Four-Bar Linkages

Rotational motion is ubiquitous in modern machine design—a lot of which boils down to the fact that electric motors rotate, and don't translate. But what if you're designing a machine that needs to move some other way? Say, follow a non-circular path, even a linear path?

Mechanical engineers have gotten around this limitation through the use of four-bar linkages. A four-bar linkage is a collection of four* bars joined together at the ends. One bar, or link, is connected to a motor or a crank or something that turns it around in a circle (the rocker), one bar is fixed, and the other two move, following a path determined by their lengths.

With this relatively simple set up, any number of complex paths can be created, allowing for all sorts of movement. Some neat examples⁶:

Crank-Rocker: takes small circular motion and amplifies it, though only in a partial arc (not a full circle).

Parallel Four-Bar: creates identical motion at two (or more) points from a single input.

Chebyshev Linkage: allows for approximations of straight line movement with rotating inputs.

(See it in action here.)

*Sometimes three, with the ground or whatever the mechanism is mounted to serving as the fourth "bar."