What two types of reduction gear teeth are in use today on propulsion reduction gears?

The use of herringbone and helical gears in propulsion reduction gears is significant due to their design characteristics that enable efficient torque transmission while minimizing noise and vibrations. Herringbone gears feature a double-sided tooth design that allows for the reduction of axial thrust, making them particularly useful in high-load applications like those found in propulsion systems. This design also promotes smoother operation and greater durability under the stresses encountered in marine environments.

Helical gears, on the other hand, have angled teeth which provide gradual engagement, allowing for more contact area between the gears. This results in quieter operation and a smoother transfer of power, key aspects for machinery in naval vessels where performance and noise are both critical factors.

The other types of gears mentioned have specific applications but do not combine the desirable traits that herringbone and helical gears offer for propulsion reduction. For example, spur and bevel gears are commonly used in various machine applications, but they might not withstand the same high-load conditions efficiently as herringbone and helical gears.