How things work
How a pen clicks
A working model of a retractable ballpoint, with the barrel cut in half. The same push sends the tip out, then back in. Nothing here is drawn by hand: the cam turns because of the shapes pressing on it.
The button
Click it
The small picture in the corner shows the other end of the pen. Fade the front ribs to see the plunger teeth behind them.
What is simplified
- The two snaps in each click are slowed down. In a real pen the spring makes them almost instant, and they are what you hear.
- The ribs are molded into the barrel. Here they are a different color, and the front half of them is left in place, so you can see what the fins slide against.
- This cam has four fins and turns an eighth of a turn per click. Other pens use different numbers of fins and teeth, but work the same way.
- The parts are plastic in most pens and have small clearances so they slide. The model has none, and the plunger rests on the cam instead of rattling.
- The ink inside the cartridge, and the join where a real barrel unscrews, are left out.
In short
A retractable pen holds its tip out or in with three parts inside the barrel: a spring near the tip, a cam that is free to turn, and a plunger under the button that can slide but not turn. Ribs molded into the barrel have deep slots and shallow notches between them. Each press pushes the cam's fins clear of the ribs, and slanted edges turn the cam a little further. When the fins land in the shallow notches, the tip stays out. After the next press they land in the deep slots, and the spring pushes the refill back in. The pen only remembers which way the cam is facing.