Use of Restoring Moment on Main Shaft Thrust Bearings for Turboprop Engines With High Predicted Values of Misalignment

During the optimization of the TP400D6 turboprop engine (powering A400M military transport aircraft), the mechanical design of the main thrust bearing has proven to be one of the most challenging topics. Main problem of this bearing has been the high misalignment values combined with the high axial load (over 17000 lbs) to which it has been designed. The scope of this paper is to go through the details of the aforementioned misalignment problem focusing on the adopted solution. This is based on deeper understanding of the internal bearing kinematics and the application of the restoring moments in mechanical calculations during the design stage. The restoring moment is the reaction force due to the change in operation contact angle because of the increase in contact pressure of the balls onto the races under a high relative rotation of the inner with respect to the outer race. It is therefore translated into a rotational stiffness function, dependant on the thrust force and misalignment. Although normally the rotational stiffness is not considered in aero engines application since values are not significant due to low value of misalignment, when misalignment is high, restoring moments together with the dependency on the flight thrust condition, must be considered as a mean of improving the analytical model predictions.