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planetary gear

planetary gear

planetary gear

is it possible to get a reduction in gear ratio, using the carrier as the input ,the reason being i need the sun gear to rotate in same direction as i turn the input
approx .75:1 or thereabouts  

RE: planetary gear

If you are talking about a conventional planetary gearset with one sun gear and one ring gear and ordinary idlers as planet gears (not a Ravigneaux and not a two-stage) then ANY input from the carrier with either the sun or ring locked, will drive the remaining element in "overdrive", faster than the input. If you want a reduction then you have to drive either the sun or ring and lock the other one, and take the output from the carrier.

If the sun gears and planet gears have the same number of teeth and you drive the ring gear and lock the sun gear, and take the output from the carrier, it will be a 1.3333:1 reduction. Is that what you are looking for? It's unclear, because by convention, ".75:1" is an overdrive, not a reduction.

RE: planetary gear

hi thankyou for the reply
my plan is to have an axle through the centre of the planetary gears with the axle attached to the planet carrier and the sun gear as the output
i would like to be able to turn the axle once(planet carrier) input and the output sun gear would turn 1.5 times
then i would also like to be able to change gear ,so that i could also have the option of a reduction for example i turn the axle (planet carrier once and the sun gear would turn 3/4 of a turn.

im not set on the above ides but my criteria is
the output must turn in same direction as the input ,and that i can get a reduction in speed and an increase in speed  by changing gear
im also not set on the ratio aslong i get a small reduction and a small increase ,
thankyou for your help, and your patience
im very new to planetary gears
regards jeremy  

RE: planetary gear

To get used to planetary gears, it helps to be able to visualise how they move.

Ever pushed a box along on some sort of logs or tubes used as rollers?
Notice how the rollers always come out at the back?

Put a pencil between two CD cases and you can slide the bottom one left and the top one right the same distance, and the the pencil just spins on the spot.

These are effectively the sortof movements that planetary gears make. With the curvature of the earth, we could even argue that rollers are plantary gears. Oops sorry if there is any accidental pun, the earth being a planet and all that!

If you could put a camera on your planet carrier, you would see one aspect of the motion clearly. However much the sun moves left of the carrier, the ring gear must move right. eg 100 teeth of movement left by the sun gear is 100 teeth right by the ring gear. But 100 teeth might be once round a ring gear and twice round for a sun gear, so the number of rotations differs.

If you fix the sun or ring gear, the roller analogy makes it easy to visualise.If you fix the planetary carrier, the CD box analogy makes it easy to visualise.

More complex motions are just combinations of the two.

Another easy motion to visualise is imagining the whole thing welded together. So regarless of number of teeth, it is possible to rotate the sun, ring and planet carrier equally.


From the roller analogy [imagine a fixed floor], you can see the rollers never move as far as the box. (The rollers move half as far.) Restated, the box moves further than the rollers. In Brian Peterson's more correct terminology ....
ANY input from the carrier with either the sun or ring locked, will drive the remaining element in "overdrive", faster than the input.

If I've managed to be of any help, maybe you can see that for yourself now. If not I could post some MPEG videos. One picture is worth 100 of my words, for sure. (bit of work involved so I am reluctant to do it.)

Is there any sort of "final drive" on your application that can be changed. Plantary gears are much easier if one of the ratios is 1:1 - the everything welded together anaology, ie the planet gears don't turn.

With VERY small planet gears, then like a box on rollers moving twice as far as the rollers themselves, a sun gear or ring gear could move about twice as far the planet carrier - a bit more or less in practice, depending on whether you used the sun or ring.

Then with a final output of 3/4 of that you'd have 1.5 times your speed (2 * 3/4 = 1.5). Or with everything rotating like locked together you'd simply have the output rotating 3/4 the speed of the input.


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