Pump includes a motor and a cam shaft coupled to the motor for rotation about a longitudinal axis of the cam shaft. The cam shaft has at least one radially-outward projection defining a helical engagement portion disposed along a length of the cam shaft. A plurality of finger plates are disposed along the length of the cam shaft, each finger plate mounted for movement in a transverse direction relative to the longitudinal axis of the cam shaft. Each finger plate has an aperture defined therein to receive the cam shaft therethrough. Each aperture has a substantially straight edge region and an opposing edge region. Engagement of the helical engagement portion with the substantially flat edge region during rotation of the cam shaft urges the finger plate transversely toward an extended position.