> Do not remember why a double gear reduction is need?
> Too fast a motor? Too low power (or Torque)?
> How to calculate the ratio when double gear?
Single-reduction is used when the amount of reduction needed is small (2:1,
3:1).
When the amount of reduction you want is larger (20:1), and/or when the
shaft speeds are very high, you want multiple gear reduction, as there is a
physical limit to how much you can get in the way of reduction out of a
single set of gears without having some very large teeth trying to mesh with
a very small drive gear.
On the high rpm issue, you want a very smooth, low-friction gearbox, and you
get that only when the reduction is done in several small increments, rather
than one. A non-smooth gearbox at high rpms tends to heat up, and hot gears
melt off their grease, and then the hot gears get hotter, and then very bad
things happen, and gears break and turn into shrapnel, and someone loses an
eye, and the day is no longer any fun.
Another reality here is that engineers are hostages of both the gear
catalogs, and the impulse to always pick a gear from the "safe" middle of
the list of close-ratio gears, rather than go out on a limb with a
high-friction option. That way, if the design has an problem, a gear from
the middle of the list is easily replaced with an adjacent one, slightly
smaller or larger.
The exception is a worm gear, which, in a single step allows very high gear
reductions, and at low friction. Worm gears are very reliable and durable,
which is why you find them in so many toy cars. Toy designers have to make
highly reliable mechanisms, and at low cost.
How to calculate? Don't know the ratios? Don't even know the number of
teeth for sure?
Gotcha covered: http://geargenerator.com
Yes, the internet has everything, even gear simulators!
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