The development trend of the structure of the step transmission is to increase the number of constant meshing gear pairs, so that helical gears can be used. The latter has a longer life and lower noise than spur gears, although they are slightly more complex to make and have axial forces at work. Therefore, in the transmission, except for the low gear and reverse gear, straight cylindrical gears have been replaced by helical cylindrical gears. But in this design, the reverse gear adopts the straight gear.