Mechanical Pen

            Although initially quite unassuming, the common mechanical ballpoint pen contains more parts than may be initially suspected.  Each of the eleven discrete pieces to this device are built to facilitate the reliable operation of this easily overlooked device.

            The primary housing of the pen is little more than an injection-molded piece of polycarbonate.  At one end there is a recessed portion with threading which is made to allow the front housing to be fastened into place.  This recessed area provides a well which allows the rubber grip to stay in place.  The rubber grip itself has small divots which likely enhance a user’s grip.  On the back of the primary housing, there is a hole and a slot which allows the rear assembly to be secured into place.  A small plastic clasp on the assembly which fits into the hole in the tube of the primary housing to be fastened into place.

Diagram of a mechanical ballpoint pen.

            The rear assembly contains the bulk of the mechanical pieces which cause the pen to protrude and retract from the tip.  By referring to the associated diagram with this document, one can approximate how the contraption functions.  Piece A fits over the tip of piece B and is inserted into piece D.  By keeping pieces A and B separated, piece B is free to rotate inside of piece D without the user feeling rotation against their finger.  Piece B presses against piece C inside of the chamber of piece D, and an alternating series of grooves cause piece C to either sit in a position which is either extended or retracted.  The ink tube not only holds the ink, but also serves to connect the front of the pen with its rear assembly.  A spring placed at the front of the ink tube causes the tube to always be pressed back against the rear assembly.  A crimped portion of the ink tube prevents the spring from being forced beyond its necessary position, and a barrier in the front housing gives a foundation upon which the spring can rest.

            Other items of note in the design of the pen are the outer metal clasp which allows the pen to be clipped into a notebook or into one’s pocket.  This piece has “teeth” which are housed inside of part D to anchor the part.  Additionally, at the tip of the pen is the actual ballpoint which allows the device to write.  A metal bead is placed inside of the metal tip, and is partially exposed.  The ink inside of the connected ink tube is drawn forward by capillary action while the ball is free to move due to the natural viscosity of the ink.