Effects of misalignments on the static and dynamic behaviour of a multiple overconstrained compliant 4-bar mechanism

Abstract

The use of overconstrained mechanisms is often avoided in precision mechanics. Misalignments in the mechanism can cause deteriorated system behaviour, such as buckling. Overconstrained designs do have several advantages, such as higher load bearing capacity and higher natural frequencies. However, these advantages are only present if the mechanism is aligned within certain tolerances. In this paper a method is introduced to identify the limits of these alignment tolerances. The method allows the calculation of the forces in the mechanism due to misalignment. The internal forces are compared to the buckling loads of the mechanism yielding the critical misalignments; the method is corroborated using a multibody simulations. Subsequently, both analyses are compared to an experimental setup; this setup measures the first three modal frequencies and identifies the buckling modes. The proposed method and multibody simulation match with each other and the experiment. However, the critical misalignments are about 20% larger in the experiment; this is mainly attributed to hardware imperfections. Due to misalignment and flatness limitations of the flexures, the undeflected stiffness in the experiment is lower than modelled. The deterioration of the support stiffness is smaller in the experiment. In the most serious case, it retains 80% of the modal frequency in the support directions. The proposed method can be used as a guideline to estimate the manufacturing and assembly tolerances of an overconstrained flexure-based mechanism.