A spur gear is a series of small collisions. Every tooth arrives across its full face width at once, and that line contact is where gearbox noise, pitting and shock loading are born. Angling the teeth solved it. In a helical gear the contact line starts at one corner of the flank and sweeps diagonally across it, so load is transferred progressively instead of slammed on. More tooth pairs share the mesh and the transmission error that drives noise falls sharply. The penalty is thrust. A helix pushes sideways as it drives, producing axial force equal to the tangential force multiplied by the tangent of the helix angle. At 30 degrees that is more than half the driving load pushing along the shaft: large thrust bearings, parasitic losses and a ceiling on usable helix angle. The herringbone answer is to mirror the helix about the mid face. Two opposed helical halves on one blank, and the axial components cancel internally as an equal and opposite pair inside the gear body. - Thrust neve...