# Another gear question.



## Captain Jerry (Apr 20, 2010)

What's the difference in an involute curve and a radius? I mean, I KNOW the difference but over the approximately 20 degree arc of a gear tooth face, how closely does a radius approximate an involute curve? Or if that's not close enough, would an eliptic curve be better. I think I have read and reread every thread on this forum that discuss gear making (and there are a lot of them) but I haven't seen this question discussed. I'm asking because for the gears that I need, I have no original to use for a pattern so I will have to generate the pattern. I have worked out a method to accurately grind a perfect small radius or an eliptic curve with accurate radius but a true involute is beyond me.

As a second point to consider, does anyone know what gear cutting standards were used in the late 1800's. I think it is interesting that the American Hoist brochure of that period makes a point of stating that all fasteners on their equipment use "American Standard" threads. Not a word about gear form except to make the claim that they are so well made that they "could never break!"

Jerry


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## Dan Rowe (Apr 20, 2010)

The second point about gear standards is they were all company standards before 1916 with the foundation of the AGMA (America Gear Manufacturer Association).

It was common with early machines to use the cycloid form of gears for power gears. This type of gear is very good if a small pinion is needed to run a much larger gear. They are now mostly used by clock makers.

Dan


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## Maryak (Apr 20, 2010)

Captain Jerry  said:
			
		

> over the approximately 20 degree arc of a gear tooth face, how closely does a radius approximate an involute curve?
> 
> As a second point to consider, does anyone know what gear cutting standards were used in the late 1800's. I think it is interesting that the American Hoist brochure of that period makes a point of stating that all fasteners on their equipment use "American Standard" threads. Not a word about gear form except to make the claim that they are so well made that they "could never break!"



The radius approximation is more than close enough for a home made gear cutter and is the recommended method in Ivan Law's book Gears and Gear Cutting. This book is worth its weight in gold and well worth buying. It is available from Amazon.

Most 19th century gears were cycloidal and this gear tooth profile is still used in the manufacture of large cast gears, e.g. the ring gears on ball mills.

Hope this helps.

Best Regards
Bob


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## Captain Jerry (Apr 20, 2010)

Dan and Bob,

Thanks for the information. Just about the time I thought I was getting a handle on it, a new term appears. I have to go Google "Cycloidal Gears"

Jerry


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## miker (Apr 20, 2010)

Might be some info of use here..

http://bbs.homeshopmachinist.net/showthread.php?t=39963


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## Dan Rowe (Apr 21, 2010)

The thread on circular pitch gears brings up a good point, it was very common to use circular pitch for early cast gears. This makes it much simpler for the pattern maker because the circular pitch is the distance measured along the pitch circle between 2 teeth.

If you have measured the gears and the math does not work out to a modern DP then the gears are most likely circular pitch.

Dan


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