Construction of a Safety Clutch with Shape Memory Actuators

This paper investigates the use of martensitic Nickel-Titanium within a safety clutch. This engineering application serves as a safety element within power trains to protect the components like motors, transmissions and the working machine against overload. The concept is based on a friction clutch that is able to transmit a defined torque. By exceeding this torque the clutch opens and the friction pads rub against each other. The increasing heat now can be used to activate the NiTi actuators. These are working under compression against steel disc springs according to the extrinsic two-way effect. The increasing heat leads to the phase transformation from martensite to austenite. This causes an elongation of the compressed NiTi-elements. The increasing stiffness leads to the movement of the clamped friction pad. The clutch opens and the transmittable torque decreases. After the cooling phase and the addicted martensitic phase transformation the initial axial force is reached and the initial torque can be transmitted. The analysis of the constructive solution of the safety clutch shows the practical behaviour of this concept. The transmittable torque can be adjusted continuously by screwing a shaft nut. With the help of measuring shafts the torque can be measured. The axial movement of the friction pad caused by the phase transformation of the NiTi can be measured with positioning sensors.