As rotary power take-offs (PTO) are showing promising advantages for commercial wave energy converters (WECs), especially as rope elements are replaced with composite polyurethane-steel tension belts, the service life predictions of the PTO are becoming a major concern in design. Using belts instead of ropes for winching elements is shown to improve Cyclic Bend Over Sheave (CBOS) performance with a smaller bending thickness for the belt. Yet, with the belt subject to three-dimensional fleet angle rotations, stress concentration and reduced contact area cannot be avoided. Earlier experimental work attempted to quantify the CBOS performance of a hybrid polyurethane-steel belt tested to up to two million bend-unbend. One way to estimate the service life of such belts is to analyze their behavior within the scope of wear and tear models. This would necessitate accurate stress predictions for possible fleet angles.
In this work, three-dimensional equivalent static finite element models are constructed to evaluate the complex stress state of polyurethane-steel belts around steel drums. Two models are established, the first is to capture the response of the aforementioned experimental work and the second is for an existing functional PTO. In both models, the belt is represented with a homogenized equivalent polyurethane model to consider the change of stiffness due to steel reinforcement. The drum is being presented with elastic-plastic steel model and allowed to rotate around its center. In both models, the following parameters are being studied: 1) friction coefficient between the belt and drum ranging from 0.15 to 0.65, 2) In-plane load misalignment up to 3 degrees, and 3) torsional fleet angles rotation of the belt up to 3 degrees. Different friction coefficients are used to define a simulation matrix with each of In-Plane load misalignment and torsional fleet angles. In this study, In-plane load misalignment and torsional fleet angles were not considered to have a high probability to happen concurrently. In fact, they are each considered as a unique idealization for the fleet angle problem.
The models are implemented in the commercial code ANSYS structural solver to simulate the stress concentrations. The resulting stress states are then used to verify the former experimental program and predict the service life of the existing functional PTO. The acquired results show a significant effect for torsional fleet angles rotation of the belt on stresses concentrations and estimated service life than In-Plane load misalignment. Thus, the author recommends considering using torsional fleet angles rotation of the belt as a numerical equivalent model in future work to predict CBOS performance and belt service life.
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.