This paper presents the concept of and a design method for a new variable-radius pulley that passively changes the effective radius of a driven pulley in a belt-driven transmission in response to a changing load torque. The proposed mechanism is based on the principle that the effective radius of the pulley is passively increased when a torque is applied to the pulley by means of a cam element and an elastic element. In a belt-driven transmission, the radial size of the driven pulley determines the relationship between the output torque and the output speed. Whereas a belt-driven transmission with a typical pulley of constant radius has a fixed maximum speed and torque, a variable-radius pulley allows the maximum speed and torque to vary, enabling more efficient driving. This paper describes the working principle of the new variable-radius pulley and presents a static analysis of the proposed mechanism. Simulations and experiments conducted to demonstrate the proposed mechanism are reported, and the results show that the proposed variableradius pulley can be used to vary the transmission ratio in accordance with the torque applied to the robot joint. (C) 2018 Elsevier Ltd. All rights reserved.