The  screw jack can be raised and lowered with a metal bar that is inserted into the jack. The operator turns the bar with his hands in a clockwise direction. This turns the screw inside the jack and makes it go up. The screw lifts the small metal cylinder and platform that are above it. As the jack goes up, whatever is placed above it will raise as well, once the jack makes contact. The bar is turned until the jack is raised to the level needed. To lower the jack the bar is turned in the opposite direction. An automatic screw jack has gears inside the jack that are connected to the screw. Theses gears are connected by other gears and bars that are turned by a power source to raise and lower the jack

 screw jack