel.

                      The fact that plastics expand/shrink more than steel with changes in temperature mean that a pressed-on plastic gear will get looser when the armature assembly heats up. Couple that with the fact that the plastic will get softer when hot -- that press-fit is likely to fail.

                      *************

                      I find it ironic that we are talking about tiny brass gears costing 6 to 8 USDollars each, when back in the day I could buy a ready-to-run T-Jet for US$2.98.

                      Yes, I am as old as dirt. Antique dirt.

                      Ed Bianchi

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                      • #12
                        Yes, but you are not laying in the dirt! Be well.

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                        • #13
                          Yes agreed, no dirt!

                          The available shaft length doesnt provide enough cross section for a plastic gear to bite. There's very little room left on the ceiling of the design. While plastic gears abound in many hobby applications, any where torque is involved, we see a hub, a shoulder, or or enough cross section in the gear's ID to get a bite.


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                          • #14
                            Originally posted by HO RacePro View Post
                            I find it ironic that we are talking about tiny brass gears costing 6 to 8 USDollars each, when back in the day I could buy a ready-to-run T-Jet for US$2.98.
                            If we pick say 1968, $2.98 is equivalent to $22.29 today. That's very close to what a new,T-Dash with body or AW Ultra-G cost. So things haven't really changed much.

                            Plus, Dash sells a full set of gears for $5.99, with cluster shaft. That's only $0.80 in 1968.

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                            • #15
                              I was just considering the following...inline chassis use a plastic pinion gear mounted directly on the armature shaft. The pinion gear on an inline is basically the equivalent of a plastic armature gear on a pancake chassis. I realize the pinion is longer and has more "grip" on the shaft, but the inline armature is going to get just as hot as a pancake armature.

                              So why does is a plastic pinion gear workable but a plastic armature gear is not?

                              Thanks...Joe

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