Measure pitch diameter of odd number of teeth gear

[RJT]

I have a proven design gear train (20 units working for years) . We are replacing one unit with standard off the shelf spur gears (per drawings and BOM) and I suspect one gear is oversize on the pitch. It is a spur gear 27 teeth, 24 pitch, 1.125 P Dia 14.5 PA. , class 6, made in China. I'm having trouble finding the pin size and measurement over pins. I need to determine if the pitch diameter is oversize. Is there a way to measure over 2 pins at the closest to 180 degree apart 2 opposing teeth, or is it done with 3 pins and a ring? Can someone supply the measurement over pins? Thanks

 

[johnoder]

1.224479" over .072 wires per Van Keuren

Two pins, from their ODD chart

 

[RJT]

Thanks, that confirms what we are finding. From now on I'm going to insist on Martin or Browning, not "or equivalent".

 

[Dan from Oakland]

Just FYI, what determines backlash in a gear is the tooth thickness at the pitch diameter. You are not measuring the pitch diameter with an over pins measurement, you are measuring tooth thickness. For the given gear, a gear that is .001" thick or thin will have an over pin measurement change by .0028".

 

[abarnsley]

Sounds like a job for...

Gear tooth calipers......

 

[Dan from Oakland]

In my experience over pin measurements are much more accurate that reading the vernier on tooth calipers with a magnifier.

 

[RJT]

Replaced suspect gear with a Martin brand, same spec, and my troubles went away. About .010 difference in measurement over pins.