Nanometer Regularity in the Mechanics of a Precision Deployable Spacecraft Structure Joint

The nonlinear microdynamics of a precision deployable space structure revolute joint incorporating angular contact ball bearings are investigated. Using a controlled-displacement force-state mapping method, it is shown that the hysteresis of thejoint approaches material damping levels asthemotions approach nanometer levels, both in extension and rotation. This suggests that structuresdeveloped from such a joint will beasymptotically linear at nanostrain motions. Moreover,thejointhasregularmicrodynamicsfordisplacementsof nanometersand rotations of microradians. No mechanical irregularities are detected to at least 5 nm of extensional resolution and 20 nrad of rotational resolution. For extensions of 1 πm or less, the axial stiffness shows negligible hysteresis, which within the resolution of the test, is comparable to material damping. In rotation, the joint exhibits nonlinear rotational behavior consistent with Todd/Johnson hysteresis loops (Todd, M. J., and Johnson, K. L., “ A Model for Coulomb Torque Hysteresis in Ball Bearings,”International Journal of Mechanical Science , Vol. 29, 1987, pp. 339 ‐354) and is repeatable down to torques of 10 π-N-m. The data support the conclusion that machined irregularities in the bearing component surfaces result in smooth, rather than irregular, microdynamic response.

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