a tubular spring is provided for receiving and pretensioning a piezoelectric or magnetostrictive actuator of an actuator unit , e . g ., for actuating a fuel injector valve in internal combustion engines , wherein the tubular spring comprises at least two types of recesses , each comprising different maximum lateral extensions in the longitudinal direction of the tubular spring . the tubular spring is designed to comprise a uniform load distribution about the circumference of the tubular spring even without welding the longitudinal sides thereof abutting one another .