A hinge pin is a piece of cylindrical metal that holds together the two arms of a hinge in such a manner that the arms can freely pivot around the pin. A basic hinge has three parts: the hinge pin, and two arms, one typically attached to a base or foundation intended not to move, and the other attached to a door, hatch or other object for which movement is desired. These are typically assembled in such a manner as to allow easy movement of the arms around the pin within a limited range of motion.
In a basic hinge, the arms have flattened portions with holes in them to allow the arms to be secured to objects with nails or screws. Opposite of these flattened portions, the arms are rolled or curled to create cylindrical holes just large enough to fit the intended hinge pin. In order to secure the arms together with the hinge pin, the curled or rolled portions of the arms are cut, with the middle section of one removed and the outer sections of the other removed, so the rolled portion of one fits into the rolled portion of the other as interlocking teeth. When the hinge pin is inserted through the rolled sections, the arms are then joined into a single unit.
The final hinge assembly may be secured in a number of ways to ensure the hinge pin does not fall out. Even a hinge pin that fits tightly into the arms will eventually fall out as the hinge is used over time. To prevent this, the hinge pin in the most basic of hinges may simply be bent at each end. In a machined hinge, the hinge pin may be capped, or the ends may be shaped into cap-like shapes. In the case of the latter example, only one end of the hinge pin is normally so shaped, thus allowing the hinge pin to be removed from the hinge if desired.
In the case of door hinge pins, it is common for the hinge pin to be designed with removal in mind. In such a case, the capped end of the hinge pin is at the top of the hinge so it will not fall out from normal use. Hinge pin removal is then possible by either prying the pin out from the top or hammering it up and out from the bottom.
Hinge pins may be replaced with new pins purchased from a hardware store, but there are several alternatives for hinge pin replacement in a pinch. If a hinge pin is lost, any cylindrical object of similar size may work as a temporary replacement, even one fashioned from wood, although such a temporary hinge pin will likely not last long. Depending on the size of the hinge and how much stress is placed on it, even a piece of wire, rope or string may serve as a temporary hinge pin.