block and tackle

block and tackle

block and tackle: see pulley.
A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.

Overview

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. The block and tackle pulley was probably invented by Archimedes.

A Block is a set of pulleys or "sheaves" all mounted on a single axle. 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. 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 (a double tackle) will be able to lift 100 lbs with only 25 lbs of tension on the hauling part of the line. In the diagram on the right the mechanical advantage of the tackles shown is as follows:

  • Gun Tackle = 2
  • Luff Tackle = 3
  • Double Tackle = 4
  • Gyn Tackle = 5
  • Threefold purchase = 6

The formula used to find the effort required to raise a given weight is:

S * P =W +frac{nW}{10}

Where:
S is the power in the hauling part.
P is the power gained by the purchase (this is the same as the number of parts at the moving block).
n is the number of sheaves in the purchase.
W is the weight lifted.
10 is the denominator of the fraction for friction. An arbitrary 10%.

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. 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, 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. Lifting from a fixed point overhead is an obvious example of such a situation.

See also

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

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