1. Jack is a light and small lifting device that lifts heavy objects in a small stroke through a top bracket or a bottom bracket. Different types of jacks have different principles. Common hydraulic jacks use Pascal's law, i. e. the pressure of the fluid is the same everywhere, so that the piston can be kept stationary. 3, and the screw jack is through the reciprocating pull handle, stack ratchet clearance rotation, through gear rotation to make the sleeve lifting and lowering, to lift the pull function.