SCREW JACK LIFTING DETAIL
SCREW JACK DETAIL
SCREW JACK ENGINE
SCREW JACKS POSITIONED ONTO A BARGE
TRANSHIPMENT OF TRAFO ONTO MODULAR TRAILER
UNLOADING OF A REACTOR BY MEANS OF SCREW JACKS
MECHANICAL SCREW (OR SPINDLE) JACKSGENERAL OVERVIEW...The heavy duty screw jacks are mechanical linear actuators able to lift up to 800 tons (4 units). They are generally used to move the work piece to a comfortable height for assembly or machine operators, but also for loading and offloading it to and from transportation equipment. The jacking units can be connected by spreader beams or simply to the jacking wings of the vessel to lift or lower.Each unit is composed by an electric motor, a gear speed reducer, the screw (or worm shaft) and a worm gear all contained in a solid and stable steel frame. The screw jacks enable to move heavy loads slowly, accurately and safely. The advantage of such system is the simplicity.ELECTRIC MOTOR...Powered by alternating current (AC) motors. The motor type determines the travel speed of the actuator. The motors are three-phase with voltage depending upon the size and lifting capacity of the jacking unit. Its power is such to provide the necessary torque to lift the rated load. A limit switch and a brake are used to stop the motor.WORM-GEAR SPEED REDUCERS...The worm-gear speed reducer is an important part of the linear actuator package. The physics of the mechanical system will not allow high travel speeds. If input to the system is too fast, heat buildup accelerates wear on the machine screw jack. A speed reducer solves this problem. The reducer operates with sliding contact between the worm and the gear. This provides for a smooth transmission of power from the motor to the machine screw jack. Safety factors in a worm-gear set are significantly higher than with other types of power transmission equipment, thereby making the chance of a catastrophic failure remote, even under the most demanding conditions. MACHINE SCREW JACKS...The worm-gear set powers a threaded rod or lifting screw, providing lift or force application. Thanks to the small lead of these screws and the inefficiencies of the mechanical system, they are self-locking. This means a load can be lifted and will not back down without power being applied. The rotating screw is a lifting screw that rotates but does not translate. An external nut or worm gear translates on the screw-extending or retracting depending on the input to the jack. The screw is built in a steel structure on heavy duty conical bearings and is made of carbon steel. The worm gear is made of a brass league, so that its internal threads will wear before the threads of the screw shaft. SAFETY...The screws are regularly inspected for wearing, but as additional safety a carbon steel nut is added to the mechanism. This nut is at the bottom of the worm gear and is lifted by this; in normal conditions the safety nut does not support any load. The failure of the threads of the worm gear will cause the load to be transferred to the safety nut or safety disc preventing the load from falling. The safety disc can replace the worm gear to complete the lift and allow replacement of the worm gear after its completion.