Compressed air driven reciprocating piston hydraulic pump

A compressed air driven reciprocating piston hydraulic pump comprising a piston installed in a cylinder hole of a cylinder member and reciprocatingly driven to and fro along an axial direction thereof, a forward motion chamber and a return motion chamber at opposite sides of the piston in the cylinder hole, a compression spring for return motion provided in the return motion chamber, a hydraulic pump mechanism including a plunger extending from a center portion of the piston toward the return motion chamber, a valve case having a partition wall portion that serves as an end wall of said forward motion chamber and fixed to an end portion of the cylinder member, an air supply opening for supply of compressed air and an air discharge outlet provided in the valve case, and a changeover valve mechanism including a main changeover valve that operates to change over repeatedly between an air supply position in which the forward motion chamber is communicated with the air supply opening and an air discharge position in which the forward motion chamber is communicated with the air discharge outlet and an auxiliary changeover valve that operates in synchrony together with the main changeover valve and changes over the position of the main changeover valve, wherein:

• said main changeover valve comprises a main valve body that has an annular valve body portion that is biased toward the air supply position by compressed air in an annular air passage communicated to the air supply opening and a piston portion defining portion that is formed integrally with the annular valve body portion, and is shiftable along the axial direction;
• the annular valve body portion has first and second annular valve faces that are formed at its two ends in its shifting direction and that alternatively contact against first and second annular valve seats of the valve case, and the piston portion defining portion comprises a piston portion that is received in a piston reception hole formed in the valve case;
• and
• the auxiliary changeover valve comprises an air intake chamber defined by the piston reception hole and the piston portion, a valve rod that passes through and slides freely in the partition wall portion of the valve case extending from a center portion of the piston towards the forward motion chamber, the air intake chamber, and the valve main body, a small diameter portion formed on the valve rod, a first valve member that is capable of sealing between the valve rod and the main valve body, and a second valve member that is capable of sealing between the valve rod and the partition wall portion;
• and built so that the auxiliary changeover valve, along with changing over the main changeover valve to its air supply position when forward motion of the piston starts by releasing a seal of the first valve member by the small diameter portion and discharging compressed air from the air intake chamber, also changes over the main changeover valve to its air discharge position when return motion of the piston starts by releasing a seal of the second valve member by the small diameter portion and intaking compressed air to the air intake chamber;
• Wherein:
• a center side portion of the main valve body close to an external circumference of the valve rod is built as an auxiliary valve body that is separate from the other portion than the center side portion;
• the auxiliary valve body comprises an auxiliary piston portion that is installed into an auxiliary piston reception hole formed in the piston portion so as to slide freely therein in an airtight manner and that faces into the air intake chamber, and a tubular portion that extends from the auxiliary piston portion toward an opposite side to the air intake chamber and that is installed in a cylindrical hole of the main valve body so as to slide freely therein;
• and
• along with an end portion of the tubular portion of the auxiliary valve body being faced into the air discharge outlet, a compression spring is provided that biases the auxiliary valve body toward the air intake chamber, and the first valve member is installed in the tubular portion.