Principles of lifting: The science of cranes

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The crane is an incredible monument to the inventiveness of the human mind. Finding the human body unable to lift and move heavy objects, our inquiring instinct means that we’ve been able to think up ways to use sound physical properties to make jobs easier.

The crane has been developed down the centuries to the finely engineered machine we see today, but all obey the same basic principles of levers, pulleys and balance.

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According to designers, there are simple principles at play when a crane is designed, manufactured and put to use. They seem obvious to the layman, but they require skill and forward thought to ensure that the machine operates safely.

Firstly, the machine must be able to life the weight of the load. Secondly, the weight of the load must not make the machine topple over.  For these to be observed, especially when a simple-looking crane needs to move a heavy load over a particularly long distance, principles of leverage, pulleys and hydraulics may have to be combined to ensure the structure works as designed.

A lever is one of the simplest ideas to make work easier. A beam over a fulcrum means that a lighter force needs to be applied to move a heavy object. This can be seen in use on cranes where a downward force as the short end of a beam over a fulcrum can lift the load a significant distance.

To make the load easier to lift once the lever has done its job, pulleys can be employed. As anybody who has studied simple physics at school will tell you, multiple pulleys mean that very heavy loads can be lifted with remarkably little force, the pay-off being that significant lengths of cable running through these pulleys will be required. The more pulleys, the less force required, but the more cable is required even for a short lift.

Principles of hydraulics can be combined with these to make the lift easier. With different cylinder sizes, relatively small forces can be multiplied into a strong lifting force, once again with the pay-off that the strong force will be over a smaller distance.

In all cases energy is essentially conserved (although some is lost to the system through noise, heat and friction), so what you put in is what you get out.

These are the most basic principles that apply to anything that is lifted. Be it picking a book up from a table, or lowering a bridge into place it’s all the same. Only on a much grander scale, obviously.

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