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Robot Assembly Cap Screw Failure
A side view of the system is depicted to the left. The robot has a beam (teal), along which a mobile traverses to move the parts from the molding machine to conveyor belts or stacking devices. The entire robot is secured to the injection-molding machine on the stationary platen (blue plate below beam) by grade 8 socket cap screws. Similar screws are used to connect the top of the riser to the robot beam. Several of these cap screws were failing during normal operations. Before the system was to be redesigned non-linear finite element analysis was performed using three-dimensional models of the main components of the robot assembly. See below. These models were subjected to static and dynamic loads. The dynamic reaction load of the robot operation was applied to the models to determine the cyclic stresses in the cap screws connecting the riser to the beam. The motor assembly weight, the mobile weight and the moving weight were applied as individual forces. Weights of other components were determined by defined geometry and properties specified by the modeling software. Two finite element models were constructed to simulate normal operations. One model simulated a fully extended robot arm for removing the part in normal operations. The second model simulated conditions when the arm was closest to the riser for delivering the part.  The simulation found that preloading (torquing) the cap screws increased the stress resistance of the system and consequently, the service life of the system would lengthen. The table on the following page shows the stress resistance of the 8 cap screws assuming preloading conditions. The best result was with 90 ksi preloading. The difference can be explained by alternating stresses that are present without preloading and not with preloading. Sufficient contact pressure prevents alternating stress and thus, a higher preload can withstand greater stress. As one of the premier testing and inspection facilities featuring state of the art equipment, Matco Associates is able to provide a unique and diverse blend of services. Virtually thousands of investigations have been performed at Matco Associates, which range from jet engines and boiler tubes, to computer components and implants. Projects are not only conducted from initiation to completion with competent and experienced personnel, but conducted in a discrete manner that ensures confidentiality as well.
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Cap screws in a robot designed for the removal of parts after they are molded in an injection-molding machine failed as a result of cyclic loading and possible vibrations in the system. Redesigning the system to minimize fatigue sensitivity was recommended. Some of the failed screws are shown at left.