Finite Element Analysis (FEA) is an accurate and powerful design analysis tool that some have begun to incorporate as a failure analysis technique. Here is short discription of FEA for those unfamiliar with this technology.

The finite element method simulates the thermal and structural behavior of a component or system subjected to a variety of loading conditions. This is accomplished by first mathematically dividing the structure into many small brick-like pieces, called elements (in 2 or 3 dimmensions) and then solving the applicable equation for these simple elements, and finally combing the elemental results to obtain the solution for the entire structure.

Thermal analyses may include convection, conduction, and radiation heat transfer, as well as various thermal transients and thermal shocks. Structural analyses may include all types of constant or cyclic loads, mechanical or thermal, along with non-linearities, such as permanent deformation of materials (plasticity), the opening or closing of contact surfaces, and friction. The results of finite element analysis simulate the actual response of a component or system very accurately.

Finite element analysis is used extensively by airospace, auto, nuclear, and small and large manufacturing industries. For this project we used the finite element method to find the root cause of a reccurring failure and redesign the system to avoid future problems.

That failure involved the fracturing of cap screws used to connect the platen to the beam in an injection-molding robot, during normal operations. The machine was designed to move parts from a molding machine to conveyor belts or stacking devices.

It was believed that the screws were fracturing as a result of cyclic loadings and fatigue. Minimizing fatigue sensitivity could be accomplished by redesigning the system. Due to the complexity of the structure and many, simultaneous loading conditions acting on the robot, the finite elements analysis was selected to simulate the failure and to improve the design.

Non-linear finite element analysis was performed using three-dimensional models of the main components of the robot assembly. These models were then subjected to static and dynamic loads. The dynamic reaction load (the most active 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 finite element simulation found that inadequate preloading (torquing) of the cap screws caused isufficient contact pressure between the riser and the beam. Consequently, the high alternating stresses resulted in a high cycle fatigue failure. Higher preloads naturally decrease the alternating stresses in the cap screws resulting a much longer operating life.