Anlysis of Ball Screw Pair Jamming Mechanism in EMAs

With the development of the more electric aircraft (MEA), electromechanical actuator (EMA) are gradually becoming a popular alternative to traditional hydraulic actuators. The electromechanical actuator (EMA) brings weight reduction, improved maintenance and becomes more feasible to realize integrated control for flight control executive system. However, regarding the utilization of EMAs in the primary control surface, the major concern is the mechanical jamming, which is critical and potentially disastrous for aircraft. The mechanical jamming mainly refers to the ball screw pair which is widely used in EMAs. Therefore, the jamming mechanism and jam-tolerant capability of the ball screw transmissions are researched and discussed in this paper to find the fact that causes jam failure.

Firstly, the structure composition and working principle of the ball screw pair is introduced, and gives possible reasons for jam phenomenon. Then the kinetic and dynamic model is built when the ball moves in the raceway and through the inverter considering elastic-plastic contact deformation. Meanwhile the load distribution and deformation compatibility between adjacent balls are considered, as the viscous damping force and centrifugal force are also contained. Accordingly, the friction self-locking condition for the ball in the raceway is proposed, which could be seen as a criterion for the occurrence of the jam. The variation of contact angle and friction coefficient in different operating conditions of load, velocity and frequency are analyzed and compared. The structure parameters as altitude intercept between the raceway and inverter, the returning spiral curve are also discussed. Finally, optimized design and improved operating parameters are suggested from the view of reducing the probability of jamming.