blender bike 6

object -> convert curve to mesh, alt + click to select ring,

ctrl + r to loop cut, scroll to increase loops

enable proportional editing, g to move loops

alt + click to select ring, ctrl + b to bevel frame

create big gear, add a circle, f to fill, i to inset, x to delete face

alt + click to select perimeter, ctrl + r to loop cut, scroll to increase loops

face select on disk, x to delete faces

e to extend the disk, add a cylinder on the disk edge

alt + click to select disk perimeter, shift + d to duplicate, p to separate

object -> convert to curve

select small cylinder, modifier > add array and curve modifier

select disk, modifier -> add Boolean modifier -> select difference 

face select gear teeth, shift + d to duplicate

alt + e -> select along normal, s to scale, ctrl + b to bevel

select teeth, add array modifier

reference:

https://www.youtube.com/watch?v=6jmz3rh3SGQ