Development of a magnetorheological brake with a slotted disc
Authors: Sarkar Chiranjit , Hirani Harish 
Published: March 2015

A conventional disc wears out and the brake pollutes the environment. Brake pad dust is reported to be the largest source of environment pollution1. The particles emanating because of wear of the brake pad pollute the environment2,3,4. In addition to pollution caused by wear particles, the friction-induced noise between the brake pad and the disc is also a major concern5. Also, localized heating occurs in a conventional disc brake. To tackle both of these problems, conventional disc brakes can be replaced with manetorheological fluid brakes. Magnetorheological fluids are materials having a shear yield stress which is a function of the magnetic field. On the application of a magnetic field, magnetorheological particles become aligned and increase the shear resistance between relatively moving surfaces. The friction between the stator and the rotor increases and fulfils the braking function, which means that magnetorheological fluids can be used as brake friction materials. A magnetorheological brake consists of a rotating disc or discs immersed in a magnetorheological fluid and enclosed in an electromagnetic casing. The torque characteristics of the magnetorheological brake in the shear mode are controlled by regulating the yield stress of the magnetorheological fluid. An increase in the yield stress increases the braking torque, which means that, the higher the yield strength of the magnetorheological fluid, the better is the performance of the magnetorheological brake. However, the major disadvantage of the shear-mode-based magnetorheological brake is its high resisting torque even in the off-state viscosity, and such magnetorheological brakes cannot be recommended for automotive applications. To obtain the performance of a conventional disc brake, experimental studies on a conventional disc brake were performed using a full-scale dynamometer. In addition to wear particles, localized heating of the disc was observed. The disc–pad interfaces were modelled to simulate the disc temperature. The values of the maximum temperature, which were obtained from simulations as well as experiments, were compared. The simulations were extended to hypothetical 360° pads, and a significant reduction in the maximum temperature was noted. Based on the idea of 360° pads, a magnetorheological brake subjected to shearing was analysed. To perform experiments on a small-scale magnetorheological brake, a test set-up was designed and developed, and it was confirmed that a magnetorheological brake subjected to shearing provides a better torque than does a conventional disc brake of the same size. An ideal magnetorheological brake should exert a zero frictional torque in the off-state condition and a controllable frictional torque in the on-state condition. An attempt was made to design such a magnetorheological brake. To overcome the disadvantage of the shear-mode-based magnetorheological brake, a new design of magnetorheological brake with a slotted disc was proposed. The design and development of the proposed magnetorheological brake, incorporating the compression-assisted shear mode, was detailed. The results of the braking torque, the temperature distribution and the off-state viscosities were plotted. Finally, comparison between the proposed magnetorheological brake and a small-scale disc brake was presented. The results show that, with an increase in the braking pressure (from 2 bar to 6 bar), the maximum value of temperature rises because of enhancement of the localized heating in the disc. The temperature rise is less in the case of the extended brake pad (360°), which was used in the magnetorheological brake. The surface temperature of the magnetorheological brake increases with increasing magnetic field. The magnetorheological fluid with 50 wt % iron particles has a less viscous torque in the off-state condition and a high-field torque under the compression mechanism in the magnetorheological brake, which fulfils the criterion for an ideal magnetorheological brake to replace a conventional disc brake of the same size.