Shape memory alloys applied to improve rotor-bearing system dynamics - an experimental investigation

tor-bearing systems have critical speeds and to pass through them is an ongoing challenge in the field of mechanical engineering. The incorporation of shape memory alloys in rotating systems has an increasing importance to improve system performance and to avoid potential damaging situations when passing through critical speeds.

In this work, the feasibility of applying shape memory alloys to a rotating system is experimentally investigated. Shape memory alloys can change their stiffness with temperature variations and thus they may change system dynamics. Shape memory alloys also exhibit hysteretic stress-strain relations which may be utilized for damping purposes.

These ideas are tested in this study on a dedicated test-rig, consisting of a rigid shaft and disc held vertically by passive magnetic bearings, where the damping is low. The bearing housings is flexibly supported by shape memory alloy helical springs, and because of high dynamic coupling between shaft and bearing housing, the shape memory alloy springs are able to reduce vibration in the shaft.

The shape memory alloy springs are characterized by force-displacement tests in different temperatures. Transients of step perturbations and mass imbalance responses of the rotor-bearing system at different temperatures and excitation frequencies are carried out to determine the dynamic behaviour of the system.

The behaviour and the performance in terms of vibration reduction and system adaptability are compared against a benchmark configuration comprised by the same system having steel springs instead of shape memory alloy springs. The experimental results clearly show that the stiffness changes and hysteretic behaviour of the shape memory alloys springs alter system dynamics both in terms of critical speeds and mode shapes.

Vibration peaks could be reduced up to 47 % during ramp-up tests compared to the system configuration with steel springs and the two first critical frequencies could be altered up to 7 % by temperature changes.