Abstract: One of the popular artefacts available in people's collections is introduced as a highly expandable and collapsible structure. Starting by removing the kinematic chains which do not influence the mechanism mobility, the magic ball mechanism is hypothetically disintegrated into several kinematic chains, and subsequently two platform-like kinematic sub-chains which have three legs and four legs respectively. The theory of a screw system and its reciprocal system is used lo examine the virtual constraints and common constraints in each of the kinematic sub-chains and to further examine the mobility of those sub-chains, Based on a systematic analysis in mechanism decomposition and in the analysis of virtual constraints, a theoretical basis for the mobility analysis of the mechanism is produced and then its characteristics of mobility are obtained. The result is verified by using the loop-connectivity approach.

Key words: Kinematic chain Platform, Mobility Screw system Metamorphic mechanism