In the aerospace industry, many gas turbine compressors rely on a tie bolt to mechanically hold together all rotating components in the compressor rotor. Maintaining this clamp load is essential to the performance of the engine. In the event of an unclamp, the engine will experience a reduction in tip clearance due to a change in rotational dynamics; increased temperatures and pressures in secondary air systems; and a decrease in critical component life. Accordingly, designers must be aware of the variables effecting compressor rotor clamp loads observed for component assembly and operational missions. During testing, an axial gas turbine engine unexpectedly experienced a compressor rotor unclamp which led to an increase in turbine temperature and front sump buffer air temperature and pressure. Further investigation revealed a thermal expansion mismatch between the tie bolt and inner gas path rim during a specific transient condition. Because of this thermal effect, the rotor will experience an unclamped condition which will result in ingesting compressor discharge air into the drum. The compressor rotor will remain unclamped until the engine is operated at a lower power setting or shut down for an extended period. This paper documents and explains the transient condition at which the engine experienced unclamp through review of test data, characterizes the design space around the tie bolt by using heat transfer and structural finite element analysis codes, and shows how robust design tools were used to find an optimized solution that eliminated the risk of thermally driven unclamp through robust design and assembly choices.

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