Vibration Measurement Instruments
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ABSTRACT How many hours of vibration testing equals how many miles (or kilometers) of transport? A simple question, but unfortunately the answer is neither simple nor straightforward. This paper examines the factors involved in attempting to construct such an equivalence, explains the methodology behind an accepted and proven approach, and discusses the issue of accelerated (time-compressed) vibration simulation in detail.
Vibration Tests There are three categories of commonly-used tests in the transport packaging field. First is the “fixed-displacement” test, also known as a repetitive shock or “bounce” test, in accordance with ASTM D999 Method A1 or A21 or similar. In this test, the specimen is placed on the table of a machine which moves with a constant one inch (25 mm) displacement, either linearly or in a circular motion. The test is usually conducted at a frequency (typically around 4.5 Hz.) where the specimen just begins to intermittently leave the table surface, as evidenced by the ability to insert a thin shim under it. Technically this is not vibration at all; the motion causes a small shock or “bounce” each time the package re-contacts the table. This test is not a simulation of actual transport – although there may be “bouncing” of packages in transport vehicles, it is not at a constant frequency and amplitude.
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