Compared with straight cylindrical gear, helical cylindrical gear has the advantages of long service life, smooth running and low noise at work, and has the disadvantages of slightly complicated manufacture and axial force at work, which is disadvantageous to bearings. The constant Mesh gears in the transmission are all helical cylindrical gears, although this will increase the number of constant mesh gears and lead to an increase in the moment of inertia of the transmission. Spur gears are used only for low gear and reverse gear. However, in this design, because the first gear is a constant mesh program, so the first gear is also used helical gear transmission program, that is, except reverse, are used helical gear transmission.