Abstract: A positive displacement pump is particularly adapted for corrosive, abrasive and viscous liquids. It employs a combination of orbital and nodding movements provided by a unique mechanism. A rotor member, mounted eccentrically on a drive shaft, fits closely within a pumping chamber which it drives in an orbital path along an inclined plate having fluid inlet and outlet ports. The resultant cyclic increase and decrease in volume of the chamber beneath the rotor member draws in fluid and discharges it under pressure. For fluids of greater viscosity (or where reduced flow rate is desired) the angle of tilt is reduced, without lessening the power afforded for pumping.