The pulley is a simple machine that reduces the force needed to lift an object by distributing its weight across multiple points of contact. A rope hoist consists of a grooved wheel, casing, and attachment point. A block and tackle is created when multiple sheaves are used together. More complex hoist systems can produce higher lifting ratios through the use of compound sheave systems.
The pulley, also known as a wire rope hoist, has long been used to reduce the amount of force required to lift an object. A rope hoist consists of a grooved wheel, a casing and some sort of attachment point, usually in the form of a hook. Pulleys work by distributing the weight of an object across multiple points of contact through the use of friction. A pulley is one of six simple machines, which are mechanical devices used to change the amount or direction of force. When two or more sheaves are used together, this is referred to as a block and tackle.
The main element of a rope hoist is the grooved wheel in which the rope, cable or chain runs. This wheel is connected to the casing by an axle which runs through the center of the wheel. The pulley system must therefore be secured in some way to be used. While there are many ways to attach a hoist to its surroundings, the most common way is via a hook located at the top of the pulley which allows the unit to be hung from an object.
The simplest and most effective wire rope hoist system, or hoist, always involves at least two hoists, which yields a strength advantage of two. To lift a 100-pound (45.36 kilograms) weight off the floor of a building in this manner, a single pulley is attached to the weight and another to the ceiling above the weight. A rope would be attached at one end to the ceiling, threaded through both pulleys and pulled by a force of 50 pounds (22.68 kilograms) on the free end to lift the weight.
More complicated rope hoist systems can produce even higher headstock ratios through the use of compound sheave systems. Many cranes use a sheave system where two movable sheaves are linked together by a single driveshaft, with the lifting hook located between the two. A third fixed pulley is mounted perpendicular to the two movable pulleys on the crane frame. The lifting cable is then connected to the frame from the side opposite the fixed pulley, as is the lifting winch. The cable runs through the first moving pulley, up to the fixed pulley, back up to the second moving pulley and up to the hoist winch to create a lift advantage of four.
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