Just about everyone has seen a crane working at a building site, but have you noticed its attached mechanisms? (Its block tackle & pulley systems) If you look carefully you’ll notice there’s a block and tackle, an indispensible piece of equipment which can be seen in numerous places and for numerous uses.
A block and tackle is a system where several pulleys, or blocks, are joined together with ropes, belts or chains to raise or move heavy weights, the word ‘pulley’ has become synonymous with the word ‘block’.
Different block and tackle systems are used according to the weight they are required to lift, working out the tackle needed requires some degree of mathematical ability but in simple terms, for those who dislike maths, it’s all to do with what is termed ‘mechanical advantage’.
If we imagine someone drawing water from a well the mechanical advantage is the ratio of the force produced, i.e. the power to lift a full bucket of water, to the force applied to it, i.e. the effort put into the lifting which, without the pulley, would only have been enough for half a bucketful.
The various tackles with their mechanical advantages are:
- Gun Tackle = 2
- Luff Tackle = 3
- Double Tackle = 4
- Gyn Tackle = 5
- Threefold purchase = 6
The mechanical advantages are worked out mathematically using an equation containing as parameters the number of blocks there are in the tackle, and the number of ropes/cables joining the blocks. How the peculiar names of the tackles are worked out is anybody’s guess!