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Jonathan_hodgkins's avatar

Working with small plastic gears?

Asked by Jonathan_hodgkins (403 points ) January 17th, 2014

I am designing something that utilizes small plastic gears. Any idea about how to figure out the math and design in order to create a workable gear system?

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5 Answers

ETpro's avatar

As long as all the gears will mesh, it’s just a matter of counting teeth. For a simple example, if one gear has 10 teeth and a second has 20, then you have a 2/1 or ½ gear ratio depending on which gear is the drive gear and which is driven. Let’s say the 10-tooth gear is hooked up to a motor turning at 100 RPM. For each complete revolution it makes, it will advance 10 teeth on the driven or output gear, so it will take 2 revolutions of the drive gear to yield one revolution of the output gear. You’ll have an output shaft turning at 50 RPM, but since the total amount of energy is conserved, halving the speed doubles the torque (with minor losses for friction, of course).

If you reversed the situation and had the 20-tooth gear on the shaft of a 100 RPM motor, the 10-tooth gear would make 2 complete revolutions for each 1 the motor makes, so it would step up the speed to 200 PRM. But again, since total energy is calculated from speed and force, the torque of the motor would be cut in half.

This math carries on as you add more gears, so you could easily build a clock that rotates its minute hand 1 time in 60 minutes with a drive motor running accurately at 6,000 RPM and a set of gears that reduces that speed by a factor of 6000/1.

jerv's avatar

You can chain them too. For instance, for 6000:1, you’d likely chain three 10:1s and a 6:1 together since. 6 * 10 * 10 * 10 = 6000. If you do it right, you can get almost any odd divisor or multiplier you need. And I’ve seen a gear-differential used to do some calculations, often involving averages.

Of course, the gears have to have the right gear pitch to mesh. My old R/C car used 48-pitch gears (42 teeth per inch of circumference), so I couldn’t use the 32-pitch gears from my R/C dune buggy.

jaytkay's avatar

I don’t have specific advice, but I think you might you might find some relevant projects with detailed instructions on the web site Hack a Day .

Paradox25's avatar

I work as industrial mechanic, and I frequently work on hobbies using small plastic gears. In fact I’m currently working on a potential invention idea using small lastic gears. Whether or not the gears are plastic or metal spur, miter, worm or compound the math is still the same. Without getting too deep I’ll use a simple article demonstrating gear ratio calculations here.

Keep in mind that gears have three important characteristics, these being the amount of teeth (t), diameter and their tooth pitch (m). It is very important that regardless of other characteristics that gears in contact with each other have the same tooth pitch.

pleiades's avatar

I mean I’d measure the function of where the thing would be used then proceed to measure the actual thing

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