A screw jack is a mechanism that lifts heavy objects. Jacks are used frequently in raising cars so that a tire can be changed. A screw jack is commonly used with cars but is also used in many other ways, including industrial machinery and even aeroplanes. They can be short, tall, fat, or thin depending on the amount of pressure they will be under and the space that they need to fit into. The jack is made out of various types of metal, but the screw itself is generally made out of lead.
The screw has a thread designed to withstand an enormous amount of pressure. This is due to the fact that it is generally holding up heavy objects for an extended amount of time. Once up, they normally self lock so that they won't fall if the operator lets go, and they hold up well to the wear of repeated use. If they are made with a ball nut, they will last longer because there is less friction created with this type of jack. However, they will not self lock. This can be dangerous and handled carefully.
The 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.