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  • #31
    The smaller axle will make the drive a bit more efficient by reducing the drag torque on the idler gear.

    The drag torque on the idler gear is the reaction force times its inner radius, so the smaller axle will produce the smaller drag torque. The difference in drag torque is proportional to the ratio of the two ID's.

    The reaction force is the force that would push the idler gear sideways if the axle was missing. It is the sum of the tangential forces on the teeth of the idler gear. One tooth is being pushed on by the armature pinion gear and another tooth is being resisted by the driven gear. Both forces very close to the same size and are pointing in the same direction -- sideways. It is the job of the idler gear's axle (or hub) to resist those forces.

    Ed Bianchi

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    • #32
      How about the durability and longevity of plastic verses brass? I guess the only examples we have are to compare the T-Jets with all brass gears to AFX/X-Traction and JL/AW T-Jets. Personally I have never had gears wear out but I probably have not cumulatively run my cars enough to wear out the gears, especially my JL cars.

      Thanks...Joe

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      • #33
        Just an FYI. I spoke with someone who is testing a T-Jet style chassis using all plastic gears, including the armature gear. So far, testing has shown none of the press fit gears are slipping and/or causing any issues. So maybe an all plastic gear set is feasible and quite workable.

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        • #34
          Always great to hear from you Grandcheapskate. I hope Santa was good to you.

          The problem with plastic gears may be long-term retention. Plastics tend to creep -- slowly deform to relax the stresses on them. That is why plastic parts are usually designed to snap together rather be press-fit. Snaps only take momentary stress while press-fits have to maintain their stresses for the life of the assembly.

          The use of metal gears instead of plastic is one reason why T-Jets that were assembled almost 60 years ago still run.

          Ed Bianchi

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          • #35
            Originally posted by Grandcheapskate View Post
            Just an FYI. I spoke with someone who is testing a T-Jet style chassis using all plastic gears, including the armature gear. So far, testing has shown none of the press fit gears are slipping and/or causing any issues. So maybe an all plastic gear set is feasible and quite workable.
            Excellent. Keep us posted.

            Always love a convention being tested.

            Merry Christmas to all.

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            • #36
              Originally posted by Axle View Post

              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.
              RT gears are fully CNC cut gears. The machine cuts the outside and the inside in the same settup. So the concentricity is as good as you can get it. This is the most important dimension on these types of gears.

              Also, the material is not brass, as the originals were. It;'s a type of bronze that is used in the machine tool industry because i has its own lubrication kind of built in, as it were.

              It also is very good at long term wear.

              Yes, they're expensive, but they are almost always a perfect mesh, they don't wear out, and they are slipperyer than stock gears.

              As far as plastic gears go, I would be interested in a machined teflon gear set. I think it would be the ultimate. you would probably need to use steel or brass inserts for the press fit applications, but you would not need to lubricate the geartrain. For that matter, while we're at it, why not an entire chassis, and gear plate from teflon?

              Now you're talkin'

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              • #37
                Originally posted by HO RacePro View Post
                The problem with plastic gears may be long-term retention. Plastics tend to creep -- slowly deform to relax the stresses on them. That is why plastic parts are usually designed to snap together rather be press-fit. Snaps only take momentary stress while press-fits have to maintain their stresses for the life of the assembly.

                The use of metal gears instead of plastic is one reason why T-Jets that were assembled almost 60 years ago still run.

                Ed Bianchi
                A/FX and magnatraction came with plastic gears and are still running today. Modern inline chassis from WUZZARD, Viper, and BSRT all come with press fit pinions and crown gears and the majority of racers use them with no problem. Many racers use smooth axles and the majority use smooth armature shafts. These cars have a lot more torque than a t-jet so I don’t see why gears of the same material would not work on T-Jets.

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                • #38
                  Look at just about any 1/32nd or 1/24th aftermarket plastic slotcar crown gear and you will see they have a brass or aluminum hub. Why? See my post #34, above.

                  We've been all over the question of plastic T-Jet armature pinion gears. Sixty years on it can be assumed that it has been tried. Probably more than once.

                  Engineers and designers are always trying to wring a penny out of production costs. The fact plastic armature pinion gears aren't standard means it didn't work. Probably a combination of small contact area and heat driving plastic creep, leading to loss of press-fit stress and failure.

                  As for Teflon -- I've never seen it used for gears. Too soft. Won't hold tolerances. Will deform too easily.

                  The best plastic I know of for gears is Nylatron -- an industrial grade of Nylon specifically designed for gears, bushings and other such applications, especially applications that have to run dry. It has its own lubricant alloyed into the tough, strong, wear-resistant Nylon. And yes, it is commonly used for slotcar gears as well as industrial gears.

                  Bronze is very commonly used for industrial gears, but it spends its entire life immersed in a bath of gear oil. You won't find it used dry because without the lube it will wear out quickly. And bronze is expensive.

                  Is bronze better than brass for T-Jet gears? Interesting question. Which would will wear faster? Dunno. Which is easiest to machine to close tolerances? Dunno that either.

                  The CNC guy probably knows best. I'd be interested to hear how he justifies making T-Jet gears out of expensive bronze when brass is so cheap and machinable. I'm always willing to learn.

                  Ed Bianchi
                  Last edited by HO RacePro; 12-25-2020, 12:13 PM.

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                  • #39
                    I have 100s of crown gears from 20 or so HO concerns, all of them are all-plastic.

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                    • #40
                      I’m not aware of any plastic HO crown gear that has a brass or aluminum hub. Who makes them?

                      I think RT-HO justifies using the material he does because it produces a product that is pretty much the standard for Fray style cars.

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                      • #41
                        I said 1/32nd or 1/24th aftermarket crown gears... I've never seen an HO crown gear with a metal hub.

                        But crown gears do not experience elevated temperatures like armature pinion gears do. Nylon starts to soften significantly around 60C (140F). By 100C (212F) its mechanical properties are shot.

                        Ed Bianchi

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                        • #42
                          I must of read your post before you edited it. In any case the majority of HO crown gears and pinion gears are some form of plastic. It is not uncommon to run these in temperatures up to 150 F with no issues. Beyond that I have run as hot as 200 F and not melted, but I prefer to stay under 150 I’ve never seen a pancake arm get that hot unless it soon goes up in smoke.

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                          • #43

                            Agreed, the whole world is plastic. One of my earlier points was that the engineers use tricks to extend the cross section, and or dog the gear to the shaft; so that it permits the use of plastic components while maintaining a safe margin of reliability.

                            Rather than re-inventing the wheel, I spent some extra time with the currently available plastic offerings, and assembled a nicely free wheeling AW chassis from spares.

                            Otto Whirled uses a gnarly 6 spline arrangement on either end of the cluster shaft, to secure the driven gear and the pinion.








                            I simply moved a good pinion to the armature. Even with cleaned acetone, the press fit is "gooshy" as you would expect on a smooth shaft. The gear cross section is .060". Typically the mesh is all over the road.

                            Certainly it's doable in the short term, putting your name on thousands is quite another matter. I purposely built her loose. The Mini has been completely "Krylon" after a coupla hundred laps on 24 volts. No drips, no runs, no errors. Plastic creeps, so do lubricants. Heating and cooling cycles exacerbate these traits. If it survives a stock set up for any duration, I have a Yellowjacket 3 ohm crank by Srigs to test the outer limits.

                            Tossing armature pinions isnt a new phenomenon. Thats why we occasionally see them soldered in hi-po aplications

                            ***********




                            Last edited by model murdering; 12-25-2020, 08:07 PM.

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                            • #44
                              Originally posted by HO RacePro View Post
                              I said 1/32nd or 1/24th aftermarket crown gears.
                              You clearly edited the post, that is not what you said.

                              Now you have changed it, why do you think the bigger scale crown gears need metal hubs, when HO ones do not?
                              Last edited by NicoRosberg.; 12-26-2020, 02:21 AM.

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                              • #45
                                Originally posted by HO RacePro View Post

                                Is bronze better than brass for T-Jet gears? Interesting question. Which would will wear faster? Dunno. Which is easiest to machine to close tolerances? Dunno that either.

                                The CNC guy probably knows best. I'd be interested to hear how he justifies making T-Jet gears out of expensive bronze when brass is so cheap and machinable. I'm always willing to learn.

                                Ed Bianchi
                                It has been years since I talked to Rick, of RTHO fame, so some of this may have changed since then.

                                When I asked about materials, he said the reason he used a bronze type alloy was when he machined the 9 tooth pinions, they would split when pressing them on the cluster shaft when making them from free machining brass.

                                He may have changed the pocces since then, IDK.

                                As far as teflon goes, Ed is probably right about the softness thingy. But maybe coatings? I know that teflon coats aluminum very well, so maybe brass also?

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