Jack is impossible as long as a market demand requires the development of its diverse forms . In fact , according to structural features can be divided into the jack rack jacks, screw jacks and hydraulic jacks three. The so-called rack jacks , it is to work through the gear lever and the human, from the 20 ton Chain Hoist weight of its relatively small , usually not more than twenty tons , but in the long term to support heavy loads , mainly used in operating conditions inconvenient places, such as railway from rail operations. The screw jack is rotated by a human through the screw vice , screw or nut sleeve as a top piece . Ordinary screw jack supported by heavy thread- locking effect , but this jack is defective, transmission efficiency is low , return slowly . From the lower screw jack screw is not self- locking effect, but fitted with brakes. Relax brakes, heavy to own rapid decline , shorten turnaround times , but this jack construct more complex . Jackscrew long-term support heavy loads , wider 20 ton chain hoist application . Hydraulic jacks are powered by electricity hydraulic pumps, hydraulic drive system , the use of the cylinder to work . Compact hydraulic jacks structure, high transmission efficiency, but easy to leak , not long-term support heavy loads. Hydraulic jacks can be divided into integral and separate . Integral pump and hydraulic cylinder into one ; separate pump and hydraulic cylinders separated in the middle associated with a high pressure hose . If long-term support should be chosen self-locking jacks, screw jacks and hydraulic jacks to further reduce the appearance or increase the height of jacking distance , can be made multi-stage telescopic 20 ton chain hoist. Jack in other ways may be classified as separate jack , horizontal jack , claw jack, sync jack , one jack , electric jacks. In short , jack combination of different forms , their purpose and function is not the same , applied to situations are not the same .