1 ST. EDITION
PULLEY Block and tackle
Rope and pulley systems. How it Works. Free body grams.
dia-
Belt and pulley systems.
Luis Eduardo RodrĂguez C.I. 24.339.311 TelĂŠfono: 555-555-5555 Fax: 555-555-5555 Correo: alguien@example.com
We can not get rid of the machines, for the simple reason that the release process would force us to get rid of that half of the human race on the planet whose existence is only possible because of the existence of the machines.. Leonardo Da Vinci
Math Inc.
A pulley is a wheel on anaxle that is designed to support movement of a cable or belt along its circumference. [1] Pulleys are used in a variety of ways to lift loads, apply forces, and to transmit power. A pulley is also called a sheave or drum and may have a groove between two flangesaround its circumference. The drive element of a pulley system can be a rope, cable, belt, or chain that runs over the pulley inside the groove.
Hero of Alexandria identified the pulley as one of six simple machines used to lift weights. [2] Pulleys are assembled to form a block and tackle in order to providemechanical advantage to apply large forces. Pulleys are also assembled as part of beltand chain drives in order to transmit power from one rota-
.A block and tackle is assembled so one block is attached to fixed mounting point and the other is attached to the moving load. Themechanical 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 mechanical advantage of each of 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 ablock and tackle.
the block and tackle assemblies shown is as follows:
Gun Tackle: 2 Luff Tackle: 3 Double Tackle: 4 Gyn Tackle: 5 Threefold purchase: 6
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.
The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch. In equilibrium, the forces on the moving block must sum to zero. In addition the tension in the rope must be the same for each of its parts. This means that the two parts of the rope supporting the moving block must each support half the load.
These are different types of pulley systems: Fixed: A fixed pulley has an axle mounted in bearings attached to a supporting structure. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. Mechanical advantage is gained by combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter. Movable: A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two. Compound: A combination of fixed and a movable pulleys forms a block and tackle. A block and tackle can have several pulleys mounted on the fixed and moving axles, further increasing the mechanical advantage.
The mechanical advantage of a pulley system can be analyzed using free body diagrams which balance the tension force in the rope with the force of gravity on the load. In an ideal system, the massless and frictionless pulleys do not dissipate energy and allow for a change of direction of a rope that does not stretch or wear. In this case, a force balance on a free body that includes the load, W, and nsupporting sections of a rope with tension T, yields:
A belt and pulley system is characterised by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axles. If the pulleys are of differing diameters, a mechanical advantage is realised. A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is approximately given by the ratio of the pitch diameter of the sheaves only, not fixed exactly by the ratio of teeth as with gears and sprockets.