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Title: Novel design and geometry for mechanical gearing
Author: Chiappetta, Erasmo
ISNI:       0000 0004 8503 7055
Awarding Body: Oxford Brookes University
Current Institution: Oxford Brookes University
Date of Award: 2018
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This thesis presents quasi-static Finite Element Methods for the analysis of the stress state occurring in a pair of loaded spur gears and aims to further research the effect of tooth profile modifications on the mechanical performance of a mating gear pair. The investigation is then extended to epicyclic transmissions as they are considered the most viable solution when the transmission of high torque level within a compact volume is required. Since, for the current study, only low speed conditions are considered, dynamic loads do not play a crucial role. Vibrations and the resulting noise might be considered negligible and consequently the design process is dictated entirely by the stress state occurring on the mating components. Gear load carrying capacity is limited by maximum contact and bending stress and their correlated failure modes. Consequently, the occurring stress state is the main criteria to characterise the load carrying capacity of a gear system. Contact and bending stresses are evaluated for multiple positions over a mesh cycle of a contacting tooth pair in order to consider the stress fluctuation as consequence of the alternation of single and double pairs of teeth in contact. The influence of gear geometrical proportions on mechanical properties of gears in mesh is studied thoroughly by means of the definition of a domain of feasible combination of geometrical parameters in order to deconstruct the well-established gear design process based on rating standards and base the defined gear geometry on operational and manufacturing constraints only. From this parametric study, suitable suggestions for enhancing the load carrying capacity of the tooth flank are made by showing that the use of non-standard geometric parameters can improve the performance of gears. As this study also aims to improve the performances of epicyclic gearings specifically for low speed-high torque operating conditions, the optimum parameters found in the preliminary parametric analysis were applied to this category of systems. The design procedure based on the area of existence of gear geometry was extended to this case which required the determination of the domain of feasible combination for gears in internal mesh with the addition of constraints addressed to epicyclic configurations. Three epicyclic systems with same boundary design conditions but different combination of geometrical parameters have been modelled and analysed by means of quasi-static FEA. The results have shown that the improvements found for the case of two mating spur gears are also valid for the case of higher order systems in which multiple contacts are simultaneously occurring. Based on these results, suitable suggestions are made for the design of gears working in epicyclic systems for an enhanced torque capacity and a volume reduction for applications characterized by low speed and high loads conditions. An alternative solution to geared systems that guarantees compactness and high torque transmission capabilities has also been investigated; it consists of a cycloidal transmission system. The parametric equations for the cycloidal profile have been determined and an executive design, then manufactured, has been produced. The preliminary quasi-static Finite Element analysis has predicted the load sharing and stress distribution among multiple components confirming the mechanical advantage of this category of transmission systems.
Supervisor: Morrey, Denise ; Durodola, John Sponsor: Norbar Torque Tools Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral