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 45-degree 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 bevel-gear differential*; the blue axle is the "spider shaft take-off." | This mechanism is a rotary form of the straight-line 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 counter-clockwise 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`. |