Medication dispensing apparatus with triple screw threads for mechanical advantage

A medication dispensing apparatus comprising:

• a housing;
• a drive member rotatably fixed during dose preparing and injecting and axially movable in a distal direction relative to said housing, said drive member including a threaded shaft;
• a fluid container defining a medicine-filled reservoir with a movable piston at one end and an outlet at the other end, said piston engagable by said drive member to be advanced toward said outlet when said drive member is moved distally;
• a nut screwable along said drive member threaded shaft;
• a screw element threadedly engaged with said housing to be screwable relative to said housing;
• a nut rotating element connected with said nut to be axially movable and rotatably fixed relative thereto, said nut rotating element rotatably fixed with said screw element when said nut rotating element and said screw element are in a first axial arrangement, said nut rotating element rotatable relative to said screw element when said nut rotating element and said screw element are in a second axial arrangement;
• a nut advancing plunger threadedly engaged with said screw element, said plunger axially movable and rotatably fixed relative to said housing;
• wherein the threading of said screw element to said housing is of a first lead, the threading of said plunger to said screw element is of a second lead, and the threading of said drive member threaded shaft is of a third lead, wherein said first lead equals a factor M times the third lead, and wherein said second lead equals a factor (M−1) times the third lead;
• wherein during dose preparing, said nut rotating element and said screw element are in said first axial arrangement, whereby a screwing motion of said nut rotating element and screw element relative to said housing screws said nut rotating element and said screw element a first axial distance from a home position, which screwing motion of said nut rotating element screws said nut along said drive member threaded shaft a second axial distance different than said first axial distance;
• and
• wherein during dose dispensing, said nut rotating element and said screw element are in said second axial arrangement, whereby a screwing motion of said screw element relative to said housing back toward said home position advances said plunger in said distal direction to axially advance said nut and thereby said drive member and said fluid container piston to dispense medicine from said outlet.