When vertical vibration testing of large test articles is required, it is common to install a head expander on the armature of a shaker. Larger test articles often have a center of gravity relatively far above the mounting surface. When combined with the armature and head expander, these test articles may exhibit multiple structural resonances within the desired test band that do not exist in the intended application. These test configuration-driven characteristics are likely to create unwanted cross-axis excitation during a vibration test.

The difficulty in controlling unwanted cross-axis motion usually increases when testing large items. Excessive cross-axis motion can "over-test" the test item, creating the risk of damaging the test item, or can limit the input in the test axis, thus jeopardizing a successful test.

Orbital Sciences commissioned the design of a guided head expander system that greatly reduces the cross-axis motion at the test article mounting surface of the head expander. The design submitted by Team Corporation couples large inertial masses to the head expander through high-stiffness, hydrostatic, self-aligning bearings. Together, the guided head expander and inertial mass structures have a first resonance higher than the test band of interest and provide high dynamic stiffness. The head expander and inertial masses are supported by a suspension system with a low first resonance, below the test band of interest. It is noteworthy that this design approach exhibits high "dynamic" stiffness and low static stiffness.

Conventional designs for this type of equipment may have relatively high cross-axis load ratings, which might suggest that such designs would provide good cross-axis motion control, but these designs often suffer from structural resonances within the test band of interest that produce unwanted cross-axis motion.

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