PSI - Issue 17

Felix Kresinsky et al. / Procedia Structural Integrity 17 (2019) 162–169 Felix Kresinsky / Structural Integrity Procedia 00 (2019) 000 – 000

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A comparison of all opened specimens shows that the keyed shaft-hub connection always fractures according to crack II at 45° to the shaft axis, but the location of the crack initiation varies. In some keyed shaft-hub connections, the crack is initiated at or near the shafts surface at the keyway rounding (Fig. 5 a). In other connections the crack starts at the bottom radius of the keyway (Fig. 5 b). The location of the crack initiation depends on the load level. With lower loads, the crack tends to begin at the keyway bottom radius. The main focus was on the failure criterion of inadmissible keyway expansion. Therefore just a few cracked specimens could be investigated and it was not possible to deduce a clear tendency from the few fractures. Further investigations in this regard are currently underway at the Institute. All previous investigations regarding the maximum permissible keyway expansion are related to keyed shaft-hub connections with low-strength shaft materials (C45+N). At keyed shaft-hub connections made of higher strength shaft materials (42CrMo4+QT, 16MnCr5E case hardening) the shaft always cracks before the shaft keyway could expand critically. The different failure criteria are shown in Fig. 6 a depending on the shaft material. The result for C45+QT is taken from Bruzek (2014). For low-strength materials, a keyed shaft-hub connection fails by reaching the maximum permissible surface pressure before the fatigue strength of the shaft. For higher strength materials, a keyed shaft-hub connection will fail due to shaft breakage. Consequently, the DIN 743 (FKM analogous) is decisive for the design of high-strength keyed shaft-hub connections. The permissible torque shown in Figure 6 a is the maximum torque of a swelling load. In addition to the different failure criteria, Figure 6 a shows that the permissible torsional moments of high-strength materials are barely higher than those of low-strength materials. The higher material quality cannot be transferred to an increased durability of the component. Hofmann (2015) has already made similar observations for keyed shaft-hub connections with bending load. 3.3. Variation of shaft material

Fig. 6. a) Different failure criteria based on the yield strength of the shaft material (d = 40 mm); b) Comparison of the experimentally determined notch effect factors with the notch effect factors according to DIN 743

The notch effect factures recalculated from the experimentally determined permissible torsional moments (cf. Bruzek (2011)) are shown with the notch effect factures according to DIN 743 (FKM analogous) in Figure 6 b. It is noticeable that the experimental values deviate towards the unsafe side, especially with the high-strength shaft materials. There is therefore an urgent need for further research here. Since the focus of the investigations was on the maximum permissible surface pressure, the shaft fractures for the materials 42CrMo4+QT and C45 (above the permissible limit load) resulted from random tests and the associated design strengths are therefore not sufficiently statistically verified. The permissible torsional moment for 16MnCr5E is statistically secured by a step tests evaluated according to Hück (1983).