US Patent 7527445 Writing instrument

A writing instrument has a writing refill slidably disposed in a tubular casing. A rotary cam mechanism extends and retracts the writing refill in response to knocking of a knocking member at a rear end of the casing. The rotary cam mechanism includes a cam cylinder inserted axially into the rear end of the casing, and projections on the cam cylinder engage with apertures in the casing to prevent axial movement of the cam cylinder inside the casing. Rotational movement of the knocking member rotates the cam cylinder and disengages the projections from the apertures, thereby permitting withdrawal of the cam cylinder from the casing and replenishment of the writing refill.