Pump having spring-loaded piston shaft assembly

Claim 1

In a pump, means including a first housing portion defining a fluid driving compartment, means including a second housing portion defining a driven fluid compartment, valve-controlled means for introducing fluid into said fluid driving compartment, piston means in said fluid driving compartment having a suction stroke in response to which said valve-controlled means is opened to cause said fluid to pass into said fluid driving compartment, said valve being of a size such that intake of said fluid into said fluid driving compartment is so restricted that on the suction stroke of said piston means, said piston means cannot completely fill said fluid driving compartment with sufficient rapidity to avoid cavitation in said fluid driving compartment, and means for preventing significant cavitation in said fluid driving compartment, said last-named means comprising a stem extending axially from said piston means, a body removably secured to the outer end of said stem, said body comprising a portion having an axial bore which is counterbored to provide a hollow chamber in said body, the junction of said bore and chamber forming a shoulder, a piston shaft slidably mounted in said axial bore and having a portion extending into said chamber and provided at one end with an abutment, a compression coil spring interposed between said abutment and said shoulder for resiliently biasing said piston shaft toward said stem, and means for causing reciprocal movement of said piston shaft, whereby on the suction stroke of said piston means, said piston shaft is first retracted through said axial bore, so that said compression coil spring is compressed while said piston means remains in a forward position relatively to said piston shaft, the pressure of said compression coil spring being less than atmospheric pressure, so that when said compression coil spring expands to its initial condition, said piston means is drawn back, thereby substantially eliminating what would otherwise have been a vacuum in said fluid driving compartment, and, in effect, substantially eliminating the possibility of cavitation in said fluid driving compartment.