Patent ID: 7592769

Claim:
An electric cylinder comprising: a first thread nut and a second thread nut, the first thread nut being a ball thread nut in which a thread groove in an arc shape in section is formed, and second thread nut being a non-ball thread nut which is rotatable in synchronization with the ball thread nut and in which a thread groove in a rectangular or trapezoidal shape in section is formed; and a thread rod inserted through the first and second thread nut, wherein in the thread rod is a double-start thread in which the two kinds of thread grooves are formed at the same pitch, the thread grooves including a thread groove in an arc shape in section meshed with the ball thread nut through balls and a thread groove in a rectangular or trapezoidal shape in section meshed with the non-ball thread nut, and the electric cylinder is configured such that, when load working in the axial direction on the thread rod is less than a predetermined value, a thread ridge of the non-ball thread nut and a thread ridge of the thread rod are separated and are out of contact with each other so that the thread rod is moved in the axial direction thereof by only the ball thread nut, and when the load working in the axial direction on the thread rod exceeds the predetermined value, the thread ridge of the non-ball thread nut and the thread ridge of the thread rod are made in contact with each other so that the thread rod is moved in the axial direction thereof by the non-ball thread nut, and wherein a width of the thread ridge of the non-ball thread nut is set smaller than a width of the thread groove for the non-ball thread nut of each thread rod so that a state is controllable and exchangeable between a state that the thread ridge of the non-ball thread nut is separated from the thread ridge of each thread rod and a state that they are in contact with each other.