Safety Clutches With Nickel-Titanium Shape Memory Alloys

The ability of Shape Memory Alloys (SMA) to remind two different macroscopic shapes and to alter between these shapes by changing their temperature, leads to innovative approaches within drive technology. Especially Nickel-Titanium (NiTi) Shape Memory Alloys offer high actuating forces and adjustment travel in combination with high cycle stability. The shape memory effect is based on the transformation between martensitic and austenitic microstructure depending on the temperature of the actuators. The transformation temperatures in the range of 20°C to 100°C make NiTi SMA attractive for engineering applications. This paper investigates the technical use of NiTi SMA as actuators within a safety clutch. Safety clutches serve in power trains as torque limiting elements with the aim to prevent destruction of the working machine or the motor. Based on the concept of a friction clutch the conceptual design of the NiTi safety clutch is developed and followed by the design and manufacturing of a prototype. The activation of the NiTi actuators occurs as a result of the frictional heat at the friction pads when the torque limit is exceeded and the clutch slips. The actuators transform from martensitic to austenitic condition. Their stiffness increases so that the actuators are able to open the clutch. This leads to a complete collapse of the torque. During the cooling phase the transformation from austenite to martensite occurs and the NiTi actuators are deformed again. The friction pads are clamped with their original force and the clutch is able to transmit the demanded torque. The mechanical dimensioning of the actuator system is figured out as well as the measurement results of the analysis on the testing bench. The variation of the input parameters like torque and speed and the variation of the actuator system itself show possibilities and frontiers of this technology.