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Gearboxes usually have a reducing ratio like 200:1, which means that they take a high-speed input and convert it into a high-torque output.

Would they work if I connected them in reverse?

Is there an approximate ratio at which a human could no longer power one in this configuration (without the use of a large lever)?

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    $\begingroup$ counter torque as a result of friction on the high-speed side will result in a pretty high torque requirement for the low speed input (which could easily exceed the low speed shaft's breaking strength) $\endgroup$ Aug 4, 2016 at 9:02
  • $\begingroup$ Are you saying that at high enough ratio between input and output gears the torque required to move the output gear would mechanically break the system? $\endgroup$
    – andrey
    Aug 4, 2016 at 18:04
  • $\begingroup$ It might break the system remeber small change in your end is a big change in the other. $\endgroup$
    – joojaa
    Aug 4, 2016 at 21:52

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It depends on the type of gearing. Gearboxes for winches and the like typically are designed (for example worm gears in the drive chain) so that they can not possibly be turned in reverse.

On the other hand a manual car/truck gearbox gets pushed in reverse every time you downshift

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Generally yes, if you use the original output shaft as the new input shaft then you would be achieving the opposite (i.e. converting high-torque input to high-speed output).

In answer to your second question, it depends on the value of torque. Theoretically if a lever is long enough you can "turn" anything. Say you limited the lever length to 1m and the fact that a human can probably utilise their whole mass with gravity plus extra effort (like jumping) to produce around 1000N of force, then they could produce 1 kN-m of torque. I'm just speculating without extra detail.

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  • $\begingroup$ How would you turn that maximum torque figure into gearbox ratio? $\endgroup$
    – andrey
    Aug 4, 2016 at 18:05
  • $\begingroup$ It depends on the torque or speed that you want on the ouput shaft. There's too many unknowns in your question but it sounds like your gearbox will reduce torque and increase speed. The factor by which speed is reduced is the ratio. Say the human was rotating the input shaft at 30RPM and the ouput shaft was rotating at 300RPM then the gear ratio is 30:300 which simplifies to 0.1:1. $\endgroup$
    – pauloz1890
    Aug 4, 2016 at 21:54
  • $\begingroup$ @andreyg without knowing internal friction its hard to say anything. If the system has very low friction you could move a lot. Problem is that you are multiplying the opposing end friction forces by the gear ratio. But we do not know the frictions so we can not speculate. Th biggest problem with setups like these is that it takes very long to get the dynamical masses going. $\endgroup$
    – joojaa
    Aug 4, 2016 at 21:59

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