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244 Making Things Move

• Crank/rocker The driven link that joins the ground and the coupler. If it
rotates through 360°, it’s a crank. If it’s limited to back-and-forth motion,
it’s a rocker.
• Follower The link opposite the crank/rocker. Confusingly, this is also
sometimes called the crank or rocker, depending on its motion.

Within the four-bar linkage family, there are more than a dozen well-known
variations. The linkage that drives windshield wipers is probably the most popular
example. A close second is the four-bar linkage that opens up inside your umbrella.

In order for a linkage to have continuous motion (at least one link can rotate a full
360°), the sum of the shortest and longest links must be less than the sum of the
remaining two links. This is called Grashof’s law, so four-bar linkages that follow the
law are called Grashof linkages.

Pantographs are linkages designed so that if a drawing is traced at one point, an
enlarged (or miniaturized) copy will be drawn by a pen fixed to another. These were
used to copy and scale line drawings hundreds of years ago. Scissor linkages are used
in large scissor lifts and small lab jacks to raise and lower platforms (see Figure 8-7).

FIGURE 8-7 Adi Marom’s short++ shoes use a scissor linkage and linear actuator to raise and
lower a platform you can stand on (credit: Adi Marom).