A screw jack is a device used to fully or partially lift a vehicle or other object off the ground. Depending on their size, these devices can be used to raise the corner of a vehicle, or to lift it several feet in the air so workers can access the bottom of the vehicle. Screw jacks are often found in machine shops, auto repair facilities and in the automotive racing industry. Many vehicles also have a screw jack included with the spare tire kit, so drivers can repair a flat tire more easily. Very large screw jack systems are even used to lift houses for foundation repair or replacement.
These devices are typically made of steel, lead or lightweight stainless steel. They are available in two basic varieties, scissor jacks and “worm gear” screw jacks. Another type of jack design, based on hydraulic principles, is primarily used in industrial and manufacturing facilities and does not operate like a screw jack.
Scissor screw jacks are the most basic model, and are typically used for flat tire repair. They have a scissor-like design that is operated using a large lead screw. The bottom of the jack rests on the ground while the top fits under the body of a car. A screw is inserted in the center of the scissor system and is turned to the right to raise the jack and lift the car. After the tire is replaced, the screw is turned to the left to lower the car back to the ground.
Worm gear screw jacks have a body shaped like a very large screw. At the top is a lifting platform, which is used to support a vehicle, house or other object. As the jack is operated using a ratcheting motion, the lifting platform is raised up the height of the screw-shaped body. A worm gear screw jack typically offers a greater level of precision and consistency than a scissor jack, and may also be capable of lifting heavier loads. This device is also known as a "machine screw jack."
While scissor screw jacks can only lift a relatively small amount of weight, they do offer a number of advantages. They are affordable and easy to use, allowing almost anyone to successfully lift a vehicle to change a tire or perform minor repairs. They are also small and lightweight, making them portable enough to transport in the trunk of a car along with a spare tire.