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).
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:
The formula used to find the effort required to raise a given weight is:
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.
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.
A tackle may be
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.
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