Our team is trying to use a 500:1 reduction on a bag motor in as little space as possible. Would we be in danger of damaging our gearboxes if we attempted to create a 500:1 reduction with two 10:1’s and a 5:1 VersaPlanetary bolted together? We know we can get the same effect by adding a sprocket, but we’re trying to conserve space.
Thanks for the help!
That reduction is above the rated specs for a VersaPlanetary with a bag motor.
Check out their load rating guide: https://content.vexrobotics.com/vexpro/pdf/VersaPlanetary-Load-Ratings-Rev4-20161121.pdf
Check out the load ratings guide
A VP would not survive 100:1 + a 5:1. You could do a 100:1 then use a 5:1 sprocket reduction instead, though.
Looks like I’m too slow
If you have the space to add the extra stage to the VP but can’t afford the large sprocket or pulley needed to get a 5:1 reduction after the gearbox, the document linked above does say you can use reductions up to 324:1 (note that you should put the largest reduction stages first and the smallest last) and then do the rest with chain, belt, or gears. A reduction that large feels iffy, but if you trust the document you can go for it.
Do you really need 1750 in-lbs stall and 26 rpm free speed? Even if you’re using this for a robot lift, that’s a 10" diameter pulley you’d be driving if you want to be anywhere close to correctly geared. Are you sure you really need 500:1 and not, like 100:1? or 200:1?
You could also do a 250:1 and a 2:1 sprocket reduction.
Question though, when you say smallest reductions “first” do you mean closest to the motor or closest to the output shaft?
That’s probably the most convenient choice.
Sorry, that wasn’t super clear. I’m defining first the same way VEXpro does when I say “first stage”: closest to the motor. The last stage, the one closest to the output, carries the most torque. For reasons that are somewhat intuitive but honestly not crystal clear to me (and apparent from VEXpro’s tables), putting a larger reduction stage in the higher-torque end of the gearbox is not advisable. I want to say it has to do with the fact that the smallest gear you’ll find in a planetary gearbox is the sun gear in a high-reduction stage (even smaller than the planet gears in a low-reduction stage, of which there are also four instead of one), and it’s better to put this small gear in the lowest-torque part of the system, but don’t quote me on that.
500:1 can be safe if you’re not planning on stalling it out. If you want a controllable arm that’s not in danger of stalling, you can go pretty high as long as you don’t exceed the 120ft-lb limit on the output and make sure not to use a 10:1 stage closest to the output.
That being said, I personally wouldn’t use a 3-stage VP (or a BAG motor for that matter). 775pro through a 70:1 or something followed by a couple gear/sprocket reductions would be my preferred method.
It may be fine if the reduction is for speed. It’ll fail if the reduction is for torque.
In other words: If you just need it slow, OK, but if you need it strong too, nope.