Spatial Straight Line Linkages by Factorization of Motion Polynomials
  We use the recently introduced factorization of motion polynomials for constructing overconstrained spatial linkages with a straight line trajectory. Unlike previous examples, the end-effector motion is not translational and the link graph is a cycle. In particular, we obtain a number of linkages with four revolute and two prismatic joints and a remarkable linkage with seven revolute joints one of whose joints performs a Darboux motion. 
Zijia Li, Josef Schicho, Hans-Peter Schr\"ocker
math.MG cs.RO math.RA