A Passive Magneto-Thermo-Mechanical Coupling Actuated by SMA Springs and MR Fluid

In the present paper, a passive magneto-thermo-mechanical coupling is conceived and developed. The system is designed to engage and disengage auxiliary devices such as a vacuum pump from an internal combustion engine. The coupling increases the transmissible torque of a magnetorheological (MR) clutch, under particular environmental conditions, i.e. for temperature below 20°C. At low temperatures the torque requirements become significantly greater than the nominal operating torque, due to the very low viscosity of the oil in the driven device (vacuum pump). The ON–OFF operation of the thermo-mechanical coupling is the result of two conventional compression springs and two opposing shape memory alloy (SMA) springs. At low temperature the conventional springs keep the coupling engaged, while, if the temperature rises over 90°C, the SMA springs disengages the coupling. At normal operation temperatures, the SMA coupling is disengaged, allowing for the MR clutch to operate as the primary clutch. The SMA springs were designed considering the particular constitutive properties of the material and the equilibrium of the system. A prototype of the mechanical coupling was manufactured considering the geometry of the MR clutch. The experimental results obtained with the prototype, in terms of relative axial position of the two halves of the coupling, appeared in fairly good agreement with the model.