block and tackle
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block and tackle:see pulleypulley,
simple machine consisting of a wheel over which a rope, belt, chain, or cable runs.
A grooved pulley wheel like that used for ropes is called a sheave. A single sheave mounted in a block and fixed in place simply changes the direction of force exerted on the rope
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Block and Tackle
a load-lifting device that consists of pulleys mounted in fixed and moving blocks and rigged with a continuous cable. A block and tackle provides a mechanical advantage or an advantage in speed. It is usually part of the mechanism for raising and extending the booms of hoisting cranes and rigging equipment. However, a block and tackle can also be used independently for raising or lowering small loads, such as ship’s boats.
In a block and tackle that is used to provide a mechanical advantage, the load is suspended from the moving block, and the pull is exerted on the part of the cable that runs from the last of the pulleys through which the cable is reeved. If frictional losses are ignored, the tensile force that acts upon the cable can be defined as the quotient obtained when the weight of the load is divided by the number of parts of the cable that support the load.
Essentially the opposite arrangement is used with a block and tackle that functions to speed up the movement of a load. This means that the force, which is usually supplied by a hydraulic or pneumatic cylinder, is applied to the moving block and the load is suspended from the running part of the cable. The advantage in speed with such a block and tackle results from the increased hoist of the load; this hoist is equal to the product obtained by multiplying the piston stroke of the power cylinder by the number of parts of the cable.
I. G. GERTSKIS
Block and Tackle
a suspended device for lifting loads. A block and tackle may be operated manually or mechanically (either electrically or pneumatically), and it may be stationary or mobile. Mobile tackles are suspended from special carriages that move on single-rail tracks.
Manually operated tackles have a housing containing the block and a hook from which the load is suspended. The upper block has a worm or gear transmission and is operated manually by means of an endless chain. A roller or welded-link chain goes around the drive sprocket (or pulley) and the moving sprocket (or pulley) of the lower block. The upper block is equipped with a brake to prevent accidental lowering of the load. Lifting capacities range from 0.25 to 10 tons, and loads may be raised 3 to 12 m.
An electrically driven block and tackle consists of a winch with a reduction gear, an electric motor, a drum or sprocket, a brake, and a hook. Lifting capacities range from 0.25 to 5 tons, and loads may be raised 3 to 18 m. The hoisting speed is 20 m/min when the hoist is operated from a control console on the floor and up to 360 m/min when it is controlled from a cab traveling together with the tackle. Current is supplied through trolley conductors or flexible cables.
Pneumatically driven tackles are used where there is a danger of fire. Compressed air is usually supplied from a compressor station or from gas cylinders delivered to the work location. The pneumatic drive ensures a smooth, stepless regulation of the hoisting speed.
Tackles are used as self-containing hoisting devices in industrial shops, storage areas, and repair shops and aboard ships. They are also used as hoists in single-rail bridge cranes, stationary and mobile wall-bracket cantilever cranes, and in single-rail hoist carriages.
REFERENCEGruzopod” emnye mashiny. Moscow, 1973.
T. A. NIKOL’SKAIA