A review of NPRD-2011 failed to yield any reliability information specific to roller screws. However, it should be noted that planetary roller screws are often used as the actuator mechanism in electro-mechanical linear actuators. NPRD-2011 provides a merged failure rate of 50.197 failures per million operating hours for military quality linear, electro-mechanical actuators. The merged data was derived from Airborne, Inhabited Attack, Airborne, Uninhabited Attack, and Naval environments. Since this data would presumably include actuator failures due to multiple root causes, as a rough approximation in the absence of better information, it may be used as an upper bound on the roller screw failure rate.
If design information is available, a limited review of literature found the following expression for the theoretical L10 life as:
L10 = (C/F)^3
where C is the basic (L10) dynamic load rating (90% of a sufficiently large sample of identical screws can be expected to attain or exceed 1,000,000 revolutions at this constant centrally acting pure axial load without fatiguing), and F is the constant mean axial load. Using this equation, L10 is expressed in millions of revolutions.
