Analysis and Synthesis of Shell Flexures

Compliant shell mechanisms are defined as spatially curved thin-walled structures able to transfer or transform motion, force or energy through elastic deflection. They are a sub-category of compliant mechanisms which also gain there motion from elastic deformation. As such they store energy during motion, in addition to providing desired kinematics. One major benefit of this attribute is that several functions of a mechanism or a machine can be integrated into a single monolithic part; this is often called function integration.

Certain force-deflection behaviour can be purposefully designed by tailoring the energy storage over the range of motion. This is useful for passive exoskeletons where shell mechanisms are used to compensate the user's body weight and thereby decrease the fatigue accumulated during work. Other applications can be medical devices which often need specific kinetics while operating in a small environment. Shell mechanisms or shell flexures provide different kinetic behaviour than their flat counterparts: the wire flexure and leaf spring flexure. These properties of shell flexures can be leveraged to create more compact force generators.

Shell mechanism research is a relatively new field, with articles introducing novel designs with a specific behaviour in mind, such as constant force or moment generators. The state of the art presents what shell mechanisms are capable of. However, the state of the art provides little guidance in how to analyse and design shell mechanisms in general. The objective of this thesis is to propose tools for the analysis and design of compliant shell mechanisms or flexures and to develop understanding of this class of mechanisms. This thesis is divided into three parts.

Part I presents the eigenscrew decomposition as a tool to understand and design the kinetics of all compliant (shell) mechanisms. Part II discusses the properties of a buckled tape spring and a method to synthesise a wide array of force-deflection behaviour. In Part III, a novel category of shell mechanisms is introduced. A curved surface is patterned with a lattice, which is able to deform in the membrane of the shell. This is opposed to other shell mechanism that work primarily through the bending of the membrane.