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Old 12-21-2013, 01:03 PM   #1
DakarNick OP
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Gear ratios

I want to increase a 20 rpm servo's speed using gears.

If I attach an 84-tooth gear to the servo and drive a 28-tooth gear with it, that multiplies the speed by 3.

If I take the driven 28-tooth gear and attach an 81-tooth gear to it (fixed with commom shaft), then have that drive another 28-tooth gear that gives me 2.9.

Please correct me if I am wrong but the first stage, 20x3=60 rpm.

The second stage, 60x2.9=174 rpm.

Correct?

Thanks!
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Old 12-21-2013, 01:44 PM   #2
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That sounds right, but is making my head hurt a little.
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Old 12-21-2013, 01:52 PM   #3
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Looks right...but are you aware that integer gear ratios like 3.0 are undesirable if they can be avoided? (integer ratios can cause noise, vibration, accelerated wear, etc.) Of course sometimes an integer ratio is required, like 2:1 for a four stroke engine cam drive, etc...

http://everything2.com/title/hunting+tooth

victor441 screwed with this post 12-21-2013 at 08:11 PM
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Old 12-21-2013, 02:25 PM   #4
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I think it will be ok because the odd number of teeth on the gear means that they will never mesh with the same teeth on the other gear.
I think its called hunitng but I cant remember for sure.

For mechanisms that drive a lot / turn often, if the gears always mesh at the same teeth, that is when wear and damage will occur. by having the gears with different numbers of teeth, the damaged / not perfectly machined teeth will not mesh with the same teeth on the other gear and so there will be no localised damage which would then lead to vibration, wear and so on

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Old 12-21-2013, 04:06 PM   #5
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Thanks, guys. I was unaware of the integer/hunting problems. I could replace the 84 with another 81 and that would be 2.9 for both. I'll check out the link, thanks!
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Old 12-27-2013, 03:43 PM   #6
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I was looking for something else, came across this book which I hadn't remembered, foudn teh information in the book about hunting gears and here are the words.

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Old 12-27-2013, 04:18 PM   #7
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Ah The Machinery's Handbook. Wonderful reference.
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Old 12-27-2013, 08:12 PM   #8
DakarNick OP
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Thanks, Hay Ewe! Take a pic of every page so I can read it

I found a few gearsets to use that won't necessarily have a hunting tooth but will also not be an integer ratio.
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Old 12-28-2013, 03:00 PM   #9
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Yep, Machinerys Handbook, 11th Edition 1943.
Total pages 1815
Pages just in the Gears section 288

You gonna hafta hunt down your own copy DakarNick!



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Old 12-28-2013, 03:42 PM   #10
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I just found it on kat.ph
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