Every pump works by the same logic: you push on the fluid until it moves. Centrifugal pumps throw fluid outward with a spinning impeller. Gear pumps trap fluid between teeth. A progressive cavity pump does it with a screw that walks through a rubber tube, and the whole trick is harder to explain in one sentence than it is to watch in a cutaway drawing.
Once you see it, though, the principle sticks with you. Here is how it works, what the parts do, and why the design behaves the way it does.
The core idea: a rotor that orbits, not spins
A PCP has two working parts. The rotor is a single-start metal screw. The stator is a rubber-lined tube with a double-start helix moulded through it, with a pitch that is twice the rotor’s pitch. The rotor is driven through a universal joint so that it does not rotate on a fixed centre line. Instead it rolls eccentrically inside the stator, its screw thread constantly meshing with the rubber thread.
That constant mesh is what seals the pump. Between the rotor and the stator there are always a series of closed pockets of fluid, each one separated from the next by a line of rubber-to-metal contact. As the rotor orbits, every pocket moves one step toward the discharge port. New pockets open at the suction, old ones close at the discharge. The fluid never accelerates, never stops, and never reverses. It just travels.
The parts and what they actually do
A complete pump unit is more than rotor and stator, and each component has a specific job:
- Rotor. The stainless steel screw that does the pushing. Its surface finish matters, because a rough rotor wears the stator lining faster.
- Stator. The rubber-lined cavity. It forms the seal and absorbs the wear. It is always the first part to need replacing.
- Universal joint. Connects the drive shaft to the rotor and absorbs the rotor’s eccentric orbit. Hidden inside the pump, easy to forget, and worth inspecting every time the stator is out.
- Pump housing and flanges. The inlet and outlet channel, with DIN, ANSI, or ISO connections to match your pipework.
- Drive motor and gearbox. Matches speed and torque to the application. Speed is the flow control, so this is also the metering dial.
Why flow is proportional to speed
Each rotor revolution moves exactly one set of pockets, and each pocket holds a fixed volume of fluid. Nothing slips, nothing leaks back, at least not while the stator is healthy. That means flow rate is locked to rotational speed. Double the speed, double the flow, with no valves or throttling needed.
This is why PCPs are used for dosing and metering. You do not need a flow meter and a control loop to hold a rate. You set the motor speed and the pump holds it, within a percent or two, for as long as the stator holds its seal.
Why the pressure depends on stage count
Each sealed pocket can hold back a certain amount of pressure before the fluid starts leaking past the rotor-stator contact. Stack more pockets, and you can hold back more pressure, which is exactly what more stator stages do. The practical numbers we build to:
- 1L single stage, up to 6 bar
- 2S two stages, up to 12 bar
- 3S three stages, up to 18 bar
- 4S four stages, up to 24 bar, with custom builds above that
It also explains the model number system. In SGP025-2S, SGP is the series, 025 is the cavity size (bigger cavity, more flow per revolution), and 2S is the stage count. Read those three pieces and you already know the pump’s flow class and pressure rating.
What this means in practice
Because the flow is continuous and the pockets are sealed, a PCP delivers a smooth, pulse-free stream, which matters for shear-sensitive fluids like flocculant and fruit pulp. Because the rotor pushes rather than throws, viscosity does not collapse the performance the way it does on a centrifugal pump. And because the pump can pull a vacuum at the suction, it self-primes without priming water or foot valves.
The trade-offs are the ones we always mention: the stator wears and must be replaced on schedule, and the pump costs more than a centrifugal of the same flow. If your fluid is thin, clean, and cheap to move, a PCP is overkill. If it is thick, gritty, or fragile, there is nothing else that does the job this gently.