![]() The gear tooth must project both below and above the rolling-cylinder surface (pitch circle), and the involute only exists outside of the base circle. ![]() The cylinders from which the strings are unwrapped are called the base circles (□□□ ) and are smaller than the pitch circles (□), which are the diameters of the original rolling cylinders. The involute of a circle is a curve that can be generated by unwrapping a taut string from a cylinder. The cycloid is still used as tooth form in some watches and clocks, but most gears use the involute of a circle for their shape. There is an infinite number of possible conjugate pairs that could be used, but only a few curves have seen practical application as gear teeth. In order for the fundamental law of gearing to be true (“the angular velocity ratio between the gears of a gearset must remain constant throughout the mesh”), the gear tooth contours on mating teeth must be conjugates of one another. RMIT University - Thierry Perret-Ellena – mail:
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