Spur Gear

Gears are machine elements used to transmit rotary motion between two shafts, normally with a constant ratio.   The pinion is the smallest gear and the larger gear is called the gear wheel..   A rack is a rectangular prism with gear teeth machined along one side- it is in effect a gear wheel with an infinite pitch circle diameter.   In practice the action of gears in transmitting motion is a cam action each pair of mating teeth acting as cams.  Gear design has evolved to such a level that throughout the motion of each contacting pair of teeth the velocity ratio of the gears is maintained fixed and the velocity ratio is still fixed as each subsequent pair of teeth come into contact.   When the teeth action is such that the driving tooth moving at constant angular velocity produces a proportional constant velocity of the driven tooth the action is termed a conjugate action.   The teeth shape universally selected for the gear teeth is the involute profile.