Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 342 included, one remained unable to guarantee against in-service crack growth likely to bring the PS about failure (see Fig. 1a). a) b) Figure 2 : (a) The joint (schematic) of the (1) propeller hub and (2) propeller shaft; (3) to (6) internal parts with (4 and 5) the sealing caps; (b) the sequences and durations of running periods; one can also see, at the ordinate, the dates and periodicity of inspecting the PS torn out in flight (see in Fig. 1a). Cracking took place either because of having the nuts (fastening the propeller) tightened unequally or as caused by the dynamic loadings (born by excessive misbalance). Effects of the said loadings revealed themselves in the propeller obliquely blown–up in flight. When insufficiently tightened, the PS flange experience tiny shifts and, consequently, local fretting-corrosion and cracking of splines occurs over their contacting areas. Experimental investigations of the dynamic stress patterns in the elements of a splined joint of an AV-72 airscrew shaft with an AI-24A engine shaft revealed the following. With the fastening nuts tightened uniformly, the level of the stresses (tensile component) applied dynamically to the fillet part of an PS remains insensitive to the momentum of tightening (in 9 to 18 kgm range) of the pin nuts pressing to one another the flanged portion and the body of an PS. Having reduced the tightening momentum from 13 to 9 kgm does not reduce the stresses in the PS fillet portion. In-service inspections of PS-to-engine junctions confirmed stability of the tightening momentums of the nuts fastening the AV-72 PS units to the AI-24 engine shafts. The cases of broken pins belonging to the splined joints ceased owing to the constructional improvements that brought about greater fatigue resistance of the pins, stronger tightening of PS to its hub, as well as greater total rigidity of the splined junction itself. Zones of extensively fretted splines shifted from the miner-module area to the middle part— between the pinholes. Nevertheless, statistically and regarding an equal running time, the PS units of an AI-24 (two series) engine, screened out on repairs in 2001 to 2003 years, showed that the above constructional improvements did not alter the cracking trends as compared with those of the ASS unites screened out in 1977 (Fig. 3).