Mechanical pencil with a fluid actuator

A writing instrument having a tip portion, comprising:

• a hollow body;
• a fluid pressure mechanism including a grip unit having therein a fluid without any leakage of said fluid, said grip unit having an outer wall radially displaceable in accordance with a gripping force exerted by a writer's fingers, said grip unit being provided at a part of said body which is held by the writer's fingers when said writing instrument is used for writing, said fluid pressure mechanism including an axially displaceable member, said axially displaceable member axially moving from an original position in a first direction away from said tip portion in response to an increase of a pressure of said fluid within said grip unit by the radial displacement of said grip unit, and said axially displaceable member restoring to said original position in accordance with a decrease of the pressure of said fluid upon a release of the gripping force;
• a writing member having a writing tip, said writing member being provided in said body and including lead;
• and
• a writing member actuating means responsive to movement of said axially displaceable member in said first direction for actuating said writing member to be in a condition in which said writing member is ready for writing, said writing member actuating means being provided in cooperation with said writing member and said axially displaceable member, said writing member actuating means comprising:
• a chuck means for chucking said lead, said chuck means being provided inside of said body and being always biased toward said tip portion and opened to release said lead from being chucked, said chuck means being movable away from said tip portion relative to said lead,
• lead holding means for always holding said lead with a predetermined holding force, said lead holding means being positioned between said chuck means and said tip portion and being substantially fixed with respect to said hollow body,
• tightening means for engaging with and disengaging from said chuck means to chuck and release said lead, said tightening means being provided at a side of said chuck means opposite said tip portion, in such a manner that said tightening means is axially movable by a constant distance,
• wherein said axially displaceable member moves said chuck means away from said tip portion when said gripping force is applied to said grip unit, said chuck means moves a predetermined distance while said chuck means is engaged with said tightening means to chuck said lead, said chuck means is advanced, while chucking said lead, by a distance through which said tightening means is moved when the gripping force applied to said grip unit is released, and thereafter the lead is released to be free from said chuck means.