When choosing a power transmission coupling for a particular application, several areas of concern must be attended to. Among these are the operative speed and horsepower, misalignment between driver and driven equipment and environmental conditions. However, neither the coupling nor any component used in the application can be considered safe from damage until an analysis of torsional vibration has been carefully completed. In the following paper, case studies of torsionally and environmentally induced failures will be presented. By presenting case histories, along with system models and the results of onsite measurements, it is hoped that the importance of these design steps are not overlooked. Additionally, causes and solutions for spline shaft wear will be discussed.