Re: Gear Dimensions

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Posted by BillS on November 05, 2008 at 18:44:59:

In Reply to: Gear Dimensions posted by Jim Cook on November 04, 2008 at 21:34:36:

If it's a spur gear, try this:

Measure the OD and count teeth.
Teeth + 2
DP = ----------------
OD

Compare what you calculate with standard DP, Module, and Circular Pitch sizes.

If it's a helical gear, try this:
Teeth + 2
DP = --------------------------
OD x COS (Helix Angle)

It's the same formula as the first one - but HA for a spur gear is 0 deg and COS (0) = 1.

Here's the caveat - OD may not be standard! If your sample is a small number of teeth (say less than 20-25) it's likely that the OD is enlarged. By good design, pinions are usually enlarged. Using gear gages is a good place to start in this case (see below).

I think the easiest way to find PA from a sample is to get some gear gages. Gear gages are thin rack form sections, each approx 3 inch long, that come in different sizes and PA's. Coarser pitches have fewer teeth; finer pitches have more teeth.
I remember getting mine from Globe Gear in DP, CP, and Module sizes. Roll the 20 PA side of the gage, then roll the 14.5 PA side. The side that rolls smoothly (no bumps) is the right PA. This will also confirm if the DP is correct - even if the OD is enlarged!

If the DP and PA are not standard, things get tricky. Also, if the sample doesn't have good teeth left, you will have to work from the other (mating) gear. Your customer may have to help by supplying the gearbox or prints, etc.

If the gear set pair is on fixed centers, take a measure over wires from sample and duplicate that on the new gear. Can save a lotta grief.

For general gear topics check out the FAQ here. And I recommend the old faithful - Machinist Handbook. A wealth of gear formulas, data, etc there.

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