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We already analyzed the mechanical loss factors of a commercially available geared motor, which consists of an induction motor with a rated power of 90W and a parallel gear reducer. The load independent loss holds some high proportion of the total loss, and is due to the grease stirring and the oil seal friction. The load dependent loss increases almost linearly with the applied torque, and is mainly caused by gear mesh friction which is relevant to the grease characteristics. Therefore, the grease has an important role in the loss reduction. In this paper, we studied the influence of grease characteristics on loss torque, that are the cone penetration and both the kinematic viscosity and the kind of base oil. The result shows that, the loss of grease stirring, or load independent part, depends on both the kinematic viscosity of base oil and the cone penetration. The loss of gear mesh friction, load dependent part, is not related to the cone penetration nor the kinematic viscosity of base oil. It is evaluated in terms of the mean friction coefficient of tooth flank, μmG. It was found 0.09 to 0.11 when using the grease with the lithium soap and the mineral base oil, and it was 0.07 to 0.08 with the aluminum complex soap and the synthetic base oil. For the loss reduction, the grease should be composed of a base oil having low kinematic viscosity and the appropriate thickener to make the cone penetration as high as desired.

Magneto-Rheological (MR) grease has been applied to a controllable clutch system, and its performance was investigated experimentally. MR fluid is known as a functional fluid whose yield shear stress can be controlled reversibly by the applied magnetic field strength. But its peculiar characteristics may change as time passes because of sedimentation of dispersed iron particles. To overcome this defect, grease has been adopted as a carrier fluid of particles. Three-dimensional structure of thickener developed in grease is expected to prevent iron particles from settling out. In this paper, a controllable clutch system was designed and constructed to investigate the performance of MR grease clutch system. It was shown experimentally that the transmitted torque could be controlled quite smoothly by the applied electric current to the coil equipped inside of clutch system, and the response time was estimated about 0.12 s at the fastest. However, the transmit torque decreases as time passes because of the effect of temperature rise. To stabilize the output torque, the electric current to coil was controlled by a simple feedback control system.