JPS Teknik AB in Färila, located in the middle of Sweden, is a company in manufacturingsaw units to harvester heads. This thesis is about the company's best selling saw unit, JPSR5500. The saw unit is powered by a hydraulic motor, the torque is transmitted from themotor into the saw unit thought a spline coupling. In this spline coupling, some of the unitshave been weared out after about 2000 harvester machine hours. When the splines wear outthe failure induce a total stop, the bolt connecting the hub and the motor shaft breaks, and thesaw unit assembly get loose. The purpose with this study is to find the root cause of thefailure and the goal is to give suggestions to improve the design to avoid failure. The method used to achieve the purpose is based on Ullman's mechanical design processwhere a root cause analysis is a central part. This analysis is built on two main pillars, aliterature study concerning previous research about wear in spline couplings and a knowledgebased study about the product with a customer focus. A sample of previous research in thistopic is about the load distribution in a spline coupling due to different load cases and angularmisalignment, wear mechanisms and the effect of washers in a pretension bolt joint with adynamic working condition. The main conclusions of this study is that the hydraulic motor axis should be extended. Theoperator's manual and assembly instructions should be updated and revised annually, thecompany should also invest in education and training for the users of the product.