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The split ratio, i.e. the ratio of stator bore radius to stator outer radius, is an important design parameter for permanent magnet (PM) linear machines, as it significantly influences the thrust force capability and efficiency. The optimal split ratio for maximum thrust force is investigated analytically for both surface-mounted PM (SPM) and interior PM (IPM) E-core linear oscillating actuators. It utilizes simple lumped magnetic circuit models and accounts for the magnetic saturation, the curvature effect, the fringing effect and the armature reaction effect which may greatly affect the accuracy of the analytical model for the optimal split ratio. The optimal split ratio for both SPM and IPM prototype actuators is found to be around 0.55. The developed analytical models are validated by both finite element analyses and measurements.