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Originally Posted by Nuson
some one is likely to come along with a better sounding answer, but the pitch diameter is the diameter where proper meshing should be. So when using gears that "mesh", but doesn't have the expected pitch diameter, the true pd is not being achieved in one, the other, or both gears. In that case, it's hard to say what ratio they're running at. Perhaps it's somewhere between the ratios using the dp of the correct gear and the substitute.
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This is not true. While it IS true you cannot calculate what the true operating pitch diameter you CAN calculate the ratio exactly.
Unless the mesh is so bad that the gear teeth pass each other the ratio is the [Nteeth-on-Gear1 / Nteeth-on-Gear2].
Joe J.
P.S. Knowing the ratio tells you the ratio of speeds exactly, but it does not allow you to know the ratio of the torques. Don't forget that the ratio of the torques is [the gear ratio X efficeincy]. This efficiency is going to drop considerably when you loose "conjugate action" (by mating involute gears with different DP's or Pressure Angles for example). Lapping or running the gears in will help in this regard, but it is no substitute for using properly matched gears to begin with. JJ