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Measuring pitch

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Glenn Brooks

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#1
Ok, so all the tutorials say gear pitch is measured as teeth per inch (TPI). But so far, I’ve never read how to actually measure TPI.

Are the number of teeth per inch measured along a 1”chord, or around the outside diameter of the gear, or something else?

I would like to determine pitch of a gear we need to replace.

Thanks much,
Glenn
 

RJSakowski

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#2
Ok, so all the tutorials say gear pitch is measured as teeth per inch (TPI). But so far, I’ve never read how to actually measure TPI.

Are the number of teeth per inch measured along a 1”chord, or around the outside diameter of the gear, or something else?

I would like to determine pitch of a gear we need to replace.

Thanks much,
Glenn
I believe that gear pitch is specified as diametral pitch which is the number of teeth in the gear divided by the pitch diameter in inches. Pitch diameter id about halfway between the minimum diameter at the root of the tooth and maximum diameter at the crest of the tooth. Diametral pitch is usually a nice number so you can usually determine it with an approximate measurement of the pitch diameter.

All gears with the same diametral pitch and pressure angle will mesh. Pressure angles are typically 14.5º and 20º.

Metric gears are determine4d by module or modulus. Module is equal to the pitch diameter in millimeters divided by the number of teeth in the gear. Gears with the same module and pressure angle will mesh.
 

Glenn Brooks

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#3
RJ, thanks much. I knew that! What was I thinking?? Anyway, easy to figure out what I need now.

Many thanks.

Glenn
 

BtoVin83

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#4
Every once in a while you will run across a gear that is odd like an oversize pinion that doesn't match the math. Gear designers will do this to match existing center distance or ratio but design a hunting tooth ratio to spread wear through all the teeth. You can also drop the number of teeth on small pinions using the same center distance to minimize undercutting the tooth profile
 

tq60

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#5
There is a trick where you do basic math and add a tooth or 2.

Not sure but have seen it a few times in discussions.

Try Google and look for "determining gear pitch" for starters.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 

Asm109

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#6
OD of gear = N+2/DP Where N is number of teeth and DP is diametral pitch
 

69Z/28-Nut

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#7
To find the pitch of any gear, measure O.D. of gear.
Then count the teeth and add 2.
Next divide this number by the O.D. of the gear.
Just the reverse of finding the diameter of a gear blank where you take the number of gear teeth needed add 2 and divide by the D.P. of the gear you are wanting to cut.

You can also purchase a gear tooth pitch set; Common sizes are
3,4,5,6,8,10,12,16,20,24,32 & 48 DP.

Uncommon set would be;
3-1/2,7,9,11,13,15,17,18,19,22,26,28,30,36 & 40 DP.

I dont owna metric set but the can be found on the internet.

They also come in coarse pitch, fine pitch, combination stub 20° PA, module pitch, circular pitch & involute spline 30° PA.

A real nice program is GearDXFSetup.exe if you can locate it on your internet.

The program does Standard and Metric Gears
 
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