Screw jack, also known as mechanical jack, is driven by human power through screw pair, screw or nut sleeve as the lifting part. Ordinary screw jacks support heavy objects by self-locking screw thread, with simple structure, low transmission efficiency and slow return journey. The screw thread of self-lowering screw jack has no self-locking function and is equipped with brake. By relaxing the brake, the weight can drop rapidly by itself and shorten the return journey time. However, the structure of this kind of jack is more complicated. The screw jack can support heavy objects for a long time, the maximum lifting weight has reached 100 tons, which is widely used. The bottom of the installation of horizontal screw, but also make the weight for a small distance transversal.