Failure of Harmonic Gears During Verification of a Two-Axis Gimbal for the  Mars Reconnaissance Orbiter Spacecraft  Michael R. Johnson', Russ Gehling**and Ray Head**  Abstract  The Mars Reconnaissance Orbiter (MRO) spacecraft has three two-axis gimbal assemblies that support  and move the High Gain Antenna and two solar array wings. The gimbal assemblies are required to move  almost continuously throughout the mission's seven-year lifetime, requiring a large number of output  revolutions for each actuator in the gimbal assemblies. The actuator for each of the six axes consists of a  two-phase brushless dc motor with a direct drive to the wave generator of a size-32 cup-type harmonic  gear. During life testing of an actuator assembly, the harmonic gear teeth failed completely, leaving the  size-32 harmonic gear with a maximum output torque capability less than 10% of its design capability.  The investigation that followed the failure revealed limitations of the heritage material choices that were  made for the harmonic gear components that had passed similar life requirements on several previous  programs. Additionally, the methods used to increase the stiffness of a standard harmonic gear  component set, while accepted practice for harmonic gears, is limited in its range. The stiffness of  harmonic gear assemblies can be increased up to a maximum stiffness point that, if exceeded,  compromises the reliability of the gear components for long life applications.  Introduction  The Mars Reconnaissance Orbiter Mission  During its two-year primary science mission, the Mars Reconnaissance Orbiter will conduct eight different  science investigations at Mars. The investigations are functionally divided into three purposes: global  mapping, regional surveying, and high-resolution targeting of specific spots on the surface. This detailed  mapping of the surface of Mars will provide future landed missions with the high resolution data required  to land safely in a desired area. The instruments on board the MRO spacecraft consist of five types:  cameras, a spectrometer, a radiometer, a radar, and engineering. Refer to Figure 1 for an overall view of  the MRO spacecraft's science deck.  Cameras  HiRlSE (High Resolution Imaging Science Experiment)  This visible-camera can reveal small-scale objects in the debris blankets of mysterious gullies and details  of geologic structure of canyons, craters, and layered deposits.  CTX (Context Camera)  This camera will provide wide area views to help provide a context for high-resolution analysis of key  spots on Mars provided by HiRlSE and CRISM.  MARC1 (Mars Color Imager)  This weather camera will monitor clouds and dust storms.  Spectrometer  CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)  This instrument splits visible and near-infrared light of its images into hundreds of "colors" that identify  minerals, especially those likely formed in the presence of water, in surface areas on Mars not much  bigger than a football field.  ' Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA  ** Lockheed Martin Space Systems, Denver, CO  Proceedings of the 38 Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  37