In this paper the dynamics of an important and frequently used robotic working process is investigated. The robot manipulator approaches, suddenly grasps and lifts a three dimensional object which is travelling on a conveyor. The spatial robot manipulator has two rigid links, rigid rotors and flexible joints. In order to model the joint flexibility, one-degree of freedom torsional springs have been used. The effects of continuous and discontinuous constraint equations are analysed. Simulation results are presented.