DIY





The Busy Shore: The Block and Tackle System

The author explains how a block and tackle system multiplies human strength and illustrates a number of uses for them.

| May/June 1973

In the matter of strength, a man's a puny creature. You cannot carry a deer in your jaw, you cannot fly two thousand miles or two feet on your own, cannot ant-like lift ten times your weight. Man's mind must multiply his trifling strength. But a race of men that has developed aerosol cheese spread leans heavily on its technology, and tends toward caterpillar tractors to move its garbage cans. A few people, watermen especially, rely more on clever thought and simple machines, and know that puny strength can be craftily applied to immense advantage. One of the oldest and simplest machines is the block and its tackle, an idea of such wonderfully spare reason that it seems magical, running uphill against the stream of nature. Its principle is just this: if one line suspends a weight, it bears all the load; if two lines suspends a weight, each bears half the load; if the lines are rendered movable, the weight can be moved with half the effort. There are a few cautions in using blocks. The sheaves must be of sufficient diameter and width for the size of the rope; too tight a turn will damage the fibre. Keep the pivot pin oiled. Figure a safely factor of five; the line you use should have a breaking strength of five times the expected load.





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