Quote:
Originally Posted by DarkRune
Hi guys,
My team's running a small scale build season in the off season, and we're going to be using mini CIMS for the pickup. The necessary ration is 20:1, but I haven't really dealt with gears before. Can any of you guys help me? Or point me in the right direction?
Thanks,
Dylan
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Gear ratios are quite simple. To gear 20:1 you state for every 20 rotations of the input gear, you want 1 rotation of the output gear.
It is also true that for a single stage reduction (2 gears, one input, one output) the Output gear must have 20 times more teeth than the input. If the input gear has 12 teeth, the output must have 240.
240 teeth is a lot. If VexPro carried a gear that size, it would be a whole foot in diameter.* That is unrealistically big, and not surprisingly, they don't sell one.
What you will need is a 2 stage reduction. That is, 2 sets of gears that, in total reduces the rotational rpm by a factor of 20.
In essence, instead of having one ratio of 1:20, you have 2 ratios, let us say 1:4, and 1:5. The CIM enters the gearbox (let's assume 5000rpm for ease), and running through the first gear ratio, is reduced from 5000 by 1/4 to 1250RPM. That gear drives a shaft, and consequentially another gear, into the 5:1 gearbox. That 1250rpm gets a reduction to 250rpm, which, what do you know, is 1/20th of the starting rpm, thus a 20:1 ratio.
There are several combinations that can achieve this, such as 10:1 and 2:1, but the premise is the same.
For your application, another great option is a planetary gearbox. The internal architecture is different and hard to easily explain, but the same purpose is achieved. VexPro sells a very good customizable planetary gearbox set that will easily get you the 20:1 you are looking for, at a reasonable price, but you will have to slightly and permanently modify your motor to work in it.
There aren't a lot of options when it comes to non-planetary reductions reaching as high as 20:1. But here are 2 links to popular planetary gearboxes used in FRC.
The
VexPro option, cheaper but requires some modification.
The
Banebots option, more expensive, and doesn't have exactly 20:1 like you want, but requires no modification