# OT - Things you might never need to know.

Gears are probably the most efficient method of mechanical power transmission we have today, and much of the reason for that is the shape that gear teeth are ground to.
Unlike the triangular or rectangular teeth you see on comical depictions of gear trains, gear teeth are actually ground to a shape called an "involute".
If you have a string wound around a cylinder, and you keep that string tight as you unwind it from the cylinder, the shape of the curve the end of that string makes is called an "involute". It's a shape similar to a spiral.
The unique advantage of involute gear teeth is that when two involute gear teeth meet, they actually ROLL over each other. That is, the point of contact between the two teeth is a ROLLING contact, not a sliding contact.
And it's this rolling contact made possible by the involute shape of the gear teeth that results in negligible loss of mechanical power to friction, and therefore the excellent mechanical efficiency of gear trains.
If gear teeth slid across each others surface, as is depicted in comical gear trains with rectangular teeth or the "peg" shaped gear teeth you might see on a TV show like "Gilligan's Island", then much of the mechanical power would be lost to friction.
Watch the gear animation at the top of this web page: 'Involute gear - Wikipedia, the free encyclopedia' (http://en.wikipedia.org/wiki/Involute_gear)
The black dot is the point of contact between any two gear teeth, and it's the involute shape of the gear teeth allows them to roll over each other, thereby transmitting power with negligible friction losses.
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nestork

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On 09/07/2014 10:27 AM, nestork wrote:

Thanks for the info
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'philo*[_2_ Wrote:

I studied cams and gears as part of my 4th year mechanical engineering course years ago at the U of Manitoba. I've forgotten most of what I learned back then, but I still remember the basic principles. I guess that applies to everything. No matter what you learn, you eventually forget the details, but you always remember the principles.
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nestork

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On 09/07/2014 12:42 PM, nestork wrote:

Well , I was an electrical engineering student so never studied gears... but I did learn that most of the principles of electrical engineering could be applied to mechanical engineering (and visa versa)
Perhaps gears and transformers are analogous as they transfer and convert power efficiently.
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On Sunday, September 7, 2014 2:23:02 PM UTC-4, philo  wrote:

I was a math major but worked evenings on a machine that cut the teeth into the gear blanks. Very interesting process.
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On 09/08/2014 11:51 AM, Pavel314 wrote:

A few of my customers were gear manufacturers so I got to see them being made.
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On Sun, 7 Sep 2014 17:27:06 +0200, nestork

I've never need to know that. But like a lot of other facts, it is interesting to know. Thanks for posting it. I'll pass it on to a couple of others that may find it of interest too.
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On Sun, 07 Sep 2014 08:27:06 -0700, nestork

I feel stupid today, but to me the Wikipedia .gif shows 'constant' speed transfer, NOT very low friction.
It looked like a lot of sliding along each tooth, but importantly there was absolutely NO variation in rotational speed for each gear's shaft.
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On 9/7/2014 11:27 AM, nestork wrote:

Years ago, I noticed "hypoid gear oil" for sale at the auto parts. I asked he twenty something behind the counter was hypoid gear oil all about. He said it had to do with the viscosity. I asked a mechanic who knows a bit out these things, and he explained that the gears were cut in the shape of a hyperbola, and that made the energy transfer more efficient. Hmm.
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Christopher A. Young
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'Stormin Mormon[_10_ Wrote:

Yes, that's a common experience. If you ask any store employee a question, and they don't know the answer, they'll think up one for you.
It's a service they provide free of charge so that their customers don't have to think up an answer themselves.
I kinda feel embarrassed offering up this information on involute gear teeth, and then finding out I was wrong, but I know that no one in here believes that I had anything but the best of intentions.
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nestork

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On 9/8/2014 9:30 AM, nestork wrote:

We know you well enough to forgive small mistayks.
Wish the guy was still there, I'd go ask for a hypoid-dermic syringe.
http://www.ffx.co.uk/Content/images/tools/AK46.V2.jpg
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Christopher A. Young
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On Mon, 08 Sep 2014 06:30:39 -0700, nestork

Don't STOP! I saved that little animated gif from Wikipedia. And for the first time, I marveled out just how difficult true gear design must be. Think about the vibration potential as the gears wear and the transferring shaft speed starts to become modulated. Arrgg! And I thought electronics was fraught with peril.
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