An interesting topic of this chapter is that of fourbars. These mechanisms can be seen in such real-world examples as locking pliers, bicycles, pistons, and oil well pumps. Fourbar mechanisms have three different inversions, meaning that there are three possible links that could be converted into the ground. This produces three other fourbars with unique motion. These are the crank and lever mechanism, double crank mechanism, and the double lever mechanism.

Crank and Lever

This inversion uses a crank attached to a coupler to drive a lever - as seen in oil well pumps. The lever is able to oscillate about a fixed point in its center where it is attached to ground. Due to the rotation of the crank, a piston or other attachment is able to produce a repeating motion.

Double Crank 

This mechanism has two cranks that are connected by a coupler. This mechanism powers locomotive wheels. The cranks rotate, which rotates the wheels of the locomotive.

Double Lever

The double-lever mechanism uses a force on a cylinder to push the mechanism up. This is ideal as an indicator - as in Watt's indicator mechanism. Watt's indicator uses the force on the indicator cylinder to push the mechanism upwards, which may have a ruler or other measuring device attached. The upwards translation corresponds to a measurement, which could be pressure of a gas, fluid, or as an actuator for a circuit breaker.