Abstract
Failures of two power take off (PTO) shafts of an aircraft have been analysed. Two shafts, one each developed by two different manufacturers failed separately during power run endurance test conducted at room temperature and ambient normal atmosphere. In both the cases, cracks were observed on the outer diaphragm disc. One shaft showed cracking in the engine side, while the other one exhibited cracks in the aircraft mounting accessory gearbox (AMAGB) side. Chemical analysis, microstructure and hardness evaluation indicate that the diaphragm material of the shafts is Ti-6Al-4V alloy used in solution treated and aged condition, as per the desired specification AMS 4928. Microstructural in-homogeneity, possibly a result of improper forging, was observed in diaphragm material of both the shafts. Additionally, surface discontinuities induced by forging and subsequent insufficient machining were noticed on the diaphragms. The diaphragms failed by fatigue with cracks possibly nucleating at surface discontinuities. Discontinuities with lower availability in one shaft led to somewhat increased life (466 million cycles) as compared to the life (104 million cycles) of the other shaft. Another possible factor contributing to lower life in the later shaft is the presence of higher quantity of nitrogen rich inclusions. Controlling factor triggering the failure of diaphragm of shaft with lower life seems to be the available high stress level along the rim periphery, while that for shaft with higher life is presence of few localized sharp surface discontinuities.
Highlights
1.1 Background Information Two power take off (PTO) shafts, one each developed by two different manufacturers separately failed during a power run endurance test carried out at room temperature and ambient normal atmosphere
In case of shaft 1 (Fig. 5a), cracks were observed on the diaphragm disc at the engine side, i.e. close to the input end
In shaft 2 (Fig. 5b), cracks are observed on the diaphragm disc at the aircraft mounting accessory gearbox (AMAGB) side, i.e. close to the output end
Summary
1.1 Background Information Two power take off (PTO) shafts, one each developed by two different manufacturers separately failed during a power run endurance test carried out at room temperature and ambient normal atmosphere. In addition to these functions, these couplings are required to dampen vibration, reduce shock or peak loads, reduce noise, protect the equipment from overload or measure the output torque from the driven equipment, to accommodate thermal expansion of the equipment or any other components etc These couplings are often classified into following groups depending on various differentiating criteria: (i) general purpose or special purpose (high performance), (ii) lubricated or non-lubricated (dry), (iii) mechanical or metallic/elastomeric etc. Use of I.D. and O.D. sections of generous radii matching the disc profile reduces local stress concentration significantly and eliminates unexpected and unpredictable fatigue failures This is to ensure the maintenance of “infinite life” what is usually rated for a flexible diaphragm coupling by material selection and design under simultaneous actions of specified torque, speed and misalignments
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