A control device 11 of a power transmission device 1 sets an allowable range of a temporal change rate of an inter-element driving force transmitted between a driving element 4 (elastic member) and a driven element 5 such that the allowable range changes in accordance with an observed value of the amount of elastic deformation of the elastic member (driving element 4), and controls the inter-element driving force, via an actuator 2, in such a way as to restrict the temporal change rate of the inter-element driving force to be within the allowable range thus set.

A secondary drive coupling may comprise at least one coupling member having a first mating portion and a second mating portion. The first mating portion is configured to engage a first section of a shaft on one side of a region of weakness of the shaft, whereas the second mating portion is configured to engage a second section of the shaft on an axially opposite side of the region of weakness. Both the first mating portion and the second mating portion are each configured to prevent rotation of the at least one coupling member about the shaft. In another embodiment, the at least one coupling member comprises a first coupling member comprising the first mating portion and a first contact surface and a separate, second coupling member comprising a second mating portion and a second contact surface that is configured to engage the first contact surface.