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Design of Mechanical Transmissions: Efficiency, Noise and Durability Optimization

Final Report Summary - DEMETRA (Design of Mechanical Transmissions: Efficiency, Noise and Durability Optimization.)

The overall objective of the EC Marie Curie IAPP project “DEMETRA” has been to enable a transfer of knowledge between partners with complementary expertise and test facilities in the field of mechanical power transmissions, which are very important for the optimal performance of mechanical structures as cars and wind turbines. Due to increasing awareness on environmental concerns, stringent targets for energy efficiency and emissions are being urgently set in the frame of worldwide agreements. Product and process innovation is required by the involved European industries and R&D parties in order to meet the targets. More specifically, industry is forced to improve its design under conflicting requirements between customers (increasing performances) and regulatory bodies (higher efficiency and sustainability). Supporting and promoting innovation is of paramount priority to allow them surviving in the globalized and competitive market. Both energy generation and usage are forced to become greener. Major players in this direction are represented by clean energy sources and automotive field to reduce pollutant gases and fuel consumption. Mechanical transmissions are crucial components in both sectors, as they are responsible for efficient dynamics and hence key to avoid energy losses (friction).
DEMETRA project has studied promising state-of-the-art modelling methodologies and advanced methodologies for two main sub-systems in a transmission (i.e. gear trains and bearings). The aim has been to combine promising sub-system models to analyse and optimise a mechanical transmission as a complete coupled dynamic system. A systematic approach was followed for the development of advanced simulation methods, from upfront until the detailed phase, supported by experimental validation. The resulting technology is perfectly in line with the efforts of the European Community to increase efficiency of industrial products and processes, while maintaining or further improving the quality.

Three main approaches are available to predict the dynamic behaviour of a mechanical transmission: analytical modelling, where the system is simplified in a few degrees of freedom using spring-damper elements for the main elasticity of the system (gears and bearings); Finite Element (FE) modelling, where the detailed geometry of the components is analysed with a 2D or 3D approach; Multi-Body Simulation (MBS) models, where components are represented in 3D. The analytical approach is useful to study very simple systems, where only few components dominate the dynamic response. On the other end of the spectrum, the FE approach offers a higher level of detail, provided that a sufficiently fine mesh is used to describe tooth surfaces. The computational cost of a full FE approach may be prohibitively large on a system level with operational loads. The Multi-Body approach offers the prospect of sufficient accuracy with feasible calculation times, and therefore has high potential in non-linear dynamic coupled simulations of mechanical transmissions. However, accurate predictions of the dynamic response can be obtained to tackle industrial applications only if all relevant phenomena in gears and bearings are correctly captured. For this purpose, DEMETRA has pursued innovations in general methods and the development of dedicated approaches to capture all the relevant phenomena (contact mechanics, tooth and body flexibility, parametric excitation, ...) as to enable accurate but yet efficient system-level simulation and optimization of mechanical transmissions.

The targeted results were achieved through the execution of a research program consisting of six technical work-packages:
WP1 - Industrial requirements: simulation and testing
The purpose of WP1 was to create strong connections and continuity with the state of the art research and industrial development context. The execution of the research tasks foreseen under this first work-package allowed to collect industrial requirements for simulation and testing of mechanical transmissions. A synthesis of industrial needs and an inventory of use cases in different industrial sectors have been made. WP1 was configured as to serve as an exchange platform between the academic knowledge and the industrial experience brought by the DEMETRA partners during the entire lifetime of the project.

WP2 and WP3 - Gear train and bearing modelling
These two work packages allowed to advance the state of the art in dynamic modelling of mechanical transmissions through the development and/or assessment of scalable-detail non-linear dynamic simulation methodologies for contact mechanics in the two core components: gear trains and bearings. Techniques that allow to include tooth micro-geometry description and to capture the real-case effects of system flexibility have been developed and assessed against state-of-the-art techniques. The investigated numerically-efficient approaches were instrumental to the representation of gears and bearings in three dimensional simulation environments, in view of enabling system-level calculations of industrial models as a required step towards reaching an optimal balance of multiple and conflicting attributes.

WP4: System-level integration
Scalable-detail models from WP2 and WP3 represented the necessary input to provide the foundations of WP4. Throughout the project, the DEMETRA partners worked in synergy to integrate the separate models for the core components (gears and bearings) with the supporting structure (from shafts to housing) and with the user/generator models, in order to create a non-linear dynamic simulation environment that is capable of supporting the development of mechanical transmissions in any phase of the design process.

WP5: Validation by testing
The overall aim of the fifth work package was to provide experimental validations of simulation models developed in the project at the level of core components and at the level of full-system behavior of mechanical transmissions. For this purpose, a precision gear test setup available in the network was used to perform detailed testing using heavily instrumented components, under controlled operating conditions expressed in terms of misalignments, applied load and speed.

WP6: Multi-attribute system optimization
The sixth work package was directly related to the outcome of integrating all the sub-systems into a non-linear simulation platform. Optimization criteria were selected for a number of case studies and strategies for the optimization at different levels (individual gear pair, gearbox, entire transmission) and with multiple objectives (mass reduction, efficiency enhancement, noise & vibration improvement, etc.) were implemented, assuming both micro and macro-geometry parameters as design variables.

The final S&T outcome of DEMETRA has been the development of comprehensive simulation methodologies capable of modelling core components and key phenomena with sufficient detail but still with high computational efficiency, as to enable component and system-level multi-attribute optimization of mechanical transmissions.
The full achievement of the scientific and technology development objectives of the DEMETRA project has been enabled by a multifaceted Transfer of Knowledge (ToK) program, in which the main action was represented by research staff exchanges between the partners: the inter-sectoral secondments of 8 researchers with different levels of experience (early stage, experienced and more experienced researchers) were implemented for an overall duration of about 92 person-months. Instrumental for the achievement of the project goals was also the recruitment of three experienced researchers from outside the DEMETRA consortium for an overall duration of 56 person-months. Thanks to this comprehensive ToK programme, enriched with frequent visits by the involved researchers and regular technical meetings, a continuous exchange of information, knowledge and best-practices between industrial and academic partners was achieved. The resulting cross-fertilization of industrial needs/expectations and state-of-the-art research methodologies enabled the involved research teams to address the multidisciplinary and challenging aspects involved in the design, analysis, verification and optimization of industrial mechanical transmissions.
The involved researchers (seconded and recruited within the project) were the main beneficiaries of the training and knowledge sharing activities that have been implemented within the DEMETRA project. With the aim of promoting their career development, they underwent a comprehensive training programme that included state of the art technical training on the main topics addressed in the project (contact mechanics, FE and MBS simulation, experimental testing, multi-objective optimization, ...) as well as training on transferrable skills development (team-working, fund-raising, language training, IPR protection and valorisation, ...). In addition, the wide exposure of the DEMETRA fellows to the international community of scientists and researchers was achieved by encouraging their participation to multiple external events (workshops, conferences, courses) and internal training and dissemination activities. Fundamental for their full career development was also their direct involvement in the DEMETRA communication programme. The latter consisted of more than 50 dissemination and outreach activities, including scientific contributions to international journals and conferences, the co-organization of international events for scientific and transferrable skills training, the organization a special session at the 2016 International Conference on Noise and Vibration Engineering (ISMA2016), the publication of articles describing the project outcome in regional and international magazines and newspapers. Worthy to mention are also two video clips that the DEMETRA fellows have prepared and that have been uploaded to the project website to further promote the project and the Marie Curie IAPP scheme, under which this project has been funded.
For more details on the training, networking and dissemination events organized in the frame of the DEMETRA project, please visit the project website at the address