Characterization of Magnetorheological Brake in Shear Mode Using High-Strength MWCNTs and Fumed Silica-Based Magnetorheological Fluids at Low Magnetic Fields

The performance of magnetorheological (MR) brakes is dependent on the MR characteristics of the braking fluid, working parameters, and magnetic fields. Due to the size limitations, it is quite difficult to use large-sized electromagnet for a high magnetic field inside an MR brake and thus working parameters indirectly affect the MR properties of MR fluid. Again, MR fluids show thermal thinning with working temperature. Therefore, in the present study, MR fluids that have stable MR properties at high temperatures and can provide better braking torque at low magnetic fields are prepared. To improve the MR properties at high temperature, multi-walled carbon nanotubes (MWCNTs) which have high thermal conductivity are used as additives, and initially, three MR fluids are synthesized by varying MWCNT fractions. The MR properties of these fluids are examined and plotted using magnetorheometer. The effective fraction of MWCNTs at which MR fluid has stable MR properties with temperature is identified. It is found that MR fluid which consists of 0.25% fractions of MWCNTs has large yield strength but only at high magnetic fields. To improve its MR properties at a lower magnetic field, 0.5% fraction of fumed silica is mixed with 0.25% fractions of MWCNTs. Then, a fabricated MR disc brake is characterized using those prepared magnetorheological fluids (MRFs). The braking torque of MRFs at different speeds is presented and compared. It is found that approximately 26% more braking torque is offered by fumed silica + MWCNTs-based MR fluid in comparison to other MRFs at 1200 RPM.