X-screw is a load-sensitive prismatic joint with variable transmission designed for use as a linear actuator(Fig.1). It relies on a modified screw/nut system in which the nut has the same pitch but slightly larger diameter than the screw. In addition, the nut is mounted in a bearing so that it is caused to rotate by the rotation of the screw(Fig.2(b)). If the nut and screw are eccentric, contact between them occurs at a single point. Since the diameter of the nut is larger than that of the screw, and their centers are offset, rotating the screw at a certain speed will cause the nut to rotate more slowly. This difference in speed produces linear motion that is much slower than that of a conventional power screw. Changing the eccentricity alters the location of the point of contact which changes the transmission ratio. The load-sensitive behavior of X-sxrew relies on the inclination of the threads(Figure 2(c)). When an axial load is applied, the wedging action of the threads tends to force the screw and nut to be concentric. X-screw has a spring mounted on the nut housing which counteracts this force. The balance between the spring force and the centering force determines the amount of eccentricity of the nut. Under small axial loads, the spring force is sufficient to push the nut to its maximum offset where the output speed is greatest, but increasing the load increases the centering force of the nut, in turn compressing the spring and decreasing the eccentricity and output speed. X-screw has the following characteristics: The transmission ratio automatically changes in response to the axial load. The transmission ratio is continuously variable from a minimum value to infinity. It is a simple, compact mechanism that relies on rolling contact and has a high load capacity. Also, the motor can be protected from overload since X-screw wil stop advancing above a certain load, although the motor can continue to rotate. With X-screw it is possible to introduce the versatility of a variable transmission into applications that have been limited to a fixed transmission up to this time.