Those who know gears all know that helical gears have higher load-bearing capacity than spur gears. Because of their good meshing properties, helical gears have a higher degree of coincidence (more teeth are involved in meshing at the same time), so the vibration is relatively small and the noise is low. But one disadvantage of helical gears is that they will generate axial force. Under heavy load, the axial force is very large. The additional axial force is detrimental to the transmission. In order to eliminate this axial force, it produces Herringbone gears.
If a gear is made into a helical gear with opposite symmetry, the axial forces of the helical gears in the two directions due to the helix angle will cancel each other, so the bearing supporting the gear shaft will not bear the axial force.
Advantages of herringbone gear:
1. The herringbone gear has a high degree of coincidence, and at least two teeth are engaged at any time.
2. The meshing process between the teeth of the herringbone cylindrical gear is an excessive process, and the force on the teeth gradually increases from small to large, and then from large to small; the herringbone gear has high load-bearing capacity and stable operation.
3. Herringbone gear has no or very small axial force due to the opposite direction of the tooth helix angle in the symmetrical direction.

Herringbone gear disadvantages:
1. A very obvious disadvantage of herringbone gears is that it is difficult to process. Herringbone gears cannot be processed by conventional gear shaping and hobbing methods.
2. To make more precise herringbone gears is so expensive that it can only be used in heavy equipment.