Features of helical gears:
1. When meshing two external gears, the rotation occurs in the opposite direction, when meshing an internal gear with an external gear the rotation occurs in the same direction.
2. Care should be taken with regard to the number of teeth on each gear when meshing a large (internal ) gear with a small (external) gear, since three types of interference can occur.
3. Usually internal gears are driven by small external gears
4. Allows for a compact design of the machine
Applications of Internal gears: planetary gear drive of high reduction ratios, clutches etc.