The efficiency of high ratio planetary gear trains (PGTs) is mainly constrained by the meshing efficiency of acting gears in each stage. Such gear wheels, predominantly with involute teeth profiles, work with a meshing efficiency of around 99%. Yet, the overall efficiency of the gearbox drops to even below 70% when gear ratios of a few hundred are exceeded. Unconventional gear profiles can solve the efficiency issue in high ratio planetary gearboxes.

Apart from easy manufacturability and insensitivity to center distance deviations, the involute gears are dominant because of the largely evolved and properly established frameworks supported by commercial design software and manufacturing methods. On the other hand, their noise levels at high speeds, incompatibility with a lower number of teeth gearing and significant wear at the start and end of meshing due to higher sliding velocities are critical disadvantages of involute gears. An alternative to involute gears which operates at a higher meshing efficiency than that of involute gears will not only enhance the overall efficiency of the high ratio gearbox but also alter the paradigm of choosing involute gears unanimously.

A review of unconventional gear profiles used in the PGTs is provided. With a set of evaluation criteria, other gear profiles that are also suitable for increasing gearbox performance are discussed. The analyses show that although a few existing unconventional gear profiles have better meshing efficiency and load-bearing capacity, they are heavily constrained by their high transmission error and susceptibility to manufacturing inaccuracies which need to be addressed in future.