Direct-drive robot arms, often utilized in manipulators and humanoid robots, face challenges related to increased distal mass and reduced backdrivability due to motors being placed at each joint. This paper presents a 4-DOF elbow-wrist mechanism for a robot arm driven by wires. The design includes the upper arm, elbow, upper forearm, lower forearm, and wrist, with all actuators concentrated in the upper arm to minimize distal mass. To achieve this, the mechanism employs rolling-contact joints, forearm rotator idlers, and agonist-antagonist wire pairs. Additionally, a differential mechanism is integrated at the wrist, allowing for yaw rotation at the forearm while enabling roll and pitch rotations at a single point, thereby eliminating inter-axis offset and mimicking human wrist motion. A prototype was fabricated and evaluated, achieving an elbow flexion of 107°, forearm yaw exceeding 90° in both directions, wrist roll of 63° and 50° in each direction, and wrist pitch of 33° in both directions. The independence of each degree of freedom was validated, and torque efficiency from the upper arm to the wrist tip was measured and analyzed.
