Turning down the OD of a gear

[hrench]

Originally Posted by chadr03

Absolutely.

Involute gears do not simply contact at the pitch diameter the contact moves from the tip to the root along the line of action as can be seen on the previous graphic. Gears all have sliding contact and most have at least one place on the line of action that has pure rolling contact. A gear mesh has three distinct actions during mesh. ...

...A shift in center distance will not change the fact that the contact will move from the tip to the root along the line of action it will simply make the line of action shorter and change the amount of backlash.

Some really great discussion here and the graphic is amazing.

I think I'll add that chadr03 pointing out that the backlash will increase with added center-to-center spacing is correct because the teeth are no longer engaging along the theoretical pitch circle (or near it as the graphic shows) but instead, further out on the tooth--where the two teeth don't fit as well together. Also note, the teeth are narrower out there, hence weaker.

Gears are built with a bit of undercut already to take up a tolerance if the centers are too close. I'm still not an advocate of moving the center distance apart.