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• # Gears and gear nomenclature

Discussion in 'The main mechanical design forum' started by Pierrick94, Mar 23, 2022.

1. ### Pierrick94New Member

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Dec 2021
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Hello,

I'm not familiar with gears and gear nomenclature and I'm trying to use the AGMA standards to calculate the bending stress at the root of the teeth in a pinion gear. I've highlighted the information on the drawing that I don't quit understand.
I understand what is the helix angle, but I wonder what is the 58' 19''?
Also, I'm guessing the DATUM (PC) DIAMETER is the pitch diameter, but I'm not 100% sure.
As for the tooth form and the T.D.P, I have no idea what that is.

If someone could help me to understand theses notions, I'd be glad!
Thanks 2.
3. ### DanaWell-Known Member

Joined:
Sep 2010
Posts:
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The first is part of the helix angle: 22 degrees 58 minutes 19 seconds or 22.9719444°. TDP is transverse diametral pitch. I'm guessing the tooth form is saying it's based on a standard 2" DP (diametral pitch) gear, dunno about FD but that 23° is suspiciously close to 22.9719444°.You're probably right about the datum diameter, probably (PC) is "pitch circle"

From Machinery's Handbook:

"
2) Diametral pitch is the ratio of the number of teeth in the gear to the number of inches in
the standard pitch diameter. It is used in this standard as a nominal specification of tooth
size.
a) Normal diametral pitch is the diametral pitch in the normal plane.
b) Transverse diametral pitch is the diametral pitch in the transverse plane.
c) Module is the ratio of the number of teeth in the gear to the number of mm in the standard
pitch diameter.
d) Normal module is the module measured in the normal plane.
e) Transverse module is the module measured in the transverse plane.
3) Pressure angle is the angle between the gear tooth profile and a radial line at the pitch
point. It is used in this standard to specify the pressure angle of the basic rack used in defining
the gear tooth profile.
a) Normal pressure angle is the pressure angle in the normal plane.
b) Transverse pressure angle is the pressure angle in the transverse plane.
4) Helix angle is the angle between the pitch helix and an element of the pitch cylinder,
unless otherwise specified.
a) Hand of helix is the direction in which the teeth twist as they recede from an observer
along the axis. A right hand helix twists clockwise and a left hand helix twists counterclockwise.
5) Standard pitch diameter is the diameter of the pitch circle. It equals the number of teeth
divided by the transverse diametral pitch."

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