Spur gears are the most commonly used gears, are used to transmit motion between parallel shafts. These impose load on bearings. Its teeth are straight and parallel to the axis. These can be either internal type or external type. An external spur gear of infinite radius, called as rack. Rack has straight-sided tooth (for involute tooth profile gears).
In helical gears, the tooth profile in transverse plane gets gradually rotated along the helix angle as we move along the axis. The hand of the helix determines whether it is a right handed helical gear or a left handed helical gear. Helical gears can be external or internal type. However, internal helical gears are not very common in external helical gears, the hands of the mating gears are of opposite hands, whereas for internal helical gear meshing with external gear, they are of the same hand. The mating gear should always have the same helix angle.
Sometimes double helical gears are also used where they have both right handed and left handed helical teeth on each gear. Normally there is a gap between the two helices. However, there are gears which have no gap between the helices double helical gear are also called chevron gears. Single helical gear impose both radial and thrust load on bearings. Double helical gear normally impose radial loads as the thrust loads due to opposite helices are in the opposite directions and, therefore, they cancel out. The spur gears can be considered as a special case of gears with helix angle equal to zero.