Pulley

A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that does not transfer power to a shaft, but is used to guide the cable or exert a force, the supporting shell is called a block, and the pulley may be called a sheave.A gun tackle has a single pulley in both the fixed and moving blocks with two rope parts supporting the load W.Separation of the pulleys in the gun tackle show the force balance that results in a rope tension of W/2.A double tackle has two pulleys in both the fixed and moving blocks with four rope parts supporting the load W.Separation of the pulleys in the double tackle show the force balance that results in a rope tension of W/4.Fixed pulleyDiagram 1: The load F on the moving pulley is balanced by the tension in two parts of the rope supporting the pulley.Movable pulleyDiagram 2: A movable pulley lifting the load W is supported by two rope parts with tension W/2.Diagram 3: The gun tackle 'rove to advantage' has the rope attached to the moving pulley. The tension in the rope is W/3 yielding an advantage of three.Diagram 3a: The Luff tackle adds a fixed pulley 'rove to disadvantage.' The tension in the rope remains W/3 yielding an advantage of three. A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that does not transfer power to a shaft, but is used to guide the cable or exert a force, the supporting shell is called a block, and the pulley may be called a sheave. A pulley may have a groove or grooves between flanges around its circumference to locate the cable or belt. The drive element of a pulley system can be a rope, cable, belt, or chain. The earliest evidence of pulleys date back to Mesopotamia in the early 2nd millennium BCE, and Ancient Egypt in the Twelfth Dynasty (1991-1802 BCE). In Roman Egypt, Hero of Alexandria (c. 10-70 CE) identified the pulley as one of six simple machines used to lift weights. Pulleys are assembled to form a block and tackle in order to provide mechanical advantage to apply large forces. Pulleys are also assembled as part of belt and chain drives in order to transmit power from one rotating shaft to another. A set of pulleys assembled so that they rotate independently on the same axle form a block. Two blocks with a rope attached to one of the blocks and threaded through the two sets of pulleys form a block and tackle. A block and tackle is assembled so one block is attached to fixed mounting point and the other is attached to the moving load. The ideal mechanical advantage of the block and tackle is equal to the number of parts of the rope that support the moving block. In the diagram on the right the ideal mechanical advantage of each of the block and tackle assemblies shown is as follows: A rope and pulley system—that is, a block and tackle—is characterised by the use of a single continuous rope to transmit a tension force around one or more pulleys to lift or move a load—the rope may be a light line or a strong cable. This system is included in the list of simple machines identified by Renaissance scientists. If the rope and pulley system does not dissipate or store energy, then its mechanical advantage is the number of parts of the rope that act on the load. This can be shown as follows. Consider the set of pulleys that form the moving block and the parts of the rope that support this block. If there are p of these parts of the rope supporting the load W, then a force balance on the moving block shows that the tension in each of the parts of the rope must be W/p. This means the input force on the rope is T=W/p. Thus, the block and tackle reduces the input force by the factor p.