Shaft-Hub Connections

The internal knurl pressure connection (IRPV) is characterised as a representative of non-positive and positive shaft-hub connection (WNV) by the transmission of high power densities in the field of drive technology. With the help of experimental and dynamic investigations of a wide range of parameters, existing design methods and failure mechanisms of the inverse method, the external knurl interference fit, are qualified and subsequently transferred into a standardised design specification. The intended research results will serve the future design of WNV, with which performance-limiting weak points in the drive train can be eliminated or significantly higher volumetric efficiency values can be achieved in new designs.

Translated with www.DeepL.com/Translator (free version)

The knowledge of the system reliability of machines and assemblies is essential for design, dimensioning and risk assessment. To determine system reliability, the basis is knowledge of the reliability or survival probability of the elements or components of the system. The survival probability is the percentage of a set of components that can withstand a certain stress. Various concepts exist for the calculation of system reliability. These require knowledge of the survival probabilities of the individual components. In a gearbox, the survival probabilities of gears, roller bearings and shafts are linked in series. However, the survival probabilities of the shafts or shaft-hub connections are still unknown. Therefore it cannot be taken into account for the calculation of system reliability. To determine the survival probability under cyclic loading, the stress acting in the component and the existing strength must be analysed. Shafts in gearboxes are usually designed to have a fatigue strength. Both the stress and the fatigue strength are not deterministic values, but are subject to a probability distribution. If the stress is a constant one-step collective, as is usually the case in laboratory tests, one can assume the survival probability of a strength value. Currently, there is no reliable knowledge of the scatter of a strength value in design concepts such as DIN 743 or the FKM guideline. However, this knowledge is necessary to select suitable safety values to avoid overdimensioning or unexpected damage. Therefore the aim of this project is to assign a scatter to the fatigue strength.

The aim of the project is to develop a new empirical calculation method for dimensioning of hypotrochoidal polygon-shaft-hub connections. By the development of empirical approaches for calculation the necessity of the application of the FE-method for the dimensioning of the shaft-hub-connections for the static torque transmission shall be eliminated and thus the practical application shall be simplified. Accompanying experimental investigations lead to calibration of examined finite-element calculations and to validate the developed calculation models. In addition they generate basic values for fatigue strength, which are also used for comparison with geometrical similar connections. In addition, the profile eccentricity and the number of tappets have to be optimized for the load cases torsion and the combined loading torsion and bending. The expected results should lead to a new standard for calculation of the polygonal connections with hypotrochoidal profiles. The industrial manufacturability of this profiles is ensured.

The procedure for the strength-side design of press-fit connections with high interference has not yet been sufficiently clarified. The reason for this was the inadequately investigated effects of material plasticizations as well as high medium stresses on the notch effect. Especially when using hollow shafts, which are becoming increasingly important, these two effects can be clearly seen. The proven design concepts DIN 743 and FKM guideline (nominal stress concept) reached the limits of their applicability here. The objective of the project was therefore to investigate the fatigue strength of highly stressed press connections with hollow shafts and to define a suitable design specification to be derived from this.
The basis for this was formed by our own analytical, experimental and numerical investigations on the two geometry variants "press-fit connections on hollow shaft" and " shouldered hollow shaft with press-fit connections". The influence of the material strength was evaluated by using the two materials C45E+N as well as the higher strength 42CrMo4S+QT. The individual loads alternating torsion and rotating bending as well as the static torsional load of the connection were considered.
It was shown that the design according to the nominal stress concept (DIN 743 or FKM guideline) using notch efficacy figures is the most suitable method for strength evaluation for the press-fit connection on a smooth hollow shaft. Due to the greater variety of parameters, however, notch factors deviating from solid shafts. For the investigated boundary conditions, the notch factors could be determined experimentally. A calculation specification for notch factors as a function of material strength was proposed. For the shouldered hollow shaft with press-fit connection it was proven that only a strength evaluation by using the "fatigue strength analysis with local stresses" in accordance with the FKM guideline is most effective. The occurring high mean stresses can be taken into account sufficiently precisely with the help of this concept. Furthermore, it could be shown that the underlying medium-stress sensitivity factor, although well reflecting the influence of the medium-stresses, has improvement potential to increase accuracy. In this respect, a proposal for incorporation into the FKM guideline was submitted.
Both for the press-fit on a straight shaft and for the shouldered hollow shaft with press-fit connection, the cause of the damage and the incipient crack position could be qualitatively conclusively explained with the developing elastic-plastic stress or strain state. In addition, the quantitative comparison with determined material-dependent stress and strain amplitudes shows good correlation. It was also demonstrated that the application of DIN 7190 for the calculation of the transferable torsional moment is also valid for the investigated hollow shafts.
The research results also provide a good basis for further research on this topic in order to increase the accuracy and for the statistical validation of the concepts mentioned and their parameters. Proposals for this have been developed.

Das IGF-Vorhaben IGF-Nr. 19149 BR der Forschungsvereinigung Forschungskuratorium Maschinenbau e.V. (FKM) wurde über die AiF im Rahmen des Programms zur Förderung der Industriellen Gemeinschaftsforschung und -entwicklung (IGF) vom Bundesministerium für Wirtschaft und Energie (BMWi) aufgrund eines Beschlusses des Deutschen Bundestages gefördert. Weitere Informationen erhalten Sie beim FKM unter info@fkm-net.de.