Gear teeth center distance

Teeth have an involute profile, a curve that is generated by the rolhng of a straight line over a circle, as shown in Fig. 29-83. Although many cui ves could be used for gear teeth, the involute is the preferred one because it allows the correc t engagement, even when the distance between the centers of the gears is not accurately held. 

In addition to gear tooth wear, center-to-center distance between shafts will create an erratic spacing and amplitude. If the shafts are too close together, the spacing will tend to be at input shaft speed, but the amplitude will drop drastically. Because the gears are deeply meshed, i.e., below the normal pitch line, the teeth will maintain contact through the entire mesh. This loss of clearance will result in lower amplitudes but will exa erate any tooth profile defect that may be present. 

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