Reliability Calculation Methodologies for Mechanisms and Actuators  Tanner Horne* and Evan Harrington*  Abstract  MIL-HDBK-217 and similar reliability calculation methodologies inadequately assess the reliability of  complex electro-mechanical components and mechanisms. This paper utilizes accepted reliability  methodology techniques to achieve realistic and technically justifiable approaches for MTBF and reliability  ratings. Additionally, this paper will address the historically overlooked effects of radiation on the  accelerated degradation of insulation systems, such as those used in motors and other wound components.  Introduction  The challenging overall system requirements for aerospace mechanisms and gearboxes have led to several  innovative and unique solutions for the application. One of the most demanding of system requirements is  the reliability and number of steps. Even when the load friction is minimal, there is considerable transmitted  torque through the gearbox to accelerate the load inertia. We will assess the methodologies used in  determination of transmitted torques, as well as several other key requirements.   Analysis of Transmitted Torques  Calculation of Load Acceleration Torques An often-overlooked implication of high torque margin in driving inertial loads is the acceleration torques at  each step of the stepper motor. These torque pulses can be significant and must be considered in the  mechanical structural and life analysis. For an example application, assume the load inertia is coupled  directly to the output shaft, supported by a separate bearing system. Also, assuming the load friction is  extremely low, and the driven load inertia is relatively high, each step of the stepper motor will accelerate  the load and this will translate a reaction torque throughout the actuator. If the motor current is limited to  maintain a maximum holding output torque this translates to a torque at low pulse rate ( TPPS-0) of at the  motor. Using the torque at low pulse rate for these calculations will provide the mean acceleration and  torque during each step. The Peak Torque and acceleration will be 41% higher than the mean calculated  values.   For applications that do not implement current limiting, nominal and maximum motor torque calculations  should be analyzed. That is, nominal voltage, resistance and temperature for the nominal case, and  minimum temperature and resistance as well as maximum torque constant and voltage for the maximum  case. The engineer needs to make sure that they have structural and endurance margin at maximum  conditions, but it is also of value to assess the nominal conditions to gage how conservative the analysis  is. Since torque margin requirements are calculated at the minimum conditional values, the maximum  conditions may result in surprising results.   The mean acceleration at the load (α L) is calculated in Eq. 1. The mathematical proof of this equation is  available by contacting the authors.   aL=(TPPS-0-FM)(N×hG)-FL JL+(JM×N2×hG)(1)  *Avior Control Technologies, Inc, Longmont CO Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 223