Purchases

Using lines and blocks to provide mechanical advantage
A purchase is any rig of rope and blocks that provides a mechanical advantage. It also refers to the amount of power gained in this manner. The terms purchase and tackle are synonymous.When using a tackle to lift heavy weights or exert a strong pull, you need to know how much purchase a certain set of blocks will provide. The rule is that the power gained is equal to the number of lines leading to and from the moving block only, including a line attached to its becket, but excluding a line attached to the load.It’s a fairly simple concept,

These are the tackle configurations the coast guard might present for identification on the exams for captain’s licenses.

but many people’s minds refuse to grapple with it; perhaps it’s more easily understood if you have a tackle in front of you—say, a single block at one end and a double block at the other.Another way to explain it is to say that the mechanical advantage of a purchase equals the number of parts pulling on the load. In this case, a part means a line leading into or away from the sheave of the moving block or attached to the block’s shell. From that, it follows that you get a greater advantage when the part on which you haul is pulling on the weight (i.e., leading from the moving block). Such a tackle is said to be rove to advantage. The emphasis here is all on the moving block for the simple reason that a fixed block adds no power—it merely changes the direction of pull.Incidentally, if you apply one tackle to the hauling part of another, the total power gained is the power of the first multiplied by the power of the second. Thus, a two-part purchase applied to the tail of a four-part purchase results in an eight-part purchase, not a six-part purchase.There is friction at every sheave the line passes over, of course, so a 3:1 mechanical advantage, for example, as calculated by the previous rule, will actually be a little less. Traditionally, friction is reckoned at one tenth of the load for every sheave. For modern blocks, with improved bearings, that is undoubtedly an excessive figure, but it does automatically provide a margin of safety in your load calculations.See also Block and Tackle; Handy-Billy.

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