Bevel gears are gears where the rod or the rotating shafts intersect.
The shafts are often mounted 90 degrees apart, but can be designed to work at other angles as well.
They are used for power transmission between intersecting shafts.
This allows a vertical rotation to be converted to a horizontal rotation.
Animation: https://youtu.be/0gbcMuVdzbY
The kua and the shoulder facilitate the vertical rotation of the core into a rotation on the arms.
This was covered on the lesson of 5th Oct 2020: http://practicalmethod.com/2020/10/chen-zhonghua-online-lesson-20201005-online-video-purchase/
