Jack
From Encyclopædia
A jack is a type of mechanism that is used to press against an object, moving it if it is free, or exerting
pressure on the object if it is fixed. Common types of jacks are the lever jack, the screw jack, and the hydraulic jack; all three produce a sizable mechanical advantage, so that the applied force of a person is multiplied, resulting in a larger force being exerted on a heavy body. The lever jack is essentially a LEVER combined with a ratchet device that prevents the object from slipping back
Down during the reverse motion of the lever. The object is thus raised a small amount each
stroke. In the screw jack, or jackscrew, the turning of a screw (by the use of a lever) causes the screw, which is fitted into a threaded base, to move upward as it rotates. A hydraulic jack makes use of the properties of an enclosed liquid to exert a large force on a piston in a large cylinder by applying a smaller force to the piston of a smaller cylinder (see
HYDRAULIC SYSTEMS).
