Gear Ratio & TPA

Discussion in 'Gun Building, Modifications & Repairs' started by MeusVox, Jan 27, 2019.

  1. MeusVox

    MeusVox Member

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    I’m going to overhaul an ICS M4, and want advice for the lowest stress combination for my specifications.

    So, a 22 TPA motor can easily pull an M120 on a 12:1 set. The stress on the gears, however, gets to be pretty impressive.
    A 16 TPA motor can easily pull an M120 on an 18:1, and here the gears are experiencing less stress due to greater mechanical advantage.

    I want to make a build that, unless there’s a grievous unseen error, functions reliably without subsequent tinkering. I already have a gun that requires that.

    So, for the lowest stress possible, and achieving 20+ RPS with an 11.1v battery, what would you recommend?
    Any motor, any ratio, and brand.

    I already have a parts list for the rest of the gun.
     
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  2. Lefse

    Lefse Well-Known Member Supporting Member

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    M120 spring on 12:1 gears is well within what they can handle, if they're shimmed properly they'll last several years before you'll see any significant wear. Heck, I've gone as high as SP150 (M170 equivalent) on 12:1 ratio gears, no problems, they would wear out relatively quickly with such high load though.

    If trigger response and smoothness are a priority I recommend combining high TPA and low gear ratio, if you want higher efficiency and a tad more longevity you could do the opposite. In one of my AK's I'm running 12:1 ratio gears with an SP110/M120 spring at 30 RPS, and this is what I consider a "workhorse" setup that doesn't require much maintenance. I open it up once a year for a quick inspection and re-grease is needed, that's it.
     
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  3. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    Put in a good 40K motor on a 18:1 and you will hit 20rps and you don't need a 11.1V - 3S.

    I have a 25rps setup on a 2S 45K - 18:1 setup
     
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  4. BOA_SP3CT3R

    BOA_SP3CT3R Moderator Staff Member Moderator Supporting Member

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    13:1 gears+9 tooth bevel(15.17:1) gear and a 16-18tpa motor works quite well. Trigger response will be snappy, though RPS may be a bit higher than you’re wanting(27~33). The gears will last a very long time and run smoothly.
     
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  5. LightIsRight

    LightIsRight Well-Known Member

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    A 16 tpa motor on 18:1 ratio gears will easily be above 20 rps when using an 11.1v. The same setup in my MK18 is giving me around 17-18 rps on a 7.4v Lipo.
     
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  6. MeusVox

    MeusVox Member

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    So, replying to all of you.

    Trigger response is very important to me.
    I’m hearing the consensus is that, while a low ratio high TPA setup may be higher stress on the gears, that ultimately it’ll still last quite awhile.
    Is that correct?
    And for trigger response, you simply can’t get that from a high speed motor because of wind-up time under load.

    I ask because I’ve always followed the logic of high torque/low ratio is king.
    I’ve never played with any ratio over 15.17:1 (nine tooth bevel).
     
  7. BOA_SP3CT3R

    BOA_SP3CT3R Moderator Staff Member Moderator Supporting Member

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    Eh not really, low TPA motors can give excellent response when paired with the right gear ratio and battery. I’ve only done 2 low TPA setups:
    Star Wei 14tpa, 15:1, M120
    Custom 15tpa, 24:1 helicals, M140

    Both were quite snappy. The 15T+24:1 setup was extremely efficient, the other was...well, not incredibly efficient...
     
  8. MeusVox

    MeusVox Member

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    Do you have any numbers, or are these “by ear”?