Speed Ratio
The teeth of a gear are distributed on the circumference of the pitch circle so that the thickness of each tooth t and the space between two teeth are the same. The pitch p of a gear, which is the distance between the equivalent points on two teeth, is equal to twice the thickness of a tooth,
The pitch of a gear GA can be computed from the number of teeth NA and the radius rA of its pitch circle
In order to mesh smoothly two gears GA and GB must have the same sized teeth and therefore they must have the same pitch p, which means
This equation shows that the ratio of the circumference, the diameters and the radii of two meshing gears is equal to the ratio of their number of teeth,
The speed ratio of two gears rolling without slipping on their pitch circles is given by,
therefore
In other words, the gear ratio, or speed ratio, is inversely proportional to ratio of the radii of the pitch circles and the number of teeth of the two gears.
Read more about this topic: Gear Ratio
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