CFD Methodology Development to Predict Lubrication Effectiveness in Electromechanical Actuators 2024-26-0466

Electromechanical actuators (EMAs) play a crucial role in aircraft electrification, offering advantages in terms of aircraft-level weight, rigging and reliability compared to hydraulic actuators. To prevent backdriving, skewed roller braking devices called "no-backs" are employed to provide braking torque. These technology components are continuing to be improved with analysis driven design innovations eg. U.S. Pat. No. 8,393,568. The no-back mechanism has the rollers skewed around their own transverse axis that allow for a combination of rolling and sliding against the stator surfaces. This friction provides the necessary braking torque that prevents the backdriving. By controlling the friction radius and analyzing the Hertzian contact stresses, the brake can be sized for the desired duty cycle. No-backs can be configured to provide braking torque for both tensile and compressive backdriving loads. This allows the motor of the EMA to consistently operate against a positive torque while overcoming the friction of the no-back mechanism. While operating, the rollers require sufficient lubrication to ensure local temperatures do not exceed limits of the components or the lubricant itself. This was analyzed by multiphase CFD modeling to predict fluid characteristics around the skewed rollers and conjugate heat transfer modeling to identify component overheating. Elasto-hydrodynamic lubrication theory was used to estimate the lubrication thickness around the rollers. The analysis revealed the formation of vapor due to the high rotational speed of the rollers with respect to the stator discs, resulting in direct metal-to-metal contact. Consequent testing of the no-back in a Tribological Testing Machine confirmed the findings with wear patterns on the stator consistent with loss of lubrication. In addition, the temperature data aligned within 5°C of predicted CFD results at key locations on the no-back. Given the high fidelity of the analysis methodology, future steps include assessing new design iterations against intended performance parameters.