The Double Rack and Sector Pinion is one is my series of gear-based mechanical projects.
There are numerous options for converting rotary motion to linear motion - the crank, or James Watt's geared alternative, or a friction drive, or similar. Such arrangements are useful for converting energy that primary exerts pressure (i.e. steam power) into useful rotary motion for driving wheels, shafts, belts, and so on.
The Double Rack and Sector Pinion has some interesting properties when compared with, say, a standard crank. For one, it can only be driven by the pinion - there is a point in the pinion's rotation when it is not engaged with either rack, so using the rack as the driving source could leave the pinion disengaged and stationary. Second, it provides linear motion at a constant speed for (almost) each half rotation of the pinion, as compared to the crank shaft's sinusoidal motion.
This incarnation of a Double Rack and Sector Pinion was modeled in Fusion 360. The pinion and rack both have a module of 3mm. The pinion is 6 teeth of a 20-tooth gear.