What Is a Progressive Cavity Pump (Eccentric Screw Pump)? Rotor-Stator Working Principle

A progressive cavity pump belongs to the positive displacement category. Unlike a centrifugal pump, which imparts kinetic energy to the fluid through an impeller, a progressive cavity pump conveys a fixed volume of fluid per revolution. The two main components that form the pumping mechanism are the rotor (rotating part) and the stator (stationary elastic element).

The name “progressive cavity” derives from the series of successive sealed cavities that form between the rotor and stator. As the rotor turns, these cavities “progress” sequentially from the suction port to the discharge port, carrying fluid in a steady, uniform manner. As a result, the flow is nearly continuous with minimal pressure fluctuation — in sharp contrast to the pulsating delivery of an air-operated diaphragm pump.

The heart of the pump is the rotor-stator pair. The rotor is a metal helical shaft with a single-start helix profile, typically manufactured from steel. The stator is a molded rubber sleeve whose inner bore has a double-start helix profile. The difference of one pitch between the rotor and stator is the key factor that creates the sealed cavities.

As the shaft rotates, the rotor’s center does not remain stationary but moves eccentrically in a small orbit inside the stator. The sealing contact line between the two surfaces continuously shifts, causing each fluid cavity to be “pushed” progressively along the axis from suction port to discharge port without back-mixing. This is why the flow is stable and pressure builds gradually and uniformly with each successive cavity stage.

The stator material is selected according to the fluid: NBR for oils and petroleum-based fluids, EPDM for water-based chemicals, Viton (FKM) for corrosive or high-temperature environments. Choosing the correct elastomer determines the service life and chemical compatibility of the pump.

The core advantage of a progressive cavity pump lies in its smooth, low-shear pumping action. Because the fluid is gently lifted within sealed cavities rather than being struck by a high-speed impeller, shear-sensitive fluids such as cream, sauce, gel, starch paste, or fermentation broth retain their structure. The pump is also self-priming, reversible, and capable of handling fluids containing solids or fibres.

Flow rate is directly proportional to rotational speed, making it highly convenient for metering applications: adjusting the flow rate simply requires changing the speed via a variable frequency drive.

On the limitation side, the rotor-stator pair is a wear element requiring periodic replacement. Important: the pump must never run dry under any circumstances. Without fluid to lubricate and cool the interface between the steel rotor and rubber stator, friction will burn the stator elastomer within a very short time. Systems therefore typically require sensors or operating procedures to ensure fluid is always present in the pump.

Thanks to its wide operating range, the progressive cavity pump is used in food and beverage processing (viscous liquids, jams, sauces), chemical handling, wastewater and sludge treatment, pharmaceuticals, and industrial metering. For fluids containing large solids, a twin-screw grinder/macerator is typically integrated at the inlet to reduce particle size before pumping.

A notable variant is the wobble pump with a floating stator. This design has fewer components, a compact construction, and is suited for viscous fluids and light solid content. Within the Nova Rotors range, wobble pumps include the R compact version and the RL with a separate shaft end and coupling, broadening the selection for simpler applications compared to the standard progressive cavity range.