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Small 4-stroke, cordless drill gearbox, flywheel

  1. Sep 29, 2008 #1
    I would like to know whether a gearbox (reducer) designed for an electric motor is suitable for the same powered 4-stroke engine.
    The most common, and therefore inexpensive, gearboxes are on drills, either cordless or wired.
    A small 4 stroke engine gives a maximum power of 750 watts. Would a gearbox off a 750 watt drill be suitable? or does the 4 stroke engine only powering 1/4 of the time mean that a gearbox off a 3000 watt electrical tool is needed?
    I also assume that the flywheel mass will have some effect on this, could anyone explain what effect the flywheel mass would have?
    Literature on cordless drills don't generally show a wattage, for the larger ones a torque of 30 nm is shown. The small 1 hp (750 watt) 4-strokes have a maximum torque of 1 nm. As I am seeking a reduction of 27:1, which would give an output torque of 26 nm, I was wondering whether these gearboxes would be OK
    Many Thanks
  2. jcsd
  3. Sep 29, 2008 #2


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    What is your application?
  4. Sep 29, 2008 #3
    Bicycle, but I am aiming at the best assistance possible from 1 litre of fuel over 1,000 km. I need the right terrain, with just 2 gradients, 0.8% downhilll (unpowered, 90% of the time) and 5% uphill (powered, 10% of the time). With the right terrain this provides a constant speed of 21 km/hr, so the engine has increased the speed from 16 km/hr to 21 km/hr and overcome the tyranny of hills.
    I am also having trouble identifying a jump start clutch at the back wheel - I can't afford to consistently lose the energy driving a chain and half a clutch away from the bike hub. The average assistance provided is only 40 watts, so a loss of 10 watts is huge, a loss of 20 watts means you may as well not bother with the engine!
  5. Sep 29, 2008 #4


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    Just double check the speeds that both are required to run at.
  6. Sep 29, 2008 #5
    Engine at 4500 - 5000 rpm (BSFC of 408 g/litre). 26" external diameter bicycle wheel at 168.7 rpm gives 21 km/hr. Reduction needed 26.7:1.
    I can get 2.5:1 from the chain drive, if necessary. So I need somewhere in the range 11:1 to 27:1 from the gearbox. Single stage planetary's go up to 12:1, unsure what they are in drills.
  7. Sep 29, 2008 #6
    From a practical standpoint, the flywheel dampens the power pulses. Kind of a "shock absorber" between the crankshaft and gears.
  8. Sep 29, 2008 #7
    Just a thought... a gearbox from a drill is likely not rated for continuous maximum force. You might want to include a healthy safety factor into your working load.
  9. Oct 8, 2008 #8
    You may want to measure the bicycle wheel, the measurement on the sidewall doesn't usually reflect the actual diameter of a complete wheel. I just thought I'd point that out because that would greatly effect your calculations.
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