Rotary Vane Vacuum Pump

The eccentricity is the pump. Everything else is containment.

1. Bore a stator 60 mm diameter and 40 mm deep in aluminium.
2. Turn a rotor 50 mm diameter, so it is 10 mm smaller than the bore.
3. Mount the rotor's shaft OFFSET so the rotor almost touches the stator at one point and leaves a 10 mm crescent opposite.
4. Set the near-contact clearance to a few hundredths — close, but not touching.
5. Position inlet and outlet ports either side of that near-contact point.

The near-contact line is the seal between inlet and outlet. If it is too loose, gas leaks straight from the outlet back to the inlet and the pump achieves nothing; if it touches, it seizes. Oil bridges that gap in service, which is why an oil-sealed pump reaches a far better vacuum than a dry one of identical geometry.

The vanes must stay in contact all the way round despite the gap constantly changing.

1. Cut two slots across the rotor's diameter, at 90 degrees to each other.
2. Make vanes from hard plastic or brass to slide freely in those slots.
3. Fit light springs behind each vane so they are pushed outward against the stator.
4. Turn the rotor by hand and confirm each vane extends and retracts smoothly through a full revolution.

Springs start the seal; centrifugal force maintains it. At speed the vanes are flung outward hard enough to seal without help, and the springs matter mainly at start-up and low speed. That is why some designs use no springs at all and simply cannot be turned slowly by hand — a detail that surprises people servicing them.

Run it dry first, then wet, and the difference is dramatic.

1. Assemble and run the pump DRY against a gauge. Record the ultimate pressure.
2. Now introduce a few millilitres of low-vapour-pressure oil at the inlet.
3. Run again and record the ultimate pressure.
4. Compare, and note the change in sound.

Every pump has a characteristic curve and a point where it stops improving.

1. Connect to a known volume and record pressure every ten seconds from atmosphere.
2. Plot pressure on a logarithmic scale against time.
3. Note the straight portion, then the knee where it flattens.
4. Compare the time taken with the Sprengel pump's from the previous blueprint.

Rotary vane vacuum pumps developed through the early twentieth century as electric motors became common and laboratories wanted vacuum without mercury or hand-cranking. Wolfgang Gaede is central to this period — he produced rotary mercury and then oil-sealed designs, and went on to invent the diffusion pump later in this batch.

The oil is doing three jobs at once and that is the design's elegance. It lubricates the vanes, seals the clearances, and fills the dead volume. Any one of those alone would justify it. Using one substance for three purposes is why the design has barely changed in a century — and it explains the pump's main drawback too, since that same oil can creep back toward the vessel as vapour, contaminating what is being evacuated.

Its place in the chain: Guericke's piston is defeated by seals and dead space; Sprengel's mercury column removes both at the cost of speed and toxicity; the rotary vane pump gets the speed back and removes the mercury by using oil to defeat dead space instead. Each generation keeps the previous one's gains and attacks its specific limit.

Its honest limits: an ultimate pressure set by the oil's own vapour pressure, so it cannot reach high vacuum however long it runs; oil backstreaming into the system; and the need to change oil as it absorbs vapours and contaminants. For pressures below its floor you need a fundamentally different mechanism, which is what the diffusion pump provides.