Independence of friction self-locking from kinematic singularity

Self-locking analysis in closed kinematic chains is sometimes likened to kinematic singularity. Here a novel approach to tackle self-locking analysis due to joints friction is exploited, that is completely different from the classical kinematic analysis based on the jacobian conditioning. It is shown that an inverse kinematic singularity always entails a self-locking phenomenon because of the general increasing of joints reactions and, then, friction forces; hence, a self-locking domain can be always identified including such a locus. On the other side, this paper is aimed at demonstrating that the aforementioned condition is not necessary: namely, self-locking may occurs also if the mechanism kinematics is well-conditioned. Then, the theoretical result is clarified performing the self-locking analysis on a simple crank-slider mechanism.