thumb|200px|A "gun tackle" rove "to disadvantage"

A block and tackle or only tackle is a system of two or more pulleys with a rope or cable threaded between them, used to provide tension and 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.

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.

Overview

thumb|upright=1.5|Various ways of rigging a tackle. All these are "rove to disadvantage" A block and tackle system amplifies the tension force in the rope to lift heavy loads. They are common on [[boats and sailing ships, where tasks are often performed manually, as well as on cranes and drilling rigs, where once rove, the tasks are performed by heavy equipment.

In the diagram shown here, the number of rope sections of the tackles shown is as follows:

  • Gun tackle: 2
  • Luff tackle: 3
  • Double tackle: 4
  • Gyn tackle: 5
  • Threefold purchase: 6

Note that in the gun tackle, double tackle and threefold purchase, both blocks have the same number of pulleys (one, two and three, respectively) whereas the Luff tackle and Gyn tackle have mis-matched blocks with differing numbers of pulleys.

Mechanical advantage

thumb|upright|A gun tackle has a single pulley in both the fixed and moving blocks with 2 rope parts (n) supporting the load (F<sub>B</sub>) of 100&nbsp;N. The mechanical advantage is 2, requiring a force of only 50&nbsp;N to lift the load.

A block and tackle is characterized by the use of a single continuous rope to transmit a tension force around one or more pulleys to lift or move a load. Its mechanical advantage is the number of parts of the rope that act on the load. The mechanical advantage of a tackle dictates how much easier it is to haul or lift the load.

If frictional losses are neglected, the mechanical advantage of a block and tackle is equal to the number of parts in the line that either attach to or run through the moving blocks—in other words, the number of supporting rope sections.

Rove to advantage

An ideal block and tackle with a moving block supported by n rope sections has the mechanical advantage (MA),

<math display="block">MA = \frac{F_B}{F_A} = n,\!</math>

where F<sub>A</sub> is the hauling (or input) force and F<sub>B</sub> is the load.

Consider the set of pulleys that form the moving block and the parts of the rope that support this block. If there are n of these parts of the rope supporting the load F<sub>B</sub>, then a force balance on the moving block shows that the tension in each of the parts of the rope must be F<sub>B</sub>/n. This means the input force on the rope is F<sub>A</sub>=F<sub>B</sub>/n. Thus, the block and tackle reduces the input force by the factor n.

thumb|upright|A double tackle has two pulleys in both the fixed and moving blocks with four rope parts (n) supporting the load (F<sub>B</sub>) of 100&nbsp;N. The mechanical advantage is 4, requiring a force of only 25&nbsp;N to lift the load.

<gallery>

Image:Pulley1a.svg|Separation of the pulleys in the gun tackle show the force balance that results in a rope tension of W/2.

Image:Pulley3a.svg|Separation of the pulleys in the double tackle show the force balance that results in a rope tension of W/4.

</gallery>

Ideal mechanical advantage correlates directly with velocity ratio. The velocity ratio of a tackle is the ratio between the velocity of the hauling line to that of the hauled load. A line with a mechanical advantage of 4 has a velocity ratio of 4:1. In other words, to raise a load at 1 metre per second, the hauling part of the rope must be pulled at 4 metres per second. Therefore, the mechanical advantage of a double tackle is 4.

Rove to disadvantage

The mechanical advantage of any tackle can be increased by interchanging the fixed and moving blocks so the rope is pulled in the direction of the load movement. In this case the block and tackle is said to be "rove to advantage."

  • "Rove to advantage" – where the pull on the rope is in the same direction as that in which the load is to be moved. The hauling part is pulled from the moving block.
  • "Rove to disadvantage" – where the pull on the rope is in the opposite direction to that in which the load is to be moved. The hauling part is pulled from the fixed block.

<math display="block">\frac{1}{\textit{eff \approx 1 + S \frac{x}{100}.</math>

This approximation is more accurate for smaller values of <math>S</math> and <math>x</math>.

<math display="block"> \textit{eff} = \frac{K^N-1}{K^S N (K-1)}.</math>

Typical <math>K</math> values are 1.04 for roller bearing sheaves and 1.09 for plain bearing sheaves (with wire rope). or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well.

Mid-line attachment

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 is a special kind of block which can be opened to engage with a bight, without the necessity to thread the rope through the block or remove the load from the end of the rope. The snatch block is also the load lifting pulley in certain arrangements, such as during use in a recovery operation.

thumb|Three vintage snatch blocks showing the ‘snatch’ closed, opened, and securely moused

A swing cheek block is a special kind of block which can be opened to engage with a bight, without the necessity to thread the rope through the block or remove the load from the end of the rope. The snatch block is also the load lifting pulley in certain arrangements, such as during use in a recovery operation

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 arboricultural 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 strongly to corners and edges, increasing risk of deformation or damage.

::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.

See also

Notes

References

<!--This article uses the Cite.php citation mechanism. If you would like more information on how to add references to this article, please see http://meta.wikimedia.org/wiki/Cite/Cite.php -->

Further reading

  • M. Oppolzer, T. Wahls: I Like To Move It. Flaschenzüge in der Seiltechnik. Hamburg 2019, .
  • Rescue Technician: Operational Readiness for Rescue Providers. St. Louis, Missouri 1998, .
  • Pulley System (Model and demonstration) HTML5 app by Walter Fendt
  • Sailing tackle animations, nomenclature, and table of mechanical advantages at Smack Dock Soundings 16