Rotordynamic characteristics of an APU gas turbine rotor-bearing system having a tie shaft

In this paper it is intended to set-up a sound model of the 60,000 rpm 100 kW prototype APU gas turbine rotor-bearing system, and particularly to inve stigate the influences of the tie shaft on the rotordynamic characteristics of the entire APU gas turbine rotor-bearing system, employing the dual sha ft model. Firstly, a mock-up APU rotor has been constructed to test and ver ify the model. Analytical natural frequency results have agreed with the co rresponding modal test ones to within 5% difference. Then, the rotordynamic characteristics of the prototype APU rotor-bearing system have been invest igated. Natural vibration and unbalance response analyses results have show n that the inner tie shaft resonance can cause high enough vibration of the outer main rotor shaft. This could be a concern as the rotor journals oper ate on very thin air film at high speed. It is concluded as a conservative design practice that the inner tie shaft should be explicitly modeled in th e rotordynamic analysis of the APU rotor-bearing system.