to the present invention wherein the lead screw 30 and the ratchet wheel 42 are not fixed together as in the previous embodiment illustrated by fig1 in this alternative embodiment the ratchet wheel 42 provides a threaded nut portion 68 which rotates about the lead screw 30 the cam 38 provides a cavity 70 which accommodates a first portion of the lead screw 30 a second portion of the lead screw 30 extends from the cavity 70 through the threaded nut portion 68 of the ratchet wheel 42 and through a through bore or cavity 72 in a retainer 74 the retainer 74 serves to constrain the ratchet wheel 42 from translating up the rotational axis x as it rotates the threaded nut portion 68 about the lead screw 30 the retainer 74 also provides a key portion 76 situated in the cavity 72 the key portion 76 rides in a keyway 78 provided in the lead screw 30 in this manner the lead screw 30 is constrained to translate but not rotate incrementally along rotational axis x the incremental advancement of the lead screw 30 along the rotational axis x with each tooth by tooth rotation of the ratchet wheel 42 causes the piston 64 to be pushed into the container 66 thereby expelling the liquid drug from the container 66 in a controlled manner fig3 shows still another embodiment of a pump drive 300 according to the present invention in this embodiment the shaft 22 of the motor 12 is mounted perpendicular to the rotational axis x the.