Anti-sedimentation mechanism of rotary magnetorheological brake integrating multi-helix microstructure

Solving the sedimentation problem of magnetorheological (MR) fluids is the key to promoting the application of MR actuators for long-term storage. Current strategies for mitigating MR fluid sedimentation focus on improving the material constitution of MR fluids. However, the sedimentation is bound to emerge over time as a result of the density mismatch between carbonyl iron particles and carrier liquids. Therefore, more robust approach to solve the particle sedimentation should be considered, such as improving the robustness of the MR actuator. This paper proposes a novel anti-sedimentation MR brake (AS-MRB) that integrates a multi-helix microstructure on the shaft surface. The protrusion-shaped multi-helix microstructure applied a combination of shear stress, pressure, friction, and centrifugal force to MR fluids. Therefore, the AS-MRB can autonomously alter the internal flow state, thus performing the self-homogenization of sedimented particles. Remarkably, vortexes with axis displacement were generated in the MR working gap during the self-homogenization process. This phenomenon facilitated the mass transfer between different fluid layers, as confirmed by the speed reduction and torque test. To accurately describe the real-time rotational speed of the AS-MRB, an impact-introduced dynamic model of the shaft was also proposed. An inductance sensor-based test system was developed to characterize the volume fraction of the particle instantaneously, which is directly pertinent to the evaluation of the anti-sedimentation performance. Severely sedimented MR fluids were introduced into the AS-MRB for the performance evaluation in harsh conditions. The results show that the maximum volume fraction difference Δφ_max of the sedimented MR fluid was decreased from 54.02 % to 2.46 % after the homogenization. The maximum shear stress τ_max of the sedimented MR fluid was increased from 11.78 % to 80.7 % of that of the initial MR fluid. These demonstrate the strong anti-sedimentation performance of the AS-MRB. Consequently, this study has developed a unique method for solving the sedimentation problem of MR fluids and creates avenues for promoting the application of MR actuators in long-term storage.