Abstract: An actuator for converting fluid energy into a mechanical force comprises an inner expansion chamber and an outer power transmission envelope which surrounds the expansion chamber and comprises a flexible but non-stretchable endless filament or multifilament which is redirected with a back-and-forth motion between two tie rod members for surrounding the expansion chamber with multiple layers, the two tie rod members being connected with the two axial ends of the inner expansion chamber. By admitting a fluid into the expansion chamber for obtaining its radially outward directed expansion the two tie rod members will be approached with respect to each other by means of the power transmission envelope to thereby produce axially directed tension forces.