Investigation of the relationship among the microstructure, rheological properties of MR grease and the speed reduction performance of a rotary micro-brake

A magnetorheological (MR) grease-based micro-brake is proposed for providing a damping torque for a miniature turbine generator when working at high wind speeds. MR greases with a magnetic particle volume fraction from 10% to 90% are prepared for the MR grease-based micro-brake. Flow curves show that the shear thinning behaviour of MR grease is enhanced by the increased volume fraction. The zero magnetic rheological behaviour of MR grease is studied with a Herschel-Bulkley model. Then, the speed reduction effect of the MR grease-based micro-brake is experimentally studied. A rotational speed difference Δn as high as 6750 r/min is achieved by applying a current of 1.01 A to the micro-brake. MR grease with lower volume fraction demonstrates a more significant speed reduction effect under low applied currents, since the chain forming behaviour is enhanced by the decreased volume fraction. Finally, a comprehensive model for the MR grease-based micro-brake is developed to reveal the relationship among the microstructure, rheological properties of the MR grease and the speed reduction performance of the MR grease-based micro-brake.