You might find this discussion interesting, John

https://www.model-engineer.co.uk/forums/topic/herringbone-gear/

… from July 2023

MichaelG.

I can recommend referring to  the previous Thread [now Topic], Martin

… and not just because I started it.

MichaelG.

Martin it is definitely where the Citroen logo came from, prior to manufacturing vehicles they were gear manufacturers and specialised in the form that was their logo.   Dave W

Prior to M. Citroen inventing the method for cutting herringbone teeth (opposed helical), gear manufacturers were partially there with stepped gears. These are simply spur gears laminated so the teeth on one overlap the gullets on its neighbour.

The double form is indeed to remove the end-thrust a single helical gear generates, and prior to Citroen’s development helical gears of opposing hands were sometimes laminated to give the same effect.

At the other end of the sophistication scale are, or were, “Knuckle” or “Hollows-&-Rounds” gears; with simple hemicylindrical teeth and gullets, relatively easy so cheap to make. With no proper rolling action, they give a lot of friction and an oscillating velocity ratio, and were suitable only for slow-moving, rough duty like simple winches. They were obsolescent by the early years of last Century, as gear-cutting advanced and customers wanted better quality.

What were those photographed, on?