Abstract: Certain aspects relate to systems and techniques for robotic medical instrument system. The medical instrument system can include a pulley and a capstan. The capstan can be supported for rotation about a rotational axis and translation along the rotational axis. A cable can be wrapped around the capstan and extending from the capstan to the pulley. The capstan can be configured to translate relative to the pulley along the rotational axis as the capstan rotates about the rotational axis.