A block and tackle^[1] is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. A pulley (also called a sheave or block) is a Wheel with a groove between two Flanges around its Circumference A rope is a length of Fibers twisted or Braided together to improve strength for pulling and Connecting. A cable is one or more Wires or Optical fibers bound together typically in a common protective jacket or sheath

Overview

This block and tackle on a davit of the Mercator is used to help lower a boat. For the name of a person see David. A davit (dāv'ĭt dā'vĭt is a structure usually made of steel which is used to lower things over an edge of a long

Seamen aboard the now-defunct USNS Southern Cross freighter rigged this block and tackle to make heavy lifts during cargo operations.

Although used in many situations, they are especially common on boats and sailing ships, where motorized aids are usually not available, and the task must be performed manually. A boat is a Watercraft of modest size designed to float or plane on water and provide transport over it Sailing ship is now used to refer to any large Wind -powered Vessel. The block and tackle pulley was invented by Archimedes. Archimedes of Syracuse ( Greek:) ( c. 287 BC – c 212 BC was a Greek mathematician, Physicist, Engineer

A Block is a set of pulleys or "sheaves" all mounted on a single axle. A pulley (also called a sheave or block) is a Wheel with a groove between two Flanges around its Circumference An axle is a central shaft for a rotating Wheel or Gear. In some cases the axle may be fixed in position with a bearing or Bushing When rope or line is run through a block or a series of blocks the whole assembly is called a Tackle. It usually is a compound machine.

The most common arrangement of block and tackle is to have a block attached to a fixed position (the fixed or standing block), and another block left to move with the load being pulled or lifted (The moving block).

Mechanical advantage

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 block, or the number of supporting ropes. In Physics and Engineering, mechanical advantage (MA is the factor by which a mechanism multiplies the force put into it For example, take a block and tackle with 2 sheaves on both the moving block and the fixed block. If one compares the blocks, one will see one block will have 4 lines running through its sheaves. The other will have 4 lines running through its sheaves (including the part of the line being pulled or hauled), with a 5th line attached to a secure point on the block. If the hauling part is coming out of the fixed block, the block and tackle will have a mechanical advantage of 4. If the tackle is reversed, so that the hauling part is coming from the moving block, the mechanical advantage is now 5.

The mechanical advantage of a tackle is relevant, because it dictates how much easier it is to haul or lift your load. A tackle with a mechanical advantage of 4 will be able to lift 100 lbs with only 25 lbs of tension on the hauling part of the line.

Mechanical advantage correlates directly with velocity ratio. The velocity ratio of a tackle refers to the relative velocities of the hauling line to 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 meter per second, 4 meters of line per second must be pulled from the hauling part of the rope.

Friction

The increased force produced by a tackle is offset by both the increased length of rope needed and the friction in the system. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e 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. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International 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^[2], or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well.

Rigging methods

A tackle may be

• "Rigged 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.
• "Rigged 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.

While rigging to advantage is obviously the most efficient use of equipment and resources, there are several reasons why rigging to disadvantage may be more desirable. The decision of which to use depends on pragmatic considerations for the total ergonomics of working with a particular situation. Ergonomics is the Scientific discipline concerned with Designing according to the human needs and the profession that applies theory principles data and methods Lifting from a fixed point overhead is an obvious example of such a situation.

See also

• Block
• Crane
• Deadeye
• Tripod
• Two six heave
• Winch

Notes

References

• Rescue Technician: Operational Readiness for Rescue Providers, edited by Claire Merrick et al. , published by Mosby, Inc. , St. Louis, Mo. , 1998, copyright held by Maryland Fire and Rescue Institute. ISBN 0-8151-8390-9 See Chapters 4 and 5, p. 41 and ff.

External links

• Fendt, Walter (March 1998). Pulley System (Model and demonstration) (JAVA applet). Retrieved on 2007-10-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony