Currently, there are fixed displacement linkage driven
pumps on the market based on the four-bar crank-slider
mechanism. However, the displacement of this linkage is
not easily varied and cannot be done without changing the
length of a moving link or moving the pumping cylinder.
An example of such as a pump is the hydraulically
cushioned variable delivery triplex pump which allows the
piston to displace a variable gas spring until a certain
force is reached, at which point it will begin to displace
working fluid. This high pressure water jet pump can only
operate at low speeds on the order of 6 rpm.
14) To avoid
adjusting a moving member of the mechanism, or the
position of the pumping cylinder, a higher order linkage is
required.