Correlation and Repeatability Improvement Between Tie Bolt Stretch and Vibration Levels Using Automated Stretch Tool and Regression Analysis

Many large commercial aircraft utilize an auxiliary power unit (APU) to provide ground and in-flight power and start assist. The gas turbine APU can have a radial flow rotor system made up of a compressor and turbine section which are joined mechanically for ease of assembly and later maintenance activity. In a typical application, these two modules are assembled using a tie-bolt, an adaptor and a curvic coupling. The tie-bolt stretch along with all the internal rotor geometric interfaces is known to greatly influence rotor generated vibration. The tie-bolt is loaded or stretched to achieve the required contact stiffness between the impeller and turbine. This contact stiffness must be maintained throughout the APU ground-flight cycle to ensure that the operational loads do not degrade the rotor condition throughout the design service life. The rotor system is mounted using two bearings, typically a ball bearing for thrust and a roller bearing. These bearings in-turn transit vibration to the APU components, housing, and surrounding aircraft hardware. In some applications, the APU also provides the bleed air to the main engine start. This model of APU is generally inclusive of an additional impeller, called a load compressor. In that case, the turbomachinery consists of the three rotors, whereas assembly features of those three rotors exist two curvic couplings, adapter and tie bolt as required. It will be shown here how the tie-bolt stretch as a manufacturing assembly operation can be related to rotor generated vibration. Further, through regression analysis an approach to optimizing this stretch a priori for a given set of hardware to minimize vibration for a production unit subject to production style detail part and assembly tolerances is presented. The investigation focuses on optimizing any specific stretched state to best position a rotor assembly in the zone between elastic and plastic limits so to actively minimize vibration to best approach analytical prediction. As a result of knowledge gained here, this approach can be used as a predictive tool for optimizing and projecting operational characteristics of a complex jet engine rotor assembly system with curvic couplings involving dis-similar homogeneous metals. Included in this investigation is the impact of straightness of a rotor assembly with multiple curvics and its impact on vibration, where minimizing or eliminating the undesired post assembly trim balance process (a.k.a. in situ balancing) or other reworking of the assemblies at final engine test was the objective.