Reliability of wear-out items in electric motors in space — A case study

Failure of electric motors can be severely detrimental to a spacecraft in orbit. Unlike electric motors (or actuators) used for ground operations, motors used on a spacecraft in orbit are rarely able to be repaired or replaced. For this reason, most spacecraft actuators are considered single point failures which, in many cases, will result in a loss or termination of the space mission. Depending upon whether an actuator is functioning as a rotary arm for a solar array, communication antenna, or as any other revolving device on-orbit, a seizure or failure of its rotating function due to wear out in the life of the mission would likely eliminate the ability of the spacecraft to generate power, communicate with ground stations, or complete its science objectives. Certain mechanical components comprise the main modes of failure of electric motors. The failure rates of these components generally increase with time and are thus termed wear-out items. In this paper, the methods of reliability prediction used by GSFC and its contractors for wear-out items in electric motors are compared and analyzed. The use of a Weibull distribution with a shape parameter value of β = 1.2 is recommended for calculation of the reliability of a bearing assembly. It is shown that assuming a constant failure rate, and more specifically implementing a life expectancy as a constant failure rate within a failure distribution for wear-out items, produces an overly conservative reliability, which can lead to over-design. As a result of this investigation, both the value of empirical test data and the importance for an extended study into determining the most accurate methods of reliability prediction of wear-out items are affirmed.