Also known as twisted spur gears, its teeth are not parallel to axis, but in the same
side with spiral line, and the spiral angles of two intermeshing gears are the same,
but
the spirals are in opposite directions. Be applied to general transmission devices,
automotive reducer and so on.

Features of helical gears:
1. High efficient, used under larger load and higher rotate speed
2. Transmission density
3. Lower Noise