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A pulley string is a length of rope or wound material that is looped around one or more pulley wheel to complete the pulley system. Pulley systems are often used to lift or move heavy objects, or to change the direction of force in a system; a pulley string is a major component of the system, though larger systems can replace the string with a belt, chain, cable, or rope for added durability and weight capacity. The size of the string will usually be dictated by the size of the pulley or pulleys, as well as the weight capacity of the entire system.
Systems that use a pulley string tend to be smaller units. They are sometimes used in scientific experiments or even for home use; a pulley string that is wound through a system designed for lifting and securing bicycles in a garage, for example, will feature a thinner string rather than a thicker cable or rope. Larger systems can be used in industrial settings, construction settings, or even on sailing vessels, though these are all likely to use heavier duty materials rather than a pulley string, since these pulleys are likely to haul significant amounts of weight.
The pulley string can be wound through numerous pulleys. Doing so will help distribute the weight of the object being lifted more efficiently, which will in turn reduce the likelihood that the string will break or the system will otherwise fail. Adding more pulleys can, however, significantly increase the amount of friction placed on the system, so if too many pulleys are placed in the system, the string can break, leading to system failure. The string must also be secured properly at one or both ends to ensure that it will not slip or release unexpectedly.
Exercise machines commonly use pulley string to allow a user to lift heavy weights safely. In many cases, the string is actually metal, though it can be fairly thin to avoid excess friction. The string or cable will be run through strategically positioned pulleys, usually more than one pulley for each exercise. This is an example of more pulleys acting converse to the intended purpose: more pulleys means the weight is distributed more evenly, and the user will not be lifting the true stated weight. Fewer pulleys means the user will get the benefit of more weight during the exercises done on the machine.