Tribological Performance Prediction of Aircraft Gas Turbine Mainshaft Ball Bearings

Abstract

The internal thrust loads in aircraft gas turbines are carried by angular-contact ball bearings, one on each of the compressor-turbine-driven shafts. There are typically two or three concentric shafts, rotating at individual speeds to generate the required aerodynamic loading with a resultant thrust load on each of the ball bearings. The designs of these bearings are based primarily on the required life or fatigue endurance. Estimation of this parameter is currently accomplished using empirically developed, factor-based calculation methods, e.g., material life and lubrication life factors. Experience has demonstrated in many instances that bearing fatigue lives predicted by this method are less than those achieved in practice. A more accurate performance prediction system would allow improved optimization of bearing designs with associated savings in engine performance, weight, and cost as well as improved reliability. As thrust-to-weight ratio increases with new engine designs, the need for more compact engines becomes more acute. To extrapolate bearing design to the more demanding applications requires a stress-based rather than a factor-based bearing life prediction method. This means that, in addition to the Hertz stresses on which current calculation methods are based, contact frictional stresses and ring hoop and residual stresses need to be included in the performance prediction system. The Ioannides-Harris fatigue life model provides the capability to consider all of these stresses in the life calculation. Moreover, in this high performance application, the potential for bearing life to be limited by ball fatigue, as well as by raceway fatigue, has been experienced; the current method accounts only for the raceway failure mode. In this paper, it is demonstrated how the Ioannides-Harris model, together with appropriate estimations of bearing frictional stresses detained from macrocontact and microcontact elastohydrodynamic lubrication theory, combined with contact heat transfer analyses, may be used to more accurately estimate ball bearing endurance in modern applications. Bearing endurance as limited by fatigue of any of the bearing load-carrying components is included in the analysis. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 6–12, 1997