Patent ID: 7581896

Claim:
A safety knock-type writing instrument, comprising: a barrel having a cartridge therein; a half gear unit provided on an exterior of the barrel and rotatably seated in a clip, wherein the half gear unit has a shape of an eccentric gear which rotates about a central axis thereof, and comprises: a first rotation guide part providing a spirally inclined slide surface so that the first protrusion of the knock unit slides along the first rotation guide part to rotate the half gear unit within a predetermined angular range; a first inclined groove part provided at a lower end of an inclined surface of the first rotation guide part to form a linearly inclined slide surface and a flat surface in a direction of an axis of rotating shafts, the first inclined groove part serving as a locking step using a height difference; a first protrusion seat provided at an end of the fiat surface of the first inclined groove part to have a height different from the first inclined groove part, the first protrusion seat having a spirally inclined slide surface and a sharp corner, thus seating and stopping the first protrusion when the cartridge is extended; a second rotation guide part provided above the sharp corner of the first protrusion seat, and having a toothed shape with a spirally inclined slide surface; a second inclined groove part which is the equal to the first inclined groove part, but has a linear inclined slide surface and a flat surface in an opposite direction to the first inclined groove part; a third rotation guide part having a slide surface so that the second protrusion of the knock unit slides along the third rotation guide part; a second protrusion seat to function as a stopper of the second protrusion; and first and second sidewalls provided outside the first and second inclined groove parts to be perpendicular to the first and second inclined groove parts, the first and second sidewalls guiding and restraining the rotation of the first protrusion within the predetermined angular range; and a knock unit positioned in the barrel, and including first and second protrusions to engage with the half gear unit; wherein the cartridge is retracted into the barrel when the first protrusion of the knock unit disengages from the half gear unit; and the half gear unit rotates in a rotating direction and an opposite rotational direction (Q), in response to reciprocating motion of the knock unit in a pushing direction (F) and a releasing direction (R).