4. The radial piston pump has a short axial package, which creates a great potential for allowing multiple units to be common-shaft mounted for displacement control.
Third, extremely high displacement density can be achieved in a radial pump by using a multi-lobe cam to create multiple pumping strokes per revolution. Not only does this improve compactness, but it also balances radial forces on the pump axle, which reduces the required bearing size and allows the displacement variation to be achieved with a small rotation angle of one pump case. Basically, ac hydraulic pump/motors can handle very high pressures and offer infinite variability.
It is easy to see how variable-displacement pumps are a key component to eliminating metering valve control through displacement control. Yet, existing variable-displacement pump architectures are heavy, have poor efficiency at low displacements, and are axially long—making common-shaft mounting challenging.
This is why ac hydraulic pump technology, which varies the flow rate by phase-shifting pairs of oscillating pistons, is a better choice. If you connect pairs of pistons of two radial piston pumps and phase-shift the case of one pump with respect to the other, it will vary the displacement. This result is a variable pump that is highly efficient, power dense, and axially short—allowing multiple pumps to be mounted on a common shaft.
James D. Van de Ven is professor of mechanical engineering at the University of Minnesota. He acknowledges the contributions of Kim A. Stelson, University of Minnesota, and Eric J. Barth and Douglas E. Adam, both of Vanderbilt University, for their contributions to this discussion.