165 rates to 0.014 rad/s (0.8 deg/s) within the GN&C so ftware. Acceleration was originally restricted only by  the CMG firmware was similarly limited to 0.0007 rad/s2 (0.04 deg/s2) by the GN&C software. These limits  were imposed with intent to potentially benefit CMG health, but still allow satisfactory vehicle attitude  control.   CMG3 showed short periods of elevated accelerome ter activity between the CMG1 failure and the CMG3  soft-failure. Concern over these signatures result ed in the implementation of several operational  limitations intended to lower the gimbal angular rate s of motion that were believed to be triggering the  disturbances. The limitations include d significant restrictions on attitude control methods that required  nominal CMG momentum desaturations, limited values for the gimbal rotation rates used to manually positioning the CMGs (especially CMG3), and temporary cessation of use of the X-POP flight attitude. XPOP in particular was found to impart cyclical gim bal rates at an order of magnitude greater than the  LVLH flight attitudes for quiescent operations     On-Orbit Failure Description and Signature 2   CMG1 “Hard” Spin Bearing Failure

At the end of May 2002; the ISS was in a high sola r beta X-POP period. The ISS attitude was transitioned  from X-POP to LVLH on June 7th, several hours prior to shuttle docking. In order to accomplish this reorientation and docking, large gimbal rates within de sign requirements were imparted to the CMG due to  desaturations. Approximately 6 ho urs after the docking at ~23:00 on GMT Day 158, 2002, CMG1’s Spin  Motor Command Current (SMCC), the indicator of the combined bearing torques, started increasing over  a 14-minute period. This increase in SMCC was later concluded to be the start of the failure. The increase  in SMCC was caused by an increase in the Hall Reso lver side bearing torque. This was confirmed by the  increase in bearing temperature that indicated the Hall Resolver side bearing may be in distress. During the course of the next 15 hours, CMG1 showed signs of good and bad bearing performance, with the ultimate failure occurring at 14:57 on GMT Day 159, 200 2. Several hours after docking, the flight control  team noted a flag that indicated the accelerometer on CMG1 had measured 0.5 Gs. The rise in SMCC and associated Hall Resolver temperature was repeated twice during the next 16 hours. Then, following a 30-minute period with no ground communication, telemetry showed that the CMG1 SMCC was at its maximum value, 1.6 Amps, and the wheel-speed was decreasing. After several seconds of maximum current and wheel-speed reduction, Fault Detecti on and Isolation software declared the CMG failed and  automatically shut down the Spin Motor. Normal  shut down would result in the flywheel speed  decelerating to 0 rad/s in approx imately 12 hours. However, during this event, the flywheel stopped 72  minutes after Spin Motor shutdown because of the b earing failed condition. During this run-down period,  the Hall Resolver bearing temperature ex ceeded the maximum telemetry value of 93 °C (199.4°F) while  the Spin Motor bearing temperature reached only ~30°C (86°F), indicating major distress in the Hall  Resolver bearing. Data was retrieved from the on-board data record ers to fill in the missing telemetry for  analysis purposes. The timeline of events is given with SMCC and spin bearing temperatures shown in Figure 5, Figure 6 and Figure 7. The CMG1 was later returned to L-3 for disassembly and formal failure investigation.   NASA/CP-2010-216272