The first reduction gear set in a turboprop engine gearbox was changed from a spur configuration to helical in order to reduce dynamic load and tooth stress. During initial strain gage testing, a low frequency modulation (approximately 70 Hz) of the tooth engagement strain was observed. The modulation had not been present during previous strain gage tests of the gears in their original spur configuration. The expected decrease in dynamic load and tooth stress was not realized due to the low frequency modulation. Post test inspection revealed indications of end loading on both the forward and aft ends of the gear teeth. Additionally, a 22/rev standing wear pattern developed on the aft face of the bull gear rim where it contacts the high-speed pinion bearing thrust collar. Detailed analysis of the strain gage data coupled with traveling wave theory identified the source of the modulation as 19/rev response of a forward traveling three nodal diameter mode of the bull gear at approximately 2,500 Hz. An analytical simulation of the high-speed pinion tooth mesh multiplied by a signal representing a forward traveling three nodal diameter response of the bull gear exactly matched the observed modulation. Design-of-experiment engine tests using proximity probes to measure bull gear vibration identified improper contact at the bull gear thrust collar as the excitation of the three nodal diameter mode. A verification strain gage test with proper contact showed no modulation in tooth engagement.

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