Vacuum pumps have been described as compressors for rarefied gases. For more than twenty-five years the scroll compressor principle has been used in designing vacuum pumps that do not rely on oil to provide their pumping action. The dry scroll vacuum pump has been particularly successful as a source of rough vacuum for analytical instruments such as liquid-chromatography or gas-chromatography mass spectrometers. Such instruments are typically located in quiet laboratory environments where the scroll vacuum pump may be the only significant source of noise. Consequently, quiet operation is a key requirement of dry scroll vacuum pumps. This paper discusses an occurrence of stick-slip squeal that developed during the design of a new model, low noise, dry scroll pump. Stick-slip phenomena and the associated noise are the result of "self-excited" oscillations related to the difference between static and sliding friction. Rapidly oscillating friction forces occurring during a portion of the orbital cycle can result in induced vibration of the participating surfaces and structure, which radiate high-pitched noise. In the case described, considerable effort was expended in developing a quiet pump design. However, a loud "chirping" noise found in a few late-stage prototypes caused a good deal of concern. It was necessary quickly to fully characterize the vibration behavior within the pump related to the noise and to develop design modifications to ensure that the noise was eliminated. The characteristics of the noise are described for the scroll pump along with the methods utilized to eliminate it from the design.
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