In this
family of diagrams, meshing gears are represented by
tangent circles. In this case the upper and lower
gears and the pinion must be bevel gears, with
teeth at a 45degree angle to the face. Notice that
since the three discs are stacked, the blue axle
must be threaded through one of the outer ones.
Such a device, usually incorporating several pinions,
is called a Simple bevelgear differential;
the blue axle is the "spider shaft takeoff."

This mechanism is a rotary form of the straightline
differential: imagine folding the purple and green racks
back and bending them into equal circular gears. The slider
folds over to a disc of the same radius; now the pinion
is attached to its circumference.
Let us count counterclockwise
rotation as positive, since it correponds, in this interpretation,
to the racks moving to the right. Since the three wheels
have the same radius, their rotational speeds a (top),
b (bottom) and c (central blue disc) must
still satisfy c=(a+b)/2.
