An engine that uses a piston, a connecting rod and a crankshaft is called a reciprocating engine. The piston travels in a back-and-forth motion. The connecting rod is attached on one side to the piston and the other side to the crank, by a round rod called the crank pin. Because the connecting rod cannot bend, the linear back-and-forth motion of the piston is transferred to the crankshaft, causing it to turn. An early use of a piston reciprocating inside a cylinder was in 1712, when Thomas Newcomen designed a steam engine to draw up well water. The earliest known use of a piston traveling back and forth in a cylinder was water pumps used in ancient Rome, on Caligula's ships.
A bell crank is a special kind of lever. It is triangular in shape, with holes near each tip of the triangle. A bolt is placed through one hole in the crank, firmly attaching it to a base yet still allowing the bell crank to swing back and forth. Long rods are connected to the other holes. These rods are usually placed 90 degrees apart from each other. When one rod is pushed or pulled, it moves the triangle. The movement of the triangle is transferred to the other rod. This mechanism is used in many airplane levers. The joystick is connected to one rod. When the pilot pushes on the joystick, it moves one rod. The reciprocating movement is transferred through the bell crank, moving another rod connected to a control surface, such as the rudder or elevator.
A spring's motion is also reciprocating. A spring stores energy. When it is compressed, it is exerting back pressure on the object compressing it. When released, it springs back, hence its name. Many pens use a spring to create reciprocating motion. When you push on the button at the top of the pen, it pushes the tip out. A small internal spring creates back pressure on the tube holding the tip. When you push on the button again, it releases the spring, so the tip retracts back into the pen.