Block and tackle
Illustrated versions of Hero's Mechanica (a book on raising heavy weights) show early block and tackle systems.
An assembly of blocks with a rope threaded through the pulleys is called tackle.
[7] A block and tackle system amplifies the tension force in the rope to lift heavy loads.
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.
Thus, the block and tackle reduces the input force by the factor n. Ideal mechanical advantage correlates directly with velocity ratio.
The mechanical advantage of any tackle can be increased by interchanging the fixed and moving blocks so the rope is attached to the moving block and the rope is pulled in the direction of the lifted load.
By adding a pulley to the fixed block of a gun tackle the direction of the pulling force is reversed though the mechanical advantage remains the same, Diagram 3a.
The decision of which to use depends on pragmatic considerations for the total ergonomics of working with a particular situation.
Reeving to disadvantage adds an extra sheave to change the direction of the pulling line to a potentially more ergonomic direction, which increases friction losses without improving the velocity ratio.
Situations in which reeving to disadvantage may be more desirable include lifting from a fixed point overhead--the additional pulley allows pulling downwards instead of upwards so that the weight of the lifter can offset the weight of the load, or allows pulling sideways, enabling multiple lifters to combine effort.
The formula used to find the effort required to raise a given weight using a block and fall:
is the ideal mechanical advantage of the system (which is the same as the number of segments of line extending from the moving block), and
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.
[citation needed] Swing cheek blocks may be roughly divided into two categories:
