Classic Machine Screw Jack, also called Classic Worm Gear Screw Jack that is the most widely used mechanical actuator for intermittent duty cycles as the actuator incorporates a precision worm gear set in a rugged casting delivering positive, precise actuation. The machine screw jacks lift and precisely position loads (1 ton to 100 tons). Upright or inverted jacks operate in tension or compression load. single lead jacks are self-locking. double lead jacks offer increased travel speed and require a brake or external locking device to hold position.