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In recent years, collision safety between a human and a robot has drawn much attention as service robots and humanoids are increasingly being used in the human environment. Safety of a robot arm can be achieved by either active or passive compliance system. Since active compliance systems with actuators are usually slow and expensive, several passive compliance systems with purely mechanical elements are proposed. Passive systems can provide faster response to collision and higher reliability than active systems. Since both positioning accuracy and collision safety are equally important, a robot arm should have very low stiffness when subjected to a collision force greater than the one causing human injury, but maintain very high stiffness otherwise. In order to implement these requirements, a safe joint mechanism composed of a linear spring, slider-crank mechanism, and 4-bar linkage is proposed in this research. Various experiments on static and dynamic collisions show high stiffness of the SJM against an external force of less than the pre-determined threshold force, but an abrupt drop in the stiffness when the external force exceeds this threshold, which guarantees positioning accuracy and collision safety.