Crossed Helical Gearing a.k.a. Screw Gears

A common refrain from my childhood was “if you keep crossing your eyes, they are going to get stuck like that!” It never did happen, but it was a real-life example of what nonparallel meant. In gearing, the axis of the pair is usually parallel, or they are intersecting. Crossed helical gears, also known as screw gears, are both nonparallel and nonintersecting.  They allow for some very unique designs.