The mass properties measurement apparatus of this invention includes a pair of opposed bearing assemblies, each comprising a fixed bearing and a movable journal, and a radially rigid connection between the movable journals located on an axis with a single degree of freedom, which is rotation. A test object support table is fixed to one journal for rotational movement therewith, the plane of the surface of the support table being perpendicular to the rotational axis. A torsion wire coincident with the rotational axis is fixed at one end to the one journal for rotation with the support table and is fixed at its other end against rotation with the support table, thereby creating a torsional pendulum which permits oscillatory movement of the support table. Means are provided for measuring successive oscillation periods of the support table for determining the center of gravity and principal axis moments of inertia of a test object on the support table. Finally, a fail-safe caging mechanism is provided. This device engages the shaft under controlled conditions if gas pressure falls below acceptable values thus protecting the bearing surfaces from damage and providing an orderly shutdown.
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