US Patent 10054167 Driveshaft with two-stage stiffness

A two-stage stiffness driveshaft includes a hollow cylinder having first and second ends and a hollow cylinder stiffness. An inner shaft having first and second ends and an inner shaft stiffness extends through the hollow cylinder. The inner shaft's first end and the hollow cylinder's first end are engaged via a rotational clearance fit. The inner shaft's second end is rotationally fixed to the hollow cylinder's second end to permit the inner shaft's first end to twist through a predetermined angle relative to the inner shaft's second end. The inner shaft's stiffness defines the driveshaft's first-stage stiffness, while the combined stiffness of the inner shaft and the hollow cylinder defines the driveshaft's second-stage stiffness. A damping element positioned between the inner shaft and the hollow cylinder controls variation in torque transmitted by the driveshaft and generates gradual transition between the first-stage stiffness and the second-stage stiffness.