SCREW JACK DEPARTMENT OF MECHANICAL ENGINEERING SCREW JACK • A screw jack is a portable device consisting of a screw mechanism used to raise or lower the load. The principle on which the screw jack works is similar to that of an inclined plane. PRINCIPLE OF SCREW JACKS • Main components of screw jacks are; trapezoidal lifting screw, worm screw, worm gear and gear housing. Worm screw is rotated manually or by motor. Worm gear is rotated by worm screw. The lifting screw moves through the rotating worm gear. The linear motion speed of lifting screw depends on thread size and rotation ratio of worm gears. TYPES OF JACKS • Jacks are of mainly two types- • Mechanical Jack • • Scissor Jack • Bottle (cylindrical) Jack • • Hydraulic Jack • MECHANICAL JACK • A mechanical jack is a device which lifts heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. • Scissor Jack Bottle (cylindrical) Jack HYDRAULIC JACK • Hydraulic jacks are often used to lift elevators in low and medium rise buildings. • Hydraulic jacks are typically used for shop work, rather than as an emergency jack to be carried with the vehicle. HYDRAULIC JACK SELF-LOCKING PROPERTY OF SCREW JACK • The large frictional forces due to the large area of sliding contact between the threads cause most screws to be "selflocking", also called "non-reciprocal" or "non-overhauling". This means that applying a torque to the shaft will cause it to turn, but no amount of axial load force against the shaft will cause it to turn back the other way, even if the applied torque is zero. CONSTRUCTION OF SCREW JACK • Screw jack consists of a screw and a nut. The nut is fixed in a cast iron frame and remains stationary. The rotation of the nut inside the frame is prevented by pressing a set screw against it. The screw is rotated in the nut by means of a handle, which passes through a hole in the head of the screw. The head carries a platform, which supports the load and remains stationary while the screw is being rotated. A washer is fixed to the other end of the screw inside the frame, which prevents the screw to be completely turned out of the nut. • ADVANTAGES • Have very high efficiency • Could be used in applications which require precise and repeatable movement • Could be easily preloaded to eliminate backlash • Smooth movement over full travel range • Can use a smaller size for same load • Has a longer life for thread • Easy maintenance • Self-locking feature
