How the mechanism displaces fluid
A rotary vane pump moves fluid without any check valves or reciprocating pistons: a rotor spins off-center inside a circular housing, and a set of vanes slide in and out of slots in the rotor as it turns, staying in contact with the housing wall under spring or centrifugal loading. As the rotor turns, the volume trapped between each pair of vanes grows on the intake side and shrinks on the discharge side — the geometry alone does the pumping.
The housing is fully enclosed, so the vane geometry that actually does the pumping isn't visible from outside — here's the mechanism the assembly above is hiding:
Where the difficulty actually lives
The working principle is simple to sketch and hard to build. Pump efficiency and reliability come down to a small number of tight clearances: the gap between vane tip and housing wall has to be small enough to seal against leak-back, but not so small that the vane binds or gouges the housing as it slides. That's a tolerancing and material problem as much as a geometry problem, and it's why the design work went well beyond the initial concept sketch — into vane slot clearance, rotor eccentricity, and how the housing bore was going to be finished.
From CAD to hardware
The pump was modeled in SolidWorks, with the rotor, housing, and vane geometry parameterized so eccentricity and vane count could be adjusted without rebuilding the model from scratch — useful for iterating on the displacement-per-revolution before committing to fabrication.