This paper presents the results of an experimental investigation on the failure behaviour of power screw linear actuators subjected to very high compressive loads. Quasi-static tests performed in laboratory have shown the presence of primary and secondary buckling failure modes. On the one hand the primary buckling is characterized by plane deflection of the inner screw, on the other hand the secondary buckling involves either spatial buckling, forcing the screw to assume a helical shape, or plane buckling of the external arm, in relation to the actual slenderness and the position of the actuator. Non linearities of the axial stiffness have been observed during the proportional phase of loading, as a consequence of the superposition of primary buckling and the lateral constraint effect opposed by the cylindrical case of the actuator to the bending deformation of the screw. Maximum deflections and longitudinal deformations have been measured as a function of the applied compressive load, whose axial and bending components have been calculated. A mathematical model of the elastic loss of stability has been developed, in order to calculate the critical load as a function of the actuator geometry.