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  1. #1

    3D printed gears for a handcar/railcar?

    Every year a friend of mine and I compete in a wacky boat race. This year we wondered if we could build a boat using the mechanism of a handcar railcar to power a propeller. My question is would 3D printed gears be able to support two guys working a hand pump powering a propeller underwater? If so, what ratio might the gears be able to support? I have no engineering or printing background but after reading a few threads here I thought this would be a good place to ask.

    Here is a video of a handcar driving mechanism
    https://www.youtube.com/watch?v=VDdp8sh4G3U
    The difference in a handcar and our hypothetical boat is that we would be oriented east and west on the boat rather than north and south like on a handcar as we would need the second gear to turn a propeller shaft behind us rather than an axle and wheels to our side.

    Thanks for any help!

  2. #2
    Super Moderator curious aardvark's Avatar
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    the answer is yes - with caveats.

    You'd need pretty big gears, so a lot of machines would probably not have large enough build volumes and you'd most probably need to make them from nylon. Otherwise in the heat of competition I can see things breaking.
    That said, if you made them big and solid enough you could probably get away with pet-g. I did make an 8mm diameter 1mm thick gear with 12 teeth that I couldn't break between my fingers. So an 8 inch diameter 4 inch thick gear would probably require a sledge hammer. As long as you got the teeth design right, yeah it should work. A propeller doesn't need as much torque as a loaded axle.
    Use metal bolts for the winding part. That you absolutely don't want made from plastic.

    Unless you own or know someone with a 3d printer - it would work out pretty expensive from a print service.

    Given that you only need 2 gears, might be easier to make them from a sandwich of plywood with thin metal sheet either side for reinforcement.
    That way you could make them with nothing more than a hack saw and a metal file.
    Last edited by curious aardvark; 04-07-2017 at 01:43 PM.

  3. #3
    Engineer Marm's Avatar
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    You're gonna need a heavy fly wheel to store the energy between pumps. On a rail car the fly wheel is the car itself moving down the track, keeping the wheels turning, and allowing the operators to maintain a smooth rhythm, slowly increasing speed..
    If you replace the car with a prop, the first downward push would quickly spin the prop, causing it to cavitate and produce no thrust. Also, because there is little to no rotational resistance on the prop, the handle of the non pusher will fly up and smack him in the mouth, which will likely knock him overboard, and attract sharks, which is usually grounds for disqualification. If he does survive the first pump, his next downward pump would again spin the prop, again resulting in cavitation and reduced thrust. Props work best under constant rpm's.

    So, in your diagram, if you replace the axle with a large fly wheel, that will offer the resistance you need to safely and efficiently pump the mechanism. Add a belt drive from the wheel to the prop shaft,and that will smooth out the rpms to the prop, resulting in longer duration and smoother thrust, drastically increasing your efficiency. With a belt drive, it would be trivial to change your standard prop RPM's too. After seeing what RPM's on the flywheel you can easily maintain for the length of the race, you can then change the pulley ratios on the fly wheel and prop shaft to suitably match the optimum RPM's on the prop you select.

    An old car wheel (tire and hub) should be sufficient for your purposes as a fly wheel. If you really want to make the fly wheel more efficient, fill the tire with concrete. And as it's a steel hub, you can easily weld on the axle to the hub, and the pulleys and gears from there. I'm assuming you were thinking 3d printing the gears to save weight, but if you add a flywheel (which you kind of have to), it will make weight saving gears kinda of pointless. Minus the welding, you can probably find everything you need for this at a junkyard for under $50.

    And since it's a wacky boat race, I'm assuming you have to build your own hull. That's gonna be an issue with a concrete tire on board. I'd go with plywood with well sealed joints.

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