Patent ID: 7713037

Claim:
A pump apparatus which provides a pump body including a cylinder, a piston reciprocably moving in the cylinder, and a piston rod having two ends, one end connected to the piston and the other end projecting outward from the cylinder, in which the piston moves up to suck fluid from outside of the cylinder into a pumping chamber in the cylinder through a suction passage and discharge fluid in a pumping driving chamber in the cylinder to outside of the cylinder through an external discharge passage, while the piston moves down to send the fluid in the pumping chamber to an external fluid-sending destination through a sending-out passage and circulate a portion of the fluid sent to the fluid-sending destination into the pumping driving chamber through a circulation passage, the pump apparatus comprising: a switching valve provided in the circulation passage; and a link mechanism for driving the switching valve and the piston rod, wherein the switching valve includes a sub-cylinder located in parallel to the cylinder of the pump body, and a plunger reciprocably moving in the sub-cylinder to bring the external discharge passage into and out of communication with the pumping driving chamber, and the link mechanism includes a connecting rod projecting outward from the sub-cylinder, and a connecting bar for connecting the piston rod and the connecting rod with each other through a first pivot and a second pivot to apply a pressure to the piston rod and the connecting rod to be reciprocated, further wherein the piston and the plunger each have dead centers on distal and proximal end sides thereof, the plunger being located at its distal dead center when the piston is located at its proximal dead center, and the plunger being located at its proximal dead center when the piston is located at its distal dead center, and the piston has a stroke amount larger than that of the plunger, the sub-cylinder has four recessed peripheral grooves formed on an inner peripheral surface thereof at predetermined distances in its axial direction and four annular seals, with each annular seal being mounted on a corresponding one of the four recessed peripheral grooves, the plunger has at least one recess provided in the middle of an outer peripheral surface thereof in its axial direction, the circulation passage connected to the fluid-sending destination is connected to a portion of the sub-cylinder between two of the annular seals on its proximal end side, the circulation passage connected to the pumping driving chamber is connected to a portion of the sub-cylinder between two of the annular seals in the middle, and the external discharge passage is connected to a portion of the sub-cylinder between two of the annular seals on its distal end side, the recess forming a gap between one of the annular seals when the plunger is located at either dead center, and the pumping driving chamber is brought into communication with the external discharge passage or the circulation passage via the gap.