Ratio and gears

[jamesmatheson]

Hi there can anyone tell me the ratio of this https://grabcad.com/library/planetary-gear-set-5
I need to have as many ratios as possible to know what motor to put in the middle

[fred_dot_u]

According to https://woodgears.ca/gear/planetary.html website, the formula is (sun)/(ring + sun) which in this case is 27/(27 + 75) which calculates out to 3.7778 to one, inversely, one to 0.2647

[jamesmatheson]

Ratio means 15:1 that is ratio
Inversely is not ratio and 0.267 is not correct at all.

[fred_dot_u]

The ratio calculates out to 3.778 to one, which would mean that one rotation on the input would generate 3.778 on the output. The inverse means that one rotation on the output provides slightly more than one quarter rotation on the input. A ratio can be expressed in any manner, not necessarily relative to unity. A five to three presentation is a legitimate ratio as is a three to five ratio. It is common to reference to unity and it is acceptable to reference unity on either side of the representation.

[Roxy]

Here is a really neat demo of planetary gears. With both a hand crank and a bit to put into an electric drill to make it go fast.
I suggest you do the 4 stage set. And if you print multiples of them in different colors, it looks really nice to see all the different colors doing their thing at each stage.

planetaryAssembly.zip

[fred_dot_u]

That assembly is explained and posted in Thingiverse:
https://www.thingiverse.com/thing:23030

One of our makerspace members built this with about a ten inch span. The ratio was astonishing. He had been spinning it with a hand crank for about a month and got one tooth progress on the final stage. Anytime someone new came into picture, he would be enlisted to crank for as long as tolerable.