Design, analysis and construction of a fully compliant mechanism using FEM and 3D printing

The Goal of this work was to closely examine an existing compliant mechanism, in the form of a pair of pliers, using the Finite Element Method (FEM) and augment the design based on the results. Finally, the new design was then manufactured using 3D printing by the Institute of Production Engineering at the TU Graz.

A compliant mechanism gains at least some of its mobility from the deflection of its flexible members and often consist only of a single piece. In contrary to conventional mechanisms, which use joints and springs to connect rigid components, the compliant approach offers unique advantages.

These include:
•    Reduced part count
•    Different production processes can be used
•    Precise motion
•    No friction wear
•    Compact and lightweight

However, there are also some disadvantages, one of them is that the design of such mechanisms is more cumbersome, due to the fact that even small changes to the geometry can have a significant impact on the performance, and material fatigue plays a greater role than in traditional designs.

This design was well suited for the analysis. The reasons for that are firstly the potential of implementing changes later on and the relative simplicity of the geometry. This simplicity enabled the simulation to be two dimensional, which greatly lowered computing time and made the geometry creation simple.

A major part of the FEA was to identify the optimal contact settings, which was done by testing different combinations of algorithms, surface detection methods, surface behaviours and tolerances.

In most cases, adding a contact pair to the systems results in convergence problems, which can either be fixed by altering the contact settings or by applying the loads over a larger time span.

For the analysis to converge, it was necessary to divide the desired load into smaller parts, called load-steps (LS). In each LS the applied loading is increased by a specified amount, the size of these steps must be chosen individually for each system and for each point in time. The necessary LS sizes were found through multiple tests.

Through many iterations and experimenting with the settings of ANSYS, it was possible to gain some insights about the stress distribution of the original design. After interpreting these results, it was clear that the maximum stress needed to be drastically reduced in order to remain under the value given by the materials tensile strength. To achieve this goal, several changes were implemented and tested.
After simulating a certain amount of different design, a final design was chosen for 3D printing. The Institute of Production Engineering at the TU Graz has agreed to cooperate in this endeavour and two versions were printed. One of them broke at the first test and the other one deformed plasticly. Both confirmed the simulation regarding the location of the maximum stress and the stress value to a certain degree.

Further research on this topic would eventually lead to a design which is able to withstand the strain during the working motion without breaking or developing plastic deformations.