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Digital Twins offer vast potential, yet many companies, particularly small and medium-sized enterprises, hesitate to implement them. This hesitation stems partly from the challenges posed by the interdisciplinary nature of creating Digital Twins. To address these challenges, this paper explores systematic approaches for the development and creation of Digital Twins, drawing on relevant methods and approaches presented in the literature. Conducting a systematic literature review, we delve into the development of Digital Twins while also considering analogous concepts, such as Cyber-Physical Systems and Product-Service Systems. The compiled literature is categorised into three main sections: holistic approaches, architecture, and models. Each category encompasses various subcategories, all of which are detailed in this paper. Through this comprehensive review, we discuss the findings and identify research gaps, shedding light on the current state of knowledge in the field of Digital Twin development. This paper aims to provide valuable insights for practitioners and researchers alike, guiding them in navigating the complexities associated with the implementation of Digital Twins.

In recent years, electrical damage phenomena in rolling and plain bearings have become increasingly important due to the growing electrification of individual mobility and renewable energy technologies. Being a major root cause, the electrically induced bearing failure has been intensively analyzed in fundamental and application-oriented research. Even though being of general importance for, e.g., electric mobility, much of the research work is published in German; this special issue shall provide a comprehensive overview in the topic of the behavior of lubricated bearings in electric circuits. The models available in the literature by Furtmann [1,2] and Gemeinder [3] for describing the electrical impedance behavior of rolling element bearings were developed for the analysis of electrically induced bearing damage [4]. The current impedance models of rolling element bearings assume operation in the elasto-hydrodynamic range according to Prashad [5] and describe the individual rolling contacts under full lubrication as plate capacitors that are electrically connected via the components of the rolling element bearing. The rolling contact between the rolling element and the bearing raceway develops a Hertzian contact surface, which represents the surface of the plate capacitor. The models of Furtmann [1,2] and Gemeinder [3] are based on the consideration of the loaded rolling elements only; Schirra extends the approach by including the load-free rolling element contacts for sensory purposes [6,7]. For journal bearings, due to the absence of rolling elements and the rather plain bearing surfaces, more sophisticated models, such as the cylinder capacitor, can be used to achieve a good agreement between model predictions and test rig measurements, e.g., for the lubricating film thickness or other operation conditions [8,9,10,11]. However, such methods cannot be used for insulating coatings, such as those found in tilting pad bearings with, e.g., PEEK thrust pads forming an

The integration of Digital Twins (DTs) is becoming increasingly important in various industries. This entails the need for a comprehensive and practical IT infrastructure framework. This paper presents a modifiable medium-level architecture that serves as a link between established reference architectures such as RAMI 4.0 and the pragmatic implementation of Digital Twins. The functionalities of an IT infrastructure are considered, and functional hardware and software components for fulfilling these are described. The proposed architecture is suitable for various deployment scenarios, including local, cloud and hybrid cloud–edge configurations. In order to improve the applicability of the medium-level architecture, a step-by-step procedure is also proposed, which helps to transfer the overarching requirements for a Digital Twin into a suitable IT infrastructure. Finally, the results are demonstrated by an exemplary application to a two-stage industrial gearbox.

The ongoing digitalization of processes and products in mechanical engineering is accompanied by an increasing demand for data. In order to provide this data, technical systems are being extended with sensory functions. To supply those sensory functions on rotating elements — such as shafts — with electrical energy, and to be able to transmit signals out of the system, sliding contacts can be used as a cost-effective and established solution. However, if electrical properties of machine elements are utilized for sensing purposes, such as condition monitoring of rolling element bearings by means of impedance measurement, sliding contacts are directly in the measurement path and can thus influence the measured impedance. The aim of this paper is to analyze the impedance of graphite and silver graphite carbon brushes under different rotational speeds, in different positions, and with different carrier frequencies. The material of the carbon brushes as well as the position have significant effects on the impedance behavior. Furthermore, carbon brushes show a significant running-in behavior. The results are discussed, and indications for use in impedance measurements are given. Silver graphite carbon brushes in axial positioning are particularly suitable for impedance measurements of sensory utilizable machine elements. Sufficient running-in time must be considered.

In Zeiten dynamischer Märkte und internationalen Wettbewerbs erlangt die Verkürzung von Produktentwicklungszyklen immer größere Bedeutung. Gleichzeitig werden auch die Möglichkeiten zur Herstellung von Prototypen zur Unterstützung der Produktentwicklung immer zahlreicher und diverser. Dieser Artikel beschäftigt sich mit der Ermittlung von Anforderungen an funktionelle Prototypen. Mithilfe dieser Prototypen sollen verlässliche Verifikationen der Einhaltung von Anforderungen an verschiedene Produktfunktionen ermöglicht werden. Als Beispiel werden dazu Produkte herangezogen, die im Spritzgussverfahren aus verschiedenen Kunststoffen hergestellt werden. Die konventionelle Verifikation dieser Produkte ist durch die Fertigung und den Einsatz von Prototypenwerkzeugen sehr zeitaufwändig und kostenintensiv. Der Einsatz additiver Fertigungsverfahren bietet die Möglichkeit, diese Faktoren zu senken. Hierbei muss jedoch sichergestellt sein, dass die Ergebnisse der Tests mit additiv gefertigten Prototypen auch auf das spritzgegossene Produkt übertragbar sind. Durch die Änderung des Fertigungsverfahrens und des verwendeten Materials treten Änderungen gewisser Eigenschaften des Prototyps im Vergleich zum finalen Produkt auf. Diese Abweichungen können sich wiederum negativ auf die Aussagefähigkeit der Ergebnisse der Verifikation mittels Prototypen auswirken, wenn die Eigenschaften essentiellen Einfluss auf die zu verifizierenden Funktionen haben. Daher wurde eine Methode – das House of Properties – entwickelt, mit der zum einen der Bedarf an Prototypen ermittelt werden kann und zum anderen die Eigenschaften des in der Entwicklung befindlichen Bauteils dahingehend untersucht werden können, welche Eigenschaften besonders kritisch auf bestimmte Funktionen wirken. Durch diese Analyse können Anforderungen an additiv gefertigte Prototypen identifiziert werden, die als Basis zur Auswahl des Fertigungsverfahrens und des Materials dienen. Die entwickelte Methode wird im Rahmen dieses Artikel