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How does an oil-sealed rotary pump work?

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McLeod gauge; vacuum technology — vacuum technology Diagram of a McLeod gauge.

An oil-sealed rotary pump functions by utilizing a rotor positioned eccentrically within a stator, which creates a crescent-shaped volume that is swept by blades or a rotary piston through an outlet valve.1

In a common two-bladed design, the blades sweep gas through the pump to the outlet. Another variation, the rotary piston pump, features a single blade integrated into a sleeve around the rotor; this blade is hollow and serves as an inlet valve, effectively closing off the pump from the system when the rotor reaches its top center position.1

The sealing oil serves a critical role in maintaining the vacuum, though it also imposes limits on the ultimate pressure attainable. These limits occur due to the leakage of gases and vapors dissolved in the oil, which can flash off when exposed to low inlet pressures, as well as the decomposition of oil caused by hot spots from friction. These pumps are frequently employed as roughing pumps or forepumps for other vacuum systems.1