A rotor retaining sleeve is mechanically necessary in the structure of high-speed SPM machines to prevent structural failure. However, the significant eddy current loss within metal sleeves leads to a reduction in electro-magnetic efficiency and serious thermal and mechanical issues. Therefore, several techniques and methods have been proposed and developed to reduce such losses. In this paper, a helical retaining sleeve is proposed to reduce losses by cutting the flow path of the eddy currents. Using FEA, a comparison is made of the electromagnetic and mechanical performance of the proposed sleeve against a conventional sleeve having the same material and dimensions. It is concluded that the eddy-current loss reduces significantly when the helical sleeve is used, whereas the mechanical stress is broadly unchanged in the central region of the rotor with only a small increase around the ends when compared with the conventional sleeve.