In this work a magnetorheological clutch prototype with a permanent magnet is presented and the torque transmitted in several operating configurations is obtained by a specifically designed test bench. The clutch prototype was designed in order to have the possibility to disengage the vacuum pump which operates the power brake in diesel engines continuously. Torque transmission and size specifications were suggested by the specific application. A permanent magnet was used in order to obtain a fail-safe operation and particular attention was focused on materials selection in order to maximize torque transmission in the engaged mode and to minimize power consumption when the clutch is disengaged.
Tests were carried out on the test bench, in which electric motors apply the driving and braking torques to the primary and secondary shaft of the clutch; the transmitted torque was measured by a torque-meter at several rotational speeds. Experimental results are discussed in the paper with reference to classical literature models. The experimental tests gave also useful indications on possible improvements in the clutch design, especially regarding the seals, which have to prevent leakages at the working fluid pressure and to keep the friction losses low.