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Large Spur Gear and Pinion Problem 2

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BEMPE16524

Mechanical
Feb 17, 2008
284
SG
Hi Guys,
I'm designing a set of gear. My problem arise when my gear is very big, more than 2.5mtr diameter.
I have to keep my pinion around 120-150mm diameter. Velocity ratio will not effect my problem.
Force on teeth is very low as not much load to carry.

When i use standard pressure angle, 14.5 or 20 degree, it gives wrong value of base diameter and dedendum diameter.
It suppose to give a dedendum dia. smaller than base diameter but instead it is the other way.

So, then i try my stupid analysis to try make different pressure angle, i tried 6 degree, it gives quite reasonable value.
but when i try to plot the involute, it doesn't seem right.

I need someone expert here. Attached is the analysis i did (sheet 3).
 
 http://files.engineering.com/getfile.aspx?folder=3d081190-d986-4991-8e76-4acda0904f4d&file=GEAR.xlsx
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Why is the root dia. supposed to be smaller than the base dia.? In your case for 20[sup]0[/sup] and 15 teeth it's true but not for 297 teeth, so what? Up to 7.25[sup]0[/sup] it's true, but not after that.
 
what are you trying to say? if not true for 297 teeth, what is the applicable formula then? please advise.
 
conantar,

You need to go and research the reasons behind a phenomenon that occurs with involute gearing generally referred to as 'undercut'. Put basically; it's a function of the number of teeth and pressure angle and how these affect the diameters that you're talking about.
A quick search with Google reveals heaps of info.

That's a neat little spread sheet that you've come up with.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
 
Conatar
A simple formula
#teeth/diametral pitch=pitch diameter.

And as long as the true involute is above the base diameter. You are good.
And It is not necessary to have the root below that base diameter. I did not look at your data. But these are elementary gear subjects.

If know the basic center distance of the pair gears. You can reverse calculate to get the correct # of teeth.

Mfgenggear
 
conantar-

As gearcutter noted, you will have undercutting with a 15T pinion unless you tweak your mesh geometry. One other option would be to go with a 20T pinion that would not undercut, plus a slightly different module and a 396T gear to maintain the same PCD.

Hope that helps.
Terry
 
Guys,

Thanks a lot for your information.

Regards,

Conantar
 
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