Internal gears – In internal spur gear the teeth is always cut on the inner diameter of ring while the outside diameter is kept smooth and they mesh with external spur wheel. The bigger wheel is called annular wheel and smaller one is called pinion. In internal gearing when gears move then shaft run in the same direction. Internal gears are hollow. The properties and teeth shape is similar as of external gears except that the internal gear had different addendum and dedendum values modified to prevent interference in internal meshes. When choosing a mating spur gear, always remember that the difference in the number of teeth between the internal gear and pinion should not be less than 15.