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A block and tackle[1][2] or only tackle [3] is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift heavy loads.

The pulleys are assembled to form blocks and then blocks are paired so that one is fixed and one moves with the load. The rope is threaded through the pulleys to provide mechanical advantage that amplifies the force applied to the rope.[4]

Hero of Alexandria described cranes formed from assemblies of pulleys in the first century. Illustrated versions of Hero's Mechanica (a book on raising heavy weights) show early block and tackle systems.[5]

Typical ${\displaystyle K}$ values are 1.04 for roller bearing sheaves and 1.09 for plain bearing sheaves (with wire rope).[11]

The increased force produced by a tackle is offset by both the increased length of rope needed and the friction in the system. In order to raise a block and tackle with a mechanical advantage of 6 a distance of 1 metre, it is necessary to pull 6 metres of rope through the blocks. Frictional losses also mean there is a practical point at which the benefit of adding a further sheave is offset by the incremental increase in friction which would require additional force to be applied in order to lift the load. Too much friction may result in the tackle not allowing the load to be released easily,[notes 1] or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well.

When installing a block on an existing line, it is often inconvenient at best to thread the rope through the block to be added.

• Open blocks have a space wide enough between the fixed cheeks to be able to slide the pulley over the rope. These can be extremely small and light while retaining significant strength due to the lack of moving parts.
• A swing cheek block a special kind of block which can be opened to engage with a bight, without

Swing-Cheek blocks may be roughly divided into two categories:

• Swing cheek pulleys: used for light loads or redirection of forces, usually with a single pulley wheel (though multiple sheaves/cheeks are not uncommon) and an attachment point (or several) for a carabiner or sling. The cheeks are not fixed or locked in position aside from the device used to secure them to the load or rigging point.

Exampl

Swing-Cheek blocks may be roughly divided into two categories:

• Swing cheek pulleys: used for light loads or redirection of forces, usually with a single pulley wheel (though multiple sheaves/cheeks are not uncommon) and an attachment point (or several) for a carabiner or sling. The cheeks are not fixed or locked in position aside from the device used to secure them to the load or rigging point.

Examples of use (in an arborocultural setting) include: tail minding/tending, and for setting a rigging point in the tree above the cut to take place - a positive rigging situation.

• Snatch or Impact Blocks: used for heavier loads and more dynamic rigging, the cheeks of these blocks are fixed in place with a pin which locks into the opposite cheek. This pin may form part of the axle for a second pulley, which is secured to the load or rigging point with a soft sling, rather than a solid device such as a shackle. This allows for more even distribution of forces to the faces where the forces will be applied, as opposed to a carabiner or shackle, where the forces are applied more str

Examples of use (in an arborocultural setting) include: tail minding/tending, and for setting a rigging point in the tree above the cut to take place - a positive rigging situation.

• Snatch or Impact Blocks: used for heavier loads and more dynamic rigging, the cheeks of these blocks are fixed in place with a pin which locks into the opposite cheek. This pin may form part of the axle for a second pulley, which is secured to the load or rigging point with a soft sling, rather than a solid device such as a shackle. This allow

Examples of use (again, in relation to tree care) may include setting a block below the current cut, resulting in a 'negative' rigging situation, in which shock loads can be significant - especially if removing large sections of vertical stem.

## Literature

• M. Oppolzer, T. Wahls: I Like To Move It. Flaschenzüge in der Seiltechnik. Hamburg 2019,