Electric machines with permanent magnet rotors are becoming increasingly popular due to the high power density that they offer relative to other configurations. Where the speed of rotation is high, the magnets are typically mounted on the surface of the rotor and retained by an outer sleeve. In the literature a variety of analytical models have been proposed to aid the mechanical design process. Many of these models contain inherent assumptions about the stress and strain field in the rotor which may not always be apparent. In this article the range of rotor stress models are presented and explained and the limitations due to the inherent assumptions are investigated. This will allow the designer of such rotors to assess mechanical performance without introducing unforeseen errors.