Given the application of gears in various industries including steel industries, studying the reasons for facture in gears before reaching the end of their lifetime is of great importance. In this study, a gearbox was investigated which included a gear and a pinion. After four years of use, the teeth of gear and pinion along with the shaft attached to the gear were fractured. The fracture had occurred suddenly and with a lot of noise. The fracture in gear and pinion were in the teeth while the fracture in shaft occurred in the keyway. At the beginning of the study, shaft and gear design equations were used to evaluate the suitability of each of the parts in the system using theoretical equations and then fracture type was determined using fracture studies and the accuracy of analytical results were determined. In the analytical study, the results showed that the fracture is due to improper design for the gear shaft leading to more than one million unites of load on the shaft leading to cracks in the keyway and misalignment between gears causing fracture. In the fracture studies, the fracture type (ductile and brittle) was determined and the accuracy of analytical results was confirmed. In the numerical results, the distribution of static strain in the fractured shaft and a redesigned shaft are investigated. Manuscript profile