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Thread: Gear Ratio Question

  1. #11
    I can't speak for others. But I aim for 20A per mm². I also rather double up thinner wires than go up a size. Thinner wire solders easier, is easier put un corners and so on.

  2. #12

  3. #13
    Thanks for the link but, I have the drive gear ratio figured out, now I'm trying to figure out what gear ratio for the weapon.

  4. #14
    Quote Originally Posted by ACEGUY View Post
    Thanks for the link but, I have the drive gear ratio figured out, now I'm trying to figure out what gear ratio for the weapon.
    For a very rough estimate, use the Run Amok Spinner Weapon Calculator. Bear in mind that it's only as good as the information you put in.

    Let's take, for example, the following:

    - 457.2x101.6x25.4 spinning bar (18'' x 4'' x 1'' because most of the online calculators are American save for this one, plus bars are a bit more simple than discs to work out)
    - Ampflow A28-400, as you suggested.
    - 8s Battery (29.6V)
    - Mild Steel Material

    Thanks to the Ampflow website, we know that the Ampflow A28-400 has the following characteristics:
    Diameter (inches) 3.0
    Length (inches) 6.7
    Peak HP 4.3
    Stall Torque (oz-in) 3560
    Efficiency 83%
    Nominal Voltage* 24V*
    RPM @ 24V 4900
    No Load Amps 4.4 4.4 2.1 2.5 3.2 2.1
    Resistance (Ohms) 0.044 0.064 0.076 0.081 0.089 0.190
    Kv (RPM/Volt) 205 257 189 287 240 237

    We know that our battery puts out 29.6V, and the Kv (RPM/Volt) is 205 - which gives us a no-load RPM of 6068, assuming 100% efficiency. In real terms, again helpfully provided by Ampflow, the efficiency of that motor is 83%, which gives the no load RPM as

    5036RPM (rounded down)

    Pop that into the calculator with a 3:1 gear ratio and you'll see that the bar above will give you a spinning bar at 1679RPM, with a tip speed (which must be under 250MPH for Robot Wars) of a hair under 90MPH and with a kinetic energy of 2.5ish KiloJoules of energy at the tip of the bar. You'll want a fast spin up time (larger gear ratio, such as 5:1) so that your weapon is up to speed before your bot hits someone else in the arena.

    2.5KJ is not enough for a heavyweight. It's barely enough for a featherweight.

    (as a guide, the feather weight that I'm designing at the moment has over 15KJ of energy)

    Hope this helps

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