Abstract: A thread locking device for securing a male threaded member to a female threaded member at a desired interengaged position. A nut, threaded internally, and a spiral spring in combination form the thread locking device. The spring is mounted in the nut with the end tabs of the spring extending radially into a slot provided in the nut. The spring is configured to also threadably engage the threads of a member, such as a shaft, to which the nut will be threadably mounted. The spring in its relaxed state will forcibly grip the thread form of the shaft to frictionally secure the nut in position on the shaft. The radially extending end tabs of the spring are forced apart by a tool to enlarge the diameter of the spring to facilitate the threadable installation of the nut and the spring to the shaft and the removal of the nut and spring from the shaft.