Investigation of natural mode measurements on shells using hand-held transducers with special attention to compressor shells

Mau-Pin Hsu, Purdue University

Abstract

Natural frequencies and modes of shells of hermetic refrigeration compressors were measured with hand-held transducers. Nodal lines were traced based on phase information and recorded photographically. The advantage of this procedure is its apparent precision. In order to assess the limitations of this approach, an analytical model was developed which simulates the interaction of human hands with a vibrating shell. For this purpose, the receptances of the fingers of the human hand were measured. It was found that these data were not appreciably influenced by the hand size of the test subjects or the magnitude of the contact pressure. These data were combined with the theoretical model of a circular cylindrical shell simulating the way an experimenter would obtain data. In order to validate the procedure, simpler cases such as the attachment of point masses, springs, or dampers were also modeled and examined. It was found that the influence of the human hand (or other attachments) is mainly a function of certain receptance ratios. The case in which only the hand-held transducer is contacting the shell and the case in which the shell is contacted by a second hand were studied. The first case results in precise nodal line traces, even for very thin shells. The second case approach may or may not give good results depending on the characteristic receptances of the shell. It was also shown that the use of a hand-held probe to which an accelerometer is attached, or the use of a magnet to attach the accelerometer to the shell, lowers the precision of the measurements. For the class of shells typically found in hermetic compressor applications, the characteristic receptances of these shells as compared to those of the human hand are such that it was concluded that the hand-held transducer technique is very acceptable.

Degree

Ph.D.

Advisors

Soedel, Purdue University.

Subject Area

Mechanical engineering

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