A jack is a mechanical device which is used as a lifting device to lift a heavy vehicle in the air so that you can access the chassis or wheels. There are jacks available that employ both a screw thread or hydraulic cylinder depending on the jack you chose. Screw jacks are most commonly found inside vehicles for emergencies because of their simple, small design. These screw jacks are a bit harder to use but they are perfect for that once in a few years task when you are stuck changing a spare tire. Hydraulic jacks are a more complex jack which uses a hydraulic cylinder to create tremendous amounts of linear force. This type of jack is normally found in homes or at mechanics because of its heavy and bulky design. To make moving this style jack easier, it incorporates swiveling wheels which allow it to be slid under the car into precise locations. Hydraulic jacks are much easier to use than most other vehicle jacks and they can be used on even the largest of vehicles. Since jacks have to lift extremely heavy loads, they are built using high quality metal which is resistant to bending and cracking.