Monday, March 10

10E2337: Block And Tackle

 Archimedes’ work with the principle of levers caused him to announce, “Give me but one firm spot on which to stand, and I will move the earth.” [The Oxford Dictionary of Quotations, Second Edition, Oxford University Press, London, 1953, p. 14.] More technically he said in On the Equilibrium of Planes, "Magnitudes are in equilibrium at distances reciprocally proportional to their weights." Quite.

Archimedes understood the concept of mechanical advantage, that is the ratio of output and input forces. Think of a simple bucket on a pulley over a well. That rope and pulley (whether pulling up or down) gives a mechanical advantage of only 1 as the force applied must be equal to the weight of the object. 

But with a fixed and movable pulley system (ie a block and tackle arrangement) there are multiple rope lengths (tackle) at work so the mechanical advantage in increased. Yes, this is mechanical engineering 101.

Block and tackle being used to lift post into place - via Knobb Hill Joinery

In a block and tackle system the pulleys rotate inside the blocks threaded with tackle, the rope that moves through the pulleys. Traditionally the blocks are made of wood, and are damned attractive if I may say so, in that rustic chic way... Combining several pulleys inside one block increases the leverage, or mechanical advantage but also causes friction on the rope. For most calculations (into which we will not delve here) we ignore the friction created from turning the pulleys, and ignore the weight of the blocks and tackle. Though as the number of sheaves (pulleys) in the block increase, increasing your mechanical advantage, friction does increase, so the direct advantage is not all gained.
Woodblock print of sailors hauling in, from Iron Men and Wooden Ships.

It is generally accepted that block and tackle arrangements were created for nautical uses (to tighten rigging, lift sails, move ships) but they are also common on the farm to move hay up to the top floor of the barn. The gable end of a barn with its overhang will often feature a block and tackle tied to the top beam.

At our old barn in Weston, VT the dads would sometimes attempt (after some liquid encouragement) to haul themselves up by the block and tackle, putting their foot through a ring on the bottom block and pulling on the rope. Usually the first pull resulted in their raised foot swinging out wide, and they would literally pull themselves off their feet and crash down to earth. Didn’t seem to stop the next man trying. Highly entertaining for the young crowd...

 This seaside structure is using the block and tackle to move lobster pots. Photo via Katie Hutchison

The objects themselves are things of beauty; rugged, and a visual cue to centuries of work done. They allow one to imagine what could be done with say 3 or 4 times your leveraged power. What would you do?

Nice vintagey examples go for big money; $200 via