It is well known that vibrations can occur in screw jack mechanisms under certain conditions, especially during downward motion. Several models have been proposed in the literature in order to explain this vibratory phenomenon due to system instability. Nevertheless, to the best of our knowledge, complete and accurate experimental results have never been carried out before. In this paper, the mechanical system made up of a screw and a nut is analyzed. Then a 2 dof model is introduced. In particular, this model shows that the system is unstable when the moment of inertia of a mass clamped to the free end of the screw is in a range between two boundary values and . These values depend on the mechanical characteristics of the system. The existence of this range is experimentally observed. Moreover, it is shown that theoretical values and are in good agreement with the experimental ones.
From a design point of view, the second main contribution of this work consists in providing a simple but effective way to avoid instability in screw jack mechanisms: in order to prevent the mechanism from vibrating (instability), it is sufficient to clamp (when allowed) an inertia mass to the free end of the screw.
