1. Field of the Invention
The present invention relates to an improvement in pumps and more particularly to those of the reciprocating piston type having radially contained packing with axially acting follower means for expanding it. Such pumps generally are applied to a variety of uses, for example to supply slurries of paint or other coating compositions to spray heads, liquid transfers and fluid injection such as is required in the petroleum industry.
2. Description of the Prior Art
Pumps of the type with which the present invention is concerned produce a discharge of fluid under pressure during reciprocatory movements of the piston in both directions. The initial suction stroke of the piston inducts fluid past a check valve into one chamber and the subsequent movement of the piston in the opposite direction transfers that liquid past a piston check valve through a smaller second chamber and out through a discharge port. The next suction stroke of the piston, in addition to inducting fluid into the first chamber as described above, decreases the volume of the second chamber forcing fluid in it out through the discharge port. An expansible packing surrounds the piston between the two chambers and a second expansible packing surrounds the piston rod adjacent the opposite end of the second chamber.
In the known Wagner pump a spring is compressed between these two packings exerting axial pressure sufficient to expand them into sealing engagement with the piston and piston rod and the surrounding casings. The force exerted by such a spring is not capable of being altered without disassembly of the pump and replacement of the spring.
The pressures under which slurries are to be discharged from such pumps vary from as high as 4000 lbs./in.sup.2 depending upon the viscosity of the slurry, the finish to be imparted to the work piece, and other factors. In order to avoid excessive wear on the piston, piston rod and surrounding casing, it is desirable to decrease the axial pressure exerted by such a spring when low discharge pressures are desired as in the case of relatively thin slurries and to increase such pressure only when high discharge pressures are required as in the case of viscous slurries, or a different spray arrangement to be employed requires various (high or low) pressures.