Patient-mounted robotic needle guidance is an emerging method of needle insertion in percutaneous ablation therapies. During needle insertion, patient-mounted robots can account for patient body movement, unlike gantry or floor mounted devices, and still increase the accuracy and precision of needle placement. Patient-mounted robots, however, require repeated sterilisation, which is often a difficult process with complex devices; overcoming this challenge is therefore key to the success of a patient mounted robot. To eliminate the need for repeated sterilization, we have developed a disposable patient-mounted robot with two rings as a kinematic structure: an angled upper ring both rotates and revolves about the lower ring. Using this structure, the robot has a clinically suitable range of needle insertion angles with a remote center of motion. To achieve disposability, our structure applies a disposable gear transmission component which detachably interfaces with non-disposable driving motors. With a manually driven prototype of the gear trains, we assessed whether the kinematic structure of the two rings can be operated only by using input pinions locating at outside of the kinematic structure. Our tests confirmed that the input pinions were able to rotate both upper and lower rings independently. We also determined a linear relationship of rotation transmission with the gear trains and determined that the rotation transmission between the pinions and the two rings were within 3 % of error from the designed value. Our robot introduces a novel approach to patient-mounted robots, and has potential to enable sterile and accurate needle guidance in percutaneous ablation therapies.