Double-Helical or Herringbone Gears:

A double-helical gear is corresponding to a pair of helical gears with equal helix angle secured together, one along with a right-hand helix and the other a left-hand helix. The teeth of two rows are separated by a groove that is needed for tool run out. The axial thrust that occurs in case of single-helical gears is removed in double helical gears. If the left & right inclinations of a double helical gear meet at a similar apex & groove is removed in it, the gear is called as herringbone gear as illustrated in Figure.