Design of shear gaps for high-speed and high-load MRF brakes and clutches

Abstract

Magnetorheological fluids (MRF) are smart fluids with the particular characteristics of changing their apparent viscosity significantly under the influence of a magnetic field. This property allows the design of mechanical devices for torque transmission, such as brakes and clutches, with a continuously adjustable and smooth torque generation. Especially the use of MRF in devices for applications with the demand on high rotational speeds and high loads can offer advantages, because more dissipation energy and thermal load peaks can be compensated by the torque generating fluid volume. However, high rotational speeds can cause a centrifugation of the particles contained in the MRF especially in idle mode when no magnetic field is applied. This can yield on the one hand to labile and unpredictable behavior of the torque response and on the other hand to a higher wear of the MRF or in the worst case to a destruction of the MRF with respect to a long-term view. For ensuring reliable braking or coupling conditions, in this contribution the development of a Taylor-Couette flow with axisymmetric toroidal vortices in axial shear gaps of MRF brakes and clutches is considered. The developing flow profiles in these shear gaps are modeled and analyzed in detail and finally design rules for axial shear gaps are introduced. Measurements at high rotational speeds up to 6000min−1 with a test actuator based on this design prove the positive influence of the vortex flow on the homogeneity of the MRF suspension and on the consistency and predictability of the braking or coupling torque even under long-term conditions.