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Mechanical vacuum pumps

Vacuum pumps may be of the reciprocal piston type, wherein a piston draws gas into a cylindrical chamber through an inlet and expels gas from the chamber through an outlet, or they may be of the rotary internal vane Gaede-type pumps, such as the one shown in U.S. Pat. No. 3,040,973 issued June 26, 1967 in the name of K. F. Wessling. The Wessling pump is a two-stage vacuum pump, with each stage comprising a stator with a rotor eccentrically mounted therein. Slidable vanes ride in radial grooves in the rotor, so that they bear against the inner wall of the stator and form a seal between the stator and the rotor. An inlet and an outlet are provided in the interior portion of each stator, so that the rotation of the rotor will draw gas into the inlet and expel gas through the outlet of the stator. The two stages of the Wessling pump are mounted side-by-side, separated by a separator plate and mounted in a hollow casing. An end plate caps one end of the pump unit assembly and an intake manifold or trap assembly is provided at the other end of the pump unit assembly and provides an inlet therefor.

The major components of both reciprocal piston pumps and rotary vane type pumps are customarily constructed from metal castings, primarily cast iron or cast aluminum. These metal castings are not entirely satisfactory, however, as they are heavy and expensive and require a substantial amount of machining in order to obtain proper tolerances for moving parts.

Another disadvantage in the metal castings is especially apparent in the rotary vane type pump. In such a pump, in order to effect a seal between the stator and the vanes, the ends of the vanes bearing against the stator gradually wear down and "seat" with the irregularities in the stator surfaces. To this end, it would be desirable that the vanes be formed from a softer metal than the stator, so that the inexpensive and easily replaceable vanes will wear down, rather than the more expensive and more difficult to replace stator. However, with cast metals it is often somewhat difficult to obtain the proper relative hardness between vanes and stator, and the heat treating required to obtain the different hardness properties is an expensive and not altogether reliable process.

2012-01-16

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