The Influence of Motion Paths and Assembly Sequences on the Stability of Assemblies

In this paper, we present an approach for the stability analysis of mechanical part disassembly considering part motion in the presence of physical forces such as gravity and friction. Our approach uses linear complementarity to analyze stability as parts are moved out of the assembly. As each part is removed from the assembly along a specified path during disassembly, we compute the contact forces between parts in the remaining assembly; positive contact forces throughout the disassembly process imply the disassembly sequence is stable (since the parts remain in contact with one another). However, if the part that is being taken out induces motion of other parts in the remaining subassembly, we conclude the disassembly sequence is unstable. Thus, we are able to simulate the entire disassembly considering physical forces and part motion, which has not previously been done. We then show the influence of part motion on stable disassembly. In contrast to prior work on disassembly that has focused either on planning part motions based on only geometric constraints, or on analyzing the stability of an assembly without considering part motions, we explore the relation between part motion and the selection of stable disassembly sequences in 2-D and 3-D. We establish conditions that characterize path-dependent assemblies, where motion paths can play a significant role in stable disassembly. Since we track the motion of all parts in an assembly, instability inducing motions can be identified and prevented by introducing appropriate fixtures by selecting alternative disassembly sequences or by changing the motion paths. We extend the stability analysis for single part disassembly to stability analysis of subassembly disassembly. We additionally show that in the presence of friction, assembly and disassembly can be noninvertible.