Correlation and Repeatability Improvement Between Tie Bolt Stretch, Vibration Levels and Process Control Using Automated Stretch Tool and Regression Analysis – Part II

Abstract An Auxiliary Power Unit (APU) is a gas turbine engine with rotor systems that often use radial flow rotor assemblies. The rotor assemblies are made up of a compressor and turbine section which are joined mechanically for assembly and maintenance ease using curvic couplings. For the single curvic style of radial gas turbine, a compressor and turbine stage are joined using a tie-bolt mechanically fixing together the stages via radial contact through the curvic coupling with an associated torque retaining adapter. Our previous paper showed the promising correlation between plane unbalance and vibration level for this single curvic coupling style assembly. This portion of the study further extended into Quality Level process control. It also explored the measured modulus of elasticity as an enabler for additional predictive process control through assembly related measured and reported parameters. This paper also extended the analysis from the single curvic coupling by applying the same regression correlation concepts to the two curvic coupling model design. The two curvic design exists in some aircraft applications where the APU provides significant mass flow of pressurized air for main engine start. This model of APU is generally inclusive of an additional impeller joined into the powerhead rotor assembly. This three-rotor assembly is then affixed using two curvic couplings all indexed using a single tie-bolt and adapter. The preliminary potential for a successful regression correlation for the two curvic model to the analytical design was demonstrated in an initial review. This continued investigation presents the initial correlation state as well as the basis for the necessary design changes to the load impeller static and dynamic unbalance state. An up-change pilot lot group and later full production sample series demonstrate that the rotor stretch tooling and process improvements can reduce vibration levels on the two curvic model. This two curvic coupling assembly is also explored to show the influence of mode shape in the vibration levels in addition to unbalance as part of this study. This correlation study Part II as applied to the two curvic coupling also goes further to include additional process control parameter exploration such as runout as one of the important drivers among others that exist in the system specifically to reduce unbalance impacts. The Automated stretch tool and process improved the resultant APU rotor vibration to remain well within the analytical predictive design intent and final test vibration limit value. For the single curvic model, additional refinement of the regression model was undertaken toward more of a real-world representation to comparable modeled results.