Feather key connections represent a prevalent type of shaft-hub connection. The primary advantages of this connection are its ease of manufacture and assembly, as well as its disassembly. The transmittable torque can be calculated using the German national standard DIN 6892, which is based on a critical pressure on the keyway of the shaft and hub or on the feather key. The focus of this analysis is on the shaft, where the pressure leads to a plastic deformation of the keyway. The present work utilizes a feather key connection with a ductile shaft and hub composed of C45+N, connected by a feather key of C45+QT. A numerical study is conducted to elucidate the plastic deformations in relation to load cycles. The analysis reveals a critical plastic deformation of the keyway, which establishes a novel metric for assessing the transmittable torque potential of a feather key connection.