A screw and nut assembly includes a movable platform, a first screw and nut set with the first screw rod affixed between two locating members and the first screw nut pivotally mounted in the movable platform, a second screw and nut set with the second screw nut pivotally mounted in the movable platform, a driving mechanism for driving first screw nut and the second screw nut simultaneously for causing the first screw nut to create a first speed and the second screw nut to create a second speed slower than and reversed to the first speed, and a first support and a second support respectively affixed to the second screw rod to support and to move with the second screw rod. Thus, the gap between the movable platform and each support is changed subject to movement of the screw nuts.