A gear train consists of one or more gear sets intended to give a specific velocity ratio, or change direction of motion. Gear and gear train types can be grouped based on their application and tooth geometry.
Spur gears are inexpensive to manufacture.
They cause no axial thrust between gears. Note that two mating helical gears must have equal helix angle but opposite hand. These are found in automotive transmissions, and any application requiring high speed rotation and good performance. They are more expensive to manufacture. Helical gears create axial thrust.